3  Module II: Structural Geology and Crustal Deformation

3.1 Key Concept Summary

Structural geology studies deformation structures at all scales, from microscopic to crustal scale. The three main tectonic regimes (contractional, extensional, and strike-slip) produce characteristic structures controlled by the orientation of principal stresses. Understanding these structures is essential for resource exploration, geotechnical applications, and interpreting Earth’s deformation history.

3.2 Self-Test Questions

3.2.1 Fundamental Concepts

Can you explain “Pumpelly’s Rule” and its significance for structural geology ?

Think about:

  • How do small-scale structures relate to large-scale structures?
  • Why is this important for interpreting limited outcrop data?
  • What are the limitations of this principle?

Can you distinguish between structural geology, tectonics, and geodynamics ?

Consider:

  • What spatial scales does each discipline focus on?
  • What questions does each try to answer?
  • How do they complement each other?

Can you describe the three main approaches to studying geological structures ?

Think about:

  • Field observations (final product)
  • Laboratory experiments (progressive deformation)
  • Numerical modeling (forces and interactions)
  • What are the strengths and limitations of each?

3.2.2 Contractional Regimes

Can you identify and describe the major structures formed in contractional settings?

Consider:

  • Thrust faults vs. reverse faults (what’s the dip angle distinction?)
  • Nappes and their significance
  • What is a décollement and why is it important?
  • Can you explain the difference between thick-skinned and thin-skinned tectonics?

Can you explain ramp-flat geometry and why it forms?

Think about:

  • Why do thrusts preferentially follow weak layers (flats)?
  • Why do they cut through competent layers (ramps)?
  • What are frontal vs. lateral vs. oblique ramps?
  • How does this geometry affect fold development?

Can you describe what a duplex structure is and how it forms?

Consider:

  • What are horses?
  • What is the difference between floor and roof thrusts?
  • Why do duplexes form?
  • What is an imbrication zone?

Can you explain the concept of an orogenic wedge and the factors controlling its geometry?

Think about:

  • What is the critical taper concept?
  • What controls the angle of the wedge (basal friction, internal strength)?
  • How does surface erosion affect wedge geometry?
  • Can you distinguish between bulldozer, gliding, extrusion, and spreading models?

Can you give real-world examples of major thrust systems and describe their characteristics ?

Examples to consider:

  • The Alps and their nappe complexes
  • The Himalayas as an active collision zone
  • Chief Mountain, Montana (older over younger rocks)
  • Your favorite example here (remember some key facts to bring up in questions)

3.2.3 Extensional Regimes

Can you explain the difference between extension, elongation, and the beta factor ?

Consider:

  • How are these quantities defined mathematically?
  • How do we measure extension in the field?
  • What is the relationship between crustal extension and lithospheric thinning?

Can you identify and describe the major structures formed in extensional settings?

Think about:

  • Normal faults and their orientation relative to stress
  • Graben vs. horst structures
  • Half-graben and their asymmetry
  • Domino systems and block rotation

Can you explain the difference between pure shear and simple shear rift models?

Consider:

  • Which produces symmetric basins and which produces asymmetric?
  • What is a metamorphic core complex and which model does it support?
  • What evidence would distinguish between these models?

Can you describe the stages of rift evolution from initial extension to passive margin?

Think about:

  • Early extension phase
  • Main stretching and thinning
  • Syn-rift sedimentation
  • Post-rift thermal subsidence
  • Breakup and ocean formation

Can you explain active vs. passive rifting ?

Consider:

  • What drives each type?
  • Which involves mantle plumes?
  • How would you recognize the difference?

Can you give real-world examples of extensional provinces?

Examples to consider:

  • East African Rift System
  • Basin and Range Province
  • North Sea rift system
  • Gullfaks Field domino system

3.2.4 Strike-Slip Regimes

Can you distinguish between transfer faults, transform faults, and transcurrent faults?

Consider:

  • What is each type’s function?
  • Where does each typically occur?
  • Which are true plate boundaries?

Can you explain the difference between right-lateral (dextral) and left-lateral (sinistral) faults?

Think about:

  • How do you determine the sense of slip?
  • What observations help identify slip sense in the field?
  • Can you think of a couple of examples of each ?

Can you describe what happens at restraining bends vs. releasing bends?

Consider:

  • Which produces local shortening and which extension?
  • What structures form at each (pop-ups, pull-apart basins)?
  • What are positive and negative flower structures?
  • Can you give real-world examples (Dead Sea, Death Valley)?

Can you explain transpression and transtension?

Think about:

  • What is the kinematic vorticity number (\(W_k\))?
  • What does \(W_k = 0\), \(1\), and intermediate values represent?
  • How is strain partitioned in transpressional zones?
  • Where do we see these settings in nature?

Can you describe the San Andreas Fault system as a key example?

Consider:

  • Total offset and age
  • Slip rate
  • Is it pure strike-slip or transpressional?
  • Major historical earthquakes
  • Why is it slightly oblique to Pacific-North America motion?

Can you explain the role of conjugate strike-slip faults in escape tectonics?

Think about:

  • How do they accommodate lateral extrusion?
  • The Himalayan example
  • Tapponnier’s plasticine experiments