Evolution Of Himalaya Research Articles

Top-to-south shear at the base of the eastern Tethyan Himalayan Sequence during the Eocene-Oligocene Himalayan orogeny

The early mountain building processes in the Himalayan orogen are still not clear because of extensive deformation and metamorphism since the Miocene. A large gently dipping ductile shear zone, referred as the Tethyan Himalayan Décollement (THD), is defined here as the sole décollement of the south-verging Tethyan fold-and-thrust belt in the Lhozag-Cuona area of the eastern Himalayan orogen. The ∼4 km-thick THD is characterized by a top-to-south shear sense, moderate T/P Barrovian to high T/P Buchan type metamorphism and Eocene-Miocene partial melting. Zircon U-Pb dating of metasedimentary rocks and granitic gneisses from the THD yields protolith ages of the Late Cambrian to Early Ordovician. Based on structural analysis, zircon U-Pb ages, monazite U-Pb ages and mica 40Ar/39Ar thermochronology, the THD was the boundary shear zone at the top of the Greater Himalayan Crystalline Complex (GHC) and accommodated the persistent north-south shortening in the Tethyan Himalayan Sequence (THS) from ∼50 Ma to 20–17 Ma. From ∼20–17 Ma, the top-to-north South Tibetan Detachment System (STDS) was predominantly activated to juxtapose the unmetamorphosed or low-grade THS over the GHC. This tectonic transition can be attributed to the roof collapse in the eastern Himalaya (younger than that of the central-western Himalaya), which triggered rapid exhumation of the GHC and the northern Tethyan Himalayan gneiss domes. Hence, the THD was the predecessor of the STDS and a prolonged pathway for leucogranitic melts from the Eocene to early Miocene. The transition from the THD-controlled crustal thickening in the Eocene and Oligocene to the STDS-controlled extrusion in the Miocene shed insights on a new synthesis of the tectonic wedging model for the Himalayan evolution.

When did the Indus River of South-Central Asia take on its “modern” drainage configuration?

Abstract For sedimentary archives to be used as a record of hinterland evolution, the factors affecting the archive must be known. In addition to tectonics, a number of factors, such as changes in climate and paleodrainage, as well as the degree of diagenesis, influence basin sediments. The Indus River delta-fan system of South-Central Asia records a history of Himalayan evolution, and both the onshore and offshore sedimentary repositories have been studied extensively to research orogenesis. However, a number of unknowns remain regarding this system. This paper seeks to elucidate the paleodrainage of the Indus River, in particular when it took on its modern drainage configuration with respect to conjoinment of the main Himalayan (Punjabi) tributary system with the Indus trunk river. We leverage the fact that the Punjabi tributary system has a significantly different provenance signature than the main trunk Indus River, draining mainly the Indian plate. Therefore, after the Punjabi tributary system joined the Indus River, the proportion of Indian plate material in the repositories downstream of the confluence should have been higher than in the upstream repository. We compared bulk Sr-Nd data and detrital zircon U-Pb data from the Cenozoic upstream peripheral foreland basin and downstream Indus delta and Indus Fan repositories. We determined that throughout Neogene times, repositories below the confluence had a higher proportion of material from the Indian plate than those above the confluence. Therefore, we conclude that the Indus River took on its current configuration, with the Punjabi tributary system draining into the Indus trunk river in the Paleogene, early in the history of the orogen. The exact time when the tributary system joined the Indus should correlate with a shift to more Indian plate input in the downstream repositories only. While the upstream repository records no change in Indian plate input from Eocene to Neogene times, a shift to increased material from the Indian plate occurs at the Eocene–Oligocene boundary in the delta, but sometime between 50 Ma and 40 Ma in the fan. Though further work is required to understand the discrepancy between the two downstream repositories, we can conclude that the tributary system joined the Indus trunk river at or before the start of the Oligocene.

