Exhumation of the Pyrenean orogen: implications for sediment discharge

Basin Research (1998) 10, 69–85

Exhumation of the Pyrenean orogen:implications for sediment dischargeR. G. Morris,* H. D. Sinclair* and A. J. Yelland†*The School of Earth Sciences, The University ofBirmingham, Edgbaston, Birmingham B15 2TT, UK†London Fission Track Research Group, Research School ofGeological and Geophysical Sciences, Birkbeck College andUniversity College London, Gower Street, London WC1E6BT, UK

ABSTRACT

Apatite fission track analyses of 21 samples from the central and eastern Pyrenees are modelledto generate time–temperature plots for the post 110±10 °C cooling history over the 40–10 Matime interval. Modelled thermal histories have been converted into exhumation plots throughthe application of the present-day geothermal gradient in the Pyrenees. The documentedgeology of the Pyrenees allows us to assume no significant extensional unroofing andsubvertical exhumation trajectories, thus enabling exhumation to be translated into erosionaldenudation. Maps of denudation have been constructed for six, 5-Myr time intervals.Denudation varied with a 20–50-km length scale, and does not appear to have been related tothe major structural zones of the mountain belt. Spatially averaged denudation rates for the sixtime intervals ranged from 34 to 61 mm kyr−1 assuming the present-day geothermal gradient.Maximum rates of 240 mm kyr−1 occurred in the interval 35–30 Ma, in the region of theQuerigut-Millas massif.

Assuming the denudation resulted primarily from erosion, the denudation maps can be usedto calculate sediment discharge through time to the neighbouring foreland basins. Using aseries of rectangular drainage basins with a 251 aspect ratio (based on modern linear mountainbelts) and a location of the main drainage divide based on the mean present-day position, it ispossible to evaluate the potential for spatial and temporal variations in sediment discharge as afunction of denudation. The results show along-strike variations in sediment dischargebetween drainage basins of 500%, and temporal variations from individual basins of >300%.A comparison of total sediment discharge per year to the Ebro and Aquitaine basins, assuminga fixed drainage divide, shows that the discharge to the south is likely to have been between 1.5and 2.8 times greater than to the north.

sediment supply derived from erosion of the mountainINTRODUCTION

belt (Flemings & Jordan, 1989; Sinclair & Allen, 1992;Johnson & Beaumont, 1995). Given the significance ofRates and magnitudes of erosional denudation are funda-

mental parameters in the mechanical, geomorphological erosional denudation to orogenic processes, little is knownabout long-term (105–107 Myr) spatial and temporaland sedimentological development of mountain belts and

their associated sedimentary basins. Mechanically, the changes in rates and magnitudes.Present-day mechanisms of weathering and erosion inremoval of material from the upper surface of a deforming

orogen causes a modification to the gravitationally mountainous terrains are well understood (Selby, 1993),but the long-term rates at which these processes denudeinduced stresses, leading to a change in strain expressed

through folding and thrusting (Dahlen & Suppe, 1988; entire drainage basins and mountain belts are moredifficult to document. Present-day rates for the denu-Beaumont et al., 1991); the geomorphology of a mountain

belt is dependent on rates of surface uplift, and dation of a drainage basin are commonly obtained frommeasurements of the sediment mass transported at theresponding rates of erosional downcutting (Penck, 1953;

Koons, 1989; Montgomery, 1994); and the sedimentolog- mouth of the trunk stream divided by the drainage basinarea (Fournier, 1960; Milliman & Meade, 1983;ical development of foreland basins is a function of the

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Summerfield & Hulton, 1994; Hovius, in press). underthrusting of the Iberian plate beneath the Europeanplate (ECORS-Pyrenees Team, 1988; Choukroune et al.,However, the projection of these results to the geological

time-scale is hampered by a number of factors such as 1989; Munoz, 1992). Continued convergence and short-ening in Oligocene to early Miocene timesanthropogenic influences on drainage systems, and stor-

age of sediment in floodplains, lakes and alluvial fans (Puigdefabregas & Souquet, 1986) established an internal,uplifted axial zone flanked by oppositely vergent fold-(Walling, 1983).

An alternative method for calculating long-term denu- thrust belts and associated foreland basins (Fig. 1). Fromnorth to south the Pyrenean orogen can be divided intodation rates is by documenting the cooling history of

dated rock samples in the mountain belt (thermochronol- five principal zones: (1) the Aquitaine retro-forelandbasin; (2) the North Pyrenean thrust belt; (3) the Axialogy), and assuming a geothermal gradient and a vertical

rock trajectory, to obtain a value for the magnitude of Zone; (4) the South Pyrenean thrust belt; and (5) theEbro pro-foreland basin (Munoz, 1992; Verges et al.,exhumation in a given time interval. Numerous thermo-

chronological methods exist, but the most applicable to 1995).The Axial Zone of the central and eastern Pyreneesstudies of denudation is apatite fission-track analysis

(AFT analysis) which records cooling from a 110±10 °C comprises an antiformal stack of Hercynian basementthrust sheets, which are named, from top to bottom,isotherm, thus documenting cooling in the upper 3–5 km

of the crust (Brown et al., 1994). This approach has been Nogueres, Orri and Rialp (Munoz, 1992). These base-ment rocks contain a series of crustal derived, Hercynianapplied to the unroofing history of numerous mountain

belts such as the Himalayas (Zeitler et al., 1982), the granitoid plutons, referred to as ‘massifs’ (Zwart, 1979;Gleizes et al., 1997). The boundary of the Axial ZoneAndes (Kohn et al., 1984) and the Alps (Hurford et al.,

1989; Hurford et al., 1991). A recent development in the with the Northern Pyrenean thrust belt is marked by azone of subvertical faulting and shearing known as theanalytical approach to fission track length distributions

(Gallagher, 1995) has enabled detailed temperature/time North Pyrenean Fault zone. During late Albian to earlyCenomanian times the North Pyrenean Fault (NPF)plots to be developed for the post 110 °C cooling history

of individual samples (see below for a more detailed acted as a sinistral strike-slip and extensional dip-slipfault system (Debroas, 1987, 1990). This strike-slipexplanation). Assuming a geothermal gradient, these

cooling curves can be translated into exhumation rates, motion acting on thinned crust resulted in the transportof deep crustal material to upper crustal levels (Munoz,and in the absence of extensional unroofing (Platt, 1986),

these exhumation rates can be equated with erosional 1992) as characterized by the presence of lower crustalgranulitic rocks and ultrabasic upper mantle rocksdenudation.

