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At the north end of the San Andreas fault off Cape Mendocino, three lithospheric plates (the Juan de Fuca, Pacific, and North American) meet at the Mendocino triple junction, where a trench meets two transform faults, the San Andreas and Mendocino faults. Along this trench to the north, the Juan de Fuca plate is subducting eastward beneath the North American plate. The geometry of this subducted plate has important implications (Jachens and Griscom, 1983) for an understanding of the Mendocino triple junction and its effects on the tectonics of California. During approximately the past 29 Ma, this triple junction has been migrating relatively northwest-ward along the coast of California from a latitude near Los Angeles (see fig. 3.11; Atwater, 1970; Atwater and Molnar, 1973). As this incipient San Andreas transform fault lengthened over time, eastward subduction continued to the north of the migrating triple junction.

During 29-23 Ma, the major fault of the San Andreas system was probably situated near the base of the continental slope, where an accreted wedge of Miocene (?) sedimentary rocks (McCulloch, 1987) accumulated between lat 35� and 40� N., presumably because of oblique subduction from transpressive forces between the plates. This now-inactive fault forms a contact between oceanIc and continental crusts (fig. 9.4) that have major differences in magnetic properties (fig. 9.3). The oceanic crust displays the typical oceanic lineated or striped magnetic pattern striking north-south and northeast, with interruptions striking east-west or southeast that are caused by transform faults. The continental crust adjacent to this inactive fault is magnetically rather smooth and featureless. The magnetic boundary between oceanic and continental crust west of the San Andreas fault is very abrupt in comparison with active subduction zones (compare the magnetic expression of the Cascadia subduction zone off Oregon in Bond and Zietz, 1987); the oceanIc stripes terminate at the base of the continental slope, even though reflection profiles show oceanic crust continuing farther east beneath the slope (McCulloch, 1987). The low convergence rate of oblique subduction and the time available since the fault became inactive may have allowed the concealed or subducted oceanic crust to heat up sufficiently beneath the continental margin to destroy the remanent magnetization that causes the stripes.

During early Miocene time (23 Ma), the motion along the transform must have been essentially strike slip and was substantially transferred to the present San Andreas fault system in central California. Without subduction east of the elongating transform, an ever-enlarging triangular hole or window (Dickinson and Snyder, 1979) developed in the slab of lithosphere subducted beneath the continent. This window model is also applicable to the time interval (29-23 Ma) but needs modification to include effects of transpression along the earlier San Andreas fault. The north boundary of this window is the subducted south edge of the Juan de Fuca plate, and hot upwelling asthenospheric material presumably occupies the window. The south edge of the Juan de Fuca plate lies beneath the North American plate at the shore about 20 km south of Cape Mendocino and can be identified by an east-west magnetic anomaly (Griscom, 1980a), as well as by the distribution of seismicity (Hutchings and others, 1981). This position coincides with a steep gravity gradient (here called the Cape Mendocino gravity anomaly) that slopes downward into a large gravity low (-50 mGal) to the north and east. The spatial coincidence between the position of the Cape Mendocino gravity anomaly at the coast and the place where the south edge of the Juan de Fuca plate passes beneath the coastline strongly suggests that this gravity anomaly reflects the south edge of the subducted plate (fig. 9.4). At least three other characteristics (Jachens and Griscom, 1983) of the anomaly support this interpretation. (1) The southeastward trend of the gravity anomaly and then its change to easterly are consistent with the directions of present and past relative motions between the Juan de Fuca and Pacific plates (Nishimura and others, 1981; Wilson, 1986). (2) The gravity anomaly broadens and is less steep toward the southeast, suggesting that its source progressively deepens in this direction; calculated depths along the anomaly to the end of the southeast-trending segment define a line plunging approximately 9� SE. with a depth of only 6 km at the coastline corresponding well to the 8-km depth estimated from aeromagnetic data (Griscom, 1980a). (3) A cross section across this anomaly, using the above depths together with reasonable densities and thicknesses for the subducted Juan de Fuca plate and the asthenospheric window fill to the southwest, produces a calculated gravity model (Jachens and Griscom, 1983) in good agreement with the observed gravity field. We draw the following conclusions from the gravity data (Jachens and Griscom, 1983).

