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.

Description: Highlights • Seismic depth imaging gives insight into the southern Hikurangi subduction zone. • Velocities reveal regional variations in compaction and drainage of input sediments. • Dewatering of subducted sediments might influence décollement strength. • Thrusts at the leading edge of deformation are upper-plate dewatering pathways. • Stratigraphic host of the décollement changes at the southern end of the margin. Abstract The southern end of New Zealand's Hikurangi subduction margin accommodates highly oblique convergence between the Pacific and Australian plates. We carry out two-dimensional (2D) seismic reflection tomography and pre-stack depth migrations on two seismic lines to gain insight into the nature of subducted sediments and upper plate faulting and dewatering at the toe of the wedge. We also investigate the NE to SW evolution of emergent upper plate thrust faulting using 47 seismic lines spanning an along-strike distance of ∼270 km. The upper sequence of sediments that ultimately gets subducted (the MES sequence) has an anomalously-low seismic velocity character. At the southwestern end of the margin, ∼150 km east of Kaikōura, the MES sequence has experienced greater compaction (for an equivalent effective vertical stress) than it has some 200 km further to the northeast. This difference is likely attributable to greater horizontal compression in the southwest caused by impingement of the Chatham Rise on the deformation front. Relationships between velocity and effective vertical stress suggest that the MES sequence is well-drained in the vicinity of frontal thrusts, corroborated by evidence for upper plate dewatering along those thrusts. Effective drainage of the MES sequence likely promotes interplate coupling on the southern Hikurangi margin. The décollement is generally hosted near a seismic reflector known as “Reflector 7”. East of Kaikōura, however, Reflector 7 becomes accreted, indicating that subduction slip at the southwestern end of the margin is no longer hosted at (or above) this reflector. Instead, the décollement steps down to a deeper stratigraphic level further inboard. Further to the SW, approximately in line with the lower Kaikōura Canyon, the offshore manifestation of subduction-driven compression ceases.

Description: We analyse reflection seismic profiles across the outer accretionary wedge at the convergent New Zealand Hikurangi margin. We identify several, in some case stacked, bottom simulating reflections (BSRs). We interpret these multiple BSRs to record changes in gas hydrate stability. With the aid of gas hydrate systems modelling, we identify two geological drivers that affect gas hydrate stability: (1.) rapid sedimentation in trough basins and (2.) uplift and erosion of thrust ridges. Rapid sedimentation in trough basins buries gas hydrates that formed above the former base of gas hydrate stability (BGHS). Locally, we observe a remnant BSR from this process, likely due to residual gas and possibly gas hydrate. The combined effects of uplift and erosion, in contrast, result in the preservation of a remnant BSR within the gas hydrate stability zone, whilst a new BSR forms locally at the present-day BGHS. However, the limited occurrence of double BSRs in seismic data and the model both suggest that the formation of a deeper BSR is limited by gas supply. Formation of significant gas hydrate at this deeper level only occurs in areas of focused gas migration. This slow formation of gas hydrate also has implications for the response to glacio-eustatic sea-level rise: gas hydrates are more likely to accumulate above the BGHS corresponding to the last glacial maximum, whereas only small amounts formed above the deeper present-day BGHS. Hence, future bottom water warming will, at least initially, not lead to significant methane release from dissociating gas hydrates in deep water.