Eastern Himalayan Syntaxis Research Articles

Recent large and strong earthquakes in the eastern Himalayas: An appraisal on seismotectonic model

Since 1934, two great earthquakes (Mw ≥8.0) and some eight large/strong earthquakes (Mw 6.5–7.8) occurred in the eastern Himalayas, from eastern Nepal to the Eastern Himalayan Syntaxis (EHS) Zone. All these events are instrumentally recorded by local as well as by the global network. From west to east these events are the 2015 two large/devastating earthquakes (Mw 7.8 and 7.3, respectively) in eastern Nepal, the 1934 and 1988 Bihar–Nepal earthquakes (Mw 8.1 and 6.8, respectively), the 2011 Sikkim (Mw 6.9), the 1967 and 2009 Bhutan (Mw 6.5 and 6.3, respectively), 1964 and 1947 Arunachal–Tibet (Mw 6.5 and 7.0, respectively), and the 1950 Assam–Tibet great (Mw 8.4) earthquakes. Most recently, on 28 April 2021, an earthquake Mw 6.1 occurred in the Assam foredeep. These events are documented as shallow (0–20 km) Main Himalayan Thrust (MHT) earthquakes in the International Seismological Center and National Earthquake Information Center—USGS catalogues. With the advent of local digital seismic networks and better precision of hypocentre locations, the eastern Himalayan earthquake source zones are found to be much different; these are not typically MHT events. Precise hypocentre locations indicate bimodal seismicity at shallower (0–20 km) as well as at deeper depths (40–70 km) in the eastern Himalayas. Seismic cross‐sections and fault plane solutions of the earthquakes do not support a uniform seismotectonic model for the entire Himalayas. All the instrumentally recorded past great earthquakes in the Himalayas and the recent destructive earthquakes in the eastern Himalayas study region are critically examined in view of the known seismotectonic model, and we suggest that each large/great earthquake occurred in its own unique complex tectonic environment. The transverse structures play a major role in generating the deeper earthquakes in the eastern Himalayas and the EHS; the shallower MHT may be flattened.

Comprehensive interpretation of the Sedongpu glacier-related mass flows in the eastern Himalayan syntaxis

Comprehensive interpretation of the Sedongpu glacier-related mass flows in the eastern Himalayan syntaxis

A modern pulse of ultrafast exhumation and diachronous crustal melting in the Nanga Parbat Massif

We combine monazite petrochronology with thermal modeling to evaluate the relative roles of crustal melting, surface denudation, and tectonics in facilitating ultrafast exhumation of the Nanga Parbat Massif in the western Himalayan syntaxis. Our results reveal diachronous melting histories between samples and a pulse of ultrafast exhumation (9 to 13 mm/year) that began ~1 Ma and was preceded by several million years of slower, but still rapid, exhumation (2 to 5 mm/year). Recent studies show that an exhumation pulse of similar timing and magnitude occurred in the eastern Himalayan syntaxis. A synchronous exhumation pulse in both Himalayan syntaxes suggests that neither erosion by rivers and/or glaciers nor a pulse of crustal melting was a primary trigger for accelerated exhumation. Rather, our results, combined with those of recent studies in the eastern syntaxis, imply that larger-scale tectonic processes impose the dominant control on the current tempo of rapid exhumation in the Himalayan syntaxes.

Spatial and temporal distribution of landslide-dammed lakes in Purlung Tsangpo

Reconstructing the ages and magnitudes of prehistoric extreme events that are recorded in landforms and sediments allows us to capture the full range of frequency-magnitudes and estimate the expected time for the repetition of a catastrophic event. The Eastern Himalayan Syntaxis and its surrounding areas are highly prone to the occurrence of river blocking. Previous studies have re-established three glacier-dammed lakes in the valley of Yurlung Tsangpo, namely, Gaga Lake, Dazhuka–Yueju Lake and Jiedexiu Lake. Here, we identified a series of sedimentary evidence containing offshore clay or silt varve, nearshore vaguely stratified silty sand and subaqueous foresets, which are associated with five landslide-dammed lakes in the lower reach of Purlung Tsangpo, including Lulang (LLDL), Dongjiu (DJDL), 102 (102DL), Jialong (JLDL) and Zedang (ZDDL). By assuming an initial catastrophic drainage, the highest elevation of exposures and original dam heights derived from dam morphology were used to represent the minimum and maximum estimates of lake levels of individual lakes. The maximum estimate of lake volume ranges from 4.3 × 109 m3 (LLDL) to 1.1 × 108 m3 (ZDDL). The 14C dating results provide the chronology of the LLDL from 24.2 ka BP to 15.4 ka BP. DJDL from 11.5 ka cal. BP to 7.0 ka cal. BP, JLDL from 4.9 ka cal. BP to 2.7 ka cal. BP. ZDDL from early Holocene to 5.8 cal. BP. Most of them occurred during deglaciation. We further used low-temperature thermochronological data to illustrate the spatial pattern of fluvial incision rates. Overall, climate-driving factors, such as rainfall intensity along with enhanced river incision post LLGM, preconditioned the occurrence of the abovementioned landslides.

