Detrital-zircon fission-track ages from the Lower Cenozoic sediments, NW Himalayan foreland basin: Clues for exhumation and denudation of the Himalaya during the India-Asia collision

doi:10.1130/B26304.1 2009;121;519-535 Geological Society of America Bulletin

A.K. Jain, Nand Lal, B. Sulemani, A.K. Awasthi, Sandeep Singh, Rajeev Kumar and Devender Kumar

Himalaya during the India-Asia collisionHimalayan foreland basin: Clues for exhumation and denudation of the Detrital-zircon fission-track ages from the Lower Cenozoic sediments, NW

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ABSTRACT

Detrital-zircon fi ssion-track (FT) ages from the Lower Cenozoic Sub-Himalayan foreland basin refl ect the progressive effects of crustal thickening and exhumation on the Himalayan source rocks as a consequence of the India-Asia collision. The oldest stratum, the transgressive marine Paleocene-Eocene Subathu Formation (57–41.5 Ma) contains ca. 50 Ma detrital-zircon P1 peak, which was derived from the Indus Tsangpo Suture Zone and the Ladakh Batholith of the Asian plate. A dominant 302.4 ± 21.9 Ma peak with a few 520 Ma grains in this formation has been derived by erosion of the zircon partial-annealing zone (ZPAZ) of 240–180 °C. As the fi rst imprint of the collision, this zone affected the Himalayan Proterozoic basement and its Tethyan sedimentary cover.

Since the detritus in the Subathu has been derived both from the Indian and Asian plates, the possible suturing of these plates took place during the Subathu sedimentation. A sudden change in the provenance is recorded in the detrital-zircon FT cooling ages in the Oligo-Miocene Dagshai and Kasauli Formations, which have dominant 30 and 25 Ma P1 peaks, respectively. We interpret a distinct unconfor-mity spanning ~10 m.y. between the Subathu

and Dagshai Formations. Since ca. 30 Ma, molassic sedimentation coincides with shifting of the source rocks to the Himalayan metamor-phic belt. This belt has sequentially undergone three distinct cooling and exhumation pulses after the ultrahigh-pressure–high-pressure (UHP-HP) metamorphism (53–50 Ma) in the extreme north and two widespread M1 and M2 metamorphisms (40–30 and 25–15 Ma) in the middle parts. These events appear to be largely responsible for the deposition of the ca. 30 Ma zircon Himalayan peak and ca. 25 and 15 Ma young Himalayan peaks, respec-tively; the latter appears within the Lower Siwalik Subgroup (13–11 Ma). During the Lower Siwalik deposition, pre-Himalayan peaks gradually decrease with the intensifi ca-tion of the Himalayan events in source rocks. In spite of uninterrupted fl uvial sedimenta-tion in the Dagshai-Kasauli–Lower Siwalik sequences since 30 Ma, breaks of ~5–7 m.y. in the zircon FT ages reveal pulsative cooling and exhumation in the well-identifi ed source areas. Although cooling and exhumation of the Himalayan source rocks remained almost uniform during the Eocene, source heteroge-neity is refl ected in fl uvial sedimentation since 37 Ma from Pakistan to Nepal in response to the India-Asia collision.

Keywords: India-Asia collision, Himalayan fore-land sediments, detrital-zircon fi ssion-track ages.

INTRODUCTION

The frontal Cenozoic Sub-Himalayan fore-land basin is composed of the Paleo-Neogene sedimentary sequence and the vast Indo-Gangetic Plains of active Holocene sediment accumulation by the Indus-Ganga-Brahmaputra river system (Fig. 1A). In these evolving basins, ~10-km-thick orogenic sediments have pre-served the denudation records of intimate inter-play between tectonics and erosion of the rising Himalaya as a result of the India-Asia collision (Parkash et al., 1980; Najman et al., 1993; Singh, 1996; Najman et al., 1997; DeCelles et al., 2000; Yin, 2006; Najman, 2006). Estimated timing of this continental collision is debatable and var-ies from ca. 65 to 50 Ma from stratigraphic, paleontological, isotopic, structural, and paleo-magnetic records (Klootwijk et al., 1992; Gar-zanti et al., 1987; Jaeger et al., 1989; de Sigoyer et al., 2000; Yin and Harrison, 2000; Steck, 2003; Najman, 2006; Yin, 2006 and references therein). Initial foreland basin development puts it at 55 Ma (DeCelles et al., 2002) or ca. 50 Ma from the stratigraphic relationships (Zhu et al., 2005). The India and Asia contact took place at 57 ± 1 Ma after the steeply subducting Indian continental crust underwent ultrahigh-pressure (UHP) metamorphism at 53.3 ± 0.7 Ma imme-diately to the south of its junction with the Asian plate in the Tso Morari region (Leech et al., 2005, 2007). This collision has caused extensive

For permission to copy, contact [email protected]© 2008 Geological Society of America

519

GSA Bulletin; March/April 2009; v. 121; no. 3/4; p. 519–535; doi: 10.1130/B26304.1; 8 fi gures; 1 table.

Detrital-zircon fi ssion-track ages from the Lower Cenozoic sediments, NW Himalayan foreland basin: Clues for exhumation and denudation of

the Himalaya during the India-Asia collision

A.K. Jain†

Department of Earth Sciences, Indian Institute of Technology Roorkee, Roorkee 247 667, India

Nand LalDepartment of Geophysics, Kurukshetra University, Kurukshetra 136 119, India

B. SulemaniA.K. AwasthiSandeep SinghRajeev KumarDepartment of Earth Sciences, Indian Institute of Technology Roorkee, Roorkee 247 667, India

Devender KumarNational Geophysical Research Institute, Uppal Road, Hyderabad 500 007, India

†E-mail: [email protected]

Jain et al.

520 Geological Society of America Bulletin, March/April 2009

TibetLhasa

Trans-Himalaya

MFT

+

+++

++

+

++

+

+ + ++ +

++

ITSZ

Indo-Gangetic PlainsHigher Him

alaya

MCT

STDZ Tethyan HimalayaLesser Himalaya

HimalayaSub-

Brahm

putra

R.

Ganga R.

Indu

s R

.

Indian craton 500 km

Fig. 1B

MBT

Cenozoic Himalayan foreland basin

Lesser Himalayan sedimentary zone

o

34

32

77

79

78

76

ZSZ

STDZ

Sutlej R.

40 Km

LH

Fig. 2

Dehra Dun

76o

Bhagirathi R.

o

Rampur

Dalhousie

JT

VT

Gangotri

Beas R.

o

INDEX

a

o

32o

MBTMandi

Sutlej R.

MCT

Himalayan Metamorphic Belt

Paleozoic & Cenozoic granitoids

Tso Morari Crystallines

Vaikrita GroupMunsiari Group

Tethyan Sedimentary Zone

Indus Tsangpo Suture Zone

Kulu

Nahan

MCT

34o78

o

Shyok Suture Zone

JT - Jutogh Thrust

Jutogh Group (Lesser Himalayan Crystallines)

Indian plate

Ladakh Batholith

Asian plate

Uttarkashi

Yamuna R

.

Indus River

MBT - Main Boundary Thrust

MCT - Main Central ThrustVT - Vaikrita ThrustSTDZ - South Tibetan Detachment Zone

ZSZ - Zanskar Shear Zone

X

Y

Shimla

Leh

N

SSZ

ITSZ

SH

Padam

Manali

Keylong

Chenab R.

o

o

Kishtwar

A

BFigure 1. (A) Cenozoic foreland basins of the Indian subcontinent and various Himalayan units as possible present-day source rock for sediments. Also shown are different well-known tectonic boundaries. (B) Geological map of the NW Himalaya showing source rocks for the Cenozoic Himalayan foreland basin. SH—Sub-Himalayan Cenozoic foreland basin. LH—Proterozoic Lesser Himalayan Sedimentary belt. HMB—Himalayan Metamorphic Belt: LHC—Lesser Himalayan Crystallines; HHC—Higher Himalayan Crystallines with ca. 850–500 Ma granitoids and Paleozoic and Late Cenozoic granites. TMC—Tso Morari Crystallines. TSZ—Tethyan Sedimentary Zone cover. Trans-Himalayan tectonic units: ITSZ—Indus Tsangpo Suture Zone; SSZ—Shyok Suture Zone. Compiled mainly from Thakur (1993), Srikantia and Bhargava (1998), Steck (2003), Jain et al. (2003, 2005), Yin (2006), and Webb et al. (2007).

Detrital fi ssion-track geochronology of NW Himalayan foreland basin

Geological Society of America Bulletin, March/April 2009 521

deformation, metamorphism, and leucogranite emplacement along the northern margin of the Indian Proterozoic continental crust. This belt has undergone exhumation and erosion during the Late Cenozoic, and consequential develop-ment of the foredeep along the southern front of the rising Himalaya (Jain et al., 2002; Najman, 2006; Yin, 2006).

Imprints of the Himalayan collision tecton-ics, their tectonothermal episodes, and ensuing cooling have been intense on the source rocks of the Cenozoic foreland sediments. Hence, most of the well-constrained fi ssion-track zircon and apatite ages from the present-day exposed tec-tonic units in the Himalaya range from Miocene to Pliocene, including the Pleistocene (Kumar, et al., 1995; Sorkhabi et al., 1996, 1997; Kumar, 1999; Jain et al., 2000; Burbank et al., 2003; Vannay et al., 2004; Thiede et al., 2005; Kumar et al., 2007; Foster and Carter, 2007; Patel et al., 2007). An enormous volume of source rocks has been eroded, and the detritus transported south-ward into the evolving Himalayan frontal fore-deep since the Paleocene; hence, evidence of the initial India-Asia collision and consequential cooling, exhumation, and denudation histories are to be searched within this basin, as well. To unravel this record, 87Sr/86Sr and ε

Nd isotopes

and dating of individual detrital zircon, mona-zite, apatite, and white mica by U-Pb, Th-Pb, 40Ar/39Ar, and fi ssion-track methods have been used within the Cenozoic foreland sequences (Cerveny et al., 1988; Copeland and Harrison, 1990; Harrison et al., 1993; Najman et al., 1993; Najman et al., 1997; DeCelles et al., 2001, 2004; Najman et al., 2001, 2004; Robinson et al., 2001; White et al., 2001, 2002; Bernet et al., 2006; van der Beek et al., 2006; Szulc et al., 2006; Foster and Carter, 2007; Patel et al., 2007). These stud-ies have established distinct variations in cool-ing and exhumation patterns of the source rocks from northernmost Indus Tsangpo Suture Zone to the Himalayan Metamorphic Belt.

