Early-Middle Miocene paleodrainage and tectonics in the ... et al GSAB 2003.pdf · Imran Khan...

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GSA Bulletin; October 2003; v. 115; no. 10; p. 1265–1277; 7 figures; 1 table; Data Repository item 2003138.

Early-Middle Miocene paleodrainage and tectonicsin the Pakistan Himalaya

Yani Najman†

Department of Geology and Geophysics, Edinburgh University, Kings Buildings, West Mains Road, Edinburgh EH9 3JW, UK

Eduardo GarzantiDipartimento di Scienze Geologiche e Geotecnologie, Universita di Milano-Bicocca, Piazza della Scienza 4, 20126 Milan, Italy

Malcolm PringleScottish Universities Environmental Research Centre, Scottish Enterprise Technology Park, Rankine Avenue, East Kilbride,G75 0QF, UK

Mike BickleDepartment of Earth Sciences, Cambridge University, Downing Street, Cambridge CB2 3EQ, UK

John StixDepartment of Earth & Planetary Sciences, McGill University, 3450 University Street, Montreal, Quebec H3A 2A7, Canada

Imran KhanSedimentary Geology Division, Geological Survey of Pakistan, Sariab Road, Quetta, Pakistan

ABSTRACT

The 18–14 Ma Kamlial Formation Hi-malayan foreland basin sedimentary rocksin the Chinji Village region, Potwar Pla-teau, Pakistan, are characterized by: (1)lithofacies indicative of deposition by alarge river; (2) a dominant magmatic arcprovenance completely unlike the ‘recycledorogen’ foreland basin deposits strati-graphically below, above, or coeval withthese rocks; and (3) subordinate contribu-tion from a rapidly exhuming source, inter-preted as either the Nanga Parbat Hara-mosh Massif or the southern margin of theAsian crust. The start of Kamlial Forma-tion deposition at this locality at 18 Mamarks a major break with the older Mur-ree Formation rocks, which were depositedby rivers draining predominantly the Hi-malayan thrust stack south of the arc. Weinterpret this change as the result of diver-sion of the paleo-Indus River to its presentposition, which crosses the Kohistan arcand Himalayas and debouches into theforeland. If the rapidly exhuming subordi-nate source region were the Nanga ParbatHaramosh Massif, then initiation of its up-lift would have resulted in significant arc

†E-mail: [email protected].

detritus to the basin as the overlying arccarapace was exhumed. As the carapacewas progressively breached, arc materialwould have become a less substantial com-ponent of detritus to the basin, consistentwith the reported petrography of the over-lying Siwalik deposits.

Keywords: Himalaya, Indus River, detritalminerals, exhumation, foreland basin, Nan-ga Parbat.

INTRODUCTION

The Himalayan orogen formed due to thecollision between India and Eurasia that beganca. 55 Ma. The sedimentary record of materialeroded from the mountain belt and preservedin foreland basins provides a history of ero-sion, tectonism, and paleodrainage in the or-ogen. This study concentrates on the forelandbasin sedimentary rocks of the Kamlial For-mation in the Chitta Parwala section, ChinjiVillage, Potwar Plateau area of Pakistan (Fig.1), deposited between 18–14 Ma (Johnson etal., 1985).

The foreland basin study area in the PotwarPlateau is now drained by the Indus River andits tributaries. Today the Indus River flowswest along the line of suture zone and thencuts south over the Himalayas, perpendicular

to the strike of the orogen, to the foreland ba-sin and finally the Indus Fan (Fig. 1). Yet, theroute of the paleo-Indus remains controversial.While some researchers consider the path ofthe Indus River to be antecedent, others sug-gest that it first cut through the Himalayan beltand debouched into the foreland basin in theEarly Miocene, or at 11 Ma, with earlier rout-ing perhaps into the Katawaz remnant oceanbasin (Abbasi and Friend, 1989; Qayyum etal., 1997, 2001; Brookfield, 1998; Shroder andBishop, 2000; Clift et al., 2001b). The sug-gested interrelationships and feedback be-tween tectonism, denudation, and drainageevolution of the Indus River (e.g., Shroder andBishop, 2000; Zeitler et al., 2001a, b) makereconstruction of the paleodrainage and tec-tonics of the region an important goal.

GEOLOGICAL BACKGROUND

Mountain Belt Evolution

The mountain belt in Pakistan consists ofthree tectonostratigraphic units that formedduring a series of orogenic events that oc-curred both prior to and during India-Asia col-lision (Fig. 1). Farthest north lies the southernmargin of the Asian crust, including the HinduKush and the Karakoram. These belts includea Paleozoic–Mesozoic succession intruded by

1266 Geological Society of America Bulletin, October 2003

NAJMAN et al.

Figure 1. Geological map showing locations for region under study and present-day drainage configuration. Area of study, Chitta Parwalasection, Chinji Village area; Potwar Plateau is indicated by star. HKS—Hazara-Kashmir syntaxis, KF—Karakoram Fault, MMT—Main Mantle Thrust, PT—Panjal Thrust, KP—Kohat Plateau, MBT—Main Boundary Thrust, MFT—Main Frontal Thrust, NS—Northern Suture, NPHM—Nanga Parbat Haramosh Massif syntaxis, PP—Potwar plateau. Inset: location of main map, as indicated bybox, K—Katawaz basin, Su—Sulaiman Range.

a Jurassic to Cretaceous batholith and affectedby metamorphic events pre- and post- India-Eurasia collision (Debon et al., 1987; Gaetaniet al., 1990; Searle, 1996; Hildebrand et al.,2001; Fraser et al., 2001). Sandwiched be-tween the Asian crust along the Northern orShyok Suture to the north and the Indian crustalong the Main Mantle Thrust (MMT) to thesouth is the Cretaceous–Eocene Kohistan Is-land Arc, intruded by the Kohistan batholith,which shows pre- and post-collisional stagesof formation. A south-to-north transect pro-vides a complete section through the arc, fromthe deeper crustal levels of the ultramafic-layered igneous complexes in the south tostructurally higher seafloor sedimentary rocksand basic andesitic and rhyolitic volcanicrocks in the north (Treloar et al., 1989; Khanet al., 1993). South of the Main Mantle Thrustis the Himalayan thrust stack consisting of In-dian continental crust cover of Paleozoic–

Mesozoic sedimentary protolith, metamor-phosed to greenschist to amphibolite gradeduring the Himalayan orogeny, locally imbri-cated with basement gneisses that retain Pro-terozoic and Phanerozoic cooling ages (Tre-loar and Rex, 1990). Cambro–Ordovician,Carboniferous, and Permian intrusions arealso found within the thrust stack (Le Fort etal., 1980; Smith et al., 1994; Anczkiewicz etal., 1998a, b; DiPietro and Isachsen, 2001).

