Intrusion of Miocene dikes into wet tephra, Kaipara Harbour, Northland, New Zealand

Abstract

Several subvertical dikes of quenched and chilled basalt intrude a submarine tephra and underlying Miocene sediments on the north shore of Okahukura peninsula, Kaipara harbour, west coast of Northland, New Zealand. Results of simple physical modelling show that a number of structures, complex at first sight, i.e. rim dikes and horns, fingers, sigmoidal and rhomboidal intrusives ( the three latter named internal intrusives ) and peperites form in sequence when the dikes stop and cool. Rim dikes and horns are formed at an early stage as dike tips are blunted as a result of consolidation of tephra when the pore water pressure dissipates. Internal intrusives and at last peperite are interpreted as volume of expelled magma consecutively to the change from liquid to the more voluminous solid+gas states of the magma as the dikes cool. The tips of the dikes act as a pressure valve because, in such sheet intrusion, an increase in the internal pressure is magnified some thirty times at the tip. After the dikes had solidified, the tephra ahead of dike tip was fluidized by vapor flow. Two dikes intruding the tephra are selected for mechanical modelling. Preliminary results show that the tephra was not cemented. It is modelled as a sand with a hyperbolic stress vs strain relationship which changes with the stress path. The dikes are treated as flat elliptical cavities. It is shown that the presence of an array of cracks in the tephra ahead of the dikes can be simulated with the Barenblatt-Dugdale model of fracture mechanics and that the width of the elliptical slit used in calculation must be wider than that of the dikes. The driving pressure of these two dikes was approximately 4 MPa and elastic displacement 25 to 30 % of total displacement. The tephra is the remnant of a dissected but structurally intact shallow submarine volcano of surtseyan type. Results of the mechanical modelling and structural and sedimentological data enable to localise the main vent of the volcano. The surtseyan cone, which consists of tholeiite, overlies unconformably all other Tertiary formations. This confirms that, along the west coast of Northland, the latest lower Miocene manifestation of high alumina volcanism occurred in Mid-Upper Altonian times ( 17.5-15.5 My ) and was basic and tholeiitic.