1. Above the south edge of the Juan de Fuca plate, the North American plate must have the shape of a thin lip that gradually thickens eastward, attaining a thickness of possibly only about 30 km at the Coast Range fault; this fault marks the east limit of the Franciscan assemblage about 130 km inland from Cape Mendocino (see chap. 3). Just south of the Juan de Fuca plate, asthenospheric material that filled the slab window should lie beneath the North American plate at a depth comparable to that of the upper surface of the Juan de Fuca plate. Because the North American plate has been moving relatively southward across this boundary for many millions of years, the top of the asthenosphere probably is shallow beneath much of the Coast Ranges in central California, and the thin west lip of the North American plate may be decoupled from much of the mantle, although some under-plating by mantle material is likely.

2. For reasons similar to conclusion 1, the lithosphere of southern California near the San Andreas fault system is thin and may be decoupled from much of the mantle.

3. Relatively thin, decoupled lithosphere may explain why deformation along the boundary between the Pacific and North American plates takes place over a zone 50 to 100 km wide rather than being restricted to the San Andreas fault, and why the plate boundary has been able to migrate eastward from the base of the continental slope to its present position at the San Andreas fault. It may also explain both why certain structural blocks southwest of the fault in southern California have been able to rotate clockwise by as much as 70�-90� during and after the Miocene (Luyendyk and others, 1985; Hornaflus and others, 1986) and how extensional basins formed between these blocks. Furthermore, it can help explain why the seismicity of the San Andreas fault generally does not extend below 12-km depth.

4. Thin, relatively cool lithosphere of the southward-moving North American plate has been continuously placed on hot upwelling asthenosphere when crossing the Juan de Fuca plate boundary. As pointed out by Lachenbruch and Sass (1980), this process can explain the heat-flow anomaly in the North American plate that peaks in the Coast Ranges about 300 km south of the latitude of Cape Mendocino (Lachenbruch and Sass, 1973). Calculations by Lachenbruch and Sass (1980) show that, given a velocity of 5 cm/yr for movement of the Pacific plate relative to the North American plate, the heat flow should increase by a factor of 2 approximately 200 km south of the edge of the Juan de Fuca plate because 4 Ma is required for the heat anomaly to reach the surface from 20-km depth. These various parameters agree with the observed heat-flow anomaly. For a heat source as deep as 20 km, the model requires the hot asthenosphere to accrete to the bottom of the North American plate and to be conveyed off southward, so that a continuous supply of vertically moving, hot asthenosphere be supplied to the bottom near the Juan de Fuca plate boundary. This hypothesized coupling involves a rather thin layer of accreting upper mantle that, in turn, is probably decoupled from underlying asthenosphere. The gravitationally predicted depth to the base of the North American plate is within the limits required by Lachenbruch and Sass, (1980) model, at least within 70 km of the San Andreas fault.

Interpretation of geologic and geophysical data for the San Andreas fault system north of San Francisco (Griscom and Jachens, 1989) suggests that eastward migration of the plate boundary from its presumed original position at the base of the continental slope to its present position at the San Andreas transform fault may have occurred by means of a series of eastward jumps of the Mendocino triple junction covering a total distance of about 150 km during the past 29 Ma. Our general model for the history of this triple junction is one of successive eastward jumps, with sustained periods at each position while significant strike-slip motion occurred on the various transform fault systems, including the San Andreas fault. We are aware, however, that the picture in detail may have been far more complex. The present position of the San Andreas fault north of San Francisco is thus interpreted to be relatively youthful. The triple junction was initially situated near the base of the continental slope at the northwest end of the Miocene (?) accreted wedge (but far to the south of its present latitude); the basal fault (McCulloch, 1987) below the subduction complex is shown as a toothed line in figure 9.4 because of the thrust component in this transform fault. The triple junction is interpreted to have been subsequently situated at the north end of the proposed Pilarcitos fault extension and then to have jumped eastward a minimum of about 100 km to the present San Andreas fault trace at what is now approximately lat 38�20' N. on the North American plate. When this jump occurred, the three faults that formed the junction all had to readjust; the simplest scenario is as follows: (1) The Mendocino fault was extended on strike farther eastward, for the distance of the jump, about 100 km; (2) a new segment of the San Andreas fault broke obliquely through the Franciscan assemblage to the northwest (severing the correlated geophysical anomalies described above) and extended from the new triple junction to the junction of the newly formed (or soon to be formed) San Gregorio fault with the Pilarcitos fault, a distance of about 250 km; and (3) the surface trace of the subduction zone north of the triple junction also jumped eastward 100 km, thus abruptly isolating a thin triangular slab of Franciscan assemblage (probably less than 15 km thick) from the North American plate. This postulated triangular slab of rocks is now gone, most likely subducted away. Further complexity is provided by the King Peak subterrane of the King Range terrane (McLaughlin and others, 1982), which is an elongate mass of ???turbidites, about 45 km long, just south of Cape Mendocino (fig. 9.4) that is believed to have been obductively accreted from the west during the early Pleistocene (McLaughlin and others, 1986). The King Peak subterrane may have been detached and transported northwestward from the San Francisco area (just south of lat 38�20' N.) as part of the Pacific plate and then reattached to the North American plate by a very recent local jump of the triple junction westward less than 35 km (McLaughlin and others, 1982); this explanation may account for the anomalously higher thermal metamorphism of this subterrane relative to the terranes that are now adjacent to it. Recent work suggests that the triple junction may be on shore at Cape Mendocino (Clarke, 1988; McLaughlin and others, 1988); if so, the King Peak subterrane may still be essentially part of the Pacific plate. The tectonic interpretation detailed above also requires that the San Gregorio-Hosgri fault first began moving and joined the present San Andreas fault at approximately the same time as or shortly after the eastward jump of the triple junction, and thus cut off the proposed northward extension of the Pilarcitos fault, after which the extension became inactive.