Collision between Asia and the Kohistanarc along the Northern or Shyok Suture tookplace between 70 and 100 Ma (Coward et al.,1986; Treloar et al., 1989; Gaetani et al.,1993). In Kohistan, cooling occurred ca. 80Ma, although a later phase of cooling in theeast is associated with the more recent rapiduplift of the Nanga Parbat Haramosh Massif(Zeitler, 1985; Treloar et al., 1989). At ca. 55Ma (Klootwijk et al., 1991), the Kohistan Arccollided with the Indian crust along the Main

Mantle Thrust, along which lies melange con-taining serpentinite and blueschists. Subse-quently, the northern margin of the Indiancrust underwent tectonic thickening, medium-to high-pressure metamorphism, plutonism,and deformation beginning at ca. 50 Ma. Be-tween 45 and 25 Ma, post-metamorphicthrusting occurred during which time therocks followed decreasing pressure-temperature paths probably due to tectonic un-roofing associated with thrusting. A period ofrapid cooling between 25 and 20 Ma is ten-tatively associated with tectonic denudation,as the overlying Kohistan Arc slid northwardon normal faults. From ca. 20 Ma, faultingceased and the Kohistan arc and Indian crusthave only undergone simple uplift and erosion(Treloar et al., 1989, 1991; Chamberlain et al.,1991; Pognante and Spencer, 1991; Chamber-lain and Zeitler, 1996; Burg et al., 1996). Theexception is the region of the Nanga Parbat

Geological Society of America Bulletin, October 2003 1267

EARLY-MIDDLE MIOCENE PALEODRAINAGE AND TECTONICS IN THE PAKISTAN HIMALAYA

TABLE 1. GENERALIZED FORELAND BASIN STRATIGRAPHY, POTWAR REGION, PAKISTAN

Formation Age

Chinji Fm., Siwalik Group ,14 Ma†

Kamlial Fm. 18–14 Ma‡

Murree Fm. Early Miocene ‘‘southern outcrops’’#, 37 Ma (Balakot Formation – ‘‘northern outcrops’’) §

Marine strata, e.g. Chorgali Fm. Eocene††

Note that in the Chitta Parwala region of study, the Murree Formation is absent and the Kamlial Formationrests unconformably on Eocene limestones.

†Burbank et al. (1996) and references therein.‡Johnson et al. (1985).§Najman et al. (2001).#Fatmi (1973), Abbasi and Friend (1989).††Shah (1977); Johnson et al. (1985).

Haramosh Massif in the western syntaxis re-gion. The Nanga Parbat Haramosh Massif isan anomalous region of the Indian crust thathas been undergoing metamorphism to gran-ulite grade and extremely rapid exhumationsince at least 10 Ma (Treloar et al., 1989; Zeit-ler, 1985; Zeitler et al., 1989, 1993). Con-straints on the timing of initiation of this eventare poor, since the Late Neogene events haveobliterated evidence of much of the massif’searlier history. However, Early Miocene min-eral ages indicate an earlier anatectic and cool-ing history (Schneider et al., 1999; Treloar etal., 2000; Pecher et al., 2002). Originally man-tled by the overthrust Kohistan arc, the NangaParbat Haramosh Massif’s major uplift hasalso affected the arc rocks above and adjacentto it.

The Foreland Basin Deposits (Table 1)

The Kamlial Formation (e.g., Wadia, 1928;Cotter, 1933; Shah, 1977; Johnson et al.,1985) forms part of more than 10 km of sed-iment that fills the Himalayan foreland basin.The Kamlial Formation succession is morethan 400 m thick and is exposed in the Kohatand Potwar Plateau regions. In the northernregions of the plateaus its lower contact withthe underlying Murree Formation is conform-able and transitional, but in the Chinji Villagestudy area, in the southern region of the Pot-war Plateau (Fig. 1), the Murree Formation isabsent and the Kamlial Formation rests un-conformably upon Eocene marine strata. TheKamlial Formation is conformably overlain bythe Chinji Formation of the Siwalik Group. Inthe region of study—Chitta Parwala section,Chinji Village area, Potwar Plateau—magne-tostratigraphic studies have dated the KamlialFormation succession at 18 to 14 Ma (Johnsonet al., 1985). Data Repository item DR11 pro-vides information on the age of the samplesused in this research and their location in thesection. The Kamlial Formation deposits con-sist of alluvial sandstones, mudstones, and ca-

1GSA Data Repository item 2003138, DR1—Sample locations and ages at Chitta Parwala section,Potwar Plateau, DR2—Petrographic composition ofanalyzed sandstone samples from the Kamlial For-mation, DR3—Dense mineral assemblages in ana-lyzed sandstone samples from the Kamlial Forma-tion, DR4—Recalculated key indices for frameworkcomposition and dense-mineral suites, DR5—Detailed description of Kamlial Formation sandstonepetrography at Chitta Parwala section, Potwar Pla-teau, DR6—All data, DR7—Ar/Ar total fusion,DR8—incremental heating data of single detritalwhite mica grains from the Kamlial Formation sedi-ments, is available on the Web at http://www.geosociety.org/pubs/ft2003.htm. Requests may alsobe sent to [email protected].

liche. In the Chinji Village study area, arena-ceous lithofacies predominate. Individualsandstone stories are on average 7–10 m thick,with a maximum thickness of 19 m. Amal-gamated multistory sandstone bodies have atotal thickness of a maximum of 58 m and anaverage of 17.6 m. Sandstone-fine unit cyclesare an average of 25 m thick. These faciesindicate deposition by a large river (Stix,1982; Willis, 1993; Hutt, 1996). Such detailedsedimentological information does not existfor Kamlial Formation rocks in other regions.Qualitative descriptions indicate that arena-ceous facies generally predominate over argil-laceous units, but with regional variations(e.g., Wadia, 1928; Cotter, 1933; Gill, 1951).

