The Namche Barwa Complex (NBC) in the eastern Himalayan syntaxis, south Tibet, is generally interpreted as the north-eastern extremity of the exposed Greater Himalayan Sequence, comprising ...Neoproterozoic to early Paleozoic sedimentary strata along the northern margin of the Indian continent. Field and petrological investigations indicate that the NBC consists mainly of orthogneiss, paragneiss, amphibolites and calc-silicate rocks. U–Pb zircon data demonstrate that the protoliths of the orthogneiss formed during late Paleoproterozoic at ca. 1610
Ma and also in early Paleozoic at ca. 490–500
Ma. The amphibolites were derived from mafic magmatic rocks formed during 1645 to 1590
Ma. Zircons in the paragneisses have highly variable inherited zircon ages ranging from the Neoarchean to early Paleozoic, with four major age populations of 2490
Ma, 1640
Ma, 990
Ma and 480
Ma. The calc-silicate rock has zircons with early Paleozoic metamorphic age of 538
Ma. Almost all the rocks of the NBC have been metamorphosed during Cenozoic with the metamorphic zircon U–Pb ages ranging from 8 to 30
Ma and a peak at 23
Ma. These, together with previous results suggest that the NBC was originally derived from an Andean-type orogeny following the Columbia supercontinent assembly, and experienced multiple reworking during the Grenvillian, Pan-African and Himalayan orogenies. We conclude that the NBC in the eastern Himalayan syntaxis was derived from different provenance and tectonic setting as compared to those of the Greater Himalayan Sequence which constitutes the high-grade metamorphic core of the western and central Himalayan orogenic belt. We thus infer that the NBC was originally part of the eastern segment of the Central Indian Tectonic Zone.
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► The Namche Barwa Complex (NBC) consists of Proterozoic to Paleozoic rocks. ► The NBC experienced the late Paleoproterozoic and early Paleozoic magmatic events. ► The NBC represents part the eastern segment of the Central Indian Tectonic Zone. ► The NBC has been subjected to the multiple tectonic evolution and reworking.
In and around the Eastern Himalaya Syntaxis (EHS), the metamorphic complex and its structural boundaries are key tectonics to investigate the kinematics and thermal evolution of the continental ...collision. We provide new structural, kinematic and geochronological data along the northeastern boundary of the EHS, namely the Jiali strike-slip shear zone. Structures and various kinematic indicators record sinistral strike-slip shearing in the zone. Based on the microstructural features, crystallographic preferred orientation patterns of quartz and their slip systems, coupled with Raman Spectroscopy of Carbonaceous Materials (RSCM) geothermometry, the ductile sinistral shearing fabrics formed at upper greenschist and amphibolite facies (>500 °C). Two hornblende, one muscovite and five biotite separates from the mylonites in the shear zone were dated by the 40Ar/39Ar method. A hornblende plateau age of 23.7 Ma is interpreted as representing or being close to the time of sinistral movement. Muscovite and biotite 40Ar/39Ar ages from 22.9 to 16.9 Ma represent cooling ages following sinistral shearing. It is concluded on the basis of these data that the Jiali sinistral strike-slip shear zone was active in latest Oligocene. This sinistral movement on the Jiali shear zone is considered to link with the Ailao Shan-Red River shear zone, and form a unique, continuous structural boundary around the EHS. The Jiali-Ailao Shan-Red River sinistral shear zone and Gaoligong dextral shear zone formed a huge conjugate strike-slip shear pair around the EHS to accommodate the N–S shortening corresponding to northward indentation of the folded Indian continent. The conjugate strike-slip shear systems also contribute to transporting Tibet southeastward in a limited scale.
•Kinematics of the Jiali strike-slip shear zone documented as sinistral motion.•The Jiali shear zone experienced sinistral strike-slip deformation prior to 24 Ma.•Conjugate strike-slip shear pairs accommodate the indentation of continent.
Newly acquired GPS data along transects across Himalaya in Eastern Himalayan Syntaxis (EHS) reveal a clockwise rotation of rigid micro-plate comprising part of Brahmaputra valley, NE Himalaya and ...Northern Myanmar that rotates about a pole located at 14.5°N, 100.8°E at an angular rate of 1.75±0.12°/Myr. The EHS is being torn-off from the main Indian Plate as a rigid block around which the kinematic clockwise rotation of Tibetan GPS sites toward the Sichuan-Yunnan region occurs in the Eurasia fixed frame. The residual velocity field of the newly acquired data estimated after removing the rotation that minimizes the GPS rates around EHS show a clear NE motion of the EHS sites, indentation of the rigid Indian plate into a less rigid area of the Eurasian plate. The most extensive EHS zones of compression and shortening are in the direction of indenter convergence, with average values ranging between ~50–100 nanostrain/year. Along the frontal segment of EHS, from NW to SE, the shortening rate is reduced from the local maximum value of 160 to ~80 nanostrain/year, thus indicating a possibly locked fault patch of Mishmi or Lohit thrusts, the southernmost part of segment activated during the large 1950 Assam earthquake, Mw 8.6.
