Along-strike variations in the thermal and tectonic...

Along-strike variations in the thermal and tectonic response ofthe continental Ecuadorian Andes to the collision with

heterogeneous oceanic crust

R.A. Spikings a;*, W. Winkler a, D. Seward a, R. Handler b

a Geologisches Institut, ETH Zentrum, CH-8092 Zurich, Switzerlandb Institute for Geology and Palaeontology, University of Salzburg, Hellbrunner Str. 34/III, A-5020 Salzburg, Austria

Received 24 July 2000; accepted 29 December 2000

Abstract

Oblique to strike geological segmentation in the Andean chain has been previously recognised at various scales and iscommonly attributed to changes in the convergence vectors of the oceanic and continental plates, as well as the upper-plate expressions of differing along-strike subducted slab age, strength and composition. We present new white micaand biotite 40Ar/39Ar and zircon and apatite fission-track data from several traverses across the Cordillera Real ofEcuador in the northern Andes that reveal distinct along-strike differences in the timing of accelerated crustal coolingduring the Cenozoic. The data record elevated cooling rates from temperatures of V380³C during V65^55 and V43^30 Ma from all sampled regions of the Cordillera Real and at V15 Ma and since V9 Ma in the northern CordilleraReal. Each cooling period was probably driven by exhumation in response to the accretion and subduction ofheterogeneous oceanic crust. Elevated cooling rates of up to V30^20³C/Myr were initiated during the Palaeocene andEocene^early Oligocene along the entire contemporaneous margin of Ecuador and were driven by the accretion of theoceanic Pallatanga Terrane and Pin¬on^Macuchi Block, respectively, onto northwestern South America. Both of thesegeological provinces originated at the southern parts of the leading and trailing boundaries of the Caribbean Plateauand accreted onto the margin during the approximately northeastward migration of the Plateau into its currentposition. Within Ecuador the development of higher topography and elevated cooling rates of up to 50³C/Myr at V15Ma and since V9 Ma are restricted to the region north of 1³30PS and is situated above the postulated subducted flat-slab section of the aseismic Carnegie Ridge. Plate convergence rate calculations suggest the Carnegie Ridge collidedwith the Ecuador Trench at V15 Ma, which caused the pre-existing coastal provinces to displace to the northeast,subsequently driving extension and marine ingressions in southern Ecuador and compression and uplift in northernEcuador. ß 2001 Elsevier Science B.V. All rights reserved.

Keywords: Northern Andes; Ecuador; Carnegie Ridge; Caribbean Plate; thermochronology; tectonics

1. Introduction

North^south, oblique to strike geological seg-mentation in the Andean chain has been identi¢ed

0012-821X / 01 / $ ^ see front matter ß 2001 Elsevier Science B.V. All rights reserved.PII: S 0 0 1 2 - 8 2 1 X ( 0 1 ) 0 0 2 2 5 - 4

* Corresponding author. Fax: +41-1632-1080;E-mail: [email protected]

EPSL 5742 22-2-01 Cyaan Magenta Geel Zwart

Earth and Planetary Science Letters 186 (2001) 57^73

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at various scales for at least 30 yr (e.g. [1,2]) andthree Andean segments (northern, central andsouthern) are traditionally recognised. Docu-mented geological variations include geomorpho-logical changes in the height and width of theAndean Range (Fig. 1), distinct changes in 20thcentury seismicity [3], the timing of Neogene vol-canic activity and di¡erences in volcanic arc width[4] and composition [5]. Along-strike variationsare generally related to changes in the conver-gence vectors of the oceanic and continentalplates, as well as upper-plate expressions of di¡er-

ing along-strike subducted slab age, strength andcomposition (e.g. [6]).

The Ecuadorian continental margin, situated inthe northern Andean segment (north of 5³S), isseparated from the central segment by a distinctchange in the convergence angle between the oce-anic Nazca and the South American (SOAM)coast line [1], otherwise referred to as the Huan-cabamba De£ection (Fig. 1). Subduction of heter-ogeneous oceanic crust beneath both the conti-nental margin and accreted oceanic plateaumaterial has occurred since the late Middle Juras-

Fig. 1. Digital elevation model of the Ecuadorian Andes (GTOPO30) with illumination from the east. Colour coded AFT agesfrom ¢ve traverses across the Cordillera Real are shown (from north to south: Tulcan (T)^La Bonita (Lb), Papallacta (P)^Baeza(Ba), Ban¬os (B)^Puyo (Pu), Cuenca (C)^Plan del Milagro (Pd), Loja (L)^Paquisha (Pa)). Young AFT ages are recorded in theAndean belt situated above the proposed extension of the Carnegie Ridge [6] and Grijalva Fracture Zone representing a buoyantoceanic slab compared to the remnants of the Farallon Plate. The elevated landscape swings seaward in the vicinity of the Gulfof Guayaquil. Simpli¢ed bathymetry, magnetic anomalies (approximate age in Ma) [6,40] and plate vectors [16] of the NazcaPlate are shown. Red symbols indicate the position of dredged and dated Galapagos Hot Spot basalts [47] and biostratigraphi-cally dated sediments (9.5^8.5 Ma), which lie on hot spot basalts at the DSDP Site 157 [48]. Inset shows the oceanic crust (lightgreen) and Galapagos Hot Spot ridges (dark green) that have formed since V25 Ma. Abbreviations: CP, Cauca Fault; CT,Chimbo^Toachi Shear Zone; GG, Gulf of Guayaquil; LS, La So¢a Fault; MB, Manabi Basin; NF, Nangaritza Fault; PB, Pro-gresso Basin; PaF, Palanda Fault; PeF, Peltetec Fault; PF, Pujili Fault; PlF, Pallatanga Fault; R, Romeral Fault; SAF, Sub-Andean Fault.


R.A. Spikings et al. / Earth and Planetary Science Letters 186 (2001) 57^7358

sic (V160 Ma), initially within the Tethyanspreading system and afterwards in the EasternPaci¢c subduction system [7^9]. The accretion ofallochthonous terranes onto the Guyana Shield atV140^120 Ma during the Peltetec Event [10] re-sulted in crustal thickening, cratonward propaga-tion and building of the Cordillera Real onto thecontinental margin of Ecuador since that time(e.g. [10]). Unroo¢ng of the Cordillera has ex-posed poly-deformed, greenschist facies and lowergrade metamorphic rocks (e.g. [10,11]).

