Lithoprobe, southern Vancouver Island: Seismic reflection sees through Wrangellia to the Juan de Fuca plate

C. J. Yorath Geological Survey of Canada, Pacific Geoscience Centre. Sidney, British Columbia VBL 4B2, Canada A.G. Green Earth Physics Branch, 1 Observatory Crescent, Ottawa, Ontario K1 A OY3, Canada R. M. Clowes Department of Geophysics and Astronomy, University of British Columbia, Vancouver, British Columbia V6T 1 W5, Canada A. Sutherland Brown 546 Newport Avenue, Victoria, British Columbia VBS 5C7, Canada M. T. Brandon Geological Survey of Canada, Pacific Geoscience Centre, Sidney, British Columbia VBL 4B2, Canada E. R. Kanasewich Institute of Earth and Planetary Physics, University of Alberta, Edmonton, Alberta T6G 2J1. Canada R. D. Hyndman Earth Physics Branch, Pacific Geoscience Centre, Sidney, British Columbia VBL 4B2, Canada C. Spencer Earth Physics Branch, 1 Observatory Crescent, Ottawa, Ontario K1 A OY3, Canada

ABSTRACT Multichannel seismic reftection profdes obtained on Vancouver Island show that above a zone of decoupling between the North American and Juan de Fuca plates, Wrangellia is underplated by two accreted terranes of probable oceanic origin. Both the zone of decoupling and Wrangellia are disrupted by easterly dipping faults, some of which are thrusts. The crustal structure of Wrangellia comprises Paleozoic and Mesozoic volcanic and sedimentary rocks disposed as roofpendants upon, and large irregular masses within, a ubiquitous Jurassic plutonic and metamorphic complex.

INTRODUCTION form ferrotholeiitic pillow lava sequence of of accreted terranes, plutonic complexes, and During the summer of 1984, multichannel Late Triassic age that separates middle and successor basins. During the past several years, seismic reflection surveys and supporting geo­ upperPaleozoic from Jurassic and younger many workers have identified the island as part logic and geophysical studies were undertaken volcanic and sedimentary rocks, most of which of Wrangellia, a terrane known to be alloch­ on southern Vancouver Island as part of the in­ have been intruded by Lower Jurassic plutons. thonous by virtue of paleomagnetic studies itial phase of a project known as Lithoprobe. Along its axis and eastern margin, northwest­ on Triassic and Jurassic volcanics (Yole and The objective of thiS work was multifold and erly aligned structural culminations expose the Irving, 1980; Irving and Yole, 1983). The included the delineation of the Juan de Fuca island's oldest rocks, the Sicker Group, in the manner of assembly of this and other adjacent plate beneath the island and the nature of the Buttle Lake, Cowichan Lake, and Nanoose up­ terranes and their accretion to the ancient con­ crust underlying Wrangellia. We describe here lifts (A, B, C, respectively, of Fig. l ). On the tinental margin is largely unknown. However, the results obtained from Line 1, which crosses west coast and beneath the continental margin, a reasonable model has recently been proposed south-central Vancouver Island (Fig. l). For an apron of Cenozoic elastic rocks (Carmanah by Monger et al. (1985) which shows the var­ further details on the history of development of Group) overlies complexly deformed Upper ious terranes of the Cordillera as having under­ the project, see Fyfe and Rust (l 981), CAN­ Triassic to Cretaceous slope and trench deposits thrust one another during accretion such that DEL (1981), and Clowes (1984). (Pacific Rim Complex) and Eocene oceanic ba­ they are now vertically juxtaposed, one above salts(Shouldice, 1971; Macleod et al., 1977; the other. Beneath these ancient terranes, the REGIONAL GEOLOGIC AND Yorath, 1980). The volcanics represent the modern Juan de Fuca plate is thought to de­ TECTONIC SETTING structurally highest slice within the subduction scend beneath the margin, first at a relatively Vancouver Island (Fig. l) is made up of Pa­ complex. On the east coast the Nanaimo shallow angle beneath the continental shelf and leozoic, Mesozoic, and Cenozoic volcanic, plu­ Group of Late Cretaceous age comprises cycli­ then more steeply beneath the island (Riddi­ tonic, sedimentary, and metamorphic rocks, cal graded sequences of conglomerate, sand­ hough, 1979; Ellis et al., 1983; Spence et al., upon which is impressed a dominant northwest­ stone, and shale that developed within a 1985). This phase of Lithoprobe was designed erly structural grain (Sutherland Brown and successor basin on W rangellia. to test the proposed terrane accretion and sub­ Yorath, 1985; Muller, 1977; Fyles, 1955). The The primary crustal architecture of Van­ duction model with the aid of a velocity struc­ main stratigraphic and structural unit is the couver Island is believed to comprise an under­ ture determinedfrom recent refraction studies Karmutsen Formation, a thick, relatively uni- thrusting oceanic plate overlain by an amalgam (Ellis et al., 1983; Spence et al, 1985).

