Geodynamic evolution of southern Costa Rica related to low ... etal_Costa Rica onset of... · Geodynamic evolution of southern Costa Rica related to low-angle subduction of the Cocos

Geodynamic evolution of southern Costa Rica related to low-angle

subduction of the Cocos Ridge: constraints from thermochronology

K. Grafe a,*, W. Frisch a, I.M. Villa b, M. Meschede a

aInstitut fur Geologie und Palaontologie, Universitat Tubingen, Sigwartstr. 10, 72076 Tubingen, GermanybMineralogisches–Petrographisches Institut, Universitat Bern, Erlachstr. 9 a, 3012 Bern, Switzerland

Received 29 August 2001; accepted 14 March 2002

Abstract

The Late Tertiary shallow subduction of the Cocos ridge under the Caribbean plate controlled the evolution of the Cordillera

de Talamanca in southeast Costa Rica, which is a mountain range that consists mainly of granitoids formed in a volcanic arc

setting. Fission track thermochronology using zircon and apatite, as well as 40Ar–39Ar and Rb–Sr age data of amphibole and

biotite in granitoid rocks constrain the thermal history of the Cordillera de Talamanca and the age of onset of subduction of the

Cocos ridge. Shallow intrusion of granitoid melts resulted in fast and isobaric cooling. Aweighted mean zircon fission track age

(13 analyses) and Rb–Sr biotite ages of about 10 Ma suggest rapid cooling and give minimum ages for granitoid emplacement.

In some cases 40Ar–39Ar and Rb–Sr apparent ages of amphibole and biotite are younger than the zircon fission track ages,

which can be attributed to partial resetting by hydrothermal alteration. Apatite fission track ages range from 4.8 to 1.7 Ma but

show no correlation with the 3090-m elevation span over which they were sampled. The apatite ages seem to indicate rapid

exhumation caused by tectonic and isostatic processes. The combination of the apatite fission track ages with subduction

parameters of the Cocos plate such as subduction angle, plate convergence rate and distance of the Cordillera de Talamanca to

the trench implies that the Cocos ridge entered the Middle America Trench between 5.5 and 3.5 Ma. D 2002 Elsevier Science

B.V. All rights reserved.

Keywords: Ridge subduction; Fission track dating; Geochronology; Thermal history; Cocos Ridge; Costa Rica

1. Introduction

The shallow subduction of oceanic ridges below

continental margins has profound effects on the geo-

dynamic evolution of the upper plate. Subduction of

oceanic ridges in the east Pacific realm occurs off

Ecuador (Carnegie ridge, Gutscher et al., 1999),

northern Chile (Nazca ridge, Barazangi and Isacks,

1976; Pilger, 1981) and southern Costa Rica (Cocos

ridge, Kolarsky et al., 1995). Gravity, seismic refrac-

tion surveys and isostatic modeling have shown that

oceanic ridges are characterized by thickened crust

and high heat flow (Detrick and Watts, 1979; Kogan,

1979). The resulting buoyancy of particularly young

ridges leads to low-angle subduction of oceanic crust

and increased coupling between the lower and upper

plates (Cross and Pilger, 1982; Behrmann et al., 1994;

Gutscher et al., 1999). Displacement of the mantle

wedge as a consequence of low-angle subduction may

result in termination of arc volcanism as observed in

0040-1951/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved.

PII: S0040 -1951 (02 )00113 -0

* Corresponding author. GeoForschungsZentrum Potsdam,

Telegrafenberg, 14473 Potsdam, Germany.

E-mail address: [email protected] (K. Grafe).

www.elsevier.com/locate/tecto

Tectonophysics 348 (2002) 187–204

Fig. 1. (A) Plate configuration of Central and South America (after Gutscher et al., 1999). Inlay refers to map B. (B) Major geologic units,

tectonic features and sedimentary basins in Costa Rica. The position of the Cocos ridge is indicated by the 1000-m bathymetric line. Inlay shows

the location of the study area with the Chirripo profile from the town of San Gerardo to Mount Chirripo, and the Buenos Aires profile, near the

town of Ujarraz.

K. Grafe et al. / Tectonophysics 348 (2002) 187–204188

northern Chile and southern Costa Rica (McGeary et

al., 1985; de Boer et al., 1995; Kolarsky et al., 1995).

Subduction of the young aseismic Cocos ridge at

the southern Middle America Trench (Fig. 1) has

produced a number of distinct geodynamic character-

istics. High-resolution seismicity data reveal that the

subduction angle is about 60j in central Costa Rica.

In southern Costa Rica, which is the location of ridge

subduction and the target area of the present study, the

data reveal an angle of 30j for the first 60 km of the

subducting plate, thereafter the plate is assumed to

continue almost horizontal (Adamek et al., 1987;

Protti et al., 1995). Paleobathymetric and stratigraphic

data document the Early to Late Pliocene emergence

of fore-arc and back-arc basins in southern Costa Rica

due to isostatic reequilibration induced by ridge sub-

duction (Collins et al., 1995). This is manifested by a

change from marine to continental sedimentation in

the outer arc of the Burica and Osa Peninsulas and the

Terraba fore-arc basin (Fig. 1). Chronostratigraphy of

sediments from the back-arc Limon basin (Fig. 1)

indicates Pleistocene uplift rates of 0.05 mm/year,

attributed to the subduction of the Cocos ridge

(McNeill et al., 2000). Uplift rates based on the age

and paleobathymetry of marine microfauna in post

Miocene shelf and slope deposits are 0.2 mm/year at

the Osa Peninsula and 0.94 mm/year at the Burica

Peninsula (Fig. 1; Corrigan et al., 1990).

