FAULT ZONE ARCHITECTURE AND STRUCTURAL DIAGENESIS AT THRUST FAULTS: ALONG STRIKE VARIATION IN DISTRIBUTION OF DEFORMATION AND FAULT-ZONE FLUID MIGRATION

Faults zones are complex structural features composed of three dimensional strain volumes that vary in scale. Fault zone architecture and the distribution of fault facies is important in understanding the affect these rocks have on permeability and fluid migration within and across the fault zone. We examine along -strike variations in a well-exposed small thrust fault system that cuts the Jurassic and Cretaceous strata at Buckhorn Wash Utah, and we link fault zone architecture/facies to tectonic stress and the rheology of the host rocks. These likely Sevier related thrust faults strike 030, with heaves of up to 600 m.

Field, microstructural, and petrographic analysis show along-strike variations in deformation features that include drag folds with hinge thickening, moderately south plunging parasitic folds, vertical calcite veins, joints, fault breccia, and fault gouge. At this locality we divide the Curtis into two units based on rock type, the lower Curtis (~110 m thick) is a thick bedded, calcite cemented, quartz sandstone and the overlying thin bedded calcite cemented glauconitic quartz siltstone, sandstone and rare limestone beds of the upper Curtis (~40 m thick).

Fault zones limited to the lower Curtis are characterized by drag folds, cataclasis and quartz and iron oxide mineralization adjacent to slip surfaces. Faults that juxtapose the lower Curtis on upper Curtis result in moderately south plunging parasitic folds with the thin bedded upper Curtis and calcite cemented fault breccia, calcite, quartz, and iron oxide mineralization, and fault gauge zones adjacent to slip surfaces. The average axial trend of folds associated with this fault zone is 12S/006, and fold tightness varies along strike. Variation in deformation features within the fault zone and changes in fault rock distributions and mineralogy are dependent on the lithologies involved and the local tectonic stresses that occurred within the fault zone.