Using Apatite Fission Track Thermochronology to date Cenozoic...
Transcript of Using Apatite Fission Track Thermochronology to date Cenozoic...
Using Apatite Fission Track Thermochronology to date Cenozoic Range Growth:
Examples from NW Argentina
Edward Sobel
OUTLINE
• Introduction to fission track dating • Orographic Barriers • Plateau-margin Topography • Marginal Sedimentary Basins
~1 Ma offset Central Naryn Fault along the Kadjerty river, Tien Shan
after Thompson et al., 2002
Neogene ss
Neogene ss
QIII(2) terrace
gravel
gravel
26 ± 2 Ma
112 ± 8 Ma
Maidan fault, Tian Shan
Fission track methodology
~10 µm
c axis
Fission tracks on an internal surface of an apatite crystal
.
c axis
Redrawn after Brown, Summerfield and Gleadow, 1994
accumulation of spontaneous fission tracksin apatite crystal
SurfaceConfined
polished section through crystal
spontaneous tracks etched
mica external detector attached
thermal neutron irradiation
induced fission tracks register in detector
induced fission tracks etched only in detector
From Gallagher, Brown and Johnson, 1998
Track Production (1)
From Gallagher, Brown and Johnson, 1998
Plan view ofseveral crystals
Mirrorimage
Grain mountshowing spontaneous tracks
in the individual apatite grains
External detector mountshowing induced tracksdefining grain outlines
A
B C
A'
B'C'
Track Production (2)
Mount (apatite) Print (mica)
200
100
60
40
20
-2
300
150
20
2 6 10 15
50 25 10 715 8 6%
Precision Index (X)
% Error on Age
200
100
60
40
300
150
20
2 6 10 15
50 25 10 715 8 6%
(b) Tightly ClusteredSingle Grain Ages
200
100
60
40
300
150
20
2 6 10 15
50 25 10 715 8 6%
(c) Broad Spread in SingleGrain Ages, with Young Cluster
(a) Reading Single GrainAges and Their Uncertainties
Single GrainFT Age,
RadialScale(Ma)
Read theAge Here
±2σ confidenceinterval
lower preci-sion grain
symbol for deposi-tional age range
all single grain ageshave ±2σ error bars ofthis length, which arenormally not shown
Almost all grains clusterwithin ±2σ swath(statistically concordent,passes chi square test)
Youngest grainscluster within ±2σswath
±2σ swath of oldestpopulation of grainsin sample
higher precisiongrain
±2σ confidenceinterval
Read theAge Here
Dumitru, 2000
Radial Plots
Precision
2 6 10 15
50 25 10 715 8 6%
300200150100604020
Confined lengthsReflected light Transmitted light
10
20
30
40
5 10 15 20
10.92 ± 0.16 µm1.64 S.D.n = 100
Track Length (µm)
Dumitru, 2000
Otway Basin Fission Track Data
. .
45°C
Outcrop(10°C)
86°CClustered
80-90°C
0 10 20
>≈110°CToo fewtracks tomeasure
Track Length(µm)
102°-110°C
90-101°C
60-70°C
40-50°C
0
20
40
0
20
40
0
20
40
Outcrop(10°C)
0
20
40
0
20
40
0
20
40N
umbe
r of T
rack
s (N
orm
aliz
ed to
100
)
0
100
50
Sample DownholeTemperature (°C)
0 50 100 150Sample Fission Track Age (Ma)
0 5 10 15Mean Track Length (µm)
Track-LengthHistograms
Histograms of Single-Grain Ages, with
Age Spectra Curves
92°CSpread
73°C12
109°C
2468
0
2000 100
2468
0
(Extrapolated)
>125°CClustered
20
Single Grain Age (Ma)
150
200
Observed SampleFission-Track Ages
- 202
>125°C10080
125150
175
65504030
2010
410
0 20 40
- 202
109°C10080
125150
175
65504030
2010
410
- 202
92°C10080
125150
175
65504030
2010
410
- 202
86°C10080
125150
175
65504030
20
