Lecture 3 - Structural Geology PDF

Structural Geology Crustal Structure and Deformation H.D.A. Reyes | Correlation 1 Structural Geology Deals with the origin, geometry, and kinematics of formation of structures Principles and Concepts:  Ductile Deformation Continuous, produces certain kinds of folds, ductile faults, cleavage, and...

Structural Geology Crustal Structure and Deformation H.D.A. Reyes | Correlation 1 Structural Geology Deals with the origin, geometry, and kinematics of formation of structures Principles and Concepts:  Ductile Deformation Continuous, produces certain kinds of folds, ductile faults, cleavage, and foliation  Brittle Deformation Discontinuous, produces other kinds of folds, brittle faults, and joints  Wilson cycle The closing and opening of ocean Pumpelly’s Rule states that small structures are key to mimic the style and orientation s of larger structures of the same generation within a particular area  Suspect terrane a rock mass in which original position is questionable with respect to the adjacent terrane or continent to which it is presently attached.  Exotic Terrane Bears no resemblance to the mass to which it is attached, and the source may be on the opposite side of a major ocean  H.D.A. Reyes | Correlation 1 Structural Geology Deals with the origin, geometry, and kinematics of formation of structures Principles and Concepts:  Non-penetrative structures – structures that occur as single features (e.g. a fault or an isolated fold)  Penetrative structures – structures that occur pervasively (e.g. cleavage, foliation and some folds) H.D.A. Reyes | Correlation 1 Structural Geology Geochronology  Radioactive decay – a process unaffected by heat, pressure, fluids, or chemical reaction.  Blocking or Closing Temperature – the temperature below which a crystal lattice traps radioactive daughter products.  Uranium – Lead Method  Probably the most reliable technique for rocks wherein ages exceed 10 million years. The most common way of applying the U-Pb method to date rock is by analyzing zircon, monazite, and sphene from crushed rock sample  Rubidium – Strontium Method  Applicable in rocks aged that exceed 100 million yrs. The age is determined by analyzing 87Rb, 87Sr, and 86Sr in a mass spectrometer.  Potassium – Argon Method  Provides information primarily on the time of uplift of a rock mass because the relatively low blocking temperatures.  Samarium – Neodynium Method  Most commonly employed to provide a model age for the time of separation of a magma from the parent mantle and, in concert with Sr isotopes, data about sea water contamination and hydrothermal alteration of mass of rocks. H.D.A. Reyes | Correlation 1 Structural Geology Equilibrium  Isostasy - is the state of gravitational equilibrium between Earth's crust and mantle such that the crust "floats" at an elevation that depends on its thickness and density.  Pratt’s Hypothesis Developed by John Henry Pratt, English mathematician and Anglican missionary, supposes that Earth’s crust has a uniform thickness below sea level with its base everywhere supporting an equal weight per unit area at a depth of compensation. In essence, this says that areas of the Earth of lesser density, such as mountain ranges, project higher above sea level than do those of greater density. The explanation for this was that the mountains resulted from the upward expansion of locally heated crustal material, which had a larger volume but a lower density after it had cooled. H.D.A. Reyes | Correlation 1 Structural Geology Equilibrium  Airy’s Hypothesis Earth’s crust is a more rigid shell floating on a more liquid substratum of greater density. Sir George Biddell Airy, an English mathematician and astronomer, assumed that the crust has a uniform density throughout. The thickness of the crustal layer is not uniform, however, and so this theory supposes that the thicker parts of the crust sink deeper into the substratum, while the thinner parts are buoyed up by it. According to this hypothesis, mountains have roots below the surface that are much larger than their surface expression. This is analogous to an iceberg floating on water, in which the greater part of the iceberg is underwater. H.D.A. Reyes | Correlation 1 Structural Geology Stress A force applied to a material that tends to change that material’s dimensions Pressure – stress where the forces act equally in all directions Differential stress – stress is not equal in all directions Types of differential stress  Tensional Stress  the stress that tends to pull something apart. It is the stress component perpendicular to a given surface, such as a fault plane, that results from forces applied perpendicular to the surface or from remote forces transmitted through the surrounding rock.  Compressional Stress  The stress that squeezes something. It is the stress component perpendicular to a given surface, such as a fault plane, that results from forces applied perpendicular to the surface or from remote forces transmitted through the surrounding rock  Shear Stress  It is the stress component perpendicular to a given surface which usually results to slippage and translation. H.D.A. Reyes | Correlation 1 H.D.A. Reyes | Correlation 1 Structural Geology Stress Principal Stress Stress vector acting across plane of zero shear stress     Normal Stress (σn) – acts perpendicular to a reference surface Shear Stress (τ) – Acts parallel to a reference surface Principal Normal Stress Components (σ1> σ2> σ3) Deviatoric Stress – The non-hydrostatic of stress, expressed as total stress with mean stress subtracted from the normal stress components. H.D.A. Reyes | Correlation 1 Structural Geology Pressure The force applied perpendicular to the surface of an object per unit area over which that force is distributed  Hydrostatic Pressure - the pressure that is exerted by a fluid at equilibrium at a given point within the fluid, due to the force of gravity.  Lithostatic Pressure - is the pressure or stress imposed on a layer of soil or rock by the weight of overlying material. H.D.A. Reyes | Correlation 1 Structural Geology Strain Deformation  the displacement field for tectonically driven particle motion and involves processes by which the particle motions are achieved.  