11 DEFORMATION OF ROCKS Deformation of rocks Folds

11 -DEFORMATION OF ROCKS

Deformation of rocks • Folds and faults are geologic structures. • Structural geology - the study of the forces that deform rocks (stress applied) and the effects of this force (strain).

Small-scale Folds Phil Dombrowski Fig. 10. 1

Small-scale Faults Tom Bean Fig. 10. 2

Stress (force per unit area) Types of directed (or differential ) stresses include: • Compression - shorten • Extension - elongate • Shear - distort shape

Differential stress

Strength • Ability of an object to resist deformation (low pressure vs. high pressure)

Strain is any change in original shape or size of an object in response to stress acting on the object

Types of deformation • Elastic • Ductile (plastic) • Brittle (rupture)

Elastic deformation Temporary change in shape or size that is recovered when the deforming force is removed Think “rubber band”

Ductile (plastic) deformation • Permanent change in shape or size that is not recovered when the stress is removed • Occurs by the slippage of atoms or small groups of atoms past each other in the deforming material, without loss of cohesion • Think “deck of cards”

Brittle deformation (rupture) • Loss of cohesion of a body under the influence of deforming stress • Sucker breaks!!! • Usually occurs along sub-planar surfaces that separate zones of coherent material

Typical stress and strain curve

Factors that affect deformation • Temperature • Pressure • Strain rate • Rock type The variation of these factors determines if a rock will fault or fold.

Effects of rock type on deformation Some rocks are stronger than others. competent: rocks that deform only under great stresses incompetent: rocks that deform under moderate to low stresses

Effects of deformation on rock type Experimental Deformation of Marble Brittle Deformation (Under low pressure) Ductile Deformation (Under high pressure) Fig. 10. 7 M. S. Patterson

Orientation of deformed rocks - way to describe the orientation of geologic structures. ATTITUDE Strike: Strike (compass) bearing of a line defined by the intersection of the plane in question and the horizontal Dip: Dip acute angle between the plane and the horizontal, measured perpendicular to strike.

Fig. 10. 4

Fig. 10. 4

Dipping Sedimentary Beds Chris Pellant Fig. 10. 3

Cockscomb Ridge, S. Utah P. L. Kresan

Cockscomb Ridge, S. Utah Strike Dip P. L. Kresan

Tectonic Forces and Resulting Deformation Fig. 10. 6

BRITTLE DEFORMATION A. Abrupt movement breaks or cracks strata B. 2 kinds of breaks: – 1. JOINTS - NO movement of blocks – 2. FAULTS - YES movement of blocks 1. Movement along STRIKE 2. Movement along DIP

Columns Formed by Jointcontrolled Weathering Terry Englander Fig. 10. 20

Joint-controlled Landscape, S. E. Utah

Faults Fractures in rocks created by earthquakes that have moved A. Dip-slip faults normal reverse thrust B. Strike-slip faults right lateral or left lateral

Hanging-wall and footwall

Dip-slip faults Motion of the fault blocks, parallel to the dip direction.

Classification of Faults hanging wall footwall cross section

Normal Fault hanging wall footwall cross section

Normal Dip-slip Fault

Reverse Fault hanging wall footwall cross section

Reverse Dip-slip Fault

Drape Fold over Reverse Fault, WY George Davis

Thrust Fault Thrust faults are low-angle reverse faults. hanging wall footwall cross section

Thrust fault

Keystone Thrust Fault, S. Nevada Cambrian Limestone Jurassic Sandstone John S. . Shelton

Lewis Thrust, Sawtooth Range, Wyoming Kurt N. Coonstenius

French Thrust, Wyoming Mississippian Limestone Cretaceous Shale Kurt N. Coonstenius

Strike-slip faults Motion of the fault blocks is parallel to the strike direction. To determine the direction of strike, put toes on the fault line & look at the direction the opposite block moved.

Left-lateral Strike Slip Fault map view

Right-lateral Strike Slip Fault map view

Strikeslip Fault Gudmundar E. Sigvaldason

Rift Valley Formed by Extension Horst Graben Horst

Wildrose Graben, Southern California

NASA/TSADO/Tom Stack

1872 Fault Scarp, Southern California

1988 Armenian Earthquake Fault Scarp Armando Cisternas

1992 Landers Earthquake Fault Scarp

PLASTIC DEFORMATION FOLDS anticline: anticline older rocks on the inside syncline: syncline older rocks on the outside (scale - from mm to tens of km)

Fold Terminology Fig. 10

Symmetrical Isoclinal Asymmetrical Overturned Recumbent Fig. 15. 22

Anticline Axial plane Bill Evarts Fig. 10. 11

Asymmetric Folds Breck Kent

Overturned Folds Phil Dombrowski Fig. 10. 1

Overturned Syncline, Israel Geological Survey of Israel Fig. 10. 13

Valley and Ridge Province of the Appalachian Mountains Fig. 10. 19

Valley and Ridge Province P. L. Kresan

Plunging Folds in the Valley and Ridge J. Shelton, Geology illustrated Fig. 10. 15

Raplee Anticline, S. E. Utah

Raplee Anticline on the San Juan River, Utah

Domes and Basins Fig. 10. 16

Sinclair Dome, Wyoming John S. Shelton Fig. 10. 17

Anticlines and Synclines Fig. 10. 9

Geologic Map and Cross Section Fig. 10. 5

Outcrops of Syncline Fig. 10. 18

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