Welcome to "the Shear Zone" - a hub for educational videos on geology, developed by Rob Butler of the University of Aberdeen. The materials focus on the interpretation of the geometry and relationships found in and between rocks - topics embraced by the term "structural geology". This involves the study of rock deformation - describing and understanding how rocks are broken, folded and distorted. And this links through the scales, from grains to plates. But it also includes broader ideas on how rock formations are organised - which is fundamental to stratigraphic understanding, deducing earth history and the processes that have shaped it. You can find videos on the interpretation of geological maps, 2D seismic profiles of the subsurface alongside developing ideas on how deformation structures, sedimentary basins, mountain belts and tectonic plates work. Supporting materials (maps, images, course notes) will go up on the companion website: .
Thanks a lot for sharing useful geological content. I have a question about, is it possible that, when normal fault reactive, its accompany also with strikes-slipe events? .
Thanks, Rob, for these great videos! Quick question: Could this be considered horizon flattening rather than seismic restoration, since the lateral distance remains unchanged? For true restoration, wouldn’t you need to account for changes in XX' distance?
Well indeed - we are flattening on the X-X' horizon, and it could be done using seismic software ... though you'd need the picks to be in too. Of course, for any restoration (extension or contraction) there will be a change in horizontal distances too... which is what would come next... but to test the basic idea of inversion and finding which faults have reactivated, I'd always do a first pass as in the film.
Fascinating video, as always Rob. I walked up Teide, past house sized boulders that had clearly rolled down the side of the mountain. And descended into an impressive lava tube near the summit. The Anaga peninsula is beautiful, such a contrast with the south of the island.
Wouldn’t the original normal faults still be listric? And how do you restore the original extension? I noticed that the distance between X and X’ was the same in both sections, but before inversion the section should be longer, no? And are the ages of the faults all the same? Or is the purpose of this exercise merely to judge the general magnitude of inversion on each fault individually, and not necessarily to restore the whole section?
Thanks for the comments. As stated - this was a quick look approach. Of course the horizontal distance between X and X' would have decreased during inversion.But restoring this would be far more elaborate. The ideais to do a quick pass as shown first to get an idea of what's happened, then focus in.
@@robbutler2095From the example, it is clear that the first extension stage (ES) is replaced by the second contraction stage (CS). In ES, the orientation of the minimum principal stress (σ3 = σh) is parallel to σ1 = σH in CS. This is indeed a textbook definition of an inversion fault, which is a type of fault that experiences several stages of movement along the same fault plane. However, regional inversion can also occur even when the orientations of σH and σh do not change significantly, but σV changes from being the largest to the smallest compared to σH and σh. Basin inversion also occurs, though not on the same fault plane. What do you think about this other type of inversion?
Apologies for creating some confusion -many of my films are made for students or amateur geologists... if you've a specific piece of jargon you'd like covering - let me know...
Thanks for an interesting vlog. I was confused by the SW corner (eg at 17:30) - I didn't know if the structure was raised or formed a dip. There seemed to be few clues on the map - the main one seemed to be shading (with the san at top left). So you described the dip (I also note the order of layers to support this) - I also tried to see the limestone as an overlay, so the limestone overlying the sandstone could have been a repeat layer, especially looking at the stream zigzagging north through the Nippenose label on the map. Finally, I looked on Google maps to see the actual depression of that area.
Thanks for the question. The outcrops in Limestone area (Limestone township) on te map is the continuation of the antiform that we see in the line of the cross-section (in Armstrong area) - the patches are rather like peas in a pod - with a small saddle of sandstone between.... hope that's what you're after.
I spent 8 weeks in 1975 as a conservation volunteer building stone walls on the track below the Devil's Kitchen. Staying on Willy's farm (Gwern Gof Isaf?). One of the hardest times of my life.
A very insightful film regarding map interpretation. Dear sir your films are not only source of information, but these are also source of inspiration for all those, who are concerned with Structural Geology and Tectonics.
Very nice job! It appears you have done this a few times. I'm good with stratigraphy but your ability to xsection folds is above average. As a teacher I know it helps to do it 5 times a day for 20yrs! Maybe slow down on a simpler map and show us again to help out those just starting out? I mean for those who are not structural geologists like me.
Hello Rob, as I've just graduated from my geology degree in Zaragoza, just wanted to thank you for your insightful work throughout these past years, as it has helped me immensely. I'll be glad to keep up with your videos in times to come.
Many thanks for this. Never saw a fossil in ten years at Loch Torridon which correlates with the development of life on earth . So different in the east of Fife where carboniferous life was very much on the move.
