Structural analysis of the Hurricane fault in the transition zone between the basin and range province and the Colorado Plateau, Washington County, Utah

Abstract

This study defines fault segments and segment boundaries along the Hurricane fault in southwestern Utah and determines the geometric and kinematic relationship to other regional structunes. Fault segment identification is critical for understanding fault processes and seismic risk; segment length is the maximum earthquake rupture length along a fault Segment boundaries may act as barriers to earthquake propagation. Normal fault segmentation only recently has received attention and has never been studied along the Hurricane fault. The fault changes strike along its length, and is thus a segmented fault. This study documents one nonconservative segment boundary and two fault segments, the Ash Creek segment and the Anderson Junction segment, based on fault geometry, scarp shape. and shortening structures in the hanging wall and footwall. A rupture along the Ash Creek segment may affect Cedar City, UT, and a rupture along the Anderson Junction segment may affect St. George, UT and a number of smaller nearby towns. Previously undocumented surface offsets were observed along both segments. Evidence that Quaternary slip along the Hurricane fault is predominately normal includes offset of geochemically identical Quaternary (?) basalt, slickenlines, hanging wall dip analysis, and the 89 rake of the 1992 StGeorge earthquake. The Hurricane fault and the Gunlock-Grand Wash fault system, which lies 50 km to the west, may be a linked system whereby the two en echelon faults form a displacement transfer zone that generates the relatively wide transition zone between the Basin and Range province and the Colorado Plateau in the region. Data from both faults that support the existence of a transfer zone include symmetric changes in stratigraphic separation, similar timing of fault motion, and balanced regional cross sections.