THE TECTONIC DEVELOPMENT OF THE EASTERN COLUMBIA RIVER FLOOD-BASALT PROVINCE

The majority of the Columbia River flood-basalt province lies in eastern Washington, northeast Oregon, and western Idaho, and consists of three structural subdivisions: the Yakima fold belt on the west, the Palouse subprovince on the east, and the Blue Mountains subprovince to the south. Nearly all the basalt erupted from fissures in the Palouse and Blue Mountains subprovinces. The Palouse subprovince is characterized by a gently southeast dipping paleoslope and has two main structural trends and one minor one: 1) a series of NE trending faults that extend from west of the Hite fault to the Mt Idaho-Syringa faults on the east. These NE faults define basins such as the Lewiston and Walla Walla basins and, 2) a series of low amplitude long wavelength folds and faults that trend NW. East-west trending fold and faults are present but less abundant compared to the NW and NE trends. The basalt dikes parallel the NW fold trend but have a more westerly trend north and west of the Hite fault.

The greatest thickness of basalt occurs in the central portion of the province, the Columbia basin, which parallels the NE structural trend. The NE structural trend can be traced outside the basalt province into NE Washington and probably resulted from accretion tectonics during the Mesozoic. This trend probably defines the edge of the North American craton beneath the basalt. The NW trend appears to be related to a preexisting structural grain that controlled the emplacement of dikes, and later folding and faulting. The NW trend parallels the main structural trend exposed in the basement rocks of the Cascades and throughout the Pacific Northwest. This trend appears to be related to tectonism during the Eocene. The E-W trend may be controlled by basement structures.

Both the NW and NE structural trends were reactivated under the present north-south compressive stress regime giving rise to the main structural features of the eastern portion of the Columbia River flood-basalt province. Subsidence along the NE faults during the eruption of the basalt produced many of the present major structural basins. The E-W folds and faults appear to be among the younger structures.