Characterizing sediment dewatering and fluid flow in accretionary systems – A rock magnetic approach on the example of IODP Site U1518

The dewatering and subsequent drainage of fluids from porous sediments in forearc regions control the frictional behavior of the plate boundary décollement and all other faults in the forearc. Site U1518 of International Ocean Discovery Program (IODP) Expedition 375 penetrated the Pāpaku fault, a shallow splay fault near the deformation front of the Hikurangi Subduction Margin (New Zealand). We present rock magnetic datasets that help to depict strain history, and provide information on permeability anisotropy as well as the locus of fluid and gas migration across the fault zone. A set of room temperature magnetic parameters was acquired on one sample per every meter of core recovered. Conspicuous are two anomalies at 304 - 312 m and 334 - 351 m (below seafloor), towards lower remanence intensity and coercivity. The upper interval coincides with an upper strand of high intensity fracturing and deformation within the Pāpaku fault, the lower interval to a sand-rich interval adjacent to a lower strand of high intensity deformation. Backscattered electron images show a significantly higher abundance of, and recrystallization of framboidal iron-sulfide minerals in this zone. We propose that the rock magnetic signature is due to the reduction of ferrimagnetic greigite (Fe3S4) to paramagnetic pyrite (FeS2). This is most likely caused by the drainage of methane-, and sulfide rich fluids/gas along high permeability zones. The results of magnetic fabric analyses conducted using the Anisotropy of Magnetic Susceptibility (AMS) indicate a sub-horizontal near fault-parallel alignment of paramagnetic clay minerals, which may have enhanced permeability anisotropy.