Asymmetric exhumation across the Pyrenean orogen: implications for the tectonic evolution of a collisional orogen

ELSEVIER Earth and Planetary Science Letters 173 (1999) 157–170www.elsevier.com/locate/epsl

Asymmetric exhumation across the Pyrenean orogen:implications for the tectonic evolution of a collisional orogen

P.G. Fitzgerald a,Ł, J.A. Munoz b, P.J. Coney a, S.L. Baldwin a

a Department of Geosciences, University of Arizona, Tucson, AZ 85721, USAb Group de Geodinamica i Analisi de Conques, Departament de Geodinamica i Geofisica, Universitat de Barcelona,

Zona Universitaria de Pedrables, Barcelona 08028, Spain

Received 17 December 1998; accepted 13 September 1999

Abstract

The Pyrenees are a collisional mountain belt formed by convergence between the Afro–Iberian and European plates.Apatite fission track thermochronology from three vertical profiles along the ECORS seismic line constrain the exhumationhistory of the Pyrenean orogen and hence tectonic models for its formation. In the Eocene there is relatively uniformexhumation across the Pyrenees, but significantly more exhumation occurs on the southern flank of the axial zone in theOligocene. The variation in exhumation patterns is controlled by a change in how convergence is accommodated withinthe Pyrenean double-wedge. Accommodation of thrusting on relict extensional features that leads to inversion dominatedthrust stacking resulted in relatively slow exhumation in the Eocene. However, subsequent crustal wedging and internaldeformation in the upper crust under the stacked duplex of antiformal nappes resulted in extremely rapid exhumationon the southern flank in the Oligocene. The Maladeta profile in the southern axial zone records extremely rapid EarlyOligocene exhumation followed by dramatic slowing or cessation of exhumation in the middle Oligocene and the formationof an apatite partial annealing zone (PAZ). This PAZ has subsequently been exhumed 2–3 km since the Middle Miocene,supporting the observations of Coney et al. [J. Geol. Soc. London 153 (1996) 9–16] that the southern flank of the rangewas buried by �2–3 km of syntectonic conglomerates in the Oligocene and subsequently re-excavated from Late Mioceneto Recent. The present-day topographic form of the Pyrenees is largely a relict of topography that formed in the Eoceneand the Oligocene. Comparison with paleoclimatic records indicates that the Eocene–Oligocene exhumation patterns arecontrolled by tectonic forces rather than resulting from an orographic effect due to uplift of the Pyrenees. 1999 ElsevierScience B.V. All rights reserved.

Keywords: tectonics; thermochronology; exhumation; Pyrenees; paleoclimatology

1. Introduction

The Pyrenean mountain belt is an intraplate col-lisional orogen formed by Late Cretaceous to EarlyMiocene convergence between the Afro–Iberian and

Ł Corresponding author. Tel.: C1 520 621 4052; Fax: C1 520621 2672; E-mail: [email protected]

European plates [1]. The Pyrenean orogen comprisesa central axial zone (AZ) of Hercynian Paleozoicbasement, flanked north and south by fold and thrustbelts developed in Mesozoic and Cenozoic sedi-mentary cover rocks. These thrust belts are in turnflanked to the north (Aquitane Basin) and to thesouth (Ebro Basin) by foreland basins (Fig. 1).

The Pyrenees are geologically well known be-

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158 P.G. Fitzgerald et al. / Earth and Planetary Science Letters 173 (1999) 157–170

Fig. 1. Regional tectonic setting of the Pyrenees located between the Aquitane and Ebro Basins. The Axial Zone of the Pyreneescomprising, in part, Hercynian basement lies between the north-vergent Northern Thrust Belt and the south-vergent South PyreneanThrust Belt. Modified from [2]. The three Hercynian plutons sampled are shown in solid black and labeled M (Maladeta), R (Riberot),and L (Lacourt).

cause of the unusual preservation of synorogenic de-posits with well exposed structural relationships thatconstrain their tectonic evolution [3–6]. These re-lationships, together with knowledge of lithosphericstructure as constrained by geophysical data, andthe absence of significant late extensional collapsestructures, make the Pyrenees a natural laboratoryto investigate orogenic processes and foreland basinformation (e.g. [5,7]). While the timing of eventsin the foreland fold and thrust belts are relativelywell constrained, the variation in timing, rate andamount of exhumation across the range is relativelyunconstrained. Quantification of the exhumation his-tory across the Pyrenees provides constraints on thevertical component of its tectonic evolution and con-strains tectonic models for its formation. Knowledge

of the rate of sediment supply as revealed by ex-humation histories is critical for understanding thestratigraphic record of foreland basins (e.g. [8]).

In this paper we present apatite fission track(AFT) thermochronology on samples from three ver-tical profiles collected from Hercynian granite mas-sifs plutons along the line of the ECORS transect,a deep-seismic reflection transect across the upliftedcentral core of the central Pyrenees. AFT resultsquantify the unusual exhumation history of this in-traplate orogen and have implications for the debateof climatic versus tectonic control on exhumation.

