South Tibetan Detachment System Research Articles

Rongbuk re-visited: Geochronology of leucogranites in the footwall of the South Tibetan Detachment System, Everest Region, Southern Tibet

The Hermit's Gorge area adjacent to the Rongbuk Glacier on the north side of Mount Everest is a critical location to establish the timing and duration of movement along the ductile strand of the South Tibetan Detachment system (STDS), a low-angle, north-dipping normal fault that bounds the upper part of the Greater Himalayan Sequence. Monazite from four leucogranite samples in the Hermit's Gorge that bracket the timing of ductile fabric development has been dated using both U/Th–Pb ID-TIMS and LA-MC-ICPMS. Results suggest that the earliest, ~16.4Ma, leucogranite sills have been folded and deformed along with the host sillimanite gneisses and calc-silicates whereas structurally higher sills and dykes that post-date fabric development are slightly younger, all within uncertainty of one another at 15.6 to 15.4Ma. Field relations combined with age data constrain ductile fabric formation associated with movement along this strand of the STDS as being on-going at 16.4Ma but had ceased prior to 15.6Ma, while brittle faulting along the STDS is younger than 15.4Ma. Combined with data from the Everest massif and surrounding region, ages of granite crystallization and ductile shearing propagated up-structural section and northward with time.

Shear Sense Analysis of the Higher Himalayan Crystalline Belt and Tectonics of the South Tibetan Detachment System, Alaknanda-Dhauli Ganga Valleys, Uttarakhand Himalaya

In the central parts of Uttarakhand Himalaya, more than 20 km thick homoclinally NE-dipping Higher Himalayan Crystalline (HHC) Belt is thrust over the Lesser Himalaya Sedimentary Belt along the Main Central Thrust (MCT), and is almost continuously exposed between Helang and Malari along the Alaknanda and Dhauli Ganga valleys. The upper contact of this belt with the Martoli Formation of the Tethyan Himalayan Sequence is demarcated by the South Tibetan Detachment System (STDS). The belt is ubiquitously marked by small-scale asymmetrical structures like S-C and S-C' foliation, porphyroclasts and porphyroblasts, mineral fishes, intrafolial folds, duplex structures, ductile-brittle shear zones, and asymmetric shear boudins. Sense of ductile to brittle- ductile shearing has been determined from these structures across the whole belt, the MCT and the STDS, and reveals two phases of shear deformation: (a) an older top-to-SW upwards phase throughout the HHC, having an overall thrust geometry (DS1), and (b) a younger superposed top-to-NE downwards phase with normal fault sense from the middle to upper parts (DS2). These shear senses provide invaluable constraints on various tectonic models currently in use for the evolution of the Himalayan metamorphics.

Crustal geoelectric structure of the Sikkim Himalaya and adjoining Gangetic foreland basin

We present the results of a broadband magnetotelluric survey along a 200-km-long profile across the Sikkim Himalaya. The data were acquired at average station spacing of 5–6km and transfer functions of 31 sites in 0.01–1000s period range have been used for 2-D joint inversion of TE and TM modes. The composite model incorporating the effect of transverse strike reveals several features that correlate with the available seismic and kinematic models of the region. A major result of the present study is that the Main Himalayan Thrust forms the base of several resistive blocks within the wedge and that a ramp structure is present south of the Main Central Thrust Zone (MCTZ). Another significant result is that the crust and mantle lithosphere beneath the MCTZ and the Higher Himalayan Crystallines (HHC) seem to be compositionally/geologically different from the lithosphere south of the MCTZ. A steep crustal-scale fault with the Moho offset of 14km is inferred to be separating these two blocks. The deep crustal seismicity could be related to this fault whereas shallow seismicity can be linked to the deformation within the wedge. The results also reveal the presence of some more conductors. We relate the conductor within the HHC to the sedimentary rocks of the Tethyan sequence exposed in a window about 40km west of the profile and north of the South Tibetan Detachment System (STDS). The conductor at90km profile location is linked to the Gondwana rocks exposed in the Rangit Window. A 4–6km thick sedimentary layer overlies the basement in the Gangetic foreland basin. We also delineate a sub-crustal conductor at 50–60km depth beneath the foreland basin at the southern end of the profile, the cause of which is not apparent and needs to be explored.