Detrital-zircon fi ssion-track analysis of indi-vidual grains has been recently applied to prov-enance studies of clastic sediments and exhuma-tion of many convergent orogenic belts (Garver and Brandon, 1994; Brandon et al., 1998; Spie-gel et al., 2000; Garver and Kamp, 2002; Bernet et al., 2004, 2006; Bernet and Garver, 2005). Targeting the eroded Himalayan source rocks from the detrital-zircon FT peaks within the foreland sediments is based on the premise that these may be correlated with (a) presently avail-able zircon FT ages in bedrock indicating rapid exhumation of deep-seated metamorphics in the core of the orogen (Bernet and Garver, 2005); (b) foreland sediments do not contain younger zircon FT ages due to their non-reset character after the deposition (Bernet et al., 2006); and

(c) cooling and exhumation pulses, deciphered from the present-day exposed source rocks, are similar to somewhat older patterns in the cover rocks, eroded during approximate time span of the foreland deposition.

Because the main source unit of the Himala-yan Metamorphic Belt is thrust along the Main Central Thrust around the Himalayan windows in the northwest, one of the important questions to be addressed through the detrital-zircon FT ages is the timing and mechanics of its exposure and providing detritus to the foreland basin. In our work, well-established concepts on detrital-zircon fi ssion-track geochronology (Brandon et al., 1998; Garver et al., 1999; Bernet et al., 2004, 2006; Stewart and Brandon, 2004; Bernet and Garver, 2005) have been adopted to visu-alize the interplay between tectonics, erosion, and exhumation of source rocks for synoro-genic Himalayan foreland sediments. Our work has been applied to establish (1) mutual strati-graphic relationships in establishing the Oligo-cene unconformity, (2) the early patterns of the Himalayan exhumation since the initiation of the India-Asia collision, (3) uneven denudation of source rocks through time, and more signifi -cantly, (4) effects of imprints of the Himalayan orogeny on pre-Cenozoic source rocks through the Paleocene–Lower Miocene.

GEOLOGICAL FRAMEWORK

Source Rock Character

In the Himalaya, the Cenozoic continental collision followed the late Mesozoic subduction of the Neo-Tethyan oceanic lithosphere along the Indus Tsangpo Suture Zone and the Shyok Suture Zone in Ladakh, the western continua-tion of the Lhasa terrane of south-central Tibet (Fig. 1A; Honegger et al., 1982; Thakur, 1993; Hodges, 2000; Jain et al., 2002; Yin, 2006). As a result, the following Himalayan and Ladakh units became sources for synorogenically deposited Sub-Himalaya foreland sediments (Figs. 1B and 2).

(1) The Indus Tsangpo Suture Zone, the southern junction between the Indian and Asian plates, delimits the passive Indian continental margin from the Andean-type Ladakh Batholith. The suture zone contains basalt of the Lower Cretaceous Dras Volcanics, fl ysch sediments of the Indus Group, and dismembered ophiolites (Honegger et al., 1982). The belt has undergone blueschist metamorphism at ca. 100–80 Ma in Ladakh (Honegger et al., 1989) and ca. 80 Ma in Pakistan (Maluski and Matte, 1984; Ancz-kiewicz et al., 2000). To the north of the Indus Tsangpo Suture Zone, the Ladakh Batholith is marked by dominant emplacement of diorite-

granodiorite with minor amounts of noritic gab-bro and granite. U-Pb zircon crystallization ages are 60.1 ± 0.9 and 58.4 ± 1 Ma (Singh et al., 2007) with minor episodes of ca. 100, 70, and 49.8 Ma (Honegger et al., 1982; Weinberg and Dunlap, 2000).

(2) With the listric-overturned geometry, the Indus Tsangpo Suture Zone is thrust southward as an ophiolite nappe, whose remanents are observed at Kiogarh (Kumaon), Spongtang, and Karzok (Jammu and Kashmir) (Gansser, 1964; Searle et al., 1997). It overrides a thick pile of low-grade metamorphosed late Proterozoic to Eocene Tethyan Sedimentary Zone of siliciclas-tics, carbonates, and volcanics and Cambrian-Ordovician granitoid intrusives (Gaetani and Garzanti, 1991; Srikantia and Bhargava, 1998; Girard and Bussy, 1999; Steck, 2003). This sequence was deposited on the northern Indian passive margin of folded Paleoproterozoic to Ordovician (?) Himalayan Metamorphic Belt, known as the Great Himalayan Crystallines or Tibetan Slab in Nepal (Thakur, 1993; Jain et al., 2002; Yin, 2006 and references therein).

(3) Folded slab of the Himalayan Metamor-phic Belt incorporates the Tso Morari Crystal-lines in the north, the Higher Himalayan Crys-tallines in the middle, and the Lesser Himalayan Crystallines in the south (Jain et al., 2003, 2005). The Himalayan Metamorphic Belt is separated from the Tethyan Sedimentary Zone by an extensional South Tibetan Detachment Zone and its equivalent, e.g., the Zanskar Shear Zone (Burchfi el et al., 1992; Herren, 1987). Along its southern margin, the belt is regionally thrust southward over the Lesser Himalayan sedimentary belt along the Main Central Thrust (Valdiya, 1980; Jain et al., 2003).

The Tso Morari Crystallines contain a domal pile of eclogitized to greenschist metamorphics and Cambro-Ordovician granitoids (de Sigoyer et al., 2000; Guillot et al., 1997; Girard and Bussy, 1999; Jain et al., 2003; Leech et al., 2005). Multichronometry of the UHP eclogite and host gneiss indicates that the north Indian continental margin subducted beneath the Ladakh terrane to 90 km at 55 ± 12 or 53.3 ± 0.7 Ma (de Sigoyer et al., 2000; Leech et al., 2005, 2007).

In the middle, the Higher Himalayan Crys-tallines are made up of three subunits (Fig. 1B) (Thakur, 1993; Jain et al., 2002; Steck, 2003; Yin, 2006): (1) the lower Paleoproterozoic Mun-siari Group of mylonitized schist and gneiss of the Kulu-Bajura nappe, (2) the upper Vaikrita Group of garnet mica schist, staurolite-kyanite schist and gneiss, sillimanite-kyanite schist and gneiss, amphibolite, calc-silicates, migmatite, and Cenozoic leucogranite, and (3) Cambrian-Ordovician granitoids. Main Central Thrust and Munsiari Thrust at the base, the Vaikrita

Jain et al.


Thrust in the middle, and the South Tibetan Detachment Zone at the top bind these subunits, respectively. In Himachal and southeastern Kashmir, the Paleozoic metasediments and the Triassic-Jurassic carbonates are either isocli-nally folded or overturned within these meta-morphics (Srikantia and Bhargava, 1998; Steck, 2003), thus making the mapping of the Hima-layan Metamorphic Belt and its Tethyan cover diffi cult as distinct units. U-Pb zircon ages from deformed granitoids of the Munsiari Group are between 2068 ± 5 and 1840 Ma (Miller et al., 2000; Singh et al., 2008). Therefore, metasedi-ments containing these intrusives are likely to be much older. The Vaikrita Group has not been precisely dated, while the uppermost intrusives along the contact with the Tethyan Sedimentary Zone yielded a U-Pb zircon age of 488 ± 4 Ma (in Marquer et al., 2000).

The Higher Himalayan Crystallines have undergone intense deformation and two meta-

morphic episodes out of which the early pro-grade M1 regional metamorphism took place at 40–30 Ma under 8–11 kbar and 600–700 °C, and the younger M2 event occurred at 25–15 Ma, 6–8 kbar, and 500–750 °C (see Hodges, 2000; Yin, 2006 and references therein). The belt suffered Miocene anataxis and melt-ing, cooling, and exhumation to occupy the core of the mountain belt. Reliably calculated cooling and exhumation rates are highly vari-able between 2.6 and 0.55 mm/yr in Miocene, ~0.4 mm/yr between Miocene and Pliocene, and up to 3 mm/yr in Pliocene-Pleistocene (see Jain et al., 2000; Thiede et al., 2005; Yin, 2006). These rates are controlled by tectonics along the Main Central Thrust, the South Tibetan Detach-ment Zone, and the domal structures, enhanced by erosion since Miocene.

Low- to medium-grade folded Lesser Hima-layan Crystalline nappes along with late Paleo-proterozoic–Early Paleozoic granitoids—the

Kashmir, Jutogh, Garhwal, and Almora—are synformally exposed farther to the south as an eroded metamorphic veneer over the Lesser Himalayan sedimentary belt (Fig. 1B; Srikantia and Bhargava, 1998).

(4) The Lesser Himalayan belt is made up of the Paleoproterozoic quartzite, slate, phyllite, dolomite, volcanics, and granitoids (Valdiya, 1980; Srikantia and Bhargava, 1998), and is overlain by the latest Proterozoic–Cambrian Krol-Tal sequence in the southern parts along with the unconformably deposited Eocene Sub-athu Formation. This belt is thrust southward over the Sub-Himalayan foreland sequence along the Main Boundary Thrust. The Lesser Himalayan belt extends farther toward the northeast beneath the overriding metamorphic nappes, and is exposed in many windows, e.g., the Kishtwar and Kulu-Rampur windows, where the Rampur volcanics, Bandal granite, and Kisht-war granitoids have yielded U-Pb zircon ages of 1800 ± 13, 1866 ± 4, and 1840 Ma, respectively (Miller et al. 2000; Singh et al., 2008).

Sub-Himalayan Cenozoic Foreland Basin

The outermost and southernmost Cenozoic foreland basin has accumulated ~10 km of predominantly fl uvial sediments, which were derived from the rising Himalaya. The belt rises abruptly above the Indo-Gangetic Plains along the Main Frontal Thrust and, in turn, is overrid-den by the Lesser Himalayan belt along the Main Boundary Thrust (Valdiya, 1980; Srikantia and Bhargava, 1998). Four major stratigraphic units characterize this belt (Figs. 2 and 3).

