Geophysical Constraints on the Plate Tectonic History of the Emerald Basin and South Tasman Ocean Crust Southwest of New Zealand

The Pacific-Antarctic-Australia-Lord Howe Rise plate circuit has long been a weak link in global plate reconstructions in the late Cretaceous and Tertiary (Molnar et. al. 1975). The lack of closure of this plate circuit is an important problem to address because the motion of the Pacific Plate relative to other major plates in the world is best constrained through its connection to Antarctica. In order to explain the lack of closure of this plate loop, it is necessary to consider addition plate motions somewhere along the circuit, most likely between East and West Antarctica or in the seafloor SW of New Zealand. Cenozoic motion between East and West Antarctica is likely responsible for much of the observed discrepancy (Cande et al. 2000); however, this problem has been difficult to address because of the lack of plate tectonic constraints in the South Pacific and seafloor SW of New Zealand. It has been estimated that seafloor spreading in the Tasman Sea ended at 54 Ma. At 45 Ma, spreading was initiated along the Macquarie Ridge Complex in a direction perpendicular to the direction of previous Tasman Sea spreading. Spreading continued along the Macquarie Ridge Complex until 30 Ma, at which point the relative motion along this boundary became progressively more oblique. The motion along the Macquarie Ridge Complex at present is predominantly right lateral strike-slip with a component of compression. Fracture zones on either side of the Macquarie Ridge Complex are easily identifiable, but the large component of strike slip motion along this boundary has made tracing these fracture zones across the boundary very difficult. Thus, the plate motions in this region have remained poorly constrained. By compiling all the currently available magnetic and mutibeam bathymetry data from this region, we have been able for the first time to confidently match fracture zones across the Macquarie Ridge Complex and thus calculate poles and rotations associated with motion along this boundary.