Geometry, Timing, and Kinematics of Neogene Extensional and Transtensional Structures of Southern Death Valley: Implications for Regional Reconstructions and a Corrective Method for Rigid Body Rotations
Chapter 1:Advancements in computing capabilities over the last decade have allowed for the routine creation of Structure from Motion-Multiview Stereo (SfM-MVS) terrain models that can serve as base for high resolution geologic mapping. Outcrops models developed from these systems are high-resolution, photo-realistic 3D base providing unprecedented capability for geometric analysis. Yet, before this technology becomes a mainstay of field geology, the potential errors associated with it must be well understood. Here, we compare orientation measurements from multi-point analyses on the SfM-MVS point clouds to those taken in the field with the objective of resolving the geometry of complex folds within the outcrop. We also analyzed two point clouds of the same exposure created from different ground-based cameras to compare the ranges of error. We found that the point clouds produced from ground-based photos exhibited significant rigid-body rotation relative to the real world despite well distributed ground control, yet the models maintained a realistic scale and internal geometry. To correct the error the model values were rotated and the discrepancy reassessed. The two point clouds produced similar results, however, the Sony compact-digital-camera-based point cloud ultimately corresponded more closely to field values. We suggest that the primary cause of the error in the point clouds was GPS-based and was enhanced by the lack of significant topographic relief in our camera positions, allowing rigid-body rotations along the axis of the photographic array. This outcome suggests that care must be taken when GPS errors are a significant fraction of the outcrop size and relatively 2D outcrops imaged by a relatively 1D image array are subject to rotation errors that are difficult to remove without high-resolution ground control. Short of using a UAV and/or RTK-GPS we show how this can be resolved simply by collecting several known orientations in the field, which can then be used to orient the model more accurately, akin to ground control points. This addition is a key step if this method is to be used for more thorough analysis and is a general method that could be used to orient virtual outcrops with no geographic reference. Chapter 2: Recent mapping in the Ibex Hills demonstrates Mesozoic contractional structures overprinted by Neogene extension and strike-slip deformation. The earliest deformation is interpreted as two phases of Mesozoic shortening, recorded primarily by overprinted fold systems. The oldest folds (F1) are tight to sub-isoclinal, approximately E trending folds with an axial planar to fanning pressure solution cleavage. F1 is overprinted by approximately N-S trending, more open folds that produce strong curvature in F1 axes and type 1 interference patterns. These folds are cut by a series of Neogene faults. The oldest identified faults consist of currently low to moderate angle, E-NE dipping normal faults which are folded about a SW-NE trending axis. That this folding postdates at least some of the extension suggests a component of syn-extensional shortening that is probably strike-slip related. Cross-cutting these east dipping faults are both NW and NE striking, high angle normal faults. In general, it appears that the NE striking high-angle faults are younger and displace the other normal faults, consistent with NW directed extension superimposed on older SW directed extension. Additionally, a major N-S striking, oblique-slip fault bounds the eastern flank of the range with slickenlines measured at rakes of <30 degrees which together with the map pattern suggests dextral-oblique movement along the east front of the range. In the central Ibex>Hills, the oblique fault clearly cross-cuts low angle normal faults, however, to the north it appears to be truncated by normal faults, potentially signifying a zone of accommodation related to the adjacent Sheephead Fault (SF), overprinting by NW directed extension, or both. Transcurrent systems have been proposed to explain extension parallel folding and the geometry of folded low angle faults followed by NW striking faulting in the Ibex Hills aligns with these models. Collectively the field data suggest an early period of SW directed extension is recorded in the Ibex Hills, which was suggested by B. Troxel. The younger NE striking faults presumably record the younger NW directed extension during or after SF movement. Chapter 3: The Ibex Hills occupies an important position in the context of southern Death Valley tectonics as it is often incorporated as an up-dip portion of the Black Mountain Detachment Fault, a critical interpretation for the structural evolution of the region. In addition, it is proximal to both the Sheephead Fault Zone and the Grand View Fault, structures which have been incorporated into regional constructions in the past. Despite these factors the Ibex Hills have not been thoroughly considered for previous reconstructions of the southern Death Valley region. In this chapter the Ibex Hills are correlated across the region using a 3D structural model created in Move by Midland Valley. Geologic mapping done in the Ibex Hills revealed a previously unmapped granitic mega-breccia which provides a minimum amount of displacement to within 10 km of the Kingston Range. Further piercing lines were provided by an intrusive unit in the northern Ibex Hills as well as the unconformity which underlies the lower Noonday Dolomite. The results of the reconstruction imply a significant amount, >25 km, of strike-slip movement along a combination of the Sheephead Fault Zone and the Grand View Fault. In addition, a component of clockwise rotation is needed to properly restore the granitic mega-breccia of the Ibex Hills. This model is also consistent with an early phase of southwest directed extension which is preserved in the Ibex Hills, followed by more recent, northwest directed extension. ^
Fleming, Zachariah Fleming, "Geometry, Timing, and Kinematics of Neogene Extensional and Transtensional Structures of Southern Death Valley: Implications for Regional Reconstructions and a Corrective Method for Rigid Body Rotations" (2018). ETD Collection for University of Texas, El Paso. AAI13421565.