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The collisional history between Greater India and the Eurasian plate has been well constrained by the study of exhumed Barrovian metamorphic sequence (BMS) rocks in the Himalayan Range. However, in the southeastern Tibetan Plateau, the collisional records have been obscured by intense, regional‐scale strike‐slip overprinting and recrystallization. Here, in BMS rocks from the Ailao Shan–Red River shear zone (ARSZ), we report the first discovery of a >250 km long, high‐pressure (high‐P) granulite belt (>1.0 GPa), identified by the presence of relict kyanite and associated decompression reaction textures. Petrological phase equilibrium modeling showed that exposed micaschists in the region represent exhumed middle crust (20–25 km, 600–670°C), while the high‐P granulite rocks are remnants of thickened lower crust (45–55 km, 800–850°C). This indicates that the northeast edge of the ARSZ experienced an additional ∼25 km of uplift and exhumation compared to the southwest side, facilitated by brittle thrusting/imbrication along the Ailao Shan fault (micaschists) and ductile extrusion along the Red River fault (granulite). Geochronological study shows that the upper portion of the BMS preserves older metamorphic ages (52–34 Ma) than the lower portion (32–29 Ma), which was attributed to spatial variation in cooling rates. Using calculated P–T–t–d paths, we also examined variation in density and seismic wave speeds for BMS in the ARSZ. Our data correlate with fieldwork conducted elsewhere within the Himalayan Range indicating that the kyanite to sillimanite transition zone may serve as a "cap" for the horizontal migration of melt within the lower crust. Plain Language Summary: Clay‐rich rocks exhibit systematic mineralogical transformations during burial to the middle and lower continental crust as temperature and pressure increase. This is often referred as a Barrovian metamorphic sequence (BMS). Exhumed BMS rocks in crustal high‐strain zones provide an excellent opportunity for revealing the dynamic processes that take place during mountain building. In the southeastern Tibetan Plateau, crustal‐scale shear zones accommodated much of the northward‐directed indentation of the Indian plate. Here, we systematically studied BMS rocks in the Ailao Shan–Red River shear zone (ARSZ) and simulated how their physical properties vary with changes of temperature and pressure. Additionally, we report the first discovery of a high‐pressure granulite belt along the northeast edge of the ARSZ that was uplifted from ∼45 to 55 km, marking an additional ∼25 km of exhumation compared to the southwest edge. Moreover, distinct metamorphic peak temperature conditions across the ALS massif may account for uneven exhumation and discrepant cooling rates across the massif. Thermal and physical properties simulation indicate that the kyanite to sillimanite transform zone may serve as a "cap" for the sub‐horizontal melt migration in the lower crust of shear zones. Key Points: High‐pressure granulite belt is reported for the first time from the northeastern Ailao Shan (ALS) massif, southeastern Tibetan PlateauWe develop integrated P‐T‐t‐d paths and ρ‐Vp profiles for Barrovian metamorphic sequence rocks in the ALS massifBulk‐rock densification across the kyanite to sillimanite transition zone may serve as a "cap" for horizontal melt flow in the lower crust [ABSTRACT FROM AUTHOR]

Amphibole‐rich rocks constitute significant components of the mid‐ to lower continental crust, particularly in active orogens characterized with thick and hot crusts. Nevertheless, experimental data on their viscosity remain scarce. We conducted axial compression deformation experiments on synthetic amphibolites under temperature and pressure conditions resembling deep sections of overthickened crust. A novel flow law for a calcic‐amphibole‐rich rock (80% amphibole +20% garnet) in the dislocation creep regime is derived from these experiments. Contrary to common assumptions, our results reveal that calcic‐amphibolite is 1‐2 orders of magnitude weaker than plagioclase‐rich amphibolite, granulite, or gabbro. A calcic‐amphibole‐rich, low viscosity deep crust may not only support the "channel flow" model proposed for the Tibetan Plateau but also explain the observed high crustal seismic anisotropy in the region. Plain Language Summary: The rarity of earthquakes despite strong deformation in the deep crust of mountain belts produced by continental collisions, such as the Himalayas, implies that the deep continental crust is rather weak, easily dissipating imposed stresses by ductile deformation. The major components of the orogenic lower crust are plagioclase, amphibole, pyroxene, and garnet. Experimental data on the ductile deformation of coarse‐grained amphibole‐rich rocks was until now missing. We present new deformation experiments, from which we derive a flow law for an amphibolitic aggregate (80% amphibole +20% garnet). These data show high volumes of amphibole should result in high ductility and, hence, low strength in the deep crust, which should be considered when modeling the mechanical behavior of the lower crust during continental collisional. An amphibole‐rich deep crust also accounts for the high crustal seismic anisotropy in southern Tibet. Key Points: Experimentally based flow law for dislocation creep of calcic‐amphibole‐rich (80 wt.%) rocksA calcic‐amphibole‐rich deep crust supports the "channel flow" model in southern TibetExperimental amphibole crystal preferred orientations (CPO) may explain deep crustal seismic anisotropy [ABSTRACT FROM AUTHOR]

