THE 100 MA EVENT IN THE SOUTHERN SEVIER BELT OF NEVADA AND CALIFORNIA

Retroarc shortening is well recognized in many Andean-style orogenic belts, and yet its causes remain controversial. Proposed causes include increased interplate coupling due to increased plate convergence rates and/or slab flattening, changes in relative plate convergence directions, terrane collisions, or batholithic wedging/sinking and intracrustal flow. The increased fidelity of retroarc deformation histories through detailed geo- and thermo-chronological studies suggests that deformation is not quasi-continuous but rather accumulates through punctuated events, which allow better comparison to potential geodynamic causes. It is increasingly apparent that the mid-Cretaceous was a period of enhanced and punctuated retroarc shortening, foreland basin subsidence, and arc magmatic flux along the western Cordillera of the U.S. Here we summarize evidence from a ca.200 km swath of the southern Sevier fold-thrust belt – from the New York Mountains of California to the Muddy Mountains of Nevada – for activity of frontal thrusts, synorogenic sedimentation, and passive uplift and exhumation of hinterland thrusts from 103 to 96 Ma. Additionally, we present U-Pb and Hf isotopic results from arc-derived (probable volcanic) zircon that show an isotopic pull down from ε_Hf +10 to -10, from ca. 110 to 100 Ma, consistent with increased continental crustal contribution through time. Increased rates of plate convergence during a mid-Cretaceous global plate reorganization, and associated slab flattening and arc migration, may provide a viable explanation for these punctuated events.