Subathu Formation The Paleocene–Middle Eocene marine trans-

gression commenced with the unconformable deposition of the Subathu Formation on the Sub-Himalaya and Proterozoic–Early Paleo-zoic Lesser Himalayan domains. Carbonaceous and coaly shale is overlain by dull green-gray fossiliferous splintery shale and siltstone, thin fossiliferous limestone, and sandstone intercala-tions. These rocks were deposited in euxinic and evaporitic lagoons and shallow tidal sea (Singh, 1978) between Middle Paleocene and lower Middle Eocene (Mathur, 1978). Felsic volca-nics, chert, serpentine schist clasts, and high-Al and Cr spinel refl ect a mixed source from the proto-Himalayan Indus Tsangpo Suture Zone (Najman and Garzanti, 2000; Bhatia and Bhar-gava, 2006). It is supported by ε

Nd values of ~−9

and 87Sr/86Sr ratios of ~0.710–0.715, which plot between fi elds of the Indus Tsangpo Suture Zone and Tethyan sedimentary cover (Najman et al., 2000). The sequence grades into variegated purple siltstone-shale alternations, grouped as

6

*

77° 0′

31° 0′

77° 15′

Subathu

30° 45′

77° 0′ 77° 15′

Subathu Formation

Lower Siwalik Subgroup

Middle Siwalik Subgroup

Pre-Cenozoic Lesser HimalayanSedimentary Belt

Dagshai Formation

Kasauli Formation

SarahanThrusts

Roads

Kasauli

26

17

4053

23

28

27

INDEX

Synclines:A-Anji D-Dagshai K-Kasauli P-Pachmunda

Anticlines:S- Sanawar

33

PSolan

1

30°45′

AK S

Main Boundary Thrust

Main B

oundary Thrust

0 5 10 km

Kalka

Upper Siwalik Subgroup

Indo-Gangetic Plains

Bilaspur Th.

Nahan Thrust

Nahan

Dharmpur

Dagshai

31° 0′

Surajpur Thrust

Dagshai Syncline

Sample Locations• Lower Siwalik Subgroup-1• Kasauli Fm.-

27 (basal), 56 (lower middle), 40 (middle), 17 (upper)

• Dagshai Fm.-53 (uppermost) 23, 6 33 (lowermost)

• Subathu Fm.-28 (upper)

Figure 2. Geological map of the Sub-Himalaya, Simla Hills, showing the loca-tion of samples used for the detrital-zircon geochronometry. Simplifi ed after Raiverman (1979) and Raiverman et al. (1983), with the observations that the Subathu-Dagshai contact lacks facies variation and is conformable.



the Passage bed sequence (Bhatia, 2000) and, in turn, by dull white sandstone; the latter was deposited as coastal barrier in a high-energy environment (Srivastava and Casshyap, 1983). Furthermore, the age of the Subathus has been estimated to range between 61.5 and 43.7 Ma (Lakshami et al., 2000), while its uppermost part may be even 41.5 Ma, magnetostratigraphi-cally (Sangode et al., 2005).

Dagshai FormationThe overlying unfossiliferous Dagshai For-

mation in Shimla Hills or the Dharamsala For-mation of Punjab (Raiverman et al., 1983) is red-colored siltstone and mudstone in the lower parts, while sandstone and caliche appear in the upper parts. The arenites contain schistose clasts, zircon, garnet, tourmaline, and epidote,

which were derived from medium-grade meta-morphosed Himalayan rocks (Sinha, 1970; Naj-man et al., 1997; Najman et al., 2004). ε

Nd and

87Sr/86Sr values range between ~−12 to −18 and ~0.755 to 0.775, respectively (Najman et al., 2000). It is likely that the Dagshai Formation was deposited either in an estuarine (Bhatia and Bhargava, 2006) or meandering silting fl uvial channels (Najman, 2006), eroding the top of the Subathus (Bera et al., 2008).

The precise contact relationship between the Subathu and Dagshai Formations remains con-troversial since the transition was postulated from marine to fl uvial environment (Bhatia, 2000 and references therein; Bhatia and Bhar-gava, 2006). On the contrary, an unconformity was proposed between these two sequences with a hiatus ranging from ~10 m.y. to <3 m.y.

(Najman et al., 1993, 2004; Bera et al., 2008). The age of the Dagshais is debatable: (1) 35.5 ± 6.7 Ma for the whole formation (Najman et al., 1994) or between <28 and 25 Ma from 40Ar/39Ar detrital micas (Najman et al., 1997), or (2) base younger than 31 ± 2 Ma from detrital-zircon FT ages (Najman et al., 2004) or 35.5 Ma magneto-stratigraphically (Lakshami et al., 2000).

Kasauli FormationThe overlying Kasauli Formation contains

~2000-m-thick, gray-green sandstone and alternating siltstone-mudstone with litharen-ites having a larger percentage of metamorphic fragments than the Subathu and Dagshai Forma-tions, and isotopic character like the latter. The formation appears to have been deposited in a migratory braided river system under humid

Thickness(km)

Stratigraphy(Age in Ma)

Upper Siwalik

MiddleSiwalik

UpperDharamsala

LowerDharamsala

Kasauli

Dagshai

Heavy minerals

Kyanite

9

8

7

6

5

4

3

2

1

0Subathu

Sillimanite

Staurolite

Garnet

Chromespinel

(12.5)

(57–41.5 Ma)

(20)

(16.5)

(11.0)

(7.0) (5.2)

(2.6)

(1.0)

(D)

(B)

(A)

(<33–27 Ma)

Lithology DepositionalEnvironment

Gray-green shale, 1st and sst, purple shale-sst at top with white sst

Cu

mu

lati

ve a

pp

roxi

mat

e th

ickn

ess

Purple-gray shale-sst micaceous near top

Micaceous gray-green sst-shale

Purple sst and shale intercalation

Massive coarse gray sst, thin shale intercalation; local conglom.

Thick boulder bed with coarse sst

10

LowerSiwalik

Fluvial flood plains silting up channels

Humid braided fluvial channels

Sinous meandering rivers in muddy flood plains

Braided channel belts of alluvial fan complexes

Coalescing alluvial fans

......

...

.

.

.. .. ... . .

... .

.

. ..

._.

. ._ __

._._ _.._.

.

.

.. .

. .. . .

..

.. . .. ..

...

.. ..

..

..

...

..

..

.

..

..ll

l

ll

ll

. l ___

__

.. ..

...(20–13 Ma)

(13–11 Ma)

(11–4.5 Ma)

(4.5–1 Ma)

(20–16.5 Ma)

(16.5–12.5 Ma)

Lagoon, tidal flats [28] ?

[01]

[17] [40] [26] [27] [53] [23] [06] [33]

(C)

Figure 3. Composite stratigraphy of the Sub-Himalayan foreland basin, NW Himalaya, showing different for-mations, their heavy minerals characters, lithology, and depositional environments. Modifi ed after White et al. (2002). Commonly acceptable ages for individual formations are approximate and given in brackets. Sections: A—Shimla Hills (Raiverman, 1979). B—Chinmun-Birdhar (Raiverman et al., 1983; White et al., 2002). C—Jwal-amukhi (Meigs et al., 1995). D—Haripur (Sangode et al., 1996). Magnetostratigraphic ages are given in italics (Meigs et al., 1995; Sangode et al., 1996; White et al., 2001). Heavy mineral suites are after Sinha (1970) and Najman and Garzanti (2000). Sample locations for detrital-zircon fi ssion-track ages are approximately shown by thick dots. Sst—sandstone.

Jain et al.


climate conditions (Singh, 1978; Najman et al., 1993). A Lower Miocene age (23–16 Ma) of this formation has been determined from fl oral and mammal remains (Arya et al., 2004). 40Ar/39Ar white mica ages constrain its age at <28 and 22 Ma (Najman et al., 1997), while no mean-ingful age can be deciphered from the detrital-zircon FT data (cf. Najman et al., 2004). As an equivalent of the Dharamsala Group, the mag-netostratigraphic age of the uppermost Kasauli is either 13 Ma (White et al., 2002) or 11.5 Ma (Lakshami et al., 2000).

Siwalik GroupThe bulk of the Sub-Himalayan basin com-

prises >~6000-m-thick, coarsening-up Siwa-lik Group. The 1800-m-thick Lower Siwalik Subgroup of <13–11 Ma (White et al., 2001, 2002) contains highly indurated, fi ne to coarse, purple-gray sandstone and interbedded brown shale (Parkash et al., 1980). Low- to medium-grade metamorphosed Himalayan source rocks are indicated by the presence of phyllite clasts, and epidote, garnet, tourmaline, zircon, rutile, chlorite, and staurolite (Sinha, 1970; White et al., 2002). The Lower Siwalik sediments were deposited by south-fl owing, highly sinuous meandering rivers in broad muddy fl ood plains (Singh, 1978; Parkash et al., 1980). The Middle Siwalik Subgroup, which is more than 2300 m thick, contains cross-bedded, medium to coarse sandstone, intercalated siltstone and shale, and was deposited between 11 and 4.5 Ma by major braided rivers with alluvial fan complexes. Coarsening-up graywacke beds have rock frag-ments ranging from 20% to 40%, with garnet, tourmaline, zircon, epidote, staurolite, zoisite, and kyanite (Fig. 3; Sinha, 1970). The Upper Siwalik Subgroup (~2300 m thick) of conglom-erate, sandstone, and mudstone was deposited as coalescing alluvial fans between 4.5 and 1 Ma (Sangode et al., 1996).

