What Drives the Shift From Orogenic Channel Flow to "Escape" Flow in Large Hot Collisional Systems? A Comparison P-T-t and Kinematic Study Between the Himalayan-Tibetan and Southern Appalachian Neoacadian Systems

The channel flow model, wherein the removal of orogenic mass and heat is accommodated by long-wavelength (>100 km) lateral crustal flow, remains as one of the most intriguing yet enigmatic ideas in modern tectonics research. Although many seemingly unrelated features in the Himalayan-Tibetan (HT) system can be explained by this model, the apparent cessation of orogen-perpendicular flow and a possible switch to orogen-parallel "escape" flow at ~15 Ma remains as a substantial challenge for this hypothesis. In the original models, cessation of channel flow is facilitated by a substantial reduction in erosion rate from >10 mm/yr to <3 mm/yr at the channel tip, driving retreat of the channel beneath Tibet. However, relatively high sedimentation rates in the Bengal-Nicobar fan and relatively short exhumation-to-deposition lag times (~2.3 Myr) since 13 Ma does not support such an erosion reduction, and thus another causal mechanism(s) must be identified to explain this geodynamic shift. Based on the original model results, channel flow also requires that heat-producing material is continually accreted to the base of the overriding orogenic plateau north of the Main Central thrust (MCT) and South Tibetan detachment systems that are interpreted to bound the channel. However, during the Miocene, shortening accommodation and consequent basal accretion moved south of and structurally beneath the MCT, as the Main Boundary and Main Frontal thrusts became active. This slip partitioning transition may have acted to suppress orogen normal channel flow by (a) moving the accretion point south of the MCT, essentially limiting accretion of material to the channel, (b) creating a strong, cold, buttressing wedge in front of the channel, and (c) reducing or eliminating slip on the MCT, thus reducing the surface directed compression of isotherms, thus increasing the thickness and strength of the upper crustal lid and limiting channel propagation to the surface. If correct, this transition may explain the spatial and temporal partitioning of channel flow and critical wedge behavior, as proposed by previous studies. A similar integrated rheologic and kinematic evolution has been interpreted in the southern Appalachian Neoacadian system, which has also been proposed to preserve evidence of crustal channel and escape flows.