台湾-阿尔泰地学断面阿尔金-龙门山剖面的地壳纵波速度结构

通过分析阿尔金—龙门山地学断面的地震资料，建立了该剖面的地壳纵波速度结构。研究结果表明，阿尔金北侧的塔里木盆地地区莫霍面为50km,而在其南侧的祁连地块莫霍面突然加深至73km，在柴达木盆地莫霍面又抬升至58km左右，然后，在松潘甘孜地块莫霍面降至70km，并呈现为台阶状向龙门山方向抬升到60km左右，最低速层，而在其南部地区则没有低速层出现，推测低速层为地壳中部的局部熔融物质，阿尔金—龙门山剖面上的两个莫霍面坳陷区分别与祁连地块和松潘—甘孜地块上的两个莫霍面坳陷区相对应，指示出这个两个地块具有较深的山根，青藏高原北部的巨厚地壳很可能是由于中生代以来发生的印度板块与亚洲板块碰撞时受到来自东西及南北方向的挤压，使地壳缩短所致。

The Crustal P-Wave Velocity Structure From Altyn Tagh to Longmen Mountains along the Taiwan-Altay Geoscience Transect

Based upon the seismic experiments along Geoscience Transect from the Altyn Tagh to the Longmen Mountains,the crustal P-wave velocity structure was derived to outline the characteristics of the crustal structure.The section shows a few significant features.The crustal thickness varies dramatically,and is consistent with tectonic settings.The Moho boundary abruptly drops to 73km depth beneath the southern Altyn Tagh from 50km below the Tarim basin,then rises again to about 58km depth beneath the Qaidam basin.Finally,the Moho drops again to about 70km underneath the Songpan-Garze Terrane and rises to 60km near the Longmen Mountains with a step-shape.Further southeast,the crust thins to 52km beneath the Sichuan basin in the southeast of the Longmen Mountains.In the north of the Kunlun fault,a low-velocity zone,which may be a layer of melted rocks due to high temperature and pressure at depth,exists in the the bottom of the middle crust.The two depressions of the Moho correlate with the Qilian and Songpan-Garze terranes,implying that these two mountains have thick roots.According to our results,it is deduced that the thick crust of the northeastern Tibetan Plateau probably is a result of east-west and northwest-southeast crustal shortening since Mesozoic time during the collision between the Asian and Indian plates.