Oceanic hotspot activity, plate rearrangementsand the generation of new spreading centres sinceat least 80 Ma has resulted in large structural,thickness and density heterogeneities in the ap-proaching and subducting oceanic slab parallelto the northern segment (Fig. 1). These include:(1) the obduction of ophiolites and the accretionof allochthonous terranes, composed of anoma-lously thick (s 10 km) oceanic plateau materialonto the continental margin during the Late Cre-taceous^Early Tertiary (e.g. [12] ; (2) the break-upof the Farallon Plate into the Nazca and Cocosplates and a change of the subducting plate vectorat the Ecuador Trench from east-northeast to eastat V25 Ma [7,8] and (3) subduction and upper^lower plate coupling of the buoyant aseismic Car-negie Ridge since the late Miocene [6]. Numerousmodels exist that speculate on the response of theupper-plate to changes in the properties of thecolliding oceanic plate (e.g. [6]).

We present the results of 40Ar/39Ar (white micaand biotite) analyses and zircon and apatite ¢s-sion-track (ZFT and AFT) analyses from traver-ses across the Cordillera Real of the EcuadorianAndes. Each of the minerals studied providesthermal history information over a given temper-ature range and when the results are integratedthey can yield temperature^time paths. The resul-tant thermal histories permit an assessment of thetiming, magnitude and duration of thermal eventsover the temperature range of V380^60³C, andmay help to de¢ne the responsible geological pro-cesses. In this context a quantitative frameworkfor the thermal and exhumation history of theCordillera Real during the Cenozoic has been es-tablished. The framework provides constraints onthe unroo¢ng history of the continental margin

and reveals systematic distinctive approximatelynorth^south oriented, along-strike trends, whichcan be compared with contemporaneous hetero-geneities in the approaching and subducting oce-anic crust.

2. Geological framework

Early Cretaceous to Recent convergence be-tween the Paci¢c oceanic and SOAM plates hasgiven rise to a series of distinguishable tectono-stratigraphic regions (Fig. 1), which for the pur-poses of this contribution are divided into an oce-anic and a continental province. The continentalprovince is partly comprised of a supra-crustalsequence of Palaeozoic^Lower Cretaceous meta-morphic rocks of the Cordillera Real, which to-gether with Cretaceous and Tertiary sediments ofthe retro-arc Amazon Foreland Basin (Fig. 1) un-conformably overlie the continental, sub-crustal,Precambrian Guyana Shield. The CordilleraReal continues northwards into Colombia and isbelieved to be composed of a series of sublineartectono-stratigraphic belts, which are referred toas the Alao, Loja and Salado terranes [10]. Oligo-cene and Miocene ignimbrities, exposed in theInter-Andean Valley (Fig. 1), obscure the westernlimit of metamorphic rocks that form the Cordil-lera Real. However, negative Bouguer anomaliesextend westward from the Cordillera across thevalley [13] and it is likely that rocks of the Cor-dillera Real form the basement in the Inter-An-dean Valley. Alternatively, the existence of a sep-arate microplate beneath the valley has beenpostulated [10].

Immediately to the east of the Cordillera Realthe proximal Amazon Foreland Basin is com-posed of steeply dipping thrust slices and largeantiforms within a series of foothill highs, whichare commonly referred to as the Sub-AndeanZone (SAZ; Fig. 1). Within the SAZ, conspicuoustectonic uplift is evident exposing Palaeozoic toTertiary basement, cover and foreland basin for-mations.

The oceanic province consists of three distin-guishable major allochthonous oceanic terranes[14], which are thought to have accreted to the


R.A. Spikings et al. / Earth and Planetary Science Letters 186 (2001) 57^73 59

continental margin in two stages. The timing andorder of accretion of these terranes is still a sub-ject of debate although Hughes and Pilatasig [12]suggest the ¢rst collision occurred between theoceanic Pallatanga Terrane (Early^Late Creta-ceous MOR basalts and peridotites) and the con-tinental margin between 85 and 65 Ma (Fig. 1).The second stage of accretion involved the colli-sion of the oceanic Late Cretaceous^Early Eocenecomposite Pin¬on^Macuchi Block with the Palla-tanga Terrane within a dextral shear regime alongthe Chimbo^Toachi Shear Zone (Fig. 1; [12]). Amiddle to Late Eocene marine turbidite sequenceis common to both the Pin¬on^Macuchi Block andthe Pallatanga Terrane [12] suggesting that theyhad accreted together by this time.

The suture between the oceanic and continentalprovinces is de¢ned by a tectonic melange, whichhosts metre scale blocks of metamorphic rocks ofthe Cordillera Real [12]. This feature is locallyreferred to as the Pujili Fault and extends south-wards where it intersects the Pallatanga Fault,which is commonly thought to intersect the Ecua-dor Trench in the region of the Gulf of Guayaquil

(Fig. 1). The suture continues northwards intoColombia where it is referred to as the Cauca^Romeral Fault System (e.g. [15]) and marks adistinct increase of 275 mgal in the Bougueranomalies traversing from east to west. Atpresent, almost orthogonal subduction of theNazca Plate beneath the SOAM Plate is occurringalong the Ecuadorian section of the boundary at aconvergence rate of 70^57 mm/yr [16,17].

3. Methods and results

Spikings et al. [18] presented ZFT and AFTdata from three traverses across the CordilleraReal of Ecuador north of 1³30PS (Fig. 1). Theirresults constrain the timing and amount of cool-ing and exhumation of regions of the northernCordillera Real through a static geothermal gra-dient. In this contribution, we present new 40Ar/39Ar data from white mica and biotite from thosesame traverses (Table 1). In addition, 26 ZFT andAFT ages are presented from two traverses southof 2³30PS, which are oriented approximately per-

Table 140Ar/39Ar age data from north of 1³30PSSample Lithology, formation, region Latitude Longitude Age þ 1c

(Ma)

