Introduction Dynamic forces within the Earth cause deformation. Chapter 10 Deformation is a general term that in geology applies to any change in the shape or volume of rock layers, such as when they are folded or fractured. Deformation occurs in building large mountain Deformation, ranges at convergent boundaries thru: Mountain Building, Emplacement of plutons Volcanism and the Continents Metamorphism Continental accretion Deformation: About Deformation Modification of About Deformation Rocks by Folding • Deformation mainly occurs near plate and Fracturing boundaries. • Field observations of deformation show us how to reconstruct geologic history. • Deformation includes faulting of rigid rocks and folding of rocks that can be bent. Lecture Outline 1. Mapping geologic structure 1. Mapping geologic structure Outcrop – what is it? 2. How rocks deform Where the underlying bedrock 3. Basic deformation structures is exposed. 4. Styles of continental deformation 4. Styles of continental deformation Basic source of geologic 5. Unraveling geologic history information in the field 1 1. Mapping geologic structure 1. Mapping geologic structure Folds Folds 1. Mapping geologic structure 1. Mapping geologic structure Measuring strike and dip ● strike is the compass direction of a rock layer as it intersects a horizontal surface Exposure of dipping bed ● dip is the amount of tilting of the layer and is measured at How much tilt (dip) & what direction (strike) right angles to strike 1. Mapping geologic structure 1. Mapping geologic structure Example of use of strike and dip W N W N on St on St cti rike d cti rike d ire irect N ire irect N ip d Dip ion W ip d Dip ion W D 45° E D 45° E S angle S angle E E S 45° S 45° Dip angle Dip angle Water trickles down slope parallel to dip. 2 Strike and Dip-The Orientation of Strike and Dip-The Orientation of Deformed Rock Layers Deformed Rock Layers – Strike and dip are measurements used to – Principle of original horizontality states that most describe a rock body's orientation with respect rocks are originally laid down horizontally. to the horizontal. –Strikeis the – When we intersection of a see rocks horizontal plane with inclined, an inclined plane. they have – Dip is the maximum been angle of an inclined deformed plane. by folding and/or fracturing. 1. Mapping geologic structure 1. Mapping geologic structure Geologic maps Geologic cross sections • geologic maps represent the rock formations exposed at Earth’s ● geologic cross sections – surface diagrams showing the features that would be visible if vertical slices were • a common scale for geologic made through part of the crust maps is 1:24,000 (1”=24,000” or 1”=2000’) 2. How do rocks deform • Dynamic forces within the Earth cause deformation. • Deformation is a general term that in geology applies to any change in the shape or volume of rock layers, such as when they are folded or fractured. • Deformation occurs in building large mountain ranges at convergent boundaries thru: – Emplacement of plutons –Volcanism – Metamorphism – Continental accretion 3 Rock Deformation - How Does it Occur? 2. How rocks deform – Applied stresses can deform or strain rocks Plate tectonic forces (on surface) until they become contorted or fracture. ● tensional forces – stretch and pull units apart, – Stresses are categorized as compressional, common at divergent boundaries tensional or shear. ● compressive forces – – While stress is a force squeeze and shorten rock units, Strain is the change in dominate at convergent margins shape that results from the stress being applied. ● shearing forces – push rock units pass each – Deformation and strain are the same thing. other in opposite directions, at transform boundaries Tension Compression Action of coincident oppositely directed Action of coincident oppositely directed forces forces acting away from each other acting towards each other Shear 2. How rocks deform Action of coincident oppositely directed Rock behavior in the laboratory forces acting parallel to each other across a surface in a couple Both ● brittle (near surface, low confining pressure) and ● ductile (deep burial, high confining pressure) 4 Under conditions 2. How rocks deform representative of the shallow crust, the marble is brittle. An undeformed sample An undeformed sample An undeformed sample Under conditions Under conditions Rock Deformation-How Does it Occur? representative of the representative of shallow crust, the the deeper crust, • Types of Strain marble is ductile. marble is brittle. – Rocks will deform elastically until they reach the elastic limit unless the force is applied quickly. – Elastic strain occurs if rocks return to their original shape when the stress is released. – Plastic strain occurs when rocks fold or fracture An undeformed sample when stress is applied and do not recover their 2. How rocks deform original shape. Rock Deformation-How Does it Occur? Rock Deformation-How Does it Occur? • Types of Strain • Types of Strain • What determines whether a rock will bend – Rocks will deform elastically until they reach the elastically, plastically or fracture? elastic limit unless the force is applied quickly. – Type of stress applied – Ductile rocks show a great amount of plastic – Pressure and strain (they bend) temperature before they fracture – Rock type – Length of time – Brittle rocks fracture after only a small amount of plastic strain. 