The aims of this study are to apply AFT analysis to (lherzolites) along the North Pyrenean Fault zone(Choukroune, 1976; Vielzeuf & Kornprobst, 1984). This21 samples across the central and eastern Pyrenean orogen

of France and Spain (Fig. 1) to evaluate the spatial and information combined with palaeogeographical recon-structions indicate that the North Pyrenean Fault is thetemporal variability in the magnitude of denudation over

the 40–10 Ma interval. Maps of denudation will be used suture zone between the Iberian and European plates(Choukroune et al., 1973; Choukroune, 1976; Mattauer,to calculate sediment discharge to the neighbouring

foreland basins through time. Initially, the observed AFT 1985).The North and South Pyrenean thrust belts comprisedata must be thermally modelled to derive cooling histor-

ies, then translated to exhumation and finally erosional thrusted Mesozoic and/or basement rocks derived fromthe European and Iberian plates, respectively. A signifi-denudation; the numerous assumptions in each step are

assessed along the way. The results will then be con- cant difference between the two thrust belts is that thenorthern one involves Hercynian basement massifssidered in terms of the implications for sediment dis-

charge to the northern and southern foreland basins. (Fischer, 1984; Baby et al., 1988; Deramond et al., 1993)which are overlain in part by an Upper Cretaceous flyschseries. The South Pyrenean thrust belt consists of south-PYRENEAN GEOLOGYward-imbricated thrust sheets comprising mainly aMesozoic platform series and Palaeogene siliciclastics.The Pyrenees mountain belt is an asymmetric, doubly

vergent orogen which developed as a consequence of The main thrust sheets along the ECORS line are, fromnorth to south, the Boixols, the Montsec and the SerresEuropean and Iberian plate collision. Palaeomagnetic

models constrain Mesozoic–Cenozoic regional kinematics Marginals. In addition, from east to west three thrust-top basins form part of the southern thrust belt, theof the Afro-Iberian and European plates, primarily associ-

ated with the opening of the central Atlantic Ocean and Tremp-Graus, the Ainsa and the Jaca basins.The timing of thrusting has been documented in thethe Bay of Biscay (Srivastava et al., 1990; Roest &

Srivastava, 1991). An initial sinistral displacement of South Pyrenean thrust belt, with initiation of thrustingin lower Eocene times (#55 Ma), and rapid thrustIberia resulted in a series of Triassic to Cretaceous

extensional to transtensional rift systems. The onset of propogation until #47 Ma (Verges et al., 1995; Verges& Burbank, 1996). The termination of thrusting has beennorthward convergence during Late Cretaceous times

inverted these east–west trending structures during documented using magnetostratigraphic dating of

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Exhumation of the Pyrenean orogen

Fig. 1. (a) Simplified structural map ofthe Pyrenees showing the main tectoniczones from north to south: theAquitaine retro-foreland basin; theNorth Pyrenean thrust belt (NPTB);the North Pyrenean Fault (NPF); theAxial Zone; the South Pyrenean thrustbelt (SPTB); the Ebro pro-forelandbasin. The ECORS-Pyrenees line andan outline of the study area (see Fig. 3)are shown for reference. (b) Structuralcross-section along the ECORS-Pyrenees line (a) (adapted from Munoz,1992) highlights the main thrustgeometries and, notably, the antiformalstack geometry of the Axial Zone. Themarked drainage divide corresponds tothe topography along the ECORS-Pyrenees line whereas the meantopographic profile is a spatiallyaveraged section from a DEM, using a1-km2 resolution, along the entirelength of the Pyrenees.

syn- and post-deformational sediments as upper north to 600 m in the south of the Ebro Basin (Lanajaet al., 1987).Oligocene (#25 Ma; Meigs et al., 1996; Hogan &

Burbank, 1996).The Aquitaine retro-foreland basin is the result of

METHODOLOGYthrust sheet loading from the North Pyrenean thrust beltand the Axial Zone (Brunet, 1986). The sedimentary Fission track analysisinfill consists of Late Cretaceous to Eocene flyschsequences which became progressively more clastic Apatite fission track analysis relies upon the quantitative

analysis of the effects of the radiometric decay of 238U.during Eocene–Oligocene times with the transition tocontinental alluvial fan systems (Bourrouilh et al., 1995). 238U undergoes spontaneous nuclear fission in which the

two, highly charged fissile products are propelled apartThe Tertiary infill of the southern part of the basin, tothe north of the North Pyrenean Frontal Thrust, averages at 180° producing a linear damage zone within the host

apatite crystal lattice. These damage zones are termed2500 m in thickness (BRGM et al., 1972).The Ebro pro-foreland basin is the flexural response fission tracks (Fleischer et al., 1975) and can be analysed

under an optical microscope (×1250 magnification) afterto the load of the South Pyrenean thrust belt and theAxial Zone (Millan et al., 1995). The stratigraphy of the a standardized chemical etching treatment on a polished

surface of the host apatite crystal (Green, 1986). SpecificEbro Basin consists of Palaeocene continental red beds,overlain by a succession of platform carbonates and laboratory techniques and procedures for AFT analysis

are well documented within the literature (Laslet et al.,evaporites of lower to middle Eocene age, to marls whichrepresented distal facies of alluvial fans and deltas during 1982; Hurford & Green, 1982, 1983; Hurford, 1990).