These AFT results also constrain the unusual post-orogenic evolution of the central Pyrenees southernflank, which besides being distinct from that of thenorthern flank, is somewhat unique amongst conver-

P.G. Fitzgerald et al. / Earth and Planetary Science Letters 173 (1999) 157–170 159

gent orogens in general. This is because the Ebroforeland basin became closed due to Late Eocene–Oligocene tectonism along its margins, allowing syn-tectonic detritus, derived primarily from the Pyre-nees, to fill the basin and then backfill across the ac-tively deforming southern Pyrenean thrust belt. Thisresulted in burial of the southern Pyrenean range withup to 2–3 km of continentally derived conglomer-ates topped by a graded depositional surface whichmerged with high-level erosion surfaces into the AZ[2]. After a period of tectonic quiescence marking theend of Pyrenean deformation in the Oligocene, therange was exhumed again to its present relief, proba-bly in the last 6 m.y. [2].

Data from an earlier Pyrenean fission track studyby Yelland [9] was modeled by Morris et al. [8] todetermine Eocene to Miocene exhumation rates. The

Fig. 2. Late Cretaceous to Present tectonic evolution of the Pyrenean crust along the ECORS line (modified from [7]). SPCU D SouthPyrenean central unit; NPU D North Pyrenean unit; SM D Serres Marginals; M D Montsec; B D Boixols; R D Rialp; O D Orri; N DNogueres; EB D Ebro Basin; AB D Aquitane Basin. Shaded portions represent lower crust. The asterisk in (a) and (b) is the position ofthe last reactivated extensional fault to accommodate convergence via thrusting. After this point contacts the point of convergence of theIberian and European plates (or singularity — labeled as S in b and c), deformation occurs via internal deformation in the upper crustbeneath the Rialp thrust sheet resulting in rapid rock uplift and rapid exhumation in the Oligocene. Plutons sampled for vertical profilesare shown in (c).

approach we use, in which vertical sampling pro-files are collected, as compared with their modelingof single samples, produces different interpretations.The different interpretations provide a good exampleof the type of information obtainable and relativeprecision in determining the exhumation history us-ing these different approaches (see below).

2. Geology of the Pyrenees

The Pyrenees are a mountain range dominated byinversion tectonics [10] due to Late Cretaceous toEocene–Oligocene thrusting along pre-existing ex-tensional structures (Fig. 2). These structures wereoriginally formed during Triassic to Cretaceous rift-ing and transtension associated with opening of the


Central Atlantic Ocean and rotation of Iberia toopen the Bay of Biscay [1]. The AZ is a com-plex south-vergent duplex structure that culminatesin an antiformal stack of three upper crustal stackedthrust sheets (Nogueres, Orri and Rialp) of Hercy-nian basement (Fig. 2). The basement antiformalstack is bounded to the north by the North Pyre-nean Fault (NPF), regarded as the boundary betweenthe Iberian plate and Europe [11]. The NPF formedinitially due to sinistral movement of Iberia withrespect to Europe in the middle Cretaceous [12,13].North of this fault zone, basement and cover rocksform north-vergent thrust sheets.

The Pyrenean orogen has a prevailing southwardvergence due to the asymmetric distribution of inher-ited intracrustal discontinuities above a northwardsubducting Iberian lower crust. Subducting lowercrust portrayed on balanced cross-sections by Munoz[7] has been imaged to 80 km beneath Europeancrust by ECORS [14], magnetotellurics [15], andseismic tomography [16]. Total structural relief onthe reconstructed Hercynian basement is ¾25 km,which given the present-day relief indicates 15–18km of erosion from the crest of the AZ antiformalstack.

The AZ formed synchronously with thrustingin the southern fold and thrust belt. The threemain southerly transported thrust sequences there[4,7] comprise the Boixols, Montsec, and SerresMarginals thrust sheets. The upper sequence of 5000m of mainly Mesozoic carbonate was deformed fromlatest Cretaceous to Early Paleocene, tectonicallydepressing the foreland into which turbidites andthen a more proximal shallow marine transitional toterrestrial sedimentary sequence were deposited. Inearly-middle Eocene the Montsec sheet of 3000 mof predominantly Late Cretaceous limestones movedbelow the upper thrust sheets, carrying them andtheir foreland basin deposits piggyback to the southrelative to northward-moving Iberian crust. Finally,in the Late Eocene through Oligocene, the SerresMarginals thrust sheets developed and deformationin the foreland area was broadly distributed without-of-sequence thrusting and reactivation of exist-ing structures [5,17–19]. The floor thrust carried allprevious thrusts and their associated foreland basindeposits piggyback southward synchronously withunderthrusting of the Rialp thrust sheet below the

Orri and Nogueres sheets and ramping down underthe AZ to the north uplifting it in a complex antifor-mal stack [7]. About one half of the total shorteningacross the Pyrenean orogen was contemporaneouswith burial of the thrust belt and exhumation of theAZ [20].

3. Sampling strategy, results and interpretation

AFTT is an established method for determiningexhumation histories of mountain belts and for con-straining their tectonic evolution (e.g. [21]). Thepower of this technique is fully exploited whensamples from vertical profiles are collected withina strategic tectonic framework to enable interpre-tation of data using the “exhumed apatite partialannealing zone (PAZ) concept” ([22] and referencestherein). This allows the conversion from the thermalreference frame to an absolute reference frame andpermits the timing, amount and rate of exhumation tobe determined [22–24]. In this technique, the shapeand slope of the AFT age profile as well as varia-tion of confined track length distributions (CTLD)provide an important basis for interpretation. Forsamples within a vertical profile characterized bypoor counting statistics or fewer tracks in the CTLD,a better assessment of their reliability is possiblecompared to samples not collected in close geo-graphic proximity and=or lacking the vertical profilesampling concept. Likewise, variation of data fromwithin vertical profiles can be better evaluated for itssignificance because the close geographic proximityof data points allows subtle trends to be recognized.

