Vergence (geology)

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

When folds are symmetrical, the vergence tells a geologist only where to find the next antiform or synform. However, in asymmetric folds, wherein one limb of a fold is longer than the other, the vergence is dominantly in the direction in which thrusting took place.

If a number of deformation phases can be recognized in a rock, each phase may have its own vergence. However, because 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 determined with respect to 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.

Vergence does not mean the direction of slip along a fault—even though it is widely misused in this way. Thus, a "southeast-vergent fault" means a fault that has been rotated toward the southeast; "southeast-directed fault" is correct word use for indicating the direction of motion along a fault.