North Himalayan Gneiss Domes Research Articles

Magnetotelluric Imaging of the Central Himalayan Crust Away From Rift Zones

AbstractImaging the crustal structure and state of the Himalayan Orogenic Belt (HOB) is of great significance for understanding crustal deformation and its tectonic response in the collision front of the Indian‐Eurasian plates. To avoid the influence from the anomalies underneath the north‐south trending rifts, a three‐dimensional electrical resistivity model across the central HOB was obtained between the Dingjie‐Shenzha and Yadong‐Gulu rifts. No lower‐crustal conductor was found in the southern Lhasa Terrane, and the lower crust with a moderate resistivity value of approximately 100 Ωm is interpreted to be the residue of previous partial melting beneath the Gangdese porphyry copper deposits. The middle crust between the Main Himalayan Thrust and Southern Tibet Detachment System is electrically resistive to the north of the north Himalayan gneiss domes (NHGD) belt and conductive to the south. The middle crust in the northern Himalayas may have undergone partial melting and crystallization during southward migration and duplexing. There is a strong deep‐shallow coupling relationship in the central HOB, where an increase in the underthrusting angle of the Indian crust has resulted in a southward turn of the NHGD belt.

Late Cenozoic cooling and evolution history of the Kangmar dome in southern Tibet: Insights from inverse thermal modeling

The North Himalayan Gneiss Domes, which are essential parts of the Cenozoic extensional structures in Southern Tibet, record the thermal and tectonic processes that occurred after the India-Asian collision and are thought to be effective structures regulating post-collision intracontinental deformation. However, it is still unclear how these domes are formed and how they contribute to the regulation process. Here, we performed detailed geological mapping, elevation transect sampling, low-temperature thermochronological testing, and 3D modeling on the Kangmar dome, which is located west of the N‒S treading Yadong-Gulu rift, and its core-cover contact fault is suspected to be the northern continuation of the South Tibetan Detachment System (STDS). Our analysis revealed a discrepancy in the deformation histories of the dome’s northern and southern portions. We proposed a model in which the core-cover contact fault of the Kangmar dome was a part of the South Tibetan Detachment System and the doming event that occurred at ∼12.2 Ma was dominated by thrust stacking of the southward mid-crustal channel flow. The rapid cooling following the middle Miocene was possibly influenced by the N‒S Trending Yadong-Gulu rift activity. The present landscape was shaped by the incision of the Nianchu River, which was accompanied by increased glacial activity during the Pleistocene. Our findings enhance the intracontinental deformation patterns following collisions and shed light on the numerous domes in Himalayas and other orogenic belts.

Structural and kinematic analysis of the Cuonadong dome, southern Tibet, China: Implications for middle-crust deformation

The Cuonadong Dome is located at the eastern part of the North Himalayan gneiss domes. Structural and kinematic analysis in the middle unit of the Cuonadong Dome provide significant new insights into the structural deformation of the middle crust in the Northern Himalaya. Field observations, microscopic kinematic and EBSD analyses, and deformation temperature estimates from the mid-crustal rocks of the Cuonadong Dome show high-strain D2 (top-to-N) kinematic fabrics with a downward-to-north progression from dominantly top-to-north shear in the garnet zone, to solely top-to-north shear in the kyanite/sillimanite zone. A combination of mineral assemblages, microstructures, and quartz crystal preferred orientation patterns indicates deformation temperatures associated with D2 kinematic fabrics increase with structural depth from ∼450 °C in garnet zone to > 630 °C in the kyanite/sillimanite zone. These values broadly overlap temperature range deduced from metamorphic petrography (450–630 °C). Microtextural relations indicate that peak metamorphism occurred post-D1 and pre-D2 deformation. Combined with the ∼1000-m-thick ductile deformation zone, the interpreted deformation temperatures yielded an average thermal field gradient of ∼ 180 °C/km over 1.05 km of structural thickness. Based on the geochronologic data, midcrustal D2 extensional deformation in the Cuonadong Dome initiated as early as early Oligocene (32 Ma) and ended during the middle Miocene (∼14 Ma). Comparative studies of D2 ductile deformation in midcrustal rocks from other North Himalayan gneiss domes and the South Tibetan Detachment System indicate that D2 shear zone in the Cuonadong Dome represent the northern part of the South Tibetan Detachment System, and highlight temporal and spatial variation in midcrustal D2 deformation patterns over the southern Tibet during the Himalayan orogeny.