Hercynian granitic plutons incorporated withinAZ thrust sheets provide attractive targets for verticalsampling profiles as they contain abundant apatite.Profiles were collected from the Orri thrust sheet(Maladeta profile), Nogueres thrust sheet (Riberot)and north of the NPF (Lacourt) (Table 1, Fig. 3).

The Maladeta profile can be divided into twoparts, an upper part in which all AFT ages are con-cordant and a lower part in which ages decreasewith decreasing elevation (Fig. 3a). The steep slopeof the upper part of the profile, long mean lengths(½14 µm) and shape of the CTLD, indicate rapidcooling. The measurement of the slope of this steeppart of the profile is poorly constrained due to the


Table 1Fission track analytical results: Pyrenees; ECORS transect vertical profiles

Sample Elev. Number of Standard track Fossil track Induced track Chi-square Rel. error Central age Mean track length Std. dev.number (m) grains density density density prob. (%) (š1¦ , Ma) (µm) (µm)

(ð106 cm�2) (ð105 cm�2) (ð106 cm�2) (ð106 cm�2) (%)

Maladeta profilePY63 2850 19 1.73 (5613) 2.930 (89) 2.980 (905) 76 <1 31š 3PY64 2735 20 1.73 (5613) 2.754 (138) 2.858 (1432) 47 8 30š 3 14:0š 0:2 (66) 1.4PY65 2605 20 1.73 (5613) 3.822 (251) 3.587 (2356) 3 24 33š 3 14:0š 0:5 (10) 1.8PY66 2495 22 1.73 (5613) 3.162 (149) 3.101 (1461) 14 22 32š 3 14:2š 0:1 (100) 1.3PY67 2355 9 1.75 (5693) 2.321 (80) 2.171 (748) 44 <1 34š 4 14:4š 0:8 (3) 1.4PY68 2210 20 1.76 (5693) 3.824 (173) 3.798 (1718) 42 4 32š 3 14:0š 0:2 (99) 1.6PY69 2080 21 1.76 (5693) 3.860 (235) 3.859 (2349) 96 <1 32š 2 14:2š 0:4 (14) 1.5PY70 1945 20 1.78 (5693) 4.004 (211) 3.989 (2102) 68 1 32š 2 13:8š 0:2 (100) 1.8PY56 1780 20 1.75 (5810) 3.478 (193) 3.559 (1975) 22 16 31š 3 13:6š 0:2 (83) 1.7PY58 1620 20 1.77 (5810) 2.405 (145) 2.621 (1580) 20 5 29š 3 13:7š 0:4 (19) 1.7PY59 1500 19 1.74 (5646) 3.330 (118) 3.956 (1402) 84 2 26š 3 12:3š 1:0 (9) 3.0PY55 1400 20 1.74 (5810) 6.294 (183) 7.601 (2210) 36 11 26š 2 13:1š 0:1 (91) 1.2PY61 1365 20 1.77 (5646) 2.481 (109) 3.195 (1404) 47 9 25š 3 13:5š 0:2 (85) 1.7PY62 1255 19 1.79 (5646) 2.150 (54) 3.193 (802) 89 <1 22š 3PY60 1125 5 1.75 (5646) 2.568 (43) 3.870 (648) 94 <1 21š 3

Riberot profilePY36 2483 20 1.93 (6429) 3.340 (163) 2.647 (1292) 98 <1 44š 4 14:2š 0:1 (85) 1.1PY37 2340 18 1.97 (6429) 3.406 (204) 3.032 (1816) 52 10 41š 3 14:2š 0:3 (13) 1.1PY38 2205 20 2.11 (6671) 2.157 (140) 1.966 (1276) 43 5 42š 4 14:5š 0:5 (7) 1.3PY39 2050 25 1.99 (6429) 2.281 (204) 1.999 (1788) 65 2 41š 3PY34 1595 25 2.07 (6528) 2.753 (350) 2.797 (3556) 36 2 37š 2 14:7š 0:1 (66) 1.1PY35 1460 20 2.09 (6671) 3.316 (238) 3.442 (2471) 88 <1 36š 2 14:2š 0:3 (14) 1.2PY33 1340 25 2.06 (6528) 2.581 (227) 2.447 (2152) 74 1 39š 3 13:7š 0:2 (23) 1.1

Lacourt profilePY2 1048 25 1.53 (5222) 4.546 (426) 2.274 (2131) 55 5 55š 3 13:7š 0:2 (105) 1.7PY3 950 24 1.55 (5222) 4.315 (447) 2.520 (2610) 1 21 49š 3 13:4š 0:2 (101) 1.8PY7 815 25 1.63 (5222) 3.818 (479) 2.240 (2810) 8 16 50š 3 13:9š 0:2 (69) 1.3PY6 645 25 1.60 (5222) 5.111 (548) 2.987 (3202) 67 2 49š 2 14:1š 0:2 (60) 1.7PY5 470 17 1.57 (5222) 14.27 (1136) 10.85 (8643) 14 7 37š 1 13:5š 0:1 (129) 1.6

