Lithospheric structure in the southern Canadian

Lithospheric structure in the southern Canadian Cordillera from a network of seismic refraction lines1 Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by University of British Columbia on 03/25/14 For personal use only.

Ron M. Clowes, Colin A. Zelt, John R. Amor, and Robert M. Ellis

Abstract: Lithospheric velocity structure and its relationship to regional tectonics and development of the southern Canadian Cordillera are derived from a synthesis of interpretations from nine in-line seismic refraction - wide-angle reflection profiles and broadside data recorded during the Lithoprobe Southern Cordillera Refraction Experiment (SCoRE) and other refraction experiments across southern British Columbia, and one profile in northwestern Washington. Consistency of the SCoRE two-dimensional models at their intersection positions is achieved through application of a simultaneous inversion of all relevant traveltime data. The cross-sectional and map presentations demonstrate the strong degree of three-dimensional heterogeneity within the crust and upper mantle. A first-order characteristic is the continuous increase in crustal velocities westward from the Foreland belt to the Insular belt. The variations do not correlate with the morphogeological belts; they do correspond with large-scale geological and (or) tectonic features and seismic reflection results. Crustal thickness varies from 30 to 48 km; a lack of comparable variation in Bouguer gravity anomalies requires significant density changes in the crust. Variations in the seismic parameters do not correlate well with variations in crustal resistivity or heat flow, suggesting that generalizations relating low resistivities, high temperatures, and low seismic velocities must be treated with caution. Seismic heterogeneities are due primarily to lithological and (or) structural variations and are superimposed on the generally low velocities attributed to the thermal regime. An upper mantle reflector beneath the mainland Cordillera is inferred to be the top of a shallow asthenosphere. Westward flow in the warm asthenosphere interacts with the cold lithosphere of the subducting Juan de Fuca plate below the central Coast belt, forming a "sink" that could provide a driving mechanism for the flow. RBsumC : La structure de vitesse dans la lithosphkre, et ses relations avec la tectonique rkgionale et le dkveloppement de la Cordilltre mkridionale au Canada, sont dkrivks d'une synthbe de l'interprktation des donnkes de neuf profils en ligne de sismique rkfraction - reflexion grand angle et de dkport lateral, d o ~ k e qui s ont kt6 enregistrkes durant la campagne du programme SCoRE (* Southern Cordillera Refraction Experiment >>)affilik au transect de la CordilEre mkridionale du projet Lithoprobe, et de d'autres levks de rkfraction effectuks au travers la rkgion mkridionale de la Colombie-Britannique et en plus d'un profil complktk dans la rkgion nord-ouest de 1'Btat de Washington. L'harmonisation des modeles bidimensiomels, dkduite du programme SCoRE ?i l'intersection des lignes, a kt6 rksolue grice i l'application de l'inversion simultanke de toutes les d o m k s pertinentes de la durke de trajet. Les reprksentations sur cartes et les coupes verticales dkmontrent l'existence d'une trts forte hktkrogknkitk tridimensionnelle dans la crofite et le manteau sugrieur. Une premiere observation est l'kvidence de l'augmentation continue des vitesses crustales vers l'ouest, en partant du Domaine de l'avant-pays et jusque dans le Domaine insulaire. On ne peut pas Ctablir de corrklation entre les variations et les domaines morphogkologiques; cependant il y a une correspondance avec les particularitks gkologiques i grande kchelle et (ou) tectoniques et avec les rksultats de sismique rkflexion. L'kpaisseur de la crofite varie de 30 ?i 48 km; l'absence dans les anomalies gravimktriques de Bouguer de variations comparables implique des changements de densitk importants dans la crofite. Les variations dans les paramktres sismiques ne prksentent pas de corrklation valable avec les variations de rbistivitk dans la croiite ou

Received January 27, 1995. Accepted July 13, 1995.

R.M. C10wes.~Lithoprobe Secretariat and Department of Geophysics and Astronomy, 6339 Stores Road, The University of British Columbia, Vancouver, BC V6T 1Z4, Canada. C.A. Zelt. Department of Geology and Geophysics, 6100 South Main Street, Rice University, Houston, TX 77005-1892, U.S .A. J.R. Amor and R.M. Ellis. Department of Geophysics and Astronomy, 2219 Main Mall, The University of British Columbia, Vancouver, BC V6T 124, Canada.

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Lithoprobe Publication 684. Corresponding author (e-mail: [email protected]).

Can. 1. Earth Sci. 32: 1485-1513 (1995). Printed in Canada / ImprimC au Canada

Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by University of British Columbia on 03/25/14 For personal use only.

Can. J. Earth Sci. Vol. 32, 1995

avec le flux thermique, ce qui indique que les gtntralisations cherchant I? relier entre elles les faibles rtsistivitts, les hautes temperatures et les faibles vitesses sismiques doivent &treenvisagks avec prudence. Les htttrogtntitts sismiques relkvent principalement des variations lithologiques et (ou) structurales, et elles surimpriment les vitesses gtntralement faibles attributes au rBgime thermique. Une rtflecteur dans le manteau superieur, apparaissant sous le corps principal de la Cordillkre, est interprttt comrne marquant la lirnite suptrieure de l'asthtnosphtre peu profonde. L'tcoulement vers l'ouest dans l'asthtnosphkre chaude suscite une interaction avec la lithosphkre froide de la plaque de subduction Juan de Fuca, qui plonge sous le Domaine c6tier central, crtant un u enfoncement *, qui peut servir de mkanisme activant l'tcoulement. [Traduit par la raaction]

