Fundamental Controls on Mid-Crustal 'Escape' Flow in Orogenic Systems

Although the classic channel flow model quite elegantly explains a number of seemingly unrelated features in the Himalayan-Tibetan system, the condition(s) that may have led to deactivation or redirection of the channel at the southern Himalayan front remains poorly understood. One very intriguing possibility is that mid- to lower-crustal efflux of mass and heat was redirected to the east as the frontal Himalaya transitioned to critical wedge-style slip along the Main Boundary and Main Frontal thrusts, implying an interesting spatial and temporal partitioning of deformation in these systems. If this ‘escape’ flow does indeed occur along the eastern Tibetan margin, what is clear from geophysical and geodetic studies is that the flow path is fundamentally controlled by the heterogeneity of basement blocks with different rheological character. Because such ‘escape’ flow may play a prominent geodynamic role in collisional shortening accommodation, we need to understand: (a) how may a crustal channel flow be deflected from orogen-normal to orogen-parallel flow, (b) are the rheological boundaries controlling the geometry of escape flow merely a function of pre-existing lithological differences or do they result from transient thermal effects such as heat advection, and (c) do the thermal and rheological conditions recorded in both the presumed ‘escape’ flow channel and in the bounding channel buttress and/or crustal lid match those conditions necessary for flow to occur in numerical simulations? Here, we propose to address these questions by testing the following hypotheses in the southern Appalachians: (1) The southern Appalachian Inner Piedmont acted as a weak crustal channel during the Neocacadian (Devonian-Mississippian) orogeny, with the channel developing in peri-Laurentian margin crust subducted beneath the accreting Carolina superterrane, and (2) The switch from northwest orogen-normal (ON) to southwest directed orogen-parallel (OP) flow of the channel, inferred from mesoscale solid-state fabrics and map-scale fold and fabric geometries, resulted from an inability of the weak channel to either overcome a much stronger upper crustal lid or drive continued deformation of the cold thrust wedge in front of the channel. Such flow deflection may represent a paleo-analog of the flow system operating beneath the eastern Tibetan plateau in the present day.