South Tibetan Detachment Research Articles

The Malari leucogranite, Garhwal Himalaya, northern India: Chemistry, age, and tectonic implications

Research Article| November 01, 2010 The Malari leucogranite, Garhwal Himalaya, northern India: Chemistry, age, and tectonic implications Himanshu K. Sachan; Himanshu K. Sachan 1Wadia Institute of Himalayan Geology, 33 Gen Mahadev Singh Road, Dehra Dun 248001, India Search for other works by this author on: GSW Google Scholar Matthew J. Kohn; Matthew J. Kohn † 2Department of Geosciences, Boise State University, Boise, Idaho 83725, USA †E-mail: mattkohn@boisestate.edu Search for other works by this author on: GSW Google Scholar Anubhooti Saxena; Anubhooti Saxena 1Wadia Institute of Himalayan Geology, 33 Gen Mahadev Singh Road, Dehra Dun 248001, India Search for other works by this author on: GSW Google Scholar Stacey L. Corrie Stacey L. Corrie 2Department of Geosciences, Boise State University, Boise, Idaho 83725, USA Search for other works by this author on: GSW Google Scholar GSA Bulletin (2010) 122 (11-12): 1865–1876. https://doi.org/10.1130/B30153.1 Article history received: 12 Aug 2009 rev-recd: 10 Nov 2009 accepted: 27 Nov 2009 first online: 08 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Himanshu K. Sachan, Matthew J. Kohn, Anubhooti Saxena, Stacey L. Corrie; The Malari leucogranite, Garhwal Himalaya, northern India: Chemistry, age, and tectonic implications. GSA Bulletin 2010;; 122 (11-12): 1865–1876. doi: https://doi.org/10.1130/B30153.1 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 In the Garhwal region, India, the Malari leucogranite cuts the South Tibetan detachment system, a large-scale normal fault system at the top of the High Himalaya. The leucogranite crosscuts ductile normal-sense shear fabrics and has experienced relatively little subsolidus brittle deformation or alteration. Its relatively evolved bulk chemical composition, high Rb/Sr ratio, and normative corundum indicate a (meta)sedimentary source, likely the underlying Greater Himalayan sequence. Zircon U-Pb ages, collected by laser-ablation inductively coupled plasma–mass spectrometry (ICP-MS) and corrected for initial U/Th disequilibrium, indicate emplacement at 19.0 ± 0.5 Ma. Thus, ductile normal shear on the South Tibetan detachment system must have ceased by 19 Ma. Studies elsewhere in the Himalaya suggest initiation of South Tibetan detachment system ductile movement not earlier than 24 Ma, and likely ca. 22 Ma. The short duration of extension (≤5 and likely ∼3 m.y.) and early cessation contrast with channel-flow models that predict long-duration ductile normal shear, and large displacements after ca. 20 Ma. Observations are instead better explained by critical taper models, in which internal weakening of the wedge, likely from partial melting, caused a brief interval of flattening and ductile extension in the rear of the wedge. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Low-angle normal faults in the compressional Himalayan orogen; Evidence from the Annapurna–Dhaulagiri Himalaya, Nepal

The Annapurna Detachment (AD) is a low-angle (∼20°–30° dip), north-dipping normal fault and ductile high-strain shear zone in calc-mylonites, and forms part of the South Tibetan Detachment (STD) that runs along 1800 km length of the Himalaya. The AD separates kyanite and sillimanite grade gneisses and tremolite + clinopyroxene ± hornblende-bearing marble–calc-silicates of the Greater Himalayan Sequence (GHS) below from unmetamorphosed Palaeozoic–Mesozoic sedimentary rocks of the Tethyan sedimentary zone above. It was active at ca. 22–18 Ma during south-vergent ductile extrusion (channel flow) and exhumation of the Himalayan mid-crust footwall. Restoration of the STD system suggests around 80–100 km of southward extrusion of the footwall gneisses relative to the Tethyan hanging-wall rocks. Folds in the hanging wall of the AD were formed prior to normal faulting, but axial planes are curved into alignment with the shear zone suggesting extrusion of the metamorphic footwall rocks beneath a passive roof fault. North-vergent recumbent backfolds in the Nilgiri–Tukuche peaks were enhanced by backsliding during footwall extrusion, although this does not indicate “orogenic collapse,” lowering of surface elevation, or decreasing crustal thickness because new material was continually being underthrust from the south. Axial planes of backfolds are curved and progressively rotate from subvertical in the north to subhorizontal immediately above the AD. Low-angle normal faults in the Himalaya were active during the Early Miocene, concomitantly with thrusting at deeper structural levels along the Main Central Thrust (MCT) zone. The passive normal faults and ductile shear zone were initiated at low angles aided by partial melting and ductile flow within the GHS. They do not indicate alternating periods of extension and compression but were active in a wholly compressional environment. The Channel Flow model for the Greater Himalaya and the passive roof fault model for the low-angle normal faults adequately explain all geological field structural and metamorphic criteria.