Essentially, the number of spontaneous fission tracksmiddle to late Eocene times (Verges et al., 1995).Continental deposition continued from Eocene to early is a function of the uranium content of the material and

the time since the crystal was exposed to temperaturesOligocene times (Agustı et al., 1987) which contributedto a total Tertiary infill of between #5000 m in the above which no tracks are preserved. The uranium

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R. G. Morris et al.

content is determined by irradiating the sample with a in the thermal modelling can be summarized in a flowchart:fluence of thermal neutrons in order to induce fission of

the less abundant 235U atoms. The ratio of spontaneousto induced track densities is applied to the isotopicabundance ratio, I, in order to calculate the amount ofuranium for a particular crystal.

For this study, 21 samples were collected (Fig. 2;Table 1) from the Axial Zone and the North Pyreneanthrust belt. The apparent ages of the samples werecalculated (Yelland, 1990, 1991) using the fission trackdating equation (Price & Walker, 1963) given by (1):

t=1

ldln C1+

ldQsIrs

lfri D (1)

where t=fission track age in years; rs/ri=spontaneous/induced fission track density ratio; ld=

238U total decayconstant, 1.55125×10−10 yr−1 ( Jaffey et al., 1971); lf=spontaneous fission decay constant for 238U; Q=thermalneutron fluence; s=thermal neutron fission cross-sectionfor 235U, 580.2×10−24 cm2 (Hannah et al., 1969);I=isotopic abundance ratio 235U/238U, 7.2527×10−3

(Cowan & Adler, 1976).

Observed fissiontrack data

Specify time:temp.model space

no. of generations

'Better fitting'models

'Goodness of fit'(misfit function)

Initial Monte Carlosimulation

Modelled coolinghistories

'Best-fit'cooling history

Confidenceregions

Genetic Algorithm

Fission tracks in apatite are initially 16±1 mm inlength, although under increasing temperature their A series of time–temperature boxes are specified in order

to define the model space. The optimum number of theselength shortens. The temperature dependence of thisprocess has been quantified for apatite (Laslett et al., time–temperature boxes was typically four, in terms of

producing good fitting solutions. Increasing the number1988; Duddy et al., 1988; Green et al., 1989). Thedistribution of track lengths therefore reflects the thermal of time–temperature boxes did not improve the model

results. The program then commences selecting discretehistory of the host rock. The rate of thermal annealingis nonlinear and increases markedly above #60 °C until points from within this model space in order of increasing

time and subsequently constructs a temperature historyat 110±10 °C all pre-existing fission tracks are com-pletely annealed. This temperature range defines the by joining the selected points by linear segments. The

program applies a genetic algorithm (GA; Gallagherpartial annealing zone (PAZ; Gleadow & Fitzgerald,1987). In addition, induced tracks in apatite undergo et al., 1991; Holland, 1992; Press et al., 1992; Gallagher

& Sambridge, 1994; Gallagher, 1995) to improve thesome annealing at room temperature over time-scales ofminutes to a few weeks (Donelick et al., 1990) such that thermal histories initially generated by a Monte Carlo

selection. The GA provides a method for exploring thethe assumed unannealed length of 16±1 mm may beunderestimated. global model space and uses information obtained during

this exploration to identify thermal histories which pos-sess a better goodness of fit with the observed data. AfterThermal modelling200 iterations for each generation, the GA combines thebetter models in the next generation to result in a betterA fission track thermal history modelling program

(Gallagher, 1995) has been used to model temperature fitting solution. This procedure was allowed to continuefor typically 20 generations such that 4000 modelledhistories for the 21 samples taken from the Pyrenean

massifs. For each sample, apparent fission track age, thermal histories were generated for each sample.Track length reduction as a function of the temperaturemean track length, standard deviation of the track length

distribution and frequencies for the track length histo- history is predicted using several annealing algorithms(Laslett et al., 1988; Carlson, 1990; Crowley et al., 1991).gram, binned at 1-mm intervals, were input into the

model (Table 1). The program applies a stochastic search Microprobe analyses of the samples modelled show arange of 0.00–0.23 wt% Cl such that the most appropriatescheme to find a near optimal thermal history, which

minimizes the data misfit between predicted results and algorithm was chosen on the basis of the chemicalcomposition of the particular apatite. The Laslettthe observed fission track parameters. The stages involved

Fig. 2. Fission track length distributions, binned at 1-mm intervals, with modelled ‘best-fit’ thermal histories for the 21 samplestaken from the Pyrenean orogen. The study area is separated into the North Pyrenean Zone (NPZ) and the Axial Zone by theNorth Pyrenean Fault (NPF). The lighter shading corresponds to the Hercynian basement and the darker shading the granitoidplutons, referred to as ‘massifs’.




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Table 1. Apatite fission track analysis data for the 21 Pyrenean samples used in the study (Yelland, 1991).

