Download Fluids, Shear Zones and Continental Rheology

Fluids, Shear Zones and Continental Rheology
Bruce Yardley, School of Earth Sciences, University of Leeds
The strength of quartz-bearing crystalline rocks is very strongly dependent on
the presence or absence of water. During progressive heating and
metamorphism, continental rocks are generally believed to contain trace
amounts of fluid at near-lithostatic pressure, giving high water activities and
ductile behaviour from relatively low temperatures. However this is a rare
geological setting, and continental rheology in general is dominated by the
behaviour of old crystalline rocks that are at a lower temperature than that of
their original formation. Hence the thermal history of continental crust is
arguably the most important influence on its rheology.
Simple equilibrium calculations, coupled with kinetic considerations,
demonstrate that the fugacity of water (and other volatile species) in
crystalline rocks of the middle and lower continental crust is extremely low,
and is not consistent with the presence of a free fluid phase. Any fluid that
successfully infiltrates will be rapidly consumed by retrograde reactions. Fluid
inclusion studies of veins in basement rocks adjacent to the Oslo graben
demonstrate precisely this process in operation as sedimentary fluids
penetrated basement in a fluid-starved environment, although the physical
mechanism is not entirely clear.
At temperatures of a few hundred degrees, silicate rocks are weak and ductile
when wet, but strong if dry. Fluid infiltration to dry crust will lead to localised
weakening and consequent deformation (i.e. shear zones), but this will be
followed by incorporation of fluid into mineral lattices, and hence
strengthening of the rock mass once more. As a result, from the perspective
of a relatively small rock mass, deformation will be sporadic as rock strengths
alternate between those of wet and dry rock in a relatively uniform stress field.
In this situation, rheology is a time-dependent variable. Viewing a larger
crustal volume however, it may be possible to take a continuum approach in
which deformation is localised into very small parts of the total volume at any
one time, and within those zones only, the rheology of wet rock is an
appropriate descriptor.
The main obstacle to progressing this approach to crustal deformation further,
is the difficulty in understanding and predicting the infiltration of water into the
middle and lower crust.