Journal Article

Effective Bulk Rheology of a Two‐Phase Subduction Shear Zone: Insights From Micromechanics‐Based Modeling and Implications for Subduction Interface Slow Slip Events

Lu, Lucy Xi, Beall, Adam, Fagereng, Åke

2024

Overview

Subduction interfaces exhibit various slip styles, including slow slip events (SSEs). We use a micromechanics‐based approach to calculate the effective rheology of a shear zone containing ellipsoidal amphibolite clasts deforming by dislocation creep within an interconnected linear‐viscous phyllosilicate‐dominated matrix. Frictional failure occurs if local stress exceeds Mohr‐Coulomb yield strength. At moderate fluid overpressure, mixed‐frictional‐viscous behavior emerges at ∼${\\sim} $ 350–560°^{\\circ}$ C, consistent with a broad zone of mixed fault slip behavior without requiring extreme fluid overpressures. Increasing stress in this transition zone promotes local frictional failure and raises bulk strain rate. If, however, the bulk strain rate increases by more than one order of magnitude, system‐wide frictional sliding becomes preferable. This strain rate increase is insufficient to explain the slip rates observed in geophysically detectable SSEs. Therefore, viscous matrix flow as modeled here cannot explain SSEs without either invoking dynamic weakening within a frictional‐viscous flow or a mechanism switch to dominantly frictional sliding. Plain Language Summary Subduction plate boundaries are locked near the Earth's surface and will release the stored energy as earthquakes. Subduction zones creep steadily and viscously at deeper depths where temperatures and pressures are high. At the depth of the transition from earthquakes to steady creep, episodic aseismic slip is often observed. Rocks from this region are mixtures of strong, fractured clasts surrounded by a weak matrix. The observations of exhumed rocks suggest that the episodic, aseismic slip may nucleate when local frictional failure occurs in strong clasts, but the surrounding weak matrix stops this failure from generating major earthquakes. However, it is unclear how much the small‐scale rock behavior could be linked to the large‐scale slip. We use a numerical model to simulate the interplay between frictional and viscous creep and calculate the overall behavior of the subduction zone plate boundary. We explore how the slip style changes with depth and determine the transition zone's depth/temperature range. In the transition zone, a small increase in stress or decrease in strength can lead to a change from pure viscous flow to frictional sliding. This study overcomes the scale challenge between the small‐scale features preserved on outcrops and the large‐scale geophysical observations. Key Points Frictional‐viscous flow in a two‐phase shear zone modeled by a multiscale approach occurs at ∼350–560°C with moderate fluid overpressure Stress loading and/or fluid pressure weakening can cause a switch from steady viscous creep to transient frictional slip Viscous creep modeled here can accommodate tectonic strain rates but not slow slip events without invoking a switch to frictional sliding

• Publisher: John Wiley & Sons, Inc,Wiley
• Subject: 2phase subduction zone / Amphibolites / bulk rheology / Deformation effects / Depth / Earth surface / Earthquakes / Failure / Geophysics / Interfaces / Internal energy / Mathematical models / melange / Micromechanics / multiscale model / Numerical models / Outcrops / Overpressure / Plate boundaries / Plates (tectonics) / Rheological properties / Rheology / Rock / Rocks / Seismic activity / Shear zone / Sliding / Slip / slow slip events / Slumping / Solifluction / Strain rate / Subduction / Subduction (geology) / Subduction zones / Transition zone / Viscous flow / Zone plates