GRAVITY ANALYSIS OF GEOTHERMAL FEATURES IN WESTERN YELLOWSTONE NATIONAL PARK

The Yellowstone volcanic complex (YVC) has been associated with a stationary plume source but recent seismic and magnetotelluric studies have been interpreted to support either a deep or shallow plume source. Models created by these studies have shown that whatever the source, a plume is responsible for the caldera structure that encompasses most the Yellowstone region. Additionally, the active plume system is responsible for the famous geothermal features within the caldera. Gravity studies have shown that the YVC is associated with a high amplitude gravity minimum that is due to a thick sequence of siliceous volcanic flows and to the higher temperatures under the caldera. The highest amplitude gravity minima occur in the northeastern portion of the caldera under the Sour Creek Dome and are interpreted to be caused by partially melted siliceous material in the lower crust. Numerous regional geophysical studies have defined the deeper crustal and mantle structure of the YVC but there have been few detailed gravity investigations of the structures within the geothermal features in the YVC. In this study, I collected 220 new gravity stations spaced between 300 and 400 meters between the Lone Star Geyser region and the Lower Geyser Basin. These stations were collected using differential GPS for station location and elevations, and processed using terrain corrections. These stations 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 near surface hydrothermal alteration due to hot, saline waters. The Upper Geyser Basin is on the edge of a gravity maximum suggesting that the alteration of the rhyolitc rocks is not as intense but detail data could not be obtained in the vicinity of Old Faithful and Castle Geysers. Recent seismic models indicate a low velocity zone to the southeast of Old Faithful and additional gravity data would provide constraints on the geometry of this zone. Construction of residual and derivative maps highlight the locations of the alteration zones.