Late Oligocene high-temperature shear zones in the core of the Higher Himalayan Crystallines (Lower Dolpo, western Nepal)

[1] A high-temperature shear zone, Toijem shear zone, with a top-to-the-SW sense of shear affects the core of the Higher Himalayan Crystallines (HHC) in western Nepal. The shear zone developed during the decompression, in the sillimanite stability field, of rocks that previously underwent relatively high-pressure metamorphism deformed under the kyanite stability field. PT conditions indicate that the footwall experienced higher pressure (∼9 kbar) than the hanging wall (∼7 kbar) and similar temperatures (675°–700°C). Monazite growth constrains the initial activity of the shear zone at 25.8 ± 0.3 Ma, before the onset of the Main Central Thrust zone, whereas the late intrusion of a crosscutting granitic dike at 17 ± 0.2 Ma limits its final activity. Monazites in kyanite-bearing gneisses from the footwall record prograde metamorphism in the HHC from ∼43 to 33 Ma. The new data confirm that exhumation of the HHC started earlier in western Nepal than in other portions of the belt and before the activity of both the South Tibetan Detachment System (STDS) and Main Central Thrust (MCT) zones. As a consequence, western Nepal represents a key area where the channel-flow-driven mechanism of exhumation, supposed to be active from Bhutan to central-eastern Nepal, does terminate. In this area, exhumation of crystalline units occurred by foreland propagation of ductile and, subsequently, brittle deformation.

Out-of-sequence deformation and expansion of the Himalayan orogenic wedge: insight from the Changgo culmination, south central Tibet

[1] The Changgo culmination, one of the North Himalayan domes in south central Tibet, consists of a multiphase granite core surrounded by a deformed metasedimentary carapace. The granitic core records general non-coaxial shear with a top-to-the-south sense shear component. The contact between the core and the carapace is a shear zone, characterized by general non-coaxial shear with a top-to-the-north shear sense, interpreted to be the northern continuation of the South Tibetan detachment system (STDS). The shear zone contains lenses of leucogranite dated at 35.4 Ma. This is interpreted to reflect Eocene crustal thickening, coeval with the earliest shortening event recorded in the carapace. The main phase of the Changgo granite crystallized at 23.5 Ma, while undeformed aplite dikes, the youngest phase observed in the granite, were intruded at 22.1 Ma. Aplite dikes crosscut the main deformation fabric within the Changgo granite; therefore, that deformation and associated south directed shearing must have ended between 23.5 Ma and 22.1 Ma. The dikes are strained within the STDS, indicating that final displacement along the STDS must post-date 22 Ma, yet be older than 18.4 Ma, the cooling age of muscovite in the shear zone. It is proposed that the exhumation of the Changgo culmination is related to tectonically driven erosion in response to crustal thickening and rebuilding of the orogenic critical taper wedge. Subsequently, deformation in the wedge migrated toward the foreland, expanding the orogenic wedge laterally and moving the locus of displacement from the Main Central thrust structurally downward to the Main Boundary thrust.

Metamorphic history of a syn‐convergent orogen‐parallel detachment: The South Tibetan detachment system, Bhutan Himalaya

AbstractThe South Tibetan detachment system (STDS) in the Himalayan orogen is an example of normal‐sense displacement on an orogen‐parallel shear zone during lithospheric contraction. Here, in situ monazite U(–Th)–Pb geochronology is combined with metamorphic pressure and temperature estimates to constrain pressure–temperature–time (P–T–t) paths for both the hangingwall and footwall rocks of a Miocene ductile component of the STDS (outer STDS) now exposed in the eastern Himalaya. The outer STDS is located south of a younger, ductile/brittle component of the STDS (inner STDS), and is characterized by structurally upward decreasing metamorphic grade corresponding to a transition from sillimanite‐bearing Greater Himalayan sequence rocks in the footwall with garnet that preserves diffusive chemical zoning to staurolite‐bearing Chekha Group rocks in the hangingwall, with garnet that records prograde chemical zoning. Monazite ages indicate that prograde garnet growth in the footwall occurred prior to partial melting at 22.6 ± 0.4 Ma, and that peak temperatures were reached following c. 20.5 Ma. In contrast, peak temperatures were reached in the Chekha Group hangingwall by c. 22 Ma. Normal‐sense (top‐to‐the‐north) shearing in both the hangingwall and footwall followed peak metamorphism from c. 23 Ma until at least c. 16 Ma. Retrograde P–T–t paths are compatible with modelled P–T–t paths for an outer STDS analogue that is isolated from the inner STDS by intervening extrusion of a dome of mid‐crustal material.

