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[1] A method for 2.5D viscoacoustic waveform tomography that can be applied to generate 2D models of velocity and attenuation from inversion of refraction waveforms on land seismic reflection data acquired along crooked roads is developed. It is particularly useful for typical crustal reflection surveys. First-arrival travel time tomography is applied using a 3D method, but with constraints on the intermediate 3D velocity model; the result is the starting model for the next step. A 2.5D frequency-domain full-waveform inversion stage parameterizes 3D geometry in the seismic source and receiver arrays, with the assumption that the velocity and attenuation models are homogeneous in the out-of-plane direction. This approach results in superior results compared to a strictly 2D approach when the acquisition line is crooked, with a moderate increase in computational cost. A case study using data acquired in the Nechako Basin in south-central British Columbia, Canada, exemplifies and validates the procedure. The velocity model derived from 2.5D waveform tomography is compared with that from a previous study in which 2D waveform tomography was applied to the same data set and with results from 3D travel time tomography. The resolution and accuracy of the velocity model from 2.5D waveform tomography are demonstrated to be greater than those from travel time or 2D waveform tomography. A model of viscoacoustic attenuation, which was not possible in the 2D case, is also generated. These models are interpreted jointly to highlight features of geological interest, such as a sedimentary basin, basement rocks, and faults, from surface to about 3 km depth.

Lithoprobe is Canada’s national, collaborative, multidisciplinary, Earth science research project established to develop a comprehensive understanding of the evolution of northern North America. It is regarded internationally as one of the most successful national geoscience projects ever undertaken. Part of Lithoprobe’s success derives from its history and organizational structure. A one-year Phase I program (1984–1985) was highly successful. Phases II to V carried the project forward until 2005, when funding terminated. Lithoprobe was organized as a decentralized research network of multidisciplinary scientific studies and collaborating scientists. About 1500 scientific publications were generated. Lithoprobe’s legacy also includes economic and social benefits. New and improved understanding of Earth history provides petroleum and mining companies with an enhanced knowledge base. Lithoprobe demonstrated the applicability of high-resolution seismic reflection studies to mineral exploration. Development of a portable seismic refraction recorder and a long-period magnetotelluric system led to technology transfer to Canadian companies. Very high-resolution seismic reflection studies, applied first by Lithoprobe, are proving effective for exploration for uranium deposits in Saskatchewan. In the cratonic areas of Canada, Lithoprobe seismic and magnetotelluric studies have provided significant new information relevant to exploration for diamonds. On the west coast of Canada, Lithoprobe studies provided data and a framework for better understanding the mega-thrust earthquake hazard in the region. Lithoprobe established a highly effective public outreach strategy that involved the print and electronic media as well as material for educational purposes. Unequivocally, Lithoprobe has enhanced the already strong reputation of the Earth sciences in Canada.

Anomalous crustal and upper mantle structure of northern Juan de Fuca plate is revealed from wide-angle seismic and gravity modelling. A 2-D velocity model is produced for refraction line II of the 1980 Vancouver Island Seismic Project (VISP80). The refraction data were recorded on three ocean bottom seismometers (OBSs) deployed at the ends and middle of a 110 km line oriented parallel to the North American continental margin. The velocity model is constructed via ray tracing and conforms to first-arrival amplitude observations and travel time picks of direct, converted and reflected phases. Between sub-sediment depths of ∼3 to ∼11 km, depths normally associated with the lower crust and upper oceanic mantle, the final model shows that compressional-wave velocities decrease significantly from southeast to northwest along the profile. At sub-sediment depths of ∼11 km at the northwestern end of the profile, P-wave velocities are as low as 7.2 km/s. A complementary 2-D gravity model using the geometry of the velocity model and velocity–density relationships characteristic of oceanic crust is produced. The high densities required to match the gravity field indicate the presence of peridotites containing 25–30% serpentine by volume, rather than excess gabbroic crust, within the deep low velocity zone. Anomalous travel time delays and unusual reflection characteristics observed from proximal seismic refraction and reflection experiments suggest a broader zone of partially serpentinized peridotites coincident with the trace of a pseudofault. We propose that partial serpentinization of the upper mantle is a consequence of slow spreading at the tip of a propagating rift.

