The stratigraphy of the Tethys Himalaya

INTRODUCTION

The Tethys Himalaya (also known as “Tibetan Zone”) is one of the major tectonic domains in the Himalayan Orogen, stretching for ~2000 km from Kashmir and the Zanskar-Spiti synclinorium in the northwest to South Tibet in the east (Fig. 1). Its northern boundary coincides with the Indus-Tsangpo Suture, separating the Tethys Himalaya from the Transhimalayan arc-trench system and the Lhasa Block to the north (Gansser, 1980), whereas the southern boundary is represented by the tectonic contact with the Greater Himalaya metamorphic rocks, commonly referred to as the South Tibetan Detachment System (Burg et al., 1984; Herren, 1987).

Fig. 1

The Tethys Himalayan sedimentary succession, one of the most complete and spectacularly exposed on Earth (Fig. 2), represents the deformed remnant of the northern margin of the Indian subcontinent. The well-preserved upper Neoproterozoic to Eocene stratigraphic record documents the geological history of northern Gondwana over more than 500 Ma (Gaetani and Garzanti, 1991; Garzanti, 1999). The succession can be subdivided into pre-Neotethyan (pre-rift stage) and Neotethyan parts. The latter includes a rift stage (lower Carboniferous to lowermost Permian) documenting the tectonic and magmatic processes culminated in continental breakup and initial spreading of Neotethyan oceanic floors (Table 1), followed by a drift stage (upper lower Permian to earliest Paleocene) recording the sedimentation and subsidence history of the northern passive continental margin of India facing the Neotethys Ocean (Table 2). Identified within the drift stage are an initial starved passive-margin phase (upper lower Permian to Middle Triassic), a Late Triassic extensional event, a Jurassic mature passive-margin phase, and a Cretaceous volcanic and drowning event. During the final collision stage (Paleocene-Eocene), obduction of the Transhimalayan active margin onto India terminated passive-margin sedimentation: the Himalayan orogeny had begun (Hu et al., 2016).

Fig. 2

The Tethys Himalaya zone is much wider in South Tibet, where it is traditionally divided into a southern part characterised by proximal shelfal sedimentation and an outer part characterised by offshore distal deposits (Hu et al., 2008). In the NW Himalaya, the Paleozoic-Mesozoic distal-margin succession wraps around the Ordovician granitic core of the Nyimaling Massif (Stutz, 1988). Tethys Himalayan strata have undergone very-low-grade to low-grade metamorphism (Garzanti & Brignoli, 1989; Garzanti et al., 1989, 1994a; Schneider and Masch, 1993; Spring et al., 1993) during initial subduction of the continental margin and thrust-belt development in the Eocene (Bonhomme & Garzanti, 1991). Sandstone composition follows the classification of Garzanti (2016, 2019) through the article.

THE PRE-RIFT PALEOZOIC HISTORY OF THE TETHYS HIMALAYA

In the Zanskar-Spiti synclinorium of the NW Himalaya, the uppermost Neoproterozoic-lower Cambrian sedimentary succession testifies to passive-margin sedimentation. Fine-grained tidal-flat siltstones and sandstones (Phe Fm.) are intercalated with, and finally replaced up-section by stromatolitic dolostones (Karsha Fm.), followed in turn by shales containing trilobites of late Middle Cambrian age overlain by thin-bedded deeper-water sandstones (Kurgiakh Fm.). An angular unconformity is overlain by clast-supported alluvial-fan conglomerates containing pebbles and cobbles mostly derived from sedimentary rocks (dolostone, red feldspatho-quartzose to quartzose sandstone and siltstone); quartzite clasts are subordinate and sandstone composition changes upward from lithic (dololithites) and quartzo-lithic to lithic-rich litho-quartzose. This up to ~800-m-thick unit (Thaple Fm.) testifies to a major Cambro-Ordovician tectonic event related to the Pan-African orogeny documented across Gondwana (Garzanti et al., 1986). Silurian limestones (Pin Fm.) and the coastal Devonian Muth Quartzarenite follow (Fig. 2; Draganits & Noffke, 2004).

In Nepal and south Tibet, the Ordovician is documented by very thick, strongly recrystallised and sparsely fossiliferous shallow-water carbonates followed by quartzarenites and calcschists (Colchen et al., 1986). In central Nepal, the Silurian and Devonian are represented by either graptolite-bearing mudrocks and tentaculitid-bearing marlstones (Bordet et al., 1967, 1971) or by mixed shallow-marine carbonate-siliciclastic units followed by dolostones and biocalcarenites capped by an ironstone interval (figures 3, 4, and 10 in Garzanti et al., 1992). This 2.5-m-thick nautiloid-bearing oolitic ironstone capping lower Frasnian coral-bearing biocalcarenites correlates with worldwide episodes of transgression, increasing subsidence, and drowning (Bond & Kominz, 1991). Grey mudrocks with intercalated sandstones and carbonates follow (Tilicho Pass Fm.; Colchen et al., 1986).

Fig. 3

Fig. 4

The top of the pre-rift succession is represented by shallow-water limestones all along the Tethys Himalaya (Givetian to Tournaisian Lipak Fm. of Zanskar and Spiti: Draganits et al. 2002; Tilicho Lake Fm. of Nepal: Bordet et al., 1967; lower Tournaisian Yaleb Fm. of South Tibet: Garzanti & Sciunnach, 1997). Widespread fossiliferous limestones deposited until the early Tournaisian are truncated by a major unconformity testifying to the onset of Neotethyan rifting (Fig. 3; Garzanti et al., 1992).