White mica :98RS19 muscovite gneiss, TL granite, lo 0³21P43QS 78³05P10QW 66 þ 1 (P)98RS26b Hbl granodiorite, Azafran granite, sa 01³24P02QS 78³20P41QW 56 þ 1 (P)98RS27 muscovite granite, Azafran granite, sa 01³24P02QS 78³20P41QW 59 þ 1 (P)Biotite :98RS14 graphitic mica schist, Upano Unit, sa 0³26P10QS 77³57P36QW 62 þ 1 (TFA)98RS26b Hbl granodiorite, Azafran granite, sa 01³24P02QS 78³20P41QW 58 þ 1 (P)98RS33 Bt schist, Azafran granite, sa 01³24P10QS 78³19P34QW 62 þ 2 (P)98RS55 rhyolite, Misahualli volcanics, SAZ 0³26P18QN 77³31P57QW 162 þ 2 (TFA)

lo, Loja Terrane; sa, Salado Terrane; SAZ, Sub-Andean Zone.TFA: total fusion age, result of a single step total fusion experiment or total age derived from a step heating experiment. P: pla-teau age, quoted for those samples that have undergone a step heating experiment and have yielded a plateau region on the spec-trum (concordant to 2c). The age is the weighted mean age of the plateau. Correction factors for interfering isotopes are: 36Ar/37Ar(Ca) = 0.0003, 39Ar/37Ar(Ca) = 0.00065 and 40Ar/39Ar(K) = 0.03. Variations in the neutron £ux were monitored with the B4Mwhite mica standard for which a 40Ar/39Ar plateau age of 18.6 þ 0.4 Ma has been reported [21]. 40Ar/39Ar analyses were carriedout using a UHV Ar-extraction line equipped with a Merchantek1 UV/IR laser ablation facility, and a VG-Isotech1 NG3600mass spectrometer. Stepwise heating analyses of samples were performed using a PC controlled defocused 25 W CO2-IR laser.The extracted gas was passed across one hot and one cold Zr^Al SAES getter and transferred into a bright Nier-type source op-erating at 4.5 kV. Measurements were recorded by an axial electron multiplier in static mode. 36Ar, 37Ar, 38Ar, 39Ar and 40Arisotope intensities were corrected for system blanks, background, post-irradiation decay of 37Ar and interfering isotopes.



pendicular to the Andean trend and structuralstrike. The complete data set, including the agesof Spikings et al. [18] is illustrated in Fig. 2.

Mineral separates for both the 40Ar/39Ar and¢ssion-track methods were extracted from wholerock specimens using conventional magnetic andheavy liquid methods. For 40Ar/39Ar dating sealedquartz vials containing the target minerals wereirradiated in the central position of the ASTRAreactor at the Austrian Research Center for 8 hwith an average neutron £ux of 8U1013 n/cm2.Ages and errors were calculated following sugges-tions by McDougall and Harrison [19] using thedecay constants of Steiger and Ja«ger [20] (Table1).

The external detector method was used to de-termine the ¢ssion-track ages of all samples in thisstudy (see [18] for a more detailed description ofthe analytical method; Table 2). When the singlegrain ages for a particular sample yielded a P(M2)parameter of 6 5% it was assumed that more

than one age population is present. Individualage populations have been resolved [22] and theages of all populations are presented in Table 2.

3.1. 40Ar/39Ar ages north of 1³30PS

Step heating of white mica from the Triassic(228 þ 3 Ma U/Pb zircon age [10]) Tres LagunasGranite 98RS19 near to the town of Papallacta innorthern Ecuador yielded a plateau age of 66 þ 1Ma (Table 1; Figs. 1 and 2). Further south, twowhite micas from the Azafran Granite (143 þ 1Ma U/Pb zircon age [10]), located between thetowns of Ban¬os and Puyo (Fig. 1), yielded plateauages of 59 þ 1 and 56 þ 1 Ma. These ages are con-cordant (within error limits) to several ZFT agesreported from the same regions [18] (Fig. 2).

Biotite 98RS55 (rhyolite) taken from a de-formed sequence of the ill-de¢ned Jurassic Misa-hualli Volcanics Unit between the towns of Tul-can and La Bonita yielded a slightly disturbed age

Fig. 2. A compilation of white mica and biotite 40Ar/39Ar and ZFT and AFT ages ( þ 1c error) from ¢ve traverses across theCordillera Real of Ecuador; including those from north of 1³30PS [18]. Horizontal shaded bars highlight those time periods whenregional scale exhumation was occurring at its greatest rates (see Section 4). Fission-track data for samples north of 1³30PS arefrom Spikings et al. [18].



Tab

le2

AF

Tan

dZ

FT

data

from

sout

hof

2³45P,

Cor

dille

raR

eal,

Ecu

ador

Sam

ple

No.

Stra

tigr

aphi

cun

it/t

erra

neL

itho

logy

Lat

itud

e(S

)L

ongi

tude

(W)

Alt

itud

eN

o.of

grai

nsSt

anda

rdtr

ack

dens

ityU

106

Spon

tane

ous

trac

kde

nsit

yU10

6

Indu

ced

trac

kde

nsit

yU10

6

P(M

2)

UF

issi

on-t

rack

age

þ1c

Mea

ntr

ack

leng

thþ

1 cSt

anda

rdde

viat

ion

(m)

(%)

(ppm

)(M

a)(W

m)

(Wm

)

Apa

tite

:C

uenc

a^P

lan

del

Mila

gro

99R

S53

Chi

guin

daU

nit,

losa

ndy

quar

tzit

e03

³00P

45Q

78³3

5P56

Q27

0021

1.03

0(5

467)

0.17

13(1

10)

3.16

8(2

034)

7338

10.8

þ1.

299

RS

55C

higu

inda

Uni

t,lo

biot

ite

schi

st03

³00P

09Q

78³3

9P15

Q33

8021

1.17

0(5

467)

0.63

97(6

89)

5.28

5(5

692)

957

27.9

þ1.

913

.49

þ0.

27(2

9)1.

43

99R

S56

TL

gran

ite,

logr

anit

e02

³59P

13Q

78³4

0P51

Q33

7518

1.10

0(5

189)

1.17

6(1

231)

4.96

6(5

199)

656

55.2

þ4.

813

.23

þ0.

17(6

9)1.

38

99R

S59

Ala

o-P

aute

Uni

t,al

schi

st02

³54P

49Q

78³4

4P07

Q27

5022

1.06

8(5

467)

0.31

96(3

17)

1.02

5(1

017)

2612

64.1

þ5.

112

.72

þ0.

36(9

)1.

08L

oja^

Paq

uish

a99

RS

37C

higu

inda

Uni

t,lo

phyl

lite

03³5

9P01Q

79³1

0P02

Q26

7026

1.08

1(5

467)

1.00

6(9

95)

3.52

6(3

487)

541

59.6

þ2.

913

.26

þ0.

27(5

5)1.

97

99R

S40

Saba

nilla

mig

mat

ite,

logr

anod

iori

te03

³58P

15Q

79³0

1P17

Q19

7520

1.04

5(5

189)

1.36

9(1

715)

6.10

4(7

648)

773

45.0

þ2.

013

.19

þ0.

