Focused fluid seepage related to variations in accretionary wedge structure, Hikurangi margin, New Zealand

Sally J Watson,Aaron Micallef,Geoffroy Lamarche,David Bowden,Ingo A Pecher,Tim Kane,Ashley A Rowden,Arne Pallentin,Philip M Barnes,Alan R Orpin,Helen Neil,John Mitchell,Susi Woelz,Jess Hillman,Gareth J Crutchley,Ben Higgs,Joshu J Mountjoy

doi:10.1130/g46666.1

Abstract

AbstractHydrogeological processes influence the morphology, mechanical behavior, and evolution of subduction margins. Fluid supply, release, migration, and drainage control fluid pressure and collectively govern the stress state, which varies between accretionary and nonaccretionary systems. We compiled over a decade of published and unpublished acoustic data sets and seafloor observations to analyze the distribution of focused fluid expulsion along the Hikurangi margin, New Zealand. The spatial coverage and quality of our data are exceptional for subduction margins globally. We found that focused fluid seepage is widespread and varies south to north with changes in subduction setting, including: wedge morphology, convergence rate, seafloor roughness, and sediment thickness on the incoming Pacific plate. Overall, focused seepage manifests most commonly above the deforming backstop, is common on thrust ridges, and is largely absent from the frontal wedge despite ubiquitous hydrate occurrences. Focused seepage distribution may reflect spatial differences in shallow permeability architecture, while diffusive fluid flow and seepage at scales below detection limits are also likely. From the spatial coincidence of fluids with major thrust faults that disrupt gas hydrate stability, we surmise that focused seepage distribution may also reflect deeper drainage of the forearc, with implications for pore-pressure regime, fault mechanics, and critical wedge stability and morphology. Because a range of subduction styles is represented by 800 km of along-strike variability, our results may have implications for understanding subduction fluid flow and seepage globally.

Highlights

Fluids—in their gaseous or liquid phase—play a critical role in controlling the dynamics of accretionary wedges by influencing pore pressure (Dahlen, 1990; Saffer and Bekins, 2006) and the mechanical behavior of faults (Sibson, 1992; Kodaira et al, 2004; Liu and Rice, 2007; Bangs et al, 2015)
The Hikurangi margin, offshore the North Island of New Zealand, exhibits evidence for widespread fluid seepage (Lewis and Marshall, 1996; Barnes et al, 2010; Greinert et al, 2010), together with variations in tectonic structure and processes capturing a range of subduction styles (Fig. 1)
The margin attains a width of 150 km, characterized by a deforming Late Cretaceous– Paleogene presubduction foundation inboard of a late Cenozoic accretionary wedge (Lewis and Pettinga, 1993; Barnes et al, 2010; Ghisetti et al, 2016)

Summary

Introduction

Fluids—in their gaseous or liquid phase—play a critical role in controlling the dynamics of accretionary wedges by influencing pore pressure (Dahlen, 1990; Saffer and Bekins, 2006) and the mechanical behavior of faults (Sibson, 1992; Kodaira et al, 2004; Liu and Rice, 2007; Bangs et al, 2015). We investigated the distribution of focused fluid seepage using an exceptional high-quality regional data set from the Hikurangi margin, including: (1) hydro-acoustic data that image active gas bubbles; (2) seafloor camera observations of chemosynthetic seep ecosystems; and (3) geomorphological and seafloor acoustic

Results

Conclusion