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

Å Fagereng,H.M Savage,M Wang,K Petronotis,L.M Wallace,R Bell,P.M Barnes,L Levay,F Meneghini,D.D Mcnamara,H Kitajima,J.K Morgan,D.M Saffer

doi:10.1130/g46367.1

Abstract

AbstractGeophysical 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.

Highlights

The shallow portion of subduction thrust faults, between the trench and the seismogenic zone, can slip in a variety of modes, including steady creep, slow-slip events (SSEs), afterslip, and coseismic slip (Saffer and Wallace, 2015; Araki et al, 2017)
Existing hypotheses seek to explain temporal and spatial variations in fault slip style: (1) contrasting material properties within the fault zone give rise to mixed behavior (Skarbek et al, 2012; Webber et al, 2018); (2) perturbations to fault stability occur through changes in loading rate (Scholz, 1998; Ikari and Kopf, 2017; Leeman et al, 2018); and (3) low effective stresses promote transitional frictional stability (Liu and Rice, 2007; Segall et al, 2010)
International Ocean Discovery Program (IODP) Expeditions 372 and 375 drilled, logged, and sampled the Pāpaku thrust at Site U1518 offshore New Zealand’s North Island (Fig. 1). This is a major, and highly active, splay fault rooted in the Hikurangi margin plate interface

Summary

Introduction

The shallow portion of subduction thrust faults, between the trench and the seismogenic zone, can slip in a variety of modes, including steady creep, slow-slip events (SSEs), afterslip, and coseismic slip (Saffer and Wallace, 2015; Araki et al, 2017). International Ocean Discovery Program (IODP) Expeditions 372 and 375 drilled, logged, and sampled the Pāpaku thrust at Site U1518 offshore New Zealand’s North Island (Fig. 1).

Results

Conclusion