The Murree Formation encompasses theBalakot Formation in the Hazara–KashmirSyntaxis to the north and extends south intothe Kohat and Potwar plateaus and east intoIndia (e.g., Pinfold, 1918; Wadia, 1928; Gill,1951; Bossart and Ottiger, 1989; Singh andSingh, 1995; Najman et al., 2002). The Ba-lakot Formation is dated by 40Ar-39Ar ages ofdetrital micas at younger than 37 Ma (Najmanet al., 2001) and the younger southern MurreeFormation outcrops at Early Miocene (Fatmi,1973; Abbasi and Friend, 1989). Lithofaciesare interpreted to record alluvial and tidal de-positional environments. Published descrip-tions of the Murree Formation are less de-tailed and quantitative compared to those ofthe Kamlial Formation at the Chinji Villagearea (see references above). Regional lithofa-cies variations are significant, but generallythe Murree Formation appears to have a fine-grained rock:sandstone ratio of $ 50%, and arecognizably higher proportion of mudstonecompared to that of the Kamlial Formationsuccession in the Chinji Village region. Wheremeasured and recorded, sandstone units in theMurree Formation are also considerably thin-ner than Kamlial Formation strata at the Chin-ji Village study area, but such quantitative rec-ords only exist for the Murree Formation inthe Hazara–Kashmir syntaxis (Balakot For-

mation; Bossart and Ottiger, 1989; Najman etal., 2002), and to the east at Jammu (Singhand Singh, 1995). No bed thicknesses are re-corded in the literature for the Murree For-mation in the Kohat and Plateau plateaus, asfar as we are aware.

Conformably overlying the Kamlial For-mation lies the Chinji Formation of the Si-walik Group, dated at its base at 14 Ma in theChitta Parwala section (Johnson et al., 1985).The Siwalik Group is deposited basin-widefrom Pakistan through Nepal to eastern India.These rocks consist of sandstones, mudstones,and conglomerate upsection. They are inter-preted as braided fluvial deposits (Burbank etal., 1996, and references therein).

Prior Provenance and PaleodrainageStudies of the Kamlial Formation

Prior provenance work on the Kamlial For-mation is restricted to a detrital zircon fission-track study (Cerveny et al., 1988); petrograph-ic and facies study of the Kamlial Formationin the Chinji Village area, Potwar Plateau(Stix, 1982; Johnson et al., 1985; Cerveny etal., 1989; Hutt, 1996; this study); and a pet-rographic study of a lithostratigraphically cor-related section in the eastern part of the KohatPlateau to the west (Abbasi and Friend, 1989;Pivnik and Wells, 1996). In addition, there isa short petrographic description of locally de-rived sedimentary rocks at the western edgeof the Kohat basin, which the authors corre-lated with the Kamlial Formation (Abbasi andKhan, 2003). In the eastern Kohat Plateau sec-tion, Kamlial Formation sandstones containabundant quartz and common sedimentary tometamorphic lithics with only minimal evi-dence of volcanic detritus. From the relativelack of arc material and the absence of dis-tinctive blue-green hornblende, (noted for itsfirst appearance at 11 Ma and interpreted asindicative of arc unroofing; Cerveny et al.,1989), Abbasi and Friend (1989) consideredthat the paleo-Indus River first cut through the


NAJMAN et al.

arc after deposition of the Kamlial Formation,since the Middle-Miocene. Kamlial Formationsandstone composition in the eastern part ofthe Kohat Plateau is in complete contrast tothe Kamlial Formation sandstones of the Chit-ta Parwala section, Potwar Plateau, whereabundant igneous material indicates dominantarc provenance since 18 Ma (Hutt, 1996, thisstudy).

Paleocurrent data in the Chinji Village Pot-war Plateau study area (Stix, 1982; Johnson etal., 1985; Hutt, 1996) demonstrate predomi-nant flow toward the east, east-southeast, andsoutheast, similar to that recorded in the over-lying Siwalik rocks in the area, but differentfrom the more southward-directed paleocur-rents further west in the Kohat Plateau. Thesesoutheast-directed orientations have been in-terpreted as indicative of either (1) localslopes on large alluvial fans, not necessarilyrepresentative of the main direction of region-al flow (Willis, 1993); or (2) axial drainage inthe Potwar Plateau region, flowing east towardthe Ganges River catchment at these times andlater (Raynolds, 1981; Beck and Burbank,1990; Burbank et al., 1996). In the second sce-nario, the drainage divide would have lainwell west of its present-day position, separat-ing a southerly-flowing river in the Sulaimanforedeep from the easterly-draining system ofthe Potwar plateau. Hutt (1996) also reporteda subsidiary set of north-directed paleocur-rents from which she interpreted the presenceof a subsidiary southern source, althoughthere is no petrographic distinction betweenthe two drainage types.

Although most attempts at reconstruction ofthe hinterland tectonics from the Pakistanforeland basin record in this area have beenrestricted to the timing of erosion of the arcbased on the appearance of blue-green horn-blende subsequent to Kamlial Formation times(e.g., Johnson et al., 1985; Cerveny et al.,1989), one notable exception is that of Cer-veny et al. (1988). They compared detrital zir-con fission-track ages with host sediment de-positional age and showed that there was onlya short lag time throughout the period 17 Mato present day. This suggests that regions ex-periencing rapid exhumation, perhaps analo-gous to the Nanga Parbat Haramosh Massif,had contributed sediment to the basin through-out this period. However, the samplingthroughout the period of study was sparse;only one Kamlial Formation sample and threeSiwalik Formation samples were analyzedfrom the Chinji area, and the study can there-fore only provide a first-order approximation.

PETROGRAPHIC AND DENSEMINERAL DATA FROM THEKAMLIAL FORMATION, CHITTAPARWALA SECTION

Petrography was determined by counting300 framework points on each of 12 samplesusing the Gazzi-Dickinson method, and wealso counted 200–250 transparent dense min-erals on 10 samples. (Results are summarizedin Figs. 2 and 3, and more detail is suppliedin Data Repository item DR2–5 [see text foot-note 1]). Analyzed samples are mostly fine-grained sandstones, ranging from 1.5–3 f inmedian diameter. Traditional ternary parame-ters and plots (QmFLt, QmPK, QpLvmLsm,LmLvLs; Dickinson, 1985; Ingersoll et al.,1993) were supplemented, specifically wherelithic grains are concerned, by an extendedspectrum of key indices. Metamorphic grainswere classified according to both compositionand metamorphic grade, which were largelyinferred from degree of recrystallization ofmica flakes (Garzanti and Vezzoli, 2003).

