UTIG Archive Projects

Principal Investigators: Millard F. Coffin, Cliff Frohlich, Paul Mann

Funded by: National Science Foundation, Award #9216873

Publications resulting from this study

Field work on Macquarie Island.

New marine geophysical data along the Macquarie Ridge Complex, the Australia-Pacific plate boundary south of New Zealand, illuminate regional neotectonics (Fig. 1). We identify tectonic spreading fabric and fracture zones, and precisely locate the Australia-Pacific plate boundary along the Macquarie Ridge Complex. We interpret a ~5-10 km 'Macquarie Fault Zone' between the two plates along a bathymetric high that extends nearly the entire length of the Australia-Pacific plate boundary south of New Zealand. We conclude that this is the active Australia-Pacific transform plate boundary. A broad zone of less intense deformation associated with the plate boundary extends ~50 km on either side of the Macquarie Fault Zone. Marine geophysical data suggest that distinct segments of the plate boundary have experienced convergence and strike-slip deformation, although teleseismic evidence overwhelmingly indicates strike-slip motion along the entire surveyed boundary today. The southern Puysegur and McDougall segments show no evidence for past underthrusting, whereas data from the Macquarie and Hjort segments strongly suggest past convergence. The present-day strike-slip plate boundary along the Macquarie Ridge Complex coincides with the relict spreading center responsible for Australia-Pacific crust in the region. Our conceptual model for the transition from seafloor spreading to strike-slip motion along the Macquarie Ridge Complex (Fig. 2) explains the decreasing length of spreading center segments and spacing between fracture zones, as well as the arcuate bend of the fracture zones that become asymptotic to the current transform plate boundary.

We evaluated teleseismically determined focal mechanisms and epicenters for earthquakes along the Macquarie Ridge Complex from 45°S to 61°S and 155°E to 168°E, a region characterized by some previous investigators as undergoing subduction initiation. From 65 centroid moment tensors reported by Harvard, we develop statistical guidelines for choosing 26 which represent better determined, more reliable focal mechanisms for tectonic analysis (Fig. 3). Although thrust mechanisms occur in the north, near Fiordland, elsewhere along the MRC the better determined mechanisms virtually all indicate that present-day motion along most of the MRC is strike-slip. This is consistent with sidescan sonar and multichannel reflection data collected between 50°S and 57°S on 1994 and 1996 cruises; the active plate boundary zone appears to be quite narrow (<5 km wide), and no active compressional features can be observed on the seafloor. If we determine a rotation pole for plate boundary motion using only slip vectors from better determined Harvard mechanisms along the MRC, the best fitting "instantaneous" pole is at 57.4°S, 179.4°E, about 2.5° north of the NUVEL-1 Australian-Pacific pole, which averages motion over the last 3.0 m.y. If the MRC pole was formerly farther south than at present, this could explain the existence of relict features associated with crustal shortening, such as bathymetric highs and troughs; yet, the absence of active features such as thrust faults, etc., suggests no ongoing compression or subduction initiation. We also carefully read arrival times for P phases for 53 earthquakes at 16 teleseismic stations, selected to represent a range of azimuths surrounding the earthquakes; we relocated these earthquakes using standard joint epicentral determination (JED) methods. While most of the better quality relocations lie on or very close to the Australia-Pacific boundary as determined on the 1994 cruise, a few epicenters occur well away from the boundary, apparently on Cenozoic fracture zones. Thus, on the Macquarie Ridge Complex and other major strike-slip boundaries (e.g., in California), it appears that the very largest earthquakes occur on the principal plate boundary fault but that other earthquakes, some quite large, may occur away from the boundary along zones of preexisting weakness.

Seafloor structure at the Macquarie Ridge Complex strongly influences the intensity and circulation pattern of ocean currents south of New Zealand. New marine geophysical data show heterogeneous sedimentary environments on Macquarie seafloor that reflect interaction of highly variable bathymetry with the Antarctic Circumpolar Current and north-flowing Antarctic Bottom Water. Acoustic backscatter, bathymetry, and seismic reflection data collected aboard R/V Rig Seismic in 1994 show five bathymetrically-constrained sedimentary provinces flanking the ridge complex: 1) northwest Macquarie hemipelagic drifts, 2) current-modified Solander submarine fan complex; 3) southwest Macquarie manganese nodule province, 4) Emerald Basin pelagic drift province, and 5) sediment-free oceanic crust related to the 53.5°S passage in the Macquarie Ridge Complex. The Late Miocene-Pliocene opening of a 53.5°S passage in the ridge complex cause a major increase in the intensity of ocean current circulation, sediment reworking, and erosion in all sedimentary provinces (Fig. 4).

