REVIEW SHEET GeoLoGY 311 FALL 2001: TOPICS 6 TO 10 1/10 GEOL311: Review sheet for midterm Lecture exam 2 The exam will be on topics 6 through 10 Since we have built on concepts introduced in topics 1 through 5 you should also be familiar with this material, although there will be no questions specifically on material in topics 1 through 5 There will be no questions in this exam on material covered by Steve Hurst athough I will get some questions from him for the final exam See you Monday morning, please bring compasses (hint: for drawing circles Topic 6- Brittle deformation 6.1 Introduction Brittle deformation= the permanent change that occurs in a solid material due to the growth of fractures and/or sliding on fractures A fracture is a surface of discontinuity(includes cracks, joints and faults) Faults result from shear stresses, and joints result from tensile stresses shear= movement parallel to plane of contact tensile= movement normal to plane of contact 6.2 Types of brittle deformation Cataclasis: Grain-scale fracturing and frictional sliding(sliding on a pre-existing fracture surface). Asperities, real area of contact Shear rupture vs Tensile crackin 6.3 The strength paradox The theoretical strength of a rock= total strength of all the bonds across the fracture plane(before it fractured). This is much large than tensile stresses actually needed to break a rock. So we have a paradox(the strength paradox) Explained b (i) Stress concentration occurs around the edges of flaws in elastic material (ii) Natural rocks are not perfect materials,(Griffith cracks) (ii)Finally, cracks propagate(they do not form instantaneously) 6.4 Causes of tensile cracking (i)Axial stretching (ii)Hydraulic fracturing (iii )Longitudinal splitting 6.5 Modes of crack surface displacement Mode l is a tensile-mode crack, (walls displaced normal to crack plane Mode Il(sliding) and Mode Ill (tearing) are shear-mode cracks and cannot grow in their own plane, so curve to become mode I cracks or spawn wing crack Faults and shear ruptures are not simply large mode Il or mode Ill cracks 6.6 Formation of shear fractures
REVIEW SHEET GEOLOGY 311 FALL 2001: TOPICS 6 TO 10 1/10 GEOL311: Review sheet for midterm Lecture exam 2 The exam will be on topics 6 through 10. Since we have built on concepts introduced in topics 1 through 5 you should also be familiar with this material, although there will be no questions specifically on material in topics 1 through 5. There will be no questions in this exam on material covered by Steve Hurst, although I will get some questions from him for the final exam. See you Monday morning, please bring compasses (hint: for drawing circles!) Alan Topic 6 – Brittle deformation 6.1 Introduction Brittle deformation = the permanent change that occurs in a solid material due to the growth of fractures and/or sliding on fractures. A fracture is a surface of discontinuity (includes cracks, joints and faults). Faults result from shear stresses, and joints result from tensile stresses: shear = movement parallel to plane of contact tensile = movement normal to plane of contact 6.2 Types of brittle deformation Cataclasis: Grain-scale fracturing and frictional sliding (sliding on a pre-existing fracture surface). Asperities, real area of contact. Shear rupture vs. Tensile cracking 6.3 The strength paradox The theoretical strength of a rock = total strength of all the bonds across the fracture plane (before it fractured). This is much large than tensile stresses actually needed to break a rock. So we have a paradox (the strength paradox). Explained by: (i) Stress concentration occurs around the edges of flaws in elastic material. (ii) Natural rocks are not perfect materials, (Griffith cracks), (iii) Finally, cracks propagate (they do not form instantaneously) 6.4 Causes of tensile cracking (i) Axial stretching: (ii) Hydraulic fracturing:. (iii) Longitudinal splitting: 6.5 Modes of crack surface displacement Mode I is a tensile-mode crack, (walls displaced normal to crack plane) Mode II (sliding) and Mode III (tearing) are shear-mode cracks and cannot grow in their own plane, so curve to become mode I cracks or spawn wing cracks. Faults and shear ruptures are not simply large mode II or mode III cracks 6.6 Formation of shear fractures
REVIEW SHEET geoloGY 311 FALL 2001: TOPICS 1 TO 5 /10 a shear fracture is a surface across which a rock loses continuity when the shear stress parallel to the surface is sufficiently large. Stage l: volume decrease and crack closure Stage IL; elastic compression(Poisson effect), continues until o (the yield strength) is reached II: new cracking -tensile microcracks begin to grow throughout the sample and wing cracks grow at the tips of shear mode cracks Dilatancy(volume increase)is associated with cracks opening Tensile cracking intensifies along a band oriented at -30 to o, then cracks coalesce to form a through-going rupture, and the sample Note distinction between o, (yield strength, single cracks will grow)and O. (failure strength: many cracks coalesce to form a through-going rupture 6. 7 Shear failure criteria Shear failire criterion describes stress state at which a shear rupture forms Coulomb criterion: os=C+uON, plots as two straight lines on Mohr diagram Failure envelope separates stable unstable stress states The Coulomb failure criterion is empirical, and there are several modifications (i)The Mohr-Coulomb criterion has a curved failure envelope (ii)Plastic yielding (von Mises criterion) plots as horizontal lines (iii) The tensile strength of a sample depends on the number, shape size and orientation of Griffith cracks, so it can vary a lot even for a single rock type Combine many criteria into a composite failure envelope 6. 8 Crack or slide? (a) Frictional sliding criteria: plot as straight lines on a Mohr diagram The best fit to experimental data is known as Byerlee's law: (b)Will a new crack form or a pre-existing crack slide? Plotting Byerlee's law and the composite shear failure envelope for a given sample, and see which would be intersected first-in general, frictional sliding will occur on fractures with angles between about 15 and 75 to o, before new cracks would initiate (c) Hydrofracture: In rocks with high permeabilities, pore fluid pressure is hydrostatic: P:=Pwgh If permeability is restricted, pore pressure may exceed hydrostatic pressure (overpressure)and may approach lithostatic pressure(P PRgh). If pore fluid pressure exceeds O, tensile stresses at the tips of cracks oriented perpendicular to o, may become sufficient for the crack to propagate Hydrofracture can occur whenever the pore fluid pressure is 203 (d)Effective pressure Pore fluid pressure acts equally in all directions, so the PFP can be subtracted from both o, and o,, which moves the mohr circle to the left The term( on-Pe) is often labelled on, and is called the effective stress opic 7-Joints and veins 7. 1 Introduction loint=planar fracture or crack in a rock, without shear displacement, i.e. naturally formed tensile crack (mode I crack) Note: joints will form normal to o,, the joint plane will contain both O, and o2 Joints are useful paleostress indicators(also significant for groundwater flow)
