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

[摘要] 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 décollement, 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 Green-bush 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 foot-wall; (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.

[学科分类] 地质学