Search

Abstract: The Qiangtang Basin, located in central Tibet, is a thick and widely developed Jurassic marine sedimentary stratum, and it is the largest marine basin on the Chinese mainland without a breakthrough in oil or gas exploration. Various forms of crustal movement related to the convergence between the Eurasian and Indian plates in the Cenozoic have played significant roles in the formation and preservation of the oil and gas resources of the Qiangtang Basin. To determine the shallow crustal structure of the Qiangtang Basin and forecast its prospects for oil and gas extraction, we reprocessed the seismic reflection data (0–6s TWT) from 11 reflection sections acquired at different times by different groups and connected them to form a 350-km shallow seismic reflection profile across the Qiangtang Basin. This profile provides reliable data on the north to south changes of the basement and the upper crustal structures of the Qiangtang Basin. We speculated that the reflective events at 3–4s TWT in the Qiangtang Basin represent the Paleozoic basement, which runs shallow beneath the central anticline. The location of the Proterozoic basement was determined from the discontinuous reflection events at approximately 4.5s TWT. The data indicate that the basements of the Qiangtang Basin are deeper in the south and shallower in the north. The shallow crustal deformations (approximately 0–3s TWT) are quite different between the north and south Qiangtang Basin. In the north Qiangtang Basin, there are strong fold deformations alternating between uplifts and depressions, while the deformations are relatively flat in the south Qiangtang Basin. Continuous arc reflections, interpreted as Paleozoic strata, were found beneath the central anticline of the Qiangtang block. A half graben on the north side of the central anticline represents a possible location for oil and gas resources. [Copyright &y& Elsevier]

Abstract: From October 2009 to May 2010, through the support of the SinoProbe project a series of deep seismic reflection profiles were conducted. In order to image deep crustal structure of central Tibet, these profiles extend from the northern Lhasa terrane to the Qiangtang terrane by crossing the Bangong–Nujiang suture (BNS). The Moho depth varies from 75.1km beneath the northmost Lhasa terrane to 68.9km beneath southmost Qiangtang terrane. There is a 6.2km sharp Moho offset across the BNS. Within 25km to the north of the BNS, the Moho rises smoothly to 62.6km. Distinct Moho reflector lies at 62.6–67.3km beneath the Qiangtang terrane. The boundary of the middle and upper crusts and that between the middle and lower crusts interpreted from INDEPTH velocity model correspond to the strong reflection horizons at about18.8km and 31.3km depth beneath south Qiangtang terrane, respectively. A north-dipping series of reflection packages in the mid- to lower crust may mark subduction of the Lhasa terrane beneath the Qiantang terrane. The central portion of the reflection profile exhibits an antiformal structure at 14.1km in the upper crust, which corresponds with the blueschist-bearing metamorphic belt. [Copyright &y& Elsevier]

• Integrated broadband and dense-nodal seismic-array investigation in northern Tibet. • Revealing significant Moho offsets indicative of deep thrust shear zones. • Imaging stark contrasts across the Haiyuan Fault indicative of strain partitioning. • Northern Tibetan plateau margin was constructed by a lithospheric-scale thrust-wedge. • Our results favor an overall oblique continental subduction model. The Tibetan plateau is one of the best field laboratories to investigate mechanisms of continental-lithosphere deformation, which contrasts sharply with the typical rigid-plate behavior of the oceanic lithosphere. Several end-member processes have been proposed for the plateau formation involving vertically coherent pure-shear shortening, decoupled upper- and lower-crustal shortening, oblique continental subduction, and crustal channelized flow. In order to test the above models, we conducted an integrated broadband and dense-nodal seismic-array investigation along two traverses across the northern Tibetan plateau margin in the Qilian Shan and its foreland region. The integrated observation provides the first coherent structural image of the lithosphere in general and shallow crust correlative to surface geology in particular in northern Tibet with an updated resolution. Our seismic imaging shows that the surface-exposed south-dipping Qilian Shan frontal thrust zone is closely associated with a low-velocity zone with concentrated seismicity in the upper crust, and along an inclined convertor can be extended downward into the middle-lower crust near a >10-km Moho offset beneath the Haiyuan Fault zone. This dipping feature and an inferred lithospheric detachment zone with oriented anisotropic fabrics constitute a lithospheric-scale thrust zone that bounds crustal-scale thick-skinned thrusts above, coupled with the development of distributed thrust shear zones that cut across the lowermost crust and uppermost mantle. We interpret the bounding lithospheric-scale thrust zone as the basal thrust of a thrust-wedge system across the northern Tibetan plateau margin. A minimum ∼26% bulk shortening strain is accommodated in the thrust-wedge system, which is compatible with the surface-geology constraints. Together with magnetotelluric data, our seismic imaging reveals stark contrasts in the crustal structure and rheology across the left-slip Haiyuan Fault. The involvement of strain partitioning across this regional structure in the thrust wedge supports the earlier proposed transpressional continental underthrusting of North China beneath the Tibetan plateau. [ABSTRACT FROM AUTHOR]

