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Denudation history of the continental margin ofwestern peninsular India since the early Mesozoic ^

reconciling apatite ¢ssion-track data with geomorphology§

Y. Gunnell a;�, K. Gallagher b, A. Carter c, M. Widdowson d, A.J. Hurford e

a Laboratoire de Ge¤ographie Physique, CNRS-UMR 8591, and De¤partement de Ge¤ographie, Universite¤ de Paris 7, Case 7001,2 Place Jussieu, 75251 Paris Cedex 05, France

b Department of Earth Science and Engineering, Imperial College London, South Kensington, London SW7 2AS, UKc London Fission Track Research Group, Research School of Geological Sciences, Birkbeck College, Gower Street,

London WC1E 6BT, UKd Department of Earth Sciences, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK

e Department of Geological Sciences, University College London, Gower Street, London WC1E 6BT, UK

Received 17 February 2003; received in revised form 3 July 2003; accepted 7 July 2003

Abstract

A comprehensive apatite fission-track (AFT) study of the passive margin of western peninsular India between 12‡and 16‡N is used to reconstruct the denudation chronology of the continental hinterland. In common with other riftedmargins, the morphology is characterised by a low-lying coastal plateau separated from an elevated inland plateau byan erosionally controlled escarpment (Western Ghats). We modelled the fission track data using the commonlyadopted annealing algorithm of Laslett et al. [Chem. Geol. 65 (1987) 1^13]. Using the default parametrisation (i.e. aninitial track length of 16.3 Wm), it was found that during the Mesozoic denudation rates remained extremely low,increasing sporadically when erosion peaked at 130 Ma (rifting with Antarctica) and 80 Ma (rifting withMadagascar). Denudation rates rose considerably during the Cenozoic, reaching maxima of ca 120 m/Myr. Suchvalues are, however, considered as major overestimates and the effects of the Seychelles rifting at 65 Ma remainsuspiciously unrecorded. We explored the consequences of changing the initial track length in this model to a value of14.5 Wm. In practice, this reduces the rapid Cenozoic denudation artefact, model peak rates during the Mesozoic aremuch more variable, and during the Cenozoic reach values an order of magnitude lower than with the original initialtrack length. The response to the Seychelles rifting event is almost immediate. Just as previous model calibrations inAFT analysis have been relatively empirical, this revised approach does not provide insights into the physicalmechanisms of low-temperature annealing. However, it is shown to agree much better with independently establishedgeomorphic, cosmogenic, stratigraphic and tectono-magmatic evidence in this and other stable shield regions in terms

0012-821X / 03 / $ ^ see front matter F 2003 Elsevier B.V. All rights reserved.doi:10.1016/S0012-821X(03)00380-7

* Corresponding author. Present address: Laboratoire de Ge¤ographie Physique, CNRS-UMR 8591, Campus du CNRS,1 Place Aristide Briand, 92190 Meudon, France. Fax: +33-1-46-64-78-81.E-mail addresses: [email protected] (Y. Gunnell), [email protected] (K. Gallagher), [email protected] (A. Carter),

[email protected] (M. Widdowson), [email protected] (A.J. Hurford).

§ Supplementary data associated with this article can be found at 10.1016/S0012-821X(03)000380-7

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Earth and Planetary Science Letters 215 (2003) 187^201

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of both the timing and the magnitude of geological events, and the geomorphic response of the landscape to them.F 2003 Elsevier B.V. All rights reserved.

Keywords: India; rifted margin; ¢ssion track; denudation rate; escarpment; geomorphology; modelling

1. Introduction

Passive margins constitute about half of theoverall length of present-day continental coast-lines and represent the transition between the rel-atively simple tectonics of the oceans and themore complex settings of continents. Onshoremorphologies at mature passive margins aremostly controlled by the denudation-related un-loading onshore, the magnitude and spatial distri-bution of the sediment load o¡shore, and the lith-ospheric strength across the margin (i.e. isostaticrebound) [1,2]. As it is primarily the post-rift de-nudational history that regulates the nature andtiming of sediment supply to the o¡shore margin,an understanding of the long-term onshore evolu-tion of these margins is of fundamental impor-tance to passive margin models.

Previous studies have used apatite ¢ssion-track(AFT) data to quantify the degree, rate, and tim-ing of denudation in a selection of passive marginsettings (e.g. [1,3^7]). To varying degrees, thesestudies also considered the wider consequencesof the inferred denudation chronology in termsof the evolution of passive margin topography.AFT is particularly suited to the problem of re-solving depths of post-rift denudation on the scaleof 6 5 km. This is a consequence of the sensitivityof annealing of ¢ssion tracks in apatite to temper-atures up to 120‡C on timescales of 1^100 Myr.Moreover, the technique is ideal in areas such asthe Indian shield, which onshore is largely devoidof direct post-rift chronostratigraphic indicators.

On the basis of extensive new AFT analysesand integrated geomorphological data, this con-tribution aims to provide a quantitative overviewof long-term denudation at the western Indianmargin, and to infer the morphotectonic evolutionof the Western Ghats. To achieve this it is neces-sary to (i) quantify the cooling history recorded inthe AFT data and (ii) infer regional denudationrates that are compatible not only with AFT data,

but with other chronostratigraphic and geomor-phological evidence.

