Geochemical and age constraints on the formation of the Gorda Escarpment and Mendocino Ridge of the Mendocino transform fault in the NE Pacifi c 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 ABSTRACT southernmost Gorda Ridge, or on a series of period of the breakup of the Juan de Fuca plate short intratransform spreading-center seg- and development of the San Andreas fault zone, The Mendocino transform fault is an ments during plate reorganization. Thus, the forming the boundary of the transform regime active, dextral strike-slip zone that separates Mendocino transform fault provides a record to the south and subduction regime to the north the Gorda plate from the Pacifi c plate in the of ridge migration, abandonment, and resid- along the North American plate margin. NE Pacifi c Ocean. The compositions of the ual volcanism of the southern Gorda Ridge Prior to this study, the Gorda Escarpment igneous rocks exposed along the southern spreading system from 23 to 11 Ma. section of the Mendocino transform fault margin of the Mendocino transform fault has not been systematically sampled beyond include tholeiitic and alkaline basalts. Major- Keywords: transform faults, geochemistry, dredge samples collected in 1964 by Krause et element, trace-element, and radiometric mid-ocean ridges, NE Pacifi c, 40Ar-39Ar dating. al. (1964). For this study, basement exposures data suggest that the rocks were generated along the entire Gorda Escarpment and eastern through fractionation of different parental INTRODUCTION part of Mendocino Ridge were examined and melts, derived by varying degrees of partial sampled during a series of remotely operated melting from different depths, at or near the Transform faults have an important role in the vehicle (ROV) dives to determine the lithol- intersection of the Mendocino transform dynamics of the global mid-ocean-ridge system. ogy, age, and origin of these transverse ridges. fault with the Gorda Ridge. There is evidence Transform faults are strike-slip faults that offset Using fi eld observations, geochemistry, and for extensive cooling and fractionation remi- active spreading centers, thus creating a change age data, we ascertained whether these rocks niscent of the transform-fault effect of Lang- in the melting regime along the mid-ocean are the result of (1) mid-ocean-ridge processes, muir and Bender (1984). Alkaline and high- ridge. In the vicinity of the fault, cold litho- near the transform zone; (2) tectonic slivering Al compositions also argue for melts from a sphere is introduced adjacent to the hot, upwell- of the Pacifi c plate; (3) relicts of the rift propa- deeper source than a normal mid-ocean-ridge ing mantle of the mid-ocean-ridge axial region. gation that created the Juan de Fuca–Gorda environment. The preferred geochemical Extensive cooling results in an increased degree plate; or (4) rocks derived from ephemeral analogue for the Mendocino transform fault of fractional crystallization and increases the intratransform spreading axes. We conclude is a failed rift system where mid-ocean-ridge depth of melting (Langmuir and Bender, 1984). that crustal formation at the southern end of basalt (MORB) compositions likely repre- Transform faults also provide tectonic windows the Gorda Ridge was complicated by waning sent basalts created at a waning spreading into crustal processes. Fault zones may expose magmatic activity associated with the chang- center before its abandonment. The MORB crustal sections of variable ages in uplifted trans- ing tectonics of the adjacent Mendocino trans- compositions were subsequently buried by verse ridges where younger, more buoyant crust form-fault boundary. younger enriched (E-MORB) and alkaline is adjacent to older, denser crust. It is within basalts derived from deeper melting and/or these transform zones that relicts of the com- GEOLOGICAL SETTING a more enriched source. We suggest that a plexities of plate tectonics may be preserved and period of rift failure, abandonment, and con- sampled. In the northeast Pacifi c Ocean (Fig. 1), The Mendocino transform fault is the major tinued alkaline volcanism occurred on the the Mendocino transform fault provides such a plate boundary between the Gorda plate (south- record into the tectonic and magmatic history ern Juan de Fuca plate; e.g., Stoddard, 1987; †Present address: Department of Earth Sciences, of the area with the preservation of rocks and Denlinger, 1992) and the Pacifi c plate (Fig. 1). University of California, Santa Cruz, Santa Cruz, structural features associated with changing It is an active zone of dextral strike-slip motion California 95064, USA. spreading-center regimes. This major structure separating the 6–8 Ma crust of the Gorda ‡Corresponding author present address: Division of Science and Environmental Policy, California consists of two transverse ridges (Gorda Escarp- plate from the 28–30 Ma crust of the Pacifi c State University, Monterey Bay, Monterey, Califor- ment to the east, and Mendocino Ridge to the plate (Atwater, 1970, 1989). The eastern part nia 93955, USA; e-mail: [email protected]. west; Fig. 1), and has existed during the entire of the Mendocino transform fault consists of GSA Bulletin; January/February 2007; v. 119; no. 1/2; p. 88–100; doi: 10.1130/B25650.1; 7 fi gures; 3 tables. 