PRESENT-DAY KINEMATICS OF THE ORDOS BLOCK AND ITS SURROUNDING AREAS FROM GPS OBSERVATIONS

doi:10.3969/j.issn.0253-4967.2020.02.005

Abstract

Abstract: Located among the South China block, Tibetan plateau, Alxa block and Yinshan orogenic belt, the Ordos block is famous for its significant kinematic features with stable tectonics of its interior but frequent large earthquakes surrounding it. After the destruction of the North China Craton, the integrity, rotation movement and kinematic relations with its margins are hotly debated. With the accumulation of active tectonics data, and paleomagnetic and GPS observations, some kinematic models have emerged to describe rotation movement of the Ordos block since the 1970's, including clockwise rotation, anticlockwise rotation, clockwise-anticlockwise-alternate rotation, and sub-block rotation, etc. All of these models are not enough to reflect the whole movement of the Ordos block, because the data used are limited to local areas.
In this study, based on denser geophysical observations, such as GPS and SKS splitting data, we analyzed present-day crustal and mantle deformation characteristics in the Ordos block and its surrounding areas. GPS baselines, strain rates, and strain time series are calculated to describe the intrablock deformation and kinematic relationship between Ordos block and its margins. SKS observations are used to study the kinematic relationship between crust and deeper mantle and their dynamic mechanisms, combined with the absolute plate motion(APM)and kinematic vorticity parameters. Our results show that the Ordos block behaves rigidly and rotates anticlockwise relative to the stable Eurasia plate(Euler pole: (50.942±1.935)°N, (115.692±0.303)°E, (0.195±0.006)°/Ma). The block interior sees a weak deformation of~5 nano/a and a velocity difference of smaller than 2mm/a, which can be totally covered by the uncertainties of GPS data. Therefore, the Ordos block is moving as a whole without clear differential movement under the effective range of resolution of the available GPS datasets. Its western and eastern margins are characterized by two strong right-lateral shearing belts, where 0.2°~0.4°/Ma of rotation is measured by the GPS baseline pairs. However, its northern and southern margins are weakly deformed with left-lateral shearing, where only 0.1°/Ma of rotation is measured. Kinematics in the northeastern Tibetan plateau and western margin of the Ordos block can be described with vertical coherence model with strong coupling between the crust and deeper mantle induced by the strong extrusion of the Tibetan plateau. The consistency between SKS fast wave direction and absolute plate motion suggests the existence of mantle flow along the Qinling orogenic belt, which may extend to the interior of the Ordos block. SKS fast wave directions are consistent with the direction of the asthenosphere flow in Shanxi Rift and Taihang Mountains, indicating that the crustal deformation of these areas is controlled by subduction of the Pacific plate to North China. The week anisotropy on SKS in the interior of Ordos block is from fossil anisotropy in the craton interior. After comparing with the absolute plate motion direction and deformation model, we deem that anisotropy in the interior of Ordos block comes from anisotropy of fossils frozen in the lithosphere. In conclusion, the Ordos block is rotating anticlockwise relative to its margins, which may comes from positive movement of its margins driven by lithospheric extrusion or mantle flow beneath, and its self-rotation is slight. This study can provide useful information for discussion of kinematics between the Ordos block and its surrounding tectonic units.

Key words: Ordos block, micro-dynamic block movement, strain time series, SKS shear wave splitting

摘要： 鄂尔多斯地块的运动学特征和动力学机制深受地学界关注。文中基于GPS数据和SKS剪切波分裂结果等地球物理资料, 分析了鄂尔多斯地块及其周缘现今的壳幔运动学特征。结果表明, 鄂尔多斯地块相对于欧亚大陆呈现逆时针旋转, 欧拉极位于俄罗斯东南部, 欧拉矢量为(50.942±1.935)°N, (115.692±0.303)°E, (0.195±0.006)°/Ma; 块体内部变形微弱, GPS速率差异<2mm/a, 应变率<5nano/a, 应变时间序列的变化范围为-10~10nano, 均在GPS的误差范围之内, 表明在现有GPS资料的有效分辨范围内, 鄂尔多斯块体内部相对完整, 不存在明显的差异运动。块体西缘和东缘活动强烈, 形成了2条明显的右旋剪切带, 旋转速率为0.2°~0.4°/Ma; 块体南缘和北缘活动较弱, 边界断裂有左旋运动性质, 旋转速率约0.1°/Ma。青藏高原东北缘和鄂尔多斯块体西缘的壳-幔变形完全一致, 满足垂直贯通模型, 变形由青藏高原东北缘强烈的推挤作用引起; 块体南部到秦岭造山带的地震各向异性与绝对板块运动方向一致, 表明该区域存在地幔流通道, 且已深入到鄂尔多斯块体内部; 山西断陷带到太行山的SKS剪切波分裂的快波偏振方向与软流圈地幔流动方向一致, 表明该区域受控于太平洋板块的俯冲作用; 鄂尔多斯块体内部微弱的SKS各向异性来自于克拉通内部“化石”的各向异性。综合上述资料分析, 鄂尔多斯地块相对于其周缘的旋转运动可能主要来自于其周缘构造带在岩石圈和软流圈作用下的主动运动, 块体的主动旋转可能比较微弱。

关键词: 鄂尔多斯块体, 块体微动态运动, 应变时间序列, SKS剪切波分裂

CLC Number:

P315.2

LI Zhang-jun, CHAI Xu-chao, GAN Wei-jun, HAO Ming, WANG Qing-liang, ZHUANG Wen-quan, YANG Fan. PRESENT-DAY KINEMATICS OF THE ORDOS BLOCK AND ITS SURROUNDING AREAS FROM GPS OBSERVATIONS[J]. SEISMOLOGY AND GEOLOGY, 2020, 42(2): 316-332.

李长军, 柴旭超, 甘卫军, 郝明, 王庆良, 庄文泉, 杨帆. 基于GPS观测的鄂尔多斯地块及其周缘现今的运动学特征[J]. 地震地质, 2020, 42(2): 316-332.

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