Anisotropy and mantle flow of the Indo-Burma subduction zone from shear wave splitting and shear wave splitting tomography

Abstract

The Indo-Burma subduction zone lies on the eastern boundary of the 1500 km long Alpine-Himalayan orogenic belt. Despite highly oblique plate motion with limited arc development, convergence is still ongoing with the potential to generate megathrust earthquakes that could affect more than 140 million people. This sub-aerial subduction system provides an excellent opportunity to study the nature of mantle deformational fabric resulting from the active convergence and collision with possible clockwise rotation of the Indian plate. Characterizing the mantle flow field helps us to better understanding the geodynamics and regional tectonics of this part of the plate boundary. Shear wave splitting is a simple yet powerful technique to investigate anisotropy and mantle strain fabric. I have used both teleseismic and local shear waves to create a detailed map of the upper mantle anisotropic fabrics using a recently deployed temporary and permanent seismic stations, across the northern and central parts of Myanmar. Along with the detailed analysis of splitting parameters, I have generated a tomographic model inverting local and teleseismic shear waves splitting results. The resulting 3D anisotropic model helps to image the prevailing mantle flow below the slab in the sub-slab region and above the slab in the mantle wedge. I found a trench parallel fast direction with high lag time (>2.0s) on the fore-arc that primarily accumulated in the sub-slab region. Such observed fabric results from the toroidal flow around the slab in response to the slab roll-back. The mantle wedge shows a complex deformational fabric with an average lag time of over 0.5s. Dextral motion along the Sagaing fault induces a dominant trench parallel fabric on the mantle wedge. Null results in the southern part of the Shan plateau suggest that back-arc upwelling may exist between the transition of the east-west fast direction on the southern Yunnan and the north-south fast direction of the western subduction margin. The nature of the transition of the mantle fabric and back-arc upwelling is not well understood due to the lack of seismic station coverage on the Shan plateau.