geochemical and age constraints on the formation of the gorda escarpment and mendocino ridge of the...

For permission to copy, contact [email protected]© 2006 Geological Society of America

ABSTRACT

The Mendocino transform fault is an active, dextral strike-slip zone that separates the Gorda plate from the Pacifi c plate in the NE Pacifi c 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 Pacifi c, 40Ar-39Ar 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 Pacifi c 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 fi eld 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 Pacifi c 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 Pacifi c 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 Pacifi c 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 Pacifi c

J.M. Kela†

D.S. Stakes‡

Monterey Bay Aquarium Research Institute, 7700 Sandholdt Road, Moss Landing, California 95039, USA

R.A. DuncanCollege 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: [email protected].

GSA Bulletin; January/February 2007; v. 119; no. 1/2; p. 88–100; doi: 10.1130/B25650.1; 7 fi gures; 3 tables.

88

The geochemistry of the Mendocino transform fault

Geological Society of America Bulletin, January/February 2007 89

two shallow transverse ridges that parallel the transform. A change in morphology occurs at ~126°W; for ~150 km to the west, the south-facing Mendocino Ridge rises to 1 km above the Pacifi c plate. This vertical offset is consis-tent with the more than 20 m.y. age difference between the Gorda plate and the Pacifi c plate. To the east of 126°W, the north-facing Gorda Escarpment is ~80 km long and consists of sedi-mented, faulted basement blocks. At the Gorda Escarpment, the older Pacifi c plate is elevated up to 1.5 km above the younger Gorda plate, a depth relationship opposite to what would be expected of normal conductive cooling with age. The Mendocino transform fault continues beyond the southern terminus of the Gorda Ridge to form the Mendocino fracture zone, which extends ~3000 km into the Pacifi c Basin.

The evolution of the Mendocino transform fault can be traced back to ca. 85 Ma as a dis-continuity within the Farallon-Pacifi c spreading

system (Atwater, 1989). The eastern end of the present-day transform fault meets the San Andreas fault at the Mendocino triple junction (MTJ; Fig. 1). The triple junction is thought to have formed at ca. 27 Ma when the Farallon spreading ridge was subducted underneath the North American continental margin. The anom-alously shallow NE corner of the Pacifi c plate at the Mendocino triple junction is the faulted “Vizcaino block,” thought to represent an accre-tionary wedge that was formed during Farallon–North American plate collision prior to 27 Ma and subsequently transferred to the Pacifi c plate after the initiation of the San Andreas transform fault (e.g., Griscom and Jachens, 1989). To the west, at 127°30′W, the Mendocino transform fault meets the Escanaba Trough, the southern-most segment of the Gorda Ridge (Fig. 1). The Gorda Ridge is a 300-km-long active spreading center (Clague and Holmes, 1987), which cre-ated the crust of the Gorda plate.

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 Mendocino Ridge is thought to be an uplifted transverse ridge of oceanic basalt that formed at the Gorda Ridge (Fisk et al., 1993), with lithology and age relationships characterized by previous ROV and submersible investigation results (Fisk et al., 1996; Duncan et al., 1994). Previous bathymetric and petrologic studies have shown that the Mendocino Ridge is com-posed of deformed basalts and crystalline rocks with a fl attened summit created by wave erosion during a time when the ocean crust was uplifted to sea level or above (Krause et al., 1964; Fisk et al., 1993). The Mendocino Ridge is presumably derived from the Gorda plate (Fisk et al., 1993; Duncan et al., 1994; Krause et al., 1998) and was transferred to the Pacifi c plate by northward migration of the Mendocino transform fault.

The eastern end of the Mendocino transform fault, the Gorda Escarpment, has been seismi-cally imaged as a series of sedimented crustal slivers. The Gorda Escarpment has been inter-preted to be a relatively young (younger than 6 Ma) structural feature (Leitner et al., 1998) produced by compression across the Mendocino transform fault. The transpressional environment is the result of misalignment of the Mendocino transform fault compared to current regional plate motions that can be related to the nonpar-allel movement of the Blanco fracture zone in

Figure 1. Regional map of the NE Pacifi c with major tectonic boundaries. The Mendocino transform zone between the Escanaba Trough and the Mendocino triple junction is com-prised of the Mendocino Ridge (just south of the Escanaba) and the Gorda Escarpment at the eastern end of the transform. The partially subducted Monterey plate is preserved off-shore central California with the failed spreading center capped by the Davidson Seamount adjacent to the Morro Ridge on the fossil Morro fracture zone.

Kela et al.

90 Geological Society of America Bulletin, January/February 2007

the north (between the Juan de Fuca and north-ern Gorda Ridges) compared to the Mendocino transform fault in the south (at the southern end of the Gorda Ridge).

SAMPLING METHODS AND SAMPLING SITES

In 2000 and 2001, eleven dives with the Mon-terey Bay Aquarium Research Institute (MBARI) ROV Tiburon were carried out to study the geo-logical evolution of the Mendocino transform fault and to collect samples for petrologic, geo-chronologic, and geochemical analysis. All new samples were collected using the robotic manip-ulator arm of the ROV. The ROV dive tracks on the Mendocino transform fault were based on a Simrad EM300 30 kHz multibeam sonar mapping survey performed by MBARI in 1998 (Fig. 2). ROV tracks were targeted on erosional gullies that were expected to provide exposures of the underlying basement rocks. Bright refl ec-tors observed on the backscatter data (see inset on Fig. 2; Stakes et al., 2002) were interpreted to represent exposed bedrock and were targeted as promising dive sites. Two of the dives were located on the Mendocino Ridge: one near the intersection of the Mendocino Ridge and the Gorda Escarpment (Dive T203), and another near the Escanaba Trough (Dive T347). These two dives were placed on the western Men-docino Ridge near the 1994–1995 dive sites of the Navy ATV (Advanced Tethered Vehicle) where outcrops of gabbro and diabase were sampled using the ROV manipulator (ATV81; ATV154; Duncan et al., 1994; Fisk et al., 1996). Historical data from dredged samples include basalts from Fisk et al. (1993: CASC8, 9; FAN 25, 33, 36). Many of the historical samples do not have complete geochemistry published, but comparisons are made where possible.

The Gorda Escarpment was sampled during eight ROV Tiburon dives (T202, T204, T205, T207, T208, T348, T349, and T353). These dive tracks were spaced as evenly as possible along the escarpment, targeting unsedimented areas as inferred from high-refl ective backscat-ter based on the EM300 data. Sampling from bottom to top of the fault scarp revealed gab-bros, dolerites, basalts, and sediments, which together comprise a complete oceanic crustal sequence in three out of the eight dives. Most of the sampling locations yielded only normal tholeiite samples, while T208 and T353 yielded both tholeiites and alkali basalts (Fig. 2). The samples collected from the Gorda Escarpment did not include any potential accretionary com-plex or continental material from the Vizcaino block (e.g., Franciscan sediments, serpentine or metabasalt, Salinian granite). Thus, the

recovered samples did not include lithologies of the continental margin just to the south. Conglomerates and carbonate-welded brec-cias were present as vertical sheets separating more intact basalt, representing lithologies that might be expected within an active fault zone. Dive T208 contained extremely well-preserved pillow basalts overlying massive basalts with columnar jointing. Dive T353 located gabbros overlain by sheeted dikes. These observations suggest that at least locally the Gorda Escarp-ment includes relatively intact slices of oceanic crust separated by near vertical shear zones of highly deformed rocks. This picture is consis-tent with the published seismic transects across the Gorda Escarpment (Trehu et al., 2003).

To supplement the Mendocino transform fault data set, six basaltic samples collected from the Morro Ridge, using the ROV Tiburon were also analyzed (T143; Tables 1, 2, 3). The Morro Ridge is a transverse ridge on the Morro transform zone and thus is a remnant of the par-tially subducted Monterey plate located offshore central California (Fig. 1). The Morro Ridge is ~20 km from the intersection of the Morro trans-form with the fossil Pacifi c spreading center at Davidson Seamount (Fig. 1; Davis et al., 2002). Thus, Morro Ridge provides samples of the Pacifi c plate at 27 Ma and a Pacifi c equivalent of the fossil ridge-transform environment.

