Chapter 11 Crags Cracks and Crumples Crustal Deformation

Chapter 11 Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building LECTURE OUTLINE earth Portrait of a Planet Third Edition © 2008 W. W. Norton & Company, Inc. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building Prepared by Ronald Parker Earlham College Department of Geosciences Richmond, Indiana

Mountains < Attractive landscape features for humans. = Provide beautiful scenery. = Refuge from the mundane. = Inspire poetry and art. < Mountains provide vivid evidence of tectonic activity. < They embody… = Uplift. = Deformation. = Metamorphism. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Mountain Belts < Mountains frequently occur in elongate, linear belts. < Mountains are constructed by tectonic plate interactions in a process called orogenesis. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Mountains < Mountain building involves… = Structural deformation. = Jointing. = Faulting. = Folding. = Partial melting. = Foliation. = Metamorphism. = Glaciation. = Erosion. = Sedimentation. < Constructive processes build mountains up; destructive processes tear them back down again. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Orogenic Belts < Mountains are born and have a finite lifespan. = Young mountains are high, steep, and growing upward. = Middle-aged mountains are dissected by erosion. = Old mountains are deeply eroded and often buried. < Ancient orogenic belts are found in continental interiors. = Orogenic continental crust is too buoyant to subduct. = Hence, if it escapes erosion it is usually preserved. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Crustal Deformation < Orogenesis causes deformation, consisting of… = Bending. = Breaking. = Tilting. = Squashing. = Stretching. = Shearing. < Deformation is a force applied to rock. < Changes in shape via deformation are called strain. < The study of deformation is called structural geology. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Deformation < Deformation strain creates geologic structures. = Joints – Fractures that have no offset. = Folds – Layers that are bent by slow plastic flow. = Faults – Fractures that are offset. = Foliation – Planar metamorphic fabric. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Deformation < Deformed terrane passes into undeformed terrane. = Undeformed (unstrained). 4 Horizontal beds. 4 Spherical sand grains. 4 Flat-lying clays. 4 No folds or faults. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Deformation < Deformed terrane passes into undeformed terrane. = Deformed (strained). 4 Tilted beds. 4 Metamorphic alteration. a. Sand = Quartzite. a. Clay = Slate, phyllite, schist, or gneiss. 4 Folding and faulting. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Deformation < Deformation results in one or all of the following. . . = Translation – Change in location. = Rotation – Change in orientation. = Distortion – Change in shape. < Deformation is often easy to see. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Strain < Changes in shape caused by deformation. = Stretching – Pulling apart. = Shortening – Compressing, squeezing. = Shear – Sliding past. < Elastic strain – Reversible change in shape. < Permanent strain – Irreversible change in shape. < Two types of permanent strain: brittle and ductile. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Deformation Types < Two major types: brittle and ductile. = Brittle deformation – Rocks break by fracturing. 4 Brittle deformation occurs in the shallow crust. < A transition between the two occurs at 10 -15 km depth. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Deformation Types < Two major types: brittle and ductile. = Ductile deformation – Rocks deform by flow and folding. 4 Ductile deformation occurs in the deeper crust. < A transition between the two occurs at 10 -15 km depth. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Causes of Deformation < Strain is the result of deformation, but what causes it? = Caused by force acting on rock, a phenomenon called stress. < Stress is the force applied across an area. = A large force per area results in much deformation. = A small force per area results in little deformation. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Causes of Deformation < Types of stress: = Compressional – Squeezing. = Tensional – Pulling apart. = Shear – Sliding past. < Tectonic collision produces horizontal compression. = Large scale. = Most common type of deformation. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Stress < Pressure – Object feels the same stress on all sides. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Stress < Compression – Squeezing (greater stress in 1 direction). = Tends to thicken material. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Stress < Extension – Pull apart (greater stress in 1 direction). = Tends to thin material. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Stress < Shear – Blocks of rock sliding past one another. = Crust is neither thickened or thinned. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Geologic Structures < Geometric features created by deformation. = Folds, faults, joints, etc. = Often preserve information about stress fields. < 3 -D structural orientation is described by strike and dip. = Strike – Horizontal intersection with a tilted surface. = Dip – Angle of surface down from the horizontal. