THE COLUMBIA RIVER BASALT GROUP VOLCANISM IN THE BLUE MOUNTAINS A MIDDLE MIOCENE REFERENCE DATUM FOR STRUCTURAL ANALYSIS

Over 35 years of Columbia River Basalt Group (CRBG) studies in the Blue Mountains and adjacent areas have provided a framework that has helped unravel the Miocene and younger evolution of the Blue Mountains. Imnaha, Grande Ronde, Picture Gorge, Wanapum and Saddle Mountains basalts are present both as dikes and flows. These lava flows erupted directly onto accreted terranes and in small isolated basins (e.g. John Day Basin), unlike the Columbia Basin to the north where the basalts continued to fill an Eocene basin with many kilometers of sediment overlying the accreted terranes. The Hite fault of eastern Washington and north-central Oregon divides the dike trends into two parts. West of the Hite fault, dikes trend N35-40W and appear to follow an older (Eocene?) structural weakness. East of the Hite fault, dikes trend N-S to N20W and appear to follow a structural weakness in the accreted terranes.

Imnaha Basalt dikes/flows occur only in the eastern part of the Blue Mountains area. Grande Ronde dikes/flows, however, are found through out most of the area and provide a datum for structural control. Wanapum Basalt dikes/flows are largely confined to the crest and north slope. The Eckler Mountain Member of the Wanapum Basalt is largely confined to the Blue Mountains. Saddle Mountains Basalt flows appear to be confined to mainly the north slope. This demonstrates that during the CRBG eruptions, and afterward, the Blue Mountains were rising as the Columbia Basin was subsiding.

The CRBG flows show that the initial relief between the Blue Mountains and surrounding areas like the Columbia Basin was not great. The CRBG flows lap onto the Blue Mountains of north-central Oregon demonstrating that this portion was a structural high. Many areas were never completely inundated by the basalt. The eastern portion, near the Idaho border, had lower relief and provided a northward path into the Columbia Basin for CRBG flows and paleoriver channels. As the Blue Mountains were rising during CRBG eruptions, fault-controlled basins like the Walla Walla, La Grande, and Lewiston basins began to develop. Following the CRBG eruptions, these tectonic patterns continued to develop providing the present structural relief.