New Zealand Journal of Geology and Geophysics abstracts
Mud volcano eruption within the emergent accretionary Hikurangi margin,
southern Hawke’s Bay, New Zealand
Jarg R. Pettinga
Natural Hazards Research Centre
Department of Geological Sciences
University of Canterbury
Private Bag 4800
Christchurch, New Zealand
Abstract 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 m3 of mud was erupted
within hours of the beginning of activity. Other minor gas seeps within a
250 m radius also showed signs of increased mud and fluid flow activity. The
main eruption vent and nearby permanent gas seep lie within a 900 m 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
m 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.
Keywords mud volcano; eruption; gas venting; mud blast;
mudflow; gas seep; tectonic melange zone; thrust faulting; highest accretionary
ridge; Hikurangi margin; Hawke’s Bay, New Zealand
G01013 Received 23 May 2001; accepted 24 September 2002; published 21
March 2003
New Zealand Journal of Geology & Geophysics, 2003, Vol. 46: 107-121
0028-8306/03/4601-0107 $7.00/0 © The Royal Society of New Zealand
2003
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