CHAPTER 2: Fission Track Thermochronometry 17m.rezaeian/thesis/thermochronometry.pdf ·...

CHAPTER 2: Fission Track Thermochronometry 17

2-1 Motivation

The juxtaposition of rocks of different ages (see Fig. 1.7) suggests that deformation within the Alborz

Mountains has occurred along a series of longitudinal thrust faults, transporting rocks that were once deep

within the Earth up to the surface. However, the timing of this deformation, and the total amount of rock

exhumation can not be fully constrained from geological outcrop relations alone. A reconstruction of the

history of mountain building in the Alborz region requires the acquisition of other records of past conditions.

The exhumation of rocks results in their cooling from elevated temperatures at depth, and in the production

and deposition of sediments. These processes offer two independent windows on past conditions within the

mountain belt, and specifically on the patterns, rates and styles of erosion, which are, in turn, intimately

linked with the mountain building processes that operate at depth.

The cooling of rocks is recorded in selected minerals within the rocks mass, some of which are

specifically sensitive to temperature changes that are likely to occur within the top 1.5-6 km of the Earth’s

crust. Thermochronological techniques use this property to reconstruct the motion of mountain building rocks

towards the surface. These techniques can help to identify episodes of rapid exhumation associated with

faulting and erosion. They can also constrain the inception and timing of offset on major individual faults

within the mountain belt.

The erosional exhumation of rocks drives a sediment flux from zones of mountain building to

adjacent depositional basins. The magnitude of this flux, the physical properties of the sediment particles, and

the patterns and environments of their deposition all contain information about the erosion of the mountain

hinterland, and the accumulation of sediment permits the tracking of source area conditions over time.

Combined, these two approaches should yield a robust insight into the history of deformation,

mountain building and erosion of the Alborz Mountains. To date, there has not been an effort to reconstruct

this history on the scale of the entire mountain belt, although some individual studies have focused on smaller

areas. In the absence of a comprehensive inventory of cooling, exhumation, erosion and deposition of rocks

in the Alborz Mountains, it is difficult to identify key points such as the onset of mountain building, and to

determine the role of the mountain belt in accommodating the convergence between Arabia and Eurasia over

geological time. Chapters 2 and 3 of this thesis aim to start this effort.

This chapter is devoted to establishing thermochronological constrains on the exhumational history

of the Alborz Mountains, by means of the analysis of fission tracks in the mineral apatite (AFT), and the U-

Th/He system in that mineral. To validate the cooling phases found by these means, and to gain a better

resolution of the spatial distribution of deformation within the Alborz Mountains, I have also performed some

Detrital Apatite Fission Track (DAFT) analyses at the scale of river catchments within the mountain belt. The

chapter starts with a review of (apatite) fission track analysis.

Rezaeian M., 2008, Coupled tectonics, erosion and climate in the Alborz Mountains, Iran. PhD thesis, University of Cambridge; 219 p.


CHAPTER 2: Fission Track Thermochronometry - AFT

18

2-2 Apatite Fission Track Thermochronometry (AFTT)

2-2-1 History

Improved understanding of how the Earth’s crust has behaved in response to climate-erosion and

tectonic perturbations remains a major goal for the Earth sciences. Thermochronology is central to this

process. It uses Time-temperature histories extracted from rock or mineral samples to understand how rates

of rock uplift (vertical motion of rock relative to the geoid) and exhumation (displacement of rock relative to

Earth's surface) have varied spatially and temporally in different geological settings (Reiners et al., 2005). A

widely used thermochronometric technique is Fission Track (FT) analysis because it is sensitive to changes in

comparatively low temperatures over geological time-scales, i.e., >106 yrs. This sensitivity to low

temperature, ~120 °C for apatite and ~350 °C for zircon is ideal for monitoring the part of Earth’s crust most

sensitive to climate-tectonic interactions (Gallagher et al., 1998; Tagami & O’Sullivan, 2005).

The origins of FT analysis, which is based on the radioactive decay of uranium by spontaneous

fission, can be traced back to 1961 when R.L. Fleischer, P.B. Price and R.M. Walker, working together at the

General Electric Research Laboratory at Schenectady, New York, explored the possibility of using solid-state

track detectors in nuclear research. In Price and Walker (1962a, b) the basic concept of FT analyses began

with the identification of fission tracks in natural minerals such as mica, zircon and apatite. These swaths of

damaged crystal lattice were caused by the fission of a radioactive uranium particle. The latent fission tracks

in each mineral were chemically etched and enlarged so they could be observed under an optical microscope,

a practice that continues to the present-day. Fission track dating as a radiometric dating method was born

when Price and Walker (1963) first suggested that the spontaneous fission decay of 238U could be used to

define a sample age. If the amount of parent uranium could be measured then, using the spontaneous fission

decay constant, the number of spontaneous fission tracks must be a function of time.

Throughout most of the 1960’s and 1970’s ages determined by the fission track technique were

largely interpreted as absolute dates that recorded rock or mineral formation events (as was the case for other

radiometric dating methods such as K-Ar and Rb-Sr). A number of studies led to the realization that fission

track ages were frequently lower than ages derived from other methods. Wagner (1968) reported substantial

differences between apatite and K-Ar biotite and hornblende ages and on this basis it was argued that apatite

ages represented the approximate time of cooling through ~100°C. At this stage it was clear that the

temperature stability fields for the preservation of fission tracks in different minerals must be clearly defined.

Laboratory experiments on the minerals apatite and sphene by Naeser and Faul (1969) yielded estimates of

the temperatures below which fission tracks were considered stable, ~100°C and 250°C for apatite and

sphene respectively. Above these temperatures, fission tracks were annealed. Independent studies by Wagner

(1968) reported virtually identical results for apatite, but importantly, also highlighted the need to consider

exposure time, i.e. the stability of fission tracks is governed by both temperature and temperature history.



19

The main transformation of fission track thermochronometry took place in the early 1980’s when a

research group (Gleadow, Green, Duddy, Laslett & co-workers) in Melbourne, Australia made a critical

advance by producing a quantitative model that described how Time-temperature combinations govern the

stability of fission tracks in apatite. The Melbourne trilogy, three key papers (Green et al., 1986; Laslett et al.,

1987; Duddy et al., 1988) set out the experimental basis for this new model which provided the foundation

for a major expansion of the method as for the first time, it enabled a measured age to be interpreted in terms

of the low-temperature thermal history of a sample. In the next section fission track thermochronometry is

described in more detail with emphasis on the mineral apatite.

2-2-2 Fission Tracks

Fission Track Analysis (FTA) is based upon the natural spontaneous radioactive fission decay of the

isotope 238

U that occurs in trace amounts in a range of different minerals. According to Fleischer et al. (1975)

a spontaneous fission of 238

U produces two highly charged heavy particles and releases about 200 MeV of

energy. The frequency of fission events is low, about 1 for every 2 x106 alpha-particle decay events. When

spontaneous fission of the uranium nucleus occurs two highly charged, approximately equal-sized fission

fragments move apart at 180° to each other, stripping electrons from atoms lying in their paths (Fig. 2.2.1).

The resultant damage trails in the host atomic lattice, referred to as a spontaneous fission track, accumulates

within a crystal over time at a slow but statistically constant rate as more and more 238

U particles decay. The

latent tracks are directly visible under the transmission electron microscope; the diameter can be increased

1000 fold by chemical etching, making the tracks visible under an optical microscope. The number of these

tracks, generally 10-20 µ in length, is a function of the initial uranium content of the sample and of time (e.g.,

Gallagher et al., 1998; Wagner, 1998; Carter, 1999).

Fig. 2.2.1: Cartoon representation of the ion spike explosion model and the formation of a fission

track in a mineral (Fleischer et al., 1975). (a) Crystal lattice with a 238

U in the centre (dark circle). (b)

Spontaneous fission of 238

U produces two highly charged heavy particles (black small circles). (c) The fission

particles slow down and come to rest, leaving a damage trail or fission track.

(a) (b) (c)



20

2-2-3 The Age Equation

Age determination consists of measuring the relative abundances of the daughter decay product, and the

parent isotope. A population (track density) of fission tracks observed in a natural mineral sample is the

product of natural spontaneous fission of the isotopes 238

U, 235

U and 232

Th present within a host mineral. Both

235U and

232Th have long half lives and therefore, for samples of Phanerozoic age, their contribution is

statistically insignificant. In practice all of the observed tracks in a sample can be regarded as originating

from 238

U. As the natural uranium consists of 238

U and 235

U in a known ratio, the 238

U abundance can be

determined by measuring 235

U.

Therefore, to obtain a measurement of the parent : daughter ratio the amount of parent, 238

U, is measured

by irradiating a sample with neutrons to induce fission in the uranium, recorded by an adjacent external

detector, normally muscovite. The resulting damage tracks in the detector (induced tracks) are counted.

Because the ratio of 235

U/238

U is constant (7.2527x10-3

) the proportion of 238

U can be readily estimated from

the abundance of 235

U. Low energy thermal neutrons are used to target 235

U since high energy neutrons,

although inducing fission in 238

U, would also cause fission in 235

U and 232

Th, thereby producing a spurious

result.

Using this approach the fission track age equation, proposed by Price and Walker (1963), and Naeser

(1967), has the basic form;

Ι+=

if

sd

d

gt

ρλ

ρσφλ

λ

1ln

1, (2.2.1)

where λd , σ, Ι and λ

f are constants;

λd = total decay constant for uranium (1.55125x10

-10 y

-1)

σ = thermal neutron capture cross section of 235

U (580.2x10-24

cm2)

φ = neutron fluence, n/cm-2

I = isotope abundance ratio of 235

U/238

U (7.2527x10-3

)

λf = spontaneous fission decay constant for

238U

g = geometry correction factor. For an internal crystal surface this is 4π and for an external surface, as in a

mica detector, it is 2π. Thus, for the external detector method g = 0.5 (4π2π).

ρs = Density of natural spontaneous fission tracks (daughter product)

ρi = Density of induced fission tracks (

235U) in a mica detector (a surrogate for the parent isotope).

2-2-4 System Calibration

λf in equation (2.2.2) should be a constant, but there is as yet no agreement on its value, largely because it

is technically difficult to measure. To overcome this problem Hurford and Green (1982), proposed an

alternative calibration system based on independently characterised age standards. This led to the ‘Zeta’



21

calibration method (Hurford & Green, 1983) which has become the standard approach to fission track age

determination (Hurford, 1990). Zeta replaces the factors λf, σ and I of equation (2.2.1):

][

d

stdi

sd

stdd

g

te

ρρ

ρλ

ζλ

−=

1 , (2.2.2)

The neutron fluence (Φ) is represented by the induced track density of a standard uranium glass mica

detector (ρd). Thus, the age equation (2.2.1) becomes;

[ ]d

i

sd

d

gt ρρ

ρζλ

λ 1ln

1+= , (2.2.3)

An important aspect of the Zeta calibration approach is that it also incorporates, and corrects for,

elements of method-based bias due to sample preparation, observation conditions and counting efficiency.

Consequently, each analyst first has to determine their own personal zeta calibration value against a specific

standard uranium glass, and for each mineral phase. In this study Durango, Fish Canyon Tuff (FCT), Mt

Dumetry apatites, CN5 and CN1 were used as age standard and glass dosimeter, respectively. Details of the

zeta calibration produced for this thesis are contained in Appendix 2.2.1.

2-2-5 Thermal Sensitivity of the Fission Track System

Fission tracks are semi-stable features that react to elevated temperature over time by progressive track

shortening. With elevated temperature a crystal lattice undergoes a process of self-repair known as annealing,

whereby displaced atoms and electrons are able to migrate back to more or less their original sites. This

process occurs at different rates depending upon exposure time and temperature. The net effect of this process

is to cause a progressive shortening of tracks that eventually, when temperatures are high enough, leads to

their total disappearance, i.e., the FT clock becomes reset.

Importantly, a decrease in fission track length causes a reduction in the probability of a track intersecting a

mineral surface and this lowers the measured track density resulting in a reduced or apparent age that has

little direct geological meaning. Consequently, to interpret fission track data properly it is essential to know if

the measured age reflects a true normal full length distribution or, is an apparent age as a result of track

shortening due to exposure to elevated temperatures (e.g., Gallagher et al., 1998; Wagner, 1998; Carter,

1999).

2-2-6 Annealing Characteristics of Apatite

To deduce from a measured track length distribution the thermal history of a sample, we need to

understand how Time-temperature controls the rate of annealing. This understanding has been achieved

through laboratory scale annealing experiments that have been extrapolated to geologcial timescales and the

predictions compared against geological samples with independently, well-constrained thermal histories.



22

Fission tracks in apatite are sensitive to comparatively low temperatures. It has been attempted to describe

this on a time-dependence basis through Arrhenius plots. In 1985, Green et al. proposed that track length

data, measured on confined tracks length (Laslett et al., 1982), provides an accurate record of track annealing.

In this study an Arrhenius plot based on data from a single apatite crystal (Durango) with uniform

composition, produced a near parallel plot that supported a single activation energy model for track

annealing. Subsequent studies using confined track length data to monitor fission-track annealing in

laboratory experiments (Duddy et al., 1988; Green et al., 1986; 1989; Laslett et al., 1987), and in the natural

geological environment (Gleadow & Duddy, 1981), have yielded a quantitative predictive model of fission

track annealing, based on the Durango apatite. This model can be used to predict partial annealing

temperatures (~60-110°C) to ~ ±10°C for time-scales between 106-10

8 years.

