Southern Tibetan Detachment System 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.

S-wave velocity characteristics and mineralization of the southern tibet detachment system-yalaxiangbo dome

A series of extensional structures, including the southern Tibet detachment system (STDS), the north-south trending rifts (NSTR), and the northern Himalayan gneiss dome (NHGD), developed from the collision and compression between the Indian and Eurasian plates. These tectonic movements were accompanied by magmatism and polymetallic mineralization. Cuona Rift (CR) is located on the STDS next to the Yalaxiangbo Dome (YD) and passes through the Zhegucuo-longzi fault (ZLF), the Lhozhag fault (LZF), the Rongbu-Gudui fault (RGF), the Cuonadong dome (CD), and the YD. The study area contains numerous metal deposits, such as rare metal ore, lead zinc ore, gold deposits, and two geothermal fields, i.e., the Cuona geothermal field (CGF) and the Gudui geothermal field (GGF). Current research on the geological structures from the STDS to the YD is mainly based on magnetotelluric and natural seismic imaging. These surveys have a low resolution, making it impossible to image the shallow crust in detail. This study obtained about a 112 km S-wave velocity profile from the STDS to the YD using the multichannel surface wave imaging method. The profile results indicated that the average thickness of the sedimentary layer from the STDS to the YD is 400–500 m, while it is more than 800 m at certain fault zones. The CD is connected to the high-velocity body below the Zhaxikang ore concentration area (ZOCA) and may have the same provenance. The thermal conductivity reveals that the CGF, the GGF, and the ZOCA have high values and a more intense thermal radiation capacity. This drives the migration and circulation of the thermal fluids in the CGF and the GGF, causing them to continuously transmit heat to the shallow surface along the fault system. The migration of the thermal fluids extracts useful elements from the geological bodies through which it flows. When these elements mix with the atmospheric infiltration water, it precipitates to form the Zhaxikang hydrothermal superimposed transformation type lead-zinc polymetallic deposit.

Genesis of the Abunabu antimony deposits in the Tethys Himalayan metallogenic belt: Evidence from He–Ar and S isotopes of stibnite

Introduction: The Abunabu antimony mining area is located between the Indus–Yarlung Tsangpo suture and the southern Tibetan detachment system. Ore deposits in the mining area provide an excellent opportunity to understand the nature and genesis of antimony mineralisation in the Tethys Himalayan metallogenic belt.Methods: In this study, we analysed the He–Ar and S isotopic compositions of stibnite-hosted fluid inclusions as a basis for investigating the sources of ore-forming fluids in the Abunabu mining area and the Tethys Himalayan metallogenic belt.Results: The analysed stibnites have 4He contents of 0.016 × 10−7–1.584 × 10–7 cm3 STP/g, 40Ar contents of 1.37 × 10−7–2.94 × 10–7 cm3 STP/g, 40Ar/36Ar ratios of 303.8–320.7, and 3He/4He (Ra) ratios of 0.021–0.351. These isotopic features indicate that the ore-forming fluids were primarily metamorphic fluids of crustal origin, with small amounts of magmatic-derived materials and modified air-saturated water with low 40Ar*/4He ratios. The δ34S values of stibnite vary within a narrow range of −4.9‰ to −3.5‰, with a mean value of −4.31‰, indicating a deep magmatic origin.Discussion: On the basis of these results and a compilation of data for sulphide deposits in the metallogenic belt, we infer that compositional variations in the He and Ar isotopes of the ore-forming fluids of each antimony deposit in the Tethys Himalayan metallogenic belt are independent of each other. This suggests that antimony deposits in the belt had similar ore-forming fluid sources and mixing processes and that differences in the metallogenic tectonic setting within the belt emerged only in the later stages of deposit evolution. Our new results and compiled data also show that antimony–gold deposits and lead–zinc–antimony polymetallic deposits in the Tethys Himalayan metallogenic belt differ in their sulphur isotopic compositions and that multiple sulphur sources were involved in each of these types of deposit.

