FRACTURING IN A REVERSE DRAG FOLD, PARASHANT CANYON, AZ

The western Grand Canyon is a natural laboratory for investigating processes of continental extension due to the great vertical exposure (> 1 km) and the relatively simple pre-extensional structure. We have undertaken a detailed field study of joint frequency in Parashant Canyon, a natural cross-section through the normal fault related Lone Mountain monocline, in order to better understand the role that joints play in accommodating extensional folding.

The Lone Mountain monocline is made up of two half-monoclinal flexures: a hanging wall fold in which dips gradually increase toward the fault from a regional dip of 2° to a maximum dip of 25° toward the fault over a distance of approximately 1.5 km and a footwall fold in which beds dip away from the fault with maximum dips of 12° near the eroded fault scarp that gradually return to a regional dip of 2° over a distance of approximately 0.5 km away from the fault. The fold is most pronounced southeast of Parashant Canyon where the fault system is made up of a master normal fault and smaller synthetic fault and less well developed to the northwest where the fault system is comprised of two parallel half-grabens.

Fracture frequency across the fold has been quantified using scanline surveys of a ~6-m thick sandstone from within the Manakacha Formation and panoramic photo interpretation of a ~30-m thick amalgamated sandstone within the Esplanade Formation. In both layers southeast of Parashant Canyon a near fault parallel joint set is well-developed in the hanging wall and nearly absent in the footwall. The frequency of this joint set increases from ~0.4/m at distances > 1 km from fault to ~1.6/m at ~150 m within the Manakacha sandstone and from 0.05/m to 0.2/m over distances from 1400 m to 700 m from the fault in the Esplanade sandstone. Joint frequency patterns are not as clear along the northwest side of the canyon where the fault pattern is more complex and folding less pronounced.

We propose that the increased fracture density may be acting to accommodate or localize folding associated with slip on the Froggy fault, a hypothesis that we plan to investigate through numerical modeling.