Fluid Pathways and Sources Driving Recurring Pockmark Formation on the Chatham Rise, New Zealand

Abstract

Pockmarks are geomorphological depressions observed in diverse marine environments, typically associated with fluid release from subsurface reservoirs. In addition to seafloor features, buried paleo-pockmarks offer valuable records of past fluid escape events. However, the processes, fluid types and conditions controlling their formation and recurrence remain poorly constrained. This study examines potential subsurface fluid sources and pathways that contribute to the cyclic formation of a prominent pockmark field located on the southwestern Chatham Rise, offshore New Zealand. Previous geochemical analyses have revealed no evidence for biogenic or thermogenic methane in the shallow subsurface, necessitating alternative interpretations of the fluid source(s). Here, we use quasi-3D multichannel seismic reflection data to examine the relationship between subsurface fluid flow pathways and spatio-temporal pockmark distribution. A polygonal fault system (PFS), characterized by low fault network connectivity and preferential fault orientations, indicates the influence of the regional stress field. Additionally, a distinct, discordant reflection with relatively high-amplitude, interpreted as a diagenetic front, is hosted within the PFS. Vertically extensive faults extend above the PFS and align spatially with basement faults, forming West–East trending half-grabens that may provide pathways for buoyancy-driven fluid migration. However, spatial pattern analysis reveals no statistically significant spatial association between pockmarks and faults, indicating primarily diffuse fluid flow in the upper subsurface. Generally, the upslope extent of pockmarks on the Chatham Rise appears to be lithologically controlled and coincides with the limit of the PFS in underlying strata. Recurring pockmark formation is interpreted to reflect a combination of compaction-related dewatering during polygonal faulting and silica diagenesis, coupled with deeply sourced, possibly CO₂-rich fluids from subducted limestones migrating along basement structures. These findings are combined into a conceptual model of the subsurface fluid flow system, offering new insights into the controls on recurring pockmark formation in non-hydrocarbon settings.