Introduction As one of the orogens in which terranes were first recognized as fundamental building blocks of the continent, the Canadian Cordillera has served as a model for interpreting geological relationships observed in many other orogens of the world. Through a wide range of geological studies, the terranes, their boundary structures, and associated North American rocks have been outlined, described, and related to the major morphogeological belts of the Cordillera (e.g., Gabrielse and Yorath 1989; Fig. 1). While the geological studies provided some fundamental insights on the processes of continental growth, they also focused attention on the need for geophysical studies of the deep structure. Lithoprobe's Southern Cordillera Transect has addressed this need through the integration of seismic reflection, seismic refraction, and electromagnetic imagery of the region with additional studies of the geology and geochemistry of near-surface rocks and with other geophysical information. This paper provides a synthesis of lithospheric velocity structure for the southern Canadian Cordillera as derived from a network of seismic refraction profiles acquired during the 1989 and 1990 Southern Cordillera Refraction Experiment (SCORE) and supplemented by results from two other experiments. A comprehensive refraction - wide-angle reflection (R -WAR) survey, comprising seven 350 km long in-line profiles, was carried out over the southern Canadian Cordillera orogen, where an extensive network of reflection lines and magnetotelluric stations also was acquired (Figs. 1, 2). Line 8 provides velocity structure along strike in the Omineca belt (Kanasewich et al. 1994); lines 1 (Zelt et al. 1992) and 10 (O'Leary et al. 1993) provide similar information along the Intermontane and Coast belts, respectively. Through a complementary study by United States scientists in 1991 (the Pacific Northwest (PacNW) experiment; Luetgert et al. 1993), an extension of Coast belt structure southward into Washington State is available (Miller et aL3). An earlier study along line NAF (Fig. 1) developed a similar velocity model (though less well constrained) for the Insular belt (McMechan and Spence 1983; Drew and Clowes 1990). Line 2 (McLean and Spence 1994) runs normal to strike, extending from the Insular belt across the Coast and Intermontane belts, and was sited to extend an earlier onshoreoffshore line (PJ'J in Fig. 1; Spence et al. 1985; Drew and Clowes 1990) into the central Cordillera. Line 3 provides K.C. Miller, G.R. Keller, J.M. Gridley. J.H.Luetgert, W.D. Mooney, and H. Thybo. Crustal structureof western Washington: Insights from wide-angle seismic data. In preparation.

two-dimensional (2-d) structure along an oblique crossing from the Insular belt to the middle of the Intermontane belt (Zelt et al. 1993). East-west line 7 crosses the Intermontane and Omineca belts (Burian k and Kanasewich 1995) while about 70 km to the south, su parallel line 9 crosses the extensive Omineca belt into the Foreland belt (Zelt and White 1995). Lines 1, 2, and 3 also form a triangle for which large sources at the ends and the middle of all lines, with an additional shot point in the middle, were recorded at all the stations along the lines as well as additional stations interior to the triangle (Fig. 1). This configuration (cf. Kanasewich et al. 1987) provides a data set suitable for three-dimensional (3-d) traveltime tomography of the region interior to the triangle (Zelt et al. 1995), which was centered over the Fraser fault, part of the boundary between the Coast and Intermontane belts. References cited above provide individual interpretations for the lines, each of which addresses major features of the Canadian Cordillera. In this paper, we extend these results in three ways. First, we have modified the individual interpretations, as necessary, by forcing consistency of the models at their intersection points through application of a simultaneous inversion of all relevant data to fit the traveltime picks within their assigned uncertainties (Zelt 1994). Second, we present a summary of the results in which data and velocity structural models are illustrated in common formats, enabling direct comparisons among profiles. Third, we extract principal features of the models (e.g., average crustal velocity, depth to Moho, Pn velocities) and combine these to show variations in these features across the Cordillera. From this synthesis of all SCoRE and associated refraction data, we relate lithospheric structure to regional tectonics and development of the southern Canadian Cordillera.

Regional tectonics The Canadian Cordilleran orogen is the product of an evolutionary history spanning 1700 Ma and encompassing a wide variety of tectonic processes. These and other aspects of the orogen are summarized by Gabrielse and Yorath (1989) and described in detail in Gabrielse and Yorath (1991a), in which the most relevant discussions relating to geotectonics are those by Gabrielse et al. (1991) and Gabrielse and Yorath (1991b). Monger et al. (1994~)also provide a comprehensive discussion of the southern Canadian Cordillera. Five major morphological belts and the offshore tectonic plates, whose present form is a result of the varying tectonic processes, define the transect region (Fig. 1). The belts include, from east to west, the Foreland thrust-and-fold belt, the

Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by University of British Columbia on 03/25/14 For personal use only. Clowes et al.



