Abstract

The Peace River Arch (PRA) is a roughly east-west striking cratonic element of the Western Canada Sedimentary Basin that cuts across the trend of underlying Paleoproterozoic basement tectonic domains at a high angle. This region preserves several distinct phases of tectonic evolution during the Phanerozoic that included an early- to mid-Paleozoic arch phase and a Carboniferous-to- Triassic embayment phase. In 1994, Lithoprobe recorded the first public-domain regional seismic-reflection survey across the PRA, a 627-km transect called the Peace River Arch Industry Seismic Experiment (PRAISE). This survey combined typical industry acquisition parameters with large shot-receiver offsets (>6 km) and long recording times (18 s) to image both bedding-scale and crustal-scale tectonic elements. The regional extent, continuity and long recording times of the PRAISE seismic profiles are exploited here to examine various aspects of basement and basin evolution from Middle Cambrian to Triassic time. Key findings of this investigation are: (1) a greater than expected role for onlap in the overall northward thinning of the Cambro-Ordovician sedimentary wedge onto the southern flank of the PRA; (2) evidence for a previously unrecognized pre-Middle Devonian deformational event that produced broad, low-relief folds in Cambro-Ordovician rocks; (3) seismic evidence that some Granite Wash deposits formed within Upper Devonian grabens that may have been precursors to Carboniferous extensional faults; (4) a spatial overlap of the southeastern arm of the Fort St. John graben with a strong, east-dipping reflection fabric developed in basement rocks of the Ksituan domain, but otherwise an absence of definitive seismic evidence to link Phanerozoic extensional faults directly with antecedent basement structures; (5) observation of strain partitioning within some northwest-trending Carboniferous extensional structures, including the Tangent and Dunvegan Faults, that exhibits characteristics of extensional forced folding; (6) evidence that westward thickening of Lower and Middle Triassic strata beneath the Upper Triassic Coplin Unconformity is primarily the result of tilting and erosion. One of the most spectacular discoveries from the reflection profiles is a sequence of exceptionally continuous, subhorizontal basement reflections interpreted as Paleoproterozoic mafic sills that intruded into the upper crust of the eastern and central PRA. This network of sills may have modified the rheology of the crust by acting as strong beams, thus providing a possible mechanism for localization of anomalous stresses that ultimately controlled the vertical tectonic evolution of the Peace River Arch.

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