Development of Williams Ridge (Kerguelen Plateau) and Broken Ridge: Tectonics, Hotspot Magmatism, Microcontinents, and Australias Extended Continental Shelf

Williams Ridge, an ~300-km-long salient extending southeast from the Central Kerguelen Plateau, and Broken Ridge, located ~2600 km to the north, are conjugate divergent margins in the southern Indian Ocean that separated at ~43 Ma. In early 2020, we acquired multichannel seismic reflection (MCS), sub-bottom profiling, multibeam bathymetry, and gravity data on these margins, as well as dredged rock samples. Our project constitutes the first-ever case study of conjugate oceanic plateau end-members, with the goal of advancing knowledge of lithospheric rifting, breakup, and initial plate separation processes. The first dedicated multibeam mapping of Williams and Broken ridges encompassed ~52,000 km2 and ~43,000 km2, respectively. Seven new RV Investigator (4) and RV Sonne (3) MCS transects of Williams Ridge totalling 945 line-km complement one legacy RV Rig Seismic MCS transect (~150 line-km); five new RV Investigator MCS transects of the conjugate portion of Broken Ridge totalling 603 line-km are the first to be acquired on that feature. Multibeam bathymetry and MCS transects of Williams Ridge show multiple ESE-WNW-striking linear ridges and troughs interpreted as horsts and grabens. In contrast, multibeam bathymetry and MCS transects of Broken Ridge show a prominent E-W scarp (Diamantina Escarpment) with a complex morphology of emanating en echelon crustal blocks and depressions at the base of the scarp. Prominent angular unconformities (middle Eocene hiatus?) characterize the sedimentary section on some ridges, and dipping reflection sequences within interpreted igneous basement suggest subaerial basalt flows. Rock dredges on the facing conjugate margin fault scarps targeted all stratigraphic levels exposing basement rocks. Nine on Williams Ridge yielded both mafic extrusive and (in situ?) continental rocks; eight on Broken Ridge yielded solely mafic extrusive rocks. The new geophysical data and geological samples may justify a new or revised submission to the United Nations Commission on the Limits of the Continental Shelf to extend Australias marine jurisdiction on and around Williams Ridge under the United Nations Convention on the Law of the Sea.