Miocene ridge impingement and the spawning of secondary ridges off Oregon, Washington and British Columbia

Tectonophysics, 69 (1980) 321-348 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

321

MIOCENE RIDGE IMPINGEMENT AND THE SPAWNING OF SECONDARY RIDGES OFF OREGON, WASHINGTON AND BRITISH COLUMBIA EDWARD FARRAR Department (Canada)

and JOHN M. DIXON

of Geological Sciences,

(Received August 2, 1979;revised

Queen’s University, Kingston, Ont. K7L 3N6 version accepted April 25,198O)

ABSTRACT Farrar, E. and Dixon, J.M., 1980. Miocene ridge impingement and the spawning of secondary ridges off Oregon, Washington and British Columbia. Tectonophysics, 69: 321-348. The East Pacific Rise is the only extant portion of the South Pacific Spreading Center, which began to impinge on the North American plate margin -30 Ma ago. The North Pacific Spreading Center, offset -1500 km to the west on the Mendocino transform fault, remained active until -9 Ma ago when it, too, impinged on the trench off the OregonWashington margin. The zones of mantle upwelling associated with the spreading centers have since migrated inland to their present sites beneath the Cascade and Garibaldi volcanic chains and the Rio Grande Rift, maintaining the - 1500 km offset that they have had since the Early Tertiary. Impingement of the North Pacific Spreading Center on the trench at anomaly 5 time juxtaposed the Pacific and American plates and the continuing relative motion between these plates across the northsouth boundary has been accommodated by secondary spreading at the Gorda-Juan de Fuca-Explorer ridge system. These obliquely spreading ridges, fed by lateral mantle flow originating at the zone of mantle upwelling beneath the Cascade belt, are migrating away from the North American coast at half the Pacific-America velocity, and no subduction is occurring along the continental margin, the Juan de Fuca sea floor being an integral part of the American plate. This scenario is consistent with the fact that the sea floor along the continental margin has the same age as that which bears the last northsouth anomaly on the Pacific plate (anomaly 5) preceding initiation of spreading at the Juan de Fuca and Explorer ridges (a fortuitous coincidence in models involving continued subduction of the Juan de Fuca sea floor). This model implies that the rate of motion between the Pacific and American plates is 8 cm/a. The Blanc0 fracture zone formed as a transform fault at the south end of the Juan de Fuca ridge and was initially collinear with the North American margin south of Cape Mendocino. The North Pacific Spreading Center impinged on this portion of the margin somewhat later than north of Cape Mendocino, and the Gorda ridge began to migrate northwestwards. Interaction among the Pacific plate south of the Mendocino fracture zone, the American plate north of the Blanc0 fracture zone, and the growing Gorda plate caused clockwise rotation of the Gorda plate, faulting of the Juan de Fuca sea floor and counterclockwise rotation of the Blanc0 fracture zone. The mantle upwelling that was formerly the site of the North Pacific Spreading Center is now situated beneath Oregon, Washington and western British Columbia and is responsible for high heat flow, basaltic volcanism, low P, velocities, high upper mantle conductivity and crustal rifting in this region. 0040-1951/80/0000~000/$02.25

@ 1980 Elsevier Scientific Publishing Company

322 INTRODUCTION

The Pacific lithospheric plate has been formed by sea floor spreading at a ridge-transform system which we will call the Pacific Spreading Center (PSC). For convenience we subdivide it into the North Pacific Spreading Center (NPSC), north of the Mendocino transform fault, and the South Pacific Spreading Center (SPSC), south of it. The East Pacific Rise, south of the Clarion fracture zone, is the present-day remnant of, the SPSC, and the Gorda, Juan de Fuca and Explorer ridges of the northeast Pacific (Fig. 1) are

PIONEER

F.Z.

Fig. 1. Map showing locations of spreading ridge crests and fracture zones in the northeast Pacific. Shading on the Pacific Ocean shows the distribution of terrigenous mud as near-surface sediment (Scripps, 1972). Solid circles show earthquake epicenters for the period 1954-1963 north of 40’N (Tobin and Sykes, 1968) and for the period 1965-1969 south of 40’N (Bolt and Miller, 1971). Large asterisks on North America give positions of the High Cascade volcanoes, and small asterisks mark smaller Quaternary volcanoes (King, 1969).

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commonly regarded to be remnant segments of the NPSC that separate the Pacific plate to the west from the last remaining fragments of the Farallon plate to the east (Menard and Atwater, 1968). The orientation and spreading direction of the Gorda, Juan de Fuca and Explorer ridge segments are interpreted to differ from those of the East Pacific Rise and of the earlier NPSC because the Farallon plate broke up after impingement of the PSC on the North American plate margin (ca. 30 Ma ago, Atwater, 1970). The Gorda, Juan de Fuca and Explorer plates are generally assumed to be actively subducting beneath the margin of North America, but the evidence for this subduction (summarized by Riddihough, 1978) is equivocal. We suggest that the NPSC has already impinged on the North American plate north of Cape Mendocino. Since the time of impingement the site of mantle upwelling has migrated northeastward beneath the North American continent, and secondary oceanic ridges, the Gorda, Juan de Fuca and Explorer ridges, have been reflected from the continental margin and are presently migrating northwestwards relative to North America. We regard the Gorda, Juan de Fuca and Explorer “plates” to be part of the American plate. Evidence in support of these contentions is presented below. The model is presented in a somewhat idealized form, with the assumption that changes in plate interaction occur instantaneously. It is intended as a working hypothesis that may serve as the basis for further study. ( THE HYPOTHESIS

One of the earliest observations connected with the hypothesis of sea floor spreading is that the offset between two ridge segments on a transform fault remains approximately constant through time (Wilson, 1965a). The magnetic anomaly pattern of the northeast Pacific (Atwater and Menard, 1970) indicates that the Pacific Spreading Center maintained a relatively constant configuration through much of the Tertiary (Fig. 2). Such changes as do occur tend to be minor and slow, such as, for example, the annealing of the Surveyor fracture zone from an offset of 150 km in a period of 15 Ma (Shih and Molnar, 1975). The Pacific Spreading Center can be represented by a generalized form consisting of two north-south segments, the NPSC and the SPSC, joined by the east-west Mendocino transform fault whose offset was -1500 km at least from anomaly 21 time (53 Ma) to anomaly 5 time (9 Ma) (Fig. 2b). We regard the mantle upwelling associated with this system to have a comparable offset but a less rectilinear geometry. The mantle upwelling that drives and feeds the spreading plates could consist of an offset series of mantle plumes or a smooth mantle roll. The overriding of the SPSC by North America (Fig. 2) beginning - 30 Ma ago, has had little influence on the overall shape of the spreading center-transform system. We ascribe this long-term stability of the PSC system to its situation above a site of mantle upwelling

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associated with deep convection (e.g., Cathles, 1975; Elsasser et al,, 1979). We have postulated that this deep-seated upwelling would persist for some time even if a continent should override it, and we have discussed elsewhere the evidence for and consequences of the interaction between the North American plate and the southern segment of the PSC in the light of this postulate (Dixon and Farrar, 1978a, b, 1979, 1980). We concluded that the site of mantle upwelling has migrated northeastwards beneath North America during the past 30 Ma to its present location beneath the Rio Grande Rift, which is collinear with the undisturbed portion of the SPSC south of the Clarion fracture zone. If the mantle upwelling associated with the PSC maintained the offset indicated by magnetic anomalies in the Pacific, the NPSC must have migrated relative to North America in concert with the SPSC. Using Atwater and Molnar’s (1973) global reconstruction of the position of the Pacific plate (and

325

1 MENDOCINO

F.Z.

