Dear All,
At the risk of being too realistic, I would like to raise a few points
regarding the perennial question of overpressures:
1. Our methods of determining 'lithostatic' pressure generally involve
assumptions - this has been mentionned in several replies. The range of
error that this potentially engenders greatly exceeds the levels of tectonic
overpressure that, according to most theoretical studies, can be supported
in deeper crust. We are in 'depth' terms attempting to measure metres with a
scale loosely calibrated in kilometres. Very simply, the minor overpressure
component is 'hidden' within our relatively inaccurate pressure determinations.
2. From 1, were disproportionately LARGE overpressures to exist, they would
show up as numerous anomalies in metamorphic pressure studies. This is not
generally the case (or have they just been overlooked?). It is likely that
the few reported anomalies are an artefact of the methods employed and/or
reflect various otherwise-undetected reequilibration processes.
3. Of course tectonic overpressure exists, but let us not become so
confident of the accuracy of our computer-generated numbers that we lose
sight of the very considerable assumptions built into the pressure
determinations.
Regards,
Brian.
At 04:09 PM 4/11/99 -0500, you wrote:
>Dear All
>
>I read that paper before and I have a copy in my file. I think it is
>totally irrelevant to this discussion. Those who are interested in
>tectonic overpressure may wish to read Mancktelow (1993 JMG).
>
>To explain tectonic overpressure very briefly, consider extruding
>viscous material bwtween 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, the total pressure at a point within
>the material would be the sum of the overburden pressure
>(density*g*depth) and the pressure due to strain rate and viscosity.
> The latter is overpressure. In extruding situations, theory predicts
>that the overpressure can be significant. Does this occur in
>nature? That is a very important question to be answered.
>
>Sincerely,
>
>Dazhi Jiang
>
>
>On 4 Nov 99, at 16:36, Eric Essene wrote:
>
>> All,
>> Mervyn Patterson wrote a paper on non-hydrostatic thermodynamics in
>> 1973 or so in Reviews of Geophysics, I think.
>> Eric
>>
>>
>>
>
>
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Professor Brian Marshall, Tel: +61-2-9514 1775
Department of Applied Geology, Fax: +61-2-9514 1755
University of Technology - Sydney, Email: [log in to unmask]
P O Box 123 Broadway,
NSW 2007,
AUSTRALIA.
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