Brett
Fault splays means divergence of more faults from less. The resulting wedge
shape, plus any sort of ductility contrast over a vertical distance, would
probably mean that there would be more shortening near the splay nucleation
location that further up. This would lead to heterogeneous bulk shortening
of the material between the splays. Truncation /dissolution against high
strain zones can still occur.
Cheers
Tim
>
>As discussed previously, the only way to solve the geometric/volume problems
>of pure shear in real rocks is to invoke discontinuities (faults) as
>bounding the zone undergoing pure shear. But, perhaps this can be done in
>zones of relatively ductile deformation that are bounded by fault splays
>within a brittle-ductile shear zone. Volume/area problems could be solved by
>truncation/dissolution against the high strain zones and at the brittle
>faults at a stage before significant flowering out occurs. of course this
>means that the amount of pure shear would be quite small. Is this feasible?
>
>Brett
>
>
>
>-----Original Message-----
>From: Tim Bell <[log in to unmask]>
>To: [log in to unmask] <[log in to unmask]>
>Date: Friday, 13 August 1999 7:53
>Subject: Re: pure shear, simple shear and other strain scenarios
>
>
>>Hi All
>>Pure shear in its "pure" form is a rather silly concept for structural
>>geologists because we are dealing with heterogeneous materials. It is such
>>an end member concept that it probably never applies to rocks. We really
>>should be using more descriptive terms like "approximately homogeneous bulk
>>shortening" for geological materials - I know - it is a horrible mouthful,
>>but at least it is possible. If a rock deformed by pure shear, just one
>>heterogeneity in the system would progressively amplify causing a flowering
>>of the strain - one only has to play with strain field diagrams of pure
>>shear and introduce one small heterogeneity to the pattern to see its
>>cumulative effects on the strain field around it as the deformation
>>continues - e.g. add just one square that becomes slightly rectangular in
>>2-D or a cube that becomes slightly distorted in 3-D to a developing
>>strain field to see this effect (see strain field diagrams in Ramsay, 1962
>>and Bell, 1981). 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!
>>
>>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!
>>Cheers
>>Tim
>>
>>Prof Tim Bell
>>School of Earth Sciences
>>James Cook University
>>Townsville
>>QLD 4811
>>Australia
>>ph: +61 7 47814766
>>fax: +61 7 47251501
>>email: [log in to unmask]
>>
>>
Prof Tim Bell
School of Earth Sciences
James Cook University
Townsville
QLD 4811
Australia
ph: +61 7 47814766
fax: +61 7 47251501
email: [log in to unmask]
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