This paper introduces how crustal thickening controls the growth of the Himalaya by summarizing the P‐T‐t evolution of the Himalayan metamorphic core. The Himalayan orogeny was divided into three stages. Stage 60–40 Ma: The Himalayan crust thickened to ∼40 km through Barrovian‐type metamorphism (15–25 °C/km), and the Himalaya rose from <0 to ∼1000 m. Stage 40–16 Ma: The crust gradually thickened to 60–70 km, resulting in abundant high‐grade metamorphism and anatexis (peak‐P, 15–25 °C/km; peak‐T, >30 °C/km). The three sub‐sheets in the Himalayan metamorphic core extruded southward sequentially through imbricate thrusts of the Eo‐Himalayan thrust, High Himalayan thrust, and Main Central thrust, and the Himalaya rose to ≥5,000 m. Stage 16–0 Ma: the mountain roots underwent localized delamination, causing asthenospheric upwelling and overprinting of the lower crust by ultra‐high‐temperature metamorphism (30–50 °C/km), and the Himalaya reached the present elevation of ∼6,000 m. Underplating and imbricate thrusting dominated the Himalaya' growth and topographic rise, conforming to the critical taper wedge model. Localized delamination of mountain roots facilitated further topographic rise. Future Himalayan metamorphic studies should focus on extreme metamorphism and major collisional events, contact metamorphism and rare metal mineralization, metamorphic decarbonation and the carbon cycle in collisional belts. [ABSTRACT FROM AUTHOR]

We present a new high‐resolution image of the crust beneath the western Himalaya‐Asia convergence zone encompassing the geographical domain of western Himalaya‐western Tibet‐Ladakh‐Karakoram‐Pamir‐Hindu Kush, using ambient noise cross‐correlations from 530 seismological stations along with surface wave observations from 1,261 earthquakes recorded over the network. The 3‐D shear wave velocity image is created using 22,726 inter‐station Rayleigh wave dispersion measurements from 5 to 60 s period at a horizontal resolution of less than 0.5° × 0.5° following the Bayesian Trans‐dimensional Tree tomography approach. The Moho beneath the Himalayas and south Tibet correlates with a velocity transition of 4.4–4.6 km/s and a reduced velocity transition of 4.0–4.2 km/s in northern Tibet and the Pamir. We used the Moho depth and the nature of high‐velocity lower crust (Vs > 4.0 km/s) to map the northern limit of the Indian crust that extends beyond the Qiangtang block in western Tibet (77–82°E) from its previously assumed boundary in the Lhasa block and till the central Pamir farther west. The velocity image reveals discontinuous low‐velocity zones (LVZs; Vs < 3.4 km/s) in the mid‐crust of western Tibet and the Pamir that do not support the existence of the channel flow model. The LVZs in the Pamir correlate with the surface distribution of gneiss domes. The lowest velocities (Vs < 3.2 km/s) are observed over the Ladakh‐Karakoram batholith and the Nanga Parbat region. The study suggests a continuation of LVZs across the Karakoram Fault at a depth below 20 km, indicating the fault's upper crustal depth extent. Plain Language Summary: The Tibetan plateau, the Himalayas, and the Pamir are the most spectacular products of the continued collision of the Indian plate with the Asian plate since about 60 Ma. To provide constraints on the geodynamic processes responsible for the creation of these features, we present a high‐resolution 3‐D shear velocity model of the underlying crust beneath the poorly studied western segment of the India‐Asia collision zone. Using crustal thickness and the nature of the high‐velocity lower crust, we map the underthrusted Indian crust extending close to the northern end of western Tibet, and till the central Pamir farther west. As a consequence of the underthrusting, the low viscosity Indian crust beneath Tibet was proposed to flow southward and extrude along the Himalayan front. We examine the validity of this "channel flow model". Our velocity model shows discontinuous low‐velocity zones (LVZs) in the middle crust of western Tibet and the Pamir that do not support the presence of a pervasive channel flow, questioning the validity of the "channel flow model". In the Pamir, the LVZs correlate with gneissic domes. Our model also shows that the Karakoram Fault is a shallow crustal feature as it does not disrupt the mid‐crustal LVZs. Key Points: The northern limit of the Indian crust extends beyond the Qiangtang block in western Tibet and till the central Pamir farther in the westLaterally discontinuous mid‐crustal low‐velocity zones do not support the pervasive channel flow modelThe Karakoram Fault is an upper crustal feature extending to a depth of 20 km [ABSTRACT FROM AUTHOR]