2. Regional tectonic, physiographic and geologicalsetting

Here we brie£y summarise the geological andgeomorphological evolution of the western Indiapassive margin. More detail is available in previ-ously published work [8]. Since the early Mesozo-ic, peninsular India has been involved in majorrifting events related to the break-up and subse-quent dispersal of eastern Gondwana, which be-gan during the latest early Jurassic (ca 180 Ma;[9]). This was followed at ca 130^120 Ma by theseparation of a fragment incorporating India andMadagascar from eastern Africa to the west, andfrom Antarctica to the east. The formation of thepresent western Indian margin was e¡ected by thelate Cretaceous separation from Madagascar (ca88 Ma; [10]) and, ¢nally, by a ridge-jump whichdetached India from the Seychelles Bank at thetime of the Deccan continental £ood basalt(CFB) (ca 65 Ma; [11]).

The onshore topography of India’s westernmargin displays many features that are character-istic of other rifted margins [8] and, in particular,the morphological succession from the coast tothe continental interior. In the study area (Fig.1), these are from west to east: (i) a low-lyingcoastal plateau (the Konkan^Kanara lowlands)with short, seaward-£owing rivers; (ii) a coast-parallel, continental-scale escarpment with reliefranging between 0.6 and 2.2 km at a distance of0^70 km inland of the present-day coastline; and(iii) an elevated inland plateau (Karnataka andMaharashtra uplands). The escarpment currentlyforms, with a few exceptions involving drainagepiracy, the main drainage divide of peninsular In-dia: eastward-draining rivers rise often within50 km of the Arabian Sea, yet discharge into

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Y. Gunnell et al. / Earth and Planetary Science Letters 215 (2003) 187^201188

the Bay of Bengal. The well-de¢ned WesternGhats escarpment is only partly associated withsurface volcanics in the north, while the geologyof the southern part is Precambrian basement.Thus, while the surface uplift may re£ect mantleprocesses (e.g. the in£uence of a plume or hot-spot), the surface expression of this potential con-tribution is geographically restricted.

Fig. 2 shows a simpli¢ed geological map of theregion featuring the near-£at-lying late Creta-ceous to early Tertiary CFB and the Precambrian

gneiss and greenstone basement complex ontowhich the lavas were extruded (Dharwar craton).The CFB provides the main regional chronostrati-graphic marker by reference to which the subse-quent morphotectonic development of the margincan be reconstructed. A gentle seaward-dippingmonocline lying 10^30 km seaward of the West-ern Ghats escarpment has been inferred fromstratigraphical and structural studies of the Dec-can lava succession. This has been interpreted as apost-eruptive but undated modi¢cation resulting

Fig. 1. Onshore topography of the western Indian margin and distribution of apatite samples. Dashed line marks the topographicdivision (panel a) used for the ‘lowland’ and ‘upland’ provinces discussed in this paper. (b) Detailed geographical distribution ofapparent AFT ages, revealing that the pattern across the two de¢ned provinces is complex for ages s 150 Ma.

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Y. Gunnell et al. / Earth and Planetary Science Letters 215 (2003) 187^201 189

from localised uplift of the lavas [12]. A numberof fault lines, however, have also been identi¢edin the onshore coastal zone, in addition to thewell-known Panvel £exure near Bombay, so thatit has also been argued that the monocline in thebasalts is a syn-eruptive roll-over structure [13].The faults, however, do not currently de¢ne relieffeatures and the monocline, whether syn-rift andstructural or post-rift and £exural, has been fairlyuniformly breached by the receding drainage sys-tems. The current escarpment is thus primarily anerosional feature [8].

3. AFT data

3.1. Sampling and analytical procedures

A suite of 128 samples was collected acrossmajor topographic variations at the western Indi-an margin between 10‡ and ca 16‡N (Fig. 1), andanalyses for 92 of these are reported here. Themain limits to sampling further north were theDeccan CFBs, which do not contain su⁄cient

apatite for analysis. In addition, the Archeangreenstone rocks forming the higher summits (ca1.9 km) of the Ghats and the plateau also tend tocomprise unsuitable lithologies. This restrictedsampling from the higher elevations of the craton,but auxiliary geomorphological evidence aided theinterpretation of the denudation chronology inthese localities.

Apatite was separated using conventionalcrushing, sieving, heavy liquids and magnetictechniques. Apatite crystals 80^500 Wm in sizewere polished and etched using HNO3 5M at20 V 1‡C for 20 s, while low uranium muscovite,used as an external detector [14], was etched using48% HF for 50 min. Samples were irradiated atthe well-thermalised RisX reactor in Denmark, us-ing the Corning CN5 glass dosimeter. Analyseswere carried out on a Zeiss Axioplan microscopeat a magni¢cation of U1250, using a dry (U100)objective. Con¢ned track length measurementswere made using a drawing tube and digitisingtablet, calibrated against a stage micrometer. Fis-sion-track ages (Table 1) were calculated using thej calibration approach [15], as recommended bythe I.U.G.S. Subcommission on Geochronology[16].

3.2. Qualitative discussion of AFT data

The major recent tectonic event in westernIndia was rifting and CFB magmatism around65 Ma. The relationship between the AFT ageand the mean track length (MTL) is a usefulway to identify discrete thermal events in a givenregion [1,17,18]. If the data form a coherent boo-merang-shaped trend, then they may be inter-preted in terms of a discrete cooling event withdi¡erential cooling, such that the oldest samplesexperienced the least cooling and the youngest themost. Provided the youngest samples have MTLss 13.5^14 Wm, then the AFT age approximatesthe timing of the cooling event, otherwise it pro-vides an upper limit to this timing. Fig. 3 showsthe relationship between the AFT age and theMTL for the data from western India.

