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Canada's Active Western Margin Geoscience Canada. Volume 3. Nunber 4 269 in the area of Washington and Oregon seemed necessary geometrically. This suggestion was reiterated by Tobin and Sykes (1968) in their study of the seismicity ofthe regionanddevelopedin established the presence and basic bathymetric trench at the foot of the Canada's Active configuration of an active spreading continental slope, the absence of a ocean ridge system ofl the west coast of clearly defined eastward dipping Beniofl Western Margin - Vancwver Island. The theory of the earthquake zone and the apparently geometrical mwement of rigid quiescent state of present volcanism The Case for lithospheric plates on a spherical earth have all contributed toward a (introducedas the 'paving stone' theory) widespreaa uncertalnfy as to whether Subduction was tirst ~ro~osedand tasted bv subddctlon 1s cbrrently taking place ~c~enziiand Parker (1967) using data (e.8.. Crosson. 1972: ~rivast&a,1973; R. P. Riddihough and R. D. Hyndnan from the N. Pacific and in the global Stacey. 1973). Past subduction, fran Victoria Geophysical Observatory, extension of the theory by Morgan several tens of m.y. ago until at leastone Earth Physics Branch, (1 968) this reglon again played a major or two m y. ago. IS more generally Department Energy, Mines and role Morgan alsoconcluded that a small accepted ana it seems to us that the Resources, block eitof theJuandeFuca ridge (the case for contemporary subduction, in a 5071 W. Saanich Rd. Juan de Fuca plate. Fig. 2) was moving large part, must beargued upona Victoria B. C. independently of the main Pacific plate. continuation into the present of the The possibility that theJuan de Fuca processes shown to be active in the Summary plate is now underthrustingNorth recent geological past. We review the evidence for subduction America was first suggested by In this article we anempt to treat at the continental margin of south- McKenzie and Parker (1967). They systematically each of themainsources western British Columbia and conclude noted that consumptionof oceanic crust of information on whether or not that, although the pattern is complex and changing, south of 50°N convergence has continued to the present day. lntmduetlon Some of the questions that most commonly arise in studies of the recent tectonics of south-western British Columbia are 'when and where has subduction taken place along the continental margin?' and 'is subduction still going on today?'. Being actively involved in both onshore and onshore studies of this region, we felt that it would be usefulto review all the important data which bear on these questions. While admitting that our special interests have led us to emphasize particular aspects of the many fields of geology and geophysics relevant tothe problem, the exercise has nevertheless considerably clarified our own ideas. It will, we hope, be similarly useful to others whether or not they agree with our conclusions that subduction has occurred and is still occurring. The ocean floor off western Canada occupies a fundamental position in the ~~~~~~kanmalymap (black, .ogamma) development of the theory of plate from Ranand Mason (1961). Arrows show tectonics. The magnetic anomaly maps most recently anomaly. subduction is taking place. Firstly, the earthquake data extend only about 50 Vancwver Island. This triple junction is relative motions between the America. years into the past and representa very of the transform fault-ridge-trench type Pacific and Juan de Fuca plates overthe short, recent, time interval. and is not necessarily stable in the terms past I0my. based largely on sea-floor defined by McKenzie and Morgan magnetic anomalies; secondly, the Present Plate Conflgurallon (1969) and may move with time. The indications of subduction at the margin The olate boundaries of the NE. Pacific transform fault is the right lateral Queen such as sedimentary structures and dip reg& have been progressively Charlotte fault which runs northwest of the oceanic basement; thirdly, the established by many studies over the along the continental slope west of the effects of subduction to be expected on past 10 years, principally by marine Queen Charlotte Islands, meeting the the continent such as arc vulcanism, geophysical surveys (bathymetry, Aleutian trench in southeastern Alaska. heat flow and gravity; and finally the magnetics, reflection seismic and heat The ridge is composed of a complex evidence from seismicity. We feel flow) and earthquake locations in the sequence of en-echelon segments however that is is important to area off thewashington, Oregon and connected by fracture zones running emphasize that each source of British Columbia coast. The plate south from the Dellwood Knolls infomation has a different timescale. boundaries based on the most recent spreading centre, through the Explorer The offshore magnetic anomalies, for dataare shown in Figure2. Ridge to the Juan de Fuca Ridge. The example, can give information back Of critical importance forthe history of trench (although not a bathymetric