Geometry and kinematics of active normal faults, South Oquirrh Mountains, Utah: implication for fault growth

Abstract

The NNW-striking South Oquirrh Mountains normal fault zone consists of four individual faults. The individual faults are arranged in a right-stepping pattern in the north and a left-stepping pattern in the south, forming a convex-shaped fault zone in map view with the apex towards the hanging wall. Late Quaternary fault activity is characterized by 2.0–4.5 m high discontinuous fault scarps developed in both late Quaternary alluvium and bedrock. The fault scarps in bedrock contain evidence of two large rupture events. The last large earthquake occurred prior to the highstand of the Bonneville lake cycle (15 ka), based on cross-cutting relationship between the Bonneville shoreline and the fault scarp, and on comparisons of fault scarp morphology. Cumulative displacement patterns inferred from range crest elevations, Bouguer gravity data in the adjacent basin and rotation of the subsidiary faults adjacent to the West Mercur fault are similar to the pattern of displacements measured across late Quaternary fault scarps; that is, the maximum displacement is near the apex of the convex-shaped fault zone, where the West Mercur fault is located, and then tapers off towards both ends of the fault zone. Fault traces that range from a few meters to tens of kilometers long are characterized by two dominant orientations (strike N11°W and N43°W): the two orientations are separated from each other by a statistically constant angle of 33° ± 3°. Slip directions concentrate on a trend of S70°W for both groups of faults, perpendicular to the average strike of the fault zone. These data indicate that the geometry of the fault surfaces is non-cylindrical and may be composed of self-similar structural ridges and troughs of variable wavelength and amplitude that are elongated parallel to slip direction.The relationships between fault geometry, displacement and geomorphology in the South Oquirrh Mountains fault zone suggests a growth model of normal faults in which apex points on the convex-shaped fault sections mark the nucleation points of primary faults. Secondary faults developed at the ends of the primary fault step into the footwall block as the fault zone grows laterally. Intersecting regions between two laterally growing fault zones are concave-shaped in map view.