Indian Passive Margin Research Articles

Genesis and tectonic implications of cumulate pyroxenites and tectonite peridotites from the Nagaland–Manipur ophiolites, Northeast India: constraints from mineralogical and geochemical characteristics

AbstractThe Nagaland–Manipur ophiolites (NMO) of Northeast India forms a part of the Tethyan ophiolites and comprises a suite of tectonite peridotites and cumulate mafic–ultramafic sequence with mafic extrusive–intrusive rocks, felsic intrusives and oceanic pelagic sediments along with minor podiform chromitites. However, sheeted dykes, which are considered as a significant component of ophiolites, are absent in the NMO. The tectonite peridotites are distinguished from the cumulate pyroxenites by the presence of pyroxene lineation, deformed bands and strained extinction in olivine, kink twin lamellae in pyroxene. Both the tectonite peridotites and cumulate pyroxenites contain aluminous spinel with Cr number [Cr# = Cr/(Cr + Al)] in the range of 0.14 to 0.29 and 0.27 to 0.48, respectively. Mg number [Mg# = Mg/(Mg + Fe2+)] in Cr‐spinel is higher in tectonite peridotites (0.71–0.76) than cumulate pyroxenites (0.44–0.53). Chondrite‐normalized rare earth elements (REE) patterns of cumulate pyroxenites exhibit depleted at light REEE (LREE) (LaN/SmN = 0.380–0.759) but flat middle REE (MREE) to heavy REE (HREE) (SmN/YbN = 0.622–0.756). However, the tectonite peridotites show gradual decrease in concentrations from HREE to MREE (SmN/YbN = 0.285–0.460) and slight increase in LREE (LaN/SmN = 0.721–2.201). The cumulate pyroxenites show strong enriched PPGE patterns and higher PGE concentrations (∑PGE = 85.8–163.5 ppb) compared with the tectonite peridotites (∑PGE = 34.8–113.0 ppb). The estimated equilibration temperature ranges from 890 to 931 °C for cumulate pyroxenites and 971 to 1156 °C for tectonite peridotites. The olivine–spinel equilibrium along with Cr‐spinel chemistry and PGE data suggests that the tectonite peridotites represent the residual mantle left after limited extraction of basaltic melts by low‐degree partial melting (<15%). Conversely, the presence of highly magnesian orthopyroxene and clinopyroxene in the cumulate pyroxenites in conjunction with their geothermometry suggests that they were formed at high pressure and temperature by magmatic fractionation from the basaltic melt. The geochemical data together with field and petrographical evidences indicate that both the tectonite peridotites and cumulate pyroxenites are essentially spinel‐bearing and devoid of plagioclase, suggesting their derivation in the mantle beyond the stability limit of plagioclase in a mid‐oceanic ridge tectonic setting. We conclude that the ultramafic sequence of the NMO was initially generated at a mid‐oceanic ridge tectonic setting close to the eastern boundary of the Indian passive margin and then thrust over the continental margin of the Indian Plate towards the west during its collisional and subduction process with/beneath the Myanmar Plate. Copyright © 2016 John Wiley & Sons, Ltd.

Giant fossil mass wasting off the coast of West India: The Nataraja submarine slide

We use two-dimensional pre-stack depth migrated seismic reflection profiles and seafloor bathymetry to describe the continental margin structure and a massive mass-transport deposit off the west coast of India. This giant slide runs from the Gujurat–Saurashtra margin to the Laxmi Basin. It is over 330 km long, a maximum of 190 km wide and its run-out basal gradient is 1.2°. We name this giant mass wasting deposit the Nataraja Submarine Slide. This slide covers 49±16×103 km2 and represents a volume of 19×103±4×103 km3, making it the second by volume of any passive margin landslide/mass-transport deposit. Seismic facies analysis allows the internal structure of the mass-transport deposit to be described as far as the toe. This slide has been able to circumvent massive seamounts, thus highlighting the capacity of the flow and its potential energy during emplacement in a funnel between the slope of the Western Indian passive margin and the Laxmi Ridge. Stratigraphically, the emplacement of the Nataraja Slide predates the main pulse of sedimentation during the late Miocene–Recent associated with the Indus Fan but follows rapid sedimentation across S and SE Asia during the Early–Middle Miocene. The margin architecture at the head of this slide is associated with a gravity-controlled fold and thrust belt that may have caused slope steepening and triggering of the slide.