REVIEW SHEET GEOLOGY 311 FALL 2001: TOPICS 1 TO 5 2/10 A shear fracture is a surface across which a rock loses continuity when the shear stress parallel to the surface is sufficiently large. Stage I: volume decrease and crack closure Stage II: elastic compression (Poisson effect), continues until σi (the yield strength) is reached. Stage III: new cracking - tensile microcracks begin to grow throughout the sample and wing cracks grow at the tips of shear mode cracks. Dilatancy (volume increase) is associated with cracks opening. Tensile cracking intensifies along a band oriented at ~ 30˚ to σ1 then cracks coalesce to form a through-going rupture, and the sample fails Note distinction between σy.(yield strength, single cracks will grow) and σf .(failure strength : many cracks coalesce to form a through-going rupture). 6.7 Shear failure criteria Shear failire criterion describes stress state at which a shear rupture forms. Coulomb criterion: σS = C + µ σN , plots as two straight lines on Mohr diagram. Failure envelope separates stable & unstable stress states The Coulomb failure criterion is empirical, and there are several modifications: (i) The Mohr-Coulomb criterion has a curved failure envelope (ii) Plastic yielding (von Mises criterion) plots as horizontal lines. (iii) The tensile strength of a sample depends on the number, shape, size and orientation of Griffith cracks, so it can vary a lot even for a single rock type. Combine many criteria into a composite failure envelope 6.8 Crack or slide? (a) Frictional sliding criteria: plot as straight lines on a Mohr diagram. The best fit to experimental data is known as Byerlee’s law: (b) Will a new crack form or a pre-existing crack slide? Plotting Byerlee’s law and the composite shear failure envelope for a given sample, and see which would be intersected first - in general, frictional sliding will occur on fractures with angles between about 15˚ and 75˚ to σ1 before new cracks would initiate. (c) Hydrofracture: In rocks with high permeabilities, pore fluid pressure is hydrostatic: Pf = ρWgh If permeability is restricted, pore pressure may exceed hydrostatic pressure (overpressure) and may approach lithostatic pressure (Pf = ρRgh). If pore fluid pressure exceeds σ3, tensile stresses at the tips of cracks oriented perpendicular to σ3 may become sufficient for the crack to propagate. Hydrofracture can occur whenever the pore fluid pressure is ≥ σ3. (d) Effective pressure: Pore fluid pressure acts equally in all directions, so the PFP can be subtracted from both σ1 and σ3, which moves the Mohr circle to the left The term (σN – PF) is often labelled σN*, and is called the effective stress. Topic 7 – Joints and veins 7.1 Introduction Joint = planar fracture or crack in a rock, without shear displacement, i.e. naturally formed tensile crack (mode I crack). Note: joints will form normal to σ3; the joint plane will contain both σ1 and σ2. Joints are useful paleostress indicators (also significant for groundwater flow)
REVIEW SHEET GeoLoGY 311 FALL 2001: TOPICS 6 TO 10 3/10 7.2 Surface morphology of joints Plumose structure: origin, mirror zone, mist zone, hackle zone, plume axis, arrest lines. Forms due to (i)rock inhomogeneity and (ii)changing stress field at crack tip as joint propagates May split into en echelon array("twist hackle") 7. 3 Characteristics of join (i) Joint sets: frequently systematic, i. e. several with a similar orientation and approximately equal spacing Joint set =a group of systematic joints Joint system= two or more joint sets(may have formed at very different times Orthogonal and conjugate joint systems. Timing relations-the first-formed set of joints will truncate later sets, because joints cannot cross a free surface (ii) Joint spacing: " stress shadow"explains why joints are closer spaced in thinner beds. spacing depends on: [1] bed thickness, [2] strain(more strain requires more joints), [3] stiffness(Youngs modulus)and[4] tensile strength 7.4 Origin of joints (i) Poisson effect(during exhumation, unloading allows vertical extension and consequent horizontal contraction), (ii)Membrane effect(during exhumation, rock layers move away from centre of Earth and radius of curvature increases) (ii) Thermal contraction e.g. during cooling of lava, gives columnar joints (iv) Bending of brittle rock layers( during folding, local stresses may be tensile (v) Hydrofracture at depth during tectonic loading, (vi) Exfoliation joints are often seen in granites and other massive homogeneous rocks 7.5 Veins Vein=a fracture containing mineralised material joint or shear displacement) Vein array=group of veins(en echelon and sigmoidal arrays give shear sense) Stockwork veins cluster of irregular veins in pervasively fractured rock bod Vein fill minerals precipitated from solution found in the vein Blocky vein fill- generally means fluid precipitated in an open cavity(only possible near the surface). Fibrous vein fill -can form by crack-seal mechanism as vein cracks, fluid pressure and solubility of dissolved minerals both drop, so mineral precipitates and seals the crack again. Pressure builds up in response to stress until crack opens again, and so on) Fibrous veins may grow at an angle to the vein wall (paleo-motion record) Veins may be syntaxial(cracking occurs at a median line in the middle of the vein, when vein fill is same mineral as host rock or antitaxial (cracking occurs at the edges of the vein, when vein fill is different mineral to host rock Topic 8-Faults and faulting 8.1 Introduction Fault= discrete surface on which measurable slip has occurred by brittle defm Slip= relative displacement between formerly adjacent points on opposite sides of a fault, measured in the fault surface Fault zone=Fault expressed as a zone of numerous small fractures. Small fractures and faults branching off a larger fault are called splays Shear zone= distributed zone of shear displacement. Includes microscopically ductile deformation processes, and cataclasis
REVIEW SHEET GEOLOGY 311 FALL 2001: TOPICS 6 TO 10 3/10 7.2 Surface morphology of joints Plumose structure: origin, mirror zone, mist zone, hackle zone, plume axis, arrest lines. Forms due to (i) rock inhomogeneity and (ii) changing stress field at crack tip as joint propagates. May split into en echelon array (“twist hackle”). 7.3 Characteristics of joints (i) Joint sets: frequently systematic, i.e. several with a similar orientation and approximately equal spacing. Joint set = a group of systematic joints Joint system = two or more joint sets (may have formed at very different times!) Orthogonal and conjugate joint systems. Timing relations – the first-formed set of joints will truncate later sets, because joints cannot cross a free surface. (ii) Joint spacing: “stress shadow” explains why joints are closer spaced in thinner beds. Spacing depends on: [1] bed thickness, [2] strain (more strain requires more joints), [3] stiffness (Young’s modulus) and [4] tensile strength. 7.4 Origin of joints (i) Poisson effect (during exhumation, unloading allows vertical extension and consequent horizontal contraction), (ii) Membrane effect (during exhumation, rock layers move away from centre of Earth and radius of curvature increases) (iii) Thermal contraction e.g. during cooling of lava, gives columnar joints (iv) Bending of brittle rock layers (during folding, local stresses may be tensile) (v) Hydrofracture at depth during tectonic loading, (vi) Exfoliation joints are often seen in granites and other massive homogeneous rocks. 