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Mid‐ocean ridge basalts (MORB) from the well‐defined Southern Explorer Ridge segment (SER) in the northeast Pacific Ocean are moderately to strongly enriched in incompatible elements. Enriched MORB were erupted at the highest and widest part of the segment (culmination) where the magma supply may be the greatest and also at the northern and southern ends of the SER. Variations in basalts' incompatible element enrichment occur over short distances and suggest that the underlying mantle is heterogeneous on a small scale. The variations also preclude the existence of a long‐lived, well‐mixed magma chamber beneath the robust culmination. Instead, magma chambers are probably temporally and spatially isolated, as evidenced by the presence of highly differentiated lavas. Less enriched (transitional) MORB were erupted along the central part of the SER, 11–27 km south of the culmination, and were supplied to the ridge separately from a less enriched pan of the mantle. A more continuous magma chamber cannot be ruled out for this section on geochemical grounds. All of the MORB from the SER have undergone significant amounts of fractional crystallization. Unlike certain segments of the East Pacific Rise, the most evolved MORB were erupted near the culmination of the ridge segment. Aphyric lavas have been recovered only from the vicinity of the culmination, while phyric lavas have erupted along the entire length of the SER. There is no correlation between composition and total crystal abundances. Despite the requirement of multiple parental magmas, most of the lavas from the culmination and the distal ends of the ridge fall on a common liquid line of descent, indicating that they formed at similar depths and extents of partial melting. The transitional MORB from the central part of the ridge have probably formed by lesser extents of partial melting and separated from their mantle source at greater depths based on abundances of Na and moderately incompatible elements such as Sm and Zr. Enriched, high‐melt fraction lavas at the magma‐starved southern end of the SER closely resemble lavas from the culmination. Either the southernmost lavas have flowed laterally within the crust from near the culmination, or there is another melting anomaly that supplies the southern end of the SER. If the latter case is true, correspondence between ridge morphology and mantle thermal state is poor.

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...

The northern end of Juan de Fuca Ridge consists of a series of basement ridges and valleys, inundated with sediment excepl for the axis of moqt recent sea-floor spreading. This axis is associated with the western of two branches of the Brunhes magnetic anomaly. The eastern branch of the magnetic anomaly is associated with a largely sediment-covered ridge, apparently produced by spreading early in the Brunhes Epoch.The intervening negative anomaly is probably caused by reversely magnetized rocks older than 0.7 m.y. Basalts dredged from the region of the northern end of Juan de

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