HYPOTHETICAL TECTONIC EVOLUTION OF THE LASAL MOUNTAINS AND THE UNCOMPAHGRE PLATEAU BASED ON UPPER MANTLE TOMOGRAPHY

The LaSal Mountains in east-central Utah, contain hypabyssal-cored laccolithic domes that were developed from the magmatic intrusions during the Mid-Tertiary time. Just to the east of the LaSal Mountains is the NW-SE trending Laramide Uncompahgre Plateau. We suggest the tectonic processes for both areas are likely to be derived from the rising low shear velocity body in the upper mantle.

In the project area WNW-ESE striking left-lateral oblique-slip are the most common faults, possibly developed by extension along the anticlinal hinge of the Uncompahgre Plateau in combination with Laramide compressional stresses of the Colorado Plateau, and upwelling of a low shear velocity body in the upper mantle throughout the Cenozoic Era. The mineralized cataclastic faults contain silicified sandstone breccias mixed with fluorite, amethyst, calcite, hematite, and barite of epithermal origin. Geochemical analysis of the mineral samples from the faults suggest the hydrothermal fluids were heated by former magmatic bodies located in the subsurface. Gravity and magnetic anomalies indicate possible mafic to ultramafic intrusive bodies are likely to be located in the upper crust under the Uncompahgre Plateau. However, the igneous rocks in the LaSal laccolith are well exposed and identified as Oligocene nosean trachyte. These intrusive bodies indicate larger chambers may be present in the crust, the possible sources of magma intrusions and hydrothermal fluids in the area.

The upper mantle tomographic data in 2D views, provided by the Incorporated Research Institutions for Seismology (IRIS) have indicated the existence of upwelling plume of low shear velocity (% dVs perturbation) at 60 – 720 KM directly below the study area. To get a better view of the mantle flows, we constructed a 3D model of low shear velocity (at dVs < 0% perturbation) in MATLAB R2015a software. The 3D model shows the existence of low shear velocity plume at 60-720 KM depth. This indicates the upwelling plume may be responsible for the tectonic and structural development of the LaSal Mountains and Uncompahgre Plateau.