Current understanding of the seismotectonics of Western Nepal Himalaya and vicinity

The study of seismic activity at some stage in 1963 to 2006 in the Western Nepal Himalaya and its adjoining regions (28–31°N and 79–82.3°E), reveal that seismicity is non-uniform in space and time. The analyses of fault-plane solutions of twenty-four earthquakes inferred that the Western part of Nepal Himalayan frontal arc is in compressed state in which seismic activity is dominated by thrust faulting. Based on orientation of P-axes, compressive stress directed north-south to northeast-southwest approximately perpendicular to the prevailing stress along the major trend of the Himalaya. Thrust faulting coupled with shallow dip of nodal planes reflects that the Indian continental lithosphere is under-thrusting at a shallow angle. This information suggests crustal shortening in north-south direction in which earthquakes are generated due to northward compression. In the adjoining Tibet parts earthquake activity is due to normal faulting with east-west extension. These might be due to the presence of a relatively strong Main Himalayan Thrust, the plate boundary fault below the Himalayas, would have favored the occurrence of thrusting. While, a weak Main Himalayan Thrust below Tibet along with initiation of the Main Central Thrust can explain South Tibetan Detachment (geodynamic process) and associated stress field in Western Nepal Himalaya and its adjoining regions.

Exhumation history of the deepest central Himalayan rocks, Ama Drime range: Key pressure-temperature-deformation-time constraints on orogenic models

[1] The Ama Drime range located at the transition between the high Himalayan range and south Tibet is a N–S active horst that offsets the South Tibetan Detachment System (STDS). Within the horst, a paragneissic unit, possibly attributed to the upper Himalayan crystalline series, overly the lower Himalayan crystalline series Ama Drime orthogneissic unit containing large metabasite layers and pods that have experienced pressure ≥1.4 GPa. Combining structural analysis with new and published pressure-temperature (P-T) estimates as well as U-Th/Pb, 39Ar/40Ar and (U-Th)/He ages, the P-T-deformation-time (P-T-D-t) paths of the main units within and on both sides of the horst are reconstructed. They imply that N–S normal faults initiated prior to 11 Ma and have accounted for a total exhumation ≤0.6 GPa (22 km) that probably occurred in two phases: the first one until ∼9 Ma and the second one since 6 to 4 Ma at a rate of ∼1 mm/yr. In the Ama Drime unit, 1 to 1.3 GPa (37 to 48 km) of exhumation occurred after partial melting since ∼30 Ma until ∼13 Ma, above the Main Central Trust (MCT) and below the STDS when these two fault systems were active together. The switch from E–W (STDS) to N-S (Ama Drime horst) normal faulting between 13 and 12 Ma occurs at the time of propagation of thrusting from the MCT to the Main Boundary Thrust. These data are in favor of a wedge extrusion or thrust system rather than a crustal flow model for the building of the Himalaya. We propose that the kinematics of south Tibet Cenozoic extension phases is fundamentally driven by the direction and rate of India underthrusting.

Crustal structure of the Tethyan Himalaya, southern Tibet: new constraints from old wide-angle seismic data

SUMMARY A wide-angle seismic profile between Peigu Tso (85.5°E) and Pumoyong Tso (90.5°E) in southern Tibet, acquired under a joint Sino-French program in 1981, passes through the South Tibetan Detachment System (STDS, the eastern segment of the profile) and the central part of the Tethyan Himalaya (the western segment). We herein reinterpret this wide-angle seismic profile, which has a total length of ∼480 km, in order to improve our understanding of the crustal structure of the Tethyan Himalaya. We identify multiple P- and S-wave wide-angle reflections within the crystalline crust and from the Moho, but cannot identify any refracting waves beneath the Moho. We model the structure of P- and S-wave velocity throughout the whole crust, while acknowledging the significant uncertainties that are inherent in many places. A low-wave speed cover sequence may be identified with the Tethyan sedimentary cover (Vp < 5.8 km s−1), from the surface to a depth of (typically) 5–10 km, which is only absent where the profile crosses the Kangmar Dome. It may be presumed that these sedimentary rocks overlay a felsic upper-crustal basement (5.8 < Vp < 6.3 km s−1), which in places extends as far down as 35 km, and which itself overlays a 5–10-km-thick mid-crustal low-velocity zone. Between a depth of 30 and 45 km at the base of the low-velocity zone and the Moho at a depth of ∼73 km, the observed velocities gradually increase from 6.2 to 6.8 km s−1. The low-velocity layer (LVL) may consist of a partial melt due to the high geothermal gradients found in the felsic rocks, or of a fluid-rich shear zone as in the ductile channel of a channel flow model. The difference in depth of the LVL between the western (central Tethyan Himalaya) and eastern segments (nearly along the STDS) at the boundary of the Dingye-Mabjia fault, leads to a north–south dip estimate of the LVL of 8.5°, which is consistent with the estimated dip of the MHT obtained from the INDEPTH-I project, and can support the channel flow model. This north–south low dip angle of subduction slab, supports that the subducting Indian lithosphere endured a transition from steep subduction to low-angle subduction, and the subduction slab flattening may be attributed to the break-off of Indian lithosphere slab beneath the Indur-Zurpo suture.

