Lesser Himalayan Sequence Research Articles

Testing Models of MCT Reactivation vs. Duplex Formation in the Kumuan and Garwhal Himalaya, India

Himalayan Journal of Sciences Vol.2(4) Special Issue 2004 pp. 110

Tethyan and Indian subduction viewed from the Himalayan high- to ultrahigh-pressure metamorphic rocks

The Himalayan range is one of the best documented continent-collisional belts and provides a natural laboratory for studying subduction processes. High-pressure and ultrahigh-pressure rocks with origins in a variety of protoliths occur in various settings: accretionary wedge, oceanic subduction zone, subducted continental margin and continental collisional zone. Ages and locations of these high-pressure and ultrahigh-pressure rocks along the Himalayan belt allow us to evaluate the evolution of this major convergent zone. (1) Cretaceous (80–100 Ma) blueschists and possibly amphibolites in the Indus Tsangpo Suture zone represent an accretionary wedge developed during the northward subduction of the Tethys Ocean beneath the Asian margin. Their exhumation occurred during the subduction of the Tethys prior to the collision between the Indian and Asian continents. (2) Eclogitic rocks with unknown age are reported at one location in the Indus Tsangpo Suture zone, east of the Nanga Parbat syntaxis. They may represent subducted Tethyan oceanic lithosphere. (3) Ultrahigh-pressure rocks on both sides of the western syntaxis (Kaghan and Tso Morari massifs) formed during the early stage of subduction/exhumation of the Indian northern margin at the time of the Paleocene–Eocene boundary. (4) Granulitized eclogites in the Lesser Himalaya Sequence in southern Tibet formed during the Paleogene underthrusting of the Indian margin beneath southern Tibet, and were exhumed in the Miocene. These metamorphic rocks provide important constraints on the geometry and evolution of the India–Asia convergent zone during the closure of the Tethys Ocean. The timing of the ultrahigh-pressure metamorphism in the Tso Morari massif indicates that the initial contact between the Indian and Asian continents likely occurred in the western syntaxis at 57 ± 1 Ma. West of the western syntaxis, the Higher Himalayan Crystallines were thinned. Rocks equivalent to the Lesser Himalayan Sequence are present north of the Main Central Thrust. Moreover, the pressure metamorphism in the Kaghan massif in the western part of the syntaxis took place later, 7 m.y. after the metamorphism in the eastern part, suggesting that the geometry of the initial contact between the Indian and Asian continents was not linear. The northern edge of the Indian continent in the western part was 300 to 350 km farther south than the area east of the Nanga Parbat syntaxis. Such “en baionnette” geometry is probably produced by north-trending transform faults that initially formed during the Late Paleozoic to Cretaceous Gondwana rifting. Farther east in the southern Tibet, the collision occurred before 50.6 ± 0.2 Ma. Finally, high-pressure to ultrahigh-pressure rocks in the western Himalaya formed and exhumed in steep subduction compared to what is now shown in tomographic images and seismologic data.

Styles of post-subduction collisional orogeny: Influence of convergence velocity, crustal rheology and radiogenic heat production

A number of models of continental collision already exist, but the role of foregoing oceanic plate subduction or variable coupling between plates still remains to be explored. In addition, heat generation by radioactive decay may be quite variable. For example, our geochemical data from low-to-high grade metamorphic rocks of the Lesser and Higher Himalayan sequences reveal measured heat production values on the order 4–5 μW/m 3, which is considerably higher than normal inferred upper crustal values (< 1.5 μW/m 3). Therefore, we performed 2D numerical experiments of oceanic-continental subduction followed by continental collision for varying continental lower crustal strength, convergence velocity and radiogenic heating rate. Oceanic subduction produces thinning and upwarping of the overriding lithospheric wedge, such as is observed in Andean-type collisional settings (Cascadia and Central Andes). After collision two main orogenic styles develop depending on the values of key parameters: either (a) slow convergence rates (weak shear heating and consequently strong rheological coupling between plates) and/or a weak lower crust generate a two-sided, thick and narrow symmetrical collisional zone, or (b) high convergence rates (strong shear heating and therefore weak rheological coupling between plates) and a strong lower crust produce a one-sided, thin and broad asymmetrical collisional zone. In this case a vertical lithospheric body divides two upper crustal wedges that develop distinct deformation patterns. In both cases, enhanced radiogenic heating favors the formation of midcrustal partially melted channels that propagate toward the surface and the pro-foreland. Partial melting triggers lateral extrusion of metamorphic rocks from different depths (15–23 kbar, > 700 °C) on top of coherent LP-LT units. The geometry of metamorphic isograds (inverted), shapes of P-T paths at different grades and exhumation rates are similar to those observed in parts of the Himalayan chain, indicating that the models capture the essence of real systems. The model results demonstrate that internal heat production rate is a crucial parameter in determining the timing of slab breakoff, polarity reversal and exhumation of the inverted metamorphic sequence with respect to the timing of partial melting. Late stages of some numerical experiments showed interesting geodynamical evolution of the collisional zone, with the appearance of polarity reversal, roll over of both plates and roll back of the subducting slab. Irrespective of these details, hundreds of km of continental lithosphere are recycled into the mantle in all models, supporting the idea that continental crustal consumption is, indeed, a common and considerable process during collision. This affects inferences about extents of convergence and/or shortening during orogenesis. Moreover, crustal recycling into the deep mantle during collisional orogeny needs to be considered in models of crustal growth and evolution.