Metamorphic P–T Evolution and In Situ Biotite Rb–Sr Geochronology of Garnet–Staurolite Schist From the Ramba Gneiss Dome in the Northern Himalaya

The North Himalayan gneiss domes provide a window for looking into the deeper crust and record abundant clues of continent collisional orogenesis. This study carried out detailed petrology, in situ LA–ICP–MS biotite Rb–Sr dating, and phase equilibrium modeling on garnet–staurolite–two-mica schist in the Ramba gneiss dome in order to constrain metamorphic P–T evolution and the timing of metamorphism. A clock-wise P–T path, involving an early prograde process that evolves from ∼540°C at ∼4.4 kbar to ∼630°C at ∼6.0 kbar, was constructed for garnet–staurolite–two-mica schist in the Ramba gneiss dome. In situ LA–ICP–MS biotite Rb–Sr analysis yielded two metamorphic ages of 37.17 ± 5.66 and 5.27 ± 3.10 Ma, corresponding to the timing of retrograde cooling and the cooling age of the dome following the thermal resetting by the emplacement of ca. 8 Ma leucogranite pluton in the core of the dome, respectively. The peak metamorphism is inferred to be older than ca. 37 Ma. Based on these results and the data previously published, the garnet–staurolite–two-mica schist recorded the Eocene crustal thickening, following the India–Asia collision and later the exhumation process.

Tectonic transformation from orogen-perpendicular to orogen-parallel extension in the North Himalayan Gneiss Domes: Evidence from a structural, kinematic, and geochronological investigation of the Ramba gneiss dome

The North Himalayan Gneiss Domes consist of a series of isolated domes cored by leucogranite and/or gneiss body. The Ramba dome core comprises gneiss and two-mica granite, mantled mainly by schist sequences, and the degree of deformation decreases outward from pervasive mylonitization to weak mylonitization. Pre-doming deformation is characterized by the formation of regional-scale top-to-the-north shearing (D1) in the Tethyan Himalayan Sequence, locally preserved strong folding (F1), and intrusion of 46.3 Ma-40.6 Ma leucogranite. The D2 deformation consists of map-scale ductile shearing fabrics and top-to-the-north detachment along the South Tibetan Detachment System (STDS) in the middle and lower units of the dome. Intrusion of 26.1–21.0 Ma syn-kinematic leucogranite veins in the schist units occurred during the D2 north-directed extensional episode. The D3 deformation was characterized by top-to-the-east shear during 12.0–10.2 Ma, concurrent with the initiation of activity in the N–S-trending Yadong–Gulu rift. The latest deformation (D4) involved the ascent of two-mica leucogranite through diapirism at about 8.17 Ma. Microstructural analyses, as well as quartz c-axis fabrics measured by EBSD (Electron backscatter diffraction), indicate deformation temperature ranges of D2 extensional shearing from 500 to 600 °C in the middle unit of the Ramba dome to >600 °C in the lower unit. These new data imply that this type of doming in the Ramba region represents a classic type of transformation from orogen-perpendicular to orogen-parallel extensional deformation during Miocene India–Asia collision.

Fluid circulation in the South Tibetan Detachment System: Evidence from fluid inclusions and oxygen isotope data of quartz veins in the Ramba Dome, North Himalayan Gneiss Domes

Detachment faults are sites of intensive fluid–rock interactions. Here, we report fluid inclusion and oxygen isotope data for quartz veins in the Ramba Dome in the North Himalayan Gneiss Domes, with an aim to constrain the origin and circulation of crustal fluids associated with the South Tibetan Detachment System (STDS). Microthermometric data for fluid inclusions in quartz indicate that the fluids were aqueous and CO2 − H2O ± CH4 ± N2‐bearing with low to moderate salinities (0.60–11.80 wt% eq. NaCl). The entrapment conditions are 295–410°C and 98–135 Mpa, indicating a forming‐depth of 8–10 km. Oxygen isotopic compositions (δ18O) of quartz measured in situ by secondary ion mass spectrometry and bulk by the BrF5 method show limited variations in individual quartz veins, but δ18Oquartz values vary from 12.07 to 18.16‰ (V‐SMOW) among veins. The corresponding δ18Ofluid values range from 7.71 to 13.80‰, based on equilibrium temperatures obtained from fluid inclusions. From the footwall to the detachment zone, δ18Ofluid values exhibit a broadly decreasing trend and indicate that the STDS dominated the fluid flux pathway in the crust, with more contributions of meteoric water in the detachment zone. We further quantified the contribution of meteoric fluids to 8–27% using a binary end‐member mixing model. These data imply that the fluids were predominantly metamorphic/magmatic in origin, and were mixed with infiltrating, isotopically light, meteoric water during extensional detachment shearing of the STDS. The meteoric water can infiltrate from the surface to 8–10 km depth.