Kinematic and thermal studies of the Leo Pargil Dome: Implications for synconvergent extension in the NW Indian Himalaya

Building of the Himalaya and Tibetan Plateau involved an interplay between crustal thickening and extension. The NW Indian Himalaya near the Leo Pargil dome contains approximately N trending brittle normal faults and NE trending normal-sense shear zones (i.e., bounding Leo Pargil dome) between the South Tibetan detachment system to the south and the Karakoram fault to the north. The Leo Pargil shear zone bounds the southwest flank of the Leo Pargil dome. Estimates of deformation temperatures and mean kinematic vorticity (Wm) from the shear zone were integrated with pressure-temperature estimates to evaluate its kinematic and thermal evolution. Oblique quartz fabrics yielded kinematic vorticity estimates indicating that the rocks within the shear zone were thinned by up to 62% during W directed shearing. Top-down-to-the-W ductile deformation is recorded at temperatures from >650°C at the deepest structural levels within the dome and from 400 to 500°C at shallower structural depths. Hanging wall rocks record much lower temperatures. Pressure-temperature data indicate that rocks in the dome were exhumed from depths of ~36 to 22 km. In contrast to other mechanisms proposed for dome formation across the Himalaya, initiation of ductile movement on the Leo Pargil shear zone occurred at midcrustal levels in a region of localized synconvergent extension at >23 Ma. The timing and kinematic setting of deformation on the Leo Pargil shear zone suggest that exhumation could be related to a localized zone of transtension near the Karakoram fault zone.

Himalayan gneiss dome formation in the middle crust and exhumation by normal faulting: New geochronology of Gianbul dome, northwestern India

A general lack of consensus about the origin of Himalayan gneiss domes hinders accurate thermomechanical modeling of the orogen. To test whether doming resulted from tectonic contraction (e.g., thrust duplex formation, antiformal bending above a thrust ramp, etc.), channel flow, or via the buoyant rise of anatectic melts, this study investigates the depth and timing of doming processes for Gianbul dome in the western Himalaya. The dome is composed of Greater Himalayan Sequence migmatite, Paleozoic orthogneiss, and metasedimentary rock cut by multiple generations of leucogranite dikes. These rocks record a major penetrative D2 deformational event characterized by a domed foliation and associated NE-SW–trending stretching lineation, and they are flanked by the top-down-to-the-SW (normal-sense) Khanjar shear zone and the top-down-to-the-NE (normal sense) Zanskar shear zone (the western equivalent of the South Tibetan detachment system). Monazite U/Th-Pb geochronology records (1) Paleozoic emplacement of the Kade orthogneiss and associated granite dikes; (2) prograde Barrovian metamorphism from 37 to 33 Ma; (3) doming driven by upper-crustal extension and positive buoyancy of decompression melts between 26 and 22 Ma; and (4) the injection of anatectic melts into the upper levels of the dome—neutralizing the effects of melt buoyancy and potentially adding strength to the host rock—by ca. 22.6 Ma on the southwestern flank and ca. 21 Ma on the northeastern flank. As shown by a northeastward decrease in 40 Ar/ 39 Ar muscovite dates from 22.4 to 20.2 Ma, ductile normal-sense displacement within the Zanskar shear zone ended by ca. 22 Ma, after which the Gianbul dome was exhumed as part of a rigid footwall block below the brittle Zanskar normal fault, tilting an estimated 5°–10°SW into its present orientation.

Switch from thrusting to normal shearing in the Zanskar shear zone, NW Himalaya: Implications for channel flow