Slip distribution and block rotation of the Indo-South China region inferred from GNSS analyses

Indo-South China lying between the Indian Plate and the rigid South China Block is characterized by intense crustal deformation with severe seismic hazards. How deformation is partitioned through slip on various structural features provides key insights into understanding the regional geodynamic processes. Based on the up-to-date GPS velocity data together with the existing active tectonic, seismological, and geophysical data, we find that the relative motion between India and South China is mainly accommodated by two parts: ∼47 mm/yr dextral slip and 15–18 mm/yr contraction to the west of the Shan Plateau, and ∼ 10 mm/yr sinistral slip and ∼ 10 mm/yr extension to the east of it. The Burmese Range takes up ∼ 22 mm/yr dextral slip and 10–13 mm/yr contraction, while the Sagaing Fault accommodates ∼ 18 mm/yr dextral slip and 5–6 mm/yr contraction. The Shan Plateau Shear Zone absorbs ∼ 8 mm/yr dextral slip and 5–6 mm/yr extension. The Chuandian Terrane from east of the Red River Fault to the Xiaojiang Fault manifests 8–10 mm/yr sinistral slip and 5–6 mm/yr extension. The block motion inversion reveals that the clockwise rotation and deformation are caused by the northeastward motion of the India Plate and the clockwise lithospheric flow around the Eastern Himalayan Syntaxis. The Shan Plateau Shear Zone may be a newly formed dextral shear zone at depth of the middle and lower crust, driving the sinistral slip and extension along the ENE-trending faults in upper crustal level. The Shan Plateau Shear Zone might have replaced the Red River Fault as the western boundary to cause the southeastward coherent movement of the Chuandian Terrane. The complex pattern of active tectonic deformation in the region results from interactions among hyper-oblique subduction beneath the Burmese Range, clockwise rotation of lithospheric material around the Eastern Himalayan Syntaxis, and the apparently rigid Sundaland Block separating the two systems in between.

Spatial variability of earthquake hazard parameters, return periods and probabilities of earthquake occurrences in the eastern Himalayan seismic belt

We present a brief atlas of the geographical distribution of earthquake hazard parameters, return periods and probabilities of exceedance of medium to large earthquakes in a hazardous region of the eastern Himalayan seismic belt. A maximum likelihood procedure is applied for the purpose that allows earthquake data during 1500–2019 containing the mixed files of both the historical as well as instrumental era. The results reveal a high activity rate (λ) in eastern Nepal and Arunachal Himalaya, and a low value in the Bhutan-middle Arunachal-Assam Himalaya. A high b-value (or β-value) associated with eastern Nepal and its vicinity, Shillong Plateau and eastern Arunachal Himalaya reveals low crustal stress, while a low b-value associated with Bhutan, middle Arunachal, western Assam and east of the Lhasa reveals high differential crustal stress. The high values of Mmax (Mw > 8.0) are perceived in eastern Nepal, eastern Arunachal Himalaya, Shillong Plateau and south of Lhasa in Tibet. The return periods and probability of exceedance for magnitude Mw 6.0, 6.5 and 7.0 during the next 10-, 20- and 50-years are calculated and correlated with the prevailing tectonics and seismicity of the area. The low return periods (10–20 years) and high probabilities (>0.8) for magnitudes Mw 6.0–6.5 are associated with eastern Nepal, Shillong-Kopli Fault-Arunachal Himalaya, Eastern Himalayan Syntaxis and Yadong-Gulu rift in Tibet revealing potential regions for future earthquake hazard. These hazard maps are highly useful to prepare earthquake hazard mitigation plan in high hazard regions.