The Kamlial Formation consists of quartz-poor, lithofeldspathic sandstones (Fig. 2). De-tritus was derived from several distinct sourc-es, including dominant volcano–plutonicrocks, subordinate sedimentary to very low-grade metasedimentary rocks, and minorhigher-grade metamorphic rocks and ophioli-tes. Detrital modes straddle the boundary be-tween ‘‘magmatic arc’’ and ‘‘recycled oro-gen’’ provenance fields in standardquartz-feldspar-lithics (QFL) plots (Dickinson,1985). As illustrated in Figure 2, this is sig-nificantly different not only from the petrog-raphy of the Murree Formation below (Naj-man and Garzanti, unpub. data) and theSiwalik Formation above ‘Gabir/Chinji sec-tion’ between 9 and 11 Ma (Critelli and In-gersoll, 1994), but also from the Kamlial For-mation of the eastern region of the KohatPlateau to the west (Abbasi and Friend, 1989).All these other clastic wedges plot in the ‘‘re-cycled orogen’’ provenance field of Dickinson(1985; Fig. 2, QFL plot). Thus, the KamlialFormation of the Chitta Parwala section is theonly Himalayan foreland basin unit studied sofar (see also DeCelles et al., 1998a, 1998b;Najman and Garzanti, 2000; White et al.,2002), apart from the Chulung La Formationof the Tethys Himalaya (Garzanti et al., 1996)and locally derived sedimentary rocks adja-cent to ophiolites (Abbasi and Khan, 2003),that is characterized by a distinct ‘‘magmaticarc’’ signature. Dense mineral assemblages in-clude garnet and epidote, with subordinatetourmaline rutile, sphene, zircon, staurolite,

chloritoid, chrome spinel, and amphiboles(hornblende, glaucophane, tremolite).

Anomalous Samples

Intercalated in the upper part of the unit, at14.9 and 14.3 Ma, are sandstones (in partic-ular, sample CP96–6A) with significantly low-er proportions of feldspars and volcanic detri-tus (lithic grains, volcanic quartz, plagioclase)and higher very low- to low-grade metapeliteand metafelsite lithics, with virtually no me-tabasite (Fig. 2). Dense minerals in sample6A, significantly different from the remainderof the Kamlial Formation samples, are domi-nated by garnet (77%) and tourmaline (18%)with some chloritoid and staurolite and neg-ligible epidotes (1%) (Fig. 3).

Unravelling Mixed Provenances

We used a simple empirical forward ap-proach to estimate the end-member propor-tions (arc and metasedimentary thrust belt) inthe mixed-source Kamlial Formation sand-stones. End-member compositions were as-signed according to two approaches, modernand ancient, using data from modern river sed-iments and ancient sandstones, respectively,eroded from the arc and Himalayan thrust beltsource regions (Garzanti et al., 2003). The re-sults obtained according to these different ap-proaches are consistent, indicating that arc tooceanic rocks of the suture zone suppliedabout two-thirds of bulk detritus contained inthe Kamlial Formation, with mostly meta-sedimentary rocks accounting for the remain-der. Arc and oceanic rocks, however, suppliedless than a quarter of detritus to anomaloussample 6A, the remainder being chiefly ac-counted for by sedimentary and metamorphicsources up to garnet grade.

Ar-Ar MICA DATA

We analyzed more than 300 individual de-trital white mica grains from the Kamlial For-mation sandstones using total fusion and in-cremental heating 40Ar–39Ar techniques similarto that employed by Richards et al. (1999) andWhite et al. (2002), but with all crystals de-gassed. Figures 4 and 5 and Data Repositoryitems DR6–8 (see text footnote 1) summarizeAr–Ar mica age data obtained from the Kam-lial Formation. For all but one of the samples(CP96–6A, depositional age 14.3 Ma), thewhite mica population is dominated by grainsof Himalayan age (,55 Ma), with the remain-



Figure 2. Detrital modes of Tertiary Himalayan sandstones. Kamlial Formation displays distinct petrographic composition withrespect to other clastic wedges of Himalayan foreland basin (data from references in text) and is closer to arc-sourced TethyanChulung La Formation. Only anomalous samples 5A and 6A straddle the boundary between ‘‘magmatic arc’’ (MA) and ‘‘recycledorogen’’ (RO) provenance fields (Dickinson, 1985; CB—‘‘continental block’’); their lithic population is indistinguishable from UpperDharamsala Formation samples, which were sourced by very low grade accreted Indian-margin sequences (White et al., 2002). Q—quartz (Qm—monocrystalline, Qp—polycrystalline); F—feldspars (P—plagioclase, K—K-spar); L—lithic fragments (including car-bonate and chert lithics); Lm—metamorphic (metamorphic grade: Lms1—slate to metasandstone, Lms2—phyllite to quartz /sericite,Lms3—quartz/mica to micaschist and gneiss); Lv—volcanic (Lvm—volcanic and metavolcanic); Ls—sedimentary (Lsm—sedimentaryand metasedimentary). 99% confidence regions of mean, calculated after Weltje (2002), are shown for typical Kamlial and broadlycoeval Kasauli 1 Dharamsala units.

der of the population spanning ages back toca. 450 Ma. Within the Himalayan-aged pop-ulation, the youngest subpopulation decreasesin age up-section through time, from a ca. 25-Ma mode in rocks deposited at 18 Ma, to aca. 14-Ma mode in sedimentary rocks depos-ited at 14 Ma. The lag time, defined as thedifference between the youngest detrital micaage and the depositional age of its host sedi-ment, is therefore short throughout the time ofdeposition of the Kamlial Formation and de-creases up-section with a lag time between 5and 7 Ma, typical for the basal two samples

of the succession and ,2 Ma for the overlyingmain part of the succession. Sample CP96–6Ais an anomalous sample. It is dominated by apre-Himalayan-aged mica population span-ning 77–443 Ma. It has a lag time of 15 m.y.,which was calculated from the rare occurrenceof Himalayan-aged micas; two grains of theolder sub-population are dated at 33 and 53Ma. There are no Neogene-aged mica grains.The anomalous mica population is coupledwith a distinctive petrography with only lim-ited contribution from the arc-suture zonesource, as described above.

