Deformation, Mountain Building, and Earth's Crust Chapter 10 Deformation, Mountain Building, and Earth's Crust

Introduction Deformation is a general term for any change in the shape or volume of a rock, such as when a rock is folded or fractured. Deformation occurs in building large mountain ranges at convergent boundaries through: Emplacement of plutons Volcanism Metamorphism Continental accretion

Rock Deformation - How Does it Occur? Stress is a force of deformation. Strain is the change in shape that results when stress is applied. Fig. 10.1a, p. 234

Rock Deformation- How Does it Occur? Stress and Strain When subjected to stress (force), ice on a pond may bend (elastic deformation), or if the stress is great enough, it will fracture, that is, the ice strained or deformed in response to stress. Fig. 10.2, p. 235

Rock Deformation- How Does it Occur? Types of Strain Compression Tension Shear

Rock Deformation- How Does it Occur? Types of Strain Compression In compression, the rocks are squeezed along the same line. Compression shortens the rock layers by folding or faulting. Fig. 10.3a, p. 236

Rock Deformation- How Does it Occur? Types of Strain Tension In tension, the forces along the same line act in opposite directions. Tension lengthens the rocks or pulls them apart; fractures and faults form. Fig. 10.3b, p. 236

Rock Deformation- How Does it Occur? Types of Strain Shear In shear, the forces act parallel to one another, but in opposite directions. Deformation occurs along closely spaced planes like the slip between cards in a deck. Fig. 10.3c, p. 236

(a) Compression (b) Tension (c) Shear stress Stepped Art Fig. 10-3, p. 236

Rock Deformation- How Does it Occur? Types of Strain Rocks will deform elastically until they reach the elastic limit unless the force is applied quickly. Elastic deformation or strain occurs if rocks return to their original shape when the stress is released. Fig. 10.4, p. 236

Rock Deformation- How Does it Occur? Types of Strain Rocks will deform elastically until they reach the elastic limit unless the force is applied quickly. Plastic deformation or strain occurs when rocks fold or fracture when stress is applied and do not recover their original shape. Fig. 10.4, p. 236

Rock Deformation- How Does it Occur? Types of Strain What determines whether a rock will bend elastically, plastically or fracture? Type of stress applied Pressure and temperature Rock type Length of time Fig. 10.4, p. 236

Rock Deformation- How Does it Occur? Types of Strain Rocks will deform elastically until they reach the elastic limit unless the force is applied quickly. Ductile rocks show a great amount of plastic strain (they bend) before they fracture. Brittle rocks fracture after only a small amount of plastic strain. Fig. 10.4, p. 236

Strike and Dip-The Orientation of Deformed Rock Layers Principle of original horizontality states that most rocks are originally laid down horizontally. When we see rocks inclined, they have been deformed by folding and/or fracturing. Fig. 10.6b, p. 238

Strike and Dip-The Orientation of Deformed Rock Layers Strike and dip describe a rock layer's orientation with respect to the horizontal. Strike is the intersection of a horizontal plane with an inclined plane. Dip is the maximum angle of an inclined plane. Fig. 10.5, p. 237

Deformation and Geologic Structures Geologic structures are rocks that have been deformed. Deformation includes fracturing and/or folding. Fig. 10.1, p. 234

Deformation and Geologic Structures Folded Rock Layers Folds are layers of rock that were once planar that are bent or crumpled. Folds form during compression and undergo plastic strain. Fig. 10.1, p. 234

Deformation and Geologic Structures Folded Rock Layers Folding occurs deep in the crust where rock behavior is ductile. There are 3 kinds of folds: Monoclines Anticlines Synclines

Deformation and Geologic Structures Folded Rock Layers Folds have an axial plane that divides the fold in half. Each half is called a limb. The axis is an imaginary line formed by the intersection of the axial plane and the folded beds. Figure 10.8, p. 238

Deformation and Geologic Structures Folded Rock Layers Monoclines A monocline is a flexure in otherwise horizontal or uniformly dipping rock layers. One limb is horizontal. The fold axis is inclined. Fig. 10.6, p. 238

