IS TOPOGRAPHIC GRADIENT THE ONLY DRIVING FORCE FOR CHANNEL FLOW IN LARGE-HOT OROGENS?

The current paradigm derived from thermomechanical modelling of orogenic systems is that a topographic gradient, such as present in the Himalaya, is required to drive Poiseuille-type, channel flow. We present 1D analytical solutions for channel flow in orogens demonstrating that lateral density gradients in the upper crust, such as would occur across a suture zone separating arc rocks from pericratonic sediments, or the buoyancy induced by partial melting of a channel flowing up an inclined plane, provide driving forces for Poiseuille flow as large as topographic gradients observed in modern mountain belts. Because a channel flowing up a ramp detaches from its lid, it could constitute an important exhumation mechanism in large hot orogens. Our calculations indicate that mid-crustal channel flow was a highly likely process in the Late Cretaceous-Paleocene setting of the southeastern Canadian Cordillera. The flow was first driven by the lateral density contrast between pericratonic sediments and the arc-related Intermontane Terrane, then by combined effect of topographic gradient and melt-induced buoyancy of the Lower Selkirk Allochthon (part of the Shuswap Complex). Flow up the underthrusting basement ramp resulted in exhumation from mid- to upper-crustal levels. Channel flow then migrated downward to involve basement and overlying cover sequence rocks. Our results indicate that syn-convergent channel flow was a viable and very likely process in large-hot orogens such as the Canadian Cordillera. The Grenville Province is also a good candidate because it consists of a complex collage of rock types, partial melting is widespread and stiff lower crustal blocks could have constituted inclined ramps. Indeed, our results based on fieldwork conducted in the Manicouagan Reservoir area led us to propose a tectonic evolution very similar as that suggested for the Canadian Cordillera.