Mixed deformation styles observed on a shallow subduction thrust, Hikurangi margin, New Zealand

Abstract

Geophysical observations show spatial and temporal variations in fault slip style on shallow subduction thrust faults, but geological signatures and underlying deformation processes remain poorly understood. International Ocean Discovery Program (IODP) Expeditions 372 and 375 investigated New Zealand’s Hikurangi margin in a region that has experienced both tsunami earthquakes and repeated slow-slip events. We report direct observations from cores that sampled the active Pāpaku splay fault at 304 m below the seafloor. This fault roots into the plate interface and comprises an 18-m-thick main fault underlain by ∼30 m of less intensely deformed footwall and an ∼10-m-thick subsidiary fault above undeformed footwall. Fault zone structures include breccias, folds, and asymmetric clasts within transposed and/or dismembered, relatively homogeneous, silty hemipelagic sediments. The data demonstrate that the fault has experienced both ductile and brittle deformation. This structural variation indicates that a range of local slip speeds can occur along shallow faults, and they are controlled by temporal, potentially far-field, changes in strain rate or effective stress.

Imaging the Shallow Subsurface Structure of the North Hikurangi Subduction Zone, New Zealand, Using 2‐D Full‐Waveform Inversion

“…Faults 5 and 6 differ from Faults 2, 3, 4, and 7 in that they appear not to be associated with a discrete low‐velocity channel around the fault zone, but rather a moderate velocity contrast between the footwall and hanging wall. Site U1518 drilled through Fault 5 (Fagereng et al, ; Figure ) and LWD sonic velocities decrease below the fault zone (see Figure c) from ~2,550 to ~1,900 m/s. This reduction in velocity was not observed within the starting velocity model, shown in yellow in Figure b but is partially recovered by FWI.…”

“…The third splay fault in the system (Fault 5 in Figure ) was drilled by Expeditions 372 and 375 (Fagereng et al, ; Pecher et al, ; Wallace et al, ). This fault is associated with a velocity decrease of ~100 m/s in the FWI model, and the LWD data show the fault (Figure b—red region) occurring above a velocity inversion between ~300 and 400 mbsf (Figure b).…”

“…Splay faults across the Hikurangi margin are primarily northwest dipping, and some have large measurable displacements within the frontal wedge (Barnes et al, ; Barnes et al, ; Ghisetti et al, ). Fluid flow along splay faults may be expected due to widespread active fluid seepage observed at the seafloor on the crests of thrust‐related bathymetric ridges offshore of Hawke's Bay and Wairarapa (Barnes et al, ; Fagereng et al, ; Greinert et al, ; Plaza‐Faverola et al, ).…”

“…Seismic reflection data reveal that several large splay faults exist within the accretionary wedge, which sole into the subduction plate boundary fault (Barker et al, 2009;Barnes et al, 2010;Barnes & Mercier de Lépinay, 1997;Ghisetti et al, 2016;Lewis & Pettinga, 1993 Barnes et al, 2018;Ghisetti et al, 2016). Fluid flow along splay faults may be expected due to widespread active fluid seepage observed at the seafloor on the crests of thrust-related bathymetric ridges offshore of Hawke's Bay and Wairarapa Fagereng et al, 2019;Greinert et al, 2010;Plaza-Faverola et al, 2016). Plaza-Faverola et al (2016) conducted a PSDM along profile 05CM-38 ( Figure 1a),~220 km to the SW of line 05CM-04, and identified splay faults branching from the décollement associated with high-amplitude reflections, some with polarity reversals, indicating a decrease in acoustic impedance.…”

Low‐Temperature Frictional Characteristics of Chlorite‐Epidote‐Amphibole Assemblages: Implications for Strength and Seismic Style of Retrograde Fault Zones

“…Furthermore, shallow, phyllosilicate‐rich faults may still host earthquake slip if high velocities are imposed by slip that nucleates elsewhere on the fault, inducing dynamic weakening mechanisms (e.g., Faulkner et al, 2011; Ujiie et al, 2013). This could lead to time‐dependent variations in slip style on the same shallow fault segment (Fagereng et al, 2019). We emphasize the importance of this temporal effect in retrograde faults, where limitations of fluid availability and permeability may lead to time‐ and space‐variable replacement of stronger, velocity‐weakening minerals with weaker, velocity‐strengthening chlorite or other clay minerals.…”