India (Tethyan Himalaya Series) in Central Myanmar: Implications for the Evolution of the Eastern Himalayan Syntaxis and the Sagaing Transform‐Fault System

AbstractIn the Katha Range of central Myanmar, lithologic tracers and pressure‐temperature‐deformation‐time data identify Cambro‐Ordovician, Indian‐affinity Tethyan Himalaya Series, located ∼700 km from their easternmost outcrop in S‐Tibet, and ∼450 km from Himalayan rocks in the Eastern Himalayan Syntaxis. Metamorphism began at ∼65 Ma, peaked at ∼45 Ma (∼510°C, 0.93 GPa), and exhumation/cooling (∼25°C/Myr) occurred until ∼30 Ma in a subduction‐early collision tectonic setting. When the Burma microplate—part of the intra‐Tethyan Incertus arc—accreted to SE‐Asia, its eastern boundary, the southern continuation of the Indus‐Yarlung suture (IYS), was reactivated as the Sagaing fault (SF), which propagated northward into Indian rocks. In the Katha rocks, this strike‐slip stage is marked by ∼4°C/Myr exhumation/cooling. Restoring the SF system defines a continental collision‐oceanic subduction transition junction, where the IYS bifurcates into the SF at the eastern edge of the Burma microplate and the Jurassic ophiolite‐Jadeite belts that include the Incertus‐arc suture.

Mechanism of crustal thickening and exhumation of southern Lhasa terrane during the Late Cretaceous: Evidence from high-pressure metamorphic rocks of the Eastern Himalayan Syntaxis

AbstractThe mechanism of Late Cretaceous crustal thickening and exhumation of the southern Lhasa terrane is critical for understanding the tectonic evolution of the Tibetan Plateau. High-pressure metamorphic rocks from the lower crust are good candidates for addressing this issue. In this study, we focus on Late Cretaceous, high-pressure, garnet-bearing amphibolites from the Nyingchi Complex of the Eastern Himalayan Syntaxis and present an integrated study of geochronology, petrography, mineral chemistry, and thermodynamic modeling. Petrographic data determine three metamorphic stages (M1–M3). The M1 stage is characterized by a peak mineral assemblage of garnet + hornblende + albite + rutile + muscovite + quartz, which is followed by a post-peak (M2) assemblage of garnet + hornblende + plagioclase + epidote + biotite + rutile + quartz. The late retrograde stage (M3) is defined by hornblende + plagioclase symplectites surrounding garnet porphyroblasts. Mineral chemistry, with thermodynamic modeling, constrains the P-T conditions of the M1–M3 stages to 14–19 kbar/660–720 °C, 8–10 kbar/650–660 °C, and <7 kbar/<600 °C, respectively. Metamorphic zircons yield a concordant age at 90 Ma, which indicates the formation of garnet-bearing amphibolites. These results indicate a P-T-t path involving near-isothermal decompression for garnetbearing amphibolites, which suggests that the Nyingchi Complex underwent peak-pressure metamorphism (M1) at 90 Ma, followed by rapid exhumation to the depth of 32–26 km along the subduction channel. Moreover, the garnet-bearing amphibolites are considered to be the product of high-pressure metamorphism of mafic crust at the base of the Gangdese belt. Hence, the crust of the Gangdese belt experienced significant crustal thickening of up to 60 km at 90 Ma.

Crustal thickness and Poisson’s ratio variations in the Siang Window and adjoining areas of the Eastern Himalayan Syntaxis

We investigated the average crustal thickness and Poisson's ratio (σ) at 8 broadband seismological stations established in the Siang Window of the Eastern Himalayan Syntaxis. The H-k stacking method has been adopted on receiver functions of teleseismic earthquakes recorded by the seismological stations. The study reveals the variation of crustal thickness from ∼38 km in the Brahmaputra Valley (Pashighat) to ∼53 km at the northern boundary of the window. The estimated Poisson’s ratios show significant variation from 0.23 to 0.308 in the window. The average crustal Poisson’s ratios (σ) estimated at each station are divided into three categories, low (σ ≤ 0.25), intermediate (0.25 ≤ σ ≤ 0.27), and high (σ ≥ 0.27). The wide variations in σ can be attributed to the local geology. The extremely low σ at Along (0.23) can be explained by the presence of SiO2 rich quartzite in the vicinity, while the extremely high σ at Gelling (0.308) suggests the presence of partially serpentinized rocks in the ophiolitic mélange. The available data of Poisson’s ratios and Moho depths in and around the study region has been plotted to portray regional variations. The Siang Window is seismically active up to ∼40 km crustal depth.