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Clowes et al. Omineca crystalline belt, the Intermontane belt, the Coast belt, and the Insular belt; offshore, a well-developed accretionary wedge and the Juan de Fuca plate dominate. In this section, we focus on the major stages and processes in the development of the southern Cordillera, principally ones that relate to its present large-scale structure, for which R- WAR surveys can provide significant information. For much of its evolutionary history, the western margin of ancestral North America developed in a passive margin setting on and adjacent to the foundation of a late Archean to Paleoproterozoic crystalline basement. In the southern Cordillera, Mesoproterozoic clastic and carbonate sequences up to 10 krn thick, the Belt-Purcell supergroup, represent an interval from 1700 to 1200 Ma that is preserved in the Omineca belt. An episode of basement rifting less than 800 Ma, perhaps similar to that of the present Atlantic margin, resulted in the generation of the Neoproterozoic Windermere supergroup, also to be found in the Omineca belt. Following further rifting in Neoproterozoic to Early Cambrian times, broad carbonate platforms developed on a westerly sloping shelf. Although passive margin evolution was dominant during the early Paleozoic through to the Late Jurassic (550 - 150 Ma), local episodic volcanism and graben development suggest rifting at various times in the outer part of the miogeocline. Between the Middle Jurassic and Paleocene (170 -60 Ma), major compressional events occurred as extensive terranes composed of Paleozoic and younger intraoceanic-arc and ocean-floor rocks were accreted to the continental margin during their convergence with North America. As a result, intense metamorphism, plutonism, and uplift took place in the Omineca belt, and the miogeocline was detached from its basement, thickened through folding and telescoping along imbricate thrust faults, and transported eastward over the edge of the craton, thereby forming the Foreland belt. The Omineca belt thus incorporates the complex tectonic boundary between the deformed miogeocline of the Foreland belt and the easternmost, Middle Jurassic accreted terranes. The boundary coincides approximately with the Kootenay arc, east of which North American rocks dominate the surface geology, and west of which the pericratonic Kootenay terrane and rocks of the most easterly accreted terrane, Quesnellia, predominate (Fig. 1). For the Cordillera west of this region, the dominant tectonic process has been terrane accretion. The collage of accreted terranes includes both large terranes with coherent stratigraphic records, such as Stikinia and Wrangellia, and disrupted terrane fragments, such as Cache Creek, Cadwallader, and Bridge River. Geological evidence indicates that most terranes of the western Cordillera were amalgamated outboard of the craton into two large composite terranes or superterranes prior to accretion (Monger et al. 1982), although recent work (Monger et al. 1994b) suggests refinements to this interpretation. The easternmost superterrane includes most of the Intermontane belt, which was thrust eastward over North American rocks in the Middle Jurassic. The westernmost composite terrane includes Wrangelia and Alexander (only identified north of 52"N) terranes, which amalgamated by the Late Jurassic and probably were thrust as a unit along and (or) beneath smaller terranes and the Intermontane belt to the east in the mid-Cretaceous. Similar

to the Omineca belt, the Coast belt represents a broad collisional boundary, in this case between the Intermontane and Insular belts. It evolved during mid- to Late Cretaceous time as a long-lived magmatic arc with voluminous granitic rocks that overprint the suture zone. In the early Tertiary, the Insular belt was underthrust by two small terranes, a process which probably included transcurrent movement as well. Between 100 and 40 Ma, large, right-lateral strike-slip faults that partly accommodated northward motion of the terranes relative to North America formed in the western part of the Cordillera. Within the transect, the Fraser fault system at the eastern edge of the Coast belt is representative. In the Early Eocene (about 56 Ma), tectonism in the southern Cordillera changed from convergence to large-scale east -west extension and crustal thinning, along with continuing strikeslip activity in the west. The extensional activity was associated with widespread magmatic arc activity. Deep levels of the crust were exposed in metamorphic core complexes as the overlying rocks moved laterally and down along major normal faults. Since the major Eocene extensional period, the interior of the Cordillera has been relatively quiescent, whereas the western margin has undergone subduction-related magmatism producing the Cascade and Garibaldi-Pemberton volcanic belts. East-dipping subduction of the Juan de Fuca plate continues today off the West Coast, further developing the large accreted sedimentary wedge that forms the western limit of North America.

Regional refraction studies prior to Lithoprobe Seismic refraction experiments were initiated in the Canadian Cordillera in the 1950's, but most of the earlier studies were carried out during the 1960's and early 1970's. The better quality profiles were reversed with shot points at each end; station spacings were typically 10 km. Data and analysis procedures enabled primarily one-dimensional (I-d) velocity depth curves to be derived, although later work led to simple 2-d crustal models. From the interpretations, three basic parameters of the crust were derived: average crustal velocity, depth to the Mohorovicic discontinuity (Moho) or crustal thickness, and the velocity of the uppermost mantle immediately below the crust (Pn velocity). Over parts of the region, a lower crustal layer with higher average velocities and seismic characteristics of the Moho (e.g., first-order velocity discontinuity or velocity transition zone) also were inferred. Berry and Forsyth (1975) provide an early summary of seismic results; Sweeney et al. (1991) present a comprehensive listing of geophysical studies in the Cordillera and the most significant results. The 1980 Vancouver Island Seismic Project WISP 80; Ellis et al. 1983), a series of refraction and reflection profiles utilizing both land-based and ocean-bottom seismographs with explosive and airgun sources, was designed to determine the 2-d velocity structure of the convergent margin and Insular belt. As the VISP 80 data have not been superseded by subsequent experiments, the present synthesis incorporates (1) the results of McMechan and Spence (1983) along strike in the Insular belt, as modified by Drew and Clowes (1990) following interpretations from Lithoprobe crustal reflection


experiments on southern Vancouver Island; and (2) the offshore-onshore 2-d structural model of Spence et al. (1985), again as modified by Drew and Clowes (1990).


Outstanding questions Since the earlier regional studies provided only the most basic crustal information, SCoRE was planned to address a number of outstanding questions relating to lithospheric structure: (1) Are differences among the five major belts, defined by their geology and morphology, reflected in differences in their subsurface crustal structure? (2) How far westward do rocks of cratonic North America extend? (3) Do the terranes identified at the surface have a significant extension into the crust? If the terrane structures are relatively deep, are there differences among the terranes? (4) While earlier studies established that crustal thickness varies over the region, being thinner in the west than in the east, what are the detailed variations in crustal thickness and in the underlying uppermost mantle velocity? (5) Similarly, the previous regional studies have indicated a difference in the nature of the crust -mantle transition across the Cordillera. What are the characteristics of the Moho and how does it vary across the transect? (6) What is the lithospheric structure of the Coast belt and what is its relationship to earlier and continuing subduction off the West Coast? (7) How do the velocity structures relate to the geometric structures interpreted from the seismic reflection data and is there consistency between the results from the two methods? (8) How does the lithospheric structure of the southern Cordillera compare with that derived from comparable studies across other parts of western North America?