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Measuring Structures < Dip is always… = Perpendicular to strike. = Measured downslope. < Linear structures measure similar properties. = Bearing (compass direction). = Plunge – Angle from the horizontal. < Strike and dip measurements are common. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Joints < Planar rock fractures without offset. < Result from tensional tectonic stresses. < Systematic joints occur in parallel sets. < Minerals can fill joints to form veins. < Joints control weathering of rock. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Faults < Planar fractures offset by movement across the break. < Faults are abundant and occur at a variety of scales. < Faults may be active or inactive. < Sudden movements along faults cause earthquakes. < Faults vary by type of stress and crustal level. = Compression. = Tension. = Shear. = Brittle (shallow). = Ductile (deep). Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Faults < Faults may offset large blocks of Earth. < The amount of offset is a measure called displacement. < The San Andreas (below) - Displacement of 100 s of kms. = The recently developed stream is offset ~ 100 m. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Fault Movement < The direction of relative block motion… = Reflects the dominant type of crustal stress. = Defines the type of fault. < All motion is relative. = To help visualize fault motion… 4 Imagine that one block is stationary (fixed in place). 4 Then, imagine that faulting moves the other block. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Fault Classification < Fault plane orientation. = Vertical. = Horizontal. = Dipping. < Relative motion of the offset blocks. = Dip slip – Blocks move parallel to fault plane dip. = Strike slip – Blocks move parallel to fault plane strike. = Oblique slip – Combination of dip-slip and strike-slip. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Fault Orientation < On a dipping fault, the blocks are classified as the… = Hanging wall block (above the fault), and the. . . = Footwall block (below the fault). < Standing in a tunnel excavated along the fault… = Your head is near the hanging wall block. = You are standing on the foot wall block. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Recognizing Faults < Continuous layers in rock are displaced across a fault. < Faults may juxtapose different kinds of rock. < Scarps may form where faults intersect the surface. < Fault friction motion may bend rocks into drag folds. < Fault-broken rocks may be more easily eroded. < Minerals may grow on fault surfaces due to fluid flow. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Recognizing Faults < Brittle faults can be distinguished from ductile faults. = Brittle fault motion results in shattered and crushed rock. a. Fault breccia – Fault zone preserving broken fragments of rock. a. Fault gouge – Fault zone preserving pulverized, powdered rock. a. Slickensides – Surface polished by fault motion. a. Slip lineations – Linear grooves on a fault preserving direction. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Recognizing Faults < Ductile faults can be distinguished from brittle faults. = Ductile fault motion results in plastically deformed rocks. 4 Rocks do not break; instead, they are intensely sheared. 4 Rocks from ductile shear zones are called mylonites. 4 Mylonites typify detachment faults in collisional orogens. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Dip-Slip Faults < Sliding is parallel to fault plane dip. < Thus, blocks move up or down the slope of the fault. < Two kinds of dip-slip fault depend on relative motion. = Reverse fault – Hanging wall moves up. 4 Thrust fault (a special type of reverse fault). = Normal fault – Hanging wall moves down. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Normal Fault < Hanging wall moves down relative to the footwall. < Accommodate crustal extension (pulling apart). < The fault below shows displacement and drag folding. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Normal Faults

Reverse and Thrust Faults < Hanging wall moves up the footwall. < Reverse faults – Fault dip is steeper than 35 o. < Thrust faults – Fault dip is less than 35 o. < Accommodate crustal shortening (compression). Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Thrust Faults < Bring old rocks up and over younger rocks. < Common at the leading edge of orogenic deformation. < Can transport thrust sheets 100 s of kilometers. < Act to shorten and thicken mountain belts. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Strike-Slip Faults < Fault motion is parallel to the strike of the fault. < Classified by the relative sense of motion. To find this… = Imagine standing on one block looking across the fault. = Which way does the opposite block move? < 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. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Fault Systems < Faults commonly occur in groups called fault systems. = Due to regional stresses that create many similar faults. = May diverge from a common horizontal detachment fault. < Thrust fault systems. = Stack fault blocks on top of one another. = Act to shorten and thicken the crust. = Result from horizontal compression. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Fault Systems < Normal fault systems. = Fault blocks slide away from one another. = Fault dips often decrease with depth, joining a detachment. = Blocks rotate on faults and create half-graben basins. = Act to stretch and thin the crust. = Result from horizontal extension (pull-apart) stress. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Folds < Layered rocks may be deformed into curves called folds. < Folds occur in a variety of shapes, sizes, and geometries. < A special terminology is used to describe folds. = Hinge – Portion of maximum curvature on a fold. = Limb – Less-curved “sides” of a fold = Axial plane – Imaginary surface defined by connecting hinges of successively nested folds. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Folds < Folds often occur in a series. < Folding may result in extremely complex geometries. < Orogenic settings produce large volumes of folded rock. < Deformed rock often experiences multiple events. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Fold Identification < Anticline – Arch-like fold; limbs dip away from the hinge. < Syncline – Trough-like fold; limbs dip toward the hinge. < Anticlines and synclines frequently alternate in series. Anticline Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: 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. = These faults do not cut through to the surface. = Instead, displacement folds overlying sedimentary cover. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Fold Identification < Folds are described by the severity of folding. = Open fold – Has a large angle between limbs. = Tight fold – Has a small angle between limbs. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Fold Identification < Folds are described by hinge geometry. = Plunging fold – Has a hinge that is tilted. = Non-plunging fold – Has a horizontal hinge. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Fold Identification < Erosion of plunging folds can create zig-zag outcrops. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Fold Identification < Folds are described by their 3 -dimensional shape. = Dome – Fold with appearance of an overturned bowl. 4 Erode to expose old rocks in center; younger rocks outside. 4 Result from crustal upwarping. = Basin – Fold shaped like a bowl. 4 Erode to expose young rocks in center; older outside. 4 Result from crustal subsidence. < Domes and basins result from vertical crustal motions. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Forming Folds < Folds develop in two ways. = Flexural folds – Layers slip as stratified rocks are bent. = Analogous to shear as a deck of cards is bent. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Forming Folds < Folds develop in two ways. = Flow folds – Form by ductile flow of hot, soft rock. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Forming Folds < Horizontal compression causes rocks to buckle. < Shear causes rocks to smear out. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Tectonic Foliation < Foliation develops via compressional deformation. < Flattening – Develops perpendicular to shortening strain. = Sand grains flatten and elongate; clays reorient. = Foliation parallels fold axial planes. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Tectonic Foliation < Foliation can develop as the result of shearing. = Foliation created as ductile rock is smeared. = Shear foliation is not perpendicular to compression. = Rocks that are sheared have a distinctive appearance. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Orogenesis and Rock Genesis < Orogenic events create many kinds of rocks. = Igneous rocks – Intrusive and extrusive. 4 Subduction-related volcanic arc. 4 Rift-related decompressional melting. = Metamorphic rocks – Regional and contact. 4 Igneous intrusion. 4 Deep burial. 4 Horizontal compression. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Orogenesis and Rock Genesis < Orogenic events create many kinds of rocks. = Sedimentary rocks – Weathering and erosion. 4 Erosional debris is shed to adjacent regions. 4 Sediments accumulate in basins created by crustal flexure. 4 Sediments can preserve evidence of mountains eroded away. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Uplift < Construction of mountains requires substantial uplift. = Mt. Everest (8. 85 km above sea level). = Comprised of marine sediments (formed below sea level). < Lofty mountains are supported by a thickened crust. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Crustal Roots < High mountains are supported by thickened lithosphere. < Thickening is caused by collisional orogenesis. = Average continental crust – 35 to 40 km thick. = Beneath orogenic belts – 50 to 70 km thick. < This thickened crust helps buoy the mountains upward. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Isostacy < Surface elevation represents a balance between forces. = Gravitational attraction – Pushes plate into the mantle. = Buoyancy – Causes plate to float higher on the mantle. < The term isostatic equilibrium describes this balance. < Isostacy is compensated after a disturbance. = Adding weight pushes the lithosphere down. = Removing weight causes isostatic rebound. < Compensation is slow, requiring asthenospheric flow. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Gravity Anomalies < Gravimeters determine departure from predicted gravity. < Geophysicists can model crustal density changes. