Structure, Deformation and Mountain Building

[1] Deformation

Stress: external forces applied to a portion of the Earth

• compressional stress
• tensional stress
• shear stress

Strain: response of the Earth to the applied stress

• elastic strain
• plastic strain
• fracture

brittle (mineral grains/rock broken) vs ductile (mineral grains 'flow') strain:

• depends on temperature, pressure, fluids, time

[2] Strike and Dip

• strike: defined as a horizontal line on a dipping plane - ONLY one possible compass direction although you can draw an infinite number of parallel lines
• dip: maximum angle measured between the horizontal and the dipping planar surface (e.g. sedimentary bed) - TRUE (maximum) dip will be perpendicular to the strike direction

[3] Folds: basically result of plastic strain

• monoclines
• anticline: an upward fold in a package of sedimentary rocks
  • if eroded after folding, map pattern shows oldest rocks in the middle
• syncline: a downward fold in a package of sedimentary rocks
  • if eroded after folding, map pattern shows youngest rocks in the middle
• axial plane (imaginary plane that divides a fold into 2 symmetrical halves), axis: line defined by intersection of axial plane and fold, limb: one side of the fold
• symmetrical, asymmetrical, overturned, recumbant: adjectives describing more about a fold than simply calling it, for instance, an 'anticline'
• plunging folds: fold where the axis is not horizontal
  • domes and basins: examples of doubly plunging folds

[4] Joints: basically result of brittle fracture

• crack with no significant motion
• compressional: squeezing
  • folded strata: space problem
  • conjugate sets
• tensional: stretching
  • columnar joints: result of shrinkage

[5] Faults: basically result of brittle fracture

• fault plane
• hanging wall block: term comes from miners working veins that have formed along a fault - bump your head on the hanging wall
• footwall block: the same miners stand with their feet on the footwall
• Dip-slip faults: motion on the fault is in the direction of the dip
  • normal (or gravity): tensional setting
  • reverse: compressional setting, high angle
  • thrust: compressional setting, low angle
• Strike-slip faults: motion on the fault parallel to the strike
  • transform fault as special case
  • right- vs left-lateral
• Oblique-slip faults: direction of motion somewhere in between strike and dip directions

[6] Mountains

• mountains, mountain ranges, mountain systems
• volcanic
• block fault: either compressional or tensional setting
  • horst (think 'hoist') and graben (think 'grab' and pull down)
• compressional: folding and thrust faults

[7] Mountain Building: Orogenesis

• orogeny: episode of mountain building including deformation, metamorphism, emplacement of plutons
• now recognized as generally related to plate boundaries

• Ocean-ocean boundaries
  • convergence
  • accretionary wedge
  • blueschist metamorphism
• Ocean-continent boundaries
  • land-based volcanic arcs, high mountains
  • Andes
  • Rocky Mountains: unusual case due to its great distance from the trench
  • deep trench offshore
  • accretionary wedge, blueschists
• Continent-continent
  • example of India and Himalayas
  • also Appalachians, Urals