UTILIZING STRUCTURE FROM MOTION PHOTOGRAMMETRY TO INTERPRET FOLDING IN THE IBEX HILLS, CA

This work utilized structure from motion (SfM) photogrammetry to create point clouds and texturized models of a complex, folded outcrop of the Noonday Formation in the Ibex Hills. Traditionally, the majority of outcrop interpretation and, certainly, data collection, would need to take place in the field. However, using the workflow presented here, less field time is needed for complex outcrops; instead, they can be modeled, investigated in the lab, and later field checks can serve as ground truth. In this workflow, field work begins with site discovery, followed by capture of georeferenced photographs with ground control placement necessary for the SfM software. Once the data are processed (we use Agisoft Photoscan), the point cloud is imported into Maptek I-Site Studio, which allows for detailed mapping of the outcrop as well as the collection of surface orientation data to supplement field measurements. In this study, the traces of the folds were mapped and surfaces extruded parallel to the fold hinge, which provides a 3D view of the geometry of the outcrop. Final model development is performed in the software Move by Midland Valley for construction of 3D surfaces.

The Ibex Hills serves as an ideal case study for this methodology due to complex folding exhibited in key outcrops within the range. Understanding the folding displayed in these outcrops will provide insight into the contractional history of the Ibex Hills and provide context for the extension that followed. The outcrop was found to exhibit complex, disharmonic folds with the majority of the fold axes plunging gently (~10-15⁰) to the ESE and, in some cases, one of the limbs is overturned. Along the eastern edge of the outcrop and in areas adjacent to the modeled outcrop, however, the folds are more steeply plunging (~60-70⁰) and their trends vary from the NE to the SE. These shifts in orientation of the folds together with outcrop visualization indicate a refolded fold system related to changes in the direction of shortening, consistent with observations to the east indicating at least two kinematically distinct thrust systems in this region.