MORPHOLOGY AND TECTONICS OF THE HJORT TRENCH, SOUTHERN MACQUARIE RIDGE COMPLEX, SOUTWEST PACIFIC

The Macquarie Ridge Complex (MRC) is the modern, submarine portion of the active, Pacific-Australian plate boundary between New Zealand's South Island and the Indian-Antarctic-Pacific triple junction. The Hjort region is the southernmost portion of the MRC and consists of an arcuate trench paired with a ridge to the east. Whereas the current tectonic regime of the MRC north of Hjort is dominantly strike slip, the Hjort region is now experiencing oblique (northeast/southwest) convergence.

Data collected over the Hjort region during the January 2000 Australian Geological Survey Organisation cruise of the R/V L'Atalante (55- 61°S, 156-163°E) include high resolution swath bathymetry and reflectivity, 6- channel seismic reflection, gravity, and magnetics. Interpretations of these data focus on sea floor morphology and underlying structure and indicate that tectonic fabric formed by seafloor spreading characterizes the crest of the MRC in Hjort region. Seamounts and volcanic edifices, however, within the spreading fabric east of the trench highlight potential hot spot or subduction-related volcanism. In addition, an axial valley within the Hjort ridge is probably the southerly extension of active strike-slip faulting from the north. Thus, in the Hjort region, strain may be partitioned between major thrust and strike slip faults.

A developing model of the Indian-Antarctic-Pacific (Hjort) triple junction over the past 40 Ma incorporates strain partitioning and the transition from plate divergence to transform motion to oblique convergence to explain the structural geology of the Hjort region.