Research Article| July 01, 2014 Switch from thrusting to normal shearing in the Zanskar shear zone, NW Himalaya: Implications for channel flow Melanie Finch; Melanie Finch † 1School of Geosciences, Monash University, Clayton, Victoria 3800, Australia †E-mail: melanie.finch@monash.edu Search for other works by this author on: GSW Google Scholar Pavlína Hasalová; Pavlína Hasalová 1School of Geosciences, Monash University, Clayton, Victoria 3800, Australia2Centre for Lithospheric Research, Czech Geological Survey, Klárov 3, 118 21 Prague 1, Czech Republic §Current address: Centre for Lithospheric Research, Czech Geological Survey, Klárov 3, 118 21, Prague 1, Czech Republic. Search for other works by this author on: GSW Google Scholar Roberto F. Weinberg; Roberto F. Weinberg 1School of Geosciences, Monash University, Clayton, Victoria 3800, Australia Search for other works by this author on: GSW Google Scholar C. Mark Fanning C. Mark Fanning 3Research School of Earth Sciences, Australian National University, Mills Road, Canberra, ACT 0200, Australia Search for other works by this author on: GSW Google Scholar GSA Bulletin (2014) 126 (7-8): 892–924. https://doi.org/10.1130/B30817.1 Article history received: 07 Nov 2012 rev-recd: 09 Aug 2013 accepted: 22 Nov 2013 first online: 08 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Melanie Finch, Pavlína Hasalová, Roberto F. Weinberg, C. Mark Fanning; Switch from thrusting to normal shearing in the Zanskar shear zone, NW Himalaya: Implications for channel flow. GSA Bulletin 2014;; 126 (7-8): 892–924. doi: https://doi.org/10.1130/B30817.1 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 SocietyGSA Bulletin Search Advanced Search Abstract The Zanskar shear zone is a ductile, normal-sense shear zone that exploited the contact between the High Himalayan Crystalline series and the Tethyan sedimentary series. The Zanskar shear zone is an extension of the South Tibetan detachment system with similar timing and nature, and, in Zanskar, it accommodated 24 km of normal movement. Early thrusting is preserved in the footwall and hanging wall and is overprinted by normal shearing. Thrusting and normal shearing were coplanar and codirectional, with SW-directed thrusting overprinted by NE-directed normal shearing—a simple inversion of movement sense. The telescoped isograds related to normal shearing define a broad pattern of colder rocks on top of hotter. However, we found preserved thrust-related metamorphic series, with hotter rocks on top of colder, severely telescoped by normal shearing. Some determinations of the amount of displacement and thinning on the Zanskar shear zone prior to the current work have assumed a steady-state crustal profile and have disregarded preexisting perturbations of isograds such as those indicated here. Miocene leucogranitic intrusions accumulated within and below the normal Zanskar shear zone. Intrusions were sheared during thrusting and normal movement, and magmatism outlasted normal shearing. We have dated monazites by U-Pb sensitive high-resolution ion microprobe (SHRIMP) from leucogranite samples that were sheared by the thrusting event, by the normal movement event, and those that postdate all shearing. Results constrain the timing of the switch from thrusting to normal movement to between 26 and 24 Ma and ca. 22 Ma. At ca. 20 Ma, normal shearing in Zanskar shear zone was no longer active, and magmatism was waning, producing late, undeformed leucogranitic dikes. Taking into account the shear zone thickness of 0.83 km, the maximum duration of normal movement of 6 m.y., and the estimated strain of γ = 28.6, we estimate the lower bound of strain rate for the Zanskar shear zone to be 1.5 × 10–13 s–1. Given the short duration of the normal shearing event and magmatism, we find little support for the hypothesis of channel flow in Zanskar. We propose instead that Miocene anatexis weakened the midcrustal levels and caused the switch from thrusting to normal movement, doming, and cooling of the anatectic core of the High Himalayan Crystalline series. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

High-temperature cooling histories of migmatites from the High Himalayan Crystallines in Sikkim, India: rapid cooling unrelated to exhumation?

