GEOLOGY OF THE OKANAGAN WATERSHED, SOUTH-CENTRAL BRITISH COLUMBIA

The geology of the map-area was compiled from maps at scales of 1:250 000 to 1:10 000 spanning 70 years. Concepts that influenced interpretation of field data evolved considerably over that time. Integration of the diverse maps was challenging; not all elements were resolved. The process was a cautionary reminder of the limitations that bias, model-driven interpretations and changing concepts impose on geological research.

The watershed extends north from the 49th parallel for 180 km along the western margin of the Shuswap Metamorphic Complex (SMC), and east-west 100 km at its widest part. The main valley was initially interpreted to contain steep normal faults. Later, mylonite zones and kinematic indicators suggested that a gently west-dipping, top-to-the-west detachment fault with displacement as much as 100 km affected Eocene and older strata. K/Ar dates indicating an Eocene thermal event restricted to the SMC were reinterpreted to date the denudation. Eocene strata dipped at all angles to the detachment.

More recently, some mylonite zones were dated as Mesozoic and some contacts between low and high grade rocks were interpreted to be depositional. Major stratigraphic belts were mapped across the valley in the northern part of the watershed. Eocene volcanics were observed in stratigraphic contact with high grade rocks as well as against steep normal faults. No simple, continuous detachment was evident. In several instances, low-angle faults diverged markedly from the valley.

In the SMC one or more episodes of deformation and metamorphism predate Eocene faulting and the ages of many of these events and the protoliths are unknown. In low grade rocks deformation occurred from the Permo-Triassic to Paleogene. Transitions between low and high grade rocks occur east and west of the valley; some are abrupt and clearly faulted. The infrastructure and suprastructure of the Eocene detachments, where they occur, may both contain high and low grade rocks. Mylonite zones seem to occur at several structural levels and cannot all be assigned an Eocene age with confidence. Several discontinuous detachment faults may die away northward into a zone of normal faults and/or descend into high grade rocks east of the valley. Mylonite and brittle fault zones accompanied by extensive hydrothermal systems, formed before, during and after a prolonged episode of Eocene volcanism.