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Concept
Chevron (geology)
Résumé
Chevron folds are a structural feature characterized by repeated well behaved folded beds with straight limbs and sharp hinges. Well developed, these folds develop repeated set of v-shaped beds. They develop in response to regional or local compressive stress. Inter-limb angles are generally 60 degrees or less. Chevron folding preferentially occurs when the bedding regularly alternates between contrasting competences. Turbidites, characterized by alternating high-competence sandstones and low-competence shales, provide the typical geological setting for chevron folds to occur.
Perpetuation of the fold structure is not geometrically limited. Given a proper stratigraphy, chevrons can persist almost indefinitely.
In response to compressional stress, geological beds fold in order to minimize dissipation of energy. Given an unconstrained bed, folding does so by correspondingly minimizing bending and thus develops a sinusoidal geometry. In a stratigraphic sequence, beds are geometrically and physically constrained by their neighbours. Similarity must be maintained. To accommodate such constraints while maintaining sinusoidal geometry, less competent layers would need to be subjected to extensive flow. Kinked, yielding and highly localized hinges with straight limbs greatly reduce the geometrical need for deformation. Chevron folds are energetically preferred to conventional sinusoidal folds as they minimize ductile flow to the expense of localized bending.
Four stages mark development of chevron folds: sinusoidal nucleation, concentric folding, straightening of limbs/sharpening of hinges, and tightening of the chevron fold. When inter-limb angles approach 60 degrees, frictional forces limit simple shear and flow deformation in less competent layers and favors pure shear of the whole stratigraphic complex. Therefore, the inter-limb angle, rapidly decreasing as a function of time given larger angles begins to stabilize as the angle nears 60 degrees. There is, however, no physical limitation on the acuteness of the fold.
Source officielle
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