GRAVITY AND MAGNETIC ANALYSIS OF THE SPOKANE DOME, WASHINGTON

The Spokane Dome (SD) is a metamorphic core complex that is the southeastern most of five Eocene-aged core complexes in north-central and eastern Washington. These metamorphic core complexes represent regions of extreme crustal extension where midcrustal metamorphic rocks are brought to the surface. The SD is along the southern edge and is the most important component of the Priest River complex (PRC) which is a region that contains igneous and high-grade metamorphic rocks that consists of lithologies ranging from Precambrian to Eocene in age. Numerous petrological and structural studies have defined the surface features of the PRC but its subsurface structural makeup is unknown due to the lack of geophysical data and studies. To investigate the subsurface structures within the SD, a gravity and magnetic analysis was undertaken. During the summer of 2005, 240 new stations were obtained along the southern and southwestern margins of the SD and these new data were merged with the sparse available data to create a Bouguer gravity anomaly map. This map indicates that the main portion of the SD is between two gravity maxima and based on preliminary residual gravity anomaly maps, this region is a gravity minimum. A similar pattern is seen on a reduced to the pole magnetic map. This minimum is associated with Cretaceous granitic rocks and the SD extends to at least south to the Spokane River. High-grade metamorphic rocks of the Spokane Dome mylonitic zone are associated with a gravity and magnetic maximum that extends to the Hauser Lake Gneiss. A two-dimensional gravity model will be constructed in order to determine the exact subsurface makeup of the SD.