SAMPLE COLLECTION AND EXPERIMENTAL PROCEDURES

Ten representative samples from the Subathu-Dagshai-Kasauli–Lower Siwalik sequences are selected for the FT analysis from Kalka-Solan, Dharampur-Subathu, and Dagshai-Nahan road sections (Fig. 2). The section starts with the syn-clinally folded Subathu, Dagshai, and Kasauli Formations, which were thrust southward over the Siwalik Group along the Bilaspur Thrust in the Kalka-Solan section (Raiverman, 1979; Raiv-erman et al., 1983). This succession is repeated, due to north-dipping Surajpur Thrust in type localities of the Subathu and Dagshai Forma-tions, as a tightly folded package. It is thrust over by the pre-Cenozoic Krol Belt along the Main

Boundary Thrust farther northeast (Fig. 2). In the Sarahan-Nahan area farther to the southeast, the Cenozoic belt widens due to imbricated Dag-shai Formation, having the Kasauli Formation in the core of the Dagshai syncline. The northerly dipping Lower Siwalik Subgroup occurs on the hanging wall of Nahan Thrust above the Middle and Upper Siwalik Subgroups.

Zircons from crushed sandstones were picked up by sieving, panning, heavy liquid, and isody-namic separations. An array of 10 × 10 grains was embedded in perfl uoroalkoxy (PFA) Tefl on pieces of 15 × 15 × 0.5 mm at 320 °C. After grind-ing, mounts were polished with diamond paste and double etched. As the samples may contain both old and young zircons, two mounts of detri-tal suite (cf. Cerveny et al., 1988) were etched in a eutectic melt of NaOH:KOH at 320 °C. One mount was etched where no grains were visibly under-etched (long-etch mount), while a second mount was etched until grains with high track density were properly etched (short-etch mount).

Zircon mounts were covered with a low-ura-nium muscovite fl ake and stacked in an alumi-num tube of 3.5 cm length and 1.5 cm diameter, and irradiated together with the Fish Canyon Tuff and Mount Dromedary age standards in the CIRUS reactor at Bhabha Atomic Research Center in Mumbai. In order to measure thermal neutron fl uence and to account for dose gradi-ent along the tube, two uranium glass standards (CN1) were kept at the bottom and top of the samples. Micas were also etched after thermal neutron irradiation. Tracks were counted on a Nikon-Optiphot microscope, using a 100× dry objective and 15× eyepieces. Zircon ages were determined using the external detector method (EDM) and ξ-calibration approach (Naeser, 1979; Hurford and Green, 1983; Jain et al., 2000), using a ξ-value of 110 ± 2.6 (2σ) for the CN1 glass (Thakur and Lal, 1993). Details of the procedures used in this work were published earlier (Jain et al., 2000). Fission-track ages from 815 detrital zircons have been generated in this work, out of which 135 grains belong to the Sub-athu Formation, 245 grains to the Dagshai For-mation, 347 grains to the Kasauli Formation, and 88 grains to the Lower Siwalik Subgroup.

DATA ANALYSIS

Detrital-zircon fi ssion-track dating has be come one of the most important techniques for deci-phering the source rock character and exhu-mation history of orogenic belts because this common accessory mineral has a FT closure temperature of 240 ± 30 °C (Hurford, 1985; Brandon et al., 1998; Garver et al., 1999; Bernet and Garver, 2005). The age of the youngest peak in the zircon population is the most important

because it is related to the most recent cooling in source rocks, prior to transport and deposition in the basin (Bernet et al., 2004, 2006).

Distribution of grain ages is decomposed into individual age components by statistical techniques to discriminate distinct populations (Galbraith, 1981; Brandon, 1996). Using the binomial peak-fi tting routine, the BINOMFIT software (Brandon, 1996) has been applied in this work to decompose the observed grain-age distribution into different peaks (Table 1). Zircon FT grain-age distribution from differ-ent formations is demonstrated on radial plots (Fig. 4) and probability density plots with full grain age spectrum, binomial-fi tted peaks, and their ages (Fig. 5). One of the most character-istic features of these formations is that the youngest zircon peak is either older than the estimated depositional age or almost of the same age for the Subathu-Dagshai-Kasauli–Lower Siwalik sequences.

Because detrital-zircons will retain their pre-depositional FT ages only when temperatures remain below 240 ± 30 °C after deposition, proper evaluation of thermal resetting is required within the sedimentary basin. Youngest grain age spectrum in an un-reset sediment provides an estimation of its depositional age, because it will not be older than its detritus, except for a very few grains with FT ages close to or younger than the depositional age (cf. Bernet et al., 2006). It has been estimated that maximum paleotem-perature for the Subathu Formation is <180 °C from illite crystallinity and vitrinite refl ectance and still lower temperatures for the younger sequences, hence these are insuffi cient to reset FT zircon ages within the basin (Solemani, 1999; Najman et al., 2004). Also, none of the samples has an age signifi cantly younger than its depositional age (Table 1). Therefore, zircon FT ages of the Dagshai-Kasauli formations have so far provided the best possible estimates of their depositional ages in their type localities.

RESULTS

Within the Sub-Himalaya, the Subathu For-mation is marked by a P1 peak of 49.4 ± 2 Ma (15.5% grains in sample no. 28), which lies within its paleontologically determined age (Table 1). An extremely large component of 84.5% of detrital zircons characterizes this for-mation with a FT peak of 302 ± 21.9 Ma and grains between 200 and 80 Ma with a few to be 520 Ma.

In the Dagshai Formation, a detrital-zircon FT P1 peak of 31.6 ± 3.9 Ma (52% grains in sample no. 33) is signifi cant in the lower parts and limits the maximum depositional age of this sequence. This is the strongest evidence available so far for



this formation. The older peaks of 333 ± 75.5 and 81.2 ± 16.5 Ma are important with consider-able reduction in number of grains in compari-son to the Subathus. Sample number 53 from the uppermost part of the formation is almost identi-cal to the lowermost sample, having a P1 peak of 30.3 ± 3.1 Ma with 43% grains. The other two Dagshai sample numbers 06 and 23 from the Dagshai-Nahan road reveal similar ages indicat-ing spatial uniformity of the zircon FT P1 peak. The ca. 30 Ma P1 peak, determined from the 245 zircon grains is, therefore, likely to be character-istic of the Dagshai Formation.

In a previous study of the Dagshai Formation, a detrital-zircon FT age of 31 ± 2 Ma was obtained from ten grains from its base, while six grains from the main section yielded 76 ± 6 and 364 ± 45 Ma central ages (Najman et al., 2004). The Dagshai-Kasauli transition zone provided 29.4 ± 2.2 and 388 Ma ages from only 15 zircons.

The basal parts of the Kasauli Forma-tion (sample no. 27) yields a P1 peak of 37.5 ± 5.3 Ma (N = 33) with peaks of 24.9 ± 2.1 and 24.1 ± 5 Ma from the lower middle and middle parts in sample numbers 26 and 40, respec-tively. These peaks become younger to 20.7 ± 3.2 (sample number 17) in the uppermost parts. The depositional age of this formation, therefore, ranges between 25 and 20 Ma within a paleontologically established age (cf. Arya et al., 2004).

In the Lower Siwalik Subgroup, an additional P1 peak of 15.4 ± 1.9 Ma (N = 38.4%) appears in sample number 1 during the 13–11 Ma sedi-mentation, whereas the 26.8 ± 3.3 Ma peak (N = 46.9%) continues uninterruptedly since the sedimentation of the lower middle part of the Kasaulis (Table 1; Fig. 5). However, older grains >50 Ma are reduced signifi cantly to 15%.

DISCUSSION

Zircon FT Annealing Temperature and Denudation Patterns

Estimation of zircon fi ssion-track annealing temperature ranges from >300 °C in the experi-ments (Bal et al., 1983; Brandon and Vance, 1992), 200 to 250 °C from the borehole data (Brandon and Vance, 1992), and 175 to 250 °C from the geological evidence (Hurford, 1985; Tagami et al., 1990). Hence, zircon fi ssion tracks may have been totally annealed at 240 ± 30 °C in a natural system for common cooling rates of 15 °C m.y.−1 (Tagami et al., 1988; Brandon et al., 1998; Bernet and Garver, 2005) in the Himala-yan source rocks, while these fi ssion tracks might have experienced lower temperatures in the zircon partial-annealing zone (ZPAZ).

At the time of initiation of the Himalayan orogeny, the Proterozoic-Paleozoic source of the Himalayan Metamorphic Belt would have retained the zircon FT ages of older thermal and/or magmatic events above the ZPAZ (Fig. 6A, point P). Within the ZPAZ, the rocks will have mixed ages (Fig. 6A, point Q), and those below this temperature will be controlled by the tim-ing of initial Himalayan events (Fig. 6A, point R). When such a terrain is cooled, exhumed, and eroded, it will yield detritus with assorted zircon ages belonging to the totally un-reset population, mixed ages from the ZPAZ, and sources affected by the Himalayan orogenesis (Fig. 6A, point S) (cf. Thomson, 1994). As an example, the north-ernmost part of the Himalayan Metamorphic Belt in the Tso Morari Crystallines has under-gone UHP-HP amphibolite-greenschist facies metamorphism at 53.3 ± 0.7, 50 ± 0.6, 47.5 ± 0.6, and 31.1 ± 0.3 Ma, respectively (Fig. 7B;

de Sigoyer et al., 2000; Leech et al., 2005, 2007). The uppermost Tethyan cover was eroded and provided detritus for the Eocene Subathu sedi-ments, which have the oldest un-reset zircon FT ages, >500 Ma. Further erosion of this cover through the ZPAZ yielded the partially reset ages between 300 and 100 Ma during exhuma-tion, while totally reset ages were attained after the Tso Morari Crystallines, lying beneath the Tethyan cover, was exhumed to lower tempera-tures after the greenschist facies conditions ca. 31 Ma and eroding the isotherm of 240 ± 30 °C. Then, the crystalline basement shed younger zircons ca. 30 Ma, when it was exhumed to the surface. Similar cooling and exhumation may be visualized when bedrock has undergone M1 and M2 metamorphism at 40–30 and 25–15 Ma, respectively, and exhumed to provide detritus to the foreland basin. Because the zircon FT P1 peak of 49.4 ± 4.1 Ma in the Subathu Forma-tion is much older than the timing of greenschist facies in the Tso Morari Crystallines, it is very likely that it has been derived from the Indus Tsangpo Suture Zone, the ophiolite nappes, and the Ladakh Batholith; the latter still retains ca. 45 Ma FT zircons (Kumar et al., 2007).