Geochronological and Petrological Constraints for Tectonic Evolution of the Central Greater Himalayan Sequence in the Kharta Area, Southern Tibet

Abstract Within the Kharta area, east of Mount Qomolangma (Everest), garnet sillimanite gneisses and granites including mafic lenses that form the Greater Himalayan Sequence (GHS) were displaced beneath the North Col Formation by the lower South Tibetan Fault (LSTF) and above the Lesser Himalayan Sequence by the Main Central Thrust (MCT). Zircons from mafic lenses, probably former dikes, were dated by thermal ionization mass spectrometry to yield discordant U‐Pb results. One sample gave a discordia line with an upper intercept age of 971 Ma, suggesting a late Proterozoic age for emplacement of the dike. Zircon SHRIMP analyses show that the garnet sillimanite gneisses and granites were derived mainly from early Paleozoic rocks produced by high‐temperature metamorphism and/or partial melting of Neoproterozoic (meta)sedimentary rocks. These crystalline rocks were buried beneath southern Tibet and experienced metamorphic conditions of 750°–800°C and 14 kbar at \documentclass{aastex} \usepackage{amsbsy} \usepa...

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.

Thermobaric structure of the Himalayan Metamorphic Belt in Kaghan Valley, Pakistan

The thermobaric structure of the Himalayan Metamorphic Belt (HMB) has been constructed along the Kaghan Valley transect, Pakistan. The HMB in this valley represents mainly the Lesser Himalayan Sequence (LHS) and Higher Himalayan Crystallines (HHC). Mineral parageneses of 474 samples, from an approximately, 80-km traverse from southwest to northeast, were examined. Microprobe analyses were carried out to quantify the mineral composition. To determine the pressure–temperature (P–T) conditions, 65 thin sections (7 pelites from LHS and 25 pelites, 9 mafic rocks/amphibolites and 19 eclogites from HHC) were selected. Based on field observations and mineral paragenesis, low-grade to high-grade metapelites, show Barrovian-type progressive metamorphic sequence, with chlorite, biotite, garnet and staurolite zones in LHS and staurolite, kyanite and sillimanite zones in HHC. By using well-calibrated geothermobarometers, P–T conditions for pelitic and mafic rocks are estimated. P–T estimates for pelitic rocks from the garnet zone indicate a condition of 534 ± 17 °C at 7.6 ± 1.2 kbar. P–T estimates for rocks from the staurolite and kyanite zones indicate average conditions of 526 ± 17 °C at 9.4 ± 1.2 kbar and 657 ± 54 °C at 10 ± 1.6 kbar, respectively. P–T conditions for mafic rocks (amphibolites) and eclogites from HHC are estimated as 645 ± 54 °C at 10.3 ± 2 kbar and 746 ± 59 °C at 15.5 ± 2.1 kbar, respectively. The coesite-bearing ultrahigh-pressure (UHP) eclogites record a peak P–T condition of 757–786 °C at 28.6 ± 0.4 kbar and retrograde P–T conditions of 825 ± 59 °C at 18.1 ± 1.7 kbar. These results suggest that HMB show a gradual increase in metamorphic grade from southwest to northeast. The P–T conditions from Pelitic and adjacent mafic rocks having identical peak conditions in the same metamorphic zone, while the structural middle in HHC reached the highest P–T condition upto the UHP grade.

