Abstract

The Mendocino transform fault is an active, dextral strike-slip zone that separates the Gorda plate from the Pacific plate in the NE Pacific Ocean. The compositions of the igneous rocks exposed along the southern margin of the Mendocino transform fault include tholeiitic and alkaline basalts. Major-element, trace-element, and radiometric data suggest that the rocks were generated through fractionation of different parental melts, derived by varying degrees of partial melting from different depths, at or near the intersection of the Mendocino transform fault with the Gorda Ridge. There is evidence for extensive cooling and fractionation reminiscent of the transform-fault effect of Langmuir and Bender (1984). Alkaline and high-Al compositions also argue for melts from a deeper source than a normal mid-ocean-ridge environment. The preferred geochemical analogue for the Mendocino transform fault is a failed rift system where mid-ocean-ridge basalt (MORB) compositions likely represent basalts created at a waning spreading center before its abandonment. The MORB compositions were subsequently buried by younger enriched (E-MORB) and alkaline basalts derived from deeper melting and/or a more enriched source. We suggest that a period of rift failure, abandonment, and continued alkaline volcanism occurred on the southernmost Gorda Ridge, or on a series of short intratransform spreading-center segments during plate reorganization. Thus, the Mendocino transform fault provides a record of ridge migration, abandonment, and residual volcanism of the southern Gorda Ridge spreading system from 23 to 11 Ma.

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