The Tuzo Wilson Volcanic Field, NE Pacific: Alkaline volcanism at a complex, diffuse, transform‐trench‐ridge triple junction

Abstract

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 km 3 . 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 A1 depletion trends and by having low relative Fe, Sc, and Cr and high relative A1 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 (-<3%) of an amphibole-bearing, LREE-and LILE-enriched beterogenous 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.Paper number 93JB01818. 0148-0227/93/93JB-01818505.00 seamounts and the TWVF lie on a small circle about the Pacific hotspot Euler pole. Current models, based on geophysical and morphological data, apply rigid plate tectonics and propose that seafloor spreading has recently been initiated at TWVF and has resulted in the creation of as much as 100 km of new crust [Carbotte et al., 1989; Keen and Hyndman, 1979; Riddihough et al., 1980; Davis and Riddihough, 1982]. This paper presents new geological, petrological, and geochemical data concerning the Tuzo Wilson Volcanic Field that contradict these previous models and suggest instead that volcanism within this zone of transition essentially reflects small amounts of partial melting associated with the development of pull-apart basins in a region of distributed strain. DESCRIPTION OF THE TRIPLE JUNCTION The region of transition between strike-slip faulting separati...

Geochemistry and 40Ar/39Ar Geochronology of Basalts from ODP Leg 145 (North Pacific Transect)

“…Samples dredged from the Tuzo Wilson Seamounts ( Fig. 1; Allan et al, 1993) have much higher trace element concentrations at a given MgO content, and are compositionally distinct from the Site 887 basalts (Fig. 7).…”

Evidence for trench-parallel mantle flow in the northern Cascade Arc from basalt geochemistry

“…Data for Tuzo Wilson seamounts, located at the northern edge of the Explorer plate (Fig. 1a), are from Allan et al (1993). Data for Bowie seamount, located northwest of the Explorer ridge on the NE Pacific plate, are from Cousens (1988).…”

Volatiles in basaltic glasses from a subglacial volcano in northern British Columbia (Canada): implications for ice sheet thickness and mantle volatiles

“…Endeavour and Explorer Ridge basalts are derived from a moderately enriched MORB source (Michael et al 1989;Karsten et al 1990). Tuzo Wilson Seamount Chain basalts are derived from a strongly enriched source (Allan et al 1993). Subglacial lavas are more differentiated than basalts from the NE Pacific ridges.…”