More recently annealing studies have concentrated on determining the significance of apatite composition.

Compositional variation as a significant control on track annealing rate has been highlighted by Green et al.

(1985) who demonstrated that apatites from the volcanoclastics of the Mesozoic Otway Basin in SE Australia

which all started with the same age as they were erupted, have variable fission track ages and mean track

lengths that both correlate with chlorine content. Subsequent studies have shown that there are a range of

other elemental substitutions, such as Mn, Sr, OH, Fe, and the rare earth elements, that may significantly

influence track-retention in apatite (Carlson et al., 1999; Barbarand et al., 2003a,b; Ketcham et al., 2007).

However, many of these substitutions are rare and in most cases chlorine content is the dominant control.

Annealing studies have introduced the concept of the Partial Annealing Zone (PAZ) or Partial Reset Zone

(PRZ) that is used in the interpretation of data. The PAZ refers to the temperature boundaries that define the

point above which all newly formed and fossil tracks are instantaneously annealed (in apatite typically above

110-120°C for exposure times > 106 yrs) and the point below which geological annealing is insignificant, i.e.

tracks are effectively stable (in apatite typically <50-60°C). Annealing models can only constrain temperature

changes between these two boundaries. Any fluctuations in temperature that occur above 110-120°C or below

60°C are not detected by track length shortening (e.g., Carter, 1999; Armstrong, 2005).

2-2-7 Dpar

Dpar, the arithmetic mean fission track etch figure diameter parallel to the crystallographic c-axis (in

µm) is an estimator of annealing rate of an individual apatite grain (Barbarand et al., 2003a). Dpar works as a

proxy for composition, and therefore track-retentively, because etching efficiency is a function of

composition; increased etch-pit size correlates with increased Cl-content. Donelick (1993) demonstrated a

correlation between grain composition and etch pit size. Dpar is not a proxy for Cl wt% but generally it

correlates with Cl and F wt% (Carlson et al., 1999; Donelick et al., 2005). Dpar and Cl wt% are considered

equal in their effectiveness as an indicator of apatite fission track annealing kinetics (Donelick et al., 2005).

Finding a function that relates measurable parameters such as Dpar to chemical composition is essential to

understanding the significance of fission track age and length data. For samples with a heterogeneous mix of



23

single grain ages, it is important to identify the kinetic components, so that an appropriate annealing model

can be used for thermal history reconstructions (Donelick et al., 2005; Ketcham, 2005a).

2-2-8 Track Length Measurement

A sample’s track length distribution is a key to understanding its thermal history and the nature of a

measured fission track age. A common approach is to use only horizontal, confined tracks (Fig 2.2.2) which

are exposed by etchant passing through either a fracture or cleavage (Tracks IN CLEavage or TINCLEs), or,

another track (Tracks IN Tracks or, TINTs). Although much rarer than surface tracks, the lengths of tracks

which are confined in both ends within the volume of the polished and etched crystal (confined tracks) show

their full etchable length and can be measured directly requiring no correction for missing section or

inclination. Although subject to forms of observation bias, particularly for the shorter tracks, confined track

length distributions are more reproducible than semi- or projected lengths, and importantly, contain detailed

information concerning the thermal history of a sample (e.g., Laslett et al., 1994; Donelick et al., 2005).

By measuring a good number of the confined tracks (~ 100), formed at different times, it is possible to

constrain a Time-temperature path below the annealing closure temperature of apatite which is 160 °C in high

Cl content or 100-110 °C in low Cl content (Ketcham et al., 1999; Ketcham, 2003).

Fig. 2.2.2: (a) A cartoon illustrating an etched mineral that reveals confined tracks of different

dimensions, i.e., tracks-in-cleavage (TINCLEs) or tracks-in-track (TINTs). (b) A top-view photograph of

etched spontaneous tracks on a polished internal surface of apatite crystal (after: Gleadow et al., 1986). Most

of the visible tracks are surface-intersecting tracks, which are used for age determination (Tagami & Sullivan,

2005).

2-2-9 Significance of Track Length Distributions

All new fission tracks start with a similar initial length of 10-20 µm and a diameter of 10 -3

µm

(Wagner, 1998). When etched initial track lengths are ~16µm in apatite and ~11µm in zircon, the precise

value depending upon mineral composition and etch conditions. The track length distribution of a sample

provides an insight into its thermal history and a means of discriminating between true and apparent ages.

Since the length of a fission track is primarily a function of the maximum temperature to which it has been

exposed (the duration of heating has a secondary influence), and because tracks are forming continuously,

(a)

(b)



24

individual tracks will experience and therefore relate to different portions of a sample's thermal history. Thus,

the cooling history of a grain or sample is recorded in its distribution of track lengths.

Figure 2.2.3 illustrates seven example thermal histories for sedimentary strata derived from a single

monocompositional source and their characteristics track length distribution (Armstrong, 2005). In

geologically rapidly cooled samples, the track length distribution is narrow with a mean length >14µm and

standard deviation <1.5µm (Fig. 2.2.3; path5). In such cases the level of annealing is minor and the reduction

in track density small so that measured ages approximate to the time of cooling. Slow protracted cooling from

temperatures above the PAZ gives rise to unimodal track length distributions that are negatively skewed with

shortened mean lengths (mostly between 10-13µm) and increased standard deviations (typically >2µm) (Fig.

2.2.3; path 4 and 6). Thermal history modelling is required when mean track length values are below 14 µm.

A bimodal distribution (Fig. 2.2.3; path 6) represents two populations of track lengths, clear evidence of a

two-stage history; tracks are shortened while the sample gets into the PAZ, but the longer tracks are formed

after cooling out of the PAZ (e.g., Gleadow et al.1986; Carter, 1999; Armstrong, 2005).

Fig. 2.2.3: A Cartoon illustrating burial/temperature histories and relevant track length distributions. Paths

1-3 show the track length distribution if heating is not beyond the apatite partial annealing zone. Paths 4 and

5 show the distribution when heating is greater than temperature in the base of the partial annealing zone.

Path 6 shows heating into the partial annealing zone followed by cooling out of it. Path 7 shows rapid heating

to temperatures in the partial annealing zone. Modified after Gleadow et al. (1983) (Armstrong, 2005).

2-2-10 Statistical Analysis of Single Grain Ages

A consequence of compositional controls on annealing is that a single rock sample may contain

grains with different FT age. Statistical tests are used to measure whether typical fission track analyses

comprising 20-30 single grains have a mixed age population. This is important as it governs how that data set

is treated for interpretation. If the age population is homogenous then the variation between grains in the

arithmetic sum of spontaneous fission tracks divided by the number of induced fission tracks (Ns/Ni) will

(a)



25

conform to a Poisson distribution. To test for homogeneity a chi-squared (χ2) test is carried out on the single

grain age data (Galbraith, 1981); if P(χ 2) > 5% then the sample is assumed to be homogenous. P(χ 2

) is the

probability of χ 2 for v degrees of freedom where v is the number of crystals –1 (Galbraith & Laslett, 1993).

The method for calculating a sample age has two parameters which describe the location and spread

of the single grain ages, the central age and associated error and the age dispersion, respectively (Galbraith &

Laslett, 1993). Samples with a heterogeneous mix of single grain ages produce age dispersion values >20%.

For homogeneous populations of grain ages dispersion values are <20%.

2-2-11 Graphical Representation of Mixed Fission Track Ages

If the chi-squared test and age dispersion indicate that a sample has a mixed age population, it is

useful to visualise the distribution of single grain ages. This helps to identify how grain compositional

differences are affecting the sample. The radial plot (Galbraith, 1990) is a means of assessing the distribution

of single grain ages in a population. Its strength is that it allows grains ages with variable precisions to be

compared graphically. On the plot the x and y coordinates of each datum, j, are given as:

jjx σ1= jCjj AAy σ)( −= , (2.2.4)

where σj is the standard error on the grain age Aj, and AC is the central age.

In the radial plot all data have a common normalized error and a straight line drawn from the origin

to any data point will have a gradient that is proportional to the age of that data point, which can be read by

projecting the line onto the z (radial) axis. Similarly, the 2σ error on a grain age is given by drawing two

straight lines from the origin to the z-axis through the 2σ error bar on the data point, which, rather than

drawing on each data point, is represented by the truncated y-axis. This means that the more precise data plot

further from the origin, allowing visual evaluation of the dataset; in a mixed age-population, age components

will tend to fall on separate straight lines (Fig. 2.2.4).

Fig. 2.2.4: Radial plots for two samples from the Alborz. FIR2 with low chi-squared and high age

dispersion values displays a mixed age population. Conversely, FIR5 with high chi-squared and low age

dispersion values displays a single age population.



26

2-2-12 Methodology and Concepts

We have seen that fission tracks in apatite crystals accumulate continuously over geological time, at a

rate dependent only upon the concentration of uranium (Ketcham et al., 2000) and that they fade mainly as a

function of time and temperature. Theoretically, the term closure temperature has been used for a mineral-

isotopic system to represent the threshold temperature range above which the radiogenic daughter product is

lost and below which the system is "blocked" against thermal disturbance and all radiogenic products are

retained. Given the fact that, the earlier formed fission tracks tend to be shorter than later formed tracks and

Cl rich apatites tend to be annealed at higher temperature than F-rich grains, the distribution of fission track

lengths, observed in a sample, represents a summation of all of the tracks formed and annealed in grains with

different composition during their residence below the total annealing temperature (Ketcham et al., 2000;

Armstrong, 2005).

In view of these complications, and to determine an AFT age and its significance (Donelick et al.,

2005) four parameters are measured:

- Spontaneous fission track density

- Induced fission track density to constrain relative uranium concentrations

- Horizontal, confined fission track lengths

- Fission track annealing kinetic parameters (Dpar or Cl wt% - weight percent)

In this study, these parameters have been measured with the exception of Cl wt% but Dpar as a

proxy, enabling estimation of cooling age and thermal history for rock samples with help of an annealing

model.

2-2-13 Modelling Approach

The geological significance of fission track ages strongly depends on the thermal history of the

samples (Wagner, 1981). Exploration of the thermal history of a sample is based on a modelling programme,

with measured FT ages, kinetic parameters and track length data as input. The modelling procedure used in

this study employs annealing models that aim to describe the quantitative relationship between the FT age

kinetic parameters, track length parameters, temperature and time. In general, annealing models are used to

predict fission track ages for predefined track length distributions, or track length statistics for randomly

generated thermal histories.

Modelling starts with predefined Time-temperature points based on available geological constrains;

for example, the depositional age of a sampled sedimentary rock. These Time-temperature points, joined by

linear tie lines, provide starting points for a random exploration of Time-temperature space. Each randomly

generated Time-temperature path predicts a set of FT ages and lengths that can be compared against the

measured data. A goodness of fit (GOF) criterion is used to asses how well predicted thermal histories match

the observed data. For example, in HeFTy, a misfit (merit) function is used for this purpose (e.g., Willett,



27

1997; Ketcham et al., 2000; Ketcham, 2005b). Based on statistical tests to measure the difference between

the predicted and observed data, this approach applies weighting factors to each data type (e.g., track-counts,

Dpar measurements, age, mean track length) to assess the overall fit. In HeFTy, I used Dpar values to tailor

the annealing model to sample compositions. For fully reset samples, the initiating time and temperature of

an analysis may be defined with help of a thermochronometer sensitive to higher temperatures, or, if this is

unavailable, set to a time greater than 50% of the AFT age and a temperature around 200 ºC. This ensures

that no fission tracks are already present prior to exhumation of the sample (Ketcham et al., 2000). Model

results are normally displayed in a time-temperature plot, where the goodness of fit of a particular model

cooling history to the data is indicated with a colour code.

2-2-14 Sample Preparation and Methodology

Apatite fission track age determinations were carried out at the University College London (UCL)

Thermochronometry laboratory, under supervision of Andrew Carter.

Apatite separation was done with standard heavy mineral separation techniques using magnetic and heavy

liquid separation; the separated crystals were embedded in epoxy resin on glass slides, ground and polished.

Spontaneous fission tracks were revealed by etching with 5 M HNO2 for 20 seconds at 20±1°C. The external

detector method (Gleadow, 1981), with low uranium muscovite sheets as detectors, was used to monitor

induced fission tracks.

The samples were irradiated at the HIFAR nuclear reactor facility in Australia. Dosimeter glasses

CN-5, containing 12 ppm of natural uranium (Hurford & Green, 1983), were used to determine the neutron

fluence. After irradiation the induced fission tracks in the mica detectors were etched by 40 % HF for 30

minutes at 20±1°C. Fission track counting, length measurements of horizontal confined tracks, and Dpar

values were determined with a Zeiss Axioplan microscope, equipped with a digitising tablet and computer-

driven stage with 1250x magnification using a dry objective. The fission track ages were calculated using the

ζ age calibration method (Hurford & Green, 1983) with a ζ value of 338±8. The Fish Canyon Tuff and

Durango age standards were used for the determination of the personal ζ value.