Cruises: R/V Rig Seismic 124 (1994); R/V Maurice Ewing 9513 (1996)

UTIG staff: M. Coffin, P. Mann, C. Massell, L. Schuur (shipboard); C. Frohlich, S. Saustrup, L. Gahagan (shore-based)

Theses:
Meckel, Timothy, 2003, Tectonics of the Hjort Region of the Macquarie Ridge Complex, Southernmost Australian-Pacific Plate Boundary, Southwest Pacific Ocean, University of Texas at Austin, PhD dissertation.

Duncan, Laurie, 2001, Sedimentary Regimes at the Macquarie Ridge Complex South of New Zealand: Interaction of Southern Ocean Circulation and Tectonism, University of Texas at Austin, MA thesis, 104p.

Massell, Christina, 1997, Macquarie Ridge Complex Neotectonics and Plate Reconstruction in the Southwest Pacific Ocean, University of Texas at Austin, MA thesis, 101p.

Publications:
Coffin, M.F., Karner, G.D., and Falvey, D.A., 1994. Research cruise yields new details of Macquarie Ridge Complex, Eos, 75, 561-564.

Collot, J.-Y., Delteil, J., Herzer, R.H., Wood, R., Lewis, K.B., and Shipboard Party, 1995. Sonic imaging reveals new plate boundary structures offshore New Zealand, Eos, 76, 1-5.

Collot, J.-Y., Lamarche, G., Wood, R.A., Delteil, J., Sosson, M., Lebrun, J.-F., and Coffin, M.F., 1995. Morphostructure of an incipient subduction zone along a transform plate boundary: Puysegur Ridge and Trench, Geology, 23, 519-522.

Delteil, J., Collot, J.-Y., Wood, R., Herzer, R., Calmant, S., Christoffel, D. Coffin, M., FerriŽre, J., Lamarche, G., Lebrun, J.-F., Mauffret, A., Pontoise, B., Popoff, M., Ruellan, E., Sosson, M., and Sutherland, R., 1995. De la faille Alpine ˆ la fosse de Puysegur (Nouvelle-ZŽlande): rŽsultats de la campagne de cartographie multifaisceaux GEODYNZ-SUD, Leg 2, Comptes Rendus des Academie de Science de Paris 320, 303-309.

Delteil, J., Collot, J.-Y., Wood, R., Herzer, R., Calmant, S., Christoffel, D., Coffin, M., Ferriere, J., Lamarche, G., Lebrun, J.-F., Mauffret, A., Pontoise, B., Popoff, M., Ruellan, E., Sosson, M., and Sutherland, R., 1996. From strike-slip faulting to oblique subduction: a survey of the Alpine Fault-Puysegur Trench Transition, New Zealand, Results of Cruise Geodynz-sud Leg 2, Marine Geophysical Researches, 18, 383-399.

Frohlich, C., Coffin, M.F., Massell, C., Mann, P., Schuur, C.L., Davis, S.D., Jones, T., and Karner, G., 1997. Constraints on Macquarie Ridge tectonics provided by Harvard focal mechanisms and teleseismic earthquake locations, Journal of Geophysical Research, 102, 5029-5041.

Schuur, C.L., Coffin, M.F., Frohlich, C., Massell, C.G., Karner, G.D., Ramsay, D., Caress, D.W., 1998, Sedimentary regimes at the Macquarie Ridge Complex: Interaction of Southern Ocean circulation and plate boundary bathymetry: Paleoceanography, v. 13, p. 646-670.

Massell, C., Coffin, M.F., Mann, P., Mosher, S., Frohlich, C., Schuur, C.L., Karner, G., Ramsay, D., and Lebrun, J.-F., 2000. Neotectonics of the Macquarie Ridge Complex, Pacific-Australia plate boundary, Journal of Geophysical Research, v. 105, n. B6, p. 13,457-13,480.

Lebrun, J. F., Karner, G., and Collot, J. Y., 1998, Fracture zone subduction and reactivation across the Puysegur ridge/trench system, southern New Zealand: Journal of Geophysical Research, v. 103, p. 7293-7313.

Meckel, TA., M.F. Coffin, S. Mosher, P. Symonds, G. Bernardel, and P. Mann, 2003, Underthrusting at the Hjort Trench, Australia-Pacific plate boundary: Incipient subduction?, Geochemistry, Geophysics, Geosystems, Volume 4, Number 12, doi:10.1029/2002GC000498. ISSN:1525-2027

Meckel, T. A., P. Mann, S. Mosher, and M.F. Coffin, 2005, Influence of cumulative convergence on lithospheric thrust fault development and topography along the Australian-Pacific plate boundary south of New Zealand, Geochemistry, Geophysics, Geosystems, vol. 6, Q09010, doi:10.1029/2005GC000914.