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Seismic imaging of the Main Frontal Thrust reveals a shallow décollement and blind thrusting


IPGP - Îlot Cuvier


Séminaires Géosciences Marines

Salle 310

Judith Hubbard


Because great earthquakes in the Himalaya have an average recurrence interval exceeding 500 years, most of what we know about past earthquakes comes from paleoseismology and tectonic geomorphology. We use active-source seismic reflection to image the subsurface in the vicinity of the fault tip of the Main Frontal Thrust (MFT) in Central Nepal. Our results, which are the first of their kind in the Himalaya, illuminate the fault system and provide direct imaging of the décollement and both emergent and blind fault strands. We show that the décollement lies at 2.5 km depth, not the 5 km commonly assumed in the literature, which implies that either previous shortening estimates are too low by a factor of two, or that there are steps in the décollement level that could segment future earthquakes. We demonstrate that some fault strands do not reach the surface, and therefore trenching these strands to study offset layers is not possible. We identify a beveled, erosional surface buried beneath younger sediments, indicating that the vertical movement of river terraces relative to the active river channel is affected by changes in base level or sediment supply, as well as tectonics. We make the first direct observations of fault dip on two strands of the MFT at depth, which allow uplift measurements from terraces to be converted to fault slip. Together, these results indicate that the use of traditional paleoseismological tools based on trenching and river terraces must be applied cautiously in Nepal and other parts of the Himalaya.