Sandstone provenance and tectonic evolution of the Xiukang Mélange from Neotethyan subduction to India–Asia collision (Yarlung-Zangbo suture, south Tibet)

The Xiukang Mélange of the Yarlung-Zangbo suture zone in south Tibet documents low efficiency of accretion along the southern active margin of Asia during Cretaceous Neotethyan subduction, followed by final development during the early Paleogene stages of the India–Asia collision. Here we present integrated petrologic, U–Pb detrital-zircon geochronology and Hf isotope data on different types of sandstone blocks in the Xiukang Mélange. Three groups of sandstone blocks with different provenance and depositional setting are distinguished by their petrographic, geochronological and isotopic fingerprints. Blocks of turbiditic quartzarenite originally sourced from the Indian continent were deposited in pre-Cretaceous time on the northernmost edge of the Indian passive margin and eventually involved into the mélange at the early stage of the India–Asia collision. Two distinct groups of volcaniclastic-sandstone blocks were derived from the central Lhasa block and Gangdese magmatic arc. One group was deposited in the trench and/or on the trench slope of the Asian margin during the early Late Cretaceous, and the other group in a syn-collisional basin just after the onset of the India–Asia collision in the Early Eocene. The largely erosional character of the Asian active margin in the Late Cretaceous is indicated by the scarcity of off-scraped trench-fill deposits and the relatively small subduction complex developed during limited episodes of accretion. The Xiukang Mélange was finally structured in the Late Paleocene/Eocene, when sandstone blocks of both Indian and Asian origin were progressively incorporated tectonically in the suture zone of the nascent Himalayan Orogen.

Carbon isotope records of the early Albian oceanic anoxic event (OAE) 1b from eastern Tethys (southern Tibet, China)

The early Albian Oceanic Anoxic Event (OAE), i.e., OAE1b, is well documented in western Tethys and in the primary North Atlantic Ocean, but has not yet been reported from eastern Tethys. In this paper, we present bulk carbon isotope data of hemipelagites to examine if it was recorded in eastern Tethys. Samples were taken from the upper Chuangdepu Member (nannofossil zone CC8) of the lower Gyabula (former Shadui) Formation at the Bangbu section, Qonggyai, southern Tibet of China. The δ13C values mainly range from −0.6‰ to 1.8‰ with a maximum of 1.87‰ and a minimum of −0.69‰. Three stages of carbon isotope evolution were distinguished with three boundaries. By the constraint of the stratigraphic sequence and nannofossil biostratigraphic zone CC8, the rapid δ13C change and correlation with western Tethys and Atlantic Ocean together suggest that these three boundaries of the carbon isotope evolution probably correspond to three subevents of the early Albian OAE1b, and the subevent levels of upper Kilian, Paquier, and Leenhardt are recorded in eastern Tethys (southern Tibet). The fact that the amount of δ13C shift is less by ∼1.5–2.0‰ in eastern Tethys than in western Tethys and Atlantic Ocean is interpreted as a result of possible cool sea surface (∼14–16 °C) of the southeastern Tethys (northern Indian passive margin of Greater India), which was probably located in a medium–high latitude during the Albian, leading to low primary productivity. The recognition of OAE-1b from Tethys Himalaya can improve our understanding of the Tethys and global paleoclimatic and paleoceanographic changes during the mid-Cretaceous.

Radiolarian biostratigrahy of a Barremian–Aptian deep marine succession on the Indian passive margin and its record of Oceanic Anoxic Event, Southern Tibet

Radiolarian biostratigrahy of a Barremian–Aptian deep marine succession on the Indian passive margin and its record of Oceanic Anoxic Event, Southern Tibet

Paleogene carbonate microfacies and sandstone provenance (Gamba area, South Tibet): Stratigraphic response to initial India–Asia continental collision

