The Himalayan passive margin from Precambrian to Cretaceous times

Sedimentary Geology, 84 (1993) 1-35 1

Elsevier Science Publishers B.V., Amsterdam

The Himalayan passive margin from Precambrian to Cretaceous times

M.E. Brookfield

Land Resource Science, Guelph Uniuersity, Guelph, Ontario N1G 2W1, Canada

(Received June 15, 1992; revised version accepted October 22, 1992)

ABSTRACT

The Himalaya passive margin should be separated into the Indian Himalaya and the Pakistan Himalaya, divided by the Nanga Parbat-Haramosh basement uplift in the northwestern syntaxes. The Indian passive margin shows a complex history involving splitting of microcontinents off the northern Gondwana margin from early Paleozoic times until the Jurassic. In contrast, the Pakistan Himalaya formed part of the stable Indian shelf until separation from Africa started in the Jurassic. Each margin has a distinct stratigraphic history until the mid-Mesozoic after which they show similar histories until the Tertiary.

The Himalaya mountains in India consist of three distinct tectonic units juxtaposed during the Neogene by southward thrusting of the northern Indian Precambrian to Mesozoic passive margin. The Lesser Himalaya on the south forms a late Precambrian passive margin usually directly overlain by late Cretaceous to Tertiary clastics: its stratigraphy is closest to that of the Indian Shield. In contrast, the High Himalaya has a thin Ordovician to Carboniferous shelf sequence deposited after early Ordovician deformation and granite intrusion. Above this, thick Permian and Mesozoic shelf sequences mark the separation of continental blocks off the northern Indian margin and the opening of the Neotethys ocean. The North Himalaya formed the slope and basin of this ocean and consists of reactivated Paleozoic gneiss domes overlain by thick Mesozoic sediments which pass, in the northeastern Himalaya, into an enormously thick sedimentary sequence resting on oceanic crust.

Paleomagnetic and structural evidence indicates at least 500 km of southward thrusting along the Main Central Thrust between the Lesser and High Himalaya. This thrusting, together with a further 250 km of erosion at the front of the nappes, has removed the entire inner shelf of the Himalaya passive margin and can explain the startling contrast between the High and Lesser Himalaya stratigraphic sequences. But there are still discrepancies, particularly in the eastern Himalaya, where Precambrian basement is juxtaposed with the North Himalaya Mesozoic slope. These discrepancies can be partly resolved by strike-slip movements roughly parallel to the Himalaya which have removed parts of the northern Indian passive margin. Such faults of the requisite orientation, displacement and age occur in southeast Asia where their cumulative displacements can add up to several thousand kilometres. Similar faults, of undoubted pre-Tertiary age, occur in the Pakistan Himalaya. Here, the passive margin shows a much simpler history of Jurassic to Miocene subsidence.

Introduction

The Himalayan mountain chain separates In- dia from Central Asia. It consists, on modern interpretations, of slices of the Indian Shield and its cover rocks which have been thrust southwards during Tertiary continental collision (Gansser, 1981). Three main thrust slices, the Lesser Hi- malaya, the High Himalaya and the North Hi- malaya separate the Indian Shield with its Ceno- zoic cover from the Indus Suture Zone (Gansser, 1980; Le Fort, 1989) (Fig. 1). These three slices

can be traced from the northwestern to the northeastern syntaxes. They are overthrust by the Indus Suture Zone, marking the line of collision between India and Asia and the destruction of the Mesozoic to early Tertiary Neotethys ocean (Seng6r, 1984).

Between the Indian Shield and Indus Suture Zone, the Himalaya is thought to show the evolution of the northern passive margin of India from Precambrian to Eocene times (e.g. Acharyya, 1990). From the Precambrian to Jurassic, India formed part of northern Gondwanaland. From

0037-0738/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved

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late Jurassic times onwards separation from Africa, Australia and Antarctica progressively isolated India as a separate continent until collision with Asia started in the Eocene.

The Lesser, High Himalaya and North Hi- malaya consist of a late Precambrian passive margin which was deformed, intruded by granites and stabilized during the latest Precambrian (550 Ma) and Lower Ordovician (500 Ma) (Le Fort et al., 1983). Between the Ordovician and the Permian, all three slices formed part of a stable continental platform. Permian rifting, accompanied by basalt eruptions separated a microcontinent off the northern Indian margin resulting in rapid subsidence of the High Himalaya and deposition of

very thick sediments on oceanic crust in the North Himalaya. However, the shelf and shell margin deposits of the High and North Himalaya disappear eastwards where the metamorphic rocks of the eastern High Himalaya are immediately juxtaposed with the thick Mesozoic slope deposits oi the North Himalaya. And westwards, the shell' margin is progressively truncated by the ~wcr- thrust of the Indus Suture Zone (Fig. l i

West of the northwestern syntaxis, m the Pak-- istan Himalaya, the Paleozoic sheff sequences progressively thin and change in charactcr, l'hesc originally inner shelf units are directly overlain b~, thick Mesozoic passive margin sediments de. posited as India rifted from Africa in thc late

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Thrust; MCT = Main Central Thrust; IS = Indus Suture Zone; J.S. = Jinsha (Jurassic) suture,

HIMALAYAN PASSIVE MARGIN FROM PRECAMBRIAN TO CRETACEOUS 3

Jurassic. The Pakistan Himalaya forms a link

between the very different histories of the northern and western Indian passive margins.

East of the northeastern syntaxis, the eastern Indian passive margin developed during early Cretaceous rifting and separation from western Australia (Powell et al., 1988; G6riir and Seng6r, 1992). However, the eastern Indian passive mar-

gin is now buried beneath thick Tertiary clastic

sediments and the history of this eastern margin can only be deciphered in western Australia.

The western and eastern margins of the Indian plate were thus blocked out as passive margins only in the Mesozoic. The northern margin has a more complex history.

The aims of this paper are: (1) to outline the

SUTURES

X' UoDer Triassic

• • Mid-Jurassic

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B Raikot Stak (~ Sarobi fault fault

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K o h, s ,/a n : / h r ~ _ ~ _ - ~ •

| ~ . . . . . . / Kohistan • K a b ~ ~ arc

Fig. 2. (A) Inferred relationship of the Indian and Pakistan Himalaya and Western Ranges. (B) Structural relationships around the

Raikot fault (Madin et al., 1989). (C) Structural relationships around the Sarobi fault (Griesbach, 1892; Andritzky, 1971).

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stratigraphic and structural history of the northern Indian passive margin; and (2) to reconstruct the development of this passive margin in terms of the wider tectonics of Gondwana break-up.

Structural divisions

The northwestern Himalayan syntaxis marks a change from southwardly directed convergence on the east, to left-lateral translation on the west (Fig. 1). The classic subdivisions of the Indian Himalaya do not extend neatly across the syntaxis, and the fashion of carrying subdivisions of the Indian Himalaya westwards into Pakistan has obscured some of the fundamental stratigraphic and structural differences (Yeats and Lawrence, 1984). At present, three segments, the Indian Himalaya, the Pakistan Himalaya and the West- ern Ranges, are separated by major oblique-slip faults (Fig. 2).

The Raikot fault lies along the western side of the Nanga Parbat-Haramosh basement uplift. Eastwards, the Indian Himalaya has been moving south and rotating clockwise over the Indian Shield since the Miocene development of the Main Central Thrust; though the start of the India-Asia collision began in mid-Eocene times (Dewey et al., 1989). The Raikot fault shows dextral slip, taking up southward thrusting along the Main Central Thrust (Madin et al., 1989). The similar Sarobi dextral fault juxtaposes the Pakistan Himalaya with the Western Ranges. Be- tween the Raikot and Sarobi faults the Pakistan Himalaya is being thrust directly southwards over the Indian Shield (Klootwijk et al., 1986).

Before collision, the Indian Himalaya and Western Ranges were distinct passive margins formed in Permian and Jurassic times, respectively, although the Indian Himalaya also contains an earlier late Precambrian to Permian history. The Indian Himalaya formed on the northern passive margin of Gondwanaland which was periodically disrupted in both Paleozoic and Mesozoic times by the separation of microcontinents and opening of ocean basins (Seng6r, 1984). The Western Ranges preserve a mostly buried passive margin sequence commencing in the late

Jurassic with the rifting of India from Africa (Besse and Courtillot, 1988). These passive margin deposits overlie Cambrian and Permian to Jurassic remnant sediments of the northern Gondwana shelf. The Mesozoic passive margin sequences have been thrust obliquely southwards during the Tertiary collision. The Pakistan Hi- malaya marks the intersection of these different margins and show the stratigraphic and structural effects of both.

Thus, because the stratigraphy and structure of the Himalaya change across the Nanga Par- bat-Haramosh massif, it is essential to describe the Indian Himalaya separately from the Pakistan Himalaya on the west. Though both Himalayas reflect the Phanerozoic history of the northern Gondwanaland passive margin, the Pakistan Hi- malaya shows distinct stratigraphic and structural differences due not only to its transitional position between the Indian and Western passive margins, but also to its rather different structural history since the start of the India-Asia collision in the Eocene.

Problems

The main problem considered in this paper is the amazing and inconsistent large-scale changes in sedimentary facies both within and across the major structural units of the Himalaya. The changes across the structural units have been known for a long times (e.g. Gansser, 1964). The changes within the units were formerly over- looked, mainly because of the relatively coherent stratigraphy of the western Indian Himalaya which is the only part with thick, easily accessible, fossiliferous sediments (Hayden, 1904). Studies in Pakistan and the northeastern Himalaya have now shown that major along-strike changes occur as noted below. For example, the Indian and Pakistan Himalaya are not simply the same passive margin rotated in opposite senses around the northwestern syntaxis. In fact, most sequences can not be traced from one end of the Himalaya to the other as hitherto assumed. And when they can they occur in different structural units.

Across strike, in the western Indian Himalaya, the three thrust slices show abrupt changes, espe-

0 M.|:. BROOKHI;:.I J-~

cially between the Lesser and High Himalaya (Fig. 3).

