Tethyan Himalayan Sequence Research Articles

Structural and geochronological evidence for the leading edge of the Greater Himalayan Crystalline complex in the central Nepal Himalaya

The Himalaya is commonly described as a three layer-two fault stack. Namely, a high-grade crystalline core featuring an inverted metamorphic field gradient, the Greater Himalayan Crystalline complex (GHC), is separated from units above and below by shear zones. The Lesser Himalayan Sequence (LHS) underlies the GHC below the Main Central thrust, and the Tethyan Himalayan Sequence (THS) overlies it along the South Tibet detachment. However, the southern Main Central thrust hanging wall consists of a lower unit dominated by a right-way-up metamorphic sequence of biotite ± garnet schists (Bhimphedi Group) and an upper unit with only anchizone metamorphism (Pulchauki Group). The Bhimphedi Group is commonly equated to the GHC, while the Pulchauki Group is well correlated to the THS. However, no shear zone separates these units. We present new structural and geochronological data along the boundary between the GHC and the Bhimphedi Group in the Kathmandu region. These data reveal an ~ 200 m thick Early–Middle Miocene top-north-northeast shear zone that we term the Galchi shear zone. We correlate this shear zone to the South Tibet detachment on the basis of lithological, metamorphic, structural, and chronological criteria. The Galchi shear zone merges with the Main Central thrust to the south, bounding the leading edge of the GHC. This result, combined with recent work in the western Himalaya, suggests that the locally preserved leading edge of the GHC is sub-parallel to the arc of the orogen and the southern Main Central thrust hanging wall is dominated by THS rocks. This orogenic architecture rules out wedge extrusion and channel flow-focused denudation kinematic models for the Himalayan orogen, but is accommodated by tectonic wedging kinematic models, including channel tunneling models with modified timing.

Cenozoic anatexis and exhumation of Tethyan Sequence rocks in the Xiao Gurla Range, Southwest Tibet

In order to advance our understanding of the suturing process between continental landmasses, a geologic and geochronologic investigation was undertaken just south of the India–Asia suture in southwestern Tibet. The focus of this study, the Xiao Gurla Range, is located near the southeastern terminus of the active, right-lateral strike-slip Karakoram fault in southwestern Tibet. The range exposes metasandstone, phyllite, schist (locally of sillimanite facies), calc-gneiss and marble, paragneiss (± pyroxene), quartzite, metagranite, and variably deformed leucogranite. These metamorphic rocks are exposed in the footwall of a domal, top-to-the-west low-angle normal (detachment) fault, structurally beneath Neogene–Quaternary basin fill and serpentinized ultramafic rocks of the Kiogar-Jungbwa ophiolite. The detachment is interpreted to be the northeastern continuation of the Gurla Mandhata detachment fault system that bounds metamorphic rocks of the Gurla Mandhata Range ~ 60 km to the southwest. U–Pb geochronology on five detrital zircon samples of schist, phyllite, and quartzite yielded maximum depositional ages that range from 644–363 Ma and age probability distributions that are more similar to Tethyan sequence rocks than Lesser Himalayan sequence rocks. A felsic gneiss yielded a metamorphic zircon age of 35.3 ± 0.8 Ma with a significant population of early Paleozoic xenocrystic core ages. The gneiss is interpreted to be the metamorphosed equivalent of the Cambro-Ordovician gneiss that is exposed near the top of the Greater Himalayan sequence. Leucogranitic bodies intruding metasedimentary footwall rocks yielded two distinct U–Pb zircon ages of ~ 23 Ma and ~ 15 Ma. Locally, rocks exposed in the hanging wall of this fault and of the southward-dipping, northward-verging Great Counter thrust to the north consist of serpentinite-bearing mélange and conglomerate of inferred Paleogene age dominated by carbonate clasts. The mélange is intruded by a 44 Ma granite and the stratigraphically highest conglomerate unit yielded detrital zircon U–Pb ages similar to Tethyan sequence rocks. We attribute the middle Eocene magmatism south of the suture to break-off of the Neo-Tethyan oceanic slab. In addition, our observations are consistent with the late Eocene shortening and crustal thickening within the Tethyan Himalayan sequence, early-middle Miocene leucogranite emplacement being related to underthrusting and melting of the Greater and possibly Lesser Himalayan sequences, and late Miocene arc-parallel extension in the hinterland of the southward propagating Himalayan thrust belt.