The Paleogene marine strata in the Gamba area of south Tibet comprise carbonates of the Zongpu Formation and siliciclastic rocks of the Enba and Zhaguo Formations, documenting the final stages of marine deposition in the Tethyan Himalaya. The ∼350-m-thick Zongpu Formation was dated as late Danian to Ypresian based on larger benthic foraminifers. Thirteen distinct microfacies identify three sedimentary environments. Mudstone, wackestone with Udoteacean algae, bioclastic-peloidal packstone, packstone with Rotaliids and green algae, floatstone with Alveolina and Orbitolites were deposited in restricted lagoonal environments. Bioclastic packstone and grainstone with Rotaliids were deposited in high-energy shoal environments. Floatstones with Nummulitids or Alveolinids were deposited in shallow open-marine environments. The Zongpu Formation was accumulated on a carbonate ramp. It documents two deepening-upward sequences separated by an unconformity corresponding to the Palaeocene/Eocene boundary and marked by a conglomerate with limestone clasts. The overlying Enba Formation comprises greenish grey calcareous shales intercalated with litho-quartzose sandstones in the upper part and capped by subaerial litho-quartzose red beds of the Zhaguo Formation. Petrographic analysis, detrital zircon geochronology and Hf isotopic data indicate that detritus in the Enba and Zhaguo Formations, deposited on the Indian passive margin, was derived from the Asian active margin in the north. These clastic units were thus deposited after the onset of the India–Asia continental collision in the early Himalayan foreland basin. Major lithological and paleoenvironmental changes occurred at three stratigraphic levels: the Jidula/Zongpu boundary (∼62Ma), the Paleocene/Eocene boundary (∼56Ma) and the Zongpu/Enba boundary (∼51Ma). Our provenance study confirms that the India–Asia collision was already under way during the deposition of the Enba Member (i.e., by ∼51Ma) and, along with facies analysis and general palaeogeographic considerations, indicates that Neo-Tethys was still wide open during the Early–Middle Paleocene. It is thus argued, consistently with previous studies, that the Paleocene/Eocene disconformity documented in the Gamba area as in the northwestern Tethyan Himalaya is likely to record flexural uplift consequent to initial underthrusting of the Indian continental margin beneath Asia at, or just a little earlier than, 56Ma.

Near N–S paleo-extension in the western Deccan region, India: Does it link strike-slip tectonics with India–Seychelles rifting?

This is the first detailed report and analyses of deformation from the W part of the Deccan large igneous province (DLIP), Maharashtra, India. This deformation, related to the India–Seychelles rifting during Late Cretaceous–Early Paleocene, was studied, and the paleostress tensors were deduced. Near N–S trending shear zones, lineaments, and faults were already reported without significant detail. An E–W extension was envisaged by the previous workers to explain the India–Seychelles rift at ~64 Ma. The direction of extension, however, does not match with their N–S brittle shear zones and also those faults (sub-vertical, ~NE–SW/~NW–SE, and few ~N–S) we report and emphasize in this work. Slickenside-bearing fault planes, brittle shear zones, and extension fractures in meso-scale enabled us to estimate the paleostress tensors (directions and relative magnitudes). The field study was complemented by remote sensing lineament analyses to map dykes and shear zones. Dykes emplaced along pre-existing ~N–S to ~NE–SW/~NW–SE shears/fractures. This information was used to derive regional paleostress trends. A ~NW–SE/NE–SW minimum compressive stress in the oldest Kalsubai Subgroup and a ~N–S direction for the younger Lonavala, Wai, and Salsette Subgroups were deciphered. Thus, a ~NW/NE to ~N–S extension is put forward that refutes the popular view of E–W India–Seychelles extension. Paleostress analyses indicate that this is an oblique rifted margin. Field criteria suggest only ~NE–SW and ~NW–SE, with some ~N–S strike-slip faults/brittle shear zones. We refer this deformation zone as the "Western Deccan Strike-slip Zone" (WDSZ). The observed deformation was matched with offshore tectonics deciphered mainly from faults interpreted on seismic profiles and from magnetic seafloor spreading anomalies. These geophysical findings too indicate oblique rifting in this part of the W Indian passive margin. We argue that the Seychelles microcontinent separated from India only after much of the DLIP erupted. Further studies of magma-rich passive margins with respect to timing and architecture of deformation and emplacement of volcanics are required.

Latest Cretaceous Himalayan tectonics: Obduction, collision or Deccan-related uplift?