THE CARBONIFEROUS/EARLIEST PERMIAN RIFT STAGE

The up to ~1.5-km-thick Mississippian to lowermost Permian rift sequence (Po Group of Zanskar and Spiti; lower Thini Chu Group of Nepal; Naxing Group of South Tibet) displays marked lateral changes in thickness and facies, indicating sedimentation in tectonically controlled basins (Garzanti et al., 1994b, 1996a), and includes dikes of bimodal alkalic rocks and volcanic detritus (Vannay & Spring, 1993; Sciunnach & Garzanti, 1997). Rift-shoulder uplift resulted in the extensive erosion of the Gondwanan margin and development of disconformities. Major stratigraphic gaps occur at the base (“rift unconformity”; Fig. 3A), in the middle (“Carboniferous/Permian hiatus” associated with glacigenic sediments in continuous sections), and at the top (“breakup unconformity”; Sciunnach & Garzanti, 1996, 2012). The entire rift sequence is even missing locally, as in western Nepal (Fuchs, 1977; figure 2 in Garzanti, 1999) or in the Pin and Parahio valleys of Spiti (Fig. 3B), where erosion cut deep into pre-rift strata, reaching down into the Devonian (Fig. 3C), Silurian (Fig. 3D), Ordovician, and even Cambrian to Precambrian basement (figure 24 in Vannay, 1993; Vannay & Steck, 1995).

In the NW Himalaya, the syn-rift succession begins with a gypsum interval containing middle Tournaisian/early Visean brachiopods, suggesting opening of rift-controlled basins with restricted circulation in arid settings (figure 5 in Gaetani et al., 1986; Draganits et al., 2002).

Fig. 5

All along the Tethys Himalaya, the Visean-Sakmarian succession consists chiefly of dark mudrocks with intercalated feldspatho-quartzose to pure quartzose white sandstones, with quartz/feldspar ratio markedly increasing from very fine to fine, medium, and coarse sandstones (Thabo Fm. of Spiti; Marsyandi Fm. of Nepal; Rakyang, Mha Zang and Chakhang Do Sur Formations of South Tibet). Four diamictite intervals containing faceted pebbles, trapezoidal cobbles, and boulders up to 1000 m³ in volume were traced across South Tibet (Fig. 4), indicating that shore ice began to form at middle southern-hemisphere latitudes as early as the Visean-Serpukhovian, triggered by basin inversion and tectonic uplift during the initial stages of Neotethyan rifting (Garzanti & Sciunnach, 1997).

After the Mississippian glacial stage, a progressive up-section transition to dominant quartzarenites is ascribed to a change to temperate-humid conditions, coupled with incipient inversion of rift basins and recycling of older terrigenous deposits. Two major transgressive events, testified by black shale marker units of lower Pennsylvanian (Bashkirian Fenestella Shales) and middle Pennsylvanian age (Moscovian Chaetetid Shales), can be correlated all along the Tethys Himalaya, from Kashmir and the Zanskar-Spiti synclinorium (Fenestella Shale and Chichong Fm.; Hayden, 1904; Garzanti et al., 1996a) to Nepal (Col Noir Shale and Bangba Fm.; Garzanti et al., 1994b) and South Tibet (Dzaghar Chu Fm. and upper Naxing Group; Garzanti et al., 1998a).

At Late Carboniferous (Kasimovian-Gzhelian) to earliest Permian times (Asselian), a renewed and more extensive glacial event was testified by deposition of glacio-marine diamictites from Kashmir to Spiti (Ganmachidam Diamictite; Garzanti et al., 1996a), eastern Manang (Braga Fm.; Bordet et al., 1975; Garzanti et al., 1994b), and South Tibet (Jilong Group; Garzanti & Sciunnach, 1997). A close relationship between the onset of the major Gondwanan glaciation and the climax of rift-related tectonic activity is indicated by the sudden influx of sedimentary detritus, documenting an active stage of basin inversion and progressive erosion of Paleozoic sedimentary successions. Bimodal (felsic to mafic) volcanic detritus increases up-section in strata containing cold-water Gondwanan faunas of Asselian age. Chromian spinels are chemically akin to those from mantle xenoliths brought to the surface by basaltic magmas (Sciunnach & Garzanti, 1997) and detrital zircons show varied typologies indicating mixed provenance from the Indian craton, anatectic granitoids of supposed Cambro-Ordovician age, and mildly alkalic rhyolites erupted during the climax of continental rifting (Caironi et al., 1996).

THE PERMIAN DRIFT STAGE

The base of the drift sequence is defined by a low-angle unconformity locally mantled by breccias, ferruginous microconglomerates, quartzose sandstones, and glaucony-rich arenites (Gechang Fm.; Gaetani et al., 1990; Garzanti et al., 1996b). These condensed transgressive horizons containing brachiopods of late Sakmarian age at the base and of late middle Permian age at the top mark a turning point in the evolution of the northern Gondwanan margin and are inferred to reflect the combined effect of sea-level rise fostered by deglaciation (Wopfner & Jin, 2009) and onset of thermal subsidence of the newly formed rifted margin.