Kyanite petrogenesis in migmatites: resolving melting and metamorphic signatures

Aluminosilicates (kyanite, sillimanite and andalusite) are useful pressure–temperature (P–T) indicators that can form in a range of rock types through different mineral reactions, including those that involve partial melting. However, the presence of xenocrystic or inherited grains may lead to spurious P–T interpretations. The morphologies, microtextural positions, cathodoluminescence responses and trace element compositions of migmatite-hosted kyanite from Eastern Bhutan were investigated to determine whether sub-solidus kyanite could be distinguished from kyanite that crystallised directly from partial melt, or from kyanite that grew peritectically during muscovite dehydration reactions. Morphology and cathodoluminescence response were found to be the most reliable petrogenetic indicators. Trace element abundances generally support petrographic evidence, but protolith bulk composition exerts a strong control over absolute element abundance in kyanite. Sample-normalised concentrations show distinctive differences between petrogenetic types, particularly for Mg, Ti, V, Cr, Mn, Fe and Ge. LA-ICP-MS element maps, particularly combined to show Cr/V, provide additional information about changing geochemical environments during kyanite growth. Most kyanite in the studied migmatitic leucosomes is of sub-solidus origin, with less widespread evidence for peritectic crystallisation. Where present, grain rims commonly crystallised directly from the melt; however, entire grains crystallised exclusively from melt are rare. The presence of kyanite in leucosomes does not, therefore, necessarily constrain the P–T conditions of melting, and the mechanism of growth should be determined before using kyanite as a P–T indicator. This finding has significant implications for the interpretation of kyanite-bearing migmatites as representing early stages of melting during Himalayan evolution.

Accretion of the NW Himalayan foreland pre‐dates Late Cenozoic climate change

AbstractConceptual models demonstrate that climate can control the structural evolution of mountain belts and that Pliocene–Quaternary global climate change modulated Himalayan wedge behaviour. A key test of this hypothesis is the timing of foreland accretion. If foreland accretion pre‐dates climate change, then climate is not the primary driver of mountain building. New apatite (U–Th)/He data from NW Himalaya foreland thrust sheets suggest that accretion had occurred by the latest 4 Ma, prior to the Pliocene–Quaternary global climate change. Climate cannot, therefore, be invoked as the principal driver of Himalayan tectonics in the late Cenozoic.Statement of significanceThis paper presents 66 new, single grain apatite (U–Th)/He (AHe) dates from the foreland basin that constrain NW Himalayan evolution through the late Cenozoic. We show that accretion of the NW sub‐Himalayan foreland was underway by 4 Ma at the latest, several million years earlier than previous estimates, and was followed by distributed, wedge‐internal deformation by 2 Ma. These and other published data present a consistent pattern and timing of deformation observed over nearly 600 km of the NW sub‐Himalaya. Our new constraints on orogen evolution provide key insights into the relative importance of climate and tectonics in driving orogenic systems.

Eohimalayan metamorphism and subsequent tectonic quiescence explained

The early evolution of the Himalayan-Tibetan orogen remains enigmatic. A wide range of observations can be interpreted to suggest that two distinct Eohimalayan and Neohimalayan episodes of development were separated by a ∼10-15 Myr interval of limited growth, or possible tectonic quiescence, apparently at odds with the continuous continental collision implied by plate tectonic convergence of India with respect to Asia. We present a numerical geodynamical model, MEA, which provides an internally self-consistent explanation for two apparently unrelated but persistent problems in early Himalayan evolution – the nature of Eohimalayan metamorphism, and the hiatus between Eo- and Neo-himalayan tectonism. In MEA, Early Eohimalayan medium pressure-medium temperature (MP-MT) metamorphism can be explained by opening of the Neotethys subduction channel adjacent to the Indus-Yarlung-Tsangpo suture shortly after India and Asia collided. Subduction channel opening, driven by Qiangtang lithosphere delamination, results in asthenospheric upwelling into the subduction channel. Upwelling provides heat for prograde metamorphism of the Tethyan Himalaya, consistent with natural and model P-T-t paths, and could have produced basaltic magmatism. In MEA, gravitationally-driven delamination of Qiangtang mantle lithosphere after ∼45 Ma pulls Lhasa terrane lithosphere to the north, causing shortening of the overlying Asian crust, and resulting in apparent indentation of Asia by India. During this Late Eohimalayan phase, MEA predicts that India and the southern part of Tibet advance northward at about the same rate, resulting in little or no growth of the Tethyan Himalaya. High temperatures persist in Indian crust owing to the increasing role of radioactive self-heating. Renewed accretion and growth of the Himalaya following subduction channel closing at ∼33 Ma lead to Neohimalayan deformation and metamorphism. We propose that the early history of the Himalaya is best explained in terms of two contrasting regimes – Early Eohimalayan accretion and metamorphism from ∼55-45 Ma, and Late Eohimalayan tectonic quiesence and continuing MP-MT metamorphism from ∼45-30 Ma.

