2016: Year in review

The Group's research centres on high-pressure, high-temperature laboratory studies of (1) ductile rheology and seismic properties of crustal and mantle materials, and (2) deformation processes associated with fault slip in seismogenic and aseismic slip regimes, including in the presence of reactive pore fluids.

Laboratory measurements of macroscopic physical properties such as seismic wave speeds and attenuation, strength, deformation rates and permeability are interpreted through microstructural studies using optical and electron microscopy. Often it is necessary to prepare, from either natural or synthetic precursors, simpler synthetic materials whose properties are amenable to more detailed interpretation than those of complex natural rocks. Our interest in Earth materials is shared by members of the School's Experimental Petrology Group, whose research focuses primarily upon the chemical aspects of the behaviour of Earth materials.

The experimental studies on fault mechanics are complemented by field-based studies, along with microstructural and isotopic studies and numerical modeling aimed at exploring coupling between deformation and fluid flow in exhumed faults, shear zones and fracture-controlled hydrothermal ore systems.

Our research has application to:

• geodynamic modelling and the interpretation of seismological models

• understanding controls on earthquake nucleation and rupture propagation

• understanding links between deformation, fluid flow and ore deposit formation.

A highlight this year has been continued experimental study of slip processes on bare quartz interfaces, as part of Kathryn Hayward's PhD.  The development of a laser interferometer, in collaboration with colleagues in the Research School of Physical Sciences and Engineering, has allowed us, for the first time, to measure slip rates during small but rapid slip events during stick-slip behaviour of laboratory faults.  This development was funded by a grant from the ANU Major Equipment Committee.  The laser interferometer now allows us to record laboratory slip events on microsecond timescales and with micron-scale resolution of displacement (Figure 1). This facility provides us with an unrivalled capacity to capture earthquakes in the laboratory at confining pressures as high as 300 MPa, temperatures up to 1000°C, and at controlled pore fluid pressures up to 275 MPa. The first results were published recently in Geology. Ongoing work will focus on study of the first milliseconds and tens of microns of seismogenic slip to explore earthquake nucleation and the dynamic weakening processes that allow some initially small ruptures to cascade into large, destructive earthquakes.  Experiments will be used to examine earthquake nucleation in both crustal and subduction interface regimes.

Figure 1. (A) Representative differential stress - displacement curves for experiments undertaken over a range of ambient temperatures.  Sliding behaviour transitions from stable sliding at ~ 500C through to low velocity, episodic slip at ambient temperatures of 650C - 800C, and episodic high velocity slip at ambient temperatures > 850C. (B) Data from interferometric sensor shows displacement as a function of time during rapid slip events.  Note that the displacement during rapid slip events increases with ambient temperature.  Multiple small slip sub-events occur within a singe event in a number of the lower temperature experiments. Inset shows an expanded view of one slip event and has been fitted to a logistic function (red). (C) Experimental slip behaviour plotted in ambient temperature - pressure space.  Lines indicate estimated boundaries between different sliding regimes.


Studies by visitor Ulrich Faul, PhD student Chris Cline, visitor Emmanuel David, Berry and Jackson, of the high-temperature rheology of Pt-sleeved, Ti-doped synthetic olivine under water-undersaturated conditions have shown that Ti-hydroxyl defects involving protonated Si vacancies are responsible for previously reported water weakening of mantle olivine (Figure 2).

Figure 2. The variation of strain rate for dislocation creep with the concentration of Ti-OH defects in nominally anhydrous olivine. Symbols denote data from multiple specimens tested under various conditions of temperature and stress – normalized to common conditions of 1200°C and 150 MPa uniaxial stress (Faul et al., EPSL, 2016).

Similarly oxidising and hydrous conditions prevailing within Pt sleeves are responsible also for reduced shear wave speeds and increased dissipation observed in parallel low-strain forced-oscillation tests - probably through enhanced grain-boundary sliding. Complementary torsional and flexural oscillation tests (Cline and Jackson) of partially molten synthetic dunite show that its mechanical behaviour can be understood in terms of the relaxation only of the shear modulus, but the method proves to be insensitive to possible relaxation also of the bulk modulus. The ambitious PhD project of Richard Skelton (co-supervised by Jackson and Andrew Walker, University of Leeds) is nearing completion with the development and application of versatile computer software for the modelling of dislocations in minerals including the simulation of dislocation slip with and without adhering point defects involving protons. Completion of another experimental PhD project, by Yang Li, involving a broadband study of the seismic properties of synthetic glass media both dry and fluid-saturated (collaborative with visitor David, Douglas Schmitt, University of Alberta, and Seiji Nakagawa, Lawrence Berkeley Laboratory), has clearly shown that ultrasonic wave speed measurements at MHz frequencies and forced oscillation tests at seismic (sub-Hz) frequencies probe different regimes of stress-induced fluid flow in low-permeability cracked glass materials with implications for interpretation of seismological models for the upper crust.

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