Fracturing of the upper crust as regional (far-field) stresses interact
with a sudden release of pressurized magmatic fluids are modelled here.
In this model a discharge is suddenly applied to the base of the model.
Because of fluid compressibility (bulk modulus) the increased pore
pressure at the base of the model is not rapidly transmitted to the top
of the model. This pore pressure diffusion leads to a complex yielding
history of the crust, of significant interest to understand fracture
history and fluid mixing in magmatic terranes. Pore pressure diffusion
effects are fully investigated in another page
The models show that upon release of pressurized fluid to the base of
the model, simulating the release of magmatic fluids, leads to rapid
pore pressure increase at the base leading to yielding in that area.
This basal yielding leads to stress release in the upper part
inhibiting yielding. With time, however, overall fluid pressure
gradually increases, leading to yielding of the entire model. When the
influx is turned off, simulating the end of fluid exsolution from a
crystallizing magma body, yielding everywhere across the model is
inhibited because pore pressures have decreased everywhere and the
crust becomes consequently stronger. Upon further deformation and
stress loading, yielding resumesdownwards from the top. Throughout the
calculation, fluids migrate upwards.
The model focuses on the upper crust, with the top at the surface and
the base at 10 km. It comprises of a single elasto-plastic rock, and
the initial pore pressure is hydrostatic and all boundaries, except the
top, are impermeable. The top has a pore pressure fixed at atmospheric
pressures at the surface of 1e5 Pa. A constant horizontal velocity is
applied to the two vertical boundaries, and at some point during
deformation, a fluid influx (discharge, dis
is initiated at
the base of the model, and turned off after a number of steps.
The figure below illustrates the evolution of the system. A movie can
be seen by clicking on the figure. The left-hand side figure shows the
state of the crust after some deformation, before any fluid discharge
at the base. The upper half of the model is undergoing yielding (red
zones) or has undergone yielding (purple areas). When discharge starts,
the base of the model yields, inhibiting yielding in the upper half
through releasing the loaded elastic stresses at the base. As pore
pressure increases, with continued fluid influx, the entire crust
weakens and yields. When fluid influx stops, there is first a short
phase where the entire model crust stops yielding. Gradually, as
stresses are loaded, yielding starts again at the top, and migrates
During early yielding, with no fluid influx at the base, fluid flows
both upwards and downwards into zones of dilation, normally at
intersections of yielding zones. When the discharge of pressurized
fluids to the base starts, fluids everywhere tend to migrate upwards,
preferentially along zones of yielding, because of their dilation
here or on Figure to see movie (AVI).
Movie starts after loading of the crustal blow and the initiation of
yielding and just before initiating fluid discharge (indicated by the
arrows at the base of the model). The diagram on the right-hand-side
depicts the change in vertical pore pressure profile with time. When
discharge is initiated, pore pressure in the lower part of the model
(righ-hand-side of the diagram) increases and the curve becomes concave
upwards. After some time, pore pressure diffuses across the model and
the pore pressure profile becomes linear and the entire block yields.
When discharge is turned off (no arrows at the base), the curve becomes
convex upwards as the block reequilibrates.