Agree with Kurt,
as long as we are able to observe a fold we may assume a buckling
instability causing that fold, which in turn requires (from theory) that
a competence contrast was in place. On the contrary, no fold, no
competence contrast and therefore a rather homogeneous shortening might
have affected such rock body. In that case we would have to rely on
other critical observations (strain markers) that would justify
conclusions on shortening/deformation.
The provided photograph shows a single layer(?) buckle fold with cusps
(strain concentration) at the inner arc while the respective outer arc
reveals lobe geometry -not too uncommon for low viscosity contrast
buckle folds, even in high P/T environments.
Hermann
On 12/12/12 9:14 AM, Stuewe, Kurt ([log in to unmask]) wrote:
> Ahmed,
> Correct me if I am wrong, but I somehow feel that your problem is mis-posed:
>
> Clearly the rheology all materials gets softer with temperature, but it is not trivial to me that the rheology CONTRAST (i.e. the ratio of the two rheologies) decreases as well.
> If you assume a viscous rheology and an effective viscosity where the viscosity is described by an Arrhenius type relationship that depends on T, then surely the CONTRAST will depend on the pre-exponent constants and the powerlaw exponent, and not necessarily on T ?
>
> Some years ago, we had an idea that pertains to your problem: We saw that both plagioclase and K-spar occur both: porphyroblasts and smeared out layers in one of the Alps largest shear zones (the Plattengneiss shear zone).
> We thought that this observation may be useful to derive come constraints on the power law exponents.
> It never really worked well, so we never published it in an international journal, but we DID present the idea at the local Geol. Soc. and it was published locally in a paper which can be downloaded at:
>
> http://www.landesmuseum.at/pdf_frei_remote/MittNatVerSt_129_0043-0053.pdf
>
> Let me know if you share our thoughts or if I am completely on a different line from what you are trying to get at.
> Good luck
> Kurt
>
> ------------------------------------------
> Prof. Dr. Kurt Stuewe
> Institut fuer Erdwissenschaften
> Universitaetsplatz 2
> A-8010 Graz
> AUSTRIA
> http://wegener.uni-graz.at
> http://www.alpengeologie.org
>
> --------------------------------------------
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> ________________________________________
> Von: Tectonics & structural geology discussion list [[log in to unmask]] im Auftrag von ahmed abusharib [[log in to unmask]]
> Gesendet: Mittwoch, 12. Dezember 2012 14:52
> An: [log in to unmask]
> Betreff: Cuspate structure !!
>
> Dear Geo-tectonic list,
>
> Working on the Tugela Terrain, Natal Fold Belt, South Africa, I came across an exposure (please, see attached photo) where a well-developed cuspate structure is still preserved.
> This folding event is contemporaneous with the stacking of Mesoproterozoic thrust sheets onto the Archean Kaapvaal craton.
> The green rock in the photo is a folded amphibolite, which at the microscopic scale show folded hornblende crystals, implying that the hornblende was stable during folding at the amphibolite facies conditions.
> With increasing T, the rheological contrast decreases, So, my question is that, at this P-T conditions does the rheological contrast still workable to form this kind of structure ? In other words, what is the limiting conditions (maximum temperature!!!) during which the rehological contrast is still active ?
>
> Cheers
> Ahmed
>
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>
> Dr. A. Abu Sharib
> Department of Geology
> School of Agricultural, Earth and Environmental Sciences
> Room No. 120
> University of Kwazulu-Natal
> Private Bag X54001
> Durban 4000
> South Africa
> Phone: +27 31 260 7321
> Mobile: 0763747431
>
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