A Critical Analysis of North-South Continuity of Landmasses across Indus-Yarlu-Tsangpo Suture Zone: Its Bearing on the Himalayan Evolution

A Critical Analysis of North-South Continuity of Landmasses across Indus-Yarlu-Tsangpo Suture Zone: Its Bearing on the Himalayan Evolution

40Ar/39Ar ages of muscovites from modern Himalayan rivers: Himalayan evolution and the relative contribution of tectonics and climate

40Ar/39Ar ages from detrital muscovites have been analyzed from six modern rivers in central and western Nepal; the size of the drainage basins associated with these samples ranges from a few square kilometers to >40,000 km2. These data, when combined with previously published ages of detrital muscovites from other modern rivers in the region, suggest that a good correspondence between normalized age and normalized topography (the comparison of t * and z *) is rare, due to either nonuniform rates of passage through the ∼400 °C isotherm or subsequent faulting in the drainage area. The closure temperature of Ar in muscovite is perhaps too high to make meaningful comparisons to modern topography in tectonic analysis of active orogens. The distribution of 40Ar/39Ar ages from detrital muscovites from the Karnali basin in western Nepal is much older than that for the Narayani basin in central Nepal. The Karnali muscovites, when combined with previously published muscovites from the Siwalik Group in western Nepal and zircon fission track ages from modern and ancient samples from the region, suggest a thermal history for western Nepal consistent with vigorous tectonics (and attendant erosion) before the middle Miocene but a significant diminution in the rate of erosion since ca. 10 Ma. 40Ar/39Ar ages of detrital muscovites from the Narayani basin in central Nepal suggest a markedly different history with an acceleration of the rate of erosion since ca. 10 Ma and reactivation of major faults; this is consistent with the abundant bedrock data from the Narayani basin. The strong difference in the erosional history of the adjacent Karnali and Narayani basins, as evidenced by the 40Ar/39Ar ages from detrital muscovites, is not likely to have been due to variations in climate, but rather due to strain partitioning within the Himalaya during and after the Miocene.

Extrusion vs. duplexing models of Himalayan mountain building 3: duplexing dominates from the Oligocene to Present

The Himalaya is a natural laboratory for studying mountain-building processes. Concepts of extrusion and duplexing have been proposed to dominate most phases of Himalayan evolution. Here, we examine the importance of these mechanisms for the evolution of the Himalayan crystalline core via an integrated investigation across the northern Kathmandu Nappe. Results reveal that a primarily top-to-the-north shear zone, the Galchi shear zone, occurs structurally above and intersects at depth with the Main Central thrust (MCT) along the northern flank of the synformal Kathmandu Nappe. Quartz c-axis fabrics confirm top-to-the-north shearing in the Galchi shear zone and yield a right-way-up deformation temperature field gradient. U-Pb zircon dating of pre-to-syn- and post-kinematic leucogranites demonstrates that the Galchi shear zone was active between 23.1 and 18.8 Ma and ceased activity before 18.8–13.8 Ma. The Galchi shear zone is correlated to the South Tibet detachment (STD) via consistent structural fabrics, lithologies, metamorphism, and timing for four transects across the northern margin of the Kathmandu Nappe. These findings are synthesized with literature results to demonstrate (1) the broad horizontality of the STD during motion and (2) the presence of the MCT-STD branch line along the Himalayan arc. The branch line indicates that the crystalline core was emplaced at depth via tectonic wedging and/or channel tunnelling-type deformation. We proceed to consider implications for the internal development of the crystalline core, particularly in the light of discovered tectonic discontinuities therein. We demonstrate the possibility that the entire crystalline core may have been developed via duplexing without significant channel tunnelling, thereby providing a new end-member model. This concept is represented in a reconstruction showing Himalayan mountain-building via duplexing from the Oligocene to Present.

