Fracture dimensions, displacements and fluid transport

There is commonly a linear relationship between the lengths of rock fractur es and their displacements, but for many fracture populations there is a ve ry large scatter in the data. In the lava flows of the rift zone in Iceland , the displacements on a fracture or fault of a given length may vary by a factor of 2-10. Similar scatter is obtained for the aperture (width)/length ratios of several hundred mineral-filled veins in a major fault zone. I pr opose that the displacement on a fracture depends mostly on the smaller of its dip and strike dimensions, referred to as. the controlling dimension. T hus, in a horizontal outcrop, fractures with the same strike dimension (out crop length) can have widely different displacements depending on whether t he displacements of individual fractures are controlled by strike or dip di mensions. During growth of a fracture, its controlling dimension may altern ate between the dip dimension and the strike dimension. The volumetric rate of flow of fluid through a rock fracture with smooth, parallel walls depen ds on the cube of the fracture aperture. This cubic law implies that when t he aperture of a fracture of a given length in a single set or population c an vary by a factor of 2-10, the corresponding volumetric rate of fluid flo w through that fracture can vary by a factor of 8-1000. A single, wide frac ture in a set of as many as several hundred fractures may thus largely domi nate the fluid transport through that set. Fracture aperture depends not on ly on the associated stress field, but also on its controlling dimension. ( C) 2000 Elsevier Science Ltd. All rights reserved.