The most spectacular décollement folds develop when a homogeneous layer of uniform thickness overlies a less viscous layer. The incompetent layer can comprise evaporates, shales, or heated lower crust (viscosity inversion with depth).
Décollement folds involve a mismatch between horizontal dimensions of layer above versus below the detachment zone, with the upper layer larger in at least one dimension. Such a dimensional mismatch can result when:
▪ the upper layer is stretched as in diapiric folds,
▪ the upper layer has broken away and slid off an uplift, or
▪ the lower layer has been shortened by thrusting or subduction (thrusting at the margins - Sichuan basin; thin-skinned décollement driven forward along a thrust plane in low-viscosity salt and gypsum deposits - Jura Mtns; subduction - Appalachians, Anti Atlas; continental telescoping and thickening shortened the lower layer in the Western Overthrust Belt of the central Rocky Mountains.)
The Sichuan basin of central China includes some of the best samples of décollement deformation.
◙ subduction zone magmas ◙
[links: images: formations: view perpendicular to thrust movement along Early Tertiary décollement folds in Tertiary strata, décollement at base of black shales (Bravaisberget Formation, Midterhukfjellet, Bellsund, Spitsbergen; décollement fold, Reed Wash area, western San Rafael Swell; folds created by "thin-skinned" thrusting over a décollement in underlying rocks, Mt. Kidd, Kananaskis Mountains, Alberta, Canada; extensive outcrops of Upper Jurassic gypsum-anhydrite décollement layer and siliciclastic strata, Sierra Madre Oriental (SMO) salient, near Galeana; décollement, Montagne Noir; close-up: décollement folds in Khosh Yeilagh fm. (N Iran); webpages: gallery of geological phenomena]