• Brantut, N.
• Meyer, G.
• Mitchell, Thomas

Abstract

With increasing depth, the rheology of rocks in the crust change from a brittle to a ductile behaviour. Geological observations of faults and rock physics models suggest that this should occur within a gradual transition zone rather than over a sharp boundary. Little is currently known about the depth and breadth of this transition and how strain rate or other parameters might influence it. Here, we address this question by deforming pre-faulted cylindrical cores of Carrara Marble in a conventional triaxial apparatus under increasing confining pressure in order to determine the relative partitioning of the total deformation between bulk strain and on-fault slip. Deformation behavior of the samples was monitored with a pair of Linear Variable Differential Transformers (LVDTs) and two pairs of strain gauges allowing for the accurate recording of the partitioning of bulk deformation in the sample against slip on the fault. We found that with increasing confining pressure, the samples transitioned from a purely frictional sliding to a purely ductile behavior over a 20 MPa wide zone. In this pressure domain, axial shortening is accommodated by both slip on the fault and bulk ductile deformation, with the contribution of slip decreasing with increasing pressure. In this transitional regime, deformation initiates as diffuse microcracking accompanied by strain hardening, which continues until the stress overcomes friction and reactivates the fault. The boundaries of this mixed behavior zone are controlled by the frictional strength of the fault and the yield stress of the sample; when the frictional strength is lower than the yield stress, the sample is frictional slip dominated. Between the yield stress and the flow stress, the sample is in the transitional mixed regime. When the frictional strength exceeds the flow stress of the rock, the sample deforms in a purely ductile manner. We also investigated the effects of strain rate and fault history on the transition zone. We found that higher strain rates promote brittle behavior and thus pushes the transition to higher pressures. Moreover, fault geometry controls the depth of the transition by having an impact on the frictional strength. We confirm the existence of a zone of the crust at depth where strain can alternatingly be localized or diffuse....

Topics

• impedance spectroscopy
• strength