Basement-cover relationships and orogenic evolution in the Central East Greenland Caledonides

Recent structural, petrological and geochronological data from the Fjord Region of central East Greenland requires revision of tectonic models for this segment of the Caledonian-Appalachian orogen. The data document the existence of a regionally extensive, orogen parallel, east-dipping extensional detachment zone (Fjord Region Detachment Zone) separating high grade, infracrustal para- and orthogneisses and migmatites in the footwall or Central Metamorphic Complex (basement) from overlying Neoproterozoic to Middle Ordovician metasedimentary rocks in the hanging wall. A tectonically disturbed depositional contact exists between the basement gneisses and overlying amphibolite grade Neoproterozoic metasedimentary rocks along the western margin (towards the foreland) of the Central Metamorphic Complex. Top-to-the-east movement on the Fjord Region Detachment Zone and associated extensional faults in the hanging wall post-dates orogen-perpendicular contractional structures in the hinterland. U-Pb and 40Ar-39Ar ages from the detachment zone and underlying infracrustal gneisses indicate that extensional movement on the detachment zone started as early as in the Late Silurian and continued into the Middle Devonian. We have found no evidence in the upper crustal level in the hinterland of an Early Devonian contractional event as indicated in some previous models. Sedimentological as well as isotopic data from the foreland suggest that extension in the hinterland was contemporaneous with folding and thrusting in the foreland. To explain these relationships we envision a scenario where upper crustal extension in the hinterland triggered upwelling of a low viscosity ("fluid") lower crust in the hinterland, represented by the eastern part of the Central Metamorphic Complex. The hinterland welt exerted enough subhorizontal stress to cause east-west shortening in the foreland. Diapiric upwelling of the inferred "fluid" lower fluid crust was facilitated by invasion of large volumes of granitic melt dated to c. 425 Ma.