Eastern Himalayan Syntaxis Research Articles

Imaging the crustal structure at the northeast corner of the indenting Indian Plate in the Eastern Himalayan Syntaxis

SUMMARYCrustal configuration beneath the indenting northeast corner of the Indian Plate in the Eastern Himalayan Syntaxis has been investigated with the help of receiver function (RF) analysis of teleseismic earthquakes recorded by 19 broad-band seismological stations. The common conversion point stacking of RFs and 1-D velocity models obtained through inversion provide new information on the intracrustal structure. The study reveals the signature of the Main Himalayan Thrust (MHT) beneath the Lohit Valley at ∼22–26 km depth. The MHT is not prominent in the Siang window plausibly due to large-scale crustal deformation related to the formation of the window and antiform folding. Unlike in the western and central Himalaya, the MHT does not play a major role in seismogenesis in the Lohit Valley and Siang Window, where seismicity is active up to the crustal depth of ∼40 km. The crustal thickness increases from ∼38 km at Pasighat in the south to ∼50 km at the northernmost station (Gelling) in the Siang window. In Lohit Valley, the crustal thickness increases from ∼40 km at Mahadevpur in the west to ∼54 km in the Tidding–Tuting suture zone, which again shallows to ∼51 km in the eastern Lohit Plutonic Complex (Walong station). The thinner crust beneath the Tidding–Tuting suture compared to the Indus Tsangpo Suture Zone of northwest Himalaya is caused due to the differences in convergence rate, higher exhumation rate and mechanisms to accommodate collision and rotational tectonics.

The extraordinary Namche Barwa sediment factory in the Eastern Himalayan Syntaxis

AbstractThe Namche Barwa Syntaxis (NBS) is one of the most productive detrital factories on Earth. Previous studies have shown that the NBS supplies large amounts of sediment to the Brahmaputra River, although the sources and controlling factors of sediment production have not been ascertained in detail. This study presents petrographic and heavy‐mineral data for 43 sand samples collected in the Yarlung and Parlung river catchments covering the entire NBS and surrounding areas. Combined with U–Pb ages of detrital zircons, our data indicate that 89 ± 11% of Yarlung River sediments downstream of the NBS are produced in the Yarlung and Parlung gorges. The annual sediment flux of the Yarlung River increases by a factor of 20 within ca. 250 km from upstream of the NBS (ca. 10 Mt) to downstream (ca. 200 Mt/a). The Yarlung and Parlung gorges, representing only ca. 1% of the Yarlung‐Brahmaputra catchment area, contribute 74 ± 9% of the total Brahmaputra sediment flux. Average interannual erosion rates in the Yarlung and Parlung gorges corresponding to these fluxes are calculated to be 9.2 ± 1.2 mm/a and 6.5 ± 2.1 mm/a respectively. Focused erosion of the Namche Barwa Complex and Yarlung Suture Zone in the gorge, where high‐grade metamorphic rocks are exposed, is a consequence of high channel steepness (ksn values up to 800–1800), high stream power and extreme events including frequent earthquakes and landslides. The coupling between surface erosion and tectonic uplift in the Yarlung Gorge is in full agreement with the tectonic aneurism model of NBS evolution.

Potential seismic landslide hazard and engineering effect in the Ya’an-Linzhi section of the Sichuan-Tibet transportation corridor, China

The Sichuan-Tibet transportation corridor is located at the eastern margin of the Qinghai-Tibet Plateau, where the complex topography and geological conditions, developed geo-hazards have severely restricted the planning and construction of major projects. For the long-term prevention and early control of regional seismic landslides, based on analyzing seismic landslide characteristics, the Newmark model was used to carry out the potential seismic landslide hazard assessment with a 50-year beyond probability 10%. The results show that the high seismic landslide hazard is mainly distributed along large active tectonic belts and deep-cut river canyons, and are significantly affected by the active tectonics. The low seismic landslide hazard is mainly distributed in the flat terrain such as the Quaternary basins, broad river valleys, and plateau planation planes. The major east-west linear projects mainly pass through five areas with high seismic landslide hazard: Luding-Kangding section, Yajiang-Xinlong (Yalong river) section, Batang-Baiyu (Jinsha river) section, Basu (Nujiang river) section, and Bomi-Linzhi (eastern Himalaya syntaxis) section. The seismic action of the Bomi-Linzhi section can also induce high-risk geo-hazard chains such as the high-level glacial lake breaks and glacial debris flows. The early prevention of seismic landslides should be strengthened in the areas with high seismic landslide hazard.