The High Himalayan Crystallines (HHCs) provide an excellent natural laboratory to study processes related to crustal melting, crustal differentiation, and the tectonic evolution of mountain belts because partial melting in these rocks occurred under well-defined tectonic boundary conditions (N–S collision of the Indian and the Eurasian plates) and the rocks have not been modified by subsequent metamorphic overprinting. We have used petrogenetic grids, kinetically constrained individual thermobarometry, pseudosection calculations, and reaction histories constrained by textural evidence to determine that the migmatites in the HHC of Sikkim attained peak P–T conditions of 750–800 °C, 9–12 kbar, followed by steep isothermal decompression to 3–5 kbar, and then isobaric cooling to ~600 °C. There may be a trend where rocks to the north [closer to the South Tibetan detachment system (STDS)] attained somewhat higher maximum pressures. The decompression may have been triggered by a reduction in density due to the production of melt (~20 vol%); minor amounts of additional melt may have been produced in individual packages of rock during decompression itself, depending on the exact geometry of the P–T path and the bulk composition of the rock. The stalling of rapid, isothermal exhumation at depths of 10–18 km (3–5 kbar) is related to metamorphic reactions that occur in these rocks. Geospeedometry indicates that at least a two-stage cooling history is required to describe the compositional zoning in all garnets. Both of these stages are rapid (several 100’s °C/my between 800 and 600 °C, followed by several 10’s °C/my between 600 and 500 °C), but there appears to be a spatial discontinuity in cooling history: Rocks to the south (closer to main central thrust) cooled more slowly than rocks to the north (closer to STDS). The boundary between these domains coincides with the discontinuity in age found in the same area by Rubatto et al. (Contrib Mineral Petrol 165:349–372, 2013). Combined with the information on petrologic phase relations, the data reveal the remarkable aspect that the rapid cooling and change of cooling rates all occurred after, rather than during, the rapid exhumation. This result underscores that high-temperature (e.g., >550 °C) cooling is a result of several processes in addition to exhumation and a one-to-one correlation of cooling and exhumation may sometimes be misplaced. Moreover, average cooling rates inferred from the closure temperatures of two isotopic systems should be interpreted judiciously in such nonlinearly cooling systems. While many aspects (e.g., isothermal decompression, isobaric cooling, duration of metamorphism, and cooling rates) of the pressure–temperature history inferred by us are consistent with the predictions of thermomechanical models that produce midcrustal channel flow, the occurrence of blocks with two different cooling histories within the HHC is not explained by currently available models. It is found that while exhumation may be initiated by surface processes such as erosion, the course of exhumation and its rate, at least below depths of ~15 km, is mostly controlled by a coupling between mechanical (density gain/loss) and chemical (metamorphic reactions) processes at depth.

Structurally-controlled melt accumulation: Himalayan migmatites and related deformation — Dhauli Ganga valley, Garhwal Himalaya

In the uppermost parts of the Higher Himalayan Crystallines (HHC) of the Great Himalaya, widespread in situ partial melting of sillimanite+K-feldspar gneiss resulted in the formation of migmatite and resultant melt accumulation near the South Tibetan Detachment System (STDS) during various deformation events along the Dhauli Ganga valley in Garhwal. The oldest migmatite phase, designated as the Me1, parallels the main foliation Sm as the stromatite layers and concordant leucogranite bands. Younger melt phases Me2, Me3 and Me5 are recorded along small-scale ductile thrusts, extensional fabric and structureless patches, respectively. It is only the Me4 melting phase that is evidenced by large-scale melt migration along cross-cutting irregular veins. These were possible conduits for migration and accumulation of melt into larger leucogranite bodies like the Malari granite (19.0± 0.5 Ma).

Upper Oligocene–Lower Miocene Gangrinboche Conglomerate in the Xigaze Area, Southern Tibet: Implications for Himalayan Uplift and Paleo-Yarlung-Zangbo Initiation

The Gangrinboche Conglomerate, exposed along the Yarlung-Zangbo suture zone, records a crucial stage of the Himalayan orogeny. The type section of these strata in the Xigaze area, southern Tibet, including the Qiuwu and Dazhuka Formations, was studied by integrated stratigraphic, sedimentologic, petrographic, and geochemical techniques. Palynological data and detrital zircon U-Pb ages indicate that the Qiuwu Formation was deposited during the latest Oligocene to the earliest Miocene (most probably ∼23 Ma), while the overlying Dazhuka Formation was deposited during 23–18 Ma. The Qiuwu Formation was deposited in a deltaic setting, and detritus was entirely derived from the Gangdese magmatic arc in the north. The Dazhuka Formation, in contrast, was deposited in mainly braided river environments and contains clasts derived from both the Gangdese arc in the north and the Himalayan orogen in the south. Clasts derived from the south first occur at the base of the Dazhuka Formation and increase in abundance upsection to become predominant at the top of the formation. This indicates active Early Miocene uplift and accelerated erosion of the Himalayan belt. Paleocurrent data from the Dazhuka Formation show westward axial sediment transport, which together with mixed provenance from both sides of the basin indicates that a paleo-Yarlung-Zangbo River running parallel to the suture zone initiated at the very start of the Miocene, although with flow direction opposite to that of the present. Flow reversal and establishment of the modern eastward-flowing course must have occurred later on in the Neogene, possibly initiating rapid uplift and focused erosion of the Namche-Barwa syntaxis. Basin subsidence at the close of the Paleogene and subsequent development of a major longitudinal paleo-Yarlung-Zangbo took place contemporaneously with initiation of the South Tibetan Detachment System and Main Central Thrust farther to the south, probably reflecting onset of the “hard collision” phase of the Himalayan orogeny.

