Alpine Fault at Gaunt Creek

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!

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 depth, and gradually dragged to the surface by diagonal uplift of the Pacific Plate over the last few million years. Vertical movements along the fault have brought up this rock material from perhaps as much as 30km or more from below the surface.
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!