Folding on the Flanks of the Southernmost Laramide Uplift, Big Bend Region, Texas

Map-scale and outcrop-scale Laramide folds measured in three widely separated areas in Sierra del Carmen (SDC) within Big Bend National Park contain consistently oriented NNW-striking axial planes and subhorizontal fold axes. A fourth area in the SE margin of the Marathon uplift contains NW-striking Laramide map-scale monoclines. Folds are on reverse fault-bounded flanks of a Laramide uplift that includes the Marathon uplift at its NW end and the El Burro-Peyotes uplift, mostly in Coahuila, at its SE end. Late Cretaceous – Middle Eocene synorogenic sediments accumulated in the flanking Tornillo basin (Lehman, 1991). In the Dog Canyon – Dagger Mountain area Laramide folds concentrated near Laramide thrust faults are concentric, open – tight folds lacking axial planar cleavage. Map-scale folds include overturned folds and synformal anticlines. Axial plane strikes average N14W. Early Late Cretaceous penecontemporaneous folds predate Laramide folding. Doming over plutons and post-35 Ma folds related to Basin and Range faulting postdate Laramide folding.

Near Ernst Tinaja, 48 km S of Dog Canyon, abundant open to tight, mainly parallel, flexural-slip folds have rounded to chevron hinges. Some layers show slight disharmonic thickening in the hinge due to flowage in thin clayey units. Competent layers thicken only slightly, if at all, in hinges. Axial planes average N8W and verge westward. Half-wavelengths of these subhorizontal to gently plunging, steeply inclined folds average 1.4 m.

Near Muskhog Spring, upright, outcrop-scale Laramide folds contain N2W axial planes and subhorizontal fold axes. Folds are abundant adjacent to the Muskhog Spring fault, a large-displacement normal fault that cross-cuts the Tertiary McKinney Hills laccolith, suggesting Laramide as well as Basin and Range fault movement.

Consistent Laramide fold orientations suggest late Cenozoic Basin and Range transtension did not involve major rotation or tilting throughout the SDC. Laramide structures described to date appear to be part of a single deformation phase.