The basal part of the drift sequence comprises the Panjal Traps continental flood basalts, discontinuously exposed along the Tethys Himalaya. These upper lower Permian lavas (Sakmarian-Artinskian, 289 ± 3 Ma; Shellnutt, 2018) are up to 2500-m-thick in Kashmir, reduced to 300 m at most in Zanskar (Fig. 5), and lacking in Spiti. Further to the east, vesicular lava flows crop out in central Nepal (Nar-Tsum Spilites: Le Fort, 1975) to South Tibet (42-m-thick Bhote Kosi Basalts: Garzanti et al.,1999). The tholeiitic lavas traced along the ≥ 2000 km newly formed Tethys Himalayan volcanic rifted margin are held to be directly related with the onset of seafloor spreading in the Neotethys Ocean.

In South Tibet, the lavas are sharply overlain by ferruginous siltstones and sandstones, passing upward to white quartzarenites containing oversized chert grains (Qubu Fm.). Middle-upper Permian mudrocks containing phosphate nodules and shelfal biocalcarenites rich in brachiopods and bryozoans follow (Qubuerga Fm.; figure 6 in Sakagami et al., 2006). In central Dolpo (Nepal; figure 10 in Sciunnach & Garzanti, 1996), the lavas are lacking and the breakup unconformity separating the white Carboniferous Atali Quartzarenite from the overlying middle Permian brachiopod-rich sands (Wordian Costiferina arenites; Fig. 6B) is mantled by an up to 20-cm-thick discontinuous lens of sparsely bioclastic ferruginous quartzarenite (Fig. 6C) containing felsic volcanic grains and subhedral red-brown chromian spinel (Fig. 6D; Sciunnach & Garzanti, 1997).

Fig. 6

In the Zanskar-Spiti synclinorium, the Panjal Traps lavas are overlain by glaucony-rich pebbly arenites yielding brachiopods of early late Permian age (Gechang Fm.), followed by phosphatic black shales rich in brachiopods deposited in open, deepening-upward shelf environments during the latest Permian (Gungri Fm.; Garzanti et al., 1996b). Deposition of glauco-phosphoritic horizons and dominance of black shales testify to warming climate at progressively lower southern-hemisphere latitudes through the Permian, correlating with oceanographic and tectono-magmatic changes in the Tethyan domain and worldwide (Dercourt et al. 1993; Ogg & von Rad 1994; Muttoni et al., 2009). Middle to upper Permian strata seal the Paleozoic succession in all studied areas, documenting the final submergence of the rift shoulders following the initial opening of Neotethys.

The configuration of the northern margin of the Gondwana supercontinent before the rifting event that culminated with sea-floor spreading in the Neotethys Ocean is highly debated. Close stratigraphic affinities indicate that India, Lhasa, and South Qiantang were attached until the early Permian (e.g., Smith & Xu, 1988; Sun, 1993; Zhai et al., 2013; Ma et al., 2024), but hypotheses diverge whether the block that detached from India was Lhasa (e.g., Allègre et al., 1984; Fan et al., 2017; Zeng et al., 2019) or South Qiangtang (e.g., Zhu et al., 2011; Zhai et al., 2013; Wang et al., 2021). The complex paleogeodynamic and paleogeographic evolution of these microcontinents and intervening oceanic branches is discussed in Hu et al. (2022).

PASSIVE-MARGIN SEDIMENTATION IN THE TRIASSIC

The long-term Permian transgressive trend, induced by thermal subsidence of the stretched crust and punctuated by higher-frequency transgressions and regressions, culminated in diachronous drowning and offshore deposition of condensed biocalcarenites rich in brachiopods and conodonts in Manang to South Tibet (Fig. 7B; “topmost biocalcarenites” of Nicora & Garzanti, 1997).

Fig. 7

Condensed carbonate deposition at bathyal depths became widespread in the Lower Triassic (Fig. 7; Tamba-Kurkur Fm.). Three intervals of ammonoid-bearing nodular limestone deposited during transgressive stages at Griesbachian/Dienerian, early/mid-Smithian, and latest Smithian/earliest Aegean times are either separated by black shale intervals or amalgamated. Total thickness decreases markedly eastwards, from 32-54 m in Spiti to 6.5-20 m in South Tibet, corresponding to very low accumulation rates (1-10 m/Ma; Garzanti et al., 1994c, 1995, 1998b).

In the Middle Triassic, ammonoid-bearing marls characterised the Tethys Himalaya, from South Tibet to the Hanse Group of the Zanskar-Spiti synclinorium (Fig. 8), where the Tamba Kurkur Fm. was locally deposited until the early late Ladinian (Mikin Fm. of Bhargava et al., 2004; Krystyn et al., 2004). The Middle Triassic to mid-Norian strata are represented by cyclically alternating shelfal marls and fossiliferous marly limestones (Hanse and Zozar Fms. in Zanskar: Gaetani et al., 1986; Hanse Group and Nimaloska Fm. in Spiti: Garzanti et al., 1995; Mukut Fm. and Tarap Shale in Nepal: Fuchs, 1977; Qudenggongba and Tarap Fms. in South Tibet: Jadoul et al., 1998).

