FRACTURE SYSTEMS IN THE UPPER CRETACEOUS NANAIMO GROUP, GULF ISLANDS, BRITISH COLUMBIA, CANADA: INITIAL INTERPRETATIONS

Regional fracture systems provide information on the timing and distribution of brittle strain for interpretation of deformational history and subsequent effects on permeability architecture. Fracture systems of the upper Cretaceous Nanaimo Group in the southern Gulf Islands, southwest British Columbia, Canada suggest possible relationships between distribution, mode and relative timing of brittle deformation with structural events and the groundwater flow system. Over 7000 fractures were measured within variable lithology for orientation, mode and kinematics, filling, alteration and density by scan line and traditional mapping techniques. The three fracture classes identified are: bedding perpendicular jointing, NE-trending extensional fracture zones, and significant fault zones. Bedding perpendicular jointing is ubiquitous, but varies in density based on lithology and location. NE trending, widely spaced (10's-1000's of metres) extensional fracture zones are large-scale joint zones and/or minor normal separation fault zones that cut regional folds. Significant fault zones are steeply to shallowly dipping zones of predominantly reverse separation, striking NW-SE, N-S and E-W with offsets from 10's of centimetres to 10's of metres oriented both parallel and oblique to bedding and regional fold axes. The fracture classes display different styles and orientations, interpreted to correlate with previously identified regional deformational events. These include, from oldest to youngest: fracturing and bedding parallel faulting related to regional Eocene-aged SW-vergent folding and thrusting; younger NW-SE and E-W oriented faulting cross-cutting Eocene folding; and youngest NE-trending extensional fracture zones. The heterogeneous spatial distribution of fracture classes is hypothesised to relate to strain partitioning due to lithologic variation, structural setting, and proximity to regional bounding faults. This distribution should affect the regional fracture-controlled groundwater system by controlling the location and orientations of permeable rock masses, providing a foundation for a regional conceptual model for groundwater flow.