Diverging interpretations and incompatible scenarios have been proposed for the early stages of Himalayan history. Numerous researchers have postulated that northern India was involved in ophiolite obduction, arc–continent, or continent–continent collision during the Late Cretaceous or Early Paleocene, but firm geological evidence was never produced. In this article we argue against orogenic events predating the Late Paleocene, when the Neotethys Ocean was still open. The Tethys Himalayan sedimentary record testifies to anorogenic evolution, primarily controlled by dynamic uplift of the passive margin prior to the massive outburst of Deccan lavas and eventually followed by thermal subsidence. Major stratigraphic gaps in pelagic sediments suggest that such tectono-magmatic episode started to affect the base of the Indian Plate in the Campanian or possibly even in the Santonian, 10 to 20Ma before the climax of Deccan flood-basalt eruptions. The abrupt increase in siliciclastic supply and accumulation rates recorded in sedimentary basins all around the Indian subcontinent during the Maastrichtian was followed by progradation of coastal quartzarenites along the northern Indian margin in the Early Paleocene. Sandstones derived from the rejuvenated craton and uplifted inner continental margin in the south are dominantly but not exclusively quartzose. Felsitic volcanic rock fragments and Cr-spinels, many of which with the same geochemical fingerprint as Deccan spinels and newly found throughout the Maastrichtian to Danian succession, resisted the combined effect of subequatorial weathering and subsequent diagenesis, and testify that detritus from Deccan basalts reached the Indian passive margin as far as South Tibet. At the close of the Early Paleocene India drifted away from the Seychelles block, and thermal subsidence led to widespread carbonate deposition along the Tethys Himalaya. This article illustrates how mega-events of magmatic upwelling followed by lithospheric cooling may control passive-margin sedimentation and stratigraphic patterns, as occurred in northern India first in the Early Cretaceous and next in the latest Cretaceous/Paleocene.

Evidence of pre-Oligocene emergence of the Indian passive margin and the timing of collision initiation between India and Eurasia

Precise knowledge of the timing of Indo-Eurasian collision is prerequisite for understanding the subsequent evolution of the HimalayanTibetan orogenic system, yet the topic remains controversial despite decades of research. We present new data for the Upper Oligocene Basgo Formation of the Indus Basin of NW India that specifi cally address the proposal that collision initiated no earlier than the EoceneOligocene boundary. The Basgo Formation has been cited as the base of the Indus Group because of its previously assumed Maastrichtian age. This age has been revised to Upper Oligocene, but the stratigraphic location has not been re-evaluated. As such, it has been used as evidence of Oligocene-aged collision between India and Eurasia. Based on age constraints in the remainder of the Indus Group, we revise the stratigraphy and place it instead toward the top of the succession. We present evidence that the zircons in the Basgo sandstones originated from the Indian passive margin. Because conglomerate clasts are known to come from the Transhimalayan batholith to the north, our data support mixed provenance and require a minimum late Oligocene age for India-Eurasia collision in the NW Indian Himalaya. (U-Th)/He cooling dates for detrital zircons from the Basgo Formation range from 52.6 to 28.25 Ma, however, implying that their most probable source, the Indian passive margin, was emergent and eroding prior to Oligocene time due to collision. These data alone do not speak to whether the Basgo Formation records pre-Oligocene collision of India and Eurasia or India and the Transhimalayan Ladakh batholith, but as of the date of this publication, there is no evidence for Oligocene collision anywhere else in the Ladakh region. Thus, we interpret our data to demonstrate terminal collision between India and Eurasia prior to Oligocene time.

U-Pb zircon crystallization age of the Muslim Bagh ophiolite: Enigmatic remains of an extensive pre-Himalayan arc