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).

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.

Geologic and U–Pb geochronologic evidence for early Paleozoic tectonism in the Dadeldhura thrust sheet, far-west Nepal Himalaya

The Dadeldhura thrust sheet inm western Nepal consists of Proterozoic–Lower Paleozoic sedimentary and plutonic rocks, and their metamorphic equivalents, that rest structurally on Proterozoic strata of the Lesser Himalayan sequence. Although regional metamorphism and ductile deformation were widespread during Tertiary thrust emplacement, relicts of early Paleozoic tectonism are preserved locally. New field and geochronologic studies, together with the findings of previous workers, indicate that this early Paleozoic tectonism included: (1) regional metamorphism to at least garnet grade, (2) regional folding of a thick metamorphic sequence into a broad east–west trending syncline, (3) outcrop-scale folding of metasedimentary rocks, (4) emplacement of Cambro–Ordovician granitic bodies during and after the metamorphism and deformation, (5) uplift and erosion of the metamorphic sequence, with garnet-grade rocks locally exposed at the surface, and (6) derivation of Ordovician conglomeratic sandstones from the early Paleozoic orogen. Similar records of metamorphism, deformation, and uplift/erosion have been found in other regions of the Himalaya, indicating that rocks of the Dadeldhura thrust sheet were originally involved in a regionally extensive orogenic system. Future tectonic models of Himalayan orogenesis must accommodate this early Paleozoic event.

Provenance of the Greater Himalayan Sequence and associated rocks: predictions of channel flow models

Abstract Numerical models for channel flow in the Himalayan—Tibetan system are compatible with many tectonic and metamorphic features of the orogen. Here we compare the provenance of crustal material in two channel flow models (HT1 and HT111) with observations from the Himalaya and southern Tibet. Thirty million years after the onset of channel flow, the entire model crust south of the India—Asia suture still consists only of ‘Indian’ material. The model Greater Himalayan Sequence (‘GHS’) is derived from Indian middle crust originating ≤1000 km south of the initial position of the suture, whereas the Lesser Himalayan Sequence (‘LHS’) is derived mainly from crust originating ≥1400 km south of the suture. Material tracking indicates little or no mixing of diverse crustal elements in the exhumed region of the model ‘GHS’, which is derived from originally contiguous materials that are transported together in the top of the channel flow zone. These results are compatible with provenance data indicating a clear distinction between GHS and LHS protoliths, with the GHS originating from a more distal position (relative to cratonic India) than the LHS. In model HT111, domes formed between the suture and the orogenic front are cored by ‘Indian’ middle crust similar to the ‘GHS’, consistent with data from the north Himalayan gneiss domes. Material tracking shows that plutons generated south of the suture should have ‘Indian’ crustal signatures, also compatible with observations. Model ‘GHS’ pressure—temperature—time (P-T-t) paths pass through the dehydration melting field between 30 and 15 Ma, consistent with observed leucogranite ages. Finally, exposure of midcrustal ‘GHS’ and ‘LHS’ material at the model erosion front is consistent with the observed appearance of sedimentary detritus in the Lesser Himalaya. We conclude that channel flow model results are compatible with provenance data from the Himalaya and southern Tibet.

Pulsed channel flow in Bhutan

Abstract We summarize our results from Bhutan and interpret the Greater Himalaya Sequence (GHS) of Bhutan, together with a portion of the underlying Lesser Himalaya Sequence, in the context of recently published channel flow models. For the GHS rocks now exposed in Bhutan the depth for beginning of muscovite dehydration melting (approximately 750°C at 11 kbar) and associated weakening of these rocks is constrained by geobarometry to be at about 35–45 km. The location of initial melting was down-dip and over 200 km to the north of Bhutan. Melt was produced and injected into ductilely deforming metamorphic rocks as they were extruded towards the south between the Main Central Thrust (MCT) and the South Tibetan Detachment zones. The lateral flow of low viscosity rocks at these depths occurred under southern Tibet between 22 Ma and 16 Ma. Subsequently, the channel rocks decompressed from 11 to 5 kbar (from 35 km to a depth of 15 km), but maintained high temperatures, between about 16 Ma and 13 Ma. The data from Bhutan are consistent with channel flow models if there were several pulses of channel flow. The first, between 22 and 16 Ma, produced the rock seen in the lower half of the GHS of Bhutan. A second pulse, which is cryptic, is inferred to have led to the uplift and exhumation of the MCT zone. A third, in central Bhutan, is exposed now as the hanging wall of the Kakhtang thrust, an out-of-sequence thrust that was active at 12–10 Ma. The latter two pulses likely broke around a plug at the head of the first pulse that was formed where the melt in the channel had solidified.