ANALYTICAL METHODS

A total of 72 igneous rock samples was col-lected and examined in thin section. Of these, 35 basaltic samples were chosen for X-ray fl uo-rescence (XRF) analyses of major elements, and inductively coupled plasma–mass spectrometer (ICP-MS) analyses of rare earth elements (REEs) based on hand specimen and microscopic exami-nation. Samples were cleaned in distilled water in a sonic bath to remove vesicle and vein fi ll-ings prior to any chemical analyses. The initial 35 samples showed primary mineralogy with less than 5% secondary minerals observed in thin section (see petrography). Of these samples, only 12 had total major-element concentrations above 98 wt% during the XRF analysis (Table 1). The 23 remaining samples had totals between 96 wt% and 98 wt%. All 35 were selected to char-acterize the igneous processes using the more immobile trace-element concentrations obtained by ICP-MS (Table 2). REEs are considered to be relatively insensitive to alteration. The 12 samples that yielded totals >98 wt% were used to describe the major-element characteristics of the sample set. The XRF and ICP-MS facilities at the GeoAnalytical Laboratory of Washington State University were used in the study. Detailed description of methods, precision, and accuracy

can be found at http://www.wsu.edu/~geology/geolab/note.html (Knaack et al., 1994; Johnson et al., 1999).

Six of the XRF analyses from the Mendocino Ridge reported in Fisk et al. (1993) are incorpo-rated for comparative purposes in this study, but only major-element data are available for those samples. The XRF and ICP-MS analyses for the Morro Ridge samples (Tables 1 and 2) were also completed at the GeoAnalytical Laboratory of Washington State University.

Sawn pieces of seven whole-rock basaltic samples, and plagioclase and groundmass con-centrates separated from an eighth sample from the Mendocino transform fault, were prepared at Oregon State University for 40Ar-39Ar incre-mental heating age determinations (Table 3). Small cores (200 mg) cut from the freshest por-tions of the sawn surfaces, and plagioclase and groundmass wrapped in Cu-foil were loaded in evacuated quartz vials for neutron irradiation. Samples were irradiated for 6 h at 1 MW power at the Oregon State University TRIGA reactor. The total fl uence of fast neutrons in produc-ing 39Ar from 39K was monitored with biotite standard FCT-3 (28.03 ± 0.16 Ma; Renne et al., 1998). After decay of short-lived radionuclides, the samples were loaded in a glass manifold above a low-blank, double-vacuum resistance furnace, where they were dropped one at a time. Each sample was heated incrementally (6–9 steps) from 600 °C to fusion. The isotopic com-position of Ar (masses 36, 37, 38, 39, and 40) at each temperature step was measured with an MAP 215/50 mass spectrometer. Further details of the experimental procedure are described in Duncan (2002).

CLASSIFICATION OF MAJOR- AND TRACE-ELEMENT GEOCHEMISTRY

The basaltic rocks from the Mendocino Ridge and the Gorda Escarpment exhibit a broad range of compositions, from tholeiitic basalts (T202, T203, T204, T207, T208, T347, T348, T349, and T353) to alkaline basalts (T208 and T353), based on their total alkali versus silica content (TAS) (Fig. 3). Based on Karsten et al.’s (1990) and the TAS classifi cation schemes, the tholei-itic basalts rocks can be further divided into nor-mal (N) mid-ocean-ridge basalts (MORBs) (Zr/Nb >25), transitional T-MORBs (Zr/Nb 16–25), and enriched E-MORBs (Zr/Nb 9–16). Basalts with Zr/Nb of ~8 are generally considered to be mildly alkalic (Davis et al., 2002). However, in this study, the E-MORBs and mildly alkalic (based on TAS) basalts of dive T208 both have Zr/Nb concentrations from 8 to 9. Dive T353 contained the most alkalic enrichment with Zr/Nb values of <8.



A

B

C

Figure 2. (A) SeaBeam bathymetry (National Oceanographic and Atmospheric Administration [NOAA] public data) with Monterey Bay Aquarium Research Institute (MBARI) ROV dive tracks (yellow and red) and numbers from the Gorda Escarpment and the western part of the Mendocino Ridge on the Mendocino transform fault in 2000 and 2001. FAN, CASC, and ATV refer to 1994–1995 dive sites of the Navy ROV (Duncan et al., 1994; Fisk et al., 1996). (B and C) Simrad EM300 30kHz multibeam bathymetry (B) and backscatter (C) maps of the Gorda Escarpment. Areas of high acoustic refl ectivity were targeted as promising dive sites. Dive tracks are shown in yellow and red. Letters N, T, E, and A refer to the basalt types present: N (normal), T (transitional), E (enriched) mid-ocean-ridge basalt (MORB) and A (alkali basalt). WF—Western Flyer.

Kela et al.


TABLE 1. MAJOR-ELEMENT DATA

Sample Longitude(°W)

Latitude(°N)

Type SiO2 Al2O3 TiO2 FeO MnO CaO MgO K2O Na2O P2O5 Total

T203-G7 126.41 40.43 N 49.53 13.99 2.07 10.67 0.252 11.12 6.89 0.62 3.07 0.23 98.45T203-G9 126.41 40.43 N 50.44 14.22 1.99 11.4 0.207 12 6.54 0.38 2.61 0.23 100.01T204-G10 125.39 39.42 N 48.98 13.12 2.88 13.08 0.236 8.71 6.29 0.57 4.03 0.28 98.17T207-G6 125.5 40.39 N 51.44 15.26 1.37 9.31 0.188 9.38 8.12 0.7 3.67 0.13 99.57T347-G16 127.61 40.42 N 50.29 14.13 2.43 11.23 0.167 10.81 6.81 0.14 2.95 0.23 99.18T204-G5 125.38 38.48 T 50.02 18.41 1.49 7.62 0.226 8.71 7.38 0.2 3.98 0.19 98.23T208-G3 125.22 40.36 E 51.08 16.89 2.09 9.67 0.191 10.47 4.6 0.86 3.63 0.48 99.96T208-G5 125.22 40.36 E 51.7 16.69 2.21 9.28 0.222 10.5 4.35 0.72 3.7 0.42 99.79T208-G19 125.21 40.36 E 50.99 16.24 2.25 9.76 0.148 11.05 4.66 0.64 3.69 0.41 99.83T143-R10 122.67 35.22 E 50.88 13.32 2.58 14.04 0.25 9.76 5.56 0.21 3.16 0.26 100.01T208-G4 125.22 40.36 A 50.19 16.75 2.09 11 0.175 10.09 4.35 1.01 3.5 0.43 99.59T208-G11 125.21 40.36 A 51 16.12 2.04 9.5 0.131 9.9 3.79 1.36 3.68 0.5 98.02T208-G18 125.21 40.36 A 50.67 17.76 2.25 7.95 0.142 10.81 2.93 1.08 3.99 0.42 98

Note: Selected major-element X-ray fl uorescence (XRF) data in wt%. Letters N, T, E, and A for rock classifi cation refer to normal (N) mid-ocean-ridge basalt (MORB), transitional (T)-MORB, enriched (E)-MORB and alkali basalt, respectively. Analyses were completed at Washington State University.

TABLE 2. TRACE-ELEMENT DATA (PPM)

Sample ID Type La Ce Nd Sm Eu Yb Nb Y Zr Zr/Nb Zr/Y Nb/Y Nb/Y(N) Ce/Y(N) Ce/Yb(N)