Apatite ages were determined using the external detector method and a kinetic automated stage. Samples were irradiated at the Oregon State University Nuclearreactor in the slow soaker position B-3 (thermal column #5) which has a Cd for Au ratio of 13.6 at the column face. Mounts were counted at 1250ð under a dry100ð objective. Ages were calculated using the zeta calibration method (zeta D 361 š 10 for dosimeter glass CN5) following the procedures of Hurford [25]and Green [26]. Analytical errors were calculated using the ‘conventional method’ [27]. Apatites separated from almost all Hercynian plutons in the Pyrenees,including Maladeta and Lacourt, are fluorapatites with near-Durango apatite composition [9]. Parentheses enclose number of tracks counted (density) or measured(track lengths). Standard and induced track densities were measured on mica external detectors (geometry factor D 0.5), and fossil track densities were measuredon internal mineral surfaces. Samples of Hercynian granitic basement rock were crushed and the apatites separated from them using conventional heavy liquid andmagnetic techniques. Apatite crystals were mounted in epoxy resin on glass slides, ground and polished to reveal an internal surface, and then etched for 20 s atroom temperature in 5 N HNO3 to reveal spontaneous fission tracks. In this paper central ages are reported rather than pooled or mean ages. The central age allowsfor non-Poissonian variations in the counts of fission tracks, providing a more robust measure of the central tendency of single-grain ages. The relative error or agedispersion (spread of the individual grain data) is given by the relative standard deviation of the central age. Where the dispersion is low (<15) the data are consistentwith a single population, and the mean=pooled ages and the central age converge and the sample should pass a chi-square test. The chi-square test performed onsingle-grain data [28] determines the probability that the counted grains belong to a single age population (within Poissonian variation). If the chi-square value isless than 5%, it is likely that the grains counted represent a mixed-age population with real age differences between single grains. Track lengths were measuredusing ‘confined’ fossil fission tracks using only those that were horizontal [29]. Tracks were measured under a 100ð dry objective using a projection tube and adigitizing tablet attached to a microcomputer. Wherever possible, 100 track lengths per sample were measured, the number being less only when insufficient suitabletracks were present in the available apatite.

relatively few samples and relatively large errors.However, if rapid exhumation had only recently be-gun [32] and advection of isotherms due to rapidexhumation had not yet reached steady state [33] theslope of an AFT age profile immediately under an

exhumed PAZ or following a period of exhumationnot rapid enough to perturb the isotherms (as in thiscase) will underestimate the true exhumation rate[32]. Other factors to consider during rapid exhuma-tion (>1 km=m.y.) are variation of exhumation rate


with respect to topography, especially if topographymaintains a stable geometry during exhumation, andthe effects of sampling from valleys versus ridgestaking into account likely depth differences to thecritical (closure temperature) isotherm which maylead to an overestimation of the exhumation ratefrom AFT age–elevation plots [34]. The essentially

vertical slope of this part of the Maladeta profileis interpreted as reflecting rapid exhumation at ¾32Ma, likely at a rate of 2–4 km=m.y. (see below)

The slope of the lower part of the Maladeta pro-file, plus the CTLD (means of �13.6 µm) indicatethat these samples have experienced more annealingcompared to samples from the upper part of the pro-


file. We interpret the lower part of the profile as theupper portion of an exhumed PAZ. A PAZ (corre-sponding to ¾60 to 110ºC for apatites of Durangocomposition) forms within a relatively stable thermaland tectonic regime, and its slope does not repre-sent an apparent exhumation rate [35]. Comparingthe measured slope of the lower part of the profile(63š23 m=m.y.) to slopes of modeled PAZs [36] in-dicate that the lower Maladeta profile would require10–28 m.y. to form if it formed within a completelystable tectonic and thermal regime; more time isneeded if a component of exhumation is included.Thus, the change in slope on the Maladeta profile at32–30 Ma marks an abrupt transition from a timeof extremely rapid exhumation to one of relativethermal and tectonic stability.

The Riberot profile yielded AFT ages rangingfrom 44 to 36 Ma with ages generally decreasingwith decreasing elevation. The slope indicates anaverage exhumation rate (173 š 80 m=m.y., š1¦ )from 44 to 36 Ma; this exhumation rate is not fastenough to perturb the isotherms (e.g. [37]). CTLD(means >14 µm, standard deviations <1.2 µm)indicate that the samples underwent relatively rapidcooling with comparatively less time spent within theapatite PAZ compared to the amount of time sincethat the samples have resided at cooler temperatureswhere annealing was minimal.

North of the NPF, the Lacourt profile only spans¾600 m relief but can be divided into two partslargely on the basis of CTLD. Although the data setis limited, we interpret the profile as straddling the

