5 Module IV: Brittle Deformation

5.1 Key Concept Summary

Brittle deformation occurs when rocks fracture rather than flow. Understanding fracture mechanics, failure criteria, and fault behavior is essential for predicting earthquakes, interpreting seismic data, and understanding fluid flow in the crust. Joints, veins, and faults record the stress history and fluid-rock interactions.

5.2 Self-Test Questions

5.2.1 Deformation Mechanisms

Can you describe the major brittle deformation mechanisms?

Consider:

Cataclastic flow: grain-scale fracturing
Grain crushing and rotation
Pressure solution
Mechanical twinning
At what conditions does each dominate?

Can you describe the major ductile deformation mechanisms?

Think about:

Diffusion creep: temperature and grain-size dependent
Dissolution creep: involves fluids
Dislocation creep: power-law relationship
Recovery and recrystallization processes
How do these differ from brittle mechanisms?

5.2.2 Fracture Fundamentals

Can you describe the four modes of fracture?

Consider:

Mode I: Opening (tensile)
Mode II: In-plane shear
Mode III: Out-of-plane shear
Mixed modes
Which modes relate to joints, and which to faults?

Can you explain Griffith crack theory and its implications?

Think about:

Why are rocks much weaker than theoretical strength?
Role of pre-existing defects and cracks
Stress concentration at crack tips
Why is tensile strength much lower than compressive strength?

Can you distinguish between tensile and shear fractures?

Tensile fractures (joints):

Perpendicular to \(\sigma_3\) (minimum principal stress)
Mode I opening
No shear displacement
Extension fractures

Shear fractures (faults):

At angle to principal stresses
Mode II and/or Mode III
Shear displacement
Angle controlled by friction

5.2.3 Failure Criteria

Can you explain and apply the Mohr-Coulomb failure criterion?

Consider:

\(\tau = C + \sigma_n \cdot \tan \phi\)
\(C\): cohesion
\(\phi\): angle of internal friction
Linear failure envelope
How to use it to predict failure

Can you explain the Coulomb-Navier criterion?

Think about:

Relates failure to principal stresses
Predicts fault orientation
Relationship to Mohr-Coulomb

Can you describe a composite failure envelope?

Consider:

Combines tensile and shear failure
Why is it more realistic for rocks?
Tensile cutoff
Transition from tensile to shear failure

Can you explain Byerlee’s Law and its significance?

Think about:

\(\tau = 0.85\sigma_n\) (low pressure, <200 MPa)
\(\tau = 50 + 0.6\sigma_n\) (high pressure)
Friction coefficient \(\mu \approx 0.6\text{-}0.85\)
Remarkably independent of rock type
Implications for crustal strength

5.2.4 Environmental Effects

Can you explain the role of pore fluid pressure?

Consider:

Effective stress: \(\sigma'_n = \sigma_n - P_f\)
Reduces normal stress on faults
Can trigger failure without changing tectonic stress
Hubbert-Rubey hypothesis for low-angle thrusts
Role in hydraulic fracturing

Can you describe how temperature affects failure?

Think about:

Reduces rock strength
Promotes ductile behavior
Brittle-ductile transition
Why are earthquakes limited to certain depths?

Can you explain the effect of strain rate on failure?

Consider:

Fast rates favor brittle behavior
Slow rates allow ductile deformation
Same rock can behave differently depending on rate
Laboratory vs. geological strain rates

Can you describe the effect of confining pressure?

Think about:

Increases strength
Suppresses brittle fracture
Promotes ductile mechanisms
Why does strength increase with depth in upper crust?

5.2.5 Joints and Veins

Can you describe the characteristics of joints?

Consider:

Extension fractures (Mode I)
No shear displacement
Perpendicular to \(\sigma_3\)
Often occur in systematic sets
Plumose structures indicate propagation direction

Can you interpret plumose structures on joint surfaces?

Think about:

Hackle marks: radial features
Arrest lines: mark pauses in propagation
Plume axis: initial fracture point
What direction did the joint propagate?

Can you explain joint spacing and its relationship to layer thickness?

Consider:

Thicker layers → wider spacing
Mechanical explanation
Implications for fracture permeability

Can you distinguish different types of joint systems?

Think about:

Systematic vs. non-systematic
Orthogonal sets
Conjugate sets
Cross-cutting relationships for relative age

Can you explain different origins of joints?

Consider:

Tectonic: regional stress
Unloading: erosion, exhumation
Cooling: thermal contraction (columnar joints)
Hydraulic: fluid pressure
How would you distinguish these in the field?

Can you describe vein characteristics and formation?

Think about:

Mineral-filled fractures
Evidence of fluid flow
Crack-seal mechanism
Fibrous veins: indicate opening direction
Why are veins important for ore deposits?

5.2.6 Faults and Fault Zones

Can you describe and use fault terminology?

Consider:

Strike: horizontal line on fault plane
Dip: angle from horizontal
Dip direction: azimuth of steepest descent
Slip: total displacement
Rake (pitch): angle of slip vector in fault plane
Throw and heave components

Can you classify faults by slip direction?

Think about:

Normal: hanging wall down (extension)
Reverse: hanging wall up (compression)
Strike-slip: lateral motion (wrench)
Oblique: combination
How do you determine sense of slip?

Can you classify faults by dip?

Consider:

Thrust: <30°
Low-angle: 30-45°
High-angle: >45°
Why is the 30° boundary significant?

Can you describe fault zone anatomy?

Think about:

Fault core: principal slip surface
Damage zone: fractured rock around fault
Fault gouge: fine-grained material
Fault breccia: angular fragments
Mylonite: ductilely deformed fault rock
Why does this matter for fluid flow?

Can you identify kinematic indicators on faults?

Consider:

Slickenlines (striations): parallel to slip
Slickensides: polished surfaces
Steps and grooves: indicate slip sense
Offset markers
How do you use these to determine slip direction?

Can you interpret faults on geological maps?

Think about:

Fault trace patterns
Stratigraphic separation
V-rule for faults in valleys
How do dip and topography interact?

Can you interpret faults in seismic sections?

Consider:

Offset reflectors
Flower structures (positive and negative)
Fault shadow zones
Growth strata (syn-tectonic sediments)

Can you measure and describe fault displacement?

Think about:

Net slip: total displacement
Throw: vertical component
Heave: horizontal component
Stratigraphic separation vs. true slip
Why is 3D analysis necessary?

5.2.7 Faults and Earthquakes

Can you describe the earthquake cycle?

Consider:

Interseismic: stress accumulation, fault locked
Coseismic: rupture and slip
Postseismic: aftershocks, viscoelastic relaxation
How long is a typical cycle?

Can you explain stick-slip behavior?

Think about:

Fault locked during interseismic period
Sudden failure when stress exceeds strength
Stress drop during earthquake
Why do some faults creep instead of having earthquakes?

Can you describe earthquake magnitude scales?

Consider:

Magnitude (\(M\)): energy released
Moment (\(M_0\)): rigidity × area × slip
Moment magnitude (\(M_w\)): most reliable scale
Why are large earthquakes less frequent?

Can you interpret earthquake focal mechanisms?

Think about:

Beach ball diagrams
Compressional (black) and tensional (white) quadrants
P, T, and N axes
What do they tell you about fault orientation and stress?
Ambiguity between fault plane and auxiliary plane

Can you explain earthquake recurrence intervals?

Consider:

Time between large earthquakes on same fault
Statistical vs. characteristic earthquake models
Why is this important for hazard assessment?