Geology Faults and Terminology Quiz

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Questions and Answers

What geological feature is indicated by layers or formations appearing cut off or offset?

Omission

Inversion

Truncation (correct)

Repetition

Which type of fault rock is characterized by fine-grained, clay-rich powder resulting from intense grinding?

Breccia

Pseudotachylite

Mylonite

Fault Gouge (correct)

What is the primary difference between breccia and mylonite in terms of their formation conditions?

Breccia is formed at high temperatures and pressures, while mylonite is formed at low depths.

Both breccia and mylonite form exclusively through gradual deformation at shallow depths.

Breccia is glassy and formed by rapid fault movement, while mylonite is cohesive and formed from clay.

Breccia forms from brittle processes, whereas mylonite forms from ductile behavior. (correct)

Which of the following features is NOT associated with slickenside surfaces?

Core stones Signup and view all the answers

What characteristic of fault rocks is affected by the presence of fluids?

They enhance ductile behavior. Signup and view all the answers

What is the primary difference between pseudotachylite and cataclasite?

Pseudotachylite is glassy due to rapid frictional heating, while cataclasite consists of crushed angular fragments. Signup and view all the answers

At what depth and temperature do rocks typically transition to ductile behavior to form mylonite?

250-300°C and 10-15 km depth Signup and view all the answers

What happens to rocks in shear zones as depth and temperature increase?

They deform by crystalplastic flow. Signup and view all the answers

How does the strain rate affect the behavior of fault rocks during deformation?

High strain rates tend to promote brittle fracture. Signup and view all the answers

Which type of fault is characterized by σ1 being vertical?

Normal Fault Signup and view all the answers

At what temperatures do ductile processes become significant in fault zones?

Above 250-300°C Signup and view all the answers

What is a characteristic feature of a reverse fault?

The smallest force acts downward. Signup and view all the answers

Which fault type typically forms at an acute angle of about 60° with respect to the principal stress axis?

Normal Fault Signup and view all the answers

What is a defining feature of slickenside surfaces?

They form from the grinding of rocks along a fault. Signup and view all the answers

What defines the angle of conjugate faults according to the conjugate angle formula?

60° Signup and view all the answers

What type of fracture forms parallel to the maximum principal stress, σ1?

Joint (Tension Gash) Signup and view all the answers

What characterizes a fault in geological terms?

A fault can be both brittle and ductile. Signup and view all the answers

Which description best fits a normal fault?

The hanging wall moves down relative to the footwall. Signup and view all the answers

What does the term 'separation' refer to in the context of faults?

The offset of rock layers along a fault. Signup and view all the answers

In a strike-slip fault, what is the primary direction of movement?

Horizontal movement along the strike direction. Signup and view all the answers

What is the hanging wall in relation to a fault?

The block above the fault plane. Signup and view all the answers

Which type of slip involves movement both vertically and horizontally?

Oblique-Slip Fault. Signup and view all the answers

What is indicated by right-lateral motion in a fault?

The opposite side of the fault moves to the right. Signup and view all the answers

What is the difference between 'throw' and 'heave' in fault terminology?

Throw refers to the vertical displacement, heave refers to the horizontal displacement. Signup and view all the answers

What characterizes a strike-slip fault?

The principal stress σ2 is vertical, allowing lateral movement. Signup and view all the answers

Which factor can influence the orientation of real-world faults?

Anisotropy of rock layers Signup and view all the answers

What is one example of how fluid pressure impacts faulting?

It decreases friction, facilitating fault movement. Signup and view all the answers

What is a common angle formed by conjugate faults?

~60° Signup and view all the answers

Which of the following best describes non-planar faults?

Irregular faults with complex geometries. Signup and view all the answers

What role does block rotation play in faulting?

It influences stress distribution and fault geometry. Signup and view all the answers

What is meant by a fault duplex?

A complex fault structure with overlapping fault segments. Signup and view all the answers

Why might older faults be reactivated under new conditions?

Changes in stress can cause older faults to slip again. Signup and view all the answers

What characterizes a ramp anticline fold?

It is a bend in the rock layers above a ramp. Signup and view all the answers

Which type of fault does NOT extend to the surface?

Blind Fault Signup and view all the answers

What is the function of antithetic faults in relation to main faults?