1

PREAENT

Fig. 2. (a) shows the present day magnetic anomaly pattern of the northeastern Pacific (after Atwater, 1970, and Pitman et al., 1974). Anomalies 21 (53 Ma), 13 (38 Ma) and 5 (9 Ma) are indicated by heavy lines to emphasize the constancy of the offset between the northern and southern segments of the Pacific Spreading Center across the Mendocino transform fault through most of the Tertiary. The overall offset (-1500 km) between these segments (and overall offset of the deep mantle upwelling) is shown for these times in generalized form (dashed lines) in (b). The present position of the site of mantle upwelling (short dashed line) has been estimated on the assumptions that (1) the southern segment should be collinear with the undisturbed Pacific Spreading Center south of the Clarion fracture zone, and (2) the offset between the southern and northern segments has remained - 1500 km. This places the northern segment beneath western Oregon and Washington and central British Columbia. Note that (b) does not show the past trajectory of the ridge-transform system relative to North America; rather, it is intended to show only its past shapes and present location.

the SPSC) relative to North America, and assuming that the offset between the NPSC and SPSC remained constant, we estimate that the NPSC would have impinged on the North American margin at approximately anomaly 5

326

b.

Fig. 3. Schematic representation in plan and cross-section of three stages (a, b, c) of the overriding of a spreading ridge by a continent. a. Oceanic plates A and B are spreading from ridge R (and its underlying mantle upwelling /.I) with velocities RVA and RVB (see vector diagram). The ridge R and the mantle upwelling site are approaching the continent with velocity CVR. The movement of a given point (P) on the mantle upwelling site, relative to the continent, is shown by the heavy arrow in the plan view. Completion of the vector triangles gives the velocities of oceanic plates A and B relative to the continent C. Thus plate I3 is being subducted beneath C with velocity CVB and plate A is moving obliquely from C with velocity CVA. The ruled lines on plates A and B in plan view are drawn parallel to these directions. The curved arrows in the cross-section show schematically the mantle upwelling @I) beneath the ridge. b. The ridge and mantle upwelling site have reached the trench and point P continues to move in the same direction (cV, and dashed arrow). Plates A and C have been juxtaposed as a result of consumption of plate B. Plate A continues to move in the direction CVA outward from the margin of plate C and a secondary spreading centre (r) is initiated to fill the void. An additional increment of relative movements is depicted in (c). The site of mantle upwelling (/J) has migrated beneath C. The point p on the secondary ridge P has moved away from the C margin with velocity cV, = (CVA/2): its spreading is fed by lateral flow of hot mantle material that originates at /_Iand passes beneath plate C including its accreted oceanic portion (shown in lighter stipple). The portion of plate A that has accreted as a result of secondary spreading at r is shown in lighter ruling. As viewed from plate C, ridge R has approached the margin and ridge r has retreated. The oceanic spreading center thus would appear to have been “reflected” by the continental margin.

327

to 4A time. It would presently lie inboard of the continental margin of northern California, Oregon, Washington and British Columbia (Fig. 2b). The commonly accepted view is that the NPSC is represented by the Gorda, Juan de Fuca and Explorer ridges and that the oceanic “plates” east of these ridges are remnants of the Farallon plate that are currently being subducted beneath North America (e.g., Atwater, 1970; Riddihough, 1977). If this were the case then the offset between the NPSC and the SPSC would have had to increase from _ 1500 km to -2000 km since anomaly 5 time (e.g. Atwater and Molnar, 1973, Fig. 1). We consider this improbable as we assume that mantle convection is deep seated and unlikely to be instantaneously affected by changes in surface plate geometry. Rather, we suggest an alternate explanation which we illustrate schematically in Fig. 3. Our model of the interaction between the NPSC and the North American continental margin north of Cape Mendocino is generalized in Fig. 3. In this treatment we allow a straight segment of a spreading ridge, R, between two oceanic plates, A and B, to impinge obliquely on a straight margin of a continental plate, C. We use a linear ridge and continental margin because complications arise if the ridge is offset by transform faults (Dixon and Farrar, 1980) and if the continental margin is irregular (see below, section on details of plate interaction). In Fig. 3a, the ridge R, which is underlain by a mantle upwelling site, 1-1,is generating plates A and B with half-spreading rate RVA (= -RVz), and the ridge is approaching the continent with velocity cV,. Completion of the vector triangles gives the subduction velocity (cVB) of plate B beneath the continent and also the velocity of plate A with respect to the continent (cV,). In this example the vector triangles have a geometry similar to that of the relative motions among the Pacific, Farallon and American plates and the NPSC, with =V, representing the northeasterly approach of the PSC towards North America (Atwater and Molnar, 1973) and cV, corresponding to the northwestward relative mqtion between the Pacific and American plates. In the schematic diagram and for the north-south linear coastline of Oregon and Washington, cVA is directed outward from the continental margin. When ridge R arrives at the subduction zone along the C margin, plates A and C are juxtaposed (Fig. 3b). Continuation ‘of the relative plate motions leads to the situation (Fig. 3c) where the continent has overriden the site of mantle upwelling, 1-1,and plate A has moved away from plate C with velocity cV,. The void produced by the separation of plates A and C is filled with material derived by lateral flow from the site of mantle upwelling (p). We have termed this process secondary spreading, exemplified by the spreading in the Gulf of California (Dixon and Farrar, 1978a, 1980) because it is not underlain by a site of deep mantle upwelling. As equal amounts of oceanic crust will accrete to the trailing edges of both plates A and C, symmetric spreading will occur and the secondary ridge r will migrate away from the continental margin at velocity cV, (=cVA/2). As viewed from the continent it would appear as if the oceanic ridge system has been “reflected” by the continental margin.

328

We suggest that the NPSC impinged on the Oregon-Washington coast 8-9 Ma ago, juxtaposing the Pacific and American plates, and spawned the Gorda, Juan de Fuca, and Explorer ridges which are reflected, secondary spreading centers that accommodate the continuing Pacific-America motion by accreting oceanic lithosphere to the trailing edges of these plates. Thus we postulate that the Juan de Fuca “plate” is actually part of the American plate, having been passively accreted to its western margin during the past 8-9 Ma. The mantle upwelling formerly associated with NPSC has been overridden by North America and now lies beneath the Cascade and Garibaldi volcanic belts. We suggest that the arrival of the NPSC at the trench off the Pacific northwest occurred about 8-9 Ma ago for three reasons. First, anomaly 5 (8.610.0 Ma, LaBrecque et al., 1977, as modified by Harrison et al., 1979) is the youngest anomaly on the Pacific plate west of the Juan de Fuca-Explorer ridges with the same northsouth trend that is characteristic of the earlier Tertiary anomalies on the Pacific plate (e.g., Barr and Chase, 1974). Subsequent anomalies trend progressively more northeasterly (see Fig. 4) because they reflect spreading between the Pacific and North American plates, albeit at an initially highly oblique angle (see below). Second, our interpretation of Atwater and Molnar’s (1973) Pacific-America relative motion suggests that the NPSC was situated at the trench at anomaly 4A-5 time (see above). Third, the sea floor immediately adjacent to the OregonWashington continental margin bears anomaly 5. We regard ridge reflection through secondary spreading to be an alternative mechanism to eduction (Dixon and Farrar, 1978a, b, 1980) for the accommodation of plate separation following ridge impingement. Eduction is the one-sided emergence of lithosphere from beneath the continent at the continental margin and was an important mode of accommodation of plate divergence following the overriding of the SPSC by the North American plate (Dixon and Farrar, op. cit.). SUPPORTING

