Evolution of amphibolite-facies structural features and boundary conditions for deformation during exhumation of high- and ultrahigh-pressure rocks, Nordøyane, Western Gneiss Region, Norway

摘要: [1] Geologic mapping and structural analysis of Baltica basement overlying thrust nappes have yielded a record the late exhumation history high-pressure rocks, where strain partitioning has preserved evidence for interpreting evolution features. The earliest these were extensional detachments juxtaposing eclogite-facies rocks against amphibolite-facies that show no metamorphism. These early are strongly overprinted complexly folded, they represent phase upper crustal extension was active during continued convergence at deeper levels. Younger more localized mylonite zones formed synchronously with tubular, sheath, isoclinal, tight open folding shows progression from WNW to ENE trends. zones, interpreted as originally subhorizontal, range in strike through 20° angle 110° 90°. Later steeply dipping under lower conditions, 75° locally truncate earlier structures. youngest lowest amphibolite 50° all Folds developed this orientation reflected orientations is progressive top west shearing. changes features been caused by boundary conditions related transtensional deformation exhumation. L > S fabrics, absence axial planar foliation, chaotic surfaces granulite folds indicate formation constrictional field. This also supported estimates finite accumulated ∼780°C 45 km similar observations previous workers. Assuming simple monoclinic transtension, measurements apparent angles 9–11° increased greater than then decreased less 20°. agree X-Y plane ∼20 depth appear reflect deformation. transtension occurred sinistral oblique provides mechanism syncollisional HP UHP metamorphic rocks.