Encompasses both rigid body rotation and rigid body translation  Can be continuous or non-continuous H.D.A. Reyes | Correlation 1 Structural Geology   Strain Aspect of shape change measured as changes in line length, changes in angular relationships between lines, or as volume changes Homogeneous strain vs. Inhomogeneous strain H.D.A. Reyes | Correlation 1 Structural Geology Strain Linear Strain  Elongation 𝑺=  Stretch  Quadratic Elongation 𝝀= H.D.A. Reyes | Correlation 1 𝒍𝟏 𝒍𝒐 𝒍𝟏 𝒍𝒐 𝟐 = 𝟏+𝜺 = 𝟏+𝜺 𝟐 =𝑺 𝟐 Structural Geology Strain Linear Strain o Elongation (ε) –the ratio of the length of the line in a deformed mass (𝑙1 ) minus the original length (𝑙0 ) to the original length, written mathematically as: 𝜀= 𝑙1 −𝑙0 𝑙0 = ∆𝑙 𝑙 ;% elongation = ∆𝑙 𝑥 𝑙 100 If 𝜀 > 0, or positive, an extension is indicated; if 𝜀 < 0, or negative, shortening is indicated. H.D.A. Reyes | Correlation 1 Structural Geology Strain Shear Strain Change in Angular Relationships o Shear strain develops when differential movement occurs along a set of parallel lines. The movement is greater parallel to horizontal lines at the tops as compared to the bottom of the rock mass. As a result, a set of lines initially normal to the horizontal lines is rotated from orthogonality by an ψ, the angular shear by: 𝛾 = tan 𝜓 H.D.A. Reyes | Correlation 1 Structural Geology Strain Dilational Strain Volume Change o It is the change in volume of a material which may occur in three different mechanisms a. Closing voids between grains – producing negative volume change b. Dissolving away part of the rock mass by pressure solution – resulting in negative volume change c. Fracturing the mass – producing positive volume change 𝑑𝑉 Δ= 𝑉 H.D.A. Reyes | Correlation 1 Structural Geology Strain Simple shear and Pure Shear  Simple shear is a three-dimensional constant-volume strain  pure shear is a three-dimensional homogeneous flattening of a body. It is an example of irrotational strain in which body is elongated in one direction while being shortened perpendicularly. H.D.A. Reyes | Correlation 1 Structural Geology Strain Strain Markers - Any deformed feature in a rock mass in which the original shape can be qualitatively compared with the present deformed shape.  Reduction Spots – A small, mostly spherical features in fine-grained sediments where the red to reddish-brown oxidized sediment has been chemically reduced to greenish color H.D.A. Reyes | Correlation 1 Structural Geology Strain Strain Markers  Pebbles – among the most frequently used strain indicator H.D.A. Reyes | Correlation 1 Structural Geology Strain Strain Markers  Ooids and pisolites – good indicator of finite strain. H.D.A. Reyes | Correlation 1 Structural Geology Strain Strain Markers  Fossils – Has been used to considerable advantage in determining finite strain.  Vesicles – Gas bubbles in volcanic rocks H.D.A. Reyes | Correlation 1 Structural Geology Strain Strain Markers  Pillow structures in volcanic rocks  Burrows H.D.A. Reyes | Correlation 1 Structural Geology Strain Strain  Flinn Diagram - a graphical representation used to plot finite strain ellipsoids. H.D.A. Reyes | Correlation 1 Structural Geology Strain Strain-Measurements Techniques  Wellman’s Method – this method is based on angular distorsion of reference lines originally aligned 90 degrees to each other H.D.A. Reyes | Correlation 1 Structural Geology Strain Strain-Measurements Techniques  𝑹𝒇 /ϕ Method – Used in elliptical deformed bodies  Center-to-Center Method – based on the distance and angular relationship between particles in an aggregate of objects with a statistically uniform initial distribution should help to determine the orientation of the strain ellipse in the deformed aggregates.  The Fry Method – a simpler version of the center-to-center method and works on the same principle. Angular relationships and distances between particles are modified according to the nature and amount of accumulated strain. H.D.A. Reyes | Correlation 1 Structural Geology Mechanical Behaviour of Rock Materials Definitions • Homogeneous materials – have properties that are the same throughout any sample of any size • Inhomogeneous material – have properties which vary with location; either in a hand specimen or in a region which leads to scale-dependent behavior of rocks • Isotropic materials – have the same properties in all directions • Anisotropic materials – properties vary with direction; layered rocks are strongly anisotropic to stress, but the degree of expression of the anisotropy depends on the direction in which the stress is oriented. H.D.A. Reyes | Correlation 1 Structural Geology Mechanical Behaviour of Rock Materials Definitions        Elastic – Strain that is easily recovered instantaneously on removal of stress Plastic - behaviour involve permanent strain that occur without loss of cohesion and is the result of chemical bonds in crystal lattices in minerals by one of the dislocation creep mechanisms Viscous – behaviour of fluids such as water or magma of any other substance with little internal structure. Yield Point – the stress beyond which a material becomes plastic. Rupture Point – Rupture Ultimate Strength The maximum ordinate in the stress-strain diagram is the ultimate strength or tensile strength. Rapture Strength Rapture strength is the strength of the material at rupture. This is also known as the breaking strength. H.D.A. Reyes | Correlation 1 Structural Geology Mechanical Behaviour of Rock Materials Definitions Ideal end-member behaviour models H.D.A. Reyes | Correlation 1 Structural Geology Mechanical Behaviour of Rock Materials Elastic (Hookean) Behaviour A material exhibits linear elastic behavior if it deforms in direct proportion to the applied stress and, after the stress is removed immediately rebounds to its original configuration.  