- 202
73°C10080
125150
175
65504030
20
- 202
45°C10080
125150
175
65504030
20
- 202
11°C10080
125150
175
65504030
20
Precision Index
Radial Plotsof Single-
Grain Ages
2468
0
2468
0
(Extrapolated)
0
20
40
0
20
40
0
20
40
0
20
40
Clustered
Well Clus-tered Single-Grain Ages
Clustered
Num
ber o
f Gra
ins
(Nor
mal
ized
to 2
0)
Single-Grain Age
Broadly SpreadSingle-Grain Ages
Error bars ±2σ
Observed SampleMean Fission-Track Lengths
FT P
artia
l Ret
entio
n Zo
ne (a
lso
know
n as
Parti
al A
nnea
ling
Zone
)
Very BroadSpread in Single-
Grain FT Ages[P(χ2)<0.1%]
WellClusteredSingle-Grain FTAges[P(χ2)>5%]
All Grains Have Zero Age(Tracks Totally Erased)(Extrapolated)
No Tracks at All(Extrapolated)
F i s s i o n T r a c k D a t a
Tc Ta
Schematic exhumation
Base of PAZ
T1young old
Base ofexhumedPAZ
apparent exhumation rate
Age Elevation
60˚C
110˚C
conceptual basis for 'exhumed'partial annealing zone (PAZ)
100˚C
'exhumed'PAZ
blockingtemperature
PAZ modernPAZ
Age (Ma)
50
0
40
20
0
50
40
0
2010
break inslope
time1 time2(=time1 + 20 My)
break inslope
Conceptual basis for exhumed partial annealing zone (PAZ)
Denali
64
161310
119 86
65
565
6
Mt McKinley (Denali) Apatite FT ages (Ma)
Slide courtesy of P. Fitzgerald
Simple bedrock exhumation
Apatite age profile collected from the west flank of Denali. The * marks the onset of rapid cooling brought about by denudation. Fitzgerald
et al., 1995
~2000 m/Ma or
~2 mm/yr
Mean length (µm) Standard deviation
Influence of Orographic Barriers
Digital elevation model of the NW Argentine Andes with superposed precipitation patterns
Strecker et al., 2007
Effect of an orographic barrier on exhumation: The Sierras Pampeanas
Modified from Trauth et al., 2000 Modified after Sobel and Strecker 2003
Precipitation (mm/yr)
3 Landscape Views
N S
N S
NS
Santa Maria basin
Santa Maria basin
Campo Arenal
Sierra Aconquija
Cumbres Calchaquies transect
transect
A
B
C
Sobel and Strecker, 2003
Modified from Kleinert and Strecker, 2001
Santa Maria Basin Strata
Summary of AFT results
.
2 3 4 5 6 7 80 12
3
4
5
Age (Ma)
length (µm)
Age (Ma)9080706050
151413121110 321Dpar (µm)
Elev
atio
n (k
m)
curve fit excludes the highest sample
Cumbres CalchaquíesAconquija
2
3
4
5
Elev
atio
n (k
m)
Cumbres CalchaquíesAconquija
Sobel and Strecker, 2003
~1.5 km/Ma
CCA1
Loebens et al., 2013
Newer Acon results
pseudosection: orange line: ZHe data offset by 3.5 km to match AFT data using 20°C/km geotherm. Blue line – minimum offfset 35°C/km geotherm
Loebens et al., 2013
Newer Acon results
pseudosection: orange line: ZHe data offset by 3.5 km to match AFT data using 20°C/km geotherm. Blue line – minimum offfset 35°C/km geotherm
Cumbres Calchaquies
AFTSolve model
Tmax = 84°C
Sobel and Strecker, 2003
.
0
25
50
75
100
125
150
175
200
020406080100120
oldesttrack
modelage
TA
CCA1
Age (my)
Wt% Cl = 0.005
T (°
C)
key
TA
temperatureconstraint
best fit
good fit(length and age ≥ 0.5
acceptable fit(length and age ≥ 0.05
Total Annealingtemperature
Schematic evolution of Cumbres Calchaquíes
Present
3 Ma
~5 Ma
~13 to ~6 Ma
rainshadow created
1st basement-derived conglomerates
Terrestrial deposits
4800 m asl
~2000 - 2500 m asl
>1600 m exhumation
150 to 1000 m asl
Sobel and Strecker, 2003