No. of Dosimeter† Spontaneous† Induced† Mean track Standard No. ofElevation crystals length deviation confined

Sample Location-Massif Easting* Northing* (m) counted rd Nd rs Ns ri Ni ‡x2 Age±1s (mm) (mm) tracks

AG1 St. Arnac-Agly 6151 553 240 20 1.808 12541 0.5654 278 4.324 2126 20–19 46.6±3.0 14.01 1.22 67AR1 Cazaux Debat-Arreau 4407 663 700 20 1.808 12541 0.3754 170 3.826 1733 10–19 35.0±2.8 14.21 1.45 80AS1 Gorges des Merens-Aston 5581 431 935 20 1.019 7053 1.157 604 7.391 3859 21–19 31.5±1.4 13.36 1.69 75AS3 Ax Bonascre-Aston 5578 450 1160 14 1.019 7053 0.9234 399 5.318 2298 12–13 34.9±1.9 13.87 1.41 100B3 Bielsa 4250 340 1020 19 1.561 11748 0.2855 121 5.526 2342 16–18 16.0±1.5 12.74 2.39 100CA3 Arrimoulat-Lesponne 4262 783 88 16 1.561 11748 0.9407 357 8.019 3043 23–15 35.9±2.5 14.3 1.43 75CAN5 Ref. Marailles-Canigou 6062 222 1718 15 1.109 7679 0.8116 426 7.43 3900 13–14 23.9±1.3 12.14 2.12 76CB1 Lago Caballers-Maladetta 4787 325 1635 15 1.808 12541 0.5585 123 5.608 1235 14–14 35.6±3.4 13 2.04 103F1 Foix-Arize 5390 735 455 20 1.019 7053 1.358 651 2.556 1225 19–19 106.3±5.3 13.28 1.69 109FTG8 Bach de Pleus-Agly 6242 479 500 15 1.109 7679 1.403 682 7.206 3503 8–14 42.6±1.9 13.19 1.45 115FTG10 Belesta-Agly 6215 472 440 10 1.109 7679 1.491 444 7.123 2121 5–9 45.8±2.5 13.38 1.43 66FTG18 Espou-Arize 5063 698 520 15 1.019 7053 1.857 751 9.384 3796 6–14 39.8±1.7 12.96 1.59 106FTG23 Gorges de Merens-Aston 5580 430 900 12 1.019 70531 1.45 464 8.424 2695 22–11 34.6±1.8 13.27 2.32 101HOS1 Col de Puymorens-Hospitalet 5572 298 1980 15 0.9766 6764 0.7722 385 5.397 2691 13–14 27.6±1.5 13.75 1.86 90HOS4 Merens les Vals 5588 394 1143 20 0.9766 6764 0.7557 337 5.126 2286 25–19 28.4±1.7 12.97 1.43 95LA7 Lac d’Aubert-Neouvielle 4206 622 2110 18 1.561 11748 0.5516 246 7.198 3210 13–17 23.6±1.6 13.26 1.53 63M3 Col de Sarille-Trois Seigneurs 5147 651 945 25 1.019 7053 0.624 490 3.288 2582 21–24 38.2±1.9 13.72 1.77 100M4 St. Sernin-Arize 5082 682 500 17 1.019 7053 1.381 568 8.241 3390 14–16 33.7±1.6 13.9 1.64 101MER3 Pont de Arene les Vals 5588 407 1015 10 0.9766 6764 1.919 537 12.06 3375 6–9 30.7±1.5 12.92 1.73 90MER4 Pont de Arene-Merens les Vals 5587 407 1000 10 0.9766 6764 1.538 588 10.86 3153 7–9 27.3±1.3 13.28 1.29 100OM3 La Rouquette-Querigut/Millas 5835 405 1330 20 1.561 11748 0.4584 240 4.248 2224 14–19 33.3±2.3 14.39 1.49 66

* Locations based on UTM topographic maps (Yelland, 1991). † rd, rs and ri are track densities as measured (106 tracks cm−2); Nd (calculated using dosimeter glass CN-5; mica external detector), Ns (internalsurface) and Ni (mica external surface) are the number of tracks counted. ‡ Chi square statistic for count data at 5% confidence interval-Degrees of freedom.

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annealing algorithm was used as there was no improve- were extracted from the modelled thermal histories(Fig. 2) and used to calculate the palaeotemperatures ofment in the goodness of fit of the models using the

Crowley F-apatite algorithm. The significance of apatite the sample at seven time slices: 40, 35, 30, 25, 20, 15and 10 Ma.composition on the interpretation of cooling histories

from AFT thermochronology in granitic plutons hasbeen documented (O’Sullivan & Parish, 1995). However, EXHUMATION OF THE PYRENEANa generalized annealing model which incorporates the OROGENeffects of apatite compositional variations is unavailable.

Exhumation from thermal historiesThe predicted fission track parameters for each mod-elled temperature history are compared to the observed In order to translate the time–temperature plots intodata by calculating the data misfit (Eq. 2). This is time–depth, a geothermal gradient must be applied. Thebasically the goodness of fit criterion. reconstruction of palaeogeothermal gradients is possible

through detailed vertical sampling along a topographicw=expG−K ageobs−agepred

dage K − A1−P(x)

WP BH (2) profile (Brown et al., 1994; Fitzgerald et al., 1995), but,as yet, this has not been achieved in the Pyrenees.

where w=the misfit function; P(x)=the significance Therefore, for the current study we have used thelevel for the null hypothesis that the modelled and present-day geothermal gradient (Pollack et al., 1993;observed track length distributions are drawn from the Zeyen & Fernandez, 1994). This has enabled us tosame distribution using the x2 statistic; WP=the weight- translate the time–temperature plots into time–depthing, 0.2, applied to P(x). plots, assuming vertical rock trajectories, for each of the

Modelled thermal histories for the Pyrenean samples sample locations thus yielding rates and magnitudes ofare shown together with the observed track length distri- exhumation in the upper 3–5 km of the mountain beltbutions. The two horizontal lines on the time–tempera- through time. Furthermore, the calculated denudationture plots correspond to 110 °C and 60 °C, which mark rates are not sufficiently rapid to have caused any substan-the upper and lower bounds, respectively, of the PAZ tial perturbation of the subsurface isotherms (Stuwe(Fig. 2). However, repeated model runs show similar et al., 1994).results in terms of stages in the cooling history and thecurves shown represent the ‘best-fit’ solutions. Erosional denudation from exhumation