15(1

00)

1.45

99R

S41

Saba

nilla

mig

mat

ite,

lom

igm

atit

e04

³01P

05Q

79³0

1P20

Q15

0020

1.00

3(5

189)

1.81

3(2

165)

9.03

9(1

0792

)10

113

39.5

þ2.

012

.53

þ0.

15(1

64)

1.9

99R

S42

Zam

ora

bath

olit

h,SA

Zgr

anod

iori

te03

³59P

02Q

78³5

2P18

Q98

025

1.09

3(5

467)

0.24

36(3

44)

0.92

07(1

300)

1211

59.6

þ5.

013

.58

þ0.

31(2

6)1.

56

99R

S43

Zam

ora

bath

olit

h,SA

Zco

arse

gran

ite

03³5

8P07Q

78³5

1P46

Q87

513

1.14

1(5

189)

0.32

50(1

56)

2.10

6(1

011)

8223

34.0

þ3.

312

.56

þ0.

48(1

6)1.

92

99R

S47

Zam

ora

bath

olit

h,SA

ZH

blph

yric

gran

ite

04³0

5P33Q

78³3

8P23

Q80

025

0.94

80(5

189)

0.14

85(2

63)

0.44

83(7

94)

116

60.5

þ5.

113

.31

þ0.

25(5

0)1.

76

Zir

con

:C

uenc

a^P

lan

del

Mila

gro

99R

S51

Upa

noU

nit,

sagr

aphi

tic

phyl

lite

03³0

0P02Q

78³3

0P34

Q30

5025

3.40

4(2

219)

4.64

8(9

98)

2.75

7(5

92)

4032

438

.5þ

2.3

99R

S52*

Chi

guin

daU

nit?

,lo

mic

asc

hist

03³0

0P07Q

78³3

3P25

Q25

007

4.48

9(2

606)

5.68

2(2

53)

3.12

1(1

39)

8527

854

.8þ

6.0

99R

S53

Chi

guin

daU

nit,

losa

ndy

quar

tzit

e03

³00P

45Q

78³3

5P56

Q28

5055

4.14

8(2

606)

9.60

9(7

180)

4.76

4(3

560)

645

957

.6þ

2.4

99R

S54

Ishp

ingo

plut

on,

lodi

orit

e03

³00P

30Q

78³3

7P30

Q34

0011

4.06

5(2

632)

9.25

2(1

140)

7.58

8(9

35)

5474

733

.3þ

1.8

99R

S55

Chi

guin

daU

nit,

loB

tpy

riti

csc

hist

03³0

0P09Q

78³3

9P15

Q33

7516

4.63

6(2

606)

7.43

4(1

052)

4.16

2(5

89)

1635

955

.5þ

3.3

99R

S56

TL

gran

ite,

logn

eiss

icgr

anit

e02

³59P

13Q

78³4

0P51

Q33

7034

3.20

9(2

219)

9.10

1(2

909)

4.11

0(1

314)

6551

247

.6þ

2.2

Pop

ulat

ions

99R

S52

Chi

guin

daU

nit?

,lo

mic

asc

hist

03³0

0P07Q

78³3

3P25

Q25

004.

489

(260

6)11

.72

(143

)0.

7377

(9)

100

6646

3þ

160

Loj

a^P

aqui

sha

99R

S37

Chi

guin

daU

nit,

loph

yllit

e03

³59P

01Q

79³1

0P02

Q26

7530

3.22

7(2

632)

6.92

8(2

096)

2.52

2(7

63)

1131

359

.4þ

3.1

99R

S38a

Chi

guin

daU

nit,

lom

icac

eous

quar

tzit

e03

³59P

38Q

79³0

8P22

Q27

7526

4.05

1(2

606)

8.48

8(3

443)

4.40

42(1

640)

4339

957

.0þ

2.4

99R

S39a

Chi

guin

daU

nit,

lom

icac

eous

quar

tzit

e03

³59P

15Q

79³0

5P38

Q26

0029

4.00

2(2

606)

8.83

2(2

737)

3.75

7(1

164)

637

663

.0þ

2.9

99R

S40

Saba

nilla

mig

mat

ite,

logr

anod

iori

te03

³58P

15Q

79³0

1P17

Q19

7523

3.45

6(2

632)

11.9

8(1

951)

4.78

9(7

80)

4255

457

.9þ

3.0

99R

S41

Saba

nilla

mig

mat

ite,

lom

igm

atit

e04

³01P

05Q

79³0

1P20

Q15

0019

4.53

8(2

606)

8.43

8(1

575)

4.19

4(7

83)

9637

061

.2þ

3.3

99R

S43

Zam

ora

bath

olit

h,SA

Zco

arse

gran

ite

03³5

8P07Q

78³5

1P46

Q87

033

3.14

3(2

219)

11.1

7(3

312)

2.32

2(6

88)

9029

610

1.0

þ5.

3

99R

S46a

Hol

linF

m.,

SAZ

quar

tzar

enit

e04

³08P

38Q

78³3

7P58

Q10

003

3.68

4(2

632)

15.6

6(4

30)

3.16

9(8

7)83

344

121

þ15

99R

S47

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spectrum with a total fusion age of 162 þ 2 Ma(Table 1). Biotite 98RS14 (graphitic schist) takenfrom the Late Jurassic Upano Unit between thetowns of Papallacta and Baeza also yielded a dis-turbed age spectrum with a total fusion age of62 þ 1 Ma. Finally, biotites from the AzafranGranite (143 þ 1 Ma U/Pb zircon age [10]) be-tween the towns of Ban¬os and Puyo yielded pla-teau ages of 62 þ 2 Ma (98RS33, granite) and58 þ 1 Ma (98RS26b, granite; Table 1).

3.2. Fission-track ages south of 2³45PS

3.2.1. Cuenca^Plan del Milagro traverseRock samples were taken from di¡erent fault

bounded units of Palaeozoic^Early Cretaceousrocks between the towns of Cuenca and Plan delMilagro (Fig. 1). AFT ages from this traverserange between V64 and V11 Ma (Table 2;Fig. 3). A majority of ZFT ages range betweenV58 and V33 Ma although sample 99RS52(mica schist) from the Palaeozoic ChiguindaUnit yielded an older population of grains, whichyielded a ¢ssion-track age of 463 þ 160 Ma (Table2). The variation in the ages shows no geographictrend or linear correlation with altitude (Fig. 3).AFT mean lengths are partially annealed rangingbetween 13.23 and 13.49 Wm (Fig. 4) with broadto medium standard deviations (Table 2).