Lithoprobe refraction reflection data

- wide-angle

SCoRE 89, comprising lines 1-6 including the triangular array, was conducted primarily across the Intermontane and Coast belts (Figs. 1, 2). Zelt et al. (1990, 1992) provide details concerning instrumentation, sources, positioning, and timing. SCoRE 90 was conducted primarily from the Intermontane belt to the Foreland belt (lines 7 -9), with one profile (line 10) recorded along strike in the Coast belt (Figs. 1,2). Burianyk et al. (1992) and Kanasewich et al. (1994) provide experimental details. For the VISP 80 and PacNW 1991 experiments, Ellis et al. (1983) and Luetgert et al. (1993) provide details. During SCoRE, more than 50 individual shot gathers were recorded, each providing a record section of data. Refracted phases that were used in the interpretations are Ps, Pg, and Pn, which represent energy refracted to the surface through the near-surface material (depths less than 2 km), upper crust, and upper mantle, respectively. The principal wide-angle reflected phases incorporated into the interpretations and the ones common to all profiles are PcP2 and PmP, reflections from a middle -lower crust boundary and the crust -mantle boundary, respectively. A number of additional wide-angle reflected phases were identified on one or more individual lines; these include reflections within the

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upper crust, from an upper -middle crust boundary (PcPI), and within the lower crust (PcP3) and upper mantle (Pm). Data quality was generally very high, but varied dependent upon shot and receiver locations. Figure 3 illustrates data quality and most of the phases mentioned through four exemplary record sections, all of which have been filtered and plotted with identical parameters. Due to low signal-to-noise ratio, inadequate shot - receiver offset, structural complexities, or perhaps the nonexistence of a particular phase in some regions, all phases are not observed on all record sections. Ps is dependent upon near-surface structure around each shot point and thus is not considered further in this paper. Identification of phases and picking of traveltimes were carried out using a variety of presentation formats to ensure reliability.

Interpretation procedures In-line profiles An iterative combination of ray-based traveltime inversion (Zelt and Smith 1992) and amplitude forward modelling (Zelt and Ellis 1988) was used to interpret the in-line data for 2-d P-wave velocity structure. Zelt et al. (1992) and Zelt and Forsyth (1994) describe the basic interpretive approach. A layered, 2-d isotropic P-wave velocity model parameterization, which allows both horizontal and vertical velocity gradients and velocity discontinuities across layer boundaries, was used. Applying a layer stripping approach, Pg was modelled to constrain the upper crust, PCP, and PcP2 were modelled to constrain the upper to middle crust, and PmP and Pn were modelled simultaneously to constrain the lower crust and upper mantle including the Moho. Figure 4 shows the many different ray types and their representative travel paths for models of lines 1 (Zelt et al. 1992) and 10 (Zelt and White 1995), as interpreted by the authors. Note in general the lack of turning rays (refraction arrivals) between depths of about 10 km and the base of the crust. Since such turning rays are most sensitive to velocity variations, the figure illustrates that velocity resolution in the middle to lower crust is poorer than elsewhere in the models. From a 1-d starting model, a 2-d model that gave a satisfactory match between observed and calculated traveltimes was derived by the inversion procedure. Amplitudes of arrivals were forward modelled to provide additional, or sometimes the only, constraints on particular features of the velocity models. For example, lines 1, 7, 8, and 10 include Moho transition zones based on amplitude modelling. Another round of traveltime inversion was then applied to compensate for the model parameters adjusted to improve the amplitude fit. In this way, amplitude modelling and traveltime inversion were alternated until a suitable match to both data types was achieved. Figure 5 shows exemplary traveltime comparisons for three of the shots from each of the lines for which data are exemplified in Fig. 3. Note the generally good comparison between the observed and calculated traveltimes. Figure 6 shows theoretical seismic sections calculated from the velocity structure models of the original interpretations for lines 1, 7, 9, and 10 in a format that allows direct comparison with the equivalent observed data of Fig. 3. The main amplitude characteristics of the data are generally well matched. However, the synthetic seismogram sections do not show the


Can. J. Earth Sci. Vol. 32, '1995

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Fig. 4. Ray tracing for all shots grouped by phases for two exemplary lines; see text for explanation of phases. Note the different vertical scales among the plots. (a) Line 1; PCP,, and PCP,, are two wide-angle reflection phases from the upper crust as observed from different shot points; Pr is from a "floating reflector" (i.e., no observed velocity contrast). (b) Line 9; Pr rays show reflections from "floating reflectors" in different parts of the model as observed at different shot points. Ray path diagrams indicate the regions of the models constrained by observations. D I S T A N C E (krn) D I S T A N C E (krn) S N S . . . . . . .-.. --- S--.. N IUU 13U ZUU Z3U UU u 3u

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Clowes et al complexities or reverberatory codas observed on the recorded data because of the limitations of the ray theoretical approach and the simplicity of the models relative to the real Earth. Some wide-angle reflections observed over limited extents along lines 8 and 9 could not be associated with any specific velocity feature. These reflections were modelled in a fundamentally different way, following derivation of the velocity structure, than the other phases. A single horizontal reflector was assigned to each phase and the final position and dip of the reflector was determined through traveltime inversion (Zelt and Forsyth 1994). Such "floating" reflectors represent a spatial image, like that obtained from near-verticalincidence reflection data, because their velocity structure is not determined, although the position of the reflectors is dependent on velocities above them. In Fig. 4b, the Pr ray group is illustrative of such phases. Ray tracing tests using perturbed versions of the final models suggest that absolute model uncertainty varies from 0.1 to 0.3 kmls for velocities and 1 to 2 km for boundary depths, dependent upon position in the model. Lateral resolution varies between 25 and 50 km, the higher values generally applying at greater depths.

