Ch10 structural geology_fall2007
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Ch10 structural geology_fall2007






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Ch10 structural geology_fall2007 Ch10 structural geology_fall2007 Presentation Transcript

  • CRUSTAL DEFORMATION and Geologic Structures
  • Deformation • Deformation involves: – Stress – the amount of force applied to a given area. – Types of Stress: –Confining Stress – stress applied equally in all directions. –Differential Stress – stress applied unequally in different directions.
  • Deformational Stress • Types of Differential Stress: (1) Compressional Stress – shortens and thickens a rock body (associated with convergent plate boundaries). (2) Tensional Stress – tends to elongate and thin or pull apart a rock unit (associated with divergent plate boundaries). (3) Shear Stress – produces a motion similar to slippage that occurs between individual playing cards when the top of the stack is moved relative to the bottom (associated with transform plate boundaries).
  • Deformation of the Earth’s Crust Caused by Tectonic Forces and Associated Differential Stresses
  • Deformation • Differential stress applied to rocks during tectonic activity causes rocks to respond via deformation. • Strain – changes in the shape or size of a rock body caused by stress. • Strained rock bodies do not retain their original configuration during deformation.
  • How Do Rocks Deform? • Rocks subjected to stresses greater than their own strength begin to deform usually by folding, flowing, or fracturing. – General Characteristics of Rock Deformation: • Elastic deformation – the rock returns to nearly its original size and shape when the stress is removed. • Once the elastic limit (strength) of a rock is surpassed, it either flows (ductile deformation) or fractures (brittle deformation).
  • • Factors that influence the strength of a rock and how it will deform: • Depth • Temperature • Confining Pressure • Rock Type • Availability of Fluids • Time How Do Rocks Deform?
  • • Rocks near the surface, where confining pressures and temperatures are low, will behave as a brittle solid and fracture once their strength is exceeded. • Rocks at depth, where confining pressures and temperatures are high, will exhibit ductile behavior or solid-state flow, in which changes occur without fracturing. How Do Rocks Deform?
  • Crustal Structures • Folds – During crustal deformation rocks are often bent into a series of wave-like undulations. – Anticlines and Synclines – Domes and Basins – Monoclines • Characteristics of Folds: • Most folds result from compressional stresses which shorten and thicken the crust. • Most of them occur in a series.
  • Anatomy of a Fold • Limbs – Refers to the two sides of a fold. • Axis (or Hinge) – A line drawn down the points of maximum curvature of each layer.• Axial Plane – An imaginary surface that divides a fold symmetrically. • Plunge – In complex folding, the axis is often inclined at an angle called plunge.
  • (A) Horizontal Anticline and (B) Plunging Anticline
  • Common Types of Folds • Anticline – upfolded or arched rock layers. • Syncline – downfolds or troughs of rock layers. Photo courtesy of J. T. Daniels Photo courtesy of Brennan T. Jordan, Department of Earth Sciences, University of South Dakota
  • Common Types of Folds • Depending on their orientation, anticlines and synclines can be described as… • Symmetrical, asymmetrical, overturned, recumbent (a type of overturned fold – “lying on its side”), or plunging.
  • Formation of Folds Insert Animation #30: Folds
  • Name the Folds Below
  • Plunging Anticlines and Synclines (Note: the outcrop pattern of an anticline points in the direction it is plunging, whereas the opposite is true for a syncline)
  • Sheep Mountain, A Plunging Anticline
  • Formation of Folds Insert Animation #30: Plunging Folds
  • Other Types of Folds • Monoclines • Large, step-like folds in otherwise horizontal sedimentary strata. • Closely associated with faulting.
  • Other Types of Folds • Dome • Upwarped displacement of rocks. • Circular or slightly elongated structure. • Oldest rocks in center, younger rocks on the flanks.
  • Other Types of Folds • Basin • Circular or slightly elongated structure. • Downwarped displacement of rocks. • Youngest rocks are found near the center, oldest rocks on the flanks.
