Mud volcano eruption within the emergent accretionary Hikurangi margin, southern Hawke's Bay, New Zealand

In September 1994 a mud volcano eruption and associated mudflow occurred adjacent to Brookby gas seep near Waimarama, southern Hawke's Bay, New Zealand. The eruption cycle lasted several weeks, although the bulk of the estimated 8-10 000 m(3) of mud was erupted within hours of the beginning of activity. Other minor gas seeps within a 250 in radius also showed signs of increased mud and fluid flow activity. The main eruption vent and nearby permanent gas seep lie within a 900 in wide northeast-trending tectonic melange zone, with incorporated floater blocks of Late Cretaceous and Paleogene formations. Structural complexity is related to late Cenozoic thrust-fault deformation of the emergent highest accretionary ridge of the Hikurangi margin frontal prism. The interpreted eruption cycle commenced with the rapid development (<1 h) of a mud dome c. 35 m in diameter and estimated 2-3 m maximum height. The dome was symmetrical and positioned over a central vent. The downslope facing sector of the dome formed by the highly cohesive, fluid montmorillonitic mud collapsed and initiated a mudflow, ultimately reaching c. 180 m downslope, blocking Kaikopu Stream. The bulk of the extruded mud flowed directly from the vent through the breached dome wall. This process appears to have taken place over several hours after collapse of the dome wall. The expelled mud is inferred to have been driven upwards by a trapped pocket of escaping gas. Effectively, the expelled mud appears to have formed a plug in the near-surface part of the vent pipe, and this is interpreted to extend to a depth of several hundred metres, trapping migrating gas, which then episodically vents to the surface. There is evidence for similar type mud eruptions in the past at the Brookby site. Once the gas pocket reached the surface, it rapidly escaped, generating a minor mud blast, as evidenced by the presence of radial mud splatters extending up to c. 80 in from the main vent. The adjacent pre-existing and permanent Brookby gas seep also showed evidence of increased activity, with more vigorous expulsion of gas and water bubbling into an enlarged pool. In the days after the main eruption phase, a smaller parasitic dome, with an associated minor mud blast aureole, developed on the margin of the breached ring-cone of the main eruption dome. Methane gas continued to "burp" from this at intervals of 15-30 s during the following weeks.