The impact of episodic fault-related folding on late Holocene degradation terraces along Waipara River, New Zealand

The Waipara River flows eastwards through growing folds in the tectonically active foothills of New Zealand's Southern Alps. In the middle Waipara reg ion, flights of degradation terraces are widespread and rise to 55 m above river channels. Ages of terrace surfaces and paleoearthquakes on four fault s are constrained by radiocarbon samples and weathering-rind dates from sur face cobbles of Torlesse Group sandstone. Terrace ages indicate rapid incis ion (c. 30-100 mm/yr) of Waipara River and three tributaries during the lat e Holocene. Cumulative-incision curves suggest a 15-25 m lowering of region al base level over the last thousand years and an additional 20-25 m of loc al incision 200-600 yr BP along Waipara River where it crosses Doctors Anti cline. Rapid river incision was strongly influenced by rock uplift on the a nticline associated with fault rupture during an earthquake 300-400 yr BP. From incision data we infer that the earthquake was preceded and followed b y aseismic fold growth. Tectonic uplift during folding was probably, at mos t, one-third of local river incision, this discrepancy may relate to the sh ort sample period and to locally elevated stream erosive power due in part to a reduction in floodplain width.