Macquarie triple junction

The Macquarie triple junction is a geologically active tectonic boundary located at 61°30′S 161°0′E at which the historic Indo-Australian plate, Pacific plate, and Antarctic plate 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, just south of New Zealand. This tectonic boundary was named in respect to the nearby Macquarie Island, 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 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 Indo-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 from 33.3 to 20.1 Mya. During this period of time the Australian-Pacific boundary underwent a transformation from mid-ocean ridge to a strike-slip fault and lastly at 20.1 Mya to a transpression 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 forming the Hjort Trench and numerous fracture zones around the Macquarie Ridge. 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. Elsewhere the Australian plate before 3 million years ago separated again from the Indian plate. Already on the Indo-Australian plate independent rotational motion had developed in a small distal portion of what is now the Australian plate, and this created a Macquire microplate. This resulted in the current state of the Macquarie triple junction and is interpreted as a ridge–fault–fault triple junction that now involves a Macquarie microplate rather than the Indo-Australian plate as was the case before 6 million years ago, Pacific plate and Antarctic plate.

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 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.

Southeast Indian Ridge The Southeast Indian Ridge 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 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 seismic 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.

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