珠江三角洲五桂山南麓断裂第四纪活动新知

NEW INSIGHT INTO THE QUATERNARY ACTIVITY OF WUGUISHAN SOUTHERN PIEDMONT FAULT IN ZHUJIANG DELTA

野外工作期间 ,在珠江三角洲五桂山南麓发现两个切割第四系的断层剖面。这是首次在珠江三角洲地区见到的断裂第四纪活动的直接地质证据。对两个剖面作了详细介绍 ,并通过三角洲第四系的分析和对比 ,确认五桂山南麓断裂在中。

A number of NE-, NW- and E-W-trending faults are developed within and around the Zhujiang Delta, controlling the formation, evolution and even the seismic activity of the Delta. The previous studies on the Zhujiang Delta focused mainly on the geometry and the nature of recent activity of the Delta. It is a pity, however, that no good surface exposure was found to reveal the cross-cutting relationship between the fault and the Quaternary deposits, so that the recent activities of the faults was deduced indirectly from the interior structures, development features and seismic activity of the Delta. In 1998 and 1999, the authors carried out field investigation in the Zhujiang Delta. During the investigation, we found at the southern foot of Wuguishan mountain 2 good surface exposures showing clearly the cross cutting relationship between the fault and the Quaternary deposits. This is the direct evidence of Quaternary faulting firstly discovered in the Zhujiang Delta area. The Wuguishan Southern Piedmont fault lies in the Zhujiang Delta, extending northeastward from Hutiaomen in the southwest, passing through Dachi, Dutou, and terminating at Yakou, having a total length of 55km. It is striking 60°, dipping southeast or northwest at an angle of over 65°. Geomorphic landform related to faulting, such as terminal facet, fault cliff, and fault valley or nek are well developed along the fault. The newly discovered 2 exposures are located at the southern foot of the Wuguishan mountain, to the west of the newly built Zhuyuan high way to the northeast of Sanxiang. The first exposure is located beside a gully in the vicinity of a stone quarry. At this exposure 3 faults can be recognized with general occurrence of 35°～45°/NW∠65°～85°, the two of which dissect the Quaternary sandy gravel and gravel beds. The second exposure is located to the south of a bedrock hill. At a cross section exposed by quarrying, a 20m wide crushed zone can be recognized between the granite and the Quaternary coarse gravel bed. It seems that the two units are in faulted contact with each other. The fault is composed of a series of compressional planes, striking 65°, and dipping northwest at an angle of 75°～85°. The lower part of the Quaternary deposits that are dissected by the 2 faults consists of yellow gravel bearing coarse grained sands, while the upper part consists of slightly bedded gravel, granule and coarse grained sands. The former is comparable with the regional mid Pleistocene alluvium (sandy gravel and pebble beds), while the latter is comparable with the regional late Pleistocene alluvium (medium fine sands and gravel beds). They are all of terrestrial origin, but span mid-late Pleistocene in ages. According to these 2 exposures, therefore, it is confirmed that the two faults were active during mid-late Pleistocene.