Intracontinental subduction and Palaeozoic inheritance of the lithosphere suggested by a teleseismic experiment across the Chinese Tien Shan (original) (raw)
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Three-dimensional elastic wave velocity structure of the western and central Tien Shan
Journal of Geophysical Research, 1993
Arrival times of compressional (P) and shear (S) waves generated by earthquakes at local and teleseismic distances and recorded by seismographs located in the westem and central Tien Shan are used to determine one-and three-dimensional elastic wave velocity structures of the crust and upper mantle beneath the mountain belt. The best fit one-dimensional structures suggest that the average depth of the Mohorovicic discontinuity in this area is 50 km. The three-dimensional structure of the upper crust reveals thick sediments within each of the major depressions in the region. A 7 km-thick wedge of sediment beneath the Chu Depression is outlined at depth by a south dipping plane of seismic activity, suggesting the presence of an active decollemont. These low velocities extend continuously to the southeast toward Issyk-Kul, suggesting a structural relationship between the two. However, rather than being consumed, it appears that Issyk-Kul is overthrusting the surrounding ranges. The low-velocity sediments in the Fergana basin reach depths of 10 km and are bounded on three sides by amorphous bands of seismicity. Velocities at midcrustal depths generally are lower beneath the central Tien Shan than beneath the western Tien Shan. This pattern becomes more evident in the uppermost mantle, with P velocity contrasts of as much as 10% across a boundary that corresponds roughly to the geographical position of the Talasso-Fergana fault. The low velocities beneath the central Tien Shan exceed 150 km depth but do not appear to be deeper than 300 km depth. There is no evidence for a lithospheric root beneath this part of the range; rather, the low velocities imply the presence of a positive buoyancy force uplifting the mountains. Evidence that this low-velocity region existed before the collision suggests that the Tien Shan may not owe its rejuvenation simply to its location at the northern edge of a strong Tarim basin but rather to an anomalous upper mantle that was easier to deform than the surrounding lithosphere. the Tien Shan can be divided into three fault bounded units [Kravchenko, 1979]. All three units consist largely of sedimentary rocks that formed during the late Proterozoic (in the north) to Cambrian (in the south). These units were accreted onto Eurasia beginning in the early Paleozoic for the
Earth and Planetary Science Letters, 2004
The Tien Shan orogenic belt is the most active intracontinental mountain belt in the world. We describe an 86-km-long N -S-trending deep seismic reflection profile (which passes through the southern Junggar basin) located on the northeastern Tien Shan piedmont. Two distinct anticlines beneath the northern margin of the Tien Shan are clearly imaged in the seismic section. In addition, we have imaged two detachment surfaces at depths of f 7 and f 16 km. The detachment surface at 16-km depth corresponds to the main detachment that converges with the steep angle reverse fault (the Junggar Southern Marginal Fault) on which the 1906 M~7.7 Manas earthquake occurred. A 12 -14-km-thick sedimentary basin is imaged beneath the southern Junggar basin near Shihezi. The crust beneath the northern margin of the Tien Shan is 50 -55-km thick, and decreases beneath the Junggar basin to 40 -45-km thick. The crustal image of the deep seismic reflection profile is consistent with models derived from nearby seismic refraction data and Bouguer gravity anomalies in the same region. The faulting associated with the 1906 Manas earthquake also fits within the structural framework imaged by the seismic reflection profile. Present-day microseismicity shows a hypocentral depth-distribution between 5 and 35 km, with a peak at 20 km. We hypothesize that the 1906 Manas earthquake initiated at a depth of f 20 km and propagated upwards, causing northward slip on the sub-horizontal detachments beneath the southern Junggar basin. Thus, in accord with regional geological mapping, the current shortening within the eastern Tien Shan is accommodated both by high-angle reverse faulting and detachment faulting that can be clearly imaged at depth in seismic reflection data. Published by Elsevier B.V. 0012-821X/$ -see front matter. Published by Elsevier B.V.
