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Ollivier Dyens Deputy Provost (Student Life and Learning) Questions/Comments Minerva Help Line Crustal Deformation and M Mountain t i Building B ildi ((orogeny)) Mountains reflect geologic processes of uplift, deformation, and metamorphism eta o p s at work; o ; tthey ey a are e vivid de evidence de ce o of tecto tectonic c act activity. ty Mt. Cook, New Zealand http://eps.mcgill.ca/~courses/c201_winter/ Crustal Deformation and M Mountain t i Building B ildi ((orogeny)) Mountain ou ta building bu d g is s a process p ocess called ca ed o orogenesis. oge es s With the exception of the large volcanoes formed over hot spots, mountains do not occur in isolation, but rather as part of linear ranges called mountain belts or orogens. The Alps formed by collision of a microplate (including Italy) with the Eurasian plate Crustal Deformation and M Mountain t i Building B ildi ((orogeny)) Mountain building g involves uplift, p , deformation,, jointing, faulting, folding, foliation, metamorphism, igneous activity, and sedimentation. Sedimentary y rocks are originally g y deposited in horizontal layers ((typically yp y with no folds or bends)) Deformation changes the character of the rocks and is often easy to see. Undeformed (unstrained) rocks, such as sedimentary rocks, display horizontal beds, with no folds or faults. Deformed (strained) rocks show tilted beds, metamorphic alteration, folding, g and faulting. g Deformation and Mountain Building (Brittle - ductile; stress and strain; faults and folds) What forces are responsible? How can it be? How do these deformations fit into a historical perspective? Deformed sedimentary strata, Rocky Mountains, British Columbia From: Lutgens and Tarbuck (1992) Essentials of Geology Deformation and Mountain Building (Brittle - ductile; stress and strain; faults and folds) Wh t forces What f are responsible? ibl ? H How can it b be? ? South Georgia Island, Antarctica From: Murck, Skinner & Mackenzie (2008) Visualizing Geology Elastic deformation Reversible or non-permanent deformation of a solid when it is stretched, bent or squeezed and the force is then removed. Stress – force applied per unit surface area Strain – change in shape or volume From: Skinner and Porter (2000) The Dynamic Earth Strain is proportional to stress. Elastic deformation of rock occurs when stress is applied slowly and under low pressure. Ductile or plastic deformation A solid exhibiting ductile deformation behaves elastically under low stress, but, beyond a certain point, elastic properties cease and ductile flow occurs. Folding occurs mostly at high temperatures and pressures - more common in metamorphic rocks Ductile or plastic deformation A solid exhibiting ductile deformation behaves elastically under low stress, but a point is reached where elastic properties cease and ductile flow occurs. From: Skinner and Porter (2000) The Dynamic Earth Brittle deformation and fracture When the limits of elastic deformation are exceeded, a solid may fracture. Brittle Versus Ductile Deformation Fractures, like ductile deformation, produce permanent and irreversible deformation. Large scale fractures in rocks are called joints or faults. faults From: Skinner and Porter (2000) The Dynamic Earth J Joints A crack along which there is little or no movement Stress, strain and fracture Development of a fault A fracture with relative movement of the rocks on both sides The h behavior b h of f a rock k depends d d on: ¾ Temperature: the higher the temperature, the weaker and less brittle a solid becomes. ¾ Confining pressure: press re an increase in confining pressure inhibits the formation of fractures. ¾ Time - Deformation/strain rate: the faster the strain rate, the more susceptible a rock is to fracture. ¾ Composition: p the mineralogical g composition p and water content of a rock determines its strength. (e.g., quartz is strong, carbonates and sheet silicates are f i l weak). fairly k) Brittle deformation and fracture Low temperatures temperatures, low pressures pressures, and high strain rates rates, conditions typical of the upper crust, enhance brittle fracture. High temperature, high pressure, and low strain rates, which are characteristic of the deeper crust and mantle, reduce the likelihood of brittle fracture and enhance the ductile properties of rocks. From: Skinner and Porter (2000) The Dynamic Earth The h behavior b h of f a rock k depends d d on: ¾ Temperature: the higher the temperature, the weaker and less brittle a solid becomes. ¾ Confining pressure: an increase