ARGUMENTS

(1) Coincidence margin

of anomaly

5 with

the

Washington-Oregon

continental

The magnetic anomaly pattern in the northeastern Pacific (Raff and Mason, 1961), as interpreted by Vine and Wilson (1965), Wilson (1965b) and Vine (1966) (Fig. 4a), shows that there was a change in the direction and a decrease in the rate of spreading in the northeast Pacific at the time of anomaly 4A-5 (-8-9 Ma ago). We associate this change with the switch from NPSC spreading between the Pacific and Farallon plates to AmericaPacific secondary spreading on the Gorda-Juan de Fuca-Explorer ridge system. The magnetic anomaly pattern also shows that the sea floor immediately offshore from Washington and Oregon was created during anomaly 5

329

Fig. 4. Pacific-America plate reconstruction for the past 8 Ma based on the Raff-Mason (1961) magnetic anomaly pattern and Silver’s (1971a) interpretation of faulting in the Cascadia basin, with the time scale modified according to LaBrecque et al. (1977) and Harrison et al. (1979). (a) shows the present-day anomaly pattern for part of the northeast Pacific (modified after Silver, 1971a).

1

135”

I

130”

I

125”

Fig. 4b.

In (b) the effect of left-lateral strike-slip faulting in the sea floor of the Cascadia basin has been removed by aligning magnetic anomalies of equivalent age (Silver, 1971a). Thus (b) represents the pattern that would exist had this faulting not occurred. This reconstruction indicates that if wholesale subduction has not occurred beneath the Pacific northwest in the past 8 Ma, the initial trend of the Blanc0 fracture zone was clockwise from its present orientation. The fracture zone (indicated by heavy stipple pattern) was collinear with the North American margin south of Cape Mendocino, and parallel to the Pacific-America relative motion direction.

331

c.

-.---PRESENT POSITION

OF

COASTLINE FLOOR

(Ma.1

86-

(Fig.rlc)

W””

Fig. 4c. (c) depicts the relative positions of the American and Pacific plates at 8 Ma ago (about the time of ridge impingement). To arrive at this reconstruction we have removed ocean floor younger than anomaly 4A, keeping the strip of anomaly 5--4A age in the Cascadia basin fixed relative to North America (i.e. no subduction) and moving it northwestwards in direction of the Queen Charlotte and San Andreas faults until it is juxtaposed with the anomaly 4A strip on the Pacific plate. The dashed arrows represent the movement of North America relative to the Pacific plate in the last 8 Ma and secondary spreading at the Explorer and Juan de Fuca ridges has formed new sea floor in the resulting gap. The Pacific--America rate suggested by this reconstruction is 8 cm/a. The Gorda plate has been omitted from the simplified reconstruction and is dealt with in Fig. 6.

332

time. The two bands of ocean crust marked by anomaly 5 trend northsouth, parallel to older anomalies in the Pacific, whereas younger anomalies have formed with a progressively more northeasterly orientation. The prevailing interpretation of the magnetic anomaly pattern is that as the NPSC approached North America the subducting Farallon plate broke up and its subduction velocity decreased so that spreading at the NPSC was slowed and reoriented after anomaly 5 time. If this interpretation is correct then it is a remarkable coincidence that anomaly 5, rather than any other anomaly, is presently poised above the subduction zone. We regard the coincidence of anomaly 5 and the continental margin to be a natural consequence of our ridge subduction and secondary spreading model, and outline our reasons below. A reconstruction (Fig. 4) of the sea floor and plate geometry of the northeast Pacific that is consistent with our model can be achieved as follows. First, the effect of two Plio-Pleistocene northeast-trending left-lateral strike slip faults in the Cascadia basin (Silver, 1971a) is removed by realigning the magnetic anomalies of equivalent age. Figure 4b thus represents the configuration that the Cascadia basin would have, had this faulting not occurred. In the restored configuration, the Blanc0 fracture zone (shown by stippled band in Fig. 4b) is collinear with the San Andreas fault trend (and parallel to the Pacific-America relative motion direction). The plate configuration at anomaly 4A time (Fig. 4c) can be produced by removing younger oceanic crust and, on the assumption of no subduction beneath the North American margin, closing the gap by moving North America and its attached oceanic crustal sliver of anomaly 5 age to the northwest along the San Andreas-Blanc0 and Queen Charlotte faults until the two bands of anomaly 4A are juxtaposed. The dashed arrows in Fig. 4c give our suggested d~placement of the American plate relative to the Pacific plate since anomaly 5, at which time the NPSC would have been adjacent to the trench. The Pacific-America rate suggested by this reconstruction is -8 cm/a. This figure is in conflict with the commonly assumed values ranging from 5.5 to 6.0 cm/a determined from magnetic anomalies in the mouth of the Gulf of California (Larson et al., 1968), an indirect global reconstruction (Atwater and Molnar, 1973), and somewhat contradictory geological evidence from the San Andreas fault in California (reviewed by Graham and Dickinson, 1978). The Juan de Fuca ridge and the San Andreas fault are approximately equidistant from the Pacific-America Euler pole, so their rates should be directly comparable. The instantaneous rates of -5.7 cm/a calculated by Minster and Jordan (1978) and by Chase (1978) rely heavily on the Larson et al. (1968) value and cannot be considered as independant estimates. The conflict between the rates may result from several possible sources. First, the Pacific-America rate of Larson et al. (1968) is based on the past 4 Ma of spreading in the Gulf of California. If the relative motion from 9 to 4 Ma were greater, the possibility exists that the average rate could have been 8

333

cm/a. This possibility is supported by Riddihough’s (1977) determination that the spreading normal to the Juan de Fuca ridge decreased - 15 to 20% about 4 Ma ago. This, together with our contention that the earlier spreading was more oblique, allows an average rate that is close to 8 cm/a. Second, it is possible that the reflection of the NPSC could have occurred over a finite interval of time rather than at an instant. If this were the case the remnant Farallon plate would have only gradually adhered to the American plate and the Pacific-America velocity required by such an interaction would be less, perhaps in accord with the published estimates. We point out that the lower published rate, if correct, implies that there has been subduction of the Juan de Fuca “plate” beneath the American margin and that the rate of this subduction has coincidentally been just sufficient to bring crust of anomaly 5 age to its present position at the “trench”. On the other hand, our higher rate explains the present location of this anomaly 5 crust without the necessity of coincidence, and is also consistent with the lack of evidence for presently active subduction along the OregonWashington continental margin. (2) Absence of a plate boundary along the Oregon-Washington

coast

Some authors have questioned the notion that there is asubduction zone beneath the Pacific northwest and Vancouver Island because of the absence of (a) a bathymetric trench and (b) an inclined seismic Benioff zone along this continental margin (e.g., Crosson, 1972). On the other hand, the Juan de Fuca-America margin is assumed by many authors to be a convergent boundary on the basis of (c). the presence of the active Cascade volcanic chain, (d) the presence of deformed sediments at the base of the continental slope, and (e) calculations of the relative velocity between the Explorer, Juan de Fuca and Gorda “plates” and North America. Our model is compatible with the former observations and provides a coherent explanation for the latter without involving wholesale subduction in the past 9 Ma since the primary spreading center was subducted and secondary spreading began.