GEOLOGY, v. 13, p. 759-762, November 1985 759 LITHOPROBE PROFILE, LINE 1 ally discontinuous and is bounded above and face geologic data where reflection quality is Line l shows clear, coherent reflectors at below by generally acousticallytransparent poor but where stratigraphic and structural in­ three main levels in the section (Fig. 2). It is sections. formation is of sufficient density and quality to noteworthy that on all four profiles the middle A preliminary interpretation of Line l is de­ allow reasonable projections to be made. In and lower zones are equally well represented. scribed in Clowes et al. (1985), Green et al. Figure 3 the lowermost reflector zone(A) is Toward the northeastern end of Line l the up­ ( l 985), and Y orath et al. ( l 985 ). The interpre­ considered to represent a zone of decoupling permost prominent band of reflectors is later- tation illustrated in Figure 3 incorporates sur- between the North American and Juan de Fuca plates, comprising a sequence (about3 km thick) of interdigitated materials of proba­ ble sedimentary and volcanic origin which we suggest is being accreted to the base of the con­ tinental plate. The overall dip of the zone is northeasterly at between 10° and l 2° from about 24 km near the coast to about 3 l km beneath central Vancouver Island beyond which resolution is lost (average velocity= 6.9 km/s). Within the zone, reflector discontinui­ ties suggest the presence of easterly dipping faults. Whether the faults penetrate the underly­ ing oceanic crust or become listric within this �o zone is unknown. At the southwestern end of the profile, structural relationships are complex and require an offshore extension of the line for their resolution; such has recently been ob­ tained and is currently being processed. The depth to the top of the zone is close to or

Figure 1. Regional geology of southern Van­ couver Island (after Muller, 1977). Dotted

llnes are courses of seismic profiles. SJF =

0 San Juan fault. LRF = Leech River fault. CLFZ

= Cowichan Lake fault zone. BRFZ = Beaufort � Range fault zone. A = Buttle Lake uplift. B = Cowichan Lake upllft. C = Nanoose upllft.

Southern Vancouver Island (after J.E.Muller)

TERTIARY LINE carmanah Group x L )()( x )( x x xi Sooke and cattace Intrusions Metchosin Volcanics

CRETACEOUS H \{ >A Nanaimo Group JURA-CRETACEOUS? -! Leech River Complex

JUR ASSIC �o Island Intrusions and Gneiss Complex �;'.;:;'.;:;:J Bonanza Group

TRIASSIC Quatsino Limestone 0 20 t?m Karmutsen Formation ORE. km UPPER PALEOZOIC ".·Z·:.-:•: Sicker Group

760 GEOLOGY, November 1985 slightly shallower than that postulated in pre­ At about this location previous models have either slices of oceanic crust or imbricated vious models for the top of the underthrusting shown a change in dip from 10° to about 30°. oceanic sediments such as are found in the core oceanic crust (e.g., Riddihough, 1979; Spence Above zone A is a thick interval with fewer, rocks of the Olympic Mountains. Several faults et al., 1985). The reflectors show some evi­ less coherent, discontinuous and discordant re­ that dislocate the lowest reflector zone appear dence of an increase in dip toward the eastern flectors. This "underplated zone" (B) may rep­ to extend upward into the underplated zone. A end of the line near where continuity is lost. resent accreted oceanic materials, comprising post-Eocene age for underplating is supported by the probable early Eocene age of the wedge of basalt identified as the Metchosin volcanics SW NE at the western end of the profile. This is dis­ o,�------� -----" ______0 c� below. A corollary to this interpretation is the necessary removal of some tens of kilometres of lithosphere that was originally a part of and beneath Wrangellia as a conse­ quence of, or prior to, the emplacement of the 5 underplated zone. - - Between the 3.5- and 5-s interval and ex­ -:=� - tending eastward across 753 of the record is -�-..;. --- another layered horizon which is interpreted as a decollement zone (C) at or beneath the base 10 10 of Wrangellia. Like the lower layered interval, this zone shows numerous reflectors, the lateral continuity of which is interrupted by easterly 0 10 20 dipping surfaces, some of which are listric km within the zone and others of which appear to H.E."1.3• penetrate into the underlying underplated zone. Most of these faultsthat arise from the de- Figure 2. Reflector geometry of Line 1, south-central Vancouver Island (unmigrated).

