Classroom presentations to accompany Understanding Earth, 3rd edition prepared by Peter Copeland and William Dupré University of Houston Chapter 10 Folds, Faults, and Other Records of Rock Deformation Deformation of Rocks Deformation of rocks • Folds and faults are geologic structures. • Structural geology is the study of the deformation of rocks and the effects of this movement. Small-scale Folds Phil Dombrowski Fig. 10.1 Small-scale Faults Tom Bean Fig. 10.2 Orientation of deformed rocks We need some way to describe the distribution of geologic structures. Strike: bearing of a line defined by the intersection of the plane in question and the horizontal 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 P.L. Kresan Cockscomb Ridge, S. Utah Strike Dip P.L. Kresan Geologic Map and Cross Section Fig. 10.5 Stress (force per unit area) Types of directed stresses include: • Compression • Extension • Shear Compression Action of coincident oppositely directed forces acting towards each other Tension Action of coincident oppositely directed forces acting away from each other Shear Action of coincident oppositely directed forces acting parallel to each other across a surface in a couple Strength • Ability of an object to resist deformation • Compressive or tensile Strain 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 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 Brittle deformation (rupture) • Loss of cohesion of a body under the influence of deforming stress • Usually occurs along sub-planar surfaces that separate zones of coherent material 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 Tectonic Forces and Resulting Deformation Fig. 10.6 Experimental Deformation of Marble Brittle Deformation Ductile Deformation Fig. 10.7 M.S. Patterson Types of folds (bent planar structures) anticline: older rocks on the inside syncline: older rocks on the outside (scale - from mm to tens of km) Anticlines and Synclines Fig. 10.9 Fold terms • axial Plane: the plane of mirror symmetry dividing the fold into two limbs • axis: line formed by the intersection of the axial plane and a bedding plane • horizontal fold: where the fold axis is horizontal • plunging fold: where the fold axis is not horizontal Fold Terminology Fig. 10.10 Bill Evarts Fig. 10.11 Symmetrical, Asymmetrical and Overturned Folds Fig. 10.12 Anticline Axial plane Bill Evarts Fig. 10.11 Asymmetric Folds Antiform Breck Kent Synform Overturned Folds Phil Dombrowski Fig. 10.1 Overturned Syncline, Israel Geological Survey of Israel Fig. 10.13 Map View of Plunging Folds Fig. 10.14 Oil Field at crest of Plunging Anticline Kurt N. Coonstenius Axial Trace of Plunging Anticline* * Note Landers Oil Field on crest of anticline Kurt N. Coonstenius Valley and Ridge Province P. L. Kresan Plunging Folds in the Valley and Ridge J. Shelton, Geology illustrated Fig. 10.15 Valley and Ridge Province of the Appalachian Mountains Fig. 10.19 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 Syncline Fig. 10.18 Drape Fold over Reverse Fault, WY George Davis Columns Formed by Jointcontrolled Weathering Terry Englander Fig. 10.20 Joint-controlled Landscape, S.E. Utah Faults Fractures in rocks created by earthquakes (hanging wall, footwall, displacement) • Dip-slip faults — normal — reverse • Strike-slip faults • Oblique-slip faults Faults may be "reactivated" History of a fault may be very long. Previously developed weakness is the most likely place to break. Reactivation may have opposite sense as before. Active = 10,000 to 100,000 years Very important for dams and reactors Dip-slip faults Motion of the fault blocks, parallel to the dip direction. Classification of Faults hanging wall footwall cross section Classification of Faults hanging wall footwall cross section Normal Fault hanging wall footwall cross section Reverse Fault hanging wall footwall cross section Thrust Fault Thrust faults are low-angle reverse faults. hanging wall footwall cross section Fig. 10.22 Fig. 10.22a Normal Dip-slip Fault Fig. 10.22b Reverse Dip-slip Fault Strike-slip faults Motion of the fault blocks, parallel to the strike direction. Left-lateral Strike Slip Fault map view Right-lateral Strike Slip Fault map view Fig. 10.22c Strikeslip Fault Gudmundar E. Sigvaldason Fig. 10.21 Fig. 10.22d Large-scale Overthrust Sheet Fig. 10.23 Keystone Thrust Fault, S. Nevada Cambrian Limestone Jurassic Sandstone John S..Shelton Fig. 10.24 Lewis Thrust, Sawtooth Range, Wyoming Kurt N. Coonstenius French Thrust, Wyoming Mississippian Limestone Cretaceous Shale Kurt N. Coonstenius Rift Valley Formed by Extension Fig. 10.25 Wildrose Graben, Southern California NASA/TSADO/Tom Stack Fig. 10.26 Stages in the Development of the Basin and Range Province in Nevada and Utah Fig. 10.27 Stages in the Development of the Basin and Range Province in Nevada and Utah Fig. 10.27 1872 Fault Scarp, Southern California 1988 Armenian Earthquake Fault Scarp Armando Cisternas Fig. 10.28 1992 Landers Earthquake Fault Scarp Dating the order of deformation Use geometry: Inclusions Cross-cutting relationships Combine with fossils and radiometric dating