ABSTRACT

Extension faults are normal cross faults that are sub-parallel to a collision front and commonly cut into the asthenosphere. During periods of uplift and high heat flow, faultblocks become mobile along the deeply cutting longitudinal faults, resulting in extension of the crust sub-parallel to the collision front. Significant is the role of longitudinal faults in pervasive dolomitization during periods of high heat flow. During the Devonian, the East Pacific Plate may have collided more than once with the North American Plate, but the effect of subduction, if any, was probably minor. However, uplift such as the Western Alberta Ridge may be a product of mid-Devonian subduction of the continent by the east Pacific Plate. During its long history, the Peace River Arch always represented a tectonic or structural anomaly in the Western Canada Basin. Its crest did not line up with the northwest-southeast trend of the basin but parallelled the regional northeast-southwest strike of the tectonic belts within the Precambrian Shield. The demise of the Peace River Arch began in the late Famennian and was completed during the Early Carboniferous with the development of the Peace River Embayment. Complete to partial removal of Famennian strata in northeast British Columbia west of 124° longitude in latest Devonian-post Wabamun time suggests a major eastward push of the East Pacific Plate. This resulted in uplift of the Peace River Arch accompanied by considerable heatflow and possible thermal thinning of the lower crust. During the Early Carboniferous, continued lithospheric stretching and extension faulting resulted in the collapse of the Peace River Arch. The collapse started with the forming of the Fort

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St. John graben and downfaulting of the northern part of the Peace River Arch and ended with the development of the Peace River Embayment. The effect of the longitudinal faults acting as extension faults may have been to re-align the axis of the newly formed Peace River Embayment in such a way that it turned parallel to the main trend (northwest-southeast) of the longitudinal faults.

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