7.5 Veins Vein = a fracture containing mineralised material (joint or shear displacement) Vein array = group of veins (en echelon and sigmoidal arrays give shear sense) Stockwork veins = cluster of irregular veins in pervasively fractured rock body. Vein fill = minerals precipitated from solution found in the vein. Blocky vein fill – generally means fluid precipitated in an open cavity (only possible near the surface). Fibrous vein fill – can form by crack-seal mechanism (as vein cracks, fluid pressure and solubility of dissolved minerals both drop, so mineral precipitates and seals the crack again. Pressure builds up in response to stress until crack opens again, and so on). Fibrous veins may grow at an angle to the vein wall (paleo-motion record.) Veins may be syntaxial (cracking occurs at a median line in the middle of the vein, when vein fill is same mineral as host rock) or antitaxial (cracking occurs at the edges of the vein, when vein fill is different mineral to host rock). Topic 8 – Faults and faulting 8.1 Introduction Fault = discrete surface on which measurable slip has occurred by brittle defm. Slip = relative displacement between formerly adjacent points on opposite sides of a fault, measured in the fault surface Fault zone = Fault expressed as a zone of numerous small fractures. Small fractures and faults branching off a larger fault are called splays Shear zone = distributed zone of shear displacement. Includes microscopically ductile deformation processes, and cataclasis
REVIEW SHEET geoloGY 311 FALL 2001: TOPICS 1 TO 5 4/10 Faults control spatial arrangement of rock units(mapping), affect topography and landscape(geomorphology), affect permeability of rocks and sediments control fluid migration), accommodate deformation and earthquakes 8.2 Fault geometry and displacenent Wall =rock adjacent to a fault surface Fault block- body of rock that moved as consequence of slip on the fault For a non-vertical fault, Hangingwall=block above the fault plane and Footwall-block below the fault plane Movement on faults can be a combination of dip-slip and strike-slip Pure dip-slip faults can be normal(h/wall down)or reverse(h/wall up) Pure strike-slip faults can be dextral (right-lateral)or sinistral (left-lateral) Oblique-slip faults combine dip-slip and strike-slip components Net slip=displacement on a fault, measured in the plane of the fault For oblique-slip faults we have to consider both the strike-lip and dip-slip components. The rake angle is the angle of the slip vector measured down from the horizontal in the fault plane Scissors fault= amount of slip changes along strike so that h/wall block rotates around an axis perpendicular to the fault surface Map symbols: Note that the ornament is always in the hangingwall block Detachment= large low-angle fault system, ( can be regionally extensional or contractional, will often change apparent nature from outcrop to outcrop the footwall is the autochthon(stuff which is still in its place or o gyved)and The hangingwall of a detachment is the allochthon(stuff which has mov Window and klippe Net slip vector completely defined by: (i)distance, (ii)orientation (plunge and bearing of offset), and (ii)sense of slip Separation=offset between a particular marker horizon from one side of the fault to another, measured along a specified line(usually not the true net slip) Fault bend=sudden change in dip and /or strike of a fault e.g. Listric faults have concave-up shapes(shallow at depth, steep near surface between footwall flat and hangingwall fla: And flat"geometry. Note distinction e.g. Thrust faults frequently display"ram e.g. Strike-slip faults often contain fault bends, which are classified as ession=combination of strike-slip compression) or releasing bends(transtension= combination of strike-slip extension) Fault terminations"-all faults have to end somewhere. An emergent fault ends at the earth's surface, a blind fault terminates inside the earth and is not seen at the surface. blind faults can be later exposed by erosion) The edge of a fault is called the tip line(separates slipped unslipped regions Faults can die out along their length in a number of ways, e.g. as a horsetail of fault splays, or in a zone of ductile deformation, Longer faults usually have more displacement 8. 3 Fault rocks Classification according to fragment size, and whether cohesive or not Fault gouge: Fine-grained(< lmm)noncohesive fault rock. If cemented by minerals precipitated from circulating groundwater it is an indurated gouge Fault breccia: Coarse noncohesive fault rock, angular rock fragments lmm (can be several m). If cemented, it is a vein-filled breccia(or indurated br Cataclasite: Cohesive fault rocks composed of crushed and rolled grain tccia
REVIEW SHEET GEOLOGY 311 FALL 2001: TOPICS 1 TO 5 4/10 Faults control spatial arrangement of rock units (mapping), affect topography and landscape (geomorphology), affect permeability of rocks and sediments control fluid migration), accommodate deformation and earthquakes. 8.2 Fault geometry and displacement Wall = rock adjacent to a fault surface. Fault block = body of rock that moved as a consequence of slip on the fault. For a non-vertical fault, Hangingwall = block above the fault plane and Footwall = block below the fault plane Movement on faults can be a combination of dip-slip and strike-slip. Pure dip-slip faults can be normal (h/wall down) or reverse (h/wall up) Pure strike-slip faults can be dextral (right-lateral) or sinistral (left-lateral) Oblique-slip faults combine dip-slip and strike-slip components. Net slip = displacement on a fault, measured in the plane of the fault. For oblique-slip faults we have to consider both the strike-lip and dip-slip components. The rake angle is the angle of the slip vector measured down from the horizontal in the fault plane. Scissors fault = amount of slip changes along strike so that h/wall block rotates around an axis perpendicular to the fault surface Map symbols: Note that the ornament is always in the hangingwall block! Detachment = large low-angle fault system, (can be regionally extensional or contractional, will often change apparent nature from outcrop to outcrop) The hangingwall of a detachment is the allochthon (stuff which has moved) and the footwall is the autochthon (stuff which is still in its place of origin) Window and klippe. Net slip vector completely defined by: (i) distance, (ii) orientation (plunge and bearing of offset), and (iii) sense of slip Separation = offset between a particular marker horizon from one side of the fault to another, measured along a specified line (usually not the true net slip). Fault bend = sudden change in dip and/or strike of a fault e.g. Listric faults have concave-up shapes (shallow at depth, steep near surface): e.g. Thrust faults frequently display “ramp and flat” geometry. Note distinction between footwall flat and hangingwall flat. e.g. Strike-slip faults often contain fault bends, which are classified as: restraining bends (transpression = combination of strike-slip & compression) or releasing bends (transtension = combination of strike-slip & extension) Fault terminations¨- all faults have to end somewhere. An emergent fault ends at the Earth’s surface. A blind fault terminates inside the Earth and is not seen at the surface. (Blind faults can be later exposed by erosion) The edge of a fault is called the tip line (separates slipped & unslipped regions): Faults can die out along their length in a number of ways, e.g. as a horsetail of fault splays, or in a zone of ductile deformation, Longer faults usually have more displacement 8.3 Fault rocks Classification according to fragment size , and whether cohesive or not. Fault gouge: Fine-grained ( 1mm (can be several m). If cemented, it is a vein-filled breccia (or indurated breccia). Cataclasite: Cohesive fault rocks composed of crushed and rolled grains