in confining pressure inhibits the formation of fractures. ¾ Time - Deformation/strain rate: the faster the strain rate, the more susceptible a rock is to fracture. ¾ Composition: The mineralogical composition and water content of a rock determines its strength. (e.g., quartz is strong, carbonates and sheet silicates are f i l weak). fairly k) Forces responsible for the deformation of rocks Most large scale crustal deformations occur very slowly, at very low strain rates, and often cannot be measured over a lifetime, whereas ... ¾Compressive forces: squeeze and shorten a body of rock (e.g., convergent margins i off plates l t or where h plates l t collide llid – subduction, orogenesis) ¾Tensional forces: stretch a bodyy of rock and pulls it apart (e.g., divergent margins or loading of a basin with sediments) ¾Shearing forces: push two sides of a body of rock in opposite directions (e.g., transform fault – San Andreas fault) Joints - a crack along which there is little or no movement – fractures along planes of weakness: bedding, foliation, mineral cleavage Unloading and breakage along planes of weakness ((e.g., p g , bedding) g) Contraction of igneous body, expansion due to thermal shock Columnar joints Joints occur in parallel sets and often control the weathering of the rock in which they occur. Classification of faults When failure of the rock occurs by the sudden dislocation of rocks along a plane - a fault results. Dip slip faults Dip-slip faults are characterised by vertical displacement of rocks on either side of the fault. Compression Shortening Extension Stretching Normal Dip-Slip Fault When you stand in a tunnel excavated along the fault, your head is near the hanging-wall block and your feet rest on the footwall block. The hanging Th h i wallll moves d down relative l ti tto th the ffootwall. t ll They accommodate crustal extension (pulling apart). The fault below shows displacement and drag folding. Dip and Strike Lake water on a dipping bed of strata defines a strike line. line Strike – direction of the fault plane at the surface Dip – angle in degrees between the horizontal plane, measured down from a plane perpendicular to the strike Horst and Graben horsts Reverse fault (Compression) Dip slip faults Dip-slip Normal fault (extension) Thrust fault (Compression) Thrust faults, Detachment fault and Mountain Building Movement on reverse faults pushes older rocks over younger ones, thereby shortening and thickening the crust. Thrust faults, Detachment fault and Mountain Building Th t faults Thrust f lt actt to t shorten h t and d thicken thi k mountain t i belts. b lt Thrusts can transport sheets of rock hundreds of kilometers and are common features at the leading edge of orogenic deformation. Thrust fault Thrust faults are low angle reverse faults with dips less than 45o. They are mostly found associated with great mountain chains where the hanging wall block may have been moved many kilometers over the footwall block. Strike-slip faults Strike-slip faults are characterised by horizontal displacement of rocks on either side of the fault. The forces (shear) involved are parallel to the strike line. Transform Fault The San Andreas Fault Sudden movements along faults are the dominant cause of earthquakes. Oblique-slip q p faults Oblique-slip faults are characterised by both horizontal and vertical displacement of rocks on either side of the fault. Forces involves are a combination bi ti off th those causing i strike-slip t ik li and d di dip-slip li faulting. f lti Compression shortening + Shear Extension stretching + Shear Folding Gradual movement, involving ductile deformation under compressive forces, leads to bending g or folding g of the crust. The term “fold” implies that a structure that originally was planar, like a sedimentary bed, has been bent. The deformation may be produced by either horizontal, vertical, or shear forces in the crust. Stress (force/area) and strain The type of force (stress) will be reflected in the style of deformation (strain). Types of stress Extension Compression Stretching Types of strain Shortening Long dimension perpendicular to the direction of shortening Long dimension parallel to the direction of stretching Shear stress and strain Shear stress develops when one side of a rock body moves sideways past the other side. 90 90° < 90° Three types of Stress From: Murck, Skinner & Mackenzie (2008) Visualizing Geology Folding is the most common form of deformation of layered rocks, its most typical manifestation is in mountain belts. Folding, as during mountain building, shortens