STRUCTURE AND INTRUSIVE STYLE ALONG A SEGMENT OF THE PASAYTEN FAULT ZONE, SOUTH-CENTRAL BRITISH COLUMBIA

The Pasayten fault zone is a ≥ 250 km long, steeply dipping tectonic boundary. Across the 1-4 km-wide fault zone in southern British Columbia, Jurassic-Cretaceous sedimentary and volcanic rocks of the Methow basin to the west are juxtaposed against Mesozoic plutonic and metamorphic rocks of the Eagle Plutonic Complex (EPC) to the east. The eastern margin of the EPC comprises a series of layered 1-2-m thick tonalite sheets that have intruded mylonitic rocks of the Upper Triassic Nicola Group, and are deformed in the Eagle shear zone. Controversial paleomagnetic data across the Pasayten fault place much of the Coast Mountains and Insular superterrane 1200-3000 km to the south at 90 Ma, leading to the Baja BC hypothesis. In contrast to the expected dextral translation, previous workers have reported sinistral motion on the Pasayten fault. Resolution requires a more complete understanding of the fault.

Field studies along a 40 km segment of the Pasayten fault concentrated on intrusive style and structural relations. EPC foliations strike parallel to the NW-trending fault, generally dip NE, and steepen towards the fault. Weak lineations in the EPC generally plunge < 30˚ to the NW and SE, but locally plunge steeply (60˚-80˚), down-dip to the NE. Greenschist-grade mylonitic muscovite-bearing rocks of the ~110 Ma Fallslake Plutonic Suite of the EPC formed within 100 m of the fault, but mylonites are more commonly cut out by brittle faulting. Cretaceous clastic rocks of the Methow basin SW of the Pasayten fault are folded, but typically strike parallel to the fault, and dip moderately to steeply NE into the fault. In the NW-striking Eagle shear zone, several outcrop-scale shear zones indicate top-to-NE reverse motion, placing the EPC with SW dipping foliations on moderately SW dipping amphibolites and biotite schists of the Nicola Group. Nicola foliations contain, shallowly (5-30˚) SE plunging lineations. Preliminary microstructural kinematic study of upper greenschist to lower-amphibolite grade Nicola rocks indicate sinistral slip, which combined with reverse shear zones in the EPC, imply sinistral transpression. Thus, this study shows no indication that the Pasayten fault and Eagle shear zone are major Baja BC faults.