REVIEW SHEET GeoLoGY 311 FALL 2001: TOPICS 6 TO 10 5/10 Pseudotachylyte: Glassy or microcrystalline material formed by melting Slickensides are fault surfaces polished by frictional sliding, often containing groove lineations(striations)caused by asperities ploughing into opposite wal There are several sense of slip indicators for brittle faults and fault zones offset markers(beware apparent offset! ) en echelon veins, fault-related folds, fiber-sheet imbrication, carrot-shaped grooves on slickensides, steps on slickensides, pinnate fractures(near a fault tip) Change in fault character with depth: (i)At the surface, faults may be characterized by Fault scarp, Fault-line scarp and notches resulting from preferential erosion (ii)At shallow depths(s about 5 km), mesoscopic faults can reactivate bedding planes or joint surfaces or fracture previously intact rock (iii) Between about 5 and 10-15 km, rocks become more ductile Brittle-plastic transition is at about 10-15 km depth(c. 250 to 350C). Below this, ductile crystal-plastic deformation mechanisms dominate and mylonite forms 8.4 Faults and folds Folds and faults often associated Fault-inception fold -deformation by folding is overprinted by faulting Fault-propagation fold -e.g. folding above and beyond a thrust fault tip line Fault-bend fold- forms passively as gravity prohibits void formation 8.5 Anderson's theory of faulting Ratio of shear stress to normal stress is a maximum on planes oriented at about 30 to o, and containing o2(e. g. shear fractures initiate at this angle) Earths surface is a free surface, therefore it is also a principal plane of stress Andersons's theory of faulting predicts basic fault geometries Thrust faulting has o, horizontal, o, horizontal, o,vertical strike-slip faulting has o, horizontal, O2 vertical, o, horizontal Note this does not always work: Frictional sliding on pre-existing surface is often easier than initiating new fractures, Fault surface may be rotated b deformation to a different orientation, anderson s theory is for isotropic homogeneous crust stress field Listric faults-concave-up faults, steep near the surface but shallow at depth. 8.6 Fluids and fault Seismic pumping- pressure gradient drives groundwater into the fault zone Fluids affect the shear stress at which faulting occurs in three ways (i) Alteration-clay minerals are weak, with low shear strength (ii) Hvdrolvtic weakening of silicate minerals(without transformation to clays (iii)Pore pressure(Puid)decreases effective normal stress(explains how large thrust sheets can move intact, without breaking Hu Rub pothesis: if Puid in the detachment zone is near to lithostatic pressure, effective normal stress approaches zero, and shear stress required for sliding becomes smaller than required for internal deformation 8.7 Fault systeins Faults usually occur in fault systems, also called fault arrays, classified according to geometric and tectonic features
REVIEW SHEET GEOLOGY 311 FALL 2001: TOPICS 6 TO 10 5/10 Pseudotachylyte: Glassy or microcrystalline material formed by melting Slickensides are fault surfaces polished by frictional sliding, often containing groove lineations (striations) caused by asperities ploughing into opposite wall. There are several sense of slip indicators for brittle faults and fault zones: offset markers (beware apparent offset!), en echelon veins, fault-related folds, fiber-sheet imbrication, carrot-shaped grooves on slickensides, steps on slickensides, pinnate fractures (near a fault tip) Change in fault character with depth: (i) At the surface, faults may be characterized by Fault scarp , Fault-line scarp and notches resulting from preferential erosion (ii) At shallow depths (≤ about 5 km), mesoscopic faults can reactivate bedding planes or joint surfaces or fracture previously intact rock (iii) Between about 5 and 10-15 km, rocks become more ductile. Brittle-plastic transition is at about 10-15 km depth (c. 250 to 350 ˚C). Below this, ductile crystal-plastic deformation mechanisms dominate, and mylonite forms 8.4 Faults and folds Folds and faults often associated. Fault-inception fold – deformation by folding is overprinted by faulting Fault-propagation fold – e.g. folding above and beyond a thrust fault tip line Fault-bend fold – forms passively as gravity prohibits void formation 8.5 Anderson’s theory of faulting Ratio of shear stress to normal stress is a maximum on planes oriented at about 30˚ to σ1 and containing σ2 (e.g. shear fractures initiate at this angle). Earth’s surface is a free surface, therefore it is also a principal plane of stress. Andersons’s theory of faulting predicts basic fault geometries: Normal faulting has σ1 vertical, σ2 horizontal, σ3 horizontal Thrust faulting has σ1 horizontal, σ2 horizontal, σ3 vertical strike-slip faulting has σ1 horizontal, σ2 vertical, σ3 horizontal Note this does not always work: Frictional sliding on pre-existing surface is often easier than initiating new fractures, Fault surface may be rotated by deformation to a different orientation, Anderson’s theory is for isotropic homogeneous crust & stress field Listric faults – concave-up faults, steep near the surface but shallow at depth. 8.6 Fluids and faulting Seismic pumping - pressure gradient drives groundwater into the fault zone. Fluids affect the shear stress at which faulting occurs in three ways: (i) Alteration - clay minerals are weak, with low shear strength (ii) Hydrolytic weakening of silicate minerals (without transformation to clays) (iii) Pore pressure (Pfluid) decreases effective normal stress (explains how large thrust sheets can move intact, without breaking). Hubbert-and-Rubey hypothesis : if Pfluid in the detachment zone is near to lithostatic pressure, effective normal stress approaches zero, and shear stress required for sliding becomes smaller than required for internal deformation. 8.7 Fault systems Faults usually occur in fault systems, also called fault arrays, classified according to geometric and tectonic features