Structural analysis of sheath folds and geochronology in the Cuonadong Dome, southern Tibet, China: New constraints on the timing of the South Tibetan detachment system and its relationship to North Himalayan Gneiss Domes

AbstractStructural analysis and new zircon U/Pb geochronology were performed in the Cuonadong Dome in order to test the hypothesis that the top‐to‐North deformation in the North Himalayan Gneiss Domes (NHGD) has been correlated with deformation along the South Tibetan detachment system (STDS). Structural analysis suggests that the X‐axes of sheath folds at the different flanks of the dome are consistent, and show average a trend and plunge of 350° and 18°. Many shear sense indicators are recorded in the ductile shear zone during the top‐to‐north deformation, including the augen structure, σ‐type and δ‐type rotating porphyroclast, S–C fabric and pressure shadow structure. The development of the sheath folds first provide a direct‐field structural evidence that represents a deeper ductile manifestation of the STDS, and further suggests that the top‐to‐north deformation in the NHGD have been correlated with deformation along the STDS. The emplacement of the synkinematic granites at ca. 32 Ma exposed at the core of the sheath folds is also interpreted as the initiation of slip along the STDS.

Gneiss Dome Formation in the Himalaya and southern Tibet

Abstract Gneiss domes in the Himalaya and southern Tibet record processes of crustal thickening, metamorphism, melting, deformation and exhumation during the convergence between the Indian and Eurasian plates. We review two types of gneiss domes: North Himalayan gneiss domes (NHGD) and later domes formed by orogen-parallel extension. Located in the southern Tibetan Plateau, the NHGD are cored by granite and gneiss, and mantled by the Tethyan sedimentary sequence. The footwall of these were extruded southwards from beneath the Tibetan Plateau and subsequently warped into a domal shape. The second class of domes were formed during displacement on normal-sense shear zones and detachments that accommodated orogen-parallel extension during the Late Miocene. In some cases, formation of these domes involved an early stage of southwards-directed extrusion prior to doming. We review evidence for orogen-parallel extension to provide context for the formation of these gneiss domes. Compilations of pressure–temperature–time–deformation data and temperature–time paths indicate differences between dome types, and we accordingly propose new terminology. Type 1 domes are characterized by doming as an artefact of post-high-temperature exhumation processes in the Middle Miocene. Type 2 domes formed in response to exhumation during orogen-parallel extension in the Late Miocene that potentially post-dates south-directed extrusion.

Post-collisional exhumation of the Indus-Yarlung suture zone and Northern Tethyan Himalaya, Saga, SW Tibet

The Cretaceous-Eocene sedimentary sequences of the Sangdanlin section at Saga, SW Tibet, within the India-Asia collisional zone, have been studied in detail for constraining the timing of initial collision. However, the post-depositional history has not been investigated. Here, we report new low temperature thermochronology data along the classic Sangdanlin section, to reveal the cooling/exhumation history of the Indus-Yarlung suture zone (IYSZ) and northern Tethyan Himalaya. Samples from the IYSZ, all yield latest Oligocene-Miocene ages, with most zircon (U-Th)/He ages ranging between ~18 and 27 Ma and apatite fission track (AFT) ages of ~12–9 Ma. Thermal history modelling indicates a consistent Middle-Late Miocene cooling episode related to post-Great Counter Thrust (GCT) incision of the Yarlung River. To the south, samples from the northern Tethyan Himalaya Mesozoic sequences yield older ZHe ages of ~32–36 Ma, signifying activity of the Tethyan Himalayan fold-thrust zone, while two granites linked to formation of the North Himalayan gneiss domes yield Late Miocene AFT ages. The regional low temperature thermochronology data reveal different cooling patterns for tributaries on the northern and southern sides of the Yarlung River. This points to the strong influence of Early Miocene tectonism along the GCT, as a major controlling factor on river drainage reorganization at this time.

