Fluid Accumulation, Migration and Anaerobic Oxidation of Methane Along a Major Splay Fault at the Hikurangi Subduction Margin (New Zealand): A Magnetic Approach

Understanding the locus of fluid flow along thrust and splay faults is important to understand the hydraulic properties of accretionary systems and fault mechanics. Here, we use rock magnetic techniques in combination with backscattered electron imaging to depict the locus of enhanced magnetic mineral alteration within the Papaku fault, an active splay fault of the subduction interface at the northern Hikurangi Margin. The Papaku fault was cored at Site U1518 during Expedition 375 of the International Ocean Discovery Program and we report room temperature magnetic parameters, complemented by first-order reversal and thermomagnetic curves in the depth interval 250-400 m below seafloor (mbsf). The similar to 60-m wide Papaku fault zone comprises two main slip zones, referred to as the upper main brittle (304-321 mbsf) and lower subsidiary (351-361 mbsf) fault zones, and an intervening zoned, termed the lower ductile deformation zone. Two narrow zones, at the top of the main brittle fault zone, and one in a sand-rich interval above the subsidiary fault zone, experienced enhanced magnetic mineral diagenesis, which resulted in the recrystallization of ferrimagnetic greigite to paramagnetic pyrite. We propose that secondary magnetic mineral diagenesis was driven by anaerobic methane oxidation within these intervals, which occurs in the presence of methane and sulfate. We relate the observed changes to the fault parallel transport of fluids which is restricted to two damage zones. Overlying compacted and clay-rich sediments likely act as a barrier to upward advective flow through the fault zone and into the hanging wall.

Automated summary

Understanding the locus of fluid flow along thrust and splay faults is crucial for comprehending the hydraulic properties of accretionary systems and fault mechanics. In this study, rock magnetic techniques were used to identify enhanced magnetic mineral alteration within the Papaku fault, revealing changes within the main brittle fault zones and subsidiary fault zones. The secondary magnetic mineral diagenesis was likely driven by anaerobic methane oxidation, with sediments acting as a barrier to prevent upward fluid flow through the fault zone.