CRUSTAL AND UPPER MANTLE STRUCTURE AND FABRICS BENEATH EASTERN IDAHO AND WESTERN OREGON: RESULTS FROM THE IDOR PASSIVE SEISMIC PROJECT

The preserved Mesozoic plate boundary between North America and exotic terranes is unique in the U.S. lower 48 States because it includes both the Salmon River suture zone – along which the composite Blue Mountains terranes accreted – and the steeply-dipping Western Idaho shear zone (WISZ), a narrow ductile fault zone that accommodated a strong strike-slip component of oblique subduction. We deployed 85 broadband seismic stations across eastern Oregon and central Idaho to study these structures in detail as part of the Earthscope IDOR project. We constructed more than 7000 receiver functions using teleseismic events recorded between 2011-13 and determined crustal architecture using the H-k grid search technique and common conversion point (CCP) stacking. Continental crust of the Grouse Creek block – now beneath the late Cretaceous Idaho Batholith and outcrops of Eocene Challis volcanics – attains thicknesses of ~35-40 km, whereas thinner crust, ~30 km to Moho, lies beneath the accreted Blue Mountains terranes. We observe a strong mid-crustal converting interface at 15-20 km depth on the western side of the Idaho Batholith, 15-20 km east of the WISZ, which we interpret to be a preserved remnant of the subduction channel of the Salmon River suture. We observe a clear ~N-S striking, offset in crustal thickness beneath the western Pahsimeroi Valley, where the Moho abruptly jumps up eastwards from ~40 km to ~30 k depth. We associate this Moho offset with the Neoproterozoic truncation of the Grouse Creek along the Farmington zone, identified at the surface to the south of the IDOR area in Utah. Upper mantle deformation fabrics, deriving from splitting intensity measurements of SK(K)S phases reveals a consistent, but subtle deviation of upper mantle deformation fabrics from the North America absolute plate motion direction. These preserved structures and deformation fabrics substantiate the importance of the Salmon River suture for the development of this portion of the North American Cordillera.