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The Leech River Complex, part of the Pacifi c Rim Terrane, is a Cretaceous metasedimentary and metaigneous assemblage on southern Vancouver Island. The Leech River Complex is fault-bounded between the Eocene Metchosin Igneous Complex to the south (part of the Crescent Terrane) and the Paleozoic to Jurassic Wrangel Terrane to the north and provides critical information on the evolution of the central part of the western North American forearc in Cretaceous through Eocene time. Single detrital zircons from the metasedimentary component, known as the Leech River Schist, give U-Pb interpreted ages that range from Precambrian to ca. 103 Ma, indicating a varied source region and a probable Early Cretaceous depositional age. U-Pb geochronology and fi eld investigations indicate at least two magmatic-metamorphic events in the Leech River Complex: one during the Late Cretaceous, and the other during the early Middle Eocene. The peraluminous Jordan River metagranodiorite, a fi ne-grained biotitic stock and related dikes intruded the Leech River Complex at ca. 88 Ma, during the older magmatic event. Metamorphic pressure-temperature conditions of 525-550 °C and 2-3 kbar are recorded in the contact aureole. The later event occurred during emplacement of the Walker Creek intrusions, a suite of peraluminous tonalite, trondjhemite, and granodiorite dikes that intruded the Complex at ca. 51 Ma and produced a similar metamorphic aureole. Both intrusive suites have

The Cowichan fold and thrust system on southern Vancouver Island comprises a linked system of folds and thrust faults in crystalline and sedimentary rocks of the Wrangellia terrane and its sedimentary cover, the Upper Cretaceous Nanaimo Group, and Upper Paleocene to Lower Eocene sandstones of the Chuckanut Formation. Apatite fission-track analysis indicates that cooling below the closure temperature of fission-track retention in apatite (100 ± 20 °C) for both the Island Plutonic Suite from the Wrangellian basement and the Nanaimo Group occurred at ~ 50–40 Ma (Middle Eocene). Thrusting is interpreted to have occurred immediately prior to this time in the Middle Eocene, but after post-Cretaceous regional burial of the Turonian to Maastrichtian Nanaimo Group and deposition of Upper Paleocene and Lower Eocene sandstones, which are involved in the deformation. These timing constraints support the hypothesized temporal linkage between contraction and uplift of Wrangellia during the Middle Eocene and continued margin-normal contraction of the Pacific Rim terrane (± the Crescent terrane) at ~ 45 Ma.

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Seismic reflection and geological studies conducted on southern Vancouver Island during 1984 show that the region has sustained considerable shortening above a widespread decollement zone. Beneath Wrangellia, a thick interval of possibly underplated pre-Upper Miocene oceanic crust overlies the modern subducting Juan de Fuca Plate. The latter is clearly seen on all record sections and comprises an undef armed layered sequence resting upon presumed oceanic crust. The Leech River Fault is identified as a surf ace dipping towards the north at 3.5°. Tleneath the fault the Eocene Metchosin Volcanics and Sooke Gabbro are seen to overlie a success ion that may have its correlatives in the core zone Olympic Mountains of northern Wash ington. Resume Des leves de sismique-reflexion et geologiques eff ectues dans le sud de l'ile de Vancouver en 1984 montrent que cette region a subi un retrecissement considerable au-dessus d'une vaste zone de decollement. Au-dessous de Wrangellia, un epais intervalle de cror1te oceanique, peut-etre sus-jacent a une plaque et forme avant le Miocene superieur, recouvre l'actuelle plaque de subduction de Juan de Fuca. On voit clairement cette derniere dans taus les diagrammes bases sur les enregistrements; elle est constituee d'une sequence stratifiee non def ormee, reposant sur la cror 1te oceanique hypothetique. On a identifie la faille de Leech River comme etant une faille superficielle dont le pendage est de 35° vers le nord. Au-dessous de la faille, les roches volcaniques de Metchosin et le gabbro de Sooke d'age Eocene recouvrent une succession dont les e1ements corre1atif s se trouvent sans doute dans la zone centrale des monts Olympiques, dans le nord de l'Etat de Washington.