Timing and conditions of peak metamorphism and cooling across the Zimithang Thrust, Arunachal Pradesh, India

The Zimithang Thrust juxtaposes two lithotectonic units of the Greater Himalayan Sequence in Arunachal Pradesh, NE India. Monazite U–Pb, muscovite 40Ar/39Ar and thermobarometric data from rocks in the hanging and footwall constrain the timing and conditions of their juxtaposition across the structure, and their subsequent cooling. Monazite grains in biotite–sillimanite gneiss in the hanging wall yield LA-ICP-MS U–Pb ages of 16 ± 0.2 to 12.7 ± 0.4 Ma. A schistose gneiss within the high strain zone yields overlapping-to-younger monazite ages of 14.9 ± 0.3 to 11.5 ± 0.3 Ma. Garnet–staurolite–mica schists in the immediate footwall yield older monazite ages of 27.3 ± 0.6 to 17.1 ± 0.2 Ma. Temperature estimates from Ti-in-biotite and garnet–biotite thermometry suggest similar peak temperatures were achieved in the hanging and footwalls (~ 525–650 °C). Elevated temperatures of ~ 700 °C appear to have been reached in the high strain zone itself and in the footwall further from the thrust. Single grain fusion 40Ar/39Ar muscovite data from samples either side of the thrust yield ages of ~ 7 Ma, suggesting that movement along the thrust juxtaposed the two units by the time the closure temperature of Ar diffusion in muscovite had been reached. These data confirm previous suggestions that major orogen-parallel out-of-sequence structures disrupt the Greater Himalayan Sequence at different times during Himalayan evolution, and highlight an eastwards-younging trend in 40Ar/39Ar muscovite cooling ages at equivalent structural levels along Himalayan strike.

A Late Miocene acceleration of exhumation in the Himalayan crystalline core

Unraveling the relative roles of erosion and tectonics in shaping the modern topography of active orogens requires datasets documenting spatial and temporal patterns of exhumation, surface uplift and climatic forcing throughout orogenic growth. Here we report the results of biotite 40Ar/ 39Ar incremental heating and single-grain laser-fusion experiments from a nearly vertical, ∼ 1000 m age-elevation transect in the central Nepalese Himalaya. Age-elevation relationships constructed from these data suggest very slow cooling in this part of the Himalayan crystalline core during the Early Miocene, accelerating to only moderate rates at ∼ 10 Ma. If we assume purely vertical exhumation and a steady-state thermal structure, the exhumation rates implied by these data are ≪ 0.1 mm/yr prior to 10 Ma and ∼ 0.5 mm/yr from ∼ 10–7 Ma. The acceleration in cooling rate at 10 Ma requires a change in kinematics that may be linked to large-scale changes in climate, or to more local tectonic perturbations. Although we do not presently have enough data to assess the relative roles of regional vs. local drivers, these data provide a new constraint on exhumation through the Miocene that must be honored by any model of Himalayan evolution.

Himalayan ultrahigh pressure rocks and warped Indian subduction plane

In this paper, we will use the knowledge of ultrahigh-pressure (UHP) metamorphic evolution as independent data to constraint the geometry and reconstruct the tectonic evolution of the early stage of the India-Asia collision. The two UHP units recognized in NW Himalaya (Kaghan in Pakistan and Tso Morari in India) belong to distal parts of the continental Indian margin subducted between 55 and 45 Ma at a minimum depth of 100 km (e.g. Guillot et al. 2003 for review), evolving simultaneously during the early Himalayan evolution. They are interpreted as the signature of the early subduction of the Indian continental plate at the Paleocene-Eocene boundary. The metamorphic conditions are synthesized in Table 1 for both units. Even if similar protoliths are involved and both UHP units record the same maximum depth of about 100 km, some differences in P-T-t conditions may be emphasized. Firstly, the temperature of the metamorphism peak is significantly lower than a minimum of 100°C in the Tso Morari unit suggesting that the subduction rate is higher (Peacock 1992) in the Eastern part. Secondly, the western Kaghan unit seems to be involved in the subduction zone 4 Ma (~ 53 Ma) after the Eastern Tso Morari unit (~ 57 Ma). The later implication of the Kaghan unit in the subduction zone is related to its greater internal localization on the Indian margin, which, combined with a lower subduction rate, induce a higher temperature peak. As the UHP units are buried and exhumed along the subduction plane (e.g. Chemenda et al. 2000), the dip angle of the subduction plane can be deduced from the geometry of the subduction and the timing of the processes. The sinus of dip angle is equal to the amount of vertical displacement (D) during an interval of time, divided by the amount of Indian plate subduction during the same time interval (H). Those two data are independently measured, D from the exhumed rocks, H from the motion of the Indian plate.

