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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