Extension fractures

Extension fractures are fractures where  the displacement is perpendicular to the fracture plane. There are two types of extension fractures: tension fractures and hydrofractures. Tension fractures form when the minimum principal compressive stress is negative and are thus mostly limited to shallow depths in areas undergoing active extension, such as rift zones (Gudmundsson, 1992). Hydrofractures, however, can form at any depth, and are thus of greater importance when it comes to permeability than tension fractures.

Hydrofractures

Hydrofractures are fractures generated by internal fluid overpressure, that is, they are driven open by any kind of crustal fluid, such as magma (dykes, sills and inclined sheets), geothermal water (mineral veins), oil, gas, and groundwater (many joints). Results from analytical modelling show that for a homogeneous host rock, the fluid overpressure in a hydrofracture generates very high crack-tip tensile stresses that often exceed the tensile strength of the host rock. It follows that hydrofractures are very important contributors to fracture network development, and hence to increase in the overall permeability. In layered rocks, however, hydrofractures tend to be arrested, especially at contacts between layers of contrasting mechanical properties (ref). The conditions that favor hydrofracture propagation, or arrest, are of vital importance for understanding the development of fracture systems.

Hydrofractures along a fault plane at Nash Point, Wales (UK)