The relationship between upper plate processes and

ridge subduction has been discussed by several

authors (Barazangi and Isacks, 1976; Pilger, 1981;

Gutscher et al., 1999). In southern Costa Rica, the

tectonic inversion of fore-arc and back-arc basins has

been attributed to horizontal contraction caused by

increased coupling between the oceanic and continen-

tal plates as a response to low-angle subduction of the

Cocos ridge (Kolarsky et al., 1995). Surface uplift of

the magmatic arc above the subducted ridge in the

Cordillera de Talamanca may have been accomplished

isostatically as a result of horizontal shortening due to

underthrusting of the Cocos ridge (Fig. 1; Drummond

et al., 1995; Kolarsky et al., 1995). Alternatively,

Abratis and Worner (2001) suggested that thermal

processes due to slab-window formation contributed

to uplift in the Cordillera de Talamanca. Ridge sub-

duction in southern Costa Rica probably led to: (1)

termination of calc-alkaline igneous activity (Drum-

mond et al., 1995), (2) erosion of the volcanic rocks

over an arc-parallel distance of about 180 km (Fig. 1;

Ballmann, 1976; de Boer et al., 1995), and (3)

emplacement of adakitic melts apparently derived

from melting of the subducted portion of the Cocos

ridge in the Cordillera de Talamanca beginning at 3.6

Ma, (Defant et al., 1991a,b, 1992; de Boer et al.,

1995; Abratis and Worner, 2001).

However, the precise correlation between the upper

plate processes and the Cocos ridge subduction remains

unclear, mainly due to a lack of geochronological data

directly dating uplift. In order to constrain the time of

incipient ridge subduction, age data on the magmatic

activity, and the precise cooling and exhumation his-

tory of the granitoid rocks of the upper plate are needed.

The age of incipient subduction of the Cocos ridge

under the southern Costa Rican magmatic arc has

been debated over the last 20 years. Collins et al.

(1995) and Meschede et al. (1998) postulated that

subduction of the Cocos ridge initiated at ca. 3.6 Ma

and between 4 and 3 Ma, respectively. Kolarsky et al.

(1995) suggested an age of 5 Ma based on the

development of thrust faults in the Terraba basin

(Fig. 1). These results clearly differ from those of

earlier studies by Lonsdale and Klitgord (1978),

Corrigan et al. (1990), and Gardner et al. (1992),

who argued that the ridge subduction occurred fairly

recently, between 0.5 and 1 Ma. The purpose of this

paper is to date the beginning of Cocos ridge sub-

duction as well as to quantify the effects caused by its

buoyancy on the respective section of the Middle

American Arc, the Cordillera de Talamanca. Fission

track thermochronology and 40Ar–39Ar and Rb–Sr

mineral ages from granitoid plutons of the Cordillera

de Talamanca are employed to unravel the subsequent

cooling history of these rocks.

2. Geological background

The Quaternary volcanic arc of Central America

extends over a length of about 1000 km from central

Mexico to central Costa Rica and comprises about 40

active or dormant volcanoes. Average spacing be-

tween the major stratovolcanoes is about 26 km,

which makes the Middle America Volcanic Arc one

of the volcanically most active convergent plate

margins on Earth (Carr and Stoiber, 1990; de Boer

et al., 1995). A volcanic gap of about 180 km in this

K. Grafe et al. / Tectonophysics 348 (2002) 187–204 189

arc extends between the active Irazu–Turrialba com-

plex in central Costa Rica and the historically active

Baru complex further southeast in the Cordillera de

Panama Occidental (Fig. 1). The section of eroded

volcanic edifices roughly coincides with the width of

the subducting Cocos ridge (Fig. 1). The northeast-

trending Cocos ridge is about 200–300 km wide and

consists of thickened oceanic crust that rises 2–2.5

km above the ocean floor of the Cocos plate (von

Huene et al., 1995). The ridge contains a 2-km-thick,

low-density volcanic layer (Bently, 1974), which

makes the Cocos ridge more buoyant than the adja-

cent Early to Middle Miocene oceanic crust (Hey,

1977; Meschede et al., 1998).

The Cordillera de Talamanca forms that segment of

the magmatic arc with the volcanic gap above the

subducting Cocos ridge. The topography of the Cor-

dillera de Talamanca is asymmetric in cross-section

whereby the southwestern flank of the Cordillera is

steeper than the northeastern one. Structurally, the

southwestern flank is dominated by reverse and nor-

mal faults, whereas thrust faults characterize the

Caribbean side of the Cordillera de Talamanca (Kolar-

sky et al., 1995). Based on petrographic, geochemical,

and geochronological information (Bellon and Tour-

non, 1978; Tournon, 1984; Defant et al., 1992,

Drummond et al., 1995), Drummond et al. (1995)

divided the volcanic and intrusive rocks of the Cor-

dillera de Talamanca into four lithologic units: (1)

Middle Oligocene tholeiitic gabbros; (2) Middle Oli-

gocene calc-alkaline plutonic rocks of the El Baru

complex; (3) Middle to Late Miocene calc-alkaline

Talamanca Intrusive Suite; (4) Plio–Pleistocene ande-

sitic and adakitic volcanic rocks, whose geochemical

characteristics are consistent with an origin from

subducted oceanic crust of the Cocos ridge.

3. Sample description

Samples for fission track thermochronology,40Ar–39Ar and Rb–Sr age determination were col-

lected from granitoid rocks of the central Cordillera de

Talamanca along two transects, in the Chirripo

National Park and near Buenos Aires, respectively

(Fig. 1). Samples weighing 5–8 kg were collected at

altitudes between 500 and 1200 m in the Buenos Aires

area (samples 77, 78, 78A, 79, 80) and at altitudes

between 1200 and 3800 m in the Chirripo area (sam-

ples 20, 22, 48, 55, 55A, 57, 58, 59). Granitoid rocks

containing magmatic hornblende and biotite occur

below 2000 m, while rocks above this elevation are

free of biotite, and hornblende is replaced by actino-

lite. The chemical composition of amphibole and

biotite was determined by electron microprobe anal-

ysis. The amphibole in samples 48, 78 and 80 are

magnesio-hornblende (nomenclature of Deer et al.,

1992) and are replaced by actinolite at grain bounda-

ries and intragranular fractures. The amphibole in

samples 55, 55A, 57, 58 and 59 are actinolite. Elec-

tron microprobe analyses and inspection of biotite in

thin sections indicate chlorite–biotite intergrowths

due to biotite chloritization in samples 48 and 55A

(Grafe, 1998).

