Benjamin J. Drenth

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, enigmatic ca. 32 Ma Pine Canyon caldera and the Chisos Mountains (Big Bend National Park, Texas). The main caldera-fi lling Pine Canyon Rhyolite, the oldest member of the South Rim Formation, is reversely magnetized, allowing it to be used as a key marker bed for determining caldera fi ll thickness. Modeling of gravity and magnetic anomalies indicates that the Pine Canyon Rhyolite is probably thicker in the northeastern part of the caldera. Lineaments in the magnetic data suggest the presence of buried faults beneath the caldera that may have led to increased downdrop in the northeast versus the southwest, allowing a thicker section of caldera fi ll to accumulate there. The Pine Canyon caldera has been interpreted as a downsag caldera because it lacks surfi cial faulting, so these inferred faults are the fi rst 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, of which few well-described examples exist, in terms of a geophysical signature. The source of a long-wavelength magnetic high over the Chisos Mountains is interpreted as a previously unknown broad intrusion, the long axis of which 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 km 3 in volume) in an area where relatively small laccoliths are ubiquitous. The intrusion most likely represents a long-lived (>1 m.y.) reservoir replenished by small batches of magma of varying composition, as refl ected in the variation of eruptive products from the Pine Canyon and Sierra Quemada calderas. The intrusion may represent the easternmost occurrence of voluminous Tertiary magmatism in the southwestern United States.

Quantitative geophysical interpretation of gravity gradient and magnetic data over a buried carbonatite: The Elk Creek deposit, Nebraska, USA

Government. *The corresponding author: mkass@usgs.gov. Quantitative geophysical interpretation of gravity gradient and magnetic data over a buried carbonatite: The Elk Creek deposit, Nebraska, USA M. Andy Kass*, Benjamin J. Drenth, Leon Foks, and Joseph Capriotti Crustal Geophysics and Geochemistry Science Center, US Geological Survey, Denver, CO, USA Center for Gravity, Electrical, and Magnetic Studies, Colorado School of Mines, Golden, CO, USA