西南天山明尧勒背斜的第四纪滑脱褶皱作用

QUATERNARY DETACHMENT FOLDING OF THE MINGYAOLE ANTICLINE,SOUTHWESTERN TIAN SHAN

通过对明尧勒活动背斜喀浪勾律克河剖面生长前地层和翼部生长地层几何结构的填图以及变形河流阶地的系统测量,结合磁性地层及释光年代学研究,认为该背斜的滑脱褶皱作用起始于距今约1·6Ma,其总体几何结构形成于褶皱作用的早期,但其生长扩展并不完全遵从自相似性特征。持续的缩短作用部分被褶皱翼部陡倾膝折带的加长所吸收(由此导致背斜波幅的增加),另一部分可能是通过不同时期褶皱翼部不同膝折带组的旋转和迁移来实现的。明尧勒背斜的持续构造抬升是背斜区河流下切形成多级基座阶地的主因。晚第四纪褶皱的生长以背斜的垂直抬升为主,主要集中在北翼近核部,背斜宽度变化不大。背斜不同时期的抬升量和抬升速率均大于其缩短量和缩短速率,表明明尧勒背斜的变形以翼旋转为主(Pobletet al.,1996)。背斜自形成以来缩短速率和抬升速率均有减小的趋势。

The preservation of muhiple, clearly deformed Late Quaternary fluvial terraces and nearly continuous exposure of underlying Neogene sediments make the Mingyaole anticline detachment fold system particularly favorable for unraveling the geomorphic and structural history of underlying fold growth within the southwestern Chinese Tian Shan Foreland Basin. We mapped the Neogene structure and Pleistocene terrace surfaces along the south-flowing Kalanggouluke He within water gap across the anticline using 1 ： 50,000 scale stereo air photographs and topographic maps. The anticline has steep （58° -70°） limbs and is box-like, characterized by broad north vergent core that rises above gently dipping outer flanks, suggestive of detachment folding. Since the growth strata are not sufficiently exposed to allow us to discern the kinematic style of fold growth, we profiled all sets of progressively deformed Pleistocene fluvial terraces plus the present rivers across the of the anticline using a laser ranging theodolite （total station） aiming to use the terrace flights as kinematic markers for the recent evolution of the folds. The terrace profiles are preserved across the steeply dipping cores and show several common features： （ 1 ） Where long profiles of terraces could be surveyed, the terraces show increased tilts with age. （2） The greatest tilts are preserved in the north limb towards the core of the fold. （3） Nearly all of the observed terrace heights are attributable to differential uplift. （4） The pre-growth strata and the terrace treads are discordant everywhere. （5） Part of the terraces are not folded across previously active axial surfaces that are now passive, and younger axial surfaces that remain active as evidenced by fold scarps. In combination with structural data, growth strata, and deformed terraces, we infer that the width of the Mingyale anticline is established early and is subsequently amplified throughout the history of the fold, but not in a purely self-similar style. Continued shortening is accommodated by vertical growth of the fold via a combination of limb rotation in the center and hinge migration on the flanks.Planar, rotated terraces indicate more recent folding is accommodated by block rotation of the limbs. In combination with paleomagnetic study and OSL dating, we infer the fold growth initiated - 1.6Ma and shortening rate and differential uplift rate decreased since then. Uplift is always greater than shortening which indicates limb rotation dominates.