The 1,600 kilometres (990 mi) long Macquarie fault zone (also known as the Macquarie Ridge, its gazetted name since 2015, the Macquarie Ridge complex or historically as the Macquarie Fault) is a major right lateral-moving transform fault along the seafloor of the south Pacific Ocean which runs from New Zealand southwestward towards the Macquarie triple junction. It is also the tectonic plate boundary between the Australian plate to the northwest and the Pacific plate to the southeast. As such it is a region of high seismic activity and recorded the largest strike-slip event on record up to 23 May 1989, of at least Mw 8.0
Geography The Macquarie fault zone extends south of New Zealand on the South Pacific Ocean's seafloor except where it surfaces as Macquarie Island. Some of it is on Tasman Sea sea floor, north of the line between the south of the Auckland Islands and Tasmania. To its west on the sea floor is the South East Tasman Basin and to its east the Solander Trough and Emerald Basin. Technically the plate boundary becomes part of the Emerald fracture zone to the south of the Hjort Trench, so the zone does not quite reach the Macquarie triple junction.
Geology The Macquarie fault zone term is also used often now to refer to a major central part of the plate convergent structures south of New Zealand that has become to be termed the Macquarie Ridge complex as the geology has been better characterised. The northern part of this complex is an extension of New Zealand's Alpine Fault which becomes the Puysegur Trench and ridge to its east. New Zealand is continental crust and the northern parts of the complex are associated with oceanic crust subducting under the continental crust of Zealandia. This north Macquarie Ridge Complex has to its west the Resolution Ridge which separates the oceanic crust of the north and south Tasman basins and defines the western margin of the Macqurie Block. The southern part of the Macquarie Ridge Complex is the Hjort Trench which also has a ridge to its east. Some authors split the central part of the Macquarie Ridge Complex into two central segments where the ridge is to the west and the trench is to the east. Slightly confusingly the 5,500 m (18,000 ft) deep McDougall Trough is the trench to the east of the ridge of the Macquarie Fault Zone, with the other northern fault zone to the troughs north being named the McDougall Fault Zone. The trough has a step over basin to the Macquarie Fault Zone which commences where the Jurru fault zone in the southern Tasman Basin intercepts the plate boundaries. In the middle of the Macquarie Fault Zone is Macquarie Island. Its rocks like that of all the zone are mid-ocean ridge basalt or ocean island basalt. The central part of Macquarie Ridge Complex has a maximum height of 433 m (1,421 ft) at Macquarie Island but the troughs to the east drop to 6,000 m (20,000 ft) below sea level. At Macquarie Island which is located about 4.5 km (2.8 mi) east of the Australian Pacific plate boundary the transform of this to the north of the island is a 15 km (9.3 mi) wide zone of closely spaced NNE- to NE-trending faults while to the south of the island north of the island the zone narrows to as little as 8 km (5.0 mi) with NNE to NNW-trending faults. The fault zone is up to 40 km (25 mi) wide, elsewhere and it often has parallel eastern and western ridge crests, as well as multiple nearby fracture zones.
Earthquakes The zone has in historic times been the source of some very large earthquakes and up until 1993 at least 20 over Mw 6.5 had been recorded. The currently seismically active area is the whole of the zone and parts to the west of its northern aspects in the northern South East Tasman Basin. However both west of Macquarie Island and to its east towards the southern tip of the Campbell Plateau there has been some historic earthquake activity outside the zone. In the Hjort Trench region the plate boundary is definitely under the stress condition of transpression as earthquake focal mechanisms are both thrust and dextral strike-slip types as found in such areas. To the south of the Hjort Trench there is relatively low grade seismic activity in the Emerald Fault Zone and towards the Macquarie triple junction. The largest strike-slip event till then on record occurred May 23, 1989, and had a magnitude of at least Mw 8.0 On 23 December 2004 a Mw 8.1 event occurred about 150 km (93 mi) to the west of the plate boundary. This event to the west of the plate boundary was close to the transition from oblique subduction at the Puysegur Trough to strike slip, with some compression, on the Macquarie Ridge. It is believed these large events are because the transition to oblique convergence with subduction is putting the area of what has been called the Macquarie Block under significant strain that is eventually relieved by major failure reactivating past transform faults in the oceanic crust of the Tasman sea floor.
Volcanics The basalts which are oceanic in the zone have been dated. To the south east of the Puysegur Trench a seamount is as young as 1.59 ± 0.26 million years. It is not yet known for sure if this relatively recent date could be related to late mid ocean ridge spreading or back arc activity. The recent dating of the adakitic Little Solander Island volcanics to the zone's far east to about 50,000 years ago suggests back arc activity from the subducted oceanic crust in the northern part of the zone may not have ceased. A northern sea mount of the McDougall Fault Zone is 10.9 ± 0.26 million years old while at the southern end there is one dated at 18.34 ± 0.53 million years old. A sea mount at the north of the smaller definition of the Macquarie Fault Zone is 14.1 ± 1.2 million years old and one beyond Macquarie Island to its south is 25.82 ± 0.18 million years old. Macquarie Island itself has multiple ages in the relatively limited range from 10.34 to 10 million years ago. A seamount near the eastern far end of the Hjort Trench is 25.90 ± 0.23 million years old.
Tectonics
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