The LITHOPROBE seismic reflection project on Vancouver Island was designed to study the large-scale structure of several accreted terranes exposed on the island and to determine the geometry and structural characteristics of the subducting Juan de Fuca plate. In this paper, we interpret two LITHOPROBE profiles from southernmost Vancouver Island that were shot across three important terrane-bounding faults—Leech River, San Juan, and Survey Mountain—to determine their subsurface geometry and relationship to deeper structures associated with modem subduction.The structure beneath the island can be divided into an upper crustal region, consisting of several accreted terranes, and a deeper region that represents a landward extension of the modern offshore subduction complex. In the upper region, the Survey Mountain and Leech River faults are imaged as northeast-dipping thrusts that separate Wrangellia, a large Mesozoic–Paleozoic terrane, from two smaller accreted terranes: the Leech River schist, Mesozoic rocks that were metamorphosed in the Late Eocene; and the Metchosin Formation, a Lower Eocene basalt and gabbro unit. The Leech River fault, which was clearly imaged on both profiles, dips 35–45 °northeast and extends to about 10 km depth. The Survey Mountain fault lies parallel to and above the Leech River fault and extends to similar depths. The San Juan fault, the western continuation of the Survey Mountain fault, was not imaged, although indirect evidence suggests that it also is a thrust fault. These faults accommodated the Late Eocene amalgamation of the Leech River and Metchosin terranes along the southern perimeter of Wrangellia. Thereafter, these terranes acted as a relatively coherent lid for a younger subduction complex that has formed during the modem (40 Ma to present) convergent regime.Within this subduction complex, the LITHOPROBE profiles show three prominent bands of differing reflectivity that dip gently northeast. These bands represent regionally extensive layers lying beneath the lid of older accreted terranes. We interpret them as having formed by underplating of oceanic materials beneath the leading edge of an overriding continental place. The upper reflective layer can be projected updip to the south, where it is exposed in the Olympic Mountains as the Core rocks, an uplifted Cenozoic subduction complex composed dominantly of accreted marine sedimentary rocks. A middle zone of low reflectivity is not exposed at the surface, but results from an adjacent refraction survey indicate it is probably composed of relatively high velocity materials (~ 7.7 km/s). We consider two possibilities for the origin of this zone: (1) a detached slab of oceanic lithosphere accreted during an episodic tectonic event or (2) an imbricated package of mafic rocks derived by continuous accretion from the top of the subducting oceanic crust. The lower reflective layer is similar in reflection character to the upper layer and, therefore, is also interpreted as consisting dominantly of accreted marine sedimentary rocks. It represents the active zone of decoupling between the overriding and underthrusting plates and, thus, delimits present accretionary processes occurring directly above the descending Juan de Fuca plate. These results provide the first direct evidence for the process of subduction underplating or subcretion and illustrate a process that is probably important in the evolution and growth of continents.

Analysis of the Lithoprobe Deep Probe and Southern Alberta Refraction Experiment data sets, focusing on the region between Deep Probe shots 43 and 55, has resulted in a continental-scale velocity structural model of the lithosphere of platformal western Laurentia reaching depths of ~150 km. Three major lithospheric blocks were investigated: (i) the Hearne Province, a typical continental Archean cratonic province lying beneath the Western Canada Sedimentary Basin; (ii) the Wyoming Province, an even older block of Phanerozoic-modified Archean crust with an enigmatic lower lithosphere; and (iii) the Yavapai–Mazatzal Province, Proterozoic terranes underlying the Colorado Plateau and Southern Rocky Mountains. In this study, the northern two of these regions are investigated with a modified ray-theoretical traveltime inversion routine that respects the spherical geometry of the Earth. The resulting crustal velocity structure, combined with supporting geological and geophysical data, reveals that the Medicine Hat block (MHB), lying between the Hearne and Wyoming provinces, is a third independent Archean crustal block. The subcrustal lithosphere along the profile is homogeneous in velocity structure, but two significant northward-dipping reflectors are apparent and interpreted as relic subduction zones associated with sutures between the three Archean blocks. The Hearne crust is typical of an Archean shield or platform both in its thickness of 34–50 km and its seismic velocity structure. The crust of the Archean MHB and Wyoming Province, which ranges in thickness from 49 to 60 km, includes a 10–30 km thick high-velocity layer, interpreted to be Proterozoic in age. Such a feature is unexpected beneath Archean crustal provinces, but if the region is considered to be the remanent marginal portion of a larger Archean continent, then the interpreted Proterozoic underplating and lack of an Archean lithospheric root can be explained. The variable topography along the reflective upper and lower boundaries of this layer, especially within the MHB, suggests considerable variability in its emplacement and subsequent tectonic history.

