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Geology of the Apex Mountain group, north and east of the Similkameen River, south-central British Columbia

University of British Columbia

The Apex Mountain Group is an east-dipping sequence of rocks that may represent part of an ancient subduction complex that formed by progressive eastward-directed underthrusting and accretion of successively younger slices of oceanic sediments and volcanic rocks. The age of the group ranges from Carboniferous to Middle to Late Triassic (Ladinian-Karnian); however, no Permian rocks were positively identified. The youngest rocks lie in the western portion of the study area and are structurally below older rocks in the east. In contrast, local stratigraphic relationships show an opposite sense of facing with their younging direction being toward the east. The Apex Mountain Group consists predominantly of pelagic chert and extruded basalt (greenstone). The depositional environment is interpreted to be a generally deep (>2 km) open-ocean basin with the volcanic rocks being generated along an actively spreading ocean ridge. Quiet conditions prevailed; however, turbidity currents were common. Local areas of high relief such as fault scarps, or ocean ridges are postulated to account for the existence of local restricted to anoxic subbasins, submarine-fan breccia, turbidite deposits, and olistoliths. At least part of the Apex Mountain Group was deposited in shallow water, possibly along an isolated seamount or high standing ridge. Earliest recognizable deposition of the Apex Mountain Group began in the Early Carboniferous. Following deposition the sediments and the associated volcanic rocks were carried eastward (present sense) into an east-dipping subduction zone where successively younger slices of rock were scraped off and accreted below older underthrust slices. Phase 1 (Fl, Fl⁺) deformation occurred during this process and in ductile zones produced tight folds with axial surfaces parallel with the direction of shear in the subduction zone. Ridge spreading, deposition, and subduction continued until the Middle or Late Triassic. During late Paleozoic time an oceanic volcanic arc probably developed east of the Apex Mountain Group and vestiges of the arc or back-arc area may be represented by the Kobau, Anarchist, and Chapperon Groups. In Late Triassic time the locus of subduction shifted to a position west of the Apex Mountain Group where renewed subduction initiated the development of the Nicola Group volcanic arc. Regional uplift and erosion may have occurred as a result of the shift and could account for a regional unconformity below local Upper Triassic clastic rocks, the latter representing detritus shed from the emerging Nicola arc and from eroded upper Paleozoic sequences. Phase 2 (F2) structures in the study area are possibly related to regional strike-slip movement in post Late Triassic-Early Jurassic time. In general, the rocks of the Apex Mountain Group have been metamorphosed to lower greenschist facies.

Text

2010-05-15

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http://hdl.handle.net/2429/24732

Geological Sciences

University of British Columbia

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