An elastic block-model was invoked to infer the average slip rates of sections around EHS and to estimate an average locking depth of ~15km. The slip rate perpendicular to the locked sector of EHS reaches 32.4mm/year and permits to roughly infer a recurrence time of ~200year for an earthquake as energetic as the 1950 Assam event.
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•New GPS data evidence•Rigid clockwise rotation of EHS with respect to India plate•Active indentation of EHS into the Eurasian plate•Differential shortening rate along the frontal EHS, high seismic hazard in its SE sector
Paleoseismological trenching at Kamlang Nagar along the Mishmi Thrust in the Eastern Himalayan Syntaxis, aided by radiocarbon analyses of charcoals yielding ages of 1111–914 B.C. to A.D. 1761–1968 ...infer primary evidences of co‐seismic surface faulting. Correlation with results of an earlier paleoseismological study indicates two successive surface rupturing earthquakes at ∼1800‐years interval, the most recent event very likely corresponding to the 1950 Assam earthquake (Mw 8.6). A dip‐slip displacement of 24.6 ± 4.6 m was estimated along a 25 ± 5°E dipping fault during the 1950 earthquake. Our study suggests dual surface faulting by the 1950 event along two orthogonal fault systems, that is, the Himalayan Frontal Thrust and the Mishmi Thrust. It thus emphasizes that, the seismicity pattern in the Mishmi Range where the locked zone of the Main Himalayan Thrust extends beyond the suture zone, is unlike that of the western and central Himalaya.
Plain Language Summary
We use paleoseismological trenching and charcoal ages ranging 1111–914 B.C. and A.D. 1761–1968 to report the evidence of surface break by the 1950 great Assam‐Tibet earthquake (Mw 8.6) in the Eastern Himalayan Syntaxis. Geological observations and correlation with results from the adjoining trench site suggest two successive surface rupturing earthquakes at an interval of 1800 years in the Mishmi Range.
Key Points
We provide the first evidence of surface rupture by the 1950 great Assam earthquake from the Mishmi Thrust in the eastern syntaxis
We suggest a dual rupture style for the 1950 earthquake along the Himalayan Frontal Thrust and Mishmi Thrust
A slip deficit of ∼1.4 m since 1950 is capable to generate an earthquake of Mw ∼ 7.7 along the Mishmi Thrust in the present day
On the eastern margin of the Himalayan orogenic belt, the rapid uplift of the Namche Barwa metamorphic terrane and significant bending of the Yarlung Zangbo suture zone occur. The formation mechanism ...and dynamics of the Eastern Himalaya Syntaxis (EHS) is still debated. In order to better understand the deep structures beneath the EHS, we deployed 35 broadband seismic stations around the Namche Barwa Mountain. The data were integrated with existing datasets for a 3-D teleseismic P-wave tomography. The results demonstrate complex deep structures and significantly contrasting Indian subduction styles in the eastern Himalaya. In the western region of the EHS, the Indian slab flatly subducts under southern Tibet and might extend to the Bangong-Nujiang Suture. In contrast, a (north)eastward steep subduction occurred in the eastern region of EHS. The contrasting subduction styles result in tearing and fragmentation of the Indian lithosphere between the flat and steep subducting slabs beneath the EHS. Consequently, the hot asthenospheric mantle may rise through the slab window, which might further lead to the rapid uplift of Namche Barwa and the formation of EHS. The lateral variation in subduction/collision mode and slab tearing induced asthenospheric mantle upwelling is similar to that observed in the Hellenide and Anatolide domains of the Tethyan orogen.
