Evolution of the Southwest Australian Rifted Continental Margin During Breakup of East Gondwana: Results From International Ocean Discovery Program Expedition 369.

International Ocean Discovery Program Expedition 369 drilled four sites on the southwestern Australian continental margin, in the deep water Mentelle Basin (MB) and on the neighboring Naturaliste Plateau (NP). The drillsites are located on continental crust that continued rifting after seafloor spreading began further north on the Perth Abyssal Plain (PAP) between magnetochrons M11r and M11n (133–132 Ma), ending when spreading began west of the NP between chrons M5n and M3n (126–124 Ma). Drilling recovered the first in situ samples of basalt flows overlying the breakup unconformity on the NP, establishing a magnetostratigraphically constrained eruption age of >131–133 Ma, and confirming a minimal late Valanginian age for the breakup unconformity (coeval with the onset of PAP seafloor spreading). Petrogenetic modeling indicates the basalts were generated by 25% melting at 1.5 GPa and a potential temperature of 1380°C–1410°C, consistent with proximity of the Kerguelen plume during breakup. Benthic foraminiferal fossils indicate that the NP remained at upper bathyal or shallower depths during the last 6 Myr of rifting and for 3–5 Myr after breakup between India and Australia. The limited subsidence is attributed to heat from the nearby Kerguelen plume and PAP spreading ridge. The margin subsided to middle bathyal depths by Albian time and to lower bathyal (NP) or greater (MB) depths by late Paleogene time. Periods of rapid sedimentation accompanied a westward jump of the PAP spreading ridge (108 Ma), rifting on the southern margin (100–84 Ma), and opening of the southern seaway between Australia and Antarctica (60–47 Ma). Plain Language Summary: The southwestern Australian margin formed during the breakup of the supercontinent Gondwana during the Late Jurassic and Early Cretaceous Periods. International Ocean Discovery Program Expedition 369 drilled four sites on the southwestern Australian rifted continental margin in order to better understand the subsidence and magmatic behavior of the margin during the final stages of rifting. Drilling shows that a widespread unconformity imaged in seismic data on the margin correlates with the onset of seafloor spreading on the Perth Abyssal Plain at 132–133 Ma. This was followed by eruption of basalts on the Naturaliste Plateau, which were generated from melting of the underlying mantle that was enhanced by the nearby Kerguelen mantle plume. Proximity of the plume kept the margin at shallow depths during rifting and for about 3–5 Myr after final separation of India and Australia at 126 Ma. The margin subsided to > 1,000 m depths between about 121 Ma and the present, with intervening periods of relatively rapid sedimentation associated with adjustments to the spreading ridge configuration in the Indian Ocean at 108 Ma, and rifting events between southwestern Australia and Antarctica at 100–84 Ma and 60–47 Ma. Key Points: Continental breakup occurred at 132–133 Ma on the Perth Abyssal Plain and at 126 Ma west of the Naturaliste PlateauVolcanism on the Naturaliste Plateau with both plume and mid‐ocean ridge compositional affinities accompanied Perth Abyssal Plain breakupLate synrift and early postrift subsidence was limited by proximity of the Kerguelen plume and Perth Abyssal Plain spreading ridge [ABSTRACT FROM AUTHOR]