distinguishing between these alternatives and unraveling the tectonics of this area cannot be fully understood until the timing of ophiolite formation is known. In this paper we present U-Pb dating of zircons from plagiogranites within the sheeted dike component of the Mus- lim Bagh ophiolite, Pakistan. These critical new data provide clear evidence of the mag- matic age of the intra-arc or backarc origin of ophiolites correlated with Himalayan arc sys- tems and help resolve Tethyan tectonics prior to the Himalayan orogeny. ABSTRACT The pre-Himalayan evolution of Tethys is fundamental in interpreting the signifi cance of India's Paleogene deceleration and the timing of the India-Eurasia collision—the archetype continental collision that governs our understanding of collisional processes. Here we date zircons from plagiogranite sheeted dikes in the Muslim Bagh ophiolite that yield a U-Pb age of 80.2 ± 1.5 Ma. This age is interpreted as dating the crystallization of plagiogranites that occur at the top of gabbro plutons as well as within sheeted dikes. These represent highly fractionated magma chamber differentiates. The plagiogranite's trace element abundances are typical of subduction petrogenesis and can be explained by the addition of slab-derived components to a depleted mantle wedge. Muslim Bagh ophiolite formation was coeval with the Spongtang Arc in a similar tectonic environment. We interpret the chain of ophiolites within western Pakistan as corollaries of the Spongtang Arc that formed ~15 Ma before they were obducted on the Indian passive margin. These form an extensive arc system that developed as oceanic crust in Tethys foundered during India's migration north. Latest Maastrichtian ophiolite-sole formation and subsequent Paleocene obduction of the arc onto India was the fi rst arc-India collision—a herald of the collisions to come later.

Tectonic evolution of the India–Asia suture zone since Middle Eocene time, Lopukangri area, south-central Tibet

Suture zones often archive complex geologic histories underscored by episodes of varying style of deformation associated with intercontinental collision. In the Lopukangri area of south-central Tibet (29°54′N, 84°24′E) field relationships between tectonic units juxtaposed by the India–Asia suture are well exposed, including Indian passive margin rocks (Tethyan Sedimentary Sequence), forearc deposits (Xigaze Group), magmatic arc rocks (Gangdese batholith and Linzizong Formation) and syncollision deposits (Eocene–Miocene conglomerates). To better understand the structural history of this area, we integrated geologic mapping with biotite 40Ar/39Ar thermochronology and zircon U–Pb geochronology. The first-order structure is a system of north-directed thrusts which are part of the Great Counter thrust (GCT) that places Indian passive margin rocks and forearc deposits on top of magmatic arc rocks and syn-tectonic conglomerates. We infer the south-directed Late Oligocene Gangdese Thrust (GT) exists at unexposed structural levels based on field mapping, cross sections, and regional correlations as it has been documented immediately to the east. A granite in the footwall has a U–Pb zircon age of 38.4 ± 0.4 Ma, interpreted to be the age of emplacement of the granite, and a younger 40Ar/39Ar biotite age of 19.7±0.1Ma. As the granite sample is situated immediately below a nonconformity with low grade greenschist facies rocks, we interpret the younger age to reflect Miocene resetting of the biotite Ar system. Syn-tectonic deposits in the Lopukangri area consist of three conglomerate units with a total thickness of ∼1.5km. The lower two units consist of cobble gravel pebble conglomerates rich in volcanic and plutonic clasts, transitioning to conglomerates with only sedimentary clasts in the upper unit. We correlate the syncollision deposits to the Eocene–Oligocene Qiuwu Formation based on field relationships, stratigraphy and petrology. Petrology and clast composition suggest the lower two units of the Qiuwu Formation had a northern provenance (Lhasa block and magmatic arc) and the upper unit had a southern provenance (Tethyan Sedimentary Sequence). Our observations are consistent with paleocurrent data from other studies which suggest a predominant south-directed paleoflow for this formation. We propose a model in which: (1) granites intrude at 38.4±0.4Ma; (2) are exhumed by erosion; (3) and buried due to regional subsidence and initial deposition of a conglomerate unit; (4) exposed by the GT at ∼27–24Ma to provide detritus; (5) buried a second time by hanging wall-derived sedimentary deposits and the GCT, then (6) exposed from a depth of ∼12–10km by a blind thrust at ∼19Ma. An alternate model describes: (1) intrusion of the granites at 38.4±0.4Ma, followed by (2) exhumation of the granites via normal faulting to provide detritus; (3) then burial by the GCT at ∼24Ma, followed by (4) exhumation via regional erosional denudation at ∼19Ma. Exposure of the GT west of Xigaze has not been confirmed. We suggest that shallower structural levels of the India-Asia suture zone are exposed to the west of the study area, compared to the east, where the GT has been previously documented. The GCT in the area is short-lived, as it is cut and offset by a Middle Miocene ∼N-striking W-dipping oblique normal fault system.