Modelling of the low-temperature thermal history, based on the apparent fission track ages , Dpar and

the confined track lengths data, was carried out using version 2 of the HeFTy Beta modelling programme

(Ketcham, 2005b). Using kinematic data and dealing with multiple kinetic populations, this program

constrains the thermal history of a sample in different ways. The two available methods for comparing the

fission track length distribution to the track length distribution predicted by the model are the Kolmogorov-

Smirnov test (or K-S Test) and Kuiper’s Statistic (Ketcham, 2005b).

Isoplot 3.00, a geochronological toolkit for Microsoft Excel (Ludwig, 2003) was utilized in order to

illustrate the Cumulative Gaussian probability distribution of refined apatite fission track ages. In this method

the probability distributions of data and associated errors are summed. In addition isoplot 3.00 includes a

partial implementation (Gaussian distributions only) of the Sambridge and Compston method (1994) for



28

0

2

4

6

8

10

12

14

16

18

20

Nu

mb

er

of

sa

mp

les

Neogene Paleogene Mesozoic Paleozoic Precambrian

deconvolving a suite of ages. The procedure uses an approach known as mixture modelling, in order to

estimate the most likely ages, proportions and number of distinct components in a given data set. Particular

attention is paid to determining errors in the estimated ages and proportions (Sambridge & Compston, 1994).

I used the BINOMFIT package (Brandon, 2002) to estimate age components in mixed, over-

dispersed fission track grain age populations in un-reset samples of Quaternary and Neogene age. The

package employs the binomial “peak-fitting” method of Galbraith and Green (1990) for decomposing FT

grain ages. The uncertainty on the peak age is given at 68% and 95% confidence intervals. BINOMFIT also

provides an iterative search of peak ages needed to qualify exhumation rates applying lag time.

2-2-15 Apatite Fission Track Thermochronometry Applied to the Alborz Mountains

2-2-15-1 Sampling Strategy

No previous work on apatite fission tracks in rock from the Alborz Mountains has been published.

Therefore, this study has used a sampling strategy designed to obtain a broad overview of the cooling history

of the mountain belt. AFT ages of the surface rocks across the Alborz Mountains were determined in order to

resolve the broad, spatial pattern and timing of major episodes of exhumation.

As a first step, I have collected 150 rock samples along 8 transects across the mountain belt, located along the

major transverse valleys draining the north flank of the Alborz to the South Caspian Basin and south flank to

Central Iran. These transects are named (from E to W) Gorgan, Firuzkuh, Haraz, Tehran, Chalus, Taleqan,

Shah Rud and Sefid Rud. Figure 2.2.5 displays the location of transects. The samples were selected based on

rock type and stratigraphy, targeting apatite bearing formations. Where possible, they were collected from the

valley floors, at ~ 5km intervals along each section. Across some of the main thrust faults, rock samples were

collected at smaller intervals. Main faults bracketed by samples include the Alamut, Astaneh, Baijan, Bashm,

Garmsar, Kabateh, Kandavan, Khazar, Kojour, Manjil, Mosha, North Alborz, North Qazvin, North Tehran,

Pishva, Qasr-e-Firuzeh, Rudbar, Shah Rud, Taleqan, Takieh, Talesh and Tarom faults.

From this sample bank, 48 samples were selected for analysis. These samples cover wide range of rock types

from fine-grained sandstone to crystalline intrusive, and stratigraphic ages from Precambrian to Neogene

(Fig. 2.2.6). The majority of samples are from Mesozoic formations predating presumed shortening and

mountain building in the region, but a spread of stratigraphic ages is required in order to differentiate between

post-emplacement crystallisation and exhumational cooling.

Fig. 2.2.6: Distribution of the stratigraphic ages

for 48 AFT samples from the Alborz Mountains.


29

Fig

. 2.2

.5:

Loca

tion m

ap f

or

the

48 A

FT

sam

ple

s over

laid

wit

h g

ener

alis

ed t

ecto

no-s

trat

igra

phic

map

of

the

Alb

orz

(A

fter

All

en e

t al.

, 2003);

som

e

geo

logic

al f

eatu

res

wer

e ad

ded

fro

m B

erber

ian e

t al.

(1985,

1996),

Ber

ber

ian &

Yea

ts (

2001),

Gues

t et

al.

(2006b),

and N

azar

i (2

006).



30

2-2-15-2 AFT ages

48 measured AFT ages range from 157±24 to 10±1 Ma (Tables 2.2.1, 2.2.2 & 2.2.3; Appendix 2.2.2),

as constrained by 877 dated single grain. The data are displayed in radial plots in Figure 2.2.7 for all samples

except for riverine Quaternary samples.

Fig. 2.2.7: Radial plots of AFT ages for Neogene and pre-Neogene samples in the Alborz (alphabetic order).



31



32



33



34



35


36

Tab

le 2

.2.1

: A

FT

ages

on f

ull

y-

and p

arti

ally

-res

et s

ample

s of

pre

-Neo

gen

e.

Sam

ple

n.

N. grain

ρ ρρρ

d

Nd

ρ ρρρs

Ns

ρ ρρρi

Ni

P(χ χχχ

2)

Re%

C

en

tral

Age

Geolo

gic

al

Age

CH

A1

20

1.0

4E

+06

5785

1.9

9E

+05

235

1.7

8E

+06

2016

35.5

6

8.7

20.5

±1.5

Ju

rass

ic

CH

A2

24

1.0

4E

+06

5766

1.7

2E

+05

148

8.5

5E

+05

716

72.0

5

11.4

35.8

±3.5

Ju

rass

ic

CH

A3

16

1.0

4E

+06

5766

1.3

8E

+05

109

1.1

4E

+06

865

1.7

9

38.8

21.7

±3.4

P

reca

mbri

an

CH

A4

26

1.2

6E

+06

8731

2.3

4E

+05

229

1.8

4E

+06

1749

0.0

8

39.7

26.6

±3

Jura

ssic

CH

A5

12

1.2

6E

+06

8731

4.2

5E

+04

17

5.8

5E

+05

241

87.7

0

15±

3.8

Ju

rass

ic

CH

A6

20

1.0

4E

+06

5785

1.5

0E

+05

164

1.5

1E

+06

1658

71.6

3

0

17.4

±1.4

C

ambri

an

CH

A7

20

1.0

4E

+06

5785

2.8

1E

+04

67

4.0

7E

+05

903

29.8

17.5

13.1

±1.7

E

o-O

ligoce

ne

CH

A8

15

1.2

6E

+06

8731

6.9

0E

+04

20

6.3

0E

+05

212

90.3

9

7.5

15.2

±9.1

E

oce

ne

FIR

1

13

1.2

6E

+06

8731

2.2

4E

+05

46

7.1

4E

+05

149

87.2

8

0

42.4

±23.3

C

reta

ceous

FIR

2

40

1.0

4E

+06

5766

3.0

0E

+05

601

4.2

4E

+05

805

0

44

130.3

±12

Jura

ssic

FIR

3

32

1.0

4E

+06

5785

4.3

6E

+05

366

2.4

3E

+06

2532

0

78.8

26.4

±4.2

Ju

rass

ic

FIR

4

10

1.0

4E

+06

5766

1.2

8E

+05

26

1.1

2E

+06

256

64.3

2.8

17.8

±3.7

C

ambri

an

FIR

5

16

1.0

4E

+06

5766

4.7

2E

+05

246

2.8

1E

+06

1422

91.4

9

0

30.3

±2.1

C

ambri

an

FIR

6

9

1.0

4E

+06

5766

1.3

3E

+05

43

9.7

4E

+05

315

69.4

7

0.1

23.9

±3.9

P

erm

ian

FIR

7

19

1.0

4E

+06

5766

2.1

6E

+05

112

2.0

2E

+06

1054

13.8

9

27

18.4

±2.2

Ju

rass

ic

HA

R1

8

1.2

6E

+06

8731

8.6

2E

+04

32

9.2

5E

+05

361

0.3

5

47.8

35±

10.2

Ju

rass

ic

HA

R2

17

1.0

4E

+06

5766

1.3

1E

+05

56

1.5

8E

+06

681

20.3

2

4.4

14.5

±2

Jura

ssic

HA

R3

8

1.2

6E

+06

8731

4.4

6E

+05

62

1.7

3E

+06

247

61.4

3

0

53.2

±7.6

Ju

rass

ic

HA

R4

11

1.0

4E

+06

5766

1.2

8E

+05

32

1.3

4E

+06

356

68.7

1.2

15.8

±2.9

Ju

rass

ic

HA

R5

17

1.2

6E

+06

8731

1.4

7E

+05

107

1.1

0E

+06

798

58.4

8

33

24.7

±6

Jura

ssic

SE

F1

20

1.1

3E

+06

6275

1.5

4E

+05

172

1.0

6E

+06

1094

6.0

7

54.8

35.9

±8.9

Ju

rass

ic

SE

F2

8

1.2

6E

+06

8731

5.4

8E

+04

15

5.5

9E

+05

163

82.5

3

0

19.6

±5.3

Ju

rass

ic

SE

F3

21

1.2

6E

+06

8731

8.9

1E

+04

75

6.9

4E

+05

633

75.7

3

0

28.8

±5.3

Ju

rass

ic

SE

F4

31

1.1

3E

+06

6275

2.3

7E

+05

301

1.4

2E

+06

1836

59.4

7

4.7

31.3

±2.0

E

o-O

ligoce

ne

SE

F5

23

1.1

3E

+06

6275

4.7

9E

+05

595

2.1

9E

+06

4454

37.5

2

15.3

41±

3.6

E

o-O

ligoce

ne

SE

F6

31

1.1

3E

+06

6275

1.8

8E

+05

443

9.0

3E

+05

2207

3.1

9

19.9

38.8

±2.5

E

o-O

ligoce

ne

SE

F7

18

1.1

3E

+06

6275

2.0

5E

+05

154

9.9

7E

+05

746

67.3

5

1.5

39.4

±3.5

E

o-O

ligoce

ne

SH

A1

20

1.0

4E

+06

5785

3.6

9E

+04

62

6.2

9E

+05

1084

37.7

7

3.4

10.1

±1.3

P

reca

mbri

an

SH

A2

20

1.0

4E

+06

5766

1.1

3E

+05

143

1.2

4E

+06

1516

66.9

1

0.8

16.6

±1.5

E

o-O

ligoce

ne

SH

A3

20

1.0

4E

+06

5785

1.5

1E

+05

126

7.9

0E

+05

713

7.8

7

25

31.7

±3.6

E

o-O

ligoce

ne

SH

A4

23

1.2

6E

+06

8731

6.6

9E

+04

102

4.2

9E

+05

665

40.6

5

6

32.6

±3.5

Ju

rass

ic

SH

A5

5

1.0

4E

+06

5785

1.1

3E

+05

27

4.6

6E

+05

121

72.7

0

39.2

±8.4

E

o-O

ligoce

ne


37

Sam

ple

n.

N. grain

ρ ρρρ

d

Nd

ρ ρρρs

Ns

ρ ρρρi

Ni

P(χ χχχ

2)

Re%

C

en

tral

Age

Geolo

gic

al

Age

TA

L1

13

1.2

6E

+06

8731

1.5

5E

+05

85

1.8

6E

+06

988

19.4

28.1

19.9

±6.9

C

ambri

an

TA

L2

21

1.2

6E

+06

8731

3.0

4E

+04

49

3.5

5E

+05

620

100

0

16.5

±3.3

E

o-O

ligoce

ne

TA

L3

7

1.0

4E

+06

5766

2.6

6E

+05

42

2.8

1E

+06

450

93.0

5

0

16.4

±2.7

Ju

rass

ic

TE

H3

14

1.0

4E

+06

5766

2.1

1E

+05

77

4.6

2E

+05

170

50.3

9

1.8

79.4

±10.9

C

reta

ceous

TE

H4

16

1.2

6E

+06

8731

1.9

8E

+06

717

2.3

7E

+06

887

35.8

6

26.2

157.4

±24.4

Ju

rass

ic

TE

H5

13

1.2

6E

+06

8731

1.8

8E

+05

43

2.1

9E

+06

493

90.0

3

0

18.5

±3

Jura

ssic

TE

H6

3

1.0

4E

+06

5785

1.9

0E

+05

11

1.1

9E

+06

65

82.7

8

0

29.8

±9.7

E

o-O

ligoce

ne

TE

H7

27

1.2

6E

+06

8731

1.0

9E

+05

114

6.5

6E

+05

690

89.3

1

0.3

35.1

±3.6

P

aleo

cene

TE

H8

8

1.4

7E

+06

8167

2.7

6E

+05

74

2.1

7E

+06

585

34.4

3

14.4

31.8

±4.3

E

oce

ne

TE

H8

7

1.0

4E

+06

5785

2.3

7E

+05

44

1.3

6E

+06

240

37.4

5

1.1

32.2

±5.3

E

oce

ne

Note

: ρ

d, ρ

s an

d ρ

i rep

rese

nt

the

dosi

met

er,

sam

ple

sponta

neo

us

and i

nduce

d t

rack

den

siti

es;

P(χ

2)

is

the

pro

bab

ilit

y o

f χ

2 f

or

v d

egre

es o

f fr

eedom

wher

e v =

no. of

cryst

als

–1;

all

ages

are

cen

tral

ages

(C

entr

al a

ge

calc

ula

tion f

rom

Gal

bra

ith &

Las

lett

, 1993).