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•A 3-D teleseismic P-wave tomography of the Eastern Himalayan Syntaxis•Contrasting subduction modes (flat versus steep) in the eastern Himalaya•Tearing of the subducting Indian slab and upwelling of the hot asthenospheric mantle
The Namche Barwa Syntaxis, as one of the most tectonically active regions, remains an appropriate place to explore the relationship between tectonics, surface processes, and landscape evolution. Two ...leading models have been proposed for the formation and evolution of this syntaxis, including the tectonic aneurysm model and the syntaxis expansion model. Here we use a multi-disciplinary approach based on low-temperature thermochronometry, numerical modeling, river profile and topographic analyses to investigate the interactions between tectonics, erosion, and landscape evolution and to test these models. Our results emphasize the presence of young cooling ages (i.e., <1Ma) along the Parlung River, to the north of the syntaxis. Using numerical modeling we argue that a recent increase in exhumation rate is required to expose these young ages. Our river analysis reveals spatial variations in channel steepness, which we interpret to reflect the rock uplift pattern. By establishing the relationship between erosion rates and topographic features, we find that erosion rates are poorly to weakly correlated with topographic features, suggesting that the landscape is still evolving. Altogether, these results seem better explained by a mechanism that involves a northward expansion of the syntaxis, which causes high rock uplift rates to the north of the syntaxis and a transient state of topography adjusting to an evolving tectonic setting.
•New thermochronometric ages the Namche Barwa Syntaxis are presented.•Rock uplift is required to expose the young ages to the north of the syntaxis.•Variations in channel steepness indicate a S-to-N gradient in rock uplift rate.•Topographic metrics are weakly coupled with erosion rate.•A northward expansion of the syntaxis is inferred.
SUMMARY
Crustal 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.
Late Quaternary river blockage events in the mountainous regions of the southeastern Tibetan Plateau, characterized by steep topographies and deeply-incised valleys, have commonly been triggered by ...tectonic and glacial activity. In this study, we described the geomorphological and sedimentological characteristics of a fluvial-lacustrine sedimentary sequence belonging to a dammed paleolake that formed along the Dongjiu River at the terminus of the Lulang Fault, on the western boundary of the Namche Barwa Syntaxis. Eight main lithofacies associations are proposed that represent four main paleoenvironments: a distal lacustrine environment, i.e. representing a lake distal environment; a lake margin environment; an ice-contact lacustrine environment; and a fluvial environment. 14C and OSL dating results indicate that the paleolake formed at least between ~7.1–3.6 ka. The paleolake covered ~2.27 km2 and contained a water volume of ~0.09 km3, with the water surface lying at an altitude of 2590 m above sea level (asl). The soft-sediment deformations induced by a paleoseismic event in the environs of the Lulang River nearby during the early Holocene, synchronous with the rock avalanche which dammed the Dongjiu River. We would therefore suggest that such a damming event might have been triggered by a paleoearthquake. Steepness index analysis of longitudinal profiles indicates that the paleodam caused by a landslide across the Dongjiu River was unable to preserve the stability of knickpoint, a migration that can potentially be related to a high river erosion rate.
•Dongjiu landslide dammed paleolake triggered by a Holocene earthquake has persisted at least ~7.1–3.6 ka.•Geomorphologic and sedimentological characteristics of the barrier dam and fluvial-lacustrine sediments.•Eight lithofacies associations revealed four main paleoenvironments during the blockage period.•Ksn indicates Dongjiu paleolandslide dam has lost the ability of to control the knickpoint stability.
•Multi-criteria decision making and machine learning models for seismic susceptibility.•Peak ground velocity-based risk assessment will aid an effective mitigation plan.•Integrated methods for ...seismic susceptibility assessment in the active foredeep basins.
The seismic susceptibility and mitigation management is paramount concern in tectonically active area like Northeastern India. This area has been devastated innumerably during the 1950 Assam great earthquake. The present study area falls in the foreland basin (Brahmaputra Valley) of Eastern Himalaya. This region is seismically vulnerable due to the tectonic complexity caused by the convergence of the Eurasian, Indian, and Burmese plates. In such, an area optimal disaster management and preparedness is necessary to define the non-linear character of seismic susceptibility, where population and unscientific urbanization have increased manifold. Therefore, for the present study, various multi-criteria decision making (MCDM) methods such as analytical hierarchy process (AHP), fuzzy-AHP (FAHP), and maximum entropy technique (MaxEnt) have been used for determining the seismic susceptibility, by assigning weightage to nine controlling factors such as: predominant frequency (f0), geology (G), vulnerability index (K), peak amplification (A0), liquefaction potential (LP), groundwater condition (WT), shear wave velocity (Vs30), peak ground acceleration (PGA), and land use/land cover (LU). The MaxEnt model exhibits the highest accuracy (87.5%) when the performance of the models was compared using the receiver operating characteristic curve (ROC) and area under the curve (AUC) value. Further, overlay analysis of best seismic susceptibility model using MaxEnt and PGV-based Japan Meteorological Agency (JMA) intensity shows that 40% the study area is in the very high and high seismic risk zone. In tectonically active areas, this kind of integration work is essential to improves the mitigation strategy and aids urban planners in designing earthquake-resistant buildings.
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