page_title: 3D fold evolution
section_title: Wind gaps
hierarchical_section_title: 3D fold evolution / Identification of 3D fold growth / Fold growth in lateral dimension / Wind gaps
caption_attribution_description: English: Diagrams showing decreasing wind gap elevation along direction of lateral fold growth
context_page_description: In geology, 3D fold evolution is the study of the full three dimensional structure of a fold as it changes in time. A fold is a common three-dimensional geological structure that is associated with strain deformation under stress. Fold evolution in three dimensions can be broadly divided into two stages, namely fold growth and fold linkage. The evolution depends on fold kinematics, causes of folding, as well as alignment and interaction of the each structure with respect to each other. There are several ways to reconstruct the evolution progress of folds, notably by using depositional evidence, geomorphological evidence and balanced restoration. Understanding the evolution of folds is important because it helps petroleum geologists to gain a better understanding on the distribution of structural traps of hydrocarbon.
context_section_description: Elevation of wind gaps decreases along the growth direction of fold and curvature of curved wind gaps convex towards the growth direction of  the fold. Multiple wind gaps can be formed from a single river if there is continuous lateral fold growth and the river keeps being deflected, by abandoning its earlier channel and forming a new one around the outside of the developing fold. In a topographic profile along fold crest, wind gaps appears as deep, narrow v-shaped valley. The incision depth of a wind gap correspond to the period of active river erosion before the channel was abandoned, i.e. the duration of its state of being a river or water gap. Since the time of abandonment of an old river channel(time of formation of a new wind gap) is the time of formation of a new river channel and the start of erosion in the new channel, the base elevation of a wind gap would approximately equals to the surface elevation of its successor, provided that they were formed from the same river. Therefore, the elevation of wind gaps must always decrease along the direction of lateral fold growth along fold crest, which is one of the strongest, although not decisive, evidence of the characteristics of fold growth. As former deflected streams, some of the wind gaps may show curvature that convex towards the direction of lateral fold growth.
  In a Digital Elevation Model(DEM), the longitudinal profile of a wind gap shows a convex-upwards trend and that of a water gap shows a concave-upwards or linear trend.
  None of the single aforementioned methodologies is decisive on determining 3D fold growth and a combination of methodologies should be used, because similar morphology can be produced by limb rotation and differential erosion across strata of varying competency, which do not implicate three-dimensional fold growth.
caption: Fig. 12: The upper diagram shows a cross-section A-A' along the fold crests of two growing folds. The lower part shows a map view of the region. WG=wind gap. Red arrows: direction of lateral fold growth; Wind gap elevation decreases along lateral fold growth direction and curvatures of wind gaps convex towards lateral fold growth direction. The 2 fold segments grow towards each other and merge. Asymmetric forked drainage patterns are delineated on the fold segments. Modified from Hetzel (2004), He and Li (2009) and Ramsey (2008).
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