The Macquarie Triple Junction is a geologically active tectonic boundary located at at which the

Indo-Australian Plate The Indo-Australian Plate is a major tectonic plate that includes the continent of Australia and the surrounding ocean and extends northwest to include the Indian subcontinent and the adjacent waters. It was formed by the fusion of the Indian an ...

Pacific Plate The Pacific Plate is an oceanic tectonic plate that lies beneath the Pacific Ocean. At , it is the largest tectonic plate. The plate first came into existence 190 million years ago, at the triple junction between the Farallon, Phoenix, and Iza ...

, and

Antarctic Plate The Antarctic Plate is a tectonic plate containing the continent of Antarctica, the Kerguelen Plateau, and some remote islands in the Southern Ocean and other surrounding oceans. After breakup from Gondwana (the southern part of the superconti ...

collide and interact. The term Triple Junction is given to particular tectonic boundaries at which three separate tectonic plates meet at a specific, singular location. The Macquarie Triple Junction is located on the seafloor of the southern region of the

Pacific Ocean The Pacific Ocean is the largest and deepest of Earth's five oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean (or, depending on definition, to Antarctica) in the south, and is bounded by the continen ...

, just south of

New Zealand New Zealand ( mi, Aotearoa ) is an island country in the southwestern Pacific Ocean. It consists of two main landmasses—the North Island () and the South Island ()—and over 700 smaller islands. It is the sixth-largest island count ...

. This tectonic boundary was named in respect to the nearby

Macquarie Island Macquarie Island is an island in the southwestern Pacific Ocean, about halfway between New Zealand and Antarctica. Regionally part of Oceania and politically a part of Tasmania, Australia, since 1900, it became a Tasmanian State Reserve in 197 ...

, which is located southeast of New Zealand.

Evolution, stability, and migration

Our understanding of the evolution of the Macquarie Triple Junction was made possible due to extensive research of the regions tectonic

magnetic anomalies In geophysics, a magnetic anomaly is a local variation in the Earth's magnetic field resulting from variations in the chemistry or magnetism of the rocks. Mapping of variation over an area is valuable in detecting structures obscured by overlying ...

as well as local fractures reconstruction. The origin of the Macquarie Triple Junction has been interpreted to have occurred 47.91 Mya (million years ago), based on Anomaly 21. Thorough reconstruction of the Macquarie Triple Junction begins at 33.3 Mya, in respect to Anomaly 13o, and can be simply described as a southeastward migration of approximately 1100 km in respect to the Australian Plate. The total migration was largely driven by the Australian–Pacific transform boundary. At 33.3 Mya, the Macquarie Triple Junction was a stable ridge–transform fault–transform fault triple junction. In reference to the Australian Plate, the triple junction moved southeast at an angle of 120° at an approximate rate of 40 km/million years. This trajectory remain relatively constant throughout the

Oligocene The Oligocene ( ) is a geologic epoch of the Paleogene Period and extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that define the epoch are well identified but the ...

from 33.3 to 20.1 Mya. During this period of time the Australian-Pacific boundary underwent a transformation from

mid-ocean ridge A mid-ocean ridge (MOR) is a seafloor mountain system formed by plate tectonics. It typically has a depth of about and rises about above the deepest portion of an ocean basin. This feature is where seafloor spreading takes place along a diverge ...

to a strike-slip fault and lastly at 20.1 Mya to a

transpression In geology, transpression is a type of strike-slip deformation that deviates from simple shear because of a simultaneous component of shortening perpendicular to the fault plane. This movement ends up resulting in oblique shear. It is generally ve ...

convergent boundary. Then 10.9 Mya, the Macquarie Triple Junction evolved into a ridge-trench-fault triple junction due to the alteration of the Australian–Pacific Boundary motion. This oblique convergent boundary instigated a clockwise rotation of the

Macquarie Ridge Complex Macquarie may refer to: People * Lachlan Macquarie, Governor of the British colony of New South Wales from 1810 to 1821. * Elizabeth Macquarie Campbell, Lachlan Macquarie's second wife Locations * Division of Macquarie, an electoral district in t ...

forming the

Hjort Trench The Hjort Trench is a linear topographic depression south of Macquarie Island in the southwest Pacific Ocean. Geologically, the depression is considered to be the seafloor expression of an ocean-ocean subduction zone, where the Australian plate ...