The South Tibet detachment shear zone in the Dinggye area: Time constraints on extrusion models of the Himalayas

We investigate the timing of end of motion along the South Tibet Detachment System (STDS), a major normal fault system that runs parallel to the Himalayan range for more than 1500 km. Near Dinggye (∼ 28°10′N, 87°40′E), the STD dips ∼ 10 ± 5° to the North and separates Paleozoic Tethyan series from Upper Himalayan Crystalline Series (UHCS). Immediately below the STD, the UHCS is highly deformed in the STD shear zone, lineations trend NNE and the shear senses are top to the NE. In micaschist, the P–T path constrained by pseudosection and garnet chemistry, shows successive metamorphic conditions of ∼ 0.6 GPa and ∼ 550 °C and 0.5 GPa and 625 °C. U/Pb dating of monazites and zircons in deformed and undeformed leucogranites suggests that ductile deformation lasted until at least ∼ 16 Ma but ended prior to ∼ 15 Ma in the STD shear zone ∼ 100 m below the detachment. Ar/Ar micas ages in the footwall span between ∼ 14.6 and 13.6 Ma, indicating rapid cooling down to ∼ 320 °C, and suggesting persistence of normal faulting, at that time. The STDS is cut and offset by the N–S trending Dinggye active normal fault which initiated prior to 11 Ma thus providing a minimum bound for the end of STDS motion. These data are interpreted as reflecting 0.3 GPa (11 km) to 0.6 GPa (22 km) of exhumation along the STDS starting prior to ∼ 16 Ma, ending between 13.6 and 11 Ma. The 1000 km long stretch of the STDS east of the Gurla Mandata probably stopped almost synchronously between 13 and 11 Ma ago, coevally with a sudden switch from NNE–SSW to E–W extension at the top of the accretionary prism, with a jump of the major thrust from the lower Main Central Thrust (MCTl) to the Main Boundary Thrust (MBT), and with a change in the India and Asia convergence direction. This synchronism is probably better explained in the frame of a thrust wedge or thrust system model than a lower channel flow model. West of the Gurla Mandata the STDS appears to stop 5 to 3 Ma earlier, possibly related to local interactions with the Karakorum fault in a way that needs to be understood.

Placing limits on channel flow: Insights from the Bhutan Himalaya

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 3 km 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 ∼ 20 km. Thin-section scale top-to-the-south shear is focused in a 2–3 km-thick zone above the MCT while the overlying ∼ 11 km-thick section displays top-to-the-north shear, suggesting asymmetric channel behavior. Strained quartz grains allow us to quantify ∼ 23–34 km 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.

Crustal shortening in convergent orogens: Insights from global positioning system (GPS) measurements in northeast India

Deformation in active mountain belts like the Himalaya is manifested over several spatial and temporal scales and collation of information across these scales is crucial to an integrated understanding of the overall deformation process in mountain belts. Computation and integration of geological shortening rates from retrodeformable balanced cross-sections and present-day convergent rates from deforming mountain belts is one way of integrating information across time-scales. The results from GPS measurements carried out in NE India indicate that about 15–20 mm/yr of convergence is being accommodated there. Balanced-cross sections from the NE Himalaya indicate about 350–500 km of shortening south of the South Tibet Detachment (STD). Geothermobarometry suggest that the rocks south of the STD deformed under peak metamorphic conditions at ∼ 22 Ma. This indicates a geological convergence rate of ∼ 16–22 mm/yr which appears to be fairly consistent with the GPS derived convergence rates. Approximately 1.5 to 3.5 mm/yr (∼ 10–20 %) of the total N-S of the present-day convergence in the NE Himalaya is accommodated in the Shillong Plateau. In addition, ∼ 8–9 mm/yr of E-W convergence is observed in the eastern and central parts of the Shillong Plateau relative to the Indo-Burman fold-thrust belt. Balanced cross-sections in the Indo-Burman wedge together with higher resolution GPS measurements are required in the future to build on the first-order results presented here.

