Structural analysis and fault segment boundary identification along the Hurricane fault in southwestern Utah

Abstract

Long normal fault zones are common in extending regions and can be divided into segments with different geometries and faulting histories based on surface observations. The boundaries between fault segments are important because they may be the sites of significant strain, may impede rupture propagation, and may greatly influence the locations of earthquakes. Because large geometric bends exist at some fault segment boundaries throughout the history of the boundary, we suggest that such bends can last throughout much of the life of the fault. We use fault segmentation concepts to define a segment boundary and parts of two fault segments along the active Hurricane normal fault, southwestern Utah, U.S.A. We use fault geometry, slip direction, shortening structures in the hanging wall and footwall, and fault scarp morphology to identify the fault segments and segment boundary. The fault strikes N13°W south of the boundary and N21°E north of the boundary. Stratigraphic separation of geochemically identical Quaternary basalt in the footwall and the hanging wall suggests that the slip vector of Quaternary displacement trends between N70°W and S18°W, and slickenlines, earthquake rakes, and dip analysis on syndeformational basalt further constrains the slip direction to N75-85°W. Relative motion on the northern segment, termed here the Ash Creek segment, is purely dip-slip, and on the southern (Anderson Junction) segment is dominantly dip-slip with a small dextral slip component. Fault scarps along the Ash Creek segment compared with offset gravels lacking scarps along the Anderson Junction segment suggest that the two segments have different Late Quaternary faulting histories, which is indicative of discrete fault segments. The Ash Creek segment is about 24 km long and the Anderson Junction segment is 19-45 km long, although the non-adjacent segment terminations remain poorly defined. The segment boundary zone consists of: (1) a small-scale hanging wall anticline that trends roughly normal to the fault; (2) a small-offset footwall thrust fault; and (3) space-filling imbricate normal faults in the hanging wall. Secondary structures suggest the segment boundary is a non-conservative boundary. Copyright © 1996 Elsevier Science Ltd.