Vergence (geology)

The meaning of vergence in modern structural geology usage is unclear. Vergence has been used to indicate the direction in which an antiform or synform is inclined or overturned^[1], and it has been used to indicate the sense of motion along a fault^[2]. These two usages of vergence are not compatible, because one means rotation, and the other displacement. Vergence in a non-geological context means rotation (e.g., convergence means rotation toward, and divergence means rotation away). The confusion has likely arisen because fold-and-thrust belts contain faults that contribute to fold rotation^[2]. Geologists might reduce confusion by stopping use of vergence to mean displacement. For fault displacement, syntax like "southeast-directed fault" could be used to indicate the direction of motion along a fault, reserving "southeast-vergent fault" for a fault that has been rotated. A "southeast-vergent" fold refers to a fold rotated toward the southeast, and does not mean a fold that has been displaced to the southeast.

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.

References