Depositional environments of Campanian–Maastrichtian successions in the Kirthar Fold Belt, southwest Pakistan: Tectonic influences on late cretaceous sedimentation across the Indian passive margin

The creation of a complex submarine morphology on the passive margin of the Indo-Pakistan continental plate during later stages of its northward drift, just prior to collision with Eurasia, is clearly demonstrated by the nature, architecture and regional distribution of sandstone bodies within the Campanian–Maastrichtian successions (Mughal Kot and Pab formations), exposed within the north–south trending Kirthar Fold Belt of southwest Pakistan. These successions range in thickness from 7 m to more than 500 m and are dominated by fine to coarse, sometimes pebbly, thin to thick-bedded sandstones with subordinate mudstones and marls, all of which were deposited on the north-facing passive margin of the Indo-Pakistan Plate. Twelve facies have been identified in these sequences and these have been grouped into nine Facies Associations, which were formed within two broadly coeval depositional systems that occupied the contiguous Southern Kirthar and Central Kirthar sub-basins. It is argued here that these sub-basins and their fill-sequences developed mainly in response to major, tectonically induced changes in both the morphology of this sector of the palaeo-margin and in the sediment-supply systems. The more proximal sequences in the east and southeast sectors of the Southern Kirthar sub-basin are dominated by fluviodeltaic deposits while more distal areas to the northwest are characterized by turbidite sandstone bodies formed in channel-levee and lobe complexes within slope and basin floor settings of a relatively confined deep marine trough. In this Southern Kirthar sub-basin, the Mughal Kot Formation comprises basin floor lobes, submarine channel-fill sand bodies and base of slope mud-rich lobes, whereas the succeeding Pab Formation reflects overall progradation of the system, being dominated by submarine slope fan lobes, channels and levee deposits. The Central Kirthar sub-basin displays a broadly east-to-west transition from shoreface (inner shelf), through shelfal delta lobes (middle shelf) and deeper shelf/ramp (outer shelf) facies associations, all formed on a gently inclined, storm- and flood-dominated clastic ramp. Synsedimentary normal faults observed in this study were controlled the depositional architecture and creation of closed deep marine southern Kirthar sub-basin during Late Cretaceous.

Thermal regime of the northwest Indian rifted margin – Comparison with predictions

We use a simple approach to estimate the present-day thermal regime along the northwestern part of the Western Indian Passive Margin, offshore Pakistan. A compilation of bottom borehole temperatures and geothermal gradients derived from new observations of bottom-simulating reflections (BSRs) allows us to constrain the relationship between the thermal regime and the known tectonic and sedimentary framework along this margin. Effects of basin and crustal structure on the estimation of thermal gradients and heat flow are discussed. A hydrate system is located within the sedimentary deep marine setting and compared to other provinces on other continental margins. We calculate the potential radiogenic contribution to the surface heat flow along a profile across the margin. Measurements across the continental shelf show intermediate thermal gradients of 38–44 °C/km. The onshore Indus Basin shows a lower range of values spanning 18–31 °C/km. The Indus Fan slope and continental rise show an increasing gradient from 37 to 55 °C/km, with higher values associated with the thick depocenter. The gradient drops to 33 °C/km along the Somnath Ridge, which is a syn-rift volcanic construct located in a landward position relative to the latest spreading center around the Cretaceous–Paleogene transition.

Cretaceous-Tertiary Carbonate Platform Evolution and the Age of the India-Asia Collision along the Ladakh Himalaya (Northwest India)

ABSTRACT The India-Asia collision resulted in the formation and uplift of the Himalaya and the enhanced uplift of the Tibetan plateau. The transition from marine to continental facies within the Indus–Yarlung Tsangpo suture zone and along the northern margin of the Indian plate provides the most accurate method of dating the closure of the Tethys Ocean separating the Indian and Asian plates. Other indirect methods of dating the collision, such as paleomagnetism, dating the UHP metamorphism along the north margin of India, dating the youngest subduction-related granites along the southern margin of Asia, and dating the postorogenic Indus Molasse Group deposits within the suture zone, cannot provide such a precise or reliable age of collision. Ophiolite obduction onto the Indian passive margin occurred during the latest Cretaceous and predated initial collision of the two continental plates. Unconformities occur beneath the Late Maastrichtian Marpo Formation and beneath the Danian Stumpata Formation on the ...