Structural setting of the Yalaxiangbo dome, SE Tibet (China)

The Yalaxiangbo gneiss dome (SE Tibet) in eastern Himalaya is one of the major metamorphic culminations in the Tethyan Himalayan Sequence, referred to as North Himalayan Gneiss Domes (NHGD). It comprises three main tectonic units separated by an upper ductile/brittle and a lower ductile detachment. The upper tectonic unit, above the upper ductile/brittle detachment, includes unmetamorphosed to low-grade metamorphic Triassic-Lower Cretaceous slate andmetapsammite of the Tethyan Himalayan Sequence. The middle tectonic unit, sandwiched between the upper and lower detachments, consists of mylonitic granite, staurolite-garnet-two mica schist and biotite-plagioclase gneiss affected by the ductile top-to-the north extensional shear of the lower detachment. The lower tectonic unit consists of mylonitic gneiss, leucogranite plutons, dikes and sills.By integrating meso-/micro-structural analyses, petrographic and Laser Ablation Inductively Coupled Plasma-Mass Spectrometer (LA-ICP-MS) in situ U-(Th)-Pb monazite geochronological data from selected samples affected by ductile shear, we constrain the activity of the lower detachment at ~ 18 Ma and the shearing along the upper detachment later than ~ 15 Ma. The kinematic, geochronology and petrologic features of the Yalaxiangbo detachments are similar to the South Tibetan Detachment system, which crops out in southern Tibet.

Early Miocene rapid exhumation in southern Tibet: Insights from P–T–t–D–magmatism path of Yardoi dome

Reconstructing the evolution of Gneiss domes within orogenic belts poses challenges because domes can form in a variety of geodynamic settings and by multiple doming mechanisms. For the North Himalayan gneiss domes (NHGD), it is debated whether they formed during shortening, extension or collapse of the plateau, and what is the spatial and temporal relationship of magmatism, metamorphism and deformation. This study investigates the Yardoi dome in southern Tibet using field mapping, petrography, phase equilibria modelling and new monazite ages. The resulting P–T–time–deformation–magmatism path for the first time reveals the spatial and temporal relationship of metamorphism, deformation and magmatism in the Yardoi dome: a) the dome mantle recorded prograde loading to kyanite-grade Barrovian metamorphic conditions of 650 ± 30 °C and 9 ± 1 kbar (M2) in the Early Miocene (18–17 Ma); b) the main top-to-the-north deformation fabric (D2) formed syn- to post-peak-metamorphism; c) the emplacement of leucorgranites related to doming is syn-metamorphism at 19–17 Ma. The link between the detachment shear zone in the Yardoi dome and the South Tibetan detachment system (STDS) is confirmed. By comparing with orogen-scale tectonic processes in the Himalaya, we suggest that north-south extension in a convergent geodynamic setting during Early Miocene accounts for formation of the Yardoi dome. In a wider tectonic context, the Early Miocene rapid exhumation of deep crustal rocks was contemporaneous with the rapid uplift of southern Tibet and the Himalayan orogen.

Synchronous granite intrusion and E–W extension in the Cuonadong dome, southern Tibet, China: evidence from field observations and thermochronologic results

The Tethys Himalaya sedimentary sequence (THS) is characterized by N–S trending extensional rifts (NSTR) and the North Himalayan gneiss domes (NHGD). The Cuonadong dome, as one of the NHGD, is located at the N–S trending Cuona rift in the eastern part of the Tethys Himalaya. The dome had undergone four major deformation events: D1, characterized by top-to-S thrust resulting in N–S shortening and vertical thickening; D2, characterized by vertical thinning and N–S extensional deformation; D3, approximately E–W extensional deformation associated with intrusion of leucogranites; and D4, late collapse structural deformation around the core of the Cuonadong dome. There are numerous granitic dykes at the contact zone between the granite and surrounding rocks in the middle unit of the Cuonadong dome. Field observations show that granite intrusion occurred in part synchronously with D3, E–W extensional deformation. U–Pb–zircon dating shows that the granite crystallization took place at ca. 17.5–15.6 Ma, while muscovite Ar–Ar geochronological data from the deformed pegmatite and mylonitic schist in the Cuonadong dome show that the onset of the E–W extensional deformation is between 16.5 and 14.8 Ma, and suggesting that granite intrusion was synchronous with the E–W ductile extensional deformation in the Cuonadong dome, southern Tibet. Combined with previously published ages of the South Tibetan Detachment System (STDS), our study shows that granite intrusion was at least locally contemporaneous with both the E–W and N–S extensional deformation, especially with the E–W extension.

