If you don't mind, what is the scale and orientation of the picture you posted?

me too, I would favor S-C bands and shearing, possibly during lava emplacement. You have thin sections from these brownish bands? Pseudotachylites...?

Yep, shear band cleavage, C-S fabric, a beautiful example!

This looks like S-C structures to me as well.

...but hard to say more without microscopic details...

I am thinking similar to Saint-Bézar... such that the mostly continuous, finer bands resemble C or C' surfaces. However, I feel like I am really squinting to define a regular orientation of the (what would be) S fabric if this is an S-C or S-C' type of structure. I should probably look at it on a proper big screen.

I'd more likely call it a set of minor, sinistral shears (without a well-defined prior foliation, C-S is really not quite correct.

And where is the scale??

I've seen similar structures in metamorphic rocks in Montana, USA but it was over 20 years ago so I can't remember anymore details

I am not sure about the lithology and degree of cementation of the host rock, they should be seen in thin section, but from this image they look like phyllosilicatic bands (a type of deformation bands). Check Fossen et al. 2007 GSL for more details.

Seems to be a two-stage solid-state microfabric formed by deformation during emplacement of teh lava-dome.(1) high-temperature foliation parallel to long axis; (2) wide-spaced pressure solution during low-temperature (small brownish layers cutting the earlier fabric. I have not seen a published paper on this sort of lava dome fabrics. Occurs often in trachyte domes (e.g., Queyrieres in the Massif Central).

I would agree with most of you: shearing bands!!!!

I vote for S/C fabrics as well—or at least progressively formed shear fabrics. If you could cut a section of the drill core parallel to the spaced foliation (which would be the C-plane), you might see if there is a slickenline-type lineation within that pane, and a plane cut parallel to the interpreted "S" plane should have a lineation mark by mineral elongation. If such lineations exist, you can use the orientations of these lineations to cut a plane normal to foliation(s) and containing these lineations, and this would be the plane in which you would expect to see the 'best' development of S/C fabrics, if indeed this is what these are. A plane cut normal to foliation and normal to say the C lineation should display symmetric fabrics, rather than the asymmetric fabrics you would see in the plane containing the lineations.

I agree, they look like shear bands (http://gsabulletin.gsapubs.org/content/120/1-2/238.full) , but, something isn't quite right. I would suggest that either they occurred while the material (tuff?) was still hot and/or in the presence of fluid. The same material undeformed might help illuminate what is part of the depositional deformation pattern versus the structural deformation.

Your picture is similar to samples with highly stretched bubbles on shear planes that have been described in welded ignimbrites from Sardinia. Author name: Laura Pioli

I agree with the shear bands (it looks a little bit too regular for pressure-solution surfaces). I would avoid the term C-S structure since it commonly implies the concurrent formation of the two planar elements (foliation S and shear surfaces C) - at least this is how it was defined (Berthé et al. 1979). This looks like a superposition of two structures, the shear bands (or a second cleavage?) over the first fabric (the one parallel to the long side of the figure), which is folded (this is never the case in proper C-S structures!). The first fabric could be solid state, but if it is a volcanic rock, why not a magmatic flow foliation? (I have seen similar very nice flow structures in some obsidians). A petrographic examination should give an answer...

Does presented figure show a plan or profile view which one?

Prior the sinistral shear bands (spaced cleavage with very fine grains), the (granitic) rock presents a foliated structure similar to SC mylonites, with rotated porphyroclasts (delta types?). They look like also sinistral rotation. Sugestion: progressive mylonites from reactivated fault zone each time shallower and increasingly colder

I would suggest mylonitic S-C structure as shown here! Can be veirfied in thin sections!

S-C fabrics in sinistral shear domain.

I was getting set to answer, but see that what I would offer has already been nicely addressed by other respondents.

I concur with the previous guys.

S-C fabrics in sinistral shear zone,with attention to core sample, you have a reverse fault zone.

Definitely looks like sinistral shear bands (C-surfaces) with some stretching parallel to S

S-C fabric well developed

It looks like S-C, BUT...

To name that secondary foliation as an "S foliation" you need to prove that is formed by aligned platy minerals; and to prove the primary foliation to be a C foliation, you need to prove that you have shear along it (Sigmoids within them, fragmental texture under microscope, striations, or displaced markers)

Can you give any description of the micro texture? (at least with a hand lens)

I agree with Pavel about a possible magmatic flow rather than classical shear bands for this quite complex structure

An uncommon but not inintelligible feature that records transition between flow and semi-brittle shear in a cooling magmatic rock. Similar or approaching structures (ie isoclinal flow folds, s-c bands and "metamorphic" foliation as well) appear in some dykes especially when the magma contains large amounts of water. Well foliated and lineated "amphibolite" dykes are not rare in some ophiolite complexes and record hydrous magma injection in active faults.

I am not so sure for C/S structure, because the supposed S feature is well cross-cut by the supposed C plan in top and bottom left of the picture. I think it is here a volcanic flow structures affected by later fracturations.

