关键词: 柯坪塔格, 山前逆断层, 晚第四纪, 古地震

Abstract: Kalpin thrust tectonic system is an important part of foreland thrust tectonic of the south Tianshan. This paper tries to discuss paleoearthuakes along the east section of the range front fault of Kalpintag during late Quaternary by doing field investigation along the fault, measuring geomorphic deformation and excavating trenches in some important sites where young alluvial fans were displaced obviously. There are 6 rows of fold-thrust faults from Puchang fault to Shanchakou and 4 rows from Shanchakou to Kalpin. Furthermore, the offset alluvial fans of Holocene have been found along two sections of the fault. One is Sanjianfang-Wudaoban section to the west of Sanchakou, another is Dakouzidaoban section to the east of Sanchakou. Both sections are located near arcuate top of thrust-sheet. We can find more than two periods of Holocene fans were displaced by thrust fault except the modern fan in both sections, and only Pleistocene fans were offset in other parts along the fault. It suggests that there are two segments to the east of Puchang fault along the range front fault of Kalpintag. 3 trenches are excavated along the Sanjianfang-Wudaoban segment and 4 events are discovered. Ages of the events are obtained by controlling deposit dating of TL samples. Referenced ages of events are 22, 14, 6.5 and 4.4ka BP and recurrence intervals are 8, 7 and 2ka, respectively. The vertical displacement of events is 1～1.2m and shortening slip 1.3～1.4m for events of long return-time, and they are 0.2～0.3m and 0.6～0.7m for events of short return-time, respectively. And one trench is excavated and 2 events since the end of late Pleistocene are exposed along Dashankoudaoban segment. Ages of the two events in this segment are a bit older than 13 and younger than 6ka BP, respectively. The recurrence interval is about 7ka. The coseismic displacement of each event is about 0.5m and the shortening slip 1.3m, respectively. It is necessary to tell that there is an uncertainty for the event age given in this paper because it is difficult to get good samples for dating. However, we can learn more on the large earthquake activity during late Quaternary along the range front fault of Kalpintag from these events.

Key words: Kalpintag, range front fault, late Quaternary, paleoearthquakes