ALPINE FAULT AT GAUNT CREEK

BY JULIAN THOMSON (GNS)
ACCESSIBILITY
EASY
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!
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, but the owner does offer visits for a fee (contact information below).
The most striking thing on 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 depth, and gradually dragged to the surface by uplift of the Pacific Plate.
The fault itself is at the base of the greenish grey cataclasite layer. It is worth having a close look at this as it has been forced up from many kilometres below the surface to get here! You can see that it has been thrust over the much younger Ice Age gravels 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).
Directions/Advisory

Access is across private farmland, just south of Whataroa. See below for contact information to organise a visit.

Accessibility: EASY

This is a commercial tour run by the local landowner, and visits must be organised in advance. They normally last about 2 hours. Visits involve a bus ride along a farm track and a short walk with a river crossing to reach the outcrop.
Contact http://alpinefaulttours.co.nz/ (or phone 0800556244) to book a trip that will depart from Whataroa.

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
Pakihi Supergoup
Links
This video describes the Deep Fault Drilling Project DFDP: https://www.youtube.com/watch?v=n9ZPq5FRmnE