4. Results

4.1. 40Ar–39Ar age determination

40Ar–39Ar age spectra of four amphibole samples

from the Chirripo region (Fig. 2) are internally dis-

cordant, indicating a complex mineralogy and crys-

tallization/alteration history. Such age spectra indicate

low-temperature amphibole growth in a variable

chemical environment (Villa et al., 1996a,b). A multi-

phase composition of the K–Ar system in Ca-bearing

minerals such as amphibole can be monitored by Ca/

K ratios, which can be calculated from the Ar isotope

data (Fig. 2). The variability of the Ca/K ratio can be

used to detect the presence of chemically different

amphibole phases, because degassing of chemically

different phases will lead to variations in the Ca/K

spectrum. The preferential loss of K as a consequence

of alteration involving actinolitization and chloritiza-

tion may increase the Ca/K ratio of a sample. Step-

heating experiments of all samples reveal that Ca/K

ratios vary during degassing of the samples, consistent

with polyphase mineral composition and possibly

with loss of K (Fig. 2).

The formation of successive amphibole generations

requires distinct thermodynamic and kinetic condi-

tions. Reequilibration of magmatic assemblages in a

low p–T environment is only possible in the presence

of fluid. We therefore suggest that retrogressive reac-

tions such as formation of actinolite and chlorite in the


Chirripo samples were triggered by hydrothermal

fluids. Other mineral reactions related to retrograde

alteration and observed in thin sections of the Chirripo

area are sericitization of feldspar and chloritization of

biotite. Fluid-driven alteration and neo-crystallization

of amphibole will be accompanied by partial loss of

Ar, resulting in partially reset ages. Therefore, the

presence of several retrograde breakdown products in

amphibole samples from the Chirripo area obstructs a

straightforward interpretation of the Ar isotope data in

terms of cooling ages. This is also obvious from the

age spectrum of sample 58 (Fig. 2d) which has geo-

logically unrealistic apparent ages of > 50 Ma in the

last degassing steps that can only be explained by the

presence of excess Ar. In summary, the amphibole40Ar–39Ar age spectra are affected by complex geo-

chemical processes. Only sample 48 displays a pla-

teau age (Fig. 2a). Since alteration features as

indicated by actinolite domains are also present in

this sample the apparent age of 8.7F 0.2 Ma has no

direct geological meaning, and can only be interpreted

as a minimum cooling age.40Ar–39Ar biotite data were obtained to better

understanding cooling of the rocks around 350 jC.40Ar–39Ar age spectra for five biotite samples from

the Chirripo area show plateau ages between 8.6 and

7.5 Ma (Fig. 2a–c). In samples 48 and 55A, different

sieve fractions were analyzed, but only sample 55A

Fig. 2. 40Ar–39Ar age spectra (lower diagram) and variation of Ca/K ratios (upper diagram) of amphibole samples (solid line) and biotite

samples (large size fraction-dotted line; small size fraction-dashed line) from the Chirripo area. Ca/K ratios refer to amphibole samples.

Abbreviations: amp—amphibole, bt—biotite. The mineral age is indicated when the calculation of a plateau age was possible.


yields a plateau age of 7.5F 0.1 Ma for both size

fractions (Fig. 2c). However, loss of K from the biotite

samples of the Chirripo area due to formation of

secondary chlorite is evidenced by the presence of

fine chlorite lamellae inside biotite and/or a chlorite

rim. Therefore, the apparent 40Ar–39Ar biotite ages

have to be interpreted as minimum cooling ages, in a

similar way as the 40Ar–39Ar amphibole data.

4.2. Rb–Sr age determination

Five samples from both study areas were inves-

tigated by the Rb–Sr method. In order to obtain a

maximum spread of the data points in the isochron

diagram, apatite was analyzed in addition to the whole

rock and biotite fractions, since apatite has a very low

Rb/Sr ratio.

Table 1

Rb–Sr and 40Ar–39Ar data

[Rb] [Sr] 87Rb/86Sr 87Sr/86Sr * Rb–Sr age 40Ar–39Ar age

Chirripo

Sample 22 (8.4F 0.1 Ma; Sri = 0.70384F 2; MSWD=0.1)

Whole rock 67 562 0.35 0.70388F 1

Apatite (80–125 Am) ND ND 0 0.70384F 1

Biotite (125–180 Am) 479 4.56 305 0.74034F 1 8.4F 0.1

Sample 48 (8.5F 0.4 Ma; Sri = 0.70386F 66; MSWD=6.1)

Whole rock 55 639 0.25 0.70386F 1

Apatite (80–125 Am) ND ND 0 0.70383F 1

Biotite 1 (80–125 Am) 366 7.67 138 0.72071F1

Biotite 2 (125–180 Am) 368 5.27 200 0.72769F 1 8.5F 0.4 8.6F 0.1

Amphibole 8.7F 0.2

Sample 55

Biotite (125–180 Am) 7.9F 0.1

Sample 55A

Biotite (63–125 Am) 7.5F 0.1

Biotite (125–180 Am) 7.5F 0.1

Buenos Aires

Sample 78 (10.1F 0.1 Ma; Sri = 0.70380F 4)

Whole rock 106 385 0.797 0.70391F1

Biotite (80–125 Am) 518 4.44 339 0.75216F 10 10.1F 0.1

Sample 78A (10.0F 0.1 Ma; Sri = 0.70379F 4)

Whole rock 114 399 0.827 0.70391F1

Biotite (80–125 Am) 524 3.48 439 0.76634F 8 10.0F 0.1

Sample 80 (9.9F 0.1 Ma; Sri = 0.70379F 4)

Whole rock 105 381 0.798 0.70391F1

Biotite (80–125 Am) 392 5.10 223 0.73522F 14 9.9F 0.1

Samples 78, 78A, 80 (10.0F 0.1 Ma; Sri = 0.70379F 4; MSWD=0.8)

10.0F 0.1

Rb and Sr concentrations are in ppm, the ages are in Ma; 87Rb/86Sr ratio for apatite is assumed to be zero; the age uncertainty is reported at the

95% confidence level. 2r input errors for the age calculations are 1% for the 87Rb/86Sr ratios and 0.005% for the 87Sr/86Sr isotope ratios. For

samples with higher within-run errors, the individual error on the measurement was chosen.

wr—whole rock, ap—apatite, bt—biotite, ND—not determined.

