STRAIN WITHIN THE ULTRAHIGH-PRESSURE WESTERN GNEISS REGION OF NORWAY RECORDED BY QUARTZ LPOS

Ultrahigh-pressure (UHP) and high-pressure (HP) tectonics are a fundamental component of continental orogenesis, but the exhumation of UHP and HP rocks remains poorly understood. As one of two giant UHP terranes worldwide, the Western Gneiss Region (WGR) of Norway is a key area for study. The WGR is characterized by regional E-W stretching lineations, subhorizontal foliations, and an eclogite-facies pressure gradient that indicates 2 times vertical thinning. Several models for the exhumation of the WGR have been proposed, including regional eclogite-facies constriction (Terry and Robinson, 2004), amphibolite-facies constriction (Krabbendam and Wain, 1997), and amphibolite-facies flattening (Walsh and Hacker, 2004). Lattice preferred orientations (LPOs) of recrystallized quartz grains across the WGR were measured from 60 samples using electron-backscatter diffraction to test these hypotheses, assessing i) deformation mechanisms (e.g., slip systems), ii) deformation temperatures, iii) distortion (i.e., the non-dilatational component of strain) and iv) sense of shear. The LPOs suggest chiefly prism- slip with minor basal- slip and prism- slip. Most fabrics are asymmetric, with shear sense predominantly top-W; few samples have symmetric fabrics. The LPOs primarily indicate plane strain, though a minority of samples exhibit varying degrees of constriction. In aggregate, these data indicate that the bulk amphibolite-facies deformation of the WGR involved top-W plane strain with 2 times vertical thinning and 2 times E-W stretching.