TIMING OF EXTENSION IN THE GREENLAND CALEDONIDES AS AN ANALOGUE FOR THE FUTURE EXHUMATION OF OVERTHICKENED CRUST BENEATH THE HIGH HIMALAYA

Synorogenic collapse is well documented along the South Tibet detachment system in the Himalayas where mid-crustal migmatites and leucogranites were exhumed beneath Tethyan metasedimentary rocks at 17-11 Ma. Ongoing Indo-Asian plate collision has produced a 70 km thick crustal section beneath the High Himalaya, raising the question of when in the future of this orogen might the present deep crust be exhumed? Recent work on the timing of extension in the ancient, collapsed collisional Greenland Caledonide orogen may offer some insight. Ductile extensional detachments in Greenland divide the regional geology into a lower, middle and upper plate. Mid-crustal leucogranites and migmatites, analogous to those in the High Himalaya, formed between 424 and 430 Ma and are thought to be synchronous with extensional detachment faults that place them in contact with overlying, low-grade Neoproterozoic to Lower Paleozoic metasedimentary rocks of the upper plate. The lower plate includes HP granulites and related anatectic melts beneath the Payer Land detachment (74°28'-74°47'N, 22°30'-23°40'W). Granulite facies metasedimentary rocks contain garnet + kyanite + K-feldspar + antiperthite (exsolved ternary feldspar) + quartz +/- biotite +/- rutile, and record PT conditions of 800 to 850°C and 1.4-1.5 GPa. U-Pb SHRIMP ages of the HP metamorphism were obtained from zircon in two samples. Metamorphic rim analyses (n=8) on zircon from a metapsammite yield a weighted mean ²⁰⁶Pb/²³⁸U age of 403 +/- 5 Ma. Rim and euhedral core analyses (n=24) of zircon from an anatectic melt derived from metapelite yield a weighted mean ²⁰⁶Pb/²³⁸U age of 404 +/- 4 Ma. This age for HP metamorphism precludes post-orogenic extension and exhumation of deep level rocks before 405 Ma, roughly coeval with the estimated age of exhumation of eclogite terranes (i.e. lower plate equivalents) in the Scandinavian Caledonides. Orogenic collapse in the Caledonides appears to have taken place in two stages: synorogenic extension dated by leucogranites followed 24-30 Ma later by post-orogenic exhumation of the deepest level. An old, well-exposed orogen such as the Caledonides preserves the entire complex history of continent-continent collision and collapse. We may have to wait another 10 to 20 Ma to see the overthickened crust beneath the present Himalayas.