The youngest ages are 55^75 Ma. The corre-sponding samples have relatively long MTLs(v 13.5 Wm) and are geographically close to the

Fig. 2. Geology of the Dharwar craton, SW India.

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present-day outcrop of the Deccan CFB, in thenorth of our study area. The other samples withyoung AFT ages generally have MTLs 6 13 Wm,and these were collected at much greater distancefrom the Deccan CFB. The majority of the sam-ples have AFT central ages ranging from 150 to300 Ma and MTLs between 11.5 and 13 Wm.These data indicate variable amounts of coolingand thus do not conform to the simple trend of asingle cooling event related to a change in region-al heat £ow as might be expected during the for-mation of a volcanic margin such as this.

3.3. Geographical variation

Previous studies of other passive margins haveindicated an important geographical control oncooling (and so AFT age and MTL) in terms ofthe position of the sampling localities within thecoastal plain^escarpment^inland plateau morpho-logical units [1,3,4]. Given that elevation tends tovary systematically with position from the rift axisor the present-day coast towards the continentalinterior, there is often a correlation between ele-vation and the AFT age. The AFT ages fromwestern India have a tendency to increase withelevation (Fig. 4), with the oldest ages being ob-

tained from the highest summits sampled(IND2308, 1450 m: 351 V 22 Ma; and IND2305,2630 m: 411 V 24 Ma). However, although rela-tively young ages are exclusively associated withsamples from low elevations (e.g. no age 6 150Ma is found above 400 m), some samples fromthe low-lying coastal zone yield relatively oldages. This observation is not exceptional in com-parison with other known high-elevation passivemargins, where a clear-cut younging of ages is notalways observed (e.g. SW Africa, SE Australia)between the upland interior and the coastal plain[1,18]. In summary, the Western Ghats escarp-ment separates two AFT age groups: one withages typically greater than 220 V 10 Ma (i.e. inthe upland plateau, with elevations generallys 400 m), and one with ages lower than220 V 10 Ma (i.e. in the coastal lowland). How-ever, this division does not simply re£ect a pro-gressive younging of ages from the continentalinterior towards the coast. In detail, the patternis clearly more complex.

The northernmost samples, close to the presentoutcrop of the Deccan CFB, have the youngestages and also show the longest MTLs, implyingthat they record a relatively discrete cooling event.Given that, within error, the AFT ages between54 and 72 Ma correlate with the Deccan CFBepisode (65 Ma), this suggests a thermal eventrelated to the formation of the Deccan CFB.However, as these samples are immediately sur-rounded by other samples at similar elevationswith AFT ages three times greater, the localised

Fig. 4. Relationship between AFT age and elevation acrossthe Dharwar craton.

Fig. 3. Relationship between AFT age and mean con¢nedtrack length (MTL). Error bars are 2c and vertical greyband indicates the time of rifting related to the Deccan CFB.The scatter and moderately short MTL for a majority ofsamples indicate a complex cooling history.

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nature e¡ectively rules out heating by a large sub-lithosphere heat source (e.g. the Deccan sourceplume). An alternative interpretation of theseyoung AFT ages is that reheating, followed byrapid cooling, is the result of burial of the Pre-cambrian basement by a now eroded lava pile. Asimilar inference has been made for data from theParana¤ CFB province in southeast Brazil [4] and,given the relatively low thermal conductivity ofbasalt (e.g. between 1.59 and 1.85 W m31 K31

for the Deccan basalt [19]), would imply between2 and 4 km of burial to achieve total annealing.Again, however, the localised nature of these fewsamples is problematic. The chemistry of theseapatites was also considered as a possible factorof deviation from the regional trend, but a com-parative microprobe analysis of those samples andothers (older ages and/or di¡erent elevations)from the same area revealed identical geochemicalsignatures (IND2033, 69 V 5 Ma: mean F 2.82wt% and mean Cl 0.01 wt% based on 16 grains;IND2030, 160 V 10 Ma: mean F 2.29 wt% andmean Cl 0.04 wt% based on 15 grains;IND2000, 245 V 11 Ma: mean F 2.05 wt% andmean Cl 0.03 wt% based on 30 grains). Conse-quently, we suggest that these samples most likelyrepresent local resetting due to either local lava£ows or late minor intrusions [20], which are com-mon along the coast but not easily identi¢ed be-cause of generalised deep lateritic weathering. Ifreheating was caused by local lava £ows, it im-plies that those samples must have been close tothe pre-eruption surface as the thermal e¡ects oflava £ows occur over a similar length scale toindividual £ow thickness [4]. In either case, theinferred cooling for these few samples is not solelyattributable to denudation.

4. A quantitative denudation chronology of theWestern Ghats margin

4.1. Thermal history modelling assumptions andprocedure

To obtain quantitative thermal and denudationhistories for the large sample population, we fol-lowed the procedures described by Gallagher [21]

and Brown et al. [6]. The thermal modelling uses aguided search (genetic algorithm) combined witha maximum likelihood criterion to identify ther-mal histories, optimal in terms of ¢tting the ob-served track counts and length data. Each samplewas modelled independently, although the sameparameter space was used to de¢ne the rangefor temperature and time values. In this studywe use the algorithm designed by Laslett et al.[22] to simulate the temperature and time depen-dence of ¢ssion-track annealing in apatite. It is anempirical model, calibrated against an extensiveseries of laboratory annealing experiments. Cur-rently, it is probably the most commonly adoptedannealing algorithm and is generally applicable tothe more common compositions of apatite (i.e.similar to the Durango apatite age standardused to calibrate the model).

