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Alpine Fault
The Alpine Fault is a geological fault that runs almost the entire length of New Zealand's South Island, being about 600 km (370 mi). long, and forms the boundary between the Pacific plate and the Australian plate. The Southern Alps have been uplifted on the fault over the last 12 million years in a series of earthquakes. However, most of the motion on the fault is strike-slip (side to side), with the Tasman district and West Coast moving north and Canterbury and Otago moving south. The average slip rates in the fault's central region are about 38 mm (1.5 in) a year, very fast by global standards. The last major earthquake on the Alpine Fault was in about 1717 AD with a great earthquake magnitude of Mw8.1± 0.1. The probability of another one occurring before 2068 was estimated at 75 percent in 2021.
The Pacific plate and Indo-Australian plate boundary forms the Macquarie Fault Zone in the Puysegur Trench off the southwestern corner of the South Island and comes onshore as the Alpine Fault just north of Milford Sound. The Alpine Fault then runs the length of the South Island just west of the Southern Alps to near Lewis Pass in the central northern section of the island. At this point, it splits into a set of smaller faults known as the Marlborough fault system. This set of faults, which includes the Wairau Fault, the Hope Fault, the Awatere Fault, and the Clarence Fault, transfer displacement between the Alpine Fault and the Hikurangi subduction zone to the north. The Hope Fault is thought to represent the primary continuation of the Alpine Fault.
The Australian plate, which is in the process of again separating from the Indo-Australian plate, is subducting towards the east south of the South Island and the Pacific plate is subducting towards the west to the north. In the middle, the Alpine Fault is a transform boundary and has both dextral (right-lateral) strike-slip movement and uplift on the southeastern side. The uplift is due to an element of convergence between the plates, meaning that the fault has a significant high-angle reverse oblique component to its displacement.
In the northern section of the fault the transition to the Marlborough Fault System reflects transfer displacement between the mainly transform plate boundary of the Alpine fault and the mainly destructive boundary further northwards from the Hikurangi Subduction Zone to the Kermadec Trench. This has resulted in a complex splaying of faults, which is associated with large earthquakes adjacent to, but off the Alpine fault itself, such the 1929 Murchison earthquake, 1968 Inangahua earthquake and 1929 Arthur's Pass earthquake.
The Alpine Fault has the greatest uplift of the Pacific plate near Aoraki / Mount Cook in its central section. Here the relative motion between the two plates averages 37–40 mm a year. This is distributed as 36–39 mm of horizontal and 6–10 mm upwards movement on the fault's plane per year.
At the southern end of the fault there is effectively no uplift component of the Pacific plate and other faults share the strain as a result of the plate collision. These include in Fiordland faults associated with the 2003 Fiordland earthquake and 2009 Dusky Sound earthquake, the Otago fault system, in Canterbury faults such as the Ostler Fault Zone and those associated with the 7.1 MW Darfield earthquake. The uplift in this South Westland region of the fault which has a dextral strike-slip rate of about 28 mm (1.1 in)/year is on the Australian plate side of it with meter scale vertical uplift every 290 years odd.
Between 25 and 12 million years ago the movement on the proto-Alpine Fault was exclusively strike-slip. The Southern Alps had not yet formed and most of New Zealand was covered in water. Then uplift slowly began as the plate motion became slightly oblique to the strike of the Alpine Fault. In the last 12 million years, the Southern Alps have been uplifted approximately 20 km (12 mi), however, as this has occurred more rain has been trapped by the mountains leading to more erosion. This, along with isostatic constraints, has kept the Southern Alps less than 4,000 m (13,000 ft) high.
Uplift on the Alpine Fault has led to the exposure of deep metamorphic rocks near the fault within the Southern Alps. This includes mylonites and the Alpine Schist, which increases in metamorphic grade towards the fault. The eroded material has formed the Canterbury Plains. The Alpine Fault is not a single structure but often splits into pure strike-slip and dip-slip components. Near the surface, the fault can have multiple rupture zones.