Fault systems impede incision of the Yarlung river into the Tibetan plateau

It is widely accepted that tectonics generally enhances river incision. However, why rivers have not incised further into orogenic plateaus to destroy terrains over long-time scales remains ambiguous. Here we hypothesize that the diverse nature of regional tectonics could have impeded river erosion, taking Yarlung River in Tibetan Plateau as a case. We constrain the incision history and effect of a tectonic rift on fluvial incision by low-temperature thermochronology. Results show focused cooling near the rift, but markedly reduced cooling in the upstream and downstream regions since ~7 Ma. This coincides with an episode of rapid exhumation of Eastern Himalaya Syntaxis downstream. We propose that these two co-phased tectonic systems resulting from accelerated extension of southern Tibetan Plateau prevented upstream migration of river knickpoints. Our study highlights that the activity of fault systems may hinder regional erosion, thereby facilitating the preservation of topography and high plateaus in active orogenic belts.

The origin and compositions of melt inclusions in an Al2SiO5‐free paragneiss from the Namche Barwa Complex in the Eastern Himalayan Syntaxis

AbstractMelt inclusions (MIs) in high‐temperature metamorphic rocks provide a unique window into crustal anatexis in collisional orogenic belts and have been widely used to characterize compositions of anatectic melts as well as melting mechanisms. In this study, MIs hosted by peritectic garnet were for the first time identified in an Al2SiO5‐free graywacke‐type paragneiss from the Namche Barwa Complex, the Eastern Himalaya, Southeast Tibet. These MIs occur as nanogranites in the rims of porphyroblastic garnet, exhibit negative crystal shapes with an average diameter of ~12 μm and consist of a mineral assemblage of biotite + quartz + plagioclase + K‐feldspar ± muscovite. Re‐homogenization experiments of these nanogranites were conducted at a pressure of 1.5 GPa and temperatures of 800°C, 850°C and 900°C and produced homogeneous glasses at 850°C. The homogenized glasses are strongly peraluminous and calc‐alkalic in composition, with 66.43–71.31 wt.% SiO2, 12.64–15.06 wt.% Al2O3, high alkaline (5.41–7.22 wt.%) and low ferromagnesian (2.72–4.46 wt.%) contents. They are lower in silica and CaO but higher in K2O compared with MI produced by fluid‐present melting of metasedimentary rocks, thus indicating fluid‐absent melting. These glasses are also characterized by enrichment of large ion lithophile elements (particularly Cs and Rb), depletion of Ba and Sr, low contents of light rare earth elements (3.6 to 33.7 ppm), high Rb/Sr ratios (6.19–37.3) and low Nb/Ta ratios (2.55–18.7). In combination with phase equilibrium modelling, these compositional features suggest that a sequential dehydration melting of muscovite and biotite was responsible for the production of MI during prograde metamorphism of the studied paragneiss. By compiling MI data published in the literature, we show that dehydration melting of metasedimentary rocks from the Himalayan orogen can produce initial melts with various peraluminous and granitic compositions.

Petrology of Bengal Fan turbidites (IODP Expeditions 353 and 354): provenance versus diagenetic control

