New insights into the structural heterogeneity and geodynamics of the Indo-Burma subduction zone from ambient noise tomography

Abstract

The absence of fine lithospheric-scale velocity models beneath Myanmar makes it difficult to understand the neotectonics and geodynamics along the Indo-Burma subduction zone. In this study, we present a high-resolution crustal and uppermost mantle 3-D shear-wave velocity (Vs) model of Myanmar to fill this knowledge gap, using ambient noise data from newly deployed seismic arrays. In the upper crust, our model reveals two thick (>10 km) N-S-elongated basins between the Indo-Burman Ranges and the Central Myanmar Basin. At middle to lower crustal depths, low velocities dominate the Indo-Burman Ranges, especially in its northern part where Vs is observed to be as low as ∼3.2 km/s in the lower crust. This feature is interpreted as sediments that were deposited west of the ranges and have since been subducted northeastward and accreted onto the overriding plate. Furthermore, our model reveals an N-S trending high-velocity anomaly beneath the Sagaing Fault, which could be explained by solidified basaltic magma that intruded upwards from the mantle where a low-velocity anomaly is imaged. In the upper mantle, the subducting Indian Plate is clearly imaged beneath Myanmar as an east-dipping high-velocity zone, overlain by a prominent wedge-shaped low-velocity body (Vs < 4.3 km/s). We interpret this low-velocity anomaly to represent partial serpentinization (19–38%) in the forearc mantle. The size and amplitude of this anomaly decrease towards the north, suggesting a northward reduction in serpentinization level within the forearc mantle, possibly related to a northward reduction of water in the subduction zone. This could be associated with lower water content in the subducting plate, as the thick sediments deposited in the north may have driven water out of the lowermost section, while the upper sedimentary section, which could still have been carrying water, would have been scraped off of the downgoing plate and accreted onto the overriding plate, forming part of the Indo-Burman Range.

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