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) 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 > 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