Ch.9 Crustal Deformation PDF

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Ch.9 Crustal Deformation Structural Geology: A Study of Earth’s Architecture - Results of tectonic activity are apparent in Earth’s mountain belts - Massive rock units show evidence of having been intensely fractured and folded - Rocks reveal history of deformation that shows they have been uplifted from much deeper levels in crust - By studying orientations of faults and folds, structural geologists can reconstruct original geologic setting and nature of forces that generated these rocks - Understanding of rock structures is important in deciphering Earth’s history - Rock fractures are sites of hydrothermal mineralization, which means they can be sites of metallic ore deposits Deformation - Every rock has limit which it will fracture of flow - Deformation - General term that refers to all changes in the original form or size of a rock body - Can produce changes in location and orientation of a rock - Most crustal deformation occurs along or near plate margins - Plate motions and interactions along plate boundaries generate tectonic forces that cause rock units to deform - Force, Stress, Strain - Force - Any influence that tends to put stationary objects in motion or change the motions of moving bodies - Stress - Used to describe the force that deform rocks - Amount of force applied to a given area - Magnitude of stress is not a function of the amount of force applied; it also relates to the area on which the force acts - Force applied to a given area - Strain - Visible result of stress - Strained bodies do not retain their original configuration during deformation - Types of Stress - Differential stress - When stress is applied unequally from different directions - Compressional stress - Diffrential stress that shortens a rock body - Associated with plate collisions tend to shorten and thicken Earth’s crust by folding and faulting - More concentrated at places where mineral grains are in contact, causing matter to migrate from areas of high stress to areas of low stress

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Tensional stress - When stress tends to elongate or pull apart a rock unit - Where plates are being rifted apart, tensional stress tend to lengthen those rock bodies located in the upper crust by displacement along fault - Differential stress can cause a rock to shear - Shearing is similar to the slippage that occurs between individual playing cards when the top of the deck is moved relative to the bottom - In near-surface environments, shearing often occurs on closely spaced parallel surfaces of weakness - At transform fault boundaries, shearing stresses produce large-scale offsets along major fault zones How Rocks Deform - When rocks are subject to stresses greater than their own, they begin to deform - They deform usually by flowing (folding) or fracturing ( faulting) - Each rock type deforms somewhat differently - General characteristics of rock deformation are determined from experiments - When stress is gradually applied, rocks first respond by deforming elastically - Changes that result from elastic deformation are recoverable like a rubber band, the rock will return to nearly its original size and shape - Temperature and Confining Pressure - Brittle Deformation/ Failure - Rocks near the surface, where temperatures and confining pressures are low, tend to behave like a brittle solid and fracture once their strength is exceeded - Ductile Deformation - A type of solid state flow that produces a change in the size and shape of an object without fracturing - Ductile deformation of a rock, strongly aided by high temperature and high confining pressure - One way this type of solid state-flow is accomplished within a rock is by gradual slippage and re-crystallization along planes of weakness within the crystal lattice - Rock Type - Crystalline rocks composed of minerals that have strong internal molecular bonds tend to fail by brittle fracture - Sedimentary rocks that are weakly cemented, or metamorphic rocks that contain zones of weakness, such as foliation, are more susceptible to ductile flow - The weakest naturally occurring solid to exhibit ductile flow on a large scale is glacial ice - Time

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One key factor that researchers are unable to duplicate in the lab is how rocks respond to small amounts of stress applied over long spans of geologic time - Forces that are unable to deform rock when initially applied may cause rock to flow if the stress is maintained over an extended time Mapping Geologic Structures - Process of deformation generate features at different scales - When conducting a study of a region, geologist identifies and describes the dominant structure - Number of mapping techniques enable geologists to reconstruct the orientation and shape of existing structures - Geologic mapping is most easily accomplished where sedimentary strata are exposed - If sedimentary rock layers are still horizontal, this tells geologists that the area is probably undisturbed structurally - Strike and Dip - Geologists use measurements called strike and dip to help determine the orientation or attitude of a rock layer, joint, or fault surface - By knowing the strike and dip of rocks at the surface, geologists can predict the nature and structure of rock units and faults that are hidden beneath the surface - Strike - The compass direction of the line produced by the intersection of an inclined rock layer or fault with a horizontal plane - Compass bearing - Generally expressed in azimuth form, as a three digit angle clockwise from north - Dip - The angle of inclination of the surface of a rock unit or fault measured from a horizontal plane - Way to visualize dip is to imagine the water will always run down the rock surface parallel to the dip direction which will always be at a 90 degree angle to strike - Data is plotted on a topographic map or aerial photograph Folds - Folds - During mountain building, flat-lying sedimentary and volcanic rocks are often bent into a series of wavelike undulations called folds - Some folds are broad flexures in which rock units hundreds of metres thick have been slightly warper - Most folds result from compressional stress that shorten and thicken the crust - Occasionally, folds are found singly, but most often they occur as series - Two sides of a fold are called limbs - Line drawn along the crest of the fold is termed axis of the fold - Axial plan is an imaginary surface that divides a fold as symmetrically as possible