The Lesser Himalaya consists of a thick pile of dominantly late Precambrian mostly clastic sediments and metasediments in several thrust sheets, emplaced southwards along the Main Boundary' Thrust over the post-orogenic elastics of the Indo-Gangetic plain and Foothills (Valdiya, 1980). The Lesser Himalaya is itself overthrust along the Main Central Thrust by the High Himalaya crystalline rocks and their sedimentary cover (Le Fort, 1989).

The High Himalaya consists, in the west, or' Precambrian metasediments intruded by Cam- bro-Ordovician granites which arc unconformably overlain by a thin Ordovician to Pcr- mian shelf sequence followed by a thick Meso- zoic-Eocene passive margin sequence.

The North Himalaya, though more deformed, resembles the High Himalaya in the west where it marks the Mesozoic shelf edge and where reactivated Cambro-Ordovician gneiss domes werc thrust southwards during the Tertiary orogeny (Stutz and Steck, 1986; Fuchs, 1989). Here, very

A S Lesser Himalaya I No Evidence

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~ CretaceOUS

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Permian

Carbonilerous

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20,000 m

B Central Crystalline Axis

ethyan Basin l

S 1 N NO Evidence I

J

Fig. 4. Two views of the original relationships of the Lesser and High Himalaya sequences. (A) From Colchen ct al. (1982). (B) From Saxena (1981).


thick Permian to Cretaceous continental margin sequences rest unconformably on basement. However, east of lake Yamzo Yumco (due south of Lhasa), part of the Mesozoic slope and oceanic basement is preserved.

There has always been a problem in comparing the Indian Shield, Lesser Himalaya and High Himalaya sequences (the North Himalaya has only been separated recently). Each cover sequence seemed to be progressively younger northwards with little in common (Fig. 3). Fur- thermore, within each structural unit, the stratigraphy is relatively uniform in the western and central Indian Himalaya. Thus, the Muth Quartzite, a mature ?Devonian shelf sandstone, extends from Kashmir to the central Himalaya with only minor changes.

Early attempts at correlation between the Lesser and High Himalaya suggested that deposition occurred in two separate but contemporary basins (Fig. 4B), or that deposition occurred on a continental margin with a very strange configuration and facies relationships: an entirely non- marine section passed northwards into a mostly marine sequence with the facies boundary re- maining static throughout the Paleozoic (Fig. 4A). Rare fossils were supposed to indicate a Paleo- zoic to Mesozoic age for the Lesser Himalaya clastic rocks and the general rarity of fossils at- tributed to non-marine sedimentation. Few of the fossils could be recollected from their supposed locations and some of the published records turn out to be very doubtful (Tewari et al., 1985), or not to be fossils at all (Agarwhal, 1974). New finds have shown that the Lesser Himalaya sediments are mostly of late Precambrian age, and that most of the Paleozoic and Mesozoic is missing (Azmi, 1983; Singh and Rai, 1983; Kumar et al., 1987).

The main stratigraphic break in the western Himalaya occurs across the Main Central Thrust. Here, the High and North Himalaya form a coherent though deformed assemblage. The stratigraphic contrasts across the Main Central Thrust abruptly decrease east of Bhutan where the major contrast is between the crystalline rocks of the High Himalaya and the thick slope clastics of the

North Himalaya. Several explanations for these contrasts are possible.

First, crustal shortening and erosion at the front of the High Himalaya has removed a large part of the original inner shelf of the Indian passive margin. This explanation requires very large southward displacements of the order of 500 km on the Main Central Thrust (Dewey et ah, 1989) and is consistent with current models of underthrusting of the Tibetan plateau by the In- dian plate (e.g. Powell, 1986) or with estimates of late Tertiary displacements and frontal erosion at the Main Central Thrust (Besse et al., 1984; Klootwijk, 1984; Brookfield, 1989). However, simple southward thrusting is not consistent with some of the measured paleomagnetic rotations, nor with the stratigraphic changes, especially in the east.

Second, the entire High Himalaya was displaced laterally by transcurrent movement along the northern Gondwanaland margin sometime during the Mesozoic. This explanation fits with known left-lateral displacements of" up to 500 km on Cretaceous faults in southeast Asia (Metcalfe, 1984; Tapponnier et ah, 1986) and may also explain the juxtaposition of the North Himalaya Mesozoic slope and the High Himalaya basement in the northeastern Himalaya. On the other hand, this juxtaposition can be explained in other ways (see below).

Third, pieces of the original passive margin have been removed prior to Tertiary collision. The Permian rifting and separation of microcontinents off the northern Gondwanaland shelf readily explains the absence, anywhere in the Himalaya, of a thick Ordovician to Permian passive margin sequence. Also, the late Jurassic separation of a microcontinent documented off northwestern Australia (Exon et al., 1982; G6ri~r and Seng6r, 1992) can account for the juxtaposition of the High Himalaya crystalline rocks with Mesozoic oceanic rocks of the North Himalaya in the northeast.

Fourth, the present tectonic boundaries have cut obliquely across the original facies boundaries of the northern Indian passive margin. This explanation can account for similar stratigraphies

8 M}: H}~ ~()KI:I}'I i t

appearing in different structural units. For example, Permian basalts and associated sediments occur in the High Himalaya in Kashmir on the northwest, but in the Lesser Himalaya in Arunachal Pradesh on the northeast.

In the following sections of this paper, I will describe the stratigraphic sequences of the various units of the Indian and Pakistan Himalaya, with a view to tracing the evolution of the northern Indian passive margin and evaluating the various explanations noted above. However, before doing so I must mention some subsidiary problems.

Few good stratigraphic correlations are available outside the western High Himalaya. In part,

this is due to lack of good marker horizons and scarcity of fossils, but in part it is duc also 1(; unjustified, purely lithostratigraphic time correlations which have obscured the actual stratigraphic diversity of the Himalaya. For example: the Blaini Formation of the western I~esser Hi- malaya was correlated with fossiliferous Permo- Carboniferous formations to the east on the basis that they both contained glacial tillites, evcn though the Blaini Formation was unfossiliferous and even though it was otherwise lithologically unlike the Permo-Carboniferous (Brookfield, 1987).

The Phanerozoic sections have rarely been at:l equately measured since the early 20th century.

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Fig. 5. (A) Stratigraphic sections of the Indian Shield and Ganga basin. Data from: Rao, 1973; Halder and Ghosh, 1981; Pareek.

1983; Sastry and Moitra, 1984. (B) Section across plains along section line of Fig. 1 (from Rao, 1973), showing thin Upper Ter t i a~

~)n Precambrian clastics.


For example, even in the famous, easily accessible and fossiliferous rocks of the High Himalaya in Kashmir, few detailed sections with accurately located fossils have been described since Middle- miss (1910). This is now being remedied though (Srikantia and Bhargava, 1983; Baud et al., 1984; Gaetani and Garzanti, 1991).

Many of the stratigraphic records of fossils are now suspect (Talent, 1990). For this reason I have omitted all data from papers with V.J. Gupta as author or co-author.

Most of the sections described here have been checked personally in the field. The only areas I have not visited are in the northeastern Hi- malaya.

Indian Himalaya

Stratigraphic sequences in each of the four tectonic units--Indian Shield, Lesser Himalaya, High Himalaya and North Himalaya--are shown in Figs. 5-9.

Indian Shield

Thin shallow marine and continental clastics, with only a few thin limestones, rest unconformably on Archean or early Proterozoic basement (Fig. 5A). Though dating is poor, these sediments and associated volcanics appear strati- graphically older than the Proterozoic to Cam- brian sediments of the Lesser Himalaya rocks to the north (Sastry and Moitra, 1984). Exploration drilling and seismic studies within the Indo- Gangetic trough show several basins, generally with northward-thickening sequences (Fig. 5B). A marked change takes place across the family of NNE-trending faults along the Aravalli range: these separate the Vindhyan/Ganga basin from the poorly known, mostly concealed, Punjab/Ra- jasthan basin to the west.

The Vindhyan sediments consist of a thick shallow marine to continental mixed clastic- carbonate shelf sequence of probably mid-Pro- terozoic to ?Vendian age (Sastry and Moitra,

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Fig. 6. Stratigraphic sections in the Lesser Himalaya, from west to east. Data sources: Jammu (Singh, 1973; Tewari et al., 1985); Simla (Bagati, 1974; Singh and Rai, 1983); Tansen (Sakai, 1983, 1985); Bhutan (Janpangi, 1974; Gansser, 1983); Arunachal Pradesh

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Fig. 8. Stratigraphic sections of the eastern High Himalaya. Data sources: Thakkhola (Border et al,, 1971; Gradstein et al., 1989): North Everest (Yin Ji-Xiang et al., 1983); Kathmandu (Bordet et al., 1960; Kumar, 1980).

1984). The upper part shows mostly northwesterly paleocurrents (Rao and Neelakanjum, 1978; Awasthi and Parkash, 1981). These probably cor-

relate with the thick deltaic clastics of the Simla Group in the Lesser Himalaya (Kumar and Brookfield, 1987).

]2 M L ~R()OKFIt t,D

Paleozoic and Mesozoic sediments do not occur in the subsurface of the Indo-Gangetic plain. They do occur south of the Narmada-Son lineament, in nor th-west trending grabens within the exposed Indian Shield. Here, Permian deposits unconformably overlie Precambrian rocks. Above basal tillites are Permian post-glacial continental coal-bearing sediments deposited by northwesterly flowing streams (Casshyap and Tewari, 1984), with a few interbedded marine horizons of early Permian age (Dickins and Shah, 1981)~ These Permian deposits resemble those of the eastern Lesser Himalaya: the two areas may have been offset by post-Permian but pre-late Cretaceous (Deccan Basalt) right-lateral movement of 600 km on the Narmada-Son lineament (Sen, 1991).

