ALBERT

Description: Rock Garden is a broad ridge system that sits atop the deforming accretionary wedge of the convergent Hikurangi Margin, where the Pacific Plate (on the east) is being subducted beneath the Australian Plate (on the west) (see Figure 1A). It is inferred that Rock Garden’s origin is owed to subduction of a seamount, where the topographic high on the down-going plate has caused localised uplift and flexural doming of the seafloor.1–3 Active deformation of the ridge is therefore likely to be extensional in nature, in response to the uplift and doming – an atypical deformation style for the regionally compressional tectonics of the subduction margin. The geology of the ridge is not well constrained, but dredge samples indicate that the ‘country rock’ probably consists of relatively well consolidated mudrocks with low primary porosity.4,5 Gas hydrates are inferred to be widespread beneath much of the Rock Garden ridge. This is based on the observation of numerous bottom simulating reflections (BSRs) in several seismic data sets.1,6,7 BSRs in gas hydrate provinces are usually attributed to gas hydrate overlying free gas.8 Therefore, such BSRs are seismic manifestations of the base of gas hydrate stability (BGHS), above which conditions are generally suited for gas hydrate formation and below which they are not. The region between the seafloor and the BGHS, which are sub-parallel to each other, is defined as the gas hydrate stability zone (GHSZ). The ridge has been a focus site for gas- and gas hydrate-related research since 1996, when Lewis and Marshall first documented methane seepage through the seafloor into the water column.9 In 2004, seismic images of BSRs and gas pockets beneath the ridge were presented and a link was made between sub-seafloor gas distribution and seafloor seepage.1 More recently, greater data coverage revealed gas migration pathways beneath several seep sites, requiring the migration of gas through the GHSZ.7 In addition to studies of gas seepage, a regional erosion mechanism associated with dynamics of the gas hydrate system has been hypothesised to explain the remarkably flat ridge-top profile that stands out amid the surrounding bathymetry of the subduction wedge (see Figure 1B).3,5,6,10 High-resolution seismic data sets have formed the basis for much of the research into Rock Garden’s gas hydrate system. The purpose of this article is to highlight some areas where focused flow of gas-charged fluids into the GHSZ is expected – a process that can benefit from, for example, localised structural deformation11 and relatively permeable sedimentary layering.12,13 From the perspective of gas hydrates as a potential alternative energy resource, these geological relationships are important because the enhanced fluid flow may lead to highly concentrated deposits as gas converts to hydrate.11,13 Recent three-phase modelling also predicts that high concentrations of hydrate are likely to form around regions of gas penetration through the GHSZ.14 Hence, we are mapping potential locations of highly concentrated gas hydrate.