REVIEW SHEET GeoLoGY 311 FALL 2001: TOPICS TO 5 6/10 Geometric classification: parallel array, anastomosing array, en echelon array, relay array, conjugate array, or random array: Usually most faults in a parallel array will dip in the same direction. Subsidiary faults parallel to major faults are called synthetic. Sometimes a fault dips in the opposite direction, in which case it is an antithetic fault Tectonic classification: Normal fault system: often form in rifts, where the lithosphere is extending, along passive margins, and along mid-ocean ridges. Two different styles,(a) horst-and-graben(planar faults)and(b) half-graben systems(listric faults) Thrust fault system: accommodate regional shortening, e. g. margins of convergent plate boundaries and collisional orogens. Often combined with folding, resulting in fold-thrust belts. Flats follow weak horizons, and ramps cut across rigid beds. Imbricate fan or duplex structure Strike-slip fault system: occur at transform plate boundaries, within plates, and as components of collisional or convergent orogens. Typically splay into many separate faults near the surface(flower structure). Transpression and transtension give either"positive"or"negative"flower structures Reactivated fault systems Fault inversion is the reversal of displacement on a fault during reactivation Thin-skinned fault system: faults occur only at shallow depths in the edimentary cover, separated from un-faulted basement by a detachment Thick-skinned fault system: faults involve basement as well as cover Topic 9-Ductile Deformation mechansims 9.1 Introduction Temperature is higher in the mid-and lower crust, so it is more ductile e? Why is the upper crust typically brittle and the lower crust typically ductile? Crystal defects:( deviations from perfection in crystal lattices) Three main types: point defects, line defects, and planar defects 9.2 Point defects point defects come in three basic forms Vacancies(where an atom is missing from the lattice, leaving a hole) Interstitial defects(where an extra atom is inserted into the lattice) Substitutional defects(where a different atom replaces what should be there inducing strain in the crystal lattice) The most important type of point defect for ductile deformation is vacancies, since whenever an atom moves to fill the vacancy, a new one is created 9.3 Movenent of point defects Point defects can move throughout the crystal by diffusion-a thermally activated process (i.e. temperature is very important There are two basic diffusion mechanisms in crystals (i)Volume diffusion(Herring-Nabarro creep): Vacancies can be created at the crystal face where the compressive stress is minimum(o,), and destroyed at the face where compressive stress is maximum(o,) (ii) Grain boundary diffusion(Coble creep): Has a lower activation energy than volume diffusion, but the grain boundary is a less direct path for diffusion
REVIEW SHEET GEOLOGY 311 FALL 2001: TOPICS 1 TO 5 6/10 Geometric classification: parallel array, anastomosing array, en echelon array, relay array, conjugate array, or random array: Usually most faults in a parallel array will dip in the same direction. Subsidiary faults parallel to major faults are called synthetic. Sometimes a fault dips in the opposite direction, in which case it is an antithetic fault: Tectonic classification: Normal fault system: often form in rifts, where the lithosphere is extending, along passive margins, and along mid-ocean ridges. Two different styles, (a) horst-and-graben (planar faults) and (b) half-graben systems (listric faults): Thrust fault system: accommodate regional shortening, e.g. margins of convergent plate boundaries and collisional orogens. Often combined with folding, resulting in fold-thrust belts. Flats follow weak horizons, and ramps cut across rigid beds. Imbricate fan or duplex structure: Strike-slip fault system: occur at transform plate boundaries, within plates, and as components of collisional or convergent orogens. Typically splay into many separate faults near the surface (flower structure). Transpression and transtension give either “positive” or “negative” flower structures. Reactivated fault systems: Fault inversion is the reversal of displacement on a fault during reactivation Thin-skinned fault system: faults occur only at shallow depths in the sedimentary cover, separated from un-faulted basement by a detachment Thick-skinned fault system: faults involve basement as well as cover. Topic 9 – Ductile Deformation mechansims 9.1 Introduction Why is the upper crust typically brittle and the lower crust typically ductile? Temperature is higher in the mid- and lower crust, so it is more ductile Crystal defects: (deviations from perfection in crystal lattices) Three main types: point defects, line defects, and planar defects. 9.2 Point defects Point defects come in three basic forms: Vacancies (where an atom is missing from the lattice, leaving a hole) Interstitial defects (where an extra atom is inserted into the lattice) Substitutional defects (where a different atom replaces what should be there, inducing strain in the crystal lattice) The most important type of point defect for ductile deformation is vacancies, since whenever an atom moves to fill the vacancy, a new one is created. 9.3 Movement of point defects Point defects can move throughout the crystal by diffusion - a thermally activated process (i.e. temperature is very important) There are two basic diffusion mechanisms in crystals: (i) Volume diffusion (Herring-Nabarro creep):Vacancies can be created at the crystal face where the compressive stress is minimum (σ3), and destroyed at the face where compressive stress is maximum (σ1) (ii) Grain boundary diffusion (Coble creep): Has a lower activation energy than volume diffusion, but the grain boundary is a less direct path for diffusion
REVIEW SHEET GeoLoGY 311 FALL 2001: TOPICS 6 TO 10 7/10 Both volume and grain boundary diffusion change the shape of the crystal 9.4 Solution creep(Pressure Solution) requires the presence of an aqueous fluid phase(often present in sed rocks) acts around grain boundaries(like Coble creep) ions dissolve at crystal faces under high compressive stress(o,) ons precipitate at crystal faces under low compressive stress(o3) result is change in shape of crystal (like the two diffusion mechanisms) may be associated volume loss if material is transported away from the crystal 9.5 Dislocations Line defects Dislocations) come in two endmembers Edge defects= extra half-plane of atoms. The dislocation propagates perpendicular to its orientation(perpendicular to the dislocation line) crew defects="strike-slip"offset of lattice; slip parallel to the dislocation line The density of dislocations in an unstrained crystal is typically of the order 10* to 10 cm". In strained crystals, this rises to 10 to 10cm Dislocations can be created through a Frank-Read source 9.6 Dislocation glide Dislocations can move through the crystal by a mechanism called dislocation glide, which involves displacement on a slip plane or a glide plane The greater number of possible slip planes, leads to more ductile behaviour Movement on glide planes leads to macroscopic deformation of the crystal Movement of the dislocation on a slip plane, may be stopped by pinning(by point defects)or tangling(by intersection wth a different dislocation) This leads to work hardening 9.7 Dislocation climb At higher temperatures, dislocations can climb from one plane to another, thereby overcoming tangling and increasing the ductility of the mineral. Known as dislocation climb, and can lead to creation of vacancy-type point defects 9.8 Recovery Dislocations and point defects both increase the internal strain energy of crystal This can be reduced by dislocation annihilation, Internal strain energy may also be reduced by recovery: process whereby edge dislocations accumulate in a le boundary, known as a"tilt boundary", dividing regions of the crystal th slightly different lattice orientations(subgrains 9.9 Recrystallization Internal strain energy remaining after recovery and subgrain formation is dissipated by recrystallization: high-angle grain boundaries form, separating relatively strain-free grains. This may occur during shearing ( dynamic more dislocations accumulate at the tilt boundary, increasing its mismate do recrystallization or in the absence of differential stress(static recrystallization 1) Rotation recrystallization occurs when a subgrain progressively rotates, (2) Boundary migration recrystallization occurs when a grain grows at the expense of its neighbour (grain with higher dislocation density is consumed)
