Magma plumbing systems: a geophysical perspective

Over the last few decades, significant advances in using geophysical techniques to image the structure of magma plumbing systems have enabled the identification of zones of melt accumulation, crystal mush development, and magma migration. Combining advanced geophysical observations with petrological and geochemical data has arguably revolutionised our understanding of, and afforded exciting new insights into, the development of entire magma plumbing systems. However, divisions between the scales and physical settings over which these geophysical, petrological, and geochemical methods are applied still remain. To characterise some of these differences and promote the benefits of further integration between these methodologies, we provide a review of geophysical techniques and discuss how they can be utilised to provide a structural context for and place physical limits on the chemical evolution of magma plumbing systems. For example, we examine how Interferometric Synthetic Aperture Radar (InSAR), coupled with Global Positioning System (GPS) and Global Navigation Satellite System (GNSS) data, and seismicity may be used to track magma migration in near real-time. We also discuss how seismic imaging, gravimetry and electromagnetic data can identify contemporary melt zones, magma reservoirs and/or crystal mushes. These techniques complement seismic reflection data and rock magnetic analyses that delimit the structure and emplacement of ancient magma plumbing systems. For each of these techniques, with the addition of full-waveform inversion (FWI), the use of Unmanned Aerial Vehicles (UAVs) and the integration of geophysics with numerical modelling, we discuss potential future directions. We show that approaching problems concerning magma plumbing systems from an integrated petrological, geochemical, and geophysical perspective will undoubtedly yield important scientific advances, providing exciting future opportunities for the volcanological community

Implicit 3D geological modelling in structurally complex environments

Geological structures are closely related to the genesis of ore deposits due to their ability to transport and trap hydrothermal fluids. Structurally-controlled mineralisation commonly shows distinctive geometries, orientations and spatial distributions that derive from associated structures. Moreover, structures such as faults and shear zones can offset, truncate and redistribute earlier mineralisation. Consequently, the analysis of these structures provides fundamental insights on the genesis of ore deposits and their evolution in a regional context. Accurate three-dimensional (3D) geological models are therefore required to visualise and analyse ore bodies and their host rock geometries in order to understand the underlying controls on mineralisation or remobilisation processes. The aim of this thesis is to understand the relationship between ductile structures (i.e., folds), brittle structures (i.e., fractures) and mineralisation. 3D implicit modelling of dense lithogeochemical drillhole datasets is employed to assess mineralisation in structurally complex environments. Such models are used to determine the geometry, size and orientation of ore bodies and first-order structures, and to understand their spatial relationships to host rocks. Working hypotheses based on such geometrical analyses can then be tested and compared to field measurements and to other independent datasets (e.g., geophysical maps). This approach is applied here to both the Navachab lode gold (Namibia) and the Currawong base metal (Victoria, Australia) deposits. In both cases, links are established between modelled ore bodies and local to regional structural patterns. The modelling also constrains the structural frameworks of the deposits and explains their temporal evolution. First order (regional) structural controls are also identified for future exploration in these areas and similar geological environments worldwide. In a third case study at Cape Liptrap and Cape Paterson (Victoria, Australia), the complex structural framework of basement folds and faults and their influence on brittle deformation in younger cover sediments are assessed by a combination of 3D implicit modelling and unmanned aerial vehicle (UAV) photogrammetry. The latter results in the computation of dense 3D point clouds and corresponding orthorectified photographs at sub-centimetre resolution. Bedding orientation measurements from Cape Liptrap are extracted from the point cloud and used to generate a 3D implicit model, which facilitates the analysis of fold geometries and the estimation of bulk shortening strain. Orthorectified photographs are used to measure and analyse fracture orientations in order to understand their relationship to the development of folds and fractures in the overlying cover rocks. This approach is applied to constrain the structural history of the southern part of the Lachlan Orogen and to elucidate the role of basement structure inheritance on subsequent basin development and deformation. The understanding of deformation-related structures and their influence on the emplacement and redistribution of mineralisation can be significantly enhanced by combining of modern modelling and mapping techniques with well-established geological methods and principles

