Introduction to Balanced Crosssections: Part 1

1
Introduction to Balanced Cross-sections Part 1

• Basics, and the Kink Method
• ESS 112
• Lab 6.5

2
What are balanced cross-sections?

• 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 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.)

3
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

4
(Tool 1 strikes and dips) (Tool 2
stratigraphy) allows the construction of both
cross-sections.
Tool 3 the original undeformed nature of the
rocks which requires that the section be viable
(retrodeformable) allowed us to choose the valid
cross-section.
5
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?)

6
Example, Part 1 Assembly of the basic data
7
Example, Part 2 Extrapolation, Interpretation

• This may be ok, but it doesnt show us the whole
  structure. If we try to image the whole thing,
  what do we learn?

8
Example, Part 3 Completing the structural picture

• By trying for the whole structural picture, we
  got a new idea maybe Fault A links in to the
  same decollement as Fault B? It works, and may
  explain the steps along Fault B these could
  result from folding of the upper plate of Fault
  A. So completeness allows this more
  sophisticated analysis.

9
Example, Part 4 Further testing of the
cross-section by retrodeforming it.

• Does it work? Yes!
• Do we learn anything else? Yes!
• Fault A has some 3 km of slip on it we should
  be able to see a consistent structure if we
  extend the section to the left.
• Fault B still steepens a bit the change in dip
  of Fault B was not solely due to motion on Fault
  A.

10
Getting down to businessAssembly of data on a
cross-section

• First, you have to select the line of section.
  Criteria
• 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.

11
Assembly of data on a cross-section

• Next steps
• Make topographic profile at true scale (vertical
  horizontal)
• Plot contacts, strikes and dips. Strikes and
  dips (sd) present two complications
• SD may be off the line of section. Project at
  the measurements elevation, or along the fold
  axis, as appropriate.
• Apparent dip. You can zip-a-dip, or

12
Apparent dip plot
13
Starting to use your brainsExtrapolation and
Interpolation

• General ideas
• Start with the stratigraphic contacts rather than
  the faults stratigraphy is generally better
  constrained and behaved.
• Things are a lot easier if the folds are parallel
  these folds have constant layer thickness
  measured perpendicular to bedding.

14
Extrapolating and Interpolating for real the
KINK METHOD

• Assumption Folds are parallel, with straight
  limbs and sharp, angular hinges (as in kink
  folds). No matter how curved a fold is, you
  could reasonably approximate it with a series of
  straight lines.

15
The KINK METHOD

• 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).

16
The KINK METHOD
17
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!
18
The KINK METHOD
19
All these rules, it must be right!

• No, not really
• A balanced cross-section is not truth. It
  follows more rigorous, reasonable rules than a
  regular cross-section, so it is more likely to be
  correct.
• Nevertheless, like any cross-section, it is a
  MODEL.
• Next week methods of retrodeformation.