Young’s Modulus (E)- is a measure of the ability of a material to withstand changes in length when under lengthwise tension or compression H.D.A. Reyes | Correlation 1 E= 𝝈 𝜺 Structural Geology Mechanical Behaviour of Rock Materials Elastic (Hookean) Behaviour  Rigidity or Shear Modulus - is the elasticity coefficient for shearing or torsion force. The ratio of shear stress and shear strain  A linear relationship exists in ideal elastic behavior between the application of stress and the resulting strain. Below the elastic limit, an elastic material rebounds instantaneously to the original shape when the stress is removed. G= H.D.A. Reyes | Correlation 1 𝜏 𝛾 Structural Geology Mechanical Behaviour of Rock Materials Elastic (Hookean) Behaviour  Poisson’s Ratio- the ratio of the proportional decrease in a lateral measurement to the proportional increase in length in a sample of material that is elastically stretched H.D.A. Reyes | Correlation 1 Structural Geology Mechanical Behaviour of Rock Materials Viscous Behavior - Fluid-like behavior  Perfect Fluid – Stationary fluids that does not transmit shear stress  Newtonian Fluids – Fluids in which there is a linearly proportional relationship between differential stress and shear strain rate.  Fluidity - The reciprocal of viscousity  Rocks at high temperatures near their melting points behave in a nearly viscous fashion, but their behavior is better described as plastic. H.D.A. Reyes | Correlation 1 Structural Geology Mechanical Behaviour of Rock Materials Permanent Deformation  Permanent strain in the form of viscous or plastic deformation occurs in a material beyond the elastic limit.  Viscous behavior occurs in fluids, whereas plastic deformation occurs in solids below their melting points.  Presence of water or other fluid may lower threshold temperatures and pressures for ductile behavior. H.D.A. Reyes | Correlation 1 Structural Geology Mechanical Behaviour of Rock Materials Permanent Deformation  Plastic (Saint-Venant) Behavior - Involves permanent deformation that affects the entire rock mass and begins at a yield stress.  After the yield point is passed, the material flows at a constant stress unless one of the two things occurs:  Strain hardening – Increased resistance to deformation as strain increases  Strain softening - the stress in the material is actually decreasing with an increase in strain. H.D.A. Reyes | Correlation 1 Structural Geology Mechanical Behaviour of Rock Materials Behavior of Crustal Rocks  Ductile-Brittle Behavior - Brittle behaviour generally dominates the upper crust. A transition occurred due to increase in temperature and pressure.  Elasticoviscous (Maxwell) Behavior H.D.A. Reyes | Correlation 1 Structural Geology Mechanical Behaviour of Rock Materials Behavior of Crustal Rocks  Elasticoviscous (Maxwell) Behavior o It is a combination of elastic and viscous behavior. It depends on the stress and strain rates. Total strain in an elasticoviscous material is given by the sum of elastic and viscous strain components 𝛾 = 𝛾𝑒 + 𝛾𝑣 or 𝛾 = 𝜏 𝐺 + 𝜏𝑡 𝜂 Where G is the shear modulus, 𝜏 is shear stress, t is time and 𝜂 is viscosity o Hence, we must consider time-dependence of deformation. Rocks that deform brittlely over a short time (milliseconds to a few years) will deform ductilely over periods of thousands to million years. H.D.A. Reyes | Correlation 1 Structural Geology Mechanical Behaviour of Rock Materials Strain Partitioning • Behavior of rocks may vary in space as well as in time; concentration of deformation into specific parts of a rock mass by different behavior or mechanisms is called strain partitioning. This may be related to differences in flow rate in a ductilely deforming mass. • Different strains result from the bulk properties of the rocks being deformed. o Relatively weak rocks (shale, salt, schist) commonly exhibit styles of deformation that contrast with those of stronger rocks . o Layers of different thickness in the same rock may also cause partitioning of mechanical behavior. Shapes and wavelengths are strongly influenced by thickness H.D.A. Reyes | Correlation 1 Structural Geology Mechanical Behaviour of Rock Materials Why is elastic deformation temporary while plastic deformation is permanent?  Elastic deformation – controlled by chemical bonds  Plastic deformation – controlled by breaking of bonds and dislocation of atoms within the structure. H.D.A. Reyes | Correlation 1 Structural Geology Mechanical Behaviour of Rock Materials Factors controlling behaviour • Temperature ↑ = Yield stress ↓ = weakens rock. • Strain rate ↑ = Flow of stress level ↑ = promotes fracturing • Presence of fluids ↑ = Yield stress ↓ = weakens rock. • Confining pressure ↑ = accumulate larger finite strain before failure, favors plastic deformation • Grain size depends on microscale deformation but in plastic regime:  Grain size ↓ = strain weakening • In frictional regime:  Grain size ↓ = strain hardening H.D.A. Reyes | Correlation 1 Deformation Mechanisms and Microstructures • • • • When strain accumulates in a deforming rock, certain deformation processes occur at the microscale that allow the rock to change its internal structure, shape or volume. Strain is accommodated through the activation of one or more microscale deformation mechanisms. Intracrystalline deformation occurs within individual mineral grains. The smallest deformation structures of this kind actually occur on the atomic scale and can only be studied with the aid of the electron microscope. When deformation mechanisms produce microstructures that affect more than one grain, such as grain boundary sliding or fracturing of mineral aggregates, we have intercrystalline deformation. Intercrystalline deformation is particularly common during brittle deformation. H.D.A. Reyes | Correlation 1 Mechanical twinning • • Stress can result in mechanical bending or kinking of the crystal lattice of some minerals, even at very low temperatures Plagioclase and calcite are common mineral examples. These intracrystalline kink structures are expressions of strain and are also known as deformation twins and the process as mechanical twinning. Calcite commonly shows a type of deformation twin formed by twin gliding. Twin gliding involves simple shear along the twin plane The kinked and unkinked parts are mirror images of one another about this plane occurring at a shear stress just above a critical value of 10MPa. This means that calcite can deform by twinning at any level in the crust, even at the surface (and by hand for demonstration purpose), as long as the critical stress is reached. H.D.A. Reyes | Correlation 1 Crystal defects • • The atomic lattice of any mineral grain, deformed or not, contains a significant number of defects. This means that the crystal has energy stored in the lattice. The more defects, the higher the stored energy. There are two main types of defects: 1. Point defect - represented by either vacancies or, less importantly, impurities in the form of extra atoms in the lattice The point defect of interest to us is the one represented by a missing atom. Movement of vacancies is called diffusion. 