REVIEW SHEET GEOLOGY 311 FALL 2001: TOPICS 6 TO 10 7/10 Both volume and grain boundary diffusion change the shape of the crystal 9.4 Solution creep (Pressure Solution) requires the presence of an aqueous fluid phase (often present in sed. rocks) acts around grain boundaries (like Coble creep) ions dissolve at crystal faces under high compressive stress (σ1) ions precipitate at crystal faces under low compressive stress (σ3) result is change in shape of crystal (like the two diffusion mechanisms) may be associated volume loss if material is transported away from the crystal 9.5 Dislocations Line defects (Dislocations) come in two endmembers: Edge defects = extra half-plane of atoms. The dislocation propagates perpendicular to its orientation (perpendicular to the dislocation line) Screw defects = “strike-slip” offset of lattice; slip parallel to the dislocation line The density of dislocations in an unstrained crystal is typically of the order 104 to 106 cm-2. In strained crystals, this rises to 108 to 1012 cm-2. Dislocations can be created through a Frank-Read source: 9.6 Dislocation glide Dislocations can move through the crystal by a mechanism called dislocation glide, which involves displacement on a slip plane or a glide plane. The greater number of possible slip planes, leads to more ductile behaviour Movement on glide planes leads to macroscopic deformation of the crystal: Movement of the dislocation on a slip plane, may be stopped by pinning (by point defects) or tangling (by intersection wth a different dislocation). This leads to work hardening, 9.7 Dislocation climb At higher temperatures, dislocations can climb from one plane to another, thereby overcoming tangling and increasing the ductility of the mineral. Known as dislocation climb, and can lead to creation of vacancy-type point defects: 9.8 Recovery Dislocations and point defects both increase the internal strain energy of crystal. This can be reduced by dislocation annihilation,.Internal strain energy may also be reduced by recovery : process whereby edge dislocations accumulate in a low-angle boundary, known as a “tilt boundary”, dividing regions of the crystal with slightly different lattice orientations (subgrains) 9.9 Recrystallization Internal strain energy remaining after recovery and subgrain formation is dissipated by recrystallization: high-angle grain boundaries form, separating relatively strain-free grains. This may occur during shearing (dynamic recrystallization) or in the absence of differential stress (static recrystallization). (1) Rotation recrystallization occurs when a subgrain progressively rotates, as more dislocations accumulate at the tilt boundary, increasing its mismatch. (2) Boundary migration recrystallization occurs when a grain grows at the expense of its neighbour (grain with higher dislocation density is consumed)
REVIEW SHEET GEoLOGY 311 FALL 2001: TOPICS 1 TO 5 8/10 Boundary migration recrystallization may cease during dynamic recrystallization if the new grains become deformed as they grow crystallized rocks typically have grains without undulose extinction(strain free), with straight grain boundaries that meet at about 120. Recrystallization is often not complete, so can see core and mantle structure, or mortar structure Dynamic recrystallization=> grain size reduction. The decrease in grain size often leads to increased strain rate for a given stress, i.e. work softening Static recrystallization=> grain size increase since it also involves a component of secondary grain growth. 9.10 Mechanical twinning Some minerals, e.g. calcite, dolomite and feldspar, may form twins in response to an applied stress. Mechanical twinning involves the glide of partial dislocations along a twin boundary, which separates two regions of a crystal lattice which are mirror images of each other Different to dislocation glide, because (i) atoms are moved by a fraction of th interatomic distance, not an integer, and (ii)the slipped portion is a mirror image of the unslipped portion, whereas in dislocation glide both portions have the same crystallographic orientation Mechanical twinning is not a steady-state deformation mechanism Shear strains up to 0.35 can be accommodated by mechanical twinning of calcite. Beyond this, another deformation mechanism must operate 9. 11 Superplastic creep Only at high temperatures and strain rates, in very fine-grained rocks Strictly known as: Grain Boundary sliding Superplasticity(gBss) Grains slide past each other without friction This only happens in very fine-grained rocks(grains typically 5 to 15 um)at high temperatures: diffusion paths are short, T is high, so diffusion is rapid 9.12 Deformation mechanisIn maps So far in this topic we have covered nine different microscopic mechanisms creep), Pressure solution, Dislocation glide, Dislocation creep, Recovery able Volume diffusion(Herring- Nabarro creep), Grain boundary diffusion(Ce Recrystallization, Mechanical twinning, and Superplastic creep All may be active to some degree over a wide range of temperature conditions Each have different activation energies, so under any given conditions of strain rate and temperature, one mechanism is likely to dominate Deformation mechanism map: on a graph of strain rate vs temperature, we can map out regions where a particular mechanism dominates these maps are for fixed grain size, pressure, and either wet ot dry the pressure solution field extends to higher temperatures under wet conditions than the grain boundary diffusion field under dry conditions Topic 10-Folds and folding 10.1 Introduction a fold is a structure formed by bending of a layer without loss of cohesion
