Tectonics of the Bhutan Himalaya: New Insights from Seismic Tomographic Images of the Lithosphere

Abstract

The present-day orogenic collision structure of the Himalayan mountain belt is the result of long-term tectonic processes of frontal upper crustal accretion, internal deformation, exhumation and thrusting along major fault zones. To provide the first detailed seismic images of the Eastern Himalaya for better understanding the lateral structural variations in this region and to provide insights into the present-day seismotectonics of the Bhutan Himalaya a temporary, passive seismic network, the GANSSER network, of 38 stations was deployed in the Kingdom of Bhutan between January 2013 and November 2014. Based on these new seismic data, we derive the main seismic lithospheric structure using different seismological methods, including receiver function imaging and upper crustal ambient noise group velocity tomography. To accurately image lateral variations and dipping seismic converters with receiver functions in this complex tectonic region, we extend a 2-D high-frequency ray approximation common conversion point stacking method to 3-D and include linear phase weighting and a quality assessment. Our results of the seismic structure reveal significant along-strike variations in the crustal collision within Bhutan and spatially correlates with variations in the seismic behavior of the Main Himalayan Thrust (MHT) and the underthrusting Indian crust. In western Bhutan, crustal thickening is indicated by a distinct downdipping of the Moho and coincides with a pronounced mid-crustal ramp at the front of the Higher Himalaya. Clustered microseismicity located along this ramp and a high shear-wave velocity anomaly atop indicate localized tectonic deformation in its hanging wall and likely stress buildup on a fully coupled MHT towards the south, as proposed by tectonic models of the long-term growth of the Himalayan orogen. In eastern Bhutan, the underthrusting of the Indian crust is imaged as sub- horizontal structure at around 50 km depth with distinct intra-crustal listric structures beneath the MHT. Seismicity in this region is concentrated at shallow depth at 12 km, likely representing a local, partially creeping fault segment of the MHT. In contrast, brittle deformation in western Bhutan is predominantly located in the underthrusting Indian crust by scattered strike-slip faulting. In correlation with the regional large-scale lithosphere collision structure in this part of the Himalayan arc, we propose that these variations in the seismic structure and behavior are primarily driven by an Indian mantle-slab in northwest of Bhutan, and its absence in the Northeast. Besides these first-order lateral variations, our results of upper crustal shear-wave velocity anomalies provide the first seismological evidence for localized duplexes of the Lesser Himalayan Sequence at the base of the Himalayan orogenic wedge related to the formation of tectonic windows such as the Paro window in western Bhutan or the Kuru-Chu half-window in eastern Bhutan. Distinct intra-crustal structures resolved by the receiver function converter in the Indian crust, such as the listric structures in eastern Bhutan, indicate internal deformation and suggest a long-term involvement of the underthrusting Indian crust in the orogeny of the Bhutan Himalaya. Beneath Sikkim and western Bhutan and its southern foreland, the occurrence of a broad zone of mid-crustal seismicity, in correlation with a low P-wave velocity anomaly, indicate an active NW-SE striking dextral fault zone in the Indian crust. We name this fault the Dhubri-Chungthang Fault Zone (DCF), which might connect the deformation front of the Himalaya with the Shillong Plateau in the foreland. A continuation of this peculiar oblique-striking fault zone across the MHT would imply a segmentation of megathurst events between eastern Nepal and western Bhutan. Strike-slip faulting along the DCF the Indian crust and beneath western Bhutan might take up parts of the convergence in this region, however, its influence on the average recurrence time of large earthquakes or on the apparent instrumental lower seismic activity in western and central Bhutan remains unclear.