Geochemistry of Holocene volcanic rocks associated with ridge subduction in Baja California, Mexico

Abstract

Although the geochemistry of magmatism associated with the subduction of oceanic lithosphere is well understood^1–6, the geochemical signature of lavas produced shortly after subduction of an oceanic spreading centre has not been characterized. The Baja California peninsula in Mexico provides an ideal scenario to study the latter process. Geophysical models indicate that, in response to oblique collision of the ancestral East Pacific Rise (EPR) with the western seaboard of North America at ∼29 Myr, subduction of the Farallon plate was succeeded by the progressive southwards development of a transform fault system^7–9. At 12.5 Myr a considerable length of the EPR was simultaneously subducted beneath Baja, thus terminating subduction processes along this segment¹⁰. By 3.5 Myr the locus of transcurrent faulting had switched from the west of Baja to within the developing Gulf of California, coupling Baja to the Pacific plate^11–13. We present here data for Holocene volcanic rocks from Baja California Norte which show that post-ridge subduction volcanism has a distinctive geochemistry. These results have important consequences for the interpretation of ancient orogenic belts.