Petrogenesis and Tectonic Implications of the Latest Cretaceous Intrusive Rocks from the Eastern Gangdese Belt, Southeast Tibet

AbstractThe latest Cretaceous magmatic activity in the eastern segment of the Lhasa terrane provides important insights for tracking the magma source and geodynamic setting of the eastern Gangdese batholith, eastward of eastern Himalayan Syntaxis. Detailed petrological, geochemical and geochronological studies of the intrusive rocks (monzodiorites and granodiorites) of the eastern Gangdese batholith are presented with monzodiorites and granodiorites giving zircon U–Pb crystallization dates of 70–66 Ma and 71–66 Ma with εHf(t) values of –4.8 to +6.2 and –1.9 to +5.3, respectively. These rocks are metaluminous to weakly peraluminous I‐type granites showing geochemically arc‐related features of enrichment in LREEs and some LILEs, e.g., Rb, Th, and U, and depletion in HREEs and some HFSEs, e.g., Nb, Ta, and Ti. The rocks are interpreted to be derived from partial melting of mantle material and juvenile crust, respectively, which are proposed to be triggered by Neo‐Tethyan slab rollback during northward subduction, with both experiencing ancient crustal contamination. The studied intrusive rocks formed in a transitional geodynamic setting caused by Neo‐Tethyan oceanic flat subduction to slab rollback beneath the eastern Gangdese belt during the latest Cretaceous.

Signature of Himalayan orogenic features in Brahmaputra River sediments, Bangladesh: Evidence from single-grain heavy mineral chemistry

The present study describes results obtained from the chemistry of detrital heavy minerals i.e. pyroxene, amphibole, biotite, garnet, epidote and Fe-Ti oxides in fluvial sediments of the northern Brahmaputra River (Bangladesh) with an aim to determine conditions of their petrogenesis and provenance. The primary and secondary genera of ferromagnesian minerals occurred in calc-alkaline and peraluminous subduction zone. In which, the garnets are Fe-rich, indicating mostly almandine component (Alm65–Pyp16–Grs8–Sps6 averagely), occurred in medium to high grade metasedimentary rocks in the Lesser Himalaya (LH), along the Main Central Thrust (MCT) and the eastern Himalayan syntaxis. Besides, the fingerprint of omphacite and actinolite owe to ascertain the co-existence of garnet developed in ultrahigh-pressure (UHP) eclogites that may also be drained from the Tso Morari massif. Augite to aegirine-augite pyroxenes emphasizes Fe enrichment in basaltic systems and high to ultrahigh grade metamorphic rocks, which are exposed in the LH, Shillong Plateau, Mikir Hills, South Tibetan Detachment System (STDS), eastern Himalayan syntaxis and Tso Morari massif. Geochemistry and thermobarometry of the primary magmatic amphiboles and biotites manifest the source of granitoid and granodiorite like bodies, and their windows are exposed in the Bomi–Chayu, Gangdese arcs and the western Arunachal Himalaya. Again, metamorphosed Fe-Ti oxide minerals are well-exposed along the NE Lesser Himalaya, where magmatic derivative of Fe-Ti oxide minerals were modified through the diffusional processes in low-grade metamorphism (534–562 °C with 10–22.1–10−21.5fo2). Integrating the aforementioned discussion with the thermochronology, it is evident that the eastern Himalayan syntaxis is the major source of sediment flux, which is carried mostly by the upper Himalayan tributaries i.e. Yigong, Parlung, Dibang and Lohit. Also, the lower Himalayan tributaries i.e. Subansiri and Manas drain the sequestered derivatives dominantly from the Arunachal Himalayan. Tso Morari eclogites (NW Himalaya) have also contribution somewhat of dense minerals to the Tsangpo-Brahmaputra River system. Thus, scrutinizing the fingerprint of single-grain detrital minerals provides key information regarding the source terrains and tectonics of the Himalayan sequences.