and thickens the crust. The complexity of folds is proportional to the degree of deformation F lds Folds Broad, open folds form in the stable interiors of continents, where the rocks are only mildly warped. Complex folds develop in mountain belts where deformation is intense. Folding g Folding shortens and thickens the crust When sedimentary rocks are buried deep within the crust, where confining pressures are great, strata can be deformed into very tight folds stacked one atop of each other. Fold Geometry A hinge is a line along which curvature is greatest. Limbs are the less curved “sides” of a fold. Th axial The i l plane l connects t hinges hi off successive i llayers. Fold g geometry: y Monocline A monocline is a fold that looks like a carpet draped over a stair step. Monoclines are generated by blind basement faults that do not cut the surface but which still wrinkle the overlying sedimentary cover. Fold geometry: Anticline An anticline (an upfold) is a fold that looks like an arch. The limbs of the fold dip away from the axis or crest of a fold. The limbs dip out and away from the hinge. Fold geometry: Syncline A syncline (downfold) is a fold that opens upward like a trough. The limbs dip inward and toward the hinge. Anticline, syncline and monocline Limbs of the fold dip away from the axis or crest of the fold Horizontal or gently dipping beds are modified by simple step-like bends. Syncline Geologists represent the hinge of a fold by strike and dip symbols on the outer layers of the fold (long lines parallel to the hinge with a shorter line pointing in the direction of the younger layers). Folding g Anticline Anticlines and Synclines Layered rock may be deformed into complex folds by tectonic compression. Folds occur in a variety of shapes, sizes, and geometries. Anticlines and synclines are usually paired. As Asymmetrical t i lA Anticline ti li (tilted beyond the vertical) A ti li Anticline Plunging and non-plunging folds Plunge Hinge: portion of the fold where the curvature is greatest. Plunge: g angle g between the fold axis (or hinge) and the horizontal. Folds do not continue forever, their ends die out much like the wrinkles on a piece of cloth. Truncated folds and map patterns After erosion, Aft i folds f ld b become ttruncated. t d They are recognized by the pattern of rock layers on the ground surface. Layers in a non-plunging fold (anticline or syncline) of sedimentaryy layers y appear pp as p parallel stripes p on the g ground. Layers in a plunging fold ( ti li ) h (anticline) have a U Ushape on the ground. Truncated folds and map patterns Geologists represent the hinge of a fold by a heavy line bordered by outward-pointing outward pointing arrows for an anticline and inward-pointing arrows for a syncline or by strike and dip symbols on the outer layers of the fold (short lines parallel to the hinge with a shorter line pointing in the direction of the younger layers). Sandstone 1 Sandstone 2 28 28 29 29 Sandstone 1 Sandstone 2 Differential resistance to erosion of adjacent strata in folded structures leads to distinctive topographic features by which folds can be recognized at the surface surface. Plunging Anticline Plunging Anticline Plunging Syncline Dome and basin formation A fold with an overturned bowl is called a dome, whereas a fold shaped like a right-side-up bowl is called a basin. Anticlinal structure Synclinal structure ( (can make k good d oilil reservoirs i if …)) ( (typically associated with old mountain belt)) Domes and basins Dome Basin Dome Isostasy and Mountain building Whereas typical continental crust has a thickness of about 35-40 km (measured to the surface of the Moho), Moho) the crust beneath some mountain belts may reach a thickness of 50-70 km (or about double its normal thickness). Crustal roots are important because, without their buoyancy, mountain ranges would not sit so high on the surface of the Earth Æ Archimedes’ Principle. The soft but solid asthenosphere p can slowly flow out of the way as the base of the lithosphere sinks under its own weight. When the buoyancy force pushing the lithosphere up equals the gravitational force pulling the lithosphere down, we have isostacy. An iceberg and its root Archimedes’ Principle – the mass of water displaced is equal to the mass of the floating iceberg. Convergence, Subduction and Orogeny The Plate Tectonics theory has taught us that mountains form in response to convergent boundary deformation, continental collisions, and rifting. If plate movements push the continent tightly against the subduction zone, compression