I agree with previous interpretations and may add that you probably see a process of strain localization whereby you first generate a sort of S fabric (which actually could be P shears) and later deformation focusses on the main shears (Y) in which grain comminution is produced as they accommodate the majority of displacement. Definitively have some thin sections done (parallel to the shown section). Also agree with the fact that the first stage may be syn cooling (allowing larger sigmoids to form) but the later shears are definitively tectonic in my opinion.

CS fabric sinistral

It looks like pressure solution cleavage in volcanics

Looks almost like solution cleavage to me as well. Truncates larger grains.

S-c fabric

"C" seems all right, but "S" is questionable considering that the rock consists mainly of glass; sinistral also ok. Definitely shear bands formed during magma (or rather lava flow) consolidation.

Dear all,

I would also propose a kind of sinistral S-C fabric.

I have no problem with the "S" as the numerous clasts appear flattened and stretched.

One can note the pressure shadows developped at the tips of the clasts.

What is the scale? What are the clasts?

Resolution of your picture is low. We must know kind of clast grains and them perimeter (may be this structure concluding of weathering), dip of fracture (may be reloading joints), and one thin section picture as cross cut from one on the fractures and grains.

Hi everybody, I do not know the porosity and the cementation degree of the sample but they look like deformation bands, in particular the central one. A thin section is needed in order to check the grains and their deformation.

To me they appears like discrete shear zones with anticlockwise movement.

It appears that the dark planes are shear planes which are discrete to continuous in the scale of photograph. it is advised to get thin section and carefully look into it to resolve the matter.

Lava flows often present shear structures due to differential flow related to variable viscosity/temperature; these present aspects similar to fragile or ductile tectonic shear zones. I have seen structures of these kind both in mafic (basalts) and silicic (trachyte) flows in the Azores. The lateral edge of basaltic lava flows frequently show shear structures, including striated planes that look exactly like faults. Also trachyte coulées may present these types of shearing.

So I would say that these are shear structures related to flow.

dear all

thanks for your interest in this question! Actually you are right, I did not put a reference to the scale! the photo shows a 55x95 mm section of the core sample: there is no glass in the sample and the clasts are from the trachyte and are always angular in shape even at high magnification.

Dear Monatanari,

Mr. Jose's answer is quite right and the angularity and euhedrallity-semieuhedrallitty of your clasts is a good sign of his answer.

best

Masoud

Dear Monatanari,

I agree with Lorenzo Petracchini, I think the best way of knowing that is

make a thin section.

Best regards,

Saleh

It seems that there are two surfaces They resemble either crenulation cleavage (with microlithon and C-domain) or shear zones (C-foliation), with internal S-foliation. Microscopic investigation can reveal if they are shear zones (e.g., finding mylonitic or cataclastic fabric along the long surfaces) or crenulation.

Dear Domenico,

I saw similar structures in some lava flows from the Aeolian Islands (mainly Vulcano, Lipari and Salina).

you can check similarities using these references:

Perugini, D., Valentini, L., & Poli, G. (2007). Insights into magma chamber processes from the analysis of size distribution of enclaves in lava flows: A case study from Vulcano Island (Southern Italy). Journal of Volcanology and Geothermal Research, 166(3), 193-203.

Piochi, M., De Astis, G., Petrelli, M., Ventura, G., Sulpizio, R., & Zanetti, A. (2009). Constraining the recent plumbing system of Vulcano (Aeolian Arc, Italy) by textural, petrological, and fractal analysis: The 1739 AD Pietre Cotte lava flow. Geochemistry, Geophysics, Geosystems, 10(1).

Ventura, G., Del Gaudio, P., & Iezzi, G. (2006). Enclaves provide new insights on the dynamics of magma mingling: A case study from Salina Island (Southern Tyrrhenian Sea, Italy). Earth and Planetary Science Letters, 243(1), 128-140.

De Campos, C. P., Perugini, D., Ertel-Ingrisch, W., Dingwell, D. B., & Poli, G. (2011). Enhancement of magma mixing efficiency by chaotic dynamics: an experimental study. Contributions to Mineralogy and Petrology, 161(6), 863-881.

Davì, M., De Rosa, R., & Holtz, F. (2010). Mafic enclaves in the rhyolitic products of Lipari historical eruptions; relationships with the coeval Vulcano magmas (Aeolian Islands, Italy). Bulletin of volcanology, 72(8), 991-1008.

Cheers

Federico

Mair K., Frye K. M., and Marone C. 2002. Influence of grain characteristics on the friction of granular shear zones.

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 107, NO. B10, 2219, doi:10.1029/2001JB000516

Can help

Similar to cleavage cracks - a consequence of the flow of the partially consolidated stream. Residual melt of the same lava healed cracks. Such cracks most permeable to secondary mineralization.

It seems that, multi-phases of lava flow affected by tectonic activity resulted in tilting and uplifting of the original lava flows

Lave flow in the first-stage, and then weakly-sheared deformation formed the thin likely C-plane.

There is a bit of S/C band. I would suggest a a bit of simple shear at the ductile/brittle boundary....?

Manuel