DETRITAL ZIRCON TRACE ELEMENT ANALYSIS OF THE UPPER CRETACEOUS-PALEOGENE FOREARC BASIN IN SOUTHERN CALIFORNIA: IMPLICATIONS FOR TECTONIC EVOLUTION

During latest Cretaceous-Paleogene time, coincident with flat subduction and the Laramide orogeny, slip on the Nacimiento fault of west-central California caused truncation of the magmatic arc and forearc basin and juxtaposition of the Nacimiento and Salinian-Transverse Ranges (STR) blocks. To help understand this event, we determined trace element (TE) concentrations of detrital zircon from the Nacimiento and STR blocks and compared these results to ones from the retroarc McCoy Mountains Formation (Barth et al., 2013). Nacimiento samples include three from the distal forearc basin and one from the Cambria slab trench-slope basin. They range from Cenomanian to Campanian and predate the Nacimiento fault. STR samples represent inboard parts of the forearc basin. Four from the NW Salinian block are Maastrichtian to Paleocene and postdate slip on the Nacimiento fault. A fifth STR sample from the Simi Hills is Campanian and predates the Nacimiento fault.

The Nacimiento, STR, and McCoy groups all include a range of arc zircon, although in differing proportions related to changes in the locus of magmatism with time. Irrespective of sample group, Early Cretaceous zircon has low U/Yb, Hf, U/Yb to Hf, and Ce/Gd compared to Late Cretaceous zircon, consistent with sources for the two age groups in the relatively primitive western versus more continental eastern batholith, respectively. Subtle differences in TE of Cretaceous zircon between the three sample groups could reflect insufficient sample size, but may also indicate different source areas along the length of the arc. In contrast, TE data for Jurassic zircon unequivocally indicate two disparate groups. Group 1 is found in forearc samples that predate the Nacimiento fault and has low U/Yb and Ce/Gd indicative of a relatively primitive arc source, probably in the western margin of the central to northern Sierra. Group 2 Jurassic zircon characterizes the McCoy Mts. Fm. and STR samples that postdate the Nacimiento fault. Group 2 TE values denote a continental arc source—exceptionally high Th/U (commonly > 0.8) points specifically to the Mojave. The combined TE data confirm previous inferences that the Nacimiento fault/Laramide orogeny had a profound impact on deposition within the southern California forearc, causing a shift from relatively outboard to inboard sources.