Subsurface fault geometries and crustal extension in the eastern Basin and Range Province, western U.S.

Abstract

We provide the first synthesis of seismic reflection data and active present-day crustal deformation for the greater Wasatch fault zone. We analyzed a number of previously unpublished seismic reflection lines, horizontal and vertical crustal velocities from continuous GPS, and surface geology to investigate the relationships between interseismic strain accumulation, subsurface fault geometry, and geologic slip rates on seismogenic faults across the eastern third of the northern Basin and Range Province. The seismic reflection data show recent activity along high-angle normal faults that become listric with depth and appear to sole into preexisting décollements, possibly reactivating them. We interpret these listric normal faults as reactivated Sevier-age structures that are connected at depth with a regionally extensive detachment horizon. These observations of subsurface structure are consistent with the mapped geology in areas that have experienced significant extension. We modeled the crustal deformation data using a buried dislocation source in a homogeneous elastic half space. The estimated model results include a low-angle dislocation (~ 8–20°) at a locking depth of ~ 7–10 km and slipping at 3.2 ± 0.2 mm/yr. Despite the model's relative simplicity, we find that the predicted location of the dislocation is consistent with the interpreted seismic reflection data, and suggests an active regionally extensive sub-horizontal surface in the eastern Basin and Range. This result may imply that this surface represents aseismic creep across a reactivated low-angle fault plane or the onset of ductile flow in the lower crust at or beneath the brittle–ductile transition zone under the present-day Basin and Range extensional regime. This result may also have implications for crustal rheology, and suggests that geodesy might, under some circumstances, serve as an appropriate tool for inferring deeper crustal structure.