PHYSICAL ANALOG MODELS OF BASEMENT FAULT SEGMENTATION AND MONOCLINAL FOLDING

Debate about the existence of reactivated basement faults underlying some or all Colorado Plateau monoclines has raised questions about the effects of basement fault geometry, slip gradient, and segmentation on monocline surface expressions. Physical analog models are being conducted to study the relationships among basement fault segmentation, monoclinal folding, and surface strain patterns. The basic model design involves a stiff plastic sheet containing pre-cut fault segments overlain by alternating colored layers of dry sand. The plastic ‘basement' sheet and the sand overburden are shortened at 6.0 cm/hr by a motor-driven moveable wall, causing the plastic sheet to both flex and fault along the pre-existing cuts, resulting in deformation of the overlying sand layers. Variations among experiments involve the lengths and orientations of pre-cut fault segments, and the strike-parallel and strike-perpendicular step-over distances between fault segments. Following deformation, models are moistened and sliced to inspect fault and fold patterns within the sand overburden.

Model results show that stepped basement fault segments create bends in the surface expressions of monoclines. In most models conjugate normal faults form along the crests of monoclines, developing earlier and showing greater displacement above lateral basement fault steps than above the centers of basement fault segments. Deformation of surface markers is consistent with increased extensional strain accommodation in step-over regions of the models. Cross sectional slices reveal thrust faults propagating from both the hanging wall and footwall tips of the basement faults. Surprisingly, some models display the greatest vertical offset and the largest amplitude of monoclinal folding near the tips of basement faults while others experience maximum displacement and fold growth at the middle of each segment.

The normal faults observed above basement fault steps in the physical models resemble a pattern of faulting associated with eastward bends in the generally northerly trace of the East Kaibab monocline of southern Utah. Further physical modeling and field studies of monoclines may help identify specific links between surface fold and fault patterns and underlying basement fault characteristics.