Seismicity and magnitude recurrence hazard assessment in Eastern Nepal, Northeast India and Tibet Himalaya

The regions of eastern Nepal, northeast India and Tibet Himalaya (26°-30°N, 85°-97°E) are one of the most earthquake hazardous areas in the Indian subcontinent. The seismicity of this region is analyzed using Gumbel's third asymptotic distribution (GIII) method of extreme values to assess the earthquake hazard. A uniform and complete seismic data covering the time span of 120 years (i.e. 1900-2019) is used for the appraisal of earthquake magnitude recurrence in different return periods. The expected largest earthquake magnitudes during the next 20 (M20), 50 (M50) and 100 (M100) years with their 90% probability of not being exceeded (PNBE) are spatially mapped to create a brief atlas of short- and long-term regional earthquake magnitude recurrence hazard maps. The obtained results of maximum magnitude during 20, 50 and 100-years are expected to exceed the values of Mw 7.1, 7.7 and 8.2, respectively in two prominent blocks of eastern Nepal and Arunachal Himalaya in the examined region. These blocks are also more prominent for the results of 20, 50 and 100 years earthquake magnitudes with 90% PNBE i.e. return periods of 190 (M190), 475 (M475) and 950 (M950) years along with two additional blocks in the Eastern Himalayan Syntaxis (EHS) and Yadong-Gulu rift near Lhasa in the Tibet Plateau. These earthquake magnitudes are expected to exceed Mw 8.5, 8.6 and 8.9 during the next 190, 475 and 950 years, respectively in these blocks. The spatial distribution of earthquake hazards in the region reveals the complex seismotectonic setup and high crustal heterogeneity.

Along‐strike variation in the shear wave crustal structure of the NE Himalayan and Indo‐Burmese arc: Evidence based on surface wave dispersion analysis

Surface wave dispersion data is used to infer the shear wave velocity structure variation in the north‐east Himalaya and Indo‐Burmese arc regions. We have used 25 earthquakes data from four groups with a magnitude range of 5.0–6.7, epicentral distance range 368–800 km, and focal depth less than 50 km. Ray paths from the earthquake location to the seismic station are transversely passing different geotectonic units of the Himalayas, Indo‐Gangetic plains, and Indo‐Burma collision zones. The weighted average dispersion curve and the path averaged shear wave velocity models are computed for the four groups located at different azimuths around Shillong seismic station. Non‐linear least‐square inversion is performed to obtain the shear wave velocity structure of the crust and uppermost mantle from joint inversion of Rayleigh and Love waves group velocities. Subsequently, Rayleigh and Love waves group velocities are inverted separately to obtain vertical and horizontal components of S‐wave velocity. Next, using these components anisotropy coefficient values at different depths for each path are also estimated. A high variation of dispersion curves and the shear wave velocity models from one group to another indicates that the region is geotectonically very complex. An approximately 80 km‐thick zone beneath the study region has shear wave velocity as low as 1.7 km/s in the uppermost crust in the southern part and ~4.7 km in the uppermost mantle beneath the Eastern Himalayan Syntaxis (EHS). Inferred velocity is also lower than that of PREM and AK135 global models with a much thicker crust beneath the study region. Radial anisotropy varies even within the northern part from the Indo‐Eurasian collision zone to EHS and northern to southern Indo‐Burma ranges. Anisotropy is comparatively stronger in the deeper part below ~40 km for the three paths, except for the EHS, where the result is contrary.

Tomographic Imaging of the Plate Geometry Beneath the Arunachal Himalaya and Burmese Subduction Zones

AbstractWe present 3‐D P and S‐wave velocity perturbation maps of the upper‐mantle beneath eastern Himalaya and Burmese subduction zones at 2° × 2° lateral resolution, using teleseismic travel‐time tomography. Collated waveforms of 5,727 teleseismic earthquakes recorded at 273 seismic stations generated an accurate data set of 86,570 P‐arrival and 85,227 S‐arrival times using multichannel cross‐correlation. Tomograms enabled imaging of the structural features down to 1,000 km depth. The subducting Indian lithospheric plate extends up to Bangong‐Nujiang Suture Zone, overturns and descends steeply beyond 200 km below the Himalayan arc. A southward plunging detached slab can be traced beyond 600 km. Results reveal no evidence for the detachment of a S‐E deflecting Indian lithospheric slab below the Burmese arc. The geometry of the slab mimics regional seismicity (up to 200 km) followed by a deeper aseismic zone related to petrological changes. No gaps are seen between the northward and eastward subducting Indian plate, hence confining the eastward escape of Tibetan‐lithospheric material between the Eastern Himalayan Syntaxis and Sichuan Basin.