Age and origin of granites in the Karakoram shear zone and Greater Himalaya Sequence, NW India

The crustal-scale Karakoram shear zone structurally distinguishes the western Himalaya from—and provides an opportunity to compare to— the central and eastern portions of the orogen. To evaluate the tectonic evolution of the western Himalaya, this paper presents granite U/Th-Pb ages and zircon Hf isotopic signatures along the two major structures in northern India: the Karakoram shear zone and the Zanskar shear zone, the westernmost limb of the South Tibetan detachment system. Leucogranites in Zanskar crystallized 27-20 Ma and exhibit Precambrian to Paleozoic inheritance and predominantly negative e Hf (t) values typical of the Greater Himalayan Sequence. Karakoram shear zone leucogran- ites have igneous crystallization ages over a prolonged period from 22 Ma to <13 Ma, contain Late Cretaceous through Paleocene inherited cores, and have e Hf (t) values from +1 to +9. These inherited ages and mostly positive e Hf (t) values compare closely to the adjacent Ladakh batholith, but low e Hf (t) values along the Karakoram shear zone suggest an input of older crustal material from the proximal Karakoram terrane or subducted Indian crust. The Zanskar Greater Himalayan Sequence contains two suites of Paleozoic granites: (1) Pan-African Cambrian- Ordovician granites at the cores of gneiss domes and (2) Mississippian-Permian granites related to magmatism associated with the Panjal Traps. Monazite ages record peak through retrograde metamorphic conditions from 27.3 ± 1.2 Ma to 17.2 ± 0.9 Ma concurrent with anatectic leucogranite crystallization. Cenozoic partial melting in the Greater Himalayan Sequence occurred contemporaneously across the Himalayan orogen, but lower degrees of partial melting and ubiquitous doming distinguish the westernmost Greater Himalayan Sequence in Zanskar.

Structural kinematics, metamorphic P–T profiles and zircon geochronology across the Greater Himalayan Crystalline Complex in south‐central Tibet: implication for a revised channel flow

AbstractA specific question about the Himalayas is whether the orogeny grew by distributed extrusion or discrete thrusting. To place firm constraints on tectonic models for the orogeny, kinematic, thermobarometric and geochronological investigations have been undertaken across the Greater Himalayan Crystalline Complex (GHC) in the Nyalam region, south‐central Tibet. The GHC in this section is divided into the lower, upper and uppermost GHC, with kinematically top‐to‐the‐south, alternating with top‐to‐the‐north shear senses. A new thrust named the Nyalam thrust is recognized between the lower and upper GHC, with a 3 kbar pressure reversion, top‐to‐the‐south thrust sense, and was active after the exhumation of the GHC. Peak temperature reached ∼749 °C in the cordierite zone, and decreased southwards to 633–667 °C in the kyanite and sillimanite‐muscovite zones, and northwards to greenschist facies at the top of the South Tibetan Detachment System (STDS). Pressure at peak temperature reached a maximum value in the kyanite zone of 9.0–12.6 kbar and decreased northwards to ∼4.1 kbar in the cordierite zone. Zircon U‐Pb ages of a sillimanite migmatite and an undeformed leucogranite dyke cutting the mylonitized rocks in the STDS reveal a long‐lived partial melting of the GHC, which initiated at 39.7–34 Ma and ceased at 14.1 Ma. Synthesizing the obtained and collected results, a revised channel flow model is proposed by considering the effect of heat advection and convection by melt and magma migration. Our new model suggests that distributed processes like channel flow dominated during the growth of the Himalayan orogen, while discrete thrusting occurred in a later period as a secondary process.

