geologic structures!geologic structures!up until now, we have focused our attention mostly on! flat-lying rocks, i.e. sedimentary or volcanic layers!not all layers on Earth are flat-lying: geologic structures!geologic structures!dynamically produced patterns or arrangements of! rock or sediment that result from forces acting! within the Earth!structures yield information about these forces!geologic structures!produced as rocks change shape or orientation from! applied stress -- force / area!structural geology is study of shapes, arrangements and!relationships among rocks and stresses that deform them!rocks, like the fence below, deform in response to stress!can bend! or break!geologic structures!“bending” or “breaking” of rocks yield different structures!bending: folds! breaking: faults!stress is force/area--hitting with a hammer!geologic structures -- key concepts!importance of area: !think of difference !between!standing on water bed !in high heels or sneakers!3 types of stress:!"1) compression: pushed together!"2) tension: pulled apart!"3) shear stress: moved horizontally past each other!strain is change in shape or volume arising from stress!geologic structures -- key concepts!…rock flattens after !being hit with hammer!stress and resulting strain!1) compression: shortening!2) tension: stretching!3) shearing stress: shear strain!strain is change in shape or volume arising from stress!translation!(no change in shape)! rotation!(no change in shape)!distortion!(shape changes) !time 1! time 2!volume change!(dilation, contraction)!4 responses possible to stress!from: Davis and Reynolds, 1996!dilation! translation!rotation! distortion!another way to look at it:!geologic structures!geologic structures reflect the type of stress applied! and its rate of application in addition to the phyiscal! properties of the rocks or sediments!how rocks respond to stress!rocks behave as elastic, brittle, or ductile bodies depending on!• amount and rate of stress application!• type of rock!• temperature & pressure!elastic: rock returns!to orginal shape when!stress is removed!(think rubber band)!brittle: rock breaks!at yield point!(strength of rock overcome)!lower T and P!rock under compression (break)!ductile: rock “flows”!at yield point!(no continuous break)!higher T and P!rock under compression!(change in shape)!from where does stress come?!• motions of tectonic plates on Earth’s surface!deformation primarily!occurs along!plate boundaries!factors that affect deformation of rock!• lithostatic pressure: is weight of overlying rock!" " ""(lithos = rock)!• heat: causes atomic bonds to weaken!" --temperatures low at shallow depths (brittle)! --temperatures high at great depths (ductile)!• time: allows stress to be applied slowly or quickly!" --initial stress may not be enough, but!" "will be given enough time (sagging bookshelves)! --stress applied quickly (snap stick) (brittle)!" --stress applied slowly (bend stick) (ductile) !• composition: controls rock response to stress!" --minerals, weaknesses in rock, fluids in pores, etc.!to understand deformation!need to know orientations!of deformed rocks!orientation of geologic structures!deformation is easiest to see!in sedimentary rocks!i.e. layers are not flat,!but are tilted or dipping!geologists measure orientation!of layers in outcrops!outcrop: where bedrock is!exposed at the surface!orientation of geologic structures!rock layers are planes - use 2 lines to define plane in space!• find 2 lines that can be used and easily measured!1) strike: intersection of plane with horizontal plane (e.g. sea level)!" --gives direction of plane relative to north!" " "i.e. N60°E!North!60°!strike!2) dip: angle that plane! is inclined relative to! the horizontal plane!" e.g. 45° to SE!45°!Earth’s surface!dip!orientation of geologic structures!measure strike !with a compass!measure dip !with an inclinometer!map!cross-section!profile along vertical plane!--allows one to see !structure in subsurface--!NS strike!30° dip to W!30!orientation of geologic structures!plot strike and dip!of layer!on a geologic map!strike and dip symbol!types of geologic structures!folds!wavelike bends in layered rock!• represent ductile deformation!• form during compression!fold divided into two limbs!by its axial plane!limbs!hinge is where fold curves!hinge!types of geologic structures!folds!can arch upward or downward!anticline: !upward arching fold!syncline: !downward arching fold!types of geologic structures!folds!have different geometries!open folds: gently dipping limbs!isoclinal folds: parallel limbs!types of geologic structures!folds!have different geometries!overturned folds: limbs dip in same direction!recumbent folds: limbs are parallel and horizontal!types of geologic structures!folds!have different orientations!plunging folds: ! hinge is not horizontal!from: http://www.stmarys.ca/academic/science/geology/structural/!folds!occur at all scales from < mm to mountains!from http://www.eos.duke.edu/geo41/geo41.htm!from: http://earth.leeds.ac.uk/learnstructure/index.htm!Appalachian Mountains!Arkansas!Ouachitas!types of geologic structures!domes and basins!domes: ! layers dip away from! central point (look at symbols)!basins: ! layers dip toward! central point (look at symbols)!outcrop patterns form rings!Ozark dome!bright pink area!in!Missouri and!Northern!Arkansas!geologic structures!folds -- anticlines and synclines -- produce characteristic!"pattern, or superposition, of layers!" "note: 1 (oldest) - 5 (youngest) layers below!top or middle!(hinge)!of anticline!at surface!has oldest!age rocks!(3)!and limbs!at surface!have!youngest!(4)!top or middle!(hinge)!of syncline!at surface!has youngest!age rocks!(5)!and limbs!at surface!have oldest!(4)!1!2!3!4!5!faults!Back to geologic structures!“bending” or “breaking” of rocks yield different structures!breaking: faults (discuss now)!bending: folds (already discussed)!how rocks respond to stress!rocks behave as elastic, brittle, or ductile bodies!brittle: rock breaks!at yield point!(strength of rock overcome)!lower T and P!…FAULTS…!rock under compression (break)!ductile: rock “flows”!at yield point!(no continuous break)!higher T and P!…FOLDS…! rock under compression!(change in shape)!geologic structures!fractures: cracks in rocks!joint --! fracture along which no movement has occurred!very common; !do not reflect