ORCID Profile
0000-0001-9836-2365
Current Organisation
University of Leeds
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Publisher: Geological Society of London
Date: 06-04-2022
Abstract: The geomorphology and sediment systems of volcanic areas can be influenced by uplift (forced folding) related to subsurface migration and accumulation of magma. Seismic geomorphological analysis presents a unique tool to study how surface morphology and subsurface magma dynamics relate, given seismic reflection data can image buried landscapes and underlying intrusions in 3D at resolutions of only a few metres–decametres. However, differential compaction of the sedimentary sequence above incompressible igneous intrusions during burial modifies palaeosurface morphology. Here we use 3D seismic reflection data from offshore NW Australia to explore how the stratigraphic record of igneous intrusion and associated ground deformation can be unravelled. We focus on a forced fold that likely formed in the Early Cretaceous to accommodate intrusion of magma, but which was later lified by burial-related differential compaction of the host sedimentary sequence. We show how: (1) marine channels and clinoforms may be deflected by syn-depositional intrusion-induced forced folds and (2) differential compaction can locally change clinoforms depth post-deposition, potentially leading to erroneous interpretation of shoreline trajectories. Our results demonstrate seismic geomorphological analysis can help us better understand how magma emplacement translates into ground deformation, and how this shapes the landform of volcanic regions.
Publisher: Volcanica
Date: 03-2021
Publisher: Oxford University Press (OUP)
Date: 06-2018
Publisher: Copernicus GmbH
Date: 27-03-2022
DOI: 10.5194/EGUSPHERE-EGU22-3238
Abstract: & & Magma transport through the Earth& #8217 s crust is commonly described to occur through interconnected planar sheet intrusions such as dykes and sills, which form so called magma plumbing systems. Elongate intrusion geometries (i.e., magma fingers and segments), hereafter referred to as & em& elements& /em& , may form during magma transport due to viscous and/or elastic instabilities at the propagating intrusion tip, and they are often observed at the outer margin of solidified sheet intrusions. Field observations, geophysical datasets, and analogue models further show that when elements grow in width, they can coalesce, indicating that planar sheet intrusions can form and grow by the amalgamation of in idual elements. Previous studies suggest that the emplacement and growth of elements is accommodated by one dominating emplacement end-member process, namely: i) tensile-elastic fracturing, ii) shear failure, or iii) viscous deformation (e.g., host rock fluidisation). However, the interplay between in idual end-member processes remains poorly understood. Here we present field observations of elongate magma fingers located at the SE margin of the Paleogene Shonkin Sag laccolith (Montana, USA) to assess how host rocks (Cretaceous Eagle Sandstone) deform to make space for the magma. We combine drone photogrammetry surveys with field mapping and microstructural analyses to describe and quantify host rock deformation in the vicinity of 37 magma fingers, and we conduct thermal modelling to further evaluate the conditions at which viscous deformation due to host rock fluidisation is feasible.& & & & Our field observations show that all three proposed end-member processes accommodated the emplacement of magma fingers at the SE margin of the Shonkin Sag laccolith. Brittle deformation, shear failure, and folding of host rock mainly occurs in the compressional regime between two adjacent magma fingers, whereas host rock fluidisation and mobilisation is predominantly observed at the cross-sectional, lateral finger tips. Our photogrammetric analyses show that up to 40 % of the finger thickness is accommodated by elastic host rock uplift. Critically, this range of host rock deformation mechanisms is observed in one outcrop at metre scale, and in some cases associated with an in idual magma finger. Thermal modelling of temperatures ahead of a propagating intrusion tip indicates that intrusion induced host rock fluidisation is only possible at low tip velocities of & #8804 10& sup& -5& /sup& m/s, which can vary depending on the emplacement depth, magma temperature, and the thermal diffusivity of the host rock.& & & & Overall, we conclude that the emplacement of magma fingers at the outer margin of the Shonkin Sag laccolith was accommodated by a combination of elastic host rock uplift and both brittle and ductile host rock deformation. Based on our field observations and thermal modelling results, we suggest that intrusion tip velocities and the resulting strain rate are key parameters that control the dominating space-making mechanisms during magma emplacement. Due to the elongate geometry of elements and the resulting different strain rates at their lateral and frontal tips, we further propose that deformation mechanisms observed at lateral tips in cross sectional outcrops are likely decoupled from those at frontal tips such that they may not be equivalent.& &