ABSTRACT High-resolution petrographic and heavy-mineral analyses of Bengal Fan turbidites from six cores drilled during IODP Expeditions 353 and 354 elucidate factors controlling their intersample compositional variability as a key to understanding sedimentary processes and erosional evolution of the Himalayan belt since the Miocene. Bengal Fan turbidites are feldspatho-quartzose to litho-feldspatho-quartzose with plagioclase > K-feldspar; slow-settling micas increase in abundance in very fine sand and coarse silt. The feldspar/quartz ratio and higher-rank metamorphic rock fragments notably increase from uppermost Miocene to Pleistocene deposits, which is ascribed to the onset of rapid exhumation of the Eastern Himalayan syntaxis since ∼ 5 Ma. The same trends are documented in Nicobar Fan turbidites, confirming that they belong to the same sedimentary system. Both Bengal and Nicobar fans record a pulse in mass accumulation rate at Tortonian times, when supply of sedimentary and very-low-grade metasedimentary detritus reflected accelerated exhumation of the Lesser Himalaya. In contrast to foreland-basin sediments, where ferromagnesian minerals have been completely dissolved in strata as young as Pliocene–Pleistocene, in both Bengal–Nicobar and Indus fans amphibole invariably represents about half of the moderately rich to rich transparent-heavy-mineral suite, demonstrating that amphibolite-facies Greater Himalaya metamorphic rocks were widely exposed in the Himalayan range well before the late Miocene and possibly since the late Oligocene, as indicated by a few sillimanite and kyanite grains in Bengal Fan sediments as old as 23 Ma and 28 Ma, respectively. Diagenetic dissolution strongly affected olivine and pyroxene in strata older than the middle and early Pleistocene, respectively, whereas amphibole decreases markedly through progressively older Miocene strata. Ferromagnesian minerals and sillimanite are almost completely dissolved in lower Miocene strata, where durable zircon, tourmaline, rutile, and apatite make up half of the strongly depleted heavy-mineral assemblage. Quaternary turbidites from the six studied cores have virtually the same compositional signatures, testifying to efficient homogenization by turbidite transport and reworking across the fan. Turbidites in western cores closer to peninsular India (U1444A and U1454B) are not different from those in eastern cores, indicating very minor supply from the subcontinent. Forward-mixing calculations based on integrated petrographic and heavy-mineral data indicate that sand supply from the Brahmaputra River to Quaternary turbidites was four times larger than supply from the Ganga River, indicating up to six times higher sediment yields and erosion rates in the Brahmaputra than in the Ganga catchment, largely reflecting superfast erosion of the Eastern Himalayan syntaxis.

A tale of an orbicule in the eastern Gangdese belt of southern Tibet: Petrographic, geochemical, and submagmatic structural perspectives on its formation

Owing to their intriguing appearance and unusual occurrence, as well as their significance in addressing magmatic processes, orbicules have attracted attention from geologists all over the world. We discovered an orbicule-bearing boulder in the Linzhi region, near the Eastern Himalayan Syntaxis, which is the first report of orbicules in the Gangdese magmatic belt, southern Tibet. The orbicules are composed of dioritic shells around monzodioritic cores, in a dioritic matrix. They typically have a single shell, composed of elongate and radially oriented hornblendes, with interstitial plagioclase. Zircon laser ablation−inductively coupled plasma−mass spectrometry U-Pb dating yields an indistinguishable age of ca. 28.8 Ma for both the orbicule core and matrix, indicating approximately coeval crystallization. Zircon Lu-Hf and whole-rock Sr-Nd isotopes reveal that the original magmas of both the orbicule matrix and orbicule core were relatively juvenile (ƐHf(t) = +2.2 to +6.0 for matrix and +1.2 to +3.9 for core; ƐNd(t) = −1.68 to −1.46 for matrix and −3.65 to −2.52 for core), similar to reported Oligocene−Miocene post-collisional adakites in the Gangdese belt. Zircon and hornblende geochemistry reveals that both the orbicule matrix and cores were generated from highly oxidized, high oxygen fugacity magmas. Hornblende composition further indicates that the original magmas of the matrix and the orbicule cores were water-rich, with H2O contents greater than 4 wt%. In combination with the whole-rock geochemistry, these results suggest that the orbicules were formed in a post-collisional setting, from magma generated by partial melting of the juvenile thickened lower crust of the Lhasa terrane triggered by convective removal of the Lhasa lithospheric root. The preferred orientation of euhedral igneous minerals in the matrix and the alignment and compaction of orbicules define a magmatic to submagmatic foliation. This, together with the subsolidus deformation microstructures exhibited by minerals within the orbicules, indicates that some flow or movement of orbicules took place under magmatic to submagmatic conditions, as the subsolidus plastic orbicules were jostled and transported within a hypersolidus liquid magma.