5 2. How rocks deform Rock Deformation - How Does it Occur? Rock behavior in the Earth’s crust • Types of Strain – Compression ● depth affects brittle v. ductile (confining pressure, & possibly heat) ● rock type affects way rocks deform (old hard rock = brittle, while newer, softer (sed. ) rock =ductile) – In compression the rocks are squeezed ● rate of deformation is a factor towards one another along the same line. ● rate of deformation is a factor – Rock layers in compression are shortened the (slow=ductle, fast=brittle, silly putty) rocks by folding or faulting Rock Deformation-How Does it Occur? Rock Deformation-How Does it Occur? • Types of Strain • Types of Strain – Tension – Shear – In shear the forces act parallel to one another, but in opposite directions – In tension the forces along the same line – Deformation occurs along closely spaced act in opposite directions. planes like the slip between cards in a – Tension lengthens the rocks or pulls deck. them apart. Deformation & Geologic Structures • Joints – Joints are fractures along which no movement has (a) Compression causes shortening of rock layers by folding or faulting. taken place. – Joints occur in almost all surface rocks. – Form in response to compression, tension, and shearing. (b) Tension lengthens rock layers and causes faulting. (c) Shear stress causes deformation by displacement along closely spaced planes. Stepped Art 6 Deformation & Geologic Structures 3. Basic deformation structures • Faults are fractures along which the opposite sides have moved relative to one another and Types of faults parallel to the fracture surface. Types of faults ● dip-slip – normal, reverse, and thrust ● strike-slip – right- and left- lateral ● oblique-slip Fig. 10.15a-c, p. 259 Deformation & Geologic Structures • Faults – Dip-slip Faults – All movement is in the direction of dip along dip- slip faults. – Dip-slip faults are categorized as normal or reverse. Fig. 10.15a, p. 259 7 Deformation & Geologic Structures Deformation & Geologic Structures • Faults • Faults – Dip-slip Faults – Normal faults – Normal faults form in response to tensional forces. Deformation & Geologic Structures Joints & Faults – Deformation by Fracturing • Faults • Faults – Normal faults – Reverse Faults Reverse Faults Fig. 10.18a-b, p. 264 Deformation & Geologic Structures Deformation & Geologic Structures • Faults – Quick review • Faults – Dip-slip faults form in tensional strain – Thrust Faults – Lewis Overthrust – Reverse faults form in response to compressional forces. – Thrust faults are a type of reverse fault that dips at less than 45 degrees, often as low as 5 Geo-inSight 5.-7. , p. 262 degrees! 8 Deformation & Geologic Structures • Faults – Strike-slip faults • Faults in which all movement is in the direction of the strike of the fault plane are known as strike-slip faults. • Strike-slip faults are classified as right-lateral or left- lateral depending on the apparent direction of the offset between blocks. Deformation & Geologic Structures Deformation & Geologic Structures • Faults • Faults – Strike-slip faults – Great Glen Fault – Strike-slip faults – San Andreas fault Geo-inSight 8. and 9. , p. 263 Geo-inSight 10. - 12. , p. 263 Deformation & Geologic Structures • Faults – Oblique-slip faults – Oblique-slip faults have both strike-slip and dip-slip components of movement. Fig. 10.16e, p. 260 9 3. Basic deformation structures Deformation & Geologic Structures • Folded Rock Layers • Folded Rock Layers – Folds are layers of rock that were once planar All folds exhibit their own fold anatomy. that are bent or crumpled. Folds have an axial plane – Folds form during compression and undergo that divides the fold in plastic strain. half. – This occurs deep in the crust where the rocks – Each half is called a limb. behave ductilely. Adjacent anticlines and synclines share a limb. – There are 3 kinds of folds: –The axis is an imaginary • Anticlines line formed by the • Synclines intersection of the axial • Monoclines plane and the folded beds. Deformation & Geologic Structures Deformation & Geologic Structures • Folded Rock Layers • Folded Rock Layers –