MAGMATISM AND EXTENSION - THE THERMAL AND MECHANICAL EFFECTS OF THE YELLOWSTONE HOTSPOT

Currently, there is a parabolic-shaped pattern of seismicity and latest Quaternary faulting which centers on the axis of the eastern Snake River Plain (SRP) and has its apex at the Yellowstone Plateau. The pattern of activity surrounds a region of aseismicity that includes the eastern SRP. Geologic evidence suggests that this pattern has migrated in tandem with rhyolitic volcanism induced by the Yellowstone hotspot during its 16 m.y. trek across western North America. Similarly there is a migration in the onset of basaltic lava flows and the emplacement of up to 12 km of midcrustal mafic intrusions that follows the track of rhyolitic volcanism. These observations suggest that the intrusion of magmas has a significant influence on the pattern of seismicity and faulting. We present a one-dimensional, finite-difference, thermomechanical model that accounts for the observed pattern of increased faulting followed by fault quiescence within the circum-eastern SRP. In this model, mafic magmas are intruded into a lithosphere that is already extending. The intrusions heat the surrounding rocks resulting in locally increased strain rates. As the intruded magmas solidify, the length of time required to return strain rates to their pre-intrusion level is then determined. The model assumes constant horizontal tectonic forces and maps strain rate as a function of yield strength and time since intrusion. We vary model parameters such as crustal thickness, initial geothermal gradient, and amount of magma intruded, in order to assess how they affect turnaround time for strain rates. The amount and timing of intruded material allowed by the model are constrained by seismic refraction data as well as by regional geochronologic and petrologic studies. We assume intruded rock rheology to be a Maryland diabase; mantle rheology we base on dry olivine; we represent the crust by both dry granite and dry anorthosite rheologies. Our results suggest that 2 to 3 m.y. appears to be a reasonable length of time for strain rates, to return to levels present before a midcrustal mafic intrusion equivalent in magnitude to the Yellowstone intrusion. This corresponds closely to the length of time between the onset of along-axis accelerated faulting and subsequent fault quiescence, assuming a hotspot velocity of 3.5 to 4.0 cm/yr relative to the North American Plate.