Zircon FT Ages and Source Rock Interpretation

A perusal of Table 1 reveals that detrital-zir-con FT peaks from the Lower Cenozoic Hima-layan basin can be categorized into distinct peaks: (1) ca. 50 Ma and older pre-Himalayan peaks, (2) the Himalayan ca. 30 Ma peak, and (3) young Himalayan ca. 25 and ca. 15 Ma peaks. These peaks were assigned to distinct exposed Himalayan sources from their known zircon FT ages older than 15 Ma (Fig. 7A) and

TABLE 1. DETRITAL-ZIRCON FISSION-TRACK AGES FROM THE SUB-HIMALAYAN FORELAND BASIN, HIMACHAL PRADESH Peaks (Ma) (% of grains) Stratigraphic

unit Sample number Age range

(Ma) 4P 3P 2P 1PLower Siwalik Formation

1 Main 9.2 to 338.5 n = 88

15.4 ± 1.9, (38.4%) W = 0.30, N = 34

26.8 ± 3.3, (46.9%) W = 0.31, N = 41

76.9 ± 17.1, (8.3%) W = 0.32, N = 07

277.3 ± 93.5, (6.4%) W = 0.45, N = 6

17 Upper

13.8 to 445.3 n = 155

20.7 ± 3.2, (18.8%) W = 0.24, N = 29

31.8 ± 2.8, (48.3%) W = 0.25, N = 75

88.5 ± 11.4, (21.1%) W = 0.34, N = 33

288.3 ± 54.6, (11.8%) W = 0.44, N = 18

40 Middle

17.7 to 467.1 n = 52

24.1 ± 5.0, (28.9%) W = 0.28, N = 15

37.5 ± 4.9, (53.8%) W = 0.29, N = 28

267.4 ± 69.5, (17.3%) W = 0.46, N = 09

____

26 Lower middle

12.4 to 476.4 n = 107

24.9 ± 2.1, (12.3%) W = 0.27, N = 13

50.2 ± 11.6, (53.2%) W = 0.30, N = 57

268.2 ± 33.2, (34.5%) W = 0.43, N = 37

____

Kasauli Formation

27 Basal

23.1 to 421.7 n = 33

37.5 ± 5.3, (59.1%) W = 0.34, N = 20

289.2 ± 77.3, (40.9%) W = 0.54, N = 13

____

____

53 Upper

16.3 to 486.3 n = 74

30.3 ± 3.1, (42.9%) W = 0.30, N = 32

82.0 ± 16.7, (17.1%) W = 0.33, N = 12

308.4 ± 43.2, (40.0%) W = 0.43, N = 30

____

23 Main

20.6 to 465.0 n = 55

31.4 ± 1.9, (36.3%) W = 0.3, N = 20

268.3 ± 31.6, (63.7%) W = 0.4, N = 35

____

____

06 Main

16.3 to 468.3 n = 69

29.2 ± 2.2, (77.4%) W = 0.28, N = 53

270.5 ± 45.1, (22.6%) W =0.37, N = 16

_____

____

Dagshai Formation

33 Lower

20.2 to 477.0 n = 47

31.6 ± 3.9, (52.0%) W = 0.33, N = 24

81.2 ± 16.5, (20.4%) W = 0.33, N = 10

333.0 ± 75.5, (27.6%) W = 0.48, N = 13

____

Subathu Formation

28 Main

32.6 to 520.0 n = 135

49.4 ± 4.1, (15.5%) W = 0.20, N = 21

302.4 ± 21.9, (84.5%) W = 0.39, N = 114

____

____

Note: P1–P4 binomial peak-fit ages determined with BINOMFIT Program (Brandon, 1996). %—Percent of grains; W—Width; n—total number of grains analyzed in each sample; N—No. of grains in each peak at 95% confidence level. Also shown are the ranges of single-grain ages in each sample.

Jain et al.


inferred cooling histories, because any abnor-mally fast cooling in the source will be associ-ated with its fast exhumation and equally accel-erated denudation of its cover (Figs. 7B–7G).

Circa 50 Ma and Older Pre-Himalayan PeaksDetrital-zircon FT peaks of ca. 50 Ma and

older pre-Himalayan ages from the Subathu, Dag-shai, and Kasauli Formations reveal that detrital grains have retained the tracks refl ecting their precollisional geological history of the bedrock. Sources for the youngest 49.4 ± 2 Ma P1 peak in the Subathu Formation appear to be the Indus Tsangpo Suture Zone, thin ophiolite nappes, and the Ladakh Batholith (Fig. 6B). Blueschist metamorphism affects the suture zone rocks ca. 100–80 Ma (Maluski and Matte, 1984; Honeg-ger et al., 1989; Anczkiewicz et al., 2000), while only one available zircon FT age is 88 ± 16 Ma (Schlup et al., 2003). Exposed Ladakh Batholith has zircon FT ages between 45.2 ± 3.1 and 41.4 ± 2.2 Ma (Sorkhabi et al., 1994; Kumar et al., 2007), while its eroded parts must have yielded older zircons during the Subathu sedimentation. Detrital character, Nd and Rb-Sr isotopes of this formation (Najman et al., 2000), and its equiva-lent Bhainskanti Formation in Nepal (Robinson et al., 2001; DeCelles et al., 2004) reveal that the Indus Tsangpo Suture Zone and Ladakh and Gangdese batholiths supplied the detritus to these sequences. All the younger sediments were dom-inated by sources from the Himalayan Metamor-phic Belt and Tethyan Sedimentary Zone (Naj-man et al., 2000; DeCelles et al., 2004).

In the Subathu Formation, the oldest zir-con FT peak of 302 ± 21.9 Ma (84.5% grains) represents the contributions from an un-reset and partially reset terrain, which is character-ized by the Paleozoic magmatic and meta-morphic events and denuded in contrast to the areas affected by the Himalayan events. A few Lower Cretaceous–Triassic zircon FT ages are recorded from the Tso Morari Crystallines and its Tethyan sedimentary cover in Zanskar and Lahaul (32–33°N:77–78°E; Schlup, 2003), indicating that the detritus has been derived from an exhumed ZPAZ (Fig. 7A). Because the Subathu detritus was derived from the sources from both the Indian and Asian plates, it appears that these plates were sutured during this period. The Himalayan orogeny was initiated since the beginning of the Subathu sedimentation ca. 57 Ma; from the start, it had very localized effects on source in terms of aerial extent and inten-sity. Detailed modeling of a recently generated U-Pb sensitive high-resolution ion microprobe (SHRIMP) zircon age led Leech et al. (2005) to visualize that the initial contact between the two plates took place ca. 57 ± 1 Ma along steep Indian continental lithosphere.

SE[z] (~RSE[age])

100

252

185

136

342

463

0

-4

-8

-12

0.87 0.12 0.09

54

73

0.22

Subathu Fm.(Sample No. 28)

4

8

12

39

108

182

306

511

16

12

8

4

0

-4

-8 1.32 0.33 0.19

13

22

64

37

20

Kasaui Fm(Sample No. 40)

FT

gra

in a

ge

(Ma)

Sta

nd

ard

ized

var

ian

ce

-9 1.51 0.38 0.22 0.15

17

26

41

159

102

65

387

249

-6

0

-3

3

6

9

12 Dagshai Fm.(Sample No. 53)

0

-4

-8

1.18 0.17 0.12

15

25

118

71

42

197

327

0.29

Kasauli Fm.(Sample No. 26)

4

8

12

16

142

223

350

0

-3

-61.64 0.23 0.16

23

36

90

57

0.41

Dagshai Fm.(Sample No. 33)

3

6

9

-12

1.32

16

12

8

4

0

-4

-8

0.33 0.19

8.6

15

74

43

25

126

214

36320 Lower Siwalik Fm

(Sample No. 1)

0

-5

-10

1.90 0.27 0.19

14

24

111

66

40

185

0.47


10

15

20

5

8.4

308

-8

182

129

257

3618

6

4

2

0

1.59 0.23 0.1623

32

45

64

91

0.4


-6

-4

-2

-10

1.12 0.16

15

10

5

0

-5

0.117.3

12

21

106

62

36

181

306

0.28


-15

0

-2

-4

1.90 0.27 0.19

24

37

145

92

59

227

354

0.47


4

6

8

10

2

Figure 4. Radial distribution plot of detrital-zircon fi ssion-track ages from the Sub-Hima-layan foreland basin, NW Himalaya. X-axis denotes standard error (SE) of log of age (z), which is approximately equal to relative standard error (RSE).



Fission-track grain age (Ma)

Kas

auli

Fo

rmat

ion

0

2

4

6

8

10

12

14

1 10 100

1000

28 Subathu P1

0

2

4

6

8

10

12

1433 Lower

P1

0

4

8

12

14

P1

Lower Siwalik

0

2

4

6

8

10

12

14

P117 Upper

0

4

8

12

16

20

P140 Middle

16

0

4

8

12

26 Lower middle

1 10 100

1000

0

4

8

12

1427 Basal

0

2

4

6

8

10

12

14

P153 Upper

0

2

4

6

8

10

12

14

P123 Main

0

2

4

6

8

10

12

14

P106 Main

P1

P1

Pro

bab

ility

Den

sity D

agsh

ai F

orm

atio

n

(15.

4 ±

1.9

Ma)

(26.

8 ±

3.3

Ma)

(76.

9 ±

17.

1 M

a)

( 27

7.3

± 9

3.5

Ma)

(30.

3 ±

3.1

Ma)

(267

.4 ±

69.

5 M

a)

(82.

0 ±

16.

7 M

a)

(268

.3 ±

31.

6 M

a)

(31.

4 ±

1.9

Ma)

(270

.5 ±

45.

1 M

a)

(29.

2 ±

2.2

Ma)

(49.

4 ±

4.1

Ma)

(302

.4 ±

21.

9 M

a)

(333

.0 ±

75.

5 M

a)

(31.

6 ±

3.9

Ma)

(81.

2 ±

16.

5 M

a)

(24.

9 ±

2.1

Ma)

(50.

2 ±

11.

6 M

a)

(268

.2 ±

33.