Constraints to Crustal Exhumation Ratesfrom Mica Population Ages

It is possible to estimate exhumation ratesfrom lag times, given an appropriate thermalstructure for the crust and ignoring radiogenicheat production (cf. Moore and England,2001). We assume that the initial crustal ther-mal profile comprises a linear gradient fromzero at the surface to 700 8C at 35 km. Belowthis, temperatures are constant at ;700 8C.The thermal evolution for exhumation of thiscrust is calculated from equation 20 of Bickle


NAJMAN et al.

Figure 3. Dense minerals in Tertiary Himalayan sandstones. During initial collision (latePaleocene/Early Eocene), detritus was derived from Indus suture zone and thrust sheetsof Indian-margin sediments (Subathu Formation). Subsequently, very low grade Indianmargin sequences were eroded to feed the Dagshai Formation, with first unroofing ofgarnet to staurolite-bearing Himalayan crystalline nappes recorded by Lower to MiddleMiocene Kasauli to Dharamsala Formations (Najman and Garzanti, 2000; White et al.,2002). Epidote-rich Kamlial suites, instead, were chiefly eroded from arc sources. Supplyfrom Himalayan metamorphic nappes is dominant only for anomalous sample 6A. Legendas in Figure 3. ZTR—zircon, tourmaline, rutile; E—epidote; S—spinel; A—amphibole;P—pyroxene; Gt—garnet; St—staurolite; Chtd—chloritoid. 99% confidence regions ofmean, calculated after Weltje (2002), are shown for Kamlial and broadly coeval Kasauli1 Dharamsala units.

and McKenzie (1987). This thermal structureis appropriate for a Barrovian-style metamor-phic crust formed as a result of crustal thick-ening (e.g., Vance and Harris, 1999; Vance etal., 2003). Vertical exhumation of such crust(700 8C maintained at 35 km depth from sur-face, surface maintained at zero temperature)establishes an equilibrium geotherm within afew Ma (Bickle and McKenzie, 1987, equa-tion 18). For a crustal thermal diffusivity of10–6 m2/s and a mica blocking temperature of350 8C, an exhumation rate of 4.5 mm/yr re-sults in an equilibrium lag time of 1.1 m.y.(similar to the lag time displayed for the ma-jority of Kamlial Formation samples, i.e.,those samples aged # 17.4 Ma), while an ex-humation rate of 1.7 mm/yr would result in anequilibrium lag time of 6 m.y. (similar to thelag times displayed by the two basal Kamlialsamples) (equation 20, Bickle and McKenzie,1987).

This difference in lag times between thebasal and main Kamlial samples may be ex-plained by (1) increasing exhumation rate ofthe source area, (2) a change in source area,

or (3) nonrepresentative sampling in the basalsamples, i.e., our analyses of ;60 grains,picked randomly, failed to incorporate any ofthe zero-aged grain population that was pres-ent. Assuming a single source (scenario 1), thedecrease in lag time occurs abruptly, within;0.3 m.y. It is interesting to calculate howrapidly minimum lag times would change ifthe exhumation rate were instantaneously in-creased. Such model curves are shown on Fig-ure 6. The curves model a range of exhuma-tion rates from 2, 1.7, 1.0, 0.5, and 0.1 mm/yr to 4.5 mm/yr at times prior to 17.4 Ma,satisfying the criteria that micas reset after theaccelerations in exhumation rate are exposedby 17.4 Ma. Figure 6 shows that, assuming asingle source for the Himalayan-aged micapopulation, if the crust was previously beingexhumed at rates . ;0.5 mm/yr, it would notbe possible to obtain the observed very rapidtransition of lag times from ca. 6 Ma to , 2Ma. Alternatively, if sources switched (sce-nario 2) or we failed to analyze the youngestmicas in the lowermost samples (scenario 3),initiation of rapid exhumation must have oc-

curred prior to ca. 20 Ma for plausible initialexhumation rates (Fig. 6). Therefore, regard-less of scenario, the data show increased ex-humation by ca. 20 Ma.

INTERPRETATIONS

Provenance

The Kamlial Formation at Chitta Parwala,Potwar Plateau, is of mixed provenance: two-thirds of detritus was supplied by an upliftedarc–trench system, with continental marginsedimentary and metasedimentary sources ac-counting for the remaining third. Relative con-tributions are reversed in samples 5A and 6A,in which continental margin detritus includinggarnet-bearing medium-grade metamorphicrocks predominate. Mica ages indicate contri-bution both from regions affected and unaf-fected by Himalayan metamorphism. Thereremains little doubt that the dominant sourceto the foreland basin by this time was the ris-ing orogen to the north. In spite of eastwardand subordinate northward-directed paleocur-rent data, we do not consider southern andwestern sources to be significant: paleocurrentand cathodolumiscence data given as evidenceof erosion from the Precambrian crystallinebasement of the southern peripheral forebulge(Hutt, 1996) can equally be explained by cre-vasse splay and erosion from an igneous arcsource. Moreover, the peripheral forebulge, oflow relief, would be unlikely to have contrib-uted significant detritus, and its petrographiccomposition and age contrast radically withthe Kamlial Formation data. To the west ofthe foreland basin lie the sedimentary andophiolitic Sulaiman ranges (Waheed andWells, 1990). While these ophiolites were ob-ducted prior to Eocene time (e.g., Treloar andIzatt, 1993; Beck et al., 1996), we do not con-sider this a likely source for the igneous-derived detritus in the Kamlial Formationsandstones of Chitta Parwala section, PotwarPlateau, because (1) the more westerly Kam-lial Formation in the Kohat Plateau shows noevidence of significant ophiolitic detritus, ex-cept for locally derived sedimentary rocks atthe western margins (Abbasi and Friend,1989; Abbasi and Khan, 2003), although wenote the regionally incomplete nature of avail-able data, and (2) based on sedimentologicalchanges at 18 Ma in the Sulaiman range fore-land basin, a major south-flowing trunk riveris interpreted as having been established atthis time (Friedman et al., 1992; Downing etal., 1993). Such paleodrainage would precludewestern-derived ophiolitic material from beingtransported to the Potwar Plateau.



