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Earth and Ocean Sciences
EOSC 210

Lecture 8: Goal: Relate the crustal forces that result from each of the three types of plate boundaries and their relationship to rock behaviour. List and describe the various ties of folds, and examples of their signicance with respect to engineering. Identify the 3 types of faults and describe the forces and motion responsible for their development. Compare and contrast the characteristics and mechanical behaviour of faults and joints, and how they affect rock mass strength. ROCKBEHAVIOUR STRESS-the force applied over a unit area (not tangible) STRAIN-how that force manifests itself in the rock (tangible) DIFFERENT STRESSES compression-pushed together. vertical lengthening, horizontal shortening tensional-pulled apart shear-pushed in opposite directions, twisting laterally PLATE BOUNDARIES FORCES -CONVERGENT- compression causes shortening of the rock layer through folding and faulting -TENSION-lengthens and causes faulting STESS-STRAIN elastic limit/yieldpoint- strain will not be recovered (property of rock).. FOLDING-DUCTILE. FAULTING-BRITTLE BRITTLE/DUCTILE DEFORMATION: depends on: 1. kind of stress (CTS) 2.amount of stress 3.temperature: lower temperature requires more stress and less strain percent for fracture 4.lithology of material being deformed (Strength) 5. strain rate. length of time for stress 6. conning pressure-depth higher pressure= higher stress IN EARTH's CRUST: rocks display increasing ductile behaviour with depth (increasing T&P) most brittle on top, most ductile at the bottom. Brittlle-ductile transition zone.. (6-15km). few rocks are brittle below 30km Ductile-FOLDING FOLDING-result from horizontal COMPRESSION of rock layers by internal forces of the earth along plate boundaries. ANTICLINE-a fold that resamples an ARCH like structure with the rock beds (or limps) dipping way from the centre of the structure. OLD- middle. YOUNG-away from axis SYNCLINE-fold that resamples a TROUGH, rocks are warped downward. YOUNG-middle. OLD-away from axis. anticline and syncline occur in pares with adjacent folds sharing limbs. GREAT PRESSURE=INCLINED AND ASYMMETRICAL ANTICLINE AND SYNCLINE (fold centre tilts away from middle) RECUMBENT FOLDS TO THRUST FAULT. important in the alps (structure is calles "nappes"). FOLD IS JUST FOLDING. FAULT IS WHEN TOP OF FOLD REACHES GROUND. BRITTLE DEFORMATION-FAULTING FAULTING-rocks break and move along a fracture (elastic-brittle deformation, fracturing-rock laters too rigid to bend/fold, happens in seconds and connected with earthquakes) TECTONIC FORCES & FAULTS 1NORMAL FAULTS-vertical motion along the fault plane so that the hanging wall is boring DOWN relative to footwall (FROM TENSIONAL STRESS). left to right drainage=canyon on footwall block and forms an alluvial fan. Nevada,USA==> regional tensional stress=series of normal faults. ex. GRAND TETON MTS in Wyoming-high angle normal faults erosional remnats. (in a series of normal fault, HORSTS are up-lifted blocks that form mountains, down dropped blocks that form valleys are called GRABENS) 2REVERSE FAULTS-vertical motion along the fault plane so that the hanging wall is moving up relative to the footwall (FROM COMPRESSION STRESS). left to right drainage=lake on footwall block. THRUST FAULTS-special class of reverse faults, low angle reverse faults with dips less than 30 degrees. due to differences in regional stress directions. EX. CANADIAN ROCKIES 3STRIKESLIP FAULTS-SHEAR STRESS-movement horizontal, thus parallels to the strike of the fault. movement is classied to what is seen by observer (father direction). EX. San Andreas FAULT ZONE-engineering hazard represent major zones of weakness (Los Angeles Aqueduct one week after lling killed 450 people because the dam was built over a fault zone and failed) -provide pathways for increased uid ow (groundwater)FAULTS AND JOINTS Faults-involve fractures along which there has been observable movement of one side relative to the other. Joints-involve fractures along which thare has been NO observable movement of one side relative to the other. FORMATION of ROCk JOINTS. (rock mass may contain several JOINT SETS) -occur as JOINT SETS. TECTONIC JOINS=tectonic stresses inducing TENSILE failure of the rock UNLOADING JOINTS- uplift and erosion removes the overlying rocks, thereby reducing the compressive load and allowing the rock to expand laterally. COOLING JOINTS-cooling of hot magma or lava introduces stress induce tensile failure See ROCK BEHAVIOUR SLIDE RQDrockqualitydesignation=overall quality of the rock mass cut by the different fracture systems BLOCKYROC=70RMR(good rock), lowstress regime, minimal but systematic ground suppo
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