Seismological Evidence for Multiple Incomplete Crustal Subduction Beneath the Himalayas and Southern Tibet

Abstract

AbstractGeological evidence has indicated that large‐scale crustal shortening can be accounted for by thrust faulting beneath the Himalayas, such as MCT and MBT. The thrust faulting near the Yarlung Zangbo suture was also observed . However, it is still unknown to what depth those thrust faults have under thrust into the crust or upper mantle. Seismological evidence indicates that thrust faults in the Himalaya and southern Tibet have extended to the depth of 80–100 km, and stopped there. The thrust faultings beneath the MCT, MBT and Yarlung Zangbo suture are closely related to multiple crustal subduction beneath the Himalayas and southern Tibet. It provides important constraints for the collision process between India and Eurasia. The incomplete crustal subduction in this region suggests that the impinging Indian crust is too light to prevent it going further down, and retreat of crustal subduction is required for the continuing northward movement of the Indian plate. The multiple crustal subductions are compatible to the multiple episodes of geological activities in the Himalaya and southern Tibet. At first, crustal subduction occurred at the Yarlung Zangbo suture, and stopped at the depth of 80–100 km. Then, it migrated to the south, other new crustal subductions would start successively at MCT and MBT, respectively. They also stopped at the depth of 80–100 km. Beside the north‐dipping seismic zones in the Himalaya and Yarlung Zangbo suture, another gentle south‐dipping seismic zone, extending from Tanggula Shan at the surface to the Moho at the Yarlung Zangbo suture was also observed. It can be interpreted to be an obduction at Tanggula Shan. The multiple incomplete crustal subduction beneath the Himalaya and the Yarlung Zangbo suture highly imply that the impinging Indian crust has been splitted apart from its upper‐most mantle, which may be subducted deeper into the Eurasian mantle.