DEEP-CRUSTAL FABRICS AND A MODEL FOR THE EXTENSIONAL COLLAPSE OF THE SOUTHWEST NORWEGIAN CALEDONIDES

The exhumed deep crustal rocks in the Western Gneiss Region (WGR) of Norway experienced Caledonian high-pressure metamorphism during the Silurian, Scandian continental collision between Baltica and Laurentia. The record of coesite-bearing eclogites and pressure-temperature estimates from the WGR demonstrate extreme burial of these rocks at P-max. Eclogite tectonite fabrics record coaxial deformation characterized by bulk horizontal shortening and vertical stretching. Many eclogites, particularly those with a high content of kyanite, quartz, phengite and clinozoisite have constrictional fabrics related to vertical stretching. Fabrics that developed during orogenic extensional collapse are of two main types. The deepest exposed sections are dominated by penetrative coaxial fabrics that are characterized by vertical flattening and horizontal, E-W, stretching. These fabrics developed during rapid decompression and were associated with, and locally enhanced by, partial melting of the deep crust. The collapse-related coaxial vertical shortening and horizontal stretching developed at granulite to amphibolite facies and is overprinted by non-coaxial deformation that formed thick mylonites along extensional detachments. The detachment zones are rooted in the coaxially deformed deep crust, and separate the exhumed deep-crustal rocks of the Lower Plate, from the rocks in the hanging-walls that are characterized by medium- to low-grade Caledonian metamorphism. Devonian basins were formed by extensional faulting in the upper crust, and the faults that controlled the sedimentation were rooted in the extensional detachments.