The best-fit models found using the GA efficientlyThe evaluation of erosional denudation from exhumationdefine the regions of the time–temperature space wheredata requires making two assumptions. Firstly, denu-the predicted fission track parameters fit the observeddation is achieved primarily through surface erosion asdata well in terms of goodness of fit. However, theseopposed to extensional unroofing (Platt, 1986). Second,models are nonunique, so statistical techniques arethe rock trajectory paths during exhumation were sub-applied to the best-fit models to define confidence regions.vertical (see Appendix).A multidimensional line search technique, using

There is no structural or metamorphic evidence toPowell’s method, is carried out on each time–temperaturesuggest that the main Pyrenean orogen has undergonepoint of the best model found using the GA to locate asignificant extension during 40–10 Ma. However, in themaximum in the likelihood function. The maximumeastern Pyrenees, there are two sets of normal faults withlikelihood involves maximizing the probability of thea roughly NE–SW and NW–SE trend infilled by middlemodel producing the observed data. This updated bestMiocene and upper Miocene to Quaternary depositsGA solution is now the local best solution and is assumed(Emporda Basin), respectively (Olivera et al., 1986; Saulato be the global best solution. Confidence regions areet al., 1994; Roca, 1996). The south-eastern portion ofthen mapped out about this solution using an approxi-the area studied includes this region of recent extension,mation to the full likelihood ratio test: a test statisticand therefore should be taken into consideration for thewhich compares the maximum likelihood values obtainedcooling histories for the samples in the Alberes-Junquera,by fitting the observed data with and without restrictionsand Agly massifs.on the parameter estimates. The function defined by this

During exhumation, rock trajectories may be influ-statistic is assumed to be distributed as x2 with twoenced by low-angle thrust geometries and will alter thedegrees of freedom (Gallagher, 1995). All except oneamount of denudation required for a given exhumationtime–temperature point are kept fixed while defining a(Appendix). In the case of thrusts dipping at anything95% confidence interval, in the form of confidence barsgreater than #10° this effect is negligible. However,on the time and temperature values, of that point (Fig. 2).with subhorizontal thrusts, the denudation of the thrustsheet can be 1.5–2 times greater than the exhumation.ResultsIn the Pyrenees, thrusting occurred during the 40–25-Ma interval studied, but is thought to have ceased byThe modelled temperature/time plots for the samples

indicate variable cooling paths across the eastern/central upper Oligocene times (Meigs et al., 1996; Hogan &Burbank, 1996). All samples were taken from HercynianPyrenees (Fig. 2). Cooling rates (Table 2) for the samples


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Table 2. Cooling rates derived from the modelled thermal histories (Fig. 2).

1. Time 1. Temp 1.−2. Cooling rate 2. Time 2. Temp 2.−3. Cooling rate 3. Time 3. Temp 3.−4. Cooling rate 4. Time 4. Temp.Sample (Ma) (°C) (°C Ma−1) (Ma) (°C) (°C Ma−1) (Ma) (°C) (°C Ma−1) (Ma) (°C)

AG1 54 130 5.3 39 51 0.3 6 42 4.7 0 14AR1 66 130 1.8 18 46 −0.5 3 53 16.0 0 5AS1 42 130 3.2 20 60 −0.1 6 62 8.0 0 14AG3 47 130 2.7 24 69 0.2 8 66 7.0 0 10B3 30 130 1.9 10 92 −2.0 9 94 9.0 0 13CA3 39 130 11.8 33 59 −0.8 23 67 2.4 0 11CAN5 57 130 0.9 8 84 7.6 3 46 10.7 0 14CB1 54 130 2.0 48 118 1.7 2 42 16.0 0 10F1 149 130 0.6 25 61 2.0 0 10FTG8 54 130 3.5 35 64 −0.1 8 67 6.5 0 15FTG10 57 130 3.6 39 66 −0.2 16 71 3.8 0 11FTG18 56 130 2.1 27 68 0.2 4 63 13.3 0 10FTG23 79 130 0.5 37 108 2.3 6 38 4.8 0 9HOS1 60 130 0.8 27 104 3.3 0 15HOS4 36 130 4.7 21 60 −0.8 3 75 20.3 0 14LA7 35 130 4.2 23 80 1.2 1 54 43.0 0 11M3 50 130 3.3 35 81 0.7 4 60 12.3 0 11M4 50 130 2.0 29 87 1.5 1 45 32.0 0 13MER3 43 130 2.7 20 69 0.0 8 69 7.9 0 6MER4 32 130 7.7 20 38 −1.7 1 70 60.0 0 10OM3 37 130 7.3 30 79 −2.1 23 64 2.6 0 5

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basement massifs that were not involved in low-angle erosional products into either the pro- or retro-forelandbasin. Therefore, the long-term position of the drainagethrusting, and we can be confident that the later exhum-

ation history (younger than #25 Ma) was not influenced divide is fundamental in understanding the sedimentsupply to the foreland basins. In terms of the mechanicsby thrust trajectory. Therefore, as with tectonic denu-

dation, rock trajectory is not considered to have played of a doubly vergent orogen such as the Pyrenees, theposition of the drainage divide represents the boundarya significant influence in the evaluation of erosional

denudation from exhumation. between the surface slopes of the pro- and retro-wedges.In a noncohesive Coulomb thrust wedge, without erosionof the upper surface the surface slope direction opposesDenudation mapsthe direction of the slope of basal detachment (Davis

To help visualize spatial and temporal patterns of denu- et al., 1983). Therefore, the point of convergence betweendation across the sampled area of the mountain belt, the the two plates beneath the mountain belt (sometimesamount of denudation has been interpolated between the termed the singularity; Willett et al., 1993) wouldsample locations by applying a standard Kriging pro- immediately underlie the main drainage divide of thecedure (linear variogram model) to generate contoured mountain belt. However, numerical models of doublymaps of total denudation. The history of denudation has vergent orogens indicate that with uniform erosion acrossbeen divided into six, 5-Myr time intervals from 40 to a mountain belt, the drainage divide will be carried10 Ma (Fig. 3). towards the retro-wedge in response to the predominant

