Idaho Slab Curtain w/ Gene Humphreys

Nick Zentner & Gene Humphreys | August 21, 2024
Nick learns some new Pacific Northwest seismic tomography from University of Oregon's Gene Humphreys.

[@dalehagglund]

A great discussion, Nick, and a big thanks to Dr Humphreys for his time, knowledge, and willingness to share with all of us.

[@sharonseal9150]

Wow - this was a very VERY helpful presentation - thanks Gene and Nick! I cannot wait to read the papers and watch this video a couple of more times. Exciting program.

[@pathorgan8643]

This one turned on light bulbs for me. The visuals presented were fantastic and it was fun to observe Nick working to knit together a story that will no doubt be polished and told. Great stuff!

[@AvanaVana]

Sorry I missed this live. I have been following Humphreys’ work on this for a long time-I remember seeing his talks at the CIDER Dynamic Earth conference several years ago, during which I first was made aware of the plethora of tomographic anomalies in the western US, both fast and slow. There’s a lot I will probably have to say about this topic, so I’ll probably just comment and add/edit this comment as I watch. ~ 12:00 here’s the problem: sure, when you do the Darin/Dorsey and/or McQuarrie/Wernicke palinspastic/paleo-reconstruction you can get a very nice, well-behaved little continental arc, with long-lived eastward-dipping subduction and a paleo trench not too far outboard of the Idaho batholith…except this all falls apart when you remember that the Idaho Batholith magmas were erupting both before and during the time that Baja BJ was translated northward along the margin. This means that there couldn’t have been subduction right off of a paleo coast going throuŷgh present-day Idaho, because during the time period from about 90-50 Ma, the massive Insular superterrane, including Wrangellia and the Mt. Stuart Granodiorite (formed at the latitude of modern-day mexico) was outboard of the continental margin instead. And for the insular superterrane/Baja BC to be translated so far so quickly northward, a tectonic regime like today’s San Andreas/Salinia/Baja CA would have been necessary-in other words, the Baja BC margin had to have been purely transcurrentin nature (strike-slip), with no component of subduction. The sheer magnitude of translation requires a San Andreas-like margin of pure transcurrent movement without subduction or rather, without oblique subduction, like today’s tectonic slivers translated by oblique subduction along parts of both the Andean and Indonesian continental arcs. Yes, the palinspastic restoration Humphreys presents around 12:00 looks lovely at 50 Ma, after Baja BC is off the north edge of the map, but how did the Idaho Batholith and all of the related plutons get emplaced due to subduction if there was a massive superterrane in the way, without any subduction underneath it (Iike Baja CA today)? It’s a huge problem and for me the central mystery of the Idaho Batholith. Also, commenting on the tomography shown around 9:00, which I remember from Humphreys’ and also Dave Stegman’s CIDER presentations, as well as from EarthScope, USArray, and IRIS data and presentations, it isn’t just the Idaho fast/negative anomaly (the “slab curtain”) that is interesting-there are a host of other features in the tomography that are really interesting that should be called out. Take a look, for example, at the fast/hot anomaly under Central Washington, which is effectively a “hole” in the flat slab that used to connect the subducting Cascadia slab and the Idaho sunken “slab curtain”…a hole likely “burned” through by the YS plume head, causing the eruption of the Columbia River Flood Basalts (it is also possible they were erupted without a plume, by asthenosphere simply escaping through a tear or window in the slab). Humphreys makes a passing reference to a remnant of the ancient connecting flat slab segment just north of this “hole” in the subcontinental mantle lithosphere. Edit: I watched more of the video and Humphreys claims that this positive/low anomaly, the so-called “Pasco Underplate” is pre-CRFB, actually an Eocene/Challis-related underplate. What is the evidence for this? It ems much more likely this is an underplate from the CRFB, just like the kind of underplate psycho we observe from LIPs throughout earth history for billions of years, for example, the Duluth Complex from under the Midcontinent Rift. According to Humphreys, the topside part of the “Pasco Underplate” would be the Teanaway Basalt, essentially. Next, observe the crazy fast/cold, roughly circular anomaly directly under the Blue Mtns. In northeastern Oregon. This represents the single fastest/coldest anomaly in the whole tomographic dataset. It is likely part of the dense crustal root of the ancient Nevadaplano, the orogenic plateau formed after the Sevier orogeny, which was so dense that it delaminated from the lighter lithosphere above and sank into the mantle below. This delaminatin or “dripduction”, causes the crust above it to rise, because the crustal root can no longer drag down the crust above it, and in its place flows hot, buoyant asthenosphere. This likely explains the relatively recent uplift of the blue mountains, their relative high elevation and geomophological youth, and the fact that they are one of the only places around where otherwise buried rocks of the ancient accreted terrane basement is exposed at high elevations. Farther south, one can observe similar anomalies in the American southwest, including one associated with the recent uplift of the Sierra Nevada mountains.

