Net Slip and Linkage Patterns of the Sevier-Toroweap Fault System

Abstract

The Sevier-Toroweap fault system is a 250+ km high-angle normal fault located in southern Utah and northern Arizona in the High Plateaus subprovince. It is one of three main normal faults that accommodate the strain concentrated on the boundary between the Basin and Range and Colorado Plateau in the Utah Transition Zone. As with all faults in the Transition Zone, the Sevier-Toroweap fault system trends N-S but has distinct bends in the fault trace that trend northwest or northeast. These characteristic changes in strike reflect the segmentation and growth history of the fault. Since fault linkage is reflected in a fault system’s net slip distribution, this study characterizes the net slip distribution along the Sevier-Toroweap fault system reveals fault linkage patterns which can improve our understanding of how this fault formed, its extent, and geometric segmentation that could impact the potential seismic hazards associated with it.Compiled surface geology and subsurface constraints were used to construct cross-sections using kinematic forward modeling techniques in order to evaluate the subsurface geometry of the fault and calculate displacement along the main trace of the fault system. Displacement constraints collected from these cross section, published USGS cross sections and maps, and other literature were compiled to produce distance vs. displacement plots. These plots demonstrate the segmentation of the Sevier-Toroweap fault system and help reduce uncertainty as to where the northern terminus of the fault system lies. The linkage history implied by variations in fault strike, locations of fault splays, and displacement gradients along strike suggest that there are three main segments of the fault, which are different than the fault sections commonly used to describe the fault in the existing literature. Additionally, the high displacements and lack of observed decrease in displacement at the northern extent of the mapped fault suggests that the northern terminus may extend over 100 km north of the northernmost continuously mapped trace of the fault system and into the Marysvale volcanic field where it is thought to continue but be obscured due to outcrop conditions and geologic complexity within the volcanic system. In light of these new observations and constraints on displacement, fault linkage, and subsurface geometry, potential earthquake hazards associated with this fault are then evaluated.