3.2.2. Loja^Paquisha traverseThe traverse between the towns of Loja and

Paquisha intersects the Loja Terrane of the Cor-dillera Real and crosses the Palanda Fault intothe SAZ, extending across the Middle JurassicZamora Batholith (Rb/Sr and K/Ar ages rangebetween 170 and 190 Ma [10]) and into sedimen-tary rocks of the Amazon Foreland Basin (Fig. 3).

Within the Loja Terrane, west of the PalandaFault, a younger population of ZFT ages lieswithin a narrow range between 63 and 57 Ma(Fig. 2). However, micaceous quartzites from thePalaeozoic Chiguinda Unit contain older zirconpopulations (Table 2) with ZFT ages of 94 þ 13Ma (99RS38) and 156 þ 17 Ma (99RS39). AFTages from the Loja Terrane range between 60 and40 Ma and have intermediate mean track lengthsranging between 12.53 and 13.26 Wm (Fig. 4).T

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The Zamora Batholith yields older ZFT agesranging between V131 and 101 Ma and are dis-tinctly older to the east of the Nangaritza Fault(Figs. 1 and 3). AFT ages from the batholith areyounger and lie between 60 and 34 Ma.

Finally, within the proximal SAZ, detrital zir-con from a quartz arenite of the Hollin Forma-tion (Fm.) yields two age populations. The young-er population has an age of 121 þ 15 Ma, which issimilar to the Aptian^Albian stratigraphic age ofthe formation (determined using microfossils[10]). However, a morphologically distinct popu-lation of older zircons yields an older age of347 þ 27 Ma (Table 2).

4. Interpretation and discussion

4.1. Periods of cooling north of 1³30PS

In order to extract thermal history informationfrom the 40Ar/39Ar data, estimates of the closuretemperatures (Tc) of each mineral phase to theK/Ar system have been made utilising experimen-tally de¢ned di¡usion parameters (e.g. [23]). Wehave no a priori knowledge of palaeo-coolingrates within the study area. Therefore, the bulkTc values for hydrous white mica and biotitephases have been calculated [24] for a large spreadof cooling rates of 50^10³C/Myr and range be-

Fig. 3. Two simpli¢ed geological sections across the southern (south of 2³45PS) Cordillera Real, Ecuador [10]. The traverses cor-respond to those shown in Fig. 1 (Cuenca^Plan del Milagro, Loja^Paquisha). Fission-track ages are shown (italic = zircon; boldplain text = apatite) with sample numbers in brackets (see also Table 2). SAZ: Sub-Andean Zone.



tween V380 and 350³C and 350 and 325³C, re-spectively.

Spikings et al. [18] presented several modelledtemperature^time envelopes that described themost likely thermal histories of regions of theCordillera Real north of 1³30PS at temperatures9 110³C. The modelling followed the approach ofGallagher [25], using the quantitative descriptionof AFT annealing kinetics described by Laslett etal. [26]. The envelopes were de¢ned by groupingtogether the 50 temperature^time paths, whichyielded predicted AFT data that most closelymatched the observed AFT data. Where ZFTages were available, the envelopes were extrapo-lated to a temperature of 260³C, which lies at themid-point of the temperature zone where tracksare partially annealed in zircon (samples 98RS55,98RS28 and 98RS14) [18,27]. Fig. 5 shows theresults of combining some of those models withthe 40Ar/39Ar data acquired in this study. The

close similarity of the 40Ar/39Ar white mica andbiotite ages (65^55 Ma; Fig. 2) over a potentialtemperature range of 380^325³C suggests thatthey record a distinctive cooling event during65^55 Ma. In addition, previously measured andun-interpreted ZFT ages (zone of partial trackannealing in zircons is between 300 and 220³C;[27]) of V60 Ma along traverses between thetowns of Papallacta and Baeza and between Ba-n¬os and Puyo (Fig. 2; [18]) are interpreted here torecord the lower temperature segment of the samecooling event (e.g. 98RS14; Fig. 5). AFT agesnorth of 1³30PS range between V40 and 10 Maand this region did not cool to temperatures6 110³C until at least V43 Ma. Rocks in thenorthern Cordillera Real cooled rapidly to tem-peratures 9 110³C at rates as high as 30³C/Myr,probably via crustal exhumation, during V43^30Ma, 25^15 Ma and 10^0 Ma (Fig. 5; [18]). Theamount of cooling and exhumation recorded atany particular time varies between sample loca-tions. However, ZFT and AFT ages from betweenthe towns of Tulcan and La Bonita in far north-ern Ecuador fall within a narrow range of V15^8Ma (Figs. 1 and 2) and this region has cooledsigni¢cantly more (up to 200³C) than regions fur-ther south since V15 Ma (Fig. 5).

The Jurassic biotite 40Ar/39Ar age from farnorthern Ecuador (sample 98RS55; 162 þ 2 Ma)is from a faulted slice of deformed rhyolites ofthe Misahualli Volcanic Unit and is distinctly old-er than other 40Ar/39Ar ages recorded in thisstudy. However, the same sample yields concor-dant ZFT and AFT ages of V15 Ma [18] andexperienced rapid cooling from temperatures ofV260 to 6 80³C at V15 Ma. The extrusionage of the Misahualli Volcanic Unit is not wellknown although K/Ar ages from further southrecord ages varying between V168 and 143 Ma,which led Litherland et al. [10] to de¢ne the unitas containing Jurassic continental volcanic rocks.Therefore, we propose that the biotite 40Ar/39Arage reported here is a result of rapid coolingfollowing the eruption. Subsequent burial wasinsu¤cient to reset the biotite K/Ar system(6V325³C) but high enough to remove pre-ex-isting ¢ssion-tracks in the zircon grains. There-fore, the 40Ar/39Ar age of 162 þ 2 Ma is consid-

Fig. 4. AFT age versus mean ¢ssion-track length for thosesamples from the Cordillera Real that yielded su¤cientlength data (including data from Spikings et al. [18]). Thedashed line separates those samples located north of 1³30PSfrom those located south of 2³45PS.



ered to be a minimum extrusion age for rhyolite98RS55.

