At 11:20 AM and 4:09 PM 1999/11/04 -0500, Dazhi Jiang wrote:
>... if we consider ductile high-strain zones with extrusion
>then the "extra" pressure as a result of flow and viscosity can be
>quite significant from theoretical predictions (Jaeger 1964, Robin
>and Cruden 1994, JSG). ...
>To explain tectonic overpressure very briefly, consider extruding
>viscous material between two rigid plates as in Jaeger (1964,
>p.141). If the experiment is performed in zero confining pressure,
>because of flow (strain rate) and viscosity, there will be pressure
>(mean stress) in the material. Now imaging a similar process
>taking place in the earth's crust,...
"Extra" pressure of 6 kbar at the base of a transpressional shear zone 1 km
thick and 10 km deep (Robin & Cruden 1994 JSG) would certainly be
significant for thermobarometry. However, several assumptions implicit in
Jaeger's equation become less tenable with increasing depth in the crust:
1. The horizontal base of the model shear zone is a "perfectly lubricated
planar surface"; this is a "rather unrealistic boundary condition" (Robin &
Cruden 1994 JSG).
2. The model shear zone is a rectangular prism with parallel walls, and it
permits extrusion only in the vertically upwards direction. This means it
must be somehow "enclosed" at the base and both ends. Natural shear zones
have "releasing bends" that permit lateral, strike-parallel extrusion of
rock, and they are likely to be underlain by ductile rock that would permit
strike-transverse extrusion as well. Both of these departures from Jaeger's
model would ease the tectonic overpressure and tend to permit the prevailing
pressure to approach its lithostatic equilibrium value.
3. Jaeger's equation postulates constant and uniform rock viscosity within
the shear zone; in a natural shear zone viscosity would decrease with depth
due to rising temperature, and this would ease the tectonic overpressure.
4. Jaeger's equation postulates rigid walls that are planar and parallel all
the way to the base of the shear zone and cannot be deformed or displaced
upwards, no matter how hard the "extra" pressure pushes downwards on the
rigid base of the shear zone. In a natural shear zone at 10 km depth, what
kind of wallrock would be strong enough to contain 6 kbar of "extra" pressure?
5. Jaeger's equation postulates a constant and uniform rate of shortening
across the shear zone. Natural shear zones are likely to grow wider at
depth as their wall-rocks soften and start to yield. This would reduce the
rate of upward extrusion and also increase the likelihood that the wallrocks
will be displaced upwards, easing the tectonic overpressure.
Accordingly, tectonic overpressure in large natural shear zones is likely
to be very much lower than 'theoretical predictions' based on Jaeger's equation.
Dugald M Carmichael PhD PEng Phone/V-mail: 613-533-6182
Dept of Geological Sciences and Geological Engineering
Queen's University FAX: 613-533-6592
Kingston ON K7L3N6 E-mail: [log in to unmask]
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