THE ROLE OF A MELT-RICH MIDDLE CRUST LAYER IN CORE COMPLEX FORMATION: EVIDENCE FROM THE SHUSWAP METAMORPHIC COMPLEX, SOUTH-CENTRAL BRITISH COLUMBIA

The Shuswap metamorphic complex underwent extension during the early Tertiary, resulting in the exhumation of high-grade, mid-crustal rocks and the formation of gneiss domes. Rocks within the study area may be divided into three crustal levels based on structural style, metamorphic grade, and lithostratigraphy. In the region between Vernon and Nakusp all three levels are exposed, providing the ideal opportunity to study the relationship between extension and gneiss dome development in a Cordilleran metamorphic core complex.

Rocks of the lowest level are exposed in the Monashee complex, which is comprised of two domes of Early Proterozoic paragneiss and orthogneiss, mantled unconformably by paragneiss of the middle level. Middle level rocks, of Proterozoic to Mesozoic age, display a marked contrast in structural style compared with the lower level. They are characterised by gently-dipping foliation, a consistently oriented stretching lineation, and abundant Paleocene to Eocene syn- to post-kinematic granitic material. Upper level rocks record much lower peak metamorphic conditions and exhibit more upright structures of mid-Jurassic age. The upper/middle level transition is characterised by a several kilometre-thick zone with a steep but continuous metamorphic gradient and a penetrative stretching lineation. Detachment faults and steep normal faults, where present, modify this transition. Pressure and temperature differences between the middle and upper levels indicate that rocks apparently separated by a depth of 10 to 15 kilometres during the late Cretaceous are presently juxtaposed across this 2 to 3 kilometre-thick zone.

We propose a model for extension in the Shuswap metamorphic complex whereby exhumation occurred primarily through removal of ductile, melt-laden mid-crustal rocks from beneath the stiff upper crust by extrusion in response to lateral pressure gradients. Ductile flow of mid-crustal rocks may be of greater importance in the formation of Cordilleran metamorphic core complexes and gneiss domes than has been previously recognised.