Wellington Fault at Mains Rock

This outcrop shows the crushing and grinding effect of fault movement on bedrock along the Wellington Fault.
The fault has been active for approximately 750,000 to one million years in this region, subjecting the local greywacke bedrock to the brittle fracturing effect of hundreds of fault ruptures (earthquakes). This creates a mixture of broken rock fragments and clay that geologists call fault gouge. (Technically if the mixture contains more than 30% large fragments it is called a fault breccia)

Having softened the bedrock in a steeply inclined fault plane descending down many kilometres into the crust, the fault has become a long line of weakness across the land surface. This easily eroded material has governed the path of the Hutt River which follows the fault along most of the length of the Hutt Valley.

Have a think about the big picture here: You are very close to, or on top of the Wellington Fault itself. This fault extends down perhaps 25 kms below you, quite possibly all the way through the Australian Plate that you are standing on, to the plate boundary fault on the upper surface of the Pacific Plate. Because the Wellington Fault, and its local companions such as the Ohariu and Wairarapa faults are in the crust above the plate boundary they are known as upper plate faults (as opposed to the plate boundary fault). The Wellington Fault extends south from where you are standing all the way through Wellington City and into Cook Strait, and also to the north through Kaitoke into the Tararuas and beyond.

Now that you have the overview, you can appreciate that with the relentless collision of the Australian and Pacific tectonic plates occuring at a rate of roughly 40mm per year in this region, the forces at work along such a fault are unimaginably huge. The rocks along the fault are usually locked together, only rupturing every 700 to 1000 years or so in large earthquakes. Typically these might move the rock 4 or 5 metres sideways in what geologist call a 'dextral' movement (ie: looking across the fault the opposite side moves to the right). There will also be perhaps 1 metre of vertical movement at the same time.

Looking closely at the exposed rocks do you think they are mainly fault gouge or fault breccia? How wide is the area of the crush zone that you can see exposed?