Lesser Himalaya

Here, thick late Precambrian to Cambrian passive margin sandstones, shales and limestones are unconformably overlain by, in places, Permian glacial and continental clastics. These. in turn, are covered by widespread late Cretaceous to Eocene shallow marine carbonates and clastics (Fig. 6). In the westernmost outcrops around Jammu, Paleocene sediments rest directly on the late Precambrian Sirban limestone (Singh, 1973). Eastwards, this stratigraphic gap is reduced with the progressive insertion of other units. Thus, in the Simla area, late Cretaceous limestones rest directly on thick late Precambrian to Cambrian deposits. Further east, Permian glacial and continental sediments and volcanics, together with Mesozoic continental deposits occur in thrust slices from Garwhal to the eastern Himalaya (Sakai, 1983; Tripathi and Singh, 1987). However, in the eastern Lesser Himalaya, Lower Permian glacial sediments and overlying rift basalts are thicker, rest on mid-Proterozoic sediments and were probably deposited closer to the rift axis.

In the pile of imbricated thrust sheets of the Lesser Himalaya (Valdiya, 1980), only nappes east of Simla have rocks between Cambrian and Pale- ocene, and then only Permian and Cretaceous. If we assume simple stacking, then the restored Lesser Himalaya units would form a northward- thickening continental margin sequence. On this

margin, the hinge lines were progressively Iurthcr and further northwards from Precambrian to Mesozoic times, but with a marked southward jump in the late Cretaceous. The eastward ha. crease in late Paleozoic stratigraphic completeness is compatible with Cenozoic clockwise rotation of the Lesser Himalaya during Tertiary collision, as determined by paleomagnetic and structural studies (Klootwijk, 1984).

The post-Cambrian stratigraphic gap is related to an early Ordovician event, involving granite intrusion, metamorphism and mild detormation, which affected the High and North Himalaya a~d other areas of Gondwanaland (e.g. eastern Aus- tralia) (Fuchs, 1967; Garzanti ct al.. t98t~: l,e For~ et al., 1983).

The absence of younger Paleozoic deposits may be due to late Carboniferous glacial erosion. Enormously thick late Carboniferous elastic sediments accumulated at the late Paleoz~ic conti. nental edge in southern Tibet (Jin Yugan and Waterhouse, 1986)while the main Gondwana- land continent was being scoured b} glaciers (Powell and Veevers, 1987).

The absence of Mesozoic deposits may be ~e-- lated to pre-late Cretaceous erosion or non-deposition. In the case of erosion, thick Mesozoic clastic sedimentary sequences would be expected to the north. Such sequences exist in the east:crn North Himalaya, but not in the western High Himalaya where the Mesozoic consists dominantly of shelf carbonates. Non-deposition is more likely: Mesozoic sediments on the Indian Shield are thin, often confined to grabens and contain many erosional breaks (Casshyap, 1979)

In general terms, the west to east increase m

late Paleozoic stratigraphic completeness indi.- cates that the eastern Lesser Himalaya lay closer to the Permian rift axis and hence to the site ~t continental splitting. Curiously, Permian sections similar to those of the eastern Lesser Himalaya are found only at the opposite end of the range, in the western High Himalaya of Kashmir.

High Himalaya

The High Himalaya Phanerozoic sections occur in two areas separated by the extension of the

H I M A L A Y A N PASSIVE M A R G I N F R O M P R E C A M B R I A N TO C R E T A C E O U S 13

Karakorum fault--an entirely late Miocene to Recent feature (Brookfield, 1992) (Figs. 7 and 8). Though generally similar, these two regions have several stratigraphic differences. In the western High Himalaya, Upper Ordovician continental clastics rest unconformably on Lower Ordovician and older sediments. In the eastern High Hi- malaya, possibly Lower Ordovician clastics pass upwards into thick Ordovician limestones without an obvious break, though there is usually a Ter- tiary shear zone below the limestones. The overlying Silurian to Permian shelf sequences are similar but thicker in the east. East of Bhutan, Phanerozoic sediments are either missing or metamorphosed to greenschists or higher facies.

Precambrian events are only dimly discernable through the Tertiary metamorphic overprint. Be- cause Tertiary metamorphism progressively reaches higher towards the east, Precambrian events are easier to study in the west. In the western High Himalaya, Precambrian rocks giving occasional ages of 1800 Ma (Trivedi et al., 1984) form a basement to thick late Precambrian to Cambrian clastic sediments overlain by thin mid- Paleozoic shelf, variable Permian, and thick Mesozoic shelf sediments (Fig. 7).

The late Precambrian to Cambrian sediments are dominantly continental shelf and slope clastics equivalent to the deltaic and shallow marine deposits of the Indian Shield and Lesser Hi- malaya (Garzanti et al., 1986). After a Cambro- Ordovician deformation phase marked by granite intrusion, deformation, mild metamorphism, uplift, and coarse Ordovician conglomerates above an unconformity, only thin shelf sediments were deposited until the Permian Le Fort, 1988; Bagati, 1991).

During the early Permian, the change from thin shelf deposits to thick basalts and then thick shelf limestones represents the onset of rifting of microcontinents off the northern Gondwanaland margin (Kapoor and Nakazawa, 1981; Seng6r, 1984). The resulting horst-graben pattern con- trolled the preservation of pre-Permian deposits and the deposition of Permian sediments. Thus, the pre-Permian stratigraphic sequences of the High Himalaya vary markedly over small areas,

though generally becoming thicker and more complete eastwards (Fig. 7). The actual break-up unconformity is marked by the onset of uniform starved shelf, phosphatic sedimentation in the Upper Permian (Gaetani and Garzanti, 1991).

In the eastern Himalaya, the sequences are strongly metamorphosed up to the Ordovician limestone and only in the frontal nappes, as at Kathmandu, can the pre-Ordovician sequence be deciphered (Fig. 8). The sequences are similar to, but thicker than, those of the western High Hi- malaya. Furthermore, from Thakkhola to North Everest the entire Phanerozoic section thickens greatly.

East of Sikkim, the Upper Paleozoic rests directly on metamorphic rocks, as in the Lesser Himalaya (Zhou Xiang et al., 1984), but there may be a major structural break in this practically unknown region.

The High Himalaya contains two passive margin megasequences that start with rifting events in the Permian and in the early Cretaceous (Gaetani and Garzanti, 1991). Both events are marked by a decrease in subsidence rates (and in places uplift) throughout the High Himalaya. The Permian marks rifting and separation of microcontinents off northern Gondwanaland. The early Cretaceous marks rifting and separation of India from Australia (Powell et al., 1988) and is marked by the eruption of the Rajmahal and related flood basalts on the northeastern Indian Shield and by abundant basaltic grains in High Himalaya shelf sandstones (G6riir, 1991).

During the late Cretaceous and early Eocene, thin shelf carbonates cover everything, though ephemeral shelf basins developed at times in the late Cretaceous and Paleocene related to tectonic events at the northern Indian margin (Brookfield, 1977; Searle et al., 1988). Like the Lesser Hi- malaya, the High Himalaya shows an eastward increase in stratigraphic completeness which is compatible with clockwise rotation during Ter- tiary nappe displacements--until the easternmost High Himalaya. Here, only metamorphic basement occurs and this is directly overthrust from the north by the oceanic Mesozoic sequences of the eastern North Himalaya.

[4 ~,'11 t~N.~)()KL'ID1D

North Himalaya

The North Himalaya is the shelf edge and slope of the Mesozoic northern Indian passive margin and consists of Tertiary gneiss domes reworking Cambro-Ordovician granitoid intrusions and thick Mesozoic passive margin clastic sediments. The North Himalaya has been highly deformed in several phases and thrust southwards over the High Himalaya. So far, the unit has only been studied in detail in Ladakh in the western Himalaya and near Xigatse in the central Himalaya (Debon et al., 1981; Burg and Chen. 1984; Burg et al., 1984; Stutz and Steck, 1986). East of Lake Yamzo Yumco in Tibet, the enormously thick section apparently rests on oceanic crust (Tibetan Geological Survey, 1981) (Fig. 9).

Though sections shown in Fig. 9 arc tentatiw:, they are sufficient to show the similarity of the High and North Himalaya sequences in the Pale- ozoic and their distinctness from the Mesozoic onwards. They also show a west to east change from thin shelf sediments to thick slope sediments representing the progressive ~weanward built-out of a Mesozoic passive margin,

In the western and central North Himalaya, Permian carbonates rest, in places, directly on Cambro-Ordovician gneiss domes, The gneiss domes are the reactivate edge of the Mesozoic continental crust and are overlain by thick continental edge Mesozoic clastics. At Kangmar, these sediments are overlain by olistostromes whose matrices contain fossils up to late Cretaceous m age (Qian Dingyu et al., 1982; Wang l,ianeheng,

1 2 NIMALING KANGMAR

Km Km 5 ~ & U,Cret. 6 i;'~ ;.',~,:~,~o~ U,Crel.-

I..'T.-.).. L.CreL I"o~,~:O \~, o'l Paleo. 4 5 "_22"" 'l . . . . . . .

Jur. ~ LCret.

--32 ~ ~ , o ~U~., "-"- 4 m Jur.

\

?

2, g

3 YAMZQ YUMCO

Km30_ ~ Paleogene

• U.CreL e8

(~ L.Cret,

21 ~ M.-U.Jur.

18 L.Jur.

U.Trias.

ophiolitic

0 ~

(note scale change)

Fig. 9. Stratigraphic sections of the North Himalaya. Data sources: Nimaling (Baud et al., 1984; Stutz and Steck, 198~): Kangma~ (Wang Yi-Gang et al., 1981: He Kezhao, 1982: Yin Ji-Xiang et a l , 1983; Chen et al., 1990): Yamzo Yumco (Tihet~l~ (}eologic~t

Survey 1~81k


1982). These olistostromes are probably related to late Cretaceous emplacement of ophiolites in

an oceanic setting prior to collision (Brookfield, 1977; Searle, 1983). Gneiss domes with their Pa- leozoic shelf cover disappear east of Kangmar.

Around lake Yamzo Yumco and eastwards over 20 km of Mesozoic passive margin deposits, with Upper Jurassic basalts, are thrust southwards over the Proterozoic rocks of the High Himalaya (Tibetan Geological Survey, 1981). U1-

trabasic masses extend almost to the southern thrust contact (Liu Zhengqian, 1988). These pas-

sive margin deposits rest on ophiolitic melanges

and were probably deposited on oceanic crust; they thin northwards.