An alternative tectonic model for the Yarlung Zangbo suture zone

The new field investigations along the Yarlung Zangbo ophiolites zone show that these series underwent low green-schist metamorphism and were then fractured and occurred as slabs in tectonic melanges, without regional tectonic polarity. No large shear zone in north-south direction has been identified between ophiolite bodies and flysch layers on both side and a conformable contact relationship can be observed locally between them. A great mass of tectonic melange has been substantiated as submarine olistolith bodies. The Mesozoic sedimentary facies and its evolution in both north and south of the ophiolite zone are corresponding in time. The ophiolite zone has often been divided into parallel branches, separated by narrow flysch slats. There is also a similarity of the Paleozoic and the basement of the High Himalaya, Lhasa and Qiangtang Terranes, and they are distinctly different from those of the Indian continent. The geologic information does not warrant a postulate that the Himalaya and Tibet were once separated by a great ocean; it is therefore consistent with an alternative tectonic model by back-arc basin collapse with its juvenile narrow oceanic crust. The real plate tectonic suture, the Neotethys might be covered under the Miocene Siwalik molasse in the southern slope of the High Himalaya range. Based on the new model, the Neotethyan ocean floor was subducted beneath the Asia since the Late Triassic. The outer continental margin of Eurasia was split from the Lhasa Terrane so that a back-arc basin came into existence. Hemi-pelagic and deep sea sediments were deposited before the Late Cretaceous flysch sedimentation, with the linear juvenile oceanic crust when back-arc volcanism occurred in the Gandese region. The Yarlung Zangbo back-arc basin was eventually eliminated when the High Himalayas were sutured onto Eurasia. The ocean floor lightly underthrusted to north and south sides, sediments of the basin were deformed as fold-thrusting. The Neotethys was eliminated during the Tertiary when India collided with Himalaya arc from the Oligocene. The crust of southern Tibet suffered penetrative shortening, the ophiolite zone became exposed during regional uplifting, and the South Tibet Detachment System, doming in southern Tibet and rifting in a north-south direction since the Middle Miocene Epoch.

Miocene structural reorganization of the South Tibetan detachment, eastern Himalaya: Implications for continental collision

In the eastern Himalaya (Bhutan), there are two distinct top-down-to-the-north segments of the South Tibetan detachment system. The outer segment is a diffuse ductile shear zone preserved as klippen in broad open synforms. New age constraints show that it was active until at least ca. 15.5 Ma and cooled by ca. 11.0 Ma, as constrained by sensitive high-resolution ion microprobe (SHRIMP) U-Pb geochronology of magmatic zircon and 40 Ar/ 39 Ar thermochronology of muscovite in ductilely deformed leucogranite sills. The inner segment is a ductile shear zone active at least until ca. 11.0 Ma (constrained by SHRIMP U-Pb geochronology of magmatic zircon) and overprinted by more recent brittle faulting. These age constraints indicate that ductile deformation continued on the South Tibetan detachment more recently in the eastern Himalaya than in central and western parts of the orogen. These improved constraints on timing of South Tibetan detachment segments allow for a more detailed reconstruction of continental collision in the eastern Himalaya in which the outer South Tibetan detachment segment was abandoned in the mid-Miocene and passively transported southward in the hanging wall of the Main Himalayan thrust (the basal detachment of the orogen), while top-to-the-north ductile to brittle shearing continued on the inner South Tibetan detachment segment. Hinterland stepping of the South Tibetan detachment to maintain an orogenic critical taper (frictional wedge model) is a possible mechanism for this tectonic reorganization of the South Tibetan detachment during the Miocene. However, our data combined with published geochronologic data for the eastern Himalaya demonstrate that foreland translation and exhumation of a midcrustal dome (viscous wedge model) is the more tenable mechanism.

Structure and deformation around the Gyirong basin, north Himalaya, and onset of the south Tibetan detachment system

Gyirong basin and its adjacent area are located at a special position in the Himalayan orogen, where the south Tibetan detachment system (STDS) and N-S trending rift converged. The north Himalayan orogen here can be divided into five petrologic-tectonic units successively from south to north: 1) the Greater Himalayan crystalline complex (GHC); 2) the STDS shear zone; 3) the Tethyan Himalayan sedimentary sequence (THS); 4) the late Cenozoic sedimentary basins, such as Gyirong and Oma basins; and 5) the Malashan gneiss dome. Structural studies show that this area experienced four stages of deformation: 1) the earlier south-directed thrusting, preserved both in the GHC and THS; 2) top-down-to-north slip along the STDS, normal faults related to this slip formed the early controlling structures of the Cenozoic basins, and the tilted pattern of the blocks between the basins indicated a north-directed slip; 3) east-west extension, the resultant N-S trending normal fault formed the eastern boundary of the basins; and 4) late gravitational collapse. Zircon SHRIMP U-Pb dating on the syn-deformational (leuco-) granite along the STDS indicates that the major activity of the STDS occurred at ca. 26 Ma, but its onset may have begun as early as ca. 36 Ma.