Upper Crustal Collapse Reconstructed the Topography and Remodeled the Fault System of the Chuandian Fragment in the Southeastern Edge of the Tibetan Plateau, Evidenced by Anisotropy of Magnetic Susceptibility Data Sets

AbstractThe steps of the southeastward stepwise‐descending topography of the Chuandian Fragment (CDF) in the southeastern edge of the Tibetan Plateau coincide with N‐S and NE‐SW trending faults that changed from thrusting to normal activity, indicating that since the Middle‐Late Miocene the CDF maintained an E‐W oriented extensional tectonic environment. However, the anisotropy of magnetic susceptibility (AMS) data sets presented herein, from the Cretaceous and Cenozoic sedimentary strata of the southeastern Tibetan Plateau and its edge, show that since ∼28.0 Ma the CDF remained within a stabilized compressional stress field, oriented NEE‐SWW. A comprehensive analysis of the AMS results, topography, fault systems, and geophysical observations indicate that the northward‐advancing eastern Himalaya syntaxis impeded the southeastward‐flowing viscous lower crust and correspondingly decreased its flux, which could have reduced the thickness of the lower crustal channel beneath the CDF. The original low‐gradient topography of the CDF formed by the viscous lower crustal flow could no longer be supported by the thinned lower crustal channel, and thus gravity drove the collapse of the uplifted CDF. This process reconstructed the topography and remodeled the preexisting fault systems of the CDF, resulting in the coexistence of the E‐W oriented extensional tectonic environment and the stabilized NEE‐SWW oriented compressional stress field of the western part of the CDF, since the Middle‐Late Miocene. Thus, the change in the status of the lower crustal channel related to block interactions provided a major geodynamic setting for shaping the topography of the southeastern edge of the Tibetan Plateau.

The Early Cretaceous Zaduo Granite, Eastern Qiangtang Terrane (China)—An Attempt to Constrain its Paleolatitude and Tectonic Implications

The Eastern Qiangtang Terrane is an orogenic-like belt around the Eastern Himalayan syntaxis (EHS). The deformation history of this terrane must be known to understand how the EHS region responded to the Lhasa-Qiangtang collision and the closure of the Bangong-Nujiang Ocean (BNO). Here, we present a new paleomagnetic investigation on an Early Cretaceous granite (∼126 Ma) in the Zaduo area, Eastern Qiangtang Terrane. Petrographic observations reflect crystallization from primary melts with only limited subsequent alteration (chloritization of biotite). Magnetite appears to be the dominant carrier of the characteristic remanent magnetization (ChRM) based on stepwise demagnetization of the natural remanent magnetization, supplemented by detailed rock magnetic measurements, including magnetization versus temperature, and acquisition curves of the isothermal and anhysteretic remanent magnetization. End-member modeling of those acquisition curves helped to constrain the paleomagnetic analysis. The inconsistent demagnetization behavior between alternating field (AF) demagnetization at high levels and thermal demagnetization was attributed to the development of gyroremanent magnetization in the AF demagnetization generated by fine-grained single domain magnetite. The ChRM directions from 92 granite samples in geographic coordinates yield an average of declination (Dg) of 2.6° and inclination (Ig) of 38.6° (precision parameter k = 51.4, and 95% confidence cone α95 = 2.1°). The amount of tilting of the granite is poorly constrained which makes proper correction rather tedious. We compared the expected bedding attitudes (Strikeexp = 43.1°, Dipexp = 46.1°) derived from published data (Huang et al., 1992;Tong et al., 2015) with the average observed bedding attitudes (Strikeobs = 54°, Dipobs = 32°) of the Middle-Upper Jurassic sandstones of the Yanshiping Group that was intruded by the Early Cretaceous granite. The discrepancy between the expected and measured bedding attitudes implies that the strata of the Yanshiping Group in the Zaduo area were already tilted prior to the intrusion of the ∼126 Ma Zaduo granite, which was attributed to the Lhasa-Qiangtang collision and the closure of the BNO. The collision led to a series of geological events, such as the tilting of the strata, the ophiolite emplacement, the development of a peripheral foreland basin, and the magmatic activity gap. The tilting/folding of the strata was generally delayed by the layer parallel shortening processes during the early stages of the deformation, thus suggesting an older Lhasa-Qiangtang collision (i.e., >126 Ma).