(a) Bathymetry

of the continental margin

The lack of a well-developed bathymetric trench and the presence of thick accumulations of largely undisturbed Late Miocene, Pliocene and Pleistocene sediments (Dehlinger et al., 1970; Kulm and Fowler, 1974) along most of the margin seems to indicate that very little motion has occurred along this plate boundary since the time of anomaly 5. Dehlinger et al. (1970) in fact interpret the Washington margin to be an analog of the Southern California Continental Borderland, and to be indicative of east-west extension, as might be expected following subduction of a spreading center (Dixon and Farrar, 1980).

(b) Evidence from seismicity The present seismicity of western Oregon, Washington and British Colum-

334

bia is characterized by hypocentres generally limited to depths of less than 50 km (Barazangi and Dorman, 1969; Crosson, 1972) and with no hint of an easterly dipping Benioff zone. Furthermore, Riddihough (1978) reports that attempts to detect microseismicity that might be expected even where hot young lithosphere is being subducted proved fruitless. He further states that the hypocenters of earthquakes inland from the coast are predominantly located in the crust and that focal mechanisms corresp,ond either to strikeslip faulting parallel to the margin or to north-south compression, patterns that we find hard to reconcile with active subduction. Rogers (1979) has found similar patterns from the seismicity of Vancouver Island. Evidence given by McKenzie and Julian (1971) suggests the presence of an east-dipping slab of high velocity material in the mantle beneath the northern Cascades but does not demonstrate presently active subduction of oceanic lithosphere. (c) Miocene to Recent continental volcanism The evolution of the Late Tertiary volcanism of the Pacific northwest, including that of the modern Cascade Range, has been reviewed by McBimey (1978). Although the Cascade Range, consisting of a linear belt of spectacular strato-volcanoes (see Fig. l), is commonly assumed to represent a typical andesitic arc situated above an active subduction zone, McBirney (1978) concludes on the basis of bulk chemical analyses, trace element concentrations and isotope ratios, that the Cascade magmas are derived principally from the asthenosphere rather than from a subducting slab although they may be contaminated by continental crust components. His data show that in central Oregon the proportions of basalt: andesite: dacite-rhyolite have changed very significantly from 10 : 45 : 45 in the Oligocene and Early Miocene to 39 : 41 : 20 in Mid-Late Miocene, to 90 : 9 : 1 in the Pliocene and finally to 85 : 13 : 2 in the Pleistocene. We regard these major changes to be consistent with our contention that the NPSC impinged on the coastline of Oregon and Washington about 9 Ma ago and that the associated zone of mantle upwelling has migrated inland since that time, to a position beneath the modem Cascade Range. The dominantly andesitic extrusives of the Oligocene-Miocene would thus represent the waning stages of volcanism associated with subduction of the Farallon plate, whereas the more recent basaltic activity would be due to passage of the zone of mantle upwelling beneath the continent. By contrast, these major changes in volcanicity are difficult, if not impossible, to reconcile with the uninterrupted subduction of normal oceanic lithosphere that is a requirement of the prevailing plate tectonic model. Souther (1970) has documented a similar change in the character of volcanism in British Columbia and the Yukon Territory. This volcanism had predominantly acid and intermediate composition during Paleocene (?) and Eocene time, and became basic in the Miocene and Pliocene (Souther, 1970, Fig. 3). The distribution of Quatemary and Recent basaltic volcanism (Fig. 1) correlates well with our estimated position of the site of mantle upwelling

335

that was once associated with the NPSC. The predominant rock type is a primitive alkali olivine basalt that presumably is derived directly from the ~thenosphere (Souther, 1970). That the volcanism has alkaline rather than tholeiitic character is consistent with the deepening of the site of magma generation that would necessarily take place as the spreading center passes beneath the continental plate. Evidence that the NPSC was in close proximity to the continental margin -10 Ma ago is provided by the Mid-Miocene basalts intercalated in the western Oregon coastal sedimentary prism at Cape Foulweather and Depoe Bay (Braislin et al., 1971). (d) Deformation at the continental margin Tiffin et al. (1972), Kulm and Fowler (1974), Seely et al. (1974) and Barnard (1978) cite evidence of deformation of Pleistocene sediments at the base of the continental slope and of recent uplift of the continental shelf off Oregon, Washington and British Colombia in support of continuing subduction of the Juan de Fuca plate. Rather than being indicative of wholesale, ongoing subduction, we suggest that this disruption and uplift of Pleistocene sediments may be due to underthrusting of portions of the Juan de Fuca sea floor as left-lateral offsets on northeast strike slip faults accumulated in the past few million years (i.e. the two faults discussed by Silver, 19’71a, and the presently active Nootka fault identified by Barr and Chase, 1974, and named by Hyndman et al., 1978; see below, section on details of place interaction). Tiffin et al. (1972) report that there is no evidence of such compression northwest of Brooks Peninsula (northern Vancouver Island), and that the deformation increases progressively to the southeast. They also report that the deformation of the Kyuquot uplift comprises two parts, one of late Oligocene to middle Miocene age and the other of post early Pliocene age. We ascribe these periods of uplift to, first, the easterly passage of the NPSC in the Miocene (e.g., DeLong et al., 1978), and second, the westerly passage of the Explorer ridge in the Pliocene-Ple~tocene (Tiffin et al., 1972). The increasing compression of the continental shelf sediments southeast of Nootka Island may be due to underthrusting by the Juan de Fuca sea floor as the offset on the Nootka fault proceeds. Barnard (1978) has studied the deformation of sediments on the Washington continental slope. He concludes that deformation has been relatively continuous throughout the Pleistocene but that the rate of sho~en~g across the outer margin is only about 0.7 cm/a, which is considerably less than the 2.3 cm/a rate of subduction of the Juan de Fuca plate that he calculated from the assumed Pacific-America and Pacific-Juan de Fuca velocities. He postulates that the remaining compression must be distributed across the rest of the margin, but indicates that evidence for such distributed deformation is not apparent. We suggest that the low sho~en~g rate of 0.7 cm/a probably represents the total rate of under-thrusting which has occurred as a result of motion on the Nootka fault.

