Physical properties and gas hydrate at a near-seafloor thrust fault,
Hikurangi Margin, New Zealand
Abstract
The Pāpaku fault zone, drilled at IODP Site U1518, is an active splay
fault in the frontal accretionary wedge of the Hikurangi Margin. In
logging-while-drilling data, the 33 m-thick fault zone exhibits mixed
modes of deformation associated with a trend of downward decreasing
density, P-wave velocity and resistivity. Methane hydrate are observed
from ~30-585 mbsf, including within and surrounding the
fault zone. Hydrate accumulations are vertically discontinuous and occur
throughout the entire logged section at low to moderate saturation in
silty and sandy cm-thick layers. We argue that the hydrate distribution
implies that the methane is not sourced from fluid flow along the fault
but instead by local diffusion. This, combined with geophysical
observations and geochemical measurements from Site U1518, suggests that
the fault is not a focused migration pathway for deeply-sourced fluids
and that the near-seafloor Pāpaku fault zone has little to no active
fluid flow.