DEXTRAL TRANSPRESSION AND GNEISS DOME FORMATION, CHUGACH METAMORPHIC COMPLEX, SOUTHERN ALASKA

The Chugach metamorphic complex (CMC), southern Alaska, is an accretionary prism that was subjected to high-temperature/low-pressure metamorphism due to late Cretaceous to Eocene ridge subduction. A distinct feature of the CMC is a large-scale, narrow, anticlinorium that exposes an amphibolite-grade gneiss core in the center of the complex that extends parallel to the former plate boundary. Formation of the gneiss core correlates with peak metamorphism in the CMC, oblique convergence of the Kula plate, and displacement along multiple dextral shear zones. Wrenching motion associated with the dextral shear zones produces large-wavelength folds in the layers of metagreywacke and metapelite overlying the gneiss core. Dextral shear appears to be distributed homogeneously throughout the gneiss core, and folds and shear structures similar to those in the overlying metasedimentary rocks, are observed at both smaller and larger scales. The southern margin of the CMC, however, shows thrust-sense shear indicators. Collectively, these observations suggest large scale slip-partitioning during formation of the gneiss core with the strike-slip component accommodated on the inboard shear zones, thrusting accommodated along the base of the complex, and distributed transpression in the interior of the complex. We suggest that distributed dextral transpression, and associated folding produced the narrow, elongated dome of the CMC gneiss core; a different origin than suggested for classic gneiss domes. The CMC represents the importance of the effects of oblique convergence and wrenching motion associated with margin-parallel strike-slip faults on the formation of gneiss domes and similar large scale structures.