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变形（lithospheric deformation）的东西向差异，本文构建了横穿柴达木盆地西部（western Qaidam Basin, WQB）与柴达木盆地东部（eastern Qaidam Basin, EQB）的两条剖面的热流变模型（thermo-rheological models）。同时阐述了其东西向地球动力变形样式（geodynamic deformation styles）的差异，涉及GPS运动（GPS motions）、震源机制（P轴）、地震各向异性（SKS波分裂）以及低速带（LVZs）。柴达木盆地西部表现为具有软弱下地壳的温暖失稳克拉通盆地（cratonic basin），其流变结构自阿尔金-柴达木边界（Altyn-Qaidam boundary）至柴达木盆地西部东北隅，由弱焦糖布丁型（crème brûlée-1）流变体制转变为强果冻三明治型（jelly sandwich-1）流变体制。结合低速带的有限分布，以及P轴（N20°E）与SKS波分裂（N110°E）的不匹配模式所揭示的强壳幔解耦（crust-mantle decoupling）现象，研究认为沿阿尔金断裂带（Altyn Tagh Fault）发生的塔里木盆地（Tarim Basin）俯冲楔入（under-thrusting）所引发的壳幔混合（crust-mantle mixing）作用，是柴达木盆地西部克拉通失稳的主要构造动力诱因（tectonic dynamic inducement），该作用大幅削弱了岩石圈强度（lithospheric strength），进而促成了浅部脆性变形（shallow brittle deformation）。然而，柴达木盆地东部则表现为典型的寒冷刚性克拉通盆地，具有果冻三明治型（jelly sandwich-2）流变体制，其强度足以维持壳幔耦合（crust-mantle coupling）。近期广角地震剖面（wide-angle seismic profiles）揭示的柴达木盆地东部下方的低速带，对岩石圈强度衰减（lithospheric strength drop）的影响十分有限。柴达木盆地东部可被视为锚定于青藏高原东北部（Northeastern Tibetan Plateau）的刚性盆地，其对软弱高原物质的北东向挤出（NE extrusion）具有极强的阻挡作用，进而引发顺时针旋转（clockwise rotation）。