Sitting between the Greater Himalayan sequence (GHS) and Lesser Himalayan sequence (LHS), the Main Central Thrust zone (MCTZ) has experienced multiple episodes of anatexis, which presents an opportunity to explore the nature of partial melting and its response to Himalayan orogenic processes. A series of deformed rocks, including migmatites, gneisses, and leucosomes were collected across the MCT at Arun Valley, eastern Nepal. We investigated the bulk rock major and trace elements, Sr-Nd isotopes, mineral chemistry, zircon geochronology and Hf isotopes, and conducted phase equilibria modeling. The protolith boundary between the GHS and LHS is recognized on the basis of Sr–Nd isotopes with ε Nd(0) of −16.7 to −8.0 for the GHS and −31.2 to −23.9 for the LHS. Samples from both the GHS and LHS have undergone partial melting, as revealed by in situ leucosomes at outcrops and melt inclusions at thin-section scale. Leucosomes separated from their host rocks are divided into four groups: those derived from hydration melting, muscovite dehydration melting, amphibole dehydration melting, and feldspar accumulation. Phase equilibria modeling results for the GHS migmatite show isothermal decompression from peak P–T conditions of 11 kbar and 795°C, accompanied by muscovite dehydration melting evolving into biotite dehydration melting. In contrast, rocks from the LHS are modeled to have undergone hydration melting at P–T conditions of 9 kbar and 685°C. Zircon U–Pb geochronology suggests that long-lived partial melting (35–13 Ma) occurred in the MCTZ. Moreover, anatectic zircon Hf isotopes show that the protoliths for partial melting changed from the GHS to the LHS with εHf(t) of −19.4 to −5.7 during the early Miocene, and lower values of −42.5 to −16.7 during the middle to late Miocene. These zircon geochemical results indicate that hydrous metasediments from the LHS were progressively accreted to the base of the GHS, resulting in hydration melting of both the GHS and LHS assisted by MCT. The timing of activity of the MCT is constrained to 25–13 Ma, coeval with movement of the South Tibetan detachment system. Integration of petrogenetic modeling, the chronology of partial melting, and metamorphic P–T paths allows us to propose that thickened Himalayan crust was heated from the middle to late Eocene, and widespread anatexis occurred during the Oligocene to middle Miocene, forming a large-scale melt channel. The hot GHS channel flow moved upward in association with the synchronous activity of the MCT system, triggered intense dehydration of LHS metasediments, resulting in fluid-present melting in both the GHS and LHS during middle to late Miocene, and the formation of leucogranite with mixture features of GHS and LHS. Furthermore, with the cooling of the melt channel, duplexing has gradually operated since the middle to late Miocene in the shallow crust. [ABSTRACT FROM AUTHOR]

The high‐grade Higher Himalayan Crystallines (HHC), located between the South Tibetan Detachment System and Main Central Thrust in the collision zone between the Indo‐Australian and Eurasian plates, have been subject to at least four significant phases of deformation and metamorphism. The earliest of those significantly predates the Cenozoic continent‐continent collision, but has been difficult to date isotopically because of later overprinting. Migmatitic paragneiss from the Badrinath Formation in the Dhauliganga Valley, northern Uttarakhand, some of the highest‐grade rocks in the HHC, preserves direct evidence of mid‐Ordovician metamorphism in the form of 465.8 ± 6.4 Ma zircon overgrowths with extremely low Th/U (0.0038–0.0074). The overgrowths have formed on ca. 2.63–0.71 Ga detrital zircon cores and are themselves overgrown by two generations of Miocene metamorphic zircon with mean Pb/U ages of ca. 21.5 and 18.2 Ma. Monazite from the same sample has a mean Pb/Th age of 19.4 ± 0.2 Ma. The oxygen isotopic compositions of the monazite (δ18O: 7.69 ± 0.08‰) and youngest zircon overgrowths (δ18O: 7.95 ± 0.12, 8.24 ± 0.09‰) are consistent with mineral growth in a metasediment, but either of the two minerals did not grow in isotopic equilibrium with each other, or the original composition of the monazite has not been preserved. If the quartz (δ18O: 13.29 ± 0.11‰) equilibrated with the youngest zircon and its composition has been preserved, then the last episode of zircon growth took place at low temperature, ca. 420°C, after the migmatization. The protolith of the Badrinath migmatite was a Neoproterozoic or Early Palaeozoic metasediment partially melted (and probably migmatized) in the Middle Ordovician. The strong planar foliation currently present in the migmatite is probably the result of mid‐crustal extrusional channel flow and HT decompressional partial melting in the Miocene. [ABSTRACT FROM AUTHOR]

Channel flow has been proposed as a mechanism to explain the formation of the Greater Himalayan Sequence that is bounded by normal sense ductile shear along the Himalayan orogen. The key requirements for channel flow are: (i) extruding middle-to-lower crust of low viscosity, and (ii) excess gravitational potential due to topography. We present scaled two-layer physical models where the effect of the gravitational potential with respect to the plate convergence rate is investigated. Viscous middle crust starts moving towards the surface where the strain rate imposed by the convergence is 30% of that arising from the lateral pressure gradient. How efficiently the low-viscosity crust extrudes is directly linked to the imposed pressure gradient. A simple correlation between the extruding rock's viscosity, the convergence rate, and the topography imposing the pressure gradient is established. The upward motion of viscous material is expected already for a mid-crustal viscosity of 1021 Pa s. This is significantly higher than previously expected, suggesting that one of the fundamental requirements for channel flow might not be necessary. [ABSTRACT FROM AUTHOR]

