Abstract:
The Porangahau Ridge is situated on the southern Hikurangi SubductionMargin. Beneath this ridge, a high-reflectivity zone (HRZ) is located above the regional level of bottom-simulating reflectors (BSRs), which usually mark the base of the gas hydrate stability zone (BGHS). Similar HRZs are often thought to mark highly concentrated gas hydrates. However, based on reflection polarities, it had been suggested that at this location, the HRZ is caused by free gas beneath a locally upwarped BGHS. The shoaling of the BGHS was proposed to be caused by advective heat flux due to expulsion of fluids originating deep in the subduction zone. This study presents an analysis of seismic reflection and ocean bottomseismometer (OBS) data across the ridge, with the goal to distinguish between the two possible causes of the HRZ based on seismic velocities: Gas hydrates would lead to elevated velocities whereas free gas would lower velocities. Data preparation includes OBS relocation, identification of corresponding arrivals in OBSs and reflection data, arrival-time picking, and traveltime modelling. For the last step, a ray-tracing simulation is performed with iterative model updating of velocity models until a final velocity model has been determined. The final model demonstrates that the HRZ is a low-velocity zone with velocities around 1.57 km/s. Such low-velocity values imply that zone with free gas is present in the HRZ. The study, therefore, supports the hypothesis that the HRZ originates from local upwarping of the BGHS beneath which free gas is present. The local upwarping is the results of a heat flow anomaly due to warm-fluid advection beneath the anticline. The ridge is located at the boundary between the deforming foundation in the west and the accretionary wedge seaward. At this location, thrust faults may facilitate fluid migration from deep in the subduction zone. As a consequence, the BGHS is shifted upward by several hundred meters above its regional level. Seismic reflection images also suggest gas migration occurs along north-dipping stratigraphic layers in the northern part of the study area potentially focussing gas migration southwards into the ridge. Remnants of a continuous reflection are present at the regional level of the BSR, suggesting that a BSR was present at this level at some stage. This observation indicates that warming from fluid expulsion is transient, suggesting fluid expulsion takes place in pulses rather than continuously.