The proposed 150-km eastward movement of the triple junction can also explain the submarine topography near Cape Mendocino, where the Continental Shelf south of the Mendocino fault extends about 130 km farther west than that directly north of the fault. The timing of the jump can be estimated from the horizontal offset of the paired geophysical anomalies, about 250 km, which translates to an age of about 5 Ma, assuming combined strike-slip rates of 4.8 cm/yr (DeMets and others, 1987; Minster and Jordan, 1978) for the San Andreas and San Gregorio faults. This age estimate is crude because it assumes that no other faults were absorbing the relative motion between the two plates. For example, simultaneous movement on the Hayward-Calaveras fault system will cause the computed age of offset to be too young. The eastward jump of the triple junction appears to be associated with a change in stress orientations in this region. The north end of the San Gregorio-Hosgri fault trends about 20� clockwise relative to the older fault traces. In addition, the northward-migrating triple junction subsequently traced out a major right-lateral bend, as shown by the present position of the San Andreas fault north of Point Arena. The central section of this bend is about 100 km long and trends 20� clockwise to the older trace. This change may correlate with the gradual change in absolute motion of the Pacific plate that occurred between 5 and 3.2 Ma (Cox and Engebretson, 1985; Pollitz, 1986), producing a change from strike slip to transpression in this region and a clockwise rotation of 20� (Harbert and Cox, 1986) in the relative-velocity vector for the plate pair, the same angle as the anomalous change in direction for both the San Gregorio fault and the right-lateral bend in the San Andreas fault north of Point Arena. This change in relative motion probably correlates with a change in strike direction of the subducting south edge of the Juan de Fuca plate, as deduced from gravity data (Jachens and Griscom, 1983). Before the jump, this strike was east-west, thus permitting eastward extension of the Mendocino transform fault without interference; after the jump, the strike of the subducting plate edge changed to S. 60� E., making later eastward fault extension more difficult.

Stratigraphic evidence for the postulated eastward jump of the triple junction about 5 Ma may be sought in the late Miocene and Pliocene stratigraphy of Deep Sea Drilling Project (DSDP) Site 173 (fig. 9.4). Depositional hiatuses occur at 5 and 4.3-3.2 Ma (Barron, 1989), whereas a study of both micropaleontology and tephra beds indicates a hiatus from about 4.4 to 2.8-Ma (Sarna-Wojcicki and others, 1987). McCulloch (1987, fig. 25) believed that the middle Pliocene deformation and minor erosion interpreted from reflection profiles correlate with this 4.4-2.8-Ma hiatus at Site 173. We suggest that the eastward jump of the triple junction about 5 Ma was shortly followed by the middle Pliocene deformation and by the hiatus at Site 173. These two correlative events were thus caused both by the jump and by the simultaneous change in the direction of relative plate motion.