Burial and exhumation history of a Lesser Himalayan schist: Recording the formation of an inverted metamorphic sequence in NW India

Coupled analysis of the pressure–temperature ( PT) evolution and accessory phase geochronology of a single sample reveals the burial-uplift history of part of the Lesser Himalaya during the Middle Miocene. Phase-equilibria calculations indicate that a peak temperature of 600–640 °C followed burial to approximately 25 km depth. Laser-ablation monazite geochronology yields a weighted mean 206Pb ⁎/ 238U age of 11.1 ± 2.0 Ma and a Tera-Wasserburg Concordia intercept age of 10.6 ± 0.9 Ma, with no distinguishable age difference between matrix and inclusion grains. Considerations of the likelihood of excess 206Pb further suggest that the crystallization age lies in the range 9–10 Ma. Textural analysis suggests that monazite grew during prograde metamorphism. Peak metamorphic conditions were followed by exhumation and cooling, forming a distinctively tight PT path closure. Both the shape of this path and its relatively young prograde phase distinguish Lesser Himalayan evolution from that typically inferred for the High Himalaya, and allow exploration of the thermal mechanisms that operated in the western Himalaya during the interval ca. 23–6 Ma. The PTt history is characteristic of footwall heating due to rapid overthrusting of hot rock (the Higher Himalaya), followed by incorporation into a thrust sheet that exhumed the sequence rapidly enough to preserve an inverted metamorphic gradient.

Emergence and evolution of Himalaya: reconstructing history in the light of recent studies

India collided with mainland Asia at 65 Ma. The pressure rose to 9-11 kbar in the collision zone. As the Indian lithosphere bent down and its upper crust buckled up as an upwarp in the period 35-45 Ma, the southern margin of Asia became the water-divide of the Himalayan rivers. A variety of Eurasian fauna migrated to the Indian landmass. The southern margin of the Himalayan province synchronously sagged to give rise to the foreland basin that was linked with the Indian sea. In this Paleocene foreland basin 48-49 Ma ago, the whales from one of the species of the immigrant terrestrial mammals evolved. The sea retreated from the Himalayan province by the early Miocene, even as the crust broke up along faults 20-22 million years ago. The basement rocks, which had attained high-grade metamorphism at 600-800°C and 6-10 kbar, were thrust up to give rise to what later became the Himādri or Great Himalaya. Differential melting of the high-grade metamorphic rocks of the Himadri extensively produced 21 ± 1 Maold granites.Rivers carried detritus generated by the denudation of the fast emerging Himalaya and deposited it in the foreland basin which turned fluvial around 23 Ma. Another fluvial foreland basin, the Siwalik, was formed at ~18 Ma in front of the rapidly rising orogen and was filled by river-borne sediments at the rate of 20-30 cm year-1in the early stage and at 50-55 cm year-1later when the Himadri was uplifted and briskly exhumed in the Late Miocene (9-7.5 Ma). The Himadri then became high enough to cause disruption of wind circulation, culminating in the onset of monsoon. The climate change that followed caused migration of a variety of quadrupeds from Africa and Eurasia, bringing about considerable faunal turnovers in the Siwalik life.Spasmodic uplift of the outer ranges of the Lesser Himalaya and tectonic convulsion in the Siwalik domain at 1.6 Ma resulted in widespread landslides with debris flows and emplacement of the Upper Siwalik Boulder Conglomerate. Strong tectonic movements at 0.8 Ma caused the partitioning of the foreland basin into the rising Siwalik Hills and the subsiding IndoGangetic Plains, and also the initiation of glaciation in the uplifted domain of the Great Himalaya. After the end of the Pleistocene ice age around 0.2 Ma, there was oscillation of dry-cold and wet-warm climates. This climatic vicissitude is recorded in the sediments of the lakes that had formed because of reactivation of faults crossing rivers and streams. Activeness of faults, continuing uplift and current seismicity imply ongoing strain-buildup in the Himalayan domain.