Simultaneous traveltime inversion of intersecting profiles The in-line data along each of the seven SCoRE lines were analyzed independently by various workers using the same basic modelling approach described above. Therefore, consistency of the models at depth beneath the four line intersection points was not achieved in an optimal manner. Since this paper presents regional 3-d variations across the southern Canadian Cordillera as sampled along the seven SCoRE lines, a reevaluation of the in-line data was undertaken to objectively ensure consistency of the models at the intersecting points using a method described by Zelt (1994). During this reevaluation, the SCoRE data have not been remodelled to the same level of detail as in the original publications for two reasons. First, the final models obtained by each worker are a result of fitting both traveltimes and amplitudes, whereas the approach of Zelt (1994) involves only traveltime inversion. Second, the data for some of the lines contain phases not observed on other lines, in particular, some intracrustal and sub-Moho reflections. Therefore, our purpose was limited to (1) showing that the traveltime data of the main phases can be fit by models that are consistent at their intersection points; (2) providing a consistent set of models from which the large-scale regional variations and trends in the southern Canadian Cordillera can be presented. The method of Zelt (1994) consists of one modification to the basic in-line modelling approach described in the previous section: data from networks of intersecting lines are modelled simultaneously, such that the model parameters at an intersection point are common to the models of each intersecting line. Thus the same quantity of data is modelled using fewer independent parameters, providing the most constrained set of 2-d models for each line and consistency at the intersection points. Since the four intersection points of the SCoRE network are widely separated with respect to the lateral resolution of the models, the velocity structure at one intersection point will not significantly influence the velocities at another point. This simplified the analysis by allowing

the new modelling to be performed around each intersection point in turn, rather than simultaneously. The four intersection points and relevant lines are (1) lines 2 and 10, (2) lines 1 and 7, (3) lines 7 and 8, and (4) lines 8 and 9. The phases used in the new modelling were Pg, PcP2, PmP, and Pn. Although amplitudes were not remodelled, the original amplitude fits will not be significantly altered, since the vertical gradients from the original set of models were maintained during the modified inversion procedure. The starting model for each line was the original model, except at the intersection point, where the average of the two models was used. To do this, an additional node was added at the intersection point in each model for each layer velocity and boundary, unless the original model contained a node within 20 km, in which case this node was moved to the intersection point. All nodes within 50 km of the intersection point were allowed to vary during the new modelling. Each iteration of the simultaneous inversion involved parameters from all depths varying until the traveltime data were fit at least as well as by the original models according to an F-test statistical criterion (see Zelt 1994).

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Velocity structural models for the southern Cordillera Interpretations of 2-d velocity structural models by the original authors have been modified where necessary to provide model consistency at the four profile intersection points following the procedure described in the previous section. The fits of the final set of consistent models is comparable to the fits obtained after each group modeled the lines independently. Maximum differences between the independently modeled structures and the consistent ones are 0.65 km in depth and 0.06 kmls in velocity, well within the estimated uncertainties of the modeling procedures. Such results indicate that the gross features of the models are robust features of the data and can be assumed to be representative of the "real" Earth. Here we review and compare the principal features and results from (1) SCoRE along-strike lines, supplemented by a line of equivalent quality in the Coast belt extending southward from the international border and by interpretation of VISP 80 data along the Insular belt; and (2) SCoRE cross-strike lines supplemented by interpretation of VISP 80 data recorded from the Juan de Fuca plate across the accreted wedge and Insular belt to the western Coast belt.

Along-strike models Figure 7 shows the along-strike velocity structural models for four of the major geomorphologial belts forming the southern Cordillera. One clear trend from this composite presentation is the increase in velocities throughout the crust from the Omineca belt to the Insular belt. Although the velocities change, the general thickness of the layers above the well-defined middle-lower crust boundary (MLC) remains in the 20-25 km range. A lower crustal layer of relatively low velocities for the depths involved is characteristic of most profiles, and crustal thickness (boundary M) is generally in the 30-35 km range. U.ppermost mantle velocities vary by about 0.2 kmls, both along individual profiles and among the profiles for which the parameter could be reliably interpreted, and are 2 8 kmls.

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asterisks) and crossing lines (black inverted triangles) are indicated at the top of each panel. Solid black lines indicate m d e l boundaries from which wide-angle reflections were recordad. Thick broken black lines indicate "floating reflectors," that is, those without an associated velocity discontinuity. (a) Omineca belt, line 8 (after Kanasewich et a]. 1994); MLC, middle-lower cmst boundary; M, Moho or Moho transition zone, lowermost lighter line is base of transition zonc (if one is interpreted) and corresponds to the forma1 definition of the base of the crust; MR, mantle reflector; GC. Gwillim Creek shear zone; MD,Monashee dkollement. (6) Intermontane belt, line 1 (after Zelt et al. 1992); labels as in (a). (c) Coast belt. line 10 (after O'Leary et al. 1993): LC, lower crust reflective boundary; other labels as in (a). (e) Insular hell, line NAF. WSP SO (after Drew and Clowes 1990); labels as in (a). (d)PacNW 1 Iine in Washington State (after Miller et JdF-C, Juan de Fuca plate crust; JdF-M. Juan de Puca plate mantle; other labels as in (a).

