Research Technical Report

Origin of Volcanic Seamounts at the Continental Margin of California Related to Changes in Plate Margins

Davis, A.S., D.A. Clague, J.B. Paduan, B.L. Cousens, and J. Huard (2010)

Geochem. Geophys. Geosyst., 11, Q05006, doi:10.1029/2010GC003064.

ABSTRACT

Volcanic samples collected with the Monterey Bay Aquarium Research Institute's ROV Tiburon from eight seamounts at the continental margin offshore central to southern California comprise a diverse suite of mainly alkalic basalt to trachyte but also include rare tholeiitic basalt and basanite. All samples experienced complex crystal fractionation probably near the crust/mantle boundary, based on the presence in some of mantle xenoliths. Incompatible trace elements, poorly correlated with isotopic compositions, suggest variable degrees of partial melting of compositionally heterogeneous mantle sources, ranging from MORB-like to relatively enriched OIB. High-precision 40Ar/39Ar ages indicate episodes of volcanic activity mainly from 16 to 7 Ma but document one eruption as recent as 2.8 Ma at San Juan Seamount. Synchronous episodes of volcanism occurred at geographically widely separated locations offshore and within the continental borderland. Collectively, the samples from these seamounts have age ranges and chemical compositions similar to those from Davidson Seamount, identified as being located atop an abandoned spreading center. These seamounts appear to have a common origin ultimately related to abandonment and partial subduction of spreading center segments when the plate boundary changed from subduction-dominated to a transform margin. They differ in composition, age, and origin from other more widespread near-ridge seamounts, which commonly have circular plans with nested calderas, and from age progressive volcanoes in linear arrays, such as the Fieberling-Guadalupe chain, that occur in the same region. Each volcanic episode represents decompression melting of discrete enriched material in the suboceanic mantle with melts rising along zones of weakness in the oceanic crust fabric. The process may be aided by transtensional tectonics related to continued faulting along the continental margin.

Citation:

Davis, A. S., D. A. Clague, J. B. Paduan, B. L. Cousens, and J. Huard. 2010. Origin of volcanic seamounts at the continental margin of California related to changes in plate margins, Geochem. Geophys. Geosyst., 11, Q05006, doi:10.1029/2010GC003064.