Poly-phase structural evolution of the northeastern Alxa Block, China: Constraining the Paleozoic-Recent history of the southern Central Asian Orogenic Belt
Jin ZhangDickson CunninghamFeng Jun QuHang Bei ZhangJinyi LiHeng ZhaoF. NiuJie HuiYun LongShuo ZhaoRongguo ZhengPing Yi Zhang
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Abstract:
The Alxa Block is a significant tectonic unit in the middle part of the southern Central Asian Orogenic Belt that was affected by multiple Paleozoic and Meso-Cenozoic deformation events. In this study, the results from detailed mapping and structural analysis coupled with new U-Pb zircon ages indicate that the northeastern Alxa Block has experienced ten deformation events since the late Paleozoic. Four separate structural domains are identified in the study area, and these domains contain intrusive and structural crosscutting relationships that allow the complex deformational history to be determined. Each deformation phase can be related to regional tectonic events associated with the consolidation of Central Asia's crust and subsequent intraplate reactivation. The first three events are tied to convergence between the Alxa Block, the North China and the Yangtze Cratons prior to and during closure of the Paleo-Asian Ocean in the Mid-Late Permian. Subsequently, sinistral displacement occurred between the Alxa Block and the North China Craton during the Triassic. Since the late Mesozoic, reactivation of the northeastern Alxa Block occurred repeatedly as an intraplate response to the subduction of the Paleo-Pacific Plate, the closure of the Mongol-Okhotsk Ocean, the collision between the Qiangtang and Lhasa blocks and the later collision between India and Eurasia. The Alxa Block provides a superb case study of how continental interior regions that evolve from plate boundaries to intraplate settings may remain susceptible to reactivation in different kinematic modes in response to distant plate margin-derived forces and internal gravitational forces that evolve through time.Keywords:
Pacific Plate
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The interaction of the Australian, South Bismarck and Solomon Sea Plates in Papua New Guinea is the source of frequent earthquakes that occur as a result of subduction and arc–continent collision. Previous investigators have drawn attention to a discontinuity in the horizontal azimuth of slip vectors along the southern boundary of the South Bismarck Plate, with those to the west of 148°E being systematically rotated ~20–30° clockwise compared to those located east of 148°E. This has led to the suggestion that relative motion may be occurring between the Huon Peninsula and New Britain or that more than two plates are acting south of the South Bismarck Plate. Global positioning system (GPS) measurements since 1991 indicate that there is no internal deformation occurring within the South Bismark Plate and that at least two distinct plates are in contact with the southern edge of the South Bismarck Plate. We show from a study of a recent earthquake dataset that the change in slip‐vector azimuth can be modelled by the interaction of the overriding South Bismarck Plate with the underthrusting Australian and Solomon Sea Plates, consistent with the GPS observations, while maintaining the South Bismarck Plate as a rigid entity. We found that a transition zone exists between 147°E and 148°E where the underlying plate changes from the Australian Plate to the Solomon Sea Plate. There are insufficient data at present to indicate whether or not a third plate, the Woodlark Plate, is also interacting directly with the South Bismarck Plate in this transition zone. Slip‐vector azimuths were used to estimate an Euler pole (6.74°S, 144.64°E), which describes the relative motion of the South Bismarck and Solomon Sea Plates along the New Britain Trench.
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This map shows details of Japan and vicinity not visible in an earlier publication, U.S. Geological Survey Scientific Investigations Map 3064. Japan and its island possessions lie across four major tectonic plates: Pacific plate, North America plate; Eurasia plate; and Philippine Sea plate. The Pacific plate is subducted into the mantle, beneath Hokkaido and northern Honshu, along the eastern margin of the Okhotsk microplate, a proposed subdivision of the North America plate (Bird, 2003). Farther south, the pacific plate is subducted beneath volcanic islands along the eastern margin of the Philippine Sea plate. This 2,200 km-long zone of subduction of the Pacific plate is responsible for the creation of the deep offshore Ogasawara and Japan trenches as well as parallel chains of islands and volcanoes, typical of the Circumpacific island arcs. Similarly, the Philippine Sea plate is itself subducting under the Eurasia plate along a zone, extending from Taiwan to southern Honshu, that comprises the Ryuku Islands and the Nansei-Shonto trench.
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There existed some blocks (micro-plates) in the oceans between Australia and Asia in the Cenozoic, when some blocks were separated from the Australian plate and moved northward and collided and sutured with some blocks that were separated from the Eurasian plate. In this period small ocean basins such as the South China Sea, Sulu Sea, Celebes Sea and Andaman Sea formed as a result of block separation and seafloor spreading, and finally the present tectonic framework formed in the Great South China Sea area. After a study of the Cenozoic tectonic history of the Great South China Sea area, the authors believe that Cenozoic tectonic activities in the Great South China Sea were not only related to collision between the Indian Plate and Eurasian Plate but also to subduction of the Pacific Plate beneath the Eurasian Plate and were also affected by the northward movement of the Australian. Plate. In the South China Sea Basin there occurred two events of seafloor spreading in the Cenozoic. The first seafloor spreading, which was oriented in a NW-SE direction, occurred before 42-35 Ma BP under the influence of the southeastward mantle flow beneath the Eurasian continent caused by India-Eurasia collision. The first seafloor spreading gave rise to the Southwest Basin of the South China Sea. The second seafloor spreading took place before 32-17 Ma BP. As the Pacific plate was subducted beneath the Eurasian plate to 700 km depth, the SE-directed flow of the upper mantle of the Eurasian continent was blocked and then turned toward the south, thus causing N-S-trending seafloor spreading in the South China Sea area, i.e. the second seafloor spreading. The second seafloor spreading resulted in the formation of the Central Basin of the South China Sea. After the Cenozoic South China Sea Basin was produced, collision between the blocks and seafloor spreading continued in the Great South China Sea area, and under the compression of these northward blocks the south margin of the South China Sea, sediments in the area were deformed, thus producing the Wanan movement (at about 10 Ma BP) on the south margin of the South China Sea.
