Focal mechanism solutions of 219 earthquakes which happened in the Red River Fault Zone(RRFZ) and its adjacent areas were collected and analyzed.Based on these analyses,the distribution of earthquakes in different segments of the RRFZ and their difference in seismic types,as well as the regional deep dynamics condition were discussed.The characteristics of the RRFZ activity in different segments were concluded as follows.(1) The northwestern segment of the RRFZ shows a characteristic of a compressional stress field as a result of the India-Tibet collision,and the fault activity is characterized by thrusting with local extension;(2) As a primary shear zone between the Indochina Plate and the South China Plate,the middle segment of the RRFZ displays a characteristic of a shear stress field,and the fault activity is characterized by strike-slip;(3) Besides dextral slip,lithosphere in the southeast segment of the RRFZ experienced extension and thinning due to the upwelling of deep material,which resulted in the dextral transtensional stress field and tense-shearing fault activity in this section.
Low-velocity layer in crustal structure has a close relationship with tectonic and dynamic settings.Many seismic profiles in South China have demonstrated that low-velocity layer generally exists in the middle crust.The onshore-offshore deep seismic experiment was implemented in the transition zone between South China and northeastern South China Sea in 2001.By analyzing the seismic data from land stations and OBSs,different seismic phases were identified,and their travel time arrivals were modeled by ray-tracing program MacRay.Then the crustal structure was obtained along OBS-2001 profile,and the low-velocity layer was identified based on its travel-time gap.The low-velocity(5.5—5.9km·s~(-1)) layer exists in the bottom of upper crust(10.0—18.0 km deep)from Xintang area to 150.0 km SE of the Nanao Island,with a thickness about 3.0—4.0 km.It has a relatively stable spacial distribution and was deduced to be the extensionof the low-velocity layer of South China.The dehydration of water-containing minerals and the presence of melted or partially melted rock in the crust maybe the main factors that reduce the P-wave velocity.
Existing evidence shows that an Oligocene erosion event occurred on the northern continental margin of the South China Sea, and the Tainan Basin area might be at the center of this event, followed by a rapid tectonic subsidence in the late Oligocene and early Miocene period. The rapid tectonic subsidence is mainly thermal‐controlled, and the effect of the Yichu Fault on the Tainan Basin is limited to the basin's eastern part. We develop a 2‐D thermal‐mechanical kinematic numerical model to explore the relationship between thermal uplift and subsequent rapid subsidence in the Tainan Basin. Our modeling indicates that the Oligocene uplift, erosion, and subsequent rapid subsidence could be caused by a thermal event, and the differential subsidence of the basement caused by thermal contraction can initiate the development of small faults. However, it also suggests that other mechanisms might be needed to jointly account for the observed erosions.
Abstract The Qiongdongnan Basin is one of the largest Cenozoic rifted basins on the northern passive margin of the South China Sea. It is well known that since the Late Miocene, approximately 10 Ma after the end of the syn‐rift phase, this basin has exhibited rapid thermal subsidence. However, detailed analysis reveals a two‐stage anomalous subsidence feature of the syn‐rift subsidence deficit and the well‐known rapid post‐rift subsidence after 10.5 Ma. Heat‐flow data show that heat flow in the central depression zone is 70–105 mW m −2 , considerably higher than the heat flow (<70 mW m −2 ) on the northern shelf. In particular, there is a NE‐trending high heat‐flow zone of >85 mW m −2 in the eastern basin. We used a numerical model of coupled geothermal processes, lithosphere thinning and depositional processes to analyse the origin of the anomalous subsidence pattern. Numerical analysis of different cases shows that the stretching factor β s based on syn‐rift sequences is less than the observed crustal stretching factor β c , and if the lithosphere is thinned with β c during the syn‐rift phase (before 21 Ma), the present basement depth can be predicted fairly accurately. Further analysis does not support crustal thinning after 21 Ma, which indicates that the syn‐rift subsidence is in deficit compared with the predicted subsidence with the crustal stretching factor β c . The observed high heat flow in the central depression zone is caused by the heating of magmatic injection equivalently at approximately 3–5 Ma, which affected the eastern basin more than the western basin, and the Neogene magmatism might be fed by the deep thermal anomaly. Our results suggest that the causes of the syn‐rift subsidence deficit and rapid post‐rift subsidence might be related. The syn‐rift subsidence deficit might be caused by the dynamic support of the influx of warmer asthenosphere material and a small‐scale thermal upwelling beneath the study area, which might have been persisting for about 10 Ma during the early post‐rift phase, and the post‐rift rapid subsidence might be the result of losing the dynamic support with the decaying or moving away of the deep thermal source, and the rapid cooling of the asthenosphere. We concluded that the excess post‐rift subsidence occurs to compensate for the syn‐rift subsidence deficit, and the deep thermal anomaly might have affected the eastern Qiongdongnan Basin since the Late Oligocene.
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