青藏高原东北缘地壳介质各向异性非均匀特征及其构造意义

本文利用径向和切向接收函数后方位角加权叠加方法分析了中国地震科学台阵探测项目二期资料, 确定了青藏高原东北缘海原断裂和西秦岭北缘断裂邻近区域地壳各向异性参数.结果显示, 研究区地壳各向异性快波方向表现出强烈的横向不均匀性, 快慢波延迟时间平均值为0.44 s; 快波方向受断裂带影响显著, 走滑断裂带附近快波方向和断层方向一致性较强, 逆冲断裂附近快波方向和断层方向差别较大或垂直; 晚中新世以来形成的海原断裂、香山—天景山断裂等多个走滑断裂附近地壳各向异性强度明显小于其他区域, 且形成时间更早的海原断裂方向和快波方向一致性更强.这种断裂附近各向异性强度和方向的分布特征也是该区域构造演化过程中逆冲断裂向走滑断裂转化的反映.陇中盆地快波各向异性结果复杂且没有明显规律性分布特征可能反映出该次级地块保留了较老构造过程中残存的化石各向异性.本研究结果表明东北缘地区高原扩展主要以连续流变模式为主, 在走滑断裂带附近存在局部侧向挤出, 但是侧向挤出影响的地壳变形范围较小.

Laterally heterogeneous crustal anisotropy in the northeastern margin of Tibetan Plateau and its tectonic implications

In this paper, we determined the crustal anisotropy parameters around the Haiyuan and West Qinling faults in the northeast margin of the Tibetan Plateau by using the azimuth weighted stacking method of radial and tangential receiver functions from seismic stations belong the ChinaArray Ⅱ projection. The fast wave direction (FD) of the crust anisotropy in the study area shows strong lateral heterogeneous, and the average delay time between the fast and slow wave is ~0.44 s. The FD is significantly affected by the fault zone. The FD near the strike-slip fault zone is consistent with the direction of the fault, while the FD is quite different or vertical near the thrust fault. The delay time of several strike-slip faults, such as Haiyuan fault, Xiangshan-Tianjingshan fault and other strike-slip faults formed since the Late Miocene is obviously less than that in other regions, and the FD near the Haiyuan fault formed earlier is more consistent with the fault direction. The distribution characteristics of anisotropic strength and direction near the fault are also the reflection of the transformation from thrust fault to strike-slip fault during the tectonic evolution of the region. The FDs in Longzhong basin are complex and have no obvious regularity, which may reflect the residual fossil anisotropy reserved in the ancient secondary block. The results indicate that the Tibetan Plateau expansion in the northeast margin is mainly controlled by continuous rheological mode, and there is local lateral extrusion near the strike-slip fault zone, but the range of crustal deformation affected by lateral extrusion is small.