Location of the Lhasa terrane in the Late Cretaceous and its implications for crustal deformation

The Cenozoic crustal deformation of the Asian continent is constrained by the location of its southern margin in the Late Cretaceous, which is, however, affected by potential remagnetization and inclination shallowing of redbeds in the Linzhou Basin. Our paleomagnetic results from the Shexing Formation redbeds in the Late Cretaceous yield a mean paleo-inclination of ~21.5°, consistent with the recalculation of previous data. Rock-magnetic, paleomagnetic and petrographic analyses indicate that the redbeds have recorded the original paleomagnetic field and have not suffered from significant inclination shallowing. The paleopole from 70 redbeds sites is at 71.8°N, 281.5°E with A 95 = 2.6°, yielding a paleolatitude of ~12.0 ± 2.6°N at ~70–91 Ma for the Linzhou Basin. This result indicates a latitudinal convergence of 1264 ± 477 km north of the sampling area within Asia since the Late Cretaceous. Combined with previous data, our results show that the southern margin of Asia was at ~11–15°N in the Late Cretaceous, aligning in a WNW-ESE direction, which is similar to its present-day orientation. A review of available data shows that the sharp bending of the southern margin of Asia around the eastern Himalayan syntaxis (EHS) is probably due to different crustal movement patterns around the EHS. The shape of the southern margin of Asia determines the arc-shaped orientation of Himalayan orogen. • The Shexing Formation redbeds have recorded the prefold magnetizatioin. • The southern margin of Asia was oriented in a WNW-ESE direction at ~12°N. • Different movement around the eastern Himalayan syntaxis make a sharp turn. • The southern margin of Asian plays a primary role in shaping the Himalayan orogen.

Elevation of zircon Hf isotope ratios during crustal anatexis: Evidence from migmatites close to the eastern Himalayan syntaxis in southeastern Tibet

The initial Hf isotope composition of zircon in high-grade metamorphic rocks is an important tracer for protolith origin as well as for the growth and evolution of continental crust. However, less attention has been paid to the behavior of Lu-Hf isotopes in peritectic reactions during partial melting of the continental crust. In this paper, we present a combined study of zircon U-Pb ages, Lu-Hf isotopes and trace elements as well as petrology and whole-rock geochemistry for migmatites close to the eastern Himalayan syntaxis in southeastern Tibet. The results show that peritectic zircons in the migmatites have significantly higher 176Hf/177Hf ratios than their protolith zircons. More interestingly, the 176Lu/177Hf ratios and the contents of high field strength elements (such as Nb, Ta, and Hf) and heavy rare earth elements are significantly elevated in the peritectic zircons compared to the protolith zircons. This suggests that garnets with high Lu/Hf ratios and other minerals (such as titanite, ilmenite, amphibole, and biotite) with moderate Lu/Hf ratios were decomposed during crustal anatexis, contributing to the Hf isotope composition of peritectic zircons. This is confirmed by petrographic observations that garnet and biotite occur as residues or remnants in mesosomes (biotite gneiss) and some leucosomes (felsic veins), but are absent in the melanosomes of the target migmatites. Therefore, the peritectic zircons were produced together with anatectic melts through peritectic reactions and acquired the elevated 176Lu/177Hf ratios compared to the inherited zircons from the protoliths if no peritectic garnet was coevally produced during the crustal anatexis. In this regard, the greatly elevated Hf isotope compositions of peritectic zircons in anatectic granites cannot faithfully reflect the Hf isotope composition of parental rocks. As such, great care must be taken when using Hf isotope ratios to trace the nature of parental rocks with respect to magmatic processes and crustal evolution.

Retrospective Analysis and Hazard Assessment of Gega Glacial Lake in the Eastern Himalayan Syntaxis

Retrospective Analysis and Hazard Assessment of Gega Glacial Lake in the Eastern Himalayan Syntaxis

New Evidence of High-Magnitude Flood(S) in the Region of Eastern Himalayan Syntaxis, Southeastern Tibet Plateau

New Evidence of High-Magnitude Flood(S) in the Region of Eastern Himalayan Syntaxis, Southeastern Tibet Plateau