Metamorphic evolution of the Tethyan Himalayan flysch in SE Tibet

Abstract The metamorphic conditions and the age of thermal overprint were determined in metapelites, metaarenites and metabasites of the Tethyan Himalayan Sequence (THS) in SE Tibet using Kübler Index and vitrinite reflectance data and applying thermobarometrical (Thermocalc and PERPLEX) and geochronological methods (illite/muscovite K–Ar and zircon and apatite (U–Th)/He chronology). The multiple folded thrust pile experienced a thermal overprint reaching locally peak conditions between the diagenetic stage ( c . 170 °C) and the amphibolite facies ( c . 600 °C at 10 kbar). Burial diagenesis and heating due to Early Cretaceous dyke emplacement triggered the growth of illite in the metapelites. Eocene collision-related peak metamorphic conditions have been reached at c . 44 Ma. During collision the different tectonic blocks of the THS were tectonically buried to different structural levels so that they experienced maximum greenschist to amphibolite facies metamorphism. Later, during Oligocene to Miocene times the entire THS underwent anchi- to epizonal metamorphic conditions, probably associated to continuous deformation in the flysch fold-thrust-system. This period terminated at c . 24–22 Ma. Adjacent to the north Himalayan metamorphic domes, the base of the THS was metamorphosed during Miocene times ( c . 13 Ma). Post-metamorphic cooling below c . 180 °C lasted until Late Miocene and took place at different times.

In-situ detrital zircon geochronology and Hf isotopic analyses from Upper Triassic Tethys sequence strata

Marine sedimentary rocks of the Tethyan Himalayan sequence exposed south of the India–Asia suture and north of the high Himalayan mountain belt are widely attributed to India's passive margin along the southern margin of the Neo-Tethys ocean. The focus of this study are the Tethyan sequence strata exposed in southern Tibet that yield U–Pb detrital zircon age probability spectra and εHf values that are in stark contrast with Tethyan sequence strata of known Indian affinity. The rocks of the Upper Triassic Songre and Nieru Formations, exposed < 10 km south of the India–Asia suture, yield populations of U–Pb zircon ages in the range of ~ 450–220 Ma. Detrital zircon crystals of this age are not known to populate India affinity Tethyan sequence strata. The youngest cluster of ages, in the range of ~ 266–224 Ma, typically have juvenile εHf (T) values (+ 5.5–+13.5). Zircon ages and εHf (T) values in this range are consistent with igneous rocks of the Lhasa terrane on the northern margin of the Neo-Tethys ocean during Triassic time or with a hypothesized juvenile arc source from within the Neo-Tethys ocean. Collectively these unexpected results highlight the current uncertainties over the nature Neo-Tethys ocean and tectonic setting of the southern Lhasa prior to the India–Asia collision. We propose that the Upper Triassic age northern Tethyan Himalayan strata studied here represent an independent terrane from the widely studied southern Tethyan Himalayan strata. These rocks where isolated from the southern Tethyan Himalayan strata during Late Triassic time. This isolation was the result of a bathymetric barrier within the Neo-Tethys ocean such as a spreading center or an inter-Tethys arc system.

Tracing the position of the South China block in Gondwana: U–Pb ages and Hf isotopes of Devonian detrital zircons

U–Pb detrital zircon geochronology from Lower Devonian quartz arenites of the northwestern margin of the Yangtze block yields dominant early Neoproterozoic (0.85–1.0 Ga), Pan-African (0.5–0.65 Ga) and middle Neoproterozoic (0.68–0.8 Ga) age populations and minor Mesoproterozoic to middle Mesoarchean (1.0–3.0 Ga) ages. Middle Mesoarchean to Mesoproterozoic rocks, however, are widespread in the South China block. Although Hf isotopic compositions show both juvenile crustal growth and crustal reworking for all the age groupings, the crust growth, essentially mantle-derived, occurred mainly around 3.1 Ga, 1.9 Ga and 1.0 Ga, respectively. Zircon typology and youngest grain ages indicate that this suite of quartz arenites was the product of multiphase reworking. Abundant magmatic zircon detritus with concordant U–Pb Grenvillian and Pan-African ages, together with accompanying various ε Hf( t) values, indicate an exotic provenance for the quartz arenite external to the South China block. Qualitative comparisons of age spectra for the late Neoproterozoic sediments of the Cathaysian Block, early Paleozoic sediments of pre-rift Tethyan Himalaya sequence in North India and lower Paleozoic sandstone from the Perth Basin in West Australia, show that they all have two the largest age clusters representing Grenvillian and Pan-African orogenic episodes. The resemblance of these age spectra and zircon typology suggests that the most likely source for the Lower Devonian quartz arenites of the South China block was the East African Orogen and Kuunga Orogen for their early Grenvillian and Pan-African populations, whereas the Hannan–Panxi arc, Jiangnan orogen, and the Yangtze block basements might have contributed to the detrital zircon grains of the Neoproterozoic and Pre-Grenvillian ages. Hf isotopic data indicate that the crustal evolution of the drainage area matches well with the episodic crust generation of Gondwana. These results imply that the previously suggested position of the SCB in Gondwana should be re-evaluated, and the South China block should be linked with North India and West Australia as a part of East Gondwana during the assembly of Gondwana, rather than a discrete continent block in the paleo-Pacific.

