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

monoclines

A monocline is a step-like geological fold without a change in dip direction across the fold hinge because the layers dip in the same direction.

This is contrasted with anticlines, in which limbs dip away (curve downward) from the hinge, and with synclines, in which the limbs dip toward the hinge (curve upward).

(images at left - click to enlarge - top, schematic of monocline; middle, looking along the eroded strata of Waterpocket Fold in Capitol Reef National Park, mid-res, hi-res; bottom Waterpocket Fold from Strike Valley Overlook, mid-res; image below right, schematic cross section through Waterpocket Fold.)

The Waterpocket Fold in Capitol Reef National Park is a classical monocline that is almost 100-miles long (160 km). It is a huge regional fold with one very steep side in otherwise nearly horizontal layers.

During the Laramide Orogeny, between 50 and 70 Ma, the rock layers on the west side of the Waterpocket Fold were elevated more than 7000 feet (> 2000 m) higher than the layers on the east. The entire Colorado Plateau was subsequently uplifted again, and erosion has exposed this fold within the last 15 to 20 million years.

[links: formations: classic monocline in which Mesozoic strata dip 45º, near Mexican Hat, Utah, 2, 3; monoclinal drape fold, Shell Canyon, Bighorn mountains; Dinosaur National Park monocline; Waterpocket Fold, Capitol Reef, Utah, and eastern flank of Waterpocket Fold, 2; Waterpocket Fold area, as viewed from Boulder Mountain; Strike Valley overlook, 2; Capitol Reef, 2, 3, 4, from the Burr Trail, Chimney Rock trail, Sunset Point; Cross sections of Strike Valley of Waterpocket Fold showing formations and their topographic expression; View of tilted beds along the Waterpocket Fold; satellite: monocline and syncline crossed by transverse stream, Capitol Reef, UT, and nearby monocline and syncline; Henry Mt. laccoliths, Capitol Reef, UT; satellite image; webpages: Capitol Reef; Folding Satellite Images Set]