GEMOC ARC National Key Centre
Macquarie Island
Macquarie Island, located approximately 1200 km southwest of New Zealand in the Southern Ocean (see below), forms the apex of the Macquarie Ridge Complex (MRC), a system of ridges and troughs along the currently active Australian-Pacific oceanic transform plate boundary between the Alpine fault of New Zealand and the Australian-Pacific-Antarctic triple junction. The island exposes the eastern side of the ~5 km high, ~50 km wide submarine ridge and lies ~4.5 km east of the major active plate boundary fault zone. Macquarie Island represents a globally unique opportunity to examine in situ oceanic crust and an active oceanic transform plate boundary as it is the only subaerial exposure of non-plume-related oceanic crust that still lies within the basin in which it formed. Thus geologic features studied on the island may be placed into a relatively well-constrained present-day plate tectonic setting.
Oceanic crust around Macquarie Island originated at three different seafloor spreading systems, the Southeast Indian, Pacific-Antarctic, and Macquarie spreading ridges. The first two spreading centres are still active, whereas crust of the Macquarie region was generated at the divergent Australian-Pacific plate boundary following break-up between the Campbell Plateau and Resolution Ridge between middle Eocene (~40 Ma) and late Miocene time (< 10 Ma from the age of Macquarie Island crust). Fracture zones in the Macquarie region crust curve and merge asymptotically into the active Australian-Pacific plate boundary, consistent with progressive clockwise rotation of the spreading axis segments with time. The cumulative length of spreading segments along the plate boundary decreased relative to that of transform faults until spreading ceased at < 10 Ma. At that time, right lateral strike slip faults began accommodating displacement along the transform plate boundary. Subsequent transpression across the Australian-Pacific transform plate boundary has led to uplift along the MRC.
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