336

Braislin et al. (1971) and Kulm and Fowler (1974) have noted the presence of a prominent angular unconformity of late Miocene age that is widespread on the Oregon continental shelf. Kulm and Fowler (1974) ascribe the uplift to intense underthrusting in mid-late Miocene time. We suggest that this uplift, like that of the Kyuquot uplift, is associated with the approach of the NPSC (DeLong et al., 1978; Farrar and Dixon, 1979) and its passage beneath the continental margin. (e) Calculations of convergence velocity The rate of convergence between the Juan de Fuca and American plates has been estimated by addition of the vectors corresponding to spreading at the Juan de Fuca ridge and to Pacific-America motion along the Queen Charlotte and San Andreas fault systems. Riddihough (1977) has considered the implications of uncertainties in azimuth and rate for each of these vectors. On the assumption that the spreading direction is normal to the magnetic anomaly pattern, and that the Pacific-America motion was constant at 5.5 cm/a at 145’, he argues that subduction of the Juan de Fuca plate has occurred, and that it continues at 3.5 cm/a in a northeasterly direction relative to North America. As he points out, this rate is critically dependent on the assumed direction of spreading at the ridge and on the Pacific-America velocity. Furthermore, his assumption that spreading was normal to the magnetic anomaly pattern seems inappropriate. We concur with Menard and Atwater (1968) that an abrupt change in the direction of spreading would be followed only gradually by a reorientation of the magnetic anomaly pattern. The anomalies produced at the obliquely spreading ridge might bear no evidence of the obliquity prior to segmentation and reorientation of the ridge and the accompanying formation of transform faults. If we assume a higher Pacific-America rate (8 cm/a, see above) and that the spreading is so oblique (at azimuth -145’) that it produces compression across the Blanc0 (as supported by fault plane solutions slightly misaligned with the Blanc0 fracture zone - Dehlinger et al., 1970) and Sovanco fracture zones (Milne et al., 1978) (two of the seismically most active transform faults on the entire ocean ridge system - Barazangi and Dorman, 1969), we can obviate the need for subduction of the Juan de Fuca plate because the America-Pacific velocity (A VP) is equal to the spreading velocity (,V,) at the Juan de Fuca ridge (see Figs. 4 and 6). (3) Asymmetry of heat flow across the Juan de Fuca ridge Dehlinger et al. (1970) give heat flow values for a profile from the Cascadia Basin off Oregon westward across the Juan de Fuca ridge. They note that heat flow values fall to below normal oceanic values a short distance west of the ridge, whereas they remain consistently above average across the Juan de Fuca sea floor to the American margin (Fig. 5). They also note a correlation between high heat flow and low upper mantle seismic velocity. This pattern

337

m

Pn VELOCITY

< 7.9 km/set.

rl

HEAT

FLOW

>2H.F.U.

[-.I

HIGH

GEOMAGNETIC

VARIATION OCEANIC *

ANOMALY.

HEAT

FLOW

N. OF 41’N

> 3.0

.

< 3.0

-

< 1.5

AND

> 1.5

H.F.U.

I

c

+

Fig. 5. Zones of low P, velocity (McGetchin et al., 1979; Smith, 1978; E.L. Procyschyn, pers. comm., 1979), high heat flow (Gough, 1974; E.L. Procyschyn, pers. comm., 1979) and high geomagnetic variation anomaly (Law and Riddihough, 1971; Gough, 1974) in western North America are depicted. Oceanic heat flow values are from Dehlinger et al. (1970). The zone of low P, velocity and high geomagnetic variation anomaly, both possibly indicative of high subcrustal temperature and partial melting, appears to correlate with the postulated sites of mantle upwelling beneath the Garibaldi and Cascade volcanic chains and the Rio Grande Rift, and mantle flow beneath the Basin and Range province (Dixon and Farrar, 1978b, 1980). The zone of high heat flow apparently does not correlate so well; rather it seems to precede the inland migration of the zone of mantle upwelling. The asymmetric pattern of oceanic heat flow on either side of the Gorda-Juan de Fuca ridge system is consistent with the postulated westward lateral flow of hot upper mantle material from the site of mantle upwelling towards the secondary spreading center.

is consistent with our model in which the secondary spreading at the GordaJuan de Fuca-Explorer ridge system is fed by mantle flow that wells up beneath the continent and passes northwestwards beneath the oceanic lithosphere east of the ridge system. Detailed studies of heat flow at the north end of the Juan de Fuca ridge

338

and on the Explorer ridge have been reported by Davis and Lister (1977) and by Hyndman et al. (1978). Although most of their observations were made too close to the ridge crests to exhibit the asymmetry of heat flow documented by Dehlinger et al. (1970), the few that were taken remote from the crests yielded heat flows that appear to be somewhat higher on the Juan de Fuca-Explorer sea floor than the Pacific plate in accord with our model. The presence of thicker sediment cover east of the ridge system may also contribute to the observed asymmetry (Davis and Lister, i977). (4) Terr~ge~ous s~d~ments of the tufts abyssaf plain The distribution of “terrigenous mud” on the northern Pacific Ocean floor is shown on the map, “Surface Sediments and Topography of the North Pacific” (Scripps, 1972) and is reproduced in Fig. 1. Such sediment is normally found bordering the continents and islands arcs, and is generally scarce in areas far from shore. There is one extensive patch of this sediment far from the coastline, on the Tufts Abyssal Plain, and this area is underlain by sea floor bearing magnetic anomalies 5a-13 west of the Juan de Fuca ridge. The Tufts abyssal sediments have been described by Hamilton (1967) as turbidites with a thickness of up to 470 m in the north, thinning to the south to 145 m, and he suggests that they have a probably age of Miocene. He classified the Tufts plain as a “fossil” abyssal plain because post-Pleistocene pelagic clay overlies the turbidites. We suggest that these sediments, presently remote from any reasonable area of proven~ce, were deposited when this area was juxtaposed with the North American margin at around the time of impingement of the NPSC. Sediments derived from this margin were deposited in a relatively proximal position west of the ridge, and have been transported to their present distal position by the Pacific-American plate motion. Such sediments are lacking on younger sea floor west of the Juan de Fuca ridge system because the ridge would have formed a barrier to sediment transport and the sediments would have been ponded in the Cascadia basin in the past 9 Ma.

(5) Other evidence of sub-con tinen tal man tie up welling In Fig. 5 we present a generalized compilation for western North America of the physical parameters that might be indicative of mantle upwelling, including P, velocity, geomagnetic variation anomaly and heat flow. In general terms the zones of low P, and high apparent upper mantle conductivity correlate with our postulated sites of mantle upwelling, the northern one beneath Oregon, Washington and central British Columbia, and the southern one beneath the Rio Grande Rift system. Northwestward lateral mantle flow from the southern segment beneath North America is responsible for the manifestations of high upper mantle temperature beneath the Basin and Range Province (Dixon and Farrar, 197813). The pattern of high heat flow

