Structural Geology Crosssection Restoration

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Structural Geology Cross-section Restoration
Department of Geology University of Texas at
Arlington
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Cross-section Terminology

• Deformed State Cross-section X-Section as it
  appears today
• Restored X-section X-Section as it appeared
  before deformation, Produced by un-deforming the
  deformed state X-S
• Admissible X-section Structures depicted in both
  the deformed state cross-section and restored
  actually occur in the area and are physically
  possible.

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Cross-section Terminology

• Reference Lines
• Pin Line A line (usually at one end of the
  deformed state X-S) that remains undeformed.
• Regional Dip The initial dip of the layers
  before deformation
• Loose Line Any line in the deformed state X-S
  that changes shape and orientation during
  restoration. It is usually the end of the X-S
  opposite the pin line or an internal fault.

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Cross-section Terminology
Terminology
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Cross-section Terminology

• Restoration Assumptions
• Area Conservation The area of a geologic unit
  (formation) remains constant during deformation.
  i.e. the area of that unit in the deformed state
  X-S must be the same as the area in the restored
  X-S.
• Bed-length Conservation The length of formation
  boundaries in the restored X-S must be the same
  as those in the deformed state X-S
• Deformation is two-dimensional and cross section
  is parallel to transport direction

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Cross-section Terminology
Restoration Assumptions
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Cross-section Terminology

• Other Constraints
• Thrust faults usually have ramps and flats. If
  the layers are initially undeformed, the faults
  always ramp up section, not down section.
• There are no gaps or overlaps in the restored
  section hanging wall cutoffs match the footwall
  cutoffs.

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Cross-section Terminology

• Other Constraints
• Fault slip usually does not increase in the
  direction of transport because it must eventually
  die out.
• Thrust faults are usually younger in the
  direction of transport, but Out of Sequence
  faults may form in late stages

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Cross-section Terminology

• Other Constraints
• Admissible Loose Lines. Restored loose lines must
  be straight or vary smoothly.

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Cross-section Terminology
Other Constraints Undeformed fault trajectories
must be admissible
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Cross-section Terminology

• Balanced Cross Section
• A X-Section that is admissible and can be
  restored to an undeformed state
• By either area or line length conservation
• That has admissible undeformed fault
  trajectories
• That has consistent bed lengths

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Cross-section Assignment
Restore the cross-sections provided and determine
whether they are viable and balanced. If not,
what needs to be changed in the cross-sections?