Rifting process of the Xihu Depression, East China Sea Basin

• Published in: Tectonophysics 2009-07, Vol.472 (1), p.135-147
• Main Authors: Li, Chun-Feng, Zhou, Zuyi, Ge, Heping, Mao, Yunxin
• Format: Article
• Language: English
• Subjects: Backstripping; China; Depression; Depth profiling; East China Sea Basin; Gravity modeling; Inversions; Nonuniform extension; Seismic engineering; Seismic phenomena; Stretching factor; Subsidence; Tectonics; Xihu Depression
• ISSN: 0040-1951; 1879-3266
• DOI: 10.1016/j.tecto.2008.04.026

Having a maximum depositional depth up to 15 km, the NE-trending Xihu Depression is evolved from a deep Paleogene continental margin rifting along the present-day East China Sea continental shelf. In this paper we use deep seismic reflection, well logging, and gravity data to synthesize early rifting process of the depression. To study the regional tectonic subsidence history we developed a computer program that performs backstripping and simultaneous inversion for stretching factor, initial lithospheric thickness and incipient age of thermal subsidence. Tectonic subsidence analyses of 40 industrial wells reveal nonuniform stretching — preferential crustal extension along the depocenter but preferential mantle extension at the western flank. Uniform extension may have occurred only somewhere between the central inversion zone and the western margin. In the early rifting stage, subdued initial subsidence or initial uplift is observed on several wells near the western margin, due possibly to flexural isostatic balance. The initial mechanical lithospheric thicknesses from subsidence inversion seem to vary from about 55 km to almost 130 km, suggesting significant longitudinal segmentation as well as transverse variation in the prerifting lithosphere. It is likely that the southern part of the Xihu Depression was developed initially on a region of relatively thin lithosphere, a statement conformable to geological evidences. NW–SE trending deep seismic profiling clearly reveals the deep basement structure of the depression, as well as the synrift and postrift sedimentary packages separated by a breakup unconformity. Crustal layer thicknesses are independently estimated from gravity modeling/inversion, tectonic subsidence analyses, and deep seismic profiling, respectively. These methods all arrive at very small present-day thicknesses of the crustal layer beneath the depocenter of the Xihu Depression.