Source localized modelling to estimate the depth of anisotropy beneath eastern Himalaya using spatial coherency of shear-wave splitting parameters

The present study focuses on the source localization and depth estimation of anisotropy beneath the eastern Himalaya ranging from Nepal in the west to Eastern Himalayan Syntaxis at the north eastern flank of the Indian region. Although previous literatures on anisotropy suggest the possible deformation patterns and their mechanisms beneath the eastern Himalayan region, a compressive analysis of the source and depth localization of anisotropy is missing, which is addressed in tghis study. A comprehensive analysis of the central depth of anisotropy beneath this complex orogeny is carried out using the Spatial coherency method. This technique is based on the Fresnel-zone principle with a more generalized use of variation factor as a function of fast axis polarization and delay time beneath a particular station. A total of 435 splitting measurements from the entire eastern Himalayan region are utilized to model the anisotropy source localization and depth estimation beneath three major tectonic domains like Nepal Himalaya, Arunachal Himalaya and the Eastern Himalayan Syntaxis to understand the source of anisotropy and the deformation mechanism. Spatial coherency analysis of splitting parameters beneath the Nepal Himalaya suggests that the source of anisotropy is at about 180 km depth, which coincides with the region's lithosphere-asthenosphere boundary (LAB). This emphasizes the asthenospheric source in controlling the deformation parallel to the absolute plate motion (APM) of the Indian plate. For both the Arunachal Himalaya and the Eastern Himalaya Syntaxis, the source of anisotropy corresponds to the depth of 135 km and 150 km respectively, suggestive of a lithospheric source in dominating the deformation patterns in these regions.

The atypical Gaoligong orocline: Its geodynamic origin and evolution

Various orocline systems around the India–Eurasia collision zone have long been recognized and studied. Different portions of the India–Eurasia boundaries represent various scales and models of orocline-forming processes, such as the Baluchistan orocline formed by multiple deformation events and the Himalayan orocline formed by a mixture of complex structural mechanisms. The curvature from the eastern Himalayan syntaxis through east Burma to west Yunnan showed a unique convex curvature toward the mantle wedge. This is different from the concave Baluchistan orocline and the Himalayan orocline. The unique geometry of the Gaoligong orocline shows an N-S trend for the northern section and a NE-SW trend for the southern section. This curvature also marks the boundary between the Tengchong and Baoshan blocks along the Santaishan suture in western Yunnan, China. Our structural reconstruction identified five deformation events: 1) D1 is km-scale upright folding, which only affected the Neoproterozoic meta-sedimentary unit, 2) D2 recumbent folding, which only developed in the southern section of the Gaoligong orocline, 3) D3 large-scale gently westward-inclined thrust folding, 4) D4 right-lateral shear belt, and 5) the D5 normal faults. Since the D3 structure is the earliest event that shows penetrative foliation development along the orocline, we consider D1 and D2 as pre-orocline-forming events. The geometry of the Gaoligong orocline is controlled by the distribution of the Ordovician basement between the Tengchong and Baoshan blocks. Both north and south sections experienced the same structural evolution since D3 (a fault-propagation fold system occurred between 40 Ma and 28 Ma), D4 (steep right-lateral shear belt occurred between 28 Ma and 15 Ma), and D5 (normal faults after 15 Ma). The curvature first developed as a shovel-like top-to-the-NE thrust plane (S3) that formed under amphibolite-facies conditions between 40 Ma and 28 Ma. The following deformation events (D4 and D5) show orocline parallel foliation development under lower metamorphic conditions, indicating that the curvature of the Gaoligong orocline is not generated by additional rotation along multiple deformation events. However, due to the lack of orocline parallel foliation development for S3, and the lack of a proper position of the indenter, the Gaoligong orocline cannot be classified as a primary orocline nor a rotational orocline. The curved geometry is an interference pattern of topography relief to the shovel-like thrust plane that developed during D3. Our new reconstructed structural evolution concludes that the Gaoligong orocline is an “atypical” orocline.

Dense-array adjoint tomography reveals lithospheric delamination and asthenosphere upwelling beneath the western Yangtze Craton

The eastern Tibetan Plateau has attracted widespread attention due to its complex topography and strong seismicity. However, the mechanism controlling the growth of this margin remains enigmatic. Here, we present detailed upper mantle structures of the easternmost and northeastern Tibetan Plateau from dense-array adjoint waveform tomography. The seismic images show mushroom-shaped low-velocity zones atop at the uppermost mantle of the eastern margin of the Tibetan Plateau and slab-shaped high-velocity bodies preserved beneath both the eastern Himalayan syntaxis and the western Sichuan basin. The seismic features suggest asthenosphere upwelling and lithospheric delamination beneath the western Yangtze Craton, which might be induced by the retreat of the subducted Indian Plate. Our study shows that the mantle dynamics of the western Yangtze Craton have played an important role in the Tibetan Plateau growth and suggests that the Yangtze Craton might be reconstructed by ongoing continental collision.

