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Structure An Introduction to Deformation Standards • Describe the composition and structure of Earth’s materials A Brief Intro to Mapping • Geologic maps – represent the rock formations exposed at Earth’s surface. • Maps have special symbols to indicate strike and dip of rock formations, and lines to mark faults Geologic Map of New Mexico Strike and Dip • Strike – the compass direction of a rock layer as it intersects with a horizontal surface • Dip – is measured at right angles to strike and is the amount of tilting of the formation (angle at which the bed is inclined from the horizontal) Geologic Cross Sections • Diagrams that show the features that would be visible if vertical slices were made through part of the crust • Shows the rocks and structures under the surface Plate Tectonics • Steady motion between plates causes deformation at plate boundaries. • Rocks are deformed by faulting and folding. • Layers of rock can be pushed into folds. • Or, rock formations can break and slip on both sides of a fracture, forming a fault. Review Types of Plate Boundaries • Convergent Plates push together • Divergent Plates pull apart • Transform Plates slide horizontally past each other Types of Plate Tectonic Force • Tensional forces – stretch and pull formations apart. Dominate at divergent boundaries. • Compressive forces – squeeze and shorten rock formations. Dominate at convergent boundaries. • Shearing forces – push two sides of a formation in opposite directions. Dominate at transform boundaries. Ductile vs. Brittle • Brittle material – a material that undergoes little deformation under increasing force until it suddenly breaks • Ductile material – a material that undergoes smooth and continuous plastic deformation under increasing force and does not spring back to its original shape when the force is removed. Brittle & Ductile Behavior in the Crust • Some rocks are brittle and others ductile. • The same rock can be brittle at shallow depths and ductile deep in the crust • A rock formation that would flow as a ductile material if deformed slowly may break as a brittle material if deformed more rapidly. Exs: silly putty and gak (in-class & video) • Rocks break more easily under tension than compression Basic Deformation Structures • • • • Faults Folds Circular Structures Joints Fault • Surface across which rock formations have been displaced • Offset can be centimeters to hundreds of kilometers • Faults are classified by their slip direction. 2 Main types: Dip-slip fault Strike-slip fault Dip-Slip Fault • There has been relative movement of the blocks (rock to either side of the fault) up or down the dip of the fault plane. Dip = tilt of the fault Caused by compressive or tensional forces. Dip-Slip Fault • 1. 2. 3. 3 types: Normal fault Reverse fault Thrust fault Fault Parts • Fault – plane along which rocks have been displaced • Hanging wall – rocks above fault plane • Foot wall – rocks below fault plane Draw & label this diagram in your notes Cross-Section View Fault Plane Hanging Wall Foot Wall Normal Fault • Dip-slip fault formed when rocks above the fault plane (on the top side of the dip) move down relative to rocks below the fault plane. • This extends the structure horizontally. • Formed by tensional forces Photo: http://www.southalabama.edu/geography/allison/gy480tour.htm Photo: http://www.webpages.uidaho.edu/~simkat/geol345_files/2010lecture12.html Reverse Fault • Rocks above fault plane move upward in relation to the rocks below. • Causes shortening of the structure. • Formed by compressive forces Photo: http://www.corbisimages.com/Enlargement/ IH011691.html http://www.flickr.com/photos/51870389@N05/4778443130/ Thrust Fault • Low-angle reverse fault – the angle of the fault plane is less than 45 • This causes the overlying block to move horizontally Photo: http://www.geology.wisc.edu/courses/g112/rock_deformation.html Strike Slip Fault • Movement is horizontal (just like movement at transform boundaries) • Caused by shearing forces • Two types: 1. Right-lateral fault 2. Left-lateral fault Right-Lateral Fault • An observer on one side of the fault sees the block on the opposite side move to the right. Fence along San Andreas Fault Photo: http://www-class.unl.edu/geol101i/images/structure%20images/fence.gif Photo: http://www.uoregon.edu/~millerm/LVSS.jpeg Left-Lateral Fault • The block on the opposite side of the fault moves to the left. Recognizing Faults in the Field • Faults may form a small cliff called a scarp Recognizing Faults in the Field • Formations that have large offsets (like on the San Andreas) differ in age and rock type • Small amounts of offset can be observed and measured Folds • • A bend in rocks Are often observed in layered rocks • Have a range of sizes • Two types: 1. Anticlines 2. Synclines Photo: http://www.science.smith.edu/~rburger/ Images/Many%20chev%20folds%20crop.jpg Photo: http://gse.umr.edu/images/folds-7.jpg Anticline • Layered rocks that fold upward into arches • Oldest rocks are in center of fold Photo: J. T. Daniels. http://www.teachingboxes.org/mountainBuilding/lessons/ foldImages/index.html Syncline • Layered rocks that fold downward into troughs. • Youngest rocks are in center of fold • Synclines look like Smiles Photo: http://geology.about.com/library/bl/images/blsyncline.htm Photo: http://gsc.nrcan.gc.ca/natmap/ cf/images/syncline440.gif Parts of Folds • Anticlines and synclines have limbs and an axial plane: Limbs – the two sides of the fold Axial plane – an imaginary surface that divides a fold as symmetrically as possible, with one limb on either side of the plane Photo: http://ess.nrcan.gc.ca/esic/gallery/images/180345.jpg Overturned Fold • Forms when deformation is intense and one limb has been tilted beyond the vertical. • Both limbs dip in the same direction, but the order of layers in the bottom limb is the reverse of their original sequence – that is, older rocks are on top of younger rocks Photo: http://bio-geo-terms.blogspot.com/2007/02/overturned-and-recumbent.html Photo:http://visualsunlimited.photoshelter.com/image/I00000omOQcPh5PY Circular Structures • In many cases, these form from upward force of rising material or downward force of sinking material • Two types: 1. Dome 2. Basin Domes • Broad, circular or oval upward bulge of rock layers • Type of anticline Basins • Bowl-shaped depression of rock layers • Type of syncline Photo: http://www.ysnet.org.my/Maliau/ public/images/maliau/satellite_image2.jpg Joints • A type of fracture, or crack, along which there has been no movement. • Found in almost every outcrop • Formed by: Tectonic forces Expansion & contraction of rocks Photo: http://piru.alexandria.ucsb.edu/collections/geography3b/p-s/ps-tab_06-10.jpg Deformation textures • As rocks along a fault plane move past each other, they grind and fragment • Brittle deformation will form fault breccias • Ductile deformation deeper in the crust will form mylonites Fault Breccias • Rocks with a broken appearance Photo:http://academic.emporia.edu/aberjame/struc_geo/ouachita/ouac30.jpg Mylonites • Minerals recrystallize and string out in bands or streaks. Photo: http://earth.boisestate.edu/home/cjnorth/images/mylonite.JPG