The contoured maps of denudation through time show rock trajectory paths from the pro- to the retro-side ofthat it is localized and does not seem to be influenced by the mountain belt (Willett et al., 1993; Batt & Braun,the major structures. Commencing at 40–35 Ma, the 1997). Also, nonuniform erosion across a mountain beltpattern of denudation shows a range of values from 0 to is likely to cause a shift in the drainage divide due to100 mm kyr−1, and does not show any major anomalies. headward back-cutting in the upper reaches of the drain-Maximum amounts of denudation occurred in the interval age basins. The relationship between the controls on the35–30 Ma and were located around the Aston and position of the drainage divide imposed by the mechanicsQuerigut-Millas massifs, with rates of up to of the orogen, and by headward erosion of river catch-240 mm kyr−1. Between 30 and 25 Ma, the magnitude ments is an important topic which has not yet been fullyof this high persisted, but decreased. At the same time, researched.a neighbouring high, of greater magnitude, developed In the Pyrenean case, the drainage divide along the#20 km to the west centred over the Aston massif; these ECORS line is offset #3 km to the north from thetwo highs in denudation persisted through to #20 Ma. underlying point of plate convergence towards the retro-Post 20 Ma the dominant high shifts further towards the side of the orogen (Fig. 1). A spatially averaged topo-east, in the region of the Agly massif; this may be related graphic cross-section from the entire length of theto late-stage extension in this region. Pyrenees shows a marked asymmetry with a southward-

The total volume of material represented in each of dipping slope which is #110 km wide and dipping at anthe denudation maps (Fig. 3) can be divided by the study average of 0.8° (the pro-wedge side) and a northward-area to yield spatially averaged denudation rates; values dipping slope which is #30 km wide with an averagefor the six time intervals range from 34 to 61 mm kyr−1

slope of 3°. The similarity in the topographic asymmetry(Fig. 4). Broadly speaking, these results indicate that of the present-day Pyrenees and the predictions fromthere is little temporal variation in long-term, spatially numerical models for doubly vergent orogens leads toaveraged denudation rates across the study area over a the belief that the present geometry reflects a long-term30-Myr interval. However, localized highs and lows characteristic of the mountain belt.in denudation occur on a 20–50-km length scale with To summarize, whilst there is no obvious asymmetryvariations in denudation rates of 0–240 mm kyr−1. In in the recorded exhumation of the Pyrenean orogen,comparison, present-day denudation rates based on sedi- long-term asymmetry of topography is likely to havement discharge for the Ebro and Garonne river drainage caused higher sediment supply to the southern Ebrobasins are 90 and 10 mm kyr−1, respectively (Hovius, in foreland basin than to the northern Aquitaine basin.press). Interestingly, denudation rates from the SwissAlps for the interval 36–10 Ma were #74 mm kyr−1,

Calculating sediment dischargebased on sediment volumes in the North Alpine forelandbasin (Sinclair, 1997). Having generated contoured maps of denudation (Fig. 3),

a map of drainage basin outlines for the same area canIMPLICATIONS FOR SEDIMENT be overlain within a GIS format, allowing calculations of

sediment discharge for individual fluvial basins throughSUPPLYtime. The area and shape of the drainage basins used inLong-term position of drainage dividethe calculation is an important factor. In order to evaluatethe potential significance of variable denudation, we haveThe main drainage divide running down the topographic

axis of a mountain belt controls the distribution of used a simple rectangular outline for the drainage basins,


R. G. Morris et al.


Exhum

ationof

theP

yreneanorogen

Fig. 3. Contoured (a) maps and (b) surface plots of denudation (km) for six time intervals. Relative highs in the amounts of denudation are shown by the darker shading with contour valuescorresponding to levels in the shaded scale bar. The positions of the ECORS-Pyrenees line and the North Pyrenean Fault (NPF) are shown as references.

©1998

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R. G. Morris et al.

palaeogeographical and isopach maps to attempt inversemodels of the mountain belt drainage systems. Inaddition, the present-day topographic characteristics of adrainage basin will be brought into consideration whenextrapolating denudation over areas that are not coveredby the denudation map.

The results indicate the potential for localized vari-ations in the amount of denudation along a mountainbelt to cause significant, along-strike variations in sedi-ment discharge into the foreland basins. The time intervalof maximum variation in sediment discharge is 35–30 Ma;this being a direct response to the localized variations inFig. 4. Plot to show the variation in spatially averaged

denudation rate for six time intervals using the present-day denudation (Fig. 3). The high rates of denudation overgeothermal gradient. the Querigut-Millas massif during this interval resulted

in a calculated sediment discharge of 0.34 million tonnesper year for the fluvial basin draining to the south duringterminating at the present-day mean position of thethis time; this value is approximately five times greaterdrainage divide, and at the mean position of the fluvialthan for the two basins to the west (Fig. 5).outlets. The aspect ratio of the drainage basins is 251,

In terms of the temporal development of discharge forwhich is an average value taken from numerous linearan individual basin, the maximum variation is found inmountain belts (Hovius, 1997). We have kept this con-the drainage basin covering the northern portion of thefiguration constant through time, and for each basinarea of the Querigut-Millas massif; here, the averagecalculated the sediment discharge (Fig. 5). For some ofsediment discharge into the Aquitaine basin from 35 tothe drainage basins, a full overlap of the basin area, and30 Ma is 3.35 times greater than for the same basinthe denudation map was not possible, and the mean valueduring the interval 30–25 Ma. Over the entire area, thefor the denudation within the overlap portion has beenevolution of sediment discharge mimics the developmentprojected into the rest of the basin area. It must beof denudation, with broadly greater values towards theemphasized that, overall, this is a simple calculationeast during the later intervals from 25 to 10 Ma. Awhich must not be viewed as an attempt to simulate thecomparison of total sediment discharge per year to theactual sediment discharge into the Ebro and Aquitaine

basins through time. Future calculations will include Ebro and Aquitaine basins, assuming a fixed drainage

Fig. 5. Plots of modelled sediment discharge (million tonnes per year, Mt a−1) using a series of rectangular drainage basins withan aspect ratio of 251 based on modern linear mountain belts (Hovius, 1996), and a drainage divide based on the mean present-day position, and which remain constant through the six, 5-Myr time intervals. Arrows indicate the magnitude of discharge fromthe drainage basins and do not relate to palaeocurrents in the basins.



