Exhumation of Ultrahigh-Pressure Rocks from the Overriding Plate of a Continent Collision - An Example from the Greenland Caledonides

Ultrahigh-pressure (UHP) metamorphism has long been known from western Norway along the subducted margin of Baltica, but has recently been discovered in Laurentian continental crust of North-East Greenland, which sits in the overriding plate of the Caledonian orogen. The late timing with respect to the collision, the spatial location some hundreds of kilometers away from the suture, and the abnormally high temperature (>950 °C) of UHP metamorphism in Greenland indicates that UHP conditions were probably attained in an intracontinental subduction setting. An earlier period of HP metamorphism indicates overthickened crust, much like that found in Tibet today, setting up conditions favorable for intracontinental subduction. The Greenland UHP rocks underwent a two-stage exhumation process similar to many UHP terranes around the globe. The tectonic setting deduced for the formation of UHP metamorphism strongly influences the possible exhumation mechanisms. The initial phase of exhumation is the most cryptic because tangible structures that might be associated with it are impossible to identify. A subduction zone geometry at least provides a pathway back to the lower or middle crust that may have been opened during a change in plate motion from convergent to divergent at the very end of the Caledonian collision. The second phase of exhumation was likely aided by simultaneous motion on conjugate strike-slip faults and low angle extensional detachments with orogen parallel displacement. The ultimate result of this combined deformation history would be lateral escape of continental crust that contained both the HP and UHP terranes. Final exhumation to the surface was accomplished by orogen parallel, high angle normal faults that form the rifted margin of the present day Atlantic Ocean.