Geochemistry of the Ganesh Himal zinc-lead deposits, central Nepal Himalaya

A dolomite hosted strata-bound high-grade (19-25%) zinc-lead sulphide deposit occurs, between 4,000 m and 5,100 m over an area of 5 km2 in the ~ 7 km wide MCT zone of the Ganesh Himal area of central Nepal. The host crystalline milky white sugary dolomite occurs in a repeated sequence of garnet-mica schist, quartzite, calc-schist and concordant amphibolite of older Lesser Himalayan sequence (Upper Nawakot Group), all showing ductile deformation. The rocks along with the ore have undergone at least three phases of deformation. A series of overturned, steep northerly dipping and NE to ENE plunging anticlines and synclines form the dominating structure, characterised by disharmonic shape of the dolomite host rock because of apparent squeezing out from limbs into the axial regions. Compared to their strike lengths, the ore bodies and the host rock bodies have long extension along the plunge direction. The ore has a very simple composition of sphalerite-galena-pyrite with a little pyrrhotite, magnetite and chalcopyrite. Chemically, it consists mainly of zinc along with iron, lead, silver and very low silica, silicates and alumina. Concentrations of trace and rare elements are very low. Ore body types vary from dissemination and bands to massive sulphide lenses, arranged en echelon parallel to the schistosity/ bedding. The rocks of the area were subjected to almandine-amphibolite facies of metamorphism, to 750±150 MPa pressure and 500 °C to 750 °C temperature conditions. The latest thermal event was as young as ~12 Ma. The lead isotope data are interpreted to establish an age of 875 to 785 Ma for the Ganesh Himal deposits, while sulphur isotope data imply an age greater than 650 Ma. The Ganesh Himal deposit appears to be Vindhyan equivalent in the Himalaya, showing highest metal values as on date. Only two out of six occurrences in the area have been explored so far. The ore reserve estimates stand at 2.4 million tonnes with 14.66% zinc, 3.01 % lead, and 23.5 g/t silver. Taking into account all the occurrences the Ganesh Himal basin might have had 861,000 tonnes zinc and 182,000 tonnes lead at the minimum. The lithologic sequence represents a shallow marine facies of deposition. The δ34 S values for Ganesh Himal sulphides indicate that the sulphur was probably produced by biogenic reduction of contemporaneous seawater sulphate. The lead isotope ratios of Ganesh Himal deposit fall on a single-stage growth curve, on which also fall many big deposits of the world, indicating that the Ganesh Himal zinc-lead deposit has high potential. The primary control of the mineralisation is stratigraphic, the present ore body configuration being controlled by south vergent folding related with the metamorphism and thrusting along the MCT. Pyrite framboids and geochemical characters of ore indicate that the mineralisation is syngenetic sedimentary, and it may have been deposited in association with a bioherm or reef in an anoxic environment. The source of metal ions is not clear yet, but the contemporaneous basic rock bodies providing the metal ions could be a possibility.

A preliminary engineering geological study in the northern part of the Taplejung Window, far eastern Lesser Nepal Himalaya

In the northern part of the Taplejung area, two tectonic units are recognised. These are the Higher Himalayan and Lesser Himalayan Sequences separated by the Main Central Thrust (MCT). The Lesser Himalayan Sequence is exposed in the Taplejung Window represented by Taplejung Formation, Mitlung Augen Gneiss, and Lingkhim Schist from the bottom to top. The main rock types in the Lesser Himalayan Sequence are phyllite, schist, metasandstone, quartzite and augen gneiss. Three Proterozoic granitic bodies are intruded into Taplejung Formation: Tamor River Granite, Amarpur Granite and Kabeli Khola Granite. The rocks of the Higher Himalaya on the hanging wall of the MCT are composed of kyanite-sillimanite bearing banded gneiss, orthogneiss, migmatite, quartzite.&#x0D; Three major landslides in the area, viz., Hangdewa, Hireba, and Paire lie along the valley slope sides of the Tamor River. Stream erosion and incision, presence of highly weathered rocks, surface and subsurface drainage are responsible in activating the landslides. Due to the presence of mainly weathered phyllites, favourable structural orientations and wet cultivation practice in the area have contributed to the development of these landslides. The Hangdewa and Hireba landslides have endangered the villages in between them and ultimately the Suketar airport. The landslides are gradually destroying more and more areas of cultivated land and property and increased loss of human lives.&#x0D; Excessive anthropogenic interference in the form of development activities like unplanned urbanization, deforestation and unfavorable agricultural practices have greatly aggravated the situation. The intensity of damages amplifies during the late part of monsoon each year. The study suggests that further detailed surface and subsurface investigation of the landslides are needed to prevent further loss of lives and property.

