About the Project
Project Background
Subduction thrust interfaces host seismic styles ranging from steady creep to the most damaging earthquakes on Earth. We do not yet understand the controls on how these faults slip. In particular, high pressure-low temperature deformation processes at the downdip end of the seismogenic zone are poorly understood. This is because they occur at depths where resolution of geophysical imaging is low and from which rocks are rarely exhumed. Yet, because earthquake magnitude is determined by rupture area, it is important to establish the controls on how deep an earthquake is likely to propagate.
Subduction zones can be divided into cold and warm end-members, based on their thermal gradient that depends primarily on convergence velocity and subducting plate age. Cold end-members host deeper subduction thrust interface earthquakes, and with current observational data seem to transition to deep creep without a pronounced zone of non-volcanic tremor that is observed in warmer end-members. An outstanding question is subduction thrust geology is therefore how deformation transfers from earthquakes to creep in cold subduction zones, where pressure suppresses fracture while low temperatures do not allow easy plastic creep.
Project Aims and Methods
Because rocks are more commonly preserved from warmer subduction zones, we know little of how deformation is accommodated at the colder end of blueschist conditions. This project aims to study blueschist facies rocks in locations where subduction-related deformation can be deciphered from deformation related to exhumation. Such locations include Ishigaki Island in the Ryukyu Arc of Japan and Alpine Corsica in the Mediterranean. They both include blueschist and/or amphibolite metamorphic units comprising schists and metamorphosed ophiolitic units, thus spanning the entire, deep frictional-viscous transition along the plate interface. More over, both complexes benefit from existing, detailed background geologic knowledge.
Questions to be addressed include, but are not limited to: (1) What are the dominant deformation structures, as a function of depth, in cold subduction zones? (2) What are the mechanisms that form these key structures? (3) What are the stress magnitudes and orientations at the frictional-viscous transition in cold subduction zones? (4) How do mineral reactions, mineral precipitation, and the evolution of porosity control “deep” fluid flow and its interplay with deformation? There is flexibility to alter and add to these questions as the project evolves.
Data to be collected will include field observations, focused on maps and structural data collected both with traditional and modern photogrammetry methods, and laboratory observations such as EBSD and chemical mapping on the Scanning Electron Microscope. From the field observations, information will be gained on the geometry of structures, from which stress and strain interpretations can be made. From the laboratory observations, active deformation mechanisms and their distribution between minerals can be inferred.
The project forms part of a larger ERC starting grant project focussed on the mechanisms of slow earthquakes, which occur at the edges of the subduction thrust interface in numerous locations world-wide. Within this project, there are opportunities to become involved with laboratory experiments and numerical models, depending on direction of the PhD project and interests of the student. In other words, while this is a field and laboratory geology project, there are opportunities for hypothesis testing using numerical or laboratory experiments. The successful candidate will have the opportunity to be part of an international team with field experience in different countries.
Candidate Requirements
The candidate must be willing and able to undertake periods of international fieldwork, both in a research group and independently. We are looking for a creative scientist with strong theoretical background, who can work within a dynamic research group, and who will participate in defining the project as it evolves. Field experience is essential, laboratory experience beneficial.
Training
Training in SEM work is provided in-house at the Cardiff microscopy facility. Field training is provided during site visits, including collaboration with overseas partners.