Last week I was in Hawkes Bay with geologist Kyle Bland, who led a field trip for teachers, students and parents of Crownthorpe School.
Hawkes Bay geology is a story of uplift along fault lines, combined with rapid erosion and deposition by rivers flowing from the inland mountain ranges. This story is etched into the geomorphology of the landscape.
The Mohaka fault last ruptured between AD 1600 and 1850, and forms an amazingly straight scar across the landscape. Like many faults in New Zealand, it is an oblique strike slip fault, including both sideways and vertical movement. If you click on the image to enlarge it you can see how streams crossing the fault have been offset by sideways movement from the last rupture.
Combined sedimentation, uplift and erosion have produced stepped terraces alongside the Ngaruroro river flowing from the Ruahine range out towards the coast.
There are many fossils to be found in the sedimentary rocks that have been uplifted and exposed. Fossil hunting Hawkes Bay style involves using a digger to get access to your specimens!
Ancient greywacke sediments are exposed in the Ruahine Range, having been uplifted by tectonic movements of the North Island fault system (Mohaka and Ruahine faults). These rocks were deposited in a trough at the edge of Gondwanaland, long before New Zealand ever existed.
In the video below, Kyle gives us a Hawkes Bay case study of landscape evolution.
Great article!! I imagine that this fault line will one day slip again. Probably overdue by now. What's your view?
The thing that gets an Australian me excited about New Zealand geology is being able to see the tectonic processes actually happen.
Where I am in Australia anything less than about 25 million years old is 'recent'. That means that different processes have often overprinted the earlier ones making reconstruction of the history of the area very very hard. All the same… I'm not sure I'd prefer to live in a stable tectonic area!