THE BIG BEND REGION AS LABORATORY FOR MAPPING THE RELATION OF STRUCTURES TO PALEOSTRESS FIELD TRAJECTORY

Tectonic stress field shape can be deduced from careful mapping and measurement of structures such as tectonic stylolites, fractures, fold axes, and other naturally occurring strain gages. Variations in structural bearing are often presumed to signify separate tectonic events. However the interaction of structures must be discerned to verify whether separate tectonic events occur or if structures are responding to local stress field variations within a single event.

Mapping in the Big Bend region reveals that structures interacted with each other to reflect the shape and relative magnitude of tectonic stress at the time of their formation. For example, tectonic stylolites, mode I fractures, and a broad anticlinal fold that later became the Terlingua push-up structure along Udden’s northern boundary of the ‘sunken block’, all reflect early Laramide tectonic stress directions in this area. As shear surfaces developed a composite stress field formed, being composed of the regional field orientation and a local stress field associated with developing shear surfaces. This composite field is preserved along the Terlingua monocline where local fold axes, fractures, and tectonic stylolites display interpreted tectonic stress field curvature. The tectonic stylolites exhibit teeth trends that are both spatially and temporally dependent upon their distance from the monocline. Similar relations between shear surfaces, mode I fracture failure, and the orientation of the paleostress field was also encountered at other locations. Such observations have helped in developing a mathematical model for stress field curvature. A similar relation of structures and interpreted stress fields has also been found in Dead Horse Graben at Boquillas Canyon.