STRUCTURAL-KINEMATIC SETTING OF THE 2015 GORKHA, NEPAL EARTHQUAKES: LESSONS FROM A CRITICALLY TAPERED OROGENIC WEDGE

The April, 2015 Gorkha earthquake (Mw=7.8) and aftershocks took place within the Himalayan thrust belt at depths of 15-18 km corresponding to the location of the Main Himalayan detachment, which marks the boundary between underthrusting India and the overlying Himalayan orogenic wedge. A restorable regional structural cross-section in central Nepal shows that the earthquake probably occurred along the basal detachment beneath the Lesser Himalayan duplex (LHD), a huge complex of north-dipping imbricate thrusts that formed between about 12 and 5 Ma. The LHD is a fundamental regional-scale structural feature of the Himalayan thrust belt. Faults within the duplex fed slip southward and upward from the basal thrust into a structurally higher roof thrust in Paleoproterozoic metasedimentary rocks of the Lesser Himalayan sequence. The fact that the Gorkha event did not rupture the surface implies that deformation was confined to the internal part of the Himalayan orogenic wedge, and that future, probably more energetic, earthquakes will eventually feed this deformation upward and southward toward the Main Frontal thrust (MFT). Growth of the Lesser Himalayan duplex may be viewed as a classic problem in orogenic wedge dynamics. The duplex could be a manifestation of internal deformation related to subcritical taper, whereas slip on the MFT may reflect either stable sliding or attainment of critical taper accompanied by self-similar growth of the orogenic wedge. Most of the duplex growth took place during Late Miocene time, and post-Miocene activation of the Main Boundary and Main Frontal thrusts, accompanied by internal blind thrust deformation farther north, are consistent with self-similar growth of a critically tapered orogenic wedge. Thus, the well-known seismic gap along the MFT in central-western Nepal is not likely to be a stable feature associated with a subcritically tapered segment of the thrust belt. Instead, this part of the thrust belt is probably critically tapered and growing self-similarly by a combination of frontal accretion and internal deformation, and seismogenic energy accumulating in the internal part of the orogenic wedge is expected to propagate to the orogenic front within a geologically brief time span.