From west to east, the North Himalaya is an oblique section through the northern Indian Mesozoic passive margin. It is apparent that, in the west, this margin has been obliquely overthrust and truncated by the Indus Suture Zone. Post-collision displacements on the Indus Suture Zone are thus greater in the west than in the east. This is the opposite to displacement on the thrust bounding the North and High Himalaya,

A B SURGHAR KALA CHITTA

RANGE RANGE

k 5 ~ M i o c e n e

cret_

2 P

011-I ° "

South

c D

ATTOCK - CHERAT RANGE KHYBER- PESHAWAR

Central North

~. r , 7 - , ~ C a r b "

ABBQETABAD F km TARBELA

- - P a l e o z o l c

• " . . - . . . . -..!...

Main Boundary thrust _ ~ P r o t

Fig. ]0. Stratigraphi¢ sections of the Pakistan Himalaya. Map and sections A to C, based on Yeats and Hussein (1987); D, from

Shah eta| . (]980); E, F, from Mart in ct aL (]962), Calkins et a]. (1975).

16 Mt: tH~t)t)KFIELi .~

where, from the facies distributions, the displacements are smaller in the west than in the east.

Conclusions

The High and North Himalaya have very little in common with the Lesser Himalaya. The only time periods represented in all three are the late Precambrian-Cambrian, Permian and late Creta- ceous-Eocene. We can therefore use only these time periods as marker horizons for working out the relationships of the Himalaya units. For com- parison with the Indian Shield we are reduced to only the Permian for certain.

The High and North Himalaya currently form a (highly deformed) northeastward-facing Meso- zoic passive margin which has been obliquely truncated by the southward emplacement of the Indus Suture Zone. Thus, only in the northeast is the Mesozoic slope and basin preserved. In the northwest, these units have been overridden ~ind destroyed.

The displacement of the High Himalaya over the Lesser Himalaya is in excess of 500 km and the facies relationships are now incompatible due to the loss of the entire Mesozoic inner shelf.

Westwards, however, across the Nanga Par- ba t -Haramosh massif, the sequences in the Pak- istan Himalaya are more easily traced across the various structural units, partly due to smaller displacements across a larger number of faults.

Pakistan Himalaya

The Pakistan Himalaya stretches between thc Raikot fault and extensions and the Sarobi fault (Fig. 2). Until recently, the structural units of the Indian Himalaya were extended across the northwestern syntaxis (e.g. Gansser, 1964; Le Fort, 1989). However, recent work in Pakistan has shown that neither the stratigraphy nor the structure is similar to the Indian Himalaya (Yeats and Lawrence, 1984; Coward et al., 1987). For this reason I have not used the terms 'Lesser' and 'High' Himalaya in Pakistan. Many more structural units are distinguishable in Pakistan where deformation seems to be distributed over more nappes showing less stratigraphic contrast: than in

the Indian Himalaya. The Pakistan Himalaya marks the transition from the Phanerozoic northern Gondwana passive margin exemplified by the Indian Himalaya to the entirely Mesozoic western lndian passive margin blocked out by rifting only in the Jurassic.

Shield

Most of the shield is buried in the Punjab/ Rajasthan basin west of the Aravalli range (Fig. 1). The late Precambrian to Paleozoic stratigraphy of both the northern Salt Rangc and the southern Rajasthan sectors is comparable with that of the High Himalaya and Lesser Himalaya to the east but unlike that of the Ganga basin (Fig. 10; section A, Surghar Range). This is the first example of a stratigraphic sequence which is found in Himalayan nappes to the east. but m a more southerly structural unit to the wcst. in this case on the shield.

In the north, the shield sections arc well-exposed in the Salt Range, a parautochthonous unit with relatively little southward displacement (Gce~ 1989). Above Cambrian clastics, mixed marine- continental sedimentation started in dac Upper Permian and the overlying Mesozoic stratigraphic sequences are like those of the High l-hmalaya in India, though with more continental intercala- tions (Gee, 1989). The major bounding tmcon- fortuities of the Salt Range show particularly the eastward onlap of the Paleocene and youngcr sequences (Fig. 11).

'SALT RANG~ y x

West i :~

JUR'-CRET

P E R M I A N - UPPER TRIASSIC

CAMBRIAN

Fig. l l. U n c o n f o r m i t i e s f rom west to ca s t in the Sail R a n g e :

loca t ion o n Fig. 10. F r o m G e e (1989!

HIMALAYAN PASSIVE MARGIN FROM PRECAMBR1AN TO CRETACEOUS 17

In the south, surface exposures are limited, but, like parts of the Lesser Himalaya in India, the Punjab/Rajas than sequences show late Pre-

cambrian to Cambrian clastics unconformably overlain by Permian glacial and post-glacial clastics. In Rajasthan, these are overlain by non- marine Triassic to early Jurassic clastic sediments and by mid-Jurassic and younger marine sediments marking the start of development and subsidence of the western Indian passive margin (Pareek, 1983).

Kala Chitta and southern Hazara

Stratigraphies are similar from the Salt Range northwards into the Potwar plateau and across the Murree and 'Main Boundary' thrusts into the Kala Chitta and southern Hazara ranges (Fig. 10; section B). The late Jurassic to Eocene sequences in southern Hazara are almost identical to those of the High Himalaya in India (Latif, 1970). Whereas the Proterozoic to mid-Jurassic sections are like those of the Central Lesser Himalaya (compare section E, Fig. 10 with section 2, Fig. 6). Mesozoic sections tend to become thicker and more marine westwards towards the western In- dian passive margin. The entire pre-Cenozoic sequence is truncated and overlapped to the east by the Paleocene Lockhart limestone.

Attock-Cherat and northern Hazara ranges

The Paleocene overlap continues northwards across the Hissartang and Panjal faults into the At tock-Chera t and northern Hazara ranges. Here, the pre-Paleocene sections are very different to those on the south (Fig. 10). The sections are poorly known because of lack of fossils but the sequences are somewhat similar to those in the frontal nappes of the Indian High Himalaya (Fig. 10, sections D and F). In the east, the Cambrian Mansehra and associated granitoids in- trude late Precambrian to Cambrian clastic sequences, including tillites, and are overlain by ?Paleogene clastics. Westwards, recent work in the Peshawar basin indicates the presence of an almost complete Paleozoic section similar to that of the Indian western High Himalaya. Thick late

Precambrian to Cambrian clastics, including a tillite, are overlain by Ordovician quartzites, Sil-

urian to Devonian shales and carbonates, Car- boniferous conglomerates, Permian greenschists and thick Triassic-Jurassic carbonates (Shah et al., 1980; Pogue et al., 1992).

The At tock-Chera t and northern Hazara ranges are overthrust from the north, along the Main Mantle Thrust, by the Kohistan complex. This is a Cretaceous island arc marking the western continuation of the Dras arc which overthrusts the western Indian High Himalaya in Ladakh (Searle et al., 1987).

Further west these ranges are truncated by southeastward thrusting of the Waziristan and Nuristan blocks, the latter consisting of Asian continental crust thrust directly over the Indian

crust of the Pakistan ranges with the Kohistan arc and Indus Suture Zone overthrust and obliterated.

Movement on the Hissartang and related faults must predate the Paleocene which truncates all earlier sections. Since these sections do not con- sistently get older to the north, the faults are not strictly thrust faults: some must have significant dip-slip a n d / o r strike slip components. There are great and abrupt changes in pre-Paleocene stratigraphy across these faults, particularly across the Hissartang, Manki and Panjal faults (Fig. 10). Together with the apparently uniform and consistent overlap of the Paleocene, this suggests major pre-Paleocene displacements. Thrusting would involve erosion of uplifted blocks. So, in the apparent absence of coarse Mesozoic clastics, the motion on the faults was probably dominantly strike-slip.

Conclusions

The Pakistan Himalaya contains analogous sequences to those in the Lesser and High Hi- malaya units of India, but in different structural units. Thus, the pre-Permian of the Salt and Kala Chitta ranges is equivalent to the Indian Lesser Himalaya. However, the former are essentially the Pakistan shelf cover thrust relatively small distances southwards along decollement surfaces, and not major nappes like the Indian Lesser

l~ N I l P;R()()K} IF~I I) Himalaya. Northwards, the pre-Permian sequences of the Attock-Cherat and northern Haz- ara ranges resemble those of the Indian High Himalaya and may have been displaced equally far south. The Permian to Eocene sections are everywhere like those of the Indian western High Himalaya. There is no sign of a post-Permian passive margin equivalent to the North Himalaya. The southward thrusting of the Indus Suture Zone thus cuts across the facies belts of the Himalaya and is oblique to the original strike along the entire northern Indian margin.

The eastward onlap of the Mesozoic in Pak- istan shows that the formation of the western Indian margin also cut obliquely northeastwards across the northern Gondwanaland shelf, though this is pretty obvious from reconstructions in any case.

Continental margin reconstructions

Before we can evaluate the sedimentary and subsidence history of the northern Himalayan passive margin, we must try and reconstruct the possible relationships of the structural units before Tertiary deformation and southward thrusting.

The first requirement is to try and remove the enormous cumulative southward thrusting, clockwise rotation and frontal erosion of the Himalaya in the late Tertiary. In the northwest, India has indented Asia by around 2,400 km since the start of collision around 50 Ma (Dewey et al., 1989). Much of this shortening was taken up by com- pression of arcs and intra-arc basins between India and Asia (Brookfield, 1992), but at least 1000 km was due to crustal shortening within the collision zone since the early Miocene (20 Ma) (Savostin et al., 1986).

Restoring the original pre-collision margin in- volves removing this 1000 km shortening and in- volves the following (Fig. 12).

(1) Removal of approximately 500 km of cumulative southward thrusting of the High Himalaya relative to the Indian Shield inferred on structural grounds in Pakistan (Coward et al., 1987). A further 500 km occurs within and along the northern edge of the Pamir (Brookfield, 1992).

.,? +,,%: j "

~ ~ ¢ e a s l of fault ](i!

