青藏高原西北缘晚新生代构造变形研究

晚新生代,印亚碰撞的远程效应使青藏高原周缘发生了强烈的构造变形和隆升作用,然而不同学者对高原强烈构造变形和隆升时代的认识却大相径庭。本文通过对青藏高原西北缘晚新生代褶皱冲断带的构造变形、沉积作用、岩浆活动与地貌响应等的综合研究,依据古新统至中新统地层的连续沉积和产状的协调一致,提出青藏高原西北缘在古新世—中新世末并未发生区域性强烈的构造变形,并基于褶皱、生长地层、楔顶沉积和冲断带中局部不整合等标定青藏高原西北缘强烈构造变形的时代为上新世—早更新世,其中最强烈的构造变形发生于西域砾岩沉积结束阶段,即约1.1～0.7Ma的昆黄运动最终使中更新世以前地层全面褶皱-抬升,形成区域性的乌苏群与西域砾岩之间的角度不整合,这为青藏高原西北缘晚新生代的构造变形提供了关键的构造地质学证据;同时,根据磷灰石裂变径迹的研究成果提出青藏高原西北缘的主要隆升可能是在上新世—早更新世通过高原边缘的边界断层系以后展式逆冲扩展作用抬升形成的,并就裂变径迹热历史模拟的剥蚀厚度提出西域砾岩很可能主要来自高原边缘地形变化最剧烈的陡坡带,支持西域砾岩属构造成因的认识。

Late Cenozoic tectonic deformation on the northwestern margin of the Qinghai-Tibet Plateau

In Late Cenozoic, the long-range effect of Indo-Asia collision may result in strong tectonic deformation and uplift of marginal area in the Qinghai-Tibet Plateau. However, different geologists have very different understanding of the age of the tectonic deformation and uplift in the Qinghai-Tibet Plateau. Hence, there is no consensus to be reached. Based on an integrated research of tectonic deformation, sedimentation, magmatic activity and physiographic response to the fold and thrust belt on the northwestern margin of the Qinghai-Tibet Plateau late Cenozoic, in combination with Paleocene to Miocene strata sedimentation′s succession and strata dips′ coherence, it is suggested that a strong regional tectonic deformation has not taken place in the northwestern margin of the Qinghai-Tibet Plateau at the Paleocene-late Miocene. Unconformity marks such as folds, growth stratum, wedge-top sediment and local unconformity contact in thrust-fold belt indicate an age of Pliocene-early Pleistocene for the strong tectonic deformation in the northwestern margin of the Qinghai-Tibet Plateau. In addition, the strongest deformation took place at the end of Xiyu conglomerate deposition, that is Kunlun-huanghe Movement at about 1.1～0.7 Ma, which finally resulted in overall folding and uplifting of strata before Pleistocene and regionally angular unconformity between Wusu Formation and Xiyu comgloerate. This provides the key structural geological evidence for tectonic deformation of the northwestern margin of the Qinghai-Tibet Plateau in late Cenozoic. Meanwhile, research of apatite fission track suggests that main uplifting of the Qinghai-Tibet Plateau likely resulted from the boundary fault of the Plateau in the form of thrusting extension in Pliocene and Pleistocene. Denudation thickness by fission track modeling indicates that the Xiyu conglomerate should come from slope zone which had suffered landform change, which supports the understanding of the tectonic genesis of the Xiyu conglomerate.