2. Line defects/dislocation - a mobile line defect that contributes to intracrystalline deformation by a mechanism called slip (movement of a dislocation front within a plane) When a crystal is deformed by plastic deformation, the dislocation density increases. Deformation adds energy to the crystal, and a high density of defects implies that the crystal is in a high-energy state. H.D.A. Reyes | Correlation 1 Diffusion creep • • • • • Diffusion creep/diffusion mass transfer is a mechanism involving migration of vacancies in crystallographic lattices. Nabarro-Herring creep is a diffusion of vacancies through crystals. Grain size is important, particularly for Nabarro–Herring creep: the smaller the grains, the higher the strain rate The rate is not high (perhaps a few centimeters per million years). However, at some point vacancies will reach a grain boundary and disappear. Because vacancies migrate towards sites of maximum stress the crystals acquire a shape fabric or strain. During this process the crystal will gain regularity and turn into a more perfect crystal. Volume diffusion requires a lot of energy, so the migration rate is highly temperature dependent: high temperatures give high vibrations in the lattice, which increases the rate. H.D.A. Reyes | Correlation 1 • Grain boundary or Coble creep is a migration of vacancies occurs preferentially along grain boundaries. It is a bit less energy demanding than Nabarro–Herring creep, and is more important in the deformation of the plastic crust. H.D.A. Reyes | Correlation 1 • • Pressure solution (or dissolution) or wet diffusion bears similarities to Coble creep, and geometrically and mathematically it can be treated in the same way. However, in the case of pressure solution, diffusion occurs along a thin film of fluid along grain boundaries. Rocks exposed to wet diffusion experience a volume reduction. Wet diffusion is the main mechanism in what is known as chemical compaction. Wet diffusion is also common in limestones, where pressure solution seams known as stylolites form H.D.A. Reyes | Correlation 1 Dislocations and dislocation creep • A dislocation is a mobile line defect that contributes to intracrystalline deformation by a mechanism called slip. Slip implies movement of a dislocation front within a slip plane. • Slip planes are relatively weak crystallographic directions controlled by the atomic structure, and are usually the plane(s) in a crystal that have the highest density of atoms. H.D.A. Reyes | Correlation 1 Important: • Volume diffusion: vacancies move through crystals (temperature and stress controlled). • Grain boundary diffusion: vacancies move along grain boundaries (temperature and stress controlled). • Pressure solution: ions move in fluid films and pore fluid (chemically and stress controlled). H.D.A. Reyes | Correlation 1 Dislocations and dislocation creep • • • • • Studies of dislocations have shown that there are two different types of dislocations. 1. Edge dislocation - which is the edge of an extra half-plane in the crystal lattice 2. Screw dislocation - dislocation line is oriented parallel to the slip direction The formation, motion and destruction of dislocations in a crystal are all contained in the term dislocation creep. The process by which edge dislocations move is called dislocation glide. Dislocation creep allows the deformation to take place at much lower differential stress than that required for brittle fracturing. Dislocation movements do not damage or weaken the mineral. H.D.A. Reyes | Correlation 1 H.D.A. Reyes | Correlation 1 Microstructures • Atomic-scale deformation structures such as dislocations can only be studied by means of electron microscopy at 10 000–100 000 times magnification. • They are referred to as microstructures and carry information about temperature, state of stress and rheological properties at the time of deformation. H.D.A. Reyes | Correlation 1 Structural Geology Joint and Shear Fractures  Joint – Fractures along which there has been no appreciable displacement parallel to the fracture and only slight movement to the fracture plane      Three types of fracture Mode 1 – Opening Mode 2 – Sliding Mode 3 – Tearing (Shear Fractures) Mode 4 – Closing H.D.A. Reyes | Correlation 1 Structural Geology Joint and Shear Fractures Types of Joints  Systematic Joints - have subparallel orientation and regular spacing  Nonsystematic Joints – joints that so not share a common orientation and those with highly curved and irregular fracture surfaces.  Joint Set – Joints that share a similar orientation in the same area  Joint System - Two or more joint set in the same area.  Plumose Joint - exhibits feathered texture  Veins - Joints filled with minerals or aggregates.  Conjugate Joints – Joints that comes in pair (Unfilled and Filled) H.D.A. Reyes | Correlation 1 Structural Geology Joint and Shear Fractures H.D.A. Reyes | Correlation 1 Structural Geology Joint and Shear Fractures Formation of Fractures Griffith Theory - States that a crack will propagate when the reduction in potential energy that occurs due to crack growth is greater than or equal to the increase in surface energy due to the creation of new free surfaces. This theory is applicable to elastic materials that fracture in a brittle fashion. H.D.A. Reyes | Correlation 1 Structural Geology Joint and Shear Fractures Joint and Fracture Mechanism  Hackle Marks – indicates zones where the joint propagate rapidly  Arrest Lines – perpendicular to the direction of propagation and forms parallel to the advancing edge of the fracture  Four categories of joints according to Terry Engelder (1985)  Tectonic joints – Forms at depth with stress originate tectonically, and horizontal compaction occurs. Forms at depth less than 3km.  Hydraulic Joints - Forms at depth in response to abnormal fluid pressure arid involving hydrofracturing. Forms during burial and vertical compaction of sediment at depths greater than 5 km, where escape of fluid hindered by low permeability, which creates locally abnormally high pore pressure.  