The Columbia River Basalt as a record of Miocene topography and the deformation that followed

In the middle Miocene, the Grande Ronde and Imnaha flows of the Columbia River Basalt buried approximately 210,000 km² of Washington, Oregon, and Idaho with hundreds of meters of basalt. These flows crossed major tectonic terrane boundaries between Paleozoic North America, Mesozoic accreted terranes in the Blue Mountains province, and the Eocene Siletzia terrane. Erupted between 17-16 Ma, the distribution of Columbia River Basalt both acts as a marker for subsequent crustal deformation and a cap, preserving characteristics of the Miocene landscape over which it flowed. This has been used to great effect in previous studies focused locally in the Inland Northwest.

To more comprehensively examine the information recorded by Columbia River Basalt distribution, we compiled and digitized mapped contacts of the Imnaha and Grande Ronde flows from previous mapping efforts. We used the resulting dataset to constrain structure-contour surfaces for contacts between members of the Columbia River Basalt and the basal unconformity below the basalts. Results show that Miocene topography and the style of post-Miocene deformation vary significantly across the study region. Crustal architecture, as defined by terrane boundaries, appears to play a significant role in the style of post-Miocene deformation whereas variations in Miocene topography show a more direct link to Cenozoic deformation events.