Cross section balancing

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Cross section balancing
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Deformed-state cross sections

• Deformed-state cross sections can be constructed
  using map data, well data, and, if you're lucky,
  seismic data.

Deformed-state section from Glacier National Park
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Deformed-state cross sections

• All cross sections involve interpretation. They
  are not data.
• Admissible cross sections Interpretation matches
  surface and subsurface data fault and fold
  geometries match expected for thin-skinned
  fold-and-thrust belts

Interpretation
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Some criteria for admissible sections

• Ramps do not dip steeper than 35
• Flats follow weak layers
• Fault geometries match the geometries of
  overlying folds
• Out-of-sequence faults are justified
• Hinter-land-dipping faults are justified

See McClay, 1991, Glossary of thrust terms
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Not admissible Does not match expected thrust
geometries
Admissible Matches expected thrust geometries
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Restored cross sections

• Restored section can be constructed that depicts
  the probable pre-deformation geometry of the
  deformed beds

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Restored cross sections

• Individual thrust sheets are "pulled back" to
  their pre-thrust positions by matching footwall
  and hanging-wall cutoffs at ramps.

HW cutoff
FW cutoff
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Viable cross section Deformed-state section that
can be restored such that bed length and/or bed
area is conserved and fault geometries are
admissible
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Balanced cross section Deformed-state section
that is both admissible and viable.
Do you think this is a balanced cross section?
Why?
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Restoring a cross section

• Pick regional pin linePoint in deformed-state
  section that is not allowed to move
• Set up stratigraphic reference frame for restored
  section. Account for changes in bed thickness
  (most platform sedimentary packages thicken
  towards hinterland)

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Restoring a cross section

• Work in order from youngest faults to oldest.
• Add local pin lines in thrust sheets as needed.

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Restoring a cross section

• Keep track of the loose line on the hinterland
  end of the section.

Acceptable loose lines must be straight or form
smooth curves. a, b, and c are acceptable. d and
e are not.
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Restoring a cross section

• Keep track of the loose line on the hinterland
  end of the section.

Unacceptable loose line, must modify section to
balance
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Modified deformed-state section and new restored
section Section is now balanced
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