[@KozmykJ]

So often I've got the impression that 'Flat Slab' stories were 'Old Skool' and somehow behind the times. Yet here is Prof. Humphries with modern data and coherent explanations as to why it might have been so. Quite refreshing. Thank you Gene and Nick for putting this out there. 👍

[@inqwit1]

What an excellent Forum for all of these conversations to arise, be examined, and discussed.

[@andrewburleson4994]

Nick, this might be too simple but as a petroleum reservoir engineer, can I offer that as slab break and curtain reoriented from slanted to vertical the rate of melt at base of slab would have increased significantly, and as it rotated (since it’s a 3D object) it first focused melt to Atlanta lobe, then further to north later. Of course you would have to add an east-west tear at another slab or plate boundary to create Challis time pattern.

[@Engineer1980]

Great episode! Professor Humphrey’s did an awesome job explaining the slab roll back phenomena on a regional basis. Love the episodes on tomography…seems like such a cutting edge research tool. Thank you Nick and Professor Humphrey!

[@justinrogers5408]

That was excellent. Thanks to both of you. The slides that Gene provided gave a great understanding of this series.

[@richardmourdock2719]

Wonderful as always. Equally fascinating and exciting stuff. I imagine Basil Tikoff's eyes lighting up with much of this. Reminds me I owe Mr. Zentner and email!

[@philsigman9088]

I love when the geology of my home state of Idaho is being discussed. Thanks guys!

[@stephencarter1442]

Light bulb he made a lot of sense and clarified things! What a great interview he is such a stand up individual.. I really appreciate all your guests who Share great People Always.!

[@cordellmathieu7017]

Can I give you more than 1 thumbs up? This is the era I'm most interested in, and you answered a lot of questions for me. Thanks.

[@Vickie-Bligh]

Yay, Nick. Thanks for all of this.

[@redskybeach]

Thank you for producing this interview and discussion for us. The ID Slab Curtain is an important component of the regional story.

[@Shelbyj13]

WOW! The 💡 went off watching this. I was really struggling with this subject, now I have a better understanding of what you have been talking about. The visuals really helped. Thank you.

[@yukigatlin9358]

Woo... Love tomography!!😃💙✨Thank you, Gene!!💞😁I'm excited that I get to enjoy reading your papers...!!

[@ivanoffw]

I really enjoyed this conversation since he explained just how large the Farallon plate was, and an explanation of possible the process of creating the basin and range. Siletzia changed the entire west coast of North America, all of the way to the Rocky Mountains.

[@gerryjamesedwards1227]

I've heard some geologists say that slab-pull is more of a driver of oceanic plate motion than the plate being pushed by the ridge. That being the case, could it be that, as Silezia began to be accreted, it began to act as a brake on the slab-pull (from the part of the Farallon plate that has sunk before the flat-slab episode), slowing subduction of the Northern half of the plate leading to the stresses that would start to tear the plate? The part of the plate South of Silezia would still be feeling the full force of the slab-pull, so I imagine the tear might have started at the junction of Silezia and the N American plate, that being where the rotational force on the slab would be greatest.

[@gerryjamesedwards1227]

Sorry, Gene covers this at around 42:00

[@cyndikarp3368]

I observe Siletzia every where I drive on the Oregon Coast. It is amazing to see all the different geology on west coast USA.

[@MGeofire]

Wow! That was a spicy meatball! Think I'll need to revisit this episode a couple times.

[@greentea7180]

Okay, this was absolutely incredible, I need more mantle tomography in my life. The Pasco underplate was really interesting in particular, I have been wondering over the years what's under the layer cake in the columbia basin, and as it turns out there is even more freakin' basalt lol. This also helped me indirectly understand how the farallon slab sinking facilitated the CRB eruption, as well as the subsequent uplift of the wallowa and bald mountain batholiths, something I hadn't quite been able to wrap my head around.

[@julescaru8591]

Great presentation gentleman, thank you !

