Evidence for a Moho-penetrating steep accretionary margin from the EarthScope Idaho-Oregon controlled-source seismic survey

Abstract:

Seismic refraction and wide-angle reflection travel times from the EarthScope IDOR controlled-source seismic survey were inverted to derive crustal velocity structure in the U.S. Cordillera of Idaho and Oregon. The P-wave velocity model and preliminary analysis of the shear waves reveal a near-vertical boundary that juxtaposes accreted island-arc terranes against Precambrian North America and the Idaho batholith. Post-accretion transpression across the western Idaho shear zone (WISZ) steepened and narrowed the original suture zone to ~5 km at the surface. The seismic data indicate that the WISZ penetrates the entire crust and offsets the Moho by ~7-8 km. The lithosphere of this region has been significantly modified since activity on the WISZ by emplacement of the Idaho batholith, Challis volcanism, the Columbia River Basalts, and Basin and Range-style extension. The crust west of the WISZ is characterized by faster P-wave velocities consistent with oceanic-arc crust and an average Moho depth of ~32 km. A very strong wide-angle reflector and higher seismic velocity are observed in the lower crust beneath the feeder dike system for the Columbia River Basalts; these features are interpreted to represent mafic magmatic underplating. In contrast, the crust east of the WISZ has a much slower P-wave velocity indicating a felsic-to-intermediate composition, and the Moho depth is close to 40 km. There are no major differences in intra-crustal reflectivity or seismic velocity between the Idaho batholith and the adjacent cratonic crust, and there is no evidence of a mafic magmatic residual beneath the Idaho batholith. The strong lateral contrasts across the WISZ provide evidence of inheritance and structural influence by this steepened boundary on the subsequent tectonic events.