HR: 0830h
AN: T21C-0469    [PDF]
TI: Topographic Signatures of Neotectonics in the Central Nepal Himalaya
AU: * Wobus, C
EM: cwobus@mit.edu
AF: Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139 United States
AU: Hodges, K
EM: kvhodges@mit.edu
AF: Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139 United States
AU: Whipple, K
EM: kxw@mit.edu
AF: Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139 United States
AB: The central Nepal Himalaya are characterized by a sharp topographic break, separating the High Himalayan peaks to the north from the more subdued topography of the Lesser Himalaya to the south. River longitudinal profiles derived from a 90-meter digital elevation model of the region consistently reveal a sharp decrease in normalized steepness indices across this transition, while no systematic changes in rock character are observed. These topographic data are consistent with a narrowly distributed decrease in rock uplift rate from north to south. In many of these drainages, $^{40}$Ar/$^{39}$Ar thermochronologic data from detrital muscovites also exhibit a profound change in cooling ages across the topographic discontinuity: samples with provenance from the north side of the discontinuity consistently record Miocene and younger cooling ages, while those derived from the southern side record Proterozoic or Paleozoic cooling ages. Field observations, including pervasive small-scale shear zones and hydrothermal activity in tributary valleys along the topographic front, are consistent with motion on a broad shear zone subparallel to the fabric of the Lesser Himalayan lithotectonic sequence. Combined, the geomorphic, thermochronologic, and field data suggest the presence of an extensive and previously unrecognized thrust-sense shear zone in the central Nepal Himalaya, approximately 20-30 km south of the trace of the principal fault of the Main Central Thrust system. Importantly, the results seem inconsistent with traditional models of the tectonic architecture of the Himalaya, which would imply similar deformational and thermal histories for rocks to the north and south of the topographic break. In this setting, river longitudinal profiles provide an important piece of data for narrowing the range of possible tectonic models for the region, and help us inform the sampling strategy for thermochronologic and structural studies.
DE: 1824 Geomorphology (1625)
DE: 8107 Continental neotectonics
DE: 9320 Asia
SC: Tectonophysics [T]
MN: 2003 Fall Meeting