Unloading Joints – Forms when more than half of the original overburden has bee removed from a rock mass  Release Joints – Form late in the history of an area and are ultimately oriented perpendicular to the original tectonic compression that formed from he dominant fabric in the rock.  Nontectonic and Quasitectonic Fractures  Sheeting – forms subparallel to surface topography, generally in massive rocks and corresponds to the unloading joint of Engelder (1985)  Columnar Joints - Response to cooling and shrinkage of congealing magma  Mud Cracks or Desiccation Cracks – Shrinkage due to evaporation of water in unconsolidated sediments H.D.A. Reyes | Correlation 1 Structural Geology  Faults A fracture having an appreciable movement parallel to the plane of the fracture. Anatomy of Faults  Fault Plane - The actual movement surface  Hanging wall - The block resting on the fault plane  Foot wall - The block beneath the fault plane  Strike - Is the direction of the line formed by the intersection of a rock surface with a horizontal plane.  Dip - The acute angle that a rock surface makes with a horizontal plane H.D.A. Reyes | Correlation 1 Structural Geology Faults Anatomy of Faults  Dip-slip Component – down or up movement parallel to the dip direction of the fault  Strike-slip Component – movement parallel to the strike  Oblique Slip – The combination of Dip-slip and Strike-slip  Net-slip Component – (True displacement) the total amount of motion measured parallel to the direction of the motion  Rake or Pitch – Angle formed. H.D.A. Reyes | Correlation 1 Structural Geology Faults Anatomy of Faults  Heave – the horizontal component of dip separation measured perpendicular to strike of the fault.  Throw – the vertical component measured in the vertical plane containing the dip.  Slickensides – polished fault surface  Slickenfiber – Alligned fibrous minerals on a movement surface  Separation – The amount of apparent offset of faulted surface such as bed or a dip measured in specified direction. H.D.A. Reyes | Correlation 1 Structural Geology Faults Anderson Classification: Normal Fault H.D.A. Reyes | Correlation 1 Structural Geology Faults Anderson Classification: Normal Fault Normal fault – dip-slip fault in which the hanging wall has move down relative to the foot wall.    Graben – a block that moved down between to subparallel normal faults that dip towards one another. Horst – consist of two subparallel fault that dip towards each other so that the block in between remains high. Listric (Lag) Faults – A normal fault that exhibits steep dip near the surface but flattened with the depth. Concave-up surface. H.D.A. Reyes | Correlation 1 Structural Geology    Faults Anderson Classification: Normal Fault Splay – branching characteristics of thrust and strike-slip faults Synthetic Fault – dip in the same direction as the master fault and join the master fault at depth. Antithetic Fault - Join the master fault at depth, but dip in the opposite direction. H.D.A. Reyes | Correlation 1 Structural Geology Faults Anderson Classification: Normal Fault Growth Faults - Commonly form in relatively unconsolidated sediments during deposition and produce thickened stratigraphic units in the downthrown block. H.D.A. Reyes | Correlation 1 Structural Geology Faults Anderson Classification: Normal Fault Folds associated with normal fault  Monocline – A bend in rock strata that are otherwise uniformly dipping or horizontal.  Drape Fold - A normal fault may break and displace between rocks but die upward into the sedimentary cover.  Drag Folds – Form because of friction along the fault surface and occur along normal faults  Reverse drag folds and Rollover Anticline – form along growth faults where the part of the downthrown block close to the fault is displaced the downward more than the parts further away . H.D.A. Reyes | Correlation 1 H.D.A. Reyes | Correlation 1 Structural Geology   Faults Anderson Classification: Thrust Fault Thrust Fault – (30 degrees or less dip angle) Hanging wall moves upward relative to the foot wall Reverse Fault – (45 degrees or more dip angle) H.D.A. Reyes | Correlation 1 Structural Geology         Faults Anderson Classification: Thrust Fault Ramp - A high-angle segment which may occur on all scales crossing units from a few cm to a km or more thick. Break Thrust – form by faulting the connecting limb of an anticline or syncline pair, overthrusting the hanging-wall anticline, and preserving the footwall syncline Shear Thrust – Formed independently of folding but not having a footwall syncline Fault-bend Folds – A thrust sheet wherein dip flattens as it passes over a ramp. Fault-propagation Fold – Forms as layers fold during propagation of a thrust through a sedimentary sequence. Drag Folds – Folds that forms near the thrust surface during movement Horse or Slice – Piece of material. The age of horse material is usually intermediate between the age of the hanging-wall and the footwall. Imbricate Thrust – The smaller faults in a group of thrust faults. H.D.A. Reyes | Correlation 1 Structural Geology    Faults Anderson Classification: Thrust Fault Duplex – Occurs where two subparallel thrust of approximately equal displacement are separated by a deformed interval that is thin relative to its total areal extent. Roof Thrust – The upper of the two master faults Floor Thrust - The lower of the two master faults H.D.A. Reyes | Correlation 1 H.D.A. Reyes | Correlation 1 Structural Geology    Faults Anderson Classification: Thrust Fault Branch Line – The line of intersection of two fault surface Erosion or Emergent Thrust – Thrust fault the is terminated at the surface due to erosion. Blind Thrust – A thrust fault that does not manifest into the surface. Features Produced by Erosion  Simple Window or Fenster – Usually develops where thrust sheet has been antiformally folded, causing part of the sheet to have a higher elevation H.D.A. Reyes | Correlation 1 Structural Geology Faults Anderson Classification: Thrust Fault Features Produced by Erosion  Klippe – Dismemberment of a thrust sheet by erosion  Allochthon – A large remnant. Also used for a large single thrust sheet  Crystalline Thrust – Involve transport of metamorphic or igneous rocks, or both, as part or all of a thrust sheet H.D.A. Reyes | Correlation 1 Structural Geology Faults Anderson Classification: Strike-slip Fault Type of Strike-slip Faults  Left-lateral Strike-slip Fault or Sinistral  Right-lateral Strike-slip Fault or Dextral  Tear Fault – Bound the edge of thrust sheets.  