Figure 4. Histograms of detrital white mica ages. Depositional ages of sediment samples given in Ma and shown with dotted arrows.Number of grains younger than 60 Ma given as percentage of total number analyzed per sample. Mica ages between 10–60 Ma sortedin 2 m.y. wide bins and shown in a linear scale; ages older than 60 Ma sorted in 10 m.y. bins and shown on a logarithmic scale.

Petrographic ConstraintsThe obvious sources for the dominant ig-

neous component are the Kohistan arc and In-dus suture zone/Main Mantle Thrust. Occur-rence of rare blueschist grains and blue sodicamphiboles support minor but significant sup-

ply from ophiolites and melange pinchedalong the suture zone. Appropriate sources forthe subsidiary sedimentary and very low tomedium grade metasedimentary detritus canbe found both north and south of the arc, inthe Karakoram and Himalayan thrust belts.

Although the present-day Himalayan thrustbelt exposes little sedimentary and low-grademetamorphic material, this does not precludeit from consideration as a source region in thepast, because higher stratigraphic and/or struc-tural levels of Indian crust cover rocks were


NAJMAN et al.

Figure 5. Ar-Ar detrital mica ages (,55 Ma Himalayan-aged grains only are shown)plotted against sample depositional age, as determined by magnetostratigraphy. Solid 1:1 line shows ‘‘zero lag time,’’ when mica age equals sediment depositional age.

Figure 6. Plot of mica lag times, maximum—black squares, minimum—black circles,where maximum and minimum represent 2s errors on youngest mica Ar-Ar ages withno account taken of errors in stratigraphic age. Solid lines represent evolution in lag timesfor crust initially exhuming at rates shown and then with exhumation accelerated to 4.5mm/yr at a time (19.7 Ma for 1.7 mm/yr to 21.2 Ma for 0.1 mm/yr). These times arechosen so that micas at their blocking temperature at the time at which exhumation ac-celerates reach the surface at 17.4 Ma and therefore have model ages between 2.3 Ma and3.8 Ma. Dashed line shows lag times for micas that closed after increase in exhumationrate. Thermal model used for calculation is described in text.

likely never buried to depths sufficient tocause metamorphism to amphibolite grade asseen in the rocks currently exposed. Moreover,we consider this region the most likely sourcefor the metasedimentary detritus because theIndian margin must have been contributingmaterial to the basin in view of its paleogeo-

graphic position south of the arc. Both lithicand dense mineral populations of samplesCP96–6A and 5A are, in fact, very similar tothose of sedimentary suites clearly derivedfrom such Indian margin units, e.g., the coevalUpper Dharamsala foreland basin deposits inIndia (White et al., 2002) (Figs. 2 and 3).

Constraints from Pre-Himalayan-AgedMicas

The pre-Himalayan micas can all be attrib-uted to an Indian crust Himalayan provenance.Lithologies with minerals of appropriate agesin the thrust belt south of the Main MantleThrust in Pakistan include Cambrian–Ordovician and Permo–Carboniferous igneousand metamorphic rocks (e.g., Treloar and Rex,1990; Smith et al., 1994; DiPietro and Isach-sen, 2001). Although there is no knownJurassic–Cretaceous event in the Himalayanthrust belt to explain the occurrence of micasof this age in the Kamlial sandstones, Treloarand Rex (1990) report mica ages of ca. 175Ma from this thrust belt that may be the resultof alteration, and White et al. (2002) report asimilar aged population of altered detrital mi-cas from the coeval Dharamsala Formationforeland basin sediments in India, which areclearly derived from the Indian margin thruststack. However, although a combination of de-tectable alteration of older mica grains andsuitable lithologies in the Himalayan thrustbelt make a contribution from other sourcesunnecessary, it is still possible that Asiansources or the Kohistan arc (with mineralcooling ages mostly between 75–90 Ma, Zei-tler, 1985; Treloar et al., 1989; Chamberlainet al., 1991) supplied a proportion of the pre-Himalayan-aged detrital mica grains.

Constraints from Himalayan-Aged MicasHimalayan-aged micas (youngest grain

aged 14 Ma) were eroded from a source ex-huming rapidly since ca. 20 Ma. Outside theNanga Parbat syntaxial region, the Indiancrust thrust stack and the Kohistan arc areboth inadmissible as sources for these grains,since micas exposed at the surface today areolder than those found in the # 17.4 Ma agedKamlial Formation rocks (Maluski and Matte,1984; Zeitler, 1985; Treloar et al.,1989; Tre-loar and Rex, 1990). Micas of suitable age arefound today in the Indian crust in the NangaParbat region and in regions of the Asian crust(e.g., Karakoram Batholith and KarakoramFault area; Dunlop et al., 1998; Searle et al.,1998).

Mica 40Ar–39Ar ages in the Nanga ParbatSyntaxis are 4–6 Ma (George et al., 1995), aresult of very rapid exhumation of the massifsince 10 Ma. Initiation of exhumation likelystarted prior to 10 Ma, as evidenced by thedecreasing mineral cooling ages across theKohistan arc: In the western region of the arc,unaffected by uplift of the Nanga Parbat Har-amosh Massif, average zircon fission-trackages range between 30 and 52 Ma. By con-trast, in the vicinity of the Nanga Parbat Har-



Figure 7. Cartoon drainageevolution maps; see text for de-tails. Note that present-dayhinterland thrust boundariesare only intended for orienta-tion and were likely different,at least to some extent, in Mio-cene times. KP—Kohat Pla-teau, PP—Potwar Plateau,CP—Chitta Parwala KamlialFormation studied section. A:Murree Formation times: Riv-ers depositing sediment to fore-land basin have catchment ar-eas predominantly located inHimalayan thrust belt andreach no further back thanarc’s southern fringe. Paleo-Indus River in Paleogene isshown as dashed line; debatecontinues about its presence(Searle et al., 1990) or absence(Sinclair and Jaffey, 2001) andinterpreted exit through Kata-waz basin rather than forelandbasin (Qayyum et al., 1997). B:Kamlial Formation times: Ini-tiation of Indus River drainageinto foreland basin and estab-lishment of position compara-ble to its modern day route.Major petrographic source forsediments at Chitta Parwala,Potwar Plateau, is Kohistanarc; source for rapidly exhum-ing micas is NPHM or Asia.More local transverse rivers,predominantly draining Hi-malayan thrust stack, are rep-resented by Kamlial Forma-tion sediments in KohatPlateau region (Abbasi andFriend, 1989) and in Potwarregion by sample CP96–6Atype compositions.

