Underthrusting at the Hjort Trench, Australian‐Pacific plate boundary: Incipient subduction?

Abstract

The southernmost portion of the Macquarie Ridge Complex (MRC) comprises the Hjort Ridge, Trench, and Plateau, the topographic expression of the Australian‐Pacific plate boundary south of New Zealand. On the basis of marine geophysical (swath bathymetry/reflectivity, seismic reflection, gravity, magnetic) data, teleseismic data, and gravity modeling, we argue that the Australian plate is actively underthrusting the Pacific plate along the Hjort Trench, but self‐sustaining subduction does not yet appear to have commenced. We interpret a crustal discontinuity in the Trench as a shallowly dipping thrust splay off a sub‐vertical transform fault in the crest of the Hjort Ridge (Hjort Fault). The Hjort Fault separates oceanic crust generated at the Southeast Indian Ridge (SEIR crust), located to the south, from oceanic crust generated at the extinct Australian‐Pacific spreading center located to the north (MRC crust). Oblique convergence on the transform has initiated a significant plate boundary decollement in the Trench, but our estimate of ∼50 km of underthrusting does not support the existence of an eastwardly dipping Australian slab below ∼20 km depth. For the length of the Hjort Ridge and Trench system, oblique convergence is partitioned between the decollement in the Trench and the Hjort Fault. South of 57.5°S, the trench decollement accommodates thrusting. North of 57.5°S, the boundary‐parallel component of convergence becomes dominant, the Trench gradually shallows, and the trench decollement evolves into a low‐angle, oblique‐slip fault. The trench decollement and strike‐slip system in the Hjort Ridge are structural boundaries that contain tectonic slivers of inferred SEIR crust of the western flank of the Hjort Ridge that currently belong to neither the Australian nor the Pacific plates.

Rapid conversion of an oceanic spreading center to a subduction zone inferred from high-precision geochronology

“…Intraoceanic subduction appears to be initiating along several sections of the Australian-Pacific plate boundary south of New Zealand, exploiting weaknesses associated with extinct spreading centers and/or fracture zones undergoing transpressional deformation (50)(51)(52). However, unlike the case in Palawan where deep underthrusting occurred within 1 My of oceanic crust formation, spreading at the boundary south of New Zealand had ceased and was followed by strike-slip deformation several million years before incipient subduction.…”

Evaluation of far-field tsunami hazard in French Polynesia based on historical data and numerical simulations

“…4). These alignments of variable length (350 to 2500 km) have a similar orientation SE-NW (N110 • -N120 • ) which corresponds to the absolute motion of the Pacific plate (about 11 cm.yr −1 N290 • , Minster and Jordan, 1978); only the Marquesas have a different orientation of N140 • . These archipelagos were created by several hotspots, now located southeast of the alignments (Duncan and McDougall, 1976).…”

“…The Vanuatu and Ryukyu trenches belong to this class as island studded propagation path (effect of the Tonga and Okinawa arcs) should protect FP from large wave heights. The incipient Hjort subduction zone is another exception to the directivity criteria, as it is considered too immature to generate large thrust earthquakes (Meckel et al, 2003).…”

Eos, Transactions, American Geophysical Union Volume 88, Number 22, 29 May 2007

“…In Australia's submission, Macquarie Island illustrates the challenge in interpreting 'natural prolongation.' The island is the subaerial exposure of the curvilinear, segmented oceanic ridge/ trench system forming the complex transform boundary between the Australian and Pacifi c plates [Meckel et al, 2003]. The submitted ECS follows this ridge/trench system well beyond 200 nautical miles, indicating that the ridge/trench system is interpreted as a natural prolongation of the island (Figure 1).…”