Faults PDF

FAULTS 2A 1. Definition of a Fault A fault is a planar fracture in rock where blocks on either side have moved parallel to the fracture. Faults can range from microscopic size to thousands of kilometers long. Characteristics: o Faults are often not perfect...

FAULTS 2A 1. Definition of a Fault A fault is a planar fracture in rock where blocks on either side have moved parallel to the fracture. Faults can range from microscopic size to thousands of kilometers long. Characteristics: o Faults are often not perfectly planar but are curved (curviplanar). o Movement along the fault can vary, for example, normal faults. o Faults are surrounded by a damage zone with less severe deformation extending into the surrounding rock (country rock), important for engineering projects. o Faults are brittle, but they may grade into ductile shear zones and can be associated with folds and other ductile strain phenomena. 2. Terminology Orientation: Describes the fault's direction using strike, dip angle, and dip direction. Footwall: The block of rock below the fault plane. Hangingwall: The block of rock above the fault plane. Slip: The actual relative displacement that occurred along the fault. Separation: The apparent displacement viewed along a specific direction (either horizontal or vertical). 3. Types of Slip Net Slip (ns): The total movement along the fault. Dip Slip (ds): Movement along the dip direction (up or down the fault plane). Strike Slip (ss): Horizontal movement along the strike direction of the fault. Specific Fault Types: Normal Fault: The hanging wall moves down relative to the footwall. Thrust (Reverse) Fault: The hanging wall moves up relative to the footwall. Strike-Slip Fault: Movement is primarily horizontal: o Dextral (right-lateral): The opposite side of the fault moves to the right. o Sinistral (left-lateral): The opposite side of the fault moves to the left.  Oblique-Slip Fault: Movement involves both vertical (dip) and horizontal (strike) components. Rotational (Scissor) Fault: One side of the fault rotates relative to the other. 4. Separation Separation refers to the offset of rock layers along a fault: o Dip Separation: The vertical displacement across the fault. o Strike Separation: The horizontal displacement across the fault. o Throw (t): The vertical component of separation. o Heave (h): The horizontal component of separation. Right-lateral vs Left-lateral: Terms used to describe the relative movement across the fault based on the horizontal separation. 5. Difference Between Slip and Separation Slip refers to the actual movement along the fault, whereas separation is the apparent movement based on how the displaced layers appear. In some cases, a dip-slip fault and a strike-slip fault can show the same apparent separation in a map view, but the actual slip differs. 6. Map Symbols for Faults Faults are represented with specific symbols in geological maps to show the direction and type of movement (e.g., arrows for strike-slip faults, tick marks for normal or thrust faults). 7. Loss and Gain of Ground Loss of Ground: Areas where the ground surface has dropped due to faulting. Gain of Ground: Areas where the ground surface has risen. 8. Fault Termination A fault typically terminates when: o It intersects another structure (e.g., tear faults in a thrust sheet or folds). o Displacement decreases to zero, forming a tip line (where the fault ends on a map or section). Splay Fault: A major fault may branch into smaller faults, which also terminate at tip lines. Blind Fault: A fault that is buried and not exposed at the surface but may cause distortion of adjacent layers (e.g., folding or monoclines). 9. Summary 1. Faults are planar to curviplanar fractures where displacement occurs. 2. Fault orientation is measured as for any plane (strike and dip). 3. The rock is divided into footwall (below) and hangingwall (above) blocks. 4. Slip refers to actual displacement; it can be categorized into net slip, strike-slip, dip- slip, oblique-slip, etc. 5. Separation is the apparent displacement viewed from a specific direction and can be described as throw, heave, strike-separation, or lateral movement. 6. Apparent displacement is influenced by the fault orientation and the relative movement. 