A CONTRAST IN CRUSTAL STRUCTURE ACROSS THE STEEP CRATONIC MARGIN OF IDAHO-OREGON, ILLUMINATED BY EARTHSCOPE IDOR CONTROLLED-SOURCE SEISMIC DATA

The EarthScope IDOR project is investigating the formation and modification of a steep tectonic boundary in the U.S. Cordillera of Idaho and Oregon. Island-arc terranes were accreted to the margin of Precambrian North America at the Salmon River suture zone, which was subsequently shortened and steeped by transpression along the western Idaho shear zone (WISZ). This created an unusually steep, narrow boundary at this latitude. The region was later modified by emplacement of the Idaho batholith, Challis volcanism, the Columbia River Basalts, and Basin and Range-style extension. The IDOR controlled-source seismic survey crossed this region with a 430-km refraction and wide-angle reflection line. Traveltime tomography and reflection modeling were used to produce a preliminary seismic velocity model of the whole crust.

The seismic velocity, mid-crustal boundaries, and crustal thickness are significantly different between the accreted terranes west of the WISZ and the Idaho batholith and Precambrian craton to the east. The crust west of the WISZ is characterized by faster velocities consistent with mafic oceanic-arc crust. Numerous wide-angle seismic reflections are observed, including a lower-crustal reflector that produces a stronger reflection amplitude than the Moho. The crust east of the WISZ has a slower velocity consistent with primarily felsic, continental-affinity crust. Beneath the Idaho batholith and Challis volcanics, several smaller-amplitude wide-angle reflections indicate complexity in mid-crustal structure. East of the WISZ the crust is 6-12 km thicker than the crust to the west, and Moho reflections near the WISZ are very weak. The contrasts across the WISZ are consistent with a lithospheric-scale, near-vertical shear zone that juxtaposes cratonic with oceanic lithosphere. Ongoing refinement of this model will produce a more detailed crustal-scale characterization of this tectonically modified edge of the North American craton.