The Tyee: Inside the Dense, Intense History of Plate Tectonics https://thetyee.ca/Culture/2023/06/22/Dense-Intense-History-Plate-Tectonics/ #bcnews #TheTyee - via @tyee@mstdn.ca #TheUnlikelyRevolutionaryofPlateTectonics #UniversityofTorontogeophysics #InternationalGeophysicalYear #MaoZedongGreatLeapForward #NorthAmericangeologists #Geologypermanentists #2011Japanearthquake #Mid-AtlanticRidge #NorthAmericaplate #2011Japantsunami #BCplatetectonics #Geologymobilists #AuthorNickEyles #JohnTuzoWilson

Wilson Cycle Phase 4 - Episode 4 - Collision and Collapse

“Tuzo was indeed an amazing man. His scientific interests were in the building of mountain ranges and moving of continents, and he certainly had a huge and long-lasting effect on Earth Science. But even more, his talent was in moving another type of mountain: those ideas in men’s and women’s minds that lock us into past patterns of behaviour and prevent visualization of new possibilities.”
~ Gordon F. West and others, from “John Tuzo Wilson: a man who moved mountains.”

‘The final stage of the Wilson Cycle involves continental-continental collision and the formation of giant mountain belts. And yet it’s how these giant mountain belts get exposed at the surface that allows us to study the Wilson Cycle in its entirety. Now we are going to explore the rocks which are formed deep within the mountain building stages of the Wilson Cycle and discover how these rocks are exposed at the surface today.
~ Professor Dougal Jerrom - intro to Episode 4

In the video below, “Wilson Cycle Phase 4 -Episode 4 -Collision and Collapse”, we finish our trip with Professor Dougal Jerrom, and the team of researchers in search of evidence for the collision and collapse of the Caledonian orogeny in the Caledonides Mountains. This series has kept the last and best for the finale, and it is a mind-blowing conclusion. I really recommend you watch the 29 minute video for yourself. It is well worth your time!

We first stop to look at some eclogites, and Norway has the best in the world - “the jewel in the crown of Norway” and are under its protection. These are ultra-high pressure metamorphic rocks that have been formed 100 km. or more deep in the subduction zone, and are especially stunning with coarse-grained “pegmatic” texture, deep green orthopyroxene, bright red garnet, and a white mineral coesite, a metamorphic index mineral indicates they underwent extremely high pressure. As Professor Jerrom says, discovering how these rocks were exposed is one of the most fascinating geology stories yet.

This starts with the subduction of oceanic crust underneath oceanic crust. The subducted slab is being pulled down by its own weight, and the two continents are approaching each other. The upper portion of the slab is still buoyant (see the model below, and the model in the video they run in the CEED lab) and doesn’t want to go down, so the slab breaks off and sinks through the mantle. The upper slab springs back up in a process called “eduction” (the reverse of subduction) and brings the deep metamorphic rocks back up where they are exposed during the collapse portion of the Wilson Cycle.

The late stages of subduction and collision build mountains as high as 3 km., but they are gravitationally unstable. The crust is thickened and getting heavier, and the upper level goes back into extension even as the lower crust is still thickening because of its own weight. This happens EVEN AS MOUNTAIN BUILDING IS STILL OCCURRING. Lots of erosion fills the basin with thick sediments up to 25 km. We see a giant detachment fault that is slowly denuding the top of the mountain of sedimentary rock that is sliding down a fault plane of metamorphosed rock. So mountains can quickly (in geological time) be stripped and the deeper crustal rocks exposed by isostatic rebound. We see this happening to day in the Himalayas Tibetan Plateau.

We are taken up close to a “textbook” active detachment fault with gouge, grain-size reduction, and fluid circulation. By studying the magnetism of the fractures of the fault and comparing it with the magnetism in this part of the world, tells us that the fault was reactivated, and also tells us about the end of the Caledonian, and the beginning of a new Wilson Cycle!

“So we’ve seen all the gems of knowledge stored in the rocks of Norway from the mountains to the fields tell us about this major cycle in Earth’s history, but it doesn’t end here because it’s happening again today with the north Atlantic opening and Norway is the place we could come back to in millions and millions of years in the future to understand the closure of that ocean and the mountain building event that would form the next Wilson Cycle.’
~ Professor Jerrom Dougal closes the series.

Wilson Cycle Phase 4 - Episode 4 - Collision and Collapse: https://youtu.be/pFuW4MtXhIM

The video introduces a paper published in the 90s that outlines the process of subduction and eduction, by Torgeir B. Anderson (yes, the same Torgeir that has been providing all the information on our journey) et al. Highly informative https://www.researchgate.net/publication/229872945_Subduction_and_eduction_of_continental_crust_major_mechanisms_during_continent-continent_collision_and_orogenic_extensional_collapse_a_model_based_on_the_south_Norwegian_Caledonides

#JohnTuzoWilson #LegendsOfGeology #TheWilsonCycle #Caledonides #Norway #WatchTheDamnVideo #PlateTectonics #SubductionAndEduction #OrogenicCollapse #eclogites #geology #Science #NorwegianGeologyRocks @geology

Wilson Cycle Phase 3: Ocean Closing

“I enjoy, and have always enjoyed, disturbing scientists.”

~ John Tuzo Wilson

“From great oceans, giant mountains will form. This is the prediction of the third phase of the Wilson Cycle where we see the closure of the vast oceans like the Iapetus Ocean, and the formation of continent to continent collision mountains.”

