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Orthorhombic faulting in the Beagle Sub-basin, North West Shelf, Australia

Abstract The Beagle Platform forms a structurally complex outboard boundary to the Mesozoic Beagle Sub-basin in the Northern Carnarvon Basin in 50–1000 m of water c. 250 km offshore on Australia's North West Shelf. North- and NE-trending conjugate fault systems link to form c. 5–10 km-wide rhomboidal horsts bound by c. 5 km-wide graben in complex orthorhombic symmetry. Interpretation of the Canning TQ3D three-dimensional (3D) seismic survey identified four populations of faults comprising: (1) latest Triassic–Early Cretaceous north–south-striking normal faults (Fault Population I); (2) latest Triassic–Early Cretaceous NE–SW-striking normal faults (Fault Population II); (3) Cretaceous polygonal faults (Fault Population III); and (4) Neogene–present-day north–south- and NE–SW-trending en echelon conjugate fault arrays (Fault Population IV). Structural interpretation, comprising fault seismic interpretation, displacement analyses and fault orientation analysis, illustrates that orthorhombic extensional faulting occurred penecontemporaneously. Three-dimensional non-plane strain with non-zero intermediate (σ2) extension magnitude controls the near-synchronous displacement of fault population I and II conjugate fault sets in orthorhombic symmetry to create the characteristic rhomboidal fault geometry. Neogene–present-day north–south- and NE–SW-striking en echelon conjugate fault arrays (Fault Population IV) form in response to oblique reactivation of these subjacent latest Triassic–Early Cretaceous polymodal faults (Fault Population I and Fault Population II). Fault Population I, II and IV together form a vertically decoupled (soft-linked) pseudo-conjugate fault system partitioned by a Cretaceous interval characterized by polygonal fault systems (Fault Population III) and monoclinic draping. The structural interpretation of the Beagle Platform illustrates a seismic-scale orthorhombic fault symmetry accommodating 3D strain and the insufficiency of plane-strain ‘Andersonian’ conjugate fault theory to resolve complex polymodal faulting evolved penecontemporaneously under a single stress regime. Subsequent oblique extension reactivation of these polymodal fault systems also demonstrates that complex soft-linked en echelon extensional conjugate fault arrays form in response to a single stress regime. This research supports the evolution of complex polymodal faulting under simple states of stress, and has implications for understanding the distribution, linkage and age of extensional faults within the Beagle Sub-basin and other extensional basins.