REVIEW SHEET GEOLOGY 311 FALL 2001: TOPICS 1 TO 5 8/10 Boundary migration recrystallization may cease during dynamic recrystallization if the new grains become deformed as they grow. Recrystallized rocks typically have grains without undulose extinction (strainfree), with straight grain boundaries that meet at about 120˚. Recrystallization is often not complete, so can see core and mantle structure, or mortar structure, Dynamic recrystallization => grain size reduction. The decrease in grain size often leads to increased strain rate for a given stress, i.e. work softening Static recrystallization => grain size increase since it also involves a component of secondary grain growth.. 9.10 Mechanical twinning Some minerals, e.g. calcite, dolomite and feldspar, may form twins in response to an applied stress. Mechanical twinning involves the glide of partial dislocations along a twin boundary, which separates two regions of a crystal lattice which are mirror images of each other. Different to dislocation glide, because (i) atoms are moved by a fraction of the interatomic distance, not an integer, and (ii) the slipped portion is a mirror image of the unslipped portion, whereas in dislocation glide both portions have the same crystallographic orientation Mechanical twinning is not a steady-state deformation mechanism Shear strains up to 0.35 can be accommodated by mechanical twinning of calcite. Beyond this, another deformation mechanism must operate. 9.11 Superplastic creep Only at high temperatures and strain rates, in very fine-grained rocks. Strictly known as: Grain Boundary Sliding Superplasticity (GBSS) Grains slide past each other without friction. This only happens in very fine-grained rocks (grains typically 5 to 15 µm) at high temperatures: diffusion paths are short, T is high, so diffusion is rapid 9.12 Deformation mechanism maps So far in this topic we have covered nine different microscopic mechanisms: Volume diffusion (Herring-Nabarro creep), Grain boundary diffusion (Coble creep), Pressure solution, Dislocation glide, Dislocation creep, Recovery, Recrystallization, Mechanical twinning, and Superplastic creep All may be active to some degree over a wide range of temperature conditions. Each have different activation energies, so under any given conditions of strain rate and temperature, one mechanism is likely to dominate. Deformation mechanism map: on a graph of strain rate vs. temperature, we can map out regions where a particular mechanism dominates. • these maps are for fixed grain size, pressure, and either wet ot dry. • the pressure solution field extends to higher temperatures under wet conditions than the grain boundary diffusion field under dry conditions. Topic 10 – Folds and folding 10.1 Introduction A fold is a structure formed by bending of a layer without loss of cohesion
REVIEW SHEET GeoLoGY 311 FALL 2001: TOPICS 6 TO 10 9/10 10.2 Fold description I-basic terms limb, hinge and inflection point hinge line axial surface and profile plai Interlimb angle wavelength, amplitude arc length; fold axis (only for ylindrical folds); crest and trough(highest and lowest points with respect to horizontal surface); Culmination and depression (high and low points of the hinge line in non-cylindrical folds) 10.3 Fold description ll-fold classification Symmetry: is the fold symmetric or asymmetric? Orientation: record strike and dip of axial plane if appropriate(cylindrical folds only), record plunge and bearing of hinge line, and describe as vertical, upright, inclined,or reclined (plunging)/recumbent(not plunging Style: (i)3-D fold shape: is the fold gentle, open, tight or isoclinal? is the hinge angular or smooth? (i) facing: is it an anticline, syncline, antiformal syncline or synformal anticline? I anticline: beds young away from the core of the fold; class 1, 2 or 3? [an isogon is a line connecting points of equal dip class Is e syncline: beds young towards the core of the fold)(iii) shape in profile plan convergent(1B= parallel fold), parallel to axial plane= class 2(similar fold), divergent =class 3. Note that there is a space problem with parallel folds(typ 1 B). To get round this, parallel folds are usually associated with nearb disharmonic folds: successive layers in a folded sequence have distinct different wavelengths and /or amplitudes] 10. 4 Fold systens and vergence Fold system=a series of related antiforms and synforms An enveloping surface may be drawn, connecting the hinge lines of adjacent folds This enveloping surface may itself be folded by higher order folds Highest order folds, are often called anticlinoria and synclinoria Asymmetric folds may be interpreted in terms of vergence sense of shear) Asymmetric folds are often second order folds within a first order fold system These smaller folds are then known as parasitic folds: often described as"S M"or"Z"folds, but you must be looking down the hinge to use these terms Note that the pattern of parasitic folds does NOt resemble a pine tree 10.5 Special fold geometries Folds with straight limbs and very angular hinges are known as kink folds if they are less than about 1 m in size, and as chevron folds if they are larger. Kink folds often develop in finely laminated rocks Doubly plunging folds have hinge lines which change curvature along their length This gives culminations and depressions If additional folds are present, there may be en echelon folds Doubly plunging and en echelon folds are non-cylindrical In a box fold, the folded layer is detached from over-and under-lying layers Ptygmatic folds look like intestines Fault-related folds were covered in topic 8, but as remember fault-propagation folds(also called drape folds and fault-bend folds above thrust ramps 10.6 Superposed folding Fold generation= group of folds formed roughly at the same time, under similar kinematic conditions
REVIEW SHEET GEOLOGY 311 FALL 2001: TOPICS 6 TO 10 9/10 10.2 Fold description I – basic terms limb, hinge and inflection point; hinge line, axial surface and profile plane; Interlimb angle, wavelength, amplitude, arc length; fold axis (only for cylindrical folds); crest and trough (highest and lowest points with respect to horizontal surface); Culmination and depression (high and low points of the hinge line in non-cylindrical folds) 10.3 Fold description II – fold classification Symmetry: is the fold symmetric or asymmetric? Orientation: record strike and dip of axial plane if appropriate (cylindrical folds only), record plunge and bearing of hinge line, and describe as vertical, upright, inclined, or reclined (plunging)/recumbent (not plunging) Style: (i) 3-D fold shape: is the fold gentle, open, tight or isoclinal? is the hinge angular or smooth? (ii) facing: is it an anticline, syncline, antiformal syncline or synformal anticline? { anticline: beds young away from the core of the fold; syncline: beds young towards the core of the fold} (iii) shape in profile plane: class 1, 2 or 3? [an isogon is a line connecting points of equal dip; class 1 = convergent (1B = parallel fold), parallel to axial plane = class 2 (similar fold), divergent = class 3. Note that there is a space problem with parallel folds (type 1 B). To get round this, parallel folds are usually associated with nearby disharmonic folds: successive layers in a folded sequence have distinctly different wavelengths and/or amplitudes] 10.4 Fold systems and vergence Fold system = a series of related antiforms and synforms An enveloping surface may be drawn, connecting the hinge lines of adjacent folds This enveloping surface may itself be folded by higher order folds Highest order folds, are often called anticlinoria and synclinoria. Asymmetric folds may be interpreted in terms of vergence (= sense of shear): Asymmetric folds are often second order folds within a first order fold system. These smaller folds are then known as parasitic folds: often described as “S”, “M” or “Z” folds, but you must be looking down the hinge to use these terms. Note that the pattern of parasitic folds does NOT resemble a pine tree !!!! 10.5 Special fold geometries Folds with straight limbs and very angular hinges are known as kink folds if they are less than about 1 m in size, and as chevron folds if they are larger. Kink folds often develop in finely laminated rocks Doubly plunging folds have hinge lines which change curvature along their length This gives culminations and depressions If additional folds are present, there may be en echelon folds. Doubly plunging and en echelon folds are non-cylindrical In a box fold, the folded layer is detached from over- and under-lying layers Ptygmatic folds look like intestines Fault-related folds were covered in topic 8, but as remember fault-propagation folds (also called drape folds) and fault-bend folds above thrust ramps 10.6 Superposed folding Fold generation = group of folds formed roughly at the same time, under similar kinematic conditions