The original calibration [22] was based on ¢t-ting the empirical model to a transformation ofthe laboratory annealing data involving a normal-isation of the measured sample track length l byan initial length l0 in the unannealed apatite. Oneof the key assumptions in developing this calibra-tion was that uncertainty around unannealedtrack lengths in apatite was negligible, withl0 = 16.3 Wm in the original model formulation.This initial length was calibrated from inducedtrack length measurements. More recent workhas shown that the initial length of induced tracksis perhaps 5% greater and that room temperatureannealing occurs over timescales of a few weeksfor induced tracks [23]. Furthermore, it also ap-pears that geological samples that have remainedat temperatures 6 40‡C have MTLs considerablyshorter than predicted from the Laslett et al. an-nealing model [24,25]. Some of these samples,such as the Durango apatite, are used as age stan-dards, whereby it is implicitly assumed that thelevel of annealing is minor. However, these ‘un-annealed’ age standards have MTLs of 14.5^15 Wm, implying that the e¡ective initial tracklength for geological timescales and spontaneoustracks may be V10% lower than that observed onlaboratory timescales with induced tracks [26^28].The consequence of these observations is that theLaslett et al. model, as originally formulated, ap-pears not to predict su⁄cient amounts of anneal-

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ing at temperatures lower than 50^60‡C. In thecontext of inferring thermal histories from geolog-ical samples, this limitation often results in arti¢-cially generating a late cooling event, typicallyimplying s 1 km of recent denudation, to bridgethe gap between the known surface sampling tem-perature and the signi¢cantly higher temperaturerequired by the Laslett et al. model to account forthe observed amount of annealing.

A de¢nitive solution to this problem is not theaim of this paper, but in principle the issue can beapproached from four directions: (i) designingnew annealing algorithms based on independentexperimental research [27], but with the problemof extrapolating laboratory experiments to geo-logical timescales ; (ii) calibrating estimated depthsof denudation against independent chronostrati-graphic and geomorphic data [29,30], but withimplicit uncertainty in the underpinning geomor-phic theories ; (iii) performing cross-calibrationsof AFT data against other radiometric tech-niques, but with the risk of having to face uncer-tainties inherent in the physical underpinnings ofseveral, rather than just one, technique [31] ; (iv)exploring new parametrisations of existing AFTannealing algorithms.

In this paper we follow the latter approach, inthat we consider the consequences of adopting aninitial track length of 14.5 Wm in addition to usingthe conventional value of 16.3 Wm. We recognisethat, as such, this approach does not directly pro-vide new insights into the physical mechanisms oflow-temperature annealing, and this is not ouraim. However, other existing model calibrationsare themselves relatively empirical [26,27], andthe entire philosophy behind thermochronologybased on age standards derives precisely fromthe elusiveness of some key physical underpin-nings [15]. Furthermore, other annealing modelssensitive to our concerns about low-temperatureannealing, and therefore potentially suited tocomparisons with our own approach (e.g. [27]),are based on a di¡erent laboratory protocol tothat used by both Laslett et al. and by us; also,the statistical procedures used in the model cali-bration have been questioned [32]. The motivationfor our approach is that, by modifying a singleparameter within an otherwise well-known model,

we explore the artefacts associated with the initialmodel relative to shorter MTL annealing, andthus compare the denudation chronology resultswith independent chronostratigraphic data [8] inorder to assess the geological viability of the mod-el results.

Having obtained an optimal data-¢tting ther-mal history for each sample independently, weneed to convert the temperature histories to cor-responding depths of denudation. We do this us-ing the following form of Fourier’s law for heatconduction:

z ¼ ðTðzÞ3T sÞkQ

ð1Þ

where z is depth, T(z) is temperature at that depth(obtained from modelling the AFT data), Ts is thesurface temperature, k is the average thermal con-ductivity of the rock down to depth z, and Q isthe surface heat £ow. Across the Dharwar craton,the present-day temperature gradients and heat£ow are fairly low and homogeneous, 11^15‡Ckm31 and 35^40 mW m32, respectively [33^35].Such low values are not unusual for stable cra-tonic shields, and as the craton is not known tohave experienced signi¢cant thermal disturbancessince at least the Pan-African orogeny, we adopt aspatially constant heat £ow of 40 mW m32 acrossthe region. We assumed a thermal conductivity of3.25 W m31 K31 for the eroded crustal section,based on reported values for the Dharwar craton[19,35] and a constant surface temperature of20‡C. The resulting geothermal gradient is alsoconstant, and equal to 12.3‡C km31.

We also considered the in£uence of heat £owvariations over time (assuming an exponential de-cay with time since continental break-up), andalso the e¡ect of changing the thermal conductiv-ity (by V 50%). The absolute values of the denu-dation rate varied in a predictable way from Eq. 1.Thus, increasing Q leads to a decrease in equiva-lent denudation depth, while increasing k leadsto an increase in equivalent depth. We do notshow the results here, but the relative variationsin the denuation chronology over time were main-tained (i.e. the peaks were still peaks). Thus, weconsider the features in the denudation chronolo-gies we show later to be robust to the adopted

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thermal parameters, particularly rift-related heat£ow, which after an initial increase tends to de-crease slowly with a time constant of about 60Myr, i.e. longer than the timescale of the varia-tions inferred here.

