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

Recent structural, petrological and geochronological data from the Fjord Re gion of central East Greenland requires revision of tectonic models for thi s segment of the Caledonian-Appalachian orogen. The data document the exist ence of a regionally extensive, orogen parallel, east-dipping extensional d etachment zone (Fjord Region Detachment Zone) separating high grade, infrac rustal para- and orthogneisses and migmatites in the footwall or Central Me tamorphic Complex (basement) from overlying Neoproterozoic to Middle Ordovi cian metasedimentary rocks in the hanging wall. A tectonically disturbed de positional contact exists between the basement gneisses and overlying amphi bolite 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 fau lts in the hanging wall post-dates orogen-perpendicular contractional struc tures in the hinterland. U-Pb and Ar-40-Ar-39 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 l evel in the hinterland of an Early Devonian contractional event as indicate d in some previous models. Sedimentological as well as isotopic data from t he foreland suggest that extension in the hinterland was contemporaneous wi th folding and thrusting in the foreland. To explain these relationships we envision a scenario where upper crustal extension in the hinterland trigge red upwelling of a low viscosity ("fluid") lower crust in the hinterland, r epresented by the eastern part of the Central Metamorphic Complex. The hint erland welt exerted enough subhorizontal stress to cause east-west shorteni ng in the foreland. Diapiric upwelling of the inferred "fluid" lower fluid crust was facilitated by invasion of large volumes of granitic melt dated t o c. 425 Ma.