Fig. 3. (a) Apatite age versus elevation graphs for the Maladeta, Riberot and Lacourt profiles (errors are š1¦ ). Representative CTLDare shown with mean, standard deviation (µm), and number of tracks measured. (b) Generalized age profile interpretation for the threeprofiles. This profile gives the overall thermal and tectonic history of the central Pyrenees as revealed in the AFT data, but it does notimply that all profiles have undergone identical histories. (c, d) ‘Thermotrack’ models [30] of the Maladeta profile with and without LateMiocene exhumation. Thermotrack uses the Laslett et al [31] preferred annealing equation and incorporates the effects of heat conductionand advection. The models indicate that post-Mid-Miocene exhumation is required to elevate the exhumed PAZ (as revealed in theMaladeta profile) to its present position. The gray envelope represents the measured Maladeta profile (š1¦ ) and the diamonds representmodeled data points every 250 m with a fixed error of š3 Ma. In (c) the model cannot replicate the abrupt change in slope at 32–30 Mamarking the transition from extremely rapid exhumation to relative stability. The important feature to note in (c) is that to generate theslope of the lower part of the Maladeta profile, relative thermal and tectonic stability is required until 10–5 Ma (modeled here using thelimiting case of 5 Ma). This allows a PAZ to develop, prior to its exhumation to its present elevation. The input of 4.2 km to exhume theMiocene PAZ is not an indication of the true amount of exhumation as the model assumes that mean surface elevation is sea-level. Forthe Maladeta profile, 2–3 km of post-orogenic exhumation is required to reveal the PAZ formed in Oligocene and Miocene times. In (d)we have modeled 4.2 km of continuous exhumation since the mid-Oligocene which prevents formation of a PAZ. The modeled profile in(d) does not fit the observed profile at all indicating that a period of relative stability prior to later exhumation must have occurred (asshown in (c)).

base of an exhumed PAZ. The upper two samplesof the profile, with slightly shorter mean lengths(�13.7 µm) and larger standard deviations (½1.7µm) indicative of more annealing, lie at the base ofan exhumed apatite PAZ. The two samples belowthese have means ½13.9 µm and standard deviations�1.7 µm, indicative of rapid cooling with somepartially annealed tracks. These are interpreted asforming the lower transition from the base of a PAZto a region where tracks are essentially annealedinstantaneously over geologic time. We place thebreak in slope representing the base of the exhumedPAZ at 50 Ma and 1 km. Note that short tracks existbelow the break in slope reflecting the fact that thetrue shape of an exhumed PAZ is curved rather thanthe idealized straight lines shown here (cf. [22]). Wesuggest that the lowermost sample (PY-5) collectedsouthwest of a prominent gorge (Gorges de Ribaute)lies off the trend of the other samples (Fig. 3a) due tofaulting (up to the southwest), an inference supportedby additional AFT data recently collected by us, butnot presented here.

In the early stages of inversion there must havebeen a significant component of horizontal transportas the Nogueres as well as Orri thrust sheets weretransported above the previously horizontal EarlyCretaceous extensional detachment fault [7]. Thrust-ing or near-horizontal transport of rocks, whetherthose rocks are above or below an apatite PAZ, willnot be recorded in the AFT data as those trajectoriesare sub-parallel to the thermal frame of reference.There is no doubt thrusting is an important pro-


cess in the formation of the Pyrenees; however, therock exhumation trajectory is near vertical. This isshown in the cross-sections of Munoz [7] in whichrock uplift and formation of the AZ antiformal stackare caused by underthrusting, and rock uplift northof the NPF is caused by wedging of European crust(Fig. 2). A discussion of the effect of thrust geometryon exhumation in the Pyrenees is given by Morris etal. [8] who conclude that for thrusts dipping>10º theeffect of sub-vertical rock trajectories on exhumationcalculations using AFTT is negligible.

4. Discussion

AFT ages young from north to south across thePyrenees. Combined with geologic constraints onthe timing of thrusting and unroofing [4,5,7,10,17]the AFT profiles indicate an asymmetric pattern withmore exhumation to the south. We can combinethe three vertical profiles to determine a generalizedinterpretative age profile for the central Pyrenees,cognizant that each stage of the thermal=exhumationhistory may not be present or may vary at eachlocality (Fig. 3b). Taken together, the three profilesindicate an erosional history resulting from rockuplift due to Iberian–European plate convergence inthe Paleogene with different crustal levels exposed ateach profile due to varying amounts of exhumation.

Onset of rapid exhumation at ¾50 Ma is recordedin the Lacourt profile. The point of convergenceof two plates under a doubly vergent convergentmountain belt has been termed the singularity [38].Geodynamic models [39,40] predict that exhumationwas initiated above the subduction point or singu-larity likely located just south of the Riberot massif.Within the AZ, the southerly vergence of thrustfaults in the antiformal stack (Fig. 2) indicates thatexhumation is not synchronous, but youngs predom-inantly to the south [7]. Thus, exhumation may havebegun slightly earlier at Riberot than at Lacourt,but was probably initiated slightly later at Maladeta.The Riberot profile records Eocene exhumation at arate of ¾173 m=m.y. from 44 to 36 Ma. Exhuma-tion at this rate was followed by extremely rapidexhumation in the Early Oligocene (starting at ¾35Ma) concentrated on the southern flank of the AZas recorded in the Maladeta profile. The change in

slope at 32–30 Ma in the Maladeta profile signalsthe dramatic slowing or cessation of exhumation inthe middle Oligocene which we interpret to markthe apparent end of major tectonic activity in theMaladeta region and likely also the central Pyrenees.The presence of an exhumed PAZ in the lower partof the Maladeta profile indicates that this profilemust have been exhumed to its present elevation<20 Ma. Forward modeling of the Maladeta pro-file indicates this post-orogenic exhumation was postMid-Miocene, with the best possible fit between 10and 5 Ma (Fig. 3c).

4.1. Amount of exhumation

The amount of exhumation can be determinedusing the elevation of the ‘break in slope’ repre-senting the exhumed base of an apatite PAZ [23].This calculation requires knowing the present-dayelevation of the ‘break in slope’, the present-daymean land surface elevation around the samplingprofile, the paleo-mean annual surface temperatureand paleo-geothermal gradient. The geothermal gra-dient used for this calculation corresponds to thetime the PAZ formed (i.e. a time of relative ther-mal and tectonic stability) and thus is characterizedby relatively undisturbed isotherms. The present-dayupper crustal (10 km) geothermal gradient in thePyrenees is ¾30ºC=km [41]. We use a paleo-meanannual surface temperature of 5ºC and assume ageothermal gradient of 25–30ºC=km for the periodspreceding initiation of exhumation in the Eocene andthe Miocene.

4.2. Eocene–Oligocene exhumation

The amount of post-Eocene exhumation at Lacourt(break in slope at 50 Ma and 1 km elevation, presentmean land-surface elevation of 0.6 km) was 3.9–4.6km (Fig. 4, Table 2). Exhumed PAZs marking the on-set of Eocene exhumation (e.g. Lacourt) have sincebeen eroded away at Riberot and Maladeta. Thus wehave no direct constraints on the timing of the onsetof Eocene exhumation in the AZ. However, geody-namic models by Beaumont et al. [40] suggest ex-humation at the change of vergence in the Pyreneandouble wedge above the subduction point or singular-ity [7] would begin in the northern AZ slightly before

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Table 2Exhumation estimates across the central Pyrenees

Maladeta Profile Riberot profile Lacourt profile

Total exhumation since Eocene (km) ¾15, stratigraphic 4.9–7.1, calculated 3.9–4.6, calculatedEocene–Oligocene exhumation (km) Total 12–13 1.7–7.1 allowable, ¾5 likely 1.7–4.6 allowable, ¾4 likely

slow, 173š 80 m=m.y., �50–35 Ma 1.4–3.8 allowable, 2–3 likely 1.7–4.5 allowable, ¾4 likely 1.7–4.5 allowable, ¾4 likelyfast, km=m.y., 35 Ma to 32–30 Ma 8.2–11.6 (35–32 Ma), ¾10 likely 0–3.4 allowable, ¾1 likely 0–0.9 allowable, ¾0 likely

Post-orogenic exhumation (since Mid-Miocene) (km) 2–3, calculated <2 (<Maladeta), <1 likely <1 (residual), <0.5 likely

Total exhumation since the Mid-Eocene at Lacourt is determined using the elevation of the ‘break in slope’ representing the base of an exhumed PAZ. For this and all othercalculations, a paleo-mean annual temperature of 5ºC and a paleo-geothermal gradient of 25–30ºC=km was used. Total exhumation has been divided into three components, anEocene component (173š 80 m=m.y. from ¾50 Ma to ¾35 Ma) based on the slope of the Riberot profile, a very rapid Oligocene component (2–4 km=m.y., ¾35 to 32–30 Ma)based on the upper part of the Maladeta profile, and a post-orogenic component (post-Mid-Miocene) based on modeling to reproduce the timing of exhumation of the lower partof the Maladeta profile and then extrapolation of the slope of the lower part of the profile (63š 23 m=m.y.) to that time (¾6 Ma) to estimate the amount of exhumation. Twoestimates are given: ‘allowable’ is an envelope of exhumation based on uncertainties in the geothermal gradient plus extrapolation of age–elevation slopes (š1¦ least squaresregression on points taking into account š1¦ error bars); ‘likely’ is what we think is a good estimate of the amount of exhumation taking all other geological factors intoaccount. Post-orogenic exhumation is only constrained by data from Maladeta. At Lacourt and Riberot post-orogenic exhumation is constrained as being less than at Maladeta orthe residual after subtracting other ‘likely’ components from the total estimated exhumation since the Mid-Eocene.


Fig. 4. (a) Simplified schematic geologic cross-section across thecentral Pyrenees near the ECORS line — simplified from [42].See Fig. 2 for crustal-scale cross-section. M, R and L refer toMaladeta, Riberot and Lacourt plutons. NPF D North PyreneanFault. (b) Estimated exhumation across the Pyrenees from FT con-straints — see Table 2 for determination of estimates and explana-tion of allowable envelopes versus likely amounts of exhumation.(c) Reconstructed age–elevation profiles to show thermal historiesand calculation of total exhumation since the Eocene.

exhumation north of the NPF. So, while it would be arelatively simple matter to extrapolate up the Riberotprofile at a rate of 173 š 80 m=m.y. to 50 Ma to cal-

culate the amount of exhumation, any estimate usingthis method would be a minimum. For calculations ofthe amount of exhumation at Riberot we model onsetof exhumation as 50 Ma, aware that this results ina slight underestimate. Using the same assumptionsas above and 50 Ma as the timing for an exhumedPAZ at Riberot (the base of which would then lie at2.9–4.4 km elevation) gives an estimate for the mini-mum amount of exhumation as 4.9–7.1 km. The steepand unconstrained slope at Maladeta does not per-mit us to extrapolate up the profile to determine theamount of exhumation since the Eocene. However,a balanced reconstruction of the eroded stratigraphyof the Orri and Nogueres thrust sheets there [7] indi-cates that a total of ¾15 km has been eroded awaysince convergence began. We also use 50 Ma for theonset of exhumation at Maladeta, although exhuma-tion may have initiated there slightly after this as theonset of exhumation likely progressed from north tosouth across the AZ [40].