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<p>The seismicity, structure and tectonics of the North Island plate boundary have been studied by means of a microearthquake traverse oriented in the direction of dip of the subducted Pacific plate and stretching from southern Hawke's Bay to northern Taranaki. The geometry of the top of the Pacific plate is inferred from a band of concentrated microearthquake activity which can be identified with the crust of the plate. The Pacific plate appears to have two knee-like bends, one between the east coast and the Ruahine Range, where the top of the plate is about 25 km deep, the other below the volcanic front, where it is about 70 km deep. The shallower bend and subsequent restraightening of the plate can be related to phase changes in the plate, while the deeper bend can be related to volcanism. Composite focal mechanisms indicate that seaward of its shallower bend the Pacific plate is being loaded by the Indian plate, whereas landward of this bend the Pacific plate is sinking under its own weight. Both composite focal mechanisms and the distribution of microseismicity in the Pacific plate suggest the existence of a major discontinuity striking down the dip of the plate and passing beneath the Tongariro volcanic centre. A conspicuous lack of microseismicity in the Indian plate in the eastern North Island revealed in this study can be related to the plates being unlocked in this region. A feature of the seismicity of the Indian plate in the region of the Wanganui Basin is the concentration of activity in the 25-42 km depth range, shallower activity being largely confined to the northeast edge of the basin, near Mt Ruapehu and Waiouru. Composite focal mechanisms suggest the 25-42 km deep activity reflects stresses set up by locking and unlocking of the plates, while the shallower activity reflects local stresses related to volcanic phenomena.</p>
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<p>The seismicity, structure and tectonics of the North Island plate boundary have been studied by means of a microearthquake traverse oriented in the direction of dip of the subducted Pacific plate and stretching from southern Hawke's Bay to northern Taranaki. The geometry of the top of the Pacific plate is inferred from a band of concentrated microearthquake activity which can be identified with the crust of the plate. The Pacific plate appears to have two knee-like bends, one between the east coast and the Ruahine Range, where the top of the plate is about 25 km deep, the other below the volcanic front, where it is about 70 km deep. The shallower bend and subsequent restraightening of the plate can be related to phase changes in the plate, while the deeper bend can be related to volcanism. Composite focal mechanisms indicate that seaward of its shallower bend the Pacific plate is being loaded by the Indian plate, whereas landward of this bend the Pacific plate is sinking under its own weight. Both composite focal mechanisms and the distribution of microseismicity in the Pacific plate suggest the existence of a major discontinuity striking down the dip of the plate and passing beneath the Tongariro volcanic centre. A conspicuous lack of microseismicity in the Indian plate in the eastern North Island revealed in this study can be related to the plates being unlocked in this region. A feature of the seismicity of the Indian plate in the region of the Wanganui Basin is the concentration of activity in the 25-42 km depth range, shallower activity being largely confined to the northeast edge of the basin, near Mt Ruapehu and Waiouru. Composite focal mechanisms suggest the 25-42 km deep activity reflects stresses set up by locking and unlocking of the plates, while the shallower activity reflects local stresses related to volcanic phenomena.</p>
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We use Global Positioning System (GPS) velocity data to model eastern Asian plate kinematics. Out of 15 stations in Korea, Russia, China, and Japan studied here, three sites considered to be on the stable interior of the hypothetical Amurian Plate showed eastward velocities as fast as ∼9–10 mm/yr with respect to the Eurasian Plate. They were stationary relative to each other to within 1 mm/yr, and these velocity vectors together with those of a few additional sites were used to accurately determine the instantaneous angular velocity (Euler) vector of the Amurian Plate. The predicted movement between the Amurian and the North American Plates is consistent with slip vectors along the eastern margin of the Japan Sea and Sakhalin, which reduces the necessity to postulate the existence of the Okhotsk Plate. The Euler vector of the Amurian Plate predicts left‐lateral movement along its boundary with the south China block, consistent with neotectonic estimates of the displacement at the Qinling fault, possibly the southern boundary of the Amurian Plate. The Amurian Plate offers a platform for models of interseismic strain buildup in southwest Japan by the Philippine Sea Plate subduction at the Nankai Trough. Slip vectors along the Baikal rift, the boundary between the Amurian and the Eurasian Plates, are largely inconsistent with the GPS‐based Euler vector, suggesting an intrinsic difficulty in using earthquake slip vectors in continental rift zones for such studies.
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The seismicity, structure and tectonics of the Indian/Pacific plate boundary in the North Island of New Zealand have been studied by means of a microearthquake traverse oriented in the direction of dip of the subducted Pacific plate and extending for about 210 km. The geometry of the top of the Pacific plate is inferred from a band of concentrated microearthquake activity approximately 10 km thick which is identified with the crust of the plate. The Pacific plate has two knee-like bends, one where the top of the plate is about 25 km deep, the other below the volcanic front, where the plate is about 70 km deep. The shallower bend and subsequent restraightening of the plate are related to phase changes in the plate, the deeper bend to volcanism. Composite focal mechanisms indicate that seaward of the shallower bend the Pacific plate is being loaded by the Indian plate, whereas landward of this bend the Pacific plate is sinking under its own weight.
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