The Kumaun and Garwhal Lesser Himalaya, India: Part 1. Structure and stratigraphy

Our understanding of the geologic evolution of the Himalaya remains incomplete, particularly in regard to structural and geochronologic details of the Proterozoic-Paleozoic Lesser Himalayan Sequence. We conducted an integrated field mapping, geochronological study, and geochemical analysis of the Lesser Himalayan Sequence strata in the Kumaun and Garwhal Himalaya of NW India (78° 00′–80°30′E). Structural observations reveal a systematic change in deformation styles from lower to higher structural levels in the Main Central Thrust footwall. In the south, and at lower structural levels, the Main Central Thrust footwall is characterized by parallel folding and sparse development of axial cleavage. Quartz microstructures indicate that deformation occurred at temperatures below 350 °C. In contrast, in the north and at higher structural levels close to the Main Central Thrust, deformation is characterized by replacement of original bedding, the development of penetrative cleavage, and schistosity locally. Folds are tight, in places isoclinal and overturned. The corresponding quartz microstructures indicate that deformation occurred above 350 °C. Dating of detrital zircons from Lesser Himalayan Sequence metasedimentary units, and igneous zircons from schistose gneisses throughout the Lesser Himalayan Sequence allow refinements to the stratigraphic framework. First, unlike has been observed in western Nepal immediately west of our study area, U-Pb zircon dating suggests the absence of Paleoproterozoic strata in the Ramgarh Thrust hanging wall. This suggests that the thrust cuts upsection laterally along strike. Second, U-Pb detrital zircon dating suggests that metasedimentary strata in the Ramgarh Thrust hanging wall are correlative with basal units of the Outer Lesser Himalayan Sequence, removing the necessity that the Ramgarh Thrust hanging wall was allochthonous with respect to its footwall Lesser Himalayan Sequence units. Schistose gneisses with U-Pb ages of ca. 1850 Ma are older than Lesser Himalayan Sequence units in the area, suggesting that they are thrust-emplaced Lesser Himalayan Sequence basement. We conclude that the Ramgarh Thrust is an out-of-sequence thrust postdating a folding event in its footwall. The earlier proposal that Neoproterozoic-Cambrian Lesser Himalayan Sequence strata are the southern extension of the Tethyan Himalayan Sequence requires that the Tons Thrust, which separates the distal and proximal facies of the Lesser Himalayan Sequence, be a major, south-directed structure with a slip magnitude of >50–100 km.

Does the Karakoram fault interrupt mid-crustal channel flow in the western Himalaya?

Variations in the volume and age of Miocene granites and in mid-crustal conductance from the northwest Himalaya to southeastern Tibet imply lateral differences in late orogenic processes. The change from west to east occurs near the Gurla Mandhata dome, where the Karakoram fault terminates and merges with the Indus–Yarlung suture zone. The ‘channel flow’ model, developed in southeastern Tibet, predicts that anatectic partial melts beneath the Tibetan plateau are gravitationally-driven south to a topographic erosional front and are exposed as leucogranites in the Greater Himalaya Sequence; upwellings of these channel granites occur as gneiss domes in the Tethyan Himalaya Sequence. Published magnetotelluric profiles show high conductivity 30–40 km deep beneath Tibet from c. 400 km north of the Main Frontal thrust south across the suture zone, beneath the Himalayan gneiss domes, and to the topographic front; this conductive middle crust corresponds to 2–4% partial melt in the northwest Himalaya and 5–12% melt in southeastern Tibet, sufficient in the latter case to weaken rock for flow. East of the Karakoram termination leucogranites are abundant and are as young as 7 Ma; west of the termination, channel granites are less abundant and no younger than 18 Ma. Middle Miocene (16–14 Ma) leucogranites are found in the Karakoram fault zone located north of the suture zone and south of the proposed anatectic melt source. The initiation of motion on the crustal-penetrating Karakoram fault at 25–21 Ma may have created a barrier to the southward flow of mid-crustal melts and acted as a vertical conduit for these same melts.

