Diffuse vs Localized Deformation in Scaly Clays: insights from experimental reactivation of natural fabrics

Abstract:

Scaly clays are among the most common fault rocks in the shallow part of major faults and may play a key role in controlling fault slip behavior. However scaly fabrics are characterized by foliations and anastomosing slip surfaces that may be highly heterogeneous, from the grain to the outcrop scale, and exhibit strong variability of lithology and structural features. Hence, they are particularly complex to reproduce and study in the laboratory. We performed direct shear experiments on intact clay-rich rocks sampled from an exhumed analogue of a shallow accretionary prism , in order to explore the strength and slip behavior complexity of natural scaly fabrics. We sheared three sets of samples: two natural scaly clays hosted in two different homogeneous lithologies and one slip zone that marks the juxtaposition of the two lithologies in the field. Experiments were carried out in a triaxial apparatus at confining pressures ranging from 10 to 150 MPa and undrained conditions. Each test consisted of two stages: a low pressure, room T shearing followed by a higher pressure and T=150°C shearing phase, to simulate the conditions of the updip limit of the seismogenic zone.

We show that at low temperature and pressure, all rocks show a strong strain hardening behavior and relatively high effective friction coefficient (μ = 0.4-0.8). At higher PT conditions, all rocks show lower effective friction, due to internal fluid pressurization induced by smectite decomposition. However, only the experiments reactivating the natural slip zone show a distinct slip weakening indicative of brittle slip behavior. Our result indicate the fundamental role of lithological contrast combined with fluid pressure from low-grade dehydration reactions to enhance slip localization and instability in natural clay-bearing faults.