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

Abstract

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 1380-1410 °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).

Petrophysical Property Modifications by Alteration in a Volcanic Sequence at IODP Site U1513, Naturaliste Plateau

“…On the other hand, the massive and non‐ to sparsely vesicular nature of Unit 1 and Unit 3 flows, combined with their greenish‐gray color and absence of subaerial oxidization, suggest eruption in a submarine setting. This changing eruptive setting can be indicative of emplacement in a subsiding basin (Harry et al., 2020). The ongoing subsidence is indicated in the deposition of overlying volcaniclastic‐rich sediments from shelf to upper bathyal depths (Lee et al., 2020).…”

Rift to Drift Transition in the Southwest Australian Deepwater Mentelle Basin

“…The oldest horizon mapped in this study is the Valanginian Unconformity. It lies underneath the basalt pile encountered at site U1513D and corresponds with breakup and onset of seafloor spreading on the PAP further north (133 -132 Ma) (Harry et al, 2020;Hall et al, 2013;Borissova et al, 2010). The unconformity surface has only been drilled in the MB at site U1515A, in the eastern part of the basin on the western flank of the Yallingup Shelf (Fig.…”

“…A similar unconformity imaged in MB seismic data has been interpreted to correlate with the Perth Basin Valanginian Unconformity. In the western MB, the Valanginian Unconformity is overlain by a basaltic sequence (correlated to the Bunbury basalts) that interfingers with volcaniclastic sandstone beds encountered at IODP Site U1513 during Expedition 369 (Huber et al, 2019b;Lee et al, 2020;Harry et al, 2020). The Bunbury basalts in the PB are overlain by the deltaic and marine Gage Sandstone and South Perth Shale in the Vlaming Sub-basin (Fig.…”

“…Another model suggests regional contamination of the upper mantle by the Kerguelen plume head (or other magmatic episodes) prior to the onset of rifting. This enriched fertile mantle was then tapped at different times and places during the rifting process (Harry et al, 2020;Kent et al, 1991;Storey et al, 1989). A third idea, similar to the rising mushroom plume model, is that the Kerguelen plume acted as a deep reservoir that fed individual rising columns, which then fed magmatism at various places and times as the Indian Ocean opened (Coffin et al, 2002).…”

“…The seismic data shows that these flows are stratigraphically and spatially widespread in the lower half of the volcaniclastic unit and are common but more spatially restricted to the region around the basin axis in the upper half of the unit. They lie between the top of Unit 5 (Aptian) and the basalts (constrained to be > 132 Ma), so are between Aptian and at least late Valanginian in age (Harry et al, 2020).…”