OBLIQUE SPREADING STRUCTURES EXPOSED IN THE MACQUARIE ISLAND OPHIOLITE, SOUTHERN OCEAN

Macquarie Island is an uplifted block of 12 – 9 m.y. old oceanic crust formed by the Australia-Pacific spreading center. Motion along this plate boundary changed progressively through time from pure N-S extension along E-W trending ridge segments to pure NNE-SSW, right-lateral strike-slip motion. The crust that makes up the island was formed during highly oblique spreading at a rate of <10 mm/yr when the magma supply was low and very little new crust was being created. This mode of spreading gave rise to a distinct set of structures, including: 1) NNW-trending sheeted dikes and dike swarms and 2) a series of NW-trending oblique-slip fault zones that cross-cut all other structures on the island at intervals of a few kilometers. Foliated gouge zones and slickenside data indicate oblique dextral/normal motion along these fault zones. Steeply dipping lava flows and sedimentary breccias exposed in the interior of the island indicate that these fault zones once existed as topographic lows on the seafloor. Post-deformation dikes and lava flows, as well as extensive hydrothermal alteration, indicate motion on these faults occurred near the spreading axis. The NW-trending fault zones and other accretionary structures reflect the partitioning of deformation during oblique spreading.