CONTRASTING TECTONIC REGIMES RECORDED BY JURASSIC AND CRETACEOUS ARC-RELATED SEDIMENTARY BASINS, SONORA, MEXICO

Jurassic-Cretaceous strata in northern Sonora record an important tectonic transition along the southwestern continental margin of North America, from development of a southwest-migrating extensional arc (ca. 200-158 Ma), through rift-basin formation in the context of regional bimodal volcanism (ca. 158-145 Ma), to subsequent rise of an east-migrating continental-margin arc (ca. 128-~55 Ma). Jurassic sedimentary basins evolved through three phases. Lower Jurassic (>192-180 Ma) forearc-basin strata are shallow- to deep-marine deposits of volcanic-lithic composition. They contain basement clasts and locally overlie middle Paleozoic strata, indicating significant forearc uplift. Middle Jurassic (~171-165 Ma) volcanic-lithic epiclastic rocks, eolian sandstone, andesite flows and ash-flow tuffs were deposited in intra-arc basins. Upper Jurassic (~156-145 Ma) shale and neovolcanic turbidites were deposited in a rift basin in which much detritus was supplied by contemporary silicic volcanism. Lower Cretaceous (~134-100 Ma) continental and shallow marine strata unconformably overlie Jurassic rocks and record post-rift thermal subsidence. Detrital zircons in these strata signal onset of major magmatism at ~128 Ma after an apparent magmatic lull of ~5-10 m.y. Within ~30 m.y., this younger arc developed a contractional backarc, indicated by upper Albian conglomerate derived from uplifts that accompanied initial shortening. Upper Cretaceous (>93-~70 Ma) sedimentary-clast conglomerate, sandstone and andesite breccia were deposited in basins bounded by basement-cored uplifts formed during inversion of the older extensional basins. A 40Ar/39Ar hornblende age (~93 Ma) on andesite breccia and feldspar cooling ages (~70 Ma) on basement rocks indicate that significant uplift occurred early in Late Cretaceous time and continued through the end of the Cretaceous. Jurassic basins formed in an extensional arc system with high relief developed on the pre-arc basement, analogous to arcs built above retreating slabs of the Mediterranean region, whereas Albian and Late Cretaceous basins formed in a Cordilleran-style (“Laramide”) contractional backarc setting.