~ Professor Dougal Jerrom, intro to Wilson Cycle Phase 3: Ocean Closing video linked below

In the 24 minute video below (a great watch), we continue our travels with Professor Dougal Jerrom of Oslo University, to look at evidence of the closing of the former Iapetus Sea in the Caledonian of Norway. The video starts with a nice recap of the other videos.

Whenever there is an ocean closure, geoscientists always look for ophiolite. Ophiolites in the geologic record provide evidence of subduction zone tectonics. We have already seen the pillow basalts, sheeted dikes (gabbros) and peridotites that form ocean crust in earlier videos in this series. An ophiolite is then a bit of oceanic crust that got swept up in the continental collisions and provides the first proof of an ocean closure.

Next we learn about melange - the sediments that were scraped up during subduction, also known as an accretionary wedge. There are also remnants of the island arc formed during subdution all accreted between the bits of continental crustal blocks that eventually collided.

Enjoy the video below where you see glimpses of the geology of the beautiful Norwegian Caledonian.

Wilson Cycle Phase 3: Ocean Closing: https://youtu.be/jYlQCbDmEVw

More on the closing of the Iapetus Sea here: https://en.m.wikipedia.org/wiki/Iapetus_Ocean

Includes Step 5 of the Wilson Cycle: https://c.im/@vickyveritas/10962360456...

Refer to this great labeled diagram of the Wilson Cycle for the components of the ocean closure and continent to continent collision and mountain building: https://c.im/@vickyveritas/109628513962823739

Note: We will see the last steps of the Wilson Cycle, the continent to continent collision and the formation of giant mountain belts, and the great grand finale of this series. Stay tuned.

#JohnTuzoWilson #TheWilsonCycle #Caledonides #IapetusOcean #Norway #PlateTectonics #Ophiolite #AccretionaryWedge #granites #SutureZones #Laurentia #Baltica #geology #ScienceMastodon #NorwegianGeologyRocks @geology

The Wilson Cycle: Intro

“If the continents have moved, then they have drifted like rafts and formed the ocean floors in their wake. It is to this wake that we should look first.”
~ John Tuzo Wilson

Canadian geophysicist and geologist, John Tuzo Wilson, posed the question (and the title of his article in 1966) - ‘Did the Atlantic close and then re-open?’ Spoiler alert - yes. That, and his many contributions to plate tectonics, including the concept of hotspots and transform faults, led to the Wilson Cycle (also known as the Supercontinent Cycle) being named after him.

The Wilson Cycle (WC) refers to the process of continent break-up and ocean-opening followed by subduction, collision, ocean-closing, and continent formation (see the diagram below). This can take tens to hundreds of millons of years (very deep time) to complete.

This quick video provides animation of the WC: https://youtu.be/I_q3sAcuzIY

Step 1 of the WC starts with a tectonically stable continent/craton, eroded down and perhaps scarred by earlier collisions. Rifting (or faulting), crustal thinning, and thermal uplift caused by tectonic stretching of the continent allows the upper mantle (plume) to rise up and fill in. This can lead to earthquakes and volcanic flows. Sometime the plume can die out leading to a failed rift, but when rifting continues, things get quite interesting.

Step 2 The fractures are deep and oriented perpendicular to the extensional direction. As the continent breaks apart, the plume develops convection cells that further the rifting and deepen the basin allowing water in. The mantle material exposed by the rifting is made of much denser (or mafic) material than continental crust (or felsic) and sinks, cools and hardens/crystallizes forming oceanic crust. A new ocean basin is created.

Step 3 The two new continents continue to drift apart; the rift becomes a young spreading ridge, and the new ocean crust sinks further into the mantle as it cools and becomes denser. Sediment is now collecting on the new ocean floors.

Step 4 The Mid Ocean Ridge (MOR) continues to create new ocean crust and the new ocean deepens as the oceanic crust matures and continues to sink into the mantle. The mature oceanic crust is much heavier than the bordering continental crust, and cracks can develop causing the oceanic crust to flex downwards forming a young subduction zone. Part of the oceanic crust is dragged deep into trench and the water-laden oceanic crust melts due to the higher temperatures of the mantle. Volcanic Island arcs are created. The rifted continental crust is now well below the surface of the ocean.

Step 5 Divergence ceases, and convergence begins. The MOR is eventually subducted, or consumed at the ocean basin margin. Associated volcanism and subduction continues, along with collision, narrowing the ocean and causing mountains to form. This is now part of the ocean-closing cycle.

Step 6 As the continents/cratons continue to collide, folding, faulting, and earthquakes occur creating new mountains (think of the Himalayas), while catching up bits of volcanic rock, oceanic crust and sedimentary rock. A new continent is formed.

Step 7 The continent matures and erodes. Rinse and repeat with continents colliding, eventually forming supercontinents, and dispersing again in a much longer and even deeper time.

The Wilson Cycle is somewhat simplified and doesn’t go into all the sorts of tectonic variations of rift zones and diversity of plate tectonics, but it was a landmark starting point and a sign of Wilson’s genius. Later geologists and geophysicists stand on the shoulders of this giant. We’ll dive further into the Wilson Cycle in future posts, and have a look at the rock types created and how geologists piece together the Wilson Cycle in real rocks. It’ll be fun she said :)

#WilsonCycle #JohnTuzoWilson #OceanicCrustFormation #Mantle #Plume #MidOceanRidge #SubductionZone #MountainForming #geology #ScienceMastodon @geology