Crustal Shear Wave Velocity Structure of Central Idaho and Eastern Oregon From Ambient Seismic Noise: Results From the IDOR Project

Abstract

We developed 3-D isotropic crustal seismic velocity models of central Idaho and eastern Oregon from the IDOR (western IDaho and eastern ORegon) Passive seismic data. Ambient noise tomography yielded crustal velocity structure from vertical component Rayleigh wave group and phase velocity measurements. Results include a strong shear wave velocity contrast—faster in accreted Blue Mountains terranes west of the western Idaho shear zone (WISZ), slower in the Idaho batholith, emplaced within the Archean Grouse Creek block east of the WISZ—restricted to the upper-to-middle crust. In deeper crust not affected by mafic underplating during Columbia River Flood Basalt magmatism, the shear wave velocity of the Mesozoic Olds Ferry continental arc terrane is indistinguishable from that of the Archean Grouse Creek block basement. Crustal columns of the Olds Ferry terrane and the Permian-Jurassic Wallowa intraoceanic arc terrane are characterized by low seismic velocities, consistent with felsic lithologies down to ∼20 km. West of the WISZ, the Bourne and Greenhorn subterranes of the Baker terrane, an accretionary complex between the arc terranes, have distinct shallow crustal seismic velocities. The Greenhorn subterrane to midcrustal depths is in an overthrust geometry relative to the Bourne subterrane. Lack of mafic lower crust in our results of the Wallowa or Olds Ferry arcs may be due to imbrication of upper crustal felsic plutonic complexes of these arcs. Shortening and thickening of the Blue Mountains arc terranes crust to >30 km, and subduction or delamination of their mafic lower crustal sections is a viable mechanism for growth of a felsic continental crust.