several tens of m.y. but their time subduction is the location of the point of 'trench') or line of convergence, is near resolution is little better than one my.: intersection of three main plates (atriple the base of the continental slope off the junction) near the northern end of coast of Vancouver Island and Washington. Not all of these plate boundaries are equally established and several are in doubt. One complication is the very recent 'ridge jumping' that has been suggested for the northern ends of both the Juan de Fuca and Explorer Ridges. For the former, reflection seismic studies COLUMBIA reported by Barr and Chase (1974) and by Davis and Lister (1976) have established that the present axis of spreading is located in the westerly of two spreading centres about 20 km Dellwood Knol apart. Both spreading centres are expressed by pronounced valleys and a positive magnetic anomaly. The eastern centre was probably active within the past 0.7 my. At the northern end of the Explorer Ridge a similar situation has been proposed by Srivastava ef a/. (1 971), on the basis of bathymetric, magnetic and heat flow data. A more radical possibility was suggested by Barr (1974), principally from the evidence of earthquake locations, that the Juan de Fuca - America boundary now extends northeast from the northern endof the Juan de Fuca Ridge and that the Queen Charlotte Fault extends south- eastwards to the same position. This configuration would make the Dellwood Knolls and Explorer Ridge spreading segments inactive. While the absence of any detectable magnetic or bathymetric Figure 2 Dossible EDIJDboundarv Dotted lineis discontinuity corresponding with an Tectonic features. AP - America plale, n,pornetca norrnern eogeol s~w~cttw Bp - Brooks Peninsula, EP - Explorerplate, D,are Dasned..ne!nlano!swestern im rot active extension of the Queen Charlotte Gvb - Garibaldivolcanic belt, JP - Juan de volcanism. Triangles - Quaternary Fault makes Barr's suggestion unlikely, Fuca plate, PP - Pacific plale, PRh - Paul volcanoes, squares -Miocene volcanic the existenceof a diffuse line of Revere fracture zone, SFz - Sovanco fracture centres. Arrows are probable piate morions epicentres trending north-east from the zone. Dashedlineeasfwards from SF2 is relative lo America plate. northern end of theJuan de Fuca Ridge WienceCanada Volume 3. Number 4 is confirmed in a recent earthquake geomagnetic reversal was presented by of between 0' and 20") From a gradual compilation (Fig. 3) and may be signifi- Vine (1968). Using a time scale derived change in anomaly direction and thus cant This is commented upon later. lrom a profile across the East Pacific ridge orientation, principally from four to Clearly, the area is of some Rise. Vine established a history going six m.y, ago, a progressive change in complexity. It was described by Vine back 15 m.y. The oldest dateable spreading direction has been implied. (1 966) as one in which there has been magnetic anomalies east of the Juan de Atwater (1 970), taking a right lateral 'faulting and a gradual stilling and re- Fuca rloge (now at tne margln and Pac~l~c-Amercanplate mollon deduce0 orientation ofthe ridge crest'. Me~rd oarallel to the Wasn~naton-Oreaon" - from theGulfot Cal~lorn~aand Juan oe and Alwater (1968) described the coast) were estimated to be about nine Fuca Ridge spreading parallel to the deformation in this offshore region as m.y. old. Asa complete record older than Blanco Fracture Zone at 5.8 cm yr-'. 'exceptionally complex both because of this liestothe west ofthe ridge System. deduced a present convergence at the a continent nearby and because of a subduction must have taken place continental margin of25cmyr-1 inaNE. relatively recent change in direction within the last nine m.y. direction. Silver (1 971 )subsequently of spreading'. A number of authors have considered the influence of a major NE. subsequently used Vine's analysis to transcurrent fault from the ridge to the Oflshon Magnetic Anomalicn and look at the predicted motions in more margin and showed that while it was Relatire PIale Motions detail. There is general agreement on active (210 0.5 m.y. ago) subduction may Through the dating of individual spreading rates but the absence of have been considerably less at the anomalies, magnetic suweys are the clecdrly defined transform faults has British Columbia margin (approximately most powerfultool available for madefhe accurate determination of one cm1yr)than tothe south (3 to 4 deducing past motions of thesea floor spreading directions more difficult. In cmlyr). Recent more detailed studies of away from spreading ridges. For the different studies spreading has been the magnetic anomaly geometry for the northeast Pacific, the first detailed assumed either to be perpendicular to swthern part of the Juan de Fuca plate combining of the magnetic anomaly the magnetic anomaly trend or parallel off Vancwver Island could have pattern with the chronology of to the Blanco fracture zone(a difference become virtually coupled to the America plate in the recent past.
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