For permission to copy, contact editing@geosociety.org © 2006 Geological Society of America 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 remi- niscent of the transform-fault effect of Lang- muir 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 repre- sent 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 con- tinued alkaline volcanism occurred on the southernmost Gorda Ridge, or on a series of short intratransform spreading-center seg- ments during plate reorganization. Thus, the Mendocino transform fault provides a record of ridge migration, abandonment, and resid- ual volcanism of the southern Gorda Ridge spreading system from 23 to 11 Ma. Keywords: transform faults, geochemistry, mid-ocean ridges, NE Pacific, 40 Ar- 39 Ar dating. INTRODUCTION Transform faults have an important role in the dynamics of the global mid-ocean-ridge system. Transform faults are strike-slip faults that offset active spreading centers, thus creating a change in the melting regime along the mid-ocean ridge. In the vicinity of the fault, cold litho- sphere is introduced adjacent to the hot, upwell- ing mantle of the mid-ocean-ridge axial region. Extensive cooling results in an increased degree of fractional crystallization and increases the depth of melting (Langmuir and Bender, 1984). Transform faults also provide tectonic windows into crustal processes. Fault zones may expose crustal sections of variable ages in uplifted trans- verse ridges where younger, more buoyant crust is adjacent to older, denser crust. It is within these transform zones that relicts of the com- plexities of plate tectonics may be preserved and sampled. In the northeast Pacific Ocean (Fig. 1), the Mendocino transform fault provides such a record into the tectonic and magmatic history of the area with the preservation of rocks and structural features associated with changing spreading-center regimes. This major structure consists of two transverse ridges (Gorda Escarp- ment to the east, and Mendocino Ridge to the west; Fig. 1), and has existed during the entire period of the breakup of the Juan de Fuca plate and development of the San Andreas fault zone, forming the boundary of the transform regime to the south and subduction regime to the north along the North American plate margin. Prior to this study, the Gorda Escarpment section of the Mendocino transform fault has not been systematically sampled beyond dredge samples collected in 1964 by Krause et al. (1964). For this study, basement exposures along the entire Gorda Escarpment and eastern part of Mendocino Ridge were examined and sampled during a series of remotely operated vehicle (ROV) dives to determine the lithol- ogy, age, and origin of these transverse ridges. Using field observations, geochemistry, and age data, we ascertained whether these rocks are the result of (1) mid-ocean-ridge processes, near the transform zone; (2) tectonic slivering of the Pacific plate; (3) relicts of the rift propa- gation that created the Juan de Fuca–Gorda plate; or (4) rocks derived from ephemeral intratransform spreading axes. We conclude that crustal formation at the southern end of the Gorda Ridge was complicated by waning magmatic activity associated with the chang- ing tectonics of the adjacent Mendocino trans- form-fault boundary. GEOLOGICAL SETTING The Mendocino transform fault is the major plate boundary between the Gorda plate (south- ern Juan de Fuca plate; e.g., Stoddard, 1987; Denlinger, 1992) and the Pacific plate (Fig. 1). It is an active zone of dextral strike-slip motion separating the 6–8 Ma crust of the Gorda plate from the 28–30 Ma crust of the Pacific plate (Atwater, 1970, 1989). The eastern part of the Mendocino transform fault consists of Geochemical and age constraints on the formation of the Gorda Escarpment and Mendocino Ridge of the Mendocino transform fault in the NE Pacific J.M. Kela † D.S. Stakes ‡ Monterey Bay Aquarium Research Institute, 7700 Sandholdt Road, Moss Landing, California 95039, USA R.A. Duncan College of Oceanic and Atmospheric Science, Oregon State University, Corvallis, Oregon 97331, USA † Present address: Department of Earth Sciences, University of California, Santa Cruz, Santa Cruz, California 95064, USA. ‡ Corresponding author present address: Division of Science and Environmental Policy, California State University, Monterey Bay, Monterey, Califor- nia 93955, USA; e-mail: debra_stakes@csumb.edu. GSA Bulletin; January/February 2007; v. 119; no. 1/2; p. 88–100; doi: 10.1130/B25650.1; 7 figures; 3 tables. 88