T208-G4 A 16.02 34.3 19.89 5.92 2.09 3.21 25.51 37.01 223.45 8.76 6.038 0.69 4.4 2.38 2.96T208-G11 A 19.37 38.64 21.78 6.32 2.17 3.47 25.98 40.05 228.82 8.81 5.713 0.65 4.14 2.47 3.1T353-G7 A 16.52 34.33 19.18 5.22 1.67 2.07 20.85 25.05 162.35 7.79 6.481 0.83 5.31 3.51 4.61T353-G15 A 15.82 32.48 18.4 4.89 1.7 1.91 19.84 23.69 150.25 7.57 6.342 0.84 5.34 3.51 4.71T203-G15 E 10.48 23.87 15.6 4.79 1.76 2.53 12.87 29.88 148.27 11.52 4.962 0.43 2.75 2.05 2.62T205-G4 E 10.79 24.36 14.99 4.41 1.48 2.36 15.22 26.93 141.8 9.32 5.266 0.57 3.61 2.32 2.87T205-G5 E 15.65 29.36 14.77 3.87 1.25 2.03 11.98 22.38 119.4 9.97 5.335 0.54 3.42 3.36 4.02T208-G9 E 17.54 37.07 21.67 6.35 2.23 3.49 26.35 39.09 230.37 8.74 5.893 0.67 4.3 2.43 2.95T208-G17 E 20.56 42.06 24.32 7.01 2.37 3.17 23.02 41.76 200.77 8.72 4.808 0.55 3.52 2.58 3.68T208-G19 E 16.77 36 21.14 6.3 2.19 3.44 26.97 39.56 234.17 8.68 5.919 0.68 4.35 2.33 2.91T353-G4 E 14.02 29.39 18.02 5.24 1.77 2.75 13.95 32.75 150.82 10.81 4.605 0.43 2.72 2.3 2.97T353-G17 E 12.68 27.48 16.7 4.8 1.62 2.31 16.4 27.77 148.29 9.04 5.340 0.59 3.77 2.54 3.31T208-G5 E 16.28 35.04 20.57 6.11 2.1 3.35 26 37.98 227.61 8.76 5.993 0.68 4.37 2.37 2.9T208-G9 E 17.54 37.07 21.67 6.35 2.23 3.49 26.35 39.09 230.37 8.74 5.893 0.67 4.3 2.43 2.95T348-G10 T 10.12 26.25 20.08 6.44 2.25 4.24 10.09 46.39 197.85 19.62 4.265 0.22 1.39 1.45 1.72T204-G5 T 7.92 17.69 11.21 3.54 1.37 2.09 6.82 23.3 125.35 18.39 5.380 0.29 1.87 1.95 2.35T202-G5 N 9.4 29.11 27.77 10.64 3.49 9.14 6.59 95.79 300.9 45.68 3.141 0.07 0.44 0.78 0.88T203-G9 N 4.5 12.93 11.73 4.6 1.66 4.26 2.78 43.35 110.19 39.57 2.542 0.06 0.41 0.76 0.84T203-G10 N 1.91 5.57 5.61 2.35 0.94 2.43 1 24.68 49.5 49.28 2.006 0.04 0.26 0.58 0.64T203-G11 N 4.2 13.09 12.43 4.89 1.76 4.52 2.42 46.16 118.35 48.86 2.564 0.05 0.33 0.73 0.8T204-G10 N 6.16 19.02 17.84 6.76 2.43 6.08 3.77 64.42 183.34 48.58 2.846 0.06 0.37 0.76 0.87T207-G11 N 3.68 9.47 7.13 2.49 1.04 2.5 2.18 25.69 78.73 36.07 3.065 0.08 0.54 0.95 1.05T347-G16 N 5.02 15.32 14.25 5.55 1.91 4.89 3.05 48.49 142.85 46.84 2.946 0.06 0.4 0.81 0.87T143-R7A E 10.38 22.04 15.71 5.11 1.75 4.11 12.45 44.19 129.22 10.38 2.924 0.28T143-R8A E 3.81 9.14 6.74 2.43 0.93 2.28 4.64 22.87 60.05 12.94 2.626 0.20T143-R13 E 8.29 18.29 12.07 3.79 1.29 2.72 10.9 29.03 99.85 9.16 3.440 0.38

Note: Selected rare earth element inductively coupled plasma–mass spectrometry (ICP-MS) data in ppm. Letters N, T, E, and A for rock classifi cation refer to normal (N) mid-ocean-ridge basalt (MORB), transitional (T)-MORB, enriched (E)-MORB and alkali basalt, respectively. Analyses were completed at Washington State University.

TABLE 3. 40Ar-39Ar INCREMENTAL HEATING AGES FOR THE MENDOCINO TRANSFORM FAULT AND MORRO RIDGE

Sample Material Total fusion age(Ma)

2σerror

Plateau age(Ma)

2σerror

N %39Arin plateau

MSWD Isochron age(Ma)

2σerror

MSWD 40Ar/36Arinitial

Comments

T203-G9 Whole rock 10.70 0.28 None developed None developed Recoil ArT205-G2 Whole rock 13.04 0.54 13.36 0.34 6/8 89 1.08 13.34 0.71 1.35 296 ± 7T207-G6 Whole rock 21.03 0.32 None developed 19.51 0.34 2.37 319 ± 5 Excess ArT208-G5 Whole rock 11.91 0.22 12.17 0.19 3/6 79 1.80 12.12 0.18 0.72 299 ± 5T208-G19 Whole rock 12.53 0.29 12.75 0.35 4/7 73 1.55 12.51 0.38 0.16 298 ± 3T347-G16 Whole rock 23.14 1.01 None developed 18.05 0.43 0.79 306 ± 2 Excess ArT348-G10 Plagioclase 716.7 2.40 None developed None developed Excess ArT348-G10 Groundmass 15.58 0.10 16.27 0.14 4/8 1.55 16.14 0.23 1.2 299 ± 6T353-G4 Whole rock 23.10 0.30 None developed None developed Recoil ArT143–18a Plagioclase 46.13 4.37 28.97 3.87 5/6 88 1.08 27.19 5.94 1.18 297 ± 5T143–18b Plagioclase 35.81 3.46 30.03 4.05 5/6 91 1.35 26.53 5.70 1.02 298 ± 4

Note: Ages calculated using biotite monitor FCT-3 (28.04 Ma) and the total decay constant is 5.543 × 10–10 yr–1. N is the number of heating steps (defi ning plateau/total); MSWD is an F-statistic that compares the variance within step ages with the variance about the plateau age (mean square of weighted deviates).



The rocks appear to form two chemical groups based on their major-element chemistry (Fig. 4). The alkali basalts and E-MORBs are low in FeO (8–11 wt%), low in TiO

2 (2–2.3 wt%),

and high in Al2O

3 (16–18.5 wt%). Their MgO

concentration varies from 3 wt% to 4.7 wt%. The N-MORBs (including data from Fisk et al., 1993) show a variety of TiO

2 concentrations

(1.1–3.9 wt%), low Al2O

3 (12.5–15.5 wt%), and

high FeO (from 9 wt% to 16 wt%). Their MgO concentration varies from 5.3 wt% to 8.2 wt%. The only T-MORB in the major-element data set has 7.5 wt% MgO, high Al

2O

3 (19 wt%),

low TiO2 (1.5 wt%), and low FeO (7.8 wt%).

The K2O content increases systematically from

N-MORBs to alkali basalts with the exception of the T-MORB and the most mafi c N-MORB. The Na

2O content is approximately the same in

the alkali basalts and E-MORBs (3.4–4 wt%) and lower in N-MORBs (2.6–3.3 wt%). Two N-MORBs form an exception with high Na

2O

(4.11 wt% and 3.7 wt%). The N-MORB with the highest Na

2O (4.11 wt%) plots within the

alkali fi eld on the TAS diagram (Fig. 3). Trace-element data (discussed below) confi rm that this sample is indeed an N-MORB, and therefore the high Na

2O is not likely to be a primary igneous

feature. The T-MORB also has high Na2O of

4.1 wt%. SiO2 and CaO are moderately variable

within each geochemical group across the range of MgO values.

The chondrite-normalized rare earth ele-ments (REE) refl ect great diversity in trace-ele-ment chemistry, with patterns that vary from

depleted in light REE with respect to heavy REE (LREE/HREE < 1; La/Sm < 1; Ce/Yb < 1) compositions (N-MORB) to enriched light REE compared to heavy REE (LREE/HREE > 1; La/Sm > 1; Ce/Yb > 1) compositions (T-MORB, E-MORB, alkaline samples) (Table 2; Fig. 5). The Mendocino Ridge samples are N-MORBs and are more depleted in LREEs than HREEs, with the exception of T203-G15, which is an E-MORB. The samples from the Gorda Escarp-ment show a variety of different compositions from N-MORBs to alkaline basalts. The REE patterns of the N-MORBs have a crosscutting relationship with the T-MORBs, E-MORBs, and alkaline basalts. The REE patterns of the T-MORBs also have crosscutting relationships with each of the other chemical groups. The E-MORBs and alkaline basalts share similar REE patterns. Many of the N-MORBs have a nega-tive Eu anomaly.

PETROGRAPHY

In thin section, the Mendocino transform fault rocks display an unusually broad variation in texture and mineralogy from basalts to gab-bros. The main crystallizing phases present in the groundmass of the holo- and hypocrystalline tholeiitic basalts are plagioclase, clinopyroxene, Fe-Ti oxides ± apatite and olivine. Sub- to euhe-dral plagioclase and clinopyroxene are the main phenocryst phases present, with minor amounts of olivine. Zoning is present in some of the pla-gioclase phenocrysts, and less frequently in the

clinopyroxenes. Subophitic intergrowth of pla-gioclase and clinopyroxene can be observed in the coarser-grained rocks, and quench textures dominate the fi ne-grained rocks. The gabbroic rocks contain mainly plagioclase, clinopyrox-ene, and minor olivine. Some gabbros contain primary amphibole where olivine is absent, which is not atypical for seafl oor gabbros (Stakes, 1991).

A majority of the alkali basalts are vesicular and contain abundant plagioclase; however, only samples from T208 have large (up to 6 mm) sub- to euhedral plagioclase feldspar xenocrysts (corroded), and phenocrysts occur mainly as crystal aggregates in these samples. These large crystals exhibit complex zoning patterns. The plagioclase phenocrysts are set in a matrix of plagioclase, clinopyroxene, and minor olivine.

