Geochemical signatures of Pacific-North American plate boundary transition, southern California
Mentor:Margaret Rusmore, Professor of Geology, Occidental College
Analysis of geochemical data from a suite of Miocene to Recent volcanic systems provides insight on the transition from a subduction zone to a transform plate boundary in southern California. Samples were collected and prepared by Dr. A.P. Barth at IUPUI, and 87Sr/86Sr ratios were processed by Dr. D.S. Coleman at UNC. Geochemical traits define three geographic provinces: a northern, a central, and a southern. The northern province is dominated by silica-undersaturated alkali basalts, with 87Sr/86Sr ratios that display a longitudinal dichotomy with low values to the west and high values to the east, supporting a model of contamination by old lithospheric mantle during generation of these basalts. The central province consists of Late Oligocene-Early Miocene silica-oversaturated olivine tholeiites. The southern, and youngest, province is comprised of silica-undersaturated olivine tholeiites. Geochemistry of the northern province signals the involvement of the North American craton along the continental boundary to the east. The age and general chemical composition of the central province magmatism is compatible with formation during Basin and Range extension. Preliminary analysis suggests magmatism in the southern province is related to mid-ocean ridge volcanism and the formation of oceanic lithosphere. Coupled with crustal and lithospheric thinning documented by recent geophysical studies (Lekic et. al, 2011, Zhu & Kanamori, 2000), the results suggest the basalts represent magmatism at the northern extent of spreading in the Gulf of California.