Geologic Motivation

The origin of Miocene flood basalts in the Pacific northwest of the United States has yet to be resolved. Although numerous competing hypotheses exist, there are two fundamentally different beliefs that share basic geochemical, petrologic, and geodynamic constraints: volcanism driven by the Yellowstone plume (Hooper et al ., 2002; Leitch and Davies, 2001; Takahashi et al ., 1998; Dodson et al ., 1997; Camp, 1995; Geist and Richards, 1993; Brandon and Goles, 1988) and magmatism associated with back-arc processes such as spreading and thermal erosion of the base of the lithosphere (Smith, 1992; Smith and Brandon, 1990; Carlson and Hart, 1987; Carlson, 1984). Evidence for and interest in a plume model dominate the literature, thus the overview of contemporary knowledge focuses on hypotheses utilizing a plume source component but also discusses inconsistencies in this view and opposing arguments.

The southeast Columbia Plateau offers a fascinating and important geologic setting in which to study many Earth processes, including intraplate volcanism, mantle convection, crustal deformation, and the geochemical evolution of flood basalts. Sandwiched between accreted oceanic lithosphere of the Columbia Embayment and Precambrian lithosphere of North America are numerous island arcs and other exotic terranes. These give the area a complicated and variable lithospheric structure. The Wallowa Mountains, located just west of Hell's Canyon, represent localized large-scale uplift over the past ~16 My. Although all major stress regimes (e.g., compression, shear, and extension) have been present in this area since the Miocene, it is unclear how these stresses may have been related to uplift (Reidel et al ., 1989; Hooper and Conrey, 1989). In fact, the substantial amount of uplift in the Wallowa Mountains (~2 km) is anomalous with respect to the relatively minor tectonic strain in the vicinity. Based on seismic imaging of the upper mantle, we propose this uplift is related, in part, to the cause of flood basalt volcanism (i.e., Columbia River Basalts) in the middle Miocene, which we hypothesize to be lithospheric delamination.

No comprehensive seismic exploration of the upper mantle in the source region for these basalts has yet been carried out. Reflection and refraction studies done in southeast Washington to determine crustal thickness and the viability of the area as a nuclear waste repository (Sobczyk, 1994; Saltus, 1993; Zervas and Crosson, 1986), gravity experiments to geophysically image the suture zone in western Idaho and southeast Washington (Mohl and Theissen , 1995) and several seismic investigations of the Cascadia subduction zone (VanDecar et al., 1990; Humphreys and Rasmussen, 1988; Crosson and Owens, 1987) offer some boundary information. This experiment, though a low-resolution, preliminary seismic study, provides data useful for interpreting regional geodynamics and for future, more detailed investigation of the upper mantle in this region.