BRITTLE ASPECTS OF EARLY TERTIARY EXTENSION IN THE CANADIAN CORDILLERA

The Lake Okanagan Fault (LOF) coincides with the boundary between Intermontane Terrane and Quesnelia in the southern Canadian Cordillera. The LOF is a complicated detachment associated with the development of a core complex, which juxtaposes Eocene hanging wall successions against the footwall igneous block. In this study we focus on the early Tertiary brittle deformation that accompanied the exhumation of the core complex.

Three deformational styles, shallow ductile shear, shallow brittle shear, and steep west-side down normal faults, formed progressively as the structure was exhumed both tectonically and erosionally. We measured the attitude of fault planes and their striations and use a combination of striae and displaced contacts, and/or recrystallization, mainly of calcite, in small pull-aparts to determination the sense of movement on faults. Meso-structures reveal at least two different stress regimes since the early Tertiary. The most significant group of faults, in most stations, is identical to the systematic joint set, which is parallel to both major extensional faults and early Tertiary dykes. This indicates that the early Tertiary deformation was dominated by WNW to ESE extension resulting in a pervasive and intense fracture set, striking NNE. A later NE-SW compressional deformation resulted in reactivation of these joints as normal, reverse or strike-slip faults, and in small and major folds.

Our new data provides a framework for the reinterpretation of map structures. The oldest early Tertiary structures include a major shallow detachment, with a west-northwest displacement. Paleothermometric data across the detachment indicate the omission of 6-10 km of crust. We document a change in footwall rheology from ductile to brittle as it was exhumed and progressively incorporated into the hanging wall. Once the footwall was largely exhumed, it was, together with both the major detachment and the hanging wall Eocene succession, deformed by brittle extensional structures that record, the inital WNW-ESE extension followed by the subsequent northeast-southwest compression. Coal-bearing fault-line sediments at the top of the Eocene hanging wall succession were subsequently exhumed by the erosion of ~3-5 km, probably during regional epeirogenesis that predates Miocene Plateau basalts.