The Monashee reflection: Re-examination of a Lithoprobe crustal-scale seismic reflection in the southern Canadian Cordillera

The Monashee reflection is a major crustal-scale, crosscutting reflection that appears on two mutually perpendicular Lithoprobe seismic profiles in the southern Omineca belt of the Canadian Cordillera. It has previously been interpreted as the down-plunge extension of a regional ductile thrust fault, the Monashee decollement, which is said to separate the Monashee complex from the overlying Selkirk allochthon. We reinterpret the Monashee reflection as a shear zone with modest, normal displacement based on recent mapping, geometric analysis of the seismic profiles, and existing geochronological and metamorphic data. Recent mapping has demonstrated that this boundary is not a shear zone with thrust geometry, but rather it is a gradational boundary that has resulted from mutual folding and transposition of rocks ascribed to the Monashee complex and Selkirk allochthon. Overprinting the transition zone in different areas are three high-strain zones: the Greenbush Lake shear band zone, Slate Mountain shear zone, and a ductile shear zone associated with the Columbia River fault. We interpret these high-strain zones as segments of a single, high-strain zone that wraps around the margins of the Thor-Odin culmination. This marginal zone is a complex, outward-dipping, normal structure, which we name the Thor-Odin high-strain zone. Three alternative three-dimensional geometric models have been developed for the Monashee reflection in order to project the reflection to the surface. We favor a model in which the surface trace of the Monashee reflection coincides with the Thor-Odin high-strain zone. Normal shear sense kinematics are interpreted for the Monashee reflection based on: ( 1) the overall geometry and fault-drag - like relationship between the Monashee reflection and reflections in the hanging wall and footwall; ( 2) offset of metamorphic and geochronological gradients, which are consistent with an extensional zone rather than with a thrust fault interpretation; and ( 3) the crosscutting nature of the Monashee reflection, which is consistent with normal structures throughout the region.