Tab

le 2

.2.2

: A

FT

ages

of

unre

set

Neo

gen

e sa

mple

s.

Sam

ple

n.

N. grain

ρ ρρρ

d

Nd

ρ ρρρs

Ns

ρ ρρρi

Ni

P(χ χχχ

2)

Re%

C

en

tral

Age

GO

R1

19

1.0

4E

+06

5766

5.5

4E

+05

345

1.3

57E

+06

876

0

47.9

66.6

±8.9

GO

R2

28

1.1

5E

+06

7934

2.1

5E

+05

191

6.7

9E

+05

738

100

63.2

70.2

±17.5

HA

R5

17

1.2

6E

+06

8731

1.4

7E

+05

107

1.1

0E

+06

798

58.4

8

33

24.7

±6

TE

H1

14

1.0

4E

+06

5766

1.8

7E

+05

132

1.2

1E

+06

848

0

69.4

29.4

±6.4

TE

H2

4

1.0

4E

+06

5785

1.0

8E

+05

14

8.8

8E

+05

115

72.1

6

0

21.4

±6.1

Note

: ρ

d, ρ

s an

d ρ

i rep

rese

nt

the

dosi

met

er, sa

mple

sponta

neo

us

and i

nduce

d t

rack

den

siti

es; P

(χ 2

) i

s th

e pro

bab

ilit

y o

f χ

2 f

or

v d

egre

es o

f fr

eedom

wher

e v =

no. of

cryst

als

–1;

all

ages

are

cen

tral

ages

(C

entr

al a

ge

calc

ula

tion f

rom

Gal

bra

ith &

Las

lett

, 1993).

Tab

le 2

.2.3

: A

FT

ages

of

unre

set

Quat

ernar

y s

ample

s.

Sam

ple

n.

N. grain

ρ ρρρ

d

Nd

ρ ρρρs

Ns

ρ ρρρi

Ni

P(χ χχχ

2)

Re%

C

en

tral

Age

AL

A1

36

1.0

4E

+06

5766

2.4

9E

+05

363

2.5

6E

+06

3822

34.2

5

11.6

16.8

±1

AL

A2

11

1.2

6E

+06

5785

1.1

0E

+05

35

1.0

6E

+06

513

0.0

2

75.6

17.1

±5.3

MO

R

41

1.0

4E

+06

5785

1.8

6E

+05

451

1.2

9E

+06

3145

0

51

26.2

±2.6

Note

: ρ

d, ρ

s an

d ρ

i rep

rese

nt

the

dosi

met

er, sa

mple

sponta

neo

us

and i

nduce

d t

rack

den

siti

es; P

(χ 2

) i

s th

e pro

bab

ilit

y o

f χ

2 f

or

v d

egre

es o

f fr

eedom

wher

e v =

no. of

cryst

als

–1;

all

ages

are

cen

tral

ages

(C

entr

al a

ge

calc

ula

tion f

rom

Gal

bra

ith &

Las

lett

, 1993).



38

The population is divided in two groups:

1) fully or partially reset samples from pre-Neogene formations, and 2) un-reset samples from Neogene-

Quaternary formations. A total of 40 samples were reset, with AFT age < stratigraphic age. The majority of

ages measured in these samples are between 10 Ma and 40 Ma, only 6 samples yielding an older age.

Four partially-reset samples of TEH3, TEH4, FIR1, and FIR2 demonstrate an over dispersion in single grain

age in radial plots; reflects in low P(χ2). FIR2 with some single grain ages around or older than stratigraphic

age, demonstrating that the samples never heated beyond the PAZ (~120 ºC).

In contrast, samples with single grain ages younger than stratigraphic age reveal fully reseting, demonstrating

that the samples heated up beyond the PAZ (~120 ºC) and were then exhumed to the surface. They are

described in the follows in two groups of pre-Tertiary and Paleogene.

Pre-Tertiary samples, including seven Paleozoic-Precambrian reset samples with geological age

>~245 Ma and sixteen reset samples, come from sandstones within the Shemshak Formation (geological age

~210-160 Ma). Young AFT ages in these rocks primarily record significant heating from surface

temperatures by post-deposition burial, and subsequent cooling during the Tertiary. This thermal history

reflects the structural and erosional development of the mountain belt during the Tertiary. Paleogene samples

consist of twelve reset plutonic-volcanic rocks (geological age 65-23 Ma), which record magmatism, but also

tectonic exhumation, mainly in the Neogene.

Eight samples from young sedimentary formations (Quaternary and Neogene) have AFT age >

stratigraphic age. These samples are interpreted to be un-reset, and their AFT ages reflect the cooling of the

sediment source rocks rather than any process at the site of sampling. In reality there may have been some

minor, unresolvable annealing after deposition of the sediments (Tables 2.2.2 & 2.2.3).

2-2-15-3 Confined Track Length Analyses and Thermal History Modelling

The majority of samples analysed in this study failed to provide statistically meaningful numbers of

confined track length measurements for thermal history modelling (ideally 50-100 measured lengths are

needed). In general this was caused by samples containing few spontaneous tracks due to their low uranium

contents. In addition, some samples also yielded low quantities of apatite grains. Nevertheless, it has been

possible to measure sufficient track lengths on five samples, each with more than 70 horizontal confined

tracks and Dpar measurements, and 16-40 single grain ages. These are now considered in more detail.

Samples SEF5 (AFT age ~41) and SEF7 (AFT age ~39) are from intrusive rocks, presumed to be of

Eocene-Oligocene age (Geological Survey of Iran, 1998a). They have volcanic type unimodal track length

distributions characterised by long mean track lengths of ~14µm and small standard deviations (close to

1µm) (Fig. 2.2.8), indicating rapid, uninterrupted cooling. In these samples the measured AFT age is

expected to approximate to the time of magmatic cooling.

Sample SHA2, is also from an Eocene-Oligocene intrusive body (Geological Survey of Iran, 2002) ,

but it has a broader, negatively skewed distribution of track lengths with a modal value of ~14.5 µm and a



39

0 2 4 6 8 10 12 14 16 18

0

5

10

15

20

25SEF7

Mean length: 13.9±0.13

S.D.: 1.098

N. tracks: 71

Nu

mb

er o

f tr

ac

ks

Track length (µµµµm)

0 2 4 6 8 10 12 14 16 18

0

5

10

15

20

25

30

35

40

45


Nu

mb

er

of

tra

ck

s

SEF5


S.D.: 1.13

N. tracks: 89

0 2 4 6 8 10 12 14 16 18

0

2

4

6

8

10

12

14

16

Nu

mb

er o

f tr

ac

ks

Track length(µµµµm)

SHA2


S.D.: 2.17

N. tracks: 84

0 2 4 6 8 10 12 14 16 18

0

5

10

15

20

25

30

Nu

mb

er o

f tr

ac

ks


TEH4


S.D.: 1.55

N. tracks: 73

0 2 4 6 8 10 12 14 16 18

0

2

4

6

8

10

12

14

16

18

20

Nu

mb

er o

f tr

ac

ks


FIR2


S.D.: 2.01

N. tracks: 94

secondary peak at ~11 µm, diagnostic of a slower, more protracted cooling through the PAZ after

emplacement (e.g., Gleadow et al., 1986; Carter, 1999).

Samples FIR2 and TEH4 are from the sedimentary Shemshak Formation (Geological Survey of Iran,

1991a & b). The former has a broad, negatively skewed distribution with a mean track length of ~13.1 µm

and a modal track length of ~15 µm. The latter has a tighter distribution of track length around a low mean

value of ~11.25 µm. These samples require modelling to determine more precisely their cooling histories.

Fig. 2.2.8: Track length distribution for five samples

of the Mesozoic sedimentary and Eocene plutonic

rocks in the Alborz Mountains.

The procedure of thermal history modelling, using the HeFTy (Beta version2) package (Ketcham, 2005b),

was outlined in section 2-2-13. In this procedure, the start of the T-t path depends heavily on the stratigraphic

and annealing history of the sample. For partially-reset samples of FIR2 and TEH4, the start time was set as

the Jurassic depositional age and the start temperature as the probable average surface temperature at the time



40

of deposition (20ºC). For the Eocene plutonic samples SHA2, SEF5 and SEF7, the start time was set at

crystallization age (40±5 Ma), and 200ºC was selected as the start temperature. All samples were brought to

the surface temperature 20ºC at the end of T-t at 0 Ma.

In addition, I considered a set of possible turning points based on independent geological information

reviewed in more detail in Chapter 3 (Table 2.2.4).

Table 2.2.4: Major geological events through the Late Cretaceous and during Cenozoic in the Alborz.

Modelled T-t paths are shown in Figure 2.2.9. Statistically good cooling paths for Eocene plutonic

samples SEF5 and SEF7 indicate rapid cooling at rates of 10-25°C/My, probably coinciding with post-

emplacement crystallisation by 30±5 Ma. These samples passed through the PAZ in a short time of at most 5

My and cooled down steadily onward. SEF5 appears to have been at or near the surface shortly after

emplacement, possibly because the intrusion reached a very high level within the crust. Likely cooling paths

for SEF7 imply a two-phase cooling process, with initial rapid cooling after emplacement, and later

progressive cooling, possibly due to erosional exhumation from a depth of several km starting around 30 Ma.

Likely cooling paths for Eocene plutonic sample SHA2 indicate a slower passage through the PAZ

over a period of about 10 My, some time after the likely crystallisation age, and continued progressive

cooling at a similar rate since then and to the present day (Fig. 2.2.9). It is possible that the cooling captured

by the modelling is largely unrelated to the emplacement and crystallisation of the sampled rocks, and instead

reflects the steady erosional exhumation of these rocks from depths below the PAZ.

The two Jurassic sandstone samples FIR2 and TEH4 display thermal histories that are very different

from the Eocene intrusive rocks. The sandstones were deposited around 200 Ma, and it is likely that they

were never exposed to temperatures above 120 ºC after deposition. They are only partially reset for this

reason. The rocks were progressively buried until ~130 Ma, and resided within the PAZ over 100My. FIR2

may have cooled rapidly in the latest Cretaceous, between 80 Ma and 60 Ma, and was brought to the surface

from a depth of no more than a couple of kilometres in the Neogene. In contrast, TEH4 is likely to have

remained deeply buried until ~30 Ma, possibly even more recent, and has been cooled from ~100°C by

erosional exhumation since then.

Other samples have not yielded sufficient measurements to support a detailed analysis of confined

track lengths and thermal history modelling. However, the combined age statistics from these samples can be

used to further probe the cooling history of the Alborz Mountains. This is done in the next section.

Epoch or Stage Age (Ma) Characteristic event Evidence

Middle Miocene 16 Subaerial relief Unconformity- conglomerate

Eo-Oligocene 34 Subaerial relief Unconformity- conglomerate

Paleocene-Eocene 61-37 Transtension-magmatism Submarine volcanism

Maastrichtian 71-65 Major compression Unconformity- conglomerate

Cenomanian 99-93 Hiatus - Intrusion -



41

Fig. 2.2.9: Representative thermal histories for five samples in the Alborz. Modelled T-t paths for

SEF5, SEF7, SHA2, FIR2 and TEH4, using HeFTy (Ketcham, 2005b). Green and red lines represent good

and acceptable cooling paths, respectively. Horizontal and vertical bars display constrains on time and

temperature, respectively.

2-2-15-4 Cooling Phases and Exhumation Rate

To explore the temporal pattern of AFT ages across the Alborz Mountains, the population of fully-

reset samples has been divided into rapid and slow cooled categories, based on Confined Track Length (CTL)

statistics, Chi-squared value and Dpar-age correlation. Fast cooled samples have Chi- squared values >5%

(95% confidence interval), average CTL >13µ (within the error) and a constant function for Dpar-AFT age.

Samples that meet these criteria have captured a geologically short and distinct phase of cooling when they

resided within the PAZ near the sampled location. Samples that don’t meet these criteria have recorded more

gradual cooling through the PAZ.

The majority of samples have cooled rapidly. These 30 samples will be used in this section. A

probability density plot of their cooling ages with associated uncertainties reveals two main peaks at ~32Ma

and ~16Ma (Fig. 2.2.10) at the Eo-Oligocene transition and in the Middle Miocene, respectively. Using

Gaussian statistics to extract the age components with their weight and error, elucidates the strength of the

two major components: 57% of the data defines a cooling episode at 15.78±0.58 Ma and 28% describe a



42

0

1

2

3

0 10 20 30 40 50 60 70

AFT age (Ma)

N.

of

sa

mp

les

Re

lativ

e p

ro

ba

bility

cooling episode at 31.7±1.3 Ma. A third, minor data component of 15% is associated with a possible cooling

episode at 39.1±2.7 Ma.

Fig. 2.2.10: (a) Combined histogram and probability density function plot for 30 rapidly cooled AFT

ages using Gaussian statistics. (b) Frequency distribution of the AFT ages for pre-Tertiary samples. (c)

Frequency distribution of the AFT ages for Paleogene samples.

The geological interpretation of these age population components relies on the sample lithologies.