Teleseismic P-wave tomography and the upper mantle structure of the central Tien Shan orogenic belt
2007
As the largest ultrahigh-pressure (UHP) metamorphic tectonic unit outcropping in the world, the Dabie-Sulu UHP metamorphic belt is considered to be one of the best areas for studying the continental dynamics. However, their continental collision and exhumation mechanism are still debated. We performed a 3D teleseismic P-wave tomography beneath the Sulu orogen for the purpose of understanding the deep structure. The tomographic results show that there is a prominently near-SN-trending low-velocity zone (LVZ) close to the Tanlu fault (TLF), indicating a slab tear of the subducted Yangtze plate (YZP) during the initial Early Triassic collision. Our results also suggest that both the Yangze crustal slab and the North China lithospheric slab were dragged downwards by the subducted oceanic slab, which constituted a 'two-sided' subduction mode. A conceptual geodynamic model is proposed to explain the exhumation of Sulu high-to UHP rocks and imply a polyphase exhumation driven by buoyancy of continental materials at different depth and upward extrusion of crustal partial melting rocks to the surface at the later stage.
Journal of Asian Earth Sciences, 2018
The modern Tian Shan range formed due to the India-Eurasia collision. Uplift of the Tian Shan is recorded notably by the sedimentary sequences and subsidence history of the coupling foreland basins. We analyzed a 353-km-long seismic profile and logging data of four wells in the southern Tian Shan foreland area to decipher its tectonic, stratigraphic, and subsidence history. The sedimentary sequences comprise the Cambrian-Silurian, Devonian-Permian, Triassic, Jurassic-Cretaceous, and Paleogene tectonostratigraphic units, overlain by the late Oligocene-Quaternary foreland basin unit. The Jidike Formation is the oldest sedimentary sequence of the foreland succession, deposited at ~26 Ma based on magnetostratigraphic constraints, indicating that uplift of the southern Tian Shan Mountains was initiated by at least ~26 Ma. In addition, the tectonic subsidence rate of the southern Tian Shan foreland basin increased significantly since ~26 Ma due to lithospheric flexure caused by building of
Gondwana Research, 2013
The South China continent has a Mesozoic intraplate orogeny in its interior and an oceanward younging in postorogenic magmatic activity. In order to determine the constraints afforded by deep structure on the formation of these characteristics, we reevaluate the distribution of crustal velocities and wide-angle seismic reflections in a 400 km-long wide-angle seismic profile between Lianxian, near Hunan Province, and Gangkou Island, near Guangzhou City, South China. The results demonstrate that to the east of the Chenzhou-Linwu Fault (CLF) (the southern segment of the Jiangshan-Shaoxing Fault), the thickness and average P-wave velocity both of the sedimentary layer and the crystalline basement display abrupt lateral variations, in contrast to layering to the west of the fault. This suggests that the deformation is well developed in the whole of the crust beneath the Cathaysia block, in agreement with seismic evidence on the eastwards migration of the orogeny and the development of a vast magmatic province. Further evidence of this phenomenon is provided in the systematic increases in seismic reflection strength from the Moho eastwards away from the boundary of the CLF, as revealed by multi-filtered (with band-pass frequency range of 1-4, 1-8, 1-12 and 1-16 Hz) wide-angle seismic images through pre-stack migration in the depth domain, and in the P-wave velocity model obtained by travel time fitting. The CLF itself penetrates with a dip angle of about 22°to the bottom of the middle part of the crust, and then penetrates with a dip angle of less than 17°in the lower crust. The systematic variation in seismic velocity, reflection strength and discrepancy of extensional factors between the crust and the lithosphere, are interpreted to be the seismic signature of the magmatic activity in the interest area, most likely caused by the intrusion of magma into the deep crust by lithospheric extension or mantle extrusion.
