SIGNIFICANCE OF THE COLUMBIA RIVER FAULT ZONE, SOUTHEASTERN CANADIAN CORDILLERA : INSIGHTS FROM STRATIGRAPHIC AND THERMOBAROMETRIC CONSTRAINTS

The Columbia River fault zone (CRFZ), located along the Columbia River Valley in southeastern British Columbia, is one of a family of extension faults that have been interpreted to account for much of the early Cenozoic crustal extension in the southern Canadian Cordillera. On the basis of lithological, structural and metamorphic discontinuities, the CRFZ has been interpreted as a fundamental crustal-scale structure with dip-slip displacement up to 20 kilometres or more; it has also been hypothesized to mark the transition between rocks of the Shuswap metamorphic complex to the west, and lower grade rocks to the east, collectively referred to as the Selkirk allochthon. Recent detailed geological mapping and thermobarometric constraints along the southern segment of the CRFZ (Upper Arrow Lake area) are, however, difficult to reconcile with published interpretations.

Rocks west of Upper Arrow Lake (infrastructure) consist mostly of Proterozoic and Paleozoic amphibolite-facies metasedimentary rocks that preserve late Cretaceous to Paleocene peak metamorphic assemblages and early Tertiary cooling ages. To the east, the suprastructure comprises upper Paleozoic and lower Mesozoic metasedimentary rocks of the biotite and garnet zones, which were deformed in mid-Jurassic time and yield Mesozoic cooling ages. The transition from infrastructure to suprastructure is marked by a strain gradient a few kilometres thick, from steeply dipping fabrics in the suprastructure, to penetratively transposed, near horizontal structures in the infrastructure. Thermobarometric estimates from metapelites in the vicinity of the CRFZ (Barrovian assemblages in the biotite through sillimanite zone) yield peak metamorphic conditions of ~525 to 670^oC and ~5.5 to 7.0 kbar and suggest an attenuated metamorphic gradient. The similarity of P-T estimates on either side of the CRFZ suggest the metamorphic succession is apparently continuous across the fault; yet, it juxtaposes areas that were metamorphosed at different times.

We interpret the CRFZ as a zone of multiple, moderately- to steeply-dipping brittle faults with limited dip-slip displacement. Foliation-parallel zones of intense transposition and tectonic thinning produced the attenuated metamorphic sequence between suprastructure and infrastructure, and juxtaposed domains with different thermal histories.