The oldest cooling phase of 39 Ma is captured in samples in SW Alborz, all of which have been collected

from outcrops of intrusive rocks of Middle Eocene age (Fig. 2.2.10c). The cooling histories of all these

samples are likely to be similar to those of SEF5 and SEF7, discussed in section 2-2-15-3, and their AFT ages

are likely to be closely associated with crystallisation cooling of the intrusives after their emplacement.

Widespread intrusive and extrusive magmatic activity occurred throughout Iran during the Middle Eocene,

and the intrusive rocks sampled in this study were formed mainly along the axial zone of a fault-controlled,

subsiding basin, now part of the western Alborz and south-central (Berberian & Berberian, 1981). SEF5,

TEH7 and TEH8 are among the AFT samples which have been dated using radiometric dating of U/Pb,

Ar/Ar, K/Ar, respectively. I have obtained a U/Pb date of 35±5 Ma for SEF5 in the SW Alborz, indicating

magmatic emplacement in the Middle-Late Eocene. 40Ar/39Ar dates of the Lavasan intrusive body (TEH7)

in NE Tehran and volcanic rocks of the Karaj Formation in E Tehran by P. Ballato (unpublished data-

0

1

2

3

4

5

6

7

8

9

0 10 20 30 40 50 60 70 80

AFT age (Ma)

N. o

f s

am

ple

s

Re

lativ

e p

rob

ab

ility

0

1

2

3

4

5

6

0 10 20 30 40 50 60 70 80

AFT age (Ma)

N.

of

sa

mp

les

Re

lativ

e p

ro

ba

bility

(a) (b)

(c)



43

Potsdam University) indicate magmatic emplacement at 38.47 ± 0.1 and volcanic activity at 36.02 ± 0.15Ma.

K/Ar dates on the Qasr-e-Firuzeh intrusive rocks by Davari (1987) indicate that magmatic emplacement

occurred at 41 ± 4 Ma (Middle Eocene). TEH8 has been collected from the same intrusive body.

The date of 39.1±2.7 Ma may reflect the peak of magmatic activity in this region, allowing for a time

lag associated with cooling to temperatures of the AFT PAZ. The magmatic activity is revealed through

several radiometric dates described in chapter 3 (section 3-3 & 3-4-2-1).

Some fast cooled Eocene intrusive rocks (Fig. 2.2.10c) have AFT ages younger than the 39 Ma event

(samples CHA7, SEF4, SHA2, TAL2, TEH8). These ages cluster closely around the two principal peaks in

the AFT age distribution. It is likely that these rocks were emplaced during the same period of magmatic

activity, but at depths below the PAZ. Therefore, their AFT ages are likely to record a cooling process that is

unrelated to the plutonic origin of these rocks. This process is shared with most rapidly cooled rocks of pre-

Tertiary age in the data set (Fig. 2.2.10b).

The two younger peaks in the AFT age distribution are dominated by samples from pre-Tertiary

formations (Fig. 2.2.10b). With one exception (SHA1-Precambrian granite), these are (meta) sedimentary

rocks for which sample TEH4, discussed in section 2-2-15-3, may serve as an example. That sample was

buried progressively after deposition, and exhumed rapidly from within the PAZ during the Neogene.

However, due to the limited burial depth TEH4 was not fully reset and it is not well suited to pin the time of

exhumation precisely. The CHA1, CHA5, CHA6, FIR4, FIR5, FIR6, FIR7, reset, pre-Tertiary samples and

7 of the Eocene intrusive samples together reveal that there may have been at least two distinct exhumation

pulses during the Neogene in the Alborz region. These peaks were captured primarily by AFTs in rocks that

were buried deeper than sample TEH4, and their attribution to exhumation rather than magmatic cooling is

corroborated by the lack of geological evidence for widespread magmatic activity at the Eo-Oligocene

transition and during the Middle Miocene. This will be discussed in more detail in Chapter 3.

If the AFT age peaks at ~32 Ma and ~16 Ma indeed represent (the onset of) two phases of rapid

exhumational cooling, then they can serve to constrain the range of average exhumation rates in the Alborz

region. Assuming that the composition-dependent annealing closure temperature of apatite is ~ 100-160 °C

(e.g., Ketcham et al., 1999; Ketcham, 2003), and taking a typical geothermal gradient of ~25º C/km, then the

average rate of exhumation implied by the bulk AFT data is 0.13-0.40 km/My. The range of averages applies

to the entire period since the Eocene-Oligocene transition, but it is clear that within this time there must have

been spells during which exhumation was considerably faster, and long episodes of slower unroofing. In

addition there is a spatial pattern to the cooling and exhumation of Alborz region. This is the subject of the

next section.

2-2-15-5 Spatial pattern of cooling ages

The spatial pattern of AFT ages is shown in Figure 2.2.11. In analyzing this pattern a first order distinction

has been made between samples with unreset, partially-reset, and fully-reset fission track population.


4

4

F

ig.

2.2

.11

: R

eset

(fu

lly-

or

par

tial

ly-)

and u

n-r

eset

AF

T a

ges

in t

he

Alb

orz

, dis

pla

yed

in

bla

ck,

red a

nd b

lue

colo

urs

, re

spec

tivel

y.



45

To recapitulate, un-reset samples have an AFT age older than their stratigraphic age, reset samples

have an AFT age younger than their stratigraphic age, and partially reset samples have a mix of older and

younger single grain AFT ages.

2-2-15-5-1 Un-reset and partially-reset samples

All un-reset samples are from rocks with Miocene or Pliocene stratigraphic ages, collected at the

periphery of the mountain belt. They may be used to constrain cooling in the source area of the sediments,

which was presumably located upslope of the sampling sites and in the interior of the mountain belt. No

attempts have been made to determine the precise location of these sources. Un-reset samples from

sedimentary formations may have an AFT age population with several components from different sources.

These components can be identified using the data analysis programme BINOMFIT (Brandon, 1996), based

on Galbraith and Green’s algorithm (Galbraith & Green, 1990; Galbraith & Laslett, 1993). BINOMFIT also

estimates the relative weight of all components of the age population. AFT data from all un-reset samples

has been analysed with BINOMFIT. An overview of results is given in Table 2.2.5.

sample Strat. age AFT1

(My)

AFT2

(My)

AFT3

(My)

AFT4

(My) (%) (%)

(%)

(%)

Lag Time

(My) N. grains

GOR1 Mio. (5.3-16.4) 68.9 174.3 - - 92 8 - - 63.6-52.5 19

GOR2 Mio.(5.3-16.4) 36.5 68 299 - 40.7 44.8 14.5 - 20-31.2 28

HAR5 Mio. (5.3-13.3) 13.3 31.6 - - 10.8 89.2 - - 0-8 17

TEH1+2 Plio.(1.8-5.3) 24.1 36.9 172.6 - 94.3 0.1 5.6 - 18.8-22.3 18

Table 2.2.5: The age components, age proportion, and the lag time for Neogene samples. Age components

are displayed from youngest to oldest, AFT1 to AFT4, and proportion of each component is indicated in %,

utilising BINOMFIT (Brandon, 1996). Lag Time (LT) is estimated by removing the youngest age component

from the stratigraphic age.

Un-reset samples GOR1 and GOR2 in the easternmost Alborz have Late Cretaceous bulk AFT ages

of 66.6 ± 8.9 Ma and 70.2 ± 17.5 Ma, respectively. Both samples have multiple age components. In GOR1,

92% of the data is associated with a cooling age of 68.9 Ma. GOR2 has a similar component, centered on

68.0 Ma, but it only contains 44.8% of the data. A further 40% of the data in this sample define a younger

cooling phase at 36.5 Ma. Combined, GOR1 and 2 are strong evidence of Late Cretaceous cooling in the

eastern Alborz. Partially reset sample FIR2, collected further west, has an older bulk AFT age, 130.3 ± 12

Ma, but thermal history modelling (section 2-2-15-3) has revealed that it was exhumed primarily during the

Late Cretaceous. Thus, the Late Cretaceous cooling phase may have affected a larger part of the Alborz

region, probably around 70 Ma. This is coincident with the onset of compression of the Neo-Tethyan domain

(see Chapter 1 and 3), and may have been caused by localized uplift and exhumation. If this is true, then a

proto-Alborz mountain range may have existed at the end of the Mesozoic. Younger AFT ages elsewhere in

the Alborz Mountains preclude an apatite record of this early cooling phase. Use of a thermochronometer



46

with sensitivity to higher temperatures may reveal the extent of the area affected by substantial Late

Cretaceous cooling.

Un-reset samples TEH1 and 2 were collected close to each other from the same Pliocene formation,

at the northern edge of the mountain belt. AFT age data for the samples has been combined for BINOMFIT

analysis, and a dominant age component (94.3%) centred on 24.1 Ma was found. The depositional age of the

formation is 1.8-5.3 Ma, implying that the lag time between cooling of the source rock in the PAZ and

deposition of the sampled formation (cf. Brandon & Vance, 1992; Bernet & Garver, 2005) is 18.8-22.3 My.

Un-reset sample HAR5 from the southern fringe of the mountain belt has a bulk age of 24.7 ± 6 Ma, but two

distinct age population components of 13.3 Ma and 31.6 Ma. The later is the more significant, comprising

89.2 % of the data, and implies a lag time between cooling and deposition of 18.3-26.3 My. Both age

components match closely the exhumational cooling phases identified in section 2-2-15-4.

The un-reset nature of the five samples collected from the fringes of the Alborz Mountains reflects

their limited burial depth after deposition. However, these samples were collected inside the zone of active

mountain building, implying that this zone has expanded since deposition of the sampled sediments, or the

sediments have been advected into it by sub-horizontal tectonic transport. All un-reset samples are from the

central to eastern segment of the Alborz Mountains. The lack of un-reset rocks further west may be an

artefact of the sampling strategy, or it may be real due to a sustained location of deformation and exhumation

between long-lived bounding faults.

The distribution of partially reset samples may shed some light on this. Four samples were partially

reset: FIR1 and 2, and TEH3 and 4. All are from the same area in the north east of the mountain belt that has

yielded most of the un-reset samples. However, they sit structurally well within the zone of mountain

building, close to the North Alborz and Khazar faults, and were collected from Jurassic and Cretaceous

formations. None has been buried below the apatite PAZ prior to exhumation, implying that total

exhumation along the northeast edge of the mountain belt has been less than 4-6 km since their deposition.

Indeed, thermal modelling of data from FIR2 (section 2-2-15-3) has constrained the likely Neogene

exhumation of that sample to about 2 km. FIR2 was collected in the immediate hanging wall of the North

Alborz Fault: the vertical offset on the eastern segment of that fault may not be more than a few km. The

North Alborz Fault and other faults in the NE Alborz have also not experienced major strike slip. By

contrast, the AFT data shows that the NE Alborz is likely to have undergone significantly less uplift than

other sections of the mountain belt.

Few samples from further west along the northern margin of the mountain belt have been processed,

and it is therefore unclear whether the zone of limited exhumation extends over the full length of the Alborz

Mountains. However, with the exception of HAR5, all samples from the southern margin of the Alborz have

been reset, implying that exhumation has been substantial even at the topographic edge of the mountain belt.

Best sample coverage in the south is from Tehran to the west, and it is apparent that in this area there has not

been a major outward migration of the deformation front in the recent geological past.



47

2-2-15-5-2 Reset samples

Most reset samples fall into one of three broad age categories, associated with the three cooling

phases identified in section 2-2-15-4, and there is a distinct spatial pattern to their distribution. Middle

Eocene cooling ages, associated with magmatic activity, are limited to the southwest Alborz, specifically the

Tarom Range and the mountains north of Qazvin (Fig. 2.2.5). Eocene rocks are the principal substrate of this

area, and AFT ages associated with magmatic cooling indicate that later exhumation has been limited to

(substantially) less than ~4-6 km.

A group of samples with cooling ages around the Eocene-Oligocene transition flanks the cluster of

Middle Eocene ages, implying that early exhumation of the western Alborz occurred mainly to the north and

east of the area of Eocene magmatic activity (Fig. 2.2.5). Another grouping of samples with AFT ages from

this first phase of exhumational cooling is located near Tehran, and includes the un-reset sample HAR5. This

group may be bounded to the west by the North Tehran Fault, but extends north across the Mosha Fault.

Importantly, both groups are confined mainly within areas with little topography above 2 km. Isolated

samples with ages around 32 Ma outside these two groups are all located outside the highest mountain areas.

Samples from the high mountains of the central Alborz generally have AFT ages associated with the

second, Middle Miocene phase of exhumational cooling. Between Karaj and Qazvin, this domain extends to

the southern fringe of the mountain belt. Notably, most of the youngest AFT ages were found to the south of

the Kandavan-Banan-Rudbar fault array, which also defines the boundary between domains dominated by

Tertiary and pre-Tertiary rocks, respectively. This array may be a long-lived structure that has controlled the

region’s kinematics throughout the Tertiary.

2-2-15-5-3 Transects

Some details of the AFT age pattern are best viewed along sections across the mountain belt.

Samples were picked along 8 N-S sections. These sections are represented in Figure 2.2.12, including mean,

minimum and maximum topographic elevation along a swath, location of major faults, position of the

sampling sites, and AFT and stratigraphic age of the sample. Salient details are reviewed from east to west.