The gabbroic and diabasic rocks typically contain greenschist-grade secondary minerals that refl ect their early hydrothermal alteration by seawater. These minerals include varying pro-portions of epidote, chlorite, amphibole, zeolite, smectite, and prehnite. The most abundant sec-ondary minerals are chlorite/smectite replace-ment of mafi c phenocrysts and albitization of plagioclase. Volcanic rocks more typically have low-temperature oxidative replacement of mafi c phases and glass to smectite/chlorite, along with vesicle fi llings of clay, carbonate, and (less com-monly) sulfi de. Highly deformed volcanic rocks collected by diamond drilling from the ROV contain extensive mineralization within the brittle fracture network. Veins of carbonate and sulfi de are abundant in some of the deformed samples. More extensively altered volcanic rocks contained epidote and chlorite replace-ment of phenocrysts along with secondary veins of carbonate. Samples containing more than 5% of visible secondary phases were generally excluded from chemical analyses. Given that chlorite/smectite and carbonate were the most signifi cant secondary phases, we also excluded samples with high volatile contents.

GEOCHRONOLOGY

Age determinations from the 40Ar-39Ar exper-iments were calculated in three ways (sum-marized in Table 3). Full data sets and plots from experiments are provided in the EarthRef Digital Archive (ERDA) at http://earthref.org/. First, age spectra (step ages versus temperature, represented by %39Ar released) were exam-ined for evidence of concordant step ages for a majority of the Ar released in each sample (called a plateau). In four samples, T205-G2 (E-MORB diabase), T208-G5 (E-MORB), T208-G19 (E-MORB), and T348-G10 (T-MORB) groundmass, good plateaus were apparent and

2

3

4

5

6

46.2 47.2 48.2 49.2 50.2 51.2 52.2 SiO

2 (wt%)

Na 2O

+K

2O

AlkalicE-MORBT-MORBN-MORBFrom Fisk 1993

EscanabaTrough

Davidson Seamount ALKALIC

THOLEIITIC

Morro Ridge

Figure 3. Total alkali versus silica (TAS) diagram for basaltic rocks on the Mendocino Ridge and the Gorda Escarpment. These rocks show a range of compositions from tholeiitic basalts to alkaline basalts. Comparisons for the alkaline series are from the California seamounts, e.g., Davidson Seamount (Davis et al., 2002). The tholeiitic series is analogous with the southern Gorda Ridge–Escanaba Trough (Davis et al., 1998), and the Morro Ridge, 27 Ma remnant of the Monterey plate. Abbreviations: N (normal), T (transitional), E (enriched) mid-ocean-ridge basalt (MORB).

Kela et al.


defi ned an age range from 12.2 to 16.3 Ma. In whole-rock samples T203-G9 (N-MORB) and T353-G4 (E-MORB), step ages decreased with increasing temperature as a result of irradiation-induced 39Ar and 37Ar recoil from K- and Ca-rich sites within these fi ne-grained basalts. In such cases, the best estimate of crystallization age was the total fusion age, obtained by sum-ming all the step compositions as if the sample had been heated to fusion in one step, compa-rable to a conventional K-Ar age. Hence, we report an age of 10.70 ± 0.28 Ma for T203-G9 and 23.10 ± 0.30 Ma for T353-G4.

Whole-rock samples T207-G6 (N-MORB) and T347-G16 (N-MORB) produced con-cave-up age spectra with no clear plateaus, which indicates contributions of undegassed (mantle-derived, “excess”) Ar at the time of

crystallization. In these cases, age calculations were derived from the correlation of the step Ar compositions (40Ar/36Ar vs. 39Ar/36Ar isochrons). For sample T207-G6 the isochron age is 19.51 ± 0.34 Ma; for sample T347-G16, the isochron age is 18.05 ± 0.43 Ma. These correlations also allowed us to determine the initial composition of Ar in the sample at crystallization, which was greater than the atmospheric value (40Ar/36Ar = 295.5). The suspected nonatmospheric initial Ar in samples T207-G6 and T347-G16 was con-fi rmed by 40Ar/36Ar intercepts of 319 ± 5 and 306 ± 2. In these cases, we accept the isochron ages as better estimates of the crystallization ages. For the four samples that produced acceptable plateau ages, the isochron ages were concor-dant with the plateau ages, and initial 40Ar/36Ar compositions were atmospheric, confi rming the

reliability of the plateau ages. The isochron ages have slightly larger fi tting uncertainties. Our new ages show that Mendocino Ridge rocks were erupted between 11 and 18 Ma, while Gorda Escarpment rocks formed between 12 and 23 Ma, which is signifi cantly younger than the age of adjoining Pacifi c plate crust (28–30 Ma), estimated from the identifi cation of marine magnetic anomalies (Atwater, 1989).

PETROGENESIS

The rocks from the Mendocino transform fault are exceptionally heterogeneous in com-position, including tholeiites and alkalic basalts with Zr/Nb = 7–47 and Ce/Yb(N) = 0.58–3.53. The alkaline nature of some of the basalts is refl ected both in the major-element data (high

48

52

2 4 6 8 10 MgO (wt%)

SiO

2 (w

t%)

Alkalic

E-MORB

T-MORB

N-MORB

From Fisk et al. 199312

16

20

2 4 6 8 10 MgO (wt%)

Al 2O

3 (w

t%)

1

4

2 4 6 8 10 MgO (wt%)

TiO

2 (w

t%)

6

11

16

2 4 6 8 10 MgO (wt%)

FeO

(w

t%)

8

11

14

2 4 6 8 10 MgO (wt%)

CaO

(w

t%)

0.0

1.5

2 4 6 8 10 MgO (wt%)

K2O

(w

t%)

2.5

3.5

4.5

2 4 6 8 10 MgO (wt%)

Na 2

O (

wt%

)

LLD

LLD

Figure 4. Selected major-element plots for the Mendocino transform fault; LLD (liquid line of descent) modeled using Petrolog. Abbrevia-tions: N (normal), T (transitional), E (enriched) mid-ocean-ridge basalt (MORB).



K2O + Na

2O), and the trace-element data (low

Zr/Nb), which support the argument that this is a primary igneous feature. The observed complex zoning patterns and corrosion of the plagioclase phenocrysts suggest magma mixing with peri-ods of disequilibrium and resorption.

The major-element variations (Fig. 4; Table 1) in Al

2O

3, FeO, Na

2O, TiO

2, and K

2O, are consis-

tent with the crystallizing phases (olivine + pla-gioclase + clinopyroxene) observed in thin sec-tion. The wide range of MgO values (2.99–8.15) and lack of olivine as a major mineral phase sug-gest extensive crystal fractionation. The normal MORB fractionation path is refl ected in the sys-tematic increase in FeO and TiO

2 with decreas-

ing MgO (Perfi t and Chadwick, 1998), a varia-tion that can be seen in the Mendocino Ridge samples (at low MgO) and most of the Gorda Escarpment samples (at high MgO). However, at low MgO the alkalic basalts and E-MORBs are depleted in FeO and TiO

2 compared to the

Mendocino Ridge trend. We suggest that this too is the result of extreme fractional crystalliza-tion. For example, high fO

2 values in the magma

(from the source or crustal contamination) would result in an early crystallization of ilmenite and magnetite, driving the fractional crystallization trend toward lower concentrations of FeO and TiO

2, as observed in these rocks. Simple models

of crystal fractionation would similarly predict a systematic increase in total REE concentra-tions, as is also seen in each of the geochemical groups in Figure 5. The N-MORBs also exhibit an increasing negative Eu anomaly due to pla-gioclase fractionation.

Liquid lines of descent were calculated using a computational crystal fractionation model Petrolog (Danyushevsky, 2001). The less-mafi c N-MORBs (MgO < 7) appear to be related to each other by fractional crystallization of oliv-ine and plagioclase at 1 × 105 Kpa (5 kbar). The proposed liquid lines of descent are shown in

Figures 4B and 4C. The most mafi c basalt of the sample set could not be related to the rest of the N-MORB samples. We were also unable to relate the T-MORB to the rest of the samples using Danyushevsky’s liquid line of descent cal-culation model. Based on the nonoverlapping REE patterns in Figure 5, the T-MORB with lower REE concentrations (T204-G5) could have potentially come from the same source as the E-MORBs and alkali basalts from dive T208. However, there is a crosscutting rela-tionship between the T-MORB (T204-G5 and T353 data [alkaline basalts]), and therefore the two cannot share the same mantle source. The second T-MORB sample presented on the REE plots cuts across all of the E-MORBs and alka-line basalts, and therefore cannot be related by a simple crystal fractionation model. The major-element compositions of alkaline basalts and E-MORBs could not be produced by fractionation of the more mafi c samples from the Mendocino

1

10

100

La Ce Pr Nd Sm Eu Tb Dy Ho Er Tm Yb 1

10

100

La Ce Pr Nd Sm Eu Tb Dy Ho Er Tm Yb

Alkali Alkali

E-MORB E-MORB

T-MORB Siqueiros Seamount

Morro Ridge Davidson Seamount

Davidson Seamount Endeavour

1

10

100

La Ce Pr Nd Sm Eu Tb Dy Ho Er Tm Yb 1

10

100

La Ce Pr Nd Sm Eu Tb Dy Ho Er Tm Yb

T-MORB N-MORB

N-MORB N-MORB

N. Gorda Ridge S. Gorda Ridge

Escanaba Trough Cleft RTI

A B

C D

Figure 5. Selected rare earth element (REE) patterns for the Mendocino transform fault. (A) Selected T (transitional)- and E (enriched)-mid-ocean-ridge basalts (MORBs) and alkali basalts of the Mendocino transform fault. (B) Comparison of the Mendocino transform fault data with other volcanic provinces in the NE and E Pacifi c, including the less alkalic members of the Davidson Seamount series and the adjacent Morro Ridge, Endeavor segment of the Juan de Fuca Ridge, and data from a seamount at the intersection of the East Pacifi c Rise and the Siqueiros transform fault. (C) Selected normal (N)-MORBs and a T-MORB from the Mendocino transform fault. (D) Comparison of the Mendocino transform fault data with Gorda Ridge N-MORBs and the Juan de Fuca Ridge Cleft segment–Blanco ridge-transform intersection (RTI).

