There are some active bottom currents on the northern continental slope of the South China Sea (SCS). Reflection seismic profiles show that the bottom current channels occur in the water depth range of 1000 to 2700 m,...There are some active bottom currents on the northern continental slope of the South China Sea (SCS). Reflection seismic profiles show that the bottom current channels occur in the water depth range of 1000 to 2700 m,extending from the NE to the SW,leading to accumulation of discontinuous drifts with higher sedimentation rates on the eastern side of the channel. The stacking pattern of the layers sug-gests that these drifts propagated southwestward,following the direction of the bottom currents. One sedimentary drift to the southeast of the Dongsha Islands has the highest sedimentation rate of 97cm/ka in the last 12 ka. The sedimentary characteristics of the sediment layers indicate that these bottom currents are most likley caused by the water movement of a branch of the West Pacific Ocean Current,which enters the northern SCS via the Bashi Strait. Once formed,the bottom currents trans-port sediments along the northern slope of SCS southwestward and finally disappear into the central basin of the SCS. Due to the bottom current activity,the deep-sea sedimentary process in the northern SCS is complex.展开更多
The Emeishan large igneous province(ELIP) in SW China is interpreted to be associated with an ancient mantle plume. Most of the constraints on the role of mantle plume in the generation of the Emeishan flood basalts w...The Emeishan large igneous province(ELIP) in SW China is interpreted to be associated with an ancient mantle plume. Most of the constraints on the role of mantle plume in the generation of the Emeishan flood basalts were provided by geological and geochemical methods, but the geophysical investigation is very limited. In order to better understand the deep structure and features of ELIP, we have studied the crustal velocity structure using the data acquired from the Lijiang-Panzhihua-Qingzhen wide-angle seismic profile. This profile crosses the three sub-zones of the ELIP(the inner, intermediate, and outer zones), divided based on the differential erosion and uplift of the Maokou limestone. The results provided by the active source seismic experiment demonstrate:(1) The average depth of the crystalline basement along the profile is about 2 km.(2) The middle crust in the Inner Zone is characterized by high-velocity anomalies, with the average velocity of 6.2-6.6 km/s, which is about 0.1– 0.2 km/s higher than the normal one. The velocity of the lower crust in the inner zone is 6.9-7.2 km/s, higher than those observed in the intermediate and outer zones(6.7-7.0 km/s). Relatively low velocity anomalies appear in the upper, middle and lower crusts near the junction of the inner zone and intermediate zone, probably due to the effect of the Xiaojiang fault(XJF).(3) The average velocity of the crust is comparatively low on both sides of XJF, especially on the east side, and the average velocity of the consolidated continental crust is also low there. This may suggest that the XJF extends at least down to 40 km deep, even beyond through the crust.(4) The depth to the Moho discontinuity decrease gradually from 47-53 km in the inner zone, via 42-50 km in the intermediate zone to 38-42 km in the outer zone. In the inner zone, the Moho uplifts locally and the(consolidated) crust is characterized by high-velocity anomalies, which are likely related to intensive magma intrusion and underplating associated with melting of plum展开更多
From the 1960 s to 1970 s, North China has been hit by a series of large earthquakes. During the past half century,geophysicists have carried out numerous surveys of the crustal and upper mantle structure, and associa...From the 1960 s to 1970 s, North China has been hit by a series of large earthquakes. During the past half century,geophysicists have carried out numerous surveys of the crustal and upper mantle structure, and associated studies in North China.They have made significant progress on several key issues in the geosciences, such as the crustal and upper mantle structure and the seismogenic environment of strong earthquakes. Deep seismic profiling results indicate a complex tectonic setting in the strong earthquake areas of North China, where a listric normal fault and a low-angle detachment in the upper crust coexist with a high-angle deep fault passing through the lower crust to the Moho beneath the hypocenter. Seismic tomography images reveal that most of the large earthquakes occurred in the transition between the high-and low-velocity zones, and the Tangshan earthquake area is