Deformation

Deformation - Crags Cracks and Crumples Crustal Deformation...

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Unformatted text preview: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building Prepared by: Ronald Parker, Senior Geologist Senior Fronterra Geosciences Fronterra Houston, Oklahoma City, Denver, Anchorage, Dallas, Midland, Aberdeen, Vienna, Buenos Aires, Neuquén www.fronterrageo.com When most people see mountains … They see an attractive landscape Beautiful scenery Refuge from the mundane Inspiration for poetry and art Geologists see more--evidence of tectonic activity: Geologists Uplift Deformation Metamorphism Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 9: Crustal Deformation and Mountain Building Mountain Belts Mountains frequently occur in elongate, linear belts Mountains are constructed by tectonic plate interactions Mountains in a process called orogenesis orogenesis Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 9: Crustal Deformation and Mountain Building Mountain building involves Deformation Tilting & Stretching Jointing & Breaking Faulting & Shearing Bending & Folding Partial melting Foliation Metamorphism Glaciation Glaciation Erosion Sedimentation Constructive processes build mountains up … Destructive processes tear them back down again Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 9: Crustal Deformation and Mountain Building What karma does your mountain have? Young mountains are high, steep and growing upward Middle-aged mountains are dissected by erosion Old-age mountains are deeply eroded and often buried Some mountains can be rejuvenated by uplift Some Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 9: Crustal Deformation and Mountain Building Rocks Have Stress, Too! Stress is the force acting on rocks Force applied per unit area across an area (like psi) A large force per area results in much deformation A small force per area results in little deformation (duh) Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 9: Crustal Deformation and Mountain Building A large force over a small area can do damage as well! Q: What exerts more pressure-per-square Q: inch when walking a 100 lb woman in high heels or a 6,000 lb elephant in bare feet? [At the moment when only the heel rests on the ground.] A: Stiletto heels have an area of about 1/16 of a square inch. Elephants, unlike humans, walk with two feet on the ground at a time. Each foot is about 40 square inches. Thus, the woman "wins" by far more than 1,500 psi versus 75 psi. 1,500 http://hypertextbook.com/facts/2003/JackGreen.shtml Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 9: Crustal Deformation and Mountain Building Results of Deformaton Results (lots of –tion words) words) Change in location – Translation Change in orientation – Rotation Change in shape -- Distortion Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 9: Crustal Deformation and Mountain Building In geologic terms … Deformation strain creates geologic structures: Joints – Fractures that have no offset Joints Fractures Folds – Layers that are bent by slow plastic flow Folds Layers Faults – Fractures along which offset occurs Faults Fractures Foliation – Planar metamorphic fabric Foliation Planar Structural geology is the study of rock deformation Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 9: Crustal Deformation and Mountain Building Types of Stress >> Resulting Strain Strain: Changes in shape or volume Strain: Changes Tensional -- Pulling apart >> Stretching, Thinning Tensional -Compressional -- Squeezing >> Shortening, Thickening Compressional -Shear – Sliding past Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 9: Crustal Deformation and Mountain Building Reminder: Pressure is not Stress Pressure – An object feels the same stress on all sides. Pressure Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 9: Crustal Deformation and Mountain Building Rock Responses to Deformation Two major responses: Brittle and ductile Two Brittle ductile Brittle deformation – Rocks break by fracturing Brittle deformation occurs in the shallow crust Ductile deformation – Rocks deform by flow and folding Ductile deformation occurs in the deeper crust Deformation type depends on temperature and pressure Transition between the two occurs at 10 to 15 km Transition Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 9: Crustal Deformation and Mountain Building Also Elastic Response Elastic response: Elastic response: After deformation, rocks return to their original shape Elastic response possible when seismic waves pass through No strain visible Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 9: Crustal Deformation and Mountain Building Geologic Structures Geometric features created by deformation Folds, faults, joints, etc. Often preserve information about stress fields Often 3-D structural orientation is described by strike and dip 3-D dip Strike – Horizontal intersection with a tilted surface Strike Horizontal Dip – Angle of surface down from the horizontal Dip Angle Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 9: Crustal Deformation and Mountain Building Measuring Structures Dip Dip always perpendicular to strike measured downslope Linear structures (formations, faults, fold axes) Linear measure similar properties measure Bearing – compass direction Plunge – Angle from the horizontal Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 9: Crustal Deformation and Mountain Building Joints Planar rock fractures without offset Planar Result from tensional tectonic stresses Systematic joints occur in parallel sets Joints control weathering of rock Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 9: Crustal Deformation and Mountain Building Faults Planar fractures offset by movement across the break Planar by Displacement can be several hundreds of kilometers (SAF) Faults are abundant and occur at a variety of scales Sudden movements along faults cause earthquakes Faults vary by type of stress and crustal level Compression Tension Shear