Paper No. 66-1
Presentation Time: 8:05 AM
TECTONIC TEMPOS OF THE NORTH AMERICA CORDILLERA RETROARC DURING THE MID-CRETACEOUS: EXPLORING INTERCONNECTIONS BETWEEN PLATE DYNAMICS, THRUST SLIP HISTORIES, FORELAND BASIN SEDIMENTATION, AND THE ARCHIVE OF ARC MAGMATISM FROM NEVADA TO WYOMING (Invited Presentation)
The North American Cordillera orogenic system evolved during protracted subduction and terrane accretion, including an episode of increased retroarc shortening in the Sevier fold-thrust belt and hinterland, foreland basin sedimentation, and magmatic arc activity from ca. 105–90 Ma. During this episode, the hinterland underwent early crustal thickening and the dominant Willard-Paris-Meade thrust sheet was transported >60 km eastward in the western Sevier belt, at the latitude of northern Nevada to Wyoming. Zircon (U-Th)/He thermochronologic data from the Willard sheet and thermal modeling indicate onset of major thrusting at ca. 125 Ma, with fault slip rates of ~1 km/myr from 125-105 Ma, increasing to ~3 km/myr from 105-92 Ma. Synorogenic strata deposited during emplacement of the Willard sheet totaled >3 km in thickness and included Aptian continental strata with widespread conglomeratic intervals, thick Albian-Cenomanian continental to marine strata recording increased flexural loading plus enhanced regional dynamic subsidence, and Turonian strata recording complex patterns as the thrust front propagated eastward. Detrital zircon provenance analysis shows a general unroofing sequence of the Willard sheet along with young grains derived from the magmatic arc. The magmatic arc experienced cycles of lulls to flare-ups and associated shifts from juvenile to more evolved isotopic signatures, as recorded by arc-derived materials deposited in retroarc strata as well as within arc rocks. U-Pb geochronologic and Hf isotopic analysis of arc-derived zircon grains within strata spanning Montana to Nevada record a 180–145 Ma Jurassic flare-up, an earlier Cretaceous lull, and a 110–85 Ma mid-Cretaceous flare-up with a pulldown to evolved εHf, consistent with pluton age and isotopic patterns in the magmatic arc that migrated inboard and interacted with thicker continental crust. The mid-Cretaceous episode of increased retroarc shortening, enhanced flexural and regional dynamic subsidence in the foreland, and magmatic flare-up occurred during an interval of increased convergence rates and reorganization of plate motion, highlighting interconnections among deformation, sedimentary, magmatic, and plate dynamic processes.