Metamorphic Evolution of the CMC, Alaska

In this project the metamorphic evolution of the Chugach Metamorphic Complex (CMC) of southern Alaska will be derived. The CMC exposes Eocene high grade metamorphic rocks that are exhumed between two major crustal scale strike-slip structures: the Border Range Fault system to the north and the Chugach - St. Elias Fault system to the south. Interestingly, the region exposing migmatites triples in width from 25 km to about 80 km over a mere 150 km distance along strike and is flanked by symmetric zones of decreasing metamorphic grade. This geometry, as well as the Eocene subduction of the Kula-Farallon ridge, raise a series of interesting speculations on the interpretation of this wedge shaped high grade body. In particular: (a) Has the CMC been metamorphosed largely in-situ and can the geometry therefore be interpreted in terms of the ridge subduction geometry? or: (b) Does the wedge shaped geometry reflect an extrusion in lateral or vertical direction that can be interpreted in terms of wedge extrusion models? Despite detailed structural work in the region, such questions cannot be addressed without detailed metamorphic work - which is missing. Here we will use classic thermobarometry, thermodynamic pseudosections and geochronology to derive the PTtD evolution of the CMC using pelitic gneisses and schists. In our efforts, we focus on the Eocene evolution of the centre and the southern margin of the CMC as we have sampled some of the northern areas in preliminary field work. Results will be interpreted in terms of geodynamic models that can account for the measured distribution of burial depth, peak metamorphic temperature and formation age. The project will operate in cooperation between Grasemann (Vienna) and Stüwe (Graz) and closely linked with US colleagues who are leading in the structural work in the CMC as well as a current neotectonics project focused on the Neogene exhumation of the CMC.

The Chugach Metamorphic Complex (CMC) of southern Alaska is a 600 km long and up to 50 km wide high grade metamorphic complex wedged between low grade sequences to the north and south. It developed in the Eocene along a 2200 km long convergent plate boundary, related to the subduction of the Pacific plates under the North American continent. The CMC is made up of metasediments with a metabasite belt along its southern part. The complex has been widely interpreted as a high temperature-low pressure complex (550C-650 C and 3 kbar), based on sparse petrological data. The inferred metamorphic conditions have given rise to the establishment of a ridge subduction model as the best geodynamic explanation for its presence. In this project, detailed petrographic, geochronological and structural work was performed to explain the tectonic evolution of the complex. To field seasons were conducted in 2008 and 2009 to sample the entire complex and map three transects across it. Formation pressures, temperatures and formation ages were determined using different petrological and geochronological methods. The results show that the rocks formed at 7-13 kbar pressure at about 20-30 kilometres depth and 650-750 C around 52 million years ago. The complex cooled and exhumed over a prolonged time period and reached the surface about 10 million years ago. In particular the pressure estimates are substantially higher than those previously calculated. Thus, the geothermal gradient implied by the ratio of peak temperature to depth is not as perturbed as previously suggested. It is also shown that there is a systematic change of metamorphic conditions across the complex: Throughout the northern schist zone, there is an increase in metamorphic conditions from 4-6 kbar and 550-680C to 13 kbar and 650-750C towards the migmatite zone. In view of these new data, the accepted ridge subduction model may be questioned and the project showed that heating, cooling and exhumation may be explained by normal conductive processes. During the course of the project, two PhD students were educated and graduated towards the end of the project. The project was conducted in cooperation with American and Australian universities and a series of publications in international journals have been completed.