History and present configuration
In the early Oligocene (30 Ma) the Farallon plate was subducting beneath North America. The Mendocino Triple Junction was originated when the north-eastward moving Pacific plate reached the North America boundary. Before early Miocene time (~20 Ma), there was strike slip motion along the San Andreas Fault , as well as northward migration of its northern termination (the MTJ). It is believed that the triple junction was along the Pioner Fracture Zone (Benz and Zandt, 1993), which is south of and pretty much parallel to the Mendocino Fracture Zone. The north piece of the subducting Farallon Plate west of North America was now the Juan de Fuca Plate. Transform faulting across the spreading center at the west boundary of the Juan de Fuca plate gave origin to several subplates.
At about 26 Ma, the spreading center between the Pioneer and the Mendocino fracture zones reached the trench. Given the impossibility for this spreading center of keeping generating crust beneath the North American plate, a gap in the slab, referred from now on as the slab window, was originated. This slab window is believed to be filled by upwelling of hot asthenospheric material.
One of the subplates originated by the breakup of the Farallon Plate around 52 Ma was the Monterey Plate (Atwater and Stock, 1998). This plate separated from the Juan de Fuca Plate at about 28 Ma, moving south east away from it and being later attached to the Pacific Plate when the Pacific-Monterey spreading center stalled at about 18 Ma (Atwater and Stock, 1998). So, part of the slab window in the area near the MTJ is due to the northward migration of the MTJ and the south-east movement of the Monterey Plate beneath North America (Figure 3). The fact that the Monterey plate got attached to the Pacific plate originated a decrease in its North-America-perpendicular velocity, and as a consequence, part of the slab window could be east of the Monterey plate beneath North-America.
Figure 3 (Modified from Atwater and Stock, 1998 and Dickinson, 1997): Cartoon of an inferred map view of the plates converging in western U.S.. The red hatched area denotes undersea regions. The North-American plate is not represented here, so areas to the right of the edge of the red region are under it. The black hatched area is the slab window, showing the gaps generated by the entrance of the spreading ridge in the subductionzone and the separation of the Monterey Plate from the Juan de Fuca Plate. The slab window could also be extended far south, due to a decrease in the velocity of the motion of the Monterey and Arguello plates with respect to the farallon plate.
In the present configuration, the Gorda Plate is a fragment of the Juan de Fuca
plate between the Mendocino and the Blanco fracture zones.
In addition to the northward migration of the MTJ due to the northward motion
of the Pacific Plate, there is also an eastward migration of the triple junction
due mainly to the lack of colinearity between the San Andreas Fault and the
Cascadia Trench. In the search of a more stable configuration, the MTJ jumps
eastward (Figure 4). The triangular wedge that results from this jump (figure
4) was part of North America (The Franciscan assemblage), but now is part of
the Gorda Plate and it is also subducted under the continent.
Figure 4 (Modified from Smith et al, 1993). Eastward Migration of the MTJ. A triangular wedge of continental material is being subducted. Arrows show the plate motions.
In the northern part of the San Andreas Fault, the interplate motion is accommodated
by strike-slip along the nearly vertical fault, and at depth shear should be
accommodated in the slab window. Further south, the deep plate boundary is believed
to be east of the San Andreas Fault, within the mantle underneath the Hayward
– Calaveras fault zone. So, in the vicinity of the San Andreas Fault and
40-60 km to the east, material of the Pacific Plate should be under the continental
North American Plate.
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