Paleomagnetic constraint on the formation of the Eastern Himalayan Syntaxis: A new late Eocene result from the Mangkang area of the eastern Tibetan Plateau

The Eastern Himalayan Syntaxis (EHS) is a major structural and geomorphological unit in Eastern Tibet; however, its mechanism of formation and the extent of its northward effect of compression remain unclear. To address these questions, we conducted a paleomagnetic and geochronologic study of the late Eocene trachyte intrusive mass in the Mangkang area of Tibet. The U-Pb SHRIMP II age estimate of the trachyte intrusive mass is ∼40 Ma, and the estimated mean paleomagnetic direction we obtained is D=27.0°, I=36.8°, k=42.9, α95=1.3°, and n=282 specimens. These results indicate that the Mangkang area was located at a paleolatitude of 20.5 ± 0.9° N during the late Eocene. A comparison of paleolatitudes of the Qiangtang block with those inferred from the apparent polar wander paths (APWP) of India and Eurasia suggests that the Eastern Himalayan Syntaxis was formed after 35 Ma when Eurasia was penetrated by ∼600 km by northward-moving India. This is consistent with geologic evidence of intra-continental shortening in the foreland of the Eastern Himalayan syntaxis. The northward counterclockwise penetration of the corner of India also resulted in the large right-lateral strike-slip system of the Sagaing fault (SGF), with ∼400-600 km of displacement between the EHS and the Shan-Thai block. Thus, the northward wedging of the EHS indicates minimal southward displacement of the Shan-Thai block. Our results link the tectonic activity of the EHS to the lateral extrusion of the southeastern margin of the Tibetan Plateau, providing a new perspective on the evolution of this region since the late Eocene.

Tracing high-pressure metamorphism in the eastern Himalayan syntaxis using detrital zircon and monazite from modern stream sediments

The timing of high-pressure (HP) metamorphism in the eastern Himalayan syntaxis is important for understanding the India-Asia collisional processes, but it remains elusive. To reveal the metamorphic history of the eastern Himalayan syntaxis, we performed a study of geochronology, trace elements, and mineral inclusions of detrital zircon and monazite from modern stream sediments in the eastern Himalayan syntaxis. Detrital zircon comprise magmatic and metamorphic domains with different zoning. Inherited magmatic zircon domains have high Th/U, low (Dy/Yb)N, and retain ages of 1798−360 Ma. Metamorphic zircon domains with low Th/U, high (Dy/Yb)N, and inclusions of garnet, kyanite, and/or clinopyroxene probably formed under HP conditions. They yield age groups of 49−35 Ma, 33−17 Ma, and 12−7 Ma. The low Th/U and low (Dy/Yb)N metamorphic zircon domains probably formed during retrogression and yield age groups of 27−16 Ma and 10−6 Ma. Detrital monazite yield age distributions similar to those of the low (Dy/Yb)N metamorphic zircon except for the 821−402 Ma inherited cores. The (Dy/Yb)N of 31.6−5.7 Ma monazite decreases with increasing Y content, which indicates that it likely formed under the retrograde stage during garnet breakdown. Based on the oldest metamorphic ages, the initial India-Asia collision occurred no later than 50−44 Ma in the eastern Himalayan syntaxis. The multimodal age patterns of the metamorphic zircon and monazite indicate that the Indian continent underwent multistage HP and retrograde metamorphism in the eastern Himalayan syntaxis. The nearly contemporaneous HP and retrograde metamorphism indicate that the Indian continent continued subducting while the earlier HP metamorphic slices detached and exhumed.

Underthrusting and Pure Shear Mechanisms Dominate the Crustal Deformation Beneath the Core of the Eastern Himalayan Syntaxis as Inferred From High‐Resolution Receiver Function Imaging

AbstractHow the crust in the core of the Eastern Himalayan Syntaxis (EHS) deforms responding to the India‐Asia collision remains ambiguous. Here we present the first high‐resolution receiver functions image of crustal structure along a new NW‐SE trending dense nodal array crossing the core of the EHS. Two sets of low velocity zones (LVZs) are clearly observed: one with a flat style beneath the western Lhasa terrane and Higher Himalaya at 18–20 km depth and the other with two west‐dipping shapes below the western Yarlung‐Zangbo suture within 10–30 km depth. These LVZs caused by partial melting and aqueous fluids are disconnected, impeding the formation of crustal flow. A discontinuous east‐dipping intra‐crustal discontinuity and a sharp Moho offset of 7 km under the Aniqiao‐Motuo shear zone are identified, suggesting that the underthrusting of the Indian lower crust and pure shear mechanisms jointly dominate crustal deformation in the core of the EHS.

