Vergence is the name given to the systematic changes in the symmetry of small structures about the axis of a large fold. Recognising and recording structural asymmetry in outcrop or drill core can allow deduction of the approximate position and geometry of the larger structure.
Structures from which vergence information can be gained are fold pairs (either S-shape or sinistral; Z-shape or dextral) or the sense of angular relationship between bedding and cleavage (once again described as dextral or sinistral). Note that, by convention, the terms dextral and sinistral refer to an angular relationship as it appears looking down on the structure.
When viewed from below, the sense of asymmetry is reversed. For example, a sinistral (S shape) fold pair becomes dextral (Z shape) when viewed from below. This means that vergence relationships can only be recognised in oriented core.
Structures showing vergence relationships are readily seen in drill core and, if the core is oriented, can be used to provide valuable data on the style, position and geometry of larger scale folding. Even if the core is not oriented, some aspects of changing vergence relationships can still indicate that a major fold axis has been crossed.
For example, cleavage at a high angle to bedding indicates a position on a fold hinge: cleavage at a relatively low angle to bedding indicates a position on the fold limb. However, if the core is oriented, the vergence observations can also indicate whether the large fold that has been crossed is an antiform or a synform, as well as providing data on the geometry of the fold itself.
Once a vergence determination has been made, it can be recorded on the log or drill section as a vector (an arrow) pointing up the hole or down the hole towards the adjacent large antiform. Arrows pointing towards each other on the hole trace will thus indicate the position of a map scale antiformal axis. Arrows pointing away from each other will then indicate the position of a synformal axis.
Author: Roger Marjoribanks
