Structure of the Orogenic Wedge in the Bhutan Himalaya: First Results from the GANSSER Seismic Experiment

Abstract:

In the Eastern Himalaya, the structure of the orogenic wedge above the subducting Indian lithosphere and even the Moho topography of the latter are poorly constrained at present. The Main Himalayan Thrust (MHT) is considered a crustal-scale detachment that marks the plate boundary separating the bottom of the orogenic wedge from the top of the Indian crust. In western and central Himalaya portions of the MHT have been imaged by receiver functions (RF) from the surface near the foothills of the Himalayan front range to mid-crustal depths. In Bhutan, however, the MHT and other major thrust geometries at depth remain a matter of debate, without direct geophysical image. Furthermore, earthquake activity in Bhutan seems to be significantly lower compared to most other parts of the Himalayan arc and there appears to be a lack of great earthquakes. This lower level of seismicity could be related to a more ductile deformation in the orogenic wedge, or, alternatively, result from a currently locked situation of the thrust system.

In this work, we use data of a densely spaced, temporary seismic network in Bhutan (GANSSER experiment) to determine lithospheric and intra-crustal discontinuities in the Eastern Himalaya with RF complemented by seismic velocities derived from simultaneous inversion of local earthquake data. Along two south-north profiles across the Himalayan range in eastern and western Bhutan the geometry of the dipping Indian Moho is defined by migrated RF images including a dip correction. Based on the characteristics of the RF, the dip and strike of the Moho is independently defined and the occurrence of intra-crustal conversion interfaces like the MHT are mapped across the orogenic wedge.

In combination with the geometry of the intra-crustal seismic structure, we use the seismic velocities and the hypocenters of local earthquakes to discuss the predominant deformation type in the orogenic wedge in Bhutan and the potential for large earthquakes in this region of the India-Asia collision.