Rg. 7. Comparison of interpreted along-strike velocity structural models including minor modifications following the intersecting profile inversion. Shot p i n t s (red


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6.9 kmls, occurs farther west at the western edge of the eastern Coast belt. Jones et al. (1992~)interpret the variation in lower crustal resistivity to be due primarily to variability in the porosity and (or) salinity associated with pore fluids in the lower crust. Lower resistivity values would normally be associated with reduced seismic velocities, but, as shown above, almost the opposite appears to be true for the lower crust of the mainland Cordillera. This suggests that different mechanisms may be controlling variations in resistivity and seismic velocity in the lower crust in the study area. While the former may be the result of varying pore fluid content and (or) salinity, the latter appears to be more sensitive to the lithological components of the lower crust. The conclusion that the MT and seismic data may not be sensitive to the same physical parameters draws support from a recent study of high reflectivity and low resistivity in the upper crust of the southeastern Omineca belt (Cook and Jones 1995), which shows that strong reflections are due to gabbroic sills, and low resistivity is caused by sulfides in the metasediments into which the sills have intruded. Cook and Jones (1995) caution that inferences drawn from spatial correlations of low resistivity and high reflectivity may not be appropriate, as their causes may arise from different geological features. To this we would add that when examined in detail, low resistivities do not necessarily correlate with low seismic velocities, as is often inferred (e.g., Marquis and Hyndman 1992). In the western Coast belt and in association with the volcanic belt, low values of lower crustal resistivity may be caused in part by the presence of small amounts of partial melt. High seismic velocities preclude any widespread extension of such features. From their study across the Insular belt, Kurtz et al. (1990) show that the "E" reflective zone (Fig. 10) coincides with a layer of relatively low resistivity ( 30 52 . m) imbedded in an otherwise highly resistive crust and is electrically connected to the region of low resistivity below the volcanic belt. The same zone is shown with low

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velocities (Fig. 8b, line PJ'J), but these have been imposed, rather than interpreted from the VISP 80 R-WAR data, since the latter provide no resolution on such a feature. Electrically, the high-density, high-velocity material surrounding the "E" zone is not distinguishable from the much lower velocities in the upper 15 km of the Insular belt.

Heat flow data Davis and Lewis (1984) and Lewis et al. (1985, 1988, 1992) provide the primary heat flux and radioactive heat generation data across the southern Cordillera and interpret these data in terms of crustal temperatures, relationships to other data, and tectonic models. Hyndman and Lewis (1995) provide a brief review of the results, whereas Majorowicz et al. (1993a, 1993b) use them to relate electrical conductivity, heat flow, and crustal temperatures. As discussed by Lewis et al. (1992), the reduced heat flow (i.e., with effects of upper crustal heat generation removed) of 63 mW/m2 flowing into the upper crust below the Ornineca belt (average heat flux of 86 mW/m2) and Intermontane belt (average heat flux of 73 mW/m2) defines this large area as a h g l e heat flow province with relatively high flux. However, the seismic properties vary considerably over the same area. When comparing values of crustal thickness, velocity of the lower crust, average crustal velocity, or upper mantle velocity (Fig. 9) with contour values of the heat flux for the same region (Fig. 3 of Majorowicz et al. 1993a), no clear correlations are observed. For example, high values of heat flow (> 100 mW/m2) in the southeastern Omineca belt correspond to relatively thick crust with very low lower crustal velocities and relatively high upper mantle velocities. In contrast, a region of high heat flow ( > 80 mW/m2) in the northeast Intermontane belt (similar reduced heat flow as in the Omineca belt) corresponds with thin crust, modest lower crustal velocities, and lower upper mantle velocities. In the southeastern Coast belt, where the crust is thick, lower crustal velocities are relatively high and upper mantle velocities are relatively low; the sparse heat flow values range from 60 to 70 mW/m2. The 3-d seismic heterogeneity of the southern Cordillera cannot be explained by variations in heat flow or crustal temperatures. This is not to say that thermal effects are negligible. The effectiveness of temperature at lowering seismic velocities is well documented (Christensen 1979; Christensen and Wepfer 1989). Generally, high heat flow and resulting high crustal temperatures may account for the overall low velocities, relative to crustal averages for North America (Mooney and Braile 1989), observed throughout the Cordillera. The seismic heterogeneities shown in this study appear to be due primarily to lithological and (or) structural variations and are superimposed on the overall low velocities that can be attributed to the thermal regime. Studies of heat flow often discuss depths to the 450 and 730°C isotherms, identified, respectively, as the approximate temperatures of the brittle-ductile transition in the crust and of the transition from a mineralogy in equilibrium with coexisting free water in the interstitial pore spaces to a dry mineralogy (Lewis et al. 1992). Figure 4 of Majorowicz et al. (1993~)shows maps of depths to the 450°C isothermal surface as estimated for observed heat fluxes, although Cook (1995) questions assumptions made in the calculation of

,


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Clowes et al.

these depths and suggests they could be much deeper. In the Omineca belt, depths to the isotherm of 12- 16 km are fairly consistent (Majorowicz et al. 1993a), but the data are sparse. In the southern Omineca belt, no specific seismic feature corresponds to this depth range, but farther north it correlates well with the top of a low-velocity zone. In the Intermontane belt, depth ranges of 15-20 km for the 450" isotherm are estimated. Wide-angle reflectors associated with the base of a modest low-velocity zone and (or) the top of the middle crust are found at 15 km depth, but no deeper characteristic features consistent with the range are interpreted. In the eastern Coast belt, away from the influence of the volcanic belt, the 450°C isothermal surface is variable, with depths of 18-22 km. The lower number corresponds with intermittent wide-angle reflectors at the top of the lower crust. Thus, there appears to be no distinct correlation between the seismic velocity structure and depth to the 450°C isotherm. Within the heat flow province comprising the Omineca belt and Intermontane belt, Lewis et al. (1992) estimate the depth to the 730°C isotherm to be in the range about 1822 km. In some parts of the belts, this range correlates with the depth to the mid-lower crustal reflective boundary (Fig. 9b), but over much of the region this boundary is at depths exceeding 23 km. Generally, depth to the top of the lower crust varies significantly throughout the region and would be difficult to relate to an isothermal surface.

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Discussion and conclusions Through SCORE and related experiments, the 3-d velocity structure of the lithosphere in the southern Canadian Cordillera has been mapped in considerable detail; it is shown to be highly variable. How do these variations relate to geotectonic characteristics of the Cordillera? Have the outstanding questions relating to lithospheric structure that were posed at the beginning of the program been answered? These questions and related aspects are considered here.

