Differences in east-west thermo-rheological structure and lithospheric deformation in the Qaidam Basin, Northern Tibetan Plateau, China

To investigate the differences in east-west lithospheric deformation in the Qaidam Basin, we present thermo-rheological models of two profiles across the western Qaidam Basin (WQB) and eastern Qaidam Basin (EQB). The differences in east-west geodynamic deformation styles are also described, involving GPS motions, focal mechanisms (P axes), seismic anisotropy (SKS-wave splitting), and low-velocity zones (LVZs). The WQB is characterized by a warm destabilized cratonic basin with a weak lower crust, and the rheological structure changes from a rather weak crème brûlée-1 regime into a strong jelly sandwich-1 regime, from the Altyn-Qaidam boundary to the northeastern corner of the WQB. Combined with the limited distribution of the LVZs and the strong crust-mantle decoupling revealed by the unmatched pattern between P axes (N20ºE) and SKS-wave splitting (N110°E), the crust-mantle mixing related to the under-thrusting of the Tarim Basin along the Altyn Tagh Fault is suggested as the primary tectonic dynamic inducement of the destabilized WQB craton, which weakens the lithospheric strength greatly and contributes to the shallow brittle deformation. However, the EQB performs as a typical cold and rigid cratonic basin characterized by a jelly sandwich-2 rheological regime, which is sufficiently strong to maintain crust-mantle coupling. The LVZs beneath the EQB revealed by recent wide-angle seismic profiles have only a small effect on the lithospheric strength drop. The EQB could be regarded as a solid basin, anchored in the NE Tibetan Plateau, which shows a strong resistance to NE extrusion of the weak plateau material, leading to clockwise rotation.

为探究柴达木盆地岩石圈东西向变形差异，本研究构建了横跨柴达木盆地西部（Western Qaidam Basin, 简称WQB）与柴达木盆地东部（Eastern Qaidam Basin, 简称EQB）的两条剖面的热流变学模型。同时阐述了东西向地球动力学变形样式的差异，涵盖GPS运动、震源机制（P轴）、地震各向异性（SKS波分裂）与低速带（low-velocity zones, 简称LVZs）等内容。柴达木盆地西部表现为暖性失稳克拉通盆地，下地壳软弱；自阿尔金-柴达木边界至柴达木盆地西部北东角，其流变结构由偏弱的焦糖布丁-1型流变体制逐渐转变为强韧的果冻三明治-1型流变体制。结合低速带的有限分布特征，以及P轴（N20°E）与SKS波分裂（N110°E）的不匹配模式所揭示的强烈壳幔解耦现象，我们认为沿阿尔金塔格断裂的塔里木盆地俯冲逆冲作用引发的壳幔混合过程，是柴达木盆地西部克拉通失稳的首要构造动力诱因，该过程大幅削弱了岩石圈强度，进而促进了浅部脆性变形的发育。与之相反，柴达木盆地东部则为典型的冷性刚性克拉通盆地，具有果冻三明治-2型流变体制，其强度足以维持壳幔耦合。近期广角地震剖面揭示的柴达木盆地东部下方低速带，仅对岩石圈强度弱化产生微弱影响。柴达木盆地东部可被视为锚定在青藏高原东北部的刚性盆地，对软弱高原物质的北东向挤出具有极强的阻挡作用，进而引发顺时针旋转。