3D MAPPING OF CLIFF FACES: A MODERN SOLUTION TO POLYPHASE DUCTILE DEFORMATION PROBLEMS

Modern technology affords a solution to what is generally the most difficult geometric problem in structural geology: unraveling the details of polyphase folding. Structure-from-Motion/Multiview Stereo (SM) Photogrammetry provides an inexpensive method for development of full-color, 3D terrain visualization at scales ranging from hand specimen to quadrangle scales. Most SM studies to date have concentrated on outcrop to hand specimen scales, but here we report on experiments in 3D mapping at macroscopic scales with an emphasis on cliff faces where conventional 2D or 2.5D approaches fail. We use three general data collection methods to develop models at the scale of several km: ground based imaging, drone based imaging, and imaging from a chartered fixed-wing aircraft. Although each method has pros and cons, proper image acquisition among all three methods lead to terrain models with resolutions ranging from sub-centimeter to decimeter level which leads to unprecedented visualizations of inaccessible, steep terrain where outcrop is near 100%. We suggest that it is critical that structural geology embrace this technology in the scale range from very large outcrop to scales of several kilometers. To support this suggestion we show examples of how this technology provided unprecedented analysis of multiply deformed metamorphic tectonites in the Panamint Mountains of eastern California. In this area we recognized years ago that at outcrop scale there were 3 phases of folding, yet conventional 2D mapping revealed minimal evidence of macroscopic structures developed during the first two, isoclinal folding events. Initial ground based SM studies revealed macroscopic isoclinal folds at one locality (Brush et al., 2018, Geosphere) but elsewhere large isoclinal folds were cryptic. Drone based SM models of cliff faces at resolutions of ~10 cm resolved this issue, however, with eye-popping visualizations of noncylindrical folds with amplitudes in the 100 m to km scale. The visualization demonstrates strongly curved fold axes that are either an interference pattern of superposed isoclinal folds or macroscopic sheath folding; the latter most likely given large object strains seen in metaconglomerates and a strong stretching axis parallel to the crests of dome/basin fold geometries.