Oligocene sequences of North-Eastern Spain (Ebro anddivide (see above), shows that the discharge to the southCampins Basins). Munchner geowiss. Abh. (A), 10, 35–42.is likely to have been between 1.5 and 2.8 times greater

B, P., C, G., S, M., D, J., B,than to the north.M.O. & D, E. J. (1988) Role des paleostructures albo-The above results demonstrate that the variation incenomaniennes dans la geometrie des chevauchements fron-denudation across the Pyrenees has the potential to causetaux nord-pyreneens. CR Acad. Sci. Paris, 306, 307–313.along-strike variations in sediment discharge between

B, G. E. & B, J. (1997) On the thermomechanicaldrainage basins of 500%, and temporal variations ofevolution of compressional orogens. Geophys. J. Int., 128,>300% from individual basins. The former has signifi-364–382.cant implications for the palaeogeography of the foreland

B, C., F, P. & H, J. (1991) Erosionalbasins, and the relative size of sediment fans within the control of active compressional orogens. In: Thrust Tectonicsbasins. The degree of temporal variations within individ- (Ed. by K. McClay), pp. 1–18. Chapman & Hall, London.ual fans has the potential to modify significantly the B, R., R, J.-P. & Z, G. (1995) The Northfluvial base-level, and control facies architecture, i.e. pro- Pyrenean Aquitaine Basin, France: evolution and hydro-to retro-gradational geometries. Future research will be carbons. AAPG Bull., 79, 831–853.aimed at integrating the stratigraphic information from BRGM, ELF-RE, ESSO, REP, SNPA (1972) Geologie du bassinthe two basins, with denudation maps generated here. d’Aquitaine. Atlas, 27 plates, scale 151 000 000.

B, R.W. (1991) Backstacking apatite fission-track ‘stra-tigraphy’: a method for resolving the erosional and isostaticCONCLUSIONSrebound components of tectonic uplift histories. Geology,19, 74–77.1 Cooling histories for 21 samples across the central and

B, R.W., S, M.A. & G, A.J.W. (1994)eastern Pyrenees are revealed through apatite fission trackApatite fission track analysis: its potential for the estimationanalysis. Allowing for a number of assumptions, theof denudation rates and implications for models of long-termmodelled thermal histories can be interpreted as a recordlandscape development. In: Process Models and Theoreticalof denudation across the mountain belt for the intervalGeomorphology (Ed. by M. J. Kirkby), pp. 23–53. John Wiley40–10 Ma.& Sons Ltd, New York.2 Maps of denudation have been constructed for six,

B, R.W., S, M.A. & G, A.J.W. (1994)5-Myr time intervals. Denudation varied with aApplication of fission track analysis to the estimation of long-20–50-km length scale, and does not appear to have beenterm denudation rates. In: Process Models and Theoretical

related to the major structural zones of the mountain belt. Geomorphology (Ed. by M. J. Kirkby), pp. 25–53. John Wiley,3 Spatially averaged denudation rates for the six time Chichester.intervals ranged from 34 to 61 mm kyr−1 assuming the B, M.F. (1986) The influence of the Pyrenees on thepresent-day geothermal gradient. Maximum rates of evolution of adjacent basins. In: The Geological Evolution of240 mm kyr−1 occurred in the interval 35–30 Ma, in the the Pyrenees (Ed. by E. Banda & S. M. Wickham),region of the Querigut-Millas massif. Tectonophysics, 129, 343–354.4 Assuming the denudation resulted primarily from ero- C, W.D. (1990) Mechanisms and kinetics of fission tracksion, the denudation maps can be used to calculate annealing. Am. Mineral., 75, 1120–1139.sediment discharge through time to the neighbouring C, P. (1976) Structure et evolution tectonique de laforeland basins. Using a series of rectangular drainage zone nord-pyreneenne (analise de la deformation dans une

portion de chaıne a schistosite subverticale). Mem. Soc. Geol.basins which remain constant through time, it is possibleFrance, 127, 116 pp.to evaluate the potential for spatial and temporal vari-

C, P.&. (1989) Ecors Team The ECORS Pyreneanations in sediment discharge as a function of denudation.deep seismic profile reflection data and the overall structureThe results show along-strike variations in sedimentof an orogenic belt. Tectonics, 8, 23–39.discharge between drainage basins of 500%, and temporal

C, G.A. & A, H.H. (1976) The variability of thevariations from individual basins of >300%.natural abundance of 235U. Geochim. Cosmochim. Acta, 40,1487–1490.

ACKNOWLEDGEMENTS C, K.D., C, M. & S, R.L. (1991)Experimental studies of annealing of etched fission tracks inTony Hurford and Andy Carter from The Londonfluorapatite. Geochim. Cosmochim. Acta, 55, 1449–1465.Fission Track Research Group at the University College

D, F.A. & S, J. (1988) Mechanics, growth, andLondon are thanked for providing assistance with pro- erosion of mountain belts. In: Processes in Continentalcessing the fission track data. We thank Kerry Gallagher Lithospheric Deformation (Ed. by S. P. Clark Jr, B. C.for a copy of Monte Trax together with Kit Johnson for Burchfiel and J. Suppe), Spec. Pap. Geol. Soc. Am., 218,comments on the thermal modelling. Thorough reviews 161–178.by Peter van der Beek and Jaume Verges helped in D, D.M., S, J. & D, F.A. (1983) Mechanics ofimproving the manuscript. fold-and-thrust belts and accretionary wedges. J. geophys.

