“Virtual shear box” experiments of stress and slip cycling within a subduction interface mélange

Abstract

What role does the progressive geometric evolution of subduction-related mélange shear zones play in the development of strain transients? We use a "virtual shear box" experiment, based on outcrop-scale observations from an ancient exhumed subduction interface-the Chrystalls Beach Complex (CBC), New Zealand-to constrain numerical models of slip processes within a meters-thick shear zone. The CBC is dominated by large, competent clasts surrounded by interconnected weak matrix. Under constant slip boundary conditions, models of the CBC produce stress cycling behavior, accompanied by mixed brittleviscous deformation. This occurs as a consequence of the reorganization of competent clasts, and the progressive development and breakdown of stress bridges as clasts mutually obstruct one another. Under constant shear stress boundary conditions, the models show periods of relative inactivity punctuated by aseismic episodic slip at rapid rates (meters per year). Such a process may contribute to the development of strain transients such as slow slip. 2 INTRODUCTION Subduction megathrust faults can exhibit a wide range of slip behaviors (e.g., Ide et al., 2007; e.g., Peng and Gomberg, 2010). Some are interseismically locked to ~20-30 km depths, accumulating stress slowly between large earthquakes, and transitioning to steady aseismic creep at greater depths where temperatures exceed those required for quartz plasticity (>350°C; e.g., Hyndman and Wang, 1993). Other megathrust faults can experience punctuated slow slip events (SSEs)characterized by aseismic creep that occurs at rates that are subseismic but faster than plate boundary averagesthat last for days to years (e.g., Miyazaki

Strength of Strained Two‐Phase Mixtures: Application to Rapid Creep and Stress Amplification in Subduction Zone Mélange

“…SSEs can be represented by slow stick-slip instabilities caused by competing weakening and strengthening rate-and-state mechanisms (Rubin, 2008), variation in pore pressure (Bernaudin & Gueydan, 2018), or alternating clast-supported and matrix-supported bulk rheology (Lavier et al, 2013;Webber et al, 2018). Our mélange models undergoing progressive jamming demonstrate that the most generalized mélange can switch between matrix support and clast support, resulting in significant η eff variation through time.…”

“…Above a critical ϕ, deformation may be accommodated by brittle faulting of strong blocks and localized strain of the intervening matrix (Fagereng & Sibson, 2010). Such localized slip can occur transiently when matrix shear pathways are blocked by interlocking clasts (e.g., Webber et al, 2018), so that a subset of clasts become load-bearing force chains in a process called 'jamming' (Cates et al, 1998). It is unclear what this critical ϕ is; estimates vary from ϕ ≈ 0.45 (Roscoe, 1952) to ≥0.8 (Handy, 1994).…”

The Impact of Rheology on the Transition From Stick‐Slip to Creep in a Semibrittle Analog

“…Several field (Fagereng & Sibson, ; Handy, ) and numerical (Jammes et al, ; Webber et al, ) studies have indicated that the brittle to viscous phase ratio controls the type of deformation. This can lead to three different deformation styles: (1) The strong, brittle phase provides a stress supporting framework leading to dominantly brittle deformation, (2) rheology is controlled by the flow of the weak, ductile phase where stronger asperities are isolated, and (3) where both phases accommodate strain by a mixture of localized brittle failure and ductile flow (Figure ).…”

“…The overall idea is that an abundant viscous fluid phase would lead to a creep signal, whereas an abundant brittle phase would lead to a stick‐slip signal. The impact of a semibrittle rheology on the transition between creep and stick‐slip has been investigated in a number of physical experiments (Burton et al, ; Moore, ; Reber et al, ) as well as theoretical (Lavier et al, ) and numerical (Jammes et al, ; Webber et al, ) studies.…”

The internal structure and composition of a plate boundary-scale serpentinite shear zone: The Livingstone Fault, New Zealand

“…Where pods are in direct contact with each another, brittle fracture networks radiate outwards from the contact zone ( Fig. 12b-iii), suggesting that local stresses in the pods were transiently elevated as they were brought in to contact (Webber et al, 2018).…”