STRAIN ACCOMMODATION ALONG THE MACQUARIE RIDGE COMPLEX: TRANSITION FROM TRANSTENSION TO TRANSPRESSION ALONG AN ACTIVE PLATE BOUNDARY

The Macquarie Ridge Complex, which defines the Australian-Pacific plate boundary south of New Zealand, has evolved from an active spreading center to a transform plate boundary over the last 40 million years. This major fault zone shows a complex history of both transtension and transpression, with strain being partitioned between oblique spreading and faulting. Marine geophysical data, including high quality sidescan sonar, swath bathymetry, seismic reflection, gravity, and magnetics, plus field data from Macquarie Island, an uplifted piece of oceanic crust and upper mantle immediately adjacent to the plate boundary, allow the evolution of this boundary to be constrained.

The Macquarie Ridge Complex resembles a 40-50 km wide, dextral shear zone. Spreading fabric (abyssal hills) that formed parallel to active ridge segments is orthogonal to fracture zones throughout the zone, but the fracture zones bend arcuately from nearly normal to sub-parallel to the plate boundary. Individual ridge segments decrease in length as spreading becomes more oblique, indicating that oblique spreading accommodated extension for most of the zone's history. After the overall zone changes from transtension to transpression, as recorded by the extension direction, a significant decrease in the segment length, and thus amount of magmatism, occurs, until magmatism ultimately ceases. Only locally is the orthogonal relationship of the spreading fabric modified by distributed deformation related to strike slip motion.

Macquarie Island records similar structures, including rotated sheeted dikes and faults related to an active ridge-transform intersection, with orientations consistent with extension subparallel to the plate boundary. Hydrothermal minerals cementing fault gouges and forming vein networks confirm formation during active spreading. Recent faults related to transform motion and uplift of the island are oriented at high angles to earlier structures, and evidence of reactivation of preexisting structures is rare.

Thus, most of the lateral motion along the fault zone was accommodated by oblique spreading, even during transpression. Only after magmatism ceased, did strike slip faulting and associated distributed deformation become an important strain accommodation mechanism.