Shear criteria

Part of "The Shear Zone" video channel. A short look at how small scale deformation structures can be used to determine the sense of movement on shear zones, in the same way as equivalent observations form the basis of understanding fault systems...

[@zacariasfloresdelcampo1330]

Thank you very much!

[@robbutler2095]

Glad you liked the short introduction.

[@hassanaleem2871]

wow.......................feeling so good to know all this

[@mji1967]

Your videos are so good. I’m going to watch them all

[@user-ij3qo6xl9s]

Thanks a lot for your so excellent examples and so suitalbe explainations with cartoon. Yeah, it is necessary to use composite markers to determine a Shear criteria in a rigion. Only absolute one shear sense is impossible, but we can choose the dominant one. I think if we can determine the possible maximum principle stress axis, there should be two possible shear planes with opposite shear orientations in the l-s tectonites. Of course only one shear plane governs this region while another is relatively poor.

[@robbutler2095]

complex shear sense indicators are indeed common, not least because of (probably) transient strain partitioning in shear zones... and of course non-plane/general shear....

[@argendarne8204]

Thank you for the interesting lecture! I have a question about the shear sense of the folded pegmatite at 14:09. Is sinistral shear sense also possible for the asymmetric fold of pegmatite in your example? like N-shape minor paracitic fold in a larger flexural fold.

[@robbutler2095]

Thanks for the question. Fold shapes need care, and consideration of where different strain intensities are distributed... as in the sketches that follow in the video.

[@buturusca]

Hi, at minute 9:18 it shows in left outcrop picture that the general shear is parallel to shear bands, but in the figure to the right the general shear is parallel to the foliation not parallel to shear bands ...am I thinking this the wrong way? thanks!

[@robbutler2095]

Well - the sense of shear in the shear bands is as shown - and, upscaling, it implies the same SENSE of shear in the shear zone as a whole. The general issue lies in deducing the orientation of the "shear plane" (see video on shear zones for further discussion, and the shear band vs s-c fabric in the current video)... In the case here (Rocher du L'Yret) the shear sense is "top to L" ... Hope this helps.

[@user-ij3qo6xl9s]

I study it again. In the workflow, the fourth step should be parallel to the lineation and perpendicular to the foliation; not "perpendicular to the lineation" as listed.

[@robbutler2095]

Actually not - see timing at 3.30 ±... we need to look across the lineation (in a direction perpendicular to it) - not parallel to lineation (i.e. down the lineation direction) in order to identify the relevant asymmetry of shear criteria. Hopefully the diagram makes in clear. Thanks for asking for clarification.

[@user-ij3qo6xl9s]

​@@robbutler2095 Thanks. Do you mean one should begin from the side across the lineation is ok, YZ plane. This YZ plane usually contains symmetric structures. Then naturally, change to find and determine shear sense by the XZ plane asymetric shear criteria from the side parallel to the lineation but perpendicular to the foliation .

[@robbutler2095]

@@user-ij3qo6xl9s Establish X-Y plane (foliation) = shear plane at high strain. Then find X (stretching lineation) = transport axis (at high strain). Then look in Y direction (so X-Z plane) to identify and interpret asymmetric structures (=shear sense). The other views may indeed contain asymmetric structures (fortuitously) that introduce ambiguity into a kinematic analysis. This approach assumes plane strain simple shear of course.... and this decision needs careful contemplation because not all natural deformations are like this!

[@user-ij3qo6xl9s]

@@robbutler2095 This workflow explanation is much clearer. That's great. Many thanks!