MULTIPLE STAGES OF CRUST FORMATION AND DEFORMATION IN THE MACQUARIE ISLAND ‘OPHIOLITE’

Macquarie Island constitutes the only ophiolite in the world that is still rooted in the oceanic basin in which it formed. It is an uplifted fragment of the Southern Tasmanian Ocean Basin, a slow-spreading, intra-oceanic rift basin that started to form in the Eocene. During the Miocene, spreading in the basin turned to transpression and the inactive spreading ridge became part of the Puysegur Ridge-Alpine Fault Zone system that now marks the Indian-Pacific Plate boundary in the region.

On the island a succession of lavas, sheeted dykes, gabbros, wehrlites, and mantle peridotites is exposed. It provides a section through oceanic lithosphere that is, at present, not matched by even the most successful legs of the oceanic drilling project. Despite generally poor outcrop conditions on the island, Macquarie Island allows the study of relations between different rock units and deformation stages in unprecedented detail.

Previous workers have interpreted the rock succession on the island as a continuous ophiolite sequence. Our recent fieldwork, however, strongly suggest that the sheeted dyke complex (and the lavas) were not formed by the same magmatic episode that created the gabbros on the island. We will discuss models that can account for the apparent multi-stage history of crust formation (i.e., multi-stage magmatism in a typical slow-spreading ridge setting, melt transport across a transform boundary, or creation of new oceanic crust within/on top of stranded block of older oceanic lithosphere).

In our presentation, we will also discuss evidence for the presence of an important ductile shear zone at the boundary between the mantle and the lower crust. This shear zone is interpreted as an on-ridge extensional detachment. As such it is a deep level equivalent of the recently found oceanic extensional detachment faults (‘oceanic core complexes’) at modern ocean ridges (e.g., Fifteen-Twenty Fracture Zone, MAR and Atlantis Bank, SWIR).