Introduction to Balanced Crosssections: Part 2

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Introduction to Balanced Cross-sections Part 2

• Checking for balance, doing it all
• ESS 112
• Lab 6

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What are balanced cross-sections, again?

• A balanced cross-section is both viable and
  admissible
• viable the cross-section can be restored to an
  unstrained state. Also called retrodeformable.
  
• (Humpty Dumpty can be put back together again.)
• admissible the cross-section utilizes only the
  types of structures that are seen in the field
  area. FOR INSTANCE WHAT GEOMETRIES OCCUR IN
  FOLD-THRUST BELTS?
• (For example, if Humpty Dumpty is only folded by
  gentle, large interlimb angle folds, you cant
  use an isoclinal fold to put him back together
  again.)

REVIEW
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The building blocks of balanced cross-sections

• 3 sets of facts are our tools
• the orientation of bedding, cleavage, and fold
  axes at specific places
• the distribution and thickness of stratigraphic
  units
• the originally undeformed nature of the rocks

REVIEW
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Whats the procedure? How do we make balanced
cross-sections?

• 4 steps
• Assembly of the basic data
• Extrapolation and interpolation
• Complete the structural picture (includes
  admissibility)
• Check for retrodeformability (i.e. is it viable?)

REVIEW
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Getting down to businessAssembly of data on a
cross-section

• First, you have to select the line of section.
• Through the region with the most complete data,
  and the least minor complications
• (Usually) make it perpendicular to the regional
  strike fold axes, so that folds will be
  symmetrical and units will show their true
  thickness, and give a reasonable estimate of
  total shortening.
• Assemble profile, data.

REVIEW
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The KINK METHOD

• Assumption Folds are parallel, with straight
  limbs and sharp, angular hinges (as in kink
  folds).
• The angle between the fold limb and the axial
  surface of the fold is called the axial angle ?.
• The axial surface bisects the angle between the
  fold limbs i.e. ?1 ?2.
• Where two axial surfaces meet, a new axial
  surface is formed, and it also satisfies the
  equal angle rule (?1 ?2).

REVIEW
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The KINK METHOD Getting Going
1 - Dip Domains select regions of near constant
dip that you think represent a straight line
section, i.e. a limb of a fold. 2 - Axial
Surfaces infer the locations of axial surfaces
between the edges of dip domains there should
be some play with where you can put the plane,
but its angle is determined by the
limbs. Extrapolate further!
REVIEW
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Moving forward

• Review of thrust rules
• Line-length balancing
• Area balancing
• Depth to Detachment
• The two main problems blank paper and weak
  techniques (the dip domains only get you so deep)
  and ways to move forward
• Four steps to make a balanced cross-section?
  Really getting down to it 18 steps.

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Thrust Rules

• Older over younger.
• Thrust fault cuts up section in transport
  direction.
• Hanging wall and footwall cutoffs must match.
• Thrust development tends to propagate towards the
  foreland.
• Thrusts in thin-skinned thrust belts sole into a
  common decollement.
• Bed length or area must be balanced.

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Line length balancing

• A cross-section is balanced when the line-lengths
  of beds are equal both in their deformed and
  undeformed states.
• So, draw a deformed section that follows the
  thrust rules, then measure the bed lengths to
  relocate faults in the undeformed state. Thrusts
  had better be dipping in the right direction, or
  its not balanced.
• strain (?L / L0) 100

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Area balancing

• A cross-section is balanced when the areas of
  beds are equal both in their deformed and
  undeformed states.
• A area of structural relief above an undeformed
  datum S shortening H depth to detachment
• A S H

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Depth to Detachment

• A S H H is depth to detachment. Usually,
  you dont know it. To get H
• If you are able to line length balance a single
  bed, you would have a value for shortening, S.
• If you have an undeformed datum, you may be able
  to calculate the area above it, A.
• Solve the equation H A / S, for depth to
  detachment.

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Depth to Detachment

• The depth to the detachment is a powerful
  constraint for your cross-section.

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The real world is hard, in two principal ways
(both to do with cross-sections, of course)

• 1 many cross-sections are significantly
  underconstrained what to do with blank paper?
• 2 drafting and balancing techniques (e.g. the
  kink method) usually only go a limited distance
  towards determining the correct solution
• If youre exclusively given artificial homework
  problems, like last weeks lab, you wont
  necessarily realize that these problems exist.
  The next two weeks will free your mind.

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How do you address these problems?

• Get more data, in the right places.
• Make more assumptions. For example, assume that
  all thrusts in a given area step up at
  approximately the same angle.
• Use more powerful techniques which are
  generally (a) specific, well-founded
  assumption(s). For example, assume the anticline
  you see at the surface results from a fault-bend
  fold. Doesnt work? Try a fault-propagation
  fold.

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powerful techniques

• Models like the fault-bend fold or
  fault-propagation fold have specific dip-domain
  predictions, and can be tested against surface
  data. If a model works out, it may provide an
  interpretive leap for your cross-section.

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Such interpretive leaps can get quite
sophisticated, and yield models with many
specific, testable predictions
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Just 18 Steps to Glory.

• 1 Compile geologic maps and subsurface data
• 2 Draw a section line parallel to the direction
  of tectonic transport
• 3 Ink in the topographic surface and geologic
  data
• 4 Project in well data, and geologic data from
  off the line of section

This is all old hat whats new?
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Ouch. Step 5 involves a whole nother section.

• 5 Draw a separate restored stratigraphic
  layer-cake or wedge using the youngest
  pre-orogenic unit as a horizontal datum
• 6 Find depth to basement and draw in the
  basement surface on the deformed section
• 7 Lightly pencil in the thickness of
  stratigraphic units above the basement to give a
  guide for the depths of thrust sheets trailing
  edges.

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Just 11 more steps to go

• 8 Draw a foreland pin line in the deformed
  section to correspond to the foreland edge of the
  restored section. (Cmon, this ones easy).
• 9 Continue surface geology to depth based on
  axial plane intersections, depth-to-detachment
  calculations, etc. If the world is so kind to
  you (if the area is not too ridiculously
  complex), have the trailing edge of each major
  thrust sheet return to regional above basement.
• 10 Fill deep holes with imbricates, horses, or
  duplexes, as seems appropriate

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Do I need a title here? 9 steps left

• 11 Given hanging wall cutoff geometries, draw
  subsurface footwall geometries to fit
• 12 Measure bed lengths from the foreland pin
  point back through the section for each horizon.
  (Or, measure only key-beds and the positions of
  local pin lines in each sheet)
• 13 Measure off the same bed lengths from the
  foreland margin of the restored stratigraphic
  wedge. This locates all the faults in the
  restored section

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Local Pin Lines
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the 5 final steps!

• 14 Check that all local pin lines, or
  well-constrained surface geometries, are
  preserved in the restored section
• 15 Check that the respective HW FW cutoffs
  match in the restored section
• 16 Measure the area of each thrust sheet in the
  deformed section and in the restored section.
  Equal? They should be.
• 17 If not, find out where your stratigraphic
  thicknesses are wrong.
• 18 expect that deep structures may require area
  balance, as bed-length details will probably be
  incomplete.