Vergence (geology)
Encyclopedia
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 sediment
ary layering in the rock.
Structural geology
Structural geology is the study of the three-dimensional distribution of rock units with respect to their deformational histories. The primary goal of structural geology is to use measurements of present-day rock geometries to uncover information about the history of deformation in the rocks, and...
, the vergence of a fold
Fold (geology)
The term fold is used in geology when one or a stack of originally flat and planar surfaces, such as sedimentary strata, are bent or curved as a result of permanent deformation. Synsedimentary folds are those due to slumping of sedimentary material before it is lithified. Folds in rocks vary in...
is the direction in which an antiform is inclined or overturned. The term vergence comes from the German
German language
German is a West Germanic language, related to and classified alongside English and Dutch. With an estimated 90 – 98 million native speakers, German is one of the world's major languages and is the most widely-spoken first language in the European Union....
vergenz, which means "overturn".
When folds are symmetric in shape, the vergence tells a geologist only where to find the next anticline or syncline
Syncline
In structural geology, a syncline is a fold, with younger layers closer to the center of the structure. A synclinorium is a large syncline with superimposed smaller folds. Synclines are typically a downward fold, termed a synformal syncline In structural geology, a syncline is a fold, with younger...
. 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
Fold (geology)
The term fold is used in geology when one or a stack of originally flat and planar surfaces, such as sedimentary strata, are bent or curved as a result of permanent deformation. Synsedimentary folds are those due to slumping of sedimentary material before it is lithified. Folds in rocks vary in...
(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
Stress (physics)
In continuum mechanics, stress is a measure of the internal forces acting within a deformable body. Quantitatively, it is a measure of the average force per unit area of a surface within the body on which internal forces act. These internal forces are a reaction to external forces applied on the body...
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 sediment
Sediment
Sediment is naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself....
ary layering in the rock.