THE COAST SHEAR ZONE REVISITED: EVIDENCE FOR EARLY TERTIARY DEXTRAL TRANSPRESSION IN B.C. AND SE ALASKA?

The Coast shear zone (CSZ) in western British Columbia and southeast Alaska is a northwest-striking ductile shear zone more than 1200 km long. The CSZ was active from ~65 to ~55Ma, with motion ending about 55 - 50 Ma. The inception of faulting is poorly constrained. This crustal shear zone has been considered a potential site of significant dextral shear, related to the northward motion of "Baja B.C.". Most studies, however, have revealed a remarkably consistent early Tertiary history of northeast-side-up contraction along the length of the shear zone. A stage of dextral slip has been recognized locally along the northern half of the shear zone, but has been considered subordinate to the contractional displacement in most places.

Compilation of map and foliation data from the CSZ reveals a consistent, strongly developed oblique relation between the strike of the mylonitic foliation and the shear zone boundary in map view. Mylonitic foliation (dominantly S) strikes ~310 and the shear zone boundary strikes ~325. This pattern is predicted in a dextral transpressive shear zone, however, thin section and hand sample views of the shear zone are not typical of transpression. Fabrics in thin section and hand sample are symmetric when viewed in horizontal faces perpendicular to S and the down-dip elongation lineation (L). Faces cut parallel to L show fabrics indicative of reverse shear. These fabrics have been interpreted as evidence of early Tertiary contraction without a dextral component. In light of the map-scale patterns and the local development of dextral strike slip fabrics along the northern shear zone, we speculate that dextral transpression affected the CSZ in early Tertiary time. If so, the CSZ may have accommodated significant northward motion of coastal B.C. Subsequent transformation of transpression into discrete dextral faults east of the CSZ and contraction on the CSZ may have occurred as the continental margin matured.