EVOLUTION OF THE NORDFJORD-SOGN DETACHMENT ZONE AND THE STYLE OF CALEDONIAN EXTENSIONAL DEFORMATION, HORNELEN REGION, NORWAY

The Nordfjord-Sogn Detachment Zone (NSDZ) can be followed along strike as a normal-sense ductile shear zone for several hundred km, and is presumabley responsible for the exhumation of the Norwegian UHP province. However, the kinematic evolution of the NSDZ is incompletely understood. The hanging wall of the NSDZ consists of several large low-grade Devonian sedimentary basins placed via normal-sense detachments structurally above a thin series of allochthonous amphibolite-grade basement-cover sequences. The footwall is primarlily composed of autochthonous Baltic Shield basement gneisses. Most of the current literature suggests that the Norwegian UHP provinces were brought to the surface in the footwall of the NSDZ. However, the presence of high-pressure eclogites in both the hanging wall and the footwal, in addition to a mid-crustal Barrovian overprint observed across the Western Gneiss Complex, suggest that the NSDZ exhumed rocks from mid-crustal levels, but not from UHP depths.

To better understand the mechanics and evolution of the NSDZ immediately south and east of the Hornelen Basin, we are employing i) field and laboratory structural geology to reveal the deformation histories of the various units, ii)thermobarometry to assess the P-T evolution of garnet-bearing lithologies, iii)electron back-scatter diffraction on quartzites to assess sense of shear and tempertures of deformation, iv) U/Pb geochronology to assess igneous and metamorphic crystallization ages; and v) detrital zircon geochronolgy of the myriad sedementary basins. The preliminary results presented here show that: 1. Caledonian metamorphic rocks must have been fully exhumed and exposed at the surface by the time that the Hornelen Basin opened. 2. Peak temperatures and pressures in the allochthonous rocks of the hanging wall of the NSDZ are 550-625 deegrees C and 12-17 kbars. 3. Ophiolites in western Norway are as old as 548 Ma. 4. Quartzites correlated to the Høyvik Group yield a maximum age of 1000 Ma. 5. Rocks previously correlated with the Høyvik Group include significant volumes of metamorphic crystalline basement.