4.3. Post-orogenic (i.e. post-mid-Miocene)exhumation

The Maladeta is the only profile that clearlyrecords post-orogenic exhumation (i.e., post-Mid-Miocene). Extrapolating the lower part of this profileto 6 Ma, taking into account the š1¦ uncertainty onthe slope (63 š 23 m=m.y.) and assuming a paleo-geothermal gradient of 25–30ºC=km, a paleo-meanannual surface temperature of 5ºC, with a presentmean land surface elevation of 1.5 km yields 2–3 kmof post-orogenic exhumation for the Maladeta profile(Table 2). Thus at the Maladeta profile, ¾12–13 kmof exhumation occurred in the Eocene–Oligoceneand 2–3 km exhumation since the Middle Miocene.Of this 12–13 km of exhumation, a maximum of1.4–3.8 km (50–35 Ma at 173 š 80 m=m.y.) willoccur at this slower rate while the majority (8.2–11.6km) occurs extremely rapidly between ¾35 and ¾32Ma (giving a rate of ¾2–4 km=m.y.).

It is difficult to separate out the possible com-ponents of rapid Oligocene exhumation versus post-orogenic exhumation (post Mid-Miocene) at eitherRiberot or Lacourt, as neither event is recorded ateither profile. A maximum of 4.5 km of exhuma-tion (50–32 at 173 š 80 m=m.y.) is permissible inEocene–Oligocene times at this slower exhumation


rate at Lacourt, possibly slightly more at Riberot.If there had been a significant rapid Oligocene ex-humation component plus a significant post-orogeniccomponent at Riberot or Lacourt, the age profileswould record this, and thus combined, these twocomponents do not exceed ¾4 km (i.e. enough ex-humation to expose pertinent AFT information atthe surface) at either profile. This suggests that theextremely rapid Oligocene exhumation as recordedat Maladeta was minor, if present at all at Lacourt,as was any post-orogenic exhumation, but both com-ponents were possible at Riberot owing to greateroverall exhumation there.

Morris et al. [8] used the genetic algorithm ap-proach of Gallagher [43] to model AFT data ofYelland [9] and thus establish exhumation patternsin the Pyrenees. This approach models single apatiteages to determine a thermal history. Morris et al.[8] modeled AFT data from throughout the Pyre-nees and assumed a constant geothermal gradient tocalculate exhumation rates. Their results suggestedthat exhumation rates along the ECORS profile re-mained relatively uniform (¾100–300 m=m.y.) fromthe Eocene to the Middle Miocene. In contrast, usingthe vertical profile approach, a two-stage Eocene–Oligocene asymmetric pattern and post-orogenic ex-humation is revealed. While the two data sets (Mor-ris et al. and this study) are compatible, the differentinterpretations reveal the strengths and weaknessesof each approach. The vertical profile approach hasthe potential to provide more precise information re-lated to the thermal history and exhumation throughtime, but can be limited in its geographical coverageas more samples must be processed to reveal theage–elevation patterns. While the single age-model-ing approach allows derivation of generalized ther-mal histories over a larger geographic area, it lacksprecision in determining the thermal and exhuma-tion history through time. Ideally, the combination ofboth approaches provides both precision and regionalcoverage.

5. Tectonic implications

The Eocene through Oligocene exhumationrecorded at the three profiles is due to convergenceof Iberia and Europe which resulted in inversion of

Triassic–Cretaceous rift basins, thrusting in fold andthrust belts and underthrusting to create the upliftedAZ antiformal stack [7]. Although convergence beganin the Late Cretaceous, exhumation is not recorded inthe Lacourt profile until ¾50 Ma. The geometry ofstructures in the AZ indicates that deformation pro-gressed southward [7], and it is likely that onset ofexhumation occurred at<50 Ma at Maladeta but>50Ma at Riberot: relatively uniform exhumation in theEocene recorded by the Riberot profile and followedby a highly asymmetric exhumation pattern in theOligocene as revealed by the Maladeta profile.

The observed pattern of exhumation across thePyrenees can be explained by the following tectonicscenario (Fig. 2), recently modeled by Beaumont etal. [40]. As deformation progresses south toward theforeland, convergence is taken up along relict Cre-taceous extensional structures. Eocene exhumation(¾173 m=m.y. at Riberot) is the erosional responseto the progressive orogenic inversion formed bythrusting along these Cretaceous extensional faults.Eventually the thrust faults can no longer accommo-date convergence as the faults successively reach thepoint of convergence (or singularity) of the Iberianand European plates. When the last extensional faultreaches the singularity, ongoing convergence is takenup by internal deformation in the upper crust of theIberian plate and wedging of the European crust,resulting in rapid rock uplift and rapid exhumation[40]. Rapid rock uplift and resulting exhumation areat a maximum on the Ebro Basin side (pro-foreland)of the singularity where the complete stack of threebasement thrust sheets occur. However, rock upliftand exhumation due to internal deformation of theupper crust located below the sole thrust is minor ornot present on the retro-basin side of the singularitywhere north-directed thrusts dominate.

