Evidence for Along-Strike Variations in the Crustal Deformation beneath the Bhutan Himalaya from Receiver Function Imaging and Seismicity

Abstract:

In the Bhutan Himalaya seismicity and geologic surface features like the Kuru Chu Spur (an embayment of the Main Central Thrust) or the Paro window indicate along-strike variations in the collisional structure. The deeper structure of the orogenic wedge and associated deformation processes, however, are poorly understood partly due to the lack of seismic images of the crust.

To better understand these differences in structure and deformation, we use data of a temporary seismic broadband network in Bhutan to image the crustal structure with receiver functions (RF). We apply an iterative 3D wave-based migration scheme including a high-frequency ray approximation, which satisfies Snell’s law for dipping interfaces. With this approach we image variably dipping intra-crustal interfaces and the Moho topography across the Bhutan Himalaya, and identify lateral variations in the orogenic structure, which we interpret jointly with a new local earthquake catalog.

In West Bhutan, RF imaging depicts a northward dipping Moho at ~50 km depth. The low-angle dip steepens north of ~27.6°N which matches well observations by wide-angle seismics in South Tibet and the hypocenter of a deep crustal earthquake recorded by our network. We also identify the Main Himalayan Thrust (MHT) at ~14 km depth in West Bhutan with a ramp-like structure north of ~27.6°N. The ramp is characterized by a negative impedance contrast in the RF signals and coincides with a concentration of seismicity. In the East, the Moho appears to be almost flat at a depth of ~50 km without clear indications of steepening towards north. Beneath the Kuru Chu Spur in East Bhutan, we observe listric-shaped structures reaching from the upper crust beneath the Lesser Himalaya down to the Moho beneath the Greater Himalaya, which we interpret as a stack of crustal material typical for an accretionary wedge. While these structures appear aseismic, a horizontal alignment of seismicity at ~12 km depth suggests an active MHT in this region.