Structural studies of Alpine eclogites as a contribution to understanding the exhumation of high-pressure rocks

Erwin Schrödinger Fellowship J 1986 Structural studies of Alpine eclogites Walter KURZ 09.10.2000 During the orogenesis of the Alps large volumina of both oceanic and continental crust have been subducted. Most of these materials have been assimilated in the mantle, however, some of these subducted complexes have been exhumed from depths of even 100 km back to the earths surface and are exposed in the Alps as high-pressure metamorphic rocks. These comprise both former oceanic crust (e.g., Zermatt-Saas ophiolite with eclogite), and partly continental upper crust (e.g., Dora Maira-Massif with coesite-bearing pyrope-quartzites). The mechanisms that allow the subduction of continental crust as well as the mechanisms of exhumation of subducted rock masses back to the earths surface are not fully examined yet and are one of the most important topics of research of tectonics recently, both in the Alps and in other orogens. During the past 20 years many models for the exhumation of high-pressure rocks have been proposed, which may be applied to the boundary conditions in the Alps. Some models assume that buoancy forces that are related to differences in rock-density result in the exhumation of these rocks (e.g., Holland and England (1979)). This requires, that eclogites, which actually have high densities, are enclosed within rather light, acidic crustal rocks, which again are surrounded by heavy rocks of the earths mantle, causing their exhumation. In these models the transition from subduction to exhumation is often explained by a singular event (e.g., the break-off of an subducted slab; von Blanckenburg & Davies, 1996). Another class of models assumes extensional tectonics for causing exhumation in terms of a change in the stress field subsequent to subduction, which causes extension, stretching, and removal of rock masses of the hangingwall (e.g. Platt 1986). A third group of models assumed material flow within the accretionary wedge, which is constrained by the geometry of the subduction channel between the upper and the lower plate, and is characterized first by downward flow, followed by upward flow ("comer flow"). The latter models generally describe continuous processes, and do not require a singular event. This project focusses on detailed structural field investigations, as w1l as microstructural, textural, and petrological analyses in order to explain the processes of exhumation of high-pressure rocks in the Alpine orogen.