BEAUFOR T RANGE FAULT ZONE SW COWICHAN LAKE NE FAULT ZONE) I I I I I 0

5

10 10 ,, TERTIARY JURASSIC UPPER PALEOZOIC 0 JUAN DE FUCA PLATE D ISLAND INTRUSIONS b;::'.·:1 SICKER GROUP - UNDIVIDE D CJ UNDERPLATED ZONE WEST COAST CRYSTALLINE - BUTTLE LAKE FM. � (':·'.:

UPPER CRETACEOUS UPPER TRIASSIC km CJ NANAIMO GROUP Dfill PARSON BAY FM. H.E.=1.3>< iJJllJ QUATSINO FM. B KARMU TSEN FM.

Figure 3. Interpretation of Line 1 with Integrated surface geology. Letters A-K: see text.

GEOLOGY, November 1985 761 collement zone, although not evident in the file where downthrown upper Paleozoic car­ Fyfe, W.S., and Rust, B.R., 1981, The next decade of shallower part of the record section, canbe pro­ bonate can readily be identified through down­ earth science research in Canadian universities: jected to the surface where they coincide with plunge surface projections. Proceedingsof the earth science workshop, 1981: Geoscience Canada, v. 8, p. 113-116. the positions of mapped faults or plutonic con­ Fyles, J.T., 1955, Geology of the Cowichan Lake tacts. At the eastern end of the zone (D) struc­ CONCLUSIONS area, Vancouver Island, British Columbia: tural relationships are complex; discrimination l. The lowermost reflector interval repre­ British Columbia Department of Mines Bulletin between the underplated and decollement zone sents a zone of decoupling between the North no. 37. Green, A.G., Berry, M.J., Spencer, C.P., Kanasewich, becomes difficult. American and Juan de Fuca plates. This E.R., Chiu, S., Clowes, R.M., Yorath, C.J., Like the lower layered sequence, the de­ layered sequence possibly comprises oceanic Stewart, D.B., Unger, J.D., and Poole, W.H., collement zone may represent interdigitated materials that are currently being underplated 1985, Recent seismic reflection studies in materials of sedimentary and volcanic origin to the continental plate. Canada, in Barazangi, M., and Brown, L., eds., that overlay and were emplaced with the un­ 2. Beneath Wrangellia two accreted crustal Deep structure of the continental crust: Results from reflection seismology: American Geo­ occur, derplated zone. At the western end of the pro­ sections the lower of possible post­ physical Union Geodynamics Series (in press). file a wedge-shaped area of short, discontinuous Eocene age and the upper of probable early Irving, E., and Yole, R.W., 1983, Paleolatitudes, ap­ reflectors (below E) is thought to represent the Eocene age. Between these two terranes, a parent displacements and internal rotations of volcanics that were penetratedby exploratory layered sequence, like the lowermost zone, and the Bonanza Volcanics (Early Jurassic) of Van­ couver Island, B.C.: Geological Association of wells (Shouldice, 1971; Yorath, 1980), which probably related to the underplated wne, has Canada/Mineralogical Association of Canada/ have been mapped magnetically as the Prome­ acted as a wne of detachment for easterly dip­ Canadian Geophysical Union Program with theus anomaly by Macleod et al. (1977) and ping faults, some with thrust separation, which Abstracts, v. 8, p. A35. which are equivalent to the lower Eocene extend into the upper crust and project to Macleod, N.S., Tiffin, D.L., Snavely, P.O., Jr., and Metchosin volcanics of southern Vancouver Is­ mapped faults and intrusive contacts at the Currie, R.G., 1977, Geologic interpretation of magnetic and gravity anomalies in the Strait of land and the Crescent Formation of northwest­ surface. Juan de Fuca, U.S.-Canada: Canadian Journal ern Washington. The upper bounding surface 3. The dominant structural style of Van­ of Earth Sciences, v. 14, p. 223-238. of the wedge is the Tofino fault (E) which couver Island is one of shortening due to east­ Monger, J.W.H., Clowes, R.M., Price, R.A., probably is coplanar with the Leech River fault erly dipping thrust faults and westerly directed Riddihough, R.P., and Woodsworth, G.J., 1985, B-2, Juan de Fuca Plate to Alberta Plains: Geo­ of southernmost Vancouver Island (Fig. 1 ). The folds. The faults that originate at deeper crustal logical Society of America, Decade of North Leech River fault is believed to have been ac­ levels partition