[@davec9244]

Thank you ALL

[@anaritamartinho1340]

Awesome, learning more about ocean slabs in the mantle🥳

[@Siletzia]

Thank you for this wonderful presentation. Dr. Humphreys tied together so many complex "big picture" events in a way that makes for a compelling story. The discussion will also help us better understand his excellent work and how it connects to the other papers in our expanding Cascade library.

[@johnplong3644]

I have been watching the 351 . Exotic Terranes and the Baja BC lectures again .

[@charlesflorian1758]

Great show. Was a bit late in watching. Glad it was a not live for this one. Charlie Brown

[@StereoSpace]

An important point that Gene makes is that the flat plate would have carried billions of tons of seawater under the crust. At high temperatures and pressure, adding water melts rocks. As the slab rolled back, the now water saturated lower crust is exposed to heat from the mantle, the lower crusts melts and buoyantly lifts water saturated magma to the surface, which explodes as a series of catastrophic caldera eruptions from the NE to the SW in the Eocene and Oligocene. Flat subduction is the only model that easily and obviously explains that.

[@pmgn8444]

Wow! Thanks Gene and Nick! Lost of interesting info! And a bit of the regional picture. Thanks for the papers, too!

[@tab46]

Greetings from North Central Montana.

[@Poppageno]

That really adds a lot of different dimensions to the big picture! Thanks Gene! Thanks Nick!

[@guiart4728]

Another great presentation!!! I just watched your video with Shawn as well. What fun! More collaboration with Shawn please! I really appreciate that these tech and research oriented guys are willing to stand in there and take cuts at curve balls and other off the beaten track ideas. It’s really great to have their complicated expertise moderated by great communication folks like you and Shawn.

[@BLSKYZ]

Glad to have another stream!

[@erickolb5108]

Very cool discussion! Thanks for taking the time to present this content to some of us fans of geology!

[@fernie5128]

Thank you gentlemen, terrific presentation and thank you Emeritus Prof Humphreys!

[@66kbm]

Sooo, The break in the slab does not provide enough "drag" to pull the slab into subduction immediately. However, extension can do so with enough force to "kick start" it? Lots of interesting stuff going on there. Thanks again Mr Nick..

[@dippyanddakota]

What a great jigsaw puzzle.

[@amyself6678]

Mid Oregon before and after Siletzia was underwater, and thin 8km ocean crust, then underplating added 10, sediment added 2, and CRB added 2. Without all this the Embayment would still be there, no terrane filled it, after Siletzia N to S blocked the coast.. I guess

[@williamw7847]

Excellent chat and presentation. The one question in my mind relates to the dynamics of the broken plate ... As it changes angle in the mantle towards the vertical, it must push (molten) mantle matreial aside. What effects would that give? Interesting, too, to consider buoyancy arguments.

[@cykratzer3463]

Visualize moving slabs on a sphere. I think we tend to think of it like rugs on a floor colliding and riding over each other. Now imagine that floor is not flat or columnar, but spherical. So faults that might be transver around the poles may shift towards thrust as those faults migrate north. And with the earth and mantle spinning, you might need to start considering how inertia may influence what's happening. There's a field of math around the conservation of energy on a sphere. Meteorologists use it to calculate shifting air masses around the globe. I wonder if any of your guests are including that in their data

[@richarddavies7419]

This session is very helpful in understanding the Absaroka volcanics- huge in area, even containing remains of a shield volcano! Thank you.

[@richarddavies7419]

Strato, not shield volcano!

[@swirvinbirds1971]

Well shoot... Now I want to know about that blue spot and the Columbia Basalts.

[@louiscraddock9853]

Greetings from Parkersburg West Virginia

[@jonathanblubaugh5049]

OK, Nick. I've been with you and Larry the Ladder since the beginning of the pandemic. This stands out as the first cogent and effective argument that can be construed as the alternative to westward subduction. However, it seems to be rooted in the need to explain Sevier and Laramide deformation very far east of the continental margin. He invokes the Shatsky Rise conjugate to do so. He has the flat slab going all the way to Mt. Rushmore. I'm all in with eastward subduction of the Sileztia spreading center and its resultant slab window. I'm all in with the hotspot temporarily capping the slab due to accretion of Sileztia followed by slab tear and rollback, widespread extension, and the CBRG and ignimbrite catastrophe. I still don't think I need a Shatsky Rise to explain Sevier or Laramide deformation. Westward subduction of Laurentia under the Ribbon Continent and margin-wide right lateral transpression do that for me.