2 M

a)

(37.

5 ±

5.3

Ma)

(289

.2 ±

77.

3 M

a)

(24.

1 ±

5.0

Ma)

(37.

5 ±

4.9

Ma)

(267

.4 ±

69.

5 M

a)

(20.

7 ±

3.2

Ma)

(31.

8 ±

2.8

Ma)

(88.

5 ±

11.

4 M

a)

(288

.3 ±

54.

6 M

a)

Figure 5. Composite probability density distribution and grain-age histograms for the detrital-zircon fi ssion-track (FT) ages from the Sub-Himalayan foreland basin using the BINOMFIT peak-fi t program. Also shown with bars are the prominent peaks and their distinct characters.

Jain et al.


ITSZ

Main Himalayan Thrust

HHC

2

LH

LB 42±2

1

2

UHP

HPAmp

Grn

3

TSZHMB

SHTMC

> 500 to < 100

ITSZ

Main Himalayan Thrust2

LH

LB

1

2UHPHP

AmpGrn

1

2

3 Tso Morari Crystallines: UHP -U-Pb SHRIMP zircon 53.3 ± 0.7 Ma (TMC) HP -U-Pb SHRIMP zircon 50 ± 0.6 Ma

Amphibolite facies: U-Pb SHRIMP zircon 47.5 ± 0.5 Ma Greenschist facies: Ar/Ar muscovite 31.1 ± 0.3 Ma

3

TSZ

HMB

SH

TMC

4

4

HHC5

5

ca. 25 ca. 15MCT

+

Kasauli Fm. +Lower Siwalik Subgr.

0 °C

0100200300400

**

Q=Parially reset zircon (ca. 300 to 100 Ma)R=Totally reset zircon ( < 40 Ma)

P=Totally un-reset zircon (>~500 Ma)

>~

Himalayan Orogeny(~ 55 Ma)

RQ

Proterozoic terrain,Tethyan SZ & ~ 500 Ma plutons

Zircon Partial Annealing Zone (ZPAZ) (240–180 °C)

SubathuFormation

S

*

P

ITSZ

Main Himalayan Thrust2

LH

LB

1

2

UHP

HPAmp

Grn

3

TSZ

HMB

SH

TMC

45-34

4

HHC

5

ca. 30 Dagshai Fm.

ITSZ

MBTMFT

Main Himalayan Thrust

MCT+

LB IP

TSZ

TMC HHC

SH

88 ± 1642 ± 2

22 ± 2to

12 ± 1STDZ

1

++ +

++

+++

IgPLHHMB

Indo-Gangetic Plains (IgP)

Sub-Himalayan Cenozoic foreland belt (SH)

Lesser Himalayan Sedimentary Belt (LH)Himalayan Metamorphic Belt (HMB)

Tso Morari Crystallines (HMB)Higher Himalayan Crystallines (HHC)Lesser Himalayan Crystallines (LHC)

Tethyan Sedimentary Zone (TSZ)

Indus Tsangpo Suture Zone (TSZ)

Ladakh Batholith (LB)

45 ± 2to

35 ± 2

45 ± 7to

36 ± 2

16 ± 2to

11 ± 112 ± 1

to11 ± 1

2 3

4

5 6

+

+

b

MFT—Main Frontal Thrust MBT—Main Boundary Thrust MCT— Main Central ThrustSTDZ—South Tibetan Detachment Zone ITSZ—Indus Tsangpo Suture Zone

X Y

ac

d

e f h

88±16

A

B

C

D

E

Higher Himalayan Crystallines (HHC): Eo- Himalayan metamorphism 40–30 Ma

Higher Himalayan Crystallines (HHC): Neo-Himalayan metamorphism 25–15 Ma

Ladakh Batholith: U-Pb SHRIMP zircon 60–58 Ma

ITSZ Nidar ophiolite: Sm-Nd ca. 140 MaBlueschist 100–80 Ma

Figure 6. Schematic sections of the possible source rocks for the Sub-Himalayan foreland basin. (A) Possible effects of the Himalayan orogeny on a Proterozoic terrain, its exhuma-tion and hypothetical path of particles through different zones. Different source rocks at the deposition of the (B) Subathu Formation, (C) Dagshai Formation, and (D) Kasauli For-mation and Lower Siwalik Subgroup. (E) Geo-logical cross-section of the NW Himalaya from the Indo–Gangetic Plains to the Ladakh Batho-lith, showing progressive unroofi ng and present-day measured zircon fi ssion-track (FT) ages in the provenance. Filled polygons: Present-day distribution of zircon FT ages (Ma) across the Himalaya. Open polygons represent events across the Himalayan units: 1—U-Pb sensitive high-resolution ion microprobe (SHRIMP) zir-con (60.1 ± 0.9 Ma) from the Ladakh Batholith as age of emplacement. 2—40Ar/ 39Ar glauco-phane age (90 ± 10 Ma) indicating blueschist metamorphism. 3—U-Pb SHRIMP zircon age (53–50 Ma) indicating ultrahigh-pressure (UHP) and high-pressure (HP) metamorphism. 4—Low amphibolite facies metamorphism in the Tethyan Sedimentary Zone at ca. 47 Ma. 5—U-Pb zircon and monazite and U-Th-Pb monazite ages between 40 and 25 Ma due to the Eo-Himalayan metamorphism. 6—Neo-Himala-yan metamorphism between 25 and 15 Ma. Data sources: a—Kumar et al. (2007); b, c, d—Schlup et al. (2003); e—Kumar (1999); f—Kumar et al. (1995); g—Kumar (1999). 1—Singh et al. (2007); 2—Honegger et al. (1989); 3—Leech et al. (2005, 2007); 4—de Sigoyer et al. (2000); 5—Vance and Harris (1999), Prince et al. (1999); 6—Walker et al. (1999). Geological cross-section is along the XY line in Figure 1B.



Age (Ma)

Age (Ma)

Rb-Sr bt400

500

600

200

300

201510 25

T °

C

Manali

FT zr

800

30 35

U-Pb m11

1113

15

Rb-Sr ms13

Th-Pb m

Ar-Ar ms

Ar-Ar bt

800

400

500

600

700

200

300

2015 25Age (Ma)

T °

C

Gangotri

16

17

17

Ar-Ar phe

Age (Ma)

Lu-Hf grt

400

500

600

200

300

800

1000

900

700

20 4010 5030 60 70

Rb-Sr ap-phe-wr

Sm-Nd grt-amp-wr

U-Pb aln Sm-Nd grt-gln-wr

Tso Morari

FT zr Ar-Ar bt

Ar-Ar ms

0

Tem

pera

ture

°C

U-Pb zr

U-Pb zr

U-Pb zr

1

2

1

1

1

1

2

2

1

1

13

Rb-Sr ms

400

500

600

200

300

2015 25 30

Rohtang

T °

C

13, 14

13, 14Rb-Sr bt

Age (Ma)

Ar-Ar ms400

500

600

200

300

2015 25 30

Lahaul

700

800

Ar-Ar bt

T °

C

35

U-Pb m

U-Pb (m, u, x)(i)(ii)

11

11, 12

11, 1211

T °

C400

500

600

200

300

201510 25 30

700

800

35

ZanskarSm-Nd

U-Pb m(i)

(iii)4

9

Age (Ma)

Rb-Sr ms Ar-Ar hbRb-Sr ms

Ar-Ar bt

Ar ms

Rb-Sr btFT zr10

(ii)

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Ar-Ar msRb-Sr bt7

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8

86

7

8

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D E

F

G

Ar-Ar ms

A

78°

76°

76°

34°

32°

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D

Kishtwar

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N

40 Km

Beas R. Mandi

Sutlej R.

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.

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GGangotri

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Uttarkashi

Dehra Dun

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STDZ

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ITSZ

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F

E

* U-Pb SHRIMP zircon dates (UHP-HP-Amp meta.)

U-Pb SHRIMP zircon Ar-Ar ms Ar-Ar bt FT zr

ca. 30 Ma ca. 25 Ma ca. 20 Ma ca. 15 Ma

41.743.4

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15.5

15

3638

*

16

2302228

20

213

2542104

20

2117

21

22

20 20

15

18

4245

3645

5434

37

35

Detrital-zircon FT PeaksFigure 7. (A) Simplifi ed geological map of the NW Himalaya showing the location of >15 Ma mineral ages and cooling paths within the Himalayan Metamorphic Belt (HMB). Cooling curves from the (B) Tso Morari Crystallines (TMC) and Higher Himalayan Crystal-lines (HHC) at (C) Zanskar, (D) Lahaul, (E) Rohtang, (F) Manali, and (G) Gangotri. Cooling curves i, ii, and iii in diagram (C) are from the upper HHC, Zanskar Shear Zone, and core, respectively. In fi gure (D), curves i and ii indicate cooling after M1 and M2 metamor-phism. Width of ellipses indicates age errors, and lengths represent conventional closure and annealing temperatures. Hachured fi elds represent approximate timing of accelerated cooling and exhumation of present-day source rocks and their eroded cover shedding the zircon fi ssion-track peaks. Data sources: 1—de Sigoyer et al. (2000); 2—Leech et al. (2005, 2007); 3—Schlup et al. (2003); 4—Vance and Harris (1999); 5—Searle et al. (1992); 6—Honegger et al. (1982); 7—Inger (1998); 8—Vance et al. (1998); 9—Noble and Searle (1995); 10—Kumar et al. (1995); 11—Walker et al. (1999); 12—Dezes et al. (1999); 13—Mehta (1977); 14—Frank et al. (1977); 15—Lal et al. (1999); 16—Searle et al. (1999); 17—Sorkhabi et al. (1999).

Jain et al.


It is noteworthy that a distinct break in zircon peak characters occurs at the Subathu-Dagshai contact. The ca. 50 Ma P1 Subathu peak does not extend into the Dagshai, which has younger peaks like the overlying formations. Domi-nance of the ZPAZ and older pre-Himalayan peaks between 330 and 80 Ma are considerably reduced in the overlying formations, and dimin-ish in Lower Siwalik time with intensifi cation of the Himalayan orogenesis and subsequent cool-ing, starting with the UHP metamorphism in the extreme northeast.

