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 onGeology
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 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 areEarthquakes
The zone has in historic times been the source of some very large earthquakes and up until 1993 at least 20 over 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 On 23 December 2004 a event occurred about 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
Present
The Macquarie Fault Zone includes a component of convergence which increases as it approaches the South Island of New Zealand where it merges into the Alpine Fault which cuts across thePast
The Macquarie Ridge is a very long, but incompletely studied extinct mid-ocean ridge system which has since experienced complex tectonic processes over about 30 million years that have differed down its length as it became the Australian–Pacific plate boundary. Not all timings derived from different methodologies concord as well as they do for easier to study tectonic structures. It is postulated that because the lithosphere would cool and strengthen usually with mid oceanic ridge formation, that if such tectonic inversion is not rapid, it does not lead to the relative simplicity seen at say the Kermadec-Tonga subduction zone. This is not the case with the Macquarie ridge. The localised to segments transpression and compression seen in the Macquarie Ridge Complex took about 10 to 15 million years. The process is summarised as: #At about 42 million years ago the South East Tasman (Macquarie) Basin opened as the Campbell Plateau and Resolution Ridge separated. #After 30 million years ago the changes in relative motion of the Pacific plate and Australian plate caused oblique spreading of the initial Macquarie mid-ocean ridge. To adjust for changes in spreading direction, shorter segments developed and the fracture zones became curved. #Sometime before present seafloor spreading and magmatism ceased when the relative motions of the Australian and Pacific plates became too oblique to sustain extension. #The Macquarie microplate rotated independent of the Australian plate starting 6.24 million years ago and formed the south western margin of the zone but it is unclear if this continues now. This last stage has been difficult to characterise as the usual approach through magnetic anomaly studies only allowed a good time for the central McDougall and Macquarie segments spreading ceasing by 24 million years ago and either end was only constrained to the 30 million years ago estimate. It is now known to have occurred differentially, with cessation of sea floor spreading happening at 25.9 million years ago in the south and possibly as recently as 1.6 million years ago near New Zealand. However it is known from elsewhere that basalt eruptions can continue for up to 3.5 million years after spreading stops so sea floor spreading in the north actually likely stopped sometime between 5 million and 1.6 million years ago if the single seamount sampled to date in this region is of mid oceanic ridge origin. And so to a degree the apparent clash with evidence that the Puysegur subduction initiated at 20 million years ago is less challenging to explain. Compositional analysis on this single sample does not exclude the possibility of later oceanic island type origin even if it seems to fit with other samples from the zone. Seafloor spreading must have ceased by 10 million years ago on the central Macquarie segment containing Macquarie Island. Since then the segments accommodated the two plates convergence by oceanic-oceanic subduction at the southern Hjort segment and at the northern Puysegur segment and by predominant strike-slip motion in the two central segments of the Macquarie Ridge structure. Maps of this tectonic activity have been modelled from 42 million years ago to the present showing the evolving spreading center, its extinction and fault zone relationships.Notes
References
Sources
* * * * {{Oceanic features of Zealandia, state=collapsed Zealandia Seismic faults of New Zealand Seismic faults of Australia Macquarie Island Plate tectonics Underwater ridges of the Pacific Ocean Seismic zones of Oceania