characterized by a low-velocity anomaly in the middle-lower crust. Comprehensive analysis of geophysical data identified that the deep seismogenic environment in the North China extensional tectonic region is generally characterized by a low-velocity anomalous belt beneath the hypocenter, inconsistency of the deep and shallow structures in the crust, a steep crustalal-scale fault,relative lower velocities in the uppermost mantle, and local Moho uplift, etc. This indicates that the lithospheric structure of North China has strong heterogeneities. Geologically, the North China region had been a stable craton named the North China Craton or in brief the NCC, containing crustal rocks as old as ～3.8 Ga. The present-day strong seismic activity and the lower velocity of the lower crust in the NCC are much different from typical stable cratons around the world. These findings provide significant evidence for the destruction of the NCC. Although deep seismic profiling and seismic tomography have greatly enhanced knowledge about the deep-seated structure and seismogenic environment, some fundamental issues still remain and require further work.展开更多
The Helan Mountains and Yinchuan Basin （HM-YB） are located at the northern end of the North-South tectonic belt, and form an intraplate tectonic deformation zone in the western margin of the North China Craton （NCC...The Helan Mountains and Yinchuan Basin （HM-YB） are located at the northern end of the North-South tectonic belt, and form an intraplate tectonic deformation zone in the western margin of the North China Craton （NCC）. The HM-YB has a complicated history of formation and evolution, and is tectonically active at the present day. It has played a dominant role in the complex geological structure and modem earthquake activities of the region. A 135-km-long deep seismic reflection profile across the HM-YB was acquired in early 2014, which provides detailed information of the lithospheric structure and faulting characteristics from near-surface to various depths in the region. The results show that the Moho gradually deepens from east to west in the depth range of 40-48 km along the profile. Significant differences are present in the crustal structure of different tectonic units, including in the distribution of seismic velocities, depths of intra-cmstal discontinuities and undulation pattern of the Moho. The deep seismic reflection profile further reveals distinct structural characteristics on the opposite sides of the Helan Mountains. To the east, The Yellow River fault, the eastern piedmont fault of the Helan Mountains, as well as multiple buried faults within the Yinchuan Basin are all normal faults and still active since the Quaternary. These faults have controlled the Cenozoic sedimentation of the basin, and display a ＂negative-flower＂ structure in the profile. To the west, the Bayanhaote fault and the western piedmont fault of the Helan Mountains are east-dipping thrust faults, which caused folding, thrusting, and structural deformation in the Mesozoic stratum of the Helan Mountains uplift zone. A deep-penetrating fault is identified in the western side of the Yinchuan Basin. It has a steep inclination cutting through the middle-lower crust and the Moho, and may be connected to the two groups of faults in the upper crest. This set of deep and shallow fault system consists of both strike-slip, thrust, a展开更多
基金Supported by the National Natural Science Foundation of China (Grant Nos. 40276019 and 40238060)the National Key Basic Research Special Foundation Project of China (Grant No. G2000078500)
文摘There are some active bottom currents on the northern continental slope of the South China Sea (SCS). Reflection seismic profiles show that the bottom current channels occur in the water depth range of 1000 to 2700 m,extending from the NE to the SW,leading to accumulation of discontinuous drifts with higher sedimentation rates on the eastern side of the channel. The stacking pattern of the layers sug-gests that these drifts propagated southwestward,following the direction of the bottom currents. One sedimentary drift to the southeast of the Dongsha Islands has the highest sedimentation rate of 97cm/ka in the last 12 ka. The sedimentary characteristics of the sediment layers indicate that these bottom currents are most likley caused by the water movement of a branch of the West Pacific Ocean Current,which enters the northern SCS via the Bashi Strait. Once formed,the bottom currents trans-port sediments along the northern slope of SCS southwestward and finally disappear into the central basin of the SCS. Due to the bottom current activity,the deep-sea sedimentary process in the northern SCS is complex.