Kela et al.


transform fault, and therefore we rely on the REE patterns to interpret their relative relation-ships. The majority of the alkali basalt data cross over E-MORB data on the REE plots, with the exception of T208.

The process of crystal fractionation clearly cannot explain the full range of compositional variation in these Gorda Escarpment suites. High fi eld strength elements (HFSE), such as Nb, Zr and Y, are highly incompatible, relatively insensitive to secondary alteration, and their ratios should remain constant during fraction-ation. Variations in the content of these elements can therefore be used to assess different mantle source regions and/or extent of partial melting. The variation of log Nb/Y versus log Zr/Y was exploited by Fitton et al. (1997) to distinguish N-MORB versus enriched mantle source regions beneath Iceland, with the plume-related com-positions from the neovolcanic zone bounded by the two parallel lines shown in Figure 6A. The lower line (referred to as ΔNb = 0) sepa-rates the enriched mantle source (+ΔNb) from the MORB source (–ΔNb). We have added the average normal MORB (N-MORB), enriched MORB (E-MORB), and oceanic-island basalt (OIB) from an enriched mantle source, from Sun and McDonough (1989), to this diagram for comparison; the MORB composition falls below, and both the E-MORB and OIB fall above, the ΔNb = 0 line. The Mendocino com-positions similarly fall along two parallel arrays on the log Nb/Y versus log Zr/Y plot (Fig. 6A). The offset between the two arrays suggests two mantle source regions, while the variation within the array refl ects variation in melt fractions, with smaller melt fractions occurring at higher values of Nb and Zr. One array of samples (E-MORBs and alkaline basalts) falls on or slightly above the ΔNb = 0 line, suggesting a more enriched source region. Most of these samples fall in the region between the average Sun and McDonough (1989) E-MORB and OIB, sug-gesting that there must be an enriched mantle component. The second array of samples falls below the ΔNb = 0 line, suggesting a depleted source typical for N-MORB. Over half of the samples from the Gorda Escarpment plot in the enriched source fi eld, and all of the rocks from the Mendocino Ridge fall in the depleted source fi eld, with the exception of T203-15, which is more enriched in Nb and Zr.

Similar separation into enriched and depleted mantle source groups is depicted on the Nb/Y(N) versus Ce/Y(N) plot (Fig. 6B), and also on the Ce/Yb(N) versus Ce plot (Fig. 6C). In addi-tion to the depleted and enriched source regions, the Ce/Yb(N) versus Ce plot also shows sev-eral subtle positive trends at different Ce/Y(N) values as Ce increases to the right. These can

be attributed to fractional crystallization, as the total REE compositions would increase with increasing fractionation of plagioclase and oliv-ine. Two of the Mendocino samples, T348-G10 and T204-G5, plot within the depleted source, low melt fraction in Figure 6A, but fall into the enriched source region on both Figures 6B and 6C. The log-log plot of Figure 7A might better distinguish between small percentages of partial melting versus distinct mantle sources, although it is clear that both processes are required to explain the full range of Mendocino samples.

The trends observed on the Ce/Yb (N) versus Ce, the large variation in Ce/Yb, and the crossing rare earth patterns require additional processes, such as variable melt percentages or different source regimes, to explain their origins. Further, the alkalic and high-Al character of the basalts suggests an increased depth of melt segregation compared to MORB. Such large variations in major- and trace-element chemistry have also been noted for seamounts on the fl anks of the East Pacifi c Rise (Niu and Batiza, 1997), which have been attributed to deeper heterogeneous source regions (Niu et al., 2002).

Langmuir and Bender (1984) suggested that the generation of magmas at an oceanic spread-ing center is profoundly impacted by proximity to a large-offset transform zone: Not only does the “cold edge effect” result in smaller magma bodies with more extensive cooling and crystal-lization within the axial magmatic system, but the presence of a transform zone also cools the subcrustal mantle, expressed by lower extents of melting and perhaps deeper melting near the ridge-transform intersection. The “trans-form-fault effect” would be refl ected in mixed magmas near the ridge-transform intersection that display a greater extent of fractionation and cooling (e.g., high FeO, TiO

2, Zr) combined

with elevated incompatible element ratios (e.g., alkali content, La/Sm, and Ce/Yb). Basalts that are high in FeO and TiO

2 but have Ce/Yb < 1

and Zr/Nb > 25 include N-MORBs from the Mendocino Ridge (Fig. 5) and from the north-ern Gorda Ridge axial valley (Fig. 5) (Davis and Clague, 1987; Keaten et al., 2001). Basalts that we refer to as T-MORB (Zr/Nb 16–25) are close in composition to basalts from the magmatically waning Escanaba Trough at the ridge-transform intersection (Davis et al., 1998) (Figs. 3 and 5). Rocks of intermediate composition, however, are less common in our suite than the strongly enriched E-MORBs and alkalic types.

Clearly the transform-fault effect has played a major role in the formation of the basalts high in FeO, TiO

2, and LREE, and possibly

even the alkali, enrichment. Comparative data of an andesite pillow basalt from the southern-most Juan de Fuca ridge-transform intersection

(Cleft-Blanco system; Stakes et al., 2006) are presented in Figure 5. The highly fractionated basalts, andesites, and dacites from the south-ernmost Cleft segment at its intersection with the Blanco transform fault are all related by extended crystal fractionation and cooling from a common source (Stakes et al., 2003, 2006; Perfi t et al., 2003; Cotsonika et al., 2005). The abundance of E-MORBs and alkali basalts, the crossing REE patterns, and high Ce/Yb populations for the Mendocino transform fault, however, suggest that magma may have been derived from melting of a deeper, more hetero-geneous, source to variable extents. This is the other mechanism implicit in the transform-fault effect—the proximity of the transform perturbs the mantle melting regime to greater depths.

DISCUSSION

A majority of the rocks collected along the Mendocino transform fault were probably formed in an ridge-transform intersection envi-ronment, in which normal mid-ocean-ridge magmatism was modifi ed by extensive cool-ing and fractionation. However, other processes were probably involved in order to explain the full range of compositions. The abundance and extent of enrichment along the Mendocino transform fault appear to be anomalous for mid-ocean-ridge or even ridge-transform intersec-tion environments (Perfi t and Chadwick, 1998). Cousens (1996) reported a variety of compo-sitions from the Juan de Fuca Ridge. These ranged from highly depleted basalts from the Heck and Heckle Seamounts adjacent to the northern part of the ridge, to alkali basalts in the Pratt-Welker Seamount chain in the Gulf of Alaska. A variety of MORB compositions from N- to E-MORBs has also been reported from the Endeavor segment of the Juan de Fuca Ridge (Gill et al., 2005). Cousens (1996) concluded that the enrichment present in the NE Pacifi c oceanic rocks may be due to the presence of hydrated, subducted oceanic crust, stored in the mantle. The LREE-enriched Endeavor samples compare well with some of the enriched Men-docino transform fault samples (Fig. 5) but do not reach LREE concentrations as high as in the Mendocino transform fault.