Early Himalayan exhumation: Isotopic constraints from the Indian foreland basin

Nd‐ and Sr‐isotopic compositions of Palaeogene foreland basin sediments are used to provide insights into early Himalayan evolution, particularly the timing of exposure of high87Sr/86Sr units, erosion of which may have caused the late Tertiary increase in oceanic Sr‐isotopic ratios. During the late Palaeocene–early Eocene, erosion was from mixed sources including suture zone rocks. Exhumation of the High Himalaya was occurring by the time of deposition of alluvial sediments after mid‐Oligocene times and this source has dominated Himalayan sediments from at least this time until the present day. The transition is interpreted to reflect exhumation of ‘basement rocks’ of the Indian plate, when the High Himalaya became a sufficient topographic barrier to separate suture zone rocks from the foreland basin. The marked rise in seawater87Sr/86Sr from 40 Ma is consistent with the erosion of a Himalayan source with a high87Sr/86Sr ratio.

Evolution métamorphique du dôme de Kangmar (Sud-Est-Xizang, Tibet): Implications pour les zones internes himalayennes

The thermobarometric study of metapelites from the Kangmar dome confirms the absence of high pressure metamorphism in the eastern part of the Himalayan belt, contrasting with the western Himalayan evolution. The shape of the P-T path and the geochronological data suggest a later evolution of the Kangmar dome (between 30 and 20 Ma), compared with the earlier evolution of the western Himalayan zone (between 55 and 45 Ma). Such a contrasted geochronological and metamorphic evolution from west to east is related to a lateral variation of the dip of the continental subduction plane and to a later India-Asia collision in the eastern part.

Laser 40Ar/39Ar dating of single detrital muscovite grains from early foreland-basin sedimentary deposits in India: Implications for early Himalayan evolution

Research Article| June 01, 1997 Laser 40Ar/39Ar dating of single detrital muscovite grains from early foreland-basin sedimentary deposits in India: Implications for early Himalayan evolution Y. M. R. Najman; Y. M. R. Najman 1Department of Geology and Geophysics, Edinburgh University, West Mains Road, Edinburgh EH9 3JW, United Kingdom2Department of Earth Sciences, Cambridge University, Downing Street, Cambridge CB2 3EQ, United Kingdom Search for other works by this author on: GSW Google Scholar M. S. Pringle; M. S. Pringle 3Scottish Universities Research and Reactor Centre, Scottish Enterprise Technology Park, East Kilbride G75 0QF, United Kingdom Search for other works by this author on: GSW Google Scholar M. R. W. Johnson; M. R. W. Johnson 1Department of Geology and Geophysics, Edinburgh University, West Mains Road, Edinburgh EH9 3JW, United Kingdom Search for other works by this author on: GSW Google Scholar A. H. F. Robertson; A. H. F. Robertson 1Department of Geology and Geophysics, Edinburgh University, West Mains Road, Edinburgh EH9 3JW, United Kingdom Search for other works by this author on: GSW Google Scholar J. R. Wijbrans J. R. Wijbrans 4Faculty of Earth Sciences, Vrije Universiteit, De Boelelaan 1085, 1081HV Amsterdam, The Netherlands Search for other works by this author on: GSW Google Scholar Geology (1997) 25 (6): 535–538. https://doi.org/10.1130/0091-7613(1997)025<0535:LAADOS>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Y. M. R. Najman, M. S. Pringle, M. R. W. Johnson, A. H. F. Robertson, J. R. Wijbrans; Laser 40Ar/39Ar dating of single detrital muscovite grains from early foreland-basin sedimentary deposits in India: Implications for early Himalayan evolution. Geology 1997;; 25 (6): 535–538. doi: https://doi.org/10.1130/0091-7613(1997)025<0535:LAADOS>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract In India, the Dagshai and overlying Kasauli Formations represent the oldest exposed continental foredeep sediments eroded from the Himalayan orogen. 40Ar/39Ar dating of individual detrital white micas from these sedimentary units has provided maximum depositional ages of <28 Ma for the Dagshai Formation at one locality and <25 Ma at a second locality, whereas deposition of the Kasauli Formation occurred after 28 Ma at two localities and after 22 Ma at a third locality. This timing suggests that, in India, the start of substantial exhumation and erosion from the rising Himalayan orogen was delayed until 28 Ma. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Structural and metamorphic evolution of the Nanga Parbat syntaxis, Pakistan Himalayas, on the Indus gorge transect: The importance of early events