Deformation and Geologic Structures Folded Rock Layers Anticlines and Synclines Anticlines are up-arched folds. The oldest rocks are in the core. Synclines are down-arched folds. The youngest rocks are in the core. Figure 10.8, p. 254

Deformation and Geologic Structures Folded Rock Layers Anticlines and Synclines Upright folds Axial plane is vertical Both limbs dip at the same angle. Figure 10.7, p. 251

Upright, non-plunging folds Fig. 10.9, p. 239

Deformation and Geologic Structures Folded Rock Layers Inclined and Overturned folds In these folds the axial plane is inclined Usually form under compression at convergent boundaries Overturned folds have both limbs dipping in the same direction Fig. 10.10a,b, p. 239

Deformation and Geologic Structures Folded Rock Layers Recumbent folds In these folds, the axial plane is horizontal or nearly horizontal. Usually form under compression at convergent boundaries Fig. 10.10c, p. 239

Deformation and Geologic Structures Folded Rock Layers Plunging folds Fold axis is not horizontal Axial plane may be vertical or inclined Fig. 10.11a,b, p. 240

Plunging fold Fig. 10.11c, p. 240

Deformation and Geologic Structures Folded Rock Layers Domes and basins Domes and basins are circular to oval structures which have rock layers occurring in age-position contexts which are the same as anticlines and synclines, respectively. Fig. 10.12, p. 241

Fig. 10.12, p. 241

Black Hills of South Dakota: A Dome Geo-Recap Fig. 1 p. 255

Deformation and Geologic Structures Joints Joints are fractures along which no movement has taken place parallel to fracture surface, although movement may occur perpendicular to the surface. They are not faults. Fig. 10.13a, p. 241

Deformation and Geologic Structures Joints Joints occur in almost all surface rocks. They form in response to compression, tension, and shearing. Joints may occur in two or three prominent sets. Fig. 10.13a, p. 241

Deformation and Geologic Structures Faults are fractures along which the opposite sides have moved relative to one another and parallel to the fracture surface. Fig. 10.14a, p. 242

Deformation and Geologic Structures Faults are fractures along which the opposite sides have moved relative to one another and parallel to the fracture surface. Fig. 10.14b, p. 242

Deformation and Geologic Structures Faults are fractures along which the opposite sides have moved relative to one another and parallel to the fracture surface. Fig. 10.14c, p. 242

Deformation and 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. 10.14a, p. 242

Deformation and Geologic Structures Faults Dip-slip Faults Normal faults form in response to tensional forces. The hanging wall moves down relative to the footwall. Fig. 10.15a, p. 243

Deformation and Geologic Structures Faults Normal faults Fig. 10.16a, p. 244

Deformation and Geologic Structures Faults Normal faults form from tension. Fig. 10.18, p. 248

Deformation and Geologic Structures Faults Dip-slip faults Reverse faults form in response to compressional forces. Fig. 10.15b, p. 243

Deformation and Geologic Structures Faults Dip-slip faults Thrust faults are a type of reverse fault that dip at less than 45 degrees, often as low as 5 degrees! Fig. 10.15c, p. 243

Deformation and Geologic Structures Faults Reverse Faults Fig. 10.16b, p. 244

Deformation and 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. Fig. 10.15d, p. 243

Deformation and Geologic Structures Faults Strike-slip faults – San Andreas fault (right- lateral) Fig. 10.17a, p. 245

Deformation and Geologic Structures Faults Oblique-slip faults Oblique-slip faults have both strike-slip and dip- slip components of movement. Fig. 10.15e, p. 243 Fig. 10.17b, p. 245

Deformation and the Origin of Mountains A mountain is an area of land that stands at least 300 meters above the surrounding country and has a restricted summit area. A mountain range is a group of linear peaks and ridges that formed together. A mountain system is a complex group of linear peaks and ridges that is composed of several mountain ranges. Mountain systems are the result of plate movements and interactions along plate boundaries.

Deformation and the Origin of Mountains The Tetons Range is part of the Rocky Mountain system. The Smoky Mountains are a range in the Appalachian mountain system.