Implicit 3D geological modelling in structurally complex environments

Geological structures are closely related to the genesis of ore deposits due to their ability to transport and trap hydrothermal fluids. Structurally-controlled mineralisation commonly shows distinctive geometries, orientations and spatial distributions that derive from associated structures. Moreover, structures such as faults and shear zones can offset, truncate and redistribute earlier mineralisation. Consequently, the analysis of these structures provides fundamental insights on the genesis of ore deposits and their evolution in a regional context. Accurate three-dimensional (3D) geological models are therefore required to visualise and analyse ore bodies and their host rock geometries in order to understand the underlying controls on mineralisation or remobilisation processes. The aim of this thesis is to understand the relationship between ductile structures (i.e., folds), brittle structures (i.e., fractures) and mineralisation. 3D implicit modelling of dense lithogeochemical drillhole datasets is employed to assess mineralisation in structurally complex environments. Such models are used to determine the geometry, size and orientation of ore bodies and first-order structures, and to understand their spatial relationships to host rocks. Working hypotheses based on such geometrical analyses can then be tested and compared to field measurements and to other independent datasets (e.g., geophysical maps). This approach is applied here to both the Navachab lode gold (Namibia) and the Currawong base metal (Victoria, Australia) deposits. In both cases, links are established between modelled ore bodies and local to regional structural patterns. The modelling also constrains the structural frameworks of the deposits and explains their temporal evolution. First order (regional) structural controls are also identified for future exploration in these areas and similar geological environments worldwide. In a third case study at Cape Liptrap and Cape Paterson (Victoria, Australia), the complex structural framework of basement folds and faults and their influence on brittle deformation in younger cover sediments are assessed by a combination of 3D implicit modelling and unmanned aerial vehicle (UAV) photogrammetry. The latter results in the computation of dense 3D point clouds and corresponding orthorectified photographs at sub-centimetre resolution. Bedding orientation measurements from Cape Liptrap are extracted from the point cloud and used to generate a 3D implicit model, which facilitates the analysis of fold geometries and the estimation of bulk shortening strain. Orthorectified photographs are used to measure and analyse fracture orientations in order to understand their relationship to the development of folds and fractures in the overlying cover rocks. This approach is applied to constrain the structural history of the southern part of the Lachlan Orogen and to elucidate the role of basement structure inheritance on subsequent basin development and deformation. The understanding of deformation-related structures and their influence on the emplacement and redistribution of mineralisation can be significantly enhanced by combining of modern modelling and mapping techniques with well-established geological methods and principles

Fold Stack (Cape Liptrap, VIC) orthophotograph basemap A3

The high resolution orthophotograph for this map was generated using a UAV photogrammetry workflow as outlined in Vollgger & Cruden 2015 (Journal of Structural Geology). <br><br>This basemap can be freely used for educational purposes (e.g., geological/structural mapping of sedimentary rocks). Students will be able to locate themselves quicker and more accurately in the field. Additionally, the high resolution basemap allows them to get a more holistic view of structural features (i.e., folds & fractures) as well as sedimentary features (i.e., slumping) that Cape Liptrap is famous for.<br><br>Note that the size of the PDF is optimized to be printed in format A3

GeoTrace and Compass rapid trace-mapping (example data)

<p>Example datasets provided for testing the GeoTrace (QGIS) and Compass (CloudCompare) plugins. The first dataset consists of two 10x10 meter orthophoto and DEM tiles from a high-resolution UAV survey of Bingie Bingie point (New South Wales, Australia) containing abundant joint traces as well as several mafic dykes. The GeoTrace plugin for QGIS can be used to extract the joint and contact traces from this dataset.</p><p>The second dataset comprises a dense-point cloud from a UAV survey of granite pinnacles at Cape Woolamai (Victoria, Australia), and can be used to test/demonstrate the Compass plugin in CloudCompare.</p

Rapid, semi-automatic fracture and contact mapping for point clouds, images and geophysical data

The advent of large digital datasets from unmanned aerial vehicle (UAV) and satellite platforms now challenges our ability to extract information across multiple scales in a timely manner, often meaning that the full value of the data is not realised. Here we adapt a least-cost-path solver and specially tailored cost functions to rapidly interpolate structural features between manually defined control points in point cloud and raster datasets. We implement the method in the geographic information system QGIS and the point cloud and mesh processing software CloudCompare. Using these implementations, the method can be applied to a variety of three-dimensional (3-D) and two-dimensional (2-D) datasets, including high-resolution aerial imagery, digital outcrop models, digital elevation models (DEMs) and geophysical grids. We demonstrate the algorithm with four diverse applications in which we extract (1) joint and contact patterns in high-resolution orthophotographs, (2) fracture patterns in a dense 3-D point cloud, (3) earthquake surface ruptures of the Greendale Fault associated with the Mw7.1 Darfield earthquake (New Zealand) from high-resolution light detection and ranging (lidar) data, and (4) oceanic fracture zones from bathymetric data of the North Atlantic. The approach improves the consistency of the interpretation process while retaining expert guidance and achieves significant improvements (35–65 %) in digitisation time compared to traditional methods. Furthermore, it opens up new possibilities for data synthesis and can quantify the agreement between datasets and an interpretation