Other possibilities for generating the alka-lic magmatic compositions observed along the Mendocino transform fault include the greater depths of melting resulting in alkaline composi-tions associated with seamount volcanism, pos-sibly near the intersection of the Gorda Ridge and the Mendocino transform fault in the past. This confi guration would be similar to the sea-mount observed now at the intersection of the East Pacifi c Rise and the Siqueiros transform



0.01

0.1

1

10

1 10Zr/Y

Nb

/Y Alkali

E-MORB

T-MORB

N-MORB

N-MORB

E-MORB

OIBSun and McDonough 1989

0

2

4

6

0 1 2 3 4Ce/Y(N)

Nb

/Y(N

)

0

1

2

3

4

5

0 15 30 45Ce (ppm)

Ce/

Yb

(N)

Toward OIB (alkali) compositions

Increasing degree of partial melting

N Nb- Depleted source

Nb+ Enriched source

DepletedEnriched

Toward OIB (alkali) compositions

Decreasing % of melting

FC

FC

FC

FC

Depleted source

Enriched source

A

B

C

Sun and McDonough 1989

Sun and McDonough 1989

Figure 6. (A) Log (Nb/Y) versus log (Zr/Y) plot distinguishes variations in mantle source characteristics using highly incompatible elements that are relatively unaffected by alteration. Parallel lines mark the limits of Icelandic plume source lavas compared to depleted mid-ocean-ridge basalt (MORB), each of which was empirically found to form tight linear arrays on this diagram defi ned by ΔNb = 1.74 + log (Nb/Y) – 1.92 log(Zr/Y) (Fitton et al., 1997). The lowermost line (ΔNb = 0) separates depleted mantle sources from enriched mantle sources. Basalts derived from each distinct source fall on a separate co-parallel array, with variation contained within each array determined solely by vari-able degrees of mantle melting and source depletion through melt extraction, as these variables are insensitive to crystal fraction-ation. Average MORB, enriched (E)-MORB, and oceanic-island basalt (OIB) composi-tions from Sun and McDonough (1989) are shown for comparison. The Mendocino data form two linear arrays with normal (N)-MORB source (ΔNb < 0) at moderate to high melt fractions and slightly enriched (ΔNb = 0) at moderate to low melt fractions. (B) Nb/Y(N) versus Ce/Y(N) plot similarly uses the variation in highly incompatible trace elements to distinguish different mantle source regions. Nb/Y(N) > 1 is characteristic of a more fertile (enriched) mantle source, and Nb/Y(N) < 1 is typical for a less fertile (depleted) mantle source. Ce/Y(N) (light rare earth element [LREE]/heavy rare earth element [HREE]) is insensitive to crystal fractionation, and thus covariation in these two parameters can be attributed to differ-ent melt percentages in the source region or different mantle sources. (C) Plot of Ce/Yb N versus Ce distinguishes basalts that are derived from a depleted (low Ce/Yb) mantle source as linear arrays of variable Ce. The positive trends at distinct Ce/Y(N) values with increasing Ce can be related to crystal fractionation (FC).

Kela et al.


fault (Batiza and Johnson, 1980; Natland and Melson, 1980; Niu and Batiza, 1997). The alka-line enrichment observed in our study compares well with the most evolved samples from the Siqueiros Seamount (Fig. 5). Melankholina et al. (1994) reported a compositional range from N-MORBs to E-MORBs in rocks collected along the Mendocino fracture zone far west of the Mendocino Ridge (165°W and 145°W). These compositions appear to fall within or close to the same compositional range as in the rocks collected on the Gorda Escarpment and the Mendocino Ridge. Interestingly, the E-MORBs reported by Melankholina et al. (1994) were all collected on a small seamount adjacent to the fault. However, no alkaline compositions were reported at this location, and there is no direct evidence to support the presence of a seamount at the intersection of the Gorda Ridge and the Mendocino transform fault.

Periods of transtension during regional plate reorganization might produce intratransform spreading centers. Such magmatism, described at the Siqueiros (Perfi t et al., 1996), the Gar-rett (Hekinian et al., 1992; Wendt et al., 1999), and the Blanco transform faults (Gaetani et al., 1995), has generally been associated with primi-tive magmas (picrites). Such picritic lavas have been explained by off-axis remelting of upper mantle that had previously been depleted in incompatible element–enriched heterogeneities during melting beneath the ridge axis (Wendt et al., 1999). The active spreading centers within the Siqueiros transform are now producing magmas that are mainly N-MORB in composi-tion (Fornari et al. 1989).

However, within an environment of active ridge migration, short intratransform spreading centers might tap deeper, less-depleted mantle sources. It is tectonically feasible that samples from the Gorda Escarpment formed on short, intratransform spreading segments. Magnetic anomalies east of 127°W become diffi cult to follow, and north of 40.2°N, which is also the southern limit of the Mendocino fracture zone, seafl oor ages are not constrained by mapped magnetic anomalies. Many patterns of seafl oor age from 40.2°N to the northern limit of the Gorda Escarpment are possible under an intra-transform spreading model.

A compelling geochemical analogue for the Gorda Escarpment chemical variations is the Morro Ridge–Davidson Seamount system in the central California borderland (Figs. 1, 3, and 5). The Morro Ridge samples are basalts cre-ated 29–30 Ma (Table 3) at a Farallon-Pacifi c spreading center near the intersection with the transform zone. These MORB compositions likely represent the basalts created at the spread-ing center before its abandonment. The failed

rift is now capped by younger alkaline volcanics associated with Davidson Seamount. Batiza and Vanko (1985) suggested that the process of rift failure for the Mathematician spreading center resulted in alkalic magmas for up to 10–15 m.y. after the spreading center was abandoned and the waning mantle melting regime retreated to greater depths. Perhaps a similar period of rift failure occurred either on the southernmost Gorda Ridge, or on a series of short intratrans-form spreading segments during plate reorgani-zation. The abandoned segments may have gone through an extended period of low-volume alka-lic volcanism, similar to that described for the Mathematician Ridge and Davidson Seamount, before it was slivered along the transform by tectonic processes and ultimately transferred to the Pacifi c plate by the northward movement of the Mendocino transform fault (Fig. 7).

The Mendocino transform fault was in exten-sion from 24 to 19 Ma, during which time mag-matism on the Gorda Ridge may have retreated from the transform fault leaving behind a seg-ment of waning magmatism and extensive crystal fractionation. This was followed by a more stable period from 19 to 10 Ma, when the stress regime across the transform changed into transpression. During this period of com-pression, alkalic magmatism could have con-tinued in decreasing abundance until the failed spreading segment was slivered onto the trans-form zone. Thus, the Gorda Escarpment does not provide a window into the Vizcaino block, but rather a record of the ridge migration, aban-donment, and residual volcanism of the south-ern Gorda Ridge–Mendocino transform system from 23 to 11 Ma.

CONCLUSIONS

ROV observations and systematic collection of seafl oor rocks have constrained the origin of oceanic crust exposed in the transverse ridges of the Mendocino transform zone. A study of major- and trace-element chemistry supple-mented by radiometric age determinations has led us to conclude that:

1. The slabs of ocean crust from either the Mendocino Ridge or the Gorda Escarpment did not originate from the Pacifi c plate, and cer-tainly those from the Gorda Escarpment do not represent an accretionary wedge. The range of ages (11–23 Ma) is younger than rocks from the adjacent Pacifi c plate.

2. The geochemistry of Gorda Escarpment rocks shows much more variability than the Mendocino Ridge; this is especially apparent in the E-MORB and alkalic basalt compositions. Variations in alteration-resistant trace-elements Nb, Y, Ce, and Zr indicate at least two mantle

Figure 7. (A) The Mendocino transform fault was in extension from 24 to 19 Ma, during which time magmatism on the Gorda Ridge (Juan de Fuca Ridge [JdF]) fi rst extended into the transform fault as a curved ridge (much like what is observed at the Cleft seg-ment of the Juan de Fuca at present day). (B) Toward 19 Ma, ridge magmatism may have moved away from the transform fault, leaving behind a segment of ceasing magma-tism within the transform zone due to the extensional regime. The period was followed by a more stable period from 19 to 12 Ma, when the stress regime across the transform changed into transpression. (C) 12–6 Ma transpression continued and became stron-ger. Magmatism completely ceased at the ridge-transform intersection (RTI), and the failed spreading segment was slivered onto the transform zone.

24-19 MaA

JdF

Rid

ge

B19-12 Ma

JdF

Rid

ge

C12-6 Ma

JdF

Rid

ge



sources with variable degrees of partial melting from each source.

3. There is evidence of extensive cooling and fractionation predicted by the transform-fault effect of Langmuir and Bender (1984). Alkaline and high-Al compositions also argue for melts from a greater depth and smaller degrees of par-tial melting than are characteristic of a normal mid-ocean-ridge environment.

4. The formation of an alkaline seamount at a ridge-transform intersection that was then cap-tured within the transform zone would explain some of the geochemical variability.

5. The transform-fault effect is a realistic explanation for the extensive fractionation needed to produce compositions high in FeO, TiO

2, and REE. However, the alkaline composi-

tions with high Ce/Yb suggest derivation from a failed spreading segment that was slivered onto the transform zone between 23 and 11 Ma. The Mendocino transform fault was in extension from 24 to 19 Ma, when magmatism may have waned adjacent to or within the nodal basin of the transform zone. From 19 to 11 Ma, when the stress regime across the transform changed into transpression, alkalic compositions continued to erupt after the spreading was abandoned and until the failed spreading segment was slivered onto the transform zone.

