Development of William’s Ridge, Kerguelen Plateau and Broken Ridge: tectonics, hotspot magmatism, microcontinents, and Australia’s Extended Continental Shelf

Coffin, Millard F1.; Whittaker, Joanne1; Daczko, Nathan2; Halpin, Jacqueline1; Bernardel, George3; Picard, Kim3; Gardner, Robyn2; Gürer, Derya4; Brune, Sascha5; Gibson, Sally6; Hoernle, Kaj7; Koppers, Antonius8; Storey, Michael9; Uenzelmann-Neben, Gabriele10; Magri, Luca1; Neuharth, Derek5; Christiansen, Sascha Høegh9; and Easton, Laura3

1Institute For Marine & Antarctic Studies, University Of Tasmania, Hobart, Australia, 2Macquarie University, Sydney, Australia, 3Geoscience Australia, Canberra, Australia, 4University of Queensland, Brisbane, Australia, 5GFZ German Research Centre for Geosciences, Potsdam, Germany, 6University of Cambridge, Cambridge, United Kingdom, 7GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany, 8Oregon State University, Corvallis, United States of America, 9Natural History Museum of Denmark, Copenhagen, Denmark, 10Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany

William’s Ridge, a ~300-km-long salient extending southeast from the Central Kerguelen Plateau, and Broken Ridge are conjugate divergent margins in the southern Indian Ocean that separated at ~43 Ma. In early 2020, scientists aboard Australia’s Marine National Facility, RV Investigator, acquired multichannel seismic reflection (MCS), sub-bottom profiling, multibeam bathymetry, and gravity data on these margins, as well as dredged rock samples, on a 57-day voyage. The research project constitutes the first-ever case study of conjugate oceanic plateau end-member tectonic plates, with the goal of advancing knowledge of lithospheric rifting, breakup, and initial plate separation processes. The first-ever dedicated multibeam mapping of William’s and Broken ridges encompassed ~52,000 km2 and ~43,000 km2, respectively. Four new RV Investigator MCS profiles (500 line-km) across William’s Ridge complement one legacy RV Rig Seismic and three new RV Sonne MCS profiles; five new RV Investigator MCS profiles (603 line-km) across the conjugate portion of Broken Ridge are the first to be acquired on that feature. Multibeam bathymetry and MCS transects of William’s Ridge show multiple linear ridges and troughs interpreted as horst and graben. 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 William’s Ridge yielded both oceanic and (in situ?) continental rocks; eight on Broken Ridge yielded solely oceanic 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 Australia’s marine jurisdiction on and around William’s Ridge under the United Nations Convention on the Law of the Sea.


Biography

Marine geophysicist Mike Coffin investigates interactions between the solid Earth and the oceanic environment. Educated at Dartmouth College (A.B.) and Columbia University (M.A., M.Phil., Ph.D.), he has pursued an international career in Australia, France, Japan, Norway, the UK, and the USA. His 35 research voyages span the global ocean.

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