Line-length restoration - a demonstration

Part of The Shear Zone Channel. This video demonstrates the mechanics of restoring structures onto an undeformed template assuming bed-length conservation. The method forms a fundamental part of the workflow for creating balanced cross-sections... diagrams that should be amenable to these types of restoration. The example comes from the Subandean fold and thrust belt of Bolivia. The diagram is available to download (and play with), from The Shear Zone website - theshearzone.wordpress.com/ex...

[@concretemathematics8146]

Thank you for showing me how to restore it.

[@robbutler2095]

Glad you found it useful. There are several other section balancing and restoration videos on the channel too....

[@katemat4445]

This is a great video - thank you for posting it. It is just what I’ve been looking for!

[@wrobs38]

Hi Rob, thanks for using this (my) section, and great demo of doing it with pen and paper rather than on a screen! One comment I’d make is that when you go about doing it in the real case, you have to treat the upper competent unit first as it is where the surface data come from. Then the shortening can be applied to the lower competent unit. This means that the eroded part in the anticline needs to be addressed before you can restore the lower competent unit. And also that means that most of the uncertainty on the shortening comes from there. Still need to publish the full section that was corrected since this short paper 👍🏻 Thanks again

[@robbutler2095]

Hi @wrobs38 (Jean-Christophe) - thanks for the comment. My view on the "where to start" and uncertainty issues with tasks like this is that all approaches should be tried... we're testing the interpretation (model) not the reality... As in the version of the section I used (from your paper), the green upper unit isn't drawn in so by adding that at the outset, the length of the unit becomes entirely arbitrary ... which for me isn't a great starting point. Whereas, the deeper structure is - in the model - fully defined. These types of issues, which I've not really addressed in this video (but is elsewhere) are key when applying restoration in practice - the approach explicitly relates to the question being asked of the structure/interpretation - and this will be specific to particular examples... Thanks again for constructing the original section - it's great to have it to promote these discussions ... and I hope you get out the modified version!

[@gmra98]

Bravo, excellent basin knowledge for future Structural Geologist

[@nawaz6700]

Thank you, Rob. I learned a lot today.

[@armgeo6246]

eventually I found it... thank you professor Rob..

[@mouadankach3492]

Thank you, @Rob Butler, for this great video. I have a question. I've seen a lot of videos that explain how to restore structures to an undeformed stage, assuming bed-length conservation. Most of these videos show the case of an inverted structure, but I've never seen a case of an extensional scenario. Is it possible?

[@robbutler2095]

It's a good question. There's no reason why layer-extensional structures can't be restored (assuming plane strain). However, when restorations of normal fault started (mid 1980s), they assumed a specific kinematic model - listric normal faulting and the "Chevron" construction (see video on listric normal faults). There were lots of other approaches building on assuming a particular fault construction method - so very different to thrust systems (originally). Though thrusts went that route with the application of idealsed fold-thrust models (and kink constructions)...

[@Martytalius]

This was so informative, Rob! Do you know where we could get a copy of this cross section?

[@robbutler2095]

I've added a link at the foot of the video's description text... The exercise is on my associated website (generally linked from the tab at the top of the channel) - the "exercises in structural geology" page as a set of things that relate to thrust belt interpretation... and more will follow.... Have fun!

[@armgeo6246]

I have one more question about the restoration output.. what I observe from your technique, is the length of the layer in folding section has to be equal to the restored flat layer? or only proportional to horizontal 'pinned' distance on your drawing? as I saw (correct me if I'm wrong) you rotate the folding paper at each point of turning and constrained them to the final horizontal distance of the cross section, so the restoration section does not imply the actual 'stretched' layer length prior to deformation.. do I interpret the concept in wrong way?? thank you for your advice and answer professor... best humbly regards.... ~from your 'non academic' student in far away in Indonesia~

[@robbutler2095]

Not sure I completely follow your question... but the act of measuring around folded layers is designed to establish its sinuous length... which, assuming no distortional strain, can be laid back to its assumed predeformed geometry (in this case, horizontal). Have a look at the "strain on cross sections" video - in the structural geology playlist.

[@armgeo6246]

@@robbutler2095 wow it must early morning in Britain... Thank you for your reply Prof.. My bad, I just didn't realize that this is a hand drawing demonstration which could be slightly miss the geometrical accuracy.. yes you had explained that to us in your other videos that the restored length will always be displayed as a longer section view with respect to horizontal axis reference, which in actual they both display equal length prior to deformation distortion. Thank you and thank you.. surely I will go to your sugested lecture in this channel.. Wish you all the best and Best regards Prof...

[@robbutler2095]

@@armgeo6246 Thanks. I'm a fan of doing these exercises by hand - so you stay in control of the decisions... while this may be less precise (rather than accurate) compared with graphical tools, given the assumptions made in real world applications (eg ideal layer cake strata, no distortional strain, uncertainty in drawing in the eroded parts, let alone choices in the subsurface) - most claimed precisions are bogus! It's always better to be accurate rather than precise.... thank you again for the comments and interest in the Shear Zone Channel.

[@katemat4445]

Hi Rob, if the whole of the cross section is deformed, can you use a loose line anywhere? Or is that only used where you find an area of undeformed layers?

[@katemat4445]

I am trying to the Mount Crandle Thrust section and it's so confusing! (Boyer and Elliot)

[@robbutler2095]

It's a great question - setting up a loose line is a judgement call - you want to select a part of the cross-section within which you think there has been no interbed slip ... or shear... best done where the strata "return to their regionals"... - a common site selection is along the axial trace of upright synforms....

[@robbutler2095]

Ah ha - an interesting section. There's no real pin at the front either. I'd start at the back - to the W side of the section and put a loose (de facto pin) line in there and reconstruct the strata from there - moving eastwards. You need to "rehang" the strat as you go - the units are wedge-shaped... so set up one horizon as a datum (eg as labelled by B&E) and measure strat thicknesses from the section, for given distances along the datum... drawing the restored template as you go.... The section as drawn by B&E is pretty unconstrained (at depth and eroded) so there's some freedom in getting the section to work....

[@katemat4445]

@@robbutler2095 Thank you! I will try that. There's a lot going on in the section. I had been working from the E side, so I will try from the W side. I can see the datum horizon that you mentioned, on their section, so I will do that - it's the horizon I had in mind as I read your message! Thank you again... back to it!

[@katemat4445]

@@robbutler2095 I've just finished having another go and it's the closest that I have got. I am slightly out but I'm not sure that's just my measuring. Thank you so much for your advice.

[@nawaz6700]

Hello Rob, any thoughts on how to make the restoration template?

[@robbutler2095]

A really good question - the template can be constructed before you start IF you assume a particular stratigraphic arrangement e.g. layer-cake strata, which what we've done with this Bolivia example. BUT for more complex (i.e. normal) situations you need to build the template as you restore the structures, using measured (or inferred) strat thicknesses - and re-hanging these for a chosen datum horizon (using a horizon that is shallow in the stratigraphy that you interpret to have been continuous and unbroken before thrusting)... It's a complex business - watching the stratigraphic relationships "grow" as you restore the section step by step. Some iteration (dynamic alteration) required as you go....