Preliminary stratigraphic and structural architecture of Bhutan: Implications for the along strike architecture of the Himalayan system

Preliminary mapping and stratigraphic correlation of Lesser Himalayan rock in eastern Bhutan using field characteristics, U–Pb detrital zircon dating, and ɛNd geochemistry define the first-order stratigraphic architecture of the Indian passive margin sequence in the eastern Himalaya. We use this new image of the lateral and vertical relationships of the original stratigraphy to determine the structural framework of the eastern Himalayan fold-thrust belt in Bhutan. We propose that Lesser Himalayan rock in Bhutan can be divided into lower Lesser Himalayan rocks with a Paleoproterozoic detrital zircon signal, and upper Lesser Himalayan rocks with detrital zircon signals of ∼ 1000–500 Ma. The ∼ 500 Ma detrital grains are from rocks in the frontal portions of the fold-thrust belt north of the Main Boundary thrust as well as directly in the footwall of the Main Central thrust in the hinterland. Our preliminary stratigraphic study coupled with mapping allows us to construct a composite balanced cross-section through the Kuru Chu valley in eastern Bhutan which provides the first image of the geometry and amount of shortening through Bhutan. The Main Frontal thrust tilts a 6 km of thick section of Neogene foreland basin deposits. These deposits are separated from folded and faulted upper Lesser Himalayan rocks by the Main Boundary thrust. North of the Main Boundary thrust, we propose two duplex systems. The southernmost duplex contains 9 repeated sections of upper Lesser Himalayan units, including the Permian Gondwana Sequence and the Cambrian (or younger) Baxa Group. The northernmost duplex is located in the footwall of the Main Central thrust, and is comprised of two repeated sections of the Proterozoic Shumar and Daling Formations. The southern boundary of this northern duplex is the Shumar thrust which acts as a roof thrust for the southern duplex system and may correlate to the Ramgarh thrust in Nepal and India. We propose that the development of the duplex systems passively folded overlying Tethyan and Greater Himalayan rocks. Minimum shortening for this part of the Himalayan fold-thrust belt is 359 km, all of which has occurred from 22 Ma to present defining a long-term shortening rate of 16 mm/yr.

Presence of two plume-related volcanic events along the Indian northern passive margin evidenced by the geochemistry of the Carboniferous Baralacha La dykes, the Permian Panjal traps and the Drakkar Po phonolites

Himalayan Journal of Sciences Vol.2(4) Special Issue 2004 pp. 113

Metamorphic history and geodynamic significance of high-grade metabasites from the ophiolitic mélange beneath the Yarlung Zangbo ophiolites, Xigaze area, Tibet

Blocks of highly foliated amphibolite are locally embedded within a serpentinite mélange underlying the Yarlung Zangbo ophiolites in the Xigaze area of southern Tibet. The ophiolites are remnants of an Early Cretaceous back-arc basin within the Permo-Cretaceous Tethys Ocean, which are exposed along in the Yarlung Zangbo Suture Zone (YZSZ). These amphibolites are interpreted as fragments of a dismembered dynamothermal sole. Three types of amphibolite are present: (1) common amphibolite with assemblages of Hbl + Pl ± Ep ± Ap ± Ttn, (2) clinopyroxene-bearing amphibolite with Hbl ± Pl ± Cpx ± Ep ± Ttn ± Qtz ± Ap and (3) garnet–clinopyroxene-bearing amphibolite characterized by the assemblages Hbl + Cpx + Grt + Pl ± Rt and Grt + Hbl + Pl (corona assemblage). In all three types, plagioclase is pseudomorphed by late albite–prehnite. Retrograde cataclastic veins containing assemblages of Prh + Ab + Ep ± Chl are also present. P–T estimates indicate that the amphibolites reached peak metamorphic conditions of 13–15 kbar and 750–875 °C. Partial replacement of pyrope-rich (up to 35 mole%) garnet by Al-tschermakite (Al 2O 3 up to 21 wt%) reflects a high pressure (≈18 kbar, 600 °C) metamorphic event followed by rapid exhumation. Soon after exhumation, the amphibolites were intruded by very fine-grained diabase dykes that were then hydrothermally altered. The field relationships and metamorphic history of the amphibolites indicate formation during inception of subduction within a back-arc basin prior to obduction of the ophiolites onto the Indian passive margin.

