TRANSITIONING FROM CHANNEL TO ‘ESCAPE’ FLOW IN LARGE HOT COLLISIONAL SYSTEMS: COMBINING STUDIES FROM THE HIMALAYAN AND SOUTHERN APPALACHIAN (NEOACADIAN) OROGENIC SYSTEMS

The concept of orogenic channel flow, wherein long-wavelength flow (100s of kilometers) of weak viscous mid- to lower-crust influences shortening accommodation and the redistribution of mass and heat in large collisional systems, remains as one of the most intriguing ideas to emerge in tectonics in the past ~20 years. This idea, which is mostly predicated on a series of whole-orogen numerical simulations, quite elegantly explains a number of seemingly unrelated features in the Himalayan-Tibetan (HT) system. However, these simulations also predict that the channel system should grow larger and more pervasive as the orogen accumulates mass and heat, but detailed petrochronologic studies in the HT system suggest that orogen-normal channel flow ceased by ~15 Ma. In the numerical simulations, cessation of channel flow to match the observations is obtained by a drastic reduction in the model erosion rate near the channel tip, which in turn causes the channel to retreat beneath the plateau. More recent interpretations based mostly on geophysical and geodetic data suggest that the flow has transitioned from orogen-normal to orogen-parallel ‘escape’ flow to the east. The present-day depth of these geophysical ‘anomalies’ beneath the Tibetan plateau, however, preclude our ability to directly examine the conditions and mechanisms that may control these hypothesized escape flow systems.

In the southern Appalachians, Hatcher and Merschat documented that an orogen-scale lineation pattern defined curved ductile flow in the exhumed high-grade rocks of the Inner Piedmont and proposed that this was the result of channel and escape flow during the Neoacadian (Devonian-Mississippian) orogeny. If correct, this terrane may yield our best opportunity to examine the mechanisms, stress conditions, P-T regimes, and timescales over which such flows may occur. Additionally, studies in this terrane should: (1) provide insight into channel boundary conditions; (2) allow us to test, both empirically and with rigorously designed 3D numerical models, the viability of using escape flow to explain observations in large hot collisional orogens, including the HT system; and (3) allow us to evaluate evidence for possible mechanisms that drive the transition from orogen-normal channel flow to orogen-parallel escape flow.