[@Mistydazzle]

Oh my goodness! Fabulous! Thank you!

[@Josh1888USU]

Gene from Eugene! Hello from Logan Utah.

[@judischarns4509]

Greetings from Boone NC

[@Eric_Hutton.1980]

Greetings from Marion, Virginia

[@12time12]

Definitely want to hear about the Columbia River flood basalts cool spot he was talking about.

[@atomdent]

Excellent discussion!

[@t48wolf]

Love it love it thank you

[@1986BBG]

Ahhh missed this one

[@kban77]

So flat slab flounders, tears. North side of tear leads to various magmatism. Southern side of tear leads to delamination?

[@Eric_Hutton.1980]

Howdy from Marion, Virginia

[@MarkRenn]

I know I'm not a real geologist. All I know is what I've learned from you. So take this with a grain of salt. I'm thinking back to your lecture series and the one you did on how the Rocky Mountains were formed. If we consider westward subduction of the Whale or the Mega-Whale, how would this slab curtain fit in with that? Because it appears that still requires eastward subduction. How do we resolve this? Or am I just thinking about two different time periods? (I think there should have been some eastward subduction starting after the westward subduction ended.)

[@Dragrath1]

I'm still skeptical of an all or nothing flat slab subduction scenario particularly south of the Cascades where is this tomographic imagery since all the papers I have read have failed to find anything west of the Aspen Anomaly. Now I can see this type of low volcanism subduction with thick crust but Hildebrand's slab failure volcanic signatures but the flat slab seems like it could fit post Sierra Nevada and possibly elsewhere as part of the post arc collision with NA following the Baja BC scenarios. As for the comment about Yellowstone the link between Siletzia and Yellowstone seems pretty much confirmed by igneous petrology with some evidence suggesting associated mid ocean ridge melt component which would suggest an Iceland like component. If Siletzia is linked to the EPR then that could provide the mechanism for the rollback/delamination with the hot upwelling mantle structure. The rotation of the crust and presumably the slab should be accounted for too and would love to see his tomographic evidence for extending the slab within this region.

[@amyself6678]

Most LIPs gush mid ocean and then 20-50 my later DO get subducted, Shatsky, Hess, both went under by LA, but Siletzia was only 10my so hot and buoyant so it stalled and led to tears. Fluke of timing,, the HS popped up neared the shoreline, not mid ocean.... I guess

[@Dragrath1]

It is probably not that simple the timing may or may not have been a fluke and even with older oceanic plateaus there are pieces of other old oceanic plateaus which have themselves become accreted for example the Caribbean Large Igneous Province which was at least partly accreted onto South America though that one shares similarities with a relatively young encounter including evidence which suggests the associated Galapagos hot spot passed wits way through the northern part of the Continent. You also have the situation where some LIP's lie along a MOR boundary like Iceland and or may be responsible for fueling/driving MOR activity for example there is evidence that the Icelandic hotspot has a connection to the High Arctic Large Igneous Province and the powerful eruptions centered around the now apparently extinct eruptive center known as Alpha ridge. This LIP appears to have moved south over time notably cleaving its way through Laurasia initiating rifting between North America and Greenland around 118 Ma before it reemerged east of Greenland around 60 Ma and formed a smaller LIP known as the North Atlantic Large Igneous Province and associated rifting of Greenland from Eurasia Granted there wasn't a subduction zone and that hotspot moved away from the Alpha ridge so maybe it isn't as applicable but it at least shows that in scenarios where the plume and the LIP are still associated while hot spots can still move the rate of motion is certainly much less allowing multiple plume pulses. The igneous petrology of Siletzia notably shows that not only is there a clear geochemical link between the Yellowstone hot spot and Siletzia but that volcanic activity with a hot upwelling hot spot Ocean Island Basalt composition continued within Siletzia itself up to around 34 Ma with a time transition towards producing more chemically evolved melts with continental contamination over time within the Upper Grays River Volcanics and a few samples from basalts previously thought to be part of the older Crescent River Formation of Siletzia. The persistence of young hot spot magmatism within the eastern portions of Siletzia even after accretion and the formation of the Cascade arc. Notably Siletzia's older oceanic portions contain some Mid ocean Ridge basalts which when combined with the apparent connections between Yellowstone and the Snake river plane and Aspen anomalies in the mantle it seems likely that Yellowstone also was formed along a or adjacent to a Mid Ocean Ridge and there is a series of associated "slab window" anomalies in the upper mantle which can be linked back to both the Juan De Fuca and Gouda ridge systems and the East Pacific Rise proper suggesting Yellowstone retains a MOR like alignment based on the tomography of the solid upper mantle and this is supported by Yellowstone being the site of the axis of rotation for NA's clockwise rotation based on GPS.