First field identification of the Cuonadong dome in southern Tibet: implications for EW extension of the North Himalayan gneiss dome

The Cuonadong dome exposes in east-southern margin of the North Himalayan gneiss domes (NHGD), which is reported first time in this study. The Cuonadong dome is located at the southern part of the Zhaxikang ore concentration area, which is divided into three tectono-lithostratigraphic units by two curved faults around the dome geometry from upper to lower (or from outer to inner): the upper unit, middle unit and lower unit, and the outer fault is Nading fault, while the inner fault is Jisong fault. The Cuonadong dome is a magmatic orthogneiss and leucogranite mantled by orthogneiss and metasedimentary rocks, which in turn are overlain by Jurassic metasedimentary and sedimentary rocks. The grades of metamorphism and structural deformation increase towards the core, which is correspondence with the Ridang Formation low-metamorphic schist, tourmaline granitic–biotite gneiss, garnet–mica gneiss and mylonitic quartz–mica gneiss. The Cuonadong dome preserves evidences for four major deformational events: firstly top-to-S thrust (D1), early approximately N–S extensional deformation (D2), main approximately E–W extensional deformation (D3), and late collapse structural deformation (D4) around the core of the Cuonadong dome, which are consistent to three groups lineation: approximately N–S-trending lineation including L1 and L2, E–W trending L3, and L4 with plunging towards outside of the dome, respectively. The formation of the Cuonadong dome was probably resulted from the main E–W extensional deformation which is a result of eastward flow of middle or lower crust from beneath Tibet accommodated by northward oblique underthrusting of Indian crust beneath Tibet. The establishment of the Cuonadong dome enhanced the E–W extension of the NHGD, which is further divided into two structural dome zones according to the different extensional directions: approximately N–S extensional North Himalayan gneiss domes (NS-NHGD) and E–W extensional North Himalayan gneiss domes (EW-NHGD). The NS-NHGD developed by a dominantly N–S contraction and locally extensional regime and keep a close relationship to the South Tibetan Detachment System, whereas the EW-NHGD formed by an E–W extensional deformation along the north–south-trending rifts.

Tectonic evolution of the middle crust in southern Tibet from structural and kinematic studies in the Lhagoi Kangri gneiss dome

Field, structural, kinematic, and deformation temperature analyses were conducted on rocks from the Lhagoi Kangri gneiss dome (southern Tibet) in order to establish the geologic history of the dome, identify major phases of deformation within the dome, and to relate these phases of deformation to the tectonic evolution of the Himalayan middle crust. The Lhagoi Kangri dome, one of a series of gneiss-cored domes in southern Tibet, records stratigraphy and structural features similar to previously studied north Himalayan gneiss domes. Field mapping reveals a sequence of rocks that comprise a cover of unmetamorphosed to amphibolite-grade siliciclastic and minor carbonate rocks overlying a core predominantly composed of foliated and lineated orthogneiss intruded by relatively undeformed granite, which also intrudes the cover rocks both concordantly and discordantly. Field observations and microstructural analyses suggest that the contact between the core and cover rocks was originally a nonconformity, but we do not rule out the possibility of subsequent slip along the surface, as has been reported for correlative structures in other domes. Lhagoi Kangri rocks were pervasively deformed during at least two major tectonic phases. The earliest deformation event (D1) resulted in shortening and thickening of crust, the record of which is largely eliminated, particularly in lower structural levels, by transposition and recrystallization during the second phase of deformation (D2). Ductile deformation during D2 is characterized at higher structural levels by crenulation cleavage that tightens with depth, while at lower structural levels D2 manifests as a distributed shear zone that records some evidence of both plane strain and coaxial flattening, possibly indicating overall heterogeneous general shear. The shear zone is ∼3 km thick and contains rocks with mostly symmetrical top-to-north and top-to-south shear sense indicators with a dominant top-to-north component at lower structural levels. Microstructural analyses and quartz c-axis fabrics indicate a range of D2 shear zone temperatures from 200 to 300 °C at the upper boundary to ≥630 °C at the lowest structural levels sampled with minimal evidence of lower temperature overprinting. The interpreted temperatures define a wide range in thermal field gradients (18–90 °C/km) that suggest that temperature indicators were locked in at relatively late stages of D2. The structural framework and kinematic history of the Lhagoi Kangri dome are similar to previously studied north Himalayan gneiss domes as well as to transects through the South Tibetan detachment system, which supports previous interpretations of structural continuity between the north Himalayan gneiss domes and other middle crustal exposures in the Himalaya. The Lhagoi Kangri distributed shear zone, in particular, may represent a deeper ductile manifestation of the South Tibetan detachment system.

