Examining Interference Folding Using Stereonet and Domain Analysis
Introduction:
Figure 1
Interference folds are formed by multiple deformational events resulting in
a range of complex fold geometries. These folds are commonly
encountered in the core of orogenic belts: so their understanding is critical
to the study of the evolution and geometry of mountain building events.
The orientations of the fold generations determine the resulting geometry
of the fold, or interference pattern. There are four basic types of
interference patterns, which are illustrated in fig 1.
By Caroline Fernandez and Kathryn Boardman
Advanced Structural Geology Course
Dr. John Hogan and Dr. Mohamed Abdelsalam
Department of Geological Sciences and Engineering
Missouri University of Science and Technology
Model:
Figure 11: 3-D representation of folding
Data:
Table 1: Results of stereonet analysis of selected domain combinations that best illustrate the early and late
folding events.
Goals:
To interpretively complete the given map, establish structural domains, and
create the proper stereonets in order to classify the interference pattern and
create a 3-D representation of the folded region.
Methods:
Discussion:
Domains were established by dividing the map by the axial traces of the
folds. For the Stereonets, domains were grouped in combinations to
represent either the early or the late generation of folding.
The original orientation of the early fold axial surface was
approximated using the stereonet analysis and an estimate of
the original fold axial trace on the map. The fold axial surface
of the late folding was taken directly from the Stereonets. The
early fold axial surface is dipping to the East and the late fold
axial surface is near vertical, dipping towards the Northeast.
The two fold generations are co-axial, but non co-planar. The
orientation relationship between the generations of folding
most closely resembles that of the type 3 interference pattern,
also known as a hook interference pattern.
Stereonets and Fold Sketches:
Maps:
Figure 2
Figure 3
Figure 5A: Stereonet for early folding, domains 1, 8
and 7.
Figure 5B: Sketch of fold from
data on figure 5A
Figure 8A: stereonet of late folding, domains 1, 2,
and 3.
Figure 8B: sketch of fold from
data on figure 8A.
Figure 4
Figure 2: is the original geologic map
of the folded region showing the
strike and dip data recorded in the
field.
Figure 5A: Stereonet of early folding, domains 2,
9, and 6.
Figure 6B: sketch of fold from
data on figure 6A.
Figure 9A: Stereonet of late folding, domains4,
8, and 9.
Figure 9B: sketch of fold
from data on figure 9A.
The figure to the right shows the
estimated orientations of the
original fold axial surfaces with
the early fold shown in red and
the late fold shown in blue. The
resulting Papermodel, once
folded into a cube, shows the
interference pattern in 3-D.
Notice how the hooked pattern
of folding correlates to the
geometry of the units in figure 3.
Conclusion:
Now that this complex fold has been broken down an studied
by fold generations, it is much easier to understand three
dimensionally. The geometry of the units from the interpretive
map and the results from conducting domain and stereonet
analysis has led us to classify the fold pattern as a type 3
interference fold.
Figure 3: is an interpretive map of
the folded region showing the
approximate contacts between the
units.
Figure 4: shows the division of the
map into structural domains by the
axial traces of the folds. The straight
lines are the axial trace of the late
folding and the curved lines are the
axial trace of the early folding.
References:
Figure 7A: Stereonet of early folding, domains 3, 4,
and 5
Figure 7B: sketch of fold from
data on figure 7B.
Figure 10A: Stereonet of late folding, domains 7, 6,
and 5.
Figure 10B: sketch of fold
from data on figure 10A.
G.H. Davis and S.J. Reynolds (1996). The structural geology of rocks and regions. 2nd Edition. Wiley
“ Superposed Folding Papermodels” http://www.fault-analysis-group.ucd.ie/SuperPosedFolds/Superposed_PM_Index.html
Ramsay, J.G., and Huber, M.I., 1987, The Techniques of Modern Structural Geology, Volume 2: Folds
And Fractures