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APPENDIX 1Atlantic from the Late Cretaceous to the present. Geology,19, 613–616. Effect of thrust geometry on denudation/

S, E., P, J., M, E., L, M., L, M., exhumation ratioB, X. & A, J. (1994) Evolucion geodinamica

In this study the amount of exhumation (vertical rockde la fosa del Emporda y las Sierras Tranversales. Acta Geol.Hispanica, 29, 55–75. trajectories) has been calculated assuming the present-


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Fig. A1. The effect of thrust geometry and topographic reliefon denudation/exhumation ratios at shallow levels. Schematiccross-sections perpendicular to the mountain belt surfaceassuming the subsurface isotherms parallel the topographicsurface. (a) Planar topographic surface where denudation/exhumation ratios are equal for both oblique and vertical(shaded block) rock trajectories. (b) Denudation/exhumationratios are not equal for oblique and vertical (shaded block) rocktrajectories when topographic relief is present.

Fig. A2. Schematic cross-section perpendicular to a planarday geothermal gradient and on the basis that the iso-mountain belt surface and assuming subsurface isothermstherms are at a fixed depth from each topographicparallel a mean surface elevation. Oblique rock trajectorieselevation. Essentially, the isotherms in the subsurfaceresult in higher denudation/exhumation ratios than verticalparallel a mean topographic surface of the mountain belt(shaded block) rock trajectories. Subscripts refer to positions on

(Fitzgerald & Gleadow, 1988). Furthermore, the calcu- the exhumed blocks: ‘f ’, front; ‘s’, sample; ‘r’, rear.lations assume that this topographic surface is planar anddoes not possess any relief. Therefore the amount ofcalculated denudation equals the amount of exhumation the topographic wavelength (Turcotte & Schubert, 1982,

p. 153). Therefore at depths which correspond to tem-(the distance from the surface to the depth of origin ofa rock), irrespective of whether the rock trajectory is peratures greater than #100 °C the isotherms tend

towards planar surfaces that are parallel to a mean surfacevertical or oblique. However, any significant relief on thetopographic surface of the mountain belt may result in elevation (Brown, 1991). However, erosion causes heat

advection which can perturb the subsurface isothermsdifferences between the amount of calculated denudationand the amount of exhumation if the rock trajectories such that they become more compressed into the topogra-

phy. It has been shown that for erosion rates ofare oblique (Fig. A1). Oblique rocks trajectories due tolow-angle thrust slices will generally result in greater 10–100 mm kyr−1 the amplitude of the perturbation is

insignificant for topography with 3 km amplitude andcalculated amounts of denudation relative to a verticalexhumation pathway (greater denudation/exhumation 20 km wavelength (Stuwe et al., 1994). The denudation

rates calculated here do not cause any significant pertur-ratio) when the thrust transports material from a ‘low’in the isothermal surface to a ‘high’ in the topographic bation of the subsurface isotherms.

If we consider the case of a horizontal isothermalsurface. Conversely, material transported from an ‘iso-thermal high’ to a ‘topographic low’ will result in a lower surface, the calculated amount of denudation only equals

the amount of exhumation if the rock trajectories arecalculated denudation/exhumation ratio.The effect of topographic variations on the subsurface vertical, irrespective of whether the topographic surface

is planar or possesses relief. A planar topographic surfaceisotherms decreases exponentially with depth. The dis-tance from the surface of the mountain belt to the is used to demonstrate how the denudation/exhumation

ratio changes as a function of increasing angles ofisotherm is proportional to the horizontal wavelength ofthe variation in surface temperature, itself a function of obliquity to the horizontal (Fig. A2).



Fig. A3. Plot of denudation/exhumation ratio as a function ofchanging rock trajectories for theschematic cross-section shown in Fig.A2. Rock trajectories parallel thebounding surfaces of thrust slices whoseangle of obliquity, with the horizontal,is progressively increased. Plots areshown for both the northern retro-wedge (b=3.0°) and the southern pro-wedge (b=0.8°).

The amount of calculated denudation for a ‘block’ of The present-day topography of the Pyrenees is markedlyasymmetric perpendicular to the trend of the mountainmaterial (assuming unit thickness in the y dimension) in

which all rock trajectories are vertical is given by belt. The northern slope is narrower (#30 km) andsteeper (#3°) than the southern slope (#110 km wide

Evertical=(xr−xf )(zs−z0) . (A1) and 0.8°). These values for the slopes of the retro- andNow consider a more realistic geometry where the pro-wedge, respectively, have been used to show how

material being exhumed is a ‘slice’ bounded by two the ratio of calculated denudation to exhumation increasesparallel thrust surfaces inclined at an angle a to the as the angle of obliquity decreases (Fig. A3). It is apparenthorizontal. In both examples, a ‘lower’ surface parallels that Eoblique results in greater calculated amounts ofthe surface of the mountain belt such that all rock denuded material than Evertical, but approaches Evertical astrajectories within the exhumed block travel an equal a increases.distance from the isotherm to the surface. In the second Despite the simple geometries of the ‘blocks’ of mate-example the calculated amount of denuded material is rial being exhumed in the above examples, they emphasizegiven by the need for due consideration of the rock trajectories,

topographic relief and the position and geometry of theisothermal surfaces. These factors have important impli-Eoblique=C(zs−z0)

sin a DC(xr−xf )

cos b D sin(a+b) . (A2)cations for the calculated amount of denudation for aspecified amount of exhumation in terms of sedimentsupply to basins.


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Exhumation of the Pyrenean orogen: implications for sediment discharge