Isochrons were calculated using the ISOPLOT data reduction progam (Ludwig, 1999).

* Errors are F 2r mean of the within-run statistics and refer to the last digits.


Moreover, apatite may be resistant to hydrother-

mal alteration, even in chemical environments where

the Rb–Sr system of the whole rock has been

disturbed. Therefore, it may indicate the initial87Sr/86Sr ratio of the system (Creaser and Gray,

1992). The closure temperature of the Rb–Sr system

in biotite is commonly estimated at temperatures

between 400 and 300 jC (Del Moro et al., 1982;

Blanckenburg et al., 1989). However, closure temper-

atures are dependent on cooling rates, grain size and

fluid budget (Hammouda and Cherniak, 2000; Kuhn

et al., 2000).

The Rb–Sr data are presented in Table 1. Two

biotite samples (22, 48) from the Chirripo area give

similar apparent Rb–Sr ages of about 8.4 Ma. Inter-

growth between biotite and chlorite suggests that the

Rb–Sr biotite ages are partially reset and should be

interpreted as minimum ages in terms of cooling. The

large dispersion of the 4-point regression line of

sample 48 (MSWD= 6.1) also indicates that the

Rb–Sr isotope system is disturbed.

Rb–Sr ages for three samples from the Buenos

Aires area (Table 1) were calculated with whole rock

and biotite data only as there was not sufficient apatite

for analysis. Concordant ages of about 10 Ma were

obtained for the three samples. In contrast to biotite

samples from the Chirripo area, biotites from the

Buenos Aires area show very little or no chlorite

intergrowth. Although sample localities are several

kilometers apart, the similar ages, identical initial Sr-

isotopic compositions and the lithological homogene-

ity of the granitoid rocks in the sampling area warrant

the calculation of a 6-point isochron including all Rb–

Sr data from the Buenos Aires area. The obtained age

of 10.0F 0.1 Ma and the low isochron dispersion

(MSWD=0.8) support the interpretation that the age

is an undisturbed cooling age.

4.3. Fission track thermochronology

Zircon and apatite fission track thermochronology

is an important tool for elucidating the low-temper-

ature evolution in orogenic belts as it constrains

cooling of the rocks below approximately 280–240

jC (zircon, Yamada et al., 1995; Foster et al., 1996;

Brandon et al., 1998; Stockhert et al., 1999) and 120–

110 jC (fluor-apatite, Laslett et al., 1987) The closure

temperature in fission track studies refers to a temper-

ature within the partial annealing zone (PAZ) where

more than 50% of the fission tracks are retained. The

PAZ is a wide temperature range with gradually

decreasing track stability towards higher temperatures.

Within the PAZ, tracks are therefore gradually

annealed as a function of time and temperature. Track

length distributions of tracks which are oriented

parallel to the surface and fully enclosed in the crystal

(confined tracks, Bhandari et al., 1971) are a diag-

nostic tool for thermal histories, because shortened

tracks indicate longer residence time or higher temper-

ature during this residence in the PAZ than long

tracks. This method is herein only applied to apatite

as its annealing conditions are better constrained than

those of zircon.

For interpretation of the zircon fission track data

we apply a closure temperature of 260F 30 jC. Thistemperature is derived from a range of 290–230 jC,which was calculated by Brandon et al. (1998) from

experimental data for geologically realistic cooling

rates (e.g., between 5 and 500 jC/Ma). It also encom-

passes values estimated from calibrations using40Ar–39Ar studies in K-feldspar and biotite (Foster

et al., 1996). Eight zircon samples from the Chirripo

area have fission track ages between 11.4F 1.4 and

8.4F 1.2 Ma. Five zircon ages determined on samples

from the Buenos Aires area range from 12.4F 1.8 to

9.4F 1.4 Ma (Table 2). Within 2r error, individual

zircon fission track ages are identical (Figs. 3 and 4).

The fission track ages of seven apatite samples

from the Chirripo area range from 4.8F 1.6 to

1.7F 0.6 Ma. Three apatite samples from the Buenos

Aires area gave ages between 4.8F 1.6 and 2.5F 1.0

Ma (Table 3; Figs. 3 and 4). The closure temperature

for fission tracks in apatite is strongly influenced by

chlorine concentration. In general, higher chlorine

content leads to higher resistance to annealing and a

higher closure temperature (Gleadow and Duddy,

1981; Green et al., 1989a,b; O’Sullivan and Parrish,

1995; Kohn and Foster, 1996). Therefore, electron

microprobe analysis of the apatite was performed

(Grafe, 1998). Seven apatite samples from the Chir-

ripo area show chlorine concentrations of 1.2–3.5

wt.%. The three apatite samples from the Buenos

Aires area have chlorine concentrations of 0.7–1.0

wt.%. Comparing these concentrations with the 0.4

wt.% chlorine content of the Durango apatite standard

the apatites from the Chirripo and Buenos Aires study


areas are chlorine-rich apatites. Due to their higher

resistance to annealing, the PAZ and the closure

temperature may be somewhat higher than the 125–

60 jC (Green et al., 1989a,b) range observed in

Durango apatite. From experimental data (Carlson et

al., 1999) from apatites with chlorine concentrations

similar to those of the present study, Ketcham et al.

(1999) calculated closure temperatures which range

from 190 to 150 jC. They also calculated the temper-

ature, at which a fission track population fully anneals

for a given heating rate, to values ranging from 230

to 170 jC (total annealing temperature). Based on

the results of their study we apply a closure temper-

ature of 170F 20 jC and a temperature of 200F 30

jC for the high temperature limit of the apatite PAZ

(Fig. 6) for the interpretation of the apatite fission

track data.

It is important to note that the apparent fission

track apatite ages were not corrected for shortened

track lengths. Such corrections do not give reliable

results, because little is known about the initial track

length in chlorine-rich apatite. A correction for short-

ened tracks would result in ages that are slightly older

than the values given in Table 3.

The track length distribution of some samples (Fig.

