Joints and Fractures
Fractures may develop an array of overlapping fractures called "en echelon extension fractures" (Top pictures). This array creates a zone of weakness that may evolve into in incipient fault in which each fracture is deformed into a sigmoide shape (far right picture). The sense of shear can be inferred from the S or Z shape of the sigmoides.
Extensional fractures (also called tension gashes) may also be associated with stylolitic joints. Stylolitic joints result from the preferential dissolution of soluble material. They develop perpendicularly to extensional fracture. On the bottom left extensional fractures are oriented NW-SE, whereas the stylolitic joints are oriented ENE-WSW. In section, stylotitic joints reveal tooths perpendicular to the joint (bottom right). These structures can be used to get information about the orientation of the principal axes of the stress ellipsoid. Sigma1 (the direction of maximum pressure) is parallel to the stylolitic tooths, and Sigma3 is perpendicular to the tension gashes. |
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