Surface wave imaging using deep reflection seismic data: a study on the Cuonadong dome

As interference waves in deep reflection data processing, surface waves are often suppressed as noise, but surface waves carry considerable underground media information, including structural information and the physical properties of rocks. Reasonable extraction and use of surface wave signals are of great significance when studying shallow characteristics. Deep reflection data are collected using large offsets, trail spacing, and explosive sources. The surface wave energy tends to be stronger, and the high-frequency surface wave signal is abundant. After extraction and inversion, the shallow shear wave velocity structure can be obtained. Near the Cuonadong dome in the southern Tibetan detachment system (STDS), a large number of leucogranites are developed in the core, containing important rare metal minerals and high metallogenic potential. However, studies regarding the shallow structure in this region are rare. In this paper, we use deep reflection data from a profile through the Cuonadong dome to obtain the S-wave velocity structure of the study area by extracting the surface wave fundamental-mode dispersion curve and inversion. Combined with regional geological and magnetotelluric data, we supposed that the thickness of the Cuonadong dome sediment layer (< 1.4 km/s) varies greatly from east to west, the thickness of the sediment layer is the deepest near the Cuona fault and Jisong fault (more than 1 km), and the core of the dome is the thinnest. Under the Cuonadong dome, there are obvious high-velocity anomalies (> 2.2 km/s), and the horizontal S-wave velocity changes greatly, which is mainly related to the destruction of magmatic activity since the Miocene. These understandings of the structure and velocity field of the Cuonadong dome can provide a powerful geophysical basis for establishing the dome structure model and searching for hidden ore bodies.Graphical abstract

Tectonic‒Thermal Coupling Metallogenic Models of Tethys Himalaya Pb‒Zn‒Sb‒Au Belt in Post-Collisional Stage

Crustal-scale extensions occurred in the Tibetan Plateau during the post-collision stage, and leucogranites, N–S and E–W faults and other tectonic-thermal events were developed in Tethys Himalaya, which formed a series of Pb–Zn–Sb–Au polymetallic deposits. The ore deposit may be distributed around the dome (with core of leucogranites), or along the N–S and E–W faults. Due to the lack of deep geophysical data, many different genesises of mineral deposit have been proposed by predecessors. This paper establishes the spatial relationship of deep tectonic-thermal events in the Tethys Himalaya Pb–Zn–Sb–Au belt by the N–S magnetotelluric (MT) profiles covering Cuonadong dome, the Southern Tibet Detachment System (STDS) and other tectonic-thermal events (length: 72 km, the basic point distance: 1 km): (a) a partial melting body was observed about 15 km below the Tethys Himalayan, which intruded in the form of leucogranites and formed domes; (b) the STDS and its secondary faults extended to deep the partial melting body. In combination of time relationship of tectonic-thermal events, a view has been presents that the Tethys Himalaya Pb–Zn–Sb–Au belt was formed in one tectonic‒thermal coupling metallogenic system in the post-collision stage. Two types of metallogenic models were formed based on whether the partial melting intruded or not: (a) the tectonic‒thermal coupling metallogenic model of leucogranites and the surrounding detachment faults of the dome (partial melting intruded in the form of leucogranites which driven ore-forming fluid to migrate in the surrounding detachment faults); (b) the tectonic‒thermal coupling metallogenic model of non-intruded partial melting and fault systems (under the extension stress, the N–S extension of Tibetan Plateau had formed the STDS and its secondary fault faults that extended to the partial melting body). This results in instantaneous low pressure, which decoupled the partial melting body and magmatic fluid and drove magmatic fluid and deep formation fluid to flow into fault system. Finally, the two fluids were mixed with atmospheric water to form ore. Also a hydrodinamical model for the long distance migration of ore-forming fluid along the fault systems within the Tethys Himalaya Pb–Zn–Sb–Au belt has been established. This study will provide a reference for subsequent geophysical prospecting in the belt.

Episodic crustal anatexis and the formation of Paiku composite leucogranitic pluton in the Malashan Gneiss Dome, Southern Tibet

The Paiku composite leucogranitic pluton in the Malashan gneiss dome within the Tethyan Himalaya consists of tourmaline leucogranite, two-mica granite and garnet-bearing leucogranite. Zircon U-Pb dating yields that (1) tourmaline leucogranite formed at 28.2±0.5 Ma and its source rock experienced simultaneous metamorphism and anatexis at 33.6±0.6 Ma; (2) two-mica granite formed at 19.8±0.5 Ma; (3) both types of leucogranite contain inherited zircon grains with an age peak at ∼480 Ma. These leucogranites show distinct geochemistry in major and trace elements as well as in Sr-Nd-Hf isotope compositions. As compared to the two-mica granites, the tourmaline ones have higher initial Sr and zircon Hf isotope compositions, indicating that they were derived from different source rocks combined with different melting reactions. Combined with available literature data, it is suggested that anatexis at ∼35 Ma along the Himalayan orogenic belt might have triggered the initial movement of the Southern Tibetan Detachment System (STDS), and led to the tectonic transition from compressive shortening to extension. Such a tectonic transition could be a dominant factor that initiates large scale decompressional melting of fertile high-grade metapelites along the Himalayan orogenic belt. Crustal anatexis at ∼28 Ma and ∼20 Ma represent large-scale melting reactions associated with the movement of the STDS.

