VERTICAL DISPLACEMENT RATES ON QUATERNARY FAULTS, WANGANUI BASIN

Wave-cut platforms of late Quaternary marine terraces between Hawera and Marton in Wanganui Basin are vertically displaced by several faults. The faults are probably high-angle normal, and strike generally northeast. Of the four faults that are known to displace surfaces of different age, two show increasing displacements with increasing age, and two show no change with age. Fault traces typically consist of discontinuous, subparallel scarps, up to 1.5 km long and up to several metres high. Some scraps display a left stepping, en echelon pattern, consistent with a component of dextral horizontal movement, though horizontal displacement of terrace risers is not evident. Average rates of vertical movement on the faults vary between 0.02 and 0.2 mm/yr, much less than rates of vertical and horizontal movement documented for many other faults in New Zealand. One fault, the Nukumaru Fault, shows a constant rate of vertical movement averaging 0.07 mm/yr for the last 2.2 million years. The faults are tentatively interpreted as bending-moment faults, associated with folding.