To quantify the regional denudation chronol-ogy, we interpolate the results for each sampleusing a nearest-neighbour approach [36], andthen spatially integrate the denudation estimatesover a speci¢ed area at discrete time intervals. Bydoing this at 1 Myr intervals and dividing by thearea over which the integration was made, weobtain an estimate of the average denudationrate as a function of time for that speci¢c area.As shown in Fig. 1, the geographical spread ofapatite samples is not evenly distributed. To as-sess the introduction of potentially spurious fea-tures as a consequence of the interpolation proce-dure, we have also calculated the meandenudation rate from the individual cooling his-tories (i.e. without spatial interpolation). The in-terpolation procedure does satisfy the data exactlyat each sample point, but the interpolation be-tween data can lead to some samples having anundue in£uence on the spatial integration. Wealso determined the weighted mean denudationchronology, where the weights were the uncertain-ties on the temperatures in the thermal history foreach individual sample. As a guide to the un-certainty on these average denudation chronolo-gies, we calculated the standard error on theweighted mean values, cwm using the followingformula:

c2wm ¼

XNi¼1

1c

2i

!31 ð2Þ

where ci is the uncertainty on an individual ther-mal history at a given time, and N is the totalnumber of thermal histories. These calculated un-certainties are considered indicative of the magni-tude of the minimum uncertainty inherent in thespatially interpolated curves. The aim of this is toexamine which features in the denudation chro-nology are robust to the interpolation and inte-gration procedure, and to the errors in the ther-mal histories (i.e. which features appear in allthree curves).

We present the results of this analysis for twosub-areas of the study region, chosen for theirmorphotectonic setting. These are referred to asthe ‘upland’ and ‘lowland’ provinces, and e¡ec-tively represent the regions seaward and landwardof the present-day escarpment (Fig. 1). The rela-tive variations between the denudation chronol-ogy curves calculated in the way described aboveare such that the spatially averaged denudationrate curves (heavy black line) are representativeof the general patterns for these two areas. Forinterpretation purposes, the spatially averaged‘upland’ and ‘lowland’ curves have thereforebeen preferred in this study unless otherwisestated. The major di¡erences in the interpretationoptions lie between the two AFT calibrations con-sidered.

4.2. Denudation chronology of the Indian riftedcontinental margin

4.2.1. Results with an initial track length of16.3 Wm

In Fig. 5a,b we show the smoothed denudationchronologies for the upland (16up) and lowland(16lo) areas using the thermal histories derivedwith the Laslett et al. Durango apatite annealingalgorithm [22], using an initial track length of 16.3Wm. The smoothing is a simple 10-point runningaverage, and serves to incorporate the fact thatthe timing of cooling events has an inherent un-certainty in addition to the uncertainty on thetemperatures. It is thus also a cautionary measurethat we advocate to avoid over-interpreting thesigni¢cance of sharp peaks in the raw data curve.For the spatially interpolated denudation curves,both regions show distinct peaks of enhanced de-nudation marking the rifting events of both theeast (Antarctica, ca 120 Ma) and the west coasts(Madagascar, ca 80 Ma; Seychelles, ca 65 Ma) ofIndia, although the peak for the latter is relativelysubdued or smeared into the 80 Ma peak.

Overall, this sequence of events is consistentwith denudation chronologies of peninsular Indiabased on geomorphology and basin analysis [8],although the inferred denudation peak at 80 Macannot be correlated in detail with o¡shore sedi-mentary sequences because existing o¡shore oil

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wells only currently reach the bottom of the Ce-nozoic, post-rift sediment pile. The denudationchronologies calculated without the interpolationalso have the inferred peak around 80 Ma in thelowland province but not in the highland region.In addition, the timing of the earlier (120 Ma)peak is a little later, occurring around 110^100Ma. Thus, all three methods involved in calculat-ing an average denudation rate chronology implyenhanced denudation around 80 Ma in the low-land area, and a major increase starting around70^50 Ma and lasting to the present day.

In terms of timing, the sharp increase in denu-dation recorded during the Neogene is problem-

atic. The AFT rates of accelerated denudation inthe Cenozoic borne out by the 16lo curve (Fig.6a) result in over-supplying the o¡shore basinswhen compared with the limited data on the sedi-ment volumes e¡ectively detected o¡shore [37].We suggest that this overestimate may be partiallyattributable to the 16.3 Wm initial track lengthadopted in the annealing model [22], which tendsto underestimate track annealing at sub-60‡Ctemperatures, and therefore to overestimate totalcooling rates in the more recent geological past.Additionally, it may of course be a re£ection ofsediment transport out of the system either asdetrital material or dissolved load, but existing

Fig. 5. A comparison of AFT denudation curves for the lowland and upland areas based on (a,b) initial track lengths of 16.3Wm; and (c,d) 14.5 Wm (rounded-o¡ appellations of 16lo and 14lo are meant to emphasise that we are exploring upper and lowerend-member values of initial track length rather than any precise value). The lowland samples are the sub-population of sampleslocated seaward of the Western Ghats escarpment (see Fig. 1). The spatial average (thick solid line) is based on interpolating theresults over each sub-area; the individual sample curve (dashed line) is the unweighted mean of the denudation chronology at in-dividual locations; the weighted sample curve (thin solid line) is the mean denudation at individual locations, weighted by the un-certainty in the inferred thermal history for each location. The shaded area is the standard error on the weighted mean estimateand is considered to indicate the magnitude of uncertainty inhererent in the modelling procedure.

