Abstract

An unusual opportunity for special processing of selected COCORP data has provided three-dimensional information on deep crustal structure related to Proterozoic rifting, Mesozoic thrusting, and Cenozoic extension in the interior of the United States Cordillera, near the Okanogan Dome, north-central Washington. Using a set of short, intersecting seismic line segments in conjunction with longer regional east-west and north-south seismic lines, we have found dramatic variation vertically in the orientation of structure throughout the crust in this region.

A lower crustal block of south-dipping structures lies above a regionally horizontal Moho found at about 34 km depth.This block is ∼13 km thick and is bounded above by a southwest-dipping structure which is discordant to structures both above and below it and is interpreted as a midcrustal thrust formed during Mesozoic to Paleocene convergence. These relationships suggest that the south- dipping lower crustal features were formed prior to, or during, Mesozoic convergence and may be associated either with Proterozoic rifting of the North American continental margin, earlier Proterozoic basement structure, or with a Mesozoic duplex structure formed along a lateral ramp. A zone of relatively few reflections occurs above the mid-crustal thrust and below a highly reflective upper crust and may represent a separate thrust slice in a basement duplex structure. The inferred duplex is cut by east-dipping reflections that project to surface exposures of mylonites and that may be an Eocene(?) normal fault traceable to mid-crustal depths. This duplex structure beneath the Okanogan Dome is analogous to the proposed crustal-scale duplex beneath the Monashee Déecollement exposed in a doubly plunging antiform 150 km to the north in the Shuswap Complex. The three-dimensional analysis, along with the southerly plunge of the antiform, indicates that a thrust at depth just south of the United States-Canadian border may be continuous with the Monashee Déecollement or with the proposed lower thrusts of the Canadian duplex. If so, then by extending the survey north across the international border, a structure deep in the crust may be traceable to the surface.

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