amosh Massif to the east, ages lie in the rangeof 11–16 Ma (Zeitler, 1985), indicating initi-ation of exhumation prior to this time. In ad-dition, mineral cooling ages of 16–20 Ma arefound in the Indian crust cover metasedimentson the margins of the Nanga Parbat syntaxis(Treloar et al., 2000; Pecher et al., 2002),which Treloar et al. consider to be dominantlya function of uplift-related exhumation, which

occurred during a crustal-scale folding eventthat defined the early stages of syntaxialgrowth. Schneider et al. (1999) report mineralcrystallization ages that suggest Early Mio-cene anatexis and pre-10 Ma movement on amajor Nanga Parbat shear zone. The NangaParbat Haramosh Massif may therefore be asuitable source for Kamlial Formation detritalmicas with zero lag time. Early exhumation of

the region at 4.5 mm/yr is at odds with boththe lack of evidence for the required ;80 kmof crust that should therefore have been de-nuded if these rates were sustained to presentday and the lack of suitably aged migmatitesin the core of syntaxis. The absence of mig-matites may be explained by (1) the nature ofthe protolith, which consists of Proterozoicgneisses already affected by prior metamor-


NAJMAN et al.

phism and therefore not amenable to melting;(2) the fact that these rates need not have beensustained continuously for the duration of ex-humation to present day; and/or (3) lateraltransport of hot crust by ductile flow thatcould have occurred.

Areas of the Karakoram fault and batholithshow evidence of rapid exhumation co-evalwith the time of deposition of the KamlialFormation, but this exhumation initiated after17 Ma (Parrish and Tirrul, 1989; Searle et al.,1989, 1992; Scharer et al., 1990; Searle andTirrul, 1991; Krol et al., 1996; Searle, 1996;Dunlap et al., 1998). Therefore, this region isan unlikely source for Kamlial Formation mi-cas eroded from a source modeled as exhum-ing rapidly since 20 Ma. Nevertheless, anAsian source cannot be completely ruled out,since the geology of the Asian crust is largelyknown only at reconnaissance level.

Provenance of Anomalous Sample CP96–6AProvenance of sample CP96–6A is drasti-

cally different from that of the other KamlialFormation samples, with predominant deriva-tion from very low to medium-grade meta-morphic and sedimentary sources with pre-dominantly pre-Himalayan aged micas. Thelack of significant arc/suture zone contributionnecessitates a dominant source south of theMain Mantle Thrust for which the obviouscandidate is the Himalayan thrust belt. Thethrust belt contains (1) micas of appropriatepre-Himalayan age, (2) suitable low-grademetamorphic lithologies, and (3) a tectonichistory consistent with a now-eroded sedi-mentary component outcropping at higherstructural or stratigraphic levels and not thrustto depths sufficient to result in amphibolite-grade metamorphism as displayed at the sur-face today. The absence from sample CP96–6A of Himalayan-aged micas in the range 23–30 Ma is perhaps a little surprising; we sug-gest that this may be due to heterogeneitywithin the thrust stack. Both the sedimentary/metasedimentary lithic and dense mineralpopulations for anomalous sample 6A (andsample 5A to a lesser extent) compare veryclosely with those of the coeval Indian crust-derived Upper Dharamsala subgroup (Figs. 2and 3), indicating provenance from the sametype of sedimentary to low-grade metasedi-mentary thrust units, and thus strengtheningthe argument for a more local Indian-margin-thrust-stack source for sample 6A.

Modern Analogue ComparisonFurther insights into the provenance of the

Kamlial Formation sandstones can be gainedindependently through comparison with detri-

tal modes of sediments carried by variousmodern tributaries of the Indus River (Gar-zanti et al., 2003). It should be remembered,however, that Miocene source rocks have bynow been largely or completely removed byerosion. Upper structural levels are conse-quently underrepresented in the high rangestoday, and deeper levels are overrepresented,with respect to the Miocene.

Comparison of the Kamlial Formationsandstones with modern sand of the IndusRiver systems shows that:

(1) the composition of the Kamlial sand-stones contrasts sharply with detritus carriedby most modern tributaries of the Indus. Onlythose rivers draining the Kohistan arc carrysuch low quartz contents, but with much lowervolcanic detritus than in the Kamlial Forma-tion. This difference can be attributed to in-creased dissection of the arc through time.Sands of the Indus main trunk river are sig-nificantly richer in quartz but compare broadlyto both modern Kohistan Rivers and KamlialFormation sandstones. The Kamlial Formationsandstones may be interpreted as paleo-Indusdeposits if major differences in compositionwith respect to the modern Indus are ascribedto progressive erosion, through the arc fromhigh level volcanics to deeper-level batholiths,and higher-grade metamorphic nappes south(e.g., Nanga Parbat), along (i.e., Kohistan andLadakh arcs), and north (i.e., Karakoram) ofthe Indus suture.

(2) Only sands of the Kaghan and JhelumRivers, draining Indian crust metamorphicrocks, have relatively low quartz and feldsparcontents coupled with abundant metapelite tometafelsite and significant terrigenous to car-bonate lithics, comparable to Kamlial samples5A and 6A. Sediments of the Jhelum River inits lower reaches also include a few volcanic-lithic grains recycled from the Tertiary fore-land basin deposits.

Reconstruction of Early-Middle MioceneTectonics and Paleodrainage of the Region

Any reconstruction of the tectonics and pa-leodrainage of the region at 18 Ma (Fig. 7)must take into account data from the KamlialFormation of the Chinji Village area, whichshows (1) sedimentological evidence of de-position by a large river; (2) substantial in-crease (compared to the older Murree For-mation) and predominance of arc-derivedmaterial; and (3) subsidiary contribution froma rapidly exhuming micaceous source, inter-preted as the Nanga Parbat Haramosh Massifor a region of Asian crust.

