7  Cross-Cutting Themes

These themes appear throughout multiple modules and represent integrative concepts:

7.1 Scale Relationships

Can you explain how structures at different scales relate to each other?

Think about:

  • Pumpelly’s Rule: small structures reflect large structures
  • Same processes at all scales?
  • Fractal nature of some structures
  • When does the rule break down?

7.2 Stress-Strain-Rheology Triangle

Can you explain the fundamental relationship: stress causes strain, rheology controls the relationship?

Consider:

  • Given stress and rheology, can you predict strain?
  • Given structures (strain), can you infer stress and rheology?
  • Temperature and pressure effects on rheology
  • Why does the same stress produce different structures in different rocks?

7.3 Brittle-Ductile Behavior

Can you explain what controls whether rocks deform in a brittle or ductile manner?

Think about:

  • Depth (pressure and temperature)
  • Temperature increases with depth → ductile
  • Pressure increases with depth → brittle (upper crust)
  • Result: brittle-ductile transition at mid-crustal depths
  • Strain rate effects
  • Rock composition differences

Can you describe the strength profile of the lithosphere?

Consider:

  • Upper crust: brittle, strength increases with depth (Byerlee’s Law)
  • Lower crust: ductile, strength decreases with depth (temperature effect)
  • Mantle lithosphere: strong or weak?
  • Asthenosphere: weak (high temperature)
  • Why is this important for plate tectonics?

7.4 Tectonic Regime Recognition

Can you identify the tectonic regime from principal stress orientations?

Think about:

  • Normal faulting: \(\sigma_1\) vertical, \(\sigma_3\) horizontal (extension)
  • Strike-slip: \(\sigma_2\) vertical, \(\sigma_1\) and \(\sigma_3\) horizontal
  • Thrust faulting: \(\sigma_3\) vertical, \(\sigma_1\) horizontal (compression)

Can you predict what structures form in each regime?

Extensional regime:

  • Normal faults
  • Graben and horst
  • Domino systems
  • Rollover folds
  • Rift basins

Compressional regime:

  • Reverse and thrust faults
  • Folds
  • Nappes and duplexes
  • Orogenic wedges

Strike-slip regime:

  • Vertical strike-slip faults
  • Flower structures
  • Pull-apart basins (releasing bends)
  • Pop-up structures (restraining bends)

7.5 Plate Boundaries and Structures

Can you match plate boundary types with their characteristic structures?

Divergent boundaries:

  • Normal faults
  • Dikes
  • Sheeted dike complexes (ophiolites)
  • Seafloor spreading fabric

Convergent boundaries:

  • Thrust faults and folds
  • Accretionary wedges
  • Metamorphic belts
  • Subduction-related structures

Transform boundaries:

  • Strike-slip faults
  • Flower structures
  • En echelon tension gashes
  • Riedel shears

Continental boundaries:

  • Complex, diffuse zones
  • Multiple structure types
  • Escape tectonics
  • Thick vs. thin crust differences

7.6 Fluid-Rock Interaction

Can you explain the role of fluids in deformation?

Consider:

  • Reduces effective stress: \(\sigma' = \sigma - P_f\)
  • Enables deformation at lower differential stress
  • Facilitates chemical reactions
  • Pressure solution and dissolution creep
  • Vein formation
  • Hydrolytic weakening of minerals

Can you explain the importance for natural resources?

Think about:

  • Ore genesis: fluids transport metals
  • Hydrocarbon migration and trapping
  • Geothermal systems
  • Groundwater flow
  • Structures control permeability

7.7 Deformation History

Can you explain how to determine the sequence of deformation events?

Consider:

  • Cross-cutting relationships
  • Overprinting of structures
  • Fold interference patterns
  • Multiple cleavages
  • Reactivated faults
  • Metamorphic overprinting

Can you describe common multi-phase deformation scenarios?

Think about:

  • \(D_1\), \(D_2\), \(D_3\) events
  • Progressive deformation vs. distinct events
  • Rotation of stress field
  • Changing tectonic regime
  • Superimposed fold patterns (Type 1, 2, 3 interference)

7.8 Three-Dimensional Thinking

Can you visualize structures in 3D from 2D maps and cross-sections?

Consider:

  • Strike and dip indicate 3D orientation
  • Apparent dip vs. true dip
  • Structure contour maps
  • Down-plunge projection
  • Stereographic projections

Can you use stereographic projections to solve structural problems?

Think about:

  • Plotting planes (as great circles) and lines
  • Finding intersections
  • Determining fold axis from limb orientations
  • Analyzing fault slip data
  • Identifying preferred orientations

7.9 Practical Applications

Can you explain how structural geology applies to resource exploration?

Hydrocarbons:

  • Anticlines as traps
  • Fault-bounded traps
  • Stratigraphic traps in fold limbs
  • Fracture networks for permeability

Minerals:

  • Ore bodies in shear zones
  • Veins in extensional structures
  • Replacement along faults
  • Controls on ore geometry

Geothermal:

  • Fracture permeability
  • Heat source proximity
  • Fluid circulation pathways

Can you explain applications to earthquake hazards?

Consider:

  • Active fault identification
  • Paleoseismology
  • Recurrence intervals
  • Seismic hazard assessment
  • Ground motion prediction
  • Critical facilities siting

Can you explain applications to engineering and geotechnical work?

Think about:

  • Slope stability
  • Tunnel design
  • Dam foundations
  • Nuclear waste storage
  • Understanding discontinuities (joints, faults)
  • Rock mass strength