Lithos, 2018
The geological evolution of the Early Paleozoic Wuyi-Yunkai orogen in South China is a scientific question of a long-standing debate. We document the presence of a NE-NEE-striking Early Paleozoic subduction-accretion shear zone, a possible mélange belt, in the northwestern Yunkai Domain. The northwestern Yunkai shear zone consists predominantly of tectonically juxtaposed fragments of Early Paleozoic flysch, arc volcanic rocks, and a forearc ophiolite. The Yunkai shear zone displays typical mélange structures in several locations; however, these structures are not continuous throughout the shear zone. The shear zone provides evidence for greenschist to amphibolite facies metamorphism and intense deformation resulting from Early Paleozoic to Early Mesozoic tectonic events. The flysch fragments are characterized by northwestward younging, northwestward thrusting, and northwestward migration of deformation and metamorphism. The arc volcanic rocks consist of basaltic-andesite, andesite, and dacite with a mainly sanukitic composition and also include Nb-enriched basalts. They are characterized by enrichment of LREEs, LILEs, Pb and depletion of HFSEs, suggesting a continental forearc or a continental arc tectonic setting. The ophiolitic fragments consist of MORB-like basalt and dolerite/gabbro. They have slightly depleted to flat LREE patterns and are characterized by enrichment of LILEs and Pb and depletion of HFSEs, indicating a continental forearc setting. Zircon U-Pb analyses yield 460-443 Ma and 455-437 Ma ages for the sanukitic volcanic rocks and ophiolitic fragments, respectively, suggesting that they formed in the Late Ordovician to Early Silurian. Both the sanukitic volcanic rocks and ophiolitic fragments possess negative to positive zircon ε Hf (t) values (−11.0 to +2.3), indicating that they may have been generated by partial melting of an old subarc mantle wedge source metasomatized by slab-derived fluids and/or melts. Recognition of the Early Paleozoic subduction-related magmatism and subduction-accretion structures in the northwestern Yunkai Domain has important implications for the tectonic history of the Wuyi-Yunkai orogen, South China. Collectively, combined with previous studies, we propose that the Huanan oceanic lithosphere began to subduct southeast-ward beneath the Yunkai terrane (arc) as early as 460 Ma, and the subduction continued between 460 and 440 Ma.
Seismic evidence for the stratified lithosphere in the south of the North China Craton
Journal of Geophysical Research: Solid Earth, 2013
The North China Craton (NCC) formed in the Paleoproterozoic and suffered cratonic destruction in the Mesozoic. It consists of the Western NCC (WNCC), the Trans-North China Orogen (TNCO) and the Eastern NCC (ENCC). We investigated the upper mantle structures in the southern NCC by using receiver functions analysis. Polarization analysis was applied to increase the quality of receiver functions. The seismic images show significant stratified structures in the upper mantle in the south of the NCC. In the south of the WNCC, the lithosphere is~280 km thick and contains a low-velocity zone at~110-220 km depth. This low-velocity zone is interpreted as an intracratonic partial melting zone. In the south of the TNCO and ENCC, there is a low-velocity zone at~60-200 km depth, which results from the partial melting during the Mesozoic cratonic destruction. Its upper boundary is interpreted as new Lithosphere-Asthenosphere Boundary at~60-100 km depth. Below this low-velocity zone, there are the remains of Archean lithosphere (RAL). Blocks of RAL stagnate in the asthenosphere and reach depths over 300 km. A high-velocity slab extends from the uppermost mantle of the WNCC into the RAL with a dip-angle of~20. This slab is interpreted as the Paleoproterozoic slab, which indicates an eastward subduction between the WNCC and the ENCC. The WNCC is stable. The lithosphere in the south of the TNCO and ENCC suffered the Mesozoic cratonic destruction, while the destruction is relatively weaker than that in the north of the ENCC.
Journal of Structural Geology, 2005
To advance our understanding of the kinematics of the Xuefengshan tectonic belt (XFSTB) and the Mesozoic tectonics of the South China Block (SCB), this paper presents new data and interpretations from our structural studies and thermochronological dating. The XFSTB is characterized by greenschist-facies metamorphism and development of S and S-L tectonic fabrics. An asymmetric positive flower structural pattern is composed of NW-WNW-and SE-ESE-dipping cleavages, faults and shear zones. Kinematic indicators indicate a dominant top-to-WNW-thrusting with a sinistral strike-slip component on the ESE-dipping shear zones and top-to-ESE-back-thrusting on the WNW-dipping shear zones. The quartz c-axis orientations of mylonitic rocks exhibit monoclinic point-maximum asymmetry, indicative of a sinistral shear sense under the hai basal gliding condition. The timing of the major deformation event has been constrained to the middle Triassic to early Jurassic (244-195 Ma) on the basis of 40 Ar/ 39 Ar geochronology and other geological observations. The structural characteristics of the XFSTB are probably related to an Indosinian oblique convergent zone, in combination with tectonic wedging and associated back thrusting, all above a low-angle, SE-dipping basal detachment. Oblique northwestward and upward movement along the basal detachment was partitioned into the NW-WNW-directed thrusting, SE-ESE-directed back-thrusting and subsidiary strike-slip movement along the NE-and NNE-trending faults and shear zones. The XFSTB may represent part of a huge structural fan between the Yangtze and Cathaysian blocks, and can be interpreted as a product of the Indosinian intracontinental collision involving a weak zone. q