关键词: 天山, 地壳缩短, 逆断裂-褶皱带, 缩短速率, 第四纪

Abstract: We present the results of selected balanced cross-sections across four active reverse fault-fold belts at the front margins of the Chinese Tianshan,an intracontinental mountain belt formed in response to the India-Eurasia continental collision. It consists of Paleozoic,Mesozoic and Cenozoic strata in active fold-and-thrust belts at two sides of Tianshan, and the stratum in active fold belt changes younger from west fold-and-thrust belt to east. The middle part's structure is complicated in Kalpin active fold-and-fault belt,the width gradually diminishes towards east. At the west of Akesu city,the Kalpin active fold-and-thrust belt vanishes, but at the east part of Akesu city,the Kuche active fold-and-thrust belt emerges on the Tarim desert. In the middle section of the Kuche active fold-and-thrust belt near Baicheng County,the structure is most complicated,and the width is also maximal. The width of Kuche fold-and-thrust belt gradually decreases towards east,and disappears near 85° longitude line. But at the northern piedmont of Tianshan,the Manas active fold-and-thrust belt presents itself approximately at the same longitude. The longitude of the west end of Turpan centre uplift active fold-and-thrust zone is approximately the longitude of the east end of the Manas active fold-and-thrust belt. The four active fold-and-thrust belts which we studied are located at the north and south sides of Tianshan Mountains. Two balanced cross-sections traverse the Kalpin active fold-and-fault belt and one crosses the Kuche fold-and-fault belt. Two balanced cross-sections cross the Manas active fold-and-thrust belt,and one balanced cross-section crosses the Turfan centre uplift zone. The crustal shortening of Kalpin fold-and-thrust belt is 40～45km,that of Kuche fold-and-thrust belt about 27～37km,and of Manas,Turpan centre uplift active fold-and-thrust belt are 8.5～10.5km and 6～7km,respectively. The four active fold-and-thrust belts at south and north Tianshan are not superposed in longitude. So their crustal shortenings approximately represent the minimal crustal shortening of Tianshan at different segments,and also reflect the reducing trend of crustal shortening from west to east. It is very difficult to calculate the crustal shortening across entire Tianshan,owing to deficiency of data about active reverse fault and strike-slip fault within Tianshan Mountains. If the time of initial deformation is the starting time of Xiyu conglomerate deposition(since 2.5Ma),and considering the shortening component on south-north direction of Bo-A NW strike-slip fault,the minimum crustal shortening rates at four segments of Tianshan would be 15.4～17.3mm/a,12.7～16.5mm/a,3.8～4.5mm/a and 2.3～2.7mm/a,respectively.

Key words: Tianshan, crustal shortening, active fold-and-thrust belt, Quaternary