Characteristics of the morphogeological belts and geotectonic features A first-order characteristic is the continuous increase in seismic velocities for both the upper -middle and lower parts of the crust westward from the Foreland belt to the Insular belt. The trend does not consistently correspond with the geologically defined morphogeological belts. However, in broad terms, it is consistent with the evolutionary history of the region. The most appropriate explanation for the trend is a decrease in felsic crustal components with a corresponding increase in mafic content, since it does not relate directly to heat flow variations. The low-velocity eastern part of the Cordillera (Foreland belt and eastern Omineca belt) corresponds with cratonic North America and overlying parautochthonous rocks. Based on seismic velocities, we cannot distinguish between the two, but the combined layer appears to extend at least to the Intermontane -Coast belt boundary. The low lower crustal velocities in the eastern Cordillera correspond with values for the upper crust below the western Canada sedimentary basin (Chandra and Cumming 1972) to the east. This could indicate that the Middle and (or) Late Proterozoic rifting of western North America, which thinned the crust, led to removal of the lower crust of the craton or

its assimilation into the mantle. Zelt and White (1995) discuss other tectonic implications of the seismic results in the eastern Cordillera. increasing velocities (and mafic content) in the central Cordillera (western Omineca belt, Intermontane belt, and eastern Coast belt) correlate with the presence of large tracts of accreted terranes, many of which have ocean arc affinities, accounting for the higher velocities. In the western Cordillera (western Coast belt and Insular belt), the highest interpreted velocities correspond to Wrangellia, a magmatic arc accreted terrane. While neither velocities nor thicknesses of crustal layers define the five belts, variations in these parameters correspond with large-scale geological and (or) tectonic features. In the Intermontane belt, the Nicola horst, first inferred on the basis of the reflection data, is a clear example. High seismic velocities from near surface to the top of the lower crust at about 23 km depth are associated with it. In a similar but more subtle fashion, slightly enhanced velocities to depths approaching 20 km correlate with the Vernon antiform in the Omineca belt. The Coast belt thrust system in the southeastern Coast belt is shown for the first time to have an associated crustal root. Because crustal velocities and depth to Moho decrease to the northwest, the results indicate that the thrust system is limited to the southeastern Coast belt where it has been defined geologically (Journeay and Friedman 1993). Based on the seismic data and magnetotelluric studies, extensive gra~toidsin the western Coast belt extend to depths of about 10 km. Wrangellia, the terrane forming the ~nsularbelt, is characterized by high crustal velocities. On this basis, its eastward extension to the eastern Coast belt and its limited extension to the northwest within the Coast belt are inferred. Offshore, the accreted wedge and the oceanic crust of the Juan de Fuca plate are defined well by interpreted R- WAR data. A reflection from the top of the lower crust, in which velocities increase by 0.1-0.5 kmls, is observed on all profiles in the mainland Cordillera. However, in the Foreland and eastern Omineca belts. where the reflector is weak and poorly imaged, it does not correlate with the well-defined top of North American basement. Within the western Omineca and eastern Intermontane belts. the same reflector does correspond well with the top of basement as interpreted from the near-vertical-incidence reflection data. Below the western Intermontane belt, the reflector continues at a depth of about 22 km, whereas both the high-quality reflection data and geochemical results indicate a substantial thinning of North American crust in its extension as far west as the Fraser fault. Since upper-middle crustal deformation interpreted from the reflection data appears to sole into the top of basement, it is tempting to suggest that the reflector at the base of the middle crust represents a crustal strength contrast that acted as a regional detachment during both compressional and extensional tectonic deformation in the southern Omineca and Intermontane belts. The existence of such a detachment may have limited the maximum structural relief during deformation to less than 25 km, as interpreted from the reflection data. This is all consistent except with respect to the seismic R-WAR data in the western Intermontane belt, which do not indicate a deepening of the detachment as inferred from other data. i he discrepancy remains unexplained.



The Coast belt and the northern Cascades in Washington are the only regions in which a lowermost crustal layer was detected by wide-angle reflection data (Figs. 7c, 7d). Within Canada, the layer is 2 -6 km thick with velocities increased by -0.2 kmls to -6.7 kmls. However, similar velocities (but no reflections) are interpreted in other areas and the average lower crustal velocity for the Coast belt is not anomalous with respect to surrounding regions (Fig. 9c). In Washington, the lowermost layer has a thickness of 512 km with velocities increased by 0.2 kmls to 7.3 kmls. Such differing results from elsewhere in the region indicate a very different lower crust, implying a major crustal break associated with the termination of the southwest Coast Mountains near the border. The thick, high-velocity lower crustal layer may have formed through extensive underplating of mafic material associated with Cascades volcanism (Miller et aL3).

