The Gorda plate, the southernmost fragment of the larger Juan de Fuca plate system, is an example of a nonrigidly deformingtectonic accommodation zone or buffer plate, absorbing deformation and allowing the surrounding larger plates to act in a more rigid fashion. Here we present a new structural analysis of the plate based on full-plate bathymetric coverage, augmented by seismic reflection data and earthquake moment tensors and locations. We interpret internal deformation of the Gorda plate as an asymmetrical flexural-slip buckle with a vertical axis, utilizing reactivation of spreading-ridge fabric normal faults as strike-slip faults. Newly formed second-generation faults crosscutting the structural grain overprint the reactivated structures. The spreading fabric faults finally begin a second phase of extension as the plate approaches the subduction zone. This model, based on fault constraints, has allowed investigation of ridge-plate–subduction interactions, and suggests that spreading-rate variations along the Gorda Ridge may be controlled by internal deformation of the plate rather than the reverse, as previously hypothesized.
Figure 1. A: Shaded-bathymetric map of Gorda plate. Relict spreading-fabric basement ridges can be clearly seen; in places ridges are strongly deformed into smooth curves and kinks, whereas other ridges appear undeformed. Note northwest-trending deep depression in center of plate, small seamounts restricted to north half of plate, and strong northeasttrending lineation aligned with notch in margin. Moment-tensor solutions are from Oregon State University moment-tensor database (http://quakes.oce.orst.edu/moment-tensor/). B: Tectonic setting of Gorda plate (FZ 5 fracture zone). C: Seismicity of Gorda plate from 1974 to 2000. Note approximately north-south–oriented band of seismicity just west of Cascadia subduction zone deformation front and cluster of events in center of plate. White circles are from California Geological Survey database (1974–August 1991), and black circles are SOSUS events (August 1991–2000).
Figure 2. A: Mapped
faults and fault-related
ridges within Gorda plate based on basement
structure and surface
morphology, overlain on
(gray lines—250 m interval).
of three structural
segments are also
shown. Black arrows indicated
of possible northwest-
folds. B, C: Uninterpreted
and interpreted enlargements
of center of plate
showing location of interpreted
strike-slip faults and
features that they appear
to offset. OSC—overlapping
Figure 3. Models of brittle deformation for Gorda plate overlain on magnetic anomalies modified from Raff and Mason (1961). Models A–F were proposed prior to collection and analysis of full-plate multibeam data. Deformation model of Gulick et al. (2001) is included in model A. Model G represents modification of Stoddard’s (1987) flexural-slip model proposed in this paper.
Discussion and Conclusions
The analysis of the new bathymetric data set, coupled with the correlation of moment tensors and earthquakes locations with mapped features, provides an opportunity to evaluate previously proposed models of Gorda deformation. Several of these models (Riddihough, 1980; Knapp, 1982; Bolt et al., 1968; Figs. 3A–3C) relied on the development of large-scale northwest-trending right-lateral strike-slip faults and rigid block movement in order to account for the more eastward location of the pre–2 Ma magnetic anomalies and variability of observed basement-ridge orientations. On the basis of the bathymetry and available seismic reflection data, there appears to be no fault or lineation that would correspond to such large-scale features. Similarly, no evidence was found to support the nearridge northwest-trending right-lateral shear zone component of the model proposed by Wilson (1989; Fig. 3D). By using both the magnetic anomaly data and regional GLORIA side-scan data, Masson et al. (1988; Fig. 3E) suggested a deformation model driven solely by clockwise rigid-block rotation of the entire plate. Continuous curvature (though kink bands like those of Masson et al.  are present) of the basement ridges, however, can be clearly seen in the bathymetry, requiring a nonrigid-block deformation mechanism. Figure 3F shows a flexural-slip buckling model first proposed by Silver (1971) and later investigated and modified by Stoddard (1987, 1991). In this model the Gorda plate is actively deforming in a flexural-slip faulting style (Yeats, 1986), but in the horizontal plane.
Our data support this type of model, with several stages of deformation (Fig. 3G). The first stage involved reactivation of relict, spreading-center–normal faults inherited from the Gorda Ridge. As crust moved eastward away from the ridge, left-lateral slip occurred along the faults, the ends of which move closer together owing to the nonparallel arrangement of the bounding fracture zones. This squeezing of the Gorda plate under northsouth compression between the older Pacific and Juan de Fuca plates accommodated flexure of the plate into an asymmetrical syncline with a vertical axis, allowing for the observed curvature and kinking of the lineations near the ridge. As deformation continued, the second-generation faults broke across the previously reactivated fabric. Just to the west of the subduction front, where the microseismic ity is at its highest level, outer-rise east-west–bending extension is superimposed on the syncline, and further extension may occur as the flexural slip buckle continues to grow. The reactivated pseudofault accommodates further north-south shortening.
Although many aspects of the deformation still remain unclear, narrowing the field of kinematic models provides an opportunity to gain a better understanding of the relative importance of each of the principal plate-driving mechanisms in creating the observed deformation patterns. Whereas in the past the case has been made for spreading-rate–induced changes in deformation, the preliminary deformation model described here may provide a mechanism for changes in spreading rate driven by the deformation. In other words, the deformation may not have to be driven by spreading-rate changes; rather, the deformation may open accommodation space along the northern Gorda Ridge as a consequence of the internal deformation, allowing a higher spreading rate. The active extensional processes and abundant and seemingly anomalous off-axis volcanism localized in the northern half of the plate suggest that accommodation space is opening at rates that exceed the spreading rate. This conclusion implies (1) that spreading rates along ridge segments associated with deforming microplates are controlled largely by internal deformation rates within the plate, which in essence makes the ridges passive features that respond to the deformation, or (2) that spreading rates reflect an interplay between ridge spreading and the deforming plate.
Investigators: Jason Chaytor, Chris
Goldfinger, Robert Dziak.
Bathymetry compilation and structural interpretation by Jason Chaytor
OSU Active Tectonics and Seafloor Mapping Lab. Earthquake locations from USGS NEIC, centroid moment tensors from Harvard CMT
database (dates are local).
Updated June 20, 2005
In June 2005, several significant earthquakes occurred in the Gorda Plate. On June 14th 2005, a Magnitude 7.2 event, and on June
16th 2005, a Magnitude 6.7 event, both strike slip, occurred. These events were consistent in strike and slip sense with previous Gorda events, and with the models presented by Chaytor et al., 2004.
Preliminary comparison of the
moment tensors to the known structures within the Gorda
plate strongly suggest the earthquakes occurred on a NE-trending
left-lateral strike-slip faults. It appears from the bathymetry
that the causitive faults are buried by sediments and situated along
the trends of mapped strike-slip faults in the epicentral areas.
3-Dimensional view of the Gorda plate shaded-bathymetry looking toward
the northeast from the location
of the Gorda Ridge - Mendocino Fracture Zone intersection, showing the
location of the June 2005
Magnitude 7.2 and 6.7 earthquakes
This research was funded by National Science Foundation grant OCE-9912292 (Goldfinger). Collection of SeaBeam bathymetric data was supportedby the National Oceanic and Atmospheric Administration Vents Program. We thank Sean Gulick and Greg Moore for helpful reviews.
Chaytor, J.D., Goldfinger, C., Dziak, R.P., and Fox, C.G., 2004, Active deformation of the Gorda plate: Constraining deformation models with new geophysical data: Geology, v. 32, p. 353-356.