[@amyself6678]

There is small chance there was 2 or 3 hotspots off Oregon, they do NOT give off unique signals. Or maybe plume had multiple chanels. Yes this seems crazy.. So whether initial Siletzia gush is same source as Nevada smaller volcanism is logical assumption but not certainty. . . .. Cratonic land seems thick enough to mask most hotspots, there could be many that passed thru and now under Montana. .. Yellowstone is big and maybe also lucked out finding cracks and thinner crust in NV to burn thru. I'm mostly guessing, but hotspots are random by nature and pop up randomly, so we can't know for sure.. . . . . Bits of LIP got left up top around Caribbean but not sure these jumped subduction to get windows and tears!! ... . I think HS meander occasionally...... I'm totally unsure

[@jmflournoy386]

you show old land static and new land moving E or NE I think new land may be moving or gathering with itself but it's old continent that is moving mostly due to Atlantic ridge spreading I thought that triple junction was near OR CA border additional eyes on preprint can't hurt may provide clarity

[@richards3075]

Hi Nick, Is there current evidence, anywhere in the world, of flat slab subduction? I think Gene needs to explain what conditions are needed for flat slab subduction. It seems there was a unique setup in western North America to justify the result of flat slab subduction.

[@BrianRLange]

Watching this got me thinking, could the clockwise rotation from Northern California northward to Canada be an after effect of the accretion of Siletsia? It would explain the Basin & Range, and other effects it seems. Great video as always.

[@jeffbybee5207]

Just an opinion but it's not so much a result of the collision but the north American plate heading southwest and the exotic trained causing northward drag as the Pacific gets pushed west northbwest

[@Dragrath1]

The clockwise rotation appears to be connected to the relative difference in plate motion of the Pacific Farallon ridge plate system and North America with the ridge slab system starting to move North as a consequence of several subduction zones in the Southern hemisphere ceasing in the Cretaceous and thus leading to a net northward pull drift as the basis for Basil's hit and run Baja BC model transition.

[@robertpahlowjr4257]

Hope I didn't miss something, but does this have anything to do with the emergence of the so-called German chocolate cake?

[@doemijmaarfriet]

Again Yellowstone turns key player in this story. We tried to get away from it, but that cannot be a coincidence, can it?

[@bbcpropaganda514]

Great presentation that requires multiple viewings My understanding is Wrangellia is a part of or consequence of the Shatsky Rise/Shatsky conjugate hitting the NA craton somewhere in California c100MA. This is an old but good presentation on what happened to NA as the main part passed under the craton. Note the hypothesis towards the end that bits of Shatsky broke off under the craton and created density anomalies. Fascinating how what you are discussing seems like a similar process for Siletz. kzbin.info/www/bejne/ZnLCppWnYrmtkLc

[@Dragrath1]

I don't think that works for Wrangelia as the various accreted terrains go back well before the timing of the Shatsky Rise. Subduction has already stayed in an oceanic setting by the early Jurassic if not earlier in what can only be an oceanic arc setting as NA was still part of Pangaea in the early stages of break up. You also have within the Wrangelia terrains what appears to be part of an older Triassic age oceanic plateau dating to the Carnian Pluvial event and also some old continental lithospheric fragments suggesting there was a mature evolved arc system of some form by the time NA becomes involved since the Paleomag excludes the rocks being in their present locations see Nick Zentner's Baja BC A to Z series to get a sense for the terrain complexity which doesn't seem to fit at all with the Shatsky model.

[@bbcpropaganda514]

​@@Dragrath1 I really need to read Nick's links-to-papers more often. This is the heart of Gene's "Reconciling Baja-BC and flat Shatsky conjugate subduction" paper which I've only scanned briefly We're really talking about the Insular super group/micro continent, which they were probably part of but which changed over time. My notes from the last time I dug into this said "Wrangalia/Insular plate" I'm starting to see how Shatsky might have caused Baja BC, though. That's the big hit, not Wrangalia, which it pushed aside before it went under, blocked subduction with a flat slab and caused shearing (which is why Wrangalia moved North on the Pacific side - the flat slab moved the shear zone eastwards)

[@donnaminar4689]

Are the Rockies dead by this time?

[@robtippin9111]

😎