En Echelon Strike-slip Fault  Pull-apart or Rhomb-graben or Rhombchasm – Right step-overs or Left step-overs  Push-up or Rhomb horsts – Right step-overs Sinistral Fault or Left step-over Dextral Fault H.D.A. Reyes | Correlation 1 Structural Geology Faults Anderson Classification: Strike-slip Fault Flowering Structure H.D.A. Reyes | Correlation 1 Andersonian classification Other terms which describe faults: 1. Radial faults – faults which converge or project toward a single point 2. Concentric faults – faults which form concentric to a point 3. Bedding faults/bedding-plane faults – faults which follow bedding or occur parallel to the orientation of bedding planes H.D.A. Reyes | Correlation 1 TRANSFER FAULTS • Transfer faults - strike-slip faults that transfer displacement from one fault to another. • In general, the term is used specifically for a particular type of strike-slip fault whose tips terminate against other faults or extension fractures. • They are bounded and cannot grow freely implying that their displacement increases relative to their length. • They occur in all scales and connect range of structures. o In small scales, they can connect open or mineral-filled extension fractures, veins, dikes, normal faults of the same or opposite dip directions, oblique faults, reverse faults and more. o In larger scales, transfer faults connect continental rift axes where they typically are associated with a change in dip for large rift faults • In mid-oceanic ridges, oceanic ridge valleys are shifted along transfer faults. They were given the name transform faults. H.D.A. Reyes | Correlation 1 H.D.A. Reyes | Correlation 1 TRANSFER FAULTS • Transform faults - large (kmscale or longer) strike-slip faults that segment plates or form plate boundaries. • They connect mid-ocean ridges to destructive plate (island arcs), or they connect two segments of a destructive plate boundary. • Transform faults that define plate boundaries can get very long. (e.g. San Andreas Fault, USA) • Large transform faults are actually fault zones rather than simple faults, and may consist of a number of more or less parallel faults of various lengths. H.D.A. Reyes | Correlation 1 H.D.A. Reyes | Correlation 1 TRANSCURRENT FAULTS • Transcurrent fault – strike-sip faults that have free-tips (they are ot constrained by other structures); their free tips move so that the fault length increases as displacement accumulates • Intraplate faults - free strike-slip faults that form within plates • Interplate faults – free strike-slip faults that occur along plate boundaries • Transcurrent faults can be expected to meet and interfere with other faults at some point during their growth history, but they will never have the special kinematic role that transform faults have. H.D.A. Reyes | Correlation 1 DEVELOPMENT AND ANATOMY OF STRIKE-SLIP FAULTS SINGLE FAULTS (SIMPLE SHEAR) • Strike-slip faults form when individual parts of the crust move at different rates along the surface of the Earth. • Several secondary structures are associated with strike-slip faults, and experiments have helped us explore some of the most important ones. RIEDEL’S CLAY EXPERIMENTS - Riedel used two stiff wooden blocks covered by a clay layer which were slid past one another . Stress was transferred to the overlying clay, which deformed progressively H.D.A. Reyes | Correlation 1 CONJUGATE STRIKE-SLIP FAULTS (PURE SHEAR) • Strike-slip faults can also form as conjugate sets, implying that they were active at about the same time under the same regional stress field. • Conjugate faults result from pure shear in the horizontal plane, where shortening in one direction is compensated by orthogonal extension in the other while no extension or contraction occurs in the vertical direction. H.D.A. Reyes | Correlation 1 FAULT BENDS AND STEPOVERS • Natural strike-slip faults show geometrical irregularities as a consequence of irregularities generated by linkage of fault segments. • When individual fault segments overlap and link, a local deviation from the general fault trend is established in the form of a fault stepover or fault bend. • Contractional or extensional structures form in such bends, depending on the sense of slip on the fault relative to the sense of stepping. - Contractional structures include stylolites, cleavages, folds and reverse faults, and they form in restraining bends. restraining bends are recognized as areas of positive relief - Subparallel reverse or oblique-slip contractional faults bounded by the two strike-slip segments can form and are called contractional strikeslip duplexes. - Releasing bends produce extensional structures such as normal faults and extension fractures. Series of parallel extensional faults bounded on both sides by strike-slip faults are called extensional strike-slip duplexes which generate negative structures. H.D.A. Reyes | Correlation 1 H.D.A. Reyes | Correlation 1 SEISMIC IMAGING AND FLOWER STRUCTURES • Pure strike-slip faults can be difficult to detect from seismic data alone, not only because most are too steep to set up reflections on seismic profiles, but also horizontal layers or layers striking parallel to the fault show no displacement in the vertical direction. • The clue is then to look for restraining and releasing bends, where vertical movements are associated with normal faults, reverse faults or folds. • A characteristic feature of such bends is their tendency to split and widen upward. These structures are called flower structures. o Flower structures that are associated with restraining bends are called positive, and those associated with releasing bends are called negative flower structures. H.D.A. Reyes | Correlation 1 H.D.A. Reyes | Correlation 1 TRANSPRESSION AND TRANSTENSION • Bends in strike-slip faults can produce local components of contraction or extension. • The type of deformation occurring in such bends is referred to as transpression and transtension. They can dominate the full length of the strike-slip fault if the fault (or shear zone) is not purely strike-slip (simple shear). It contains an additional