Sigmoidal tension gashes form along zones of ductile shear. "They are generally S- or Z-shaped, depending on the sense of shear along the zone." (Twiss and Moores p.40). This type of extensional fracture is usually mineral-filled and form en echelon arrays along the shear zone. En echelon features are generally defined as a segmenting set of features that are overlapping or have a staggered arrangement. " En Echelon tension fractures(gashes) may indicate the direction of displacement by two features: 1) Sigmoidal profiles of fractures, and 2) offset direction of vein-filled fractures " (Maley p.49).
The sigmoidal profiles of fractures are cross-sectional views that aid in determining the direction of rotation. The profiles resemble an S-shaped feature with the tips pointing in the direction of rotation, either clockwise or counter-clockwise. In Idaho, west of Ketchum, is a good example of several sets of sigmoidal, quartz-filled, en echelon fractures in quartzite (Maley p.50).
The gash fractures form as extensional fractures that are perpendicular to the minimum compressive stress, sigma 3 (see diagram). The fractures may be rotated by ductile deformation either during or after formation. As gash fractures develop at different times of ductile shearing they show different amounts of rotation. Since sigma 3 is normal to the unrotated portion of the gash fracture, either the tips of the sigmoidal fracture or the newly formed fractures tells the stress orientation. The youngest fractures may not show any or very little rotation. (see diagram).
References cited:
Tina Alder 2/10/97 Return to Structural Geology Notebook index page.>