Hayward, Kathryn Suzanne
Description
An experimental study has been undertaken to explore the
strength, mechanical behaviour and microstructural evolution of
bare interfaces in quartz sandstone during slip. These
experiments were designed to simulate fault processes with
increasing depth in the continental crust. Two main aspects have
been explored: (1) the effect of temperature and confining
pressure on the behaviour and stability of favourably-oriented
faults, and (2) the influence of...[Show more] reactivation angle on the
mechanical behaviour and associated microstructural evolution of
a fault zone. Experiments were conducted on Fontainebleau
sandstone using a triaxial deformation apparatus, at normal
stresses comparable to that in the continental seismogenic regime
and over small slip displacements.
The first suite of experiments was conducted at temperatures of
400-927°C and confining pressures of 50-200MPa. Experiments
reveal complex transitions in fault behaviour between stick-slip
and stable sliding regimes. Mechanical results are coupled with
microstructural analysis using multiple techniques (including
high resolution FE-SEM, and FIB-TEM) that provide insights into
fault surface processes down to the nano-scale. Significant
findings include the identification of a partially amorphous
layer formed during aseismic creep and the generation of
pure-silica frictional melt (pseudotachylyte) during high
temperature seismic slip events. The pseudotachylyte is
recognisable by the formation drawn-out glass filaments and
fractured glass patches on the fault surfaces, forming a
discontinuous layer up to 2µm thick and covering 10-60% of the
fault surface. At normal stresses > 200MPa, frictional melt
develops within the first 50µm of rapid slip, correlating with
changes in slip acceleration and velocity. High temperature
hydrothermal treatment of melt-covered fault surfaces indicates
that the pseudotachylyte has a short lifespan (<1 hour) in the
presence of high temperature, reactive fluids.
The second suite of experiments explores reactivation of fault
surfaces inclined between 25º and 70º to the maximum shortening
direction, representing faults that vary from optimally-oriented
to severely-misoriented for failure. These faults have been
reactivated in both dry and fluid-saturated conditions, using two
different loading mechanisms. ‘Stress-driven failure’
involves increasing the axial load at constant rate until
failure, whereas ‘fluid-driven failure’ is achieved by
maintaining a constant axial load and increasing pore fluid
pressure until slip occurs. While the initial reactivation of
faults obeys frictional theory, continued reactivation is
strongly influenced by the microstructural evolution of the fault
surface, most notably through the development of frictional melt.
Rapid-slip events form a locally-continuous layer of frictional
melt in both the dry and water-saturated samples. The presence of
pseudotachylyte increases fault cohesive strength through a
process termed ‘melt-welding’. Melt-welded regions serve as a
nucleation point for the development of off-fault damage and on
the most unfavourably-oriented faults, cause lock-up and the
failure of a new, more favourably-oriented fault.
This work provides new insights into the behaviour and
microstructural development of fault surfaces during the early
stages of seismic instability. These results have implications
for the interpretation of slip processes in natural fault zones,
and also more generally for understanding slip mechanics,
weakening distances and coseismic fault strength within the
continental seismogenic regime.
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