Along‐strike variations in the Himalayan orogenic wedge structure in Bhutan from ambient seismic noise tomography

Abstract

AbstractThe geological units and tectonic structure exposed in the Bhutan Himalaya document significant regional variations, expressed primarily as tectonic windows and klippen. The along‐strike variations of these structures and their metamorphic grade are usually associated with the formation of local duplexes in the underlying tectonic units. To investigate these variations and their extent in depth, we image the isotropic shear‐wave velocity structure of the orogenic wedge by ambient noise tomography. Group velocities are extracted from cross correlations of ambient seismic noise, recorded by the temporary GANSSER network in Bhutan. The upper crustal structure beneath Bhutan is mapped down to 18 km depth by directly inverting Rayleigh‐wave group velocity measurements in the period range between 2 and 20 s with a ray tracing based inversion approach. Our results reveal several distinct high shear‐wave velocity anomalies ( km/s) and reflect the along‐strike variations in the upper crustal structure in relation to the alternating tectonic windows and klippen at the surface. In correlation with the surface geology in the northern part of Bhutan, we interpret shallow high shear‐wave velocity anomalies as quarzite‐dominated rocks or felsic migmatites with large intrusions of leucogranites. High‐velocity anomalies in the orogenic wedge in eastern and western Bhutan correlate with the local geometry of the Main Himalayan Thrust and provide evidence for the formation and depth extent of localized duplexes of quartzite dominated lithology in association with the formation of tectonic windows in the Bhutan Himalaya.