Deformation and the Origin of Mountains Mountain Building Mountain building can involve faulting and folding, but can arise without these types of deformation. Ways mountain form: Volcanism Erosion: mesas and buttes Compression Block-faulting - tension Fig. 10.18a, p. 248

Deformation and the Origin of Mountains Plate Tectonics and Mountain Building Orogeny – an episode of mountain building Most orogenies are produced along convergent plate boundaries where one plate is subducted beneath another or where two continents collide.

Deformation and the Origin of Mountains Plate Tectonics and Mountain Building Orogeny – an episode of mountain building Orogenies are accompanied by the emplacement of batholiths, metamorphism and thickening of the Earth’s crust.

Deformation and the Origin of Mountains Plate Tectonics and Mountain Building Orogeny – an episode of mountain building Sedimentary rocks that formed in marine environments are often found emplaced high in the mountains as a result of orogenies.

Deformation and the Origin of Mountains Plate Tectonics and Mountain Building Orogenies are also closely associated with: Mass Wasting, including land-slides Glaciers Running Water Erosion is responsible for carving the most majestic peaks!

Deformation and the Origin of Mountains Plate Tectonics and Mountain Building Orogenies at Oceanic-Oceanic Plate Boundaries Fig. 10.19a, p. 249

Deformation and the Origin of Mountains Plate Tectonics and Mountain Building Orogenesis along oceanic-oceanic plate boundaries includes deformation, igneous activity, island arc formation, and metamorphism. Fig. 10.19b, p. 249

Deformation and the Origin of Mountains Plate Tectonics and Mountain Building Orogenies at Oceanic-Continental Plate Boundaries Subduction of oceanic lithosphere along an oceanic-continental plate boundary also results in orogeny. Fig. 10.19c, p. 249

Deformation and the Origin of Mountains Plate Tectonics and Mountain Building Orogenies at Oceanic-Continental Plate Boundaries Fig. 10.19d, p. 249

Deformation and the Origin of Mountains Plate Tectonics and Mountain Building Andes Mountains of South America Fig. 10.20, p. 250

Passive continental margin lithosphere Passive continental margin Sea level Oceanic Asthenosphere Sediments Oceanic lithosphere Continental Asthenosphere Sea level Active continental margin Oceanic lithosphere Continental Deformation Sea level Asthenosphere Stepped Art Fig. 10-20, p. 250

Deformation and the Origin of Mountains Plate Tectonics and Mountain Building Continent-Continent Boundaries Mountain systems, such as the Himalayas, occur within continents, distant from present plate boundaries as the result of continent-continent collisions and suturing. Fig. 10.21, p. 251

Deformation and the Origin of Mountains Terranes and the Origin of Mountains Continental accretion – a process of adding material to a continent Includes preexisting crust, as well as new plutons and volcanic rocks

Deformation and the Origin of Mountains Terranes and the Origin of Mountains Terranes are exotic pieces, fragments of seamounts or small pieces of continents that get transported on the plates. Common along convergent oceanic- continental plate boundaries

Earth’s Crust Continental crust is less dense and much thicker than oceanic crust. This helps explain why mountains on land stand higher on continents than in ocean basins.

Earth’s Crust Floating continents? Gravity allows the continents to "float" on the asthenosphere. A gravimeter detects gravity anomalies. Fig. 10.22, p. 253

Earth’s Crust Principle of Isostasy – Earth’s crust floats on the denser mantle. Airy and Pratt’s Models Airy model states that mountains have a low density root that allows them to project both far above and far below the surface. Pratt model states that mountains are high because they are less dense than the adjacent rocks (like the mantle and the ocean crust). Both models explain the behavior of the crust.

Earth’s Crust Principle of Isostasy Fig. 10.23, p. 253

Earth’s Crust Isostatic Rebound – When large glaciers melt or mountains erode away, the crust rises back up to its equilibrium level. Rebound occurs slowly Fig. 10.24, p. 254

Earth’s Crust Isostatic Rebound The ice sheets that covered Alaska and most of Europe 10,000 years ago have melted away but the continents are still rebounding! Fig. 10.24, p. 254

Earth’s Crust Isostatic Rebound Scandinavia is rebounding at about 1 meter per century! Fig. 10.24, p. 254

Mountains Continental crust Mantle Low-density mountain root Deposition Uplift Subsidence Erosion Transport Stepped Art Fig. 10-24, p. 254

End of Chapter 10