Crustal channel flows: 2. Numerical models with implications for metamorphism in the Himalayan‐Tibetan orogen

Results from a thermal‐mechanical model (HT1) that includes midcrustal channel flow are compatible with many features of the Himalayan‐Tibetan system. Radioactive self‐heating and rheological weakening of thickened model orogenic crust lead to the formation of a hot, low‐viscosity midcrustal channel and a broad plateau. Channel material, corresponding to the Greater Himalayan Sequence (GHS), flows outward from beneath the plateau in response to topographically induced differential pressure. At the plateau flank it is exhumed by focused surface denudation and juxtaposed with cooler, newly accreted material corresponding to the Lesser Himalayan Sequence (LHS). The model channel is bounded by coeval thrust and normal sense ductile shear zones, interpreted to represent the Main Central Thrust (MCT) zone and South Tibetan Detachment system, respectively. Inverted metamorphism associated with the model MCT zone results from distributed ductile shear along the MCT and extrusion of the hot channel. A variety of model P‐T‐t path styles, resembling those observed in the GHS and LHS, are produced for points traveling through contrasting tectonic regimes that coexist in different parts of the model. Predicted times of peak metamorphism, cooling, and erosion of metamorphic facies are generally compatible with observations, although model GHS cooling ages are too young. The times of M1 and M2 metamorphic “events” observed in the GHS correspond to model times of maximum burial and maximum heating, respectively. The results highlight the need to integrate tectonics and metamorphism in continental collision models and demonstrate the importance of lateral transport of both heat and material in large hot orogens.

Crustal channel flows: 1. Numerical models with applications to the tectonics of the Himalayan‐Tibetan orogen

Plane strain, thermal‐mechanical numerical models are used to examine the development of midcrustal channel flows in large hot orogens. In the models, radioactive self‐heating reduces the viscosity of tectonically thickened crust and increases its susceptibility to large‐scale horizontal flow. Channels can be exhumed and exposed by denudation focused on the high‐relief transition between plateau and foreland. We interpret the Himalaya to have evolved in this manner. Channel flows are poorly developed if the channel has a ductile rheology based on wet quartz flow laws, and well developed if there is an additional reduction in viscosity to 1019 Pa s. This reduction occurs from 700°C to 750°C in the models and is attributed to a small percentage of in situ partial melt (“melt weakening”). Model HT1 provides an internally consistent explanation for the tectonic evolution of many features of the Himalayan‐Tibetan orogenic system. Erosional exhumation exposes the migmatitic channel, equivalent to the Greater Himalayan Sequence (GHS), between coeval normal and thrust sense ductile shear zones, corresponding to the South Tibetan Detachment and the Main Central Thrust systems. Outward flow of unstable upper crust rotates these shears to low dip angles. In the model both the GHS and the Lesser Himalayan Sequence are derived from Indian crust, with the latter from much farther south. Similar models exhibit a range of tectonic styles, including the formation of domes resembling north Himalayan gneiss domes. Model results are relatively insensitive to channel heterogeneities and to variations in the behavior of the mantle lithosphere beneath the model plateau.

ヒマラヤ衝突帯の温度圧力インバージョン

Prograde P-T paths were obtained by Gibbs method using zonings of garnet in the Annapurna area of central Nepal Himalaya. The results show different P-T paths in the uppermost and lower Lesser Himalayan Sequence (LHS), at the footwall of the Main Central Thrust (MCT). The uppermost LHS garnet shows paths with adiabatic compression (ΔP=90 MPa, ΔT=7°C) followed by heating (ΔT=20e), whereas the lower LHS garnet simply shows a sub-adiabatic compression (ΔP=350 MPa, ΔT=70°C) path. Tectonically, adiabatic compre-ssion is supposed to correspond with subduction, and subsequent heating with thrusting along the MCT. Heating by the MCT faulting seems to have occurred in a narrow zone of approx. 100 m in thickness.