7. Faults either terminate against other structures or gradually taper off along a tip line. 8. Faulting can result in loss or gain of ground, especially relevant in mining and construction. FAULTS 2B 1. Recognizing a Fault Faults can be identified by several physical features: Topographic Lineaments: Sharp linear features in the landscape, often visible as: o Fault Scarps: Steep steps or cliffs caused by displacement along a fault. Common in arid regions, they are usually only a few meters high but can be as much as 15 meters. Truncation and Offset of Geological Contacts: Layers or formations appear cut off or offset where a fault displaces them. Inconsistent Stratigraphic Patterns: Unusual rock layer sequences indicate a fault, with two main types: o Repetition: Normal rock sequence appears repeated (e.g., ABCABC). o Omission: Part of the normal rock sequence appears missing (e.g., ABC missing D, then EFG). Slickenside Surfaces: Smooth, polished surfaces created by movement along a fault, often with: o Striations: Linear grooves or scratches indicating movement direction. o Slickenfibres: Mineral fibers that grow during fault movement, also showing movement direction.  Fault Rocks: Rocks created or modified by faulting, each with unique characteristics. 2. Types of Fault Rocks Fault rocks form through the physical processes of fault movement: Breccia: Made of broken, crushed rock fragments of different sizes, typically found near the surface or in less confined faults. Fault Gouge: Fine-grained, clay-rich powder formed by intense grinding of rock fragments; usually soft and less cohesive. Cataclasite: A cohesive, often foliated rock with angular fragments in a fine-grained matrix formed by crushing. Pseudotachylite: A glassy melt rock formed by frictional heating due to rapid fault movement (like during earthquakes); can indicate ancient seismic activity. Mylonite: A type of fault rock formed through ductile flow at higher temperatures and pressures, where minerals deform plastically rather than fracturing. 3. Why Fault Rocks Vary Fault rock characteristics depend on depth, temperature, pressure, fluid presence, and strain rate: Depth and Temperature: At shallower depths, fault rocks tend to be brittle (e.g., breccia, gouge). At greater depths and temperatures (250-300°C or 10-15 km depth in continental crust), rocks transition to ductile behavior, forming mylonite in shear zones. Pressure (P) and Fluid Presence: Higher pressures and the presence of fluids can influence fault rocks by enhancing ductile behavior and altering rock composition. Strain Rate: Rapid strain rates tend to favor brittle fracture (e.g., pseudotachylite from rapid slip), while slower strain rates allow for ductile deformation. 4. Shear Zones and Depth Shear Zones: These are ductile equivalents of faults and become wider with increasing depth and temperature. Instead of breaking, rocks in shear zones deform by crystalplastic flow, creating mylonites and other ductile fault rocks. 5. Summary 1. Faults can be recognized by scarps, topographic features, offsets, stratigraphic inconsistencies, slickenside surfaces, and fault rocks. 2. Fault rocks include breccia, gouge, cataclasite, pseudotachylite, and mylonite, each formed under different faulting conditions. 3. Comminution (breaking and grinding into finer grains) is common in fault zones.  4. Fault rock types vary with depth, temperature, pressure, and fluid presence. 5. Ductile processes become significant at temperatures above 250-300°C and depths around 10-15 km, creating shear zones and crystalplastic flow structures. FAULTS 2C 1. Kinematics of Faulting Kinematics studies the orientation and movement on faults relative to the principal stress axes (σ1, σ2, σ3): o Principal Stress Axes: ▪ σ1: Maximum compressive stress. ▪ σ2: Intermediate stress. ▪ σ3: Minimum stress. Conjugate Faults: Faults form at an angle to each other, creating a conjugate pair with opposite slip directions. o Angle of Faults: Faults form an acute angle (~60°) with σ1 bisecting this angle. 𝛗 o Conjugate Angle Formula: 2θ = 60˚, θ = 45˚- 2 where φ is the internal angle of friction (~30°). Other Fractures in Faulting**: o Joint (Tension Gash): Forms parallel to σ1, where rock is pulled apart. o Stylolite (Dissolution Surface): Forms parallel to σ3, where rock dissolves under pressure. o Late Joint: Forms perpendicular to σ1, often due to unloading after faulting. 2. Anderson’s Fault Classification Anderson’s Theory: Fault type depends on which principal stress (σ1, σ2, σ3) is vertical, as Earth’s surface can’t have horizontal shear stresses. This classification identifies three main types: 1. Normal Fault: ▪ σ1 is vertical (largest force pushes down). ▪ Forms at a ~60° angle. ▪ Typical in divergent settings (e.g., rift zones).  2. Reverse/Thrust Fault: ▪ σ3 is vertical (smallest force is downward). ▪ Forms at a ~30° angle. ▪ Common in convergent boundaries (e.g., mountain belts). 