Folds

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Folds

• Can range in complexity from simple cylindrical
  folds to intricately complex, 3-D folds.
• Simple folds can be described using the geometric
  constructs
• Axial plane
• Fold axis

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Folds

• Axial plane (surface) imaginary plane (surface)
  that bisects the fold into two mirror image
  halves.
• Fold axis line of intersection of axial surface
  with the crest/trough of a fold.
• Axial trace line of intersection of axial
  surface with the ground surface (what you draw on
  a map)

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Folds

• Folds are classified according to the orientation
  of their axial surfaces and axes and the degree
  of symmetry
• Plunging folds are those where the axis is not
  horizontal

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Map Patterns of Folds

• Plunging folds in map view resemble what they
  would look like in cross-section.
• To approximate the cross-sectional shape, use
  down-plunge viewing.

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Map Patterns of Folds

• Depends on
• Size of fold relative to map area (scale).
• Selection of map units.
• Topography
• Fold geometry

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Map Patterns of Folds

• Folded strata of different resistance to erosion
  will show up in the topography.
• Laws of Vs still work, but dont be fooled by
  Vs caused by plunging fold closures.

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Map Patterns of Folds

• In low relief areas where a bed of uniform
  thickness is folded, differences in map
  thicknesses between limbs indicates different
  dips and asymmetry.

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Map Patterns of Folds

• Isoclinal folds have parallel limbs that dip in
  the same direction.
• Where cut by streams, the Vs on both limbs of
  isoclinal folds point in the same direction.
• Fold hinges of isoclinal folds are sharp and
  narrow.

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Cross-Sections of Folds

• Freehand method
• Just like a normal cross-section.
• Locate contacts at surface and extrapolate
  underground.
• Eyeball contacts and structures with smooth
  lines.
• Layers have uniform thickness.

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Cross-Sections of Folds

• Balanced cross-section
• Aim is to be able to restore length of contacts
  and areas of beds to their original
  (pre-deformation) condition.
• Bed length before bed length after.
• Area before area after.
• Cuttoff angles of faults are constant before and
  after displacement.

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Tracing Folds

• Use same technique as for homoclines.
• Only works well for horizontal (non-plunging)
  folds that are uniform in cross-sectional shape.

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Structure Contour Maps of Folds

• Very useful for showing shape of folds.
• Need subsurface data to make.
• Show the elevation (or depth) of a particular
  surface (bed or contact) within the fold.

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Mapping Folds

• Contacts will be curved
• Opposing dips
• Small-scale folds within the limbs (S and Z
  parasitic folds)
• Cleavage parallel to axial plane

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Faults

• Faults are discrete breaks or broad zones across
  which crust has been displaced.
• Faults at shallow levels are brittle (breccia,
  gouge, etc.).
• Those at depth are ductile (foliations, etc.)

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Faults

• Since they are zones of broken up rock, they do
  not expose well at the surface.
• Typically covered by surficial deposits or
  manifest as topographic features.
• Faults usually are detected by stratigraphic
  relationships.

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Faults

• Faults can have vertical (dip-slip) or horizontal
  (strike-slip) movement. Most have some of both
  (oblique-slip).
• Dip-slip motion will create fault scarps.
• Strike-slip motion can create a variety of
  features, depending on the geometry.

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Faults

• Displacement amount and direction of movement
  (e.g. slip).
• Trace intersection of fault surface and ground
  surface.
• Hanging wall vs. footwall.
• Normal vs. reverse vs. thrust vs. strike-slip.

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Faults

• Slip components
• Throw is vertical
• Heave is horizontal
• Stratigraphic throw thickness of section missing
  at any point along a fault plane.

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Faults

• Separation distance between markers on each
  block in a specified direction...
• Strike
• Dip
• Oblique
• Vertical
• Offset
• NOT THE SAME AS SLIP. Separation is often just
  apparent (due to topography, erosion, etc.)

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Faults

• When horizontal strata cut by a dip-slip or
  oblique fault, result is change in elevation of
  strata directly proportional to the dip-slip.
• When dipping strata are cut, complex map patterns
  can result, especially after erosion has occurred.

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Faults

• Over time, scarps will disappear, yet the fault
  relationships remain.
• So for old faults, topography may have nothing to
  do with faulting. For instance, upthrown block
  usually more prone to erosion.
• Rule position of contacts of dipping beds
  migrates in the direction of dip.

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Faults

• Steep faults (high-angle) cut across topography
  in relatively straight lines (perfectly straight
  if the fault is vertical).
• Shallowly-dipping (low-angle) faults form
  irregular patterns that weave in and out across
  topography.

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Steep Faults

• Planar and have dips over 50
• Strike-slip (usually vertical)
• Normal
• Reverse

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Strike-Slip Faults

• Some are localized with beginnings and ends.
• Some are tears due to other deformation.
• Some are plate boundaries.
• Many form en echelon. Geometry and slip
  direction determine possible map relationships
  along a strike-slip zone.
• Restraining bends
• Pull-apart basins

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Steep Normal Faults

• Straight traces.
• Typical geometries
• Graben
• Horst
• Steps
• En echelon

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Low-Angle Faults

• Thrust faults (and normal faults too)
• Low angle reverse (and normal) faults.
• Can occur in folds.
• Often imbricated.
• Large lateral displacement (10-100 km) compared
  to vertical displacement.
• Low angles, even horizontal (where takes
  advantage of weak horizons).
• Ramp geometries common i.e. dip of the fault can
  change.

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Mapping Faults

• Not often well-exposed, but often associated with
  distinctive weathering/erosion, gullies, straight
  sections of channels, slope breaks.
• Out-of-place units
• Repetition or deletion of units
• Angular discordance
• Sudden changes in lithologies
• Abrupt changes in structure/orientation of units
• Brecciation and alteration (can be used to tell
  slip direction)
• Fractures and slickenlines (can be used to tell
  slip direction)
• Springs and vegetation patterns