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Geology and tectonic setting of the Kamloops group, South-central British Columbia Ewing, Thomas Edward


The Kamloops Group is a widespread assemblage of Eocene volcanic and sedimentary rocks in south-central British Columbia. Detailed mapping of the type area near Kamloops has resulted in its subdivision into two formations and thirteen formal and informal members. The Tranquille Formation, 0-450 metres thick, consists of lacustrine sediments which grade upward into pillowed flows, hyaloclastite breccia and aquagene tuff. The overlying Dewdrop Flats Formation, with nine members, consists of up to 1000 metres of basalt to andesite phreatic breccia, flow breccia and flat-lying flows. In one large and four minor volcanic cones, basal phreatic volcanic rocks pass upward into subaerial flows and breccia. A fault zone of inferred strike-slip displacement to the southwest, and a complex reverse-faulted zone to the south of the area, localized deposition of the Tranquille Formation. Later fault activity created the complex Tranquille Canyon graben, in part filled with Dewdrop Flats Formation volcanics. Reconnaissance of the Kamloops Group throughout the Thompson-Okanagan region, and detailed mapping at McAbee and Savona, have shown that most Kamloops Group sections consist of a lower sedimentary and volcanosedimentary unit, followed by thicker, dominantly basalt to andesite, flow and breccia units. Thick sedimentary accumulations were localized in zones of extension within a network of strike-slip faults. The Kamloops Group is a high-alkali calc-alkaline volcanic suite dominated by augite - pigeonite - labradorite andesite and basalt, with unusually high K, Sr and Ba. Initial strontium isotopic ratios distinguish a boundary between 'old' crust upper mantle to the east and 'young' or Rb-depleted materials to the west. Petrographic and chemical data are consistent with magma genesis by partial melting of alkali-enriched peridotite between 40 and 75 km depth, with subsequent deep- and shallow-level fractional crystallization producing the observed volcanic chemistry. Compilation of Paleogene geology and geochronometry in the Pacific Northwest shows the Kamloops Group to be part of a robust calc-alkaline volcanic arc extending from Wyoming to Alaska. Superimposed on this arc were dextral, strike-slip faults, sedimentary basins and reset metamorphic terranes. These elements formed a tectonic, network which accommodated 90 to 450 kilometres of right-lateral displacement between coastal British Columbia and North America. A tentative plate-tectonic reconstruction is based on the compilation. A northeast-dipping subduction zone, active along the entire coast of the Paleocene Pacific Northwest, ceased to be active after 53 Ma. The transform motion between Pacific and North American plates was distributed inland, driving the Eocene tectonic activity, while the remnant subducted slab gave rise to the Eocene magmatic arc. Transform motion later became localized along the continental margin, as the east-dipping subduction zone south of 49° latitude intiated the Cascade volcanic arc in the Late Eocene and Oligocene.

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