on the continent side of the volcanic arc generates a fold-thrust belt. Mountain Building Numerous thrust faults and associated folds develop, accommodating significant horizontal shortening and thickening of the crust. Andes: Foldbelt Mountains Convergence along the western coast of South America has generated a fold-thrust belt on the eastern side of the Andes. Foldbelt mountains consist of two basic components: a complex core of igneous igneous, metamorphic and volcanic rocks and a sequence of folded and faulted sedimentary rocks. From: Renton (1994) Physical Geology A foldbelt also formed on the eastern side of the North American Cordillera during the Mesozoic and Cenozoic (over ) the last 250 Ma). Mount Kidd, the end of the Lewis Thrust Fault From: Murck, Skinner & Mackenzie (2008) Visualizing Geology Convergence, Subduction and Orogeny A foldbelt also formed on the eastern side of the North American Cordillera during the Mesozoic and Cenozoic (over the last 250 Ma). During convergent convergent-margin margin orogeny, orogeny offshore islands islands, volcanic arcs and small fragments of continental crust may drift into the convergent margin. These blocks – exotic terrane - are too buoyant to subduct and accrete to the continent. Exotic terranes make up the western half of the North American Cordillera During convergent-margin orogeny, offshore islands, volcanic arcs and small fragments of continental crust may drift into the convergent margin. These blocks are too buoyant to subduct and d accrete t to t the th continent. ti t Continent-continent collision and orogeny During collision, intense compression generates fold-thrust fold thrust belts on the margins of the orogen. Regional metamorphism in an orogenic belt In the interior of the orogen, where one continent overrides the edge of the other, high-grade metamorphism occurs, accompanied by the formation of flow folds and foliation. The crust below the orogen thickens to as much as twice its original thickness. Unmetamorphosed shale Hornfels Migmatite Gneiss Schist Slate The collision of India with Asia Rocks R k att d depth th iin th the orogen heat h t up and db become so weak k th thatt th the mountain t i belt b lt may collapse and spread out sideways. The Tibetan Plateau may have formed when crust, thickened during the collision of India with Asia, spread to the northeast. Ganges Plain Himalaya Mountains 10 24 38 55 Suture between Indian and Asian Plates Indian Plate 71 Asian plate Mt. Everest Thrust faults Normal fault Tibet plateau Multiple continental collisions The Appalachian Mountains were first formed during the continental collision that ) Since their formation, the Appalachians have been created Rodinia ((1 Ga). reduced by erosion and re-uplifted (creation of Gondwana/Pangaea) by orogenic forces that affected the entire eastern portion of North America. From: Renton (1994) Physical Geology Tectonic evolution of the Appalachian Mountains 1000-1300 Mya Rodinia Volcanic island arc? 600 Mya Exotic terrain, piece of crust? 450-470 Mya 420-370 Mya 300-350 Mya 250 220 M 250-220 Mya Pangaea Relief map of the Valley and Ridge P i Province, Pennsylvania P l i tto Virginia Vi i i Divergence g and Rift Mountains When rifts form, uplift occurs because, as the lithosphere thins, hot asthenosphere rises, making the remaining lithosphere warmer and less dense As rifting continues dense. continues, stretching causes normal faulting in the brittle crust above. This produces sediment-filled basins separated by narrow, elongate mountain ranges Æ fault-block mountains (e.g., African Rift). Basin and Range Mountain Belt: southwestern U.S.A. U S A (Utah, (Utah Nevada & Arizona) Fault-block mountains From: Renton (1994) Physical Geology Digital elevation map of North America A craton consists of crust that has not been affected by orogeny for a long time, typically 1 billion years. Hence, they are cool, strong and stable. Shield: where Precambrian (>500 Ma) metamorphic and igneous rocks crop out at the surface (e.g. Greenville). Platform: where a relatively thin layer of Phanerozoic (<500 Ma) sediment cover the Precambrian rocks. The Canadian Shield The Canadian Shield, which occupies twothirds of Canada, contains traces of Himalaya-like collision zones, Andean-like convergent boundaries boundaries, and East African African-like like rifts, all formed more than 1 billion years ago. These orogens are so old that erosion has worn away the original topography, exposing deep crustal rocks at the Earth’s surface.
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