Fig. 12. Cartoon restoration of the northern Indium passive

margin before Lower Cretaceous break-up. Times of spread-

ing are shown with direction of spreading; rifting started

earlier. Data from: Rabinowitz et al., 1983; Powell et al., 198,~:

Scotese et al., 1988. Also shown are: areas removed by erosion

at the front of the High Himalaya during southward thrusting,

paleomagnetically determined southward movemcat of tile

High Himalaya thrust sheets (from Klootwijk, [984); lault

separating High Himalaya with thick Phanerozoic shelf st:

quences from areas to the east with only Permian and younge~

deposits. Inferred pre-collision location of the main tectonic

units shown: P.H. = Pakistan Himalaya; Lit.-= Indian ffigh

Himalaya; L . H . - Lesser [ t imataya

The progressive westward truncation and disap- pearance of the North Himalayan unit shows that large overthrusts have taken place along the In- dus Suture Zone and Main Mantle Thrust m Pakistan. The bulk of the southward displacement apparently progressively shifts to the Mare Central and Main Boundary Thrust in the India Himalaya (Molnar, 1984). Since the Me~zoic passive margin is preserved m the northeastern North Himalaya and is juxtaposed with High Hi- malayan basement, then almost the entire 1000 km of shortening in the eastern Himalaya must have occurred along the North Himalayan, Main Central and Main Boundary thrusts.

(2) Removal of at least 50% internal shortening of the Mesozoic passive margin by restoring the stacked continental edge back to its original thickness. In the northwestern Himalaya in Zan- skar, near the original continental edge, the measured shortening is 60%, from 250 to 100 km (Searle et al., 1988). This shortening apparently

H I M A L A Y A N PASSIVE M A R G I N F R O M P R E C A M B R I A N T O C R E T A C E O U S 19

decreases eastwards or is transferred to the Main Central and Main Boundary thrusts on the south.

(3) Removal of the oroclinal bending of the Himalayan arc inferred from paleomagnetism (Klootwijk et al., 1986).

(4) Restoration of the approximately 250 km of crust and cover removed by erosion from the front of the High Himalaya since the early Miocene (Brookfield, 1989). This erosion partly explains the strong stratigraphic contrast between the High Himalaya and areas to the south. The entire inner shelf of the northern Indian passive margin has been removed as can be seen by showing the equivalent amount of erosion on an analogous modern passive margin (Fig. 13).

The second requirement is to constrain the outline of the northeastern Indian margin, now highly distorted in the eastern syntaxis. This can be done by placing India in its pre-drift position off Antarctica and Australia. The Australian margin is of most interest here. Spreading oc-

curred off northwestern Australia in the Upper Jurassic (around 155 Ma) and off western Aus- tralia in the Lower Cretaceous (around 125 Ma) (Powell et al., 1988; G6riir and Seng6r, 1992). The latter blocked out the eastern Indian margin and the transform offsets suggest that the northeastern Indian margin formed a saw-tooth pattern (Royer and Sandwell, 1989). The result shows a possible configuration and paleogeography for the northern Indian margin before the start of collision in mid-Eocene times (Fig. 12). Until Mesozoic fragmentation, the northern Indian margin apparently ran roughly east-west between Australia and Arabia as suggested by Crawford (1974) and discussed in more detail by Veevers et al. (1975) and G6riir and Seng6r (1992).

These continental outlines can be used as a basis for relating the sedimentary and subsidence histories of the northern Indian margin to the major events affecting northern Gondwanaland. Four main rifting periods mark major changes in

I Lesser I removed by High Himalaya I Nor th H ima laya

H ima laya t frontal I erosion Hinge zone

' ', I Coastal plain .. : . ~1 Continenlal shelf + Cl°~tine ntal

I -.,=--- Fall line I | Eocene Cost Late ' I B, /O,igocene MSL I :.;~1~, ,Tz ~ . i . . . . . . . . . . :.'1: ". : - " '. : " ' . . Plio-Pleistocene

%%'k.. "~ I ~ar~ I ~~.,'." " . 'X~'~ Early ' : .!, ' " ~'Oligocene

I . .

'!it i \ 0

km 1 1 km ~:~I | J~r~

Vet L exag., ~3y~- I~ " -"tO ,/~=~ ""'~" " 30:1 3'~.~ k / ~ -

"~- ooo

Unstretched continental crust ~ Slretched crust -', ","~'.~:'- ~Z I Oceanic crust

Fig. 13. Eastern Atlantic passive margin (from Sheridan, 1976) showing analogous positions for tectonic units of northern Indian

passive margin and analogous region lost by frontal erosion at the Main Central Thrust .

20 MI; f31,~OOKFIEI l)

the evolution of the northern Indian passive margin. A late Precambrian rift was followed by early Ordovician deformation and formation of a stable shelf. In the Phanerozoic three rifting episodes variously affect the northwestern, northeastern or entire northern margin depending on the location of the rifts. However, since common stratigraphic units are found only in the Upper Cretaceous- Eocene, Permian and Upper Precambrian- Cambrian sediments, these times must form the basis of reconstructions.

(1) The Lower Cretaceous to Eocene marks the separation of India from Australia and the northward movement of India. These events are most marked in the poorly studied northeastern Himalaya. In the northeastern North Himalaya thick Lower Cretaceous volcaniclastic sandstones overlie Upper Jurassic basalts (these Upper Jurassic volcanics may be related to the separation of a microcontinent off northwestern Aus- tralian around 155 Ma as noted above). Lower Cretaceous rift volcanics occur in northeastern India. To the west, as far as Ladakh, the break-up unconformity is marked by starved Cenomanian phosphatic sediments above volcaniclastic Ap-

tian-Albian quartz sandstones (Garzanti et at., 1987). However, only Upper Cretaceous limestones are represented in all Himalaya tectonic units and may be related to the most widespread of the Cretaceous marine transgressions.

During the late Cretaceous-Eocene India was drifting northwards and its position is reasonably well-constrained by both oceanic anomalies and paleomagnetism (Klootwijk, 1984; Dewey et al., 1989). During this period a consistent picture of a subsiding continental margin can be reconstructed (Fig. 14). In the Indian Himalaya, the late Cretaceous deep water, outer shelf or slope Chikkim Limestone of the High Himalaya passes southwards into the shallow water Upper Creta- ceous Tal Shell Limestone of the Lesser Hi- malaya. An analogous change occurs in the Pak- istan sector. During the late Cretaceous to early Eocene, a very consistent facies pattern occurs along the western part of the northern Indian margin. Here, minor shelf edge uplifts periodically shed clastic sediments into early foreland basins occupying the High and Lesser Himalaya and the Indian Shield (Samanta, 1968; Wells, 1983). These basins are most obvious in Pakistan

\ _~_ Asian arc /

^

equator ~

v ) 7 ) ' .or,,.lma,ay.

t . ~" i ~ . / ~ Lesser Himalaya

)'Z°~'~ / I ~' /". no information / . - ' . ' l t~,~ ~ ! j - " - . . . . . . . . . J

/ / , J . . " ' f ( . . 7 t . 7 " ..r~:: : spreading /. " " ~ / 8 5 MZ t /~, ),?.'~"~155 Ma .: .-. / ~ ' ~ " "'L':I ~ -~'~" (~125 Ma

66.2 Ma ~ - - . _ ~ = Chron 29 . ,

Basal P a l e o c e n e ~ ,

Fig. 14. Paleogeographic reconstruction for the earliest Paleocene (location from Besse and Courtillot, 1988)


where they lie across the Hissartang and other faults. Eastwards, the basins have been progressively destroyed by southward thrusting and erosion on the Himalayan frontal thrusts (Sahni and Kumar, 1974) and they disappear before Nepal. The reason for these shelf edge uplifts is contro- versial. Some people think that late Cretaceous- Paleocene events are related to ophiolite emplacement in an oceanic setting (Brookfield, 1977; Searle, 1983). Others think that ophiolite emplacement did not commence until the Eocene (Colchen et al., 1986). However, since India lay isolated in the centre of the Indian Ocean, any changes have to be related to eustasy and/or oceanic tectonics. Paleocene arcs lay just north of the Indian margin at this time (Brookfield, 1992). The start of shelf edge uplift corresponds with major changes in spreading in the Indian Ocean around 75 Ma (Royer et al., 1988).

Despite the shelf edge uplifts, a consistent pattern of Upper Cretaceous-Lower Eocene shelf sedimentation can be reconstructed across the northern Indian passive margin, even with the disruption of large displacements and frontal erosion on the Himalayan thrusts. Since no such reconstructions can be made for earlier periods, this suggests that simple Tertiary thrusting is not

adequate to entirely account for the pre-Creta- ceous passive margin configurations.

(2) The Upper Triassic to Lower Cretaceous marks the rifting of India from Africa and the formation of the western Indian passive margin. These events should be most marked in the northwest Himalaya and in Pakistan; however, in the Pakistan Himalaya, the Mesozoic is thin or absent except in the extreme west (Fig. 10). In Ladakh, north of the Mesozoic shelf along the southern edge of the Indus Suture Zone, Upper Triassic alkaline lavas are overlain by thick Juras- sic passive margin clastics (Honegger et al., 1982) (Fig. 6, section 4). On the western Indian High Himalaya shelf, the Triassic and Jurassic shelf sequences generally thin towards the east (Fig. 7). However, Upper Triassic basalts and thick Trias- sic to Lower Cretaceous clastics are also found at Kangmar in the central North Himalaya, north of the inordinately thick Triassic-Jurassic sequence of North Everest (Fig. 8). Both these localities are presently a long way east of the site of Africa/India rifting. Mesozoic sediments are ba- sically confined to the North and High Himalaya and Indian Shield. These problems are discussed more fully in (3) below, since no reasonable reconstruction can be made for this period because

LOWER P'=RMIA. J

/~,'~ leoZOiC paSsive margin remOV.rdt~ng """ ..... : A/ ~. Pa Permian, TriassiC, jurasSic .... "" ~ ~ • . . ;

I . . . . . . I 1 ( ~ ) ) S ~ o n ~nonmarine I ~ ~ / (I [ ] rift basalts

Fig. ]5. Palcogcographic reconstruction for the Permian. Additional data from: Murris, ]980; Dickins and Shah, ]981; Wopfncr, 1981; Veevers , 1984.

