GRAVITY INVESTIGATIONS IN THE UPPER AND LOWER GEYSER BASINS, YELLOWSTONE NATIONAL PARK

The Yellowstone volcanic complex (YVC) of northwestern Wyoming is part of a larger magmatic system that extends southwest through the Snake River Plain in Idaho. The YVC has long been associated with a stationary hot spot source, however, recent seismic tomography and magnetotelluric studies have been interpreted to support both a stationary hot spot and nonhot spot sources. Gravity studies have been important in determining the crustal structure of the caldera with the latest caldera complex being associated with a regional gravity minimum. The highest amplitude gravity minimum occurs in the northeastern portion of the caldera under the Sour Creek Dome and is interpreted to be caused by partially melted siliceous material in the lower crust. Despite all the regional gravity interpretations, there is a lack of detailed data except in a few geothermal basins that were part of investigations to determine how gravity changes over time. Additionally, there have been no detailed gravity investigations of the geothermal features in the YVC. In this study, I collected 210 gravity stations spaced between 300 and 400 meters between the Lone Star Geyser region and the Lower Geyser Basin. These stations collected using differential GPS and processed using terrain corrections were merged with existing data to produce a complete Bouguer gravity anomaly map. This map indicates that the Mallard Dome Lake Dome, the Lone Star Geyser region and the northern section of the Lower Geyser Baisn are associated with gravity minima probably associated with deep hydrothermal alteration. The entire region from the northern boundary of the Lower Geyser Basin and the southern edge of the Upper Geyser Basin is characterized by a gravity minimum. This suggests that this region is underlain by a thick region of altered rhyolite similar to that exposed in the Yellowstone Grand Canyon. However, there are several exceptions including the Upper Geyser Basin is on the edge of a gravity maximum suggests that the deeper alteration of the rhyolitc rocks is not as intense but additional data are needed to confirm this observation.