High-magnitude glacial debris flows in small basins in Himalayas have a significant impact on landscape. The Peilong catchment, a tributary of the Parlung Zangbo river in southeastern Tibet, was chosen as a case study of topographic response to multi-period glacial debris flows. There are few large debris flow records in the catchment before 1983, but four large-scale glacial debris flows with peak discharge up to 8195 m3/s blocked the river during 1983–1985 and in 2015. A combination of field survey, examination of historical records and interpretation of multi-period remote sensing images was used to assess triggering factors and geomorphic impact of the events. The results show that the debris flows during 1983 and 1985 may be attributed to seismic events in 1981 and 1982, while the event in 2015 resulted from large amount of landslide deposits caused by glacier retreat during 1993–2013 and high precipitation in 2015. In the upper-midstream broad valley, erosion and accumulation of the debris flows changed the channel morphology, resulting in course diversion. In the lower-midstream narrow valley, lateral erosion of debris flows induced a large number of landslides but had little impact on the channel longitudinal profile. The ability of massive glacial debris flows to change valley topography is more than ten times that of regular water flows. The landscape of the accumulation fan at the outlet of the valley is controlled by the interaction between the sediment transportation capacity of debris flows and erosional capacity of the main river. The sediment transport capacity of the Peilong river is greater than the delivery capacity of the Parlung Zangbo river, resulting in continuous aggradation of the confluence zone. [ABSTRACT FROM AUTHOR]

The Himalayas are the archetype of continental collision, where a number of long-standing fundamental problems persist in the Greater Himalayan Sequence (GHS): (1) contemporaneous reverse and normal faulting, (2) inversion of metamorphic grade, (3) origin of high- (HP) and ultrahigh-pressure (UHP) rocks, (4) mode of ductile extrusion and exhumation of HP and UHP rocks close to the GHS hanging wall, (5) flow kinematics in the subduction channel, and (6) tectonic overpressure, here defined as TOP D P=PL where P is total (dynamic) pressure and PL is lithostatic pressure. In this study we couple Himalayan geodynamics to numerical simulations to show how one single model, upward-tapering channel (UTC) flow, can be used to find a unified explanation for the evidence. The UTC simulates a flat-ramp geometry of the main underthrust faults, as proposed for many sections across the Himalayan continental subduction. Based on the current knowledge of the Himalayan subduction channel geometry and geological/ geophysical data, the simulations predict that a UTC can be responsible for high TOP (> 2). TOP increases exponentially with a decrease in UTC mouth width, and with an increase in underthrusting velocity and channel viscosity. The highest overpressure occurs at depths < or < 15°, respectively). Viscous deformable walls do not affect overpressure significantly enough for a viscosity contrast (viscosity walls to viscosity channel) of the order of 1000 or 100. TOP in a UTC, however, is only possible if the condition at the bottom boundary is no-outlet pressure; otherwise it behaves as a leaking boundary that cannot retain dynamic pressure. However, the cold, thick, and strong lithospheres forming the Indian and Eurasian plates are a good argument against a leaking bottom boundary in a flat-ramp geometry, and therefore it is possible for overpressure to reach high values in the GHS. [ABSTRACT FROM AUTHOR]

The channel-flow model for the Greater Himalayan Sequence (GHS) of the Himalayan orogen involves a partially molten, rheologically weak, midcrustal layer "flowing" southward relative to the upper and lower crust during late Oligocene-Miocene. Flow was driven by topographic overburden, underthrusting, and focused erosion. We present new structural and thermobarometric analyses from the GHS in the Annapurna-Dhaulagiri Himalaya, central Nepal; these data suggest that during exhumation, the GHS cooled, strengthened, and transformed from a weak "active channel" to a strong "channel plug" at greater depths than elsewhere in the Himalaya. After strengthening, continued convergence resulted in localized top-southwest (top-SW) shortening on the South Tibetan detachment system (STDS). The GHS in the Annapurna-Dhaulagiri Himalaya displays several geological features that distinguish it from other Himalayan regions. These include reduced volumes of leucogranite and migmatite, no evidence for partial melting within the sillimanite stability field, reduced structural thickness, and latestage top-southwest shortening in the STDS. New and previously published structural and thermobarometric constraints suggest that the channel-flow model can be applied to mid-Eocene-early Miocene mid-crustal evolution of the GHS in the Annapurna-Dhaulagiri Himalaya. However, pressure-temperature-time (PTt) constraints indicate that following peak conditions, the GHS in this region did not undergo rapid isothermal exhumation and widespread sillimanite-grade decompression melting, as commonly recorded elsewhere in the Himalaya. Instead, lower-than-typical structural thickness and melt volumes suggest that the upper part of the GHS (Upper Greater Himalayan Sequence [UGHS]--the proposed channel) had a greater viscosity than in other Himalayan regions. We suggest that viscosity-limited, subdued channel flow prevented exhumation on an isothermal trajectory and forced the UGHS to exhume slowly. These findings are distinct from other regions in the Himalaya. As such, we describe the mid-crustal evolution of the GHS in the Annapurna-Dhaulagiri Himalaya as an atypical example of channel flow during the Himalayan orogeny. [ABSTRACT FROM AUTHOR]

