TECTONICS OF LATE JURASSIC-EARLY CRETACEOUS BASIN DEVELOPMENT, NORTH-CENTRAL SONORA, MEXICO

Ongoing stratigraphic and geochronologic analysis provides important new evidence for development of a Late Jurassic rift basin, termed the Altar-Cucurpe or Sonora basin, on the southwestern margin of North America. The basin lies directly northeast of the Caborca terrane, a block of Proterozoic North American basement. The basin stratigraphy unconformably overlies a substrate of interbedded silicic volcanic rocks and eolian quartzose sandstone with detrital zircons dated with LA-MC-ICPMS as young as 169 Ma (Bajocian). We provisionally divide the Jurassic stratigraphy into three units. (1) An unnamed lower interval, as much as 1000 m thick, consists of subaerial and subaqueous basalt, lacustrine carbonate, and volcanogenic conglomerate and indicates initial rift-basin development near the beginning of the Late Jurassic. (2) The Cucurpe Formation includes 800-1000 m of ammonite-bearing marine shale, turbidites derived from the subjacent Middle Jurassic volcanic arc terrane, tuffaceous sandstone rich in subaqueously erupted pumice, and abundant fall tuffs, one with a SHRIMP U-Pb zircon age of 151.4+/-1.5 Ma (Kimmeridgian). These rocks indicate fully marine conditions and coeval explosive volcanism, both subaerial and intrabasinal. (3) The La Colgada Formation, overlying and partly equivalent to the Cucurpe, includes 200 m of shallow-marine sandstone and silicic tuffs with SHRIMP U-Pb zircon ages of 152.1+/-1.7 and 150.6+/-1.2 Ma (late Kimmeridgian and early Tithonian, respectively) and represents late fill of the rift basin. An Early Cretaceous post-rift succession of red beds and carbonate strata unconformably overlie the Jurassic section and overlap the Caborca block. The Jurassic basin records extension of the SW continental margin as the locus of arc magmatism shifted westward to the present Peninsular Ranges in the Late Jurassic, probably due to rapid westward retreat of a fringing arc system and development of a back arc basin between the arc and continental margin.