SYNOROGENIC COLLAPSE - A PERSPECTIVE FROM THE MIDDLE CRUST, THE PROTEROZOIC GRENVILLE OROGEN

Structural, petrological, and geochronological studies of the middle to late Proterozoic Grenville orogen in Ontario, Canada, indicate that a major extensional fault developed synchronously with late thrusting. This fault zone was initiated during peak metamorphism and extended into the crust to depths of at least 25 kilometers. The temporal and spatial relations among faulting, metamorphism, and regional compression indicate that synorogenic collapse initiated because the crust exceeded the maximum physiographic height and thickness that could be supported by its rheology. Comparison of Grenville with recent Himalayan orogenic activity suggests that during Proterozoic times physiographic height, crustal thickness, and crustal strength were similar to modem conditions in orogenic belts.