The presence of hot, weak crust is a central component of recent hypotheses that seek to explain the evolution of continent-continent collisions, and in particular may play an important role in accommodating the >3000 km of convergence within the Himalaya–Tibetan collision over the last ∼55 Myr. Models that implicate flow of semi-viscous midcrustal rocks south toward the front of the Himalayan orogen, ‘channel flow’, are able to account for many geologic observations in the Himalaya, while alternative models of collision, particularly ‘thrust-wedge taper’, demonstrate that much of the observed geology could have formed in the absence of a low-viscosity mid-crustal layer. Several recent studies, synthesized here, have prompted a shift from initial assumptions that channel flow and thrust-wedge taper processes are by definition mutually exclusive. These new studies reveal the presence of several tectonometamorphic discontinuities in the midcrust that appear to reflect a continuum of deformation in which both channel- and wedge-type processes operate in spatially and temporally distinct domains within the orogen, and further, that the system may migrate back and forth between these types of behavior. This continuum of deformation styles within the collisional system is of crucial importance for explaining the evolution of the Himalayan orogen and, hence, for understanding the evolution of Earth's many continent-continent collision zones. [ABSTRACT FROM AUTHOR]

Based on metamorphic studies of the Yadong high-pressure (HP) granulite and multiple thermochronological investigations of granitoids from both upper and lower parts, the Yadong section in the eastern Himalaya constrains the Cenozoic tectonic evolution of the Greater Himalayan Sequence (GHS). The Yadong HP granulite, located at the top of the GHS, underwent a peak-stage HP granulite facies metamorphism and two stages of retrograde metamorphism. Granulite and hornblende facies retrograde metamorphism took place at 48.5 and 31.8 Ma, respectively, marking the time of exhumation of the subducted Indian slab to lower and middle crustal levels. Subsequently, an average young zircon U–Pb age obtained from the Yadong HP granulite indicated that this unit was captured by its surroundings in a partially molten condition at 16.9 Ma. In addition, three granitoids from both the lower and the upper parts of the GHS yielded biotite 40Ar/39Ar ages of 11.0, 11.3, and 11.5 million years. These consistent ages suggest that the GHS along the Yadong section was laterally extruded and synchronously cooled to ∼300°C at ∼11.3 Ma. Furthermore, the granitic gneisses yield apatite fission track ages of ∼7 million years, documenting the cooling of the GHS to ∼110°C. A two-stage model describes the Cenozoic tectonic evolution of the GHS: (1) the Indian slab had subducted under Tibet before ∼55 Ma, and was exhumed to the lower crust (50-40 km) at 48.5 Ma, and to the middle crust (22-15 km) at 31.8 Ma; and (2) the partial melting occurred at middle crustal levels during the period 31.8 to 16.9 Ma, causing channel flow. In the late stage, the GHS was laterally extruded by ductile mid-crustal flow during the period 16.9 to ∼7 Ma, characterized by a fast cooling rate of ∼2 mm per year. [ABSTRACT FROM AUTHOR]

Partial melting of deep continental crust may occur during either prograde heating or decompression. Although the effect of temperature on crustal melting has been widely investigated, few experimental studies have addressed the question of the influence of pressure on crustal anatexis. To understand the influence of decreasing pressure on partial melting processes, the thermodynamic approach of isochemical phase diagrams has been applied to garnet–K-feldspar–kyanite–sillimanite anatectic gneisses (Barun Gneiss) from the Higher Himalayan Crystallines (HHC) of eastern Nepal. The main melt-producing reactions, the amount of melt produced during heating vs decompression, and the effects of melt loss on the mineral assemblages and compositions have been investigated along four ideal P–T trajectories, dominated by either heating or decompression. Based on these results, the observed microstructures and mineral compositions of the Barun Gneiss have been interpreted in terms of melt-producing vs melt-consuming reactions (e.g. growth of peritectic garnet with preserved ‘nanogranite’ inclusions vs microstructures related to back-reactions between solids and melt), and used to derive the metamorphic evolution of the studied samples. The P–T pseudosection modelling predicts that at least 15–20 vol. % of melt was produced at peak P–T conditions through dehydration melting of both muscovite and biotite, and that melt production was mainly triggered by heating, with or without the combined effect of decompression. The preserved granulitic peak metamorphic assemblage, however, is consistent with a significant loss of most of this melt. The P–T evolution inferred for samples from different, strategically located, structural levels of the Barun Gneiss is consistent with the expectations of a ‘channel flow’ model, including: (1) the clockwise shape of the P–T paths; (2) the estimated P at peak T (new data: 10–8 kbar at 800°C; model: 13–7 kbar at 800°C); (3) the decreasing P structurally upward, which defines a ‘normal’ metamorphic sequence, in contrast to the inverted metamorphic sequence occurring in the lowermost Main Central Thrust Zone; (4) the nearly isothermal exhumation of the structurally lowest sample, reflecting the progressive exhumation of rocks that have been entrained in the deep, high-T region of the channel, versus the nearly isobaric heating of the structurally uppermost sample, reflecting the evolution of those rocks that flowed outwards with the underlying channel. [ABSTRACT FROM PUBLISHER]

