Orogenic uplift and collapse, crustal thickness, fabrics and metamorphic phase changes: the role of eclogites

Coesite-bearing eclogites in several deep crustal metamorphic assemblages now exposed in extensionally-collapsed orogens indicate the tectonic denudation of more than 90 km of crustal rocks and pre-collapsed crustal thicknesses of at least 120 km. For mountain ranges and orogenic plateaux up to 5 km in elevation and average crustal densities of about 2.8, crustal thickness cannot exceed about 80 km unless pre-shortening crustal/lithosphere thickness ratios were less than 0.135 or some way can be found to preferentially thicken the lithosphere mantle. This problem can be avoided and very thick orogenic crusts built up if granulite facies rocks transform to denser eclogite facies during shortening. A simple numerical model based upon field observations in southwestern Norway is presented in which coherent regional-scale transformation of lower crustal rocks to eclogite facies during lithospheric shortening is followed by heating, transformation of eclogite to amphibolite and granulite, extension, and crustal thinning by coaxial then non-coaxial mechanisms. -from Authors