The asymmetric exhumation pattern presentedhere invalidates lithospheric models that call forsymmetric exhumation centered on the NPF or mod-els with lower crustal stacking below the northernAZ [44,45]. We consider that the asymmetric ex-humation pattern is much more likely to have re-sulted from tectonic forces as described above ratherthan as an orographic response (e.g. [46]) to grow-ing Eocene–Oligocene topography. The Ebro Basinwas characterized by a warm and humid climate inthe Eocene [47] when it was open to the Atlantic


Ocean. Once it became closed to the Atlantic dueto uplift of the Cantabrian Ranges [48], climate be-came arid with reduced rainfall [47]. As it is today,the prevailing wind direction for the Pyrenees in theOligocene was probably from the northwest and thusthe northern flank rather than the southern flank ofthe Pyrenees should have undergone orographic in-duced exhumation (e.g. [49]). The southern flank ofthe AZ was probably in a rain shadow that was lesslikely to facilitate rapid erosion unless controlled byother (e.g. tectonic) forces.

The Maladeta profile records abrupt slowing orcessation of exhumation at 32–30 Ma with ex-humation maintained at very low rates until laterpost-orogenic exhumation. No simple relationshipexists between elevation and exhumation rate aslandscape sensitivity, style of landscape evolution,local drainage patterns, and climate must also beconsidered [50], as well as the persistence of localrelief. However, one would not expect exhumationto slow so dramatically, as once the tectonic driv-ing force (i.e. convergence) had ceased, the isostaticresponse to erosion of the existing relief should con-tinue to drive exhumation and rock uplift, followedby a gradual reduction in the rate of erosion as reliefin the mountains was reduced. We suggest that theabrupt slowing of the erosion rate at ¾30 Ma wasdue to a change in base level and a significant reduc-tion of local relief. This is consistent with the modelof Coney et al. [2] which suggests that closure ofthe Ebro Basin resulted in burial of the active foldand thrust belt, followed by complete filling of thebasin, burial of paleo-drainages and canyons alongthe southern flank of the AZ and onlapping of thesesyntectonic deposits onto the erosion surface nowpreserved high on the southern flank of the AZ. Thisresulted in syn-tectonic burial of the Ebro Basin andpartial burial of the southern flank of the Pyreneesresulting in a change in baselevel.

The preservation of an exhumed PAZ in theMaladeta profile indicates relative tectonic and ther-mal stability from ¾30 to at least 20 Ma (Fig. 3). Asdiscussed above, it is likely that relative stability ex-isted until between 10 and 5 Ma in order for the PAZto develop to the extent it is revealed in the Maladetaprofile. The Maladeta profile thus records the se-quence of events as postulated by Coney et al. [2]:erosion of a rapidly uplifted AZ in the Oligocene,

filling of the closed Ebro Basin, burial of a paleo-landscape on the southern flank of the Pyrenees,followed by a long period of relative quiescenceand subsequent re-excavation of that landscape. TheAFT data records when rapid exhumation of the AZceases (32–30 Ma), the formation of a PAZ dur-ing that time of relative stability (from 30 to 10–5Ma), when re-excavation began (10–5 Ma, from for-ward modeling) and its magnitude (2–3 km). Coneyet al. [2] proposed that Miocene–Pliocene re-exca-vation of the south flank of the Pyrenees was acombination of Miocene rifting offshore of the Cata-lan Coast Ranges to open the Valenica trough andthe Late Miocene Messinian desiccation crisis. TheMessinian crisis lowered regional baselevels creat-ing a vigorous proto-Ebro River system which eithereroded headward across the Catalan Coast Rangesor was superimposed across these ranges, eventu-ally capturing, dissecting and evacuating the closedEbro Basin and the southern flank of the Pyreneesin the last 6 Ma. Thus, the 2–3 km of post-orogenicexhumation at the Maladeta profile is most likelydominated by re-excavation of the southern flank ofthe range.

6. Conclusions

Data from the AFT profiles indicate that exhuma-tion across the Pyrenees was asymmetric. Uplift ofthe range was due to convergence in the Eoceneand Oligocene. The observed Eocene–Oligocene ex-humation patterns result from the change in tectonicstyle of the Pyrenean double-wedge from inversiondominated thrust stacking to crustal wedging and in-ternal deformation. Accommodation of thrusting onrelict extensional features resulted in relatively slowexhumation; however, subsequent internal deforma-tion in the upper crust under the stacked duplex ofantiformal nappes led to extremely rapid exhumationon the southern flank of the AZ. Sediment supply tothe Ebro foreland basin was mainly controlled by thechanging tectonic style rather than by the orograph-ically induced climatic changes due to Pyreneanuplift during the Paleogene. The AFT data supportthe observations of Coney et al. [2] that the southernflank of the range was buried by up to 2–3 km ofsyntectonic conglomerates in the Oligocene and was


subsequently re-excavated since the Late Miocene.Thus, the present-day topographic form of the Pyre-nees is largely a relict of the topography that formedin the Eocene and Oligocene.

Acknowledgements

Support for this project was from National Sci-ence Foundation Grant EAR-9506454. JAM ac-knowledges partial funding by project PB97-0882-C03-03. We thank Darlene Coney for assistance inthe field and John Dohrenwend for help with DEMdata used to construct Fig. 4a. Reviews by KerryGallagher, Sean Willett and Hugh Morris helpedclarify and improve this paper. This study is dedi-cated to the memory of Peter Coney who died 20thFebruary 1999. His depth and breath of knowledgeand his contributions to understanding fundamentaltectonic processes have been an influence not only tous, but also to countless other geoscientists. [RV]

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Documents

Asymmetric exhumation across the Pyrenean orogen: implications for the tectonic evolution of a collisional orogen