Abstract:
Northland Allochthon at Parua Bay and north Ocean Beach, east of Whangarei, consists of Late Cretaceous-Oligocene Mangakahia and Motatau Complex units. At both localities, deformed allochthon was emplaced onto autochthonous early Miocene basal Waitemata Group (which rests unconformably on Mesozoic Waipapa Terrane rocks) along a shear zone indicating westward or southwestward transport. This contact was subsequently deformed by eastwest-trending folds, causing northward overturning of units at Parua Bay and tight infolding between two blocks of autochthon at north Ocean Beach. At north Ocean Beach, intrusion of a dike/sill related to the Miocene Kauri Mountain Pluton predated the infolding. Other folds with similarly oriented axes and the same northward vergence have been found south of Whangarei Harbour. These north-verging, east-west-trending structures represent a north-south shortening after the emplacement of the Northland Allochthon not previously recognised. They indicate that the "autochthonous" blocks in the region were unusually mobile and may in fact be parautochthonous. The significance of the northward vergence of these late folds in the Whangarei area is not yet clear. Possibly equivalent structures indicating similar late north-south shortening are distributed throughout the northern North Island, but most commonly verge south.