E–W extension at 19 Ma in the Kung Co area, S. Tibet: Evidence for contemporaneous E–W and N–S extension in the Himalayan orogen

Active faulting in southern Tibet consists of N–S trending extensional faults and linked strike-slip faults, which are an expression of regional E–W extension. A second type of extensional deformation associated with N–S movement is also recognized. This extension is expressed as a series of shear zones and normal faults in the High Himalayas – the Southern Tibetan Detachment System – and mid-crustal rocks exposed in metamorphic domes. Reported constraints on the timing of movements associated with these two phases of extension indicate that N–S extension predates the onset of E–W extension. However, only a few studies have provided clear constraints on the timing of E–W extension and the extent to which the two kinematically distinct domains of extension were contemporaneous is unclear.The Kung Co fault in southern Tibet is a major N–S trending normal fault. The associated E–W extension is locally expressed as high-strain ductile deformation. Both field and microstructural observations show that this deformation occurred synchronously with granite intrusion. Previously reported U–Pb zircon dating shows granite crystallization took place at around 19Ma, implying that ductile E–W extension in the Kung Co area was also active at around 19Ma. This is the oldest documented example of E–W extension in Tibet and shows that E–W extension was at least locally contemporaneous with N–S extension to the south at shallower crustal levels. Simultaneous mid-crustal N–S extension and upper crustal E–W extension may be explained by southward flow of Tibetan crust with a divergent radial component.

Kinematics and vorticity in Kangmar Dome, southern Tibet: Testing midcrustal channel flow models for the Himalaya

[1] Kinematic, kinematic vorticity (Wm), and deformation temperature analyses were completed to test the hypothesis that midcrustal rocks exposed in the core of the Kangmar gneiss dome, southern Tibet record ductile deformation patterns of a “frozen” segment of a southward flowing midcrustal channel. Microscopic and mesoscopic kinematic indicators exhibit a downward transition from a subequal mix of top-north and top-south shear in garnet zone rocks to dominantly top-north shear in staurolite/kyanite zone and deeper rocks. Kinematic vorticity values indicate an increase in pure shear component with depth from ∼48% pure shear in chloritoid zone rocks through ∼62% in garnet zone to staurolite/kyanite zone rocks to ∼68% pure shear in an orthogneiss, the deepest exposed rocks. Deformation temperatures inferred from grain-scale microstructures and quartz lattice preferred orientations increase from ∼300°C–400°C in chloritoid zone rocks to ≥600°C in the deepest exposed rocks. These temperatures are equivalent to temperatures derived from garnet-biotite thermobarometry, indicating that Wm was recorded during peak metamorphism. This ductile deformation zone was cut by the brittle southern Tibetan detachment system (STDS) that juxtaposed metasedimentary rocks upon the orthogneiss. On the basis of these relations, midcrustal rocks in the core of Kangmar Dome record: (1) general shear (vertical thinning and N–S horizontal extension) with a component of top-north shear during peak metamorphism within a ductile shear zone corresponding to the northern and deeper portion of the STDS, (2) an increase in pure shear with structural depth, a consequence of an increase in lithostatic load, and (3) displacement of the high-temperature shear zone by the brittle STDS. Our data are compatible with the deformation patterns predicted for the top part of a southward flowing midcrustal channel.

