V21A-3007
Unraveling the unusual morphology of the Cretaceous Dirck Hartog extinct mid-ocean ridge

Tuesday, 15 December 2015
Poster Hall (Moscone South)
Sally Joan Watson1, Joanne M Whittaker1, Jacqueline Halpin1, Simon Williams2, Luke A Milan3, Nathan R Daczko4 and Derek A Wyman5, (1)University of Tasmania, Hobart, Australia, (2)University of Sydney, Sydney, Australia, (3)University of New England Australia, Division of Earth Sciences, School of Environmental & Rural Science, Armidale, Australia, (4)Macquarie University, GEMOC and CCFS, Department of Earth and Planetary Sciences, Sydney, Australia, (5)University of Sydney, School of Geosciences, Sydney, Australia
Abstract:
The Perth Abyssal Plain (PAP), offshore southwest Australia formed during Mesozoic East Gondwana breakup and Kerguelen plume activity. This study combines petrographic and geochemical data from the first samples ever to be dredged from the flanks of the Dirck Hartog Ridge (DHR), a prominent linear bathymetric feature in the central PAP, with new bathymetric profiles across the PAP to better constrain the formation of the early Indian Ocean floor.

The DHR exhibits high relief and distinctive asymmetry that is unusual compared to most active or extinct spreading centres and likely results from compression and deformation of the recently extinct DHR during changes in relative motion of the Indian plate (110 - 100 Ma). Exhumation of gabbros in the southern DHR and an increase in seafloor roughness towards the centre of the PAP, likely result from a half spreading rate decrease from 35 mm/yr (based on magnetic reversals) to 24 mm/yr at ~114 Ma. The results support a slowdown of spreading prior to full cessation at ~102 Ma.

The composition of basaltic samples varies along the DHR: from sub-alkaline dolerites with incompatible element concentrations most similar to depleted-to-normal mid-ocean ridge basalts in the south, to alkali basalts similar to ocean island basalts in the north. Therefore, magma sources and degrees of partial melting varied in space and time, a result supporting the interpretation that the DHR is an extinct spreading ridge rather than a pseudofault. The enriched alkali basalt signatures may be attributed to melting of a heterogeneous mantle or to the influence of the Kerguelen plume over distances greater than 1000 km.

The results demonstrate the significance of regional tectonic plate motions on the formation and deformation of young ocean crust, and provide insight into the unique DHR morphology.

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