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Basement Sucks
Bruce Yardley, Sarah Gleeson
School of Earth Sciences, University of Leeds, Leeds LS2 9JT, UK
Ingrid-Anne Munz
IFE, PO Box 40, N-2007 Kjeller, Norway
That groundwaters can penetrate crystalline rocks to mid-crustal depths was first
appreciated from stable isotope studies of altered granite batholiths, and
reinforced by examples of water inflows encountered during deep crustal drilling.
Nevertheless, fluid flow modelling of sedimentary basins has often treated
underlying crystalline basement rocks as impermeable. The purpose of this
contribution is to explore examples of fluid influxes into crystalline basement from
overlying sedimentary basins or the surface itself, and to explore their potential
significance, if any.
Basement rocks are potentially large sinks for water because they are
dehydrated and consume water in incipient retrograde reactions. Such reactions
mean that fluid pressure can be below hydrostatic values throughout the stable
crust. However basement rocks are also generally highly impermeable so the
effectiveness of water infiltration may be minimal. There are well-documented
examples of free convection of surface waters resulting in temperatures at the
base of the cell of the order of 250°C, but higher temperature examples unrelated
to magmatism often preserve extensive fluid inclusion evidence of
overpressuring. This is surprising since there must have been an open flowing
system for the initial infiltration to take place. We have suggested previously that
fluid-filled fractures became sealed, isolated and pressurised, possibly providing
an opportunity for further downward injection of fluid in favourable circumstances.
In the Modum area of southern Norway, basinal brines penetrated high grade
basement rocks and reacted with wall rocks to hydrate them; in the process
sufficient water was removed from the overpressured fluid to double its salinity,
reaching salt saturation at temperatures around 300°C. At the same time,
infiltrating hydrocarbons were cracked to methane and bitumen. In contrast to
geothermal systems, the basement here has been able to destroy much of the
fluid that has penetrated it, and remains only locally altered.
Many crustal shear zones are also sites of fluid infiltration, but here it is coupled
to deformation. Retrograde hydration causes weakening of many rock types, and
if the volume of rock affected is sufficiently large, deformation focussed on sites
of initial hydration results in the generation of sites of low fluid pressure into
which further fluid is drawn. Thus in a particular stress regime, there may be a
threshold beyond which fluid infiltration becomes self-accelerating through the
promotion of deformation, subject to the restraint that most retrograde reactions
result in an increase in solid volume and hence a reduction in permeability. This
contribution will explore the evidence for deep penetration of surface fluids into
basement, and the consequences for crustal rheology.