East-west extension in the NW Indian Himalaya

Research Article| September 01, 2010 East-west extension in the NW Indian Himalaya Esther Hintersberger; Esther Hintersberger * 1Institut für Geowissenschaften, Universität Potsdam, Karl-Liebknecht-Strasse 24, 14476 Potsdam, Germany *E-mail: estherh@geo.uni-potsdam.de Search for other works by this author on: GSW Google Scholar Rasmus Ch. Thiede; Rasmus Ch. Thiede † 1Institut für Geowissenschaften, Universität Potsdam, Karl-Liebknecht-Strasse 24, 14476 Potsdam, Germany †Current address: ETH Zürich, Geologisches Institut, NO E 45, Sonneggstrasse 5, 8092 Zürich, Switzerland; e-mail: rasmus.thiede@erdw.ethz.ch Search for other works by this author on: GSW Google Scholar Manfred R. Strecker; Manfred R. Strecker § 1Institut für Geowissenschaften, Universität Potsdam, Karl-Liebknecht-Strasse 24, 14476 Potsdam, Germany §E-mail: strecker@geo.uni-potsdam.de Search for other works by this author on: GSW Google Scholar Bradley R. Hacker Bradley R. Hacker # 2Institute for Crustal Studies, University of California, Santa Barbara, California 93106-9630, USA #E-mail: hacker@geol.ucsb.edu Search for other works by this author on: GSW Google Scholar GSA Bulletin (2010) 122 (9-10): 1499–1515. https://doi.org/10.1130/B26589.1 Article history received: 01 Dec 2008 rev-recd: 01 Jun 2009 accepted: 01 Jul 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 Esther Hintersberger, Rasmus Ch. Thiede, Manfred R. Strecker, Bradley R. Hacker; East-west extension in the NW Indian Himalaya. GSA Bulletin 2010;; 122 (9-10): 1499–1515. doi: https://doi.org/10.1130/B26589.1 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 Explaining the presence of normal faults in overall compressive settings is a challenging problem in understanding the tectonics of active mountain belts. The Himalayan-Tibetan orogenic system is an excellent setting to approach this problem because it preserves one of the most dramatic records of long-term, contemporaneous shortening and extension. Over the past decades, several studies have described extensional features, not only in the Tibetan Plateau, but also in the Himalaya. For a long time, the favored model explained the function of the Southern Tibetan detachment system, a major fault zone in the Himalaya, as a decoupling horizon between the regime of crustal shortening forming the Himalayan wedge to the south and the extensional regime of the Tibetan Plateau to the north. However, in recent years, increasing evidence has shown that N-S–trending normal faults in the Central Himalaya crosscut not only the Southern Tibetan detachment system, but also the Main Central thrust.Here, we present new structural data and geologic evidence collected within the NW Indian Himalaya and combine them with previously published seismicity data sets in order to document pervasive E-W extension accommodated along N-S–trending faults extending as far south as the footwall of the Main Central thrust. We conducted a kinematic analysis of fault striations on brittle faults, documented and mapped fault scarps in Quaternary sedimentary deposits using satellite imagery, and made field observations in the Greater Sutlej region (Spiti, Lahul, Kinnaur) and the Garhwal Himalaya. Studies of extensional features within the regionally NW-SE–trending NW Indian Himalaya provide the advantage that arc-parallel and E-W extension can be separated, in contrast to the Central Himalaya. Therefore, our observations of E-W extension in the Indian NW Himalaya are well suited to test the applicability of current tectonic models for the whole Himalaya. We favor the interpretation of E-W extension in the NW Indian Himalaya as a propagation of extension driven by collapse of the Tibetan Plateau. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Early Oligocene anatexis in the Yardoi gneiss dome, southern Tibet and geological implications

The Yardoi gneiss dome is located to the easternmost of the North Himalayan Gneiss Dome (NHGD), southern Tibet. It consists of metapelite, garnet amphibolite, granite and leucogranite, and is a key subject to constrain the formation and tectonic evolution of NHGD. SHRIMP zircon U/Pb data on the leucogranite yield an age of 35.3±1.1 Ma, which is substantially older than that of the similar leucogranites to the west. Sr and Nd isotope systematics indicate that this leucogranite was derived from partial melting of the mixed garnet amphibolite and metapelite. Our data suggest that (1) during the early stage of Himalayan magmatism, amphibolite dehydration melting overwhelmed that of the metapelite; and (2) such a melting at middle-lower crust might be a major factor that initiated the movement along the Southern Tibetan Detachment System (STDS).

Onset of mid-crustal extensional flow in southern Tibet: Evidence from U/Pb zircon ages

New ion microprobe U/Pb dates from zircon in deformed orthogneiss and migmatite and an undeformed granite in Mabja Dome are the first to constrain the timing of peak metamorphism, and onset and duration of mid-crustal ductile extension, in southern Tibet at 35.0 ± 0.8 Ma and ∼12–19 million years. The structural, metamorphic, and intrusive his tories in mid-crustal rocks exposed in these north Himalayan gneiss domes are similar to those recorded in the Greater Himalayan sequence, suggesting that middle crust was continuous from beneath southern Tibet southward to the high Himalaya. Strain compatibility indicates that 35 Ma ductile extension in mid-crustal rocks of southern Tibet was accommodated to the south at shallow crustal levels via normal slip along the southern Tibetan detachment system, the oldest age estimate for slip along this normal fault zone, and extrusion of its footwall. If gravitational collapse is an additional important process driving extension, then southernmost Tibet may have been at or near maximum elevation by late Eocene–early Oligocene time.