4.2. Periods of cooling south of 2³45PS

AFT ages from south of 2³45PS range between64 and 11 Ma (Table 2; Fig. 2) suggesting that thepresent land surface was variably cooling throughthe V110³C isotherm during and prior to thistime period. All of the track lengths are partiallyannealed (6V14.5 Wm) although a cluster of rel-atively long lengths occurs at V60^55 Ma (Fig.4). The AFT data for those samples with morethan 50 track length measurements has been mod-elled (e.g. [18]) to quantify the timing and amountof cooling at speci¢c sample locations at temper-atures 9110³C. Modelling followed the approachof Gallagher [25], using the quantitative descrip-tion of AFT annealing kinetics described by Las-lett et al. [26]. Most of the ZFT ages range be-tween 63 and 33 Ma (Fig. 2) and these have been

combined with the AFT thermal models whereZFT and AFT data exists for the same sampleto generate the temperature^time envelopesshown in Fig. 5. The partial retention zone for¢ssion-tracks is generally considered to lie be-tween V300 and 220³C for zircons [27] and be-tween V110 and 60³C for apatites of commonlyoccurring compositions. Deviations from theselimits can occur within compositionally exoticphases; more in-depth discussions of track an-nealing kinetics are presented elsewhere (e.g. [28]).

Most of the modelled samples, from both theCordillera Real (99RS37, 99RS40, 99RS56) andthe SAZ (99RS47), cooled at elevated rates duringthe period spanning 65^45 Ma and were at tem-peratures 960³C by 40 Ma. Higher average cool-ing rates of up to V20³C/Myr are calculated forthe period 65^55 Ma (cooling from 260 to 60³C).However, several samples yield AFT ages youngerthan 40 Ma (e.g. 99RS55; Fig. 2) and were cool-ing through V110³C at V35^30 Ma (Fig. 5).

Fig. 5. Temperature^time envelopes for representative samples north of 1³30PS and south of 2³45PS. Each envelope was producedby modelling the AFT data [25,26] and combining the result with concordant (same ages within error) white mica and biotite40Ar/39Ar and ZFT track ages. Temperature^time envelopes are only shown for those samples which yielded the most amount ofdata (i.e. both 40Ar/39Ar and ¢ssion-track data) and the most precise data (i.e. more than 50 ¢ssion-track length measurements).For the ¢ssion-track systems, the shaded zones show the region where tracks partially anneal in apatite [26] and zircon [27]. Forthe 40Ar/39Ar system, the shaded zone is that de¢ned by the range of closure temperatures for cooling rates between 10 and50³C/Myr. APAZ: apatite partial annealing zone, ZPAZ: zircon partial annealing zone, Bt: biotite, WM: white mica.



99RS53 is the only sample from south of 2³45PSthat yields a Miocene AFT age (11 þ 1 Ma; Table2; Fig. 3) and cooled through the V110³C iso-therm signi¢cantly later than other samples fromthis region.

Several ZFT ages from south of 2³45PS are old-er than the 40Ar/39Ar and ¢ssion-track ages fromall other sampled regions (Table 2; Fig. 2). ZFTages from the Zamora Batholith (131^101 Ma)post-date its crystallisation age (190^170 Ma;[10]) by at least 40 Myr and are probably mixedages that were partially reset by pre-Cenozoic aswell as Cenozoic cooling events that cannot bequanti¢ed with the current data set. Metasedi-ments 99RS38, 39 and 52, of the Palaeozoic Chi-guinda Unit contain older zircon populations,which can be statistically distinguished fromyounger populations within the same sample (Ta-ble 2). The ages of the older populations fromthese samples range between 94 and 13 Ma and463 and 160 Ma and have not been totally resetby Cenozoic events. The zircons in these popula-tions are more rounded and darker, suggestingthey were derived from a distinctly di¡erent andprobably older source region than the more pris-tine zircon crystals that yield Cenozoic ZFT ages.They may also be compositionally distinct fromthe younger zircon population and may requirehigher track annealing temperatures, accountingfor the preservation of older ZFT ages. Theform of the pre-Cenozoic T^t path cannot be de-rived for any of the older populations.

4.3. Along-strike thermal history variations andtheir relation to the contemporaneous oceanicdomain

4.3.1. 65^30 MaThe thermal history paths derived in this study

show that the entire Ecuadorian Cordillera Realexperienced two pulses of cooling, which occurredat similar rates during V65^30 Ma. Concordantwhite mica, biotite and ZFT ages, as well as mod-elling of AFT track data suggests that coolingrates increased during 65^55 Ma and at V43^30Ma (Figs. 4 and 5). Evidence for the cooling his-tory from temperatures lower than V300³C fromfar northern Ecuador (between the towns of Tul-

can and La Bonita) has been lost by erosion andtherefore cooling during 65^30 Ma in that loca-tion cannot be con¢rmed with the current dataset.

Continental red beds of the Tena Fm., weredeposited to the east of the Cordillera Real withinthe developing Amazon Foreland Basin duringthe Maastrichtian^Palaeocene (72^57 Ma) andwere the ¢rst rocks to be sourced from the Cor-dillera Real [29], suggesting that the Cordillerawas uplifting at this time. In support of this,Litherland et al. [11] attribute reset and mixedmuscovite and biotite K/Ar ages (90^50 Ma, e.g.[10,30]), from pre-Cretaceous schists of the Lojaand Salado Terranes to resetting caused by shear-ing and heating during a Late Cretaceous^EarlyTertiary metamorphic event. This loosely con-strained period may also have caused the Cordil-lera Real to be uplifted [10]. At a similar time,turbidites of the Yunguilla Unit were being partlyshed o¡ to the west from the Cordillera Real anddeposited on oceanic crust of the Pallatanga Ter-rane [12] during the Maastrichtian. Based on thisinformation, Hughes and Pilatasig [12] suggest thePallatanga Terrane accreted against the continentduring the Campanian (83^74 Ma) along the Pu-jili Fault (Fig. 1).

The Late Eocene accretion of the compositePin¬on^Macuchi Block along the Chimbo^ToachiShear Zone is more tightly constrained by a depo-sitional unconformity that spans the Late Eo-cene^Miocene period [31]. Furthermore, subse-quent changes in the composition of volcanicrocks, as well as a K/Ar age of sheared horn-blendes from an intrusion within the Chimbo^Toachi Shear Zone of V48 Ma may representthe earliest stage of dextral shearing [12] associ-ated with this accretionary phase.

