The Nature of Faulting Along the Margins of the Fitzroy Trough, Canning Basin, and Implications for the Tectonic Development of the Trough

The Fitzroy Trough is a northwest/southeast trending series of half graben which formed along the northeast margin of the Canning Basin. New deep seismic reflection data are used to revise the tectonic model of the trough. The trough formed by periods of crustal extension in the latest Middle to Late Devonian, the Carboniferous and the Permian. Estimates of crustal extension based on a structural analysis of the sedimentary strata are 20-25 km, 10 and 5 km in each of these periods, respectively. This represents an extension of approximately 50%, broadly consistent with the amount of crustal thinning beneath the trough. The crust may also have extended during the Ordovician. Where the deep seismic line crossed the trough, extension occurred on two half graben — one under the anticline at Mt Wynne and the other on the Fenton Fault. A third half graben began to form by backstepping of the Fenton Fault, but failed to evolve as a major structure before extension ceased. The half graben are bounded on their southwest sides by listric normal faults which sole onto a sub-horizontal detachment surface at about 7 s two-way time (15-17 km). The Pinnacle Fault, previously thought to be a major bounding fault on the northeast margin of the trough, now appears to be antithetic to the major listric normal faults bounding the southwest sides of the half graben. It detached into the top of the Ordovician/Silurian sequence. Transpression during the Jurassic warped the sediments in the trough into a series of broad synclines and anticlines. The anticlines were focussed on and accentuated pre-existing local highs. Mounding of the basal sediments in the trough, possibly accentuated by slip along salt layers, deformed younger overlying sediments along the northeast side of the trough. It produced movement of the basal sediments up dip out of the trough towards the northeast margin, and possibly also into the core of the Mt Wynne Anticline.