A detailed seismic refraction survey was carried out over the Endeavour Segment of the Juan de Fuca Ridge, a medium-rate spreading center which lies off western North America, to investigate the creation and evolution of oceanic crust. A bathymetric high and the presence of hydrothermal vents suggested that the study area was the most recent locus of spreading. Travel time and amplitude data from 15 in-line air gun/ocean bottom seismometer profiles were forward modeled using asymptotic ray theory to obtain two-dimensional velocity models consisting of four primary layers which correlate well with classic models of oceanic crust. Significant lateral variations in thicknesses and velocities on the scale of a few to 10 km are superimposed on this basic velocity stmcture, but they appear to be random rather than distributed symmetrically about the ridge. We attribute them to fracturing which causes porosity changes, hydrothermal circulation which fills voids and fractures with alteration products, and variations in magmatic and/or deformational processes at the spreading center. Layer 2A is found to have low (2.6-2.8 km/s) velocities, to average 0.4 km in thickness with variations up to 0.2 km, and to be bounded at its base by a sharp velocity increase to 4.8 km/s. Along the axial ridge, velocities 0.4-0.6 km/s higher than average are interpreted for layers 2B and 2C, but these values are confined to a 2-km-wide zone centered below the ridge. Velocities along ridge-parallel lines offset 10 km are normal, indicating that maturation to off-ridge structure has occurred within at most 0.3 Ma. Layer 3 velocities decrease by 0.1-0.2 km/s for arrivals traveling along and under the axial ridge, perhaps caused by higher temperatures. However, we find no anomalously low velocities beneath the ridge, indicating that no large crustal magma chamber exists. On the basis of this study, we conclude that magmatic accretion is a fully three-dimensional process within ridge segments such as Endcavour Ridge. INTRODUCWION The Juan de Fuca Ridge (JDFR) system comprises three active spreading centers which extend from Cape Mendocino off northern California to the southern tip of the Queen Charlotte Islands in the northeast Pacific Ocean. The 500-km-long central JDFR spreading center is located between 44øN and 49øN (Figure 1) and has a full spreading rate of 6 cm/yr [Riddihough, 1984]. The Endeavour Ridge is a 90-km-long segment of the JDFR characterized by an along-axis, 500-m-high bathymetric bulge, where hydrothermal vent sites and a well-developed axial rift valley are manifest (Figures 2 and 3). These characteristics suggested that the region of the rift centered about 48øN may be the locale of a magma chamber. In order to determine the three-dimensional (3-D) crustal structure in this region and to investigate the processes by which the crust forms and ages, a detailed seismic refraction survey (SEISRIDG 85) was carried out using arrays of ocean bottom seismographs (OBS) with explosive and air gun sources. The explosive/air gun compo...

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.

The Vancouver Island Seismic Project was conducted in 1980 to study the structure of the subducting oceanic Juan de Fuca plate and the overriding continental America plate. The principal seismic refraction line (line I) was a 350‐km onshore‐offshore profile perpendicular to the continental margin. An array of 32 receivers was located on the America plate on the mainland and across Vancouver Island and extended offshore with three ocean bottom seismometers (OBS's). Two shots were fired at the eastern end of the line, and 17 shots were located along the westernmost 100 km of the profile. Control for the interpretation of the onshore‐offshore profile was provided by a reversed refraction profile along the length of Vancouver Island and by a marine refraction profile recorded on the OBS'S. Modeling of the seismic structure of this complex region utilized an iterative inversion method for travel times from explosions in which shots at several locations are recorded on the same set of receivers and utilized an algorithim based on asymptotic ray theory for the calculation of synthetic seismograms through two‐dimensional media. The major features of the refraction structural model are that (1) the oceanic lithosphere dips at 3° or less beneath the continental slope, so the bend in the subducting slab occurs landward of the foot of the slope. (2) the oceanic lithosphere dips at 14°–16° beneath the continental shelf until it passes beneath the continental Moho at 37 km depth below western Vancouver Island, (3) an upper mantle reflector may correspond to the base of the subducting lithosphere, and (4) a segment of high‐velocity material above the downgoing crust, with velocity 7.7 km/s and depth range 20–25 km, may represent a remnant of subducted lithosphere, perhaps detached when the subduction zone jumped westward to its present position.

Lithoprobe's Southern Alberta Refraction Experiment, SAREX, extends 800 km from east-central Alberta to central Montana. It was designed to investigate crustal velocity structure of the Archean domains underlying the Western Canada Sedimentary Basin. From north to south, SAREX crosses the Loverna domain of the Hearne Province, the Vulcan structure, the Medicine Hat block (previously considered part of the Hearne Province), the Great Falls tectonic zone, and the northern Wyoming Province. Ten shot points along the profile in Canada were recorded on 521 seismographs deployed at 1 km intervals. To extend the line, an additional 140 seismographs were deployed at intervals of 1.25–2.50 km in Montana. Data interpretation used an iterative application of damped least-squares inversion of traveltime picks and forward modeling. Results show different velocity structures for the major blocks (Loverna, Medicine Hat, and Wyoming), indicating that each is distinct. Wavy undulations in the velocity structure of the Loverna block may be associated with internal crustal deformation. The most prominent feature of the model is a thick (10–25 km) lower crustal layer with high velocities (7.5–7.9 km/s) underlying the Medicine Hat and Wyoming blocks. Based on data from lower crustal xenoliths in the region, this layer is interpreted to be the result of Paleoproterozoic magmatic underplating. Crustal thickness varies from 40 km in the north to almost 60 km in the south, where the high-velocity layer is thickest. Uppermost mantle velocities range from 8.05 to 8.2 km/s, with the higher values below the thicker crust. Results from SAREX and other recent studies are synthesized to develop a schematic representation of Archean to Paleoproterozoic tectonic development for the region encompassing the profile. Tectonic processes associated with this development include collisions of continental blocks, subduction, crustal thickening, and magmatic underplating.

No abstract

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.