CONSTRAINING THE GEOMETRY OF A SHALLOW CRUSTAL INTRUSION: NEW EVIDENCE FROM MT. ELLSWORTH, HENRY MOUNTAINS, UT

Surface displacements resulting from upper-crustal intrusions are a paramount concern for communities and facilities located near active volcanic areas (e.g. Campi Flegrei, Yucca Mtn.). Study of active intrusions is limited to remote observations through geophysical/geodetic methods. While surface displacements due to intrusions can be measured easily, intrusion shapes and depths are often based on simplified assumptions (e.g. spheres and ellipsoids). These models benefit from data constraining both the geometries of the individual intrusions, and the kinematics and mechanics of deformation within the overlying strata.

Mount Ellsworth, a sub-volcanic intrusion within the Henry Mtns., is an ideal natural laboratory for the study of near surface intrusions because it was emplaced into relatively flat-lying stratigraphy ~24 Ma when the stress field was largely isotropic. Previous studies of Mt. Ellsworth (i.e. Hunt, 1953; Jackson and Pollard, 1988) present competing emplacement models (e.g. single vs. multiple batch construction) based on slightly different geologic maps. Through a combination of 1:5000 scale field mapping and profile-oriented gravity study, we have produced new detailed geologic maps and cross sections of Mt. Ellsworth. These were intended to be used to test emplacement models. However, the results suggest that the initial goals set out for this study could not be realized due to the lack of exposed intrusion, the relatively small dimensions of the exposed sills, and the largely uncapped nature of the mountain. Mapping results demonstrate, however, that some of the assumptions of the models theorized by previous researchers were inappropriately applied to Mt. Ellsworth. These assumptions include the thickness and separation of stratigraphic units, the size and distribution of sills and smaller intrusions, structural attitudes of beds and sills, and the presence of exposure of the main body of the intrusion. Based on balanced cross sections constructed using our mapping and assuming a floor within the Permian White Rim Sandstone, the Mt. Ellsworth intrusion can be constraint to be ~2 km thick with laccolithic geometry, elongate in the NE-SW extending ~8 km NE-SW and ~6 km in ESE-WNW.