IMAGE OF YELLOWSTONE MAGMATIC HISTORY AND LOWER CRUSTAL OUTFLOW

We have combined three sets of seismic measurements from all available seismic networks to form a new subsurface image of the Yellowstone hotspot. Our most interesting result is that the mid-crustal sill-complex (MCS) has been resolved at a 10 km thickness and the MCS ‘layer' dips down to the SW consistent with the eastern Snake River Plain (ESRP) subsidence history. The MCS is underlain be a wedge of low velocity lower crust that pinches out between the 2.1 Ma and 4-6 Ma calderas. Most fascinating is that this low velocity lower crust extends 40-60 km laterally from either side of the ESRP. We suggest that this low velocity lower crust was emplaced via lateral flow of magmatically heated lower crust beneath the ESRP. In contrast to the low velocity lower crust beneath the 4-6 Ma calderas, the lower crust beneath the Yellowstone Caldera is not low velocity, but high velocity. Our results shows that the high velocity lower crust beneath the Yellowstone Caldera is part of the large region of high velocity lower crust that comprises most of the northern Wyoming province. If true, then the high velocity x lower crust is apparently not able to density trap the input magma to accumulate heat. The need for wholesale outflow of the hotspot track lower crust is confirmed by our crustal thickness map which shows that the eastern Snake River Plain crust (40-44 km) is only 2 km thicker than its adjacent margins (38-43). Beneath the Wyoming province, a thick crust (49-54 km) is found with a pervasive high velocity lower crust; thinner crust (36-39 km) is found in the SW Montana. Below the ESRP, a very low velocity mantle (3.9 km at 80 km depth) is found that requires mantle that is >70° warmer than normal. This constraint in conjunction with the low velocity pipe that extends to 500 km depth, and a depressed the 410 km discontinuity, strongly suggests that a small upper mantle plume is present.