Abstract:

Since the Cenozoic era, the ongoing collision between the Indian Plate and the Eurasian Plate has formed the largest and youngest continent-continent orogenic belt on Earth. The Pamir Plateau, located at the western end of the India-Eurasia collision zone, is one of the most tectonically active and structurally complex regions globally. It is characterized by widespread folds and faults, frequent M≥7.0 earthquakes, and numerous intermediate to deep-focus earthquakes, making it an ideal natural laboratory for studying plate tectonics and orogenic processes. Investigating the velocity and anisotropic structure of the uppermost mantle beneath the Pamir Plateau and its surrounding regions is of great significance for understanding the tectonic deformation characteristics and dynamic mechanisms of continent-continent collisions. The Pn-wave travel-time tomography method is an effective approach for studying the physical properties of the uppermost mantle, it offers several advantages: 1)Pn-wave ray paths are concentrated in the uppermost mantle, and there is a relatively abundant record of arrival times; 2)this method can simultaneously obtain mutually constrained velocity and anisotropy structures of the uppermost mantle; and 3)although it only provides lateral velocity and azimuthal anisotropy information for the uppermost mantle, it offers higher resolution and accuracy, and the results are not influenced by deeper mantle structures. In this study, using 363 414 Pn-wave arrival times recorded from 103 190 events at 471 stations, we performed Pn-wave travel-time tomography to obtain higher-resolution seismic velocity and anisotropy images of the uppermost mantle beneath the Pamir Plateau and adjacent regions compared to previous studies. The tomography results exhibit a clear correlation with geological structures, with the following key findings: 1)Lower Pn-wave velocities are observed in tectonically active regions such as the Pamir Plateau, Tienshan, Hindu Kush, West Kunlun, and Altyn Tagh, while higher velocities are found in stable ancient blocks like the Indian Plate, Tarim Basin, Tajik Basin, Junggar Basin, and Fergana Basin. 2)The India-Eurasia collision has generated strong Pn-wave azimuthal anisotropy. On both sides of the Pamir collision zone, in the Indian Plate, Tarim Basin, and Tajik Basin, the anisotropy axes align with the plate motion directions. In contrast, in the central part of the collision zone, the anisotropy axes are nearly perpendicular to the maximum compressive stress and crustal motion directions. A similar anisotropy pattern is observed on both sides of the Tienshan. Pn-wave velocities primarily reflect the properties of the uppermost mantle, which is predominantly composed of peridotite. Temperature has a more significant influence on velocity than pressure, and temperature variations are closely linked to tectonic activity. Typically, stable cratonic regions exhibit higher Pn-wave velocities, while tectonically active or volcanic regions with significant fluid activity show lower velocities. The velocity differences between active and stable blocks are mainly attributed to temperature variations. Comparing the surface boundary between the Eurasian and Indian Plates with the high-velocity anomalies observed in tomography reveals that the high-velocity anomaly of the Indian Plate extends approximately 200km northward beneath the Tibetan plateau. Pn-wave azimuthal anisotropy is generally attributed to the preferred orientation of olivine crystals caused by mantle deformation. The anisotropy direction of the Indian Plate is predominantly north-south, consistent with its GPS motion and maximum compressive stress directions. From the Tajik Basin to the western Tarim Basin, the fast-axis anisotropy direction gradually shifts from NW to NE, aligning well with the maximum compressive stress and GPS directions. This is primarily due to the simple shear between the crust and upper mantle caused by crustal shortening and uplift, while the lithospheric mantle subducts during intense plate collision. Similarly, in the Fergana Basin, northern Tarim Basin, and Junggar Basin, the anisotropy directions are nearly north-south, consistent with GPS and maximum compressive stress directions. In regions of intense deformation, such as the Hindu Kush, Pamir Plateau, West Kunlun, Altyn Tagh, and Tienshan orogenic belts, the anisotropy directions are perpendicular to the maximum compressive stress directions, indicating strong pure shear deformation in the uppermost mantle. This suggests that in continent-continent collision zones, not only does the crust undergo significant shortening and uplift, but the uppermost mantle also experiences substantial compressional deformation. Finally, we propose an improved dynamic model of continent-continent collision to elucidate the collision process between the Indian Plate and the Tarim and Tajik Basins, as well as the mechanisms of anisotropy formation.

Key words: Pamirs, uppermost mantle, Pn-wave, velocity and anisotropy

摘要：

帕米尔高原位于印度-欧亚碰撞带西端, 研究帕米尔高原及其周边地区的上地幔的速度和各向异性结构对于认识陆-陆碰撞的构造变形特征及动力学机制具有重要意义。Pn波的射线路径集中在上地幔顶部, 其横向速度变化能够显示上地幔构造活动差异, 而其方位各向异性结构则能够显示出地幔物质的运动及形变特征。文中利用363 414个Pn波到时数据, 通过对Pn波到时进行层析成像, 获得了帕米尔高原及其邻近区域上地幔顶部的高分辨率地震波速度和各向异性图像。层析成像结果与地质构造显示出明显的相关性, 主要结果表明: 1)帕米尔高原、 天山、 兴都库什、 西昆仑、 阿尔金山等构造活动区的Pn波速较低, 而印度板块、 塔里木盆地、 塔吉克盆地、 准噶尔盆地、 费尔干纳盆地等古老稳定地块显示出高Pn波速度特征。2)印度-欧亚板块碰撞产生较强的Pn波各向异性, 在帕米尔高原碰撞区两侧的印度板块、 塔里木盆地和塔吉克盆地, 各向异性方向与板块运动方向一致, 而在碰撞区的中间部位, 各向异性方向与最大压应力方向及地壳相对运动方向几乎垂直, 可能是由于碰撞带上地幔的纯剪切变形所致。天山两侧也存在类似的特征。

关键词: 帕米尔高原, 上地幔顶部, Pn波, 速度和各向异性