Alpine Fault at Gaunt Creek

BY JULIAN THOMSON (GEOLOGIST)
Accessibility: EASY
Deep Fault Drilling Project Observatory container, J.Thomson / GNS Science
Here you can get right up close to the plate boundary fault that runs the length of the South Island. You can actually stand with each foot on a separate tectonic plate!
Information panel at Gaunt Creek, J.Thomson / GNS Science
This is a classic exposure of the Alpine Fault. It is the line along which the Australian and Pacific tectonic plates are being pushed together. Most of the movement along the fault is horizontal (sideways), but there is also a vertical component. Movement occurs during very large earthquakes approximately every 300 years, with the last one happening 300 years ago in 1717AD.
This site is on private land, A visit comes highly recommended as this is a classic location where you can actually stand astride a plate boundary fault!
The top of the Australian Plate seen from above Gaunt Creek, J.Thomson / GNS Science
You are at the foot of the range-front of the Southern Alps which are close by to the east of where you are standing. They have been pushed up by many and continued earthquakes occurring on the fault. When you look at the outcrop, the most striking thing at first sight is the colour of the rock material along the fault. This greenish grey clay is known as cataclasite. It has been crushed and altered (metamorphosed) at perhaps 20 or 25 kms depth, and gradually dragged to the surface by diagonal uplift of the Pacific Plate over the last few million years.
The fault itself is easily seen at the base of the greenish grey cataclasite layer. It is worth having a close look at this crushed and fragmented material as it has been forced up from kilometres below the surface to get here! You can see that it has been thrust over the much younger Ice Age gravels (perhaps 15 or 16,000 years old) which are quite different in appearance. Higher up the slope the fault runs horizontally from left to right (west to east) across the cliff.
The locked metal container a few minutes walk upstream houses geophysical monitoring equipment that has been put in place in a shallow (120m) borehole. This was drilled through the fault as phase one of the Deep Fault Drilling Project (DFDP 1). A powerful flood damaged the container which has been bent severely by large boulder impacts, but is still in place!
Directions/Advisory

Access is a rough 4wd track across private farmland, just south of Whataroa.

Google Directions

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Accessibility: EASY

At present (2021) visits are mostly for knowledgeable self guided groups with permission from the landowner.

Features
Sedimentary Metamorphic Active Fault Rock Deformation Landform
Geological Age
The fault itself has been active for about 20 million years, but the gravels through which it passes at this locality were deposited towards the end of the last ice age about 15 to 16 thousand years ago.
Zealandia Evolution Sequence
Pākihi Supergoup: 5 million years ago – present
Links
This video describes the Deep Fault Drilling Project DFDP: https://www.youtube.com/watch?v=n9ZPq5FRmnE