摘要
利用中国地震科学探测台阵、小江流动台阵、固定台站等共516个地震台站记录资料,通过波形相关方法获取了111 718个远震P波到时数据,运用层析成像技术研究获得了青藏高原东南缘深达650 km的上地幔三维P波速度结构。结果表明,四川盆地岩石圈具有克拉通地区的高速结构特征;青藏高原东部地区表现为活动地块常见的低速异常;四川盆地西南部下方岩石圈可能受到青藏高原软流圈物质的侵蚀。在四川盆地南部及以南地区,由北向南从浅到深连续分布的高速异常体,可能是岩石圈底部拆沉造成的。岩石圈拆沉作用可能导致上地幔热物质上涌,地壳介质在热作用下力学强度降低,受青藏高原地壳物质侧向挤出作用,导致该地区陡峭的地形地貌和较强的地震活动性。攀枝花附近存在明显的高速异常,可能与三叠纪地幔柱成因的岩浆上升过程中镁铁质和超镁铁质岩浆侵入岩石圈有关。
In this paper, we collected 111718 P-wave traveltimes of 888 tele-earthquakes recorded by 516 temporary and permanent seismic stations in the study area. Using the highest density of seismic station distribution up to now, we got the fine P-wave velocity structure deep to 670 km of the southeast of the Tibetan Plateau with application of FMTT technique. The result shows that high-velocity anomaly exists in the lithosphere of Sichuan Basin, and low-velocity anomaly in the east of the Tibetan Plateau. We educe this phenomenon is caused by the downward material flow from the asthenosphere of the Tibetan Plateau eroding the base of Sichuan Basin lithosphere. There is a continuous high-velocity anomaly dipping south in the south of the Sichuan Basin, which may be caused by the detachment of the lithosphere base. The detachment of the lithosphere brings about the upwelling of upper-mantle hot material, which decreases the mechanical strength of crust and together with the eastward intrusion of the Tibetan Plateau material gives rise to the steep topography and high seismic activity. The P-wave velocity is higher in Panzhihua than the surrounding area above the depth of 50 km. According to the geological setting of this place, we conclude that it is caused by the intrusion of mafic and ultramafic magmas during the Permain-Triassic magmatism activity which created the Emeishan Large Igneous Province.