Himalayan Ca. 30 Ma PeakThe detrital-zircon FT P1 peak of 31.6

± 3.9 Ma (52% in sample no. 33) characterizes the lower part of the Dagshai Formation, while the ca. 30 Ma P1 peak defi nes this formation overall. The Himalayan Metamorphic Belt and its cover underwent an initial metamorphism during the early Cenozoic, when the Tso Morari Crystallines experienced the UHP to greenschist facies metamorphism between ca. 53 and 31 Ma (Fig. 7B; de Sigoyer et al., 2000; Leech et al., 2005, 2007). Many FT zircon ages fall between 45 ± 7 and 34 ± 2 Ma (Schlup et al., 2003), while its Tethyan cover contains ages between 42.5 ± 4.8 and 27.9 ± 2.7 Ma (Schlup, 2003). These units are exposed in the Sutlej-Indus-Chenab catchment and appear to be the most likely source for this FT peak in the Himalayan foreland (Figs. 6C and 7B). These sources have cooled from a greenschist facies temperature of 350 ± 50 °C to a zircon FT annealing tempera-ture (see Leech et al., 2007) at an accelerated rate of ~25 °C/m.y. ca. 35–30 Ma and reached the surface to produce detritus during the Dag-shai and Kasauli sedimentation.

Young Himalayan Ca. 25 and 15 Ma PeaksWithin the Kasauli Formation, P1 peaks of

24.9 ± 2.1 and 24.1 ± 5 Ma are identifi ed and become younger to 20.7 ± 3.2 Ma in the upper-most part. In the Lower Siwalik Subgroup, another P1 peak of 15.4 ± 1.9 Ma appears during the 13–11 Ma sedimentation, while the ca. 25 Ma peak extends uninterruptedly (Table 1; Fig. 5).

The Higher Himalayan Crystallines under-went Middle Eocene to Early Oligocene (40–30 Ma) M1 and M2 regional metamorphism and leucogranite emplacement during Oligocene–Early Miocene (25–15 Ma) (Figs. 7C–7G; Le Fort, 1996; Dezes et al., 1999; Walker et al., 1999). Evidence for the M1 metamorphism abounds from the Eocene-Oligocene Sm-Nd garnet ages in Lahaul, Zanskar, and Garh-wal (Fig. 7C, curve i; Vance and Harris, 1999; Prince et al., 1999), and garnets of 44–36 and 33–28 Ma in the Everest region (Foster et al., 2000). This event is supported by U-Th-Pb

monazite inclusion ages in garnet (32–29 Ma) from Lahaul (Fig. 7D, curve i; Walker et al., 1999), 44 and 36 Ma near the bottom and top in Garhwal, and monazite growths in matrix at 36–25 Ma in Garhwal (Foster et al., 2000), ca. 32 Ma (Simpson et al., 2000), 45 ± 2.8 Ma (Cat-los et al., 2002), and 44.5 ± 0.9–33.5 ± 1.2 Ma (Catlos et al., 2002). After this metamorphism, the belt cooled differentially: (1) to ~350 °C at 28 Ma in the upper parts in Zanskar (Fig. 7C, curve i; Vance et al., 1998), (2) to 500 and 300 °C ca. 25–23 Ma within the Zanskar Shear Zone along the top (Fig. 7C, curve ii; Vance et al., 1998), (3) from 800 to 300 °C in the core during 20 and 18 Ma (Fig. 7C, curve iii; Vance et al., 1998), and (4) ca. 23–20 Ma in parts of Lahaul and Manali (Fig. 7D, curve i; Fig. 7F). Therefore, the Higher Himalayan Crystallines cooled at ~30 °C/m.y. during the Kasauli sedi-mentation (25–20 Ma) and provided a zircon P1 peak of 25 Ma (Figs. 6D, 7C, 7D, and 7F).

The younger M2 metamorphism of the Higher Himalayan Crystallines also involves melting, decompression, and leucogranite generation at 24–12 Ma (Harrison et al., 1997; Harris and Ayres, 1998; Walker et al., 1999; Hodges, 2000). Within the catchment of the Chenab, Ravi, Beas, Sutlej, and Ganga Rivers, Th-Pb and U-Pb monazite and zircon ages are between 23 and 20 Ma (Figs. 7C, 7D, 7F, and 7G; Noble and Searle, 1995; Harri-son et al., 1997; Dezes et al., 1999; Walker et al., 1999), whereas Rb-Sr and 40Ar/39Ar mica ages cluster between 25 and 17 Ma (Frank et al., 1977; Mehta, 1977; Metcalfe, 1993; Inger, 1998; Dezes et al., 1999; Walker et al., 1999; Stephenson et al., 2001). In Lahaul and Zanskar of Himachal, monazite grew during the M2 metamorphism and emplacement of leucogranite at 22–21 Ma. It was followed by instant cooling to 350 ± 50 °C at ca. 21–20 Ma and then to 300 ± 50 °C at ca. 20 Ma (Fig. 7D, curve ii) at ~200 °C/m.y. during a short span of 2 m.y. in early Miocene (Noble and Searle, 1995; Dezes et al., 1999; Walker et al., 1999; Yin, 2006). Elsewhere, cooling of ~100 °C/m.y. is recorded from ~750 ± 50 to 300 ± 50 °C between 25 and 20 Ma in Zanskar and Garhwal (Fig. 7C, curve iii; Fig. 7G; Sorkhabi et al., 1996, 1997; Inger, 1998; Searle et al., 1999). Such episodes have possibly resulted from exhu-mation and erosion of the Higher Himalayan Crystallines due to movements along the Main Central Thrust over the Lesser Himalayan win-dows and may be responsible for the ca. 20 Ma P1 peak and large litharenite and garnet in the upper Kasauli Formation.

During the Lower Siwalik sedimentation, the Higher Himalayan Crystallines were fur-ther exhumed and yielded the FT zircon peak of 15 Ma (Fig. 6E). The central and uppermost parts of this belt still retain these ages between

16.3 ± 1.6 and 10.7 ± 1.1 Ma along the Sutlej Valley (Vannay et al., 2004; Thiede et al., 2005), 12.26 ± 0.67 and 10.81 ± 0.76 Ma along the Chenab Valley (Kumar et al., 1995), and 22.3 ± 2.3 and 11.8 ± 0.7 Ma near Kulu and Beas Valleys (Fig. 7A; Kumar, 1999). The Higher Himalayan Cyrstalline belt provides cooling rates of ~20 °C/m.y. between 20 and 15 Ma (40Ar/39Ar white mica and zircon FT ages) and an exhumation rate of ~0.6 mm/yr so that it was eroded to produce detritus for the Lower Siwalik Subgroup (Figs. 7E and 7F). Thus, our assump-tion of constant geothermal gradients of 30 °C/km for source rocks have yielded exhumation rates to 6.6–3.3 mm/yr during 22–20 Ma, from 0.3 mm/yr at ca. 30 Ma, followed by decelera-tion to 0.6 mm/yr at ca. 15 Ma.

LATERAL VARIATION OF EXHUMATION IN THE HIMALAYA AND CONTROL ON FORELAND SEDIMENTATION

Although the India-Asia collision has been documented as the most signifi cant tectonic event during the late Mesozoic-Cenozoic, its precise timing remains tentative between 65 and 50 Ma due to variable data sets including the strati-graphic records (see DeCelles et al., 1998, 2002; Yin and Harrison, 2000; Steck, 2003; Zhu et al., 2005; Leech et al., 2005, 2007; Najman, 2006; Yin, 2006 and references therein). The eastern region was occupied either by shallow uplifts or seas during the Miocene while signifi cant uplift began in the western Himalaya, thus indicat-ing variations in its orogenesis along the strike (Uddin and Lundberg, 1999). However, Najman (2006) noted collisional synchroneity from west to the east from similarity in detrital record in the foreland. It is, therefore, worthwhile to reexamine the detrital ages from the Cenozoic Himalayan foreland basin and their implications (Fig. 8).

The detrital-zircon FT data, presented in this work, clearly reveal distinct cooling and exhumation patterns in the source rocks, fol-lowing the initiation of marine sedimentation ca. 57 Ma. Between India and Nepal the Eocene Subathu and Bhainskati Formations bear strong resemblance in their youngest detrital-zircon FT peaks of 49.2 ± 2 and 45 ± 5 Ma as well as older 340–120 Ma zircons (Fig. 8). Therefore, their detritus has been derived by uniform synchro-nous denudation of the Ladakh-Gangdese arc, Himalayan metamorphics, and Tethyan sedi-ments. These source rocks are located both on the Indian and Asian plates; hence, they provide undisputed evidence of their suturing during the early collisional stage ( Najman et al., 2005).

The fl uvial Dagshai and Kasauli Formations (30 and 13 Ma) exhibit ca. 30 and 25 Ma zircon



FT steady peaks, respectively, due to denudation of the Himalayan Metamorphic Belt and its cover after the UHP-HP and M1 metamorphism. In Nepal, the Dumri Formation (21–16 Ma), uncon-formably overlying the Bhainskanti Forma tion, contains zircons of 32–30 Ma (Najman et al., 2005) in contrast to the ca. 25 Ma peak in the equivalent Kasauli Formation of Himachal, thus providing evidence for heterogeneous cooling and exhumation in the Himalayan sources.

Zircon FT ages from the upper Kasauli and Lower Siwalik sequences are distinct with the arrival of the 15 Ma P1 peak and reduction of the pre-Himalayan peaks in the latter. During the Lower Siwalik (ca. 13–11 Ma), zircon FT ages are almost totally reset by two metamor-phic events in the exposed Himalayan Paleopro-terozoic–Early Paleozoic bedrock. When these data are compared with the Pakistan Siwaliks, one fi nds that the zircon FT ages are between 17 and 16 Ma for the sediments of 13–11 Ma, and range between 28.5 ± 2 and 7 ± 1 Ma for the whole group, deposited between 20 and 2 Ma (Cerveny et al., 1988; also Fig. 13 in Bernet and Garver, 2005). Enhanced exhumation is inferred from the upsection decreasing lag time from 8.5 to 1 Ma of young moving peaks and decreasing age of older zircons (Bernet and Garver, 2005).

