LEWIS, KEITH B.
National Institute of Water
and Atmospheric Research Ltd. (NIWA), P. O. Box 14901, Wellington, New
Zealand
Abstract: Seeps on New Zealand's Obliquely Convergent Margins
New Zealand sits astride a complex trench-transform-trench system with oppositefacing, obliquely convergent margins to northeast (Hikurangi Margin) and to southwest (Puysegur Margin). To date, over twenty offshore seeps or seep zones have been identified on the basis of distinctive faunas, carbonate chimneys, carbonate crusts, pockmarks, swath backscatter anomalies that suggest mud volcanoes, diapiric seismic structures, and plumes.
Most records are from the Hikurangi oblique subduction to very oblique collision zone. One is from the less extensively explored Puysegur Margin. Several are more remote from the actively deforming plate boundary zone and appear to be related to the Taranaki gas field and to seepage from low sea-level aquifers.
On the active margins, most seeps are from imbricating upper slope ridges
of presubduction margin sediments. They are characterised by new species
of Calyptogena, some live, Brachymodidlus-like mussels, vestimentiferan
tube worms, and unusual limpets and other gastropods. Depleted 13C
concentrations indicate the presence of hydrocarbons with a thermogenic
origin. It is inferred that fluids are derived by compressional dewatering,
mineral dewatering and chemical alteration of organic materials in trench
and pelagic sediments subducted to about 6 km beneath the continental margin.
Steeper, more dilational faults and instability of the methane clathrate
barrier facilitate percolation to the upper slope. In addition, there are
seep zones, presumed to be formed by compressional dewatering, at the toe
of the accretionary slope and in front of massive debris avalanches and
debris flows, some of which result from slope instability in the wake of
subducting seamounts.
AAPG Search and Discovery Article #90920©1999 AAPG Pacific Section Meeting, Monterey, California