GEOPHYSICAL INVESTIGATION OF SUBSURFACE MAGMA FEEDERS CONNECTED TO THE TRACHYTE MESA LACCOLITH IN THE HENRY MOUNTAINS OF SOUTH-CENTRAL UTAH

The Trachyte Mesa Laccolith (TML) is Tertiary in age and intrudes Jurassic Entrada Sandstone of the Colorado Plateau in south-central Utah. Several lines of evidence indicate that this laccolith was fed horizontally from magma fingers/sheets originating from a large central intrusion (Mt. Hillers) in the Henry Mountains intrusive complex. Evidence includes: 1) The TML is highly elongate and is oriented along an line that can be traced back to Mt. Hillers, 2) Magnetic lineations from the TML are strongly oriented parallel to this line, and 3) An irregular tube-like “feeder” is partially exposed and located between the TML and Mt. Hillers.

The purpose of this study was to use shallow-based geophysical tools to locate the hypothesized magma fingers feeding the TML. Our methods involved the use of Ground Penetrating Radar (GPR), Seismic Reflection (SR), and Magnetic Anomalies (MA). Data was collected along four traverses trending perpendicular to the line between Mt. Hillers and the TML. Traverses were made over a flat-lying alluvial plain adjacent to the TML. The GPR system used was a TerraSIRch SIR System-3000 with a 200 MHz antenna. The seismic source was 12 gage shells loaded in a Betsy seisgun. Each seismic line had 12 attached geophones with 4.8 m intervals between each geophone. Shot locations were taken at the beginning, middle and end of each line. Magnetic anomaly data was acquired every 5 m using the gradiometer method.

Data analysis reveals that the GPR system was ineffective. Data seems to have been recovered from too shallow a level. We also encountered significant problems dragging the antenna over the sagebrush. Preliminary analysis of the SR data reveal the presence of highly reflective interfaces approximately 12.2-15.2 m deep, consistent with our projected depth to the base of the TML. The most prominent reflector can be traced between traverses and is along the line between Mt. Hillers and the TML. These data are consistent with MA studies by Nugent et al., (2004) over the same area. Their data revealed three anomalies that could be traced between traverses. We conclude that the SR data, combined with the MA data, are at least consistent with the TML being fed from magma “fingers” from Mt. Hillers.