Microfractures: A review

Microfractures are small, high-aspect-ratio cracks in rock that result from application of differential stresses. Although the term has been used to refer to larger features in the petroleum engineering and geophysics literature, in geologic parlance the term refers to fractures visible only under magnification, having lengths of millimeters or less and widths generally less than 0.1 mm. Nevertheless, populations of these structures typically encompass a wide size range and in some cases they form the small-size fraction of fracture arrays that include much larger factures. In geologic settings, microfractures commonly form as Mode I (opening) fractures where the minimum principal stress exceeds the elastic tensile strength creating a narrow opening displacement; in isotropic rocks such fractures mark the plane perpendicular to the least compressive principal stress during fracture growth. These planar or curviplanar openings provide an opportunity for fluids and/or gases to enter the created cavity. Cement deposits or crack closure may trap fluids or gases, leaving mineral precipitates and a track of enclosed fluids and gases. In transmitted light these precipitates frequently manifest as fluid-inclusion planes (FIPs). Cathodoluminescence (CL) images show that many are cement-filled microveins. Microfractures can be used to assess the paleostress history or fluid movement history of a rock body. Also, because sudden opening produces acoustic emissions, microfractures created in the laboratory can be used to assess the rock-failure process. Here we review recent discoveries made using microfractures, including fracture patterns, strain, fracture growth and size-scaling, evolution of stresses around propagating faults (process zones), far-field tectonic stresses, and insights into the state of stress leading to earthquakes.