Thanks a nice video, however a massive subduction of this kind is extremely unlikely at least in the well known form in which it is presented and taught in most colleges. The problem is that it could be interpreted completely in other way, they are just the slabs from the ancient "continental" crusts in different orientations mainly under the Pangean pieces (and previous slabs form resurfaced crusts from the prior times) that form the now continental floors (or exist beyond them), and there are many misfittings with "subduction ideas" when you look to world´s examples. The occurrence of the seismic activity prove just that contact and stress is being released in the interface between the plates, they hold each side, they have friction, the plates did move laterally as a result of planetary expansion and changes in isostasy through time, and thus the tomographies do not prove beyond reasonable doubt any massive horizontal subduction. The process is also impossible, how lighter materials will sink into heavier and denser ones? What force will push them so massively under even more lighter continental slabs and where are the scars of the stress that such a massive collision will cause on the allegedly conveyor belt in the oceans? It just is absent, major failures are perpendicular to the rifts and some parallel features occur along the main ridges but none of them are clear near the "massive subduction zones". Both ideas are well established in the mainstream geological scheme while being mythological and against the laws of physics. Some vertical subduction do happens and lithospheric leaks do sink continental pieces, you can observe it in some places on the continental emerged areas, and in Zelandia. I had seen many seismic tomographies and look at many papers and it clearly seems that facts are being forced into these failures of the mainstream tectonic theory by dogma and academic inertia. Theories sometimes drives the research and genial ideas sometimes take a long road in order to be accepted and widely recongnized. As many are convinced of it happening (massive subduction and conveyor belts), they interpret observations in a wrong way, it is a mischieving auto maintaining circle. Now consider the thickness of the plates involved, many cases are in the ratios of 1:12 which would be already strange enough, but we have deep continental roots and thicknesses of 1:47 in some areas of the Andes, for instance. That means that a thin oceanic crust of 11-28 km at the best, is expected to curve itself and go down about 350-650 kms to go under the continent for unknown real reasons... How will a thin layer will collide with such a massive wall and swiftly move beyond it? Ancient resurfaced slabs of prior continental-planetary pre-oceanic-expansionary are a much better explanation for what we can see in the tomographic and seismic data. For if it isn´t enough to consider those ideas, we have the ages of the sea floor “precisely telling the story of how all the sides of the chronological floors met each other” and when removed carefully in corresponding ages in smaller globes we arrive to the hard truth and fact that all the edges of the continents (which are just pieces of the Pangean time crust opened by the new oceanic ridges and ocean floors) match each other at 99% precision. James Maxlow from Curtin published a brilliant PhD Thesis reconstructing all the fitting and giving a wide range of other hard proofs on several scientific fields some 28 years ago. The proofs ensembles well with Croizat´s biogeographical ideas, panbiogeographic tracks and nodes, biogeographic patterns in Gondwanaland, biodiversity resemblances, rock match-correspondances in correspondent continental sides, geological formations, the worlds hydrocarbon doughnut (pre-mesozoic, mainly late Paleozoic doughnut on then tropical areas), and even geodetical data. Once you realize the amazing match, it result unnecessary to think about massive horizontal subduction mainly because it will directly contradict the match and it goes against physics, and also, the later (expansion matches) are so real and overwhelming that I find difficult to understand the slow rate of acceptance of the whole idea and the facts behind it. It is even a well distributed phenomena in the Solar System and had been mapped in Ganymede and other places, and expansion is also a main feature of the known universe... so expansionary plate tectonics without myths might be a good idea. M Sc David Brailovsky Mexico City
A key point about subduction is that it's not about crust (oceanic or continental) but lithosphere - the mantle component thereof. This is colder than the underlying asthenosphere therefore it is denser - and, in the subduction model, it this density difference that drives the the down-going slab. Note also that the shallow earthquakes at trenches are thrust-sense - and the measured motions associated with these (e.g. Tohoku) are consistent with these being convergent plate boundaries... and its oceanic lithosphere that's going down... i.e. subduction. There are (and have been) vigorous debates across the earth sciences about all of this that are pretty much resolved by increasing data. I don't recognise your characterisation of the scientific community.
Congratulations and thank you very much Rob Butler. Your classes are very didactic and have helped me to better prepare my own. Where could I get this seismic section so I can use it as an exercise in class?