Thus, the crustal slices exposed on the trans-verse ridges of the Mendocino transform fault provide a record of the history of the south-ernmost Gorda Ridge during a period of ridge migration, abandonment, and residual volca-nism from 23 to 11 Ma.

ACKNOWLEDGMENTS

We are grateful for the skill and patience of the ROV Tiburon pilots and the crew of the R/V Western Flyer for the two highly successful fi eld programs. Mike Perfi t provided extensive suggestions and con-stant encouragement for this paper. Alicé Davis pro-vided suggestions for the interpretation of the geo-chemical data. John Chadwick, Doug Wilson, and an anonymous reviewer greatly improved this paper. The authors thank Associate Editor Rodney Metcalf for the signifi cant contribution of his time and ideas that guided this paper through the revision process. Support for Kela was provided by a Monterey Bay Aquarium Research Institute (MBARI) student internship. The fi eld program was jointly supported by MBARI funds from the David and Lucile Packard Foundation (to Stakes) and from the National Ocean-ographic and Atmospheric Administration (NOAA) Undersea Research Program (NURP) (to Duncan). T. Ramirez and A. Gough assisted with illustrations and graphics.

REFERENCES CITED

Atwater, T.M., 1970, Implications of plate tectonics for the Cenozoic tectonic evolution of western North Amer-ica: Geological Society of America Bulletin, v. 81, p. 3513–3536.

Atwater, T.M., 1989, Plate tectonic history of the northeast Pacifi c and western North America, in Winterer, E.L., et al., ed., The eastern Pacifi c Ocean and Hawaii: Boul-der, Geological Society of America, The Geology of North America, vol. N, p. 21–72.

Batiza, R., and Johnson, J.R., 1980, Trace element and iso-topic evidence for magma mixing in alkalic and tran-sitional basalts near the East Pacifi c Rise at 8°N, in Rosendahl, B.R., and Hekinian, R., eds., Initial Reports of the Deep Sea Drilling Project, Volume 54: Washing-ton, D.C., U.S. Government Printing Offi ce, p. 63–69.

Batiza, R., and Vanko, D., 1985, Petrologic evolution of large failed rifts in the eastern Pacifi c: Petrology of volcanic and plutonic rocks from the Mathematician Ridge area and the Guadalupe Trough: Journal of Petrology, v. 26, p. 564–602.

Clague, D.A., and Holmes, M.L., 1987, Geology, petrology, and mineral potential of the Gorda Ridge, in Scholl, D.W., Grantz, A., and Vedder, J.G., eds., Geology and Resource Potential of the Continental Margin of West-ern North America and Adjacent Ocean Basins—Beau-fort Sea to Baja California: Houston, Circum-Pacifi c Council for Energy and Mineral Resources, Earth Sci-ence Series, p. 563–580.

Cotsonika, L.A., Perfi t, M.R., Stakes, D.S., and Ridley, W.I., 2005, The occurrence and origin of andesites and dacites from the southern Juan de Fuca Ridge [abs.]: Eos (Transactions, American Geophysical Union), Spring Meeting, no. 86(18), abstract V13A-04.

Cousens, B.L., 1996, Depleted and enriched upper mantle sources for basaltic rocks from diverse tectonic envi-ronments in the northeast Pacifi c Ocean: The genera-tion of oceanic alkaline vs. tholeiitic basalts, in Basu, A., and Hart, S., eds., Earth processes: Reading the iso-topic code: American Geophysical Union Geophysical Monograph 95, p. 207–231.

Danyushevsky, L., 2001, The effect of small amounts of H

2O on crystallization of mid-ocean ridge and backarc

basin magmas: Journal of Volcanology and Geother-mal Research, v. 110, p. 265–280, doi: 10.1016/S0377-0273(01)00213-X.

Davis, A.S., and Clague, D.A., 1987, Geochemistry, mineralogy and petrogenesis of basalt from the Gorda Ridge: Journal of Geophysical Research, v. 92, p. 10,467–10,483.

Davis, A.S., Clague, D.A., and White, W.M., 1998, Geo-chemistry of basalt from Escanaba Trough: Evidence for sediment contamination: Journal of Petrology, v. 39, p. 841–858, doi: 10.1093/petrology/39.5.841.

Davis, A.S., Clague, D.A., Bohrson, W.A., Dalrymple, G.B., and Greene, G.H., 2002, Seamounts at the continen-tal margin of California: A different kind of oceanic intraplate volcanism: Geological Society of America Bulletin, v. 114, p. 316–333, doi: 10.1130/0016-7606(2002)114<0316:SATCMO>2.0.CO;2.

Denlinger, R.P., 1992, A model for large-scale plastic yield of the Gorda deformation zone: Journal of Geophysical Research, v. 97, p. 15,415–15,423.

Duncan, R.A., 2002, A time frame for construction of the Ker-guelen Plateau and Broken Ridge: Journal of Petrology, v. 43, p. 1109–1119, doi: 10.1093/petrology/43.7.1109.

Duncan, R.A., Fisk, M.R., Carey, A.G., Jr., Lund, D., Doug-las, L., Wilson, D.S., and Krause, D., 1994, Origin and emergence of the Mendocino Ridge [abs.]: Eos (Trans-actions, American Geophysical Union), v. 75, p. 475.

Embley, R.W., and Wilson, D.S., 1992, Morphology of the Blanco transform fault zone, NE Pacifi c: Implications for its tectonic evolution: Marine Geophysical Researches, v. 14, p. 25–45, doi: 10.1007/BF01674064.

Fisk, M.R., Duncan, R.A., Fox, C.G., and Witter, J.B., 1993, Emergence and petrology of the Mendocino Ridge: Marine Geophysical Researches, v. 15, p. 283–296, doi: 10.1007/BF01982386.

Fisk, M.R., Duncan, R.A., Carey, A.G., Jr., Nielsen, R.L., Chen, Y.J., Sours-Page, R., Sprtel, F., and Weber, M., 1996, Plate boundary effects at Mendocino Ridge: Eos (Transactions, American Geophysical Union), v. 77, Supplement, p. S270–271.

Fitton, J.G., Saunders, A.D., Norry, M.J., Hardarson, B.S., and Taylor, R.N., 1997, Thermal and chemical struc-ture of the Iceland plume: Earth and Planetary Sci-ence Letters, v. 153, p. 197–208, doi: 10.1016/S0012-821X(97)00170-2.

Fornari, D.J., Edwards, M.H., Gallo, D.G., Madsen, G.E., Perfi t, M.R., and Shor, A.N., 1989, Structure and topography of the Siqueiros transform fault system: Evidence for the development of intra-transform spreading centers: Marine Geophysical Researches, v. 11, p. 263–299, doi: 10.1007/BF00282579.

Gaetani, G.A., DeLong, S.E., and Wark, D.A., 1995, Petro-genesis of basalts from the Blanco Trough, northeast Pacifi c: Inferences from off-axis melt generation: Jour-nal of Geophysical Research, v. 100, p. 4197–4214, doi: 10.1029/94JB02774.

Gill, J.B., Kela, J.M., Ramos, F., Michael, P., Woodcock, J., and Stakes, D.S., 2005, The geology and geochemistry of the Endeavour segment of the Juan de Fuca Ridge [abs.]: Eos (Transactions, American Geophysical Union), v. 75, p. 475.

Griscom, A., and Jachens, R.C., 1989, Tectonic history of the north portion of the San Andreas fault system, Cali-fornia, inferred from gravity and magnetic anomalies: Journal of Geophysical Research, v. 94, p. 3089–3099.

Hekinian, R., Bideau, D., Cannat, M., Francheteau, J., and Hebert, R., 1992, Volcanic activity and crust-mantle exposure in the ultrafast Garrett transform fault near 13 degrees 28′S in the Pacifi c: Earth and Planetary Sci-ence Letters, v. 108, p. 259–275, doi: 10.1016/0012-821X(92)90027-S.

Hey, R.N., and Wilson, D.S., 1982, Propagating rift explana-tion for the tectonic evolution of the northeast Pacifi c; the psuedomovie: Earth and Planetary Science Letters, v. 58, p. 167–188, doi: 10.1016/0012-821X(82)90192-3.

Johnson, D.M., Hooper, P.R., and Conrey, R.M., 1999, XRF Analysis of rocks and minerals for major and trace elements on a single low dilution Li-tetraborate fused bead, Advances in X-ray Analysis: GeoAnalytical Lab, Washington State University, v. 41, p. 843–867, http://www.wsu.edu/~geology/geolab/note/xrf.html.