Early Proterozoic granite of the Taplejung Window, far eastern Lesser Nepal Himalaya

The Lesser Himalayan Sequence of the Taplejung Window in the far eastern Nepal Himalaya can be divided into Taplejung Formation, Mitlung Augen Gneiss and Linkhim Schist (from bottom to top respectively). The window is a large domal shaped anticline plunging to the east.&#x0D; Two-mica granite bodies (Amarpur Granite, Kabeli Khola Granite and Tamor River Granite) have intruded the metasediments of Taplejung Formation. The granite bodies are discordant to subconcordant in relation to the country rocks. Quartz, alkali feldspar, plagioclase, muscovite, biotite and tourmaline are the main constituent minerals of the granite. Generally, the core of granite bodies is undeformed, whereas the marginal part is gneissfied with S-C mylonitic texture showing the top to south sense of shear. This sense of shear is related to the movement along the Main Central Thrust (MCT). All the samples from the granitic bodies fall under the granite field in the normative quartz-alkali feldspar-plagioclase (QAP) triangular diagram. The mineral composition shows that the granite is peraluminous in nature. The Kabeli Khola Granite has yielded a 40Ar/ 39Ar muscovite age older than 1.6 Ga indicating its magmatic age. The granites of the study area can also be correlated with the 1.8 Ga Ulleri type augen gneiss of central Nepal.

Silicate versus carbonate weathering in the Himalaya: a comparison of the Arun and Seti River watersheds

We studied the water chemistry of two large and geologically differing Himalayan watersheds in order to maximize the contrast between silicate versus carbonate weathering effects on river chemistry. Our previous research involved the Seti River of westernmost Nepal, geologically typical of many rivers in Nepal in draining mixed carbonate/silicate lithologies, including abundant carbonate rocks of the Lesser Himalayan Sequence. For comparison, the Arun River was chosen for study because south of the Himalayan front it drains almost exclusively Greater and Lesser Himalayan silicate rocks. Despite this dominance of silicate rocks, carbonate weathering—probably of metamorphic calcite in Great Himalayan paragneisses—is clearly in evidence in many Arun watersheds. Weathering of silicate rocks exposed all along the Arun south of the range front has a small impact on mainstem river chemistry. The mainstems of both the Seti and Arun systems are dominated by weathering of carbonate rocks, although the contribution of silicate weathering is more visible in Arun mainstem chemistry. The carbonate weathering source to the Arun mainstem is probably both limestone in the Tethyan Sequence widely exposed in northern headwaters of the system, and metamorphic calcite within the Greater Himalayan Sequence. A number of small watersheds along the Arun and Seti appear to be carbonate-free. They probably provide the best constraints to date on the Ca/Na and Mg/Na ratios of waters draining Himalayan silicate rocks, two critical parameters for calculation of CO 2 consumption by silicate weathering in the Himalaya. The observed Ca/Na and Mg/Na ratios would produce slightly higher estimates of silicate weathering fluxes than previous studies. The geologic contrasts between the Seti and the Arun produce large differences in the 87Sr/ 86Sr ratio of each mainstem, unlike the major element chemistry. The Seti mainstem displays much higher 87Sr/ 86Sr ratios than the Arun mainstem, the opposite of the expected relationship since radiogenic silicate rocks of the Greater and Lesser Himalaya are so widely exposed along the Arun. 87Sr/ 86Sr ratios of the Arun mainstem never exceed 0.734 and show little downstream change as the mainstem passes through silicate rocks of the Greater and Lesser Himalaya. 87Sr/ 86Sr ratios of the Seti mainstem increase sharply from 0.725 to 0.785 when the river enters the belt of metacarbonate rocks of the northern Lesser Himalayan Sequence, a pattern also displayed by other Himalayan rivers such as the Kali Gandaki and Bhotse Khola. Metacarbonate rocks, including those of the Lesser Himalaya, are a major source of radiogenic Sr in modern Himalayan Rivers and probably have been key players in elevating marine 87Sr/ 86Sr ratios since the Early Miocene.