Brittle (shallow) Brittle Ductile (deep) Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 9: Crustal Deformation and Mountain Building Fault Orientation On a dipping fault, the blocks are classified as Hanging wall block (above the fault) Footwall block (below the fault) Standing in a tunnel or mine excavated along the fault Your head is near the hanging wall block You are standing on the footwall block You Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 9: Crustal Deformation and Mountain Building Fault Classification Fault (plane) geometry varies – Vertical, horizontal, dipping Relative motion of Hanging Wall and Footwall blocks Relative Dip-slip – Blocks move parallel to fault-plane dip (vertical) Dip-slip Blocks Strike-slip – Blocks move parallel to fault-plane strike Strike-slip (horizontal) (horizontal) Oblique-slip – Combination Oblique-slip Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 9: Crustal Deformation and Mountain Building Dip Slip Faults Sliding is parallel to fault-plane dip Sliding Thus, blocks move up or down the slope of the fault Two kinds of dip-slip fault, depending on relative motion: Reverse Fault – Hanging wall moves up Thrust fault (a special type of reverse fault) Thrust Normal fault – Hanging wall moves down Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 9: Crustal Deformation and Mountain Building Slip down the Dip: Normal Fault Hanging wall moves down relative to the footwall Hanging down Accommodate crustal extension (pulling apart) The fault below shows displacement and drag-folding Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 9: Crustal Deformation and Mountain Building Slip Up the Dip: Reverse & Thrust Faults Hanging wall moves up the footwall Hanging up Reverse faults – Fault dip is steeper than 45o Thrust faults – Fault dip is less than 45o Accommodate crustal shortening (compression) Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 9: Crustal Deformation and Mountain Building Strike-Slip Faults Strike-Slip Fault motion is parallel to the strike of the fault Fault Classified by the relative sense of motion Right lateral – Opposite block moves to observer’s right Left lateral – Opposite block moves to observer’s left Large strike-slip faults may slice the entire lithosphere Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 9: Crustal Deformation and Mountain Building Fault Recognition Continuous features are offset by a fault Continuous Faults may juxtapose different kinds of rock Faults Landscape of human features (streams, fences, etc.) Landscape Scarps may form where faults intersect the surface Scarps Fault friction motion may bend rocks into drag folds Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 9: Crustal Deformation and Mountain Building Fault Recognition Shattered fault breccias are preferentially eroded Shattered Fault motion creates slickensides lineations Fault lineations Minerals may grow on fault surfaces due to fluid flow Minerals Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 9: Crustal Deformation and Mountain Building Folds Layered rocks may be deformed into curves called folds Result of compression and ductile rock behavior A special terminology is used to describe folds special Hinge – Portion of maximum curvature on a fold. Hinge Portion Limb – Less curved “sides” of a fold Limb Less Axial plane – Imaginary surface defined by connecting Axial hinges of successively nested folds Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 9: Crustal Deformation and Mountain Building Folds Folds often occur in a series Folds Orogenic settings produce large volumes of folded rock Orogenic with extremely complex geometries Deformed rock often experiences multiple orogenic / Deformed metamorphic events Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 9: Crustal Deformation and Mountain Building Fold Identification Anticline – Arch-like fold; limbs dip away from the hinge Anticline Syncline – Trough-like fold; limbs dip toward the hinge Syncline Trough-like Anticlines and synclines frequently alternate in series Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 9: Crustal Deformation and Mountain Building Fold Identification Monocline – A fold like a carpet draped over a stairstep Generated by blind faults in the basement rock Generated which do not cut through to the surface which Instead, displacement folds overlying sedimentary cover Instead, Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 9: Crustal Deformation and Mountain Building Fold Identification Folds are described by hinge geometry Folds Plunging fold – Has a hinge that is tilted Plunging fold Non-plunging fold – Has a horizontal hinge Non-plunging Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 9: Crustal Deformation and Mountain Building Fold Identification Sheep Mountain, Wyoming, is a plunging fold Sheep Resistant sandstones form ridges Weaker shales erode away Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 9: Crustal Deformation and Mountain Building Fold Identification Folds are described by their three-dimensional shape Folds Dome – Fold with appearance of an overturned bowl Erode to expose older rocks in center and younger rocks Erode outside outside Basin – Fold shaped like a bowl Erode to expose younger rocks in center and older outside Erode and Domes and basins result from vertical crustal motions Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 9: Crustal Deformation and Mountain Building Forming Folds Horizontal compression causes rocks to buckle Horizontal Shear causes rocks to smear out Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 9: Crustal Deformation and Mountain Building Forming Folds When horizontal layers move over step faults, they fold Deep faulting may create a monocline in overlying beds Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 9: Crustal Deformation and Mountain Building Together with folding … Foliation develops via compressional deformation Flattening – Develops perpendicular to shortening strain Foliation parallels fold axial planes Foliation Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 9: Crustal Deformation and Mountain Building Make a Fist! Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 9: Crustal Deformation and Mountain Building Crustal Roots Construction of mountains requires substantial uplift High mountains are supported by thickened lithosphere, High mostly the result of plate collision (convergence) Average continental crust – 35 to 40 km thick Beneath orogenic belts – 50 to 70 km thick This thickened crust helps buoy the mountains upward Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 9: Crustal Deformation and Mountain Building Isostasy Surface elevation represents a balance between forces The term isostatic equilibrium describes this balance The describes Isostasy is compensated after a disturbance Isostasy Gravitational attraction – Pulls plates into the mantle Buoyancy – Floats plates on top of the mantle Adding weight pushes the lithosphere down Removing weight causes isostatic rebound Compensation is slow, requiring asthenospheric flow Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 9: Crustal Deformation and Mountain Building Orogenic Collapse The Himalayas are the maximum height possible. Why? The There is an upper limit to mountain heights There Erosion accelerates with height Weight of high mountains overwhelms rock strength Deep, hot rocks eventually flow out from beneath mountains Deep, The mountains then collapse downward like soft cheese Uplift, erosion, and collapse exhume deep crustal rocks Uplift, Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 9: Crustal Deformation and Mountain Building Causes of Orogenesis Mountain building is driven by plate tectonics Rifting Continental collisions Convergent plate boundaries Orogenic phases may Orogenic last several hundred Ma last Ancient mountains are Ancient deeply dissected by erosion erosion Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 9: Crustal Deformation and Mountain Building Causes of Orogenesis Continental rifting Continental crust is uplifted in rift settings Thinned crust is less heavy; mantle responds isostatically Decompressional melting adds asthenospheric magma Increased heat flow from magma expands and uplifts rocks Increased Rifting creates linear-fault block mountains and linear basins Rifting Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 9: Crustal Deformation and Mountain Building Causes of Orogenesis Continental collisions Oceanic lithosphere can completely subduct This closes the preexisting ocean basin Brings two blocks of continental crust together Buoyant continental crust will not subduct Instead, subduction is extinguished Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 9: Crustal Deformation and Mountain Building Causes of Orogenesis Continent – continent collision … Creates a broad welt of crustal thickening Thickening due to thrust faulting and folding Center of belt consists of high-grade metamorphic rocks Fold and thrust belts extend outward on either side The resulting high mountains may eventually collapse The Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 9: Crustal Deformation and Mountain Building Orogenesis Through Messy Convergence Convergence Subduction-related volcanic arcs grow on overriding plate Subduction-related Accretionary prisms (off-scraped sediment) grow upward Thrust faults stack up on the far side of the orogen Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 9: Crustal Deformation and Mountain Building Orogenesis Through Messy Convergence Convergent boundaries Convergent Island fragments of continental Island lithosphere can enter trench These are too buoyant to These subduct subduct Added to the overriding Added plate plate Called exotic terranes Called – Geologic history differs from that of surroundings Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 9: Crustal Deformation and Mountain Building Cratons A craton is crust that hasn’t been deformed in 1 Ga craton Low geothermal gradient; cool, strong, and stable crust Two cratonic provinces Two Shields – Outcropping Shields Pre-Cambrian igneous and metamorphic rocks Platforms – Shields Platforms covered by layers of Phanerozoic strata Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 9: Crustal Deformation and Mountain Building Cratonic Platforms Sedimentary rocks covering Sedimentary Precambrian basement Precambrian Exhibit domes and basins Vertical crustal adjustment Stresses transmitted from Stresses active margin to interior active Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 9: Crustal Deformation and Mountain Building Modern Orogenesis Modern instrumentation can measure mountain growth Global positioning systems (GPS) measure rates of Global Horizontal compression Vertical uplift http://geon.unavco.org/unavco/UNAVCODataViewer.php Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 9: Crustal Deformation and Mountain Building Forgive me, but … All my faults are stress-related Stress (action, force) >> Strain (result, response) Compressional >> thickening & shortening, Compressional compression compression Tensional >> lengthening & thinning, extension Shear >> translation & displacement Material response: Material Elastic, ductile/plastic, and brittle deformation How are they represented on a stress-strain curve? How Factors that influence deformational behavior Factors (temperature, pressure, rock type, time,...) (temperature, Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 9: Crustal Deformation and Mountain Building Rocks have attitude Undisturbed, horizontal Tilted – Faulted - Folded FOLDS: Syncline vs. anticline Can you distinguish an anticline from a syncline? FAULTS: Normal fault (dip-slip): extension Reverse fault (dip-slip): compression Thrust fault Transform fault (strike-slip): shear stress Can you identify a fault type from a block diagram or a Can map view? map Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 9: Crustal Deformation and Mountain Building ...
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