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Fold Systems: The Concepts of Vergence and Facing. |
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The facing of a fold system refers to the geographic direction of younging (shown with an arrow) of the long limbs of its parasitic folds. In a fold system, such as the one presented below, the facing is changing as one crosses major fold hinges. Indeed from west to east, this fold system is facing west upward, then east downward, then west upward, and east.
The concept of vergence refers to the general sense of shear involved in the development of asymmetric folds. The fold systems shown below is verging east as the asymmetry is the result of a shearing toward the east. Unfortunately the world vergence is also used to refer to the direction of the next anticlinal closure.
Changes in fold asymmetry, cleavage-bedding relationships, facing, and shear vergence occurs across fold hinges. Determination of two of these criteria constrain the two others. |
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