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Alpine Fault
The Alpine Fault is a geological fault that runs almost the entire length of New Zealand's South Island, being about 600 km (370 mi). long, and forms the boundary between the Pacific plate and the Australian plate. The Southern Alps have been uplifted on the fault over the last 12 million years in a series of earthquakes. However, most of the motion on the fault is strike-slip (side to side), with the Tasman district and West Coast moving north and Canterbury and Otago moving south. The average slip rates in the fault's central region are about 38 mm (1.5 in) a year, very fast by global standards. The last major earthquake on the Alpine Fault was in about 1717 AD with a great earthquake magnitude of Mw8.1± 0.1. The probability of another one occurring before 2068 was estimated at 75 percent in 2021.
The Pacific plate and Indo-Australian plate boundary forms the Macquarie Fault Zone in the Puysegur Trench off the southwestern corner of the South Island and comes onshore as the Alpine Fault just north of Milford Sound. The Alpine Fault then runs the length of the South Island just west of the Southern Alps to near Lewis Pass in the central northern section of the island. At this point, it splits into a set of smaller faults known as the Marlborough fault system. This set of faults, which includes the Wairau Fault, the Hope Fault, the Awatere Fault, and the Clarence Fault, transfer displacement between the Alpine Fault and the Hikurangi subduction zone to the north. The Hope Fault is thought to represent the primary continuation of the Alpine Fault.
The Australian plate, which is in the process of again separating from the Indo-Australian plate, is subducting towards the east south of the South Island and the Pacific plate is subducting towards the west to the north. In the middle, the Alpine Fault is a transform boundary and has both dextral (right-lateral) strike-slip movement and uplift on the southeastern side. The uplift is due to an element of convergence between the plates, meaning that the fault has a significant high-angle reverse oblique component to its displacement.
In the northern section of the fault the transition to the Marlborough Fault System reflects transfer displacement between the mainly transform plate boundary of the Alpine fault and the mainly destructive boundary further northwards from the Hikurangi Subduction Zone to the Kermadec Trench. This has resulted in a complex splaying of faults, which is associated with large earthquakes adjacent to, but off the Alpine fault itself, such the 1929 Murchison earthquake, 1968 Inangahua earthquake and 1929 Arthur's Pass earthquake.
The Alpine Fault has the greatest uplift of the Pacific plate near Aoraki / Mount Cook in its central section. Here the relative motion between the two plates averages 37–40 mm a year. This is distributed as 36–39 mm of horizontal and 6–10 mm upwards movement on the fault's plane per year.
At the southern end of the fault there is effectively no uplift component of the Pacific plate and other faults share the strain as a result of the plate collision. These include in Fiordland faults associated with the 2003 Fiordland earthquake and 2009 Dusky Sound earthquake, the Otago fault system, in Canterbury faults such as the Ostler Fault Zone and those associated with the 7.1 MW Darfield earthquake. The uplift in this South Westland region of the fault which has a dextral strike-slip rate of about 28 mm (1.1 in)/year is on the Australian plate side of it with meter scale vertical uplift every 290 years odd.
Between 25 and 12 million years ago the movement on the proto-Alpine Fault was exclusively strike-slip. The Southern Alps had not yet formed and most of New Zealand was covered in water. Then uplift slowly began as the plate motion became slightly oblique to the strike of the Alpine Fault. In the last 12 million years, the Southern Alps have been uplifted approximately 20 km (12 mi), however, as this has occurred more rain has been trapped by the mountains leading to more erosion. This, along with isostatic constraints, has kept the Southern Alps less than 4,000 m (13,000 ft) high.
Uplift on the Alpine Fault has led to the exposure of deep metamorphic rocks near the fault within the Southern Alps. This includes mylonites and the Alpine Schist, which increases in metamorphic grade towards the fault. The eroded material has formed the Canterbury Plains. The Alpine Fault is not a single structure but often splits into pure strike-slip and dip-slip components. Near the surface, the fault can have multiple rupture zones.