5) has been investigated in order to determine the

time–temperature paths of apatite through the PAZ.

The confined track length distributions in samples

from both study areas show a large proportion of

shortened tracks (Fig. 5). Up to one-third of the

confined tracks are between 5 and 10 Am long. For

the Chirripo samples the track length distribution

diagrams (Fig. 5a–f) show a pronounced maximum

of track lengths at values between 14 and 16 Am,

indicating rapid final cooling. For the Buenos Aires

sample 80 (Fig. 5g), the track length distribution is

broader, with the majority of track lengths between 12

and 15 Am, suggesting that cooling through the PAZ

was slightly slower than in the Chirripo area.

5. Cooling history of the Cordillera de Talamanca

Knowledge of the time–temperature history of the

Cordillera de Talamanca is important for elucidating

the active upper plate tectonics during subduction of

the Cocos ridge. Geochronological data from this

study provides new insight into the response of the

upper plate to ridge subduction.

Table 2

Zircon fission track counting parameters and age determinations

Sample Alt.

(m)

Number

of grains

qs� 105

(cm� 2)

Ns qi� 105

(cm� 2)

Ni Pv2

(%)

U

(ppm)

qd� 105

(cm� 2)

Nd Age (Ma)F 2r

Chirripo

20 2050 19 31.86 509 131.1 2094 10.1 960 5.010 9898 9.1F1.2

22 1250 20 25.32 503 83.54 1660 9.2 612 5.010 9898 11.4F 1.4

48 1230 20 19.99 514 71.67 1843 0 525 5.010 9898 10.7F 2.0

55 1780 19 8.80 204 34.34 796 37.2 236 5.330 3658 10.3F 1.6

55A 1820 20 6.99 189 26.70 722 6.3 184 5.330 10,519 10.6F 2.2

57 2090 18 27.20 392 119.5 1722 33.6 823 5.330 10,519 9.2F 1.0

58 3820 16 21.41 372 101.0 1754 10.6 695 5.330 10,519 8.4F 1.2

59 3690 7 25.83 156 103.0 622 80.4 709 5.330 10,519 10.1F1.8

Buenos Aires

77 530 20 18.36 493 66.45 1784 0 458 5.330 10,519 10.9F 2.0

78 730 20 16.71 475 67.12 1908 18.5 462 5.330 10,519 10.0F 1.2

78A 730 19 15.10 306 49.01 993 39.9 337 5.330 10,519 12.4F 1.8

79 1120 20 16.46 429 65.59 1709 27.3 452 5.330 10,519 10.1F1.2

80 780 20 11.47 299 49.51 1290 13.4 341 5.330 10,519 9.4F 1.4

Alt.—Altitude of sample locality; qs and qi—spontaneous and induced track densities in the zircon sample; qd—density of induced tracks in the

dosimeter glass; Ns and Ni—number of the counted spontaneous and induced tracks in the zircon sample; Nd—number of counted tracks in the

dosimeter glass CN 2; Pv2 is the probability of obtaining v2 values for t degrees (t= number of mineral grains� 1) of freedom; U—uranium

concentration in sample.

The ages are reported as central ages (Galbraith and Laslett, 1985).


5.1. High-temperature cooling history

The intrusion depth of the Miocene granitoid rocks

of the Cordillera de Talamanca was calculated using the

aluminum-in-hornblende geobarometer (Johnson and

Rutherford, 1989) on sample 48 from the Chirripo area

and on samples 78 and 80 from the Buenos Aires area.

The results indicate shallow intrusion depths of < 5 km.

Emplacement depth can only be estimated since the

samples record pressures between 0.4 and 1.2 kb,

which fall outside the experimentally calibrated range

of 2–6 kb (Johnson and Rutherford, 1989). However,

our results are in good agreement with the previously

suggested epizonal intrusion of melts at temperatures

between 805 and 860 jC (Drummond et al., 1995).

Modeling the temperature evolution for conductive

and convective cooling mechanisms for plutons with

similar sizes and intrusion depths to those of the

Cordillera de Talamanca shows that thermal reequili-

bration occurs on a time scale of less than 1 Ma and

increases the geothermal gradient in the country rock

(Norton and Knight, 1977; Harrison and Clarke,

1977).

Stages of cooling are indicated by the Rb–Sr

biotite and the zircon fission track ages. The emplace-

ment of the granitoid rocks into the upper crust,

subsequent rapid cooling and the similarity of the

individual zircon fission track ages, warrants the

calculation of a weighted average of the individual

zircon fission track ages for each study area. The

Fig. 3. (a) Distance and elevation of the sample localities for a southwest–northeast transect along the southwestern slope of Chirripo mountain.

(b) Diagram depicts the apatite fission track ages (filled circles), zircon fission track ages (squares), Rb–Sr ages of biotite (diamonds),40Ar–39Ar ages of biotite (triangles), and amphibole samples (filled triangles). Shaded areas show the weighted mean apatite age of 3.1F1.4

Ma and the weighted mean zircon age of 9.7F 0.9 Ma. Errors are quoted at the 95% confidence level.


weighted average zircon age for the Chirripo area is

9.7F 0.9 Ma (2r; MSWD=2.1) and for the Buenos

Aires area 10.3F 1.2 Ma (2r; MSWD= 2.0). The

Rb–Sr biotite ages of about 10 Ma of the Buenos

Aires area and the weighted average zircon ages of

about 10 Ma in both areas indicate cooling to at least

260 jC at about 10 Ma and provide a minimum age

estimate for emplacement and crystallization of gran-

itoid rocks in the Cordillera de Talamanca. Taking the

cooling models for plutons into account, the actual

age of emplacement of the granitoid rocks is probably

between 11 and 10 Ma.

