Large Earthquakes in the Macquarie Ridge Complex: Transitional Tectonics and Subduction Initiation

Abstract

While most aspects of subduction have been extensively studied, the process of subduction initiation lacks an observational foundation. The Macquarie Ridge complex (MRC) forms the Pacific-Australia plate boundary between New Zealand to the north and the Pacific-Australia-Antarctica triple junction to the south. The MRC consists of alternating troughs and rises and is characterized by a transitional tectonic environment in which subduction initiation presently occurs. There is a high seismicity level with 15 large earthquakes (M > 7) in this century. Our seismological investigation is centered on the largest event since 1943: the 25 MAY 1981 earthquake. Love, Rayleigh, and P waves are inverted to find: a faulting geometry of right-lateral strike-slip along the local trend of the Macquarie Ridge (N30~ a seismic moment of 5 x 1027 dyn cm (M w = 7.7); a double event rupture process with a fault length of less than 100 km to the southwest of the epicenter and a fault depth of less than 20 km. Three smaller thrust earthquakes occurred previous to the 1981 event along the 1981 rupture zone; their shallow-dipping thrust planes are virtually adjacent to the 1981 vertical fault plane. Oblique convergence in this region is thus accommodated by a dual rupture mode of several small thrust events and a large strike-slip event. Our study of other large MRC earthquakes, plus those of other investigators, produces focal mechanisms for 15 earthquakes distributed along the entire MRC; thrust and right-lateral strike-slip events are scattered throughout the MRC. Thus, all of the MRC is characterized by oblique convergence and the dual rupture mode. The "true" best-fit rotation pole for the Pacific-Australia motion is close to the Minster & Jordan RM2 pole for the Pacific-India motion. Southward migration of the rotation pole has caused the recent transition to oblique convergence in the northern MRC. We propose a subduction initiation process that is akin to crack propagation; the 1981 earthquake rupture area is identified as the "crack-tip" region that separates a disconnected mosaic of small thrust faults to the south from a horizontally continuous thrust interface to the north along the Puysegur trench. A different mechanism of subduction initiation occurs in the southernmost Hjort trench region at the triple junction. Newly created oceanic lithosphere has been subducted just to the north of the triple junction. The entire MRC is a "soft" plate boundary that must accommodate the plate motion mismatch between two major spreading centers (Antarctica-Australia and Pacific-Antarctica). The persistence of spreading motion at the two major spreading centers and the consequent evolution of the three-plate system cause the present-day oblique convergence and subduction initiation in the Macquarie Ridge complex.

Deformation from the convergence of oceanic lithosphere into Yap trench and its implications for early-stage subduction

“…A potential difficulty with the latter argument, however, is that the two plates involved in the incipient subduction may not have had substantial density contrast. Yet there are a number of cases where subduction has occurred apparently without large density contrasts (Ruff et al, 1989) and well-documented cases of subduction polarity reversals (McCaffrey et al, 1980;Hill and Hegarty, 1988). Recently Niu et al (2003) proposed the edges of oceanic plateaus may be suitable site for subduction initiation because they have markedly lower density compared with the surrounding oceanic lithosphere.…”

Macquarie Island: Its geology, structural history, and the timing and tectonic setting of its N-MORB to E-MORB magmatism

“…The ridge is seismically very active (average return period is 1 yr for earthquakes of magnitude Ͼ6.2; an earthquake of magnitude 7.8 was recorded in 1943 (Jones and McCue, 1988). The seismicity is shallow and probably limited to the crust (Ruff et al, 1989), and earthquake fault-plane solutions for the Macquarie segment suggest subhorizontal, east-west-oriented principal stress axes and right-lateral strike-slip movement along fault planes parallel to the ridge (Jones and McCue, 1988). Australian-Pacific plate-motion vectors calculated from modeling of global plate motions (DeMets et al, 1994) intersect the active plate boundary along the Macquarie segment of the Macquarie Ridge Complex near Macquarie Island at ϳ14Њ (gray arrow, Fig.…”

“…The transcurrent to compressional relative motion along the boundary accommodates mismatch in the Southeast Indian Ridge and Pacific-Antarctic Ridge spreading rates (Johnson and Molnar, 1972;Ruff et al, 1989).…”

Sedimentation at Plate Boundaries in Transition

“…14.1): the Mussau Trench in the Pacific (Seno et al, 1993), the Japan Sea (El-Fiky and Kato, 2006), the Puysegur Ridge off South Island New Zealand (Gurnis et al, 2004;Stern, 2004), and further south along the Macquarie Ridge complex (Ruff et al, 1989 The Mussau Trench is thought to have formed along a former fracture zone in response to plate/ microplate reorganization near the triple junction between the Caroline, Indo-Australian, and Pacific Plates (Hegarty et al, 1983). There is seismic and geophysical evidence for compressional tectonics along the intraoceanic Mussau Trench (Fig.…”