anticline

Anticlines are upwardly convex folded structures that form arches with the youngest (last deposited) strata at the hinge.

Anticlines are A-shaped structures that develop during crustal deformation as the result of compression that accompanies orogenic mountain building.

(images at left - click to enlarge - top, schematic of an anticline; botttom, road cut exposure of anticlinal fold with syncline to right)

Anticlines are of particular interest to oil-exploration geologists because the Earth's largest oilfields occur in large, gentle anticlines in thick sedimentary rock sequences.

Click here for an image that shows an anticline with a hinge that has a flat top and two steep limbs, creating a box-like shape. Folds with this shape are called box folds. Both synclines and anticlines can be box folds.

[links: images: formations: anticline; anticline and syncline near Calico Ghost Town, Yermo, California; anticline; Teton anticline; anticline; satellite: anticlines near Paradox, UT, and anticline crossed by transverse stream, and Uncompahgre uplift; plunging anticlines and synclines north of Moab, UT; plunging anticlines and synclines, Dinosaur National Monument, UT]

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]