西秦岭北缘断层的多期变形演化及其构造动力学意义

西秦岭北缘断层是青藏高原东北缘新生代盆地与西秦岭地块之间的边界断层，其构造变形的几何学-运动学特征和变形历史等研究对于重建青藏高原东北缘新生代以来的构造变形时空动力学过程，限定新生代盆地构造属性，揭示印度板块-欧亚板块碰撞汇聚的远程构造响应和青藏高原东北缘隆升等重大科学问题具有重要地质约束。本文通过对西秦岭北缘新生代盆地南边界F1断层的断层岩类型及分带、构造要素的几何学-运动学特征等较详细的构造解析，辨认出F1断层6期构造变形：第一期为北西西走向、向北倾斜的韧性-韧脆性伸展正断层作用；第二期为北西西走向、向北陡倾或近直立的高角度逆冲断层作用，指示近南北向挤压缩短作用；第三期为走向近南北、向东或向西陡倾的对冲断层作用，指示了近东西向的挤压缩短作用；第四期为北东向右旋和北西向左旋的走滑共轭断层系统，指示了近东西向的挤压作用；第五期为断层面近直立的北东向左旋、北西向右旋的共轭破裂系统，指示了近南北向挤压作用；第六期为断层面近直立的近东西向左旋和近南北向右旋走滑断层构成了几何学-运动学协调的共轭破裂系统，指示了北东向挤压作用。结合西秦岭北缘渐新世-中新世沉积盆地具有断陷盆地沉积序列特征和上新世具有类磨拉石的冲洪积扇粗砾岩特征以及F1断层多期变形对新生代盆地沉积地层的控制和改造作用分析，认为F1断层第一期韧性-脆韧性伸展正断作用始于渐新世，控制了渐新世-中新世伸展断陷盆地沉积；F1断层第二期高角度逆冲缩短变形使得渐新世-中新世断陷盆地封闭、靠近F1断层的底部砾岩层卷入了挤压逆冲断层作用，断层拖曳使地层产状翘起变陡，这期变形持续到上新世冲洪积扇粗砾岩出现；F1断层第三期与第四期虽然都为近东西向挤压，但第三期为东西向对冲挤出，而第四期为北东向和北西向斜向走滑挤出，其动力学机制是否与青藏高原东北缘西部地壳增厚隆升诱发的中-下地壳向东流动拖曳导致的上地壳东西向挤压缩短尚待证实，由于第三和第四期变形的构造形迹在上新统韩家沟砾岩不存在，因此，这两期变形的时代只能是发生在中新世末期或上新世早期；第五期北东和北西向共轭破裂系统和第六期南北向和东西向共轭破裂系统在渐新统-中新统沉积地层和上新统粗砾岩地层中都存在，其时代无疑是上新世末期或第四纪以来的构造变形，但第五期共轭断层指示的最大主压应力为近南北向，而第六期最大主压应力为北东-南西向，两者夹角约30°，指示两期变形最大主应力方向发生了30°的顺时针旋转，这可能与青藏高原东北缘变形重组过程中块体旋转有关。上述F1断层丰富且复杂的构造变形形迹揭示的断层变形方式和历史演变对于澄清青藏高原东北缘新生代红层盆地构造属性认识上的分歧和高原变形是均匀增厚变形和块体沿断层挤出滑移地壳变形机制的争论等提供了重要的构造依据。

The multiple deformation of the northern margin fault of the western Qinling and its tectonic dynamic significance

The northern margin fault of the West Qinling is a boundary fault between the Cenozoic basin and the West Qinling block in the northeast margin of Qinghai-Tibet Plateau. Its deformation history and geometric-kinematic characteristics could provide the constraints to the tectonic attributes of Cenozoic basins and spatial-temporal dynamics of crustal uplift and deformation of the northeast margin of the Qinghai-Tibet Plateau which could help understanding the remote tectonic response to the Indian-Eurasian collisional convergence. Based on the detailed structural analysis for the fault rock zoning, the morphology and geometric-kinematic characteristics of various structural elements, and the superposition and cutting relationship between different structural elements of the southern boundary F1 fault in the northern margin of West Qinling, the six phases of structural deformation with different properties and geometric-kinematic characteristics was identified in the F1 fault zone as follows:the first phase is characterized by NWW trending with northward dipping, ductile or ductile brittle extensional normal faulting by north-south tectonic extension; the second phase is characterized by NWW trending with north steep dip, brittle extrusion thrusting by north-south tectonic compression; the third phase is characterized by the north-south strike with west dipping thrust faulting formed by east-west compression; the fourth phase characterized by strike-slipping conjugate fault system with minor oblique thrusting in which NE trending fault is dextral and NW trending is sinistral formed by east-west compression as above; the fifth phase formed a conjugate fault system formed by north-south compression with sinistral NE trending faulting and dextral NW trending faulting, both fault planes is vertical; the sixth phase formed sinistral EW trending strike-slipping fault and NS trending dextral strike-slipping fault which constitute a conjugate fracture system with geometric-kinematic coordination formed by NE-SW compression. On the above F1 fault deformation analysis, in combination with the rifting basin sedimentary sequence of Oligocene-Miocene sedimentary basin, Pliocene alluvial-fluvial fan coarse conglomerate with molasses deposit, and different phases of faulting play different role with the controlling basin deposit or tectonic involved sedimentary strata in the northern margin of West Qinling, it is suggested that the first phase extensional normal faulting of F1 fault developed at Oligocene which controlled the deposition of Oligocene-Miocene extensional faulted basin; the second phase shortening-high angle thrusting deformation of F1 fault led to closure of Oligocene-Miocene basin and involved in compressive thrusting deformation and uplift, until the emergence of Pliocene alluvial-fluvial fan coarse conglomerate, therefore, this phase of deformation of F1 fault should start the end of Miocene epoch or Early Pliocene and continued to the end of Pliocene; although both the third and fourth phases deformation of F1 fault formed by E-W compression, the third phase faulting take on inverse faulting with SN trending, east-dip or west-dip fault plane, while the fourth phase take on strike-slipping conjugate fault system with minor oblique thrusting. Whether its dynamic mechanism is related to the short east-west compression of the upper crust caused by the eastward flow of the middle and lower crust induced by the crustal thickening and uplift in the west of the northeast margin of the Qinghai-Tibet Plateau remains to be confirmed. Since this structural deformation has not been found in the Pliocene Hanjiagou conglomerate, it is suggested that the third and fourth phases deformation of F1 fault maybe developed in the Early Pliocene; the fifth phase deformation with the NE and NW trending conjugate fracture systems and the sixth phase deformation with N-S and E-W conjugate fracture systems of F1 fault, both developed in the Oligocene-Miocene sedimentary strata and Pliocene coarse conglomerate strata; undoubtedly, the deformation occurred after Pliocene and before Quaternary. In addition, the direction of the maximum principal compressive stress forming two phase conjugate fracture systems are different, fifth phase with N-S direction and the sixth phase with NE-SW direction which indicate that the maximum principal compressive stress rotates clockwise by 30° from fifth phase to sixth phase. This rotation of the maximum principal compressive stress may be related to the block rotation in the expansion process of the northeast margin of the Qinghai-Tibet Plateau. The fault deformation modes and historical evolution revealed by the rich and complex structural deformation traces of F1 fault provide an important structural evidences for clarifying the controversial understanding of the tectonic attributes of the Cenozoic red bed basin on the northeast margin of the Qinghai-Tibet Plateau and the issues about the plateau deformation mechanism, that is, uniform thickening model or block strike-slipping extrusion mechanism along the fault.