New Zealand Journal of Geology and Geophysics abstracts

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Physical geology and eruptive history of the Matahina Ignimbrite, Taupo Volcanic Zone, North Island, New Zealand

ROY A. BAILEY

U.S. Geological Survey
MS-910, Menlo Park
California 94025, U.S.A.

R. GARRY CARR

170 Ladies Mile
Ellerslie
Auckland 5, New Zealand

Abstract  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 km² area mainly east of the caldera, ranges in thickness from 5 to 200 m, and has a outflow volume of c. 120 km³, equivalent to c. 75 km³ 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.

Keywords  Matahina Ignimbrite; Central Volcanic Region; Taupo Volcanic Zone; Haroharo Caldera; Waiohau Fault; North Island Shear Belt; ignimbrite; welding zonation; crystallisation zonation; devitrification; subaqueous pyroclastic flow; liquefaction; fluidisation

New Zealand Journal of Geology and Geophysics, 1994, Vol. 37: 319-344

0028-8306/94/3703-0319 $2.50/0 (c) The Royal Society of New Zealand 1994

PDF file of entire paper: medium quality (11222K); (scanned from paper original: notes about this process)

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