Sedimentation record in the Konkan–Kerala Basin: implications for the evolution of the Western Ghats and the Western Indian passive margin

ABSTRACTThe Konkan and Kerala Basins constitute a major depocentre for sediment from the onshore hinterland of Western India and as such provide a valuable record of the timing and magnitude of Cenozoic denudation along the continental margin. This paper presents an analysis of sedimentation in the Konkan–Kerala Basin, coupled with a mass balance study, and numerical modelling of flexural responses to onshore denudational unloading and offshore sediment loading in order to test competing conceptual models for the development of high‐elevation passive margins. The Konkan–Kerala Basin contains an estimated 109 000 km3 of Cenozoic clastic sediment, a volume difficult to reconcile with the denudation of a downwarped rift flank onshore, and more consistent with denudation of an elevated rift flank. We infer from modelling of the isostatic response of the lithosphere to sediment loading offshore and denudation onshore infer that flexure is an important component in the development of the Western Indian Margin. There is evidence for two major pulses in sedimentation: an early phase in the Palaeocene, and a second beginning in the Pliocene. The Palaeocene increase in sedimentation can be interpreted in terms of a denudational response to the rifting between India and the Seychelles, whereas the mechanism responsible for the Pliocene pulse is more enigmatic.

Crustal architecture of the Himalayan metamorphic front in eastern Nepal

The Himalayan Metamorphic Front consists of two basinal sequences deposited on the Indian passive margin, the Mesoproterozoic Lesser Himalayan Sequence and the Neoproterozoic–Cambrian Greater Himalayan Sequence. The current paradigm is that the unconformity between these two basinal sequences coincides with a crustal-scale thrust that has been called the Main Central Thrust, and that this acted as the fundamental structure that controlled the architecture of the Himalayan Metamorphic Front. Geological mapping of eastern Nepal and eight detailed stratigraphic, kinematic, strain and metamorphic profiles through the Himalayan Metamorphic Front define the crustal architecture. In eastern Nepal the unconformity does not coincide with a discrete structural or metamorphic discontinuity and is not a discrete high strain zone. In recognition of this, we introduce the term Himalayan Unconformity to distinguish it from high strain zones in the Himalayan Metamorphic Front. The fundamental structure that controls orogen architecture in eastern Nepal occurs at higher structural levels within the Greater Himalayan Sequence and we suggest the name; High Himal Thrust. This 100–400 m thick mylonite zone marks a sharp deformation discontinuity associated with a steep metamorphic transition, and separates the Upper-Plate from the Lower-Plate in the Himalayan Metamorphic Front. The high- T/moderate- P metamorphism at ∼ 20–24 Ma in the Upper-Plate reflects extrusion of material between the High Himal Thrust and the South Tibet Detachment System at the top of the section. The Lower-Plate is a broad schistose zone of inverted, diachronous moderate- T/high- P metamorphic rocks formed between ∼ 18 and 6 Ma. The High Himal Thrust is laterally continuous into Sikkim and Bhutan where it also occurs at higher structural levels than the Himalayan Unconformity and Main Central Thrust (as originally defined). To the west in central Nepal, the Upper-Plate/Lower-Plate boundary has been placed at lower structural levels, coinciding with the Himalayan Unconformity and has been named the Main Central Thrust, above the originally defined Main Central Thrust (or Ramgarh Thrust).

The Carboniferous Baralacha La basaltic dykes (Upper Lahul, Ladakh) : remnants of an early rifting event along the Indian northern plate

Abstract The Lower Carboniferous Baralacha La basaltic dykes were emplaced along transtensional faults. The basalts exhibit tholeiitic and alkaline affinities. The tholeiites are TiO2-poor, moderately enriched in light rare earth (LREE), and display Nb and Ta negative and Th positive anomalies. The alkali basalts, compared to the tholeiites, have higher TiO2, rare earth and highly incompatible trace element contents and greater LREE enrichments. The Nd and Pb isotope compositions of the Baralacha La basalts suggest that they derive from the partial melting of an enriched OIB mantle source, characterized by a HIMU component, and contaminated by the lower continental crust. The Baralacha La dyke swarm represent the remnants of an early rifting event on the northern Indian passive margin.