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Carbonate and shale fault gouge : experimental insights into the role of gouge composition, temperature and pore fluid pressure on the mechanical strength of faults Haywood, Jennifer

Abstract

Carbonates and shales are common in fold and thrust belts worldwide: carbonates typically comprise the hanging wall of fault zones and the shale forms the footwall. Generally, a cataclasite is developed in both the carbonate and shale materials, demonstrating that strain is accommodated in both rock types. Despite the wide occurrence of carbonate and shale cataclasites, little is known about the rheological behavior of these composites. The results of two suites of triaxial frictional sliding experiments designed to analyze the effects of composition, temperature, pore fluid pressure and forcing block composition on gouge strength and stability are presented. Experiments were conducted at 70 MPa effective confining pressure and displacement rates varied between 1 to 100 μm s⁻¹. Gouge material was created from quartz-bearing phyllosilicate-rich shale combined in various volumetric proportions with reagent grade calcite powder with an average grain size of ~5 μm. Experiments were performed on each endmember composition as well as 75%, 50% and 25% mixtures of shale and calcite. At room temperature (T), saturated conditions strain localization in the composite gouges causes significant weakening relative to the strong carbonate endmember. At 150°C and 15 MPa pore fluid pressure (Pf), the carbonate gouge undergoes significant strain weakening followed by the evolution to stick-slip sliding. Microstructure analysis indicates that deformation in the shale endmember gouge is distributed across the gouge zone. In the composite gouges, the fine grained carbonate facilitates phyllosilicate rotation and strain localization. In the carbonate gouge, strain localizes in R₁ and shear zone boundary parallel Y shears. Results show that in the absence of elevated T and Pf, the carbonate hanging wall cataclasite is strong relative to the underlying shale footwall cataclasite. At these conditions strain is most likely to localize in the shale or shale-rich composites. Elevated T and Pf promote strain localization and seismic faulting in the carbonate cataclasite. The coefficient of friction values determined for shale carbonate composites are less than the μ = .85 value predicted by Byerlee for rocks deformed at less than 200 MPa normal stress and should replace Byerlee’s value in numerical thrust sheet models.

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