Karsten, J.L., Delaney, J.R., Rhodes, J.M., and Liias, R., 1990, Spatial and temporal evolution of magmatic systems beneath the Endeavour segment, Juan de Fuca Ridge: Tectonic and petrologic constraints: Journal of Geophysical Research, v. 95, p. 19,235–19,256.

Keaten, R., Davis, A.S., and Clague, D.A., 2001, An along axis study of basaltic glass from the northern Gorda Ridge [abs.]: Eos (Transactions, American Geophysi-cal Union), v. 82, p. 47.

Knaack, C., Cornelius, S.B., and Hooper, P.R., 1994, Trace element analyses of rocks and minerals by ICP-MS: GeoAnalytical Lab, Washington State University, http://www.wsu.edu/~geology/geolab/note/icpms.html.

Krause, D.C., Menard, H.W., and Smith, S.M., 1964, Topog-raphy and lithology of the Mendocino Ridge: Journal of Marine Research, v. 22, p. 236–249.

Krause, D.C., Duncan, R.A., and Fisk, M.R., 1998, Origin of the Mendocino Ridge, NE Pacifi c, through obduc-tion of the Juan de Fuca Plate [abs.]: Eos (Transactions, American Geophysical Union), v. 79, p. 859.

Langmuir, C.H., and Bender, J.F., 1984, The geochemistry of oceanic basalts in the vicinity of transform faults: Observations and implications: Earth and Planetary Science Letters, v. 69, p. 107–127, doi: 10.1016/0012-821X(84)90077-3.

Leitner, B., Trehu, A.M., and Godfrey, N.J., 1998, Crustal structure of the Vizcaino block and Gorda Escarpment, offshore northern California, and implications for post-subduction deformation of a paleoaccretionary margin: Journal of Geophysical Research, v. 103, p. 23,795–23,812, doi: 10.1029/98JB02050.

Melankholina, E.N., Lyapunov, S.M., Baranov, B.V., Kononov, M.V., Rudnik, G.B., Saidova, Kh.M., Tik-honov, L.V., and Shmidt, O.A., 1994, Compositional variations of oceanic basalts from the Mendocino fault, Pacifi c Ocean: Geotectonics (English translation), v. 28, p. 226–237.

Natland, J.H., and Melson, W.G., 1980, Compositions of basaltic glasses from the East Pacifi c Rise and Siqueiros fracture zone, near 9°N, in Initial Report of Deep Sea Drilling Project, Volume 54: Washington, D.C., U.S. Government Printing Offi ce, p. 705–723.

Niu, Y., and Batiza, R., 1997, Trace element evidence from seamounts for recycled ocean crust in the eastern Pacifi c: Earth and Planetary Science Letters, v. 148, p. 471–483, doi: 10.1016/S0012-821X(97)00048-4.

Kela et al.


Niu, Y., Regelous, M., Wendt, I.J., Batiza, R., and O’Hara, M.J., 2002, Geochemistry of near-EPR seamounts: Importance of source vs. process and the origin of enriched mantle component: Earth and Planetary Sci-ence Letters, v. 199, p. 327–345, doi: 10.1016/S0012-821X(02)00591-5.

Perfi t, M.R., and Chadwick, W.W., 1998, Magmatism at mid-ocean ridges: Constraints from volcanological and geo-chemical investigations, in Buck, W.R., Delaney, P.T., Karson, J.A., and LaGabrielle, Y., eds., Faulting and magmatism at mid-ocean ridges: American Geophysi-cal Union Geophysical Monograph 106, p. 59–116.

Perfi t M.R., Fornari, D.J., Ridley, W.I., Kirk, P.A., Casey, D.J., Kastens, K.A., Edwards, M., Shuster, R., and Paradis, S., 1996, Recent volcanism in the Siqueiros transform fault: Picritic basalts and implications for MORB magma gen-esis: Earth and Planetary Science Letters, v. 141, p. 91–108, doi: 10.1016/0012-821X(96)00052-0.

Perfi t, M.R., Stakes, D.S., Tivey, M., Kulp, S., Ridley, W.I., and Ramirez, T.M., 2003, Magma genesis and crustal formation of the southern Juan de Fuca Ridge (JdFR): Results of fi ne-scale sampling, in Contributions of the EGS/AGU/EUG Joint Assembly: Nice, France, 6–11 April 2003, Geophysical Research Abstracts, v. 5, abs. EAE03-A-07287.

Renne, P.R., Swisher, C.C., Deino, A.L., Karner, D.B., Owens, T.L., and DePaolo, D.J., 1998, Intercalibra-tion of standards, absolute ages and uncertainties in 40Ar/39Ar dating: Chemical Geology, v. 145, p. 117–152, doi: 10.1016/S0009-2541(97)00159-9.

Stakes, D.S., 1991, Oxygen and hydrogen isotope compo-sitions of oceanic plutonic rocks: High temperature deformation and metamorphism of oceanic layer 3, in

Taylor, H.P., Jr., O’Neil, J.R., and Kaplan, I., eds., Sta-ble isotope geochemistry: A tribute to Samuel Epstein: Geochemical Society [London] Special Publication 3, p. 77–90.

Stakes, D.S., Trehu, A.M., Goffredi, S.K., Naehr, T.H., and Duncan, R., 2002, Mass wasting, methane venting and biological communities on the Mendocino transform fault: Geology, v. 30, p. 407–410, doi: 10.1130/0091-7613(2002)030<0407:MWMVAB>2.0.CO;2.

Stakes, D., Perfi t, M., Wheat, C., Ramirez, T., Koski, R., and Hein, J., 2003, Evidence of off-axis volcanism and hydrothermal venting along the Cleft segment of the southern Juan de Fuca Ridge, in Contributions of the EGS/AGU/EUG Joint Assembly: Nice, France, 6–11 April 2003, Geophysical Research Abstracts, v. 5, abs. EAE03-A-04666.

Stakes, D.S., Perfi t, M., Tivey, M.A., Caress, D.W., Ramirez, T.M., and Maher, N., 2006, The Cleft revealed—Geo-logic, magnetic and morphologic evidence for construc-tion of upper oceanic crust along the southern Juan de Fuca Ridge: Geochemistry, Geophysics, Geosystems, v. 7, Q04003, doi: 10.1029/2005GC001038.

Stoddard, P.R., 1987, A kinematic model for the Gorda plate: Journal of Geophysical Research, v. 92, p. 11,524–11,532.

Sun, S.S., and McDonough, W.F., 1989, Chemical and iso-topic systematics of oceanic basalts: Implications for mantle composition and processes, in Saunders, A.D., and Norry, M.J., eds., Magmatism in the ocean basins: Geological Society [London] Special Publication 42, p. 313–345.

Trehu, A.M., Lendl, C., Leitner, B., Meltzer, A., Gulick, S., Holl, J., Levander, A., Henstock, T., Beaudoin, B.,

Godfrey, N., Hole, J., Klemperer, S., Clarke, S., Luet-gert, J., and Mooney, W.D., 1995, Pulling the rug out from under California: Seismic images of the Men-docino triple junction region [abs.]: Eos (Transactions, American Geophysical Union), v. 76, no. 38, p. 369, 380–381.

Trehu, A.M., Stakes, D.S., Bartlett, C., Chevalier, J., Duncan, R.A., Goffredi, S., Potter, S.M., and Salamy, K.A., 2003, Seismic and seafl oor evidence for free gas, gas hydrates and fl uid seeps on the transform margin offshore Cape Mendocino: Journal of Geophysical Research, v. 108, B5, p. 2263, doi: 10.1029/2001JB001679.

Wendt, J.I., Regelous, M., Niu, Y., Hekinian, R., and Coller-son, K.D., 1999, Geochemistry of lavas from the Gar-rett transform fault: Insights into mantle heterogeneity beneath the eastern Pacifi c: Earth and Planetary Sci-ence Letters, v. 173, p. 271–284, doi: 10.1016/S0012-821X(99)00236-8.

Wilson, D.S., 1986, A kinematic model for the Gorda defor-mation zone as a diffuse southern boundary of the Juan de Fuca plate: Journal of Geophysical Research, v. 91, p. 10,259–10,269.

Wilson, D.S., 1988, Tectonic history of the Juan de Fuca Ridge over the last 40 million years: Journal of Geo-physical Research, v. 93, p. 11,863–11,876.

Wilson, D.S., 1989, Deformation of the so-called Gorda plate: Journal of Geophysical Research, v. 94, p. 3065–3075.

MANUSCRIPT RECEIVED 28 MAY 2004REVISED MANUSCRIPT RECEIVED 18 JANUARY 2006MANUSCRIPT ACCEPTED 19 MARCH 2006

Printed in the USA

geochemical and age constraints on the formation of the gorda escarpment and mendocino ridge of the...

Documents