沿阿穆尔板块西边界的活动断层(蒙古领土)

Active faulting along the western boundary of the Amur plate(territory of Mongolia)

阿穆尔板块西部边界在蒙古境内的空间位置尚不清楚,并且活动断层构造及其沿线地壳的应力状态研究较少。本文在沿此边界的三个区域——杭爱—肯特构造鞍部、布尔古特地块(鄂尔浑—土拉交汇处)和色楞格地块(包括色楞格凹陷和布伦—努鲁隆起),利用空间图像解译、地形起伏度分析、地质构造资料以及构造压裂和沿裂缝位移资料重建构造古应力,对活动断层进行研究。研究表明,活动断裂继承了古生代和中生代古构造的非均质性。这些断层沿着板块边界并不是单一的带,而是成簇的。它们的运动取决于走向:亚纬向断层是具有一定逆分量的左旋走滑断层,北西向断层是逆断层或逆冲断层,通常具有右旋走滑分量,海底断层是右旋走滑断层,北东向断层是正断层。位于色楞格凹陷和杭爱东部的断裂构造的活动始于上新世。逆断层和走滑断层与上新世情况不符,但多与更新世地貌相符,表明其活动年代较晚,为更新世时期。利用构造断裂和沿断裂的位移,重建活动断裂带变形末阶段的应力应变状态,结果表明断裂在最大挤压轴的北北东和北东方向上以压缩和走滑为主。只有在色楞格凹陷内,以扩张和走滑类型的应力张量为主,且在最小挤压轴的北西走向尤为显著。在南部,杭爱东部(鄂尔浑地堑)内有1个以扩张机制为主的局部区域,说明蒙古中部断裂在更新世—全新世阶段的活动以及现代地震活动主要受与印度斯坦和欧亚大陆汇聚过程相关的东北方向的附加水平挤压的控制。使研究区地壳产生走滑变形、贝加尔湖裂谷发散活动以及阿穆尔板块东南运动的另一个因素是东南方向软流圈流动对岩石圈底部的影响。阿穆尔板块和蒙古地块之间的边界在构造结构上是零碎的,代表了覆盖整个蒙古西部变形带的边缘部分。

The spatial position of the western boundary of the Amur plate within the territory of Mongolia is still not clear;active fault tectonics and the stress state of the Earth’s crust along it have been poorly studied.Within three regions along this border—the Hangay-Khentiy tectonic saddle,the Burgut block(Orhon-Tola interfluve)and the Selenga block,which includes the Selenga depression and the Buren-Nuruu uplift,studies of active faults were carried out using space imagery interpretation,relief analysis,geological structural data and reconstruction of tectonic paleostresses from tectonic fracturing and displacement along with fractures data.It is shown that active faults inherit ancient structural heterogeneities of the Paleozoic and Mesozoic ages.The faults do not form a single zone along the plate boundary,but form clusters.Their kinematics depend on the strike:sublatitudinal faults are left-lateral strike-slip faults with an obligatory reverse component;NW-strike faults are reverse faults or thrusts,most often with a right-lateral strike-slip component;submeridional faults are right-lateral strike-slip faults;and NE-strike faults are normal faults.The activation of fault structures localized in the Selenga depression and in the eastern part of the Hangay began in the Pliocene.Revers and strike-slip faults are not conformal to the Pliocene,and often to the Pleistocene relief,which indicates a younger,Late Pleistocene,age of their activation.Reconstructions of the stress-strain state of the last stage of deformation in zones of active faults,using tectonic fracturing and displacements along fractures,indicate the predominance of compression and strike-slip conditions with the N-NE and NE direction of the axis of maximum compression.Only within the Selenga depression is the prevalence of stress tensors of extension and strike-slip types with the NW strike of the axis of minimum compression noted.To the south,a local area with a predominance of the extension regime is located within the Eastern Hangay(Orhon graben).It is concluded that the activation of faults in the central part of Mongolia at the Pleistocene-Holocene stage,as well as modern seismicity,are mainly controlled by additional horizontal compression in the NE direction associated with the process of convergence of Hindustan and Eurasia.An additional factor that allows the implementation of strike-slip deformations in the crust of the study area and explains the divergent movements in the Baikal Rift,as well as the SE movement of the Amur plate,is the impact on the base of the lithosphere of the asthenospheric flow in the SE direction.The boundary between the Amur plate and the Mongolian block in the tectonic structure is expressed fragmentarily and represents the marginal part of the deformation zone covering the whole of Western Mongolia.