The structure and development of the continent-ocean transition zone of the Exmouth Plateau and Cuvier margin, Northwest Australia: Implications for extensional strain partitioning

Unlike continental rifts, where the presence of wedge-shaped syn-rift sediment packages highlights the importance of crustal brittle deformation during rifting, many passive margins appear to be dominated by regional syn-rift sagging, indicating that faulting, if it exists, does not control the generation of accommodation. Instead, the sagging implies that lower crust and lithospheric mantle are preferentially thinned. If extension is geometrically balanced throughout the lithosphere, the depth-dependent extension represented by the preferential thinning requires a much broader extended lower crust and lithospheric mantle relative to the upper crust, causing a strain imbalance that presumably is located in the continent-ocean transition zone. By studying the crustal structure and stratigraphy of two adjacent margin segments offshore Northwest Australia, the Exmouth Plateau and Cuvier margin, and using the obtained results as input for a quantitative basin model, the mode, timing, amplitude and distribution of extension across the margins were determined and used to test the strain partitioning hypothesis and to map the required strain balance.; The Exmouth Plateau is characterized by a steep gradient in crustal velocity that marks the boundary between a wide continent-ocean transition zone (COTZ) and the western edge of a broad plateau. The margin was affected by multi-stage rifting. During the final phase that led to breakup, the crust underlying the plateau was affected by extensive thinning of the lower crust and mantle lithosphere, while accompanying brittle deformation of the upper crust was only minor. In order to preserve the strain balance across the margin, the modeling requires a 400 km wide continent-ocean transition zone, consisting mostly of exhumed lithospheric mantle, which has been serpentinized and magmatically altered. Such a relationship is consistent with the development of the Iberian-Newfoundland conjugate margins. In contrast, thinning was focused in a small region along the Cuvier margin, leading to a narrow transition between a high-velocity continental crust and thicker than normal oceanic crust.