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o¡shore data provide few high-resolution con-straints on this issue.

4.2.2. Results with an initial track length of14.5 Wm

In Fig. 5c,d, we show smoothed denudationcurves for the upland (14up) and lowland (14lo),where we adopted an initial track length of 14.5Wm. There are few similarities between these re-sults and those obtained from the 16.3 Wm value.Here, (i) during the Mesozoic (200^100 Ma), con-spicuous denudation peaks in the highland area,i.e. that closest to the eastern margin of India,re£ect high denudation rates in response to theonset of rifting of Gondwana around 170 Ma[9]. The products of this episode are to be foundin the thick clastic ¢lls of the early Jurassic riftsand aulacogens along the eastern margin of India.Denudation resumed at ca 120 Ma, as a responseto the rifting-apart of Antarctica and GreaterIndia. Clearly, denudation related to base-levelchanges in the Bay of Bengal signi¢cantly a¡ectedthe current Western Ghats region, which lies ca800 km from the eastern seaboard, suggesting thatthe denudation signal reached far into the interiorof Greater India via an easterly peninsular drain-age already in existence at the time. (ii) The cur-rent lowland area shows a sharp rise in denu-dation rates (ca 120 m/Myr) during the lateCretaceous (80^70 Ma: Madagascar rifting), alsoapparent though more subdued (ca 60 m/Myr) inthe upland area. (iii) In the coastal zone, the ma-jor denudation event at 80^70 Ma merges with theSeychelles rifting event at 65 Ma before denuda-tion rates de¢nitively decline after 50 Ma. In theuplands, this event forms only a secondary peak(ca 20 m/Myr) at 50 Ma, which is occulted in theadjacent lowland region by the much larger denu-dation peak occurring at that time (Fig. 6b). In-creased erosion in middle Eocene times inferredfrom the Bombay o¡shore stratigraphic recordhas been attributed to a variety of triggering phe-nomena involving far-¢eld lithospheric stresses(e.g. acceleration of hotspot ridge-push [38]) ;short-lived excess melting when the Re¤unionplume is understood to have crossed the CentralIndian Ridge at 53^48 Ma [39] ; or the long-rangeimpact of the so-called ‘soft’ Himalayan collision

[40]. Although the AFT data cannot discriminatebetween these various causative mechanisms, wesuggest here that the denudation recorded at thecoast may simply re£ect a delayed response to theSeychelles rifting of drainage basins eroding theplateau edge from the west. (iv) During the lateCenozoic, denudation curves for the sub-popula-tions of samples collected respectively landwardand seaward of the Ghats escarpment are simi-larly low in magnitude (5^15 m/Myr) and indicatea signi¢cant fall in denudation rates. The slightfall in rates after 20 Ma in the lowland zone(Fig. 5c) suggests that the coastal belt had becomea base-level plain by that time. The low-relief,low-estuarine river gradients, and the penetrationof ocean tides sometimes as far as the foot of thegreat escarpment is consistent with this. The slightrise in rates during that same time interval in theupland region (Fig. 5d) may re£ect increased ero-sion caused by drainage piracy by west-£owingrivers receding into the plateau at the WesternGhats escarpment. Deranged drainage, re£ectiveof recent river capture, is generalised between13 and 15‡N latitude [8].

In summary, from the comparison between the16 and 14 curves of Fig. 5, the most signi¢cantdi¡erences arise clearly from the underlying di¡er-ences in the two model calibrations. These di¡er-ences concern the timing of events, but most of allthe magnitude of denudation: (i) the start of en-hanced denudation during the Cenozoic occursearlier for the shorter intial track length and cor-relates with the time of rifting from Madagascarfollowed by the Seychelles, while the longer initialtrack length does not bring out the Seychellesrifting event and implies a delay of about 20 Myrafter it. (ii) The results also imply that, althoughthe Cenozoic denudation chronologies of thehighland and lowland areas are very similar with-in each modelling protocol, suggesting that eachapproach is internally consistent, the maximumCenozoic denudation rates di¡er by an order ofmagnitude.

5. Discussion

The Cenozoic denudation patterns are the most

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relevant to understanding the morphology of thepresent-day Indian landscape. Here we focus onthe contrast in denudation patterns o¡ered by thetwo model parametrisations presented above, andexamine these in the light of independent geolog-ical and geomorphological data.