component of shortening or extension perpendicular to the fault plane. • Transpression is the spectrum of combinations of strike-slip and pure contraction. • Transtension is the combinations of strike slip with extension. H.D.A. Reyes | Correlation 1 THE PHILIPPINE FAULT ZONE • The Philippine Fault Zone (PFZ) is a ~1250 km NNWtrending left lateral strikeslip fault which transects the Philippine archipelago from N to S. • The Philippine Fault Zone is a major inter-related system of faults throughout the whole of the Philippine Archipelago, primarily caused by tectonic forces compressing the Philippines into the Philippine Mobile Belt. H.D.A. Reyes | Correlation 1 HISTORICAL EARTHQUAKES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 1990 Luzon Earthquake (7.8) 1645 Luzon Earthquake (7.5) 1968 Casiguran Earthquake (7.3) 2013 Bohol Earthquake (7.2) 1994 Mindoro Earthquake (7.1) 2012 Visayas Earthquake (6.9) 2002 Mindanao Earthquake 1983 Luzon Earthquake (6.5) 1976 Moro Gulf earthquake & tsunami (8.0) 2012 Samar Earthquake (7.6) H.D.A. Reyes | Correlation 1 H.D.A. Reyes | Correlation 1 Criteria for faulting • Existence of a fault at a particular locality is commonly demonstrated by the recognition of certain features and diagnostic characteristics. • If faults are active, we need only two criteria: 1. Earthquakes or aseismic creep and 2. Associated ground breakage. H.D.A. Reyes | Correlation 1 Criteria for faulting Since most faults are inactive, we must use other means of recognizing them: 1. Repetition and omission of stratigraphic units, or the displacement (offset) of a recognizable marker. H.D.A. Reyes | Correlation 1 Criteria for faulting 2. Truncation of structures, beds, or rock units against some feature. Truncation of structures on both sides of a discontinuity would most likely indicate that the boundary is a fault . H.D.A. Reyes | Correlation 1 Criteria for faulting 3. Occurrence of fault rocks mylonite and cataclasite (or both), along a suspected fault zone is another good criterion for faulting. 4. Abundant veins, silicification or other mineralization along a fracture zone suggest that a faulting has occurred, but mineralization is a poor criterion because it may occur in fractures having no offset. H.D.A. Reyes | Correlation 1 Criteria for faulting 5. Drag, which is produced along a fault as units appear to be pulled into a fault during movement. H.D.A. Reyes | Correlation 1 Criteria for faulting 6. Slickensides and slickenlines along a fault surface often indicate movement but do not prove significant faulting because they may form on bedding surfaces during certain types of folding or joints that have a small shear displacement. H.D.A. Reyes | Correlation 1 Criteria for faulting 7. Fault scarp form where a topographic surface is offset by dipslip motion along a fault and directly indicate the movement sense of the fault. H.D.A. Reyes | Correlation 1 Criteria for faulting 8. A fault scarp may evolve through time into a fault-line scarp by differential erosion along the fault which will lower the level of the topographic surface and may remove a resistant layer in the hanging wall. o Resequent fault-line scarp – a fault-line scarp which erosion preserves the original facing direction of the fault scarp o Obsequent fault-line scarp – a fault-line scarp that through erosion of a resistant layer faces opposite the direction fault scarp. An incorrect motion sense would be inferred from the topography alone H.D.A. Reyes | Correlation 1 Role of fluids • M. King Hubbert and William W. Rubey (1959) demonstrated that fluids plays an important role in faulting. • They recognized that the “lubricating” effect of fluid in a fault zone is really a buoyancy effect that reduces the shear stress necessary to permit the fault to slip, as the fluid pressure reduces the normal stress on the fault plane. • The resulting stress is effective normal stress (S), determined by subtracting the fluid pressure (P) from the normal stress (𝜎𝑛 ). • The effect of fluid on movement is illustrated vividly by landslides and snow avalanches. Conditions favoring landslides may exist for centuries in an area without initiating landslides, then the influx of a large amount of water into the susceptible materials may reduce the effective normal stress and trigger a landslide. H.D.A. Reyes | Correlation 1 Movement mechanisms • • • Movement on faults occurs in at least 2 different ways: 1. Stick slip (unstable frictional sliding) – involves sudden movement on the fault after long-term accumulation of stress (Scholz, 1990); this mechanism and the accompanying elastic rebound are what we think causes earthquakes 2. Stable sliding (continuous creep) – involves uninterrupted motion along the fault, so that stress is relieved continuously and does not accumulate; the difference in behavior may be produced along segments of the same active fault undergoing stable sliding where groundwater is abundant, but other segments may move by stick slip Withdrawal of groundwater may cause near-surface segments of active faults to switch mechanisms, from stable sliding to stick slip, thereby increasing the earthquake hazard. Pumping fluid into a fault zone has been proposed as a way to relieve accumulated elastic strain energy and reduce the likelihood of a large earthquake. H.D.A. Reyes | Correlation 1 Structural Geology Folds Folding – is the bending or warping of rock strata caused by compressive stress. The structure that develops is called a fold. H.D.A. Reyes | Correlation 1 Structural Geology Folds H.D.A. Reyes | Correlation 1 Structural Geology Types of fold: Monocline – a double flexure connecting strata at one level with the same strata at another level. H.D.A. Reyes | Correlation 1 Structural Geology Anticline – an arch-shaped fold Syncline – a trough-shaped fold. H.D.A. Reyes | Correlation 1 Structural Geology Fold nomenclature: a) Limbs – the two sides or legs of the fold b) Axis – the direction of an imaginary line connecting the points of maximum flexure of the fold c) Axial plane – an imaginary plane containing all the fold axes within a deformed layer of rock layers. The axes of most folds are inclined. The angle of dip of its axis is the plunge. H.D.A. Reyes | Correlation 1 Structural Geology Anticlines and synclines are symmetrical if their