The Siwalik Group from western Nepal con-tains a detrital-zircon FT, young static peak between 18.3 ± 1.9 and 14.4 ± 1.6 Ma (ca. 16 Ma for the whole group) and the moving peak of 13.0 ± 2.3 Ma during the deposition spanning 13–11 Ma. For the whole group, deposited during 14.1 and 1 Ma, moving peaks between 13.0 ± 2.3 and 5.4 ± 1.4 Ma, young static peaks between 23.9 ± 4 and 13.5 ± 3.3 Ma, and old static peaks between 216.6 ± 135.7 and 81.3 ± 24.4 Ma are recorded with the retention of constant lag time of ~4 m.y. (Bernet et al., 2006). Both the Shimla Hills and Nepal areas, located ~500 km away from each other, contain a static peak of ca. 16 Ma, while younger moving peaks did not appear in the northwest during this period. The ca. 25 Ma peak is missing in Pakistan, prominently observed in India, and is present only in the eastern Siwa-lik section of Tinau Khola, western Nepal. It may be linked to complete resetting of older M1 metamorphic events in the Karnali catchment by the M2 metamorphism after which source rocks cooled and exhumed for the 16 Ma peak.

Other Mineral Cooling Ages from the Himalayan Foreland Basin

Available multi-thermochronometry data from the foreland basin also demonstrate that the Himalayan source rocks were denuded non-uniformly during cooling through 750 ± 50 to 300 ± 50 °C—the blocking temperatures of

U-Th-Pb monazite and Rb-Sr and 40Ar/39Ar bio-tite, respectively, and provide insight into cool-ing of these rocks through higher temperatures.

U-Th-Pb detrital monazites of ca. 37–28 and 1300–400 Ma from the Dharamsala and Lower Siwalik sequences (20–12.8 Ma) in Himachal provide distinct clues to the sources such as the Himalayan M1 metamorphosed terrains, Cambrian-Ordovician granites, and protoliths of late Mesoproterozoic to Neoproterozoic (Fig. 8; White et al., 2001). Denudation within 20–10 m.y. of the fi rst metamorphism at ca. 40 Ma records cooling rates between 60 and 40 °C/m.y. for a period between 30 and 20 Ma (White et al., 2001). In addition, 40Ar/39Ar detrital white micas from the Dharamsala cluster between 26 and 22 Ma, while older grains range from 50 to 950 Ma (White et al., 2002). The Lower Siwalik sand-stones (13–11 Ma) have 22–20 Ma micas with a few fl akes being 500 Ma. Those from the Kasauli Formation (25–13 Ma) are of 27.7 and 22.1 Ma. The underlying Dagshai Formation has young mica peaks of 28 and 24.7 Ma (Fig. 8; Najman et al., 1993) and old fl akes from ca. 345–115 Ma. The Dumri Formation (21–16 Ma) has younger micas (ca. 20 Ma) (DeCelles et al., 2001) in Nepal. These are somewhat younger (between 19.5 and 12.1 Ma) within the Siwalik Group of western Nepal (Szulc et al., 2006). Farther west in Pakistan, the oldest fl uvial Balakot For-mation, unconformably overlying the Eocene Patala Formation, contains three mica peaks of ca. 37, 150–140, and 450–400 Ma with a few fl akes of 1100 and 1600 Ma (Fig. 8; Najman et al., 2001, 2002).

It is, therefore, evident that metamorphosed Himalayan bedrocks that cooled through 40Ar/39Ar closure temperature of white mica (350 ± 50 °C) were not totally reset until the Lower Siwalik sedimentation and provided evi-dence for older Pan-African orogenesis. White micas from Pakistan to Nepal became younger by ca. 17 Ma, between 37 and 20 Ma in the post-Eocene oldest fl uvial succession, and revealed uneven cooling in the Himalayan metamorphic source rocks during late Eocene through Oligo-cene-Miocene as a consequence of ongoing col-lision tectonics. Similar uneven denudation pat-terns are discernible in detrital-zircon FT peaks between Pakistan, India, and Nepal. The 25 and 20 Ma peaks of the Kasauli Formation are miss-ing from its Nepalese equivalent, the Dumri For-mation (ca. 20–16 Ma) (cf. Najman et al., 2005), which is characterized by the ca. 30 Ma peak, like the Dagshai Formation of Himachal. The overlying Lower Siwalik Subgroup in Pakistan, India, and Nepal, deposited between ca. 14 and 9 Ma, contains zircon FT peaks of 17–16, 15.4 ± 1.9, and 18.9 ± 3.2, respectively, and is nearly similar within the error limits. Nonetheless,

these peaks do not compare with each other in the younger Siwaliks and the river sands in Pakistan and Nepal.

CONCLUSIONS

Detrital-zircon fi ssion-track ages from the Lower Cenozoic Sub-Himalayan foreland basin of Himachal Pradesh reveal progressive cooling and exhumation of the Himalayan source rocks after various metamorphic events. Uniform cooling, exhumation, and synchronous denuda-tion are manifested in the detrital-zircon FT age record of late Paleocene–early Middle Eocene Subathu and Bhainskanti Formations between India and Nepal due to early collisional-stage suturing of the Indian and Asian plates. As a consequence, the 49 and 45 Ma zircon FT grains in these formations along with their petrographic and isotopic character support the denudation of both the Ladakh-Gangdese and Himalayan source rocks across the Indus Tsangpo Suture Zone. Within the Indian plate, the Proterozoic-Paleozoic metamorphic and magmatic rocks and the Paleo-Mesozoic Tethyan sedimentary cover have undergone the UHP-HP, amphibolite-greenschist facies metamorphism during early Eocene and resulted in the development of a ZPAZ. Denudation of these bedrocks during the early Eocene through the zone of un-reset zircons and the ZPAZ provided a dominant zircon peak of ca. 300 Ma and a wide scatter up to 520 Ma.

A distinctive major change in the Himalayan sources is decipherable since the Dagshai sedi-mentation ca. 30 Ma with an increased zircon supply from the sources that were reset during the Himalayan orogenesis. Although the marine sedimentation in the south started almost syn-chronously after the initiation of the India-Asia collision, a regional unconformity was undis-putedly established in the foreland basin with the initiation of the fl uvial sedimentation, and thus, indicated subsequent heterogeneous cool-ing, exhumation, and denudation due to colli-sion. This uneven period of nondeposition spans between 14 and 25 m.y. from Pakistan to Nepal and is ~10 m.y. in India between the Subathu and Dagshai Formations.

Targeting distinct Himalayan source rocks for the Lower Cenozoic foreland sequences has been possible from their known mineral ages and distinct cooling patterns during the time span of deposition of the sediments when these were denuded. The ca. 50 Ma P1 zircon peak in the Subathu Formation vanishes with the com-mencement of the Dagshai sedimentation and the appearance of the younger ca. 30 Ma peak. It characterizes the Dagshai-Kasauli sediments, and appears to have been derived by denuding the Tethyan Sedimentary Zone and UHP to

Jain et al.


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.



greenschist facies metamorphosed Tso Morari Crystallines. This terrain provided accelerated cooling rates ~25 °C/m.y. during a span of ~17 m.y. from 47 to 30 Ma.

Also characterizing the Kasauli Formation is another ca. 25 Ma zircon FT peak, which appears to have been derived from the Higher Himala-yan Crystallines after their M1 metamorphism at ca. 40–30 Ma and cooling of ~30 °C/m.y. ca. 30–25 Ma. Extremely fast cooling between 200 and 100 °C/m.y. is recorded in early Miocene in parts of Zanskar and Garhwal, following the M2 metamorphism and leucogranite emplacement, and may be responsible for a localized ca. 20 Ma P1 peak in upper parts of the Kasauli Forma-tion. Furthermore, the ca. 15 Ma P1 peak within the Lower Siwaliks is derived from the Higher Himalayan Crystallines, metamorphosed dur-ing the M2 phase, and reveals cooling rates of 20 °C/m.y. for this belt between 20 and 15 Ma. Therefore, long-term cooling rates ~25 °C/m.y. have remained almost unchanged over a time span from ca. 45 to 15 Ma.

Although fl uvial sedimentation in the Hima-layan foreland continued uninterruptedly since ca. 30 Ma (ca. 37 Ma in Pakistan) until present, distinct breaks in the youngest detrital-zircon FT peaks are decipherable in each stratigraphic unit by ca. 5–7 m.y.. The ca. 30 Ma P1 Dagshai peak yields to ca. 25 and 20 Ma peaks in the Kasauli, and in turn, to 15 Ma in the Lower Siwalik times. It appears to be controlled by three distinct met-amorphic events in the source rock—the UHP to greenschist facies metamorphism, the M1 and the M2 phases due to the ongoing India-Asia collision, and crustal thickening. These events have reset the Proterozoic–Early Paleo-zoic Himalayan source rocks by the time of the Lower Siwalik sedimentation (ca. 13–11 Ma) as far as the zircon FT ages are concerned with almost absence of pre-Himalayan peaks.

ACKNOWLEDGMENTS

This work was conceived as a part of a major research effort to understand the tectonic evolution of the NW Himalaya under the HIMPROBE Program, funded by the Department of Science and Technol-ogy, New Delhi, for which we are thankful to K.R. Gupta. Continuous prolonged discussions with I.B. Singh, S.K. Tandon, S.G. Sangode, Kishor Kumar, and O.N. Bhargava have helped us in understanding the mutual relationships between the formations of the foreland basin. ONB has also guided us to some criti-cal localities in the fi eld, although he does not share the viewpoints expressed in this work. B.S. is thank-ful to Indian Council for Cultural Relations (ICCR), New Delhi, for the fellowship during the Ph.D. pro-gram. M. Schlup is thanked for making the copy of the Ph.D. thesis available to us. Critical and elaborate comments by Matthias Bernet on an earlier version of this paper and later by An Yin, Peter DeCelles, and Peter Copeland have helped us in vastly improving the paper from its original version.

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Detrital-zircon fission-track ages from the Lower Cenozoic sediments, NW Himalayan foreland basin: Clues for exhumation and denudation of the Himalaya during the India-Asia collision

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