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so do your other videos, I automatically subscribed.

I’m not quite sure what the point of the question is. The hinge line doesn’t have a strike. It has a trend and a plunge, which is given by the position of the pole to the pi-plane. If the question refers to the strike of the axial surface, the answer would be that this direction must be determined in the field or from geological maps.

You're welcome

​@@Geology_clipsmaybe you can explain why does the oblique subduction exist, it baffles my mind

@@iamthewalrus3655 Imagine an oceanic plate being pushed westwards by seafloor spreading, creating a westwards movement vector. If this plate is subducted underneath a stationary continent with a north-south continental margin, the subduction will be frontal (dip-slip). If the continental margin is oriented in any other direction (NE-SW, NNW-SSE, etc.) the subduction will be oblique to the continental margin. If the continental plate is non-stationary (most are), it's a bit more complicated. In principle, the magnitude and direction of the continental and oceanic plates' movement vectors and the orientation of the overriding plate's margin will determine whether the subduction is frontal (dip-slip, oblique, or whether there is a strike-slip movement along the plate margin (e.g. San Andreas Fault). I hope that helps.

Welcome!

All orientations are given as dip azimuth and dip angles (Clar readings).

I understand this. However, even after adding 90 to the great circle strike value on my stereonet then it plotting it as a separate line, it still does not match the best cylindrical fit. Why could this be the case?

The strike of the axial surface must be obtained from field observation or from geological maps. It cannot be determined graphically using the stereonet.

​@@Geology_clipsHello, can you please provide some guidance how to do that? Did you maybe refer to it in another video?

On a geological map you might see the typical lobes of fold closures where layers bend around the hinge and curve into the direction of the opposite limb. A line perpendicular to the folded layer in the hinge, and parallel to the limbs, is the best guess for the strike direction of the axial surface. If you are in the field try and find a view point in which you can see onto the pi-plane of the fold 9view direction parallel to the hinge and the axial surface). Then you can take a compass bearing and obtain a reasonably accurate estimate of the AS strike. In such situations you also can align your compass with the orientation of the AS and get a good estimate of its orientation. If you are lucky and the there is an axial plane cleavage developed, you can measure the AS directly, but take care to do this always in the centre of the fold hinge. @@nils6097

@@Geology_clips First of all, many thanks for the quick and comprehensive reply. I finally got what you meant. I hope you have a nice day.

That's explained at 10 min 30 sec to 12 min in the video.

The surface of limbs of natural folds is always uneven and therefore compass readings show some variation for each limb. This results in some scattering of great circles or poles to planes. Average orientations of the limbs are therefore used to construct representative hinge line orientations and axial surfaces.

Deer, Howie, Zussman, 2013. Introduction to Rock Forming Minerals.

From field observation or from geological maps.

@@Geology_clips What is meant by the strike of the axial surface? I have lots of strike and dip data of the ascending limbs of an anticline, which i'm plotting on stereonet, how do I get the strike of the axial surface?

@@karlcarmody9935 For the geometric analysis you need data from both limbs . The axial surface ideally can be measured or estimated in the hinge zone. Otherwise fold closures of the fold itself or other folds nearby should be visible on geological maps. The best estimate for the strike of the axial surface one can get in such cases is by connecting the hinges of different layers on the map.

A nice introductory text would be Rey's book: www.researchgate.net/publication/299135987_Introduction_to_Structural_Geology

Thank you so much 😀

This is from Rick Allmendinger's software collection : www.rickallmendinger.net/

Glad it was helpful!