LEARNING FROM BENDY MONOCLINES: FIELD, ANALOG, AND NUMERICAL MODELING

Monoclines separate uplifts and basins, and are the most important structural features on the Colorado Plateau. Previous studies of the geometry and kinematics of monoclines, mostly in cross section view, show that at least some Plateau monoclines result from reverse reactivation of Precambrian normal faults. Only a few studies have considered 3D monocline parameters.

We investigated a common feature of Plateau monoclines: local bends in map view, where the strike of the dipping limb can change by as much as 60° between more regional segments. The Hogback monocline of NW New Mexico displays several bends along 120 km, including one near Waterflow, NM. There, the strike of the dipping limb changes from the regional trend (060°) to 015° and back to 060°. Dips of the steep limb are as high as 35°.

In the field, we measured orientations of bedding, joints and shear fractures. Joints define sets parallel, normal, and oblique to the limb dip. However, the precise angular relations vary by location along the bend. To better understand these variations, and investigate 3D kinematics, we completed both numerical and analog modeling.

Our numerical modeling used elastic dislocation forward models. We considered different options for basement faults (S-shaped, relay, breached relay), fault dip, and obliquity of shortening directions. We evaluated the model results by comparing the overall monocline geometry and the predicted fracture distributions to field observations. Our analog models deformed wet clay above an S-shaped basement fault, with variable shortening directions. The clay model geometries and fractures were compared to field and numerical models.

Numerical and analog models of deformation driven by oblique shortening across an S-shaped basement fault were consistent with each other and with field observations. Models with shortening directions of 150-330°(normal to the regional monocline strike) and 120-300° produced oblique fractures within the bend, but did not match field observations. Models with shortening directions of 000-360° produced oblique fracture patterns across the complete study area and closely matched fracture patterns observed in the field.

Our model results provide insights to the origin of monocline bends, and also reveal an oblique shortening direction for the Hogback monocline.