USING THE COLUMBIA RIVER BASALTS TO CHARACTERIZE SYN- AND POST-ERUPTIVE DEFORMATION OF THE WALLOWA MOUNTAINS AND SURROUNDING REGIONS, NORTHEAST OREGON

The impressive and young (<17 Ma) Wallowa and Blue Mountains of northeastern Oregon are found in a tectonically quiescent part of the western United States. The timing and driving force behind the uplift of these mountains is poorly constrained. The rapid, voluminous outpouring of the Grande Ronde basalts of the Miocene Columbia River Basalt Group (CRBG) in <1 Ma provide a surface from which deformation can be measured. Grande Ronde basalt erupted onto a relatively flat, planar surface formed by flows of Imnaha basalt, which had flooded the existing topography. The emplacement ages of Grande Ronde and Imnaha flows is well constrained by radiometric dating and paleomagnetism.

Using published geologic and topographic maps, we entered the distribution of Grande Ronde flows into a geographical information system (GIS) and created a series of surfaces that define the relative deformation of the basalt flows. We distinguished temporal variations in the pattern of deformation by comparing flows of different age. These techniques allow us to reconstruct syn- and post-eruptive deformation of these flows.

The kilometer-scale uplift of the Wallowa and Blue Mountains and downwarp of the Troy Basin syncline, Baker and La Grande grabens create an approximately ring-shaped pattern of deformation. This deformation began around the time of basalt emplacement causing thinning and syn-eruptive deformation of flows towards topographic highs. The center of this broad area of uplift coincides with the Grande Ronde and Cornucopia dike swarms suggesting a link between the source of CRBG and broad deformation in the surrounding area. The timing, location, and magnitude of the uplift suggests that increased mantle buoyancy caused by basalt removal may be the primary deformation mechanism, implying that the CRBG mantle source was beneath the Wallowa Mountains area.