Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download geologic structures
Document related concepts
no text concepts found
Transcript
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! rocks, like the fence below, deform in response to stress! can bend! or break! structural geology is study of shapes, arrangements and! relationships among rocks and stresses that deform them! geologic structures! “bending” or “breaking” of rocks yield different structures! bending: folds! breaking: faults! geologic structures -- key concepts! stress is force/area--hitting with a hammer! 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! geologic structures -- key concepts! strain is change in shape or volume arising from stress! …rock flattens after ! being hit with hammer! stress and resulting strain! 1) compression: shortening! 2) tension: stretching! strain is change in shape or volume arising from stress! 3) shearing stress: shear strain! 4 responses possible to stress! time 1! time 2! translation! (no change in shape)! rotation! (no change in shape)! distortion! (shape changes) ! volume change! (dilation, contraction)! another way to look at it:! dilation! translation! rotation! distortion! from: Davis and Reynolds, 1996! 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! rock under compression (break)! • 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! 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.! orientation of geologic structures! to understand deformation! need to know orientations! of deformed rocks! 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! Earth’s surface! 45°! dip! orientation of geologic structures! measure strike ! with a compass! measure dip ! with an inclinometer! orientation of geologic structures! plot strike and dip! of layer! on a geologic map! strike and dip symbol! 30! NS strike! 30° dip to W! map! cross-section! profile along vertical plane! --allows one to see ! structure in subsurface--! types of geologic structures! folds! wavelike bends in layered rock! • represent ductile deformation! • form during compression! hinge! fold divided into two limbs! by its axial plane! limbs! hinge is where fold curves! types of geologic structures! folds! can arch upward or downward! syncline: ! downward arching fold! anticline: ! upward 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! folds! occur at all scales from < mm to mountains! ! from: http://www.stmarys.ca/academic/science/geology/structural/ 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! outcrop patterns form rings! domes: ! layers dip away from! central point (look at symbols)! basins: ! layers dip toward! central point (look at symbols)! 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 syncline! at surface! has youngest! age rocks! (5)! and limbs! at surface! have oldest! (4)! 5! 4! 2! 3! 1! top or middle! (hinge)! of anticline! at surface! has oldest! age rocks! (3)! and limbs! at surface! have! youngest! (4)! faults! Back to geologic structures! “bending” or “breaking” of rocks yield different structures! bending: folds (already discussed)! breaking: faults (discuss now)! 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…! ductile: rock “flows”! at yield point! (no continuous break)! higher T and P! …FOLDS…! rock under compression (break)! 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 much strain! geologic structures! joints: form perpendicular to tension direction! cracks open to form joints! joints control erosion and can make spectacular landscapes! …water gets into the openings…! Bryce Canyon! geologic structures! fractures: cracks in rocks! fault --! fracture along which movement has occurred;! considered active if motion ! has occurred < 11,000 years! recognize by juxtaposition of! different types of rocks! or offset layers! faults: have noticeable movement across them! fault zones: can be wide with crushed rock in them! faults: examples! large faults! San Andreas Fault! Garlock Fault! Garlock! San Andreas! from: http://www.geo.duke.edu/geo42/st.html! faults: examples! offset ditch! along! San Andreas Fault! in 1975! faults: examples! same ditch! along! San Andreas Fault! in 1992! geologic structures! types of faults! use strike and dip to classify faults! dip-slip fault: slip parallel to dip (up or down plane)! strike-slip fault: slip parallel to strike (horizontal)! geologic structures! types of faults! dip-slip is not unique (up or down?)! strike-slip is not unique (left or right?)! need another description! can label opposite! sides of a ! dip-slip fault! --imagine standing in! a hole along fault --! brown is block! above your head! --hanging wall--! pink is block below your feet --footwall--! types of faults: dip-slip! hanging wall footwall reverse fault! dip-slip fault with motion! of hanging wall up! the fault plane! normal fault! dip-slip fault with motion! of hanging wall down! the fault plane! types of faults: strike-slip! strike-slip faults are vertical! …cannot use hanging wall and footwall…! (one side is not above the other)! stand on one side of fault! and look across at the other! to label the type of fault! right lateral! left lateral! types of faults: normal (dip-slip)! form during tensional stress (extension)! • extend crust (horizontal)! • thin crust (vertical)! “think pulling taffy”! graben ! downdropped ! block! horst ! high block ! between! grabens! hanging wall ! moves down ! fault plane! normal faults! normal faults: fault-block mountains! • develop where crust is stretched by tensional stresses! • form from high horsts with intervening low grabens! Basin and Range: ! western US! horst! horst! graben! topography of Nevada! types of faults: reverse (dip-slip)! form during compressional stress (shortening)! • shorten crust (horizontal)! • thicken crust (vertical)! thrust fault ! reverse fault that! has a shallow dip! hanging wall ! moves up ! fault plane! reverse fault! thrust faults! northern Rockies! thrust faults are low angle reverse faults, ! which have dips < 30°! thrust faults! normal vs. reverse faults: key concepts! original unfaulted rectangle! --has width AB--! (distance between X and Y)! normal faulted rectangle! --has width AB’--! (distance between X and Y)! longer than original length AB! reverse faulted rectangle! --has width AB”--! (distance between X and Y)! shorter than original length AB! normal vs. reverse faults: key concepts! motion of hanging wall down fault plane leads to! motion of hanging wall up fault plane leads to! omission of section, i.e.! repetition of section, i.e.! • younger layers over older! (gray layer is missing along yellow line)! • older layers over younger! (gray layer is repeated along yellow line)! types of faults: strike-slip! form during shearing stress! faults are vertical: no hanging wall or footwall! neither shortening or extension occurs! offset surface features such as streams and valleys! strike-slip fault: offset streams! strike-slip fault: San Andreas! from: Shelton! from: http://www.gps.caltech.edu/~sieh/research! types of faults: oblique-slip! have both strike-slip (horizontal) and dip-slip (vertical) motion! footwall! oblique slip! hanging wall! hanging wall! footwall! oblique slip! geologic structures: importance for petroleum! form traps! --locations where petroleum accumulates! trap ! in! anticline! gas and oil! migrate up! through! permeable! layers and! are trapped! by! impermeable! units! eroded anticline will not serve as trap! …no impermeable cap to trap oil or gas! geologic structures: importance for petroleum! trap along! fault! key is having! impermeable unit! above ! permeable unit! traps along sedimentary features!