Fig. 8

Accumulation rates remained low through the Middle Triassic, when 60–70 m of sediment were deposited at rates of 4–5 m/Ma, and began to increase rapidly in the mid-Carnian, to reach values as high as 100 m/Ma for the uppermost Carnian. This drastic increase in sediment supply is recorded all along the Tethys Himalaya, from the Zanskar-Spiti synclinorium (figure 20 in Garzanti et al., 1995) to South Tibet (Jadoul et al., 1998), and points to an extensional tectonic event beginning in the Carnian and documented from western India (Biswas, 1987) to as far as western Australia (von Rad et al., 1992). Total subsidence was greatly enhanced by sediment loading during rapid aggradation and outbuilding of the continental terrace.

LATE TRIASSIC RIFTING

Sedimentation rates increased further in the Norian, with accumulation of shallow-water carbonates in the NW Himalaya (Zozar Fm.) and of mudrocks with phosphatic nodules with intercalated feldspatho-quartzose sandstones in Nepal and South Tibet (Tarap Fm.: Garzanti et al., 1995; Jadoul et al., 1998).

During the Late Triassic, a large volume of terrigenous sediment was supplied from two independent sources to proximal and distal parts of the Indian passive margin. In South Tibet, proximal shelfal sandstones are characterised by feldspatho-quartzose to quartzose composition and strongly negative εNd(0) values down to –18, indicating provenance from peninsular India (Wang et al., 2025). More distal units, instead, consist of lithic-rich litho-quartzose sediments with bimodal (felsic and mafic) volcanic rock fragments showing alkalic signature, together with zircon grains yielding latest Carnian/early Norian U–Pb ages (229–223 Ma) and thus testifying to penecontemporaneous magmatic activity in an intraplate extensional setting (Meng et al., 2021).

Farther offshore in Tibet, the very thick turbidites of the Langjexue Group comprise litho-quartzose sandstones with volcanic rock fragments and plagioclase grains. Such a composition, the moderately negative εNd(0) values, and the 400–200 Ma age population of magmatic zircons yielding uniform εHf(t) values between −5 and +10 are incompatible with provenance from the Indian subcontinent. Long-distance supply from a long-lived magmatic-arc terrane located in the east and most likely represented by the Gondwanide Orogen of eastern Australia was thus suggested (Wang et al., 2016).

In the NW Himalaya, the deep-sea turbidites of the Lamayuru Group show mixed sources. Quartzose sandstones containing zircon grains yielding common Early Paleozoic to Precambrian ages indicate provenance from peninsular India. The intercalated lithic layers, instead, consist chiefly of basaltic rock fragments and plagioclase, lack quartz, and contain Cr-spinel geochemically akin to intraplate basalts and zircon grains with exclusive Middle to Late Triassic ages (peak at 235 Ma). Local provenance from intraplate volcanic rocks is indicated (Meng et al., 2025).

Such a magmatic activity recorded in Tethys Himalayan strata is plausibly related to block faulting and volcanism in NW Australia (von Rad et al., 1992). This suggests that a major rift developed along the northern margin of eastern Gondwana, feeding large amounts of detritus from the east and inducing accelerated tectonic subsidence and increased sedimentation rates along the northern Indian margin as far west as the NW Himalaya. This major rifting phase is plausibly related to a new stage of seafloor spreading in the Neotethys Ocean, when the Lhasa Block was torn off eastern Gondwana (Wang et al., 2025).

The Norian-Rhaetian is marked by deposition of coastal sandstones with intercalated oo-bioclastic hybrid arenites, chamosite-goethite ironstones, mudrocks and limestones (Fig. 9A,B,C,D; “Quartzite Series”: Alaror Group in the Zanskar-Spiti synclinorium, Zhamure Sandstone in Nepal and Tibet). Detrital modes are markedly size-dependent, ranging from mostly feldspar-rich feldspatho-quartzose in very fine sandstones to pure quartzose in medium sandstones (Jadoul et al., 1990). Megalodon-bearing limestones document the early establishment of the Kioto carbonate platform in the NW Himalaya (Fig. 9E).

Fig. 9

THE JURASSIC MATURE PASSIVE-MARGIN STAGE

The deposition of Kioto Group platform carbonates became widespread in the Lower Jurassic, with the characteristic Lithiothis-bearing horizons (Fig. 9F; Jadoul et al., 1998). In the middle part of the Kioto Group, the appearance of dark marly mudrocks intercalated with frequent storm layers document a significant facies change. This distinct transgressive interval is traced from Zanskar (lithofacies f of Jadoul et al., 1990) to South Tibet, where it marks the abrupt transition between the Pupuga and Nieniexiongla (Lanongla) formations (Han et al., 2016; units K1 and K2 of Jadoul et al., 1998). In this interval, a relevant negative shift in δ¹³C corresponds with the early Toarcian “oceanic anoxic event” (Wignall et al., 2006; Han et al., 2018), an abrupt environmental change connected with the emplacement of the Karoo-Ferrar large igneous province between southern Africa and Antarctica (Pálfy & Smith, 2000; Burgess et al., 2015).

In the Middle Jurassic, accumulation rates progressively decreased to less than 10 m/Ma all along the Tethys Himalaya, reflecting reduced thermal subsidence during the mature passive-margin stage (Fig. 10A). Paleomagnetic evidence suggests that lowermost Middle Jurassic strata were deposited at subtropical latitudes (32 ± 3°S; Jiao et al., 2023). A drastic paleogeographic change was recorded around the Bajocian/Bathonian boundary, when the drowning of the Kioto carbonate platform, marked by a disconformity or paraconformity from Zanskar to South Tibet, was followed by deposition of interbedded peloidal quartzarenites, marls, and “lumachelle” layers rich in belemnites, ostreids, brachiopods, and ammonoids on a storm-dominated shelf (Fig. 10B,C; Laptal Formation; Heim & Gansser, 1939).

