RIFT TO DRIFT TRANSITION IN THE SOUTHWEST AUSTRALIAN DEEPWATER MENTELLE BASIN

The deepwater Mentelle Basin formed off of the southwest margin of Australia during Late Jurassic and Early Cretaceous breakup of Gondwana. It is underlain by highly extended continental crust and is bordered to the west by the less extended Naturaliste Plateau, and to the east by the Yallingup Shelf. The Yallingup Shelf is a structural high that separates the Mentelle Basin from the coeval Perth Basin that lies beneath the continental shelf and coastal plain. This study reports the depositional, subsidence, and tectonic histories of the Mentelle Basin during the syn-rift to post-rift transition based on seismic reflection data and boreholes drilled by the Deep Sea Drilling Project and International Ocean Discovery Program (IODP). Depth structure and isochore maps were generated for five horizons deposited between breakup on the Perth Abyssal Plain to the north at ca. 136 Ma and breakup west of the Naturaliste Plateau at ca. 126 Ma. These correspond with 1) the top of Lithostratigraphic Unit 5 (LSU5; 134-126 Ma) encountered on the western edge of the Mentelle Basin at IODP Site U1513, and its more easterly age equivalents; 2) the top of a 135.4 ± 4.0 Ma basalt sequence underlaying LSU5 at Site U1513; 3) the top of a series of high amplitude reflectors within the LSU5 stratigraphic interval that are interpreted to indicate ongoing volcanism in the central and eastern parts of the Mentelle Basin during deposition of LSU5; and 4) reflectors marking the shallowest and deepest interpreted positions of the early Hauterivian unconformity that is associated with breakup further North on the Perth Abyssal Plain. Key findings are: 1) syn-rift magmatism lasted from Valanginian through middle Hauterivian and spanned the breadth of the Mentelle Basin. 2) Volcanism in the Mentelle Basin continued after breakup on the Perth Abyssal Plain to the north (136 Ma) until breakup on the Mentelle Basin/Naturaliste Plateau portion of the margin at 126 Ma. During this time the Mentelle Basin and Naturaliste Plateau remained at shallow enough depths to permit subaerial eruption of flows extending ca. 2-20 km distances and formation and weathering of small (0.5 – 2.5 km wide) volcanic cones. An abrupt transition to deepwater claystone deposition above LSU5 indicates rapid subsidence following breakup west of the Naturaliste Plateau after 126 Ma.