THE TUZO-WILSON-VOLCANIC-FIELD, NE PACIFIC - ALKALINE VOLCANISM AT A COMPLEX, DIFFUSE, TRANSFORM-TRENCH-RIDGE TRIPLE JUNCTION

The Tuzo Wilson Volcanic Field (TWVF) consists of Quaternary alkaline volcanics erupted at the complex, diffuse triple junction between the Explorer, Pacific, and North American plates. It occurs in a region of distributed strain that lies between right-lateral, strike-slip faulting at the Queen Charlotte fault, seafloor spreading at the Explorer Ridge, and subduction at the Cascadia subduction zone. The TWVF is contained within a poorly defined graben structure and consists of two 500- to 700-m-high composite seamount volcanoes surrounded by numerous smaller vents, with a total edifice volume of about 12 km3. The erupted volcanics are large ion lithophile elements (LILE) and light rare earth element (LREE) enriched alkali basalts, hawaiites, mugearites, and benmoreiites. These volcanics also differ from mid-ocean ridge basalts by lacking Fe enrichment and Al depletion trends and by having low relative Fe, Sc, and Cr and high relative Al for a given MgO content, reflecting significant high-pressure pyroxene fractionation. They are petrologically similar to other alkaline volcanics found capping near-ridge seamounts in the east Pacific. Petrogenetic modeling combined with analysis of Sr, Nd, and Pb isotopic data shows that TWVF volcanism is ephemeral in nature and that the TWVF lavas were derived by small amounts of melting (less-than-or-equal-to 3%) of an amphibole-bearing, LREE- and LILE-enriched heterogenous mantle similar to that underlying the adjacent Explorer spreading ridge. The new volcanological, petrological, and geochemical data presented constrain both the petrogenetic origin of TWVF lavas and the tectonic processes occurring in the triple junction region. Current wisdom states that the TWVF represents either a site of seafloor spreading or a hotspot/mantle plume. Our new data are incompatible with these models and instead are consistent with other geophysical data in suggesting that the TWVF represents minor volcanism associated with pull-apart structures developing between parallel strike-slip faults in a region of distributed strain. In short, the TWVF represents ''leaky transform'' volcanism in an oceanic setting.