Fine velocity structures and deep processes in crust and mantle of the Qinling orogenic belt and the adjacent North China craton and Yangtze craton
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摘要: 秦岭造山带与其南北两侧华北克拉通和扬子克拉通属三大构造单元,不论其各构造单元体还是其界带构造均甚为复杂,并受到多期次构造运动的制约,形成了大陆内部特异的造山过程.尽管在这一地域曾做过大量的地表地质工作和一些相关的地球物理工作,但对其壳、幔精细结构、深层动力过程,特别是同步穿越华北克拉通、秦岭-大巴造山带和扬子克拉通系统的耦合研究甚少.为了研究和探索该地域的壳、幔精细速度结构和其形成的深层过程,专门布置了一条北起榆林,向南经咸阳、宁陕直抵涪陵长达1000 km的高精度地震宽角反射、折射波场探测剖面.通过剖面辖区高分辨率的数据采集,数据处理、反演和壳、幔层、块精细速度结构,发现剖面辖区深部壳、幔结构存在特异的速度和结构变化,并厘定了一系列的新认识.研究结果表明:(1)秦岭—大巴造山带具有同一基底,其形成乃为结晶基底隆升所致,即它的形成仅涉及到上地壳的受力变形和空间状态.造山带与其南、北两侧的前陆盆地为陆内造山过程中同一深层过程的产物,但其沉积速率和形态却不相同.华北克拉通与秦岭造山带之间前陆盆地Bfc拉张为该区Moho界面的局部隆升所致.(2)首次提出了沿1000 km长剖面连续的沉积建造、结晶基底、上地壳、下地壳和上地幔顶部的层、块速度结构和各界面的起伏变化与空间状态.基于地震波边界场响应厘定了华北克拉通、秦岭—大巴造山带和扬子克拉通的分区界带.论述了三大构造单元各自的内部结构和其相邻界域的速度变化特征.(3)该区大陆内部速度结构和不同类型断裂分布及层序在华北克拉通、秦岭—大巴造山带、扬子克拉通三大块体地域存在显著差异.不同规模、层次与产状的断裂分布反映出它们在变形行为和机制上及所受构造运动的制约上均存在明显的差异.Abstract: The three tectonic units, Qinling-Daba orogenic belt, North China craton and Yangtz craton, are very complicated in whether each tectonic unit system or each boundary structure. Restricted by many times of tectonic movements, the Qinling orogen and its adjacent areas have experiencedspecial orogenic processes of continental interior.Although some related work has been done in this area, there are little studies on the fine structure and deep dynamical processesin the crust and mantle and,particularly for the system coupling research of the North China craton, QinLing-Daba orogenic belt and Yangtze craton. In order to fill this gap, we deployeda high-precision wide-angle seismic reflection and refraction wave field exploration survey profile across this region. It began from Yulin north of Ningshan in the north and extended southward to Fuling with a length of 1000 km. Through high-resolution data acquisition,inversion and characterization of the crust and mantle fine velocity structure, we found the specific velocity structure of deep crust and mantle along the profile and obtain the following new knowledge. (1) With the same basement, the Qinling-Daba orogenic belt was resulted from the uplift of the crystalline basement. In other words, the formation of the orogen was only related to the forced deformation and the spatial state of the upper crust. The orogenic belt and its forelands of the both sides were the products of the same deep process in the intracontinental orogenic process, while local uplift of the Moho interface led to the extension of the foreland basin Bfcbetween the North China craton and Qinling orogen. (2)The continuous sedimentary formation, crystalline basement, upper crust, lower crust, interval velocity structure and block velocity structure of uppermost mantle, relief change and space state at each interface are first put forward along the 1000 km long profile.We define the partition zones among the North China craton, Qinling-Daba orogenic belt and Yangtze craton on the basis of the seismic wave boundary response. In this paper, we also discuss internal structures of the three tectonic units and the velocity variation features of the adjacent areas. (3) There are significant differences between the North China craton, Qinling-Daba orogen and Yangtze craton in velocity structure of the continental area, Q structure, distribution of different faults and sequences.The distribution of the faults with varied scales, depths and attitudes displays the marked differencesin theirformation, deformation behavior, mechanisms and constraints by tectonic movements.
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