Fig. 10

Marked lateral variations in thickness, associated with the occurrence of huge olistoliths of Norian to lower Middle Jurassic carbonates in the Lamayuru continental-rise succession (Bassoullet et al., 1981), testify to a major block-faulting event leading to slope instability and fault-escarpment retreat, recorded along the southern Neotethyan margin as far as Australia and possibly related to rifting of India-Madagascar from Africa followed by sea-floor spreading in the Somali, Mozambique, and Argo basins (Patriat et al., 1982; von Rad et al., 1992).

The disconformity at the top of the Kioto carbonate platform may be directly onlapped by transgressive strata of the upper Bathonian to middle Callovian Ferruginous Oolite Formation (Fig. 10D), which represents a widespread, condensed oolitic ironstone horizon traced all along the Tethys Himalaya (Garzanti et al., 1989). Iron ooids consisting of concentric dark layers of francolite (carbonate fluorapatite) and bright layers of chamosite formed on the seafloor during continued resuspension and vertical oscillations of the chemocline (Fig. 10E). Precipitation of francolite under anoxic conditions reflects oversaturation of phosphorous related to intensified upwelling, high biogenous productivity and degradation of organic matter, whereas chamosite formation under suboxic conditions relates to continental weathering and erosion leading to increased iron input to the ocean during a transgressive stage characterised by low sedimentation rate and scarce oxygenation at the seafloor (Han et al., 2023).

Extensive upwelling along the Tethys Himalayan continental margin contributed significantly to the transition from shallow-marine carbonates to organic-rich black shales deposited under reducing conditions in the Late Jurassic (Fig. 10B). These outer shelf, ammonoid-rich black mudrocks (Spiti Shale; Stoliczka, 1866) represent a most characteristic and long-established stratigraphic interval traced all over the Tethys Himalaya. Thick storm layers with a basal rudite interval consisting of packed belemnite rostra may occur in the lower part (Belemnopsis gerardi beds), as well as very fine quartzose sandstones with alkali feldspars and a few volcanic rock fragments pointing to an incipient extensional stage, plausibly related to the initial opening of the proto-Indian Ocean. In large parts of Nepal, the Spiti Shale is the youngest stratigraphic unit exposed at the core of tight synclines.

EARLY CRETACEOUS VOLCANISM AND MID-CRETACEOUS DROWNING

A drastic change in sedimentation from offshore shales to locally coal-bearing deltaic and shelfal siliciclastic rocks took place close to the Jurassic/Cretaceous boundary (Fig. 11). Along the Tethys Himalaya, the Lower Cretaceous is represented by locally bioclastic sandstones alternating with dark grey/greenish siltstones (Giumal Sandstone in the Zanskar-Spiti synclinorium; Chukh Group in Nepal; Wölong volcaniclastics in South Tibet). Condensed horizons with immature glaucony, phosphate nodules, or silicate peloids occur.

Fig. 11

In the NW Himalaya, a lower unit consisting chiefly of quartz-rich feldspatho-quartzose to quartzose sandstones (Fig. 11A,B; Takh Fm.) is capped by a glauconitic horizon overlain in turn by feldspatho-quartzo-lithic to lithic-rich litho-quartzose volcaniclastic sandstones (Fig, 11C,D) interbedded with feldspatho-quartzose to up to very coarse-grained pure quartzose sandstones in proximal areas (Pingdon La Fm.; Baud et al., 1984; Garzanti, 1992, 1993a). A few ammonoid-bearing intervals occurring in the lower and upper parts of the Giumal Sandstone allowed to document the upper Berriasian/lower Valanginian and upper Aptian/lower Albian in the Spiti Valley (Giumaliceras and Cleoniceras assemblages of Bertle & Suttner, 2021). Eleven ammonoid-bearing intervals ranging in age from Berriasian to early Aptian were reported from the same locality by Pandey & Pathak (2015).

In the Thakkhola Graben of central Nepal, a basal quartzarenite unit (Dangardzong Fm.) is disconformably followed by deltaic quartzo-lithic to lithic-rich litho-quartzose sandstones and conglomerates containing wood logs (Kagbeni Fm.), overlain in turn by dark shelfal mudrocks and quartzo-feldspatho-lithic volcaniclastic sandstones locally glauconitic and yielding bivalves and ammonoids of Aptian age (Dzong Fm.; Bordet et al., 1971; Garzanti & Pagni Frette, 1991; Gibling et al., 1994).

In South Tibet, radiolarian-rich phosphatic volcanic arenites are overlain by coarsening-upward sequences of locally bioclastic sandstones showing hummocky lamination or intensely burrowed, alternating with intervals of dark grey/greenish siltstones containing carbonate concretions and locally ammonoids (Wölong Volcaniclastics; Jadoul et al., 1998; Hu et al., 2010). Paleomagnetic evidence suggests that these strata were deposited at middle-high southern-hemisphere latitudes (Huang et al., 2015). A swarm of 5-50 m-long and 20-150 cm-wide clastic dikes, injected along normal faults and filled by volcaniclastic sandstone containing zircons with youngest ages clustering at 124 ± 2 Ma, testify to liquefaction of underlying strata induced by earthquakes associated with regional extension and magmatic activity (Guo et al., 2021).

