Southern Rio Grande Rift Kinematics and Geochronology, Big Bend Fold Geometry, and the Use of Real Data in Geoscience Education: Contributions to the Understanding of Big Bend Geology and to Public Geoscientific Literacy

Abstract

The Big Bend region of Trans-Pecos Texas preserves a rich record of Rio Grande rifting and Laramide-age contraction that provide intrigue to national and state park visitors and field trips for undergraduate geology students. Despite its well-exposed geologic record, the Big Bend region remains an underutilized natural laboratory for studying these two tectonic events. This dissertation combines structural studies of Rio Grande rift and Laramide structures with public outreach and geoscience education studies focused on the Big Bend region.

In Chapter 2, existing extensional-kinematic studies from parts of this region are compiled with new fault kinematic and U-Pb geochronologic data from normal and strike-slip fault surfaces across the Sunken Block and Black Gap grabens of the southern Rio Grande rift. These data delineate an early and long-lived NE-SW orientation of principal stretch (S1) lasting from at least 30.1+3.1 Ma to at least 13.7+0.9 Ma. Clockwise rotation of S1 to NW-SE followed the early orientation after 13.7+0.9 Ma, agreeing with a widely cited regional clockwise shift in tension and extension at ~10 Ma reported in the northern Rio Grande rift and the Basin and Range province. The locations documenting this progressive rotation of S1 in the southern Rio Grande rift also suggest that later faulting narrows and is constrained to the Sunken Block graben.

Chapter 3 focuses on a footwall syncline under a Laramide thrust fault that is exposed in cross-section in a cliff wall at the end of the Dog Canyon trail in Big Bend National Park. This structure provides an opportunity to test fold models and is also societally relevant because it is a popular hiking trail for park visitors. Structural analysis reveals evidence of three deformation events preserved in minor structures within the folded rock. First and second in this geologic history were orthogonal flexure and horizontal contraction related to fault-propagation folding in front of and under the west-southwest-verging Santiago thrust fault. The third deformational event was horizontal extension interpreted to be associated with late Cenozoic Rio Grande rifting. This geologic history and an explanation of the structural analysis behind it are presented as a panel to be installed by the National Park Service as an interpretive wayside exhibit in front of the cliff on the Dog Canyon trail.

Chapter 4 presents a geoscience education study, involving undergraduate students of all majors in core-science geology classes at two universities near the Big Bend region. For this study, students interacted with either locally or globally sourced geoscientific data within in-class quantitative exercises and were surveyed to query the outcomes of their engagement, knowledge-retention, critical-thinking skill, and perceptions of the relevance of the exercise topics to human society. The group exposed to local data returned higher scores and rankings than those exposed to global data after four of the five exercises, indicating a lead in each of those outcomes but most clearly in critical-thinking skill and in their perception of the relevance of the geoscientific topic.