Was Himalayan normal faulting triggered by initiation of the Ramgarh–Munsiari thrust and development of the Lesser Himalayan duplex?

The Ramgarh–Munsiari thrust is a major orogen-scale fault that extends for more than 1,500 km along strike in the Himalayan fold-thrust belt. The fault can be traced along the Himalayan arc from Himachal Pradesh, India, in the west to eastern Bhutan. The fault is located within the Lesser Himalayan tectonostratigraphic zone, and it translated Paleoproterozoic Lesser Himalayan rocks more than 100 km toward the foreland. The Ramgarh–Munsiari thrust is always located in the proximal footwall of the Main Central thrust. Northern exposures (toward the hinterland) of the thrust sheet occur in the footwall of the Main Central thrust at the base of the high Himalaya, and southern exposures (toward the foreland) occur between the Main Boundary thrust and Greater Himalayan klippen. Although the metamorphic grade of rocks within the Ramgarh–Munsiari thrust sheet is not significantly different from that of Greater Himalayan rock in the hanging wall of the overlying Main Central thrust sheet, the tectonostratigraphic origin of the two different thrust sheets is markedly different. The Ramgarh–Munsiari thrust became active in early Miocene time and acted as the roof thrust for a duplex system within Lesser Himalayan rocks. The process of slip transfer from the Main Central thrust to the Ramgarh–Munsiari thrust in early Miocene time and subsequent development of the Lesser Himalayan duplex may have played a role in triggering normal faulting along the South Tibetan Detachment system.

Kinematics of the Dadeldhura klippe shear zones (W Nepal): implications for the foreland evolution of the Himalayan metamorphic core

AbstractMapping combined with structural analyses in the foreland edge of the metamorphic core of the Himalayas in SW Nepal highlights the existence of two north‐dipping shear zones with opposite sense of shear. Here, the metamorphic core is mainly affected by non‐coaxial top‐to‐the‐south sense of shear at temperatures between 450 °C and 550 °C that switch to a top‐to‐the‐north sense of shear at the top of the metamorphic core. We regionally correlate this upper shear zone with the South Tibetan detachment system. Ar‐dating on white mica indicates that both shear zones operated between 23 Ma and 17 Ma. Restoration of the folded South Tibetan detachment in far western Nepal yields a minimum dip‐slip distance of 190 km, compatible with predictions made by models of extrusion of a weak mid‐crustal channel. Our results support an orogenic model in which channel flow in the hinterland coexisted with thrust wedge mechanics in the foreland.

The South Tibetan detachment system facilitates ultra rapid cooling of granulite‐facies rocks in Sikkim Himalaya

ABSTRACTThe eastern Himalaya is characterized by a region of granulites and local granulitized eclogites that have been exhumed via isothermal decompression from lower crustal depths during the India‐Asia collision. Spatially, most of these regions are proximal to the South Tibetan detachment system, an orogen‐parallel normal‐sense detachment system that operated during the Miocene, suggesting that it played a role in their exhumation. Here we use geo‐ and thermochronological methods to study the deformation and cooling history of footwall rocks of the South Tibetan detachment system in northern Sikkim, India. These data demonstrate that the South Tibetan detachment system was active in Sikkim between 23.6 and ~13 Ma, and that footwall rocks cooled rapidly from ~700 to ~120 °C between ~15‐13 Ma. While active, the South Tibetan detachment system exhumed rocks from mid‐crustal depths, but an additional heat source such as strain heating, advected melt and/or crustal thinning is required to explain the observed isothermal decompression. Cessation of movement on the South Tibetan detachment system produced rapid cooling of the footwall as isotherms relaxed. A regional comparison of temperature‐time data for the eastern South Tibetan detachment system indicates a lack of synchronicity between the Sa'er‐Sikkim‐Yadong section and the NW Bhutan section. To accommodate this requires either strike‐slip tear faulting or local out‐of‐sequence thrusting in the younger segment of the orogen.

