Reconnaissance assessments of three contractional fault-related folds along the Quebradas Back Country Byway, Socorro County, New Mexico
— Stephen C. Hook and Peter A. Brennan

Contractional kink folds developed in Paleozoic strata along the Quebradas Back Country Byway, Socorro County, New Mexico, are interpreted using pattern recognition as simple fault-bend and simple-step fault-propagation folds with easterly slip. These fold types are present at outcrop scale (<30 m across and a few meters of structural relief) to sub-regional scale (>5 km along strike and >10 m of structural relief) along the Byway. Cross sections of the folds are exposed along canyon walls and in hillsides. These interpretations imply that the folds developed above bends in nonplanar, low-angle thrust faults in individual thrust sheets and did not originate along high-angle reverse faults at great depth.

A simple fault-bend fold is a balanced, geometric model of deformation in which the causative fault is throughgoing, steps up from a lower detachment to an upper detachment, and has a flat-ramp-flat geometry, above which an open, roughly symmetrical fold with relatively gently dipping limbs is developed; more slip enters than exits the fold; and the front limb is steeper and shorter than the back limb. A simple- step fault-propagation fold is a balanced, geometric model of deformation in which the fault steps up from a bedding-parallel detachment and has a flat-ramp geometry: a closed, asymmetrical fold with a short, steep to overturned front limb and a longer, more gently dipping back limb lies above the ramp. In initial stages, no slip exits the fold, bed offset decreases to zero upward along the fault, the front synclinal axis terminates at the fault tip, and folding occurs at the fault tip located in the core of the fold.

NMGS Field Trip Stop #2 (Day 1) offers a great example of an outcrop-scale, cross-sectional view of folds and faults that developed in Permian strata near the southern end of the Byway. This outcrop (called Fold #1 in this paper) contains a complex mixture of many of the structural styles present in the Quebradas region: faults range from low-angle, Laramide thrust faults to a high-angle, Neogene normal fault; contractional folds include a fault-propagation fold and fault-bend folds. Most of the faults have <2 m of displacement. Folds have developed only above bends in thrust faults comprised of nonplanar segments or at the tip line of a thrust fault. Offset of beds along planar faults was by rigid block translation; the lack of fault-plane bends resulted in beds being moved up or down the faults without dip change or drag. Fold #1 exhibits many of the characteristics of a simple-step fault-propagation fold.

NMGS Field Trip Stop #3 (Day 1) is in the Tajo structural zone, which developed in Pennsylvanian and Permian strata near the middle of the Quebradas region. Fold #2 is a large anticline developed in Pennsylvanian strata that has a steep, highly overturned front limb and a long, planar back limb with a well-defined back syncline. The fold’s causative fault is not exposed. Based on fold-shape parameters, we interpret this structure as a fault-propagation fold whose causative fault stepped up easterly at ~15° from a detachment. Slip on this fault created a large fault-propagation fold with an overturned front limb. The fold was unable to grow self-similarly as slip increased, and the fault broke through the fold along a zone of weakness below its anticlinal axis. Our interpretation is illustrated by a generic fault-propagation fold model with high-angle breakthrough.

Fold #3 (included in Day 1 Field Trip Stop #3) is an outcrop-scale, kink fold with a slightly overturned front limb developed in Permian strata in front of—to the east of—the Tajo folds. This small kink fold is preserved as a breached anticline-syncline pair. The planar back limb and back syncline of the fold form a conical hill capped by resistant gray carbonate strata. The back limb dips ~28°W. The crest of the fold and upper part of the front limb are eroded, but two thin, slightly overturned sandstone beds in the valley to the east of the hill correlate to the two resistant sandstone beds in the back limb. The reconstructed fold pattern is typical of the core of a classic fault-propagation fold.

Bed thickness is conserved in these folds, which are interpreted to have developed through bedding-plane slip and kink-band migration in the brittle domain. Folding did not involve gypsum or ductile deformation. All assessments are presented as first-order, reconnaissance interpretations that are intended to provide an alternative approach to the published understanding of the complex structural geology of the Quebradas region. These assessments are not all inclusive and should be supplemented with geologic map observations and related interpretations.