Contrasting Crustal Structure Across a Steep Accretionary Margin in Idaho-Oregon, Illuminated by EarthScope IDOR Controlled-Source Seismic Data

Abstract:

The U.S. Cordillera in Idaho and Oregon is the location of an unusually steep tectonic boundary between relatively juvenile accreted island-arc terranes and Precambrian North American craton. Accretion occurred in the Mesozoic, forming the Salmon River suture zone. Following accretion, transpressional deformation in the Cretaceous shortened and steepened the suture zone along the western Idaho shear zone (WISZ) into a narrow boundary less than 10km wide at the surface. The region was later modified by emplacement of the Idaho batholith, Challis volcanism, the Columbia River Basalts, and Basin and Range-style extension. The EarthScope IDOR project acquired a controlled-source seismic survey across this region with a 430-km refraction and wide-angle reflection line. It utilized 2555 vertical component seismometer stations to record 8 explosive shots, as well as many hours of continuous recording for background seismicity and ambient noise. Travel-time tomography and reflection modeling have been used to produce a preliminary seismic velocity model of the whole crust.

There are significant differences between the seismic velocity, mid-crustal boundaries, and crustal thickness seen in the accreted terranes west of the WISZ and those seen in 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, while the crust east of the WISZ has a slower velocity consistent with primarily felsic, continental-affinity crust. Numerous wide-angle seismic reflections are observed on both sides of the WISZ. To the west, there is a lower-crustal reflector that produces a stronger reflection amplitude than the Moho. Beneath the Idaho batholith and Challis volcanics to the east, several smaller-amplitude wide-angle reflections indicate complexity in mid-crustal structure. Thickness of the crust east of the WISZ is 6-12 km greater 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 juxtaposing cratonic and oceanic lithosphere. This reflection and travel-time tomography model will produce a detailed crustal-scale characterization of this tectonically modified edge of the North American craton.