关键词: 天山, 滑脱褶皱, 生长地层, 磁性地层, 河流阶地, 翼旋转, 膝折带迁移

Abstract: The preservation of multiple,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∶50000 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.

Key words: Tian Shan, detachment fold, growth strata, magnetostratigraphy, strath terrace, limb rotation, hinge migration