【摘要】 西秦岭位于青藏高原东北缘重力梯度带内,是高原物质向北、向东扩展的前缘,其新生代以来地质构造-地貌过程应该是印度板块—欧亚板块的碰撞造山过程和高原隆升过程的一部分。通过对西秦岭内部中—新生代沉积、变形及地貌记录的初步综合分析,得出如下初步认识:(1)根据西秦岭中—新生代红层沉积岩石组合和构造变形特征,可以分为晚侏罗世—早白垩世、晚白垩世—古近纪和新近纪三个构造层,分别对应于西秦岭新生代3个构造演化阶段。(2)西秦岭晚白垩世—古近纪构造层的褶皱缩短和区域断裂带的逆冲推覆发生在古近纪末期—新近纪初期,与整个青藏高原主要逆冲推覆构造事件同步,说明印度板块与欧亚板块碰撞的构造应力在古近纪末已波及至西秦岭。(3)西秦岭新近纪以来经历了一个构造相对稳定的侵蚀夷平期,于3.6Ma之前形成了以晚白垩世—古近纪构造层侵蚀面、前新生代碳酸盐地层的岩溶夷平面为标志的主夷平面以及夷平面发育过程中形成新近纪近水平的、以红色粘土岩为主要特征的细碎屑沉积。这一夷平面可以作为高原组成部分的西秦岭隆升的基准面。该夷平面现今高程自西向东逐渐降低,反映了西秦岭隆升呈现自西向东连续的扩展。(4)青藏高原南部构造变形方式在中新世发生了由逆冲推覆-褶皱缩短向伸展走滑的构造转换,而在西秦岭内部却并未发生这样的构造转换,仍然以逆冲构造为主,只是西秦岭北缘的边界断层在中—晚更新世才发生逆冲-左旋走滑作用,这可能指示了青藏高原东北缘晚新生代构造变形的走滑作用只是构造块体边界与构造挤压应力方向下非正交的应力分解所致,同时也可能反映了作为西秦岭块体整体滑移和块体内部的收缩变形并行不悖。(5)由GPS观测数据确定的区域位移场应该指示了现今西秦岭块体的整体缓慢的向东移动,地震机制解确定的构造应力是下地壳向东蠕动拖曳脆性上地壳的整体运动,西秦岭地壳厚度由西向东逐渐增厚是西部由于南北向缩短增厚的下地壳向东扩展流动的结果,增厚地壳的均衡抬升是西秦岭地貌面高度变化的内在原因。更多还原

【Abstract】 The Cenozoic tectonic and geomorphological processes of the west Qinling as a part of Qinghai-Tibetan plateau could provide some constraints on the uplift and deformation model of the Qinghai-Tibetan plateau.Based on the analysis of the Cenozoic sedimentary strata and its deformation features,regional fault framework and its kinematics,and geomorphological records,we could reach the following preliminary understanding.(1)Based on their rock composition,spatial distribution and tectonic deformation characteristics,we can divide the Late Mesozoic-Cenozoic red bed sedimentary strata in the west Qinling into the Late Jurassic-Early Cretaceous,the Late Cretaceous-Paleogene and the Neogene three tectonic strata units,which corresponds to three tectonic evolution stages.(2)The fault-thrusting and fold-shortening of K2-E strata in the west Qinling occurred at the end of Eocene,synchronous with the crust shortening and thrusting in the Qinghai-Tibetan plateau,which may indicate that the compressive tectonic stress from the collisional convergence between the India plate and the Eurasia plate have been transmitted to the west Qinling.(3)The west Qinling had undergone a relatively tectonic stable period since the Neogene,in which extensive erosion and planation occurred,and finally main planation surface and associated Neogene sedimentary strata consisting of red conglomerate and red clay were formed before 3.6 Ma.This planation surface at present-day decreased gradually from 3200 m in the west to 1200 m in the east,which can be interpreted as continuous uplift in the eastward expansion of the Qinghai-Tibetan plateau.(4)The structural deformation transition from thrusting-shortening to extension-strike slip at 13-14 Ma or 8 Ma in the south Tibet have not appeared widely in the west Qinling,whereas fault thrusting predominated the total Cenozoic tectonic history and left-lateral strike-slip only occurred in the northern boundary fault of the west Qinling,which may indicate that block slip and thrusting coexisted in the west Qinling.(5)The regional displacement field determined by the GPS observational data and tectonic stress directions deduced by earthquake mechanism solution showed eastward motion of the west Qinling block and E-directed tectonic stress,which may reflect that eastward creep of low crust drags upper crust movement and uplift.The gradient zone of the crust thickness of the west Qinling could be interpreted as the result of eastward expansion creep of the thickened lower crust,which controlled main landform features of the west Qinling.更多还原