REVIEW SHEET geoloGY 311 FALL 2001: TOPICS 1 TO 5 10/10 Superposed folds are classed according to the relative orientations of the two fold generations. There are four basic combinations which give rise to four different fold interference patterns Type O involves parallel fold generations, tightening the original folds Type 1 gives"dome and basin "geometry Type 2 is known as"mushroom"geometry, and type 3 as"refolded folds 10.7 Fold mechanics During passive folding, the layering has no mechanical significance. This occur in rocks of high mean ductility and where layers have low ductility contrast. During active folding, also known as flexural folding, the layering has mechanical significance: presence of layers with different competency directly affects the strain pattern, and different layers behave differentl (e.g. sandstone is more competent than shale, and resists flow more/ow Competency is a relative term comparing the resistance of rocks to flo is oblique to the layering(e- g basin formation, monocline formation); During o Two dynamic conditions for flexural folding: During bending, the applied force buckling, the applied force is oriented parallel to layering competent layers-a low viscosity contrast results in a component of laver In buckle folds the arc length of the more competent layer is proportional to thickness and also depends on the viscosity contrast between more and les thickening, as well as folding 10.8 Kinematic models of foldi (i) Flexural slip folding: folding accommodated by slip between layers e.g. bend a deck of cards. Involves no strain within the plane of the layers (i.e plane strain, X>Y=1>Z). Amount of slip between layers is greatest at the inflection point and least at the hinge. Fold is cylindrical and parallel. Typically occurs in rocks of low mean ductility e g chevron folds and kink folds (ii) Neutral-surface folding: this has a similar fold geometry to flexural slip folding, but the strain pattern is very different. e.g. bend a metal bar(!) There is a surface within the fold where there is no strain, separating the core and the exterior. This neutral surface may cross layers within the fold The fold is cylindrical and parallel(bed thickness does not ch (ii) Shear folding: is a passive folding mechanism, e.g. shear a deck of cards The slip surface and slip vector are not parallel to layering(cf flexural slip) Produces similar folds(class 2). Mylonitic foliation planes in shear zones may act as the shear planes (iv) Fold shape modification: we have seen kinematic models for parallel folds and similar folds, but have not produced types 1A, 1C or 3. If we take a type 1B fold(parallel fold) and shorten it by a superimposed homogeneous strain, then it is possible to generate type 1 C folds. At higher strains the fold will increasingly look like type 2( similar fold) Note: this is a comprehensive list. You should go this and see what you remember, and review anything you dont! Also, please read the textbook The exam will be short answer questions, will start promptly at 10 am, and will end promptly at 10.50 am Bring compasses(for drawing circles), pens and pencils Y ou will not need a calculator
REVIEW SHEET GEOLOGY 311 FALL 2001: TOPICS 1 TO 5 10/10 Superposed folds are classed according to the relative orientations of the two fold generations. There are four basic combinations which give rise to four different fold interference patterns: Type 0 involves parallel fold generations, tightening the original folds. Type 1 gives “dome and basin” geometry. Type 2 is known as “mushroom” geometry, and type 3 as “refolded folds” 10.7 Fold mechanics During passive folding, the layering has no mechanical significance. This occurs in rocks of high mean ductility, and where layers have low ductility contrast. During active folding, also known as flexural folding, the layering has mechanical significance: presence of layers with different competency directly affects the strain pattern, and different layers behave differently. Competency is a relative term comparing the resistance of rocks to flow (e.g. sandstone is more competent than shale, and resists flow more). Two dynamic conditions for flexural folding: During bending, the applied force is oblique to the layering (e.g. basin formation, monocline formation); During buckling, the applied force is oriented parallel to layering In buckle folds the arc length of the more competent layer is proportional to its thickness, and also depends on the viscosity contrast between more and less competent layers - a low viscosity contrast results in a component of layer thickening, as well as folding. 10.8 Kinematic models of folding (i) Flexural slip folding: folding accommodated by slip between layers e.g. bend a deck of cards. Involves no strain within the plane of the layers (i.e. plane strain, X>Y=1>Z). Amount of slip between layers is greatest at the inflection point and least at the hinge. Fold is cylindrical and parallel. Typically occurs in rocks of low mean ductility, e.g. chevron folds and kink folds (ii) Neutral-surface folding: this has a similar fold geometry to flexural slip folding, but the strain pattern is very different. e.g. bend a metal bar (!) There is a surface within the fold where there is no strain, separating the core and the exterior. This neutral surface may cross layers within the fold. The fold is cylindrical and parallel (bed thickness does not change, class 1B) (iii) Shear folding: is a passive folding mechanism, e.g. shear a deck of cards.. The slip surface and slip vector are not parallel to layering (c.f. flexural slip) Produces similar folds (class 2). Mylonitic foliation planes in shear zones may act as the shear planes (iv) Fold shape modification: we have seen kinematic models for parallel folds and similar folds, but have not produced types 1A, 1C or 3. If we take a type 1B fold (parallel fold) and shorten it by a superimposed homogeneous strain, then it is possible to generate type 1 C folds. At higher strains the fold will increasingly look like type 2 (similar fold). Note: this is a comprehensive list. You should go this and see what you remember, and review anything you don’t! Also, please read the textbook! The exam will be short answer questions, will start promptly at 10 am, and will end promptly at 10.50 am. Bring compasses (for drawing circles), pens and pencils. You will NOT need a calculator