Characterizing sediment dewatering and constraining spatially limited fluid flux in accretionary systems. A rock magnetic approach.

The dewatering and subsequent drainage of fluids from porous sediments in forearc regions controls heat flux and the frictional behavior of the plate boundary decollement and all other forearc faults. Here we present new rock magnetic datasets that help to depict the strain history and locus of fluid and gas migration across a shallow subduction thrust near the deformation front of the Hikurangi subduction margin (New Zealand). Site U1518 of International Ocean Discovery Program (IODP) Expedition 375 penetrated hanging-wall, the roughly 60 m thick fault-zone, and footwall sequences of the Pāpaku fault up to a maximum depth of 504 mbsf.

Rock magnetic investigations include the measurement of Anisotropy of Magnetic Susceptibility (AMS), static three-axis alternating field demagnetization (AFD), magnetic hysteresis, anhysteretic remanence acquisition (ARM) and S-ratio measurement. The datasets are presented for an interval between 275 and 375 mbsf, and encompass both fault-zone and directly adjacent sequences.

Throughout most of the sedimentary sequence, samples yield intensities of the natural remanent magnetization (NRM) between 10^-5 and 10^-6 Am²/kg. Magnetic coercivities range from 40 to 60 mT. During static AFD samples acquired a gyroremanent magnetization. These observations indicate the presence of authigenic greigite (Fe₃S₄). In two intervals, between 304 and 312, and 334 - 351 mbsf, samples yield distinctively lower remanence intensities (~ 10^-7 Am²/kg) and lower coercivities around 20 mT. The upper interval coincides with the onset of brittle deformation at the top of the fault-zone. In the same interval AMS results change abruptly. We propose that the rock magnetic signature is due to the reduction of ferrimagnetic greigite to paramagnetic pyrite (FeS₂). This is most likely caused by the drainage of methane-, and sulfide rich fluids/gas along the upper fault-zone and supports interpretations that the fault zone acts as effective conduit. A continued transport of fluids/gases could have promoted a self-sustaining weakening and strain decoupling with episodic high pore-fluid pressure within localized parts of the fault-zone.