The Gorgan and Firuzkuh sections in the eastern Alborz show a domain of un-reset and partially reset

rocks with old cooling ages that extends into the hanging wall of the North Alborz Fault, but appears to

terminate abruptly between sites FIR2 and FIR3. Ten km separates these two sites, and it is likely that a

structure with a large amount of dip slip is located in between. Further south, AFT ages are uniformly young,

where data are available.

At the northern end of the Haraz section, HAR1 is fully-reset, but its AFT age and the dispersion of

single grain ages indicate that the domain of slow (and shallow) exhumation of the NE Alborz extends into

this section. As in the Firuzkuh section, there is a significant jump to younger cooling ages within the north

flank of the mountain belt (HAR2). HAR3, located just south of Mount Damavand, in the highest part of the



48

range, has an anomalously old age for a sample in this structural and topographic position. It remains

unexplained.

The partially-reset domain of the NE Alborz is again picked up in the Tehran section (TEH1-3),

where TEH4 has an artificially old AFT age due to single grain age dispersion. It has experienced at the most

4-6 km of exhumation during the Neogene, in the hanging wall of the North Alborz Fault. However, fully

reset rocks have only been recovered from locations south of the Kojour Fault, and this structure may extend

significantly further east to explain the jump in AFT ages observed in the Firuzkuh and Haraz sections.

Three locations with AFT ages close to the Eocene-Oligocene transition in the south flank of the mountain

belt are all to the south of the Mosha Fault, which may form the southern boundary of the fast exhuming,

hingh interior of the Alborz Mountains.

In the Chalus section, the young AFT age of CHA1 indicates at least 4-6 km of vertical offset on the

North Alborz Fault since the earliest Miocene. However, the Kandavan Fault, located almost on the main

divide of the mountain belt, appears to be the northern limit of a domain with uniform, high exhumation rates,

reflected in Miocene AFT ages. At the position of this section too, the North Tehran Fault has stepped out to

the southern front of the mountain belt, and it has taken over from the Mosha Fault as the main structure

bounding the domain with youngest AFT ages to the south.

The zone of rapid exhumation extends west into the Taleqan section along the south flank of the

mountain belt. No samples from north of the Kandavan Fault are available in this section, and the role of the

fault as a bounding structure can not be assessed. As in the Chalus section, though, the southern boundary of

the domain with Miocene AFT ages appears to be located at the mountain front, now defined by the Mosha

Fault.

The Shah Rud section represents a dramatic break from the trends observed further east. In this

section, relatively old, Eocene AFT ages are found in the south, associated with Tertiary plutonic rocks. In

the north flank of the Shah Rud valley, a sharp decrease in AFT age towards the north over a short distance

may signal offset along a major structure, which could well be aligned with the Kandavan Fault. However,

now it is the northern block that has been exhumed fastest. Sample SHA1, at the northern edge of the

mountain belt, has the youngest of all AFT ages in this study. Remarkably, the rapid exhumation of this

ancient granite has not resulted in the construction of a major topographic feature.

Finally, the Sefid Rud section repeats the pattern of the Shah Rud section to the east, with Middle

Eocene AFT ages in the south, and a jump to Neogene ages along the topographic depression carved out by

the river’s trunk stream. The age difference between SEF2 and SEF3 may be due in part to the 1.5 km

difference in elevation between these two samples. However, SEF1 is from a low elevation and its Eocene

age must reflect a return to slow exhumation of the northern mountain front.



49

0

20

40

60

80

Kh

aza

r F

.

N.

A.

F.

AF

T a

ge (M

a)

0 20 40 60 80 100

0

500

1,000

1,500

2,000

2,500

3,000

3,500

GOR2

Ele

vati

on

(m

)

Kilometer south

GOR1

As

tan

eh

F.

FIR1

FIR2

FIR3

FIR4

FIR5 FIR6

Cam

brian

Ju

rassic

Ju

rassic

Cre

taceo

us

Jura

ssic

Perm

ian

Kh

azar F

.

N. A

. F

.

Dik

tash

F.

Bash

m F

.

Fir

uzk

uh

F.

Sem

nan

F.

Cam

brian

FIR7

0

20

40

60

80

100

120

140

AF

T a

ge

(Ma

)

0 20 40 60 80 100 120 140

0

500

1,000

1,500

2,000

2,500

3,000

3,500

4,000

Ele

va

tio

n (

m)

Kilometer south

(b) FIRUZKUH

(a) GORGAN

N S



50

Ju

rassic

Ju

rassic

Ju

rassic

Ju

rassic

Kh

aza

r F

.

N.

A.

F.

Ba

ija

n F

.

Mo

sh

a F

.

Pis

hv

a F

.

Ga

rm

sa

r F

.N

eo

ge

ne

0 20 40 60 80 100 120 140

0

1,000

2,000

3,000

4,000

5,000

HAR5

HAR4HAR3

HAR2

Ele

vati

on

(m

)

Kilometer south

HAR1

0

10

20

30

40

50

60

AF

T a

ge (M

a)

Ne

og

en

eN

eo

ge

ne

Cre

tace

ous

Ju

rassic

Ju

rassic

EO

-Olig

oce

ne

Pa

leo

ce

ne

Qa

sr-e

-Fir

uze

h F

.

N.T

. F

.

Mo

sh

a F

.

Ba

ija

n F

.

Ko

jou

r F

.

N.

A.

F.

TEH8

TEH7

TEH6

TEH5

TEH4

TEH3

TEH2

Kilometer south

TEH1

Kh

aza

r F

.

Eo

ce

ne

0

40

80

120

160

200

AF

T a

ge

(Ma

)

0 20 40 60 80 100 120

0

500

1,000

1,500

2,000

2,500

3,000

3,500

4,000

4,500

Ele

vati

on

(m

)

(c) HARAZ

(d) TEHRAN

N S



51

Ju

rassic Ju

rassic

Ju

rassic

Ju

rassic

Pre

ca

mb

ria

n

Ca

mb

ria

n

Eo

- O

ligo

ce

ne

Kh

azar F

.

N.A

.F.

Ko

jou

r F

.

Kan

davan

F.

Tale

qan

F.

N.T

.F.

N.T

.F.

Mo

sh

a F

.

Eo

ce

ne

10

20

30

40

AF

T a

ge (M

a)

0 20 40 60 80 100 120

0

500

1,000

1,500

2,000

2,500

3,000

3,500

4,000

4,500

CHA8

CHA7

CHA6

CHA5

CHA4

CHA3

CHA2

Kilometer south

Ele

vati

on

(m

)

CHA1

E

o-O

ligo

ce

ne

Ca

mb

ria

n

Kh

azar F

.

Nu

sh

a F

.

Ka

nd

av

an

F.

Ta

kie

h F

.

Ta

leq

an

F.

TAL3

TAL2

TAL1

Mo

sh

a F

.

Ju

rassic

0 20 40 60 80 100 120

0

1,000

2,000

3,000

4,000

Ele

vati

on

(m

)

Kilometer south

0

10

20

30

AF

T a

ge (M

a)

(e) CHALUS

(f) TALEQAN

N S



52

Kh

azar F

.

Jura

ssic

Pre

cam

brian

Eo-O

ligo

ce

ne

Eo-O

ligoce

ne

Ma

njil F

.

Sh

ah

Ru

d F

.

N. Q

azvin

F.

Eo

-Olig

ocen

e

0 20 40 60 80 100 120 140

0

500

1,000

1,500

2,000

2,500

3,000

3,500

4,000

SHA5

SHA4

SHA3

SHA1

Ele

va

tio

n (

m)

Kilometer south

SHA2

0

10

20

30

40

50

AF

T a

ge (M

a)

Eo

-Olig

oce

ne

Ju

rassic

Ju

rassic

Ju

rassic

Eo

-Olig

oce

ne

Eo

-Olig

oce

ne

Eo

-Olig

oce

ne

Ka

ba

teh

F.

Ru

db

ar F

.

S.

Ta

lesh

F.

Ma

nji

l F

.

SEF1

SEF2

SEF3

SEF4

SEF6SEF6

SEF5

Ta

ro

m F

.

0 20 40 60 80 100 120 140

0

500

1,000

1,500

2,000

2,500

3,000

Ele

vati

on

(m

)

Kilometer south

0

10

20

30

40

50

60

AF

T a

ge (M

a)

(g) SHAH RUD

(h) SEFID RUD

N S



53

0 20 40 60 80 100 120 140 160 180 200

0

500

1,000

1,500

2,000

2,500

3,000 South flank

North flank

Ele

va

tio

n (

m)

AFT age (Ma)

Fig. 2.2.12: Swath profile (15-20km width) of maximum and minimum elevation along eight

transections east to west: (a)Gorgan, (b) Firuzkuh, (c) Haraz, (d)Tehran, (e) Chalus, (f) Taleqan, (g) Shah

Rud and (h) Sefid Rud, which overlaid with AFT ages within the error (white circles) and stratigraphic ages,

sample location (black diamond), and main faults (dotted line) on each profile. Riverine samples are not

located here. NAF and NTF are standing for North Alborz Fault and North Tehran Fault, respectively.

Labelling system is on the basis of transection name, starting from northern and ending in the southern part of

each cross section (see Fig. 2.2.5).

2-2-16 Age-Elevation Correlation

AFT ages for samples collected at short horizontal distances along a transect with a substantial

vertical differential, for example going up a tall mountain side, can contain information about past

exhumation rates, and the timing of formation of relief. For a given rock body which has been subject to

substantial exhumation, apatite age invariably increases with topographic altitude due to the earlier cooling of

the upper rocks below the temperature at which tracks are retained. Thus, for a limited lateral distance, the

difference in elevation divided by the difference in apatite age provides a direct measure of exhumation. In

this study, samples were collected mostly along the main transverse valleys parallel to the direction tectonic

convergence, and close to the valley floors. This sampling pattern doesn’t permit a systematic investigation of

age-elevation correlations. However, some preliminary observations can be made.

Fig. 2.2.13: Age-elevation correlation for the entire 48 AFT ages in the Alborz.

In Figure 2.2.13 the entire dataset is plotted in an age-elevation diagram, distinguishing between data

from the north and south flank of the Alborz Mountains. No age-elevation correlation is apparent at this scale.

However, the pattern implies a contrast between north flank, with a large spread in AFT ages from 10 to 157

My, and the south flank with Tertiary AFT ages only, as seen in section 2-2-15-5-2.



54

Alternatively, it could be useful to distinguish between the fringes of the mountain belt, with limited

relief, and the zone around the main divide, where elevation and relief are greatest. An anti-correlation

between AFT ages and elevation is apparent in the axial zone of the mountain belt, but not along exterior

profiles (Fig. 2.2.14-section b). The anti-correlation may have been caused by systematically higher rates of

exhumation in areas with high elevation, decreasing relief of the range on the AFT time scale, or the role of

paleo-topography in which the modern valleys are in approximately the same positions as they were at the

beginning of the exhumation (e.g., House & Farley, 1998; Reiners, 2007).

Although other explanations can not be excluded with the data at hand, the broad spatial pattern of

AFT cooling ages suggests that gradients in exhumation rate are likely to be responsible for the

anticorrelation of cooling age and elevation in the axial Alborz Mountains. In this explanation, the pairing of

high elevation with young cooling age can only be achieved if the highest topography has formed in the areas

with the highest rock uplift rates.

Occasionally, the available data can be used in more detail. A well defined positive age-elevation

relationship over a short distance is seen for SEF5 and SEF4 collected from one plutonic body. SEF5 (~41

Ma) was collected 611m higher than SEF4 (~31 Ma). This implies 0.06 mm/year exhumation on average

between 30 and 40 Ma.

2-2-17 Detrital Apatite Fission Track (DAFT)

River sands are produced by processes driving exhumation upstream of a sampling point. If the sand

is well mixed and the apatites are evenly distributed in the source catchment, then its bulk AFT age, reflects

the average exhumation rate of the catchment. Detrital apatite fission track analyses have been applied to

understand the thermal history of source terrains (Carter, 1999), and AFT cooling ages provide a direct link

between long-term sediment supply and sediment accumulation (Bernet & Garver, 2005).

River bed load has not been systematically sampled throughout the Alborz Mountains, but a few

samples have been collected to determine the feasibility of this approach (see Fig 2.2.5 for location and Table

2.2.3 for the AFT age attributes). Sample MOR is from the coastal plain west of the Sefid Rud delta, in the

western most part of the Alborz Mountains. Samples ALA1 was collected where the runoff from the Alam

Kuh granite batholith enters into the Kelar Dasht basin, perched in the north flank of the central Alborz

(catchment area ~330 km2). ALA2 is from a tributary of the Shah Rud with its headwaters also in the Alam

Kuh massif (catchment area ~450 km2). These samples have been treated as the un-reset samples discussed

in section 2-2-15-5-1, and BINOMFIT (Brandon, 1996) was used to determine the components of their age

population (Table 2.2.6).

The single grain AFT age statistics of MOR are dominated by a strong peak at 31.6 Ma (59%),

additional peaks in the Middle and Late Miocene, and a hint of cooling in the Late Cretaceous (Tables 2.2.3

& 2.2.6). Apart from the 8.2 Ma grains, all age components are closely aligned with cooling phases identified

in previous sections.


