DETAILED GRAVITY ANALYSIS OF THE OLD FAITHFUL GEYSER REGION, YELLOWSTONE NATIONAL PARK, WYOMING

The Yellowstone volcanic complex (YVC) has been associated with a stationary plume source and this plume system has provided the heat for the world famous geothermal features within the Yellowstone caldera. Gravity studies have long been used to understand the lithospheric structure of YVC but detailed gravity surveys of the geothermal areas have been lacking. Since 2016, I have been collecting detailed gravity data in the western portions of the YVC within the Upper, Midway and Lower geyser basins. The most detailed survey has been within the Old Faithful geyser region where over 200 data points have been collected. The data were collected using differential GPS methods in order to obtain centimeter level elevation data. The data have been processed into complete Bouguer gravity anomalies using 10 m and 2 m DEM models. The resultant data were analyzed using wavelength filtering and derivative methods in order to better define the density structure of the basins. While the more regional anomaly maps indicate that the Mallard Lake Dome, the Lone Star Geyser region and the northern section of the Lower Geyser Basin are associated with gravity minima that may be related to more hydrothermally altered volcanic rocks, the detailed data collected within the Old Faithful geyser region indicates that there exists a gravity minimum to the west of the geyser in the region between the visitor center and Old Faithful lodge. This region is to the west of the velocity minimum determined by surface wave models of the area. To the northeast of the geyser, a gravity maximum exists in the same location of a velocity maximum. This velocity minimum has been explained to be caused by hot water filled fractures that might be the source of water in Old Faithful. The gravity minimum extends to the northwest of the maximum amplitude of this minimum in a region with a number of small scale thermal features. This region might contain an additional reservoir of water or be a region of altered volcanic material. New data collected in the summers of 2019 and 2020 will further define this anomaly.