NEW MAPPING OF A MID TO LATE MIOCENE CALC-ALKALINE VOLCANIC FIELD IN NE OREGON

Detailed mapping of the northernmost part of the Powder River Volcanic Field (PRVF) provides new information on the evolution of calc-alkaline lavas in this important magmatic and structural transition zone. Regionally, the PRVF volcanism occurred between the bi-modal, rhyolite-dominated volcanism of the Oregon-Idaho graben to the south and the flood basalt-dominated volcanism of the Columbia River Basalt Group (CRBG) to the north.

Previously included in the CRBG, the north and east highlands of Oregon’s Grande Ronde Valley (GRV) consist of a series of mid to late Miocene lava flows ranging in composition from phonolitic basanite to dacite. Widespread diktytaxitic olivine basalt flows, the first PRVF lavas to be erupted in this region, flowed across a slightly tilted unconformity consisting of Grande Ronde Basalt (the most extensive unit of the CRBG) and pre-Tertiary basement (Wallowa Terrane). Subsequent volcanism produced localized basaltic to andesitic flows, a small composite volcano at Mt Harris, and laterally extensive and voluminous (~4 km³) flows of aphyric andesite and dacite. These thick, cliff-forming aphyric flows are interdigitated with an olivine-phyric basanite flow, which has yielded a ⁴⁰Ar/³⁹Ar whole rock age of 10.8±0.19 Ma. Mapped PRVF and CRBG dikes and vents follow a roughly northwest-southeast trend in this area, but, with the exception of Mt. Harris, do not form well defined volcanoes.

Waning stages of PRVF volcanism was marked by localized eruptions of strontium-rich, amphibole-phyric lavas. These lavas are exposed at the northeast end of the GRV, where small shield volcanoes, lava flows, and cinder cones form the Elgin Volcanic Complex (EVC). EVC lavas range in age from ~10 Ma to 2 Ma.

The PRVF is disrupted by a series of late Miocene, north-northwest-oriented extensional basins situated south of the Olympic-Wallowa Lineament (OWL). PRVF exposures in and adjacent to the GRV, the northernmost basin, provide only ambiguous evidence of right lateral strike-slip movement associated with the OWL. Faulting along the southeastern highlands to the GRV roughly parallels the trend of extension while faults to the northeast are rotated more northward. Basin margin faults are predominantly high-angle normal faults that display considerable (>500 m) vertical displacement.