Marlborough Fault SystemLandscape evolution and thermochronology

The Awatere Fault cuts a clear line across the hills. It last ruptured in the 1848 Marlborough Earthquake.

The Marlborough Fault System (MFS) takes up the Pacific/Australian plate boundary motion through the northern South Island. A subset of the faults that make up the MFS ruptured during the 2016 Kaikōura earthquake.

Overview

The MFS link subduction offshore North Island with oblique continental collision along the western edge of South Island. The rocks of the MFS are in the process of being transferred from the Pacific plate to the Australian plate. This is occurring over several millions of years as the plate boundary shifts to the southeast.

The project

The Marlborough Fault System is currently a strike-slip system

The Marlborough Fault System (MFS) links the two major segments of the Pacific/Australian plate boundary – the Alpine Fault and the Hikurangi Subduction Zone – through the northern South Island. Unlike those fault systems, the MFS is made up of several active faults. A subset of these ruptured during the 2016 Kaikōura earthquake.

  1. The four major faults of the MFS are the Wairau Fault, which is a continuation of the Alpine fault, the Awatere, Clarence and Hope faults. These faults are approximately parallel to each other and bound a series of parallel mountain ranges including the inland and seaward Kaikōura Ranges.
  2. None of these major faults ruptured during the Kaikoura earthquake. Instead, a whole series of nearby faults broke, one of the reasons why Kaikoura was considered one of the most complex earthquakes ever.
  3. These faults take up strike-slip motion today as the relatively plate boundary vector is approximately parallel to the strike of the faults. They show up in the landscape as lines along the edges of the major river valleys.

The distinctive landscape of the MFS contains clues to its geological history

Analysis of the landscape as well as thermochronological dating of the rocks now exposed at the surface give us clues as to the geological history of this region. Prior to the strike-slip deformation taking place today, the area was a region of mountain building. Tapuae-o-Uenuku, in the Inland Kaikōura Range, is the highest point in New Zealand north of the Aoraki Mount Cook region, just higher than Ruapehu.

  • Today the Marlborough faults are mostly strike-slip, however the high topography of the Kaikōura Ranges tells us that in the past must of the movement on these faults was reverse uplifting these mountain ranges.
  • Volcanic rocks older than 100 million years within the Clarence fault tell us that the Clarence fault has been active in some form or another for at least 100 million years.
  • The elevation of the Inland Kaikōura Ranges and fission track ages of ~20 million years tell us that they have existed for about the last 20 million years.
  • The seaward Kaikōura Ranges have younger rocks and were uplifted more recently.

What don’t we know about the MFS?

This part of the plate boundary has not been studied in as much detail as either the Alpine Fault or the Hikurangi Subduction Zone. There are several very interesting scientific questions we don’t yet know the answers to:

  • How does the plate boundary link through Cook Strait from the Hikurangi Subduction Zone to the MFS?
  • Is there an active subduction interface beneath any of the MFS?
  • Why didn’t the Hope Fault break during the Kaikōura earthquake?
  • Could a large earthquake on the Alpine Fault (or the Hikurangi subduction) propagate north (south) through the MFS.

These questions are the focus of ongoing research at GNS Science and elsewhere.

Upton Phaedra 2317

Phaedra Upton Land and Marine Geoscience Theme Leader

Phaedra is a geodynamic modeller who researches a wide range of problems in tectonics. She is adept at using numerical models in collaboration with geologists from a range of subdisciplines to produce insights into a large variety of processes including faulting, fluid flow, heat transfer, drainage evolution, placer gold deposition and the relationship between tectonics and genetics. As a Theme Leader at GNS Science, she practises authentic and collaborative leadership. She promotes diversity of thought and inclusivity as vital to achieving our scientific goals. Phaedra was the 2020 New Zealand Geosciences Hochstetter Lecturer.

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