339

does not correspond so well with the proposed sites of mantle upwelling; rather it appears to precede the inland migration of the upwelling. Souther (1970) has noted the presence of a zone of east-west extension, indicated by northsouth normal faults younger than Mid-Miocene, in western British Columbia. He has also summarized the geophysical evidence, including shallow seismicity, thin crust, low P, velocities, high upper mantle conductivity, and high heat flow in this region. He concludes that these data are suggestive of the presence of convective mantle upwelling beneath central British Columbia, initiated in Miocene time, a conclusion with which we heartily concur. Heat flow measurements by Blackwell (1969) and Hyndman (1976) substantiate the presence of the zone of high heat flow inland, in contrast to the low heat flow of the coastal belt. Hyndman (1976) also notes the correlation between high heat flow and low Bouguer gravity as reported by Stacey (1973). While Hyndman (1976) relates these observations to the presence of active subduction, they are equally consistent with our hypothesis that the primary mantle upwelling formerly associated with the NPSC is now situated beneath central Oregon, Washington and British Columbia, rather than offshore beneath the Juan de Fuca Ridge system. DETAILS

OF PLATE

INTERACTION

A detailed reconstruction of the plate interactions associated with the Gorda-Juan de Fuca-Explorer ridge system sheds light on the nature of the Blanc0 fracture zone, the rotation of the Gorda plate, and the origin of leftlateral strike slip faulting of the Juan de Fuca sea floor that produced slight under-thrusting and disruption of Pleistocene sediments at the Oregon-Washington margin. Figure 6 shows schematically five stages in the evolution of this area, spanning the past 15 Ma. 15 Ma ago (Fig. 6a) the NPSC was oriented north-south and lay slightly west of the Oregon-Washington coast. It had a small left-hand offset on the Sedna fracture zone north of the Mendocino transform fault. The Farallon plate was being subducted beneath North America between the Mendocino transform and the north end of the ridge. By 9 Ma ago (Fig. 6b) the northern part of the NPSC had impinged on the north-south portion of the North American margin, north of Cape Mendocino, converting this portion of the margin to a Pacific-America plate boundary which, by -8 Ma, was spreading in the direction given by the Pacific-America relative motion. The initial Pacific-America spreading at this Juan de Fuca-Explorer ridge system would have been highly oblique to the ridge crest, and, as it accreted material to the edge of the American plate, the ridge migrated northwestward relative to the continent. South of Cape Mendocino and north of the Mendocino transform creation and subduction of the Farallon plate continued, and the NPSC maintained its northeasterly progress towards North America. By 5 Ma ago (Fig. 6c), the south end of the reflected portion of the ridge abutted a transform fault, with the Pacific-America trend, that joined it to

5Ma

2.5 Ma

----

?N

Present

1

Fig. 6. Five stages showing the origin and evolution of the Gorda, Juan de Fuca and Explorer ridge system. a. Tbe North Pacific spreading center with a small offset of the Sedna fracture zone approaches the North American margin (15 Ma). The Farallon plate is being subducted beneath the continent. b. At about 9 Ma the Farallon plate fragment north of the Sedna fracture zone ceases its subduction and becomes welded to North America. This initiates secondary spreading between the Pacific and American plates in the direction indicated by the arrows. South of Cape Mendocino, because of the orientation of the coast, continued subduction is possible and Pacific-Farallon spreading proceeds. c, By 5 Ma the reflected ridge north of the Blanc0 Fracture zone has begun to segment into the Explorer and Juan de Fuca segments, while south of the fracture zone the last extant fragment of the Farallon plate has become attached to North America and the Gorda ridge has begun to reflect. Northwestward motion of the Pacific plate relative to this fragment causes its clockwise rotation and consequent underthrusting as indicated by the thrust symbols (Silver, 1971b.). Between 8 Ma and 5 Ma a small fragment of the Pacific plate (northwest of Cape Mendocino) must have been sheared off the Pacific plate along the Blanc0 fracture zone and become attached to the American plate. d. By 2.5 Ma continued rotation of the Gorda plate has caused the first of two northeasttrending strikeslip faults in the Cascadia Basin and rotation of the Blanc0 fracture zone out of collinearity with the San Andreas fault system. e, Between 2.5 Ma and the present, continued Pacific-America motion on the Blanc0 fracture zone, now no longer parallel to the relative motion, has producted the second left-lateral offset in the Cascadia basin and the presently active Nootka fault at the north end of the Juan de Fuca ridge. The Explorer-Juan de Fuca ridge segments continue to migrate northwestwards relative to North America and reorient themselves toward perpendicularity to the spreading direction.

341

the American margin at Cape Mendocino. This transform fault is the Blanc0 fracture zone. When it formed it was collinear with the Pacific-America plate boundary southeast of Cape Mendocino, a plate boundary that eventually became the San Andreas fault system. The reflected ridge crest north of the Blanc0 fracture zone had become segmented and the segments had gradually reoriented themselves clockwise (Menard and Atwater, 1968) towards an azimuth that was more nearly perpendicular to the PacificAmerica direction. As this readjustment takes place, offsets on transform faults such as the Sovanco must gradually increase until the ridge segments are perpendicular to the spreading direction. By about this time subduction of the small remaining fragment of the Farallon plate, south of Cape Mendocino, had ceased, and the ridge had begun to reflect. The couple produced by continuing northwestward motion of the Pacific plate relative to North America was causing progressive rotation of the remaining portion of the Farallon plate (now called the Gorda plate). As it was rotated the Gorda plate was being under-thrust beneath the Pacific plate at the Mendocino transform fault (Seeber et al., 1970; Riddihough, 1978), it was being internally deformed as shown by the curved magnetic anomaly pattern (Raff and Mason, 1961; Silver, 1971b) and the Gorda ridge was spreading more rapidly at its north end than at the south. Were it not for the fact that the Gorda plate is being pushed northward by the Mendocino transform fault and is rotating as it impinges on the American plate; the situation depicted in Fig. 6c would be stable. By 2.5 Ma (Fig. 6d), the Gorda plate had grown on its west side at the Gorda ridge and had continued to be rotated and internally deformed by the torque produced by the Pacific-America relative motion. The effect of continued rotation was that the Juan de Fuca sea floor has been offset -50 km by a northeast trending left-lateral fault that propagated from near the north end of the Gorda ridge (Silver, 1971a; see Fig. 4). Although we have shown the resulting offset of the Blanco fault as a step, we suggest that the Blanc0 fracture would continuously re-form as a through-going feature, albeit with an orientation that progressively changes counterclockwise, out of collinearity with the Pacific-America (i.e., San Andreas fault) trend. North of the Blanc0 fracture zone the ridge segments continued to reorient themselves clockwise. Between 2.5 Ma and the present (Fig. 6e) a second fault (Fig. 4) has propagated northeasterly across the Juan de Fuca sea floor (Silver, 1971a) as a result of continued Pacific-America motion. The trend of the Blanc0 fracture zone has become increasingly discordant with that of the San Andreas system. This discordance is strongly supported by the pattern of internal deformation in the Gorda plate which according to Silver (1971b), apparently is undergoing left-lateral simple shear and flexural-slip folding as indicated by modem seismicity, fault scarps and pattern of the magnetic anomalies. Alternatively, Riddihough (1980) has interpreted the Gorda plate magnetic anomalies and topography as indicative that the Gorda plate is split