Highly fractionated Late Eocene (~ 35 Ma) leucogranite in the Xiaru Dome, Tethyan Himalaya, South Tibet

The Xiaru dome is located in the middle section of the North Himalayan Gneiss Domes belt in southern Tibet. The leucogranite, which crops out in the core of the Xiaru dome, is a typical medium-grained garnet+tourmaline+muscovite leucogranite. U–(Th)–Pb dating of zircon and monazite from the leucogranite yielded ages of approximately 35Ma. This finding supports a growing body of evidence indicating that an extensive magmatic event occurred during the late Eocene in the Himalayas. This leucogranite is strongly peraluminous with A/CNK values of 1.08–1.52 and characterized by evolved geochemical composition with high contents of SiO2 and alkali elements; low levels of CaO, MgO, TiO2, and FeOT; enriched large-ion lithophile elements (such as Rb); and depleted of high-field-strength elements (such as Nb, Zr, and Hf). The non-CHARAC (CHarge-And-Radius-Controlled) trace element behaviors, which are typical of a highly fractionated granite system, were recorded in the whole rock and the accessory minerals of the Xiaru leucogranite. Furthermore, the magmatic zircon overgrowths have extremely high content of Hf, consistent with those from the highly fractionated aqueous-like fluid system. In addition, whole-rock geochemical fractionation trends were observed, which can be explained by crystal fractionation of biotite, K-feldspar, zircon, xenotime, and monazite. These geochemical features indicate that the Xiaru leucogranite is a typical highly fractionated granite. The geochronological and geochemical features of the inherited zircons from the Xiaru leucogranite show a close affinity to those of the country rocks, suggesting a certain degree of assimilation from the country rocks during melt ascent and emplacement. Although a restricted range of εHf(t) values from −12.8 to −6.6 with Hf TDM2 model ages of 1.2–1.6Ga was obtained from the late Eocene zircons, it is invalid to constrain the source of the parental magma due to the strong fractionation and assimilation processes.

Andean-type orogeny in the Himalayas of south Tibet: Implications for early Paleozoic tectonics along the Indian margin of Gondwana

The granitic gneisses in the cores of north Himalayan gneiss domes (NHGD) and at the crest of the high Himalayas form part of the greater Himalayan crystalline complex (GHC). These rocks are characterized by the assemblage of quartz, plagioclase, K-feldspar, biotite, muscovite and minor garnet. Here we present results from LA-ICPMS zircon U–Pb dating of six granitic gneiss samples, which reveal formation ages of ca. 500–473Ma and suggest a long-lived magmatism during early Paleozoic in the GHC. Geochemical data show that these rocks are characterized by high SiO2 (70.93–74.59wt.%), K2O (4.22–5.91wt.%), A/CNK values (>1.1) and low Na2O/K2O (0.42–0.83), and enrichment in Rb, Th, U, depletion in Ba, Nb, Ta, Sr, Zr, and strong negative Eu anomalies. These features suggest that the protoliths of the gneisses are high potassium calc-alkaline and peraluminous S-type granites derived from partial melting of crustal materials. The high initial 87Sr/86Sr (>0.706) and negative εNd(t) (−6.2 to −10.6) of the granitic gneisses compare with those of the metasedimentary rocks in the GHC, indicating that the granites were generated from partial melting of the sedimentary protoliths. Based on the geochronological, whole-rock geochemical and Sr–Nd isotopic data presented in this study, we suggest that the granites formed in a back-arc setting with a continental arc-affinity related to the subduction of the Proto-Tethyan Oceanic lithosphere. The mafic magmatic underplating, triggered by the subduction, rollback and the break off of Proto-Tethyan Ocean slab, resulted in partial melting of the crust in the GHC. Integrating results from previous studies, we propose an Andean-type orogeny along the margin of the Gondwana.

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.

