MODELED EFFECTS OF DEEP-SEATED FAULT OFFSET AND STRIKE ON SUPRACRUSTAL GEOMETRIES IN BASEMENT-CORED UPLIFTS

The Precambrian basement of the Colorado Plateau contains a network of ancient faults, reactivated during late Mesozoic to early Cenozoic horizontal compression. The resulting Laramide basement-cored uplifts show diverse fault/fold patterns in the Paleozoic and Mesozoic sedimentary cover. In this study, physical analog models were conducted to investigate how differences in the strike and displacement of pre-existing basement faults affect the development of secondary structures in overlying sediment. Each model contained a 45º-dipping basement fault with a strike orientation of 90°, 75°, 60° or 45° relative to the horizontal shortening direction. The rigid basement was constructed of high-density polyethylene (HDPE) blocks. Overlying sedimentary cover was modeled using eight individually colored layers of 2φ sand, each 0.5 cm thick. Each model configuration was deformed at a rate of 4.0 cm/hr to a total horizontal displacement of 1.0 and 2.0 cm, for a total of eight models. After deformation, models were sliced in a series of 2 cm spaced cross sections and ~2 mm spaced horizontal sections (map views) to reveal fault and fold patterns in the sand layers. Fault strikes in the sedimentary cover generally paralleled that of the basement fault in all models. Models with 1.0 cm of displacement developed a single fault plane propagating approximately 1.5 cm into the sedimentary cover; while the overlying layers deformed in a broad open fold. All models with 2.0 cm of displacement, excluding the 45º basement block fault, developed multiple fault planes that propagated to the model surface forming structural terraces and fault-bounded lenses, suggesting that basement fault strike affects the formation of these structures. Fold and fault patterns in the models resemble structures associated with many Laramide uplifts on the Colorado Plateau and may be useful in understanding basement fault orientation, amount of displacement, and progressive fault and fold growth associated with basement-cored uplifts.