Himalayan mid-crustal ramp

Abstract

The presence of the Himalayan mid-crustal ramp under the southern Higher Himalaya has been proposed by several investigators, on the basis of analyses of microseismicity, seismic reflection, gravity anomaly, uplift rate and inferred subsurface geological data. We analyse focal depths and fault plane solutions of the moderate thrust earthquakes of the Himalayan seismic belt derived from body wave inversion, along with the hypocentral parameters of reliably located earthquakes whose focal depths have been estimated on the basis of ISC-reported surface reflected phases. These earthquakes occurred in a narrow belt of about 25 km width along the northern Lesser and southern Higher Himalaya. A more or less systematic increase in the focal depths and dips of the inferred fault planes in north-northeast direction along the four cross-sections across the Himalaya along with the available evidences, supports the presence of this ramp under the southern Higher Himalaya. These earthquakes occur on and above the mid-crustal ramp that lies in the transition zone between the seismically active detachment under the Outer and Lesser Himalaya and the aseismically slipping detachment under the Higher and Tethys Himalaya.