Abstract. New marine geophysical data along the Macquarie Ridge Complex, the AustraliaPacific plate boundary south of New Zealand, illuminate regional neotectonics. We identify tectonic spreading fabric and fracture zones and precisely locate the Australia-Pacific plate boundary along the Macquarie Ridge Complex. We interpret a-•5-10 km wide Macquarie Fault Zone between the two plates along a bathymetric high that extends nearly the entire length of the Australia-Pacific plate boundary south of New Zealand. We conclude that this is the active Australia-Pacific strike-slip plate boundary. Arcuate fracture zones become asymptotic as they approach the plate boundary. A broad zone of less intense deformation associated with the plate boundary extends -50 km on either side of the Macquarie Fault Zone. Marine geophysical data suggest that distinct segments of the plate boundary have experienced convergence and strike-slip deformation, although teleseismic evidence overwhelmingly indicates strike-slip motion along the entire surveyed boundary today. The McDougall and southernmost Puysegur segments show no evidence for past underthrusting, whereas data from the Macquarie and Hjort segments strongly suggest past convergence. The present-day strike-slip plate boundary along the Macquarie Ridge Complex coincides with the relict spreading center responsible for Australia-Pacific crest in the region. Our conceptual model for the transition from seafloor spreading to strike-slip motion along the Macquarie Ridge Complex addresses the decreasing length of spreading center segments and spacing between fracture zones, as well as the arcuate bend of the fracture zones that become asymptotic to the current transform plate boundary.

No abstract

No abstract

No abstract

The southernmost portion of the Macquarie Ridge Complex (MRC) comprises the Hjort Ridge, Trench, and Plateau, the topographic expression of the Australian‐Pacific plate boundary south of New Zealand. On the basis of marine geophysical (swath bathymetry/reflectivity, seismic reflection, gravity, magnetic) data, teleseismic data, and gravity modeling, we argue that the Australian plate is actively underthrusting the Pacific plate along the Hjort Trench, but self‐sustaining subduction does not yet appear to have commenced. We interpret a crustal discontinuity in the Trench as a shallowly dipping thrust splay off a sub‐vertical transform fault in the crest of the Hjort Ridge (Hjort Fault). The Hjort Fault separates oceanic crust generated at the Southeast Indian Ridge (SEIR crust), located to the south, from oceanic crust generated at the extinct Australian‐Pacific spreading center located to the north (MRC crust). Oblique convergence on the transform has initiated a significant plate boundary decollement in the Trench, but our estimate of ∼50 km of underthrusting does not support the existence of an eastwardly dipping Australian slab below ∼20 km depth. For the length of the Hjort Ridge and Trench system, oblique convergence is partitioned between the decollement in the Trench and the Hjort Fault. South of 57.5°S, the trench decollement accommodates thrusting. North of 57.5°S, the boundary‐parallel component of convergence becomes dominant, the Trench gradually shallows, and the trench decollement evolves into a low‐angle, oblique‐slip fault. The trench decollement and strike‐slip system in the Hjort Ridge are structural boundaries that contain tectonic slivers of inferred SEIR crust of the western flank of the Hjort Ridge that currently belong to neither the Australian nor the Pacific plates.