Quantitative Analysis of Shear Zone Structures at Cap de Creus (NE Spain) – Insights from High Resolution, Remote Sensing Mapping Techniques

Abstract

Recent advances in Unmanned Aerial Systems (UAS) and Computer-based Structure From Motion (SFM) enable the digitization, reconstruction, archiving, and quantitative analysis of Geologic outcrops in a digital environment. Reconstructed models represent a snapshot in time of an outcrop. Digital Orthophotos, 3D models, Surface Maps, 2D Vector files, and 3D Polygon maps are created by the geometrical alignment and reconstruction of thousands of high-resolution aerial and ground photographs. In this research, a Photogrammetry-based workflow is evaluated in multiple case studies on Shear Zones, Pegmatites, Aplite Dykes, and Diorite Enclaves within the Cap de Creus Peninsula in NE, Spain. The scope of this research is to assess the potential of these techniques to further our understanding of the deformation history of the study area and, in the process, develop valuable strategies and workflows to extract quantitative strain measurements. While the structural deformation history of the Cap de Creus area has been studied in detail, deformation on a mesoscale until today has not been possible due to technological constraints. Deformation markers and finite strain are extracted from the reconstructed models (XYZ) and orthophotos (XY) by applying specialized software that compiles the obtained data into a readable format, rose diagrams, stereonets, heat maps, and polar plots. These calculated datasets are calibrated and compared to field measurements to constrain the deformation conditions. Quantitative datasets are combined with high-resolution UAS surface maps to reconstruct and model the progressive deformation phases present at Cap de Creus. The resulting 2D maps and 3D numerical models represent a Geologist's best interpretation of the subsurface evolution of the late Variscan mountain-building phase in NE Iberia. This thesis demonstrates how UAS-SFM workflows can deliver high-quality results that compare and, more importantly, enhance traditional methods, enabling (i) the acquisition of larger datasets faster and more efficiently over a much larger area, (ii) Post-fieldwork quality control on field measurements, (iii) remove random human and equipment error, (iv) save valuable time during fieldwork by enabling base maps acquired in the field to be enhanced with digital measurements, enabling the Geologist to work more efficiently. The different case studies explored in Cap de Creus exemplify how powerful reconstructed field outcrops scaled to accurate world dimensions are and how they enable enhanced data acquisition in a digital environment and the ability to make and test new observations indefinitely. Digital maps of the Late Variscan Roses Granodiorite are compiled into 2D and 3D models with a Ground Sampling Distance (GSD) of 3.31mm/pix, enabling the acquisition of 1017 diorite enclaves and hundreds of intruded aplite dykes in just a few hours for quantitative strain analysis. This thesis also demonstrates the importance of using a concise and targeted approach to UAS-based mapping. Albeit mapping vast areas is possible, the resources required for reconstruction, the amount of data generated, and the negative impact on resolution constrain its usefulness in Geologic quantitative analysis. It is preferable instead to focus UAS mapping on a specific outcrop where visual markers are easily identifiable and can be used to quantify deformation. Quantitative analysis of the Late Variscan Roses Granodiorite provides new strain estimates for the mafic enclave deformation using the Rf - Φ method, 40% shortening under a close to three-dimensional homogeneous flattening and 50% shear strain of the retrograde greenschists shear zones which are in accordance with traditional and manual techniques for measuring strain. The ability to digitally measure a large population of stain markers furthers the precision and constraints on deformation.