Middle to Upper Jurassic plu­ American Geology, Continent/Ocean Transect tive during the late Eocene (Brandon, 1984, tonic rocks that were exhumed in post-Eocene 7, 2 sheets, 21 p. text. 1985). This wedge of volcanics is an important time. These granitic rocks enclose large vol­ Muller, J.E., 1977, Evolution of the Pacific margin, accreted element in the architecture of the con­ umes of late Paleozoic and younger rocks, Vancouver Island and adjacent regions: Canadian Journal of Earth Sciences, v. 9, tinental margin and is further illustrated in re­ which, at the surface, appear as roof pendants p. 2062-2085. flection profiles on southern Vancouver Island above the fault-bounded plutons. At the eastern Riddihough, R.P., 1979, Gravity and structure of an (R. M. Clowes et al., in prep.). end of the profile, westerly directed high-level active margin-British Columbia and Washing­ The several other faults that displace parts of thrust faults juxtapose older against younger ton: Canadian Journal of Earth Sciences, v. 16, p. 350-363. the decollement zone appear to control the sequences. Shouldice, D.H., 1971, Geology of the western structural level of Lower Jurassic granitic rocks. Canadian continental shelf: Canadian Society Some show thrust separation, which may ex­ REFERENCES CITED of Petroleum Geologists Bulletin, v. 19, p. 405-424. plain the uplift of central and western Van­ Brandon, M.T., 1984, Deformational proc= affect­ Spence, G.D., Clowes, R.M., and Ellis, R.M., 1985, couver Island that resulted in the exposure of ing unlithified sediments at active margins: A Seismic structure across the active subduction the Island Intrusions and the associated meta­ field study and a structural model [Ph.D. thesis]: wne of western Canada: Journal of Geophysical Seattle, University of Washington, 160 p. morphic rocks of the West Coast Crystalline Research (in press). -1985, Mesowic melange of the Pacific Rim Sutherland Brown, A., and Yorath, C.J., 1985, Complex. This may have taken place in post­ Complex, western Vancouver Island: Geological Lithoprobe profile across southern Vancouver Eocene time as a consequence of the emplace­ Society of America, Cordilleran Section Meet­ Island: Geology and tectonics: Geological ment of the wedge of Eocene volcanics ing, Vancouver, B.C., Guidebook, 28 p. Society of America, Cordilleran Section Meet­ CANDEL, 1981, Lithoprobe: Geoscience studies of (Metchosin volcanics) and the underplated ing, Vancouver, B.C., Guidebook, 23 p. the third dimension-A coordinated national zone. Mesowic stratified rocks (F, G), pre­ Yole, R.W., and Irving, E., 1980, Displacement of geoscience project for the 1980s: Geoscience Vancouver Island, paleomagnetic evidence from served between thrust traces, are thus roof Canada, v. 8, p. 117-125. the Karmutsen Formation: Canadian Journal of pendants above a regionally ubiquitous Lower Clowes, R.M., 1984, Phase 1 Lithoprobe-A co­ Earth Sciences, v. 17, p. 1210-1228. 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R.D., 1985, Lithoprobe Phase 1: Southern Van­ Ellis, R.M., Spence, G.D., Clowes, R.M., Waldron, couver Island: Preliminary analyses of reflection Above this zone, easterly dipping thrust faults D.A., Jones, 1.F., Green, A.G., Forsyth, seismic profiles and surface geological studies, in (K) juxtapose Upper Triassic volcanics against D.A., Mair, J.A., Berry, MJ., Mereu, R.F., Current research: Geological Survey of Canada Jurassic granite, and Paleozoic volcaniclastic Kanasewich, E.R., Cumming, G.L., Hajnal, Z., Paper 85- lA, p. 543-554. and carbonate rocks against Upper Cretaceous Hyndman, R.D., McMechan, G.A., and Loncarevic, B.D., 1983, The Vancouver Island sandstone, conglomerate, and shale. These rela­ seismic project: CO-CRUST onshore-offshore Manuscript received May 6, 1985 tionships are known almost entirely from sur­ study of a convergent margin: Canadian Journal Revised manuscript received July 22, 1985 face geologic data; corroborating reflections are of Earth Sciences, v. 20, p. 719-741. Manuscript accepted August 6, 1985 absent except at the easternmost end of the pro-

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