Oligocene-early Miocene rapid exhumation along the Xianshuihe fault system: Implications for the growth of the Southeastern Tibetan Plateau

• 24 new thermochronometric ages from the bend section of the Xianshuihe fault system are reported. • New data indicate migration of the locus of fast exhumation towards the fault zone from ∼25 to ∼10 Ma. • Oligocene-early Miocene fast exhumation along the fault system resulted from crustal shortening. • southeastern Tibetan Plateau; Xianshuihe fault; low-temperature thermochronology; • Oligocene-Early Miocene; rapid exhumation; Plateau growt. Large-scale strike-slip faults in the southeastern Tibetan Plateau are considered to have accommodated significant crustal extrusion and deformation during the plateau growth. However, the long-term deformation and exhumation history along the large-scale strike-slip faults is still unclear. Here, we present new thermochronological datasets from the near-field of the bend section of the Xianshuihe fault system, they reveal migration of the loci which show rapid exhumation between ∼25 Ma and ∼10 Ma towards the fault zone. Our new results clearly show a phase of rapid Oligocene-early Miocene basement exhumation, indicating contemporaneous crustal shortening along the fault. Through synthesizing the evidence of Oligocene-early Miocene fast exhumation along the Xianshuihe fault system, and synchronous deformation along adjacent strike-slip and thrust faults in the region, we propose that the southeastern Tibetan Plateau may have reached a remarkable high-elevation topography during this period. This was accompanied by significant crustal shortening under a contractional regime, with northeastward advance of the eastern Himalayan syntaxis and southeastward crustal extrusion originated from the plateau hinterland. Our findings are generally in agreement with sedimentary records in the marginal sea basins and paleo-altimetry studies.

Characteristics of Radiogenic Heat Production of Widely Distributed Granitoids in Western Sichuan, Southeast Tibetan Plateau

Abstract Investigating the genesis of geothermal resources requires a thorough understanding of the heat source mechanism, which is also a vital basis for the efficient exploration and utilization of geothermal resources. Situated in the eastern Himalayan syntax, western Sichuan is considered to be one of the main concentration regions of high-temperature geothermal resources in China. To date, various studies have been carried out to reveal the heat source and genesis of the abundant high-temperature resources in this area; however, studies on the contribution of the radioactive heat generated by the widely distributed granitoids to the high-temperature geothermal resources remain scarce. In order to resolve this knowledge gap, we attempted to obtain evidence from the geochemical data published in the literature in the past few decades. A total of 548 radiogenic heat production rate data were determined. The statistical data indicate that the average concentrations of the heat-producing elements U, Th, and K are 6.09±5.22 ppm, 26.74±16.78 ppm, and 3.51±0.82%, respectively. The calculated heat production values of the granitoids vary from 0.52 to 10.86 μW/m3, yielding an arithmetic average value of 3.74±2.15 μW/m3, which is higher than that of global Mesozoic–Cenozoic granites (3.09±1.62 μW/m3). Based on the heat production values, the capacity of the granitic batholiths to store heat was assessed, and the Dongcuo pluton was found to be the largest heat reservoir (382.88×1013 J/a). The distribution of the crustal heat flow was examined using the calculated heat production data and the stratigraphic structure obtained via deep seismic sounding in the study area. The results indicate that the crustal heat flow is 48.3–56.2 mW/m2, which is mainly contributed by the radioactive decay in the granitoids in the upper crust. The fact that it accounts for nearly half of the regional background heat flow indicates that the radiogenic heat from the granitoids is an important heat source for the formation of the thermal anomaly and the high-temperature geothermal resources in the study area. Thus, the results obtained in this study highlight the importance of the widely distributed granitoids to high-temperature geothermal resources in western Sichuan.