Crustal thickness, the Moho, and upper mantle velocities Depth to Moho, and hence crustal thickness, varies substantially across the southern Cordillera (Fig. 9f). No correlation of this depth with the major belts exists, with the possible exception of the Omineca belt - Intermontane belt boundary where a westward thinning by 2-3 km occurs. One significant feature of the variations is the consistent trend to greater crustal thickness on the west side of the Fraser fault, as shown in Fig. lob. At 50°N the Fraser fault coincides with the belt boundary, but to the south, the offset correlates closely with the southern extension of the fault, the Straight Creek fault (Wheeler and McFeely 1991). This is further evidence that the strike-slip fault system extends at least to the base of the crust, as proposed by Jones et al. (1992b), on the basis of modelling of MT data, rather than terminates at midcrust, as suggested by one interpretation of the reflection data (Varsek et al. 1993). North from 50°, the Fraser fault strikes 10" west of north, but the trend of crustal thickness change is 10" east of north, so the correspondence in that region is questionable. The thickened crust interpreted below the Coast belt thrust system provides the first evidence for a crustal root below this feature, which is of limited areal extent. Below the eastern Insular belt, depth-to-Moho data indicate that the crust is relatively thin. In the northwestern Cascades, crustal thickness reaches 48 km, due to a greatly thickened lower crust. Across the Cordillera, the Moho is interpreted as either a thin transition zone, with high velocities of 7.4 -7.7 kmls and usually less than 2 km thick (but reaching 3 km along part of line I), or a discontinuous boundary. Both these characteristics and Moho depths are consistent with the reflection data, which often show the Moho as a clear reflection of a few cycles (< 1 km) duration. As with other seismic parameters, upper mantle velocities vary significantly across the Cordillera (Fig. 9g), but primarily range between 7.8 and 8.0 kmls. Such values are lower than those for most regions of North America, but comparable to those in other parts of the North American Cordillera (Braile et al. 1989; Beaudoin et al. 1994). The lowest values, 7.65 -7.80 kmls, are found in the southernmost Coast belt and its southward extension into the Cascade ranges. The highest Pn velocity of 8.15 kmls is interpreted along the eastern Coast belt, but the value is not well constrained.

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Pn velocity variations do not correlate with heat flow variations.

Subcrustal lithosphere One of the unexpected results from the SCOREprogram was the detection of an upper mantle reflector of wide-angle energy, which is interpreted as the top of a shallow asthenosphere below the Omineca belt and Intermontane belt (Fig. lob). When combined with depth and position of the cold lithosphere of the subducting Juan de Fuca plate, the region below the central Coast belt appears to be the locus of "collision" of lateral flow from the warm continental asthenosphere to the east and downgoing oceanic plate from the west. The two features form a mantle "sink" for both types of material, perhaps providing a mechanism to drive asthenospheric flow below the mainland Cordillera. General characteristics and comparisons Bouguer gravity anomalies throughout the mainland Cordillera are consistently low and show no correlation with crustal thickness, indicating that significant lateral changes in crustal, and possibly mantle, density must exit. Magnetotelluric studies indicate low resistivities associated with the lower crust, although values vary by an order of magnitude and correspond with belt boundaries (Jones et al. 1992a). Low resistivity values are often associated with reduced seismic velocities (Marquis and Hyndman 1992), but these relationships do not hold for the mainland Cordillera. This suggests that caution must be used in generalizations concerning the relationship. The mainland Cordillera, specifically the Omineca belt and Intermontane belt, is considered to represent one province of high heat flow, although significant variations occur over the region (Lewis et al. 1992). These variations and crustal isotherms of 450 and 730°C (considered to represent the brittle -ductile transition and the transition from a wet to dry mineralogy, respectively) derived from them do not correspond to the 3-d seismic heterogeneity of the region. The latter is due primarily to lithological and (or) structural variations and is superimposed on the generally low velocities, which can be attributed to the thermal regime. As discussed by Kanasewich et al. (1994) and Burianyk and Kanasewich (1995), the eastern and central Cordillera have many geophysical and geological characteristics that are anomalous with most of North America and similar to the Basin and Range province of the southwestern United States. Low average crustal velocities, thin crust, low upper mantle velocities, high heat flow, a shallow asthenosphere, and extensional tectonics are among the similar features. These are all consistent with a mantle heat source that could be associated with mantle upwelling (Gough 1986). For the western Canadian Cordillera (Coast belt and Insular belt), Fuis and Clowes (1993) provide a comprehensive comparison with transects across southern Alaska and central California. They note similarities and differences in potential field signatures, deep structure from both reflection and R- WAR data and surface geology. In Alaska and Canada, distinctive underplated assemblages underly a Mesozoic accretionary prism and prism backstop and are truncated seaward by a Cenozoic accretionary prism; California shows no such underplating. In Alaska and California, the Mesozoic

Clowes et al.


prism is configured as a landward-verging tectonic wedge, but in Canada the prism (Pacific Rim terrane) is thrust below and along the backstop of Wrangellia. Fuis and Clowes (1993) suggest that similarities among the regions might,be expected, because all three were dominated by grossly similar plate interactions in the late Mesozoic and early Cenozoic, and they speculate about tectonic processes that created the three regions.

Summary A network of seismic refraction lines provides unprecedented coverage of the seismic velocity structure of the lithosphere in the southern Canadian Cordillera. Prominent cross-strike variations are identified, but substantive variations also occur along strike; the Cordillera truly exhibits 3-d heterogeneity. Variations do not correlate with the principal morphogeologial belts, but do correlate with geological or tectonic features. The northwest Cascades region of Washington has a velocity structure significantly different than that in Canada. When compared with other data, variations in seismic parameters also do not correlate well with Bouguer gravity values, resistivities from magnetotelluric data, or heat flow data. Additional support is provided for a thin lithosphere beneath the central Cordillera. A mantle "sink," involving downflow of the shallow Cordilleran asthenosphere juxtaposed against the subducting Juan de Fuca plate, is proposed below the Coast belt. We hope that the overview and discussion presented here will stimulate further development of ideas and concepts regarding lithospheric evolution of the Cordillera.

Acknowledgments We wish to thank the individual scientists who provided us with digital copies of their velocity interpretations and (or) prepublication manuscripts: M. J.A. Buriany k and E. R. Kanasewich at the University of Alberta, N.A. McLean and G.D. Spence at the University of Victoria, D.J. White at the Geological Survey of Canada, K.C. Miller and G.R. Keller at the University of Texas, El Paso, and B.C. Zelt at the University of British Columbia. SCoRE was funded principally by the Natural Sciences and Engineering Research Council of Canada (NSERC) Collaborative Special Projects and Programs grant to Lithoprobe and by the Geological Survey of Canada. Additional support for this study has been provided by NSERC Research and Infrastructure grants to R.M.C. and R.M.E.

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