The Marlborough Fault System is a set of four large dextral strike-slip faults and other related structures in the northern part of South Island

South Island

The South Island is the larger of the two major islands of New Zealand, the other being the more populous North Island. It is bordered to the north by Cook Strait, to the west by the Tasman Sea, to the south and east by the Pacific Ocean...

, New Zealand

New Zealand

New Zealand is an island country in the south-western Pacific Ocean comprising two main landmasses and numerous smaller islands. The country is situated some east of Australia across the Tasman Sea, and roughly south of the Pacific island nations of New Caledonia, Fiji, and Tonga...

, which transfer displacement between the mainly transform plate

Plate tectonics

Plate tectonics is a scientific theory that describes the large scale motions of Earth's lithosphere...

 boundary of the Alpine fault

Alpine Fault

The Alpine Fault is a geological fault, more specifically known as a right-lateral strike-slip fault, that runs almost the entire length of New Zealand's South Island. It forms a transform boundary between the Pacific Plate and the Indo-Australian Plate. Earthquakes along the fault, and the...

 and the mainly destructive boundary of the Kermadec Trench

Kermadec Trench

The Kermadec trench is one of Earth's deepest oceanic trenches, reaching a depth of . Formed by the subduction of the Pacific Plate under the Indo-Australian Plate, it runs over a thousand kilometres parallel with and to the east of the Kermadec Ridge and island arc, from near the northeastern tip...

, and together form the boundary between the Australian

Indo-Australian Plate

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

 and Pacific Plate

Pacific Plate

The Pacific Plate is an oceanic tectonic plate that lies beneath the Pacific Ocean. At 103 million square kilometres, it is the largest tectonic plate....

s.

Geometry

The Marlborough Fault System consists of four main dominantly strike-slip fault strands, which together carry almost all of the displacement associated with the plate boundary. Other smaller faults form as splays of these main faults or accommodate deformation of the crust between them, such as the Newton and Hura Faults at the western end of the Hope Fault and the Jordan Thrust that formed the Seaward Kaikoura Range

Kaikoura Ranges

The Kaikoura Ranges are two parallel ranges of mountains in the northeast of the South Island of New Zealand.Formed along New Zealand's Marlborough Fault System, they can be seen as the northernmost extension of the Southern Alps in the South Island....

. The dextral strike-slip across this zone has also involved clockwise rotation of the intervening fault blocks of about 20° since the early Pliocene

Pliocene

The Pliocene Epoch is the period in the geologic timescale that extends from 5.332 million to 2.588 million years before present. It is the second and youngest epoch of the Neogene Period in the Cenozoic Era. The Pliocene follows the Miocene Epoch and is followed by the Pleistocene Epoch...

.

Development

The Marlborough Fault System formed at about 5 Ma, during the early Pliocene

Pliocene

The Pliocene Epoch is the period in the geologic timescale that extends from 5.332 million to 2.588 million years before present. It is the second and youngest epoch of the Neogene Period in the Cenozoic Era. The Pliocene follows the Miocene Epoch and is followed by the Pleistocene Epoch...

, in response to a change in plate motions. The new plate vector was significantly oblique to the Alpine Fault, causing an increased amount of convergence. The set of strike-slip faults formed to accommodate this change by taking up most of the strike-slip component.

Faults

There are four main fault strands, although many other smaller faults, of either strike-slip or thrust type are known.

Hope Fault

The Hope Fault forms the southernmost part of the Marlborough Fault System. The estimated slip-rate during the Holocene

Holocene

The Holocene is a geological epoch which began at the end of the Pleistocene and continues to the present. The Holocene is part of the Quaternary period. Its name comes from the Greek words and , meaning "entirely recent"...

 is 20–25 mm/yr, just over half of the plate boundary displacement. At its northeastern end it links into the Jordan Thrust and most of the displacement is transferred onto that structure. It takes its name from the Hope River, which runs along one of the central fault segments.

Clarence Fault

The Clarence Fault runs from close to the Alpine Fault to about 10 km west of Ward

Ward, New Zealand

Ward is a small town in Marlborough, New Zealand. It is located on State Highway 1, 20 kilometres south of Seddon. Kaikoura is 82 km to the southwest...

, where it appears to terminate abruptly. A Holocene slip-rate of 3.5–5.0 mm/yr is estimated for this fault. At the surface the displacement appears to be nearly pure horizontal, but continuous uplift of the neighbouring Inner Kaikoura Range over the same period, suggests that some of the dip-slip component thought to be present at depth on the fault zone is transferred onto thrust or reverse faults under the range. An extra 10° of clockwise rotation has been recognised within the block that lies northeast of the tip of the Clarence fault. It takes its name from the Clarence River

Clarence River, New Zealand

Clarence River is located on South Island of New Zealand. It is 160 kilometres long.For its first 50 kilometres, the river runs in a generally southeastern direction. It then turns northeast, running down a long straight valley between the Inland and Seaward Kaikoura Ranges...

, which follows the fault trace in the northeastern section of the fault.

Awatere Fault

It is formed of two main segments; the Molesworth section to the southwest and the Eastern section to the northeast. The estimated recent slip-rate for the Molesworth section is 4.4 mm/yr. It takes its name from the Awatere River

Awatere River

The Awatere River is a large river flowing through Marlborough, New Zealand. Flowing along the trace of the active Awatere Fault, it runs northeast through a straight valley to the west of the Inland Kaikoura mountains...

 whose valley follows the fault trace along some of its length.

Wairau Fault

The Wairau Fault is sometimes regarded as a direct continuation of the Alpine Fault and may be referred to as the Alpine-Wairau Fault. It takes its name from the Wairau River

Wairau River

The Wairau River is one of the longest rivers in New Zealand's South Island. It flows for 170 kilometres from the Spenser Mountains , firstly in a northwards direction and then northeast down a long, straight valley in inland Marlborough.The river's lower reaches are noted for the surrounding...

, which follows the fault trace for most of its length. It has an estimated slip-rate of 3–5 mm/yr.

Seismicity

All parts of the Marlborough Fault System are currently seismically active. Historical earthquakes (since European settlement) have occurred on both the Hope and Awatere Faults and on the smaller Poulter Fault. Studies of the geomorphology and the use of trenching across fault strands, has identified many earthquakes that occurred during the Holocene on many parts of the fault system. The Hope Fault, which has the fastest slip rate is characterised by the shortest recurrence interval.