The SNORCLE refraction – wide-angle reflection (R/WAR) experiment, SNORE'97, included four individual lines along the three transect corridors. A combination of SNORE'97 results with those from earlier studies permits generation of a 2000 km long lithospheric velocity model that extends from the Archean Slave craton to the present Pacific basin. Using this model and coincident near-vertical incidence (NVI) reflection data and geological information, an interpreted cross section that exemplifies 4 Ga of lithospheric development is generated. The velocity structural models correlate well with the reflection sections and provide additional structural, compositional, and thermal constraints. Geological structures and some faults are defined in the upper crust. At a larger scale, the seismic data identify a variety of orogenic styles ranging from thin- to thick-skinned accretion in the Cordillera and crustal-scale tectonic wedging associated with both Paleoproterozoic and Mesozoic collisions. Models of Poisson's ratio support the NVI interpretation that a thick wedge of cratonic metasediments underlies the eastern accreted Cordilleran terranes. Despite the variety of ages, orogenic styles, and tectono-magmatic deformations that are spanned by the seismic corridors, the Moho remains remarkably flat and shallow (33–36 km) across the majority of the transect. Significant variations only occur at major tectonic boundaries. Laterally variable crustal velocities are consistently slower beneath the Cordillera than beneath the cratonic crust. This is consistent with the high temperatures (800–900 °C) required by the slow upper mantle velocities (7.8–7.9 km/s) observed beneath much of the Cordillera. Heterogeneity of the lithospheric mantle is indicated by wide-angle reflections below the Precambrian domains and the western Cordillera.

The Trans-Hudson Orogen (THO) is the world's largest Paleoproterozoic orogenic belt. Data from three refraction profiles are used to investigate its lithospheric structure in Saskatchewan and Manitoba. R1 crosses the orogen from the Hearne craton on the west to the Superior craton on the east; R2 and R3 are along the orogen. P-wave velocity structural models are generated using a ray-based technique. On line R1, higher crustal velocities in its eastern part coincide with rocks of the Flin Flon – Namew gneiss complex. Depth to Moho is in the 40–45 km range and equates to that from the reflection data, including a small crustal root below the Sask minicontinent. Along lines R2 and R3, depth to Moho varies from about 40 km up to 55 km at the north end of R2 and south end of R3. In general, variations in crustal velocity and depth to Moho do not correlate with the location and extent of geological domains; they appear to reflect the complex deformation and metamorphic history of the crustal rocks. Mantle velocities are high, ~8.2 km/s. However a limited area shows prominent velocity anisotropy, with values of 8.6 km/s along R2 and R3 and 8.1 km/s along R1. We speculate that the observed anisotropy represents an ~100-km-wide mantle suture zone resulting from the collision of Archean plates. The suture zone accommodated limited extensional deformation, associated with a counterclockwise rotation of the Superior plate, to generate the anisotropy. In this model, the lithospheric mantle of the THO internal domains and Sask craton are detached.

The lithospheric structure of the transition from the craton to the Cordillera in northeastern British Columbia is interpreted from inversion of seismic refraction – wide-angle reflection data along a 460-km profile, and from 3-d (3-dimensional) inversion and 2.5-d forward modelling of Bouguer gravity data. The seismic profile extends westward from the sediment-covered edge of cratonic North America across the Foreland and Omineca morphogeological belts to the eastern boundary of accreted terranes, beyond the Tintina Fault. Across the ancient cratonic margin, the resultant models reveal a westward-thickening package of low upper crustal velocities (6.2 km/s and less) and low densities to almost 20 km depth below the Western Canada Sedimentary Basin, overlying a west-facing ramp of higher velocities and densities in the middle and lower crust. These features are inferred to represent passive-margin sediments deposited on the ancient rifted margin during the mid-to-late Proterozoic and early Paleozoic. A wedge-shaped high-velocity (7.3 km/s) crustal layer at the base of the crust beneath the edge of cratonic North America is interpreted to be the result of magmatic underplating during rifting. In the Cordilleran Foreland Belt, high velocities (6.4 km/s) in the upper 5 km of the crust indicate rocks upthrust from the middle crust. A narrow trench of low velocities in the near-surface, which is imaged ~20 km to the west of the inferred location of the Tintina Fault, is interpreted to represent the actual location of the fault or a major splay. From east to west, the Moho decreases in depth from ~40 km to ~34 km below the rifted margin of ancestral North America, then defines a small root at ~38 km depth below the high topography and upper crustal velocities of the eastern Foreland Belt, and gradually shallows to ~34 km beneath the Omineca belt. An enigmatic laterally heterogeneous upper mantle has anomalously high velocities (up to 8.3 km/s) beneath the Foreland Belt, flanked by regions of low velocities (7.7–7.8 km/s). Results indicate that the location of the Cordilleran deformation front west of the ramped cratonic margin directly affected the tectonic evolution of the region.

