AEROMAGNETIC MAPPING OF THE STRUCTURE OF PINE CANYON CALDERA AND CHISOS MOUNTAINS INTRUSION, BIG BEND NATIONAL PARK, TEXAS

Analysis of aeromagnetic and gravity data reveals new details of the structure, igneous geology, and temporal evolution of the prominent ~32 Ma Pine Canyon caldera and the Chisos Mountains, Big Bend National Park, Texas. The main caldera-filling Pine Canyon Rhyolite, the oldest member of the South Rim Formation, is shown to be reversely magnetized, allowing it to be used as a key marker bed for determining caldera fill thickness. Modeling of gravity and magnetic anomalies indicates that the Pine Canyon Rhyolite is likely thicker in the northeastern portion of the caldera. Lineaments in the magnetic data suggest buried faults beneath the caldera that may have led to increased downdrop in the northeast vs. the southwest, allowing a thicker section of caldera fill to accumulate there. The Pine Canyon caldera has been interpreted as a downsag caldera because it lacks surficial faulting, so these inferred faults are the first mapped features there that could be responsible for caldera collapse. The caldera boundary correlates well with the margins of a gravity low. General features of the caldera match well with basic models of downsag calderas, meaning that the Pine Canyon caldera may be a classic example of downsagging. The source of a long-wavelength magnetic high over the Chisos Mountains is interpreted as a previously unknown broad intrusion whose long axis trends parallel to a major crustal boundary related to the Ouachita orogeny or an even earlier Precambrian margin. This feature represents the largest intrusion (28-34 km diameter, 1-4 km thick, 700-3000 km3 in volume) in an area where relatively small laccoliths are ubiquitous. The intrusion most likely represents a long-lived reservoir replenished by small batches of magma of varying composition as reflected in the variation of volcanic rocks in the Chisos Mountains. The intrusion may represent the easternmost occurrence of voluminous Tertiary magmatism in the southwestern United States.