西秦岭晚新生代构造变形的几何图像、运动学特征及其动力机制

Geometrical imagery and kinematic dissipation of the late Cenozoic active faults in the West Qinling Belt:Implications for the growth of the Tibetan Plateau

西秦岭位于东西向展布的秦岭-大别-苏鲁中央造山带与南北向展布的贺兰山-龙门山-川滇地震带构成的巨型“十字”构造区的交汇点,是中国大陆中部“西秦岭-松潘构造结”的重要组成部分。西秦岭晚新生代的构造变形与青藏高原的侧向扩展过程密切相关。该区构造变形的几何图像、运动特征及其深部动力学机制对于揭示青藏高原东北部的动力过程及强震活动具有重要意义。西秦岭地区主要断裂晚新生代以来的滑动速率及跨断裂GPS应变速率的结果表明,这一时期西秦岭构造带发生了明显的构造活动方式转换,主要的构造变形过程是通过其内部一系列低滑动速率的断裂活动以及断裂之间隆起山脉与盆地的变形,共同承担着自东昆仑断裂向西秦岭断裂之间的转换平衡。在调节这种构造转换过程中,西秦岭地区以“连续变形”为特征,即区域内的应变是以多条相对低滑动速率断裂的弥散变形遍布全区,并且西秦岭及其周缘块体的旋转作用也吸收了部分变形分量。综合已查明的区域构造活动特征、新生代岩浆活动、地球物理资料以及现今地貌特征可知,西秦岭在特提斯构造域的影响下,岩石圈的结构存在明显的流变学分层,一方面,西秦岭的上地壳保留了主造山期的地质构造形态,但中—下地壳的弱化使得莫霍面之上的圈层解耦,深部可流动的岩石圈地幔不但改变了陆内造山带的结构,同时也控制了现今上地壳连续变形的发育;另一方面,西秦岭内部的中强震主要发生在高速(或高阻)与低速(或低阻)的构造边界带附近。这种独特的流变学结构导致西秦岭在青藏高原向北生长和侧向扩展的过程中,不同阶段的构造变形过程是截然不同的。因此,进一步深入研究西秦岭地区的晚新生代构造转换过程及其机制,不仅对于理解青藏高原东北部的动力过程具有重要意义,更有助于深入认识南北地震构造带中段未来的强震危险性。

The West Qinling Belt(WQB)situated in the central China continent,is an enormous structure on the crustal scale,which is not controlled only by the Tethyan tectonic domain but is more complex,involving additional tectonic domains.The composite WQB as the coordinate system,which underwent five major episodes of accretion and collision between discrete continental blocks,has distinct geological and geophysical structure,geomorphology and environment,characterized by complex structures,complex forming processes and mixed materials.Moderate-strong earthquakes occurred frequently in the WQB in recent years,attesting its tectonic activity.Numerous results from the studies related to active fault geological and geodesic observations gave us new insights into present-day crustal deformation characteristics and its dynamic mechanism and helped us in exploring the control effect of active tectonic system on significant earthquake events in the WQB.Two groups of faults striking in different direction(NWW-trending and NEE-trending)within the WQB have played significant roles in the tectonic deformation and the transference slip along the east end of the east Kunlun fault since the Quaternary.Recent results suggest that the<2 mm/a slip rate at the tip of the east Kunlun fault is absorbed by low slip rate faults,crustal shortening,basin formation,mountain uplift and block rotation in the WQB.Whereas deformation in the shallow brittle crust does not occur on a major fault,deformation of a continuous medium at depth best describes the present-day tectonics of the WQB.Regionally,mantle magmatism,geophysical and geological data show that the actively deforming WQB crust is dominated by main mountain building contraction shortening strain in the upper crust,decoupled plastic deformation in the lower crust and extrusion of the mantle lithosphere below to the high-strain domains in the crust above,and such a transition zone(high and low velocity/resistivity anomalies)is relatively easy to accumulate stress,leading to occurrence of major earthquake in this area.