Crustal structure beneath the transition from the Coast belt to the Intermontane superterrane of the northern Canadian Cordillera is interpreted from the inversion of refraction and wide-angle reflection seismic data. The profile traverses an accretionary suture zone (Coast Plutonic Complex) to continental crust deformed by the transpressive collision (Stikine terrane). Using data acquired by the Accrete onshore/offshore experiment and by a partially overlapping Lithoprobe onshore experiment, P wave travel time inversion and forward amplitude modeling are employed to determine crustal velocity structure. The model exhibits a well-defined transition throughout the crust that distinguishes the Coast Plutonic Complex (CPC) from Stikinia. Average crustal velocities beneath the CPC (6.45 km/s) are considerably faster than those beneath Stikinia (6.25 km/s). Crustal thickness also changes across the transition; thin crust beneath the Coast belt (30–32 km) thickens beneath Stikinia (35–37 km). The observations within the Coast belt are consistent with a tectonic history most recently dominated by extensional deformation. Primary structural control could be associated with either Neogene extension and/or the processes that are responsible for exhuming the Coast belt during the early Paleogene and have been inferred from geological studies. Slow mantle velocities (7.8–7.9 km/s) beneath the entire profile are indicative of high upper mantle temperatures. Comparison with the southern Cordilleran Coast belt reveals similar velocity structure within the massive plutonic complexes. However, substantial differences between the northern and southern Coast belts emphasize along-strike variations in terranes, orogen geometry and postorogenic tectonics.

Three seismic refraction/wide-angle reflection profiles were recorded over the Trans-Hudson Orogen. The profiles display high quality coherent seismic signals associated with the Moho and lithospheric mantle to depths in excess of 160 km. As outlined by wide-angle reflections, depth to Moho varies between 40–54 km with a number of well-defined structural reliefs. Immediately beneath the Moho, in the central part of the orogen, an anomalous high-velocity zone (∼8.45 km/s) with lateral dimensions of 100 km (north-south) by 130 km (east-west) and maximum thickness of 50 km was delineated by mantle refraction arrivals. This zone may represent a locally preserved remnant of older mantle from micro-continental collisions. Mantle discontinuities with some structural relief are interpreted at average depths of 75 and 158 km. Based on wide-angle reflections at offsets beyond 400 km, they bound a layered zone with variable velocities, probably representing imbricated sequences of peridotite and eclogite. We postulate that these unusual features of the upper mantle are the result of lithospheric convergence of bounding Archean cratons and closure of the intervening ocean basin.

Seismic refraction/wide-angle reflection data were recorded on a triangular array in southwestern British Columbia centered on the boundary between the Coast Belt to the southwest and the Intermontane Belt to the northeast. The experiment, part of the Lithoprobe Southern Cordillera transect, enabled determination of the three-dimensional (3-D) velocity structure of the crust and upper mantle. An algorithm for the inversion of wide-angle seismic data to determine 3-D velocity structure and depth to reflecting interfaces is developed. The algorithm is based on existing procedures for the inversion and forward modeling of first arrival travel times and forward modeling of reflection travel times, including (1) forward modeling using a 3-D finite difference algorithm; and (2) a simple velocity model parameterization for the inversion which eliminates the need to solve a large system of equations. The existing procedure is extended to allow (1) the inversion of reflection times to solve for depth to a reflecting interface and/or velocity structure; (2) the inversion of first arrival travel times to solve for depth to a refracting interface; and (3) layer stripping. Application of the algorithm to southern Cordillera data uses Pg to constrain upper crustal velocity structure, PmP to constrain lower crustal velocity structure and depth to Moho, and Pn to constrain upper mantle velocities and depth to Moho. The 3-D velocity model for the southwestern Canadian Cordillera is characterized by (1) significant lateral velocity variations at all depths that do not, in general, correlate with surface geological features or gravity data; (2) a relatively high velocity middle and lower crust in the southwestern part of the study area which correlates with a strong relative gravity high and outlines the eastern extent of lower Wrangellia, an accreted terrane forming the Insular Belt to the west; (3) a narrow zone of slower velocity in the lower crust and change in crustal thickness associated with the Fraser Fault system, lending additional support to the view that it is a crustal penetrating fault; (4) an average upper mantle velocity of 7.85 km/s; and (5) a depth to Moho of 33–36 km in the Intermontane Belt and 36–38 km throughout most of the Coast Belt, decreasing in the west to 33 km near the Insular-Coast contact. Horizontal velocity structure slices and an interpreted cross section based on these and other results show the complexity of crustal structure in the region.

