Interpretation of Geophysical Anomalies over the Pine Canyon Caldera, Big Bend National Park, Texas

The Big Bend National Park region of Trans-Pecos Texas has experienced a protracted tectonic evolution, including Oligocene volcanism in the Chisos Mountains. One of the most important volcanic centers is the Pine Canyon caldera, which erupted the South Rim Formation about 32 Ma. This and other igneous structures in the park are poorly understood, however recently acquired high-resolution aeromagnetic data, gravity data, and geochronologic constraints reveal new details of the structure, igneous geology, and temporal evolution of the Pine Canyon caldera. The oldest member of the South Rim Formation, the Pine Canyon Rhyolite, is the main caldera-filling unit and is shown by direct and indirect observations and modeling to be reversely magnetized. This interpretation is supported by a new absolute age date that corresponds to a time period when the Earth's magnetic field was reversely polarized. Modeling of gravity and magnetic anomalies and quantitative comparison between synthetic terrain effects and measured magnetic anomalies indicates that the Pine Canyon Rhyolite has attenuated magnetization intensities and densities in the southwestern portion of the caldera, most likely caused by hydrothermal alteration. A long wavelength magnetic high can be interpreted as being sourced by a broad subcaldera laccolith. While largely unconstrained, this feature may represent the largest laccolithic intrusion in an area where laccoliths are ubiquitous. Other magnetic anomalies are interpreted to be due to shallowly buried and previously unrecognized intrusions around the Pine Canyon caldera.