The Great Xing’an Range is characterized by a complex crustal history involving the successive development of several Paleozoic to Mesozoic tectonic domains. A set of new different geophysical data, including deep seismic reflection profile, seismic refraction/wide-angle reflection profile, and magnetotelluric survey, were conducted recently to establish the lithosphere structure and geodynamic framework of the Great Xing’an Range and adjacent basins. Results based on the interpretation of the integrated geophysical data include the discovery of the “crocodile” style assembly fabric of the Songnen and Xing’an blocks; the intrusion of the Great Xing’an Range continental crust into the Songnen block while the mantle lithosphere of the Songnen block beneath the Xing’an block. The Great Xing’an Range experienced the delamination and the underplating, which is very common in the eastern part of Central Asian Orogenic Belt. The building of the Great Xing’an Range could divide into two phases: (a) the crustal shortening and overlapping resulted from the compression between the Songnen and Xing’an blocks in late Devonian to early Carboniferous; (b) the vertical building by the upwelling and underplating of the magmas in Mesozoic and Cenozoic. [ABSTRACT FROM AUTHOR]

Abstract: The 2008 Wenchuan earthquake and aftershocks occurred along the northeast-trending Longmenshan fault zone in the eastern margin of the Tibetan plateau. The Tibetan plateau has the strongest negative Bouguer gravity anomaly zone in China and is surrounded by the great gravity horizontal gradient belt. The horizontal gradient belt of the observed gravity anomaly in the Longmenshan area is a part of this giant gravity gradient belt. The Longmenshan fault zone is located to the east of this belt. The horizontal gradient belt of the residual gravity anomaly, obtained by removing large effects of sedimentary basin and variations in the crustal thickness, well matches the Longmenshan fault zone. But this belt is located to the east of the horizontal gradient belt of the observed gravity anomalies. The deviation of the two horizontal gradient belts increases from the southwest to the northeast with a maximum of about 40–50km. A significant difference in density exists in the lower crust and the uppermost mantle between the Songpan-Ganzê block and the Sichuan basin block. The Songpan-Ganzê block is less dense than the Sichuan basin block in the lower crust as well as in the uppermost mantle. The boundary between the two blocks is located to the west of the Wenchuan-Maoxian, Yinxiu-Beichuan, and Anxian-Guanxian faults approximately. The fault plane crosses the lower crust and uppermost mantle. The rigid Sichuan basin block acts as a resistant for the pushing from the Songpan-Ganzê block. Far-field effects of the collision between the Indian and Eurasian plates, might lead to thrust of some brittle layers in the upper crust along the detachment, in the middle crust of the Songpan-Ganzê block. When movement on a large and deep crustal mega-thrust occurs, earthquakes strike the Longmen Shan margin of the Tibetan Plateau. In the Guanxian-Beichuan segment in the southern Longmenshan fault zone, push from the Songpan-Ganzê block is perpendicular to the density boundary, which corresponds to the location of the horizontal gradient belt of the residual gravity. But in the Guanxian-Beichuan segment in the northern Longmenshan fault zone, the push is oblique to the location of this gradient belt. Such a relationship could lead to nearly equal amounts of thrust and right-lateral slip along the Beichuan-Qingchuan segment, and in contrast to nearly pure thrust along the Guanxian-Beichuan segment. An unusually large negative magnetic anomaly occurs around Qingchuan-Wenchuan-Guanxian. The computed magnetic model shows that some crystalline complexes with reversal magnetization might exist in the crust, which might be related to extrusion of the metamorphic basement rocks in the lower to middle crust. [Copyright &y& Elsevier]

• The causes of the error of the electrical and geographic locations of the IYS. • The division of the Lhasa terrane according to the electrical structure. • The double-sided subduction of the Bangong-Nujiang Tethyan Ocean (BNT) Ocean. • The smooth subduction of the Longmu Tso-Shuanghu Tethyan Ocean (LST). The Himalaya-Tibetan Plateau that is situated in the southwest China is an ideal natural field laboratory for continental dynamics. In this study, through the careful processing and analyzing of the magnetotellurics data, the reliable 2-D electrical model was obtained to examine the relationship between the lithospheric electrical structure and dynamics in the central Himalaya-Tibetan Plateau. Six conductors mainly result from the partial melting and aqueous fluids, and three resistors are related to different tectonic evolution in the model. Combined with other observations, we summarize that the upwelling hot material (partial melting), aqueous fluids, subduction erosion and crushed zone contribute to the error between the electrical location and the geographic location of the Indus-Yarlung Zangbo suture. The subducted Indian plate is shown by the different angles and different locations of its frontier in different areas. The electrical structure beneath the Lhasa terrane corresponds well with the geological features of an old, locally reworked crust dominantly in the central Lhasa terrane with two young juvenile crustal blocks surrounding the central terrane, and it also shows the relationship among the dynamics, magmatism and mineralization in the Gangdise metallogenic belt. Meanwhile, we indicate that the subducted polarity of the Bangong-Nujiang Tethyan Ocean lithosphere was double-sided, and the smooth subduction of the Longmu Tso-Shuanghu Tethyan Ocean lithosphere may cut through the Moho surface approximately 20 km. Above all, these results can provide helpful constrains to understand the evolution mechanism of the Himalaya-Tibetan Plateau. [ABSTRACT FROM AUTHOR]