Combined onshore-offshore seismic refraction/ wide-angle reflection data have been acquired across Newfoundland, eastern Canada, to investigate the structural architecture of the northern Appalachians, particularly of distinct crustal zones recognized from earlier LITHOPROBE vertical incidence studies. A western crustal unit, correlated with the Grenville province of the Laurentian plate margin thins from 44 to 40 km and a portion of the lower crust becomes highly reflective with velocities of 7.2 km/s. In central Newfoundland, beneath the central mobile belt, the crust thins to 35 km or less and is marked by average continental velocities, not exceeding 7.0 km/s in the lower crust. Further east, in a crustal unit underlying the Avalon zone and associated with the Gondwanan plate margin, the crust is 40 km thick, and has velocities of 6.8 km/s in the lower crust. Explanations for the thin crust beneath the central mobile belt include (1) post-orogenic isostatic readjustment associated with a density in the mantle which is lower beneath this part of the orogen than beneath the margin, (2) mechanical thinning at the base of the crust during orogenic collapse perhaps caused by delamination, and (3) transformation by phase change of a gabbroic lower crust to eclogite which seismologically would be difficult to distinguish from mantle. Except for a single profile in western Newfoundland, velocities in the crust are of typical continental affinity with lower-crustal velocities less than 7.0 km/s. This indicates that there was no significant magmatic underplating under the Newfoundland Appalachians during Mesozoic rifting of the Atlantic Ocean as proposed elsewhere for the New England Appalachians. A mid-crustal velocity discontinuity observed in the Newfoundland region does not coincide with any consistent reflection pattern on vertical incidence profiles. However, we suggest that localized velocity heterogeneities at mid-crustal depths correspond to organized vertical incidence reflections.

Travel time inversion and amplitude modeling of a 350-km Lithoprobe seismic refraction/wide-angle reflection profile determined the velocity structure of the crust and upper mantle along strike in the Omineca Belt of the Canadian Cordillera. The upper crust to 12–18 km depth has velocities from 5.6 to 6.2 km s−1, and two shear zones, the Monashee Decollement and Gwillim Creek Shear Zone, are imaged by the wide-angle reflections and velocity trends. Minor velocity differences on either side of the Monashee Decollement may be related to separate rock origins. Prominent reflections define the boundaries of a low-velocity midcrustal layer from 10–15 km to 20–25 km depth with velocities less than 6.1 km s−1. The low velocities of the midcrust, associated with high electrical conductivities and high heat flow, may be considered as support for the hypothesis of fluids in the Cordilleran crust, though other possibilities, such as the effect of high temperatures on rock velocities are possible. In the lower crust velocities range from 6.4–6.5 km s−1 at the top of the lower crust to 6.6–6.8 km s−1 at its base. The Moho is very clearly defined by the refraction/wide-angle reflection data and has a gentle southerly dip. Crustal thicknesses are 35–37 km. A thin crust-mantle transition zone of 1–2 km thickness in which velocities vary between 7.6 and 7.7 km s−1 is consistent with coincident reflection data. Upper mantle velocities range from 7.9 to 8.1 km s−1 with indications from the data of upper mantle layering. In comparison with neighboring regions, the Omineca Belt has an anomalously thin crust, low crustal velocities, and a low-velocity upper mantle, similar only to the Basin and Range province. The velocity structure may partly mirror the temperature profile which has overprinted the geological signature of the region as measured by the seismic refraction method. The characteristics of a thin crust and lithosphere, along with low velocities from midcrust to mantle suggests that both the Basin and Range and the southern Canadian Cordillera are currently being heated from a source within the mantle.

We applied an iterative combination of two-dimensional traveltime inversion and amplitude forward modelling to seismic refraction data along a 350 km along-strike profile in the Coast Belt of the southern Canadian Cordillera to determine crust and upper mantle P-wave velocity structure. The crustal model features a thin (0.5–3.0 km) near-surface layer with an average velocity of 4.4 km/s, and upper-, middle-, and lower-crustal strata which are each approximately 10 km thick and have velocities ranging from 6.2 to 6.7 km/s. The Moho appears as a 2 km thick transitional layer with an average depth of 35 km and overlies an upper mantle with a poorly constrained velocity of over 8 km/s. Other interpretations indicate that this profile lies within a collision zone between the Insular superterrane and the ancient North American margin and propose two collision-zone models: (i) crustal delamination, whereby the Insular superterrane was displaced along east-vergent faults over the terranes below; and (ii) crustal wedging, in which interfingering of Insular rocks occurs throughout the crust. The latter model involves thick layers of Insular material beneath the Coast Belt profile, but crustal velocities indicate predominantly non-Insular material, thereby favoring the crustal delamination model. Comparisons of the velocity model with data from the proximate reflection lines show that the top of the Moho transition zone corresponds with the reflection Moho. Comparisons with other studies suggest that likely sources for intracrustal wide-angle reflections observed in the refraction data are structural features, lithological contrasts, and transition zones surrounding a region of layered porosity in the crust.