Miocene to Recent exhumation of the central Himalaya determined from combined detrital zircon fission‐track and U/Pb analysis of Siwalik sediments, western Nepal

ABSTRACTFission‐track (FT) analysis of detrital zircon from synorogenic sediment is a well‐established tool to examine the cooling and exhumation history of convergent mountain belts, but has so far not been used to determine the long‐term evolution of the central Himalaya. This study presents FT analysis of detrital zircon from 22 sandstone and modern sediment samples that were collected along three stratigraphic sections within the Miocene to Pliocene Siwalik Group, and from modern rivers, in western and central Nepal. The results provide evidence for widespread cooling in the Nepalese Himalaya at about 16.0±1.4 Ma, and continuous exhumation at a rate of about 1.4±0.2 km Myr−1 thereafter. The ∼16 Ma cooling is likely related to a combination of tectonic and erosional activity, including movement on the Main Central thrust and Southern Tibetan Detachment system, as well as emplacement of the Ramgarh thrust on Lesser Himalayan sedimentary and meta‐sedimentary units. The continuous exhumation signal following the ∼16 Ma cooling event is seen in connection with ongoing tectonic uplift, river incision and erosion of lower Lesser Himalayan rocks exposed below the MCT and Higher Himalayan rocks in the hanging wall of the MCT, controlled by orographic precipitation and crustal extrusion. Provenance analysis, to distinguish between Higher Himalayan and Lesser Himalayan zircon sources, is based on double dating of individual zircons with the FT and U/Pb methods. Zircons with pre‐Himalayan FT cooling ages may be derived from either nonmetamorphic parts of the Tethyan sedimentary succession or Higher Himalayan protolith that formerly covered the Dadeldhura and Ramgarh thrust sheets, but that have been removed by erosion. Both the Higher and Lesser Himalaya appear to be sources for the zircons that record either ∼16 Ma cooling or the continuous exhumation afterwards.

Mineral chemistry, P-T-t paths and exhumation processes of mafic granulites in Dinggye, Southern Tibet

The mafic granulites in Dinggye, as various scale lense-shaped enclaves within the high Himalayan crystalline rock series, occur along mylonitic foliations at the junction between the Southern Tibetan Detachment System (STDS) and the Xainza-Dinggye normal fault system. The main lithological assemblage comprises garnet plagioclase pyroxenite, garnet two-pyroxene granulite, pyroxene garnet amphibolite and so on. The detailed petrological analyses show that these mafic granulites underwent at least four-stage metamorphic evolution. The first metamor-phic stage, the garnet+clinopyroxene+quart mineral assemblage (M1) was probably formed un-der eclogite facies, the second stage, the plagioclase+clinopyroxene symplectite mineral as-semblage (M2) was produced under high-pressure granulite facies by the early decompressive breakdown of M1 mineral assemblage, the third stage, the plagioclase+clinopyroxene+ hypersthene symplectite mineral assemblage (M3) was formed at granulite facies by the late period decompressive breakdown of M1 and M2 mineral assemblages and the final stage, pla-gioclase+hornblende mineral assemblage (M4) was formed by hydrolysis of earlier mineral as-semblages during late uplifting. The detailed mineral composition analyses suggest that garnets and clinopyroxenes within M1 and M2 mineral assemblages display similar compositions to the equivalents in the B and C types of eclogites, whereas the M3 clinopyroxenes are akin to these of the same kind of minerals in the granulite. These mineral chemistry features and P-T es-timates calculated by mineral thermometers and barometers indicate that the early stage relic porphyroblasts (M1) could be formed at the eclogite facies, the early decompressive breakdown (M2) occurred at the high-pressures granulite facies of 1.35―1.48 GPa and 625―675℃, the M3 mineral assemblage recorded the granulite facies of 0.7―0.95 GPa and 775―900℃ and M4 plagioglase+hornblende retrograde mineral assemblage was produced under the amphibolite facies metamorphism with pressure of 0.4 to 0.75 GPa and temperature at between 660 and 700℃. These construct P-T paths from crustal subduction overthickening to tectonic uplift tectonothermal evolution. The mineral chemical characteristics and P-T condition at every metamorphic stage of these granulites indicate that these rocks experienced the eclogite facies metamorphism during the early stage. Subsequently, these mafic granulites underwent the three-stage exhumation of the eclogite facies tectonic uplift, isostatic uplift related to the trans-formation from eclogite/high-pressure granulite to granulite facies and extensional uplift.