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Erosional Sculpting < Mountains are steep and jagged from erosion. < Mountains reflect a balance between uplift and erosion. < Rock structures can affect erosion. = Resistant layers form cliffs. = Easily eroded rocks form slopes. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Orogenic Collapse < The Himalayas are the maximum height possible. Why? < There is an upper limit to mountain heights. = Erosion accelerates with height. = Weight of high mountains overwhelms rock strength. 4 Deep, hot rocks eventually flow out from beneath mountains. 4 The mountains then collapse downward like soft cheese. < Uplift, erosion, and collapse exhume deep crustal rocks. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Causes of Orogenesis < Mountain building is driven by plate tectonics. = Convergent plate boundaries. = Continental collisions. = Rifting. < Orogenic phases may last several hundred Ma. < Ancient mountains are deeply dissected by erosion. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Causes of Orogenesis < Convergent tectonic boundaries create mountains. = Subduction-related volcanic arcs grow on overriding plate. = Accretionary prisms (off-scraped sediment) grow upward. = Compression stacks thrust faults on the far side of mountain belt. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Causes of Orogenesis < Island fragments of continental lithosphere won’t subduct. < Added to the overriding plate. < Exotic terranes – Geologic history differs from surroundings. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Causes of Orogenesis < Continental collisions. = Oceanic lithosphere can completely subduct. 4 This closes the pre-existing ocean basin. 4 Brings two blocks of continental crust together. = Buoyant continental crust will not subduct. = Instead, subduction is extinguished. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Causes of Orogenesis < Continent-on-continent collision… = Creates a broad welt of crustal thickening. 4 Thickening due to thrust faulting and flow folding. 4 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. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Sand Model of Orogenesis

Causes of Orogenesis < Continental rifting. = Continental crust is uplifted in rift settings. 4 Thinned crust is less heavy; mantle responds isostatically. 4 Decompressional melting adds asthenospheric magma. 4 Increased heat flow from magma expands and uplifts rocks. 4 Rifting creates linear fault block mountains and linear basins. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Cratons < A craton is crust that hasn’t been deformed in 1 Ga. < Low-geothermal gradient; cool, strong, and stable crust. < Two cratonic provinces. = Shields – Outcropping Precambrian igneous and metamorphic rocks. = Platforms – Shields covered by layers of Phanerozoic strata. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Cratonic Platforms < Sedimentary rocks covering Pre-Cambrian basement. < Exhibit domes and basins. = From vertical crustal adjustment. = Created by stresses transmitted to from active margin to interior. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Case Study - Appalachians < A complex orogenic belt formed by 3 orogenic pulses. < Preserves a complete Wilson cycle. < The Appalachians today are eroded remnants. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Appalachians < A giant orogenic belt existed before the Appalachians. = The Grenville orogeny (1. 1 Ga) formed a supercontinent. = By 600 Ma, much of this orogenic belt had eroded away. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Appalachians < The Grenville orogenic belt rifted apart 600 Ma. < This formed a new ocean (the proto-Atlantic). = Eastern North America developed as a passive margin. = A thick pile of sediments accumulated along this margin. = An east-dipping subduction zone built up an island arc. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Appalachians < Subduction carried the margin into the island arc. < The collision resulted in the Taconic orogeny 420 Ma. < A doubly dipping subduction zone developed. < Exotic blocks of continental crust were carried in. < These blocks were added to the margin during the Acadian orogeny 370 Ma. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Appalachians < East-dipping subduction continued to close the ocean. < Alleghenian orogeny (270 Ma): Africa collided with N. Am. = Created a huge fold and thrust belt and mountain range. = Assembled the supercontinent of Pangaea. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Appalachians < Pangaea began to rift apart 180 Ma. < Faulting and stretching thinned the lithosphere. < Rifting led to development of a divergent margin. < Sea-floor spreading created the Atlantic Ocean. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Modern Orogenesis < Modern instrumentation can measure mountain growth. < Global positioning systems (GPS) measure rates of… = Horizontal compression. = Vertical uplift. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

This concludes the Chapter 11 Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building LECTURE OUTLINE earth Portrait of a Planet Third Edition © 2008 W. W. Norton & Company, Inc. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building