The Nankai Trough has a 1300-year record of generating large earthquakes(M_w>8)with a recurrence time of～100-200 a,but unfortunately,we don’t know the frequency of earthquakes before.Results from recent expeditions of NanTroSEIZE Stage 1 IODP 314/315/316 recovered many cycles of turbidites since late Pleistocene,which are triggered during the past earthquake shaking.Our purpose is
Tao Jiang~1,Shenghua Li~2,Chunfeng Li~3,Xinong Xie~1,Xin Su~4,Jianye Ren~1 1.Key Laboratory of Tectonics and Petroleum Resources,China University of Geosciences(Wuhan),Wuhan 430074,China. 2.Department of Earth Sciences,University of Hong Kong,Pokfullam Road,Hong Kong,China 3.School of Ocean and Earth Sciences,Tongji University,Shanghai 200092,China 4.School of Marine Geosciences,China University of Geosciences(Beijing),Beijing 100083,China
This study examined the geochemical features of pore water in the diapiric area of the Yinggehai Basin, northwestern South China Sea, and illuminated the origin and evolution of basin fluids. Pore water with low salinity occurs in marine sediments in the diapiric area even without meteoric water infiltration. The presence of low-salinity water within deep, overpressured compartments is assumed to be due to smectite-illite transformation. Howerver, in shallow portions（less than 2 000 m） of diapiric areas with normal pressure, pore water has a much wider variation and much lower salinity than that in the overpressured intervals. Its total dissolved solid（TDS） content is ~5 336 to 35 939 mg/L. Moreover, smectite and chlorite content sharply decreases as kaolinite and illite content increase in shallower intervals. The geochemical variation of pore water in diapiric structures indicates the expulsion of low-salinity, overpressured fluids along vertical faults. Strong injection of hot fluids from deep overpressured sediments results in rapid clay mineral transformation in shallow reservoirs. Consequently, fluid mixing due to fluid expulsion from deeper overpressured deposits leads to variation in salinity and ionic composition as well as some diagenetic reactions. This includes transformation of clay minerals caused by the higher temperatur of deeper hot fluids, e.g., the transfromation of smectite to illite and chlorite to kaolinite. Therefore, variations in salinity and ionic compositions in various pressured systems provide a clue to flow pathways and associated diagenetic reactions.
The study of new seismic data permits the identification of sediment gravity flows in terms of internal architecture and the distribution on shelf and abyssal setting in the Qiongdongnan Basin （QDNB）. Six gravity flow types are recognized： （1） turbidite channels with a truncational basal and concordant overburden relationship along the shelf edge and slope, comprising laterally-shifting and vertically-aggrading channel complexes; （2） slides with a spoon-shaped morphology slip steps on the shelf-break and generated from the deformation of poorly-consolidated and high water content sediments; （3） slumps are limited on the shelf slope, triggered either by an anomalous slope gradient or by fault activity; （4） turbidite sheet complexes （TSC） were ascribed to the basin-floor fan and slope fan origin, occasionally feeding the deep marine deposits by turbidity currents; （5） sediment waves occurring in the lower slope-basin floor, and covering an area of approximately 400 km2, were generated beneath currents flowing across the sea bed; and （6） the central canyon in the deep water area represents an exceptive type of gravity flow composed of an association of debris flow, turbidite channels, and TSC. It presents planar multisegment and vertical multiphase characteristics. Turbidite associated with good petrophysical property in the canyon could be treated as a potential exploration target in the QDNB.
SU MingXIE XinongLI JunliangJIANG TaoZHANG ChengHE YunlongTIAN ShanshanZHANG Cuimei
