EVOLVING MID-CRUSTAL FLOW ALONG THE SOUTHERN MARGIN OF THE TIBETAN PLATEAU; AMA DRIME AND MOUNT EVEREST MASSIFS, TIBET

Channel flow models, as applied to the Himalayan-Tibet orogen, propose that south-directed flow of mid-crustal material from beneath the Tibetan plateau is driven by gradients in lithostatic pressure that resulted from Tertiary collision. The structures that bound the proposed mid-crustal channel have received considerable attention and the Pressure-Temperature-time history of these rocks, which spans ~38-12 Ma in the Mt Everest region, is well established. Geophysical and isotopic data define: 1) a zone of partial melting beneath the Tibetan plateau and; 2) active gradients in lithostatic pressure between the Tibetan plateau and the foreland. Modern, focused denudation along the range-front is well constrained and models demonstrate its capacity to draw warm mid-crustal rocks toward the surface. However, the faults and shear zones that bound this proposed mid-crustal channel have been inactive since the middle Miocene. Thus a temporal disconnect exists between the models and the timing of activity on geologic structures that are capable of accommodating extrusion and exhumation of mid-crustal rocks.

Our interdisciplinary research establishes links between geophysical, geomorphic and geologic datasets. The cessation of crustal melting in the uppermost portion of the ~35 km thick GHS at ~17 Ma was followed by melting in the deepest structural positions currently exposed in the Mt Everest region at 11-13 Ma. Anatexis at this time was limited to the lower portion of the mid-crust (exposed in the Ama Drime Massif) where it created a low-viscosity channel that was linked to the onset of orogen-parallel extension. These rocks were exhumed to the surface via two oppositely dipping shear zones and detachment systems suggesting that mid- crustal flow on the southern margin of the Tibetan plateau is linked to the upper crust via a network of relatively narrow shear zones and fault systems during south-directed and orogen-parallel mid-crustal flow. We propose that localization of melting in the upper or lower portion of the mid-crust is the dominant control on the bulk rheology of the mid-crust. Our data also suggests that the transition from south-directed to orogen-parallel extension was partially related to the onset of deeper crustal flow that presumably extends into the more interior portions of the Tibetan plateau.