88 For permission to copy, contact [email protected] © 2006 Geological Society of America The geochemistry of the Mendocino transform fault A major plate reorganization of the Pacifi c- Farallon spreading system occurred at ca. 30 Ma (Atwater, 1989). On the basis of seafl oor mag- netic anomaly data, Wilson (1988) concluded that north of the Mendocino transform fault there was a transition from stable ridge-trans- form fault confi gurations to periods of rift propagation (Hey and Wilson, 1982), which led to abandonment of parts of the spreading ridge at 30 Ma and at 19 Ma. The Mendocino transform fault experienced a period of transten- sion between 24 and 19 Ma, slowly changing to transpression after 19 Ma. The Mendocino transform fault has likely experienced intervals with intratransform spreading centers, in the same style as the Blanco (Embley and Wilson, 1992), Siqueiros (Perfi t et al., 1996), and Gar- rett transform faults (Wendt et al., 1999). The Gorda Ridge has a complex history due to changes in spreading rate, intraplate deforma- tion of the Gorda plate, and reorientation of the ridge axis (Wilson, 1986, 1989). Given the com- plex tectonic history, either normal spreading or abandoned ridge segments within the transform system are plausible possibilities for the interval of 30–10 Ma. Multichannel seismic data (Trehu et al., 1995, 2003) and bathymetric data characterize the Mendocino transform fault as a series of east-west crustal slivers with signifi cant vertical relief between the Gorda and Pacifi c plates. The Figure 1. Regional map of the NE Pacifi c with major tectonic boundaries. The Mendocino Mendocino Ridge is thought to be an uplifted transform zone between the Escanaba Trough and the Mendocino triple junction is com- transverse ridge of oceanic basalt that formed prised of the Mendocino Ridge (just south of the Escanaba) and the Gorda Escarpment at at the Gorda Ridge (Fisk et al., 1993), with the eastern end of the transform. The partially subducted Monterey plate is preserved off- lithology and age relationships characterized shore central California with the failed spreading center capped by the Davidson Seamount by previous ROV and submersible investigation adjacent to the Morro Ridge on the fossil Morro fracture zone. results (Fisk et al., 1996; Duncan et al., 1994). Previous bathymetric and petrologic studies have shown that the Mendocino Ridge is com- two shallow transverse ridges that parallel the system (Atwater, 1989). The eastern end of posed of deformed basalts and crystalline rocks transform. A change in morphology occurs at the present-day transform fault meets the San with a fl attened summit created by wave erosion ~126°W; for ~150 km to the west, the south- Andreas fault at the Mendocino triple junction during a time when the ocean crust was uplifted facing Mendocino Ridge rises to 1 km above (MTJ; Fig. 1). The triple junction is thought to to sea level or above (Krause et al., 1964; Fisk et the Pacifi c plate. This vertical offset is consis- have formed at ca. 27 Ma when the Farallon al., 1993). The Mendocino Ridge is presumably tent with the more than 20 m.y. age difference spreading ridge was subducted underneath the derived from the Gorda plate (Fisk et al., 1993; between the Gorda plate and the Pacifi c plate. North American continental margin. The anom- Duncan et al., 1994; Krause et al., 1998) and To the east of 126°W, the north-facing Gorda alously shallow NE corner of the Pacifi c plate was transferred to the Pacifi c plate by northward Escarpment is ~80 km long and consists of sedi- at the Mendocino triple junction is the faulted migration of the Mendocino transform fault. mented, faulted basement blocks. At the Gorda “Vizcaino block,” thought to represent an accre- The eastern end of the Mendocino transform Escarpment, the older Pacifi c plate is elevated tionary wedge that was formed during Farallon– fault, the Gorda Escarpment, has been seismi- up to 1.5 km above the younger Gorda plate, a North American plate collision prior to 27 Ma cally imaged as a series of sedimented crustal depth relationship opposite to what would be and subsequently transferred to the Pacifi c plate slivers. The Gorda Escarpment has been inter- expected of normal conductive cooling with after the initiation of the San Andreas transform preted to be a relatively young (younger than age.