Volcanic detritus reached South Tibet (Tithonian) earlier than Nepal (Valanginian) and much earlier than the NW Himalaya (Albian). The U-Pb ages of zircon grains found in Lower Cretaceous units range from ~110 to ~150 Ma and cluster at ~130 Ma (Hu et al., 2015a), thus covering the period of magmatic activity in the Comei province of southeastern Tibet and of the Rajmahal province of eastern India (Baksi et al., 1987; Zhu et al. 2009; Singh et al., 2020). The intraplate character of volcanic source rocks is documented by prevalent mafic detritus including pebbles with alkalic geochemical signature (Dürr & Gibling, 1994), associated up-section with microlitic to felsitic and microgranitoid rock fragments suggesting both incipent unroofing of plutonic roots and changing character of magmatic activity (Garzanti, 1999; Hu et al., 2010). The westward delay in the onset of volcaniclastic sedimentation testified in the stratigraphic record, still poorly understood, may be explained with either progressive westward migration of extensional/transtensional tectonic and magmatic activity or gradual progradation of sediment fed from the main volcanic centres in the east (Hu et al., 2015a). These tectonic and magmatic events accompanied the successive detachment of India from Africa, Australia, and Madagascar during the final breakup of the Gondwana supercontinent and initial opening of the Indian Ocean.

Volcaniclastic sedimentation ended synchronously in the latest Albian all along the Tethys Himalaya (Premoli Silva et al., 1991). Drowning of the clastic shelf is testified by widespread composite condensed sections represented by glauconitic and channelised glauco-phosphorite intervals, which span the latest Albian to latest Cenomanian and are up to 35 m-thick overall in the inner part of the continental margin (Fig. 12A; Garzanti et al., 1989). These peculiar horizons, documenting sediment starvation during transgression plausibly induced by crustal cooling at the end of the Early Cretaceous volcanic event, correlate with oceanic anoxic events OAE1d (Fig. 12B; ~100 Ma; latest Albian Rotalipora appenninica foraminiferal zone; Watkins et al., 2005) and OAE2 (Fig. 12C; ~ 94 Ma; uppermost Cenomanian “Bonarelli level” of the Apennines, base of Whiteinella archaeocretacea foraminiferal Zone; Mitchell et al., 2008). These events, testifying to sluggish circulation, poor ventilation, and increased fertility in the oceans at a time of warm “greenhouse” climate and global fast seafloor spreading, took place during the long Cretaceous period of normal magnetic polarity, suggesting a link with aperiodic releases of heat from deep in the mantle that profoundly altered the atmosphere and hydrosphere, thus dramatically affecting biological and sedimentary systems (“superplumes” of Larson, 1991; Garzanti, 1993b).

Fig. 12

In the Zanskar-Spiti synclinorium, the sharply overlying grey (Chikkim Fm.) to multicoloured pelagic limestones (Fatu La Fm.) span the Turonian to Campanian, locally reaching into the lower Maastrichtian (Fig. 13A; Premoli Silva et al., 1991) and are paraconformably followed by thick bioclastic marls and quartzose siltstones (Kangi La Fm.) capped by the shallow-water Marpo Limestone (Fig. 13B,C,D). In South Tibet, a paraconformity concealing a hiatus corresponding to most of the Campanian separates Santonian pelagic marly limestones (Jiubao Fm.) from the overlying thick marly limestones, storm-deposited quartzarenites, rudist biostromes, and marls (Zhepure Shanpo Fm.; Willems et al., 1996; Hu et al., 2012). These regressive sequences document a pulse of terrigenous supply and sharply increased accumulation rates during the Maastrichtian, associated with the initial impingement of the Deccan plume head at the base of the Indian lithosphere since the Campanian (Garzanti & Hu 2015; Li et al., 2020).

Fig. 13

THE PALEOCENE-EOCENE COLLISION STAGE

The Paleocene begins with deposition of coastal quartzarenites capped by condensed glauconitic layers (Fig. 14A,B; Danian Stumpata Quartzarenite in Zanskar; Jidula Fm. in South Tibet), overlain in the inner margin by thick shallow-marine carbonates with rich benthic foraminiferal faunas spanning the Selandian and Thanetian (Dibling Limestone in the Zanskar-Spiti synclinorium: Nicora et al., 1987; Zongpu Formation in South Tibet: Willems et al., 1996; Hu et al., 2012; Li et al., 2017). In the outer margin in Zanskar, Selandian to Thanetian strata of the Lingshed Nappe (Fig. 14), overthrust by the Spongtang Ophiolite, comprise marly limestones with black chert nodules and marls with planktonic foraminifera documenting offshore sedimentation (Fig. 14C; Shinge La Fm.). A major disconformity locally associated with intrabasinal carbonate conglomerates and straddling the Paleocene/Eocene boundary is documented both in Zanskar (Fig. 14D) and South Tibet. Long interpreted as the far-field effect of collision onset and ascribed to flexural uplift of the margin caused by an orogenic wave propagating from the suture zone (Garzanti et al., 1987), the unconformity may also reflect climate change during the Paleocene/Eocene thermal maximum (Li et al., 2017). Drowning of the carbonate platform and subsequent demise of Neotehyan seaways took place at different Ypresian to Lutetian times along strike (Blondeau et al., 1986; Nicora et al., 1987; Li et al., 2015; Zhang et al., 2024), followed by unconformable deltaic deposition of volcaniclastic green and red beds fed from the Transhimalayan arc-trench system and thus documenting the final closure of Neotethyan seaways (Fig. 15; Garzanti et al., 1987; Najman et al., 2010, 2017).

