Relations between hinterland and foreland shortening: Sevier orogeny, central North American Cordillera

The tectonic relations between foreland and hinterland deformation in nonco llisional orogens are critical to understanding the overall development of orogens. The classic central Cordilleran foreland fold-and-thrust belt in t he United States (Late Jurassic to early Tertiary Sevier belt) and the more internal zones to the west (central Nevada thrust belt) provide data criti cal to understanding the development of internal and external parts of orog ens. The Garden Valley thrust system, part of the central Nevada thrust bel t, crops out in south-central Nevada within a region generally considered t o be the hinterland of the Jurassic to Eocene Sevier thrust belt. The thrus t system consists of at least four principal thrust plates composed of stra ta as young as Pennsylvanian in age that are unconformably overlain by rock s as old as Oligocene, suggesting that contraction occurred between those t imes. New U/Pb,dates on intrusions that postdate contraction, combined with new paleomagnetic data showing significant tilting of one area prior to in trusion, suggest that regionally these thrusts were active before similar t o 85-100 Ma. The thrust faults are characterized by long, relatively steepl y dipping ramps and associated folds that are broad and open to close, upri ght and overturned. Although now fragmented by Cenozoic crustal extension, individual thrusts can be correlated from range to range for tens to hundre ds of kilometers along strike. We correlate the structurally lowest thrust of the Garden Valley thrust system, the Golden Gate-Mount Irish thrust, sou thward with the Gass Peak thrust of southern Nevada. This correlation carri es the following regional implications. At least some of the slip across Ju rassic to mid-Cretaceous foreland thrusts in southern Nevada continues nort hward along the central Nevada thrust belt rather than northeastward into U tah. This continuation is consistent with age relations, which indicate tha t thrusts in the type Sevier belt in central Utah are synchronous with or y ounger than the youngest thrusts in southern Nevada. This in turn implies t hat geometrically similar Sevier belt thrusts in Utah must die out southwar d before they reach Nevada, that slip along the southern Nevada thrusts is partitioned between central Nevada and Utah thrusts, or that the Utah thrus ts persist into southeastern Nevada but are located east of the longitude o f the central Nevada thrust belt. As a result of overall cratonward migrati on of thrusting, the central Nevada thrust belt probably formed the Cordill eran foreland fold-thrust belt early in the shortening event but later lay in the hinterland of the Sevier fold-thrust belt of Idaho-Wyoming-Utah.