Geometry and Visualization of Folds

Abstract

Reconstructing the geometry of geologic folds, and subsequently expressing this geometry visually, are necessary for understanding the kinematic and mechanical properties of folds. Novel technology for acquiring 3D geometric data, including drone-based photography and structure-from-motion photogrammetry, enables data collection at fold outcrops that were previously difficult to study in the field. This new technology introduces opportunities to enhance kinematic and mechanical models of folds from field data, but also requires the exploration of new methods for reconstructing and visualizing fold form from 3D point clouds. I present work involving point-cloud data collected from a drone-based photographic survey in the Bear Valley Strip Mine (Shamokin, Pennsylvania), where the 30-meter-high Whaleback Anticline and adjacent folds are exquisitely exposed in three dimensions. While the strip mine has been a popular field trip destination for half a century, it has not been previously possible to make detailed observations throughout the folded surface or making a detailed geometric reconstruction. In Chapter 1, I provide the inspiration and roadmap of this work. In Chapter 2, I propose a new methodology of interpolating fold form with non-uniform rational basis splines to construct a smooth, continuous, mathematically operable fold surface from noisy and discontinuous point-cloud data of the fold train at the Bear Valley Strip Mine. In Chapters 3 and 4, I introduce the Structural Geology Query Toolkit – an open-source software package for building first-person-perspective field-geology simulations in which a student or researcher can visualize and interrogate geologic surfaces, like the folded sandstone surface at the Bear Valley Strip Mine, generated from 3D scans. Finally, in Chapter 5, I leverage 3D models of the Whaleback and the Structural Geology Query Toolkit to determine that mesoscale extensional faults accommodate <6% strain evenly distributed throughout the Whaleback’s surface, which contributes to our understanding of the late-stage kinematics of buckle folding. Although the methods and new tools discussed in this dissertation revolve around data collected at the Bear Valley Strip Mine, they are intended to be applicable to other field sites where 3D scans of outcrops are possible.