Fig. 14

Fig. 15

In the inner margin in Zanskar, lagoonal marls intercalated with biocalcarenites, greenish siltstones, and locally pure quartz sandstones dated as early Ypresian (Fig. 15A,B; lithozone C of Nicora et al., 1987) are unconformably overlain by thick red floodplain siltstones (Fig. 15C) containing caliche nodules (Fig. 15E) and encasing small high-sinuosity fluvial paleochannels filled by obliquely laminated red sandstones with scoured base and basal rip-up clast lag (Fig. 15D; Chulung La Fm.). These quartzo-feldspatho-lithic to feldspatho-quartzo-lithic volcaniclastic sandstones chiefly consist of plagioclase and dominantly andesitic-rhyodacitic lithics (Fig. 15F), testifying to provenance from arc-related volcanic and volcaniclastic rocks of the Transhimalayan active continental margin. Recycling of older arc-derived turbidites is suggested by the occurrence of a few terrigenous rock fragments and of reworked Globotruncana released from interbedded hemipelagites. The presence of serpentine-schist grains (Fig. 15G) and of a heavy-mineral suite including common red to yellow-brown Cr-spinel (figure 8 in Critelli & Garzanti, 1994; Hu et al., 2014) indicate contribution from the Transhimalayan ophiolitic mèlange. The occurrence of prismatic zircon yielding youngest ages of ~54 Ma constrains the maximum age for arrival of volcanic detritus shed from Asia and deposited onto India, and hence certifies the complete closure and filling of Neotethyan seaways by the early Eocene in the NW Himalaya (Najman et al., 2017).

In the outer margin, obliquely laminated nummulitic limestones (Fig. 14D; Ypresian Kesi Fm.) are capped by a marine hardground overlain by shallow-marine siltstones (Fig. 14E,F; Kong Fm), considered to be either a proximal equivalent of lithozone C (uppermost Dibling Fm.) or a distal equivalent of the Chulung La Formation. The occurrence of evaporite nodules in Upper Paleocene strata of both proximal and distal parts of the Tethys Himalayan margin, followed by the deposition of lower Eocene redbeds with caliche paleosols, suggests that climate turned to semiarid because of both drift toward northern tropical latitudes and disappearance of marine sources of humidity (Nicora et al., 1987).

An analogous development of sedimentary facies characterizes the Tingri and Gamba synclines of South Tibet, the Enba/Youxia green beds and overlying Zhaguo/Shenkeza red beds being correlative with the Kong and Chulung La Formations of the Zanskar Himalaya, respectively (Najman et al., 2010; Zhang et al., 2012; Li et al., 2015). In South Tibet, the arrival of the distal edge of the Tethys Himalayan passive margin at the Transhimalayan trench, and consequent initiation of continental subduction beneath ophiolitic forearc lithosphere, has been constrained by radiometric dating of tuff layers coupled with detrital zircon chronostratigraphy and radiolarian and coccolith biostratigraphy in the Sangdanlin and Mubala sections (located south and west of Saga, respectively; Fig. 1) to have occurred between 59 and 61 Ma (Selandian; DeCelles et al., 2014; Hu et al., 2015b; An et al., 2021).

The Yarlung Tsangpo Ophiolite is stratigraphically proved to represent the Asian forearc basement (An et al., 2014; Wang et al., 2017). Consequently, the onset of collision between the Tethys Himalaya passive margin and the Transhimalayan arc-trench system does represent the onset of collision between India and Asia (Gansser, 1980; Garzanti, 2008; Hu et al., 2016; Najman et al., 2017). The final closure of the Neotethys Ocean (i.e., consumption of oceanic lithosphere at a point) with onset of subduction of the Indian continental margin beneath Asia (i.e., India/Asia collision onset) and ophiolite obduction onto India, therefore, represent one and the same event, which took place at middle Paleocene (Selandian) time.

SUMMARY

The upper Neoproterozoic to Eocene stratigraphic succession of the Tethys Himalaya documents the long sedimentary evolution of the northern margin of peninsular India (Table 3). In the NW Himalaya, the Paleozoic succession begins with lower-middle Cambrian siliciclastic-carbonate-siliciclastic passive-margin strata overlain with angular unconformity by Ordovician alluvial-fan conglomerates testifying to a major tectonic event related to the Pan-African orogeny. In Nepal and South Tibet, instead, the Ordovician is documented by strongly recrystallised carbonates followed by quartzarenites and calcschists. The Silurian and Devonian are represented by mixed carbonate-siliciclastic shallow-marine units overlain by the coastal Muth Quartzarenite in the NW Himalaya and by graptolite-bearing mudrocks followed by marls, dolostones capped by an ironstone interval, and grey mudrocks in central Nepal. At the top of the pre-rift succession, lower Tournaisian fossiliferous limestones are truncated by a major unconformity testifying to the onset of Neotethyan rifting, culminated in the middle early Permian to breakup and initial opening of the Neotethys Ocean.

