Non-linear growth kinematics of opening-mode fractures

Abstract

Fracture aperture is commonly assumed to be a linear function of fracture length, stress, and elastic material properties. Under constant stress, fracture opening displacement may change without concurrent length or height growth if the material effectively weakens after initial linear elastic fracture growth by changes in elastic properties or by non-elastic deformation processes. To investigate the kinematics of fracture opening and its dependence on length and height growth, we reconstructed the opening history of three opening-mode fractures that are bridged by crack-seal quartz cement. Similar crack-seal cement bridges had been interpreted to form by repeated incremental fracture opening and subsequent precipitation of quartz cement. Using scanning electron microscope cathodoluminescence imaging, we determined the thickness and number of crack-seal cement layers as a function of position along fracture length and height. Observed trends in crack-seal cement layer thickness and number of fracture opening increments are consistent with non-linear fracture growth kinematics, consisting of an initial stage of fast fracture propagation relative to aperture growth, followed by a stage of slow propagation and pronounced aperture growth. Consistent with earlier fluid inclusion observations indicating fracture opening and propagation occurring over 40–50 m.y., we interpret the second stage of pronounced aperture growth and slow propagation to result from fracture opening strain accommodated by solution-precipitation creep and concurrent slow, possibly subcritical, fracture propagation.