Exhumation of the Main Central Thrust from Lower Crustal Depths, Eastern Bhutan Himalaya

AbstractGeothermometry and mineral assemblages show an increase of temperature structurally upwards across the Main Central Thrust (MCT); however, peak metamorphic pressures are similar across the boundary, and correspond to depths of 35–45 km. Garnet‐bearing samples from the uppermost Lesser Himalayan sequence (LHS) yield metamorphic conditions of 650–675 °C and 9–13 kbar. Staurolite‐kyanite schists, about 30 m above the MCT, yield P‐T conditions near 650 °C, 8–10 kbar. Kyanite‐bearing migmatites from the Greater Himalayan sequence (GHS) yield pressures of 10–14 kbar at 750–800 °C. Top‐to‐the‐south shearing is synchronous with, and postdates peak metamorphic mineral growth.Metamorphic monazite from a deformed and metamorphosed Proterozoic gneiss within the upper LHS yield U/Pb ages of 20–18 Ma. Staurolite‐kyanite schists within the GHS, a few metres above the MCT, yield monazite ages of c. 22 ± 1 Ma. We interpret these ages to reflect that prograde metamorphism and deformation within the Main Central Thrust Zone (MCTZ) was underway by c. 23 Ma. U/Pb crystallization ages of monazite and xenotime in a deformed kyanite‐bearing leucogranite and kyanite‐garnet migmatites about 2 km above the MCT suggest crystallization of partial melts at 18–16 Ma. Higher in the hanging wall, south‐verging shear bands filled with leucogranite and pegmatite yield U/Pb crystallization ages for monazite and xenotime of 14–15 Ma, and a 1–2 km thick leucogranite sill is 13.4 ± 0.2 Ma. Thus, metamorphism, plutonism and deformation within the GHS continued until at least 13 Ma. P‐T conditions at this time are estimated to be 500–600 °C and near 5 kbar. From these data we infer that the exhumation of the MCT zone from 35 to 45 km to around 18 km, occurred from 18 to 16 to c. 13 Ma, yielding an average exhumation rate of 3–9 mm year−1. This process of exhumation may reflect the ductile extrusion (by channel flow) of the MCTZ from between the overlying Tibetan Plateau and the underthrusting Indian plate, coupled with rapid erosion.

Tectonic impact on the fluvial deposits of Plio-Pleistocene Himalayan foreland basin, India

The Himalayan foreland basin comprises a number of subbasins developed in various sedimentary environments during the Plio-Pleistocene phases of Himalayan orogeny. This paper describes the evolutionary processes that led to the development of the Himalayan foreland basin in the Dehra Dun and Subathu subbasins. The Dehra Dun subbasin essentially consists of conglomerate facies whereas Subathu subbasin is primarily composed of sandstone–mudstone intercalated with conglomerate. The two subbasins were sites of braided river systems. Evidence from chronostratigraphy, provenance and tectonic and climatic conditions suggests that differential tectonic movement, sedimentation rate and basin subsidence, coupled with climatic variation, were responsible for the contrasting fluvial facies in these subbasins. Following the merger of the two subbasins at about 5.23 Ma, the conglomerate facies assemblage of the Dehra Dun subbasin passes laterally into a sandstone–mudstone–conglomerate facies assemblage of the Subathu subbasin. Thickly bedded, amalgamated sheet conglomerate of Dehra Dun subbasin has a high sedimentation rate (0.70 mm/year) between 5.23 and 4.8 Ma, with a slow rate of basin subsidence. In contrast, a slow sedimentation rate (0.39 mm/year) in coeval sediments of the Subathu subbasin, with relatively fast subsidence, produced a succession of thick multistorey sandstone body with mature paleosols. Petrographic and clast composition data from Dehra Dun subbasin suggest that the detritus was derived from the pre-Tertiary Lesser Himalayan sequence lying in the north, during an active tectonic phase of the Main Boundary Thrust. Between 4.8 and 1.77 Ma, progradation of posttectonic conglomerate of the Dehra Dun subbasin continued during a quiescent phase of the Main Boundary Thrust. However, as a consequence of partitioning of Subathu subbasin along the Intra-Foreland Thrust, interfingering of buff (piedmont drainage) and grey (trunk drainage) sandstones took place. In contrast, the syntectonic conglomerate of Subathu subbasin originated from the uplifted proximal part of the foreland basin (Tertiary belt) during the active phase of Intra-Foreland Thrust around 1.77 Ma.