342

into two subplates by a NW-SE trending right-lateral fault which extends between Cape Mendocino and the Gorda Ridge at 42’N. The southern Gorda sub-plate is interpreted to be no longer converging with North America, but to be moving northwesterly. We view this pattern of deformation and the presently very active seismicity on the Blanc0 fracture zone as indications of the tendency towards reestablishment of collinearity of the Blanc0 and San Andreas fault, as suggested by Bolt et al. (1968), after which the northeast half of the Gorda plate will be part of the American plate, the southwest half will be part of the Pacific plate, and spreading at the Gorda ridge will cease. The lack of appreciable present-day seismicity on the portion of the Blanc0 fracture zone between the north end of the Gorda ridge and Cape Blanc0 suggests that the Gorda sea floor is no longer rotating relative to the Juan de Fuca sea floor, but rather is yielding to the northward movement of the Mendocino transform fault by deforming internally. The two northeast trending faults in the Juan de Fuca sea floor (Silver, 1971a) are now inactive, but a third fault with the same orientation and sense of movement is presently the locus of deformation caused by misalignment between the Blanc0 and Mendocino transforms and the Pacific-America direction. This fault is the Nootka fault (Barr and Chase, 1974; Hyndman et al., 1978), which trends northeast from the north end of the Juan de Fuca ridge to beneath Vancouver Island. The northeastward displacement of the Juan de Fuca sea floor along the three left-lateral faults may have been the cause of the observed local disruption and uplift of Late Pliocene and Pleistocene sediments along the Oregon-Washington-British Columbia continental margin (Tiffin et al., 1972; Kulm and Fowler, 1974; Barnard, 1978). It is also conceivable that underthrusting of this sort could have contributed to the Pleistocene volcanism in the Cascades. Throughout the past 8 Ma, while the Gorda-Juan de Fuca-Explorer secondary ridge segments have been migrating northwestwards relative to North America, the primary mantle upwelling of the East Pacific Rise has continued its northeastward progress beneath the continent to its present position beneath the Cascade and Garibaldi volcanic belts. “FOSSIL”

SECONDARY

SPREADING

CENTERS

OFF THE CALIFORNIA

COAST

It is generally accepted that the SPSC has already impinged on the North American margin south of the Mendocino transform fault (Atwater, 1970; Dickinson and Snyder, 1979) and we have discussed the consequences of continued spreading at this site of mantle upwelling following its overriding by the continent (Dixon and Farrar, 1978a, b, 1979,198O). This part of the margin might therefore be expected to have reflected a ridge. The magnetic anomaly pattern of the sea floor west of California is shown in Fig. 7. South and southwest of San Francisco are two areas in which the linear magnetic anomalies trend northeastwards. Atwater (1970) has suggested that these

343

40°N

35’N

reversed

observed

Fig. 7. The Mason and Raff (1961) magnetic anomaly map of the sea floor off central California, with anomaly numbers assigned by Atwater (1970), is interpreted to suggest the presence of two possible fossil secondary spreading centers (indicated by curved arrows). In the northern patch of northeast trending anomalies the line of symmetry is east of Atwater’s (1970) anomaly 7 (?) and in the southern patch it is between her anomalies 7 and 6c (?) as shown by the two magnetic profiles on lines a-u and b-b’. The anomaly numbers in the profiles are assigned by us. Secondary spreading would have commenced at about 30 Ma ago, and would have ceased at about 25 Ma. See text. Dotted line on map shows approximate positions of 2 km isobath. P.F.Z. = Pioneer fracture zone. Magnetic data for the profiles were obtained on magnetic tape from NOAA.

344

mark ocean floor accreted at the SPSC as it approaches the continental margin, the trend being northeasterly because the Farallon plate was breaking up and being dragged southeastwards by the American plate. We speculate that Atwater’s (1970) identification of magnetic anomalies that are progressively younger towards the coast in these two zones may be in error. We suggest that these anomalies may have been produced by symmetrical spreading at two segments of a secondary reflected ridge in a manner analogous to that described above for the Juan de Fuca system. We tentatively suggest that the southern fossil reflected spreading center lie between her anomalies 7 and 6c (?) (Fig. 7), and that the anomalies southeast of the postulated fossil ridge crest are symmetric to those to the northwest (see magnetic profiles on lines a-e’ and b--b’, Fig. 7). Similarly, we suggest that the fossil ridge crest in the northern group of northeast-trending anomalies may lie to the east of her anomaly 7 (?). This implies that the secondary spreading would have commenced at about 30 Ma and ceased at about 25 Ma. The symmetry of these magnetic anomalies and the possibility that this area is a result of secondary spreading are evaluated by Harris (1980). Cessation of this spreading would have been followed by either transform motion at the continental margin or eduction of Pacific plate from beneath the continent (Dixon and Far&r, 1978a, b, 1980). CONCLUSIONS

Consideration of the magnetic anomaly pattern of the northeast Pacific ocean floor suggests that the North Pacific Spreading Center, north of the Mendocino transform fault, impinged on the North American coast about 8-9 Ma ago, and that the mantle .upwelling associated with it is now active beneath the Cascade and Garibaldi volcanic belts of Oregon, Washington and British Columbia. The Gorda, Juan de Fuca and Explorer ridges are interpreted to be secondary spreading ridges that were reflected from the continental margin at the time of impingement of the NPSC and are now migrating northwestwards relative to North America. Accordingly, the oceanic lithosphere east of these ridges is interpreted to be part of the American plate, and subduction along the continental margin ceased at the time of ridge reflection. The spreading at the Gorda, Juan de Fuca and Explorer ridges is fed by northwestward mantle flow emanating from the site of mantle upwelling that is now beneath the continent. The location of this upwelling is indicated by recently active, mantlederived volcanics, in places by high heat flow, by low P, seismic velocity, and by rifting of the continental crust. A reconstruction of Pacific-Farallon-American plate interaction suggests that the Blanc0 fracture zone formed initially as an extension of the PacificAmerica plate boundary south of Cape Mendocino (a boundary which later became the San Andreas fault system). The Blanc0 fracture zone has been rotated counterclockwise out of this trend because of northward movement

345

of the Mendocino transform and Gorda plate that caused left-lateral offset of the southern Juan de Fuca plate along northeast-trending faults. The ridge reflection model as presented is of necessity schematic and idealized, with changes in plate interactions assumed to take place instantaneously rather than gradually. The under-thrusting associated with breakup of the Juan de Fuca sea floor might be regarded as a form of subduction, and probably indicates that the remnant Farallon plate is adhering only gradually to the American plate. We have chosen this idealized presentation for clarity and because Herron et al. (1979) have shown that the spreading of the Chile ridge has remained undisturbed during its approach towards and entry into the Peru-Chile trench. We realize, however, that the correct interpretation may well lie somewhere between our ridge reflection model and the prevailing hypothesis of continued spreading and subduction of the Juan de Fuca plate. The most important aspect of the model is that a site of primary mantle upwelling may become separated from its oceanic surface manifestation. Recognition of the existence of secondary spreading, remote from the site of primary mantle upwelling, has significant implications regarding the nature of the mantle convection and the dynamics of plate motion. ACKNOWLEDGEMENTS

We are grateful to D.M. Carmichael and H. Helmstaedt for their enthusiastic encouragement during the period of development of our model. We thank D.M. Carmichael for his thorough review of the first draft of this paper, and E.L. Procyschyn for providing some of his unpublished compliations of geophysical data for western North America. E.L. Procyschyn and G.A. Rartlett assisted by providing helpful discussion and relevant references. Sandra Harris provided the computer-plotted magnetic anomaly profiles for Fig. 7 from magnetic tapes obtained from NOAA. Christopher Peck assisted with the drafting. We acknowledge financial support from the National Science and Engineering Research Council of Canada. REFERENCES Atwater, T., 1970. Implications of plate tectonics for the Cenozoic evolution of western North America. Geol. Sot. Am. Bull., 81: 3513-3536. Atwater, T. and Menard, H.W., 1970. Magnetic lineations in the north-east Pacific. Earth Planet. Sci, Lett., 7: 445-450. Atwater, T. and Molnar, P., 1973. Relative motion of the Pacific and North American plates deduced from sea-floor spreading in the Atlantic, Indian and South Pacific oceans. Proc. Conf. on Tectonic Problems of the San Andreas Fault System. Stanford Univ. Publ., 13: 136-148. Barazangi, M. and Dorman, J., 1969. World seismicity map compiled from ESSA Coast and Geodetic Survey epicenter data, 1961-1967. Bull. Seismol. Sot. Am., 59: 369-380.

