INFLUENCE OF NEOPROTEROZOIC TO PALEOZOIC STRATIGRAPHIC ARCHITECTURE ON TECTONIC EVOLUTION OF THE SEVIER AND LARAMIDE BELTS

The crustal architecture of Neoproterozoic to Paleozoic sedimentary cover rocks and underlying basement, the primary building materials for the Sevier and Laramide belts, display regional variations that partly controlled subsequent deformation. The sedimentary cover comprises thick passive margin strata deposited along the previously rifted western margin of Laurentia, and thinner strata deposited eastward on cratonic basement. The passive margin evolved during (i) an early phase of rifting and volcanism with deposition of diamictite-bearing strata, (ii) an early phase of broad subsidence with deposition of siliciclastic strata, (iii) final rifting, volcanism, and transition to drift with a change from mostly siliciclastic to carbonate-rich strata. The Wasatch hinge-zone marking the transition to passive margin strata trended overall N-S to NE-SW from northern Utah to southern Nevada, and had bends that influenced subsequent development of salients in the Sevier belt. Neoproterozoic to lower Cambrian strata thicken westward from <0.3 km east of the Wasatch hinge-zone, to ~3–4 km in western areas where micaceous strata formed the basal decollement for large-displacement, wide-spaced western thrusts of the Sevier belt. Mid-Cambrian to Devonian carbonate-rich strata thicken westward from <2 km east of the Wasatch hinge-zone, to ~4–6 km in western areas, and include Cambrian shale that formed the basal decollement for closer-spaced eastern thrusts of the Sevier belt. Mississippian to Permian strata display more complex variations, partly related to early terrane accretion along the former plate margin. Subsequently, passive margin strata were located closer to the Cordilleran plate margin in southern Nevada where western thrusts were active during the Late Jurassic, whereas western thrusts in northern Utah were not active until the Early Cretaceous as the orogenic front propagated eastward. Thinner Paleozoic strata in the foreland lacked a regional decollement and overlay cratonic basement with a variety of Precambrian fabrics that partly controlled locations of Laramide basement-cored arches. Local intracratonic rift basins that presaged regional rifting, including the Uinta trough in the foreland, contain thick Neoproterozoic strata and were locally inverted during Laramide shortening.