folding

Geological folding involves the plastic deformation (bending, buckling) of a single or multiple (stack) strata, such as sediments and rocks, which were originally planar horizontal surfaces. Although even brittle rocks may undergo plastic deformation when stresses are applied over considerable periods of time (low strain rate). Beyond plastic deformation, rocks fail structurally and faulting occurs.

Folds may be isolated or may occur in extensive fold trains, and folding may range from the microscopic scale to mountain-sized folds in orogenic belts. Folds are classified according to size, fold shape, tightness, and dip of the axial plane.

Folding can occur by flexural slip, buckling, or mass displacement under under varying conditions of stress, hydrostatic pressure, pore pressure, and temperature.


Slumping of material before deformation causes synsedimentary folds. Folding orientation can produce:

▪ anticlines

▪ synclines

▪ domes

▪ basins

▪ décollement folds

▪ monoclines

▪ overturned and recumbent folds

▪ slumps

▪ ptygmatic folds

links: images: plunging anticlines and synclines north of Moab, UT; Folding Satellite Images gallery; Capitol Reef, UT, satellite image and false color image of a monocline and syncline crossed by transverse stream; Capitol Reef Fold, Capitol Reef from ground, 3D anaglyphic images (needs 3D glasses), wp; anticline, 2; folds, 2; fold axes; fold and unconformity; kink fold; kink fold in mountains; refolded; overturned fold; synclinal; close folding and lineations, White Mountains, NH; complex folds in shale and sandstone, Cumana, Venezuela; ductile thrust faulting and large-scale fold nappes, 2, and complex fold interference patterns produced during formation of the fold nappes; Lower Palaeozoic Gondwanan sediments (ca. 500 My) metamorphosed and folded during Variscan Orogenesis (ca 350 My), Aiguilles Rouges, Lac d'Emosson; folding in schist; chevron fold, road cut Kingston-Rhinecliff Bridge, NY; fold, 2, road cut, Catskill, NY; California fold; sheath fold with strongly curved hinge line; glaciotectonic fold, Melabakkar, and close up, 2, 3; ptygmatic folding in quartz vein in Archean metasediments; folded Cambrian limestones and shales, Bay of Islands, Newfoundland ; large folds (Osceola) indicate laterally directed strain away from 4 km distant transient crater (Weaubeau structure of southwestern Missouri); folds in the Barranquin Formation, northeastern Venezuela; webpages: Geology Gallery : Folds : Geological Structures : Secondary Rock Structures : Mouser

phacoliths

Phacoliths are concordant plutonic bodies that lie parallel to the bedding plane or foliation of folded country rock. They occur along the crests of anticlines or the troughs of synclines in folded sedimentary strata.

Rarely, the body of a phacolith may extend as a sill from the crest of an anticline through the trough of an adjacent syncline, such that it has an S shape in cross section. The hinge of folds in intensely folded terrains are areas of reduced pressure, so are potential sites for magma migration and emplacement.

[links: images: close-up: autolith of biotite-rich, enclave bearing, porphyritic granite in another more phenocrystic granite, Wolf Mountain Intrusion (Phacolith), Proterozoic Llano Uplift, central Texas]

syncline

A syncline is a concave geological fold, with layers that dip downward toward the center of the structure. This arrangement is opposite to that of an arching anticline.

Provided that the syncline has not been overturned, strata within synclines have progressively younger rock layers toward the center of the syncline, with the youngest layer at the fold's center or hinge, mirrored by the same layers in reverse sequence on the opposite side of the hinge. Elongate circular or circular fold patterns create basin structures.

Folding typically develops during crustal deformation as the result of compression that accompanies orogenic mountain building.

Strata folded as the Rocky Mountains formed. Near Mount Withrow in northeastern B.C., a prominent syncline is outlined by beds of sandstone that form ridges because they are resistant to weathering. A low area at the centre of the fold is underlain by shale, which weathers very readily. The syncline stretches off into the distance.

Wyoming's Powder River Basin is another notable example of synclinal folding.


[ links: images: formations: synclinal folds outlined by ridges of resistant sandstone, Rocky Mountains near Mt. Withrow; syncline core in Miocene sandstone and shale from the western Tarim Basin, China; Syncline near Indus Suture, Tibet; syncline and unconformity; Mojave syncline that formed at a bend in a strike-slip fault in Rainbow Basin, 2, aka Barstow syncline; Mexican Hat Syncline, UT; overturned syncline in Taconic slates, VT; Scotch Hill syncline, VT; synclinal; sycline in Lockhart Basin, UT; 3D Camargo syncline in central Bolivia (3D glasses needed); Camargo syncline, 2; Camargo syncline center foreground and the San Juan de Oro surface unconformably over it in the background, view looking south with LandSat image draped over the topography; syncline exposed in Monarch Canyon in the Funeral Mountains, Death Valley; Tremont syncline in Llewellyn Formation, PA, Alleghanian Orogeny 290-250Ma; syncline and some accommodation structures near Bude, Cornwall; The Glen Lake Syncline; anticline and syncline and complicated folds near Calico Ghost Town, Yermo, California; Sideling Hill, western Washington County, Maryland Syncline; syncline in Maryland I-68 roadcut, 2, 3, 4, 5; anticline-syncline pair, Canadian Rockies; Gibson Peak syncline; tightly folded syncline that formed as a drag fold along the boundary of a thick, Miocene, andesite dike (upper left), Monterey Shale at Crescent Bay in Laguna Beach; Monument Fold, Colorado River; syncline, 2; satellite: Tindouf syncline; plunging anticlines and synclines, Dinosaur National Monument, UT; monocline and syncline; Henry Mt. laccoliths, Capitol Reef, UT; monocline and syncline crossed by transverse stream, Capitol Reef, UT; Atenango Del Rio Syncline, pseudo false color, false color composite, principle components, digital elevation model; diagram: fold types; websites: Fault Block Mountains Folded Mountains Upwarped Mountains Accreted Mountains; Geology of the Sideling Hill Road Cut; Camargo Syncline Geology]