HAUMURIAN (C 66-80 MA) HALF-GRABEN DEVELOPMENT AND DEFORMATION, MID WAIPARA, NORTH CANTERBURY, NEW-ZEALAND

In mid Waipara, North Canterbury, geological mapping has defined the outcrop distribution of Haumurian-aged (c. 66-80 Ma) sedimentary rocks, which locally thicken adjacent to the ESE-striking Birch Fault and are interpreted to have been deposited in a half-graben. Late Cretaceous strata fine upwards from basal conglomerate to muddy quartzose sandstone and reflect a transgressive transition from fluvial to shallow marine environments. These data are in accord with basins documented on the Chatham Rise and suggest that Late Cretaceous extension in Canterbury developed at least as far west as the foothills of the Southern Alps. Locally, in mid Waipara, half-graben development was associated with mild NNE extension (c. 2-3%). Crustal stretching occurred in Canterbury up to c. 30 Ma after continental separation of Gondwana, which is anomalous with respect to the continental breakup model and warrants further investigation.