The Queen Charlotte Basin consists of up to 6 km of Tertiary clastic sediments in a complex sequence of fault-bounded subbasins. The tectonic evolution of the basin in still being debated, with recent interpretations including distributed strike-slip extension, oblique or en echelon rifting, simple extension orthogonal to the plate margin, and block faulting and vertical tectonics. A combined seismic reflection and refraction survey was carried out in 1988 to investigate the structure and tectonic evolution of the basin and underlying crust. While the marine multichannel reflection data were being collected, refracted and wide-angle reflected energy from the large air-gun array was recorded at surrounding land sites in both two-dimensional (in-line) and three-dimensional (broadside) geometries. The broadside refraction data recorded on the Queen Charlotte Islands provide good three-dimensional coverage of western Hecate Strait. These data are modelled to determine the three-dimensional structure of the Queen Charlotte Basin. The reflection data indicate that the sedimentary Queen Charlotte Basin beneath the shotpoints varies rapidly in thickness and is highly three-dimensional. First-arrival traveltimes from the broadside refraction data are inverted to find the three-dimensional structure of the basement interface beneath the shots and out of the planes of the reflection sections. A map of basement depth is derived for a region several kilometres wide adjacent to the reflection lines. Basin thickness varies rapidly between ~ 200 m and ~ 6 km. The model is consistent with the seismic reflection and potential field data sets. Although most of the basin is modelled as sediments overlying rocks with crustal velocities, a thick sequence of interbedded sedimentary and volcanic rocks is interpreted to underlie the shot lines in one region that lies east of the central Queen Charlotte Islands. Four major faults are also interpreted. These are based on sharp vertical relief of over 2 km on the map of basement depth. The orientation and topography across the faults and the small lateral scale and large topographic changes of the subbasins support the distributed strike-slip extension evolutionary model for the basin.

As part of a multidisciplinary investigation of the structure and tectonics of the Queen Charlotte Basin and underlying crust, deep multichannel seismic reflection and coincident crustal refraction data were collected in 1988. Energy from the reflection air-gun array source was recorded at land sites at offsets appropriate to record crustal refraction and wide-angle reflection data. Refraction data recorded in a broadside geometry provide good three-dimensional coverage of western Hecate Strait. These data are modelled using tomographic inversion techniques to determine the three-dimensional velocity structure of the crust in this region. The one-dimensional average velocity increases rapidly with depth to 6.5 km/s at 7 km depth. Velocities from 7 to at least 12 km depth remain approximately constant and are associated with rocks of the Wrangellia terrane. Significant lateral velocity variations, including large differences in near-surface velocities attributable to surface features, relatively low velocities representing interbedded Tertiary sediments and volcanics, and a deep high-velocity anomaly that may represent the root of an igneous intrusion, are mapped. Wide-angle reflections from the Moho are used to determine the thickness of the crust. The Moho is at 29 km depth beneath the east coast of the Queen Charlotte Islands. This is deeper than the Moho observed below Queen Charlotte Sound and as deep as, or deeper than, that below Hecate Strait. Crustal thinning during Tertiary extension was thus greatest beneath the surface expression of the Queen Charlotte Basin, leaving the crust under the islands considerably thicker than under the basin. In an alternate or additional explanation, compression at the continental margin during the last 4 Ma may have been taken up by thickening or underplating of the continental crust beneath the islands. If the Pacific plate is subducting beneath the islands, the Moho observations constrain the slab to dip greater than 20–26°.

