CRETACEOUS - EARLY TERTIARY TECTONIC EVOLUTION OF THE VALHALLA COMPLEX, SOUTHERN OMINECA BELT, BRITISH COLUMBIA

The 20 x 80 km Valhalla complex comprises sheets of orthogneiss and amphibolite-facies metasedimentary rocks that were intruded by Paleocene - Eocene lacoliths. The sheets are gently domed, and are disposed in three culminations; the Valhalla dome, Passmore dome and the Southern Valhalla complex. The Valhalla complex constitutes the lower plate of the Valkyr - Slocan -Champion Lakes extensional shear zone system. Deeper levels in the complex are characterized by structures that were mainly formed during Cretaceous to Paleocene crustal shortening and thickening of the orogen. The predominant structure is a diachronous transposition foliation. It is overprinted in places by polyphase folding and zones of intense strain. Geochronological and structural studies constrain the spatial distribution and timing of shear zones and transposition foliation. In the Southern Valhalla complex, displacement on Early Tertiary extensional faults is <5 km and a downward transition from Middle Jurassic upper plate structures into Cretaceous transposition foliation is exposed without a significant tectonic break. Foliations that range in age over 120 m.y. are all statistically parallel.

Where emplacement of magmas or melt is localized at particular structural levels, deformation may have been strongly thermally controlled and confined to particular levels. It is clear that the presence and orientation of pre-existing structures has served to influence disposition of magma and guide younger structures. In Valhalla and Passmore domes the peak of metamorphism and anatexis at conditions of ~ 800 oC and 8 kbar occurred at ca. 70 Ma and was concomitant with thrusting on the Gwillim Creek shear zone. Cretaceous high-grade deformation was followed by the intrusion of Late Paleocene and Early Eocene granite and pegmatite, localization of extensional shear zones along the upper margin of the zone of intrusion, and exhumation and cooling of the complex. With the exception of late steep brittle faults and late dykes, all of the rocks and structures in Valhalla complex, as well as structures as young as the 58 Ma Valkyr shear zone are arched; therefore, the doming is a young feature that occurred in the Early Tertiary. Doming may be related in part to motion on structures beneath the Valhalla complex that accommodated the last stage of Early Tertiary shortening in the Foreland Belt.