AbstractField relationships are as important as radiometric age data for unravelling the history of polycyclic metamorphic regions. Field and petrographic data were used to investigate such a region, the Nanga Parbat syntaxis ‐ a major N‐trending transverse structure in the Pakistan Himalayas. Continental rocks of the Indian plate are exposed in the core of this structure due to uplift relative to the surrounding and structurally higher amphibolites of the overthrust Kohistan Island Arc. In the Indus gorge, the Indian plate gneisses are migmatitic, highly deformed and cut by Himalayan granite sheets. Field relationships show that the migmatization pre‐dates the emplacement of a suite of basic intrusive sheets (now amphibolites) which are themselves unlikely to be Himalayan; these amphibolite sheets are cut by the Himalayan granite sheets. There is therefore no genetic link between the granites and the migmatization: the latter might be Precambrian. During the pre‐Himalayan evolution, an eastern group of metapelitic migmatites underwent biotite dehydration melting at granulite facies conditions, leading to the development of assemblages with garnet, orthoclase and kyanite. Melt was not completely lost and back‐reacted during cooling to give biotite‐kyanite coronas around garnets. A western group of orthogneisses is characterized by foliated muscovite and biotite; orthoclase is never found with either garnet or aluminium silicate. In this second group, partial melting and small‐scale segregation gave rise to the migmatitic appearance, but melt was not lost and back‐reacted fully to regenerate the micas.The migmatites and basic sheets were strongly deformed at amphibolite facies during Himalayan collision, producing LS tectonites, mylonitic in places. This strong fabric was then tightly folded on large scale. The NNE trending fold is slightly oblique to the earlier N‐S stretching lineation; this has been reoriented on the steep limbs, but its plunge remains shallow. No new fabric is seen associated with this folding. On the western margin, steeply plunging lineations are associated with shear bands in gneisses, indicating relative uplift of the syntaxis. In contrast, on the steep eastern margin, relative uplift of the syntaxis appears to have been due to rotation without the development of new fabrics. There is no evidence for further melting during this Himalayan evolution, so the granite sheets cannot have been derived locally. This study highlights the importance of distinguishing pre‐Himalayan and Himalayan events in this polymetamorphic region, and the usefulness of field relationships in addition to geochronological constraints in such a task.

Palaeomagnetic dating of the earliest continental Himalayan foredeep sediments: Implications for Himalayan evolution

Between the time of the India-Eurasia collision (50–45 Ma) [1] and the climax of crustal shortening and thrust stacking in the Himalaya when the Main Central Thrust (MCT) was active (21 Ma) [1,2] there is a ca. 30 My gap about which little is known. This paper aims to shed light on this period by dating the initiation of major erosion from the rising Himalaya and the probable start of uplift, a significant event in the orogen's history. This was achieved by accurately dating, for the first time, the Dagshai Formation sediments of northern India, which are interpreted as early Himalayan foreland basin deposits that record initial large-scale erosion of the orogen [3]. Oriented hand samples were collected from six sites and analysed, using palaeomagnetic techniques. Both polarities are represented and the remanence passes a fold test. Fitting the measured palaeolatitude to that expected for the Indian plate dates the Dagshai Formation at 35.5 Ma ± 6.7 Ma, and this is taken as the time when the embryonic Himalaya began to be strongly eroded and regionally uplifted.

