Abstract

At Pinchi Lake, the Pinchi Fault Zone separates the early Mesozoic Takla Group to the east from the late Paleozoic Cache Creek Group to the west. Between these regions a complex fault system involves a series of elongate fault-bounded blocks of contrasting lithology and metamorphic grade. These blocks consist of: (a) highly deformed aragonite–dolomite limestone and blueschist, (b) pumpellyite–aragonite greenstone, (c) a harzburgite–gabbro–diabase–basalt ophiolite sequence, (d) serpentinized alpine ultramafite, and (e) Cretaceous (?) conglomerate. The blueschist probably formed at 8–12 kbar (8 × 105–12 × 105 kPa) and 225–325 °C during a penetrative early deformation which was closely followed by a later deformation associated with a Late Triassic uplift and cooling event. The ophiolite sequence is overlain by Late Triassic sediments which locally contain aragonite suggesting that at least part of the Takla Group may have also undergone high pressure – low temperature metamorphism.The evolution of the 450 km fault zone is discussed and a model is proposed which involves right lateral transform faulting on the Pinchi Fault and underthrusting along northerly dipping subduction zones during the Late Triassic. The blueschist formed at high pressures in such a subduction zone and leaked to the surface in zones of low pressure along an active transform fault.

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