Publisher: Frontiers Media SA
Date: 22-08-2019
Publisher: California Digital Library (CDL)
Date: 22-03-2023
DOI: 10.31223/X5W63P
Abstract: Unravelling magma flow in ancient sheet intrusions is critical to understanding how magma pathways develop and feed volcanic eruptions. Analyzing the shape preferred orientation of minerals in intrusive rocks can provide information on magma flow, because crystals may align parallel to the primary flow direction. Anisotropy of magnetic susceptibility (AMS) is an established method to quantify such shape preferred orientations in igneous sheet intrusions with weak or cryptic fabrics. However, use of AMS data to characterize how magma flows within the in idual building blocks of sheet intrusions (i.e., magma fingers and segments), hereafter referred to as elements, has received much less attention. Here we use a high spatial resolution s ling strategy to quantify the AMS fabric of the Eocene Shonkin Sag laccolith (Montana, USA) and associated elongate magma fingers. Our results suggest that magnetic fabrics across the main laccolith reflect sub-horizontal magma flow, and inferred flow directions are consistent with an underlying NE-SW striking feeder dyke. Within the magma fingers, we interpret systematic changes in magnetic fabric shape and orientation to reflect the interaction between competing forces occurring during finger-parallel magma flow (i.e., simple shear) and horizontal and vertical inflation (i.e., pure shear flattening). For ex le, we highlight how local crossflow of magma between coalesced fingers increases the complexity of magma flow kinematics and related fabrics. Despite these complexities, the AMS data in coalesced magma fingers maintain their internal flow- and inflation-related fabrics, which suggests that magma flow within the fingers remains channelized after coalescence. Given that many sheet intrusions consist of amalgamated elements, our findings highlight the need to carefully consider element distribution and s le locations when interpreting magma flow based on AMS measurements.
Publisher: California Digital Library (CDL)
Date: 31-01-2022
DOI: 10.31223/X5659D
Abstract: The propagating margins of igneous sills (and other sheet intrusions) may ide into laterally and/or vertically separated sections, which later inflate and coalesce. These components elongate parallel to and thus record the magma flow direction, and can form either due to fracture segmentation (i.e., ‘segments’) or brittle and/or non-brittle deformation of the host rock (i.e., ‘magma fingers’). Seismic reflection data can image entire sills or sill-complexes in 3D, and their resolution is often sufficient to allow us to identify these distinct elongate components and thereby map magma flow patterns over entire intrusion networks. Yet seismic resolution is limited so we typically cannot discern the centimeter-to-meter scale host rock deformation structures that would allow the origin of these components to be interpreted. Here, we introduce a new term that defines the components (i.e., ‘elements’) of sheet-like igneous intrusions, without linking their description to emplacement mechanisms. Using 3D seismic reflection data from offshore NW Australia, we quantify the 3D geometry of these elements and their connectors within two sills and discuss how their shape may relate to emplacement processes. Based on seismic attribute analyses and our measurements of their 3D geometry, we conclude that the mapped elements likely formed by non-elastic brittle and/or non-brittle deformation ahead of the advancing sill tip, implying they are magma fingers. We show that thickness varies across sills, and across distinct elements, which we infer to represent flow localization and subsequent thickening of restricted areas. The quantification of element geometries is useful for comparisons between different subsurface and field-based datasets, spanning a range of host rock types and tectonic settings. This in turn facilitates the testing of magma emplacement mechanisms and predictions from numerical and physical analogue experiments.
Location: United Kingdom of Great Britain and Northern Ireland
Location: United Kingdom of Great Britain and Northern Ireland
Location: United Kingdom of Great Britain and Northern Ireland
Location: United Kingdom of Great Britain and Northern Ireland
Location: United Kingdom of Great Britain and Northern Ireland
Location: United Kingdom of Great Britain and Northern Ireland
No related grants have been discovered for Craig Magee.