Nd isotopic compositions of the Tethyan Himalayan Sequence in southeastern Tibet

The Himalayan orogen consists of three major lithologic units that are separated by two major north-dipping faults: the Lesser Himalayan Sequence (LHS) below the Main Central Thrust (MCT), the Greater Himalayan Crystalline Complex (GHC) above the MCT, and the Tethyan Himalayan Sequence (THS) juxtaposed by the South Tibet Detachment fault (STD) over the GHC. Due to widespread metamorphism and intense deformation, differentiating the above three lithologic units is often difficult. This problem has been overcome by the use of Sm-Nd isotopic analysis. The previous studies suggested that the LHS can be clearly distinguished from the GHC and THS by their Nd isotope compositions. However, the lack of detailed and systematic Sm-Nd isotopic studies of the THS across the Himalaya in general has made differentiation of this unit from the nearby GHC impossible, as the two appear to share overlapping Nd compositions and model ages. To address this problem, we systematically sampled and analyzed Nd isotopes of the THS in southeastern Tibet directly north of Bhutan. Our study identifies two distinctive fields in a ɛ Nd-T DM plot. The first is defined by the ɛ Nd(210 Ma) values of −3.45 to −7.34 and T DM values of 1.15 to 1.29 Ga from a Late Triassic turbidite sequence, which are broadly similar to those obtained from the Lhasa block. The second field is derived from the Early Cretaceous meta-sedimentary rocks with ɛ Nd(130 Ma) values from −15.24 to −16.61 and T DM values from 1.63 to 2.00 Ga; these values are similar to those obtained from the Greater Himalayan Crystalline Complex in Bhutan directly south of our sampling traverse, which has ɛ Nd(130 Ma) values of −10.89 to −16.32 and Nd model ages (T DM) of 1.73 to 2.20 Ga. From the above observations, we suggest that the Late Triassic strata of the southeast Tibetan THS were derived from the Lhasa block in the north, while the Early Cretaceous strata of the THS were derived from a source similar to the High Himalayan Crystalline Complex or Indian craton in the south. Our interpretation is consistent with the existing palaeocurrent data and provenance analysis of the Late Triassic strata in southeastern Tibet, which indicate the sediments derived from a northern source. Thus, we suggest that the Lhasa terrane and the Indian craton were close to one another in the Late Triassic and were separated by a rift valley across which a large submarine fan was transported southward and deposited on the future northern margin of the Indian continent.

Geodynamics of Chamba Nappe, western Himalaya

Chamba Nappe is a SW-directed thrust sheet, which is composed of the Tethyan Sequence and was exhumed by thrusting along the Chamba thrust that is equivalent to the Panjal Thrust or the Main Central Thrust (MCT) over the Lesser Himalaya Sequence. Internal deformation within Chamba nappe is responsible for a large amount of crustal thickening and shortening (Sharma and Bhola 2004). The Chamba nappe is folded by regional scale fold viz the Chamba-Bharmaur syncline, Tandi syncline and Tisa anticline (Frank 1995, Sharma et al. 2003, Thakur 1998) and is the result of southwestward propagation of cover rocks on their basement (HHC) due to topographic uplift. The southward propagating thrust system also led to the development of major folds DF 1 , DF 2 and minor DF 3 folds in a single progressive deformation. DF1 is the main compression phase, which corresponds to early SW-directed movements associated with the formation of the Chamba Nappe. Deformation associated with DF2 phase propagated gradually from the Indus Suture Zone towards the SW as the Indian plate was progressively subducted below Asia along the Indus Suture Zone. DF3 last phase is marked by kinkband formation as a result of brittle deformation in the slate/ phyllites sequence due to amplification of Chamba nappe. These deformations resulted with the southwestwarddirected exhumation of the Tethyan Himalayan Sequence over the Higher Himalayan Crystalline. The southwestward propagation of the Chamba Nappe also affected the basal series of the Tethys Himalaya, which will ultimately form the High Himalayan Crystalline Sequence. In initial stage, mainly Proterozoic, basal series were still welded to the Tethys Himalaya, but as deformation proceeded, and as the Tethys Himalaya formed an increasingly thick stack of rocks, the lower part of the Tethys Himalaya became ductile enough to accommodate the compressive deformation through ductile shearing (Robyr et al. 2002). From that time onwards, the compressive forces were essentially concentrated along a ductile shear zone at the base of the Chamba nappe. Movements along Panjal thrust/Chamba thrust (MCT-1) zone contributed to the further underthrusting and metamorphism of the basal proterozoic series, which leads to the ultimate metamorphic differentiation between the Tethys Himalaya and the High Himalayan Crystalline Sequence.

