Insights into dyke emplacement mechanisms using new high-resolution 3D digital outcrop methods [article]

Gregory Dering
2020
Dykes are critical for the transport of magma through the crust. Their geometries, connectivity and internal textures preserve important information on dyke emplacement, linkage and magma dynamics. Unfortunately, our ability to develop new insights is limited by the resolution and continuity of data that can be extracted from the field. The emergence of digital photogrammetry along with Unmanned Aerial Vehicles (UAVs, a.k.a. drones) have expanded our capability to acquire data from geological
more » ... tcrops with a precision and sampling density not previously feasible. Using these methods, dyke-related features can be measured across five orders of magnitude (mm to km). UAV-based imagery enables the observation of fine-scale textural features (phenocrysts, chilled zones, stepped margins) that are difficult to quantify on the ground. However, the significance of results depends on 3D model accuracy and resolution, which therefore are tracked and reported. Here, these techniques are reviewed and new approaches for the structural analysis of dykes are derived, which are applied to outstanding exposures of dyke swarms from Albany, southwestern Australia, and the Sierra Nevada, USA. At Albany, a combination of ultra-high resolution, 3D photogrammetric models (3-4 mm per pixel) and field mapping enabled the identification of a spatial relationship between dyke-parallel faulting, jointing and dyke swarm emplacement. The data show mutually overprinting dyke-fault relationships and a symmetrical increase in the number of faults and joints towards the swarm, indicating that dyke emplacement and faulting were coeval. The identification of coeval dyke-parallel faulting is not expected but matches recent microseismicity surveys indicating shear failure along fault planes parallel to the orientation of intruding dykes. We present the first explanation for such dyke-parallel shear failure where, during dyke emplacement, the elastic properties of the host-rock change due to jointing, leading to local stress reorientation and the triggering of dyke-parallel shear fractures. Dyke-parallel shear failure may be more common than previously thought, especially during upper crustal intrusion events. In the second study, the role of crystallisation and lateral linkage between dyke segments were examined. Three-dimensional dyke aperture measurements at exceptionally high resolution (3-5 mm per pixel), over strike distances up to ~100 m, yielded 20,000 measurements from 40 discrete dyke segments. This exceptionally dense and precise digital sampling method reveals flat-topped aperture-length profiles and a surprising fine-scale oscillation in dyke aperture. The data confirm the finding of previous studies that solidified dyke tips are commonly much blunter than predicted from linear elastic fracture mechanics. A new model for dyke linkage is presented that accounts for blunt tips between segments, second-order dyke aperture variations and evolution from tapered to flat topped aperture-length profiles. We suggest vertical channels of seismicity identified around laterally propagating dykes are the product of catastrophic linkage between segments, rather than volatile escape or vertical magma migration. iv Table of contents Chapter 1: Introduction 1.1 Context of this thesis 1.2 Aims of this thesis 1.3 Thesis organization 1.3.1 Thesis by papers and authorship 1.3.1.1 Chapter II 1.3.1.2 Chapter III 1.3.1.3 Chapter IV 1.3.1.4 Chapter V 1.3.2 Appendices 1.3.1.1 Supporting references 1.3.1.2 Lower crustal intrusion networks 1.3.1.3 Coauthor contributions external to thesis 1.4 References Chapter 2: Review of drones, photogrammetry and emerging sensor technology for the study of dykes: Best practises and future potential Abstract 2.1 Introduction 2.2 A practical guide for collecting high resolution data from volcanological outcrops 2.2.1 Digital photogrammetry 2.2.2 Best practise in UAV platform selection 2.2.3 Target definition and sampling resolution 2.2.4 Use of ground control points 2.3 Case studies 2.3.1 Example of mapping dyke-induced deformation 2.3.2 Example of high resolution aperture measurements 2.3.2.1 Extraction of dyke aperture data from non-uniform surfaces 2.3.2.2 Aperture result from an andesite dyke 2.3.3 Example of subtle lithologic control on dyke geometry 2.3.4 Summary 2.4 Emerging techniques and technologies 2.4.1 Sensors 2.4.2 Digital outcrop analysis 2.4.2.1 Automated and semi-automated fracture and lithology mapping 2.4.2.2 Structural analysis techniques 2.5 Discussion v 2.5.1 UAV technologies as a step-change in field observation 2.5.2 Summary of survey quality reporting 2.5.3 Insights into volcanic processes using UAV-photogrammetry data 2.5.4 Educational value 2.6 Conclusion 2.7 References Chapter 3: Evidence for dyke-parallel shear during syn-intrusion fracturing Abstract 3.1 Introduction 3.2 Dyke-induced faulting, fracturing, and seismicity 3.2.1 Mechanical predictions 3.2.2 Geophysical observations 3.3 Geological Setting 3.3.1 Albany Fraser Orogen 3.3.2 Post-Albany-Fraser Orogen mafic intrusions 3.3.3 Vancouver Peninsula locality 3.4 Fracture and dyke mapping 3.4.1 UAV-assisted photogrammetry 3.5 Brittle deformation around dykes 3.5.1 Fracture populations 3.5.2 Pre-and syn-emplacement dyke-parallel shear 3.5.3 Decametre scanline results 3.5.4 Metre-scale scanline results 3.6 Discussion 3.6.1 Dyke-induced fracture zones 3.6.2 Conceptual model of dyke-related damage corridors 3.6.3 Implications for interpretation of dyke-induced seismicity 3.7 Conclusion 3.8 References Appendix 3.1: Photogrammetric survey methods and measurement error Appendix 3.2: Scanline data Appendix 3.3: Geologic map of Mistaken Island locality vi Chapter 4: Dyke propagation and scaling properties revealed from undulating apertures, captured by UAV Abstract 4.1 Introduction 4.2 Geological setting 4.2.1 Study site 4.3 Methodology 4.3.1 Digital outcrop reconstruction 4.3.2 High resolution aperture measurements 4.4 Tip Zones 4.5 Aperture results 4.5.1 Blunt and flat-topped aperture profiles 4.5.2 Dyke segments 4.6 Discussion 4.6.1 Flat tops and blunt ends diverge from predicted aperture profiles 4.6.2 A breakout model for dyke growth 4.6.3 Aperture as a record of conduit evolution 4.6.4 Implications for the geophysical monitoring of dyke propagation 4.6.5 Implications for scaling relationships 4.7 Conclusion 4.8 References Appendix 4.1: Supplemental dyke aperture data Appendix 4.2: Physical parameters for aperture profile models Chapter 5: Conclusion 5.1 Original research outcomes 5.1.1 Digital outcrop methodologies 5.1.2 Dyke-induced fracture corridors 5.1.3 Aperture variation 5.2 Future research 5.3 References 3 Chap. 1 mechanisms of dyke growth by measuring variation in dyke aperture at very high resolution. Chapter 5 summarises the main findings of the thesis and their implications for dyke swarm emplacement. Chapter 2 Chapter 2 is titled "Review of drones, photogrammetry and emerging sensor technology for the study of
doi:10.26182/5eab97f21c63d fatcat:xa74min3wjadnltvfjt3l4jm2u