The Mississippian to lower Permian rift sequence, bracketed between the “rift” and “breakup” unconformities and up to ~1.5 km-thick but locally missing altogether, consists of alternating black mudrocks and white feldspatho-quartzose to pure quartzose sandstones. Four diamictite intervals containing dropstones and boulders up to 1000 m³ in volume traced across South Tibet testify to formation of shore ice in the Visean-Serpukhovian. Two fossiliferous transgressive intervals are documented in the Bashkirian (Fenestella Shales) and Moscovian (Chaetetid Shales), followed by renewed extensive glaciation at Kasimovian-Gzhelian to Asselian times, when glacio-marine diamictites containing volcanic detritus were deposited. Waxing of ice caps on the rift shoulders was triggered by tectonic uplift at middle-high southern latitudes.

The base of the drift sequence is defined by a low-angle unconformity mantled by condensed horizons of fossiliferous glaucony-rich arenites of mid-Sakmarian age. These transgressive sediments reflect the combined effect of sea-level rise during deglaciation and onset of thermal subsidence of the newly formed rifted margin. In the late early Permian, continental flood basalts up to 2.5 km-thick in Kashmir, traced to Zanskar and exposed again from central Nepal to South Tibet, were emplaced at the time of inferred onset of seafloor spreading in the Neotethys Ocean. Middle to upper Permian strata seal the Paleozoic succession in all studied areas, documenting the final submergence of the rift shoulders. The overall transgressive trend, documented by glauco-phosphoritic horizons and dominance of phosphatic black shales testify to warming climate at progressively lower latitudes in the southern hemisphere, correlating with oceanographic and tectono-magmatic changes in the Tethyan domain and worldwide.

Deposition of condensed ammonoid-bearing nodular limestones alternating with black shale intervals became widespread in the Lower Triassic, followed by fossiliferous marls and marly limestones in the Middle Triassic. The marked increase in sediment thickness documented at Late Triassic times was related to a major extensional event. Proximal southern units contain feldspatho-quartzose to quartzose detritus indicating supply from peninsular India, whereas distal strata in the north both in Zanskar and South Tibet comprise lithic-rich litho-quartzose sandstones containing bimodal alkalic volcanic detritus with zircon grains yielding Carnian/Norian U–Pb ages. Penecontemporaneous magmatic activity associated with a major intraplate extensional tectonic episode plausibly associated with separation of the Lhasa Block from eastern Gondwana is suggested.

At the close of the Triassic, quartz-rich coastal sandstones are followed by Megalodon-bearing carbonates in the NW Himalaya. In the Lower Jurassic, the widespread Kioto carbonate platform was deposited at decreasing accumulation rates, reflecting reduced thermal subsidence during the mature passive-margin stage. Transgressive dark mudrocks with storm layers traced from Zanskar to South Tibet document the early Toarcian “oceanic anoxic event” associated with the emplacement of the Karoo-Ferrar large igneous province in southern Gondwana.

The disconformity at the top of the Kioto platform, related with rifting and incipient opening of the Indian Ocean, is onlapped by upper Middle Jurassic condensed horizons with francolite-chamosite-goethite ooids traced along the Tethys Himalaya. Upwelling contributed to the deposition of ammonoid-rich Spiti black shales in the Late Jurassic.

Lower Cretaceous sandstones indicate renewed tectonic activity, with composition changing up-section from quartz-rich feldspatho-quartzose to quartzo-lithic volcaniclastic. Volcanic detritus arrived as early as the Tithonian in South Tibet, in the Valanginian in Nepal, and only in the Albian in Zanskar, documenting intraplate magmatic activity coeval with the Comei and Rajmahal provinces of SE Tibet and E India during final breakup of the Gondwana supercontinent. Volcaniclastic sedimentation ended synchronously in the late Albian all along the Tethys Himalaya, when the drowned clastic shelf was mantled by glauco-phosphorites documenting sediment starvation during latest Albian and latest Cenomanian “oceanic anoxic events” OAE1d and OAE2. Above, Upper Cretaceous grey to multicoloured pelagic foraminiferal oozes are followed by bioclastic marls and quartz rich siltstones or sandstones documenting accelerated sediment supply and regression during initial impingement of the Deccan plume head at the base of Indian lithosphere.

In the Paleocene, after the onset of the India/Asia collision at 60 ± 1 Ma, shallow-marine Selandian to Thanetian carbonates with rich benthic foraminifera overlie Danian quartzarenites capped by a condensed glauconitic layer deposited at sub-equatorial latitudes. A major disconformity developed at the Paleocene/Eocene boundary during the PETM event from the Zanskar Range to South Tibet has long been ascribed to uplift of the inner margin caused by a flexural wave propagating from the suture zone. Drowning of the carbonate platform and demise of Neotehyan seaways occurred diachronously in the Ypresian-Lutetian. The unconformably overlying green and red volcaniclastic sediments, containing abundant andesitic-rhyodacitic detritus and serpentine-schist and Cr-spinel grains, testify to provenance from the uplifting Transhimalayan arc-trench system and thus certify the final closure of the Neotethys Ocean, shortly followed by obduction of forearc ophiolites onto India and progressive development of the Himalayan thrust belt.