SAM BOWRING’S ROLE IN REDEFINING U-PB GEOCHRONOLOGY IN THE SOUTHWESTERN USA (Invited Presentation)

Sam started to sample New Mexico basement at NMTech with Kent Condie (MS 1978-1980) and dated them at Kansas with Randy Van Schmus (PhD 1979-1984). Bowring et al. (1982) remains the best reference for some NM rocks and mountain ranges. Early papers (Karlstrom, Bowring, Conway, 1987; Karlstrom and Bowring, 1988; 1991, 1993, and Bowring and Karlstrom, 1990) were included in, and influenced by, Hoffman’s (1988) United Plates of America synthesis. These papers developed the “accretionary orogen” model by proposing that a 1000-km-wide zone of new lithosphere was added progressively to North America from 1.8-1.0 Ga. Cordilleran suspect terrane concepts were applied to define “blocks” with different tectonic histories. Lithospheric-scale thinking included distinctions among processes such as: differentiation of continental (felsic) compositional provinces from the mantle (following Bennett and DePaolo, 1987), assembly of blocks via plate tectonic processes, and stabilization of thin oceanic lithospheric tectonic elements into continental lithosphere. The new precise U-Pb geochronology and thermochronology were the key to unraveling each of these processes but so was Sam’s emphasis that tectonic context and geologic mapping of dated rocks was paramount. Sam and his students redefined the Paleoproterozoic geochronology of the Arizona Transition Zone (Chamberlain and Bowring, 1990), the Paleoproterozoic geochronology of eastern Grand Canyon (Hawkins et al., 1996), the recognition and tectonic significance of the Payson ophiolite (Dann, 1991;1997), and the age of lower crustal xenoliths in the Four Corners region (Crowley et al., 2006) and along the Cheyenne belt (Farmer et al., 1995). The effort to combine precise dating to understand interactions of deformation, metamorphism and plutonism in the deep crust led to a decades-long 3-way collaboration (e.g. Williams and Karlstrom, 1997; Karlstrom et al., 2007). Sam’s precise dates on the Unkar Group and Chuar Group ashes in Grand Canyon remain the best constraints on the age of these successions that provide a record of the assembly and breakup of Rodinia (Karlstrom et al., 2003; Timmons et al., 2005). The accretionary orogen model persists today, continually discussed and refined, but thoroughly influenced by Sam’s dating and his philosophy of geochronology.