Within Colombia, Late Cretaceous thrustingand metamorphism of the Early Cretaceous oce-anic Calima and continental Tahami terranes andthe Campanian accretion of the Amaime Terrane[2,14,15] was synchronous with the accretion ofthe Pallatanga Terrane in Ecuador (Fig. 1). LateCretaceous extrusion ages of basalts in the Palla-tanga Terrane [12] and the oceanic Calima Ter-rane in Colombia [32], are similar to those of theCaribbean Plateau [33]. Furthermore, geochemi-



Fig. 6. Reconstruction of the Caribbean Plateau from 84 to 44 Ma (modi¢ed from [35]). GHS: Galapagos Hot Spot. Schematicpostulated extensions to the Great Arc of the Antilles and the Central American Arc have been made to include the Pin¬on^Macuchi Block (P.M) and the Pallatanga Terrane (PT).



cal signatures of basement basalts of the Pallatan-ga Terrane and the Caribbean Plateau are similar[34]. These similarities, combined with plate re-constructions that assume the Caribbean Plateauformed over the Galapagos Hot Spot and subse-quently drifted to its current position (e.g. [35]),suggest that the allochthonous basement of north-western South America may have been at leastpartly derived from the Caribbean Plateau. Thetiming of accretion suggests that the present dayPallatanga and Calima terranes may have beenderived from the southern segment of the GreatArc of the Antilles (e.g. [36]), which collided withthe leading edge of the approaching CaribbeanPlateau at V84^70 Ma (Fig. 6). Furthermore,we suggest that pre-accretionary formations ofocean island arc a¤nity within the Pin¬on^Macu-chi Block (e.g. [12]) may have originally been partof the trailing Costa Rica/Panama ocean islandarc of the Caribbean Plateau that now formsmuch of Central America (Fig. 6).

Isostatic re-equilibration following a compres-sive and probably crustal thickening period willprolong the duration of elevated exhumation ratesbeyond the period of peak compressive stress dur-ing a collision event. Therefore, the periods ofaccelerated cooling identi¢ed here (65^55 and43^30 Ma) were probably driven by the accretionof the Pallatanga Terrane and the Pin¬on^MacuchiBlock to the continental margin, but post-date theregional maximum compressive stress by 6 10Myr (e.g. [37]). In addition, subduction of oceaniccrust beneath the leading edge of the CaribbeanPlateau began between V84 and 70 Ma [36] andmay have increased regional geothermal gradientsprior to the increased Late Cretaceous^Palaeo-cene (65^55 Ma) cooling rates identi¢ed here.Therefore, a component of the cooling may be aconsequence of thermal relaxation from a highertemperature thermal regime that started to coolwithin Ecuador at V65 Ma.

A paucity of palaeo-magnetic and precise struc-tural data renders it di¤cult to reconstruct therelative position of the NW SOAM margin andthe allochthonous blocks at the time of these col-lision events. Shear indicators in the Pujili Faultand the Chimbo^Toachi Shear Zone suggest adextral sense of movement [12], possibly driven

by the east-northeastward displacement of theCaribbean Plateau into its current position rela-tive to South America. Dextral displacement ofthe oceanic region towards the northeast, utilisingthe Pallatanga [38] and La So¢a faults [39] per-sists today at a rate of V1 cm/yr [16]. Assuming aconstant displacement rate, the measured slip rateis equivalent to 150 km northeastward movementsince the middle Miocene. Therefore, it is likelythat the oceanic terranes accreted against SouthAmerica further south than their current position,possibly somewhere in the vicinity of the Huanca-bamba De£ection (Fig. 1). The accretion eventsdrove increased cooling rates, probably via ero-sional exhumation, along the entire contempora-neous Ecuadorian margin both during the Palae-ocene and Eocene^early Oligocene.

4.3.2. 30^0 MaFrom 30 Ma to present, most of the Cordillera

Real north of 1³30PS cooled signi¢cantly moreand was exhumed from greater depths than theregion south of 2³45PS after 30 Ma (Fig. 5). Ther-mal relaxation following Oligocene volcanismcombined with increased rates of crustal exhuma-tion generated particularly high cooling rates inthe north during 23^15 Ma (12^50³C/Myr) and10^0 Ma (9 50³C/Myr; [18]). South of 2³45PS,the 23^15 Ma cooling `event' is generally not ob-served and any cooling that occurred after 10 Mawas at slower rates than that experienced north of1³30PS (Fig. 5). Furthermore, within the regionnorth of 1³30PS, far northern Ecuador experi-enced distinctly higher cooling and exhumationrates both at V15 Ma (950³C/Myr) and sinceV10 Ma than more southern regions (Fig. 5;[18]). These distinct along-strike di¡erences incooling and exhumation histories since 30 Macorrelate closely with heterogeneities in the sub-ducting oceanic plate, as well as increasing con-vergence rates between the Nazca and SOAMplates during V25^20 Ma [8].

At the present day the subducted Nazca Platebetween the Malpelo Fossil Rift and the GrijalvaFracture Zone is composed of the CarnegieRidge, oceanic crust younger than 25 Myr oldand the Grijalva Fracture Zone system that de-limits Nazca crust (6 25 Ma) from the older Far-



allon Crust (s 25 Ma; Fig. 1). Within the sameregion, the sea £oor is up to 2km shallower thanregions to the north and south [6,40]. Modernseismicity patterns combined with the trend ofadakitic [41] and other active volcanic centreslead Gutscher et al. [6] to propose that the buoy-ant Carnegie Ridge extends beneath the SOAMmargin with a £at-slab geometry. Their proposedlimits, which separate the £at-slab from steepersubducting slab segments via lithospheric tearsare shown in Fig. 1 and may extend up to 500km inland from the trench. At present, the south-ern limit of the £at-slab segment is located atV1³S within the Cordillera Real, whilst its north-ern limit is at V3³N [6]. Within Ecuador, thiscorresponds with the younger cooling history ob-served to the north of 1³30PS revealed through the¢ssion-track data in the present study. Further-more, the youngest AFT ages, generated by deep-er levels of exhumation since V15 Ma, are lo-cated directly above the proposed crest of theridge (Fig. 1).

The timing of collision of the Carnegie Ridgewith the North Andean margin has been a matterof conjecture for over 20 yr. The opinion of, forexample, Lonsdale and Klitgord [40] that under-thrusting has been occurring since only V1^3 Magenerally prevails in the literature. However, morerecent lines of evidence suggest that collision oc-curred earlier. These are:

1. Variations in coastal morphology, generatedby the southward migration of the CarnegieRidge led Gutscher et al. [6] to propose thatthe Carnegie Ridge collided with the trenchsince 8 Ma, and caused the northeastward driftof the allochthonous oceanic terranes at ratesof 1^2 cm/yr [42].

2. On the basis of seismicity patterns, Gutscher etal. [6] showed that the Carnegie Ridge can betraced at least 120^500 km east of the trenchbeneath the continental plate, which at currentplate convergence rates (Fig. 1) implies thatcollision occurred prior to 3 Ma.