and numerous fracture zones around the Macquarie Ridge Complex.Lodolo, E. and F. Coren (1994). "The Westernmost Pacific Antarctic plate boundary in the vicinity of the Macquarie triple junction." (In C.A. Ricci, ed. Terra Antarctica, vol.1). pp. 158–161 This rotation also transpired into the Macquarie Triple Junction changing its migration path to an angle of 150° and rate of 34 km/million years in reference to the Australian Plate, making the migration direction southward. Between 5.9 and 2.6 Mya, the Macquarie Triple Junction evolved back into a Ridge–Transform Fault–Transform Fault triple junction as the convergence at the Hjort Trench diminished and Antarctic–Pacific spreading boundary changed back into a transform fault. This is the current state of the Macquarie Triple Junction and is interpreted as a stable ridge–fault–fault triple junction. Evolution of MTJ

Local tectonics

Emerald Fracture Zone

The Emerald Fracture Zone is the westernmost portion of the Pacific-Antarctic Ridge and is a young leaky transform fault zone no older than 2.197-2.229 Ma. This zone was formed during a change in the Pacific-Antarctic Plate Boundary between 3.4 and 3.86 Ma during a transformation of the Pacific-Antarctic Plate Boundary. This transformation was due to the change in the absolute motion of the Pacific Plate in response to

Louisville hotspot The Louisville hotspot is a volcanic hotspot responsible for the volcanic activity that has formed the Louisville Ridge in the southern Pacific Ocean. Location The Louisville hotspot is believed to lie close to the Pacific-Antarctic Ridge, altho ...

activity. The alteration of the Pacific Plate ‘s motion caused a left-lateral strike-slip fault to form at the Pacific-Antarctic Boundary. This strike-slip fault is located near the triple junction along a sharp bend in the westernmost region of the Pacific-Antarctic Boundary. This sharp bend is now the locality of the Emerald Fracture Zone formed from a release bend configuration as seen in

transtension Transtension is the state in which a rock mass or area of the Earth's crust experiences both ''extensive'' and ''transtensive'' shear. As such, transtensional regions are characterised by both extensional structures ( normal faults, grabens) and wr ...

Southeast Indian Ridge

The

Southeast Indian Ridge The Southeast Indian Ridge (SEIR) is a mid-ocean ridge in the southern Indian Ocean. A divergent tectonic plate boundary stretching almost between the Rodrigues Triple Junction () in the Indian Ocean and the Macquarie Triple Junction () in the P ...

is the divergent boundary that separates the Indo-Australian and Antarctic Plates. This boundary has experienced a vast right-lateral transform fault called the Balleny Fault Zone, which is also thought to be caused in response to the formation of the Emerald Fracture Zone. This large offset in the Southeast Indian Ridge is thought to have produced a significant difference in crustal thickness within the Australian Plate influencing the Hjort Trench formation.

Hjort Trench

The Hjort Trench is the southernmost portion of the Macquarie Ridge Complex and has been identified as an oceanic-oceanic subduction zone. This

trench A trench is a type of excavation or in the ground that is generally deeper than it is wide (as opposed to a wider gully, or ditch), and narrow compared with its length (as opposed to a simple hole or pit). In geology, trenches result from eros ...

is found in a region of diagonal convergence produced by the transform fault evolution of the Emerald Fracture Zone. Due to these transpressive plate movements this trench has frequent seismicity events generally less than 20 km depth, which suggest underthrusting of the Indo-Australian Plate underneath the Pacific Plate. This region of underthrusting may eventually evolve into a self-sustaining subduction zone, though the Hjort Trench is thought to be an example of an oceanic subduction zone initiated in response to transform fault development.

Studies of the Macquarie Triple Junction

The understanding of the Macquarie Triple Junction is primarily known due to the study of the seismicity, gravitational, magnetic and bathymetric data of the region. Initial studies took place during the early 1970s by the Eltanin Cruises, which took bathymetric and magnetic tracks in order to interpret the general sea floor topography and sea-floor spreading rates. Additional surveys have been taken during 1988–1991 by multiple cruises of the OGS-Explora. These surveys consist of approximately 6300 km the regions seismicity, gravitational signatures, and additional magnetic and bathymetric surveys, significantly contributing to the understanding of the Macquarie Triple Junction. From analysis of the data obtained from the OGS-Explora, a major change in the Pacific-Antarctic plate motion has been interpreted, instigating the compressional region of the Macquarie Ridge.

Overview of relevant plate boundaries

Indo-Australian Plate and Antarctic Plate boundary

The Indo-Australian Plate and Antarctic Plate Boundary is an active divergent boundary known as the

. The Southeast Indian Ridge ranges approximately 2000 kilometers across the southern region of the

Indian Ocean The Indian Ocean is the third-largest of the world's five oceanic divisions, covering or ~19.8% of the water on Earth's surface. It is bounded by Asia to the north, Africa to the west and Australia to the east. To the south it is bounded by th ...