5.1. Denudation in the uplands

The original annealing model calibration (16up,Fig. 5a) implies that s 4 km of crustal materialwas removed from the upland region during theCenozoic (since ca 50 Ma). Denudation rates roseto ca 120 m/Myr, i.e. roughly an order of magni-tude more than in the Mesozoic. In contrast, theshorter initial track length (14up, Fig. 5c) implies6 1 km of mean Cenozoic denudation since50 Ma. This depth of denudation is closer tothat inferred from the relief between the currentmean elevation of the Karnataka plateau surface

(0.6^0.9 km) and the many residual bedrock sum-mits encountered on the plateau (ca 1.9 km).Although such topographic di¡erences are no ac-curate measure per se of e¡ective depths of denu-dation because residual bedrock landforms arealso likely to have a history of surface denuda-tion, there is growing evidence from Australia andWest Africa, supported by multiple chronometricstudies [31,41], that long-term erosion of bedrockand laterite summits is typically 6 2 m/Myr. Insubstance, those studies reveal that depths ofpost-Mesozoic denudation in stable cratonic envi-ronments are not much greater than the currentlyobserved residual di¡erences in topography.Pending further cross-calibration of our Indian¢ssion-track data with U^Th/He and cosmogenicnuclides (in progress) to verify this in greater de-tail, it may thus be tentatively inferred that themaximum observed relative relief on the Karna-taka plateau is the result of di¡erential erosionoperating since the late Cretaceous between theresistant bedrock residuals and the more weath-erable (and most widely sampled for apatite) to-nalitic gneisses that now form the extensive up-land plain.

5.2. Denudation in the coastal zone

Post-break-up denudation values are generallyexpected to be considerably higher across thecoastal lowland and at the escarpment face of arift than on the backslope [42]. The results of thisstudy show that the largest signal of di¡erentialdenudation between the upland and lowlandprovinces occurred at ca 100^80 Ma (Fig. 6a) orbetween 90 and 45 Ma, depending on the initialtrack length calibration being used. The overlapcovering the rifting period between India and Ma-dagascar is a sign that this was a major period ofrift-£ank denudation. Although the anticipatedsedimentary products have not been encounteredby o¡shore drilling exploration because currentdrilling depths only reach the roof of ca 65 Myrold basalt £ows through post-rift sediments [8,37],this new evidence is the ¢rst indication so far pro-vided that signi¢cant volumes of syn-rift, possiblypetroleum-bearing sediment supplied by the Indi-an margin but sealed by the Deccan basalts may

Fig. 6. Summary of denudation histories in the WesternGhats region according to (a) the conventional [22] and (b)our revised approaches. Shaded areas indicate the di¡erentialbetween denudation rates landward and seaward of theGhats escarpment. Shaded bars indicate the known riftingevents at both Indian passive margins during the break-up ofGondwana.

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lie o¡shore. The shorter initial track length mod-el, however, also suggests that rapid denudationcontinued into the Palaeocene. This is con¢rmedby o¡shore terrigenous input recorded in explora-tion wells [37], a fact which brings out as suspectthe occultation by the conventional model (Fig.6a) of any rapid denudation occurring in responseto post-Seychelles rifting.

During the Cenozoic, the curves show that de-nudation was either considerable (Fig. 6a), or rel-atively small (Fig. 6b), again depending on theparametrisation used. The very high rates (140m/Myr) during the last 30 Myr as given by the16.3 Wm model are comparable to rates in activeorogens and at odds with all other evidence in theregion. The much lower Cenozoic rates a¡ordedby the 14.5 Wm model call for quite a di¡erent setof interpretations, with a possible analogue in SWAfrica [1]. In contrast to previous models for theevolution of passive margin topography (e.g.[42]), where high estimated erosion rates in thecoastal zone impose high di¡erentials of denuda-tion and therefore high magnitudes of £exural re-bound, our results imply that relatively little mod-i¢cation of the topography occurred during laterCenozoic times: denudation in the lowlands wasnot signi¢cantly higher than in the uplands. InIndia, we tentatively interpret this situation asbeing a direct consequence of the relatively smallsize of the subcontinent. Unlike passive marginsbounding vast, sometimes internally drained con-tinental hinterlands, peninsular India is relativelynarrow. The Karnataka uplands are externallydrained towards the Bay of Bengal, which is sit-uated 600^800 km from the Western Ghats head-waters. This con¢guration may be su⁄cient toexplain the sensitivity of peninsular India tobase-level changes (eustatic or otherwise), andhence the relatively high denudation rates (by cra-tonic standards) recorded landward of the escarp-ment. This may not be unique to India: the inte-riors of South Africa and Brazil also have ahistory of external drainage and denudation viathe Orange and Parana¤^Sa‹o Francisco River ba-sins, respectively, but because of its smaller scalethe Indian setting is possibly a more simple exam-ple. In summary, the major di¡erences arisingfrom the competing models are that (i) escarp-

ment growth ceased at 40^20 Ma (14lo) insteadof at 60 Ma (16lo); and (ii) the relatively long-term steady state of the escarpment after theserespective dates was achieved in a context of ex-ceedingly high rates on either side of it accordingto Fig. 6a, but much more reasonable and realisti-cally £uctuating rates according to Fig. 6b.

6. Concluding statements

The AFT denudation chronologies described inthis paper are as elsewhere model-dependent, bothin terms of timing and magnitudes of denudation.In India, it appears that results derived with aninitial track length of 14.5 Wm provide closeragreement with existing geological data thanthose suggested by the more widely adopted Las-lett et al. model with an initial track length of16.3 Wm. While the longer initial track lengthhas been applied in earlier studies with no appar-ent problems (e.g. Brazil [4]), the inference of rap-id, recent cooling when using the longer initialtrack length becomes most acute when dealingwith data with relatively old AFT ages (s 200Ma), with moderately short MTLs (e.g. 11^13 Wm), which is typical of the data from Indiapresented here. It appears logical to believe thatthis might re£ect the nature of the data, i.e. when¢ssion-track ages are younger, the spuriousness ofthe rapid cooling problem should be less acute.The generality of this point nevertheless demandsto be tested in a suitably wide range of settings, asrapid recent cooling may in some cases be real(supported by independent geological evidence)rather than merely an algorithm-related artefact.It is important to clarify that the annealing mod-els are empirical, based on ¢tting primarily labo-ratory timescale data, and subsequently extrapo-lating these results to geological timescales. Giventhese limitations, we believe the 14.5 Wm modeloverall leads to geologically more robust conclu-sions about patterns of denudation for westernIndia. We identify three main arguments in favourof this.

