Discussion:

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.

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.

References:

Examining Interference Folding Using Stereonet and Domain AnalysisBy Caroline Fernandez and Kathryn Boardman

Advanced Structural Geology CourseDr. John Hogan and Dr. Mohamed Abdelsalam

Department of Geological Sciences and EngineeringMissouri University of Science and Technology

Data:

Stereonets and Fold Sketches:

Introduction:

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.

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:

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.

Maps:

Model:

“ Superposed Folding Papermodels” http://www.fault-analysis-group.ucd.ie/SuperPosedFolds/Superposed_PM_Index.htmlRamsay, J.G., and Huber, M.I., 1987, The Techniques of Modern Structural Geology, Volume 2: Folds And Fractures

G.H. Davis and S.J. Reynolds (1996). The structural geology of rocks and regions. 2nd Edition. Wiley

Figure 1

Figure 2: is the original geologic map of the folded region showing the strike and dip data recorded in the field.

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.

Figure 2

Figure 3

Figure 4

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.

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 5A: Stereonet of early folding, domains 2, 9, and 6.

Figure 6B: sketch of fold from data on figure 6A.

Figure 7A: Stereonet of early folding, domains 3, 4, and 5

Figure 7B: sketch of fold from data on figure 7B.

Figure 9A: Stereonet of late folding, domains4, 8, and 9.

Figure 9B: sketch of fold from data on figure 9A.

Figure 10A: Stereonet of late folding, domains 7, 6, and 5.

Figure 10B: sketch of fold from data on figure 10A.

Table 1: Results of stereonet analysis of selected domain combinations that best illustrate the early and late folding events.

Figure 11: 3-D representation of folding