Fracture aperture profiles as indicators of fracture growth environments : an integrated study of fracture aperture growth in the Campito Formation of eastern California
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Processes of fracture formation control flow of fluid in the subsurface and the mechanical properties of the brittle crust. I investigate whether fracture aperture profiles describing the opening displacement along their height or length reflect fracture growth conditions and mechanisms. I hypothesize that aperture profiles of fractures growing under different environments and by different mechanisms have different ellipticity. I measured fracture profiles for quartz cemented opening-mode fractures in low-grade metamorphic sandstone of the Campito Formation, eastern California, and quantified their ellipticity using curve fitting methods based on the Lamé function. I compare the Lamé parameter n against structural and inferred environmental parameters, including fracture orientation, fracture tip characteristics, locations of fracture tips relative to layer boundaries, texture of fracture cements, and temperature during fracture growth as determined through fluid inclusion homogenization temperatures. Lamé n values range from 0.72 to 3.20, with the majority falling between 1 and 1.5. Fracture with n < 2 correlate with higher fluid inclusion homogenization temperatures between 270°C and 315°C, and are preferentially filled with blocky quartz cement. Fractures with fluid inclusion temperatures in the range of 150°C and 250°C have n~ 2 and preferentially contain crack-seal quartz cement. I observe no correlation between n and fracture orientation, and aperture/length ratio, although fractures with wide apertures have n<2. To explain fractures with n<2 we propose a fracture growth mechanism consisting of elastic opening increments followed by stages of stress dissipation in the fracture tip regions. Kinematic models suggest that rapid length or height growth with slow aperture growth results in fracture aperture profiles with n~1. Fracture aperture profiles with n>2 require opening increments with blunt tips and slow length or height growth relative to aperture growth. Our model is consistent with faster length growth resulting from reduced fracture toughness with increasing temperature, leading to low n at higher temperatures. The absence of crack-seal cement in fractures with low n may relate to overall fast fracture growth at higher temperatures, with aperture growth exceeding rates of synkinematic crack-seal quartz cementation. Fractures with n>2 require tip blunting that I attribute to enhanced solution-precipitation creep by stress concentration at the fracture tip.
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