High Resolution, Ground-Based Magnetic Surveys Over Exposed and Buried Tertiary Intrusions within Laramide Folds and Faults, Big Bend Region, TX

We present results of an extensive, ground-based, high-resolution magnetic survey across Tertiary intrusions in northern Big Bend National Park, TX. Mapping at 1:10,000-scale in the South Persimmon Gap laccolith (SPGL), Dog Canyon area, and Dagger Mountain areas has delineated numerous intrusions. Some cross-cut a Laramide thrust fault and a Laramide map-scale fold; others are cross-cut by Basin and Range high-angle faults. Pending isotopic dates from the SPGL and several Dog Canyon area intrusions of the intrusive rocks will constrain the timing of Laramide and Basin and Range faulting and folding in this area.

Although previous high resolution aero-magnetic surveys (e.g. Scott, 2004; Drenth and Finn, 2007) reveal several exposed and unexposed intrusions in the surrounding area, these surveys were typically performed at a 400m flight-line elevation, which does not provide enough resolution to delineate the small-scale, structures of our study. Furthermore, these high-resolution aeromagnetic surveys do not extend to our map area. We present high resolution, ground-based magnetic data which helps us to constrain cross cutting relationships of potentially folded and/or faulted intrusive bodies. We present data as well as results of modeling the magnetic data to test and refine interpretations of relative timing between intrusions and structures.

Dagger Mountain is a NW-striking anticlinal box fold x km SW of Dog Canyon. Maxwell and others (1967) suggested Dagger Mountain might be cored by a laccolith while Moustafa (1988) interpreted DM folds to be caused by a blind thrust within a zone of convergent wrench faulting. Alternatively, post-Laramide laccolith may have preferentially intruded into and warped a Laramide fold, similar to the way the Solitario laccolith deformed the Terlingua-Fresno monocline. Several magnetic profiles as well as structural data will test these hypotheses.