蒙古-贝加尔裂谷区地壳应变场及其地球动力学涵义

蒙古-贝加尔地区是现今构造最活跃的大陆地区之一,其地壳构造运动及变形对我们理解大陆动力学问题具有重要的科学意义.基于融合的这一区域的GPS速度场,本文计算了该区应变率场和应变能变化率场.结果显示,蒙古褶皱带以南区域表现为NNE-SSW方向的压缩状态,主压应变率约为-2.0×10^-9/a,剪应变及面膨胀均较弱,表明蒙古褶皱带比较稳定.贝加尔裂谷整体处于拉张状态且伴有较强的剪应变和面膨胀,暗示可能有多种机制控制裂谷的张裂过程.蒙古高原西部有两条高应变率的构造带,结合深部存在地幔热柱等证据,我们认为这两条构造带及所围限的区域共同构成Amurian板块的西部边界—一条弥散变形的边界带.蒙古-贝加尔地区剪应变分布与0~40 km的地震活动性基本一致,表明该地区形变在地壳尺度耦合程度较高.地幔对流拖曳力场与主应变轴方向及应变率场的一致性表明,地幔对流可能是蒙古—贝加尔地区区域构造动力学过程主要控制因素之一.

Crustal strain field of the Mongolia-Baikal rift zone and its geodynamic implications

The tectonics and crustal deformation of the Mongolia-Baikal Rift Zone (MBRZ), which is one of the most active tectonic zones in continents, is significant for understanding continental dynamics. Based on combined GPS velocities of MBRZ, the strain rate field and the strain energy variation were calculated. It is found that the Mongolian fold belt is characterized by a NNE-SSW compression with a small rate of 2.0×10^-9/a, indicating the Mongolian fold belt is stable in current tectonics. The Baikal Rift Zone (BRZ) is being extended with a maximum strain rate of 2.5×10^-8/a and high shear strain and dilation are found along BRZ, which implies that the rifting of BRZ may be controlled by various mechanisms. Two high-strain belts are found in western Mongolia,combining the evidence of mantle plumes in deep region, we suggest that the two belts and the enclosed region could be regarded as the west boundary of the Amurian plate-a diffused deformation boundary zone. The consistency between the shear strain pattern and the seismicity with hypocenter depth less than 40 km indicates a high level of coupling of deformation within crust. The consistency between the mantle convection-generated drag force and the principle axis of strain and strain rate suggests that mantle convection might be one of the controlling factors for regional tectonic process in MBRZ.