MIGRATING EXHUMATION OF MID-CRUSTAL ROCKS IN COLLISIONAL OROGENS: PROPOSAL OF A NEW FAULT MODEL FROM THE SOLUKHUMBU HIMALAYA

Most collisional orogens around the world expose regions of mid-crustal rocks that were exhumed during orogenic processes. In the case of the Himalaya, there is a laterally continuous belt of largely amphibolite facies rocks known as the Greater Himalayan Sequence (GHS). Early work of Heim and Gansser (1939) recognized this belt and the fault zone bounding its southern limit, the Main Central Thrust (MCT). Since that time, numerous studies have recognized a laterally continuous extensional structure that bounds the north margin of the GHS, the South Tibetan Detachment System (STDS), inspiring a number of models (e.g., channel flow and its hybrids) to explain the exhumation or “unroofing” of the mid-crustal GHS between the MCT and the STDS. It is now recognized that these models need to consider the role of the multiple shear zones or metamorphic discontinuities that exist between the MCT and the STDS. We have identified several distributed shear zones within the GHS in the Solukhumbu Himalaya and Sagarmatha National Park of eastern Nepal and are currently investigating their geometry, sense of motion and age relative to the MCT and STDS. Not surprisingly, preliminary evidence suggests multiple generations of shearing. There are several normal-sense shear zones (down-to-the-north) in the core of the GHS near Namche Bazaar and Khumjung that cut older contractile fabrics, ranging from sub-parallel to highly oblique relative to older thrust fabrics. Early work on one shear zone in the southern part of the GHS revealed young (~4-9Ma) cooling ages (Hubbard and Harrison, 1989). The GHS also hosts discrete zones of brittle deformation, suggesting that faulting and shearing have been active processes potentially for the past 20Ma. We propose a kinematic model for mid-crustal exhumation that involves progressive forelandward unroofing of the GHS, broadly concurrent with progressive south-directed footwall imbrication of the Himalayan frontal thrust system over the subducting Indian plate. This is a working, testable hypothesis we call “migrating exhumation” which we believe has analogues in other recent convergent orogens and provides an actualistic alternative to various channel flow models for the GHS. Future work will focus on timing among GHS shear zones to understand the interplay of convergence and exhumation.