基金supported by the National Basic Research Program of China(Grant No.2011CB808904)the National Natural Science Foundation of China(Grants Nos.41274070,41474068)
文摘The Emeishan large igneous province(ELIP) in SW China is interpreted to be associated with an ancient mantle plume. Most of the constraints on the role of mantle plume in the generation of the Emeishan flood basalts were provided by geological and geochemical methods, but the geophysical investigation is very limited. In order to better understand the deep structure and features of ELIP, we have studied the crustal velocity structure using the data acquired from the Lijiang-Panzhihua-Qingzhen wide-angle seismic profile. This profile crosses the three sub-zones of the ELIP(the inner, intermediate, and outer zones), divided based on the differential erosion and uplift of the Maokou limestone. The results provided by the active source seismic experiment demonstrate:(1) The average depth of the crystalline basement along the profile is about 2 km.(2) The middle crust in the Inner Zone is characterized by high-velocity anomalies, with the average velocity of 6.2-6.6 km/s, which is about 0.1– 0.2 km/s higher than the normal one. The velocity of the lower crust in the inner zone is 6.9-7.2 km/s, higher than those observed in the intermediate and outer zones(6.7-7.0 km/s). Relatively low velocity anomalies appear in the upper, middle and lower crusts near the junction of the inner zone and intermediate zone, probably due to the effect of the Xiaojiang fault(XJF).(3) The average velocity of the crust is comparatively low on both sides of XJF, especially on the east side, and the average velocity of the consolidated continental crust is also low there. This may suggest that the XJF extends at least down to 40 km deep, even beyond through the crust.(4) The depth to the Moho discontinuity decrease gradually from 47-53 km in the inner zone, via 42-50 km in the intermediate zone to 38-42 km in the outer zone. In the inner zone, the Moho uplifts locally and the(consolidated) crust is characterized by high-velocity anomalies, which are likely related to intensive magma intrusion and underplating associated with melting of plum
基金supported by the National Natural Science Foundation of China (Grant Nos. 91014006, 90914005 & 41474073)
文摘From the 1960 s to 1970 s, North China has been hit by a series of large earthquakes. During the past half century,geophysicists have carried out numerous surveys of the crustal and upper mantle structure, and associated studies in North China.They have made significant progress on several key issues in the geosciences, such as the crustal and upper mantle structure and the seismogenic environment of strong earthquakes. Deep seismic profiling results indicate a complex tectonic setting in the strong earthquake areas of North China, where a listric normal fault and a low-angle detachment in the upper crust coexist with a high-angle deep fault passing through the lower crust to the Moho beneath the hypocenter. Seismic tomography images reveal that most of the large earthquakes occurred in the transition between the high-and low-velocity zones, and the Tangshan earthquake area is characterized by a low-velocity anomaly in the middle-lower crust. Comprehensive analysis of geophysical data identified that the deep seismogenic environment in the North China extensional tectonic region is generally characterized by a low-velocity anomalous belt beneath the hypocenter, inconsistency of the deep and shallow structures in the crust, a steep crustalal-scale fault,relative lower velocities in the uppermost mantle, and local Moho uplift, etc. This indicates that the lithospheric structure of North China has strong heterogeneities. Geologically, the North China region had been a stable craton named the North China Craton or in brief the NCC, containing crustal rocks as old as ～3.8 Ga. The present-day strong seismic activity and the lower velocity of the lower crust in the NCC are much different from typical stable cratons around the world. These findings provide significant evidence for the destruction of the NCC. Although deep seismic profiling and seismic tomography have greatly enhanced knowledge about the deep-seated structure and seismogenic environment, some fundamental issues still remain and require further work.
基金supported by the National Natural Science Foundation of China(Grant No.91214205)the Special Scientific Research of Seismological Industry(Grant No.201408023)
文摘The Helan Mountains and Yinchuan Basin （HM-YB） are located at the northern end of the North-South tectonic belt, and form an intraplate tectonic deformation zone in the western margin of the North China Craton （NCC）. The HM-YB has a complicated history of formation and evolution, and is tectonically active at the present day. It has played a dominant role in the complex geological structure and modem earthquake activities of the region. A 135-km-long deep seismic reflection profile across the HM-YB was acquired in early 2014, which provides detailed information of the lithospheric structure and faulting characteristics from near-surface to various depths in the region. The results show that the Moho gradually deepens from east to west in the depth range of 40-48 km along the profile. Significant differences are present in the crustal structure of different tectonic units, including in the distribution of seismic velocities, depths of intra-cmstal discontinuities and undulation pattern of the Moho. The deep seismic reflection profile further reveals distinct structural characteristics on the opposite sides of the Helan Mountains. To the east, The Yellow River fault, the eastern piedmont fault of the Helan Mountains, as well as multiple buried faults within the Yinchuan Basin are all normal faults and still active since the Quaternary. These faults have controlled the Cenozoic sedimentation of the basin, and display a ＂negative-flower＂ structure in the profile. To the west, the Bayanhaote fault and the western piedmont fault of the Helan Mountains are east-dipping thrust faults, which caused folding, thrusting, and structural deformation in the Mesozoic stratum of the Helan Mountains uplift zone. A deep-penetrating fault is identified in the western side of the Yinchuan Basin. It has a steep inclination cutting through the middle-lower crust and the Moho, and may be connected to the two groups of faults in the upper crest. This set of deep and shallow fault system consists of both strike-slip, thrust, a