Seismic refraction data recorded along a 330 km cross-strike profile through the eastern Insular and southernmost Coast belts of the Canadian Cordillera are interpreted using an iterative combination of traveltime inversion and amplitude forward modelling. The resultant model is characterized by large lateral variations in velocity. The most significant of these variations is a decrease in upper and middle crustal velocities to the east of the surface trace of the Harrison fault, which likely represents the transition from crust of the Insular superterrane to that of the Intermontane superterrane. This interpretation is consistent with some present geological models that place the possible (probable) location of the suture between the two superterranes less than 20 km east of the Harrison fault. Velocities at the base of the upper crust average 6.4 and 6.2 km/s west and east of the fault, respectively. Mid-crustal velocities average 6.6–6.9 km/s to the west and 6.35–6.45 km/s to the east of the fault. Lower crustal velocities also decrease slightly to the east. Other features of the velocity model include (i) a thin near-surface layer with velocities between 2.5 and 6.1 km/s; (ii) upper crustal thickness of 12.5 km, thinning to 8 km at the eastern boundary of the Western Coast Belt (WCB); (iii) high velocity (6.6–6.9 km/s) mid-crustal layer west of the Harrison fault extending to 21 km depth; (iv) high-velocity (6.75–7.1 km/s) lower crustal layer; (v) low-velocity gradient upper mantle with depth to Moho at 34–37 km beneath most of the Coast Belt, decreasing to 30 km beneath the eastern Insular Belt, a depth much less than previous estimates. The inferred crustal velocity structure beneath the WCB is consistent with the three-layer electrical conductivity structure for this area derived from magnetotelluric surveys. The association of high resistivities with the upper crust suggests that the upper 8–12 km represents the massive cover of plutonic rocks which characterizes the WCB. Middle and lower crustal velocities beneath the WCB are consistent with Wrangellian velocities found beneath Vancouver Island, suggesting Wrangellia may extend at depth eastward as far as the Harrison fault.

As part of the Lithoprobe Southern Cordillera transect, seismic refraction data were recorded along a 330 km long strike profile in the Intermontane belt. An iterative combination of two-dimensional traveltime inversion and amplitude forward modelling was used to interpret crust and upper mantle P-wave velocity structure. This region is characterized by (i) a thin near-surface layer with large variations in velocity between 2.8 and 5.4 km/s, and low-velocity regions that correlate well with surface expressions of Tertiary sedimentary and volcanic rocks; (ii) an upper and middle crust with low average velocity gradient, possibly a weak low-velocity zone, and lateral velocity variations between 6.0 and 6.4 km/s; (iii) a distinctive lower crust characterized by significantly higher average velocities relative to midcrustal values beginning at 23 km depth, approximately 8 km thick with average velocities of 6.5 and 6.7 km/s at top and base; (iv) a depth to Moho, as defined by wide-angle reflections, that averages 33 km with variations up to 2 km; and (v) a Moho transition zone of depth extent 1–3 km, below which lies the upper mantle with velocities decreasing from 7.9 km/s in the south to 7.7 km/s in the north. Where the refraction line obliquely crosses a Lithoprobe deep seismic-reflection profile, good agreement is obtained between the interpreted reflection section and the derived velocity structure model. In particular, depths to wide-angle reflectors in the upper crust agree with depths to prominent reflection events, and Moho depths agree within 1 km. From this comparison, the upper and middle crust probably comprise the upper part of the Quesnellia terrane. The lower crust from the refraction interpretation does not show the division into two components, parautochthonous and cratonic North America, that is inferred from the reflection data, indicating that their physical properties are not significantly different within the resolution of the refraction data. Based on these interpretations, the lower lithosphere of Quesnellia is absent and presumably was recycled in the mantle. At a depth of ~ 16 km below the Moho, an upper mantle reflector may represent the base of the present lithosphere.

A procedure has been developed to interpret densely sampled broadside seismic refraction data recorded from a large air gun array. First arrival travel times are inverted to find the structure on an interface beneath the shot line. Travel times are calculated for three-dimensional velocity models using a rapid finite difference algorithm, adapted to allow variable sampling of the model and the determination of rays. A simple inversion parameterization eliminates the need for matrix inversion. The complete inversion procedure is computationally rapid yet allows the determination of detailed three-dimensional structure. Broadside refraction data recorded in the Queen Charlotte Basin, offshore western Canada, during a multichannel reflection experiment are used to demonstrate the procedure. The data are inverted for the basement interface beneath the shot line, defining a rapidly varying thickness of sedimentary basin fill. The results of the inversion stimulate a reinterpretation of the reflection data and identify a new major basement fault. Structure out of the plane of the reflection section is determined, including the strike of the fault and other nearby features.

The Strait of Georgia, a topographic depression between Vancouver Island and the mainland of British Columbia, is considered to be the boundary between two tectonic provinces: the Coast Plutonic Complex on the east and the Insular Belt to the west. The allochthonous nature of the Insular Belt has been established, mainly on the basis of paleomagnetic measurements. Various tectonic models to explain the geological differences between the two provinces have been proposed. One of these suggests that the boundary is an old transform fault zone and is represented currently by a thrust fault along the eastern side of the Strait of Georgia. Other models propose that the Coast Plutonic Complex is a feature superimposed by tectonic and metamorphic events after the accretion of the Insular Belt. Such models do not require a major crustal discontinuity along the Strait of Georgia.In May 1982, a seismic refraction survey using a 32 L air gun and a radio telemetering sonobuoy system was carried out in the Strait of Georgia with the objective of investigating the nature of this boundary and determining the upper crustal structure. Three reversed profiles across the strait were shot; these are supplemented by several high-resolution reflection profiles from previous experiments. Two-dimensional models of the crustal structure across the strait have been constructed using a forward modelling ray trace and synthetic seismogram algorithm to match the travel times and amplitude characteristics of the data.Three basic layers or strata form the models, for which the maximum depth of reliability is 3 km. The first layer consists of unconsolidated sediments and Pleistocene glacial deposits, and the second represents Late Cretaceous – early Tertiary basin fill sediments that form the Nanaimo Group, the Burrard–Kitsilano formations, and the Chuckanut Formation. The third layer is likely the extension of the Coast Plutonic Complex beneath the strait, but the westerly limit of this unit is undetermined because of seismic properties similar to those of the Insular Belt volcanics. A local fault is located ~15 km northeast of Galiano Island on the west side of the strait. However, our study shows no evidence for a major fault along the strait. Thus those aspects of tectonic models that require the existence of a major transform or transcurrent fault boundary along the Strait of Georgia. may have to be revised.

Summary. The Winona Basin is a young deep water basin along the base of the continental slope off central British Columbia which is filled primarily with turbidite sediments. Seismic reflection profiles in the basin indicate that in places the sediment thickness is greater than 4.0 s two-way travel time, and simple interpretations of seismic refraction data indicate that velocities up to 6.0 kni s-* exist within the sediment section. Because the turbidite sediments are acoustically layered with large velocity contrasts between layers, the sediments are probably highly anisotropic. A simple model to account for this has been used to estimate the errors involved in using refraction data to determine layer thicknesses and average velocities. If the effects of anisotropy are ignored, as they usually are, values for thickness and velocity may be overestimated by as much as 10-20 per cent. Even the corrected velocities are still high, however, and the inferred high densities and high degrees of lithification are remarkable when the youth of the sediments is considered. Nearly the entire depositional history is contained within the Pleistocene, and the age of the sediments whose velocity exceeds 4 km s-l is estimated to be only 1 .O Myr old. Corrected velocities in the deepest part of the basin, where the sediment section is over 6 kin thick, exceed 5 km s-'. The nature of samples dredged from the basin (mudstones to coarse-grained sandstones and conglomerates) suggests a mechanism for initial diagenesis: The mudstones are moderately porous, 40-60 per cent, and possess little carbonate, whereas the coarser grained rocks are all well cemented with calcium carbonate, with carbonate contents ranging up to 37 per cent. Measured porosities are as low as 8 per cent, but porosities calculated on a carbonate-free basis range from 35 to 55 per cent. It appears that the fine-grained sediments are first consolidated primarily by gravitational and/or tectonic compaction, and the coarsegrained sediments are cemented by carbonate which is precipitated from pore fluids escaping through the highly permeable coarse-grained layers. As lithification deeper in the basin continues, pore fluids must continue to escape; tectonic fracturing may facilitate this.

One of three explanations of a prominent time delay in the 6.5 km/s branch of a recent seismic refraction survey in the Rocky Mountain Trench suggested a high-angle crustal fault crossing the trench near Radium, British Columbia. If the density contrast between basement and cover rocks is 0.1 g/cm3, a gravity anomaly of approximately 18 mGal should be observed. To test the fault hypothesis, a gravity survey has been carried out in and adjacent to the trench in the Radium area. The resultant data are not consistent with the proposed fault model. The principal feature of the data is a pronounced low, which coincides with the trench throughout the survey area. The low is due to Cenozoic fill and interpretation by two-dimensional modelling indicates the thickness of fill is about 550 m to the north and 420 m to the south of Radium. As a result of this survey, the two alternative hypotheses to explain the seismic data must be reconsidered. These are (1) the existence of a crustal low velocity zone, and (2) a major deformation of the basement and overlying rocks due to the trench being an ancient zone of weakness, which coincides with the western limit of the continental Precambrian craton. As reflections from the top of a low velocity zone are not observed, the second alternative is preferred.

Summary. Multichannel seismic reflection sections recorded across Vancouver Island have revealed two extensive zones of deep seismic reflections that dip gently to the northeast, and a number of moderate northeasterly dipping reflections that can be traced to the surface where major faults are exposed. Based on an integrated interpretation of these data with information from gravity, heat flow, seismicity, seismic refraction, magnetotelluric and geological studies it is concluded that the lower zone of gently dipping reflections is due to underplated oceanic sediments and igneous rocks associated with the current subduction of the Juan de Fuca plate, and that the upper zone represents a similar sequence of accreted rocks associated with an earlier episode of subduction. The high densitylhigh velocity material between the two reflection zones is either an underplated slab of oceanic lithosphere or an imbricated package of mafic rocks. Reprocessing of data from two of the seismic lines has produced a remarkable image of the terrane bounding Leech River fault, with its dip undulating from > 60" near the surface to 20" at 3 km depth and -38" at 6 km depth.

The active margin between the continental North American plate and oceanic Pacific plate west of the Queen Charlotte Islands was the site of an extensive onshore–offshore seismic refraction project in 1983. An airgun line shot over two ocean-bottom seismographs (OBS's) and a 32-charge explosion line recorded on the two OBS's and eight land-based seismographs (LBS's) deployed across northern Moresby Island were selected to study the structure of the predominantly transform Queen Charlotte Fault Zone and the associated offshore terrace. Two-dimensional ray tracing and synthetic seismogram modelling produced a pronounced laterally varying velocity structural model showing three major crustal components (oceanic, terrace, and continental) separated by an outer, crustally pervasive fault and active Queen Charlotte Fault, respectively. The 3 km thick block-faulted upper terrace unit, overlain by deformed sediments, is indistinguishable from adjacent oceanic sediments and upper crustal basalts located to the west. The upper part of the 10–17 km thick lower terrace unit has anomalously low velocities relative to the adjacent oceanic and continental crustal units. A high gradient increases terrace velocity rapidly with depth until the contrast becomes negligible at approximately 17 km depth. Changes in depth to Moho beneath the terrace suggest an increase in eastward Moho dip from 2–5 °observed west of the terrace to 19 °below it. Tectonic mechanisms proposed to explain the anomalous terrace structure involve sediment accretion during subduction of oceanic lithosphere, alternating or combined with compressive upthrusting of material along near-vertical fault planes during periods of active transform motion.

Summary. During the summer of 1982 the Canadian Consortium for Crustal Reconnaissance using Seismic Techniques (COCRUST) conducted a major long-range seismic refraction and wide-angle reflection experiment across the Grenville province of the Canadian Shield. Three seismic lines each approximately 300 km in length were located (i) along the Ottawa-Bonnechere graben, (ii) perpendicular to the graben and (iii) perpendicular to the Crenville Front. Geological evidence indicates that the graben may have originated from a failed arm of the St Lawrence rift system, and the Grenville Front marks the boundary between the Grenville province and the much older Superior province. Other geological features of the Grenville province that were traversed by the profiles were the Central Metasedimentary belt, and the Central Gneiss belt. Analysis of the data involved conventional traveltime procedures coupled with the use of synthetic seismogram analysis using programs that were written to handle laterally heterogeneous structures. Results from the survey indicate a variation in near surface seismic velocity

Summary. An 80-km seismic refraction line was recorded on an array of three ocean bottom seismometers located north-west of the northern tip of Explorer ridge and parallel to Revere-Dellwood fracture zone in the northeast Pacific Ocean. The explosion data were supplemented at shorter distances by seismograms obtained using a 16-litre airgun as source. The combined use of rotated SV component and polarization filtered record sections enabled identification and timing of the refracted S-wave on most sections. The travel time-distance relations for both P- and S-waves are interpreted in the intercept time (tau) and ray parameter domain using tau inversion to give extremal bounds for velocity- depth curves. A linearized inversion technique is also applied to give the smoothest velocity-depth profiles consistent with the travel time data. Amplitude analysis using disc ray theory synthetic seismograms further refine the P-wave velocity-depth models. The final P- and S-wave velocity-depth profiles show a general increase of velocity with depth and no distinct structural discontinuities, not even at the MohoroviCid discontinuity. From the velocity profdes, values of Poisson’s ratio as a function of depth are determined. In the upper crust, these vary considerably among the three data sets, but in the lower crust they are more consistent, ranging from 0.26 to 0.31 with an average value of about 0.29. Across the M-discontinuity, the values, whch are not well constrained, decrease to between 0.25 and 0.26. Better resolution of the velocity-depth profiles is required to determine Poisson’s ratio sufficiently well for a meaningful geological interpretation to be made. We find a normal oceanic crustal thickness of approximately 6.5 km and an anomalously low P, velocity of 7.4 km s-’. The existence of an abnormally thck crust (8-1Okm) on the opposite side of the ridge in the Explorer plate, determined in other studies, contrasts markedly with the results of this research. Such a contrast lends support to the proposals that spreading along Explorer ridge is