THE CHUGACH METAMORPHIC COMPLEX OF SOUTHERN ALASKA: A TEST FOR WEDGE EXTRUSION MODELS?

High-grade metamorphic rocks are exposed in many orogenic wedges. They form at great depths and were then exhumed to the surface during collision. Several so-called extrusion models have been proposed to explain such exhumation: The rocks may have been extruded (a) as a rigid body between bounding fault zones, (b) by internal ductile channel flow, or (c) by general shear. Each of these models predicts a distinct relationship between the exhumation depth of the rocks and the width of the metamorphic complex at the surface. However, most of these models were developed in the Himalayan crystalline complex, which does not vary enough in width laterally to allow a straight forward testing of the predictions of the different models.

The Chugach Metamorphic Complex (CMC) of southern Alaska consists of high-grade metamorphic rocks (including migmatites) which developed in Cretaceous flysch sediments during an Eocene metamorphic event. The CMC is bound by major reverse and strike-slip fault zones, and its width triples from 25 km in the east to 80 km in the west over a distance of 150 km. The CMC may therefore be an ideal testing ground for different extrusion models.

Despite its interesting geometry, only little is known about the Eocene history, including the metamorphic conditions, the age of metamorphism and the exhumation history of the CMC. In a recently granted project we are currently embarking on a study to investigate transects across the CMC in parts of different width, in order to analyse the structural, chronological and metamorphic field gradients across the wedge. In this contribution we present ideas how along strike variations in field gradients in parts of different widths of the wedge can be used to constrain the different extrusion models.