EXHUMATION OF HP AND UHP ROCKS RELATED TO EARLY DEVONIAN THRUST AND EXTENSIONAL FAULTS, NORTHERN WESTERN GNEISS REGION, NORWAY

The northern Western Gneiss Region contains a UHP domain with microdiamonds, coesite pseudomorphs, and P estimates 4-6 GPa. It is dominated by near orogen-parallel sub-horizontal ductile sinistral shear and top-SW extensional fabrics overprinted by amphibolite-facies mylonites, greatly obscuring earlier features related to subduction and exhumation, and creating complicating segmentation. Nevertheless, preserved transverse top-SE eclogite-facies fabrics are identified in Nordøyane, equated with subduction and earliest exhumation. Also identified are zones with top-NW transport in early extension. Robust U-Pb zircon chronology indicates peak UHP eclogite-facies crystallization at 415-410 Ma, followed by pegmatite crystallization at 395 Ma in neck lines of boudins produced in ductile subhorizontal extension, thus limiting the process to 15-20 million years.

Farther north the Agdenes Extensional Detachment is identified by a break in titanite U-Pb geochronology. Basement gneisses below contain Proterozoic igneous titanite fully reset at 395Ma. Ordovician granitoids above contain igneous titanite barely influenced by Devonian recrystallization, implying removal of a significant section on a very large-scale detachment. Rocks both above and below are overprinted by the same subhorizontal sinistral extensional fabric, obscuring fabrics related to the Detachment.

The east of the Region in Trollheimen escaped the strong subhorizontal overprint, and shows this sequence: 1) Early emplacement of thrust nappes of Lower and Middle Allochthons over Baltican basement and Late Neoproterozoic quartzite cover. 2) Major SE-directed recumbent folding of the entire thrust-imbricated sequence including basement. 3) Major out-of-sequence SE-directed thrusting (Storli Thrust) of the recumbent folded sequence over deeper less deformed lower basement gneisses and unconformable Neoproterozoic quartzite cover. This thrust has minimum transport of 80 km across strike. Upper basement contains boudins of eclogite and garnet-corona gabbro lacking in lower basement. We suggest that crustal imbrication by the Storli Thrust, and similar thrusts 220 km N, 100 km W, provided gravitational potential to trigger the overlying Agdenes Detachment and exhumation of a large region of eclogite-bearing rocks.