22 : : l , t . I ] R ( . } ~ . ) K t . l t I .I )

there are practically no Mesozoic deposits either in the Lesser Himalaya or on the northern Indian Shield.

(3) The Permian to Upper Triassic marks the rifting of the northern Indian margin while it formed an integral part of Gondwanaland. Dur- ing this period, rifting and separation of microcontinents off the northern Indian margin cre- ated the Neotethys ocean (Seng6r, 1984) and transformed the interior shelf environment of the Himalaya into a subsiding passive margin. The break-up unconformity is marked in the High Himalaya by the transgression of latest Permian phosphatic sandstones across the grabens (Gaetani and Garzanti, 1991).

Evidence for Permian rifting is found across the entire Himalaya and into the shield (Figs. 6-10) and this period represents perhaps the most significant change along the northern Indian margin prior to Tertiary collision (Fig. 15).

The Indian Shield, south of the Narmada-Son lineament, has rifts containing continental deposits with thin marine interbeds (Casshyap, 1979). These are overlain by thick non-marine Mesozoic sediments which are mostly absent in the Lesser Himalaya, where only a few localities have evidence for even thin non-marine Mesozoic sediments (Sakai, 1983).

The Lesser Himalaya formed a stable non-depositional shelf after the early Ordovician. Any hinge line of the mid-Paleozoic northern Indian passive margin must have been far to the north of the Lesser Himalaya. Post-Cambrian deposition only started with the start of Permian rifting. Despite, however, the rapid subsidence and thick marine sedimentary accumulation recorded in the High Himalaya Permian to Cretaceous to the north, only thin Permian and late Cretaceous sediments were deposited in the Lesser Hi- malaya. In only a few areas in the central and eastern Himalaya do we see thick continental Permian sediments, and these are more like the Indian Shield graben sequences than they are to the thick High Himalaya marine sequences. The Permian progressively disappears westwards below the transgressive Upper Cretaceous (Fig. 6).

In the High Himalaya the differential preservation of Ordovician to Carboniferous sections is

due to Permian rifting and graben formation. "Fhe Permian rifts, basalt volcanics and coarse sediments lie across the Ordovician to early Permian shelf deposits, while the Paleozoic passivc margin has now been removed to southern Afghanistan. the Central Pamir and Tibet (Brookfietd, 1992). In Kashmir, very rapid Permian facic~ changcs can be detected over distances of several tens ol kilometres and pre-Permian erosion has bevelled the Spiti stratigraphic sequence down to the De- vonian in places (Fuchs, 1982a; Ciaetani and Garzanti, 1991). The Permian to Cretaceous facies contrast between the Lesser and ttigh Hi- malaya imply a wide late Paleozoic separation. In part this may be due to large Tertiary thrust displacements, frontal erosion and clockwise rotation of the Indian High Himalaya (Fig. 12). But. there are a number of problems with this.

First, the stratigraphic contrast between the Lesser and High Himalaya is much greater m the Permian than in the late Cretaceous to Eocene times. This implies Mesozoic fault displacements.

Second, Permian rift sequences occur in the western High Himalaya, in the eastern l.esser Himalaya and in the Indian Shield south of the Narmada-Son lineament. They do not occur m intervening areas and the present distribution forms a weird zig-zag pattern (Fig. 15i.

Third, in the Lesser Himalaya, thc Permian t~ Cretaceous sections are progressively over- stepped northwestwards below the Upper Creta- ceous, towards the Mesozoic passive margin---the reverse of what it should be if Tertiary southward thrusting was the only control on facies distributions.

These factors indicate that post-Permian but pre-late Cretaceous displacements have occurred between the various northern Indian units. Due to Tertiary deformation it is difficult |o refer the amount and sense of movement on the northern Indian margin itself.

Nevertheless, large left-lateral displacements of several hundred kilometres occurred m sl~i Asia during the Mesozoic (Brookfield, 1q92) and may also have affected the northern Indian margin. Simply juxtaposing the almost identical western High Himalaya Kashmir sequence and the eastern Lesser Himalaya Arunachal Pradesh se-

HIMALAYAN PASSIVE MARGIN FROM PRECAMBRIAN TO CRETACt 'OUS 23

quence gives a left-lateral displacement of about

2,000 kin. Sen (1991) with similar methods infers 600 km of Mesozoic right-lateral movement on

the Narmada-Son lineament. In the eastern High Himalaya, across a north-

easterly trending fault east of Kangmar, the Per- mian rests unconformably on metamorphic rocks and is overthrust by the North Himalaya consisting of thick Mesozoic clastics, including ophiolitic slices. In fact, east of Kangmar, the characteristic western High and North Himalaya stratigraphies are absent. One explanation for this sudden change is related to the pattern of early Creta- ceous spreading off western Australia (Fig. 13). The transform offsets of the Australian margin fit into a large re-entrant of the northeastern Hi- malaya. Due to early Cretaceous spreading, the entire northeastern Himalaya would consist of oceanic crust juxtaposed with the truncated Per- mian to Cretaceous passive margin. During Ter- tiary thrusting this re-entrant would preserve the northern Himalaya passive margin slope and basin sequences which to the west have been practically obliterated by subduction and erosion during southward emplacement of the southern Tibetan Tertiary magmatic arc. But, the passive margin slope sequence starts with the Upper Triassic: it should start with the Lower Cretaceous if it is simply the fill of the re-entrant. So, if the re-entrant model is correct then lateral movements must have moved the Upper Triassic and younger passive margin sequence to juxtapose it with the re-entrant.

This early Cretaceous (around 110 Ma) phase marks a major reorganization in spreading patterns in the Indian ocean. Madagascar and India become attached to Africa and spreading shifts to the eastern Indian Ocean between India, Aus- tralia and Antarctica (Besse and Courtillot, 1988).

(4) A late Precambrian rifting episode is apparent in the Himalaya, but has been obscured by Cambro-Ordovician and Tertiary deformation. This period is difficult to reconstruct, mainly because we do not know the nature and significance of the Cambro-Ordovician (500 Ma) intrusive, metamorphic and deformation event. Was this event a collision or continental edge orogeny, or was it simply an internal thermal event? Never-

theless, since the late Precambrian to Cambrian

is represented in three tectonic units (Shield, Lesser and High Himalaya) it is possible to make

some attempt at reconstruction. During this period, the Lesser and High Hi-

malaya formed a rapidly subsiding passive margin on which thick clastic and carbonate wedges were deposited. This margin formed in the late Pro- terozoic by rifting and prograded, like the late Proterozoic western Canadian Rocky Mountain margin, by clastic deposition in large delta com- plexes (Devlin et al., 1985). A modern analogy is the present Mississippi delta building out over thinned continental and oceanic crust in the Gulf of Mexico.

In many areas of the Lesser Himalaya, there is evidence of late Proterozoic rifting and even some evidence for possible oceanic crust since thick basaltic pillow lavas occur at the bases of some sequences (Raza, 1981; Bhat, 1987). In the Simla area, very rapid fault-block subsidence occurred in late Proterozoic times. Continental sediments and lavas overlie a carbonate platform sequence and are followed by deeper water clastic sediments with thick carbonate breccias being shed off fault scarps into the basins (Kumar and Brookfield, 1987). After deposition of the Blaini glacial sequence, the margin became a stable subsiding shelf with the deposition of the thick Krol Limestone and succeeding Cambrian Tal Quartzites (Kumar et al., 1987) (Fig. 6).

These Lesser Himalaya late Precambrian- Cambrian sequences are very thick, always ex- ceeding 6 km, along the entire length of the Himalaya. They are much thicker than any possible Indian Shield equivalents in the Vindhyans (Barman and Verma, 1982) (Fig. 5). And this great increase in thickness takes place abruptly across the Main Boundary Thrust (Sakai, 1985). In both Pakistan and India, the Cambrian sections abruptly change from dominantly clastic and phosphatic in the Himalaya to dominantly carbonate and evaporitic on the Indian Shield. The shield sections form part of the extensive Cam- brian evaporitic shelf extending across the Ara- bian shelf (Virdi, 1990). One explanation for this is that the Main Boundary Thrust is a reactivated late Precambrian normal fault (see also Bhat,

2 4 M ~ PlI~()OKFIIi l ] ~

1984). Another, not incompatible explanation is that large thrust and/or strike-slip displacements (comparable to those on the Main Central Thrust) also occurred on the Main Boundary Thrust. Such displacements are very likely in view of the startling contrast between the Lesser Himalaya and Indian Shield subsurface late Precambrian stratigraphy.

The Cambrian phosphatic deposits at the top of the Krol Limestone and at the base of the succeeding Tal Formation allow a tie in with the High Himalaya sequences. Though in places metamorphosed, in Spiti and elsewhere, thick Cambrian deltaic clastics show progradation of the northern Indian passive margin in the High Himalaya, while the Lesser Himalaya had already evolved into a shallow stable shelf area.

In the High Himalaya, coarse continental and shallow marine Ordovician deposits lie unconformably on the deformed late Proterozoic to Cambrian sediments (Le Fort et al., 1983). This widespread Cambro-Ordovician episode is found throughout the High and North Himalaya and in the northern Hazara unit of Pakistan. It may be compared with other 'Hercynian'-type events whose significance is also debatable; for example, the Silurian-Devonian granitoid intrusions of the Southern Uplands of Scotland (Halliday et al., 1980). Nevertheless, this Cambro-Ordovician episode seems to have stabilized the northern Indian margin which remained a very stable shelf, with only thin sedimentary accumulations, throughout the mid-Paleozoic (cf. Garzanti et al., 1986). There is no sign in any of the tectonic units of the edge of the mid-Paleozoic passive margin: they have presumably been removed by Permian rifting.

The various events which have formed and modified the northern Indian passive margin should be reflected in sedimentation rates and subsidence of the margin. These should provide further information on, and confirmation of, the inferred tectonic events.

Sedimentation rates and subsidence

Sedimentation rates for the various sections can easily be worked out for various time inter-

vals. Since most of the sediments were deposited near mean sea-level, thick sedimentary accumulations necessitate subsidence in excess of that pro- vided by sediment loading. Studies in the past twenty years have shown that the subsidence of passive margins can be related to cooling of hot upwelling asthenosphere after early rifting has declined (Mackenzie, 1978). This model accounts for the fact that many continental margin basins subside exponentially with the same time con~ stant as oceanic lithosphere, since both result from the cooling of lithosphere of the same thickness (Mackenzie, 1978). Such models have been refined to take into account variations in lithosphere thickness, behaviour, eustatic sea-level changes, paleodepth, and compaction (summaI.~ in Jackson and Galloway, 1984), and basin modelling can now be very sophisticated (e.g. Watts and Thorne, 1984), especially when detailed stratigraphy and sedimentology are available (e.g. Bond and Kominz, 1984).

Two main factors control the subsidence of passive continental margins. First, cooling causes the lithosphere to subside; and second, sedimentation loads the lithosphere allowing more sediment to be deposited. The subsidence due to cooling can be determined if the effects of sedimentation, water paleodepth, eustatic sea-level changes and compaction are removed. According to Keen (1979), who omits compaction, the tectonic subsidence (Y) is given by:

(Pro - p,~)S pint: Y = + h w - - .......

P m - - P,* P m -- P w

where Pm, Ps, Pw are the densities of mantlc, sediment and water respectively (here 3.3, 2.3 and 1.0); S is sediment thickness; h,~ is paleodepth of water, and E is eustatic height of sea-level above present mean sea-level.

Because the Himalayan stratigraphic data is often unreliable, particularly in regard to thickness and age, there is no justification for elabo- rate modelling of isostatic and tectonic subsidence, especially since I use sediment thickness of 500 m or more for calculation. Because of this, I simply assume sediment loading of a locally supported crust, with the depositional surface always close to mean sea-level which is kept con-

H I M A L A Y A N PASSIVE M A R G I N F R O M P R E C A M B R I A N T O C R E T A ( ~ E O U S 25

km o

1

2

3

4

5

6

7

8

9

lo

11

12

65 MESOZOIC

130

,0,o 200 i

245 PALAEOZOIC

285 £ 360 420

3oo I 4oo I i i

500

c3 o 500

i

530 PROTEROZOIC

650 I ?

(Vendian) I ?

600 I I

/ ' / / l / / Shield (Ganga basin) / / /

~ _ - / - / . - - . . . . .

" z / / /.

Arunachal Pradesh / / / Simla ~ 7

~ Bhutan / o Tansen e/

700 I

i "

- i ?

Fig. 16, Cumulative sediment thickness curves for the Indian Shield and Indian Lesser Himalaya.

stant. Most sequences cons idered here are low-

land con t inen ta l or shallow mar ine (with the ex-

cept ion of some Triassic and late Cretaceous

uni ts and the Nor th Himalaya sequences) .

Changes in sea-level due to eustat ic effects are

probably l imited to 100 m (Hallam, 1978); five

km 0

4

5

6

9

1O

1

1',

65 MESOZOIC

130

100 2001

245

eo

3o01

PALAEOZOIC

285 £ 360 420

400 D

I

530

500 { o

500 i f

P e s h a w a r ~ ~

g Hazara ~ t ~ 1 /

PROTEROZOIC 650

I ?

(Vendian) I ?

60o I I /

/ i

i i

Fig. 17. Cumulative sediment thickness curves for Pakistan Shield, and Himalaya,

700 I

A? d"

/ /

26 xl i i{R()OKFI[{I i,

times smaller than the 500 m sediment and subsidence units used here. Most compaction occurs in the upper few metres of sections and shales reach porosities of 10-20% within the upper 1 km of section (Perrier and Quiblier, 1974). Varia- tions due to depth of deposition, sea-level changes and compaction are thus close to an order of magnitude less than the stratigraphic thicknesses considered here and are usually limited to less than 100 m (Jackson and Galloway, 1984). These can therefore be ignored. Keen's (1979) formula thus simplifies to:

(p,,, - t ~ ) S y - P m - f )~

Bhat (1982, 1984) has already applied these concepts to the late Paleozoic to Tertiary evolution of the Himalaya both on a regional scale to the Kashmir basins and to the Himalayan passive margin. However, Bhat followed the earlier view that the late Precambrian-Cambrian of the Lesser Himalaya was Paleozoic-Mesozoic and that the stratigraphic sequences were relatively uniform along the Himalayan belts. Gaetani and Garzanti

(1991) provide a more detailed view ot the sedimentation and subsidence history of the northwestern Himalayan margin in Zanska>

Figures 16 to 19 show plots of total sediment thicknesses for each tectonic unit with scvcrai localities plotted. These plots approxmmtc total n e t accumulation since thc datum poinL f h e y do not indicate any periods of uplift and erosion. which are simply noted on the figures where part of the section are separated by unconformities. The simple calculation of Y (above) removes the effects of isostatic sediment loading a ,d the re. suiting tectonic subsidence is plotted for some examples in Fig. 20.

Asymptotic sections of the curves ca , often bc related to the onset of continental riIting and passive margin formation. R)r example in the late Precambrian-Cambrian of the High Himalaya (Figs. 18-20).

In both the High and Lesser Hirnalaya. lhc greatest sedimentation rate (and subsidence) occurs in the late Precambrian. when tilting and collapse of a continental margin is recorded m the Lesser Himalaya (Kumar and Brookfield.

km 0

1

2

3

4

5

6

7

8

9

1(3

1

1:

65 245 MESOZOIC ~ PALAEOZOIC

130 285 o 360 420

i 4oo o i E 2 0 ~ i i2 2 z> o

100 200 300 I 500 i I I I I -

Nor th Kashmir

E v e r e s t ~

J / / NorthZanskar /

530

5OO

rj

© o

PROTEROZOlC

65O

(Vendian) I

600 . ~ _ _ I

/ /

/ /

/

Fig. 18. C u m u l a t i v e s e d i m e n t t h i ckness cu rves for the I n d i a n w e s t e r n H i g h H i m a l a y a .

700 1


1987) and in the High Himalaya (Garzanti et al., 1987). However, no sediments accumulated on

the Indian Shield (Fig. 20). Extrapolating the High Himalaya curves back-

wards to zero thickness using the methods of Bond et al. (1984) gives ages of around 550-650 Ma for the start of thermal subsidence after rifting. This fits quite well with some of the inferred ages of various metamorphosed basaltic units of the Lesser Himalaya (Jiwan and Prasad, 1981; Raza, 1981) and corresponds with the main phase of break-up of the late Proterozoic supercontinent (Bond et al., 1984). Subsidence is greatest in the eastern part of the High Himalaya (Fig. 19) and less in the more westerly sections, sug- gesting that the eastern Himalaya lay closer to the continental margin. During the Cambro- Ordovician this margin was transformed into a stable shelf and only thin Ordovician to Permian sediments were deposited: the locus of sedimentation shifted far to the north, to areas which were separated from northern Gondwanaland during late Paleozoic rifting.

The Lower Permian rifting, basalt eruption and consequent separation of large microcontinents off the northern Gondwanaland shelf is not reflected in most of the subsidence and thickness curves and is apparent only in the Lesser Hi- malaya (Fig. 20). The great increase in subsidence rate in the Upper Carboniferous predates Per- mian rifting, but can be explained by isostatic depression of the northern Gondwanaland crust by glaciers. The maximum extent and presumably thickness of glaciers covering the Indian Shield occurred at this time. In like manner, the lack of thick Permian deposits can be explained by glacial rebound during the Permian deglaciation of the Indian Shield. Glacial unloading would cause sediment bypassing of the northern Gondwanaland shelf. And the vast sediment load from the melt- ing Permian glaciers would thus be dumped directly into the opening ocean to the north, where enormous thicknesses of Permian clastics actually accumulated in the Karakorum and Southern Ti- bet (Norin, 1946).

As in the late Precambrian, the subsidence rates diminish from east to west and are much smaller in the extreme northwest, in Zanskar, but

there are some interesting variations. North Ever- est, Spiti and Kashmir show generally similar

thickness curves. On the other hand, Zanskar and Peshawar, to the north and west, show thinner sections and lower rates of subsidence from Per- mian times on, with the most rapid subsidence occurring in the Triassic-Jurassic. These areas seem to reflect only the Upper Triassic-Jurassic development of the Western margin and then only to a minor extent. Though it has a much thinner Permian rift basalt section, Zanskar is north of Kashmir closer to the future ocean, so why does it not show the same or even greater rates of subsidence? It must either be underlain by thicker continental crust and supposedly was once further south of the Permian rift axis, or has been displaced laterally, or represents the Meso-

blESQZOIC ~ PALAEOZOIC

130 285 ~ 36Q 420 500

a_ z > km 100 2 (3 4 5 0

t L ,t i

.~...v

K a t h m a n d u

K a n g m a r /

/ /

Nor th Everes t / ~ o

/ o.O ~ O ~D f O /

Thakkhola d ¢

/ [ _ O / ~ o - O - / o"

/ O .

/ ' [] /

n~'g /

15 ! Yamzo Yumco [ ]

2O

25

Fig. 19. C u m u l a t i v e s e d i m e n t th ickness curves for the Ind ian

e a s t e rn H i g h and N o r t h H i m a l a y a .

28 M.E BR()OKFIELD

zoic slope with thinner sediments deposited in deeper water (compare the Atlantic slope of Fig. 13). The Mesozoic sediments seem mostly shallow water sediments (Gaetani et al., 1986), in which case the slope explanation must be rejected. It is not yet possible to decide between the other alternatives.

Nevertheless, as Bhat (1984) noted, the rapid Triassic subsidence of the Himalaya (not compen- sated by sedimentation as deep shelf and slope condensed Triassic sediments are present) can not be entirely due to simple thermal decay. This subsidence corresponds with a Triassic phase of alkaline magmatism in the western High Hi- malaya in Zanskar (Honegger et al., 1982) and in the eastern North Himalaya at Kangmar (Fig. 9), and with large carbonate olistoliths being shed northwards into Triassic-Jurassic flysch basins in Zanskar and elsewhere (Andrews-Speed and Brookfield, 1982).

