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 ( σ 2 ) 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.

Abstract The Perth Basin is a major tectonic province along the western margin of the Australian continent. Basin morphology is controlled by north-striking faults formed during Permian rifting and reactivated during later tectonic events, notably during continental break-up in Late Jurassic–Early Cretaceous time. Transfer structures, including those normal and oblique to the major faults, compartmentalized the basin into segments of distinctive character. East-west transfer faults, perpendicular to the basin trend, were active throughout the rift stage of basin development and are recognized only in the northernmost onshore part of the Perth Basin, corresponding to the depocentre for Permian sediment accumulation. Northerly trending normal faults change in character and/or terminate at these east-west structures. The NW-striking transfer zones influenced deformational features formed during the Late Jurassic–Early Cretaceous break-up. No continuous fault plane has been identified with these zones in the sedimentary sequences. They are characterized by the termination and/or swing of major normal faults at the transfer zones. Sinistral strike-slip movement of at least 16 km is recognized across the Abrolhos Transfer Zone on the basis of offset in the trend of the Beagle Fault system. The orientation, age of activation, and position of these zones are similar to those of transform faults in the adjoining Indian Ocean, suggesting that the two structures are contiguous.