HISTORY OF THE MAIN CENTRAL THRUST SYSTEM IN THE MARSYANDI VALLEY, CENTRAL NEPAL: EVIDENCE FOR STEADY-STATE OROGENESIS IN THE HIMALAYA?

The Main Central thrust (MCT) system, one of the largest deformational features in the Himalayan orogen, developed between 22-25 Ma and has accommodated over 100 km of shortening. Exactly how that shortening has been distributed over time is an outstanding problem in Himalayan tectonics.

Traditionally, most displacement on the system has been assigned an Early Miocene age. However, the discovery of Late Miocene-Late Pliocene 40Ar/39Ar cooling ages near the MCT in the 1980's suggested the possibility that post-Early Miocene slip might have occurred. The strongest evidence in support of this hypothesis comes from similar Th-Pb ages for monazite inclusions in garnets from the Lesser Himalayan sequence (Harrison et al., 1997, EPSL; Catlos et al., 2001, JGR). Catlos and co-workers inferred 30 km or more of slip on the MCT system over the past 3 m.y. However, the specific faults responsible for this slip were not identified.

We have surveyed new roadcuts along the Marsyandi valley (~84º23'E) that provide the best exposures of the MCT in the central Himalaya. Two strands of the MCT system have been mapped here by previous workers. The structurally highest (MCT I) is at the Greater-Lesser Himalayan tectonostratigraphic contact, whereas the lower one (MCT II) has been inferred to separate upper and lower parts of the Lesser Himalayan sequence. The new exposures, beginning structurally above the mapped position of MCT II and extending just beyond the position of MCT I, display strong evidence for multiple episodes of south-directed thrusting under a range of temperature conditions.The predominate ductile features are greenschist facies mylonites in schists and orthogneisses indicating ~S20W, thrust-sense displacement. We interpret these as Pliocene fabrics. The most unexpected result of our work was the observation of numerous younger, N-dipping, brittle thrust faults. The largest of these occur a few km south of MCT I. These previously unmapped faults correspond well with an apparent discontinuity in metamorphic conditions in the Lesser Himalayan sequence. No compelling evidence was found for late slip along MCT I.

Long-term slip on the MCT system has important implications for Himalayan orogenesis. Our model focuses on the role of dissipation of excess potential energy stored in the Tibetan plateau to explain the persistence of the MCT and other structural systems.