Age and style of deformation and stratal thinning at the transition, Wasatch Plateau to Great Basin, central Utah

Abstract

The Salina area of central Utah contains an unusually complete Tertiary stratigraphic record, affording unique opportunities to study details of the Tertiary geologic history. Stratigraphic and structural studies reveal Neogene ages for tectonically significant parts of all major structures such as the 80-km-long Sanpete-Sevier Valley anticline (SSVA) and its flanking structures, the Wasatch monocline and Sevier Valley syncline. Newly identified critical components of the Neogene history in the SSVA include: (1) a protracted Eocene through early Miocene period of quasi-continuous sedimentation and relative structural quiescence that buried an early phase of the anticline; (2) east-trending folds in rocks of the anticlinal core; (3) widespread plutons in the southern part of the core; (4) strike-slip faults; (5) major thinning of strata (vertical structural convergence) between uplifted Jurassic rocks of the anticlinal core and downdropped superjacent Tertiary rocks; and (6) a widespread zone of altered rock at the contact between core rocks and overlying Tertiary rocks. We interpret the thinning and widespread alteration (5 and 6) to be linked through protracted processes of extensional faulting (possibly including detachment faulting), fluid flow, massive dissolution, and dissolution collapse at the contact between the Jurassic core rocks and overlying Tertiary rocks. We suggest that the SSVA, one of several diapir-like elongate uplifts in central Utah, is somewhat analogous to flank ridges that develop at landslide margins by outward and upward flow of mechanically weak clay-rich rock or gouge. By analogy, the SSVA results from outward and upward movement of the relatively incompetent Arapien Shale from beneath the adjacent Sevier Valley. If the structures formed during these newly recognized components of Neogene history are palinspastically restored, much of the evidence for contractional deformation at shallow structural levels is eliminated; this affects resource assessments based on models of that deformation. Also, the association of extensional faulting with non-tectonic dissolution collapse and with lateral motion of mechanically weak rocks implies shallow penetration and non-seismogenic to weak seismogenic behavior of some normal fault systems in central Utah.