They develop in the opposite direction to the main fault’s movement. Signup and view all the answers

What is a characteristic of listric faults?

They flatten with depth. Signup and view all the answers

In the context of fault movement, what does a Normal Drag Fold signify?

It results from layers bending in the direction of fault slip. Signup and view all the answers

Which of the following describes the relationship between synthetic faults and main faults?

They are parallel to and follow the same movement as the main fault. Signup and view all the answers

What is the main purpose of analyzing folds related to faults?

To understand the fault's kinematics and stress direction. Signup and view all the answers

Riedel faults are commonly found in which type of fault zones?

Strike-slip fault zones. Signup and view all the answers

Study Notes

Fault Definition

A fault is a fracture in the Earth's crust where rocks on opposite sides have moved parallel to the fracture.
Faults can exist at varying scales, from microscopic to thousands of kilometers long.
Faults are often curved, not completely flat, and surrounded by a zone of damage extending into the surrounding rock.
Faults are brittle, but can transition into ductile shear zones and associate with folds and other ductile strain phenomena.

Fault 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).

Fault Types by 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.
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:
- Dextral (right-lateral): The opposite side of the fault moves to the right.
- 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.

Fault Separation

Dip Separation: The vertical displacement across the fault.
Strike Separation: The horizontal displacement across the fault.
Throw (t): The vertical component of separation.
Heave (h): The horizontal component of separation.

Fault Recognition

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:
- Repetition: Normal rock sequence appears repeated (e.g., ABCABC).
- 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:
- Striations: Linear grooves or scratches indicating movement direction.
- Slickenfibres: Mineral fibers that grow during fault movement, also showing movement direction.
Fault Rocks: Rocks created or modified by faulting, each with unique characteristics.

Fault Rock Types

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.

Fault Rock Formation

Depth and Temperature: At shallower depths, fault rocks tend to be brittle (e.g., breccia, gouge). At greater depths and temperatures, 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.

Shear Zones and Depth

Shear Zones: These are ductile equivalents of faults and become wider with increasing depth and temperature. Rocks in shear zones deform by crystalplastic flow, creating mylonites and other ductile fault rocks.

Fault Kinematics

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.
Angle of Faults: Faults form an acute angle (~60°) with σ1 bisecting this angle.
Conjugate Angle Formula: 2θ = 60˚, θ = 45˚- φ/2 where φ is the internal angle of friction (~30°).

Other Fractures Associated with Faulting

Joint (Tension Gash): Forms parallel to σ1, where rock is pulled apart.
Stylolite (Dissolution Surface): Forms parallel to σ3, where rock dissolves under pressure.
Late Joint: Forms perpendicular to σ1, often due to unloading after faulting.

Anderson's Fault Classification

Normal Fault: σ1 is vertical (largest force pushes down). Forms at a ~60° angle. Typical in divergent settings.
Reverse/Thrust Fault: σ3 is vertical (smallest force is downward). Forms at a ~30° angle. Common in convergent boundaries.
Strike-Slip Fault: σ2 is vertical (intermediate force is downward). Faults are vertical (~90° dip). Found in transform boundaries.

Complexities in Fault Orientation

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.

Role of Fluid Pressure in Faulting

Fluid pressure in fault zones decreases confining pressure (σN), which allows faults to slip more easily.

Analysis of Fault Zones

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).

Non-Planar Faults

Flat-and-Ramp Geometry: The fault alternates between flat and steep (ramp) sections. This often leads to the formation of a ramp anticline fold.
Fault Bends: Changes in fault direction can form bends, affecting the stress and strain patterns around the fault.
Fault Terminations:
- Tip Line/Tip Point: The endpoint of a fault where displacement drops to zero.
- Splay Fault: A branch off the main fault that usually ends in a dead end.
- Blind Fault: A fault that doesn’t reach the surface.
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:
- Roll-Over Antiform: A fold in the hanging wall above the listric fault.
- Antithetic Faults: Smaller faults that develop in the opposite direction to relieve space constraints.

Secondary Faults

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:
- R1 (Synthetic Riedel): Same movement direction as the main fault.
- R2 (Antithetic Riedel): Opposite movement direction.

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.

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Test your knowledge on faults in geology with this informative quiz. Explore the definitions, terminology, and types of faults based on their characteristics and movements. Perfect for students studying geology or earth sciences.