Eohimalayan fold and thrust belt: Implications for the geodynamic evolution of the NW‐Himalaya (India)

In the Himalayas the Eocene deformation phase is called the Eohimalayan event. By means of a forward and backward balanced cross section along a classical traverse through the Tethyan Himalaya we constrain Eohimalayan SW vergent folding and SW directed thrusting in a belt above a basal thrust at 10 km depth. This thrust is located at the structural position of the Southern Tibetan Detachment System with a NE directed normal sense displacement suggesting Miocene reactivation of a major Eocene contractional fault. Stepwise palinspastic restoration demonstrates that this Eocene contractional fault has been subsequently openly folded due to southwestward propagation of the Himalayan Thrust System.

Structural characteristics of middle and southern Xainza- Dinggye Normal Fault System and its relationship to South-ern Tibetan Detachment System

The Xainza-Dinggye Normal Fault System (XDNFS) is a large-scale nearly-north-south trending extensional structure across central and southern Tibet. Its middle segment developed in Tethys Himalaya with features of earlier magmatic core complex and later normal faults dipping moderately to northwest-west. The magmatic core complex is made up by mylonitic leucogrante with a low-angle detachment fault on the top of it and overlain by lower-grade meta-sedimentary rocks. The structural pattern of the southern segment of XDNFS take the shape of a detachment fault dipping to southeast-east with the High-Himalayan rock series as the lower plate. The Southern Tibetan Detachment System (STDS) is expressed as a ductile shear zone composed of mylonitic leucogranite in the studied area of this note. STDS was cut by the later XDNFS, which presents that nearly-east-west striking STDS is not the controlling or adjusting structure of the nearly-north-south trending extensional structures. The origin of nearly-north-south trending extensional structures in Tibet may be the result of deform ational partition of north-south compression.

Evidence for steady fault-accommodated strain in the High Himalaya: progressive fault rotation of the southern Tibet detachment system in NW Bhutan

AbstractWe present fault analyses from the exhumed middle crustal slab of the High Himalaya in eastern Lunana in NW Bhutan. Fault planes from within two-mica, tourma-line-bearing leucogranites, leucogranitic rocks and migmatites indicate a complex brittle fault pattern with two distinct fault groups. A first group of faults (D1) characterized by chlorite, quartz and tourmaline slickenfibres is mainly defined by steeply SSE-dipping oblique-slip normal faults, and by shallowly NNW-dipping normal faults. A second, younger group of faults (D2) characterized by cataclasis products comprises strike-slip faults displaying conjugate patterns and E- and W-dipping conjugate normal faults, all which indicate E-W extension. Cross-cutting relationships amongst the D1 fault group demonstrate that progressively steeper members of the fault group become younger within the NNW-dipping faults and become older within the SSE-dipping faults. These are all post-dated by the D2 fault group. The D1 fault group indicates that the slab experienced ongoing NNW-SSE extension (i.e. flow) via brittle fault accommodation, contemporaneous with fault rotation. This may reflect rotation of the entire upper orogen due to movement over deeply located major ramp structures formed by out-of-sequence thrusting (Kakhtang Thrust) within the High Himalayan Slab of the Bhutan Himalaya.

Evolution of the Kangmar Dome, southern Tibet: Structural, petrologic, and thermochronologic constraints