This study presents a Geographic Information System (GIS)-based distributed rainfall-runoff model for simulating surface flows in small to large watersheds during isolated storm events. The model takes into account the amount of interception storage to be filled using a modified Merriam (1960) approach before estimating infiltration by the Smith and Parlange (1978) method. The mechanics of overland and channel flow are modelled by the kinematic wave approximation of the Saint Venant equations which are then numerically solved by the weighted four-point implicit finite difference method. In this modelling the watershed was discretized into overland planes and channels using the algorithms proposed by Garbrecht and Martz (1999). The model code was first validated by comparing the model output with an analytical solution for a hypothetical plane. Then the model was tested in a medium-sized semi-forested watershed of Pathri Rao located in the Shivalik ranges of the Garhwal Himalayas, India. Initially, a local sensitivity analysis was performed to identify the parameters to which the model outputs like runoff volume, peak flow and time to peak flow are sensitive. Before going for model validation, calibration was performed using the Ordered-Physics-based Parameter Adjustment (OPPA) method. The proposed Physically Based Distributed (PBD) model was then evaluated both at the watershed outlet as well as at the internal gauging station, making this study a first of its kind in Indian watersheds. The results of performance evaluation indicate that the model has simulated the runoff hydrographs reasonably well within the watershed as well as at the watershed outlet with the same set of calibrated parameters. The model also simulates, realistically, the temporal variation of the spatial distribution of runoff over the watershed and the same has been illustrated graphically. Copyright © 2011 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The Higher Himalayan Shear Zone (HHSZ) contains a ductile top-to-N/NE shear zone-the South Tibetan detachment system-lower (STDS) and an out-of-sequence thrust (OOST) as well as a top-to-N/NE normal shear at its northern boundary and ubiquitously distributed compressional top-to-S/SW shear throughout the shear zone. The OOST that was active between 22 Ma and the Holocene, varies in thickness from 50 m to 6 km and in throw from 1.4 to 20 km. Channel flow analogue models of this structural geology were performed in this work. A Newtonian viscous polymer (PDMS) was pushed through a horizontal channel leading to an inclined channel with parallel and upward-diverging boundaries analogous to the HHSZ and allowed to extrude to the free surface. A top-to-N/NE shear zone equivalent to the STDS developed spontaneously. This also indirectly connotes an independent flow confined to the southern part of the HHSZ gave rise to the STDS. The PDMS originally inside the horizontal channel extruded at a faster rate through the upper part of the inclined channel. The lower boundary of this faster PDMS defined the OOST. The model OOST originated at the corner and reached the vent at positions similar to the natural prototype some time after the channel flow began. The genesis of the OOST seems to be unrelated to any rheologic contrast or climatic effects. Profound variations in the flow parameters along the HHSZ and the extrusive force probably resulted in variations in the timing, location, thickness and slip parameters of the OOST. Variation in pressure gradient within the model horizontal channel, however, could not be matched with the natural prototype. Channel flow alone presumably did not result in southward propagation of deformation in the Himalaya. [ABSTRACT FROM AUTHOR]

Abstract: It is generally believed that Miocene adakitic rocks in the Lhasa terrane (block) of Southern Tibet were derived from a thickened lower crust, and that their compositions will therefore reflect the geochemical characteristics of their lower-crustal source(s). Adakitic rocks in the western part of the Lhasa terrane possess geochemical and Sr–Nd isotopic characteristics that are clearly different from those in the eastern part of the Lhasa terrane; the former display a more enriched Nd–Sr isotopic signature together with an older model age relative to the latter, suggesting that the compositions of the lower crust under the western and eastern parts of the Lhasa terrane also differ. In addition, the adakitic rocks in the Himalayas on the southern side of the Indus–Yalu suture (IYS) have similar geochemical characteristics and Sr–Nd isotopic compositions to the adakitic rocks of the western Lhasa terrane, but differ from rocks in the eastern Lhasa terrane and from igneous rocks derived from crust beneath the Himalayas. We suggest that the lower (and/or middle) crustal materials beneath the western Lhasa terrane probably extend to the southern side of the IYS due to channel flow. If mid-lower crustal flow has occurred below Southern Tibet, the southeastward-moving anatectic mid-lower crustal material was probably derived from below the western part of the Lhasa terrane rather than the eastern. Moreover, the emplacement ages of adakitic rocks from Mayum, on the southern side of the IYS, imply that southeastward ductile flow of mid-lower crust beneath western Lhasa may have occurred as early as 17Ma. [Copyright &y& Elsevier]

