AERIAL EXTENT OF THE COASTAL DRAINAGE SYSTEM IN THE WAKE OF THE LARAMIDE COLLAPSE OF THE CALIFORNIA ARC

Saleeby’s (2003, Tectonics) synthesis of the Laramide geology of coastal S California (including fragments of Salinia) suggested the tectonic demise of the forearc and western arc elements, accompanied by the foundering of the E continental arc into a continental borderland that ultimately accumulated 1000s of meters of Maastrichtian to Recent sediments. An important consequence of the formation of this borderland was the expansion of the coastal drainage system. Prior to this time, K foreland deposits in Utah were receiving abundant arc-derived zircon. After the collapse, coastal deposits (esp. middle Eocene to early Miocene Sespe Fm.) began to receive exotic detritus in their gravel fraction, in particular orthoquartzites from at least as far inland as the Mojave and central Arizona regions (Howard, 2000, GSAB). Wernicke et al. (2010, 2012, GSA Abstr.) showed that certain orthoquartzite clasts in the upper part of the Sespe Fm. have a combination of petrographic, geochemical, paleomagnetic and detrital zircon signatures that are unique to the Shinumo Quartzite, a ca. 1.1 Ga orthoquartzite known only from the Upper Granite Gorge region of E Grand Canyon. This discovery corroborates earlier thermochronometric analyses of the Grand Canyon (Flowers et al., 2008, GSAB), and also suggests that (1) E Grand Canyon had already been cut by 20 Ma; (2) a precursor “Arizona River” drainage system was supplying detritus to the coastal borderlands; and (3) the post-Laramide drainage divide had locally migrated as far east as the Cordilleran foreland (Wernicke, 2011, GSAB). A commonly cited criticism for this “Arizona River hypothesis” is that the detrital zircon signature in the Sespe Fm. and other coastal deposits contain minimal amounts of zircon of Neoprot-Plz age, which is abundant in Upper Plz and Mz sandstones on the Colorado Plateau (e.g. Spafford et al., 2009; UCLA thesis). However, quantitative analyses of the system, considered from the point of view of either the source region or the Sespe Formation itself, predict that the fraction of Neoprot-Plz zircons in the Sespe derived from Grand Canyon to be ca. 0.2 to 0.3%, many times lower than their observed frequency (n = 5/446). This result underscores the importance of per mil-level detrital zircon fractions, which are often dismissed as “negligible” or “virtually zero.”