Both Peshawar and Zanskar are overthrust by the Cretaceous oceanic arc nappes of Kohistan and Dras. The Himalayan Mesozoic passive margin together with its northwesterly orientated major structures is also obliquely truncated by these nappes. Thus, as noted previously, a large section

of the Himalayan passive margin has either been subducted beneath these oceanic nappes or was removed prior to collision. The eastern High and North Himalaya thus lay closer to the Mesozoic continental edge than did the northwest Eli- malaya.

The contrast between North Everest, Spiti, Kashmir, and Zanskar, Peshawar does not persist into the early Tertiary, after initial collision between India and Asia. Where data is adequate, from North Everest to southern Zanskar to Pesh- war, there is a progressive and consistent decrease in early Tertiary foredeep sediment thickness, compatible with a northeast to southwest collision.

Thus, the sediment thickness and subsidence data confirm the oblique truncation, not only of the Mesozoic passive margin but also of the early Tertiary collision structures, by Neogene southward thrusting along the main Himalayan thrusts.

Conclusions

The Himalaya forms an obliquc section through a Mesozoic passive margin. This margin was orientated northwest during Paleogene colli-

65 245 530 MESOZOIC ~ PALAEOZOIC PROTEROZOIC

130 285 £ 360 420 500 650 g

m ~ o E (Vendian)

too 20o 40o zoo 600 t ~,oo I I J J J I I / ~

,n iao .ieiO f , . . " / __ / North Everest . . . . . . . . . E] " /

/ ..rw-. ".o-~ - -o-- - / . Tansen Z , ~ . . /~ '~ '~>" - /

Spiti ..o-" . - f ~ - " ~ ' / o-- ~ .rn , ' ' '~ EI""D"" /

TECTONIC SUBSIDENCE

Yamzo Yumco

RIFTINGA Z~ L'~

A ritting z~ rifting glaciation deformation rifting collision rifting

h,- .assive m.rgl__Ln . . . . _ oa.s,. margin

Fig. 20. Tectonic subsidence curves of selected localities.

/


sion and was then overthrust from the north and

internally deformed during the Neogene formation of the Himalaya. This is obvious at the front of the Indus Suture Zone which progressively cuts westwards across the northern Indian passive margin, from oceanic and slope deposits in the northeast to shelf deposits in the northwest. The result is that, from east to west within each of the three main units of the Himalaya, the Mesozoic stratigraphies show progressively more landward and thinner sequences. In addition, the oblique truncation of the margin by Neogene thrusting causes similar sequences to occur in different structural units. This is most apparent in the

northwest, where for example the stratigraphic sequences on the shield in the Salt Range are found in the Lesser Himalaya of India.

This oblique collision, subsequent southward thrusting of up to 1000 km and frontal erosion can explain a lot of the stratigraphic contrast between the Himalayan structural units for Meso- zoic and younger periods.

By back-tracking the displacements of the In- dian Shield relative to Eurasia and restoring the Himalayan thrust sheets based on facies distributions and paleomagnetism, we can make some attempt at reconstructing the deformation history of the northern Indian margin since the Eocene, and hence reconstruct the Upper Cretaceous to Eocene passive margin.

In the late Cretaceous to Eocene, the Indian Shield, Lesser Himalaya, High and North Hi- malaya outline a consistent passive margin, with outer shelf uplifts heralding the Tertiary collision. The Tertiary clockwise rotation and southward thrusting of especially the western High Himalaya has not completely disrupted the previous facies patterns, i.e. these movements were not enough to entirely cut out the very broad late Creta- ceous-Eocene inner shelf.

For pre-late Cretaceous times, these displacements are not sufficient to explain the facies contrasts. In Pakistan, the Upper Cretaceous- Paleocene transgressions cut major faults which had already displaced Paleozoic deposits into incompatible facies relationships (Fig. 10) (the Mesozoic is absent on the north). In India, Bhat (1987) explained the Lesser/High Himalaya con-

trasts by invoking reversal of motion on Permian rift faults during Tertiary deformation (cf. Jack- son, 1980). These reversals may explain some of

the minor thickness and facies changes across some faults, e.g. in the late Precambrian at Simla (Kumar and Brookfield, 1987), but the contrast and lack of common features is just too great in most places. In the Pakistan Himalaya, any movement would have to predate the Upper Creta- ceous. In the northwest Indian Himalaya in Kash- mir and Zanskar, the High Himalaya has thick late Precambrian and Paleozoic to mid-Mesozoic shelf deposits including thick Permian basalts which stretch across several hundred kilometres. Yet south of the Pit Panjal, in Jammu, these are juxtaposed with Lesser Himalaya sections in which Lower Eocene limestones rest directly on mid- Proterozoic carbonates: the entire Kashmir Pale- ozoic-Mesozoic section is missing. The removal of the Kashmir Paleozoic-Mesozoic section must predate Tertiary collision since the Eocene limestones are not noticeably different across the fault.

The strongest stratigraphic contrasts across the Main Central Thrust, in fact, occurs in the w e s t -

e rn Himalaya--the supposedly least allochthonous section from Tertiary displacements. In Kash- mir, as noted above, thick Phanerozoic deposits are juxtaposed across the Main Central Thrust against only Precambrian rocks. This area was, in fact, the pivot point for Tertiary clockwise thrust movements to the east and anticlockwise thrust movements to the west. And yet across the Nanga Parbat-Haramosh massif, in the Pakistan Hi- malaya, the same changes occur across pre-late Cretaceous faults (Fig. 10).

By contrast, in the e a s t e r n Himalaya, the supposedly most allochthonous area, the very location of the Main Central Thrust is uncertain due to lack of stratigraphic contrast. On both sides of the fault are low-grade ?Precambrian rocks overlain by Permo-Carboniferous rift deposits. The main change in this area takes place between the High and North Himalaya where the metamorphic rocks are juxtaposed with the Mesozoic slope and basin sequence noted above. This can be partly explained by the zig-zag continental outline of the northern Indian margin formed by Lower

30 M , I . B R O ( J K F IE:I.D

Cretaceous rifting from western Australia, but some things do not fit. For example, the particularly thick late Triassic-Jurassic of the North Himalaya section should be early Cretaceous and younger. If the Himalaya in this area were trans- ported even further south than is envisaged here, then the northeastern Himalayan re-entrant could be related to an earlier period of rifting along the northern Indian passive margin marked by the eruption of Upper Triassic alkaline basalts and start of deposition of thick passive margin clastics recorded in the western High and North Hi- malaya.

On a simple analogy with the Atlantic passive margin (Fig. 13) (Sheridan, 1976; Watts and Thorne, 1984), the western High Himalaya sequences are at least 450 km seaward and north of a fall line separating deposition from erosion. The Lesser Himalaya would need to be south of any fall line since they have no equivalent sediments. This model indicates that extremely large displacements, of the order of at least 500 km, are necessary along the Main Central Thrust to explain the pre-late Cretaceous Mesozoic passive margin of the western Himalaya. Not only an inner shelf, but also the landward hinge would have to be overthrust in the western Himalaya, where the main displacements occur at the Main Central Thrust and at the Indus Suture Zone.

This is not the case in the eastern Himalaya, where the entire Mesozoic shelf sequence is missing, but where the main contrast occurs between the High and the North Himalaya, not across the Main Central and Main Boundary thrusts on the south. Also, the Mesozoic passive margin and slope is at least partly preserved in the North Himalaya, so the Indus Suture Zone has not been thrust so far south.

The pre-Cretaceous facies patterns require: (1) the removal of a section of the northeastern shelf, in order to juxtapose the North Himalayan oceanic nappes with the High Himalayan basement; (2) lateral displacement of the North and High Himalaya relative to areas on the south, in order to account for the facies changes and subsidence ra tes- - th is is particularly obvious for the l o w e r Permian rift sequences (Fig. 15).

The details of any such displacement can not

yet be worked out; however, a reasonable candi- date for the piece of shelf removed is the Shan block of Burma (Seng6r, 1987; Metcalfe, 19901.

Such movements must have ceased by the late Cretaceous, when the northern Indian passive margin forms a consistent pattern after allowance is made for Tertiary deformation. Transcurrent displacements of the requisite magnitudc o c curred in the late Jurassic-Cretaceous when rapid left-lateral movement of Afr ica / India took place

relative to Eurasia and spreading occurred at the Exmouth plateau off northwestern Australia (Exon et al., 1982; Savostin et al., 1986; Scotesc ct al., 1988; GOriir and Seng6r, 19921. At least 500 km of left-lateral movement occurred o , each of the Red River and Mae Ping faults of Indochina (Bunopas, 1982; Tapponnier et al., t9861. Thesc movements are of the right direction and magnitude to juxtapose an originally more easterly High Himalaya against the northern Indian margin. Such a limited lateral insertion may also explain the facies patterns of the late Precambrian to Cambrian.

So, it appears that the High Himalaya is not an exotic block, but one that has been laterally displaced at least somewhat relative to its adjacent shield.

The strong stratigraphic contrasts can mostly be explained by three factors: (1) the rcmoval of portions of the margin during Mesozoic rifting; (2) left-lateral displacements during the Meso- zoic; and (3) erosional removal of much of the Phanerozoic inner shelf during Tertiary thrusting.

The recognition of abundant exotic pieces in both the North American western Cordillera and Appalachian mountains is based partly on such stratigraphic contrasts (Saleeby, 1983: Williams and Hatcher, 1983), but strong stratigraphic contrasts need not imply huge lateral motions.

Acknowledgements

I thank: N.E.R.C. Canada and tile National Geographic Society for grants to do field work in the Himalaya and Central Asia; the Centre of Advanced Study in Geology, Panjab University. Chandigarh, and colleagues there, tier facilities. discussions and help during joint studies, t also

HIMALAYAN PASSIVE MARGIN FROM PRIECAMBRIAN 1"O CRETACEOUS 31

gratefully acknowledge discussions and criticisms of the manuscript by C.P. Andrews-Speed, A. Baud, G. Fuchs, M. Gaetani, N. G6riir, R. Ku- mar and K. Pogue.

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The Himalayan passive margin from Precambrian to Cretaceous times