THE ROLE OF TECTONIC INHERITANCE, PLATE–REORGANIZATION, AND MAGMA FLARE-UPS IN THE EVOLUTION OF SEVIER OROGENY

The temporal and spatial distribution of strain associated with the Sevier Orogeny in western North America is significantly different in the southern end of the belt, at the latitude of Las Vegas, than that further to the north at the latitude of Salt Lake City. Until this study, reasons for these differences were speculative as a lack of constraints on the timing of movement on thrusts in the intervening region hindered correlation between the southern and northern deformation belts. We determined a crystallization age of 100.18 ± 0.04 Ma for zircons extracted from a recently recognized ash fall tuff near the base of synorogenic Iron Springs Formation at Three Peaks, Iron County, Utah. Field relationships and this age constrain movement on the Iron Springs thrust and the end of the hiatus represented by the sub-Cretaceous unconformity in the critical intervening area to latest Albian/earliest Cenomanian. Movement on the Iron Springs thrust and other Sevier thrusts at ~100 Ma is synchronous with a period of global plate reorganization and potentially increased plate convergence rates on the western margin of North America leading to a Cordilleran arc flare-up event that triggered widespread thrusting across the retroarc Sevier deformation belts. Based on these results, we suggest temporal and spatial variation along the strike of the orogenic belt are inherited from variation in the thickness of the rifted Precambrian craton. The Las Vegas recess of the Sevier fold and thrust belt mirrors the promontory of thicker Precambrian craton which impeded eastward thrust propagation sooner (e.g., ~99 Ma Keystone thrust) than further to the north. In contrast, the Salt Lake salient mirrors the presence of an embayment in the thicker Precambrian craton where thrusts remained active longer (ca. 100 My), over a wider deformation belt (~500 km), and propagated further inland (e.g., 56 Ma Hogsback thrust). The Paragonah Lineament, interpreted as an inherited transform fault that forms the boundary between the promotory and embayment in the Precambrian craton coincides with the location of the Iron Springs Thrust. Along strike extension of thrusts appear to terminate along this lineament indicating it may represent a potential internal boundary between the northern and southern Sevier Orogeny deformation belts as well.