55

Fig

. 2.2

.14:

Lon

git

udin

al A

FT

age-

topo

gra

phic

ele

vat

ion i

n c

ross

sec

tions

thro

ugh n

ort

her

n, ce

ntr

al a

nd s

outh

ern A

lborz

(a,

b, c

resp

ecti

vel

y),

over

laid

by S

RT

M i

mag

e of

the

Alb

orz

. E

levat

ion a

nd A

FT

ages

(w

ithin

the

erro

r) a

re d

ispla

yed

in w

hit

e ci

rcle

s an

d b

lack

rec

tangle

s, r

espec

tivel

y.



56

Importantly, the 67 Ma trace implies that exhumational cooling at the onset of compression in the

Neo-Tethyan domain may have extended beyond the east Alborz and into NW Iran.

The catchment area of ALA1, in the north flank of Alam Kuh, experienced a strong exhumation phase at 15

Ma (72% of signal), but parts of the area underwent less recent exhumation, permitting an Early Miocene (22

Ma) age component to be preserved. The source area of ALA2, in the west flank of Alam Kuh, has different

age components, revealing considerable complexity within the Alam Kuh area. Half of the signal records

cooling around 29 Ma, possibly related to regional exhumation around the Eocene-Oligocene transition. The

other half of the signal defines a Mid-Pliocene cooling phase, which has not been seen in other AFT data.

This is likely due to the fact that insufficient exhumation has occurred for AFT to record post-Miocene

cooling elsewhere. In the next section a very low temperature themochronometer is used to probe the most

recent exhumation of the Alborz Mountains in more detail.

sample Strat. age AFT1

(My)

AFT2

(My)

AFT3

(My)

AFT4

(My) (%) (%)

(%) (%)

Lag Time

(My)

MOR Qua.(0) 8.2 14.1 31.6 67 18.4 15.5 59 7.1 8.2

ALA1 Qua.(0) 15.1 21.9 - - 72.4 27.6 - - 15.1

ALA2 Qua.(0) 4.5 28.7 - - 47.1 52.9 - - 4.5

Table 2.2.6: The age components, age proportion, and the lag time for Quaternary riverine samples.

Age components are displayed from youngest to oldest, AFT1 to AFT4, and proportion of each component is

indicated in %, utilising BINOMFIT (Brandon, 1996). Lag Time (LT) is equal the youngest age component.


CHAPTER 2: Fission Track Thermochronometry - AHe

57

2-3 Apatite (U-Th)/He (AHe)

2-3-1 Introduction

Apatite Fission Track (AFT) dating which provides constraints on the exhumation of rocks from

4-6 km depth (for common geothermal gradients), is not an efficient tool to explore the most recent

exhumation of active areas. The U-Th/He system in apatite has a sensitivity range between 40 and 80 °C

(Farley & Stockli, 2002), resolving the exhumation of rocks from as little as 1.5-2.5 km. In this section, I

introduce the temporal-spatial pattern of He data across the Alborz Mountains, and combine radiogenic

helium thermochronometry, with apatite fission track data to complete the investigation of recent cooling

and exhumation patterns in the mountain belt.

2-3-2 Apatite (U-Th)/He thermochronometry

Following early studies by Rutherford and Strutt (1905) on the production of He4

from uranium

and thorium series decay in rocks and minerals, it was soon realised that the method was unreliable for

dating of the formation of geological materials, as helium could easily diffuse and escape from its host,

resulting in unreasonably young ages (e.g., Hurley, 1954). It was not until the mid 1980’s that a new

wave of interest in the U-Th/He system occurred, when Zeitler (1987) proposed that helium ages for

apatite could be interpreted as the time of cooling through very low temperatures, ~100°C. This closure

temperature has subsequently been refined by further diffusion measurements to 60-70°C (Farley et al.,

1996, Wolf et al., 1996, Farley, 2002; Shuster et al., 2006). The ease with which helium can diffuse out

of U-Th bearing minerals such as apatite, could be turned to advantage. Rather than absolute dating, the

U-Th/He system could be used to monitor rock cooling histories driven by exhumation in the same way

that apatite fission track analysis is used. The lower closure temperature (~ 60-70°C for apatite) entails a

better sensitivity compared to apatite fission tracks for changes in exhumation behaviour at shallow levels

in the Earth’s crust (typically 1.5-2.5km), and, therefore, to surface process (Harrison & Zeitler, 2005;

Reiners, 2005). Over the next decade analytical techniques and data interpretation were improved (e.g.,

Lippolt et al., 1994; Farley et al., 1996; Wolf et al., 1996; Warnock et al., 1997; Farley, 2000), and as a

result of improved understanding of helium diffusion, U-Th/He dating has now matured to be a routine

part of low-temperature exhumation studies (e.g., Farley & Clark, 2006).

U-Th/He thermochronometry utilizes the accumulation of alpha particles ( He4

) produced by

alpha decay (i.e. ejection of He4

nuclei from the parent nuclide) of 238

U, 235

U and 232

Th (in some

instances also Sm or other comparatively scarce alpha-emitting isotopes). This He4

may be retained

within the mineral, or lost by diffusion to the grain margin, which occurs as a function of temperature.

The amount of helium produced in a mineral is (Farley, 2002; Ehlers & Farely, 2003; Ketcham, 2005a):

]1)[exp(6]1)[exp(7]1)[exp(8 232

232

235

235

238

2384 −+−+−= tThtUtUHe λλλ , (2.3.1)

where He4

, 238

U, 235

U and 232

Th are the present day atoms, λ ‘s are their relevant decay constants and t is

the accumulation time or helium age. The coefficients preceding the uranium and thorium abundances



58

account for the number of α particles emitted within each decay series (Farley et al., 1996; Reiners,

2002).

Alpha particle ejection introduces a complication in U-Th/He dating. Each alpha decay within the

U and Th series has a characteristic energy (Ziegler, 1977), alpha particles are emitted with high kinetic

energies and typically require tens of microns to come to rest within solid matter which is called the

stopping distance. The mean stopping distance is a function of the initial energy of alpha particles, and the

elemental composition and density of the medium through which they travel; it is shortest for the 238

U

decay chain followed by 235

U and 232

Th. Material density has the most control on the stopping distance for

any given energy so that for common U-Th bearing minerals as the density increases, from apatite (3.2

g/cm3), to titanite (3.6 g/cm

3) to zircon (4.4 g/cm

3), the average stopping distance in the

238U chain drops

from 19.7 to 17.8 to 16.6 µm, respectively (Farley et al., 1996).

For apatite, the approximate stopping distance is taken as ~20 µm; therefore, an alpha particle

will come to rest on the surface of a sphere of radius 20 µm, centred around the site of the parent nucleus.

This is illustrated in Figure 2.3.1, where there is full retention in the core of a crystal, decreasing to 50%

retention at the edge of the crystal. If a parent nucleus lies within one stopping distance of the crystal

edge, then there is a possibility that the alpha particle may be ejected from the crystal. This rises to a 50%

probability when the parent nuclide is found on the crystal edge, assuming that all vectors have a similar

chance of use.

Decay from U-Th located in fluids or minerals outside of the crystal can also lead to implantation

of helium into the crystal (Fig. 2.3.1). In general, this affect can be ignored, given that for most rocks the

spacing between parent-rich accessory crystals is large compared to the stopping distance (Farley, 2002).

Careful inspection of mineral relationships in thin section provides a further check.

For a normal grain geometry, only the outermost ~20 µm of a crystal will be affected by alpha

ejection. Therefore, either chemical or mechanical removal of the outermost layers of the grains, could be

seen as a way of eliminating this effect. However, the He diffusion domain in minerals such as apatite is

the grain itself (Bahr et al., 1994; Reiners & Farley, 1999; Farley, 2000; Farley, 2002). Removal of the

outermost portion will lead to a bias in the age of the remaining crystal toward erroneously high values,

because of lower He concentration in the grain interior as a result of diffusive transport (Reiners &

Farley, 1999). Nevertheless for some applications, such as dating of rapid cooling events, this approach

may be appropriate (Farley, 2002).

Fig. 2.3.1: The effect of α-ejection

on helium retention across a crystal with

100 µm diameter (after Farley et al.,

1996). (a) Three possibilities within the

crystal, α-retention, α-ejection from the

crystal and α-implantation to crystal. (b)

α-retention change from core to rim along

A-B.



59

A better approach to account for the effects of alpha ejection is a correction factor based on the

grain size and shape. Farley (1996) found that the key element in controlling alpha ejection is the surface-

to-volume ratio, and they offered an empirical expression, known as the FT correction, that allows

corrections to be readily made for grains of different geometries. This works, if the distribution of U and

Th is homogeneous throughout the grain, i.e. no zoning, and if no implantation of helium from the

surrounding matrix has occurred. Thus when a helium age is measured, it should be corrected for alpha

ejection effects using:

Corrected Age =Measured Age/ FT

The larger the crystal, the smaller the correction required. As discussed above, each parent isotope has a

different stopping distance for a particular medium, but calculating the mean FT, separately for each

parent, does not differ substantially from using a single mean stopping distance for all parents (Farley et

al., 1996).

Despite numerous apparent successes, an increasing number of studies have found helium ages

that seem to be inconsistent with AFT ages form the same sample. Conventional interpretation requires

all AFT ages to be older than apatite helium ages as the latter system has a lower closure temperature.

However, in some cases the reverse has been found (Farley & Clark, 2006; Green et al., 2006; Spencer et

al., 2006). Such ages have been disregarded as it was assumed they were related to a change in the He

retention properties of apatite (Green et al., 2006; Hansen & Reiners, 2006), the presence of micro-

inclusions, U/Th inhomogeneity and alpha implantation (Spencer et al., 2006), or anomalous AFT

annealing behaviour at low temperatures due to non-thermal processes (Hendriks & Redfield 2005 - 2006,

Hansen & Reiners, 2006). However, recently it has been proposed that radiation damage caused by

bombardment of the apatite lattice by alpha particles produces lots of small holes into which helium

collects. These radiation damage holes require a higher temperature to cause the helium to diffuse out;

hence, the closure temperature increases (Schuster et al., 2006). This means that the apatite grain can no

longer be viewed as a single diffusion domain. This model requires a change in diffusion behaviour

linked to the accumulation of radiation damage, i.e., grain age. Thus, it predicts that apatite helium

closure temperatures will increase with helium age.

The presence of small (<15 µm) U-Th-rich inclusions of zircon and monazite, xenotime or fluid

inclusions within the apatite grains is seen as the main difficulty in He dating (Lippolt, 1994). Such

inclusions are known to implant significant amounts of helium into the surrounding apatite. As zircon is

not dissolved by the standard dissolution techniques used on apatite, the end result is a mismatch between

parent and daughter isotope concentrations. The inclusions contribute He to the analysis but not U and Th,

leading to anomalously old ages. Therefore, selection of crystals is an extremely important first step in the

analytical process. It is necessary to exclude any grains containing mineral or fluid inclusions (possibility

of primordial trapped parentless helium). Microscopic examination of grains using transmitted light with

crossed polarizer, re-heating protocols and age reproducibility are all used to demonstrate the quality of

an apatite He age (Farley et al., 1996; Farley, 2002; Ehlers & Farley, 2003).

Apatite grains with U/Th zonation should also be avoided, because, the ejection correction

assumes a homogeneous spatial distribution of U and Th (Ehlers & Farley, 2003). Homogeneity is usually



60

assumed, but examination of fission track uranium maps (induced tracks in mica detectors) can help to

identify zoned grains.

For this study, all crystals were handpicked under polarised light with a binocular microscope at

x150 magnification. A digital picture of every selected grain was taken and the length and width of the

crystal were measured. Selected grains were typically between 100-250 µm long and 60-150 µm wide.

Once identified and recorded, selected crystals were placed in 0.8x1 mm pure platinum cylinders for

outgassing. This procedure helps to prevent volatization and loss of U and in some cases Th, which has

previously been reported during He extraction. Another advantage is that the platinum cylinder serves as

a good heat conductor ensuring uniform heating (Farley, 2002). The enveloped grains were then loaded

into stainless steel capsules to facilitate handling and retrieval from the furnace after He outgassing.

Apatite U-Th/He age determinations were carried out at the UCL-Birkbeck Thermochronometry

laboratories in London. The analytical technique includes vacuum extraction of He through resistance

furnace heating followed by purification and analysis by mass spectrometry. After loading the platinum

enveloped apatite grains in stainless steel capsules, the procedure has two stages. In the first stage,

samples are heated to ~980°C for 15 minutes. The liberated gas is purified on hot and cold TiZr getters,

and 4He abundance is then measured relative to a 99.9% pure

3He spike in a quadrupole mass

spectrometer (Baltzers) equipped with a Faraday detector. Following measurement, the sample is re-

heated for a further 15 minutes to ensure complete extraction of helium. Helium blank levels throughout

the period of analysis are kept at less than 0.1% of the total 4He released by the sample. If the blank

exceeds this value, another reheat is performed until this value is achieved. The reproducibility of the 3He

spike is determined daily against an accurately known standard 4He.

In the second stage (performed by Andrew Carter), grains are retrieved from the vacuum system,

dissolved in 5% HNO3 acid, spiked with 230

Th and 235

U; U and Th and the concentration measured by

Inductively Coupled Plasma Mass Spectrometry ICP-MS (Agilent 7500).