In the Chirripo area, the weighted mean zircon age

of 9.7F 0.9 Ma is older than the apparent 40Ar–39Ar

and the Rb–Sr biotite ages of 8.6–7.5 Ma and the40Ar–39Ar amphibole age of about 8.7 Ma. As the

fission track closure temperature of zircon is lower

than the commonly accepted closure temperatures for

Rb–Sr and K–Ar in amphibole and biotite, this

obviously interferes with any straightforward inter-

pretation of the 40Ar–39Ar and Rb–Sr ages as cooling

ages. Thus, other mechanisms influencing the isotopic

signatures of the granitoid rocks need to be con-

sidered. Chloritization of biotite and formation of

actinolite in the Chirripo samples point to the fact

that low-temperature alteration affected the K–Ar and

the Rb–Sr systems. Using the geothermometer des-

cribed by Cathelineau and Nieva (1985) on chlorite–

biotite intergrowths in sample 48 indicates formation

of chlorite at approximately 227 jC. The possibility ofchlorite formation at temperatures from 325 to 155 jCwas also documented by Battaglia (1999). These

Fig. 4. (a) Distance and elevation of the sample localities for a north–south transect in the Buenos Aires area. (b) Diagram depicts the apatite

fission track ages (filled circles), zircon fission track ages (squares), and the Rb–Sr ages for biotite samples (diamonds). Shaded areas show the

weighted mean apatite age of 3.0F 1.4 Ma and the weighted mean zircon age of 10.3F 1.2 Ma. Errors are quoted at the 95% confidence level.


calculations show that alteration can occur at temper-

atures lower than the closure temperature of fission

tracks in zircon.

Clauer (1981) and Clauer et al. (1982) concluded

that low-temperature alteration of biotite results in a

preferential loss of radiogenic 87Sr relative to Rb, in an

Table 3

Apatite fission track counting parameters and age determinations

Sample Alt.

(m)

Number

of

grains

qs� 105

(cm� 2)

Ns qi� 105

(cm� 2)

Ni Pv2

(%)

U

(ppm)

qd� 105

(cm� 2)

Nd Age (Ma)

F 2rMean track

length

(AmF S.D.)

(no. of tracks)

Cl (wt.%)

Chirripo

20 2050 50 0.2894 56 6.743 1305 26.9 17 5.000 9863 3.8F 1.0 13.3F 3.6 (60) 1.7F 0.1

22 1250 45 0.3339 61 6.953 1351 64.8 17 5.000 9863 4.0F 1.0 13.8F 2.8 (87) 1.4F 0.1

48 1230 50 0.2297 69 6.936 2084 23.8 18 4.830 9543 2.8F 0.8 13.5F 3.0 (48) 1.7F 0.1

55A 1820 50 0.4812 83 0.153 2645 0.7 40 4.830 9543 2.5F 0.8 n.d. 1.1F 0.2

57 2090 50 0.3195 44 5.642 777 52.0 15 4.830 9543 4.8F 1.6 14.0F 3.4 (78) 1.4F 0.1

58 3820 50 0.1969 36 9.802 1792 51.2 25 4.830 9543 1.7F 0.6 14.1F 3.5 (21) 3.5F 0.3

Buenos Aires

78 730 50 0.1443 30 5.379 1118 5.5 13 5.060 10,054 2.5F 1.0 n.d. 1.0F 0.2

79 1120 50 0.1832 26 7.025 997 0.1 17 5.060 10,054 2.6F 1.2 n.d. 0.8F 0.1

80 780 50 0.3662 75 7.500 1536 0 19 5.060 10,054 4.8F 1.6 12.8F 2.9 (60) 0.7F 0.1

Alt.—Altitude of sample locality; Cl—chlorine concentration (mean) was determined by microprobe analysis (based on 23 oxygen atoms) on

10–15 grains per sample; errors on chlorine concentration are reported as standard deviation of the mean; qs and qi—spontaneous and induced

track densities in the apatite sample; qd—density of induced tracks in the dosimeter glass; Ns and Ni—number of the counted spontaneous and

induced tracks; Nd—number of counted tracks in the dosimeter glass CN 5; Pv2 is the probability of obtaining v2 values for t degrees (t=number of mineral grains� 1) of freedom; U—uranium concentration in sample; S.D.—standard deviation of the mean; n.d.—not determined.

The ages are reported as central ages (Galbraith and Laslett, 1985).

Fig. 5. Fission track length distribution diagrams for apatite samples from the Chirripo area (a– f ) and from the Buenos Aires area (g);

Abbreviations: n—number of tracks counted, l—mean track length, S.D.—standard deviation.


exchange of radiogenic Sr against Sr derived from the

environment, and in preferential loss of radiogenic40Ar relative to K. Apparent ages then tend to be

gradually reset, as a function of the degree of alter-

ation, and have no direct geological significance. The

shifts in apparent Rb–Sr and K–Ar ages of biotite

caused by alteration are fairly systematic in a sense that

the 40Ar–39Ar and Rb–Sr ages are reset to similar

degrees. This is exemplified by the similar apparent

ages of about 8.6 Ma obtained for both, the 40Ar–39Ar

and the Rb–Sr system on biotite sample 48 (Table 1).

Therefore, we conclude that the 40Ar–39Ar and Rb–Sr

data from the Chirripo area represent disturbed ages

due to partial resetting by hydrothermal alteration.With

our data it is impossible to precisely date the alteration,

which may have occurred as a prolonged process.

Assuming that resetting of the biotite ages was partial,

hydrothermal alteration must have continued to times

younger than about 7.5 Ma, which is the youngest

biotite age in the Chirripo area, since apparent ages tend

to become younger during alteration. In the Buenos

Aires area, the three identical Rb–Sr biotite ages and

the weighted mean fission track zircon age indicate that

alteration did not affect the samples from this area.

5.2. Low-temperature cooling history

The low-temperature cooling history is derived

from apatite fission track thermochronology. Apart

from one group of related papers, there is little pub-

lished of experimentally determined annealing data for

apatite with as high chlorine content as documented

herein (Carlson et al., 1999; Donelick et al., 1999;

Ketcham et al., 1999). This precludes rigorous model-

ing of the sample cooling histories. The closure tem-

perature for the high-chlorine apatites analyzed in this

study is estimated to 170F 20 jC (see discussion

above). The large proportion of shortened tracks in

both sampling areas indicate either prolonged resi-

dence of the samples in the PAZ, or an episode of

reheating of the apatites. In the case of a prolonged

residence in the PAZ (Fig. 6, model path a), the samples

probably resided there up to one third of the time since

the samples entered the PAZ, since shortened tracks

make up about one third of the total number of confined

tracks. This suggests that the samples entered the PAZ

at about 5 Ma (Fig. 6). If the samples experienced

reheating (Fig. 6, model path b), the thermal pulse

could have been due to adakite magmatism beginning

at about 3.6 Ma (de Boer et al., 1995). With the current

data we cannot distinguish between these two scenar-

ios. However, in any case, the samples resided for

significant times at temperatures below the high tem-

perature limit of the PAZ before final cooling through

the apatite fission track closure temperature.

