Summary
Macquarie Island, located along the Australian-Pacific transform plate boundary~1200 km south of New Zealand, consists of uplifted oceanic crust that still lies within the ocean basin where it formed. A lower crustal and upper mantle section of sheeted dikes, gabbro and peridotite on the northern end of the island is juxtaposed with a volcanic section along the Finch-Langdon fault zone. We propose that this fault zone formed near an active ridge-transform intersection prior to motion along the plate boundary changing from dominantly divergent to mainly transform. The Finch-Langdon fault zone trends NW, but is composed of multiple segments (0.5- 2.25 km long), of ENE- or WNW-trending and NNE- or NNW-trending, nearly orthogonal faults, and oblique NW-trending faults. Fault planes and associated veins within the major fault zone contain hydrothermal minerals, such as epidote and prehnite, both as mineral slickenlines and as gouge cement; one primary fault exposure shows sulfide alteration. Most fault planes dip steeply with subhorizontal to shallowly plunging mineral slickenlines, indicating oblique or strike slip motion. Locally both right and left lateral motion is observed. Sheeted dikes adjacent to the fault zone generally trend WNW, subparallel to one set of fault segments. Marine geophysical data along the Australian-Pacific plate boundary indicate that an oceanic fracture zone lies just offshore to the east of Macquarie Island; nearly orthogonal tectonic spreading fabric farther east is oriented subparallel to sheeted dikes north of the Finch-Langdon fault zone. The Finch-Langdon fault pattern is compatible with fracture zone trends and spreading fabric orientations in the surrounding submarine oceanic crust, and resembles geometries described within transform zones near their junctions with spreading ridges. These geometrical aspects, coupled with the extensive hydrothermal alteration along the Finch-Langdon fault zone, are consistent