In most of my samples stretching lineations are subparallel to the dip of c-planes. However,I have some samples where pucker axis lineations within mylonitic metapelites are stretched in a direction which is sub-parallel to the strike of mylonitic c-planes giving rise to very prominent stretching lineations. In the latter case, can I suspect greater involvement of pure shear which produced the stretching lineations and c-planes both at the same time??
You should analyzed not only pure- vs simple-shear dominated deformation but also deformation mechanisms and conditions. The activation of the different slip-systems in quartz is strongly controlled by temperature, among other processes. Depending on the deformation conditions, different slip planes are activated, and this could explain your data (e.g., low-T pure-shear dominated deformation). How are the a-planes distributed?. You could have a look on the following papers:
Law. 2014. Deformation thermometry based on quartz c-axis fabrics and recrystallization microstructures: A review. Journal of Structural Geology.
Stipp et al. 2002. The eastern Tonale fault zone: a ‘natural laboratory’ for crystal plastic deformation of quartz over a temperature range from 250 to 700 °C. Journal of Structural Geology.
Yes, Mr. Oriolo, I 've already started to measure the deformation condition for strain rate by EBSD technique!...thanks for citing the names of papers!..:)
Nirmalya
1. Are you sure the structures you've described belong to the same deformation episode ?
2. If "1" is positive, are both dip & strike parallel stretching lineations equally well-develop in the same place/volume of rock ?
3. If "2" is positive & produced by simple shear plus flattening wouldn't be odd that they are at high-angles to each-other with no stretching lineations showing 'intermediate' positions between the orientations of the 2 'end-member' (dip- and strike-parallei) ?
4. How could 3 be true ? Would this be a special case of simple shear and simultaneous (& bipolar) chocolate-tablet boudinage...simultaneously taking place, something I find difficulto to inderstand, or..."1" is NOT correct? So...back to "1" !?
Sorry that's what occured to me just now...I'm not sure is's of any help !? Luis
Dr. Luis,
Actually mylonitic C-planes dip consistently towards NNW and very shallowly. Likewise S-planes dip shallowly towards NW. Stretching lineations trend roughly E-W mainly in the southern sector but show NW/NNW trend in many rocks from the northern sector. Can the situations in (3) be viewed to be produced in high mountain belt in the fault zone and immediate hangingwall collapsing (pure shear dominated ) simultaneously/alternating with the extrusion of the footwall (simple shear dominated)?
Dear Nirmalya,
1) Do you have any other structures to combine (fold axis etc.)?
What is the angle relationship between S-C surfaces? What temperature do these angles indicating? What structural depth? Can you have both semiductile-semibrittle-britlle coexistence of structures considering your lithologies? What kinds of minerals do you have and are they elongated or recrystallized? Can you calculate duration of extention using minerals? Is there possibility, the C surfaces formation be evolutionary stage of an earlier stretching in the area? Then small declinations could be slight shifting of the stress field during the time. Declinations of some (up to 10) degrees between L and C-dip I think they are normal or expected.
2) Are you sure that the linations in the first and the second case are the same? Is it possible to be entirely different (e.g crenulation, stretching, both pervasive or non pervasive)?
I give you some questions you should take into your consideration (if you already haven't )
Regards
Ioannis
Dear Nirmalya,
S and C planes (or shear bands as they used to be called) may display variable angles to one another, especially as they are traced into areas of higher strain. The intersection of s and C planes may also rotate into higher strain areas (perhaps producing some of the relationships you describe). In addition, new sets of shear bands develop that overprint the older rotated sets. I wrote a paper many years ago looking at geometric relationships between S-C fabrics (shear bands), mineral lineations, and rotation of c intersections (see attachment).
Note also that layer-normal differential shearing may cause extensional structures (including S-C fabrics) to develop obliquely to the overall transport direction marked by the mineral lineations (Check out Fig. 2 of the attached Alsop & Holdsworth 2007 paper)
I hope this may help.
Ian
Dr. Ioannis
1) I have noticed pucker axis lineations are sub-parallel with the set of E-W stretching lineations on the c-planes?
a. The S and C surface intersection has not always been noticed depending upon the strain amount as Lister and Snoke, 1984 suggested. Is it also temperature dependent?
b. How to get structural depth?...I have only sampled horizontal transect for a 12 km length where sequence is shallowly homoclinal towards north.
c. Though I'm only working on ductile shear zone...I have noticed some random intersection of brittle-semibrittle faults which are possibly younger.
d. Biotite obviously form the stretching lineations. Quartz are recrystallized only partially by bulging mechanism with SGR only subordinate.
e. I 've already calculated strain-rate by Takahshi's fractal method. To get the duration I have to get total strain (dividing strain by its rate would give me duration)---which however we can not calculate by Fry plot or geometrical method since the rock and veins are dynamically recrystallized though only partially. However, if you follow my updates, you would notice some one suggested me neutron diffraction technique....however I'm quite unsure of applying this....so I'm trying to resolve the issue by EBSD...could you please suggest me something.
f. C surface is parallel with the folds which developed prior to normal shearing...again there was a phase of thrusting which was co-planar with the normal-sense C-surface.
g. I think E-W stretching lineation on the north dipping C planes were the result of pure shear (which may be little earlier) whereas N-S stretching lineation set on north dipping C-planes was the result of simple shear. I suppose earlier set was the consequence of active mountain belt collapse, whereas the second set was the result of rapid tectonic uplift. Could you please highlight something?
Regards and much thanks!
Nirmalya
Dear Nirmalya,
About the structural depth, you can evaluate it, using several concepts such as: pervasive or non-pervasive deformation (in general more pervasive=deeper), minerals indicating specific P/T conditions which indicates facies of metamorphism, kind of schistosity, types of folds, dip angle of the ductile zone (in general, the more horizontal=deeper), tectonic transposition (shows low str. level) etc. You have to see what you have and then exclude contradictive observations.
About the timing of the linations you want to compare, keep in mind that different minerals have different ductile limits (in terms of temperature, e.g Voll, G., 1976. Recrystallization of quartz, biotite and feldspars from Erstfeld to the Leventina
Nappe, Swiss Alps, and its geological significance Schweiz. mineral. petrogr. Mitt). With more carefull observations not only within the sh. zone but in the surrounding formations, you might see different minerals deformed under the same stress field, to suffer brittle or ductile deformation.
About your final (g) point: If I correctly understood, I don't think you could have a prior clear lination on the C surfaces. If this lination was older it should be subjected to cross-cut relationships. Clearly ( from your description) L // to C strike, is at least simultaneous or later. So, with all respect to your work, I think you should re-evaluate your wider structural frame.
I hope I haven't miss any critical information and I've helped you
Kind regards
Ioannis
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