INTERMEDIATE DEPTH DRILLING OF THE SNAKE RIVER PLAIN: TRACKING THE YELLOWSTONE HOTSPOT (?) THROUGH SPACE AND TIME: A CALL TO ACTION

Mantle plumes are thought to play a crucial role in the Earth’s thermal and tectonic evolution; plume heads are influential in forming plate boundaries, while plume tails may form incipient rifts. Nonetheless, the influence of mantle plumes on continental dynamics is poorly understood. Most researchers believe that Yellowstone is the world’s best example of a plume beneath continental crust, making it a unique place to test the plume hypothesis. In southern Idaho, the hotspot track is marked by rhyolitic eruptive centers that underlie basalt of the Snake River Plain (SRP). The plume model has been questioned by studies that suggest the magmatism may be explained by localized upwelling associated with edge effects of plate motion and the descending slabs. This poses a first order question as to whether mantle plumes exist or not, and if they do what their effect is on the evolution of continental lithosphere.

Existing shallow drill holes provide details of basalt stratigraphy locally at the INEEL site. We propose a series of 6 to 8 intermediate depth (2-3 km) drill holes taken along the axis of the SRP that will specifically target the origin and evolution of alleged plume-related volcanism in both space and time. Samples from each hole will allow us to examine the geochemical and isotopic characteristics of rhyolites and basalts through time at each site, while the series of holes will allow comparison of coeval lavas erupted at different locations, and estimation of the eruptive volume thru time. Holes drilled in the WSRP graben will help define the relationship between this structure and the ESRP plume track. These drill holes will complement geophysical studies of continental dynamics, as well as current studies centered on Yellowstone. Additional components of a targeted drilling program include paleoclimate studies of North America during the Pliocene—Pleistocene through sampling of lacustrine sediments, hydrology of the SRP aquifer, fluid flow at deeper crustal levels, and the impact of heat flow and the chemical reequilibration of rocks and fluids at depth. This project will be an ideal test of plume hypothesis. If a plume origin is supported, it may be generalized to plume processes and plume-lithosphere interactions globally. Alternatively, it may validate recent proposals that eschew a plume origin for these features.