GEOPHYSICAL MAPPING OF DIKES IN THE EASTERN UINTA MOUNTAINS, NORTHEASTERN UTAH

The WNW orientation of the Uinta Mountains of northeastern Utah, eastern Colorado, and southern Wyoming is unique in the western United States and is generally believed to result from the rotation of the Colorado Plateau during the Eocene. However, Ritzma (1983) mapped and dated sparsely-exposed diorite-gabbro dikes of Cambrian-Silurian age intruding the Uinta Mountain Group quartzite in the eastern Uinta Mountains. These dikes had an orientation (N65W – N70W) nearly identical to the anticlinal fold axis of the mountain range, suggesting that the orientation reflects pre-Eocene structure, such as a Precambrian suture zone. The objective of this study was to use geophysical methods to extend the mapping of dikes in the vicinity of Chepeta Lake and Rose Ridge in the eastern Uinta Mountains. The objective was addressed by carrying out total magnetic field surveys using the Geometrics G-856 Proton Precession Magnetometer, geoelectric (electrical resistivity, induced polarization, self-potential) sounding and profiling using the Iris Instruments Syscal Junior Resistivity System, and magnetic susceptibility surveys using the SM-20 and the Bartington MS3 Magnetic Susceptibility Meters. Data interpretation was carried out using the Interpex IX2D-GM Magnetic Interpretation and the IX1D 1-D Sounding Inversion software packages. The total magnetic anomaly data was consistent with continuation of a dike that was exposed for only 186 m in the vicinity of Rose Ridge for at least an additional 2150 m to the west along strike (N68W), including offsetting by a possible right-lateral fault perpendicular to the strike of the dike. The dike signature included both normally and reversely magnetized sections with the exposed portion of the dike being reversely magnetized. Resistivity sounding inversions were very similar along the dike and in quartzite with extensive fracturing associated with thermokarst, while quartzite that was not affected by thermokarst showed significantly higher subsurface resistivity. This result made it possible to use resistivity profiling to map the zones of fractured quartzite on either side of the dike, which were wider on the downdip side (~200 m) than the updip side (~50 m). Further interpretation will be reported at the meeting.