3. Strike-Slip Fault: ▪ σ2 is vertical (intermediate force is downward). ▪ Faults are vertical (~90° dip). ▪ Found in transform boundaries where lateral movement occurs. 3. Complexities in Fault Orientation Real-world faults often deviate from Anderson’s simplified model due to: Anisotropy: Rock layers and structures can influence fault direction. Block Rotations: Faulting can rotate blocks, affecting stress and fault geometry. Reactivation of Pre-existing Faults: Older faults can slip again in new stress conditions. Fault Hierarchies: Large fault systems may contain smaller faults with different orientations or types, influenced by varying stress and temperature conditions in the crust. 4. Role of Fluid Pressure in Faulting Fluid Pressure in fault zones decreases confining pressure (σN), which allows faults to slip more easily. o Example: In the Witwatersrand goldfields of South Africa, fluid pressure helped lower friction, facilitating fault movement. 5. Summary 1. Fault kinematics follow predictable patterns, with conjugate faults at a ~60° angle and σ1 bisecting the acute angle. 2. Anderson’s classification defines faults by which principal stress is vertical: σ1 for normal faults, σ3 for reverse faults, and σ2 for strike-slip faults. 3. Faults typically dip at 30° (thrust), 60° (normal), and 90° (strike-slip), but variations arise due to geological complexities. 4. Fluids in fault zones are crucial as they reduce friction, aiding in fault movement. FAULTS 2D 1. Analysis of Fault Zones Fault zones can have complex structures that go beyond simple, planar faults. Key features include: Non-Planar Faults: Faults with curved or irregular surfaces. Secondary Faults: Smaller faults that develop in association with a larger fault. Folds Related to Faults: Folds that form due to fault movement. Growth of Faults: The process by which faults extend or accumulate displacement over time. Fault Duplexes: Complex fault structures with multiple overlapping fault segments. Hybrid Fault Features: Combination structures such as transtension (extension and strike-slip) and transpression (compression and strike-slip). 2. Non-Planar Faults Non-planar faults have irregular geometries and include several types: Flat-and-Ramp Geometry: The fault alternates between flat and steep (ramp) sections. This often leads to the formation of a ramp anticline fold (a bend in the rock layers above the ramp). Fault Bends: Changes in fault direction can form bends, affecting the stress and strain patterns around the fault. Fault Terminations: o Tip Line/Tip Point: The endpoint of a fault where displacement drops to zero. o Splay Fault: A branch off the main fault that usually ends in a dead end, with displacement decreasing to zero at the tip line. o Blind Fault: A fault that doesn’t reach the surface, making it hard to detect, although nearby layers may be distorted. Listric Faults: Concave-upward faults that steeply dip near the surface but flatten with depth. This creates a “space problem” that is often solved by: o Roll-Over Antiform: A fold in the hanging wall above the listric fault. o Antithetic Faults: Smaller faults that develop in the opposite direction to relieve space constraints. 3. Secondary Faults Secondary faults form due to the influence of a main fault and have different movements or orientations: Synthetic Faults: Parallel to the main fault and follow its movement.  Antithetic Faults: Opposite to the main fault’s movement. Riedel Faults: Common in strike-slip fault zones. They form small fractures at ~30° to the main fault and include: o R1 (Synthetic Riedel): Same movement direction as the main fault. o R2 (Antithetic Riedel): Opposite movement direction. 4. Folds Related to Faults Fault movement often causes folds to form in adjacent rock layers: Normal Drag Fold: Forms in normal faulting as layers bend in the direction of fault slip. Reverse Drag Fold: Forms in reverse faulting as layers bend against the direction of fault slip. These folds indicate how the rock layers have responded to fault movement and can help in understanding the fault’s kinematics. 5. Summary 1. Fault zones include non-planar faults, secondary faults, fault-related folds, and complex hybrid features. 2. Non-planar faults have various shapes, such as flat-ramp and listric faults, with unique structural adaptations like roll-over antiforms. 3. Secondary faults include synthetic and antithetic faults, with Riedel faults forming in response to strike-slip movements. 4. Fault movement can cause drag folds in rock layers, providing clues to the fault’s motion and stress direction. FAULTS 2E