Seismic Anisotropy and Mantle Flow Constrained by Shear Wave Splitting in Central Myanmar

This study represents the first campaign‐style teleseismic shear wave splitting (SWS) investigation of central Myanmar, an area that is tectonically controlled by the oblique subduction of the Indian Plate underneath the Eurasian Plate. The resulting 678 well‐defined and 247 null SWS measurements obtained from recently deployed 71 broadband seismic stations show that the Indo‐Burma Ranges (IBR) possess mostly N‐S fast orientations that are parallel to the trend of the depth contours of the subducted slab. Relative to the global average of 1.0 s, extremely large splitting times with station‐averaged values ranging from 1.28 to 2.79 s and an area‐averaged value of 2.09 ± 0.55 s are observed in the IBR. In contrast, the Central Basin (CB) and the Shan Plateau (SP) are characterized by slightly larger than normal splitting times. The fast orientations observed in the CB are mostly NE‐SW in the northern part of the study area, N‐S in the central part, and NW‐SE in the southern part. The fast orientations change from nearly N‐S along the N‐S oriented Sagaing Fault, to NW‐SE in the central and eastern portions of the SP. These observations, together with SWS measurements using local S events, crustal anisotropy measurements using P‐to‐S receiver functions, and the estimated depth of the source of anisotropy using the spatial coherency of the splitting parameters, suggest the presence of a trench‐parallel sub‐slab flow system driven by slab rollback, a trench‐perpendicular corner flow, and a trench‐parallel flow possibly entering the mantle wedge through a slab window or gap. Myanmar is located at the boundary between the Indian Plate and the Eurasian Plate. Here, the Indian Plate moves northward at a rate that is faster than most other tectonic plates on Earth and subducts obliquely beneath the Eurasian Plate. This subduction not only causes a strong deformation of the Earth's surface, forming the approximately 1,250 km long, N–S trending Indo‐Burma Ranges, but also results in pervasive crustal deformation and possibly modulates the mantle flow field in the area. Mostly due to the limited coverage by broadband seismic stations until recently, crustal deformation and mantle flow beneath Myanmar were poorly understood. In this study, we used data from 71 seismic stations that we deployed in central Myanmar to analyze seismic azimuthal anisotropy, that is, the directional dependence of the velocity of seismic waves in a medium, at different depths. Based on the established relationship between seismic anisotropy and mantle flow, we proposed a model to explain the observations. Our model implies the presence of a trench‐parallel mantle flow beneath the subducted Indian Plate. Above the plate, there are two flow systems with trench‐parallel and trench‐orthogonal orientations, respectively, with spatially varying strengths. N‐S anisotropy and large XKS splitting times (∼3 s) on the Indo‐Burma Ranges are from trench parallel flow and lithospheric shorteningXKS and local S results indicate trench‐parallel flow and corner flow coexist in the mantle wedge beneath the Central BasinE‐W oriented corner flow explains the splitting measurements in the Shan Plateau N‐S anisotropy and large XKS splitting times (∼3 s) on the Indo‐Burma Ranges are from trench parallel flow and lithospheric shortening XKS and local S results indicate trench‐parallel flow and corner flow coexist in the mantle wedge beneath the Central Basin E‐W oriented corner flow explains the splitting measurements in the Shan Plateau