AN ALTERNATIVE MODEL FOR BIMODAL VOLCANISM IN THE SOUTHERN CASCADIA BACK-ARC REGION SINCE ~12 MA

Trench-normal corner flow during rollback of the Farallon slab appears to be a reasonable model for basalt generation and associated NW migration of rhyolite volcanism along the Oregon High Lava Plains (HLP) since ~12 Ma. The presence of a coherent slab required by this model, however, conflicts with seismic tomography that delineates a slab hole or highly fragmented slab beneath eastern Oregon. An alternative model described here builds upon the earlier ideas of Jordan (2005), where upwelling since ~ 12 Ma is attributed to the emplacement of plume-modified mantle from the Yellowstone hotspot.

The numerical hotspot model of Lowry et al. (2000) generates dynamic uplift across a 1000-km-long axis that parallels plate motion, with swell elevations decreasing from Yellowstone to the SW into NE California. Model buoyancy derives from a thermal structure similar in strength and dimensions to a seismically defined layer of low-density mantle at ~200-50 km depth beneath SE Oregon, northern Nevada, and southern Idaho. This thermal feature narrows, shallows, and intensifies to the NE before deepening into the low-velocity plume tail beneath Yellowstone. It appears to derive from long-lived plume flux, with mantle flow direction and velocity influenced by plume buoyancy into an upside-down drainage pattern at the base of the lithosphere, from a depth of 60-80 km beneath the Snake River Plain to ~35 km beneath the HLP. An estimated range in volume flux for the melt source (15.39 m³ s^-1 to 30.78 m³ s^-1) is lower than the total flux for most oceanic plumes, but sufficiently high since 12 Ma to produce the equivalent of a 20-to-40 km thick layer extending over 295,000 km². This is equal in area to the low-density layer currently underlying the northern Basin and Range and HLP. Low-K tholeiites in the HLP are chemically equivalent to EMORB and consistent with plume material diluted by thermomechanical erosion of depleted mantle. Since 2 Ma, westward flow of this plume-modified mantle has been directed through narrow flow-line channels delineated by finger-like trends of low-density mantle and the surface alignment of Quaternary eruption sites. These channels have allowed low-density mantle to accumulate against the Cascades arc, thus providing a heated mantle source for mafic magmatism in the Newberry and Medicine Lake volcanic fields.