3. Lonsdale and Klitgord, [40] assumed very lowmean plate convergence rates of 55 mm/yr [43]between the Nazca Plate and the northernSOAM continent.

4. Late Miocene^Pliocene uplift, recorded byshallowing upward trends in the Ecuadorianforearc (Manabi and Progreso basins) withinthe western Pin¬on^Macuchi Block led Daly[44] to propose that the initial collision oc-curred at V8 Ma.

Global plate reconstructions [7,8,17] show thatthe Nazca Plate has converged almost orthogo-nally with the SOAM Plate at rates as high asV120 mm/yr since the break-up of the FarallonPlate at 25 Ma. The eastward push from the EastPaci¢c Rise, combined with north^south orientedspreading along the Malpelo and Galapagos riftshas given rise to the bifurcation of the Cocos andCarnegie ridges, which formed over the Galapa-gos Hot Spot (Fig. 1). Plateau basalts of the Gal-apagos Hot Spot rest upon oceanic crust of theNazca Plate, which has been dated at V22 Ma(Fig. 1; [40,45]) and may be used as a minimumreference starting time for eastward movement ofthe Carnegie Ridge. However, other workers pro-posed that the Galapagos Hot Spot may havedriven the break-up of the Farallon Plate (e.g.[43]) at V25 Ma.

Plate convergence rate calculations [7,8,17] forthe time spanning 25^20, 20^10 and 10^0 Ma are50^150 (the higher rate is recorded at 12³S), 80^140 and 57^80 mm/yr, respectively. Radiogenicand micropalaeontologic dating of CarnegieRidge basalts [46] and their overlying sediments,respectively (DSDP site 157l; Fig. 1; [47]), suggestthat the Carnegie Ridge has drifted eastwards at arate of 67^75 mm/yr since 9 Ma [46], which com-pares well with proposed rates [7,8,17] for thesame time interval. Utilising the minimum andmaximum convergence rates since 22 Ma [8], theCarnegie Ridge would have collided with theEcuador Trench at some time between 15 and9 Ma, respectively. Within Ecuador, this time pe-riod coincides with the development of the Man-abi and Progreso pull-apart basins over the coast-al region (Fig. 1; e.g. [44]) and extension insouthern Ecuador causing shallow marine ingres-sions into the southern Inter-Andean valley dur-ing 15^10 Ma [48]. These have been attributed tothe north-northeast directed dextral displacementof the allochthonous oceanic blocks since 15 Ma



[49,50], which removed the crustal support for thehinterland to the east of the present day Gulf ofGuayaquil, resulting in extension and collapse.The same tectonic rearrangement is probably re-sponsible for a sudden increase in exhumationrates in far northern Ecuador at V15 Ma. Colli-sion of the anomalously thick and buoyant Car-negie Ridge at V15 Ma may have provided thedriving force for the northeastward displacement,causing extension and subsidence in the southduring 15^10 Ma [48] and compression and upliftin the north.

The restriction of rapid exhumation rates to thenorth is consistent with higher and more juvenileland surfaces composed of metamorphic rocks(excluding the Quaternary and Recent s 5000 mhigh volcanic peaks), which have been uplifted bythe Carnegie Ridge collision north of V2³45PS(Fig. 1). The elevated landscape swings seawardin the vicinity of the Gulf of Guayaquil, crossingmajor fault zones such as the Peltetec Fault and isapproximately parallel with the Grijalva FractureZone and the previously proposed lithospherictears (Fig. 1; [6]). This trend suggests that thereis a deep crustal in£uence on the evolving topog-raphy.

Elevated cooling rates commencing at V9 Maare commonly recorded north of 1³30PS (Fig. 5)and cooling at this time is probably also partlyresponsible for the single Upper Miocene AFTage from south of 2³45PS (99RS53; 11 þ 1 Ma).Cooling at this time may be a result of increasedexhumation rates that were driven by an in-crease in compressive stress and buoyancy of thecontinental margin, which developed as a conse-quence of coupling between the continental crustand the subducting £at-slab of the CarnegieRidge.

5. Conclusions

The location of along-strike variations in theCenozoic thermal history of the Cordillera Realare consistent with the collision and subduction ofcontemporary heterogeneities (varying crustal age,density and thickness) in the approaching oceanicplate.

1. The collision of ocean island arcs that mayhave developed at the boundaries of the Car-ibbean Plate with the SOAM continent a¡ectedthe entire margin of the Northern Andean Seg-ment (north of 5³S) as far south as southern-most Ecuador. These collision events resultedin elevated cooling rates of up to 20³C/Myr inthe Cordillera Real during extended time peri-ods of 65^55 and 43^30 Ma.

2. The role of the Carnegie Ridge in the post-middle Miocene thermochronological and ex-humational history of Ecuador seems to havebeen of paramount importance. NorthernEcuador (north of 1³30PS) has experienced sig-ni¢cantly more cooling and exhumation sinceV30 Ma than the region south of 2³45PS. Col-lision of the Carnegie Ridge with the Ecuador-ian Trench at V15 Ma resulted in elevatedcooling and exhumation rates in the north atV15 Ma and extension in the south. Subse-quent £at-slab coupling of the Carnegie Ridgewith the continental plate may have causedisostatic disequilibrium and a compressionalstress regime, resulting in signi¢cantly in-creased exhumation rates since 9 Ma. Litho-spheric tears and steeper slab subduction, to-gether with reactivation of shear zonesseparating continental from oceanic basementmay have isolated southern Ecuador from thecompressive regime that prevailed in the north.

3. The elevated topography in northern Ecuadorswings seaward in the vicinity of the Gulf ofGuayaquil and is approximately parallel withthe Grijalva Fracture Zone and the location ofpreviously proposed lithospheric tears [6], sug-gesting a deep crustal in£uence on the evolvingtopography. The most juvenile land surfaces liedirectly above the proposed extent of the crestof the subducted ridge.

Acknowledgements

The authors would like to express their thanksto Richard Hughes and Kevin Burke for severaluseful discussions on the con¢guration and recon-struction of western Ecuador and the Caribbeanregion, respectively. Field sampling bene¢ted from



the assistance and support of Diego Villagomez(EPN-Quito). The manuscript was improved bythe thorough and careful reviews of James Kel-logg and Barry Kohn. Funding for the projectwas provided by the Swiss National ScienceFoundation, project numbers 21-050844.97 and20-56794-99.[SK]

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