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.

Phase 1 of LITHOPROBE involved co-ordinated geophysical and geological studies on southern Vancouver Island conducted from 1984 to 1986. Geological studies, the focus of this report, included 1:50 000 scale mapping as well as stratigraphic, geochemical, and structural studies in a corridor centred on Port Alberni and on southernmost Vancouver Island in the vicinity of Sooke. In the Port Alberni region the geological architecture of Wrangellia is expressed by three volcanic/plutonic successions separated by shallow marine carbonate and clastic deposits, the total ranging in age from mid-Paleozoic to Early Jurassic. Post accretionary clastic sediments of Late Cretaceous and Tertiary ages overlie Wrangellia and the adjacent Pacific Rim and Crescent terranes. The Alberni region is partitioned by northwesterly trending thrust faults and the Cowichan Anticlinorium, the latter dislocated by northerly trending faults and hypabyssal intrusions. The Metchosin Igneous Complex of southernmost Vancouver Island is part of the Crescent Terrane. It consists of layered gabbro, sheeted dykes, pillow basalts and flows forming a partial ophiolite. The complex represents an emergent volcanic island formed in a marginal basin setting. It was transported northwestward by and emplaced beneath the Leech River Fault. Integrated geological and seismic reflection data indicate that the Wrangellian crust of Vancouver Island is largely granitic rocks of the Island Intrusions with pendants and enclaves of Paleozoic to Lower Jurassic volcanic and sedimentary rocks. Prominent inclined reflectors in the seismic records are identified with the surface traces of thrust faults.

No abstract

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

A large, previously unstudied amber deposit in British Columbia dating from the Early to Middle Eocene (50−55 Ma) provides a noteworthy new source of terrestrial invertebrates and other life forms. This deposit contains what are likely the earliest unequivocal ants (members of the family Formicidae), including extinct representatives of Technomyrmex Mayr 1872, Leptothorax Mayr 1855, and Dolichoderus Lund 1831. Discovering Technomyrmex and a corydiinid cockroach, both of which are currently restricted to tropical regions, confirms earlier evidence of warm paleoclimates and past biogeographic distributions in the early Paleogene. Chemical analysis of the amber indicates that the source tree was an araucarian belonging to or near the genus Agathis Salisbury 1807, and demonstrates that this genus survived into the Tertiary in the Northern Hemisphere, since previous records revealed Agathis as a component only of the Cretaceous forests in North America. Comparing the Hat Creek fossil assemblages in this deposit with those from the well-studied western Canadian Late Cretaceous amber deposits offers a unique opportunity to study extinction and speciation events on both sides of the Cretaceous–Tertiary boundary.