Reported He ages have been corrected for alpha ejection effects based on measured grain

dimensions (Farley et al., 1996) using the procedure of Gautheron et al., 2006. Each age typically

comprises 3-4 replicates the mean of which is reported in Table 2.3.1 (A full description in Appendix

2.3.1). The estimated analytical uncertainty for apatite U-Th/He ages based on the Durango age standard

is 7% (2σ). This uncertainty value has been used on a sample unless the standard deviation from the

sample replicate ages is higher in which case the latter is used.

Sample

n.

N.

grain

He

nmol/g

U

ppm

Th

ppm

FT He age

My

Error (± 2σ)

My

Std.

dev.

CHA1 17 0.70 21.64 29.79 0.80 5.6 0.2 0.3

SEF4 11 1.06 18.52 38.62 0.75 10.1 0.4 2.3

SEF5 10 2.85 20.18 47.19 0.75 17.3 0.6 1.9

TAL2 11 0.49 4.58 22.21 0.80 11.6 0.4 0.3

Table 2.3.1: U-Th/He ages were performed by furnace heating for He extraction and ICP-MS for

U-Th determinations at the University and Birkbeck College of London. The estimated analytical

uncertainty for He ages is about 7% (2 σ). Standard deviation on ages is error, when it is higher than the

analytical uncertainty. Figure 2.3.4 is referred for the location of samples.



61

6

5

4

3

2

1

00 5 10 15 20 25 30 35 40 45

CHA1

SEF4

SEF5

TAL2

De

pth

(km

)

Time (My)

2-3-3 Apatite (U-Th)/He thermochronometry applied to the Alborz Mountains

Over the last decade, the Alborz Mountains have been the subject of a number of apatite U-Th/He

studies. Here, result form these studies (Axen et al., 2001a & b, Davidson et al., 2004, Guest et al.,

2006c) are combined with new data.

2-3-3-1 New data

Most samples discussed in the AFT sections did not contain a good number of apatite grains of

sufficient quality to yield reliable results with U-Th/He thermochronometry. Only four available samples

have the required quality, and produced an AHe age younger the AFT age. These samples are SEF4 and

SEF5 from a pluton in the southwestern Alborz, CHA1 from the central north Alborz, and TAL2 from a

granitoid body in the mountain range to the south of the Shah Rud valley. The AHe ages of these

samples are given in Table 2.3.1.

The AFT and AHe methods have complementary thermal sensitivity, and by combining the two

techniques low-temperature thermal histories of individual samples can be defined (e.g., Stockli et al.,

2000). In Figure 2.3.2 AFT and AHe ages are presented for the four samples, together with the assumed

depth ranges of the AFT PAZ (4-6km) and AHe closure (1.5-2.5 km). In this plot, SEF4 and SEF5 appear

to have had steady, low exhumation rates of 0.14 and 0.12 mm/yr, respectively, since they passed through

the AFT PAZ shortly after magmatic emplacement (this and all other exhumation rates in this section

have been calculated with an assumed uniform geothermal gradient of 25-30 °C/km). The exhumation

rate of CHA1 appears to have been uniform but faster, at 0.20 mm/yr, but exhumation may have sped up

after the Miocene. In contrast, TAL2 probably underwent relatively fast exhumation through the AFT

PAZ (0.75 mm/yr) and slower exhumation since then. In agreement with other AFT data (section 2-2-15),

this analysis implies relatively low exhumation rates in the Tarom Range of the SW Alborz, relatively

high rates in Shah Rud and intermediate rates in the Chalus area.

Fig. 2.3.2: Combined AHe and AFT data for four samples exploring the rate of exhumation in

two thermal windows recorded by two methods.

Samples SEF4 and SEF5 were collected from the same pluton at a vertical distance of 611 m.

This has been used, together with the AFT data presented earlier, to reconstruct the cooling history of the

pluton in more detail. Specifically, exhumation rates of the pluton have been estimated for the age ranges



62

0 5 10 15 20 25 30 35 40 450.00

0.05

0.10

0.15

0.20

0.25

Time (My)

Ex

hu

mati

on

rate

(m

m/y

r)

0-10 Ma, 0-17 Ma, 10-17 Ma, 10-31 Ma, 17-41 Ma, and 31-41 Ma. As an example of the approach, 611

m of exhumation occurred between 17 Ma and 10 Ma, as indicated by the AHe ages of samples SEF4 and

5, and assuming that all recorded cooling was due to exhumation. This gives a paleo-exhumation rate of

0.08 mm/year for this time interval. In Figure 2.3.3, all paleo-exhumation estimates are given with time

and rate ranges shown as bars. It appears that the pluton, which was emplaced in the Middle Eocene

(Section 2-2-15-4 & Chapter 3), underwent an initial exhumational cooling phase during the Oligocene,

and a second phase of faster exhumation since the Late Miocene. In contrast to the regional pattern of the

Alborz, exhumation of the pluton appears to have been relatively slow during the Middle Miocene.

Fig. 2.3.3: Combined exhumation rates for SEF4 and SEF5 calculated in different time intervals.

The thermal history of sample SEF5 has also been modelled using AFT data only (Section 2-2-

15-3). All thermal histories with a reasonable or good fit of the available data show a rapid exhumation to

very near the surface, and passage through the AHe closure zone well before the AHe age of 17.3 ± 0.6

Ma. Specification of a temperature-age constraint according to this AHe date does not significantly

change the model result. Thus, there is a fundamental discrepancy between the thermal history modelling

for sample SEF5, based on AFT data, and the independently determined AHe age of the sample. Both

techniques are notoriously error prone and the discrepancy should serve as a note of caution in the use of

their results.

2-3-3-2 Published data

Published AHe studies of exhumational cooling have focused on the Tehran, Karaj-Chalus, and

Alam Kuh areas, covering a limited part of the central-west Alborz (Axen et al., 2001a & b, Guest et al.,

2006c). In addition, Davidson et al. (2004) have published a series of AHe ages of lava flows associated

with the Damavand eruptive centre. All published ages are shown with geographic location in Figure

2.3.4. Guest et al. (2006c) have also published U-Th/He ages for zircons for several locations. The

zircon helium closure temperature is commonly assumed to be around 180-200°C (Reiners, 2005), but

some studies have put it as low as 128°C.



63

Near Lahijan on the SW Caspian coast, Guest et al. (2006c) have worked on a Late Precambrian

granite body (U/Pb age 552 ± 6 Ma). Their U-Th/He age of 17.2-13.3 Ma for apatite is older than the

AFT age I have obtained for the same body (10.1 ± 1.3 Ma, Section 2-2-15). The granite body is large,

with considerable relief, and it is not clear where exactly Guest has collected his samples. Therefore,

differential exhumation is a possible cause of the apparent conflict of dates. Regardless, both results

indicate substantial exhumation of the Lahijan granite during the Neogene. However, U-Th/He ages of

162-133 Ma for zircons from the same location (Guest et al., 2006c) appear to limit the Neogene

exhumation to little more than the depth of the AFT PAZ.

At the base of the Nusha pluton in the western central Alborz (U/Pb ages of 97 ± 2, 98 ± 1 and

100 ± 1.7 Ma) Axen et al. (2001b) and Guest et al. (2006c) have obtained U-Th/He ages of 6.7-2.8 Ma

for apatite, and 33.4-12.6 Ma for zircon. Detrital sample ALA2 was collected on a stream with drainage

from the south flank of the Nusha pluton. The strongest component of its AFT age distribution is 28.7

Ma (section 2-2-17). This may well reflect the same exhumational cooling phase that has been recorded

by the zircon U-Th/He ages of Guest et al. (2006c). The other major age component of ALA2 (4.5 Ma)

(section 2-2-17) coincides with the young cooling phase detected by Guest et al. (2006c). However, it

should be noted that the sample site of ALA2 also receives sediment from the Alam Kuh pluton further to

the east. The Alam Kuh granite intruded at 6.8 ± 0.1 Ma (U/Pb and 40Ar/39Ar) and cooled rapidly to

~70 °C by ca. 6 Ma (apatite U-Th/He) (Axen et al., 2001a).

Meanwhile, the Akapol pluton, further east, intruded at 56.6 ± 2 Ma, and thermal modelling

indicates that it may have cooled to ~150ºC by ca. 40 Ma, staying near that temperature until at least 25

Ma. Detrital AFT sample ALA1 is composed mainly of material from the Akapol area, and indicates

further exhumational cooling in two steps around 22 Ma and 15 Ma. Axen et al. (2001a) have found

younger apatite U-Th/He ages of 5.9 ± 0.3 Ma and 4.4 ± 0.2 Ma.

Four U-Th/He dates for Eocene volcanics and Neogene detrital rocks in the uppermost Shah Rud

catchment are between 3.4 ± 0.2 Ma and 6.1 ± 0.3 Ma (Guest et al., 2006c).

Eocene rock of the Karaj Formation above an active thrust with Pliocene-Pleistocene footwall

strata in the area north of Tehran have an apatite U-Th/He age of ~11.3 Ma (Axen et al., 2001b). Seven

further U-Th/He dates for Paleozoic, Mesozoic and Eocene rocks from several locations N-NW of Tehran

record cooling to below 40-70°C between 4.0 ± 0.2 Ma and 6.5 ± 0.3 Ma (Guest et al., 2006c).



64

Fig. 2.3.4: Apatite U-Th/He dates overlaid the geological map of the Alborz (after Allen et al.,

2003a). The AHe data complied from Axen et al., 2001a, 2001b; Davidson et al., 2004; Guest et al.,

2006c; data from the present work are underlined.

2-3-3-3 AHe statistics and the recent exhumation of the Alborz Mountains

In total, 30 U-Th/He ages have been produced for the Alborz Mountains, covering the central and

western part of the mountain belt. The probability density distribution of these ages has one strong peak

between 4 Ma and 6 Ma (Fig. 2.3.5). Gaussian statistics have been used to analyse the age components of

this distribution with their weight and error. 77% of the data defines a cooling episode at 4.4±0.04, 13%

of the data describes a cooling episode at 10.82±0.25 Ma, and 10% is associated with a possible cooling

episode at 16.94±0.42 Ma. The latter may be equivalent to the Middle Miocene cooling phase recorded

by AFT. Data from Damavand volcano have been excluded from this analysis, because they record

Quaternary volcanism rather than exhumation.

The U-Th/He ages are not uniformely distributed throughout the western and central Alborz.

They have been obtained mainly in the interior of the mountain belt, where young AFT ages have been

found, and are therefore not necessarily representative for the Alborz as a whole. Moreover, there is a

spatial trend with Middle Miocene AHe ages in the fringes of the westernmost part of the mountain belt

(Tarom and Lahijan plutons), and Pliocene AHe ages further to the east and near the highest topography

of the Alborz. This pattern appears to track the distribution of AFT ages in the mountain belt.

The AHe age data suggests that a further cooling phase has followed exhumational cooling

around the Eocene-Oligocene boundary and during the Middle Miocene. This Early Pliocene cooling

phase is not associated with any pronounced magmatic activity in the Alborz, and it has been recorded in

sedimentary as well as older plutonic rocks. This strongly suggests that the cooling was driven by

exhumation. Assuming that He diffusion in apatite is sensitive to temperatures 30-70 °C (e.g., Wolf et



65

0

2

4

6

8

10

12

14

16

2 4 6 8 10 12 14 16 18 20

AHeFT age (Ma)

N.

of

sa

mp

les

Re

lativ

e p

ro

ba

bility

al., 1998; Farley, 2000), and using a uniform geothermal gradient of 25-30º/km, the rate of exhumation

implied by the 4.4 Ma age peak is 0.34-0.57 mm/year, averaged over the period 4.4 Ma-present. This

value is may biased towards the higher rates, due to an uneven sampling resulted in a population

composed of younger ages from the core of the mountain belt.

Fig. 2.3.5: Combined histogram and probability density function plot for 30 AHe cooling ages.

2-3-4 Summary

Knowledge of the temporal and spatial pattern of exhumation is a prerequisite for understanding

the evolution of orogenic systems. In the Alborz Mountains this has been achieved using Apatite Fission

Track analysis and Apatite U-Th/He analysis. These analyses have yielded a broad overview of the

Tertiary cooling history of the mountain belt.

Specifically, five major cooling phases have been identified with age peaks centred on 70-65 Ma,

39.1±2.7 Ma, 31.7±1.3 Ma, 15.78±0.58, and 4.4±0.04. The Middle Eocene cooling phase is limited to

magmatic intrusions of that age, and AFT is likely to have recorded initial, magmatic cooling. All other

cooling phases have a broad geographic coverage and are recorded by apatite thermochronometers in a

range of rock types. These cooling phases are interpreted to be caused by exhumation.

The highest exhumation rates are found in the interior of the central western part of the mountain

belt. Further west, slower exhumation has been sufficient to expose rocks with Neogene reset fission track

populations at the surface, but in the northeast of the Alborz, Tertiary exhumation has been limited to less

than 4-6 km.

Most of the Alborz region has experienced at least 4-6 km of exhumation during the Neogene.

This has produced an enormous volume of sediment, some of which has been preserved in basins that are

now exposed along the flanks of the mountain belt, and beyond. These sediments provide an independent

record of the exhumation and topographic evolution of the Alborz Mountains. This is the subject of

Chapter 3.


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