Higher Himalayan Shear Zone, Sutlej section: structural geology and extrusion mechanism by various combinations of simple shear, pure shear and channel flow in shifting modes

The Higher Himalayan Shear Zone (HHSZ) in the Sutlej section reveals (1) top-to-SW ductile shearing, (2) top-to-NE ductile shearing in the upper- and the lower strands of the South Tibetan Detachment System (STDSU, STDSL), and (3) top-to-SW brittle shearing corroborated by trapezoid-shaped minerals in micro-scale. In the proposed extrusion model of the HHSZ, the E1-phase during 25–19 Ma is marked by simple shearing of the upper sub-channel defined by the upper strand of the Main Central Thrust (MCTU) and the top of STDSU as the lower- and the upper boundaries, respectively. Subsequently, the E2a-pulse during 15–14 Ma was characterized by simple shear, pure shear, and channel flow of the entire HHSZ. Finally, the E2b-pulse during 14–12 Ma observed simple shearing and channel flow of the lower sub-channel defined by the lower strand of the Main Central Thrust (MCTL) and the top of the STDSL as the lower- and the upper boundaries, respectively. The model explains the constraints of thicknesses of the STDSU and the STDSL along with spatially variable extrusion rate and the inverted metamorphism of the HHSZ. The model predicts (1) shear strain after ductile extrusion to be maximum at the boundaries of the HHSZ, which crudely matches with the existing data. The other speculations that cannot be checked are (2) uniform shear strain from the MCTU to the top of the HHSZ in the E1-phase; (3) fastest rates of extrusion of the lower boundaries of the STDSU and the STDSL during the E2a- and E2b-pulses, respectively; and (4) variable thickness of the STDSL and rare absence of the STDSU. Non-parabolic shear fabrics of the HHSZ possibly indicate heterogeneous strain. The top-to-SW brittle shearing around 12 Ma augmented the ductile extruded rocks to arrive a shallower depth. The brittle–ductile extension leading to boudinage possibly did not enhance the extrusion.

Metallogenesis of the Tibetan collisional orogen: A review and introduction to the special issue

Mineral deposits associated with continental collision are abundant in many orogenic systems. However, the metallogenesis of collisional orogens is often poorly understood, due to the lack of systematic studies on the genetic links between collisional processes and ore formation in collisional orogenic belts. This paper reviews the key metallogenic settings and resultant collision-related ore deposits in the Tibetan Orogen, created by Indo-Asian collision starting in the early Cenozoic. The resulting synthesis leads us to propose a new conceptual framework for Tibetan metallogenic systems, which may aid in deciphering relationships among ore types in other comparable collisional belts. This framework includes three principal metallogenic epochs in the Tibetan orogen, and metallogenesis in: (1) a main-collisional convergent setting (∼ 65–41 Ma); (2) a late-collisional transform structural setting (∼ 40–26 Ma); and (3) a post-collisional crustal extension setting (∼ 25–0 Ma), each forming more than three distinct types of ore deposits in the Tibetan orogen. The main-collisional metaollognesis took place in a convergent setting, i.e., a collisional zone, characterized by collision-related crustal shortening and thickening, associated syn-peak metamorphism and two distinct magmatic series (Paleocene–Eocene crust-derived low- fO 2 granitoids generated by crustal anatexis and Eocene high- fO 2 granitoids formed by MASH processes at the base of the Tibetan crust). Metallogenesis during this period formed Sn–W–rare metal deposits related to the low- fO 2 granitoids, skarn-hosted Cu–Au polymetallic deposits related to high- fO 2 granitoids, and orogenic-type Au deposits formed by CO 2-dominant metamorphic fluids. Late-collisional metallogenesis occurred mainly in a transform structural setting dominated by Cenozoic strike-slip faulting, shearing, thrust systems, and associated potassic magmatism in eastern Tibet, and formed the most economically-significant metallogenic province in the orogen. Four significant ore-forming systems are recognized in the transform zone: porphyry Cu–Mo–Au systems associated with potassic adakitic melts and controlled by Cenozoic strike-slip faults; orogenic-type Au systems related to large-scale left-slip ductile shearing; REE-bearing systems associated with lithospheric mantle-derived carbonatite–alkalic complexes; and Zn–Pb–Cu–Ag systems related to basinal brines and controlled by Cenozoic thrust structures and subsequent strike-slip faults developed in the Tertiary foreland basin. Post-collisional metallogenesis occurred in a crustal extension setting, characterized by lithospheric mantle thinning or delamination at depth, crustal shortening at a lower structural level and synchronal extension at shallower levels. The resulting ore-forming systems include: (1) porphyry Cu–Mo ore systems related to high-K adakitic stocks derived from the newly-formed thickened mafic lower-crust; (2) vein-type Sb–Au ore systems controlled by the south Tibetan detachment system (STDs) and the metamorphic core complex or thermal dome intruded by lecuogranite intrusions; (3) hydrothermal Pb–Zn–Ag ore systems controlled by the intersections of N–S-striking normal faults with E–W-trending thrust faults; and (4) spring-type Cs–Au ore systems related to geothermal activity driven by partial melting of the upper crust. Associated ore deposits lie mostly within the mid-Miocene Gangdese tectono-magmatic belt, in which the scavenging role of fluids derived from evolved magma systems or dewatering of rift basins, and finally discharging at intersections of the orogen-transverse and -parallel faults are extremely important for formation of the low-temperature hydrothermal deposits. Based on the synthesis of deposits in the Tibetan orogen and comparison with the metallogenesis of other orogenic systems, a more complete classification for these collision-related deposits can be proposed.