Soft sediment deformation structures and their implications for Late Quaternary seismicity on the South Tibetan Detachment System, Central Himalaya (Uttarakhand), India

The South Tibetan Detachment System (STDS) defines the lithological and tectonic boundary between the Higher Himalayan crystallines and the Tethyan sedimentaries. Earlier studies have suggested that the STDS has been dormant since its inception during the Miocene along with the Main Central Thrust (MCT). However, recent studies indicate that the STDS was active during the Pleistocene–Holocene period. We provide additional support for this more recent activity based on the occurrence of seismically induced Soft Sediment Deformation Structures (SSDS) preserved in relict lake sediments in the Dhauli Ganga, Gori Ganga and Kali Ganga river basins of the Central Himalaya.The relict lakes are located on the hanging wall of the STDS. An optical chronology of the lake sediments brackets the seismically induced SSDS between 20ka and 11ka with a major seismic event of magnitude >6.5 occurring between 17ka and 13.5ka. Since MCT and STDS are considered to be the coupled structures, our observation supports the hypothesis that the STDS is providing accommodation space to the strain gradient arising due to the north–south compression in the Himalaya during the late Quaternary.

Higher Himalaya in the Bhagirathi section (NW Himalaya, India): its structures, backthrusts and extrusion mechanism by both channel flow and critical taper mechanisms

The Higher Himalayan Crystalline (HHC) in the Bhagirathi river section (India) on fieldwork reveals two extensional ductile top-to-N/NE shear sub-zones—the ‘South Tibetan Detachment System’ and the ‘Basal Detachment’—besides a preceding top-to-S/SW ductile shear. A top-to-N/NE brittle shear was identified as backthrusts from the HHC (except its northern portion) that occur repeatedly adjacent to numerous top-to-S/SW brittle shears as fore-thrusts. The northern portion of the HHC—the Gangotri Granite—exhibits infrequent total six extensional and compressional brittle shear senses. The backthrusts could be due to a low friction between the lower boundary of the HHC (i.e. the Main Central Thrust-Zone) and the partially molten hot rock materials of the HHC. Subduction of the Eurasian plate towards S/SW below the Indian plate more extensively in the Garhwal sector could be the second possible reason. Presence of two ductile extensional shear sub-zones may indicate channel flow (or several exhumation mechanisms) of the HHC in a shifting mode (similar to Mukherjee et al. in Int J Earth Sci 101:253–272, 2012). The top-to-S/SW extensional brittle shear exclusively within the upper (northern portion) of the HHC and a top-to-S/SW brittle shear within the remainder of it is a possible indicator of critical taper deformation mechanism. Thus, this work provides the field evidences of possibly both channel flow and critical taper conditions from a Higher Himalayan section, besides that by Larson et al. (Geol Soc Am Bull 122:1116–1134, 2010).

Middle-Miocene transformation of tectonic regime in the Himalayan orogen

Understanding the multiple tectonic transformations during the Himalayan orogeny is significant in evaluating the evolution of Himalayan orogen. In the Gyirong area in south Tibet, deformed leucogranitic veins in the biotite-plagioclase gneisses of Greater Himalayan crystalline complex (GHC) constitute south-vergent asymmetric folds. The reconstruction of the veins shows that they experienced two generations of deformation under different tectonic regimes: an earlier top-to-north extension and a later top-to-south thrusting, implying a tectonic transformation from N-S extension to N-S shortening. Zircons LA-ICP-MS U-Pb dating of the leucogranite shows that it was emplaced during 21.03-18.7 Ma. The data suggest that the tectonic transformation occurred after 18.7 Ma. The chronological data of South Tibet detachment system (STDS) and North-South trending rift (NSTR) from Gyirong and other areas indicate that the Himalayan orogeny was in a period of tectonic transformation from N-S extension to N-S shortening during 19-13 Ma. The transformation of tectonic regime was probably controlled by the India-Asia convergence rate. An increase in the convergence rate resulted in N-S shortening of the orogen, thrusting and folding, with coeval formation of the NSTR in Tibet. A decrease in the convergence rate led to N-S extension and reactivation of the STDS.