  • Crustal Structures • Faults – Fractures in rocks along which appreciable displacement has taken place. • Fault Zone – Displacements along multiple interconnected faults. • Sudden movements along faults are the cause of most earthquakes.
  • Types of Faults • Classified by their relative movement which can be Horizontal, Vertical, or Oblique.
  • Summary of Fault Types • Dip-Slip Faults: • Normal (gravity) – associated with divergent plate boundaries. • Reverse and Thrust – associated with convergent plate boundaries. • Strike-Slip Faults: • Lateral (right and left) – associated with transform plate boundaries.
  • Dip-Slip Faults • Movement is mainly parallel to the dip of the fault surface. • Parts of a dip-slip fault include the hanging wall (rock surface above the fault) and the footwall (rock surface below the fault).
  • Dip-Slip Faults • Normal Fault (gravity) Dip-Slip Faults – Hanging wall block moves down relative to the footwall block. – Tensional stress – Accommodate lengthening or extension and thinning of the crust. – Associated with divergent plate boundaries. – Most are small with displacements of a meter or so. – Larger scale normal faults are associated with structures called fault-block mountains (Teton Range in Wyoming, Basin and Range Province in Nevada).
  • Formation of Normal Faults Insert Animation #29: Faults – Normal
  • Normal Faulting – Fault Block Mountains • Fault-Block Mountains – Basin and Range Province in Nevada – topography generated by a system of roughly north to south trending normal faults. • Movements along these faults have produced alternating uplifted blocks called horsts (form elevated ranges) and down-dropped blocks called grabens (form basins). • Half-Grabens – a tilted fault block in which the higher side forms mountainous topography and the lower side forms a basin that fills with sediment. • Detachment Fault – nearly horizontal fault extending up to hundreds of kilometers into the subsurface. Smaller faults are connected to this larger fault. Boundary between ductile and brittle deformation.
  • Dip-Slip Faults • Reverse and Thrust Dip-Slip Faults – Hanging wall block moves up relative to the footwall block. – Reverse faults have dips greater than 45o – Thrust faults have dips less than 45o . • Strong compressional stress. • Accommodate shortening and thickening of the crust. • Associated with convergent plate boundaries.
  • Formation of Reverse Faults Insert Animation #29: Faults – Reverse
  • Idealized Development of Lewis Overthrust Fault near Glacier National Park
  • Strike-Slip Faults • Dominant displacement is horizontal and parallel to the strike of the fault. • May produce broad zones of roughly parallel fractures up too several kilometers in width. • Shear stress. • Associated with transform plate boundaries.
  • Formation of Strike-Slip Faults Insert Animation #29: Faults – Strike-Slip
  • Types of Strike-Slip Faults • Right-Lateral – as you face the fault, the opposite side of the fault moves to the right. • Left-Lateral – as you face the fault, the opposite side of the fault moves to the left. Animations: Right-Lateral Strike-Slip Fault
  • Types of Strike-Slip Faults • Transform Fault – Large strike-slip fault that cuts through accommodates motion between two large crustal plates. – Example: San Andreas Fault System
  • Name the Type of Fault Below
  • Name the Type of Fault Below
  • Name the Type of Fault Below Insert Animation #28: Exposing Metamorphic Rock
  • Name the Type of Fault Below
  • Name the Type of Fault Below
  • Name the Structure
  • Name the Structure
  • Mapping Geologic Structures • Geologists measure the orientation or attitude of a rock layers or fault/fracture surfaces in order to describe and map geologic structures that result from deformation.
  • Mapping Geologic Structures – Strike (Trend) • The compass direction of the line produced by the intersection of an inclined rock layer or fault with a horizontal plane. • Generally expressed an an angle relative to north. • Example: N10ºE
  • Mapping Geologic Structures – Dip (Inclination) • The angle of inclination of the surface of a rock unit or fault measured from a horizontal plane. • Includes both an inclination and a direction toward which the rock is inclined. • Example: 30ºSE
  • A Geologic Map Showing Strike and Dip of Structures By knowing the strike and dip, geologists can predict the nature of rock structures hidden beneath the surface.
  • Geologist Measuring the Dip of Strata in a Roadcut