iPlex Lunch spring-2015

The Eocene was an interesting and dynamic time in Sonora, Mexico. The effects of “flat-slab subduction” of the Farallon Plate beneath North America were especially pronounced. Uplift and exhumation accompanied by pronounced crustal contraction drove two significant tectonic imprints that are well-preserved in the geologic record: 1) a system of three rivers drained westward from Sonora/Chihuahua highlands to the Pacific Ocean, and 2) development of the Sonoran orogenic gold belt. Besides the Ballenas River and related Poway conglomerate originally described by Abbott and Smith (1978), two additional Eocene river drainages are identified at more northerly and southerly latitudes. Each of the three rivers tapped a different provenance area as recorded by distinctive conglomerate clast assemblages and detrital zircon populations. Because these rivers flowed orthogonally across the future trace of the San Andreas transform system, they provide important constraints on the total plate-boundary slip since Eocene time. Disposition of certain fault strands of the transform system are deduced from offset channels and sedimentary packages. Evolution of the river system coincided with a crustal shortening event manifested by a northwest-trending fold-thrust belt and widespread Eocene cooling ages. Orogenic gold is localized in sheared quartz veins along thrust faults that preserve Eocene 40Ar/39Ar ages on syntectonic white mica (Iriondo, 2008). The source of the gold-bearing fluids is still controversial but spatial association with Late Jurassic faults in Sonora appears to be a common link.