Cambrian trilobite faunas from India: a multivariate and computer-graphic reappraisal and its paleogeographic implications

Cambrian trilobite faunas from northern India provide data critical for assessing earliest Phanerozoic paleogeography and for constraining tectonic models of Himalayan evolution. Previous investigations suggest that Indian Middle Cambrian trilobite faunas, collected from basins 500 km apart, are strikingly different. The Kashmir fauna, in the west, shows supposed faunal affinities with northern China, while the Spiti fauna, in the east, was considered of European affinity. This counterintuitive faunal distribution in adjacent basins might suggest that the area was made up of several micro-contininents during Cambrian time. Although frequently neglected, this interpretation has major implications for models of Himalayan mountain building, as most models assume a passive northern margin of India throughout the Phanerozoic.Original type material from Kashmir and Spiti and fresh collections from intermediate localities in the Zanskar valley have been evaluated using a new statistical/computer-graphic method which removes the effects of tectonic deformation. Results show that the supposed taxonomic distinctness of the Kashmiri and Spiti faunas is largely superficial. Previous lack of appreciation of deformation has lead to: 1. over-estimation of the number of taxa present; 2. misidentification of many taxa; 3. spurious correlations with other faunas; 4. inaccurate age estimates. The revised assessment indicates that: 1. approximately 12 species of polymerid trilobites are present in the Middle Cambrian of north India; 2. 3 polymerid trilobite species are common to both Kashmir and Spiti faunas; 3. much of the Spiti fauna is significantly older than the Kashmiri fauna; 4. faunal differences between coeval deposits from the two basins are best explained as biofacies differences related to an offshore proximality trend; 5. species show patterns of developmental flexibility similar to that recently reported in other Cambrian trilobites. The faunas do not suggest that Kashmir and Spiti were part of separate continents during the Cambrian.The morphology of Middle Cambrian faunas from Kashmir, Zanskar and Spiti suggests that all polymerid species were benthic and share morphotypes characteristic of trilobites from slope environments. Lithologic evidence suggesting that Kashmiri faunas were deeper water than those from Spiti is complimented by the presence in Kashmir of the atheloptic trilobite Bailiella, which is characteristic of deeper waters. Faunas from Kashmir, Zanskar and Spiti, which lie within the Tethyan belt of the Indian Himalaya, share closest affinity with those described from north China, north Vietnam and south China. They also show affinity with faunas from Iran. This pattern is consistent with recent paleogeographic reconstructions which place these regions in close proximity and at similar latitudes. Most of the Cambrian within the Tethyan Himalaya is of middle Middle Cambrian age.The recognition of developmental flexibility as a general characteristic of Cambrian trilobites suggests many groups may be taxonomically over-split. Over-splitting may have lead to widespread over-estimation of faunal provinciality during Cambrian times.

Metamorphic evolution of the High Himalayan Crystallines in SE Zanskar, India

ABSTRACT The High Himalayan Crystallines (HHC) of SE Zanskar consist of biotite paragneisses, of orthogneisses that derive from early‐Palaeozoic granitoids, of minor metabasics and of post‐metamorphic leucogranites of Miocene age.Two main metamorphic events have been documented in the HHC. The first event occurred at P= 12.0 ± 0.5 kbar and T= 750 ± 50° C in rare metabasics intruded by early‐Palaeozoic granitoids. In the biotite paragneisses, thermobarometric estimates of the first event point to comparable T at P 4–5 kbar lower. The first event is followed by a pervasive syn‐tectonic crystallization characterized by lower P and T. On the basis of the cooling ages of the metamorphic minerals and on the geological evidence, the second event is referred to the Tertiary Himalayan crystallization. Further petrological and geochronological studies are necessary to prove whether a few mineral relics ascribed to the first event define a polyphase Himalayan evolution or if they record the incomplete obliteration of an older history during the Himalayan event.The HHC of SE Zanskar show a decrease in metamorphic grade from the middle structural levels upward, close to the Kade unit, and downward, close to the Lesser Himalaya (from sillimanite‐K‐feldspar‐biotite‐bearing assemblages to kyanite‐staurolite‐muscovite‐bearing assemblages). This metamorphic zonation is probably a consequence of the polyphase history of intracontinental thrusts and of the tectonic emplacement of hot crustal slabs within shallower and colder thrust sheets at relatively late stages of the continental collision between India and Eurasia.