Style, timing and distribution of tectonic deformation across the Exmouth Plateau, northwest Australia, determined from stratal architecture and quantitative basin modelling

Abstract

By combining a kinematic and flexural model for the deformation of the lithosphere with sequence stratigraphy, the distribution amplitude, depth-partitioning and interaction of the tectonic events responsible for the formation of the Exmouth Plateau, northwest Australia, have been determined. The style of deformation varied continually as a function of space and time. Initially, the deformation was characterized by a broadly distributed late Permian event. During late Triassic-middle Jurassic time, deformation became more localized and formed a series of sub-basins; the Exmouth, Barrow and Dampier Sub-basins. This localized phase of deformation was followed by a substantially more regional deformation event in the Tithonian-Valanginian that generated large, post-Valanginian regional subsidence across the Exmouth Plateau, with only minor accompanying brittle deformation and erosion. After the initiation of seafloor spreading, a phase of inversion occurred, which correlated with the reorganization of the Indo-Australian plates during magnetic anomaly M0-34 time. This inversion induced minor reactivation of fault systems within the sub-basins and adjacent areas. The style and distribution of each tectonic event are recorded by a diagnostic stratal architecture. For example, the late Permian event is recorded by the deposition of the broadly distributed and monotonous sequences of the Locker Shales and Mungaroo Formation. These units are more appropriately described as intracratonic in character. In contrast, the localized early Triassic and Callovian rifting events are characterized by regressive packages of thick shales (the upper and lower Dingo claystones). Crucial in the development of the upper and lower Dingo claystones was the segmented nature of the border faults that delineated the Exmouth, Barrow and Dampier Sub-basins. En echelon fault patterns and a basin synform structure, within which the pre-rift stratigraphy dips subparallel to or away from the fault trace, strongly implies a wrench influence in the development of the northern sub-basins (central Barrow and Dampier Sub-basins). This wrenching is consistent with the difference in regional trend of both the central Barrow and Dampier Sub-basins relative to the spreading fabric within either the Argo and/or Cuvier oceanic crust. The regional distribution and amplitude of the post-Valanginian subsidence is not consistent with the minor amounts of Tithonian-Valanginian brittle upper crustal extension observed on the margin. Facies and microfossil analyses suggest that, prior to extension, large portions of the platform were emergent, or at shallow water depths. To match the distribution and magnitude of the post-Valanginian thermal-type subsidence requires significant lower crustal and mantle extension across the Exmouth Plateau Basin. Such a distribution of extension implies the existence of an intracrustal detachment having a ramp-flat-ramp geometry that effectively thinned the lower crust and lithospheric mantle. A large injection of heat should accompany the Tithonian-Valanginian extension, being governed by the geometry of the detachment, and the distribution and amplitude of the lower plate extension. Implications for the generation of excessive tholeiitic magmatism during rifting support the notion that margin magmatism can occur within rift environments in the absence of a plume.