Structural, thermobarometric, and thermochronologic investigations of the Kangmar Dome, southern Tibet, suggest that both extensional and contractional deformational histories are preserved within the dome. The dome is cored by an orthogneiss which is mantled by staurolite + kyanite zone metasedimentary rocks; metamorphic grade dies out up section and is defined by a series of concentric kyanite‐in, staurolite‐in, garnet‐in, and chloritoid‐in isograds. Three major deformational events, two older penetrative events and a younger doming event, are preserved. The oldest event, D1, resulted in approximately E‐W trending tight to isoclinal folds of bedding with an associated moderately to steeply north dipping axial planar foliation, S1. The second event, D2, resulted in a high strain mylonitic foliation, S2, which defines the domal structure, and an associated approximately N‐S trending stretching and mineral alignment lineation. Shear sense during formation of S2 varied from dominantly top S shear on the south dipping flank of the dome to top N shear on the north dipping flank. The central part of the dome exhibits either opposing shear sense indicators or symmetric fabrics. Microtextural relations indicate that peak metamorphism occurred post‐D1 and pre‐ to early D2 deformation. Quantitative thermobarometry yields peak metamorphic conditions of ∼445°C and 370 MPa in garnet zone rocks, increasing to 625°C and 860 MPa in staurolite + kyanite zone rocks. Pressures and temperatures increase with depth and northward within a single structural horizon across the dome and the apparent gradient in pressure is ∼20% of the expected gradient, suggesting that the rocks were subvertically shortened after the pressure gradient was frozen in. Mica 40Ar/39Ar thermochronology yields 15.24 ± 0.05 to 10.94 ± 0.30 Ma cooling ages that increase with depth and young northward within a single structural horizon across the dome. Diffusion modeling of potassium feldspar 40Ar/39Ar spectra yield rapid cooling rates (∼10–30°C/Myr) between ∼11.5 and 10 Ma and apatite fission track ages range from 7.9 ± 3.0 to 4.1 ± 1.9 Ma, with a mean age of ∼5.5 Ma. Both data sets show symmetric cooling across the dome between ∼11 and 5.5 Ma. The S2 mylonitic foliation, peak metamorphic isobars and isotherms, and mica 40Ar/39Ar isochrons are domed, whereas potassium feldspar 40Ar/39Ar and apatite fission track isochrons are not, suggesting that doming occurred at ∼11 Ma. Our data do not support simple, end‐member metamorphic core complex‐type extension, diapirism, or duplex models for gneiss dome formation. Rather, we suggest that the formation of extensional fabrics occurred within a zone of coaxial strain in the root zone of the Southern Tibetan Detachment System (STDS), implying that normal slip along the STDS and extensional fabrics within the Kangmar Dome were the result of gravitational collapse of overthickened crust. Subsequent doming during the middle Miocene is attributed to thrusting upward and southward over a north dipping ramp above cold Tethyan sediments. Middle Miocene thrust faulting in the Kangmar Dome region is synchronous with continued normal slip along the STDS and thrust motion along the Renbu Zedong thrust fault, suggesting that extension and contraction was occurring simultaneously within southern Tibet.

Southern Tibet Detachment System at Khula Kangri, Eastern Himalaya: A Large-Area, Shallow Detachment Stretching into Bhutan?

A new map and cross sections of the Khula Kangri and Kankar Pünzum-Monlakarchung High Himalayan ranges in the Tibet-Bhutan frontier area are presented from integration of unpublished mapping from the summit section of Khula Kangri and new remote sensing together with previous mapping. The ranges define an orographic bifurcation of the High Himalaya that results in a north-south repetition of the main geological section and coincides with the morphological repetition. The Southern Tibet Detachment System that juxtaposes the Tethyan sedimentary rocks against the gneisses and granites of the High Himalayan Crystalline can be continuously traced around both ranges and is not imbricated. Postdetachment kilometer-scale flexure and faulting account for the features of the observed bifurcation. The true map extent of the Khula Kangri and Monlakarchung-Passalum granite batholiths is now apparent. We propose that the two plutons are part of the same originally continuous body.

Petrologic and structural controls on geomorphology of prehistoric Tsergo Ri slope failure, Langtang Himal, Nepal

