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 N orway experienced Caledonian high-pressure metamorphism during the Sil urian, Scandian continental collision between Baltica and Laurentia. T he record of coesite-bearing eclogites and pressure-temperature estima tes from the WGR demonstrate extreme burial of these rocks at P-max. E clogite tectonite fabrics record coaxial deformation characterized by bulk horizontal shortening and vertical stretching. Many eclogites, pa rticularly those with a high content of kyanite, quartz, phengite and clinozoisite have constrictional fabrics related to vertical stretchin g. Fabrics that developed during orogenic extensional collapse are of two main types. The deepest exposed sections are dominated by penetrat ive coaxial fabrics that are characterized by vertical flattening and horizontal, E-W, stretching. These fabrics developed during rapid deco mpression and were associated with, and locally enhanced by, partial m elting of the deep crust. The collapse-related coaxial vertical shorte ning and horizontal stretching developed at granulite to amphibolite f acies and is overprinted by non-coaxial deformation that formed thick mylonites along extensional detachments. The detachment zones are root ed 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 metamorphis m. Devonian basins were formed by extensional faulting in the upper cr ust, and the faults that controlled the sedimentation were rooted in t he extensional detachments.