Post-collisional Sb and Au mineralization related to the South Tibetan detachment system, Himalayan orogen

More than 50 Sb–Au deposits and occurrences have recently been found in an E–W-striking structural–thermal dome zone, related to the South Tibetan detachment system (STDS) in the Himalayan orogen. They are mainly distributed around the thermal domes intruded by mid-Miocene leucogranite bodies, and show a metallic zoning varying from Au, Sb–Au, to Sb mineralization from the domes outwards. At least three mineralization styles are recognized, i.e., Sb-, Sb–Au-, and Au-styles of deposits, all hosted in the Mesozoic, Tethyan passive continental-margin sequences. The Shalagang-style Sb deposits, controlled by N–S-striking normal faults and E–W-striking bedding faults, are usually composed of quartz–stibnite veins with open-space filling structure. Low homogenization temperatures (135–367 °C) and low salinities (0.5–12.6 wt.% NaCl equiv.), as well as highly variable calculated δ 18O H2O values (− 11.5–12.3‰) and very low δD values (− 140 to − 166‰), suggest an ore-forming fluid dominated by geothermal or meteoric water. The Mazhala-style Sb–Au deposits were controlled by small-scale bedding faults and their intersections with N–S-striking normal faults. They consist of gold-bearing quartz–stibnite veins with minor disseminated ores, and are associated with silicification, sericitization, chloritization and carbonatization. CO 2-rich fluid inclusions in Au-bearing gangue quartz, calculated δ 18O H2O values (5.4–11.5‰) and measured δD values (− 72 to − 119‰) for precipitation of gangue quartz suggest a mixed ore-forming fluid containing both meteoric and magmatic waters. Langkazi-style Au deposits, controlled by detachment faults and normal faults along the margin of the thermal dome, consist of Au-bearing quartz veins and disseminated ores with intensive silicification, chloritization and sericitization. Gangue quartz from Au-bearing quartz veins yielded a limited range of δ 18O H2O values (1.8–8.2‰) and δD values (− 52 to − 83‰), suggesting an ore-forming fluid dominated by magmatic water. The genetic links among these deposits, the granite-intruded thermal domes and ore-controlled fault systems suggest a mid-Miocene metallogenic epoch. The spatial zonation in metallic associations, and the variation in compositions and sources of ore-forming fluids indicate that these Sb and Au deposits resulted from a post-collisional dome-centered geothermal system, driven by mid-Miocene granite magmas related to the STDS. Rb–Sr and S isotopic data indicate that the scavenging of the convective geothermal fluids through Mesozoic permeable strata play an extremely significant role in concentrating metallic Sb and minor Au, and forming Sb, Sb–Au, and Au deposits in southern Tibet.

Crustal melt granites and migmatites along the Himalaya: melt source, segregation, transport and granite emplacement mechanisms

ABSTRACTIndia–Asia collision resulted in crustal thickening and shortening, metamorphism and partial melting along the 2200 km-long Himalayan range. In the core of the Greater Himalaya, widespread in situ partial melting in sillimanite+K-feldspar gneisses resulted in formation of migmatites and Ms+Bt+Grt+Tur±Crd±Sil leucogranites, mainly by muscovite dehydration melting. Melting occurred at shallow depths (4–6 kbar; 15–20 km depth) in the middle crust, but not in the lower crust. 87Sr/86Sr ratios of leucogranites are very high (0·74–0·79) and heterogeneous, indicating a 100 crustal protolith. Melts were sourced from fertile muscovite-bearing pelites and quartzo-feldspathic gneisses of the Neo-Proterozoic Haimanta–Cheka Formations. Melting was induced through a combination of thermal relaxation due to crustal thickening and from high internal heat production rates within the Proterozoic source rocks in the middle crust. Himalayan granites have highly radiogenic Pb isotopes and extremely high uranium concentrations. Little or no heat was derived either from the mantle or from shear heating along thrust faults. Mid-crustal melting triggered southward ductile extrusion (channel flow) of a mid-crustal layer bounded by a crustal-scale thrust fault and shear zone (Main Central Thrust; MCT) along the base, and a low-angle ductile shear zone and normal fault (South Tibetan Detachment; STD) along the top. Multi-system thermochronology (U–Pb, Sm–Nd, 40Ar–39Ar and fission track dating) show that partial melting spanned ̃24–15 Ma and triggered mid-crustal flow between the simultaneously active shear zones of the MCT and STD. Granite melting was restricted in both time (Early Miocene) and space (middle crust) along the entire length of the Himalaya. Melts were channelled up via hydraulic fracturing into sheeted sill complexes from the underthrust Indian plate source beneath southern Tibet, and intruded for up to 100 km parallel to the foliation in the host sillimanite gneisses. Crystallisation of the leucogranites was immediately followed by rapid exhumation, cooling and enhanced erosion during the Early–Middle Miocene.