. The Southeast Indian Ridge has a complex driving force which is due to the interaction of the Amsterdam-St. Paul Plateau, a developed hot spot in the western portion of the Southeast Indian Ridge, and the

mid-oceanic ridge A mid-ocean ridge (MOR) is a seafloor mountain system formed by plate tectonics. It typically has a depth of about and rises about above the deepest portion of an ocean basin. This feature is where seafloor spreading takes place along a diverge ...

(MOR). The Amsterdam-St. Paul Plateau along with the Southeast Indian Ridge produce new

oceanic crust Oceanic crust is the uppermost layer of the oceanic portion of the tectonic plates. It is composed of the upper oceanic crust, with pillow lavas and a dike complex, and the lower oceanic crust, composed of troctolite, gabbro and ultramafic cumu ...

further separating the Indo-Australian and Antarctic plates at an intermediate tectonic rate of 65 mm/yr.

Pacific Plate and Antarctic Plate boundary

The Pacific-Antarctic Plate Boundary is another active divergent boundary known as the

Pacific-Antarctic Ridge The Pacific-Antarctic Ridge (PAR) is a Divergent boundary, divergent tectonic plate boundary located on the seafloor of the South Pacific Ocean, separating the Pacific Plate from the Antarctic Plate. It is regarded as the southern section of the ...

(PAR). The Pacific-Antarctic Ridge is the southwest region of the

East Pacific Rise The East Pacific Rise is a mid-ocean rise (termed an oceanic rise and not a mid-ocean ridge due to its higher rate of spreading that results in less elevation increase and more regular terrain), a divergent tectonic plate boundary located along ...

, the mid-oceanic ridge located at the base of the Pacific Ocean. The PAR is divergent boundary driven by the interaction of a MOR and deep

mantle plumes A mantle plume is a proposed mechanism of convection within the Earth's mantle, hypothesized to explain anomalous volcanism. Because the plume head partially melts on reaching shallow depths, a plume is often invoked as the cause of volcanic hots ...

located in the eastern portion of the East Pacific Rise. These deep mantle plumes however, have given the Pacific Plate a left lateral force vector creating a transform boundary in the western Pacific-Antarctic Plate Boundary in the vicinity of the Macquarie Triple Junction, forming the Emerald Fracture Zone.

Indo-Australian Plate and Pacific Plate boundary

The Indo-Australian Plate and Pacific Plate boundary is the most complex boundary of the Macquarie Triple Junction region, due to the unique collision of the two plates creating two convergent boundaries separated by a transform boundary. The

Puysegur Trench The deepStern, Robert J. (2004)Subduction initiation: spontaneous and induced Earth and Planetary Science Letters 226, 275–292. . Puysegur Trench is a deep cleft in the floor of the south Tasman Sea formed by the subduction of the Indo-Austral ...

, which includes the Fjord Trench, is the southern region of the boundary closest to the Macquarie Triple Junction. The Puysegur Trench formed as the Indo-Australian Plate subducted beneath the Pacific plate. The Puysegur Trench ranges approximately 800 kilometers in length, from the most southern tip of the New Zealand Islands to the Macquarie Triple Junction. The Puysegur Trench makes contact with the

Macquarie Fault Zone The Macquarie Fault Zone is a major right lateral-moving transform fault along the seafloor of the south Pacific Ocean which runs from New Zealand southwestward to the Macquarie Triple Junction. It is also the tectonic plate boundary between the ...

, which is associated with the

Alpine Fault The Alpine Fault is a geological fault that runs almost the entire length of New Zealand's South Island (c. 480 km) and forms the boundary between the Pacific Plate and the Indo-Australian Plate. The Southern Alps have been uplifted on the fa ...

. The Alpine Fault is the right-lateral transform fault boundary separating the Puysegur Trench and the northern

Kermadec Trench The Kermadec Trench is a linear ocean trench in the south Pacific Ocean. It stretches about from the Louisville Seamount Chain in the north (26°S) to the Hikurangi Plateau in the south (37°S), north-east of New Zealand's North Island. Together ...

. The Alpine Fault runs through the majority of the southern island of New Zealand and is associated with New Zealand's frequent and intense earthquake history. The last major region of the Indo-Australian Plate and Pacific Plate Boundary is the Kermadec-Tonga subduction zone at which the Pacific Plate is subducted beneath the Indo-Australian Plate, in opposition of the Puysegur Trench. This convergent boundary has a rate of subduction of approximately 5.5-7.4 cm/yr.

References

{{Reflist, 30em Triple junctions Plate tectonics Macquarie Island