Retrospective analysis and hazard assessment of Gega glacial lake in the eastern Himalayan syntaxis

The frequency of glacier related hazards like Glacial Lake Outburst Flood (GLOF) is increasing in the High Mountain Asia due to global climate warming. The extreme floods generated by glacial lake outburst aggravate the risk to people, towns and infrastructure in downstream regions. However, little attention is paid to the destructive potential of the lakes in the southeastern margin of the Tibet in spite of recent rapid increases in population, infrastructure, and economic investment. With retrospective investigation of the Gega lake, we found historical overtopping flood(s) impacted by landslides based on the lake dam consists of landslide deposits on both sides and bedrock in the middle, manifesting the GLOF risk in future. BRCH-J model is used to simulate the flow hydrographs after dam break, then, the HEC-RAS software is used to simulate the propagation of outburst floods. Finally, the GLOF risk is evaluated with a semi-quantitative approach considering overtopping and piping failure scenarios. The peak discharge after dam failure would reach up to 17,000 ​m3 ​s−1 (at dam site), which is about two times greater than the average seasonal flood peak near Gega village in the trunk river. Piping failure attains a larger peak discharge and quicker peak time (∼0.13 ​h) compared to the overtopping failure (∼0.5 ​h). The sensitivity analysis of materials properties (like internal friction angle and porosity etc.) shows geotechnical properties of the dam material affect the magnitude of peak discharge. The 1D steady flood routing of the peak flow indicates the flow depth is larger than 35 ​m at the Yarlung Tsangpo - Gega confluence. The extreme flow through narrow and steep terrain could even trigger debris flow along the Gega stream. The results can be useful for mitigating risk of GLOFs in the eastern Himalayan syntaxis and neighboring alpine areas.

Present-day fault kinematic around the eastern Himalayan Syntaxis and probable viscoelastic relaxation perturbation following the 1950 Mw 8.7 Assam earthquake

Strike-slip and tensile/dip-slip rates around the EHS region from the elastic block model. • An elastic block model was constructed to analyze the fault slip rates and regional kinematic around the Eastern Himalayan Syntaxis (EHS). • The postseismic viscoelastic relaxation effects caused by the 1950 Assam Mw 8.7 earthquake is still significant even after 65 years. • The interseismic slip rate across the eastern Himalaya derived from pure elastic solution may be overestimated if ignoring the viscoelastic relaxation effects. The Eastern Himalayan Syntaxis (EHS) played important role in accommodating the intracontinental deformation and material flow in southeastern Tibet. Quantifying the regional kinematics around the EHS is critical for understanding the deformation mechanism of the Tibetan Plateau. In this study, we compiled all available GPS measurements and constructed an elastic block model to analyze the fault kinematics around the EHS. The results suggest that the deformation characteristics around the EHS can be well explained by a linear spherical block model that considers the intra-block homogeneous strain. The fault slip rates from the block model agree well with the late Quaternary slip rates, suggesting that the GPS-derived slip rates can generally represent the long-term slip rates. However, some relatively large residuals, especially in the frontal EHS region, are still detectable, suggesting that various other factors, such as the postseismic viscoelastic relaxation, may also influence the deformation. We simulated the postseismic viscoelastic relaxation deformation caused by the 1950 Mw 8.7 Assam earthquake endured as the largest intracontinental earthquake ever recorded. The results indicate significant postseismic viscoelastic deformation due to the Assam earthquake even after 65 years. The interseismic convergence rates across the Mishmi thrust and the Arunachal Himalaya derived from pure elastic solution may be overestimated if ignoring the viscoelastic relaxation effects. The spatial variations of convergence rates across the central and eastern Himalayas could be partly related to the transient deformation following great Himalayan earthquakes.

Magmatic records of Gondwana assembly and break-up in the eastern Himalayan syntaxis, northeast India