First, in terms of timing, one major di¡erencebetween the 16.3 Wm and the 14.5 Wm models isthe start of enhanced denudation after the main

EPSL 6761 22-9-03


late Cretaceous rifting and CFB event. The con-ventional model predicts a delay of about 20 Myr(Fig. 5a) before the denudation rate increases no-ticeably. In general, the time of increased denuda-tion across a passive margin will be a re£ection ofthe connectivity of the existing drainage systemswith a potentially new base level, associated withrifting and continental break-up. It is di⁄cult toexplain in geomorphological terms such a 20 Myrhiatus seaward of the Western Ghats escarpment.The 16lo curve implies that rifting of the Sey-chelles plate at 65 Ma had no signi¢cant geomor-phological e¡ect upon the non-volcanic portion ofthe Western Ghats margin. However, if the 14locurve (Fig. 5c) is considered, the increase in de-nudation rates is sustained throughout the periodencompassing the close succession of establishedrifting events at 90^80 and 65 Ma (e.g. [10,11]).We conclude that the 14.5 Wm model at least of-fers a more satisfactory ¢t to independent geolog-ical data than the more commonly adopted Las-lett et al. [22] parametrisation with an initial tracklength of 16.3 Wm.

Second, in terms of denudation magnitudes,major discrepancies are observed between the de-nudation peaks at successive intervals of the geo-morphic history. Landward of the current West-ern Ghats escarpment, the 14.5 Wm model impliespre-CFB denudation rates of ca 50 m/Myr. Suchvalues appear to be justi¢ed for a small continentsuch as India, which has experienced rifting eventsin relatively close succession (e.g. with Antarcticaonly 40 Myr before it rifted from Madagascar).More strikingly, seaward of the escarpment, pre-CFB discrepancies between models are even moreextreme, with about twice as much denudation at90^80 Ma for 16lo relative to 14lo. The post-CFBperiod landward of the escarpment is where esti-mated rates between the two models diverge themost, i.e. by a maximum factor of 10^12. Seawardof the Western Ghats, the post-CFB denudationestimates also di¡er by the same factor.

The third and most compelling reason for pre-ferring the shorter initial track length calibrationis that it allows for di¡erential denudation be-tween the coastal and the upland regions duringthe Cenozoic as late as 20 Ma (shaded areas inFig. 6b). In contrast, the conventional model cal-

ibration (Fig. 6a) implies that the Western Ghatsescarpment is an extremely ancient landform,which developed during the peak of denudationat 100^80 Ma, was formed by 65 Ma, and hasremained self-similar and in a steady statethroughout the Cenozoic as denudation rates oneither side of it remained identical. Irrespective ofthe denudation magnitudes involved, the notionof identical denudation rates on either side ofthe escarpment is quite obviously incompatiblewith the very existence of the escarpment itselfas it also extends northward, where it is carvedout of the Deccan Traps, which were not yet inexistence at 80 Ma. The 14.5 Wm model, in con-trast, allows for (i) an even larger denudation dif-ferential (100 m/Myr) during the late Cretaceous,a proportion of which compensates the trend dur-ing the early Cretaceous when denudation in thecurrent upland zone was higher than in the cur-rent coastal zone; followed by (ii) a continuationof higher rates into the late Eocene near the coast,after which denudation di¡erentials diminish andprobably only represent minor morphological ad-justments. This fall in erosion rates after the pre-Eocene discharge of coarse clastic sequences intothe o¡shore basin is in agreement with the knownstratigraphy of the basin [8,37], with its s 1 kmthick Eocene to Middle Miocene limestone plat-forms re£ecting low clastic input from the conti-nent. Late to post-Miocene sediments consist of¢ne shale and clay, mostly re£ecting the erosionof deep weathered mantles routed via the coastalrivers from the escarpment face and the plateau inrelation to gorge recession and recent drainagepiracies in the context of a highly aggressive mon-soon climate [43].

In summary, until further progress in calibrat-ing AFT annealing models is achieved, we suggestthat the shorter initial track length model used inthis study generates denudation curves that are inbetter agreement with geomorphic, stratigraphicand tectonomagmatic data than the conventionalmodel calibration used in most previous AFTstudies. We do not claim that this model is there-fore better than the existing published models, butit does highlight the need for re¢ning the latterthrough the incorporation of annealing data rele-vant to geological timescales. The quality and ap-

EPSL 6761 22-9-03


plicability of laboratory annealing models shouldfurther bene¢t from testing against independentconstraints provided by well-understood naturalsettings, especially when AFT datasets such asthis are geographically extensive and of a highanalytical quality.

Acknowledgements

The authors acknowledge the ¢nancial supportprovided by NERC (Grant GR9/963) in the UK,INSU (PNSE 99N51/0354) in Paris and CNRS-UMR 6042 in Clermont-Ferrand (France). Wethank Paul Green for helping us improve a pre-vious draft of this paper, and Peter van der Beekand two anonymous reviewers for their construc-tive comments.[BARD]

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