Mountain Building Orogeny in Precollision Hot Backarcs: North American Cordillera, India-Tibet, and Grenville Province

In this article I discuss the new perspectives on continental collisional orogeny from recent evidence that most continental subduction zones have 200- to 1,000-km-wide uniformly hot backarcs. Such hot backarcs are involved in most current and past continental collisions. I give the examples of the current India-Asia and ancient Grenville orogens. (1) In continental collision, at least one side of the closing ocean has a backarc sufficiently hot and weak to be deformed by normal plate driving forces, in contrast to cool stable lithosphere that is too strong. Most deformation is concentrated on the subduction zone side of collision zones. (2) Precollision backarcs are hot enough to generate a ductile detachment in the lower crust, facilitating subsequent thrusting of the incoming stable continent under the upper-middle crust of the backarc. (3) In hot backarcs there is a precollision low-viscosity channel in the lower crust that facilitates lateral flow from beneath collision-generated thickened crust. In continental collision orogenic belts, the high temperatures indicated by ductile deformation, metamorphism, and igneous activity are usually ascribed to heating by the orogenic process. I argue that the primary heat source is the uniformly hot backarc that predates collision. Most continental backarcs have temperatures of 800-850 degrees C at a 35-km Moho, compared to 450 degrees C for normal stable crust, and lithosphere thicknesses of 60-70km compared to about 200km. High temperatures that weaken the lithosphere may not be a consequence of orogeny but rather a precollisional requirement for most orogenic deformation and crustal thickening. Plain Language Summary This article discusses the nature of mountain building when an ocean closes and continents collide, especially the consequences of the crust being very hot and weak on one side of the collision. Examples are the modern India-Asia collision and the ancient 1000-Ma-old collision represented in the Grenville Geological Province of eastern North America.