MIOCENE TO PLEISTOCENE RHYOLITE-BASALT VOLCANISM AND TECTONISM, WESTERN SNAKE RIVER PLAIN AND OWYHEE MOUNTAINS, SW IDAHO

Bimodal basalt-rhyolite volcanism started at about 17 Ma in the Owyhee Mts adjacent to the western Snake River Plain (WSRP). Basalt erupted from fissures at 17-16 Ma and was followed by rhyolite domes and flows at 16-15 Ma. Subsequent east-west extension faulted and tilted the basalt and rhyolite. Later, mainly between 11.7 and 11.0 Ma, rhyolite eruptions along its SW margin accompanied the initial development of the NW-SE-elongate WSRP rift. Between 11.7 and 11.4 Ma phenocryst-rich rhyolite erupted from an E-W-trending row of vents and flowed northward into a lake in the paleo-WSRP rift, where it broke into large blocks that were silicified and brecciated. Then, between 11.4 and 11.0 Ma small, phenocryst-poor, rhyolite dikes and domes erupted into standing water along the WSRP margin. Two phenocryst-poor subaerial rhyolite flows from a NW-SE-trending fissure system also erupted at 11.4-11.0 Ma. At several km NE from their vents these flows also entered standing water.

Between 9 and 7 Ma a large volume of basalt was erupted into the WSRP rift. Much came up into a rather deep lake in the rift basin as it was widening in a NE-SW direction. Between 2.2 and 0.4 Ma, more basalt erupted in the WSRP rift as the lake gradually drained. During this time the style of volcanism changed from subaqueous to emergent phreatomagmatic, then to subaerial. As the style changed there was a concomitant basalt composition change. The 9-7 Ma basalts mainly are low-K olivine tholeiites. The earliest 2.2-0.4 Ma basalts are similar olivine tholeiites, but commonly have a little more K. As time passed the basalts became Fe-enriched, so that by about 0.8 Ma nearly all eruptions were ferrobasalt. Then, the composition changed to basalt enriched in alkalis but low in Fe. Intermittent faulting and tilting of basalt flows and lake sediments occurred in the WSRP to as late as a million years ago, and perhaps to the present.