Abstract: The orogenic superstructure (SS) and infrastructure (IS) constitute two levels of a mountain belt with contrasting structural styles. In the Nepal Himalaya, N-verging back folds, which oppose the orogenic vergence, dominate the SS. Competing explanations for these folds are tested using centrifuge analogue models. Modelling suggests that SS folding occurs during bulk shortening accompanied by IS thickening before IS flow. Focused erosion then instigates IS lateral flow and stretching, decoupling of the SS, and transposition of the lower SS into a detachment zone. Decoupling at the IS–SS interface separates an SS dominated by older folds and an IS characterised by younger horizontal transposition and stretching of early folds. Extrusive ductile flow of the IS locally modifies fold vergence in the SS. The fold asymmetry is thus controlled by the efficiency of coupling between IS and SS; a low viscosity at the IS–SS interface favours complete decoupling and hinders modification of fold vergence, whereas a higher viscosity IS-SS interface favours fold vergence modification. Modelling supports a tectonic scenario in which Himalayan hinterland-verging folds are the product of early shortening of the SS followed by local modification of fold geometry when the IS subsequently stretches and flows during focused erosion and melt-enhanced IS weakening. [Copyright &y& Elsevier]

Thermobarometric data and compositional zoning of garnet show the discontinuities of both metamorphic pressure conditions at peak- T and P–T paths across the Main Central Thrust (MCT), which juxtaposes the high-grade Higher Himalayan Crystalline Sequences (HHCS) over the low-grade Lesser Himalaya Sequences (LHS) in far-eastern Nepal. Maximum recorded pressure conditions occur just above the MCT (∼11 kbar), and decrease southward to ∼6 kbar in the garnet zone and northward to ∼7 kbar in the kyanite ± staurolite zone. The inferred nearly isothermal loading path for the LHS in the staurolite zone may have resulted from the underthrusting of the LHS beneath the HHCS. In contrast, the increasing temperature path during both loading and decompression (i.e. clockwise path) from the lowermost HHCS in the staurolite to kyanite ± staurolite transitional zone indicates that the rocks were fairly rapidly buried and exhumed. Exhumation of the lowermost HHCS from deeper crustal depths than the flanking regions, recording a high field pressure gradient (∼1.2–1.6 kbar km−1) near the MCT, is perhaps caused by ductile extrusion along the MCT, not the emplacement along a single thrust, resulting in the P–T path discontinuities. These observations are consistent with the overall scheme of the model of channel flow, in which the outward flowing ‘HHCS’ and inward flowing ‘LHS’ are juxtaposed against each other and are rapidly extruded together along the ‘MCT’. A rapid exhumation by channel flow in this area is also suggested by a nearly isothermal decompression path inferred from cordierite corona surrounding garnet in gneiss of the upper HHCS. However, peak metamorphic temperatures show a progressive increase of temperature structurally upward (∼570–740 °C) near the MCT and roughly isothermal conditions (∼710–810 °C) in the upper structural levels of the HHCS. The observed field temperature gradient is much lower than those predicted in channel flow models. However, the discrepancy could be resolved by taking into account heat advection by melt and/or fluid migration, as these can produce low or nearly no field temperature gradient in the exhumed midcrust, as observed in nature. [ABSTRACT FROM AUTHOR]

Abstract: Along the majority of the Himalayan orogenic belt, high-grade metamorphic rocks of the Greater Himalaya (GH) are separated from lower-grade metasedimentary rocks by two shear zones, the basal, top-to-the-south Main Central Thrust (MCT), and the upper, top-to-the-north South Tibetan Detachment (STD). Seemingly pervasive ductile deformation within GH rocks and a lack of exposed hanging wall or footwall cut-offs for the MCT and STD have permitted models that predict 100''s of km of coeval displacement on both structures, extruding the GH section via gravitational loading from Tibet and focused erosion along the Himalayan topographic front. We present new mapping, stratigraphic columns, mineral assemblages, U/Pb zircon ages, and a suite of structural data from GH and Tethyan Himalayan (TH) rocks in eastern and central Bhutan. Our observations highlight significant, ∼250–300°C along-strike changes in the deformation temperature conditions recorded by GH rocks. In eastern Bhutan, TH rocks are separated from GH rocks displaying ubiquitous partial melt textures by a top-to-the-north sense shear zone correlated with the STD. However, in central Bhutan, limited partial melt textures are only present near the base of the GH section, a biotite–muscovite–garnet mineral assemblage persists from 0.2 to 3km above the MCT through TH rocks, and distinct lithologies interfinger at the GH–TH contact, which suggests that TH strata are in depositional contact above GH strata. The same strata exposed on both sides of the STD limits slip along this structure to ∼20km. Thin-section scale top-to-the-south shear is focused in a 2–3km-thick zone above the MCT while the overlying ∼11km-thick section displays top-to-the-north shear, suggesting asymmetric channel behavior. Strained quartz grains allow us to quantify ∼23–34km of top-to-the-north shear. Our data suggest that the GH–TH section in central Bhutan acted as a cool, viscous, low-displacement channel, and when compared to sections in eastern Bhutan highlights dramatic changes in temperature conditions, viscosity and possibly displacement along strike. However, we suggest that the erosion rates needed to remove low-viscosity material limit channel flow even in regions containing significant partial melt, and that the magnitude of channel flow is small compared to the total mass balance of the system. [Copyright &y& Elsevier]