High relief (Δ h up to 2600 m) and spatial orientation of discontinuities, such as leucogranitic intrusions, mylonitic and pseudotachylitic horizons and related ore structures, were the preparatory causal factors for the giant prehistoric Tsergo Ri landslide (High Himalayan Crystalline, Langtang Himal, Nepal). Other controlling factors include overthrusting, detachment faulting, and paleoseismics related to the Main Central Thrust (MCT) and Southern Tibet Detachment System (STDS), neotectonically generated structures, and predesigned form of the scarp (broken crest) and the direction of movement towards the WSW. Coherently displacement of several 10 9 m 3 materials caused frictional fusion. Landslide dynamics and specific morphologic conditions created four different types of sliding surfaces (in chronological sequence): (i) primary—at the basement, (ii) secondary—parallel or subparallel to (i), (iii) tertiary—vertical to (i) and (ii), and (iv) quarternary sliding surfaces—local variety of (iii). (Note: ordinal numeric terms are not in time-stratigraphic sense.) These planes represented the main discontinuities conducting different syn-event stresses in the sliding mass and for further landslide dynamics. Spatial difference and widespread distribution of sliding planes and sets of discontinuities all over the landslide area caused different classes of rock quality, which subsequently determined specific erosional processes within the landslide accumulation area. The subsequent glacial and fluvial erosional processes were controlled by the correlating preexisting lithotectonic patterns, the hyalomylonitic and/or breccious sliding planes and the recent morphologic features or shapes of Tsergo Ri. (i) The SE part is gradationally brecciated to the top and includes Tsergo Ri itself. The residual mass of the bulk has been eroded almost isometrically as far as the primary sliding plane crops out along the intact gneissic basement. Differences in rock quality along a horizon of secondary sliding planes indicate a steplike break in the relief after the last main glaciation. (ii) The earlier-halted NW part still preserved micromountainous shape of compact sliding blocks (Phushung I and II, Kyimoshung), separated by landslide-dynamic-triggered faults (tertiary and quarternary sliding planes). Primary and secondary sliding planes, almost covered by Holocene sediments, produced an insignificant morphology. (iii) The highest brecciated strike–slip fault, generated during collision with the obstacle of Pangshungtramo Peak in the southwest, is the deeply eroded Dranglung Chu valley, which has been kept morphologically active in a major way until recent times. (iv) Except for the collision obstacle of Pangshungtramo Peak with recent semi-active erosion and a small secondary rockslide (block of Tsangbu) north of it, all surrounding parts show typical features of high altitude erosion.

When did the roof collapse? Late Miocene north-south extension in the high Himalaya revealed by Th-Pb monazite dating of the Khula Kangri granite

Research Article| June 01, 1997 When did the roof collapse? Late Miocene north-south extension in the high Himalaya revealed by Th-Pb monazite dating of the Khula Kangri granite M. A. Edwards; M. A. Edwards 1Department of Geological Sciences, State University of New York, Albany, New York 12222 Search for other works by this author on: GSW Google Scholar T. M. Harrison T. M. Harrison 2Department of Earth and Space Sciences and Institute for Geophysics and Planetary Physics, University of California, Los Angeles, California 90024 Search for other works by this author on: GSW Google Scholar Geology (1997) 25 (6): 543–546. https://doi.org/10.1130/0091-7613(1997)025<0543:WDTRCL>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation M. A. Edwards, T. M. Harrison; When did the roof collapse? Late Miocene north-south extension in the high Himalaya revealed by Th-Pb monazite dating of the Khula Kangri granite. Geology 1997;; 25 (6): 543–546. doi: https://doi.org/10.1130/0091-7613(1997)025<0543:WDTRCL>2.3.CO;2 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 SocietyGeology Search Advanced Search Abstract Th-Pb ion microprobe measurements made on 12 monazite grains from the Khula Kangri granite, Tibet-Bhutan frontier, are interpreted to indicate that crystallization occurred at 12.5 ± 0.4 Ma. The leucogranite is cut by the Gonto La detachment, part of the Southern Tibet detachment system that has allowed upper-level, north-directed extension of the Himalayan orogen. Significant orogen-normal extension in southern Tibet appears to have continued 8–10 m.y. later than previously recognized. This is the first reported crystallization age for a leucogranite east of the Yadong cross structure, an apparent 70 km offset of the high Himalaya and Southern Tibet detachment. West of the Yadong cross structure, reliable ages for high Himalaya events (major Main Central thrust slip, granite generation and emplacement, attainment of critical topography, and major detachment extension) group between ca. 24 and 19 Ma. We interpret the west-to-east change across the Yadong cross structure to be due to either (1) an abrupt, ∼10 m.y. younging of principal high Himalayan events or (2) a deeper (thus younger) exposed part of the footwall of the southern Tibet detachment. Near Khula Kangri, the Southern Tibet detachment is cut by the highly oblique Yadong-Gulu rift; a manifestation of Tibet plateau east-west extension. Integrated estimates of magnitude, and rate, of detachment displacement suggest that the observed postcrystallization north-directed extension lasted for 1–3 m.y., after which time the Yadong-Gulu rift formed. This interpretation is consistent with initiation of east-west extension of Tibet at ca. 8 Ma. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.