Back to list of UTIG abstract submissions, Fall Agu 2003

T.A. Meckel¹, P. Mann, S. Mosher, M.F. Coffin, G. Bernardel, and P. Symonds

¹Now at: tameckel@colby.edu

Deformation along the Australian-Pacific transpressional plate boundary south of New Zealand (Macquarie Ridge Complex - MRC) since ~5.9 Ma (anomaly 3Ay) has been accommodated by vertically displaced crust and variable amounts of strike slip faulting and/or underthrusting. Variable convergent histories result from the interplay of the close proximity of a drifting pole of rotation and the curvilinear trace of the plate boundary fault. We apply geomorphologic techniques to swath bathymetry data to quantify the amount of interplate deformation along three, distinctly oriented, 300-350-km long segments of the plate boundary since 5.9 Ma. Between 51°S and 61°S we calculate volumes of crust within 70 km of the plate boundary that are vertically displaced from the average seafloor depth occurring beyond the zone of active deformation. These volumes are compared to: (1) variable convergence angles, (2) the cumulative amount of perpendicular convergence predicted by stage pole rotations, and (3) active faulting at the seafloor in order to characterize the geodynamic evolution of the three different regions. Displaced volumes (anomalously shallow and deep bathymetry) adjacent to the plate boundary are proportional to the cumulative amount of plate boundary-normal convergence since 5.9 Ma. We integrate our results with four gravity transects across the central and southern MRC, illustrating the lithospheric-scale morphologic and structural evolution as underthrusting initiates at this obliquely convergent oceanic plate boundary.

Along-strike morphologic changes in the MRC can be attributed primarily to the variable angles of convergence since 5.9 Ma. A convergence angle of ~25° marks the transition from strike-slip dominated faulting and large displaced crustal volumes to partitioned underthrusting and strike-slip faulting, and relatively low displaced volumes. Regions where convergence angles have not exceeded 25º correspond to significant bathymetric troughs, representing failed attempts to develop thrust faults. Active thrust faults and deeper bathymetric trenches occur where convergence has been >25º since 5.9 Ma. Thus the MRC can be used to help constrain some of the convergence characteristics necessary for establishing underthrusting at obliquely convergent oceanic lithosphere settings.