A model for the tectonic setting of the Columbia River basalt eruptions

Abstract

A review of the structures mapped across the Columbia River flood-basalt province reveals a consistent strain pattern from the beginning of the eruptions of the Columbia River Basalt Group (CRBG) until the end of the eruptions of Grande Ronde Basalt (-17.5 to 15.5 Ma). The observed strain is one of north-northwest shortening and west-southwest extension. The degree of strain is small (i.e., extension «1 percent) and resulted in north-northwest tensional fissures (feeder dikes), approximately east-west folds associated with steep reverse faults, and northwest (right-lateral) and northeast (left-lateral) strike-slip faults. This strain pattern is present from the Brothers fault zone in central Oregon to the northern margin of the CRBG in northern Washington and varies only in its intensity, a factor that can be correlated with the nature of the underlying crust. From approximately 15.5 Ma to the present the same stress pattern continued to deform the flows of the CRBG as they formed north of the Olympic-Wallowa Lineament (OWL), which bisects the Columbia Plateau in a west-northwest direction. South of the OWL the structural pattern changed abruptly at -15.5 Ma. North-northwest feeder dikes, east-west folds, and northwest and northeast strike-slip faults are replaced by well-developed north-northwest-trending grabens, which indicate a much greater degree of crustal extension (-20 percent) accompanied by crustal thinning. The change in strain corresponds to a change in the type of volcanicity: from the pre-15.5 Ma flood eruptions of tholeiitic basalt to post-15.5 Ma small volumes of olivine basalt and intermediate to alkalic and silicic compositions erupted locally along the graben faults.The increased crustal extension south of the OWL at -15.5 Ma implies right-lateral strike-slip motion along that zone, and it is shown that structures previously mapped along the OWL are similar to those mapped along the Brothers fault zone to the south. In both zones the structures are consistent with their interpretation as right-lateral megashears. A tentative model is introduced in which the deformation pattern of the whole Columbia River flood-basalt province is related to oblique subduction to the west and the back-arc spreading associated with the Basin and Range crustal extension to the east.

Subduction zone and crustal dynamics of western Washington; a tectonic model for earthquake hazards evaluation

“…As noted in a tectonic analysis by Hooper and Conrey (1989), evidence for crustal thinning on the south side of the OWL relative to the north suggests right-lateral motion. The OWL represents a fundamental crust boundary, probably a suture zone, with the north side anchored in thick continental of the North Cascades on the NW and the Idaho batholith on the SE (Hooper and Conrey, 1989). Thick Miocene basalt flows form a rigid plate, with the maximum thickness centered in a ponded region (Pasco Basin), just south of the OWL.…”

“…Thick Miocene basalt flows form a rigid plate, with the maximum thickness centered in a ponded region (Pasco Basin), just south of the OWL. We interpret that this shallow crust rigid plate is being pushed along like a hockey puck over the relatively thick layer of sedimentary rocks (2-5 km thick) that occur beneath the basalts (Hooper and Conrey, 1989), accentuating the NW shear caused by Basin and Range spreading. On the north side of the OWL, the thick crustal anchor of the North Cascades means that near-surface translation probably does not occur in very noticeable fashion.…”

Primary and ‘forced folds’ of the Columbia River basalt province, eastern Washington, USA

“…2). The Hite fault zone (HF) appears to displace the suture zone by left-lateral strike-slip motion (Reidel et al 1994;Sobczyk 1994;Hooper et al 1995), before the suture zone turns north again to form the eastern margin of the Pasco basin (PB). The Blue Mountains anticline, trending westsouthwest on the south side of the OWL and south of the Pasco basin, is known to be prebasalt (Riddihough et al 1986) because the ridge formed a divide which separated the Picture Gorge and Grande Ronde basalts during their contemporaneous eruptions at approximately 16 Ma.…”

“…North of the suture zone, across the Palouse slope, the basalts on the craton remain essentially undeformed. This presumably reflects a difference in the thickness and strength of the older craton and the younger accreted terranes Hooper & Conrey 1989).…”

Surficial geology and neotectonics of selected areas of western Idaho and northeastern Oregon

“…The entire region has subsequently been buried by voluminous flows of Miocene Columbia River Basalt Group basalt (Reidel and Hooper, 1989). Miocene to recent faulting (Hooper and Conrey, 1989) has uplifted the region into a series of dissected plateaus, broken by occasional fault-controlled basins and ranges. The western part of the region is dominated by northwest-trending normal faults (Walker and MacLeod, 1991), and the eastern part of the region is dominated by north-trending normal faults (Mitchell and Bennett, 1979;Fitzgerald, 1981).…”