Mechanical evolution, fluid pressure generation, and shallow slow slip in subductionzones

To unlock the enigmatic physical processes behind a recently discovered spectrum of slow earthquake phenomena, where faults slip at speeds intermediate between steady creep and earthquakes, the ERC Project ‘MICA’ aims to define testable hypotheses based on geological observations in active and ancient fault zones, and test these inferences in numerical models with realistic lab-defined stress-strength relationships. The potential outcome is to understand the physical mechanism(s) of slow earthquakes, and thus bridge the knowledge gap in understanding what controls the speed at which faults slip.

Traditionally, major tectonic faults were thought to accommodate displacement by either slow, continuous creep, or episodic, potentially damaging earthquakes. This old paradigm of two end-member fault behaviours is now replaced by a new notion that fault slip velocities span a continuum from millimetres per year to metres per second (Peng and Gomberg, 2010). Recently, slow slip events have been discovered at shallow depths in subduction zones, notably shallower than the seismogenic zone (e.g. Wallace et al., 2016). Interest in these events is highlighted by upcoming International Ocean Discovery Program (IODP) projects to drill an area of shallow slow slip in the Hikurangi Margin, New Zealand. This PhD project aims to explore structures that may form in response to shallow slow slip, and the rheology of the sediments and sedimentary rocks within which slow slip occurs.

This student will work on classic sites of the Gwna melange on Anglesey, which represents an analogue for ocean floor sediments deformed by non-coaxial shear in the presence of fluids at subgreenschist conditions, and thus provides an analogue for the shallow slow slip zone. Detailed mapping of coastal outcrops will elucidate the internal geometry and relative viscosities in zones of low pressure, low temperature shear deformation. A visit to collaborators in Japan will provide additional examples, and further sites may be added if need arises.

Field and microstructural observations are coupled with experiments using samples taken from the field, and performed at a range of pressure, temperature, and fluid pressure conditions that simulate the evolution of the sediments with increasing P, T, and strain. To this end, the student will perform experiments with Dr. A. Niemeijer in Utrecht, and microstructural studies at Cardiff University. Experimental and natural microstructures can be compared to investigate the likely conditions that prevailed in the natural example, as well as the P-T-strain (and therefore time) evolution of the samples. This data set will form an invaluable resource when interpreting drilling data from the Hikurangi margin, and elucidate the mechanisms that are active where enigmatic, shallow slow slip events occur.