How do faults grow above dykes?

Lead Research Organisation: University of Leeds
Department Name: School of Earth and Environment

Abstract

Magma travels through Earth's crust to the surface, where it erupts at volcanoes, along vertical paths that have a sheet-like shape (dykes). When dykes are injected, either vertically or laterally, they fracture and push apart the surrounding rock, producing small earthquakes. Continued dyke injection causes fractures to develop into faults, where rock on one side of the crack starts to slip passed the other. Fault slip can pull down and extend or push up rock directly above the dyke, sometimes deforming Earth's surface. Monitoring earthquakes and ground deformation generated by dyke-induced faults can therefore tell us where dykes are injecting, providing us warning of possible eruptions. Studies of injecting dykes and dyke-induced faulting in Ethiopia show that they can also aid continent fragmentation, although these structures have yet to be found along the margins of continents where break-up once occurred. In addition, satellite images of planets (e.g. Mars) indicate that dyke-induced faults deform their surface. It is thus clear that dyke injection and dyke-induced faulting plays and has played a major role in shaping the volcanic and/or tectonic history and surface morphology of Earth and other planets.

To understand how dykes and dyke-induced faults control different volcanic, tectonic, and planetary processes, we first need to identify how faults grow above dykes in three-dimensions. However, seismicity and ground deformation related to active dyke injection, which cannot directly be observed, are rarely captured using geophysical techniques and only a small part of a dyke-induced fault can be studied at the surface. Conversely, where ancient dykes are exposed at Earth's surface, erosion of the overlying rocks has often removed dyke-induced faults and the earthquakes that accompanied dyke injection have long-since ceased. To circumvent these problems, many computer and sandbox models have been developed to try and replicate fault growth above dykes. These models have produced numerous hypotheses for dyke-induced fault growth, but without examination of the 3D structure of natural dykes and dyke-induced faults, they cannot be tested. Therefore, despite over 40 years of research, we still do not understand the true 3D structure or evolution of dykes and dyke-induced faults.

I have recently identified the first series of ancient dykes and dyke-induced faults to be observed in seismic reflection data, which provide 3D X-ray like images of Earth's subsurface, from the margins of a continent (NW Australia). These data present a unique and exciting opportunity to study the 3D structure of dykes and dyke-induced faults. By measuring offset of sedimentary rocks across faults, which record how slip accumulated, I will be able to test previous model predictions of dyke-induced fault growth. Because the processes driving dyke injection and faulting offshore of NW Australia have long-since ceased, I will also study active dyke-induced faults breaking the surface in Ethiopia. I will specifically use high-resolution, aerial Light Detection and Ranging (LiDAR) images collected in 2009 and 2012 to identify how faults grew and interacted during a single dyke injection event in 2010. Results from these analyses will be used to design of new analogue models that will replicate dyke injection and dyke-induced faulting in 3D, under different tectonic settings (e.g. extension), and using more realistic rock/magma characteristics. This cross-disciplinary research will reveal how faults grow above dykes, raising important implications for our understanding of: (i) how we can use dyke-induced fault activity to assess potential eruptions; (ii) the role dykes and dyke-induced faults play in the break-up of continents; (iii) whether dykes and dyke-induced faults influence the evolution of continental margins, which host most of the world's oil and gas; and (iv) dyke and fault structure beneath the surface of other planets (e.g. Mars).

Planned Impact

1. Ethiopian government and decision makers
Volcano monitoring worldwide commonly relies on analysing seismicity or satellite-/surface-based techniques, which map ground deformation, associated with subsurface magma movement. Within Ethiopia since 2008, the task for monitoring, assessing, and mitigating natural hazards has belonged to the Geohazards Investigation Core Process (GHICP) at the Geological Society of Ethiopia, who disseminate information to a range of public and private sector stakeholders (e.g., Ethiopian Roads Authority). Following the release of a World Bank report in 2011, which classified volcanic hazards in Ethiopia as high risk due to the lack of volcano monitoring, Biggs et al. (2011) advocated the extended use of InSAR to monitor ground deformation and track magma movement in and around Ethiopian volcanoes; this aspect of GHICP is still being developed. Results from this study will inform natural hazard assessment in the Dabbahu magma segment and elsewhere in Ethiopia by constraining how dyke-induced fault slip and associated seismicity and/or surface deformation relate to dyke intrusion. Understanding dyke and dyke-induced normal fault hazards, which affect the surface geomorphology, is also important when considering infrastructure development (e.g. road planning). This aspect of the project is expected to provide a short to long term impact.

2. Hydrocarbon industry
Frontier sedimentary basins, where little exploration has previously been conducted but there is the potential for finding significant reserves, are an increasing focus for the hydrocarbon industry. Exploration and extraction in these frontier areas is challenging, with the geological history of those located along volcanic rifted margins typically complicated by the presence of extensive igneous systems. Whilst a substantial research has focused on understanding sill-complexes and lava sequences in these settings, particularly with a view to improve seismic imaging and drilling procedures, dykes have largely been ignored because they are rarely imaged in seismic reflection data. However, the presence of dykes can positively or negatively impact petroleum systems. By providing insights into the distribution and scale of dyke swarms potentially located along volcanic margins, as well as developing a method to map dyke swarms through the identification of dyke-induced normal faults, this study will benefit: (1) seismic acquisition companies, particularly those specialising in acquiring regional 2D datasets across the continent-ocean transition zone where dyke swarms are expected, by providing constraints that can be used to design bespoke data collection and processing strategies (e.g. suitable seismic velocities for areas host in dyke swarms); and (2) de-risking potential exploration targets in frontier basins, within or near the continent-ocean transition zone, which may be affected by dykes. This aspect of the project is expected to provide a mid- to long term impact.

3. General public within the UK
Volcanism and tectonic activity, both on Earth and other planetary bodies, readily capture the public imagination, particularly amongst high-school students. For example, filming of several television programmes focusing on the geological evolution of continental breakup in Ethiopia (e.g., The Great Rift: Africa's Wild Heart, BBC2) highlights that this research will be of direct interest to the general public. Work emanating from this study thus presents an excellent opportunity to engage school and college students on how various branches of science (e.g., physics, chemistry, and geology) can be used to study natural phenomena. By targeting GSCE students before they make crucial A-level choices, I will take the opportunity to communicate the importance of maths and core science subjects to those who are interested in pursuing a career in Earth Sciences. This aspect of the project is expected to provide a short term impact.

Publications

10 25 50
 
Description Lapworth Lecture (University of Birmingham) 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Public/other audiences
Results and Impact Invited to deliver the Lapworth Lecture at the University of Birmingham; a one hour seminar on 'Rethinking volcanology' attended by the general public, students within the School of Earth, Geography, and Environmental Science at the university, and academic staff. Good questions and discussion afterwards, with several students contacting me about a related PhD project afterwards
Year(s) Of Engagement Activity 2018
 
Description Talks on 'How do faults grow above dykes?' at VMSG and TSG special interest groups annual meetings 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Postgraduate students
Results and Impact Delivered two 15 minute presentations at the VMSG (Volcanic and Magmatic Studies Group) and Tectonic Studies Group (TSG) annual meetings, which were each attended by ~150-200 people, the majority of which postgraduate students. My talk sparked lots of questions and debate, which promoted interest of the topic and highlighted potential collaborations
Year(s) Of Engagement Activity 2019