Three‐dimensional geometry of the Gorda Plate beneath northern California

Abstract

The isostatic residual gravity field over northern California displays a gravity gradient interpreted to reflect the south edge of the Gorda plate where it is subducted eastward beneath the North American plate. The locus of points of maximum slope defines a line trending S60°E from a coastal point approximately 20 km south of Cape Mendocino, a point where the buried plate boundary is inferred from magnetic and seismicity data. Southeast from the coast for a distance of 120 km the gravity anomaly parallels the strike of the Blanco fracture zone and the present direction of relative motion between the Pacific and northern Gorda plates. Calculations from the form of the anomaly yield depth estimates that fit an east–southeast plunge of approximately 9° for the top of the Gorda south edge. The sense of the anomaly (higher gravity to the south) supports the hypothesis that a window developed in the subducted slab east of the San Andreas fault and south of the Gorda plate. South of the Gorda boundary the base of the North American plate is thus in contact with hot material from the asthenosphere that invaded the window. Because the overlying North American plate has been moving relatively south across the Gorda boundary, the North American plate beneath the Coast Ranges east of the San Andreas fault in central California may be decoupled from the underlying material at a depth slightly deeper than the depth to the top of the boundary at the time the North American plate passed over it.

Seismotectonics of the San Andreas Fault System between Point Arena and Cape Mendocino in northern California: Implications for the development and evolution of a young transform

“…Jachens and Griscom [1983] show that the top of the Gorda slab is at about 7-8 km depth beneath Cape Mendocino and deepens to a depth of 20 km, about 120 km to the southeast near where the Bartlett Springs fault terminates (Figure 1). Interpretation of the gravity data for slab position east of the Bartlett Springs fault is difficult because the signature of the slab is masked by the overlying Great Valley Sequence [Jachens and Griscom, 1983] Source regions for which 1-D velocity models and related station delays were developed include the Maacama and the Bartlett Springs fault zones, regions east and west of the Bartlett Springs fault zone, the northern termination of the Maacama and Bartlett Springs faults, the Point Arena and coastal areas, and the Cape Mendocino region (Figure 2). Each source region was divided into specific subregions to better image the spatial variation in seismicity within the study area [Hill et al, 1990] and to model potential east-west variations in the mineral/textural assemblages of the Franciscan Complex [Jennings et al, 1977].…”

Seismicity and crustal structure at the Mendocino triple junction, Northern California

“…This southeasterly trend reflects the direction of Gorda plate motion parallel to the Blanco Fracture zone while the change in strike further inland correlates well in time" with the change of plate motion described in chapter 2.5. An eastward dip of approximately 10°for the downgoing slab is reported by various studies [Jachens and Griscom, 1983;Cockerham, 1984;McPherson, 1989;Smith et al, 1993;Verdonck and Zandt, 1994]. Further inland, the downgoing slab was imaged at depths up to 270 Km [Benz et al, 1992], with remnants of the ancestral Farallon plate even as deep as 400 Km [Romanowicz, 1980].…”

“…Furthermore, a southward decrease in spreading rates along the Gorda ridge [Rlddihough, 1980;Wilson, 1989] and decreasing rates in time [Wilson, 1986] define deviations from rigid plate motions. As a consequence of these changes, a [Jachens and Griscom, 1983;Wilson, 1986;McPherson, 1989;Smith et al, 1993], the space-problem is accommodated within the Gorda plate since it is the youngest and, thus, weakest within the system.…”

“…The Coast Ranges are underlain by the Franciscan Complex, an uplifted accretionary wedge that formed during the Mesozoic and early Tertiary [Jachens and Griscom, 1983;Castillo and Ellsworth, 1993]. Forearc metasedimentary and metavolcanic rocks, granitic plutons and Tertiary volcanic rocks are typical for the Coast Ranges and Klammath Mountains [Benz et al, 1992].…”

“…Black dashed line shows the aftershock zone of the 1980 Eureka earthquake. Red dashed line is the surface projection of the southern edge of the Gorda plate [after Jachens and Griscom, 1983]. Note that seismicity west of 235°is not complete (see text for explanation).…”