Structure of the upper mantle and transition zone beneath Southeast Asia from traveltime tomography

摘要

Tomographic images of the mantle beneath East Asia were obtained from the inversion of traveltime data from global and regional seismograph networks and from temporary arrays on and around the Tibetan plateau. Our results are consistent with previous studies but the unprecedented resolution of mantle heterogeneity provides new insight into the large-scale tectonic framework of the continental India-Asia collision in the western part of the study region and subduction of the oceanic lithosphere in the east. In the realm of continental collision, west of ～100°E, a relatively slow P-wave speed characterizes the upper mantle beneath much of the Tibetan plateau but the wave speed is high beneath cratonic India, the southern and western part of the Tibetan plateau, Hindu-Kush, and the Tian Shan. In the subduction realm, east of ～110°E, the main structures are (i) pronounced low-wave-speed anomalies at a depth of between 100 and 400 km beneath Asia's southeastern seaboard and the back-arc regions of ongoing subduction; (ii) narrow, fast anomalies in the upper mantle beneath major subduction zones; and (iii) widespread fast anomalies at a depth of 500-700 km beneath the Sea of Japan, the northern part of the Philippine Sea plate, and southeastern China. If the latter anomalies represent stagnant slabs, their fragmented nature and large lateral extent suggest that they are produced by different episodes of subduction beneath western Pacific island arcs, along the old SE margin of Asia, or during the Mesozoic collision of cratonic units in Southeast Asia. Attribution to ancient subduction systems implies that slab fragments can reside in the transition zone for (at least) several tens of millions of years; Shallow, slow anomalies beneath the Red River fault region connect to deep anomalies beneath the South China fold belt and South China Sea, suggesting a causal relationship between the evolution of the continental lithosphere of SW China and deeper mantle processes. Between the collision and the subduction realms, tomography reveals high-wave-speed continental roots beneath the western part of the North China craton (Ordos block) and the South China, or Yangtze, craton (Sichuan Basin) to a depth of ～300 km.