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Types of Folds - Two most common types are … - 1. Anticlines - Most commonly formed by the up-folding, or arching of rock layers - Sometimes spectacularly displayed where highways have been cut through deformed strata - 2. Synclines - Commonly found in association with anticlines - Down folds - Limb of an anticline can at times be a limb of the adjacent syncline - Basic folds are described as symmetric when the limbs are mirror images of each other and asymmetric when they are not - An asymmetric fold is said to be overturned if one limb is titled beyond the vertical - An overturned fold can also lie on its side so that the axial plan of the fold would actually be horizontal - Some folds plunge as the axis of thee fold descends into the ground - Outcrop pattern of an anticline points in the direction it is plunging, whereas the opposite is true for a syncline - In real world, folds are intimately coupled with faults Domes and Basins - Broad up-warps in basement rock can deform the overlying cover of sedimentary strata and generate large folds - Dome - When warping produces a circular or elongated structure - Can also be formed by the intrusion of magma (laccoliths) - Basins - When down-warped structures have a similar shape - Few current basins may have been the result of giant asteroid impacts - Large basins contain sedimentary beds sloping at low angles, they are usually identified by the age of the rocks composing them - Youngest rocks are found near the centre, the oldest rocks are at the flanks

Joints - Joints - A fracture in rock along which no significant displacement has occurred - Among most common rock structures resulting from brittle deformation are fractures called joints - No appreciable displacement has occurred - Although some joints have a random orientation, most occur in roughly parallel groups - Columnar joints form when igneous rocks cool and develop shrinkage fractures that produce elongate, pillar like columns

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Sheeting produces a pattern of gently curved joints that develop more or less parallel to the surface of large exposed igneous bodies Most joints are produced when rocks in the outermost crust are deformed Tensional and shearing stresses cause the rock to fail by brittle fracture Many rocks are broken by 2 or 3 sets of intersecting joints that slice the rock into numerous regularly shaped blocks where chemical weathering can occur In many areas groundwater movement and dissolution are controlled by joint pattern System of joints can influence the direction that stream courses follow

Faults - Faults - Fractures in the crust along which displacement has taken place - Occasionally, small faults can be recognized in road cuts where sedimentary beds have been offset a few metres - Large faults have displacements of hundreds of km and consists of interconnecting fault surfaces - Sudden movements along faults cause most earthquakes - Many faults are inactive remnants of past deformation - Along active faults, rock is often broken and pulverized as crustal blocks on opposite sides of a fault slip past one another - Fault gouge - Loosely coherent, clayey material that results from opposite sides a faults slipping past one another - Slickensides - Occur when some fault surfaces the rocks become highly polished and striated, or grooved as the crustal blocks slide past one another - Can provide geologists with evidence for the direction of the most recent displacement along the fault - Dip- Slip Faults - Faults in which the movement is primarily parallel to the dip or inclination of the fault surface - Up and down displacements along dip slip faults can produce long, low cliffs called fault scarps - Produced by displacements that generate earthquakes - Hanging wall - The rock surface immediately above the fault - Footwall - The rock surface below the fault - Two major types of dip-slip faults - 1. Normal Faults - A fault in which the rock above the fault plane has moved down relative to the rock below - 2. Reverse Faults

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A fault in which the material above the fault plane moves up in relation to the material below

Thrust fault - When a reverse fault has an angle of dip less than 45 degrees - Normal Faults - Dip slip faults are classified as normal faults when the hanging wall block moves down relative to the footwall block - Because of the downward motion of the hanging wall, normal faults accommodate lengthening, or extensions, and thinning of the crust - Normal faulting is prevalent at spreading centres where plate divergence occurs - Graben - A central block - Bounded by normal faults and drops as the plates separate - Produce an elongate valley bounded by relatively uplifted structures called horsts - Represent tensional stress that pull the crust apart - The pulling apart can be accomplished either by uplifting that causes the surfaces to stretch and break - Reverse and Thrust Faults - Reverse faults and thrust faults dip less than 45 degrees - They are dip slip faults where the hanging wall block moves up relative to the footwall block - Because the hanging wall block moves up and over the footwall block, reverse and thrust faults accommodate the shortening of the crust - Most high angle reverse faults are small and accommodate local displacements in regions dominated by other types of faulting - Thrust faulting is most pronounced in subduction zones and other convergent boundaries where plates are colliding and compressional forces dominate Strike- Slip Faults - Faults in which the dominant displacement is horizontal and parallel to the strike of the fault surface are called strike slip faults - Produce a linear trace that is visible over a great distance - Large strike slip faults consist of a zone of roughly parallel fractures - Crushed and broken rocks produced during faulting are more easily eroded, often producing linear valleys or troughs that mark the locations of strike slip faults - Many major strike slip faults cut through the lithosphere and accommodate motion between two large lithospheric plates - Transform fault - A special kind of strike slip fault - Cut the oceanic lithosphere and link spreading oceanic ridges - San Andreas fault

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