Depth-dependent Extension of the Continental Lithosphere: A Relook at Seismic Refraction Results from the Exmouth Plateau and Cuvier Margin (Invited)

Accepted paradigms for the formation of passive continental margins and thinning of continental lithosphere in extension have been seriously challenged by the discovery of exhumed continental mantle rocks and a deficit of magmas in ocean continent transitions, the existence of top-basement detachments, and the recognition of an “extension discrepancy” across many passive continental margins. These observations are consistent with vertical partitioning of extension of the continental lithosphere. There are other explanations, however, that suggest that this extension discrepancy relates instead to complex fault and crustal block geometries that may not be fully imaged. This may lead to an underestimation of the amount of extension. In contrast to depth-dependent extension, such a scenario implies that pre-rift and early syn-rift reservoir and source rocks are likely to be widely scattered across ultra-deep margins. There is little unambiguous evidence for multiple-fault rotations across many margins. Thus, the process by which crust thins from 30 km to less than 10 km remains elusive, as does demonstrating a balance for the vertical partitioning of extension. Continental breakup between the Northwest Australian Exmouth Plateau and Cuvier margin from Greater India occurred during the Valanginian and yet, the pre-Valanginian depositional packages across the plateau and Cuvier margin do not confirm to our notion of syn-rift units as characterized by normal faulting, divergence of seismic reflectors, and sediment wedge geometries. Rather, regional sagging and a notable lack of faulting of the appropriate age define the accommodation responsible for the Tithonian-Valanginian Barrow Group and the thin sediment cover and general basement rollover of the Cuvier margin. There is no evidence of multiple fault rotations or early volcanic intrusions. In fact, for the Cuvier margin, there is no evidence or any faulting whatsoever - only a regional truncation surface of Valanginian age and an abrupt transition to oceanic crust. This truncation most likely sourced the Barrow Group sediments of the Exmouth Plateau. The velocity structure of the Exmouth Plateau suggests extreme thinning of the lower crust relative to the upper crust and for the Cuvier margin, the lower crustal iso-velocity lines appear to be decapitated and exhumed, hinting at the presence of a top basement detachment in thinning the crust. The balance of lower and upper plate extension across the Exmouth Plateau implies that the adjacent seafloor should be characterized by exhumed lower crust and continental mantle. While there is an abrupt increase in seismic crustal velocities in this adjacent region, seismic reflection data are nevertheless inconsistent with the hypothesis of an exhumed lower crust and mantle. Rather, this region is characterized by 1) a small number of major normal faults that effectively terminate the upper crust, and 2) seaward dipping sequences that comprise the entire oceanic crust. The lack of the conjugate margin is a clear impediment to understanding fully the extensional balance of the Northwest Australian margin.