THE TECTONIC EVOLUTION OF THE NORTHERN COLUMBIA RIVER FLOOD-BASALT PROVINCE

Although the Columbia River Flood-Basalt Province (CRB) covers more than 200,000 km² of the Pacific Northwest, greater than 75% of the basalts erupted in the northern part of the province (WA, northern OR and ID). The province consists of three tectonic elements: the Palouse Slope (PS), underlain by stable cratonic rocks; the Yakima fold belt (YFB), underlain by thick early Cenozoic sediments deposited on accreted terranes; and the Blue Mountains uplift which is cored by accreted terranes. Tectonism began prior to the eruption of the Grande Ronde Basalt, the main phase of CRB, and continued after the eruptions ceased. Subsidence dominated the northern province with no evidence of any initial uplift phase prior to the eruptions, as would be expected from a plume head advancing from the south. Subsidence is characterized by a series of basins that formed along the suture between the craton and accreted terranes during the basalt eruptions. The YFB area began subsiding in late Paleocene or earliest Eocene. Subsidence continued into the Miocene as 4-5 km of basalt accumulated contemporaneous with development of the Yakima folds. Only 1-2 km of basalt accumulated on the adjacent stable craton; however, the simultaneous development of a pervasive set of NW-SE low amplitude folds influenced the distribution of lava flows. The rates of fold growth, basin subsidence, and basalt eruptions are closely correlated in the YFB, as well as the amount of clock-wise rotation of the paleomagnetic poles recorded in the basalt. Although the large volumes of the eruptive pulses (as much as 5,000 to 10,000 km³) suggest a plume, the location of the main CRB eruptive activity over 400 km north of the Yellowstone plume track, as well as the region's tectonic history, suggests that other factors like plate convergence had a significant influence.