Hi all,
I am working on a sheared granitic rock. I have measured the LPO pattern of the quartz grains of this rock by EBSD. The thin section which has been analysed by EBSD is a section perpendicular to the shear foliation. However, as stretching lineation was not found in this rock in outcrop-scale I am not sure that the section of my study is XZ section or YZ section.
Does anybody has any idea how to understand the slip system in this case?
Does your EBSD data yield an organized fabric? Your LPO data are 3-dimensional so, if it looks like you have an organized fabric you can rotate the data to find the xz section.
Hi Proloy,
Depending onyour sample mineralogy you could use Anisotropy of Magnetic Susceptibility.
You could also use the method Kyle mentioned, however, this goes with a priori assumptions (that the crystals align in a certain way parallel to the lineation). If you had any obliquity between the lineation and one of the samples, you wouldn't detect it. It's kind of a circular argument.
Cheers,
Erwin.
Dear Proloy,
at first: if you have a high pure shear component identification of a stretching lineation may be difficult.
I would also recommend to use AMS for fabric analysis. In addition to the comment of Erwin with the AMS you preferentially analyse the paramagnetic or - if present - the ferrimagnetic fabric, which may be parallel with or oblique to the EBSD quartz fabric. An overview on the different methods for granite fabric analyses is provided in the textbook of Nédélec & Bouchez (2015) Granites - Petrology, Structure, Geological Setting, and Metallogeny.
Hope this is of some help
Jens
Hi Proloy,
I'd go with Kyle's suggestion over AMS. AMS might help you but you wont know until youve spent a long time doing the analyses as a simple AMS measurement isnt enough to determine the XZ plane. You'd need to demonstrate that the AMS was controlled by a deformation fabric and then youd need to demonstrate that the AMS fabric and the quartz LPO record the same deformation - its more time consuming than you might imagine and does not always produce a robust answer.
You can see the lengths that we went to to correlate AMS and strain fabrics in our Tectonics article:
Article Orogen-parallel deformation of the Himalayan mid-crust: Insi...
In addition to Kyle's suggestion Id suggest that you look closely at the quartz and feldspar microstructures and any metamorphic assemblages and try to estimate the range of deformation temperatures. This will give you some idea of what the most active quartz slip systems were to produce the LPO and will then give you an idea as to what X and Z should be parallel or sub-parallel. Also consider that different strain geometries can give different LPO for deformation at similar temperatures.
All of this will be mute if you dont have an organised quartz LPO. If its a mess with no clearly defined point maxima or girldes then you're unlikely to be able to figure it out.
The following articles may help:
Article Thermo-kinematic evolution of the Annapurna-Dhaulagiri Himal...
https://www.researchgate.net/publication/301804917_Parsons_et_al_2016_GGG_Supporting_Information_S1
Article Strain Within the Ultrahigh‐Pressure Western Gneiss Region o...
a detailed Shape Preferred Orientation (SPO) analysis of the XY plane under the microscope might reveal the preferred orientation of minerals (especially mica/pyroxene even quartz ribbons, if any). Thereafter you can cut section parallel to the direction of major ellipse of the SPO and carry out EBSD. It will take time but so will AMS and AMs may not give you the desired result for deformed rocks. Since you have already done the EBSD, I will concur with the approach suggested by Kyle and Andrew.
Cheers
Thank you all. I don't have the AMS facility so I have to rely on the thin sections. The analyzed grains indeed have strong LPO and concentrated at the periphery (near lineation of the pole figure). I followed the method as Koushik da mentioned. Lineation is clearly visible on the foliation plane (XY plane) and defined by stretched quartz grains which is not at all visible in outcrop-scale. Following this I found that the section which I used during EBSD analyses was YZ section and that is why this type of result is coming from EBSD. I can imagine what would be the orientation of the C-axis in the XZ section as Kyle mentioned but I think I have to do EBSD again on the XZ section to confirm this.
If you have one c-axis point maxima located at the edge of your pole figure (i.e. 00/090 or 00/270) and if your EBSD data is definitely in the YZ plane then a c-axis maxima parallel to Y is typically considered to reflect prism-a slip. If the c-axis maxima is concentrating parallel to Z then its probably basal-a slip. Some microstructure context from optical microscopy might help you figure this out.
If you are only concerned with the crystal axes orientation data then you can rotate your ebsd data from YZ to XZ reference frame using MTEX (a MATLAB application) with no need to run the EBSD again on a new sample.
I was not concerned about MTEX. Thank you for informing me. However the c-axis maxima is located at 90 and 270 and have spreads towards 0 and 90. I think rhomb slip is also effective in these quartz grains.
Dear Proloy,
if you just need to identify the active slip systems, I would suggest you to have a look to misorientation axes distributions (the so called Rotation Axis distribution in Channel5) in crystal coordinate system. 1) they are much more reliable for the identification of active slip systems compared to c-axis CPOs. 2) Given that they are plotted in crystal coordinate system, they won't be affected by thin-section/sample orientation in the kinematic reference frame. I'm not so familiar with Mtex but I'm sure that it will be quite easy to plot them out from it.
For misorientation-related analysis you may have a look at these papers:
- Lloyd, G., 2000. Microstructural evolution in a mylonitic quartz simple shear zone: the significant roles of dauphine twinning and misorientation.
- Wheeler et al., 2001. The petrological significance of misorientations between grains.
cheers
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