柴达木盆地西部新生代构造演化及其对青藏高原北部生长过程的制约

柴达木盆地是青藏高原北部的一个中新生代山间陆相含油气盆地,盆地内新生代地层的构造变形记录了青藏高原北部生长、地壳缩短及其形成过程的重要信息.本文运用高精度卫星影像资料、地球物理资料剖面和磁性地层年代学数据等多种学科资料的综合研究,重点对柴达木盆地西部逆冲-褶皱构造带的形成机制和演化过程进行了详细的解析.研究结果表明:(1)由北向南依次发育分布的红三旱、尖顶山-黑梁子、南翼山和油砂山褶皱构造带均由不对称的直立褶皱或同斜褶皱构成,并且显示出背斜相对紧闭、向斜宽缓的”侏罗山式”褶皱特征,表明其下部滑脱构造带的存在;(2)红三旱、尖顶山-黑梁子逆冲-褶皱构造SW翼缓NE翼陡的不对称褶皱形态显示出是由南向北的逆冲作用形成的;两翼相对较对称的南翼山褶皱形态是由NE-SW向双向逆冲作用形成的;SW翼陡(或地层倒转)NE翼缓的油砂山褶皱带是由NE-SW向双向逆冲挤出作用形成的反映出由北向南的逆冲作用的存在;(3)红三旱、尖顶山-黑梁子和南翼山褶皱构造带的初始生长地层依次为始新统下干柴沟组、上新统狮子沟组和更新统七个泉组,高精度磁性地层限定其沉积时代依次为～39.5Ma、～8.2Ma和～2.5Ma,这不仅代表了这些褶皱的初始形成时代,而且代表了其逆冲断裂的形成时代;油砂山褶皱构造带中七个泉组初始生长地层以及上地表发育的一系列现代水系发生了弯曲,表明该逆冲-褶皱构造带从～2.5Ma形成以来一直持续到现在迄今仍在生长;红山旱地区近SN向的直立褶皱以及柴西地区似穹窿状的叠加褶皱,反映出阿尔金断裂带走滑过程中伴随的近EW向挤压的结果;(4)综合柴西地区逆冲-褶皱带构造地貌、生长地层、地球物理剖面、磁性地层年代学等证据,表明柴西存在的一系列逆冲-褶皱带是由南向北的滑脱构造产生,具有后退式生长演化特征,表明印度/欧亚板块碰撞以来,～40Ma其远程效应已到达柴达木盆地北部,并形成红山旱逆冲-褶皱构造带,随后的持续挤压,高原北部呈现出局部向南后退生长特征,依次形成尖顶山、南翼山和油砂山逆冲-褶皱带,其中～2.5Ma以来强烈的近南北向挤压作用产生的南翼山和油砂山逆冲-褶皱带构成了现今的”英雄岭”;～ 8Ma以来的阿尔金断裂带的强烈走滑活动波及到了柴达木盆地.

Cenozoic tectonic evolution of the western Qaidam Basin and its constrain on the growth of the northern Tibetan Plateau

The Qaidam Basin, a Cenozoic petroliferous basin, lies on the margin of the northern Tibetan Plateau. It records a lot of information about uplift and expansion of the northern Tibetan Plateau. High resolution satellite images, geophysical data and magnetostratigraphy chronology data are used to analyze the mechanism and development of fold belts in the western Qaidam Basin. It reveals that: (1) Hongsanhan, Jiandingshan-Heiliangzi, Nanyishan and Youshashan fold belts are all composed of asymmetric vertical or homoclinal folds related to thrusting, of which, just as the Jura-type fold, the tight anticline alternates with gentle syncline, indicating a decollement structure at depth; (2) Folds with gentle SW limb and sharp NE limb like Hongsanhan and Jiandingshan-Heiliangzi fold belts are formed by northeastward thrusting; the southwestward thrusting creates a fold with sharp SW limb and gentle NE limb, like Youshashan fold; the relative symmetric Nanyishan fold belt results from NW-SE thrusting; (3) The growth strata on the Hongsanhan, Jiandingshan-Heiliangzi and Nanyishan fold belts are Xiaganchaigou, Shizigou and Qigequan formations, with the high-resolution magnetostratigraphic age of ~39.5Ma, ~8.2Ma and ~2.5Ma, respectively. The initial time of growth strata presents the initial formation age of the fold belt as well as the age of thrusting. A series of similar drainage systems on the Youshashan fold belt is bending along the regional tectonic strike. This implies that the ongoing growth is taking placed in the Youshashan region. The SN vertical Hongsanhan fold, along with dome-like superimposed fold in the western Qaidam Basin, indicates a EW compression associated with the strike-slip of the Altyn Tagh fault; (4) Based on the tectonic geomorphology, growth strata, seismic reflection profiles and magnetostratigraphy, the main fold belts in the western Qaidam Basin result from southward propagating during Cenozoic. The backward propagation model indicates that Indian-Asian collision stress has reached the northern part of the Qaidam Basin at ~40Ma, formed the Hongsanhan thrust-fold belt. With the continuous compressing, Jiandingshan, Nanyishan and Youshashan fold belts come into being one by one, of which, the latter two fold belts, the composition of the present "Yingxiongling", are formed under the strong S-N compression since ~2.5Ma. Since ~8Ma, the intensive activity of the Altyn Tagh fault had affected the entire Qaidam basin.