Flattening the Bhutan Himalaya

A detailed thermobarometric transect of 35 samples across the Greater and Tethyan Himalayan sequences in central Bhutan demonstrates a tectonostratigraphically-intact section with uniform apparent thermal and baric field gradients of 20±2°C/km and 0.57±0.08kbar/km. Pressure–temperature paths determined from chemically-zoned garnets in 6 samples demonstrate that these P–T conditions correspond with maximum pressures. The super-lithostatic baric gradient cannot be explained by pre- to syn-metamorphic tectonic processes, or by extension within an inclined slab. Instead the data imply 50% post-peak-metamorphic flattening of the Himalayan metamorphic core, accommodated by distributed, top-to-the-north shear, consistent with microstructural analysis. Orogenic flattening best explains the development of the South Tibetan Detachment System as a strain incompatibility feature rather than a structure bounding the top of a tectonically-inserted wedge, and helps reconcile debate attributing first-order Himalayan structural features to either wedge failure (“critical taper”) or pipe-flow (“channel flow”).

Ductile deformation within Upper Himalaya Crystalline Sequence and geological implications, in Nyalam area, Southern Tibet

The South Tibet Detachment System (STDS) is a flat normal fault that separates the Upper Himalaya Crystalline Sequence (UHCS) below from the Tethyan Sedimentary Sequence (TSS) above. Timing of deformations related to the STDS is critical to understand the mechanism and evolution of the Himalaya collision zone. The Nyalam detachment (ND) (∼86°E) locates in the middle portion of STDS (81°–89°E). Dating of deformed leucocratic dykes that are most probably syntectonic at different depth beneath the ND, allow us to constrain the timing of deformation. (1) Dyke T11N37 located ∼3500 m structurally below the ND emplaced at 27.4±0.2 Ma; (2) Dyke T11N32 located ∼1400 m structurally below the ND emplaced at 22.0±0.3 Ma; (3) T11N25 located within the top to the north STD shear zone, ∼150 m structurally below the ND, emplaced at 17.1±0.2 Ma. Combining ND footwall cooling history and T11N25 deformation temperature, we indicate a probable onset of top to the north deformation at ∼16 Ma at this location. These results show an upward younging of the probable timing of onset of the deformation at different structural distance below the ND. We then propose a new model for deformation migration below the ND with deformation starting by pure shear deformation at depth prior to ∼27.5 Ma that migrates upward at a rate of ∼ 0.3 mm/a until ∼18 Ma when deformation switches to top to the north shearing in the South Tibet Detachment shear zone (STDsz). As deformation on the ND stops at 14-13 Ma this would imply that significant top to the North motion would be limited to less than 5 Ma and would jeopardize the importance of lower channel flow.

New insight into the South Tibetan detachment system: Not a single progressive deformation

Low‐angle normal faults (LANF), typically regarded as accommodating crustal or lithospheric extension, may also form during lithospheric shortening. The best‐studied system of syn‐contractional LANFs is the South Tibetan detachment system, a network of low‐angle normal sense faults and shear zones that formed coevally with and parallel to south‐vergent thrusts during lithospheric shortening accompanying development of the Himalayan orogen. In the eastern Himalaya, there are several across‐strike exposures of the South Tibetan detachment system. We present new structural and thermometry data from the eastern Himalaya that demonstrate that the South Tibetan detachment system cannot have formed as a single progressive structure. We characterize and distinguish two distinct structural and tectonic components within the currently recognized system: (1) an extensive diffuse, sheared layer that formed the boundary between strong upper crust and weak, southward‐flowing middle crust, and (2) a network of brittle‐ductile LANFs that locally exhume, partly excise and overprint the earlier mylonite zone at the topographic break between the Himalayan orogen and the Tibetan plateau. The sheared layer, not a LANF, formed the boundary between upper and middle crust during ductile flow of the middle crust and is extensively exposed in the Himalaya at the base of klippen of upper crustal rocks preserved in Bhutan, along the crest of the Himalaya where it has been excised and exhumed by the brittle‐ductile extrusion LANFs, and bounding the cores of the North Himalayan gneiss domes.