Some of the most hard arguments against pure shear:
Prof. Tim said:
Rocks are full of heterogeneities from the way a single
grain deforms versus another, upwards making pure shear impossible. Apart
from this, rocks deforming by pure shear in the lower crust have to have
faults on ALL boundaries!
And also said that:
So we should at least start with terms that describe what we mean. If we
get away from simple end member models that are unrealistic in rocks
perhaps we will make some progress in conceptual understanding of
deformation processes. Perhaps this is a bit puritanical, but my answer to
Brett Davis' question would be NO! Pure shear never occurs in rocks!
More moderate Falk said:
I think the problem may in part be the terminology itself.
And also said:
we are irrestistibly drawn to the subject of stress anyway
because (a) in physics we are interested in the relation of cause and effect,
and (b) if deformation is an effect, there must be a cause, which in all
likelihood is stress, whatever stress is. Restricting the discussion to the
properties of the effect is unsatisfying. If the stress-strain relation is
anything but clear as Rob correctly points out, this may be due to
insufficient
understanding of stress, rather than strain.
After this comments, other mailers start to send some arguments in favour
of quasipurish shear (other one like to said sub-simple shear). So just
now start to talk about geology because also I believe that in nature
nothing is pure....., sorry: nothing is simple.
It is a pity that the photograph of Paul is not enough clear, but I also I
think that in lower crust coaxial deformation is dominant over tangential
deformation. Of course, that rock heterogeneity must be accommodated by
other strain mechanism, but in terms of bulk deformations, this deformation
are quantitatively minor.
My main arguments are two:
Mainly, absence of stretching lineation on rock formed in lower crust (so
their fabrics are mainly planar S or L-S).
And second, If we study in detail the kinematic of the shear bands in
heterogeneous mylonites formed in lower crust conditions we can not found
one shear sense, however we found usually many different indicators.
I have more argument in the paper: Mylonitic foliation developed by
heterogeneous pure shear under high grade conditions in quartzofeldespatic
rocks (Chímparra gneiss formation, Cabo Ortegal, NW Spain. (Basament
Tectonics 11, 17-34, 1996) and also in my field experience in other lower
crust out crops, where usually we found flattening accommodate by simple
shear.
So my vote is for the geological existence of quasi-pure shear like a
important deformation mechanism in the lower crust.
Francisco J. Fernández
Dpto de Geología
Área de Geodinámica
Universidad de Oviedo
C/Arias de Velasco SN
33005 Oviedo. Spain
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if God does not throw dice in physics, why would we settle for just pure
shear
in geoscience?
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http://www.geol.uniovi.es/Users/FJFdez.html
phone: 34-985103150
FAX 34-985103103
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