HR: 14:35h
AN: T23C-04    [Abstracts]
TI: Exhuming Large UHP Terranes: Insights From an Amphibolite to Ultrahigh-Pressure Transition in Western Norway.
AU: * Young, D
EM: davey@umail.ucsb.edu
AF: Department of Geological Sciences, University of California, Santa Barbara, CA 93106 United States
AU: Hacker, B
EM: hacker@geol.ucsb.edu
AF: Department of Geological Sciences, University of California, Santa Barbara, CA 93106 United States
AU: Corfu, F
EM: fernando.corfu@geo.uio.no
AF: Department of Geosciences, University of Oslo, Oslo, N-0316 Norway
AB: Various mechanisms have been proposed to account for the exhumation of continental ultrahigh-pressure (UHP) terranes; these include (i) lithosphere-scale normal faulting; (ii) extrusion of a `pip' or `sliver' along a basal thrust; and (iii) buoyant return of continental lithosphere after slab detachment. However, critical aspects of these models, most notably the existence of a sole thrust beneath the UHP crust, are yet to find compelling support in studies of large UHP terranes ($>$5000 km$^{2}$). We present the results of detailed structural analysis, eclogite thermobarometry and single grain U/Pb zircon geochronology across the southern boundary of the Nordfjord UHP province, Western Gneiss Region, Norway. Specifically, we find that peak metamorphic grade increases steadily over a 40 km distance from high-P amphibolite facies to the coesite-eclogite facies. This is accompanied by a continuous evolution from hornblende and epidote-bearing, prograde-zoned, incompletely recrystallized eclogites to blastic, high variance, unzoned UHP assemblages. Most importantly, this transition is not disrupted by any significant structural or metamorphic breaks. We conclude from these observations that the UHP transition results from in-situ prograde metamorphism, to increasingly greater depths northwestward. These data do not support thrust emplacement of the UHP rocks over lower pressure crust in the Nordfjord area. Furthermore, late-orogenic extrusion of the HP/UHP terrane is not proven by seismic evidence of a buried decollement beneath the WGR, or by simultaneous thrusting in foreland regions. Therefore, we argue that the HP/UHP rocks in the Nordfjord area remained essentially attached to the Baltica craton, and were exhumed by a combination of extensional unroofing and erosion, possibly coupled with flexural rebound of the continental lithosphere after slab failure at greater depth.
DE: 8102 Continental contractional orogenic belts
DE: 3660 Metamorphic petrology
DE: 1035 Geochronology
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting