R. M. Ellis

S WAVE VELOCITY STRUCTURE OF THE NORTHERN CASCADIA SUBDUCTION ZONE

Teleseismic receiver functions from an array of portable broadband seismograph stations located in southwestern British Columbia are interpreted to estimate the S wave velocity structure to upper mantle depths across the northern Cascadia subduction zone. At our westernmost station on central Vancouver Island, a prominent low-velocity zone (ΔVs = −1 km s−1) with a high Poissons ratio is estimated at 36–41 km depth. This feature correlates with the reflective “E” zone observed in LITHOPROBE reflection data, a region that also exhibits high electrical conductivity and has recently been interpreted as a fluid-saturated shear zone above the subducting Juan de Fuca (JdF) plate. Analysis of data at our stations to the NE permits this zone to be mapped into the upper mantle, 54 km beneath the British Columbia mainland, and approximately 250 km from the locus of subduction. The subducting oceanic crust is imaged at 47–53 km depth dipping 15°± 5° in the direction N30°E ± 20° beneath central Vancouver Island. The dip angle increases to 22° ± 5° at a depth of 60–65 km beneath the Strait of Georgia. The results of this analysis provide the first definitive evidence for the location of the subducting plate in this region and indicate that the seismicity at depth occurs within the oceanic crust. The dip direction of N30°E at each station provides new evidence that the JdF plate is arched upward as it subducts in this region. A low-velocity zone that coincides with the “C” reflectors beneath Vancouver Island is interpreted to extend east to the British Columbia mainland near the base of the North American crust. The top of this zone, near 20–26 km depth, lies near the lower limit of shallow seismicity. The continental Moho is estimated to be 36 km below the Strait of Georgia.

Lithospheric assembly and modification of the SE Canadian Shield: Abitibi-Grenville teleseismic experiment

This paper presents the results of a joint Lithoprobe-Incorporated Research Institutions for Seismology (IRIS)/Program for Array Seismic Studies of the Continental Lithosphere (PASSCAL) teleseismic experiment that investigates portions of the Grenville and Superior Provinces of the Canadian Shield along the Quebec-Ontario border. Data from a 600-km-long, N-S array of 28 broadband seismographs deployed between May and October 1996 have been supplemented with additional recordings from an earlier 1994 deployment and from stations of the Canadian National Seismograph Network and the Southern Ontario Seismic Network. Relative delay times of P and S waves from 123 and 40 teleseismic events, respectively, have been inverted for velocity perturbations in the upper mantle and reveal a low-velocity, NW-SE striking corridor that crosses the southern portion of the line at latitude 46°N and lies between 50 and 300 km depth. Multievent S K S-splitting results yield an average delay time of 0.57±0.22 s and a direction of fast polarization of N93°E±18°, which is consistent with an earlier interpretation as being due to fossil strain fields related to the last major regional tectonic event. Subtle variations in splitting parameters over the low-velocity corridor may suggest an associated disruption in mantle fabric. Profiling of radial receiver functions reveals large and abrupt variations in Moho topography, specifically, a gradual thickening in crust from 40 to 45 km between latitudes 45°N and 46°N, which is followed by an abrupt thinning to 35 km at 46.6°N, some 65 km southeast of the Grenville Front. This structure is interpreted as a subduction suture extending the full length of the Front and punctuating a major pre-Grenvillian (Archean-Proterozoic) episode of lithospheric assembly in the southeast Canadian Shield. The low-velocity mantle corridor, by contrast, is better explained as the extension of the Monteregian-White Mountain-New England seamount hotspot track below the craton and is here postulated to represent interaction of the Great Meteor plume with zones of weakness within the craton developed during earlier rifting episodes.

A reconnaissance teleseismic study of the upper mantle and transition zone beneath the Archean Slave craton in NW Canada

Abstract The objective of this study is to investigate upper mantle structure below the Archean Slave craton, site of the oldest known rocks on Earth and numerous diamondiferous kimberlites, and thence to gain an understanding of early craton formation and kimberlite genesis. To this aim, a temporary array, consisting of 13 sites equipped with broadband seismometers, recorded teleseisms between November 1996 and May 1998. This data set has been augmented with recordings from the Yellowknife seismic array. Our three most robust observations and their interpretation are: (1) P-wave travel-time tomography reveals the oldest part of the craton, the Central Slave Basement Complex, to be underlain by the fastest seismic velocities, indicating that this block remains distinct well into mantle depths; (2) receiver function analysis requires only the Moho as major S-wave velocity discontinuity and points to a fairly constant crustal thickness throughout the Slave province; and (3) SKS splitting analysis shows little variation in delay times and fast polarization directions across the array, leading us to conclude that the present-day plate motion of North America is the primary cause for mantle fabric beneath the entire array. Furthermore, the data let us rule out the possibility that the Mackenzie plume had any seismologically detectable effect on the Slave lithosphere. More speculative results of our investigation, namely a possible genetic link between a low seismic velocity anomaly at depth with the overlying Lac de Gras kimberlite field, and a possible Archean origin of two shallow low-velocity anomalies, will require further investigation.

Inversion of three‐dimensional wide‐angle seismic data from the southwestern Canadian Cordillera

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.

Shear wave constraints on a deep crustal reflective zone beneath Vancouver Island

Analysis of teleseismic receiver functions recorded in the northern Cascadia subduction zone show that large P-to-S converted phases are generated at the boundaries of a dipping layer of deep crustal reflectors (the “E” zone) beneath central Vancouver Island. These data provide the first shear wave constraints on this layer which is clearly observed in seismic reflection sections and is coincident with a region of low electrical resistivity interpreted from a magnetotelluric survey. Modeling of the amplitudes, polarities, and arrival times of the P-to-S converted phases demonstrates that the E zone is a prominent low-velocity layer, with an S velocity 1.0±0.2 km/s lower than the regions above and below. The E zone dips 7°±5° in the direction N10°E±20° and extends from depths of 36.5 to 41 km at ALB-B on central Vancouver Island. The estimated S velocity contrast, when considered with refraction P velocities, yields a Poissons ratio of 0.34 for this zone. Given the uncertainty in ΔVp and ΔVs, this value may range from 0.27 to 0.38. The low P and S velocities and high Poissons ratio combined with electrical resistivity measurements suggest a porosity of 0.1–1.9% and crack aspect ratios of 0.001–0.01, values consistent with a region containing thin, fluid-saturated cracks and thus supportive of the interpretation of this feature as a major shear zone.

Crustal structure of NW British Columbia and SE Alaska from seismic wide‐angle studies: Coast Plutonic Complex to Stikinia

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.

Lithospheric mantle structure beneath the Trans‐Hudson Orogen and the origin of diamondiferous kimberlites

An array of seismographs was deployed over the central Trans-Hudson Orogen from July 1991 to January 1992 and from October 1994 to July 1996 with the objective of characterizing subcrustal lithospheric structure in a region of diamondiferous kimberlite occurrence using tomographic imaging techniques. The two-dimensional array was located in south central Saskatchewan and consisted of 17 stations with an average spacing of 100 km. We obtained relative travel time residuals for 321 teleseismic events and inverted them for subcrustal velocity variations. The ray coverage affords resolution from 60 to 400 km depth. Our results reveal heterogeneities in mantle velocity that deviate by up to ±1.5% from the iasp91 Earth model. The most pronounced low-velocity anomaly is quasi-cylindrical, 120 km in diameter and extends to ∼220 km depth. This feature is partly surrounded by a region of high velocity which penetrates to slightly greater depths. Cretaceous diamondiferous kimberlites and high concentrations of kimberlitic minerals in glacial tills occur above or near the rims of the low velocity anomalies. In addition, correlations exist between a long-wavelength gravity low and the high-velocity region, as well as between high heat flow and low mantle velocities in the southern portions of the study area. Taken together, these observations are consistent with the interpretation of the imaged anomalies as due to thermomechanical erosion of the lithospheric keel of the Sask craton during the Cretaceous by plume activity or Rayleigh-Taylor-like instability within the asthenosphere. The diamondiferous kimberlites are viewed as a direct consequence of this process. Low levels of heterogeneity below 250 km depth are interpreted to be indicative of effective homogenization in a convecting asthenosphere.

Crustal velocity structure of the Omineca Belt, southeastern Canadian Cordillera

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.

The northern limit of the subducted Juan de Fuca plate system

Analysis of data recorded at an array of three-component broadband seismograph stations deployed on northern Vancouver Island and the adjacent British Columbia mainland, at the northern end of the Cascadia subduction zone, provides the first constraints on the S wave velocity structure of this region and permits us to define the northern limit of the subducted Juan de Fuca plate system. During a 2-year period, more than 80 teleseisms were recorded at our five stations. The method of receiver function analysis was used to constrain the S velocity structure to upper mantle depths. Beneath the northern three stations, a relatively simple continental crust is interpreted with a well-defined Moho near 37–39 km depth. An upper crustal S velocity discontinuity at these stations is interpreted as the top of the high-velocity rocks of the Wrangellia terrane. In contrast, more complicated structure dominated by pronounced low-velocity zones dipping to the NE are interpreted beneath our southern two stations. The shallower low-velocity zone is 6–8 km thick, has an S velocity contrast of 0.6–1.1 km/s, and lies within the continental crust. This feature is similar to a pronounced low-velocity layer (the E zone) imaged beneath southern Vancouver Island. The deeper low-velocity zone is interpreted as the subducted oceanic crust. We interpret the pronounced change in S velocity structure that we observe as the northern limit of the subducted oceanic plate beneath Vancouver Island. This change coincides with significant changes in topography, heat flow, gravity, and geochemistry.

Mantle involvement in lithospheric collision: Seismic evidence from the Trans-Hudson Orogen, western Canada

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.