Pre-Miocene deformation of the Himalayan superstructure, Hidden valley, central Nepal

Abstract: Throughout the Himalaya, the Tethyan sedimentary sequence forms the detached carapace, or superstructure, to exhumed mid-crustal rocks. In central Nepal, low metamorphic-grade Cambrian to Jurassic rocks of this carapace preserve five phases of deformation, structurally dominated by second phase (D 2 ) north-verging back folds that oppose the vergence of the orogen. The folds are overprinted and cross-cut by the c . 22 Ma South Tibetan detachment system and the Manaslu leucogranite. New structural mapping in Hidden valley provides further constraints on the character of D 2 : megascopically, large asymmetric north-verging folds define D 2 ; microscopically, it is characterized by an axial-planar cleavage. Balanced cross-sections and bed-length restoration of F 2 folds indicate a minimum of 32–38% shortening and 180% thickening during D 2 . These data indicate that north-verging folds played a significant role in pre-Miocene crustal thickening of the Himalayan superstructure. The formation of these folds is compatible with wedge extrusion or channel flow of the underlying mid-crustal rocks, whereby their geometry reflects early coupling between the upper and middle crust.

Stratigraphic correlation of Cambrian–Ordovician deposits along the Himalaya: Implications for the age and nature of rocks in the Mount Everest region

Research Article| March 01, 2009 Stratigraphic correlation of Cambrian–Ordovician deposits along the Himalaya: Implications for the age and nature of rocks in the Mount Everest region Paul M. Myrow; Paul M. Myrow † 1Department of Geology, Colorado College, Colorado Springs, Colorado 80903, USA †E-mail: pmyrow@coloradocollege.edu. Search for other works by this author on: GSW Google Scholar Nigel C. Hughes; Nigel C. Hughes 2Department of Earth Sciences, University of California, Riverside, California 92521, USA Search for other works by this author on: GSW Google Scholar Michael P. Searle; Michael P. Searle 3Department of Earth Sciences, Oxford University, Parks Road, Oxford, OX1 3PR, UK Search for other works by this author on: GSW Google Scholar C.M. Fanning; C.M. Fanning 4Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200, Australia Search for other works by this author on: GSW Google Scholar S.-C. Peng; S.-C. Peng 5State Key Laboratory on Paleobiology and Stratigraphy, Nanjing Institute of Geology and Paleontology, Nanjing, 210008, China Search for other works by this author on: GSW Google Scholar S.K. Parcha S.K. Parcha 6Wadia Institute of Himalayan Geology, Dehra Dun, Uttranchal, 248001, India Search for other works by this author on: GSW Google Scholar GSA Bulletin (2009) 121 (3-4): 323–332. https://doi.org/10.1130/B26384.1 Article history received: 04 Dec 2007 rev-recd: 06 Jul 2008 accepted: 09 Jul 2008 first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Paul M. Myrow, Nigel C. Hughes, Michael P. Searle, C.M. Fanning, S.-C. Peng, S.K. Parcha; Stratigraphic correlation of Cambrian–Ordovician deposits along the Himalaya: Implications for the age and nature of rocks in the Mount Everest region. GSA Bulletin 2009;; 121 (3-4): 323–332. doi: https://doi.org/10.1130/B26384.1 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 depositional age and stratigraphic correlations of metamorphosed and variably deformed rocks of Mount Everest are poorly known because of limited recovery of diagnostic fossils. Detailed study of Cambrian and Ordovician strata from along the length of the Himalaya has produced a coherent stratigraphy that stretches from northern India to Tibet. Our work also demonstrates that the North Col Formation rocks (= Everest series), between the Qomolangma and Lhotse detachments of the South Tibetan detachment system, still locally preserve sedimentary textures and primary stratigraphy that match those within Cambrian strata ~1100 km to the west in northern India. This demonstrates a coherency of depositional systems and stratigraphic architecture for Cambrian deposits along much of the Himalaya Tethyan margin. It also allows, for the first time, identification of precise depositional ages of several units in the Everest region, in particular, the Yellow Band carbonate and directly underlying siliciclastic strata, which are both shown to be late Middle Cambrian in age. Detrital zircon data presented herein for a sample from these siliciclastic strata contain a similar age spectrum to those from Middle Cambrian strata in northern India, as well as grains as young as ca. 526 Ma, both of which support the depositional age and continuity of depositional systems along the length of the Himalaya. Highly fractured rocks of the Ordovician lower Chiatsun Group in the hanging wall of the South Tibetan detachment system in Nyalam, 75 km to the west of Everest, correlate with Ordovician strata of the Mount Qomolangma Formation on Mount Everest. Our correlations indicate that the base of the summit pyramid of Everest, the foot of the “Third Step,” is composed of a 60-m-thick, white-weathering thrombolite bed. The top of this ancient microbial deposit crops out only 70 m below the summit of Mount Everest. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.