265 Structural Geology: Active and Ancient Deformation in North America and around the World (Posters)

Tuesday, 27 September 2016
9:00 AM - 6:30 PM
Exhibit Hall E/F

VISUAL AND QUANTITATIVE COMPARISON OF STRUCTURAL DATA FROM THE WASATCH MONOCLINE (CENTRAL UTAH) USING TWO COMPUTER PROGRAMS: STEREONET AND R

Paper number: 265-16
Program: Technical Sessions
Time: 9:00 AM – 6:30 PM

Authors

Abstract

Lower-hemisphere plots and directional statistics are indispensable tools for structural geologists, and multiple software packages have been developed to serve that need. This presentation compares the application Stereonet (Allmendinger and Cardozo) to the R library geologyGeometry (Davis), using two case studies of structural data collected along the Wasatch monocline of central Utah.

The first case study focuses on strata that onlap the monocline limb, forming a distinct angular unconformity. Plots of poles to bedding distinctly show the angular difference between the older, folded monocline strata and the younger onlapping strata. We use bootstrapping to construct 95% confidence regions for the means of the two populations. The two regions are separated by 11.5°. Such statistical calculations can quantify the significance and effect size of an angular unconformity.

The second case study analyzes structural data (fractures, bedding) from several cross-strike transects along the length of the monocline. Stereonet lets us perform a visual fold test, to determine whether the fractures were pre- or post-monocline. R allows us to quantify the dispersion of the fold test, which is valuable in automating the test, for example. R can also perform orientation statistics on the unfolding rotations. For example, it can perform a regression to quantify geographic variation along the length and cross-strike of the monocline. These calculations inform our conclusions about stress axes.

Our comparison of the two programs found that purely visual analysis of data sets can overlook potentially significant trends in the data and oversimplify interpretations. The use of targeted statistical techniques when evaluating geographic and temporal trends improves the reliability of the results. The two techniques, however, should be used in conjunction with one another for full structural data collection strategy and analysis.

Geological Society of America Abstracts with Programs. Vol. 48, No. 7

doi: 10.1130/abs/2016AM-286886

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