Back to list of UTIG presentations at Fall Agu

Nathan R. Daczko^1,2, Karah L. Wertz^1,2, Sharon Mosher¹, Millard F. Coffin^2,3,4

¹Department of Geological Sciences, University of Texas at Austin
²Institute for Geophysics, University of Texas at Austin
³Ocean Research Institute, University of Tokyo
⁴Institute for Frontier Research on Earth Evolution, Japan Marine Science and Technology Centre

The Australia-Pacific transform plate boundary fault zone along the Macquarie and McDougall segments of the Macquarie Ridge Complex, south of New Zealand, is characterized by dominantly normal faults and pull-apart basins, in apparent conflict with the regional transpressional tectonic setting. We propose that present day curvature of the transform is inherited from a preexisting divergent plate boundary and that the overall extensional kinematics shown by faults along the main plate boundary trace and exposed on Macquarie Island result from local stresses related to right steps in the plate boundary faults and not to the current transpressional setting.

Transpression along the Australian-Pacific transform plate boundary has resulted in uplift of the ~1500 km long Macquarie Ridge Complex. Macquarie Island, the only exposure of the complex, sits atop a 5 km high, 50 km wide submarine ridge of oceanic crust and lies ~4.5 km east of the major active plate boundary fault zone. Thus, Macquarie Island and the surrounding sea floor provide a unique opportunity to study an active oceanic transform fault using complimentary datasets that include marine geophysics and land based geology. Mapping of recent faults affecting the topography of Macquarie Island shows the island is extensively cut by high angle normal faults forming pull-apart basins. Furthermore, evidence for reverse motion is rare. Using marine geophysical data, including bathymetry, reflectivity and seismic reflection data, collected along the Australian-Pacific plate boundary to the north and south of the island, we have defined a 5-15 km wide plate boundary zone. Within this zone a series of en echelon faults step to the right along the main plate boundary trace. At the right stepping fault terminations, elongate depressions (up to 10 km wide and 1.2 km deep) parallel the plate boundary. We interpret the depressions as responding to local stresses located at extensional relay zones or pull-apart basins.