The Indian sub-continent was an integral part of the Gondwana supercontinent with multiple magmatic episodes during the Gondwana assembly and break-up events. However, most of these vital records to understand the past magmatism were obliterated during the Himalayan orogeny due to the subduction of the Indian plate. In this contribution, we attempt to tackle this issue by investigating the Abor magmatic rocks from the eastern Himalayan syntaxis, which are likely to represent the leftover fragments of the eastern Gondwana continental margin. This study uses zircon U-Pb dating, whole-rock geochemistry, and Sr-Nd isotopic ratios data of the mafic intrusive and felsic volcanic rocks of the Abor magmatism. The mafic intrusive rocks have zircon ages of 500–473 Ma, while the felsic rocks yield ages of 145–132 Ma, indicating two temporally separated episodes of magmatism. The mafic intrusives are sub-alkaline/tholeiitic (Nb/Y < 0.65), with high TiO2 (1.63–3.42 wt%) and ocean island basalt to enriched-mid oceanic ridge basalt affinities. A relatively narrow range of initial 87Sr/86Sr (0.703887–0.705513), 143Nd/144Nd (0.511978–0.512118), and εNd(t) (-0.323–+2.43) of the mafic intrusives suggest fractional crystallization with negligible crustal contamination, generated by low degree (∼3–13 %) partial melting of a primitive mantle (garnet and spinel lherzolite). The felsic rocks display low MgO (0.38–1.17 wt%), CaO (1.06–5.31 wt%), LREE and LILE (Rb, K, Pb) enrichment, depletion in HREE, Sr, Nb, Ti, with strong negative Eu-anomaly (0.48–0.73), high initial 87Sr/86Sr (0.707878–0.717650), and negative εNd(t) (-14.35 to −9.21), suggesting A-type felsic magmatism. The older mafic intrusions were thus attributed to the events of the Gondwana assembly and were inferred to form in an extensional passive margin during the early Paleozoic. However, the younger felsic rocks were likely to have been generated by the interaction of the upwelling Kerguelen mantle plume and the pre-existing crust during the initiation of the eastern Gondwana break-up during the early Cretaceous. Our new findings reveal that two episodic magmatic events related to the eastern Gondwana assembly and the subsequent Gondwana break-up are responsible for the magmatism in the Siang window of eastern Himalayan syntaxis, northeast India.

Crustal deformation and dynamics of the south-eastern tibetan plateau from stress fields and geodesy

South-eastern Tibet rotates clockwise around the eastern Himalayan syntaxis due to the eastward extrusion of the eastern Tibetan lithosphere and it attracts much attention as a window for studying the dynamics of continental collision between India and Eurasia. Competing geodynamic models to describe the deformation of the eastern Tibet have been the debating subject and the dynamic processes responsible for plateau evolution remain poorly understood, partly because the mechanical state at depth and its relationship with surficial deformation are unclear. In this context, a continuous east-west oriented tensional zone was identified at the south-eastern edge of the Tibetan Plateau based on seismogenic stress field and global positioning system data. The profiles of surficial velocity field reveal a relatively speedy motion belt parallel to the Xianshuihe fault extending from north to south in the rotating region east of the tensional zone. Segmented features of the profiles around the extensional zone indicate its close relation with flow channels delineated from magnetotelluric images. Therefore, the deformation at the south-eastern edge of the Tibetan Plateau is mechanically coupled within the crustal depth. We propose a complex regional kinematics with a localised speedy belt and a continuously extensional zone, where material coupling and deformation compatibility were revealed within the crustal depth.

Low temperature eclogite facies rocks discovered in the Eastern Himalayan Syntaxis: Poly‐cyclic metamorphic evolution and implications

AbstractWe report the first occurrence of poly‐cyclic high‐pressure low‐temperature (HP‐LT) rocks from the easternmost Indus‐Yarlung suture zone, formed during subduction of Neo‐Tethyan oceanic lithosphere. Petrology, mineral composition and P–T pseudosection modelling reveal two low‐temperature eclogite facies metamorphic events with an initial high‐pressure P–T condition of 16.4–18.7 kbar and 510–520°C, exhumation to 10.5–12.0 kbar and 580–590°C and a subsequent second high‐pressure P–T condition of ~16 kbar and ~560°C and exhumation to ≤9 kbar and ≤600°C. This history implies a complex ‘yo‐yo type’ P–T path. In situ monazite dating and textural relationships show that late‐stage exhumation, cooling and garnet breakdown occurred at c. ~25–22 Ma. We interpret the first burial event to represent subduction of the Neo‐Tethys Ocean at the eastern Indus‐Yarlung suture zone. Initial exhumation, reburial and final exhumation represent material transport in a large‐scale convective circulation system in the subduction channel. Convective overturn in the subduction channel evidently serves both as a mechanism to produce poly‐cyclic metamorphism and to exhume LT eclogite facies rocks.