DETERMINING THE BASEMENT FAULT STRUCTURE OF THE HOGBACK MONOCLINE IN NORTHWEST, NM USING FRACTURE PATTERNS AND GEOMECHANICAL MODELING

Monoclines on the Colorado Plateau are well understood in cross-section, but changes along strike are poorly understood. The Hogback monocline, on the Navajo Nation in northwest New Mexico, has multiple bends in map view. This study uses geomechanical modeling (T7) and field data (fracture patterns) to explore the possible geometries of faults that created the bends. We hypothesize that under the Hogback monocline there are three possible basement fault systems: an S-bend fault, two parallel faults (relay) or two parallel faults connected by an oblique fault (breached relay). These different models should show differing fracture patterns based on differing stress and strain directions.

In the field, we measured fracture sets along the monocline in the Cliff House Sandstone. We observed four sets: one parallel to the strike of bedding, two sets oblique to the strike of bedding, and one parallel to bedding dip. Fracture geometry changes as bedding orientation changes, notably around the bends. Slickenlines on some oblique fractures suggest that they are strike-slip shear fractures rather than joints.

We built three scenarios in T7 (elastic dislocation modeling, based on rock properties and structure to predict deformation). Each scenario was designed to match the geometry observed in map and cross-section view. The monocline has three bends: southern (changing strike from 014° to 057°), middle (changing from 057° to 018°), and northern (changing from 018° to 055°). Bends connecting straight segments are 2750 m long and are offset by 1650 m (northern bend) and 1900 m (southern bend). Throw on the faults was determined to be 180 m via cross sections. We plan to test each scenario with different boundary strain conditions and shortening directions and compare predicted fracture patterns to those observed in outcrop.

Understanding timing and distribution of fractures is important for predicting the movement of oil, gas, and water, and for assessment of local subsurface risks.

Any persons wishing to conduct geological investigations on the Navajo Nation including visiting the sites described in this study must first apply for and receive a permit from the Minerals Department – Telephone (928) 871-6587.