Most importantly, both apatite ages and track

length distributions of the samples are similar, regard-

less of topographic elevation of sample localities

(Figs. 3–5). This observation and the overlap of age

error intervals justify the use of weighted average

apatite ages instead of individual apatite ages. The

weighted average of samples from the Chirripo area is

3.1F1.4 Ma (2r; MSWD=1.5) and for those from

the Buenos Aires area 3.0F 1.4 Ma (2r; MSWD=

0.8). The similar ages and track length distributions

indicate that all samples experienced a similar thermal

history. The abundance of shortened tracks point to a

prolonged residence time in the PAZ or reheating of

the samples. Rapid exhumation occurred thereafter, as

indicated by the large amount of long tracks. Slow

exhumation can be ruled out as it would have resulted

in correlations between topography, age and track

length distribution. Rapid exhumation took place at

about 3 Ma, as indicated by the weighted mean

apatite ages.

With our data, we cannot establish the mechanisms

for exposing samples with similar ages and thermal

histories at different altitudes. A possible cause for the

lack of correlation between apatite fission track ages

and topography can be that during fast exhumation the

isotherms became more parallel to topography (Stuwe

et al., 1994; Mancktelow and Grasemann, 1997).

Differential displacement within the granitoids may

account for the missing correlation, too. Fault kine-

matic, seismic and geodetic data from the northern part

of the Cordillera de Talamanca show that NW striking

faults may accommodate uplift and exposure within

the northern Cordillera de Talamanca (Marshall et al.,

2000). The authors also suggest that in response to

shallow subduction of the Cocos ridge, deformation

propagated from the fore-arc area into the magmatic

arc. In contrast to the northern part of the Cordillera de

Talamanca, tectonic structures in the central part can-

not be mapped due to dense vegetation. However, it is

likely that deformation caused by the horizontal short-

ening due to increased basal traction from the shal-


lowly subducting Cocos ridge is not only restricted to

the northern part of the Cordillera de Talamanca, but

similarly affects the central part. Isostatic reequilibra-

tion of the buoyant shallowly subducting Cocos ridge

probably contributed to the exhumation of the cordil-

lera. Rapid exhumation at about 3 Ma accounts very

well for the geochronological data obtained in this

study, such as (1) the lack of a fission track apatite age–

elevation correlation; (2) the identical weighted mean

fission track apatite ages of 3 Ma in different study

areas; (3) the similar fission track length distribution in

samples with up to 2500-m difference in elevation.

Considering the geologic setting in southern Costa

Rica we attribute this exhumation to the arrival of the

buoyant Cocos ridge underneath the Cordillera de

Talamanca which probably caused isostatic reequili-

bration and subhorizontal shortening.

6. Tectonic model and conclusions

The significance of the Cocos ridge subduction to

the upper plate geodynamic evolution is delineated

in Fig. 7. During the Late Miocene, cold, dense

oceanic crust was subducted below the western mar-

gin of the Caribbean plate (Donnelly, 1985), accom-

panied by the formation of the magmatic arc (Fig.

7A). Zircon fission track and Rb–Sr biotite ages

combined with geobarometry suggest shallow

emplacement of granitoid melts in the Cordillera de

Talamanca between 11 and 10 Ma. At this time, the

subduction angle was probably about 60j, as cur-

rently observed below central Costa Rica (Protti et

al., 1995).

The geochronological data obtained in this study

allows the calculation of the onset of ridge subduction

Fig. 6. Time– temperature path for the granitoid rocks from the Chirripo and Buenos Aires study areas. The closure temperature estimate is

350F 50 jC for the Rb–Sr system in biotite, 260F 30 jC for fission tracks in zircon, and 170F 20 jC for fission tracks in chlorine-rich

apatite. The shaded area indicates the PAZ range (200–60 jC) for chlorine-rich apatite samples. Cooling through path (a) models the shortened

tracks by prolonged residence in the PAZ, whereas in path (b) shortened tracks are generated by reheating.


using the following parameters: (1) the final cooling

event at about 3 Ma as suggested by apatite fission

track ages; (2) the distance of ca. 130 km between the

Cordillera de Talamanca and the present-day trench;

(3) the present-day convergence rate between the

Cocos and Caribbean plates (9.6 cm/year; DeMets et

al., 1990; Seno et al., 1993); and (4) the current

subduction angle of 30j (Protti et al., 1995). Calcu-

lations using these parameters show that the Cocos

ridge was located below the Cordillera de Talamanca

about 1.5 Ma after it entered the trench (Fig. 7B).

Considering the uncertainties on the subduction

geometry parameters and the age of final cooling of

the rocks, the uncertainty on the beginning of Cocos

ridge subduction is calculated to be about 1 Ma,

suggesting that ridge subduction began between 5.5

and 3.5 Ma.

For the Early Pliocene the following scenario is

proposed: At about 3 Ma, ca. 150 km of the Cocos

ridge was subducted at a low-angle, replacing the

former asthenospheric wedge underneath the Cordil-

lera de Talamanca (Fig. 7B). Subsequent exhumation

of the granitoid rocks of the Cordillera de Talamanca

occurred by horizontal shortening and isostatic reequi-

libration.

