Seafloor Overthrusting Creates Ductilely Deformed Fault Rocks in Marine Sediments at the Hikurangi Margin: Implications for Fault Zone Evolution and Mechanics at IODP Site U1518

IODP Site U1518, drilled during Exp 372 and 375, penetrated a major thrust fault rising from a megathrust that experiences slow slip along the Hikurangi margin. This thrust fault has hosted >4 km of displacement. Although intersected at only 300 mbsf within moderate porosity clay-rich sediments, the fault zone exhibits intense tectonic ductile deformation with characteristics of crustal shear zones, including (1) finely banded mudstones, contorted into decimeter-scale folds; (2) elongate mudstone clasts with grain tail complexes; (3) stacked and truncated silt beds in distorted mudstones; and (4) soft sediment injections. Locally, these ductile features are overprinted by brittle structures, including normal faults, fracture arrays, and breccias. The wall rocks also exhibit ductile deformation, including shear banding, asymmetric clasts, and injection features. The hanging wall exhibits a brittle overprint, with small-offset normal faults and locally pervasive filled fractures, increasing in intensity toward to the fault. Brittle deformation is less common and more localized in the footwall.

Intense tectonically-derived ductile deformation has not been reported in previously drilled accretionary faults, but can be readily explained by the kinematics of this thrust fault. The large offset along this fault requires that the hanging wall overthrusted the seafloor. Thus, at Site U1518 a hanging wall ramp was superimposed upon a footwall flat trapping high porosity, undeformed, and previously unburied sediments beneath the fault zone. The porous sediments underwent ductile shearing during hanging wall emplacement, and fluidized under low effective stresses, injecting into more cohesive wall rocks. Fault kinematics also explain the asymmetry in deformation: hanging wall strata underwent folding and brittle deformation as they moved through kink folds above the fault bend. In contrast, footwall strata retained their planar orientations and experienced primarily ductile compaction. Over time, pore fluids were expelled and sediments consolidated under the hanging wall load, causing progressive strengthening of the fault zone. The kinematic and mechanical evolution of this frontal thrust has implications for fault zone processes today in response to slow slip along the megathrust fault.