limbs have approximately equal dips. If one limb is steeper than the other, the fold is asymmetrical. Overturned fold – An aysmmetrical fold in which one limb has been tilted beyond the vertical. H.D.A. Reyes | Correlation 1 Structural Geology Recumbent fold – a fold in which the axial plane has been overturned. Isoclinal fold – fold in which the beds on both limbs are nearly parallel, whether upright, overturned or recumbent. H.D.A. Reyes | Correlation 1 Structural Geology The sedimentary rocks covering much of the continental interiors have been mildly warped into: Domes – circular or elliptical structural or topographic highs in which beds dip away to all directions; when eroded, the oldest rocks are exposed at the center. Basins – circular or elliptical structural or topographic lows or downwarps in which beds dip towards the center; when eroded, the youngest rocks are exposed at the center. A warped plane Outcrop pattern of an eroded dome and basin structure H.D.A. Reyes | Correlation 1 FOLD MECHANISMS AND ACCOMPANYING PHENOMENA FOLD MECHANISMS: 1. FLEXURAL SLIP o Layers play a dominant role in folding at low temperature and pressure at shallow depths of the Earth. o For layers to maintain constant thickness during folding of a mass of uniformly layered strong rocks (competent layers), they must slip past one another. o Where anisotropy in successive layers occur, flexural slip still occurs, which may produce crumpling in the hinges of the folds, thickening of the layers without ductile flow of the material. o If flexural slip does occur, the fibers may be oriented perpendicular to fold hinge lines but may not be perpendicular because of the variations in the direction of motion between layers. H.D.A. Reyes | Correlation 1 H.D.A. Reyes | Correlation 1 2. FLEXURAL FLOW o Some layers flow ductilely while others remain brittle and buckle. o Requires moderate to high ductility contrast between layers. As a result, some strong layers may not undergo appreciable thickness changes, and the stronger layers control buckling and fold geometry. o Fold amplitude and wavelength may be controlled by the original thickness, spacing, and strength of the strong layers, but they will also be modified by the amount of ductile strain parallel to the axial surfaces H.D.A. Reyes | Correlation 1 H.D.A. Reyes | Correlation 1 3. BENDING o It is a mechanism that involves application of force across layers. o Generally, it produces folds that are very gentle with large interlimb angles. o (Ramberg, 1963 and Ramsay, 1967) Involves passive flow across layers in response to transverse forces and the way folds are generated. o Folds produced by bending are common in continental interiors where vertical forces may be directed at high angles to originally horizontal bending producing broad domes, basins, swells and arches so common in cratons. o In rocks subjected to bending, layers are bent like an elastic beam that has been supported at the ends and loaded in the middle. H.D.A. Reyes | Correlation 1 H.D.A. Reyes | Correlation 1 4. BUCKLING or ACTIVE FOLDING (CLASS 1B FOLDS) o Force is applied parallel to layering in rocks and the easiest direction of relief is normal to the direction of force of application. The layers are said to buckle, and the structures formed are called buckle folds. o Flexural slip commonly accompanies buckling at low T & P resulting to sinusoidal parallel and parallel-concentric folds. o Buckling of a stiff layer (with high viscosity, strength and competence) embedded in a less stiff medium may occur at higher temperature and pressure without flexural slip. Results are similar-like folds H.D.A. Reyes | Correlation 1 H.D.A. Reyes | Correlation 1 5. PASSIVE SLIP (CLASS 2 FOLDS) o Described as slip at an angle to layering that results in new cleavage or schistosity that accumulates movement parallel to the new surface. o Folds related to this movement mechanism is called passive slip or shear folds. o Bedding or compositional layering serves only as a strain marker that records displacement parallel to the cleavage or schistosity o Passive folding produces harmonic folds where the where the influence on the fold shape. H.D.A. Reyes | Correlation 1 H.D.A. Reyes | Correlation 1 H.D.A. Reyes | Correlation 1 6. KINK and FOLDING CHEVRON o Form in regular geometric forms, either as isolated structures or as conjugate (paired )sets. o Common in well-laminated and anisotropic rocks rich in phyllosilicate minerals. o Kink bands are centimeter- to decimeter-wide zones or bands with sharp boundaries across which the foliation is abruptly rotated. o Kink bands are closely related to chevron folds but differ in terms of symmetry. o Chevron folds initiate with their axial surface perpendicular to the shortening direction while kink bands form oblique to this direction, typically in conjugate pairs. H.D.A. Reyes | Correlation 1 7. PASSIVE FLOW (PASSIVE AMPLIFICATION) o Involves uniform ductile flow of an entire rock mass, with layering (bedding, foliation or gneissic banding) serving only as a strain marker. o There must be little or no ductility contrast between layers, even if the composition differs markedly, and there must be flow across the layering. o The mechanism grades into flexural flow as ductility contrast increases and the stronger layers begin to influence the shape of the folds. o Most likely mechanism for ideal similar folds. 8. COMBINED MECHANISMS o Fold mechanisms compete with one another at various T & P and in various rock types. o Passive flow and flexural flow may compete at higher temperature in normal silicate rocks. o In a rock mass being folded under near-surface conditions, flexural slip and buckling may be initiated. o Cleavage forming mechanisms may take over and produce shortening and flow parallel to the axial surfaces of the folds, occurring in the outer parts of metamorphic cores o Buckling plus flexural slip is probably the most usual combination of folding processes H.D.A. Reyes | Correlation 1 Structural Geology References Hatcher, R.D., Structural Geology: Principles, Concepts and Problems, 2nd Edition, Prentice Hall, New Jersey Hugget, R.J., Fundamentals of Geomorpholoy, 3rd Edition, Routledge, New York Fossen, H., Structural Geology, 2010, Cambridge University Press, New York H.D.A. Reyes | Correlation 1