How do faults grow above dykes?

点击翻译按钮获取中文摘要

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).