GEOLOGIC MAP OF DAGGER MOUNTAIN: IMPLICATIONS FOR THE LARAMIDE IN WEST TEXAS

Preliminary geologic field mapping integrated with previously published data suggest Laramide shortening in west Texas persisted later than previously suggested. Flat slab subduction of the Farallon plate beneath the western margin of North America ~75-30 Ma resulted in widespread shortening prior to <30 Ma Basin and Range extension. In west Texas, previous work suggests shortening ended ~50 Ma (although Laramide compressional stress persisted until ~31 Ma) and was followed by arc-volcanism (Henry et al., 1991; James and Henry, 1991) and later Basin and Range Extension. Dagger Mountain is a doubly plunging anticline that accommodated northeast directed shortening in Big Bend National Park, west Texas. Preliminary geologic mapping was conducted in the Dagger Mountain area to evaluate the relative age of shortening, extension, and intrusive bodies. Field mapping results document the following key relationships: 1) Dagger Mountain consists of Cretaceous sedimentary rocks and intruded by two Paleogene mafic sills; 2) Dagger Mountain is folded into a northwest trending anticline; 3) small scale (<500m) northeast-striking normal faults crosscut the mafic sills (46 Ma or younger based on regional correlations) on the southwest flank of the Dagger Mountain anticline; and 4) large scale (>5 km) northwest-striking (Basin and Range) normal faults crosscut the crest of the anticline. The small scale northeast striking normal faults crosscutting the mafic sills are interpreted to be the result of hinge-parallel extension synchronous with Laramide northeast-directed shortening. Therefore, relative age relationships show Laramide shortening must be younger than 46 Ma. These observations support geochronology results from other workers in west Texas and southern New Mexico that suggest Laramide shortening persisted later than ~50 Ma, possibly until ~31 Ma. Finally, these results suggest flat-slab subduction of the Farallon Plate persisted beneath the west Texas region later than previously suggested.