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

• Growth of index minerals reveals a time relation between crystallization and deformation. • P-T & T-M H2O pseudosections suggest the peak metamorphic conditions as ~7.5 kbar & ~750 °C. • Channel flow model has been proposed for the exhumation of Sikkim Himalaya. The Sikkim Himalaya is mostly constituted of Proterozoic metapelites and metavolcanics of the Daling Group and high-grade gneisses and metasediments of the Darjeeling Group. Metapelitic schists and gneisses of the Sikkim Himalaya show an inverted sequence of metamorphic rocks ranging from lower greenschist facies (chlorite zone) to upper amphibolite facies (sillimanite + K-feldspar zone). The relative X Mg in the minerals varies as: garnet > staurolite > biotite > muscovite. The P-T evolution of the study area has been constrained through the use of an internally consistent winTWQ programme and Perple_X software in the MnNCKFMASHTO model system. The combination of these two approaches demonstrates that the schists and gneisses have experienced a range of pressure and temperature with increasing grade. Constraints on the interpretation of mineral reactions and metamorphic history provided by pseudosection modelling for mineral proportions suggest the peak metamorphic temperature as ~750 °C and pressure ~7.5 kbar. The proposed prograde P-T path implies that the rocks from the study area may have resulted from a single metamorphic event with continuous changes in pressure and temperature. The conclusions made on the basis of the study of metamorphic event in the area and the regeneration of the reaction history adheres to classical Barrovian type metamorphism. The tectonic implication of such a metamorphic evolution is also discussed in the present work. Finally, we conclude that the possible explanation for the exhumation in this section of Himalaya may most likely be explained with the help of the channel flow model. [ABSTRACT FROM AUTHOR]

Several competing models have been proposed to explain the connection between contractional structures, dominating the southern margins of the Himalayan-Tibetan orogen system, and extension, typical of the Tibetan plateau. Cenozoic E-W extension is documented in the Tibetan Plateau through strike-slip faults and grabens, while N-S extension has been widely documented along the South Tibetan detachment zone (STDZ), near the Tibet-to-Himalaya boundary. However, several arc-parallel extensional features extend as far south as the STDZ and the High Himalaya, and it is growingly recognized that extension along these features initiated in the mid-Miocene. Several authors identify the strain partitioning model as the most accurate in describing several features present at the Himalaya-Tibet transition, predicting concomitant strike-slip deformation and OP extension along the arc, even near its center where convergence obliquity = 0. If they are kinetically linked, they will share a common onset. We test this concept by looking at the long-term exhumation history of the Thakkhola-Mustang graben, the most central of the grabens, where we obtained detrital apatite fission track and (U-Th-Sm)/He thermochronology from modern river sand. We interpret the thermochronological dataset using a 2D numerical model of the thermal evolution of the crust around a normal fault, using an optimization method that minimizes the misfit between observed and predicted age distributions. The obtained history of extension in the Thakkhola-Mustang graben is consistent with other, independent stratigraphic constraints. From the relationship between the two different thermochronometers, we identify two distinct exhumation events, an earlier event which terminated by 20 Ma and a younger event which initiated between 13 and 11 Ma and accelerated after 10 Ma. We link the first event to regional exhumation related to motion along the STDZ, while the younger event marks the initiation of E-W extension in the Thakkhola-Mustang graben. Onset of extension is constrained by an increase in age density in the apatite (U-Th-Sm)/He thermochronometer distribution and is confirmed by the results of the thermo-kinematic modeling.The timing of extension initiation in this region is contemporaneous with motion on other orogen parallel structures in other parts of the Himalayas, suggesting that the cessation of extrusion along the STDZ and onset of orogen-parallel extension correspond to a transition from an extrusion-dominated mode driven by the weight of the Tibetan Plateau to an underthrusting-dominated mode driven by subduction of the Indian plate. [ABSTRACT FROM AUTHOR]