Tectonic Events Indicated by Late Oligocene Slumped Deposits from the South China Sea
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摘要: 南海北部ODP1148站晚渐新世至早中新世沉积以滑塌堆积和长时间沉积缺失为主要特征.由构造活动引起的沉积间断始于渐新世中期28.5 Ma至早中新世23 Ma左右结束.主间断面位于25 Ma, 亦即滑塌沉积层的底界.4次沉积间断总共造成至少3 Ma沉积记录的缺失.综合岩性、古生物年代测定、地球化学等分析结果, 表明南海晚渐新世的海底扩张模式呈多次跳跃式, 并以“25 Ma事件”为型变高峰.这一系列构造活动是欧亚、澳大利亚、菲律宾-太平洋板块相互作用的结果, 直接导致南海向前期裂谷更发育, 红河大断裂左擦拉张更强的南部扩张的转型.1148站的滑塌沉积为此次南海扩张转型提供了直接的证据.Abstract: The late Oligocene to earliest Miocene deposition at ODP Site 1148 from the northern South China Sea is characterized by slumps and long sedimentation breaks. Tectonic-driven hiatuses occurred from the mid-Oligocene 28 Ma to early Miocene 23 Ma, with the main hiatus falling at 25 Ma marked by the base of the slump. The four hiatuses recognized together erased at least 3 Ma of the late Oligocene sediment record.A synthesis of lithological, biostratigraphic and geochemical resultsindicates a stepwise seafloor spreading mode for the late Oligocene South China Sea, climaxing at the"25 Ma transitional event". This series of tectonic events must have resulted from interactions between Eurasian, Australian and Philippine-Pacific plates, subsequently leading to a change in seafloor spreading toward the south where rifting had been more prominent and extension force was stronger, propably as a direct response to the sinistral strike-slip and extension by the Red River fault. The slumped deposits from Site 1148 provide the direct evidence of this spreading transition in the late Oligocene South China Sea.
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Key words:
- South China Sea /
- seafloor spreading /
- ODP Site 1148 /
- Oligocene /
- plate tectonics /
- slump /
- lithobiostratigraphy
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图 1 南海地区板块构造示意图(据Coblentz et al., 1995修改)
图 2 1148站滑塌层沉积结构
Fig. 2. Core photographs of Site 1148 slumped deposits showing sediment and depositional structure details
图 3 1148站浮游有孔虫和超微化石生物地层年龄与沉积间断测定(Li et al., 2004)
Fig. 3. Age determination of Site 1148 sediments and unconformities based on planktonic foraminifer and nannofossil biostratigraphy
图 4 (a) 地震剖面上的双反射面; (b) 440~500 mcd井段的录井曲线(Wang et al., 2000)
mcd.合成深度(m); mbsf.海底深度(m); 47X~54X为延伸式取心井段系号; 灰色区为钻井取心
Fig. 4. Seismic profile showing the double reflector (a); Various log records showing dramatic changes between 440-500 mcd (b)
图 5 微量元素比值和Nd同位素变化(Li et al., 2003)
Fig. 5. Downcore variations of selected trace element ratios, Nd isotope and accumulation rate
图 6 南海与邻区新生代主要构造事件比较
资料来源于Pigram and Davies (1987)、Briais et al. (1993)、蒋仲雄等(1994)、Copeland (1997)和Hall (2002)
Fig. 6. Comparison of major tectonic events between the South China Sea and adjacent areas
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