The leading edge of the Greater Himalayan Crystalline complex revealed in the NW Indian Himalaya: Implications for the evolution of the Himalayan orogen

The three Himalayan lithologic units, the Lesser Himalayan Sequence, the Greater Himalayan Crystalline complex, and the Tethyan Himalayan Sequence, have a specific structural correlation with the Main Central thrust and South Tibet detachment in the central Himalaya. There, the Main Central thrust places the Greater Himalayan Crystalline complex over the Lesser Himalayan Sequence, and the South Tibet detachment places the Tethyan Himalayan Sequence over the Greater Himalayan Crystallines. Although this division has formed the basis for all Himalayan tectonic models, it fails to explain aspects of the geology of the western Himalaya where the Main Central thrust places the Tethyan Himalayan Sequence directly above the Lesser Himalayan Sequence. Our mapping in NW India shows that this relationship results from southward merging of the Main Central thrust and South Tibet detachment. This finding, in conjunction with observed alternating shear senses on the South Tibet detachment, is inconsistent with the wedge-extrusion and erosion-induced channel-flow models (both require only top-to-the-N motion on the South Tibet detachment) but is consistent with a tectonic-wedging model.

Cenozoic tectonic evolution of the Himalayan orogen as constrained by along-strike variation of structural geometry, exhumation history, and foreland sedimentation

Despite a long research history over the past 150 years, the geometry, kinematics, and dynamic evolution of the Himalayan orogen remain poorly understood. This is mainly due to continued emphasis on the two-dimensionality of the Himalayan orogenic architecture and extrapolation of geologic relationships from a few well-studied but small areas to the rest of the orogen. Confusion and misconception are also widespread in the Himalayan literature in terms of the geographic, stratigraphic, and structural divisions. To clarify these issues and to provide a new platform for those who are interested in studying the geologic development of this spectacular mountain belt, I systematically review the essential observations relevant to the along-strike variation of the Himalayan geologic framework and its role in Cenozoic Himalayan exhumation, metamorphism and foreland sedimentation. A main focus of my synthesis is to elucidate the emplacement history of the high-grade Greater Himalayan Crystalline Complex (GHC) that occupies the core of the orogen. Because the north-dipping Main Central Thrust (MCT) above and South Tibet Detachment (STD) below bound the GHC in most parts of the Himalaya, it is critical to determine the relationship between them in map and cross-section views. The exposed map pattern in the central Himalaya (i.e., Nepal) indicates that the MCT has a flat-ramp geometry. The thrust flat in the south carries a 2–15-km-thick slab of the GHC over the Lesser Himalayan Sequence (LHS) and creates a large hanging-wall fault-bend fold continuing > 100 km south of the MCT ramp zone. In the western Himalayan orogen at the longitude ∼ 77°E, the MCT exhibits a major lateral ramp (the Mandi ramp). West of this ramp, the MCT places the low-grade Tethyan Himalayan Sequence (THS) over the low-grade LHS, whereas east of the ramp, the MCT places the high-grade GHC over the low-grade LHS. This along-strike change in stratigraphic juxtaposition and metamorphic grade across the MCT indicates a westward decrease in its slip magnitude, possibly a result of a westward decrease in total crustal shortening along the Himalayan orogen. Everywhere exposed, the STD follows roughly the same stratigraphic horizon at the base of the THS, exhibiting a long (> 100 km) hanging-wall flat. This relationship suggests that the STD may have initiated along a preexisting lithologic contact or the subhorizontal brittle–ductile transition zone in the middle crust. Although the STD has the THS in its hanging wall everywhere in the Himalayan orogen, no THS footwall cutoffs have been identified. This has made slip estimates of the STD exceedingly difficult. The southernmost trace of the STD either merges with the MCT (e.g., in Zanskar) or lies within 1–2 km of the MCT frontal trace (e.g., in Bhutan), suggesting that the MCT may join the STD in their up-dip directions to the south. This geometry, largely neglected by the existing models, has important implications for the deformation, exhumation, and sedimentation history of the entire Himalayan orogen.