Tectonics of the arc cannot explain these

data. Exhumation of the arc had decreased tothe extent that denudation only by passive ero-sion was occurring by this time (Chamberlainet al., 1991). Furthermore, the arc had onlymade a subordinate contribution to basin de-tritus even during its earlier, more rapid phaseof exhumation coeval with Murree Formationsedimentation. Shifting of a preexisting drain-age can also be precluded. Although ourknowledge of foreland basin characteristics isregionally incomplete, available data provideno evidence of large, arc-derived rivers duringMurree Formation times. Likewise, Kamlialsedimentary rocks along strike in the easternpart of the Kohat Plateau do not exhibit amagmatic arc signature, suggesting that thecause of these changes emanates more from asingle ‘‘point-source’’ rather than a regionallyextensive tectonic unit providing an apron ofsediment to the basin. We propose the follow-ing: (Fig. 7):

Murree Formation Times (Early Miocene,$18 Ma)

Rivers draining to the foreland basin hadcatchment areas predominantly within the Hi-malayan thrust stack. Only a minor proportionof the drainage basins extended back into thearc. Correspondingly, the highest proportionof detritus was derived from the Indian crustHimalayan thrust belt.

Kamlial Formation Times (18–14 Ma)Enlargement of the drainage basin deep into

the arc, perhaps to the northern margin of thearc or beyond, occurred at this time. Thiswould account for the observed increase in arcmaterial as well as subordinate input from arapidly exhuming northern source. This sub-ordinate source is interpreted as the NangaParbat Haramosh Massif or a region of Asiancrust. Initiation of rapid exhumation of theNanga Parbat Haramosh Massif at this timewould result in a rapid massive influx to thebasin of material from the passively upliftedoverlying arc carapace, as seen at 18 Ma. Thepossible delayed response in exhumation ofzero lag time micas could be due to the timerequired for the short lag time micas to appearat the surface.

Interpretation of the Kamlial Formation atChitta Parwala as the expression of the firstdiversion of the paleo-Indus River to its pres-ent position, i.e., crossing the arc and Hi-malayas through the Nanga Parbat HaramoshMassif and debouching into the foreland, isconsistent with the provenance data and sed-imentological evidence of a large river. Brook-field (1998) considers the pronounced changein direction of the Indus, from east–west along



the suture zone to south across the arc massif,to be an elbow of capture reflecting streampiracy. Shroder and Bishop (2000) postulatedthat the river piracy, which brought the paleo-Indus to the foreland basin, was routed alongthe tectonic depressions caused by extensionalcollapse of the orogen from southwest to northof Nanga Parbat and obliquely transverse tothe Kohistan–Ladakh arc. Formation of thesetectonic lineaments, such as the Main MantleThrust, along which it is proposed the piratedriver would have flowed, was complete by ca.20–18 Ma (Treloar et al., 1989; Treloar et al.,1991; Chamberlain et al., 1991; Chamberlainand Zeitler, 1996), consistent with the start ofdeposition of the Kamlial Formation rocks at18 Ma.

Whether the Kamlial deposits at Chitta Par-wala, Potwar Plateau, represent the main trunkdrainage of the paleo-Indus, or a major trib-utary draining the arc as far north as the Nan-ga Parbat Haramosh Massif, requires morealong-strike data to establish. It is true that theKamlial Formation rocks along-strike in theKohat Plateau, very close to where the mod-ern Indus flows, more closely resemble the re-cent Indus Fan detritus (Suczek and Ingersoll,1985). On the other hand, differences betweenthe Kamlial Formation of the Potwar Plateauand deposits at Potwar and those of the mod-ern Indus Fan could be ascribed to progressivebreaching of the arc and orogenic sources. Themodern day Indus routes through the NangaParbat Haramosh Massif and may have flowedacross the Potwar Plateau, close to the ChittaParwala locality, prior to westward displace-ment consequent to Pliocene and later upliftof the Salt Range (Baker et al., 1988; Gee andGee, 1989; Burbank and Beck, 1989). Thus,although the exact position of the trunk riverremains uncertain, all data suggest depositionin the area by a river system that cut deep intothe orogen at 18 Ma. The river is likely thepaleo-Indus, consistent with (1) the timing ofsignificantly more pronounced channel andlevee complexes in the Indus Fan after theEarly Miocene (Clift et al., 2001a); (2) the in-terpreted switch of the course of the lowerreaches of the paleo-Indus in the late EarlyMiocene from debouching into the Katawazbasin to its present-day position into the fore-land basin (Qayyum et al., 1997); and (3) thefacies shift from coastal marine to southerlydirected fluvial sediments in the Sulaimanforedeep, along which the Indus would haveflowed, at 18 Ma (Friedman et al., 1992;Downing et al., 1993).

Siwalik Formation Times (#14 Ma)Much lower proportions of plagioclase, vol-

canic, and metavolcanic/metabasite lithics,

and higher proportions of quartz and meta-sedimentary detritus in the overlying SiwalikFormation of the Gabhir-Chinji Village region(Critelli and Ingersoll, 1994) indicate that arccontribution decreased in this area in post-Kamlial Formation times. The first appearanceof pleochroic orthopyroxenes and arc-derived,blue-green hornblende (Johnson et al., 1985)indicates erosion from the higher volcanic lev-els being eroded previously, into the deeperlevel amphibolite-granulite roots of the arc.We suggest that these changes reflect progres-sive breaching of the arc carapace with time,accompanied by deeper erosion into the Indiancrust Nanga Parbat Haramosh Massif syntaxis.

ACKNOWLEDGMENTS

We thank P. Treloar and T. Argles for constructivecomments, S. Critelli for providing original, de-tailed point-count data of the Siwalik Formation,and P. Friend for further discussion of the resultsfrom J. Hutt’s unpublished Ph.D. thesis. S. Canclini,N. White, and T. Schildgen provided help with min-eral separation and analyses, and E. Laws providedfield assistance. This research was funded by RoyalSociety Dorothy Hodgkin and Royal Society of Ed-inburgh/BP Fellowships awarded to the first author,the Natural Environment Research Council IsotopeGeosciences Laboratory support to the Ar-Ar facil-ity at Scottish Universities Environmental ResearchCentre, and the Cofin Ministero dell’ Istruzione,dell’ Universita e della Ricerca 2001 Fund to P.C.Pertusati. Constructive reviews from P. Copeland, P.Heller, R. Ingersoll, and R. Raynolds improved thispaper.

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