Fractures

Fractures

FRACTURE: is defined by Twiss and Moores (1992) as “..surfaces along which rocks or minerals have broken; they are therefore surfaces across which the material has lost cohesion”

Characteristics of fractures according to Pollard and Aydin (1988)

Ø fractures have two parallel surfaces that meet at the fracture front

Ø these surfaces are approximately planar

the relative displacement of originally adjacent points across the fractures is small compared to the fracture length.

Types of Fractures

Extensional Fracture

In extensional fractures the Fracture plane is oriented parallel to σ1 and σ 2 and perpendicular to σ 3.

Three types of fractures have been identified:

n Mode I fractures (joints) it is the extensional fractures and formed by opening with no displacement parallel to the fracture surface.

n Mode II and Mode III are shear fractures. These are faults like fractures one of them is strike -slip and the other is dip-slip

Same fracture can exhibit both mode II and mode III in different parts of the region.

* Conjugate  fractures : paired fracture systems, formed in the same time, and produced by tension or shear. Many of them intersect at an acute angle which will be bisected by the  

* Curved   fractures : occur frequently and may be caused by the textural and compositional differences within a thick bed or large rock mass or they may a result of changes in stress direction or analysis.

Characteristics of Fractures

n Plumose structure: is the structures formed on the joint surface during its propagation and provides information about the joint propagation direction.

Hackle marks: indicate zones where the joint  propagate rapidly.

Arrest line: forms perpendicular to the direction of propagation and is parallel to the advancing edge of fractures.

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