T74 Orogeny: Relating Rigid Plates to Diffuse Lithospheric Deformation II

Sunday, 31 October 2010
1:30 PM - 5:30 PM
Lobby F

STRAIN PARTITIONING IN AN OBLIQUE CONVERGENT OROGEN: APPLICATION TO THE WESTERN HIMALAYAN SYNTAXIS AND KARAKORAM FAULT

Paper number: T74-5
Program: Technical Sessions
Time: 2:35 PM – 2:50 PM

Authors

Abstract

Many orogens feature regions of oblique convergence, accommodated by either oblique thrust faulting or a system of partitioned strike-slip and near-orthogonal thrust faults. Analogue and numerical modeling studies suggest that the transition in orogen deformation behavior between oblique thrusting and partitioned slip is primarily a function the angle of obliquity, such that partitioned behavior is not observed below ~45°. In the Himalayan orogen, the angle of convergence obliquity varies along strike, such that it is greatest near the syntaxes. The western Himalayan syntaxis exhibits an apparent obliquity of ~40°, yet the geometry of the bounding thrust and interior strike-slip (Karakoram) faults, geodetic data and paleostress measurements suggest strain is partitioned. Here we present results from 3-D thermomechanical numerical modeling experiments of the collision of two continental plates with an oblique interface. The model uses a frictional plastic and power-law creeping viscous crustal rheology, and crustal deformation results from kinematic velocity boundary conditions that drive subduction of the mantle lithosphere. These boundary conditions generate a finite-width, doubly vergent orogen with orthogonal convergence at 50 mm/y on either end of a central oblique section featuring an obliquity angle of 40-65°. In addition, model topography either develops in the absence of surface erosion or subject to total erosion to investigate the role of topographic feedback in the orogen deformation behavior.

We do not observe strain partitioning in models with a 45° obliquity angle irrespective of the surface topography, but both strike-slip and near-pure thrust shear zones are seen in models with an obliquity of 65°, even without surface topography. This suggests that factors other than the angle of obliquity and topographic growth, such as ductile strain weakening of the shear zones, may have played an important role in the development of large-scale strike-slip faults like the Karakoram Fault. Another potential factor is the Tibetan Plateau, which features significant gravitational potential energy that enhances orogen-normal spreading. Current experiments are exploring the potential for large plateaus to aid in partitioning strain in oblique convergent orogens.