346 Barnard, W.D., 1978. The Washington continental slope: Quarternary tectonics and sedimentation. Marine Geol., 27: 79-114. Barr, S.M. and Chase, R.L., 1974. Geology of the northern end of Juan de Fuca Ridge and sea-floor spreading. Can. J. Earth Sci., 11: 1384-1406. Blackwell, D.D., 1969. Heat-flow determinations in the northwestern United States. J. Geophys. Res., 74: 992-1007. Bolt, B.A. and Miller, R.D., 1971. Seismicity of northern and central California. Bull. Seismol. Sot. Am., 61: 1831-1847. Bolt, B.A., Lomnitz, C. and McEvilly, T.V., 1968. Seismological evidence on the tectonics of central and northern California and the Mendocino escarpment. Bull. Seismol. Sot. Am., 58: 1725-1767. Braislin, D.B., Hastings, D.D. and Snavely, P.D., Jr., 1971. Petroleum potential of western Oregon and Washington and adjacent continental margin. Am. Assoc. Pet. Geol., Mem., 15 (1): 229-238. Cathles, L.M., 1975. The Viscosity of the Earth’s Mantle. Princeton University Press, Princeton, N.J., 386 pp. Chase, C.G., 1978. Plate kinematics: the Americas, East Africa, and the rest of the world. Earth Planet. Sci. Lett., 37: 355-368. Crosson, R.S., 1972. Small earthquakes, structure, and tectonics of the Puget Sound region. Bull. Seismol. Sot. Am. 62: 1133-1171. Davis, E.E. and Lister, C.R.B., 1977. Heat flow measured over the Juan de Fuca Ridge: evidence for widespread hydrothermal circulation in a highly heat transportive crust. J. Geophys. Res., 82: 4845-4860. Dehlinger, P., Couch, R.W., McManus, D.A. and Gemperle, M., 1970. Northwest Pacific structure. In: A.E. Maxwell (Editor), The Sea. Wiley-Interscience, New York, N.Y., 4 (2): 133-188. DeLong, S.E., Fox, P.J. and McDowell, F.W., 1978. Subduction of the Kula ridge at the Aleutian trench. Geol. Sot. Am. Bull., 89: 83-95. Dickinson, W.R. and Snyder, W.S., 1979. Geometry of triple junctions related to San Andreas transform. J. Geophys. Res., 84: 561-572. Dixon, J.M. and Farrar, E., 1978a. Ridge subduction, eduction, and the mechanism of uplift of blueschists (Abstr). EOS, Trans. Am. Geophys. Union, 59, p. 323. Dixon, J.M. and Farrar, E., 1978b. Ridge subduction, eduction and the Neogene tectonics of the S.W. U.S.A. (Abstr.). Geol. Sot. Am. Abstr. Progr., 10: 390. Dixon, J.M. and Farrar, E., 19791 Sierra Nevada uplift due to grounding of North America on the underlying Pacific plate (Abstr.). EOS, Trans. Am. Geophys. Union, 60: 395. Dixon, J.M. and Farrar, E., 1980. Ridge subduction, eduction and the Neogene tectonics of southwestern North America. Tectonophysics, 67: 81-99. Elsasser, W.M., Olson, P. and Marsh, P.D., 1979. The depth of mantle convection. J. Geophys. Res., 84: 147-155. Farrar, E. and Dixon, J.M., 1979. Subduction of the Kula ridge at the Aleutian trench: Discussion. Geol. Sot. Am. Bull., 90: 699-700. Gough, D.I., 1974. Electrical conductivity under western North America in relation to heat flow, seismology, and structure. J. Geomagn. Geoelectr., 26: 105-123. Graham, S.A. and Dickinson, W.R., 1978. Evidence for 115 kilometers of right slip on the San Gregorio-Hosgri fault trend. Science, 199: 179-l 81. Hamilton, E.L., 1967. Marine geology of abyssal plains in the Gulf of Alaska. J. Geophys. Res., 72: 4189-4213. Harris, S., 1980. Secondary Spreading and Oligocene Plate Tectonics off the California Coast. M.Sc. Thesis, Queen’s University, in prep. Harrison, G.C.A., McDougall, I. and Watkins, M.D., 1979. A geomagnetic field reversal time scale back to 13.0 million years before present. Earth Planet. Sci. Lett., 42: 143-152.

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348 eliminated the Surveyor transform fault. J. Geophys. Res., 80: 4815-4822. Silver, E.A., 1971a. Small plate tectonics in the northeastern Pacific. Geol. Sot. Am. Bull., 82: 3491-3496. Silver, E.A., 1971b. Tectonics of the Mendocino triple junction. Geol. Sot. Am. Bull., 82: 2965-2978. Smith, R.B., 1978. Seismicity, crustal structure, and intraplate tectonics of the interior of the western Cordillera. In: R.B. Smith and G.P. Eaton (Editors), Cenozoic Tectonics and Regional Geophysics of the Western Cordillera. Geol. Sot. Am., Mem., 152: 111-144. Souther, J.G., 1970. Volcanism and its relationship to recent crustal movements in the Canadian Cordillera. Can. J. Earth Sci., 7: 553-568. Stacey, R.A., 1973. Gravity anomalies, crustal structure, and plate tectonics in the Canadian Cordillera. Can. J. Earth Sci., 10: 615-628. Tiffin, D.L., Cameron, B.E.B. and Murray, J.W., 1972. Tectonics and depositional history of the continental margin off Vancouver Island, British Columbia. Can. J. Earth Sci., 9: 280-296. Tobin, D.G. and Sykes, L.R., 1968. Seismicity and tectonics of the northeast Pacific Ocean. J. Geophys. Res., 73: 3821-3845. Vine, F.J., 1966. Spreading of the ocean floor: new evidence. Science, 154: 1405-1415. Vine, F.J. and Wilson, J.T., 1965. Magnetic anomalies over a young oceanic ridge off Vancouver Island. Science, 150: 485-489. Wilson, J.T., 1965a. A new class of faults and their bearing on continental drift, Nature, 207: 343-347. Wilson, J.T., 1965b. Transform faults, oceanic ridges and magnetic anomalies southwest of Vancouver Island. Science, 150: 482-485.