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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.

Data from the Coast Range ophiolite and its tectonic outliers in the northern California Coast Ranges suggest that the lower part of the ophiolite formed 169 to 163 Ma in a forearc or back arc setting at equatorial latitudes. Beginning about 156 Ma and continuing until 145 Ma, arc magmatism was superimposed on the ophiolite, and concurrently, a transform developed along the arc axis or in the back arc area. Rapid northward translation of this rifted active magmatic arc to middle latitudes culminated in its accretion to the California margin of North America at about 145 Ma. This Late Jurassic episode of translation, arc magmatism, and accretion coincided with the Nevadan orogeny and a proposed major plate reorganization in the eastern Pacific basin. The high rate of poleward motion necessary to translate the Coast Range ophiolite to middle latitudes during this time implies that the ophiolite traveled north on a fast‐moving plate of the eastern Pacific basin, here termed plate X. Plate X probably was driven by a cryptic ridge east‐northeast of the Pacific‐Farallon‐Izanagi ridge triple junction. Structural relations indicate that following Late Jurassic time, parts of the Coast Range ophiolite were displaced from the west side of the Great Valley province and incorporated into the Central belt of the Franciscan Complex along steep‐dipping to low‐angle reverse faults having dominant components of dextral shear. A northwest trending eastern zone of these right‐laterally displaced outliers shows strong affinities to the main Coast Range ophiolite of the northwestern Sacramento Valley (the Elder Creek terrane), in that the outliers include ophiolitic breccias of Oxfordian to Kimmeridgian age. A southwestern zone of outliers lacks ophiolitic breccia and instead includes latest Oxfordian or Kimmeridgian to Tithonian, arc‐derived volcanic rocks like those found in the Del Puerto and Stanley Mountain terranes of the main ophiolite. Whereas outliers of the northeastern outlier zone are right‐laterally displaced no more than 260 km from the western side of the Sacramento Valley, outliers of the southwestern zone are displaced a minimum of 169 to 249 km. This displacement occurred between about 60 and 52 Ma. Ophiolitic rocks in the Decatur terrane of western Washington that have recently been correlated with the Coast Range ophiolite and the Great Valley sequence of California were apparently displaced at least 950 to 1200 km from the west side of the Great Valley between early Tertiary and Early Cretaceous time. Derived rates of northward translation for the ophiolite outliers in California are in the range of 1 to 4 cm/yr. Rates for the Decatur terrane are in the range of 2.5 to 4 cm/yr if translation was initiated 90 Ma, but as much as 11.9 to 15 cm/yr if it was not initiated until 60 Ma. The lower rates for the Decatur terrane are consistent with the rates derived for the California outliers and with the northward component of relative motion between the Farallon and North American plates from 90 to 50 Ma. The higher rat...

East of the Aleutian trench, a linear magnetic high extends for 500 km along the continental slope of Alaska. The high also extends 220 km to the west of the trench and is here interpreted to be subducted beneath the Alaska continental shelf. Based upon new magnetic data near the trench, recontoured older data to the southeast, plus dredged samples and magnetic model studies, we interpret the anomaly to be caused by the thickened south edge of a relatively flat slab of highly magnetic, Eocene, formerly oceanic crust. Depth calculations on the anomaly show that this subducting continental margin floored by Eocene crust is down-warped to the west at the Aleutian subduction fault by a local monocline having vertical offset of about 4 km and probably striking about N30°E.

Interpretation and modeling of the magnetic field of central California indicate that the magnetic basement of the forearc deposits of the Great Valley sequence extends westward beneath the coeval subduction-related rocks of the Franciscan Complex. The basement surface slopes gently to the west, reaching midcrustal depths (15-19 km) at distances of 50-100 km west of the Great Valley. This magnetic basement is disrupted by the Hayward-Rodgers Creek Fault system and is cut by the San Andreas Fault at the south end of the Great Valley and possibly throughout much of central California. The widespread presence of the Great Valley basement beneath rocks of the Franciscan Complex implies that the basement is more extensive than proposed in earlier interpretations based on seismic studies near the Franciscan Complex-Great Valley sequence contact. This result forces major modifications to ideas concerning this fossil subduction complex and other subduction zones. The eastern boundary fault of the Franciscan Complex (Coast Range Fault) is not (and never was) a subduction zone thrust fault but rather was originally a roof thrust (wedge-roof fault) formed above the eastward wedging mass of Franciscan Complex intruded along the top of the basement beneath the Great Valley deposits. This tectonic interpretation offers a solution for the question of how high-pressure metamorphic rocks of the Franciscan Complex were juxtaposed at the Coast Range Fault against low-pressure metamorphic rocks of the Great Valley sequence. This interpretation also implies an older flat-lying thrust fault (wedge-floor fault) that forms the top of magnetic basement between the active San Andreas and Hayward Faults at depths of 15-17 km. This older thrust fault may today transfer strain between the two young strike-slip faults, possibly explaining the apparent coupling of major nineteenth century earthquakes on these two faults. The former east dipping subduction zone along which the rocks of the Franciscan Complex accumulated must lie west of the western limit of the Great Valley magnetic basement. across the Coast Range Fault or successor faults [Bailey et al., 1964; Jayko and Blake, 1986] for a distance of more than 500 km (Figure 1). In early applications of plate tectonic principles to continental geology this tectonic contact was interpreted as a fossil subduction zone fault of composite slip associated with the eastward subduction of Pacific Ocean crust beneath western North America [Hamilton, 1969; Ernst, 1970] or as a great thrust fault along which rocks of the Franciscan Complex were dragged below rocks of the Great Valley sequence [Bailey et al., 1970]. Both interpretations implied an east dipping thrust fault (the Coast Range Fault, now locally modified by later fault activity [Jayko and Blake, 1986]) at the structural top of the Franciscan Complex. This fault was believed to extend well into the mantle and to truncate the westward subsurface extent of the Great This paper is not subject to U.S.