Vergence (geology)

For other uses, see Vergence (disambiguation).

In structural geology, the vergence of a fold is the direction in which an antiform is inclined or overturned. The term vergence comes from the German vergenz, which means "overturn".

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

If a number of deformation phases can be recognized in a rock, each phase will have its own vergence. However, since each succeeding phase deforms structures (and geometries) of older phases, the vergence of each of the older phases may be of little use.

Geometrically the vergence is the up-dip direction of the axial plane of the fold. The axial plane will physically coincide with the axial plane foliation, if folding deformation of the rock is sufficient to cause a foliation.

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