Eocene high grade metamorphism and crustal anatexis in the North Himalaya Gneiss Domes, Southern Tibet

Determination of the timing and geochemical nature of early metamorphic and anatectic events in the Himalayan orogen may provide key insights into the physical and chemical behavior of lower crustal materials during the early stage of tectonic evolution in large-scale collisional belts. The Yardoi gneiss dome is the easternmost dome of the North Himalayan Gneiss Domes (NHGD), and contains three types of amphibolites with distinct mineral assemblage, elemental and radiogenic isotope geochemistry, as well as various types of gneisses. SHRIMP zircon U/Pb analyses on the garnet amphibolite and garnet-bearing biotite granitic gneiss yield ages of nearly peak metamorphism at 45.0±1.0 Ma and 47.±1.8 Ma, respectively, which are 2 to 4 Ma older than the age for partial melting in migmatitic garnet amphibolite (43.5±1.3 Ma). Available data have demonstrated that ultra-high pressure metamorphism in the Tethyan Himalaya occurred at ∼55 Ma, and high amphibolite facies to granulite facies metamorphism at 45 to 47 Ma. In addition, partial melting at thickened crustal conditions occurred at 43.5±1.3 Ma, which led to the formation of high Sr/Y ratios two-mica granites. The high-grade metamorphic rocks in the NHGD may represent the subducted front of the Indian continental lithosphere. In large collisional belts, fertile components in crustal materials could melt and form granitic melts with relatively high Na/K and Sr/Y ratios under thickened crustal conditions, significantly different from those formed by decompressional melting during rapid exhumation.

Tectonic implications of felsic tuffs within the Lower Miocene Gangrinboche conglomerates, southern Tibet

Felsic tuffs are interbedded with the Gangrinboche conglomerates adjacent to the Yarlung Tsangpo suture zone in southern Tibet. Laser Ar/Ar dating of mineral separates indicates they are of Early Miocene age. Such tuffs are most likely an eruptive manifestation of geochemically indistinguishable coeval felsic (adakitic) intrusions that are widely reported across southern Tibet. The considerable lateral (E–W) extent of the Gangrinboche conglomerates and their depositional setting indicates sediment accumulation in an overall N–S compressional regime and thereby places important constraints on the tectonic setting in which magmatism initiated. The conglomerates were folded and truncated during back-thrusting associated with development of the north-directed Great Counter thrust. As N–S trending rifts associated with E–W extension of the Tibetan Plateau cut both the conglomerates and this thrust system it can be inferred that post-collisional volcanism is unlikely to have been genetically linked to later E–W extension. Early Miocene slab break-off beneath Tibet provides a model that appears to be consistent with petrogenesis of the associated magmatic suite, which requires a lowermost crust or lithospheric mantle generation, molasse accumulation, and uplift and emplacement of North Himalayan gneiss domes.

Structures, kinematics, thermochronology and tectonic evolution of the Ramba gneiss dome in the northern Himalaya

The Ramba gneiss dome, one of the north Himalayan gneiss domes, is composed of three tectono-lithologic units separated by an upper and a lower detachment fault. Low-grade metamorphic Tethyan Himalayan sedimentary sequence formed the upper unit above the brittle upper detachment fault. Mylonitic gneiss and a leucogranite pluton made up the lower unit beneath the ductile lower detachment fault. Mylonitic middle-grade garnet-, staurolite- and andalusite-schist constituted the middle unit between the two faults, which may be that the basal part of the upper unit experienced detachment shear. The Ramba dome underwent three episodes of deformation in its tectonic evolution. The first episode was a top-down-to-north-northwest sliding possibly related to the activity of the south Tibetan detachment system (STDS). The second episode was the dominant deformation related to a east–west extension, which resulted in a unique top-down-to-east kinematics and the major tectonic features of the dome. The third episode was a collapse sliding toward the outsides of the dome. The Ramba gneiss dome is possibly a result of the east–west extension and magmatic diapir. The lower detachment fault is probably the main detachment fault separating the sedimentary sequence from the crystalline basement during the east–west extension in the dominant deformation episode. The diapir of the leucogranite pluton formed the doming shape of the Ramba gneiss dome. This pluton intruded in the core of the dome in a late stage of the dominant deformation, and its Ar–Ar cooling ages are about 6 Myr. This indicates that the dominant deformation of the dome happened at the same time of the east–west extension represented by the north–south trending rifts throughout the northern Himalaya and southern Tibet. Therefore, the formation of the Ramba gneiss dome should be related to this east–west extension.