U-Pb and Ar-40/Ar-39 constraints on the Fjord Region Detachment Zone: a long-lived extensional fault in the central East Greenland Caledonides

The high strain zone separating infracrustal ortho- and paragneisses from s upracrustal cover rocks in the East Greenland Caledonides has been interpre ted variously as (1) a combined Grenvillian and Caledonian thrust, (2) a Ve ndian extensional shear zone reactivated as a Caledonian thrust, (3) a Cale donian extensional detachment reactivated as a late Caledonian thrust or (4 ) a late Caledonian extensional detachment. In this study we present new ki nematic and geochronological data (U-Pb and Ar-40/Ar-39) from a well-studie d segment of the high strain zone in Kejser Franz Joseph Fjord, demonstrati ng top-to-the-east, normal-sense displacement. Syntectonic peraluminous gra nites in the infrastructure are interpreted to have formed by decompression al anatexis (c. 430-425 Ma) and were later deformed to ultramyloniles, cata clasites and psudotachylites along the high strain zone. Extension related fabrics overprint earlier and are synchronous with contractional top-to-the -NW thrust Faults and associated structures. Muscovite records progressivel y younger Ar-40/Ar-39 cooling ages (from 408 to 388 Ma) downward in the ext ensional footwall, whereas feldspar from the mylonites yield a closure age of 349 Ma. Foreland-directed shortening in the East Greenland Caledonides l asted until at least Early Devonian times, while orogenic extensional colla pse initiated along the zone of high strain during the Late Silurian in an overall collisional setting. Upper crustal thinning continued well into the Devonian along increasingly more narrow and brittle extensional faults.