Ridge Reorientation Mechanisms and Tectonic Processes Along the Macquarie Ridge Complex Portion of the Australia-Pacific Plate Boundary

Analysis of side scan, bathymetry, magnetic anomaly and gravity data for the Macquarie Ridge Complex portion of the Australia-Pacific plate boundary south of New Zealand documents an ~100o change in spreading direction between 40 and 6 Ma prior to truncation by the present day transform fault. We present a reconstruction that correlates arcuate fracture zones on the Australian and Pacific plates and compare them to synthetic flowlines generated from 32 Ma to 6 Ma using the most recently resolved poles. Our analysis shows that this major change in spreading direction, caused by continuous migration of the proximal pole of rotation since 33 Ma, resulted in dynamic interaction and transfer of crust between the two plates, consistent with continuous transpression. In our reconstruction area, spreading following rifting was initially accommodated by 7 ridge segments ranging in width from 40 to 125km and offset by 7-25km. By the end of spreading, only three narrower segments, 5 to 50km in width offset by 117-160 km, were still active. Synthetic flowlines predict 80% shortening of the combined ridge segment widths approaching the plate boundary to maintain strain compatibility, whereas only 69% shortening is observed. Regionally available side scan and bathymetry data show an overall "Zed" pattern and fanning spreading fabric within regions of high curvature, indicating differential asymmetric spreading during gradual reorientation of spreading axes. However, away from the rifted margins, the width of crust between correlative fracture zones on the two plates differs, requiring crustal modification after spreading. This modification, the decreasing width and complete disappearance of ridge segments approaching the present day plate boundary, plus seafloor morphology and magnetic anomaly patterns, all support ridge (rift) propagation into preexisting crust. Comparison of individual fracture zones with corresponding synthetic flowlines show them nearly identical closest to rifted margins and diverging with less curvature approaching the plate boundary. Modification of fracture zones by shearing during transform motion since 6 Ma is not likely because fracture zones show less deflection into the plate boundary than the synthetic flowlines. Thus, spreading ridge segments responded to the large change in orientation by a combination of gradual rotation and unidirectional propagation into adjacent crust, decreasing the predicted curvature magnitude, coupled with faster failing of the opposite end of the ridge, thereby decreasing the width of the spreading segments.