Vergence (geology)

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For other uses, see Vergence (disambiguation).
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In structural geology, the vergence of a deformed rock is the direction in which the next antiform can be found. Geometrically the vergence is the up-dip direction of the bedding perpendicular to the planes of the regional foliation in the rock.

The regional foliation is (normally) the dominant foliation in the rock, the bedding is the sedimentary layering in the rock.

If a fold (in the bedding) is formed in the same deformation phase as the regional foliation, the foliation-planes will be oriented parallel to the plane of the fold axes (i.e. perpendicular to the principle stress direction). Therefore, the angle between bedding and foliation is dependent on the rocks' position in the fold.

When folds are symmetric in shape, the vergence tells a geologist only where to find the next anticline or syncline. However, in asymmetric folding, which occurs in thrust systems, one limb of a fold is longer than the other. On average, the vergence will then occur dominantly in the direction in which thrusting took place.

If more deformation phases can be recognized in a rock, each phase will have its own vergence. However, since every next phase deformes structures (and geometries) of older phases, the vergences of the older phases may have little use.

The term vergence comes from the German vergenz, which means "overturn".