Understanding the strain evolution, fault zone permeability and methane flux along a subduction splay fault, Hikurangi subduction margin (New Zealand). A rock magnetic approach.

Abstract:

The compaction of clay-rich and porous sediments in young accretionary systems is accompanied by significant pore-water expulsion. At the same time, the development of a clay- and shear fabric controls the permeability structure throughout accretionary prism, plate boundary décollement and all other faults in the forearc. These processes fundamentally control the fault mechanic and slip behavior.

Here we present new rock magnetic datasets that help to depict the strain evolution, and provide information on permeability anisotropy as well as the locus of fluid and gas migration across a shallow subduction thrust sampled at IODP Site U1518 (Hikurangi Subduction Margin, New Zealand). Anisotropy of magnetic susceptibility (AMS) and a set of room temperature magnetic parameters were acquired on one sample per every meter of core recovered. AMS characterizes the alignment of magnetic particles along shape-preferred orientations, and our results demonstrate that strain is decoupled between hanging- and footwall into margin parallel and perpendicular components.

Conspicuous are also 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 fault zone, 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 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. AMS measurements indicated a sub-horizontal alignment of clay-fabric within the center of the fault zone. This likely contributes to low permeabilities in this zone and hampers the upwards flow of methane (and other fluids) across the fault zone and into the hanging wall. Fault zone weakening caused by fluid flow may also contribute to the observed decoupling of strain components.