DAGGER MOUNTAIN, BIG BEND NATIONAL PARK, WEST TEXAS DOES NOT OVERLIE A LACCOLITH

Geologic mapping at 1:10,000 scale and high-resolution ground-based magnetometer surveys show that the Dagger Mountain doubly-plunging anticline is a Laramide structure and part of a Type 1 interference figure (Ramsay, 1967) passively intruded by post-tectonic plutons. The Dagger Mountain structure is within the eastern margins of the Cordilleran orogen and Basin and Range province. Abundant 48 – 17 Ma plutons (Henry and McDowell, 1986) intrude the Big Bend region in diverse ways: some are laccoliths that strongly deform margins (such as Solitario, Christmas Mountains, Packsaddle Mountain, McKinney Hills) while others passively intruded (such as Rosillos pluton, South Persimmon Gap). Dagger Mountain is a 5 km-long ridge containing continuous exposures of four Cretaceous Formations intruded by numerous small Tertiary sills and plutons. Several lines of evidence suggest that the doubly-plunging anticline exposed throughout Dagger Mountain is primarily a Cordilleran fold not caused by a buried laccolith. First, its axial plane orientation, N32W 87NE is similar to the N14W average strike of Cordilleran axial planes throughout the region. Second, its axial plane orientation is similar to the N11W 88W axial plane orientation of an adjacent 3 km-long doubly plunging syncline. Third, the double-plunges of both structures result from broad NE-striking folds that could be a second Laramide phase. Fourth, mafic sills and dikes on Dagger Mountain do not reorient their country rock. Adjacent outcrop-scale fold orientations are not related to pluton orientations. Fifth, NNW- and NW-trending high-angle Basin and Range faults cross-cutting Dagger Mountain do not significantly reorient earlier structures. Magnetometer surveys do show a long-wavelength magnetic anomaly centered under Dagger Mountain possibly indicating a small, deeply buried pluton. This work describes a previously little-recognized NE-trending folding phase and emphasizes the necessity of extending detailed mapping outside pluton margins before interpreting emplacement mechanisms.