PHYSICAL GEOLOGY AND ERUPTIVE HISTORY OF THE MATAHINA IGNIMBRITE, TAUPO VOLCANIC ZONE, NORTH-ISLAND, NEW-ZEALAND

The Matahina Ignimbrite is a 280 ka ash-flow sheet that erupted from Haroharo Caldera in the Okataina Volcanic Centre, northern Taupo Volcanic Zone, North Island, New Zealand. The ignimbrite underlies a 2000 km(2) area mainly east of the caldera, ranges in thickness from 5 to 200 m, and has a outflow volume of c. 120 km(3), equivalent to c. 75 km(3) of magma. It is a multiple-flow, compound cooling unit consisting of a basal tephra (fallout) member and three ash-flow members, designated lower, middle, and upper, that record three eruptive pulses separated by brief time intervals, estimated from cooling and compaction modelling to range from 20 to 60 days. Distribution of coarse lithic clasts, together with local interbedded co-ignimbrite lag breccias and tephra layers east of the Puhipuhi Basin, confirm Haroharo Caldera as the eruptive source. Over most of its extent on the Kaingaroa Plateau, the outflow sheet thickens eastward away from its source and attains its greatest thickness in the elongate, north-trending, fault-angle trough formed between the gently east sloping surface of the plateau and the western front of the Ikawhenua Range. Difference in thickness of the ignimbrite across the fault bounding the west front of the Ikawhenua Range suggests that during the short time interval between emplacement of the lower and middle ash-flow members, a major tectonic event caused at least 10 m displacement on the fault locally. Circumstantial evidence supporting this early syneruptive tectonic event is found along the Bay of Plenty coast where penecontemporaneous liquefaction structures, possibly seismically induced, occur in the distal subaqueous facies of the lower ash-flow member. The presence of such liquefaction structures in the Matahina and other coastal New Zealand ignimbrites suggests a possible close association between tectonism and major ignimbrite eruptions.