The results from this study on the onset of Cocos

ridge subduction correlates well with the results from

other studies such as the paleobathymetric uplift ages

of 3.6 and 1.6 Ma for the Burica Peninsula and the

Limon basin, respectively (Collins et al., 1995) and

the development of thrust faults in the fore-arc basins

attributed to increased coupling of the upper and

lower plate starting at about 5 Ma (Kolarsky et al.,

1995). The cessation of calc-alkaline magmatism has

been explained by the shallowing of the subduction

angle (Drummond et al., 1995). Adakite volcanism

starting at 3.6 Ma was attributed to the presence of the

Cocos ridge underneath the Cordillera de Talamanca,

(Defant and Drummond, 1990; de Boer et al., 1995;

Abratis and Worner, 2001) supporting the results of

our study.

Our study on the time–temperature evolution and

the exhumation history of the Cordillera de Talamanca

provides a minimum emplacement age for granitoid

rocks in the Cordillera de Talamanca of about 10 Ma

based on weighted mean fission track zircon ages of

9.7F 0.9 Ma for the Chirripo area and of 10.3F 1.2

Ma for the Buenos Aires area, and Rb–Sr biotite ages

of 10 Ma for the Buenos Aires area. The onset of

Cocos ridge subduction between 5.5 and 3.5 Ma can

be inferred from the apatite fission track weighted

mean ages, which are consistent with independent

Fig. 7. Tectonic models for the geodynamic evolution of the south

Costa Rican arc. (A) Model shows the tectonic setting in the Late

Miocene: old and cold oceanic crust subducts at an subduction angle

of about 60j. (B) Model depicts the tectonic setting at ca. 3 Ma: The

subducted portion of the Cocos ridge reached the Cordillera de

Talamanca and induced exhumation in the mountain range by

isostatic reequilibration and tectonic processes.


evidence from the tectonic, magmatic and volcano-

logic record. The final cooling of the upper crustal

rocks is mainly governed by isostatic rebound and

tectonic processes due to the shallowly subducting

Cocos ridge.

Acknowledgements

This project was supported by the Deutsche

Forschungsgemeinschaft. M. Satir provided access

to the radiogenic isotope facilities. E. Hegner assisted

with the Rb–Sr dating. We thank R. Altherr for

providing access to the electron microprobe and H.-P.

Meier for assistance with microprobe work. The

manuscript benefited from fruitful discussions with J.

Glodny and E.R. Sobel. Careful and constructive

reviews by B.P. Kohn and T. Calmus significantly

improved the manuscript.

Appendix A. Analytical methods

A.1. 40Ar–39Ar dating

Amphiboles and biotites were separated using

standard mineral separation techniques. From each

amphibole sample, 40–80 mg was handpicked to

achieve visual purity. Biotite was purified by grinding

in a mortar with alcohol. Then, 30–40 mg of each

biotite sample was then handpicked to achieve visual

purity. The samples were step-heated in a double

vacuum resistance oven connected to a MAPk 215-

50B mass spectrometer. The isotope data were cor-

rected for mass spectrometer background, mass frac-

tionation, and 37Ar decay. The ages are corrected for

interfering isotopes, as described in Belluso et al.

(2000).

A.2. Rb–Sr dating

Rb and Sr concentrations of whole rock samples

were determined by X-ray fluorescence analysis,

those of the biotite samples by the isotope dilution

method. For biotite and whole rock isotope analysis,

40–50 mg of each sample was dissolved in HF–

HClO4 in PFA vessels by open-beaker dissolution.

The second step of sample decomposition was done in

Teflon bombs at 180 jC for 1 week, to ensure

decomposition of refractory phases such as zircon.

Apatite samples were dissolved in 2 N HNO3. Biotite

samples were spiked with a 84Sr–87Rb isotope tracer

solution before decomposition. Four analyses of the

NBS-987 Sr-standard gave an average 87Sr/86Sr value

of 0.710244F 0.000010. The isotopic compositions

were measured on a Finnigan MAT 262 mass spec-

trometer at the University of Tubingen. Sr was ana-

lyzed using a single W filament and a Ta activator

solution. Isotope analysis was done using a dynamic

multi-cup measuring routine which monitors for inter-

fering Rb. 87Sr/86Sr was corrected for instrumental

mass fractionation with a 86Sr/88Sr ratio of 0.1194.87Rb/85Rb ratios were determined on a Re-double-

filament configuration and using a static data collec-

tion procedure. A fractionation factor of 0.3% per

atomic mass unit was applied to correct for mass

fractionation. For a more detailed description of the

analytical methods used, see Hegner et al. (1995).

A.3. Fission track dating

Apatite and zircon fission track ages were deter-

mined using the external detector method. Apatite

grains were embedded in epoxy, then polished, and

etched with 1% HNO3 for 210 s at room temperature.

Zircon grains were embedded in Teflon, then pol-

ished, and etched in a KOH–LiOH eutectic melt at

205 jC for periods between 40 and 87 h. The mineral

mounts were covered with low-uranium muscovite

detectors and irradiated together with neutron dose

monitor glasses (CN 2 for zircon, and CN 5 for

apatite). Samples were irradiated at the Risø reactor

in Roskilde, Denmark. The thermal neutron doses

were determined from the track densities in muscovite

detectors that covered the glass standards during

irradiation. Microscopic analysis was done under oil

immersion at 1250�magnification. Ages were cal-

culated using the zeta calibration method (Hurford

and Green, 1983). All ages are reported as central ages

(Galbraith and Laslett, 1985). Zeta values of 354F 9

for apatite and of 151F 3 for zircon were used for the

age calculations. The following age standards were

used for the determination of the zeta values: Fish

Canyon Tuff (apatite and zircon), Durango (apatite),

and Tardree Rhyolite (zircon). In total, 13 apatite and

zircon samples were analyzed. Data from four out of


13 apatite samples were discarded because of a very

high number of dislocations in the crystal lattice,

which strongly interfered with the proper identifica-

tion of spontaneous fission tracks. Lengths of con-

fined tracks could only be measured on seven out of

13 apatite samples, despite examining up to eight

mounts per apatite sample for confined track measure-

ments. The apatite track length distribution measure-

ments were carried out on sub-horizontal tracks

wholly included in the host mineral using faces

parallel to the c-axis of the mineral. Determination

of confined fission track lengths on the other six

samples was not possible because either (1) the track

density was too low, (2) the dislocation density was

too high, or (3) there was not enough material to

permit the measurement of a representative number of

confined tracks.

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