Vergence (geology)

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 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.

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