Sorry - the line isn't generally available. But you can see it published with interpretations in this paper - open access (and references therein): Bond, C.E., Alcalde, J., Butler, R.W., McDermott, K. and Carbonell, R., 2023. Continent‐Ocean Transition or Boundary? Crowd‐Sourced Seismic Interpretations of the East‐India Passive Margin. Tectonics, 42(8), p.e2022TC007624. agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2022TC007624
You've been caught out by language differences at places such as 8:08. Tryfan is not pronounced 'Triffan' with an accented first syllable. It's more like 'Truvaan', with the emphasis on the second syllable. In Welsh, the 'f' is an English 'v'. The letter 'ff' is pronounced like an English 'f'. (And yes, there are a number of Welsh alphabetical letters which are made up of two Latin letters. They are pronounced differently to single letters. These include ch, dd, ff, ll, rh and th. And, once you understand how the alphabet works, the pronunciation is usually exactly according to the spelling.
Talk of regulation is rather optimistic given the exploits of corporate profiteering in evidence to day. Thanks for the trip down memory lane which was the 1970's for me.
The surface is assumed to be planar in this example. If you take the three points and their coordinates, X, Y Z (Z being the elevation above sea level and not the depth from the surface), then you have a linear regression problem which is just three simultaneous equations in X and Y, e.g. aX1 + bY1 = Z1; aX2+bY2=Z2; and aX3+bY3 = Z3. This can be solved using the method one learned in high school, or by matrices. Software like Golden Software's "Surfer" will do this very easily. If the surface is not planar and you have more than three control points, then you can use a curved surface like a quadratic or cubic surface to fit the data more precisely.
Indeed and there are several options for software. But I think it's always best to first do these things manually and graphically when introduced to techniques like this - so you can see what you're doing... and indeed even when experienced from time to time to make sure the software is doing what you think it is... same true for applications like plotting stereonets etc.
I worked in this place for four years and loved it. Great geology, great food, nice people living there and great colleagues too. Even made a major discovery there using geophysics (TEM & IP). The water in the Confesionarios pit had pH2 so we couldn't wash our cars with it!
@@robbutler2095 Drinking water was supplied from some unknown location by a local with a donkey cart. The water for drilling was another headache and one invoice was so high the boss exploded and said it would have been cheaper to use wine. We had a regular supply of geophysics students from Leicester and Delft every summer, geology students from Cambridge occasionally. Those were the days!
All part of the planet sorting itself into layers with the heaviest at the bottom and lightest at the top. I wonder if mantle plumes fit into this somewhere - in the big picture (not in the Alps!)
as you say, not this topic.... but the Earth is dynamic, a heat engine - so convection in various forms plays a key part - subduction being one, plumes another...
Thanks for the question: veins and blobs whether pegmatitic or quartz etc can show these types of flanking structures (indicative of rotations) - regardless of metamorphic grade... I've seen them in all sorts of different settings....
Rob, very enjoyable little series with a lot of information to digest. Couple of queries for me: (a) The diagram you 'suggest' for the original distribution (pre-subduction) of Monte Rosa, Combin & Zermatt-Saas - how then does the Monte Rosa get subducted ahead of the Combin? Does the Combin get emplaced into the accretionary wedge and then get subducted/ buried collisionally after Monte Rosa? (b) In terms of western US flat-slab subduction could the strongly bonded subducted material remain bonded and bouyant and simply force the descending Farallon plate to not descend and 'attach' itself to the base of the NA crust? Have enjoyed a lot of your YT vids especially the areas I did some of my mapping (Skye) and field training (Tryfan) on. Thanks
Thanks for the questions. The Combin for me stays relatively shallow and MR subducts beneath it before coming back up.. as in the rather simple animation (c 28.30).. AS for western US - can't really say, except that bonded buoyant crust is the conventional view for clogging subduction zones during continental collision...
I had access to physical copies... but Yes - view via the BGS maps portal which has a treasure trove of historical maps. www.bgs.ac.uk/information-hub/bgs-maps-portal/
Good video Rob. There's an excellent book about William Smith and the making of his map:. The Map That Changed the World, by Simon Winchester. I still find it amazing how similar it is to today's version.
Thank you for producing this interesting video. The way you and others have worked out where all the rock types originated off what would have then been the South of Gondwana (where Madagascar is today) is very impressive. So too is the story of their metamorphosis.
Thanks - the fundamental research laid out in the film was obviously done by others - but putting the story together is interesting. It was a nice prelude to my films on Morocco and on the Rio Tinto stuff...
Fair observation. They do - but they're not huge at that stage... some could be due to differential compaction - but just because a basin forms, then goes into thermal subsidence it doesn't make it immune to further deformation....
Perfect view of the 'Indian Face' of Tryfan in profile at about 7.56min mark behind Rob. Rotate your screen 90° anticlockwise, and you will see the Indian and his headress. Spent many weeks at Og. Cott. as a youth, never studied the geolgy of the area properly, we just wanted to climb or kayak! Thanks for this insight.
