STRIKE-SLIP FAULTING IN THE GREENLAND CALEDONIDES – OBLIQUE COLLISION OR ESCAPE TECTONICS?

The Greenland Caledonides, as part of Laurentia, are situated in the overriding plate of the collision with Baltica. A major sinistral strike-slip fault system, comprised of the Storstrømmen shear zone (north of 76°N) and the Western fault zone (south of 75°), runs approximately N-S along the length of the Greenland Caledonides. This fault system, together with the west directed thrusts in the foreland, has been interpreted as transpression resulting from Silurian to Devonian oblique collision between Baltica and Laurentia. This model ignores a significant dextral strike-slip fault, the Germania Land shear zone, which lies outboard of the Storstrømmen shear zone. Both structures cut the North-East Greenland eclogite province, which represents an overthickened crustal welt analogous to the Tibetan plateau. The Germania Land shear zone separates ≈400 Ma high-pressure (HP) gneisses and eclogites in the west from 360 Ma ultrahigh-pressure (UHP) gneisses and eclogites in the east. This fault zone may have originally been the locus of intracratonic subduction that allowed a UHP terrane to form in the overriding plate of the collision. Our U-Pb geochronology of zircon and titanite from both of these structures indicates that the shear zones were active from 370-340 Ma, well into the Carboniferous, and probably played some role in exhumation of both the HP and UHP rocks. We view the strike-slip systems as structures that allowed material to escape laterally northward (present geography) as the continent-continent collision waned. We postulate that the present day northern margins of Greenland and Norway marked the edge of the continents, allowing material to be swept into an oceanic area similar to the extrusion of material out of Tibet and into the South China Sea in the present day India-Asia collision.