Agust Gudmundsson

All tectonic rock fractures develop in response to local stresses. The stress conditions for fracture formation has been analysed theoretically and experimentally and used to explain fracture patterns and activity in the field. How the local stresses themselves are generated, and then modified through fracture development, has received less attention. Here I show how local stress fields arise because of stress concentrations around rocks with elastic properties that differ from those of the surrounding rocks. Large-scale stress raisers include cavities (such as magma chambers) and elastic inclusions (such as fault zones). One main mechanical effect of tectonic fracture formation and development is to lower the raised stresses and thereby reduce the local stress gradient. Using dikes in Santorini (Greece) as an example of fluid-driven extension fractures, I show how their magmatic overpressure during emplacement tends to bring the local stresses closer to isotropic. Similarly, using normal, reverse, and strike-slip faults in Iceland as examples, I show how their formation and subsequent slips bring the local stress field closer to isotropic. Based on these examples, and a general theoretical framework, it is concluded that all tectonic fracture formation tends to bring the state of stress towards isotropic, thereby reducing the local stress gradients.