Jeff Tepper: Slab Window Thinking (excerpt...with cliffhanger ending)

[@grandparocky]

Looking forward to many more hours of conversations about this subject and being a Witness to this science work as it happens. Thanks Nick and Jeff for sharing!

[@alanrobbo6980]

Thank you Nick and all the Guest speakers, for bringing geology alive, and I’d like to wish everyone all over the world a very prosperous and happy New Year.

[@montananative2414]

A great way to end the discussion and I can hardly wait for the conclusion. Bravo to you Nick and Jeff....Happy New Years!!!

[@cdineaglecollapsecenter4672]

Really appreciate you guys continuing to wrestle with this on screen! I'm not sure I understand why a rollback of a plate wouldn't create arc magmas as it rolled back, unless it's because it has been depleted of fluids by the time it rolls back?

[@teddwayne]

thank you both !

[@gregmoore2386]

Happy New Year from New Zealand. Started down this rabbit hole inhaling as much as I could about my own region whereupon the algorithm lead me to your fabulous videos. Really appreciate your time and effort.

[@tastiger91]

Happy New Year people.

[@xwiick]

Thanks for all the hard work on these videos!

[@pmgn8444]

Happy New Year Nick and Jeff!

[@wtpauley]

Nick, we need maps with clockwise rotation and Frasier / straight creek fault (and any other well known faults) restored... then add dates and locations. This would paint a better picture, I think.

[@ArthurDearinger]

I ENJOY YOUR TEACHING!

[@marieagun3520]

So, is it possible for the break to be incomplete? Could the southern end of the curtain still be attached to the Farrallon plate?

[@sharonseal9150]

Well that was a tantalizing ending! Thanks for sharing this discussion Nick! So excited to hear that our Wenatchee volcanics will be part of an episode, and now I need to locate Golden Horn on a map again. I hope we end up with some color coded technicolor mountain maps at some point in the series - that would be so cool.

[@charlesflorian1758]

Another Nw Years Surprise! Thanks -You and family have a Good New Year. 😅

[@scottowens1535]

Hey your on it!! We get it !! Go there men and ladies!!! Do it, I'm hearing you stuck Nick! Foreward Man Foreward you've done well and there's some smart students listening!!🎉 Hey happy new year everyone and peace too you

[@Guytron95]

Well, that answers my question from episode G. The underlying dynamics and timing windows were such that many of these magmas may have mixed sources along the way, it seems to me. But mostly I'm just glad I can remember "mafic enclaves" now. Happy New Year, everybody!

[@gabsy6443]

I am surprised that I know what mafic means now 🤔

[@Dragrath1]

Regarding the Gray's river from the Petrogenisis of Siletzia paper by Ciborowski et al 2020 in Geochemistry provides a pretty convincing case against either a slab window or subduction related processes for the melting here because these units geochemically have an Ocean Island Basalt signature meaning it is enriched in elements depleted in the upper mantle and crust. So either it is pure melting of parts of Siletzia via some process or it is melting caused by the Yellowstone mantle plume. The formation is composed of pillow basalts indicating a submarine deposition and eventual transition to tholeiitic subaerial basalts andesites which largely rules out crustal contamination or subduction related melting proceses. That is about as definite mantle plume derived melt as you can get so close to the subduction zone. Also if the paper in questions ages are right the transition from the Grays River volcanics to Camps Adakite track seems to work out both in time and space assuming Northward translation is still occurring at least it is within the bunds of jumping seen in other mantle plume tracks in geologically complex environments such as Iceland the Kerguelen or to a lesser extent the Galapagos. The authors also conclude that the primitive basalts they focused on in their paper show a gradual evolution of sampled upper mantle in terms melt composition from the Crescent formation to the Siletz river formation and Grays River form a purely oceanic to increasing continental sediment influence which can be followed through into the Yellowstone track and snake river plain so yu can't forget that the plume which formed Siletzia is still present and active. It isn't just slab window or subduction there is a 3rd option the Yellowstone hotspot. Seems Teper and his students are as far as I can tell alone in their slab window interpretation from the papers I've looked up via google not sure how they explain the OIB signatures

[@reginebellefontaine4936]

Thank you Jeff Tepper and Nick Zentner ! You got me hooked ! I want to know more, and understand how you can have the Farallon moving north and have a ridge in southern Oregon... A ridge with which other plate ? Pacific ? Is this the end of the subduction underneath California ? Is this the beginning of the Cascadia subduction zone ? Did the unbroken southern part of the Farallon slab sink underneath Washington and Oregon as it moved north and is it now the Cascadia slab ? So many questions... And more to come, because with all these magmas everywhere coming from different sources even after the trench is reestablished, the beginning of the Cascades does not look boring to me !

[@cyndikarp3368]

Clarifying interview helped.

[@jacotacomorocco]

Awesome I Love a Cliffhanger!

[@wildwolfwind6557]

Would what Jeff said about Klondike (no arc signature /with basalt) be a negative Nd / Sr (or negative Hf) and definitive enough for a gap of sorts allowing mantle to reach the surface and exciting & important enough to remember by thinking 'what would you do-oo-oo for a Klondike bar'?

[@timbyrne914]

Regarding the MOR offset ideas toward the end: couldn't you also have a transtentional basin along the ridge with more diffuse weaknesses and MORB infiltration? Then the slab window might have something like a N focus along one ridge offset, a S focus along the another, and an area between them with some MOR like behavior in a pull-apart basin.

[@fez3606]

I'm still missing the puzzle piece that tells me why younger lavas are not seen among the older ones in the eastern area of the rollback window. Is it due to 'underplating' from the older (53ma) lavas erupted at the beginning of the rollback?

[@BudKnocka]

The YHS would make spreading ridges as it made its way east and the transform fault would carry the speeding ridge north while being subducted

[@Dragrath1]

Yeah you can't forget the Yellowstone hot spot is still in the region though its hard to say exactly where given that things have moved around and the geology is quite complex. In fact based on the Petrogenesis of Siletzia article Ciborowski et al 2020 The Grays River formation mentioned near the end as the enigmatic "near trench magmas" they were talking about near the end apparently have a tholeiitic Ocean Island Basalt(OIB) chemical signature meaning the melt source is enriched in elements depleted in the crust and upper mantle thus the magmas that fed these volcanics must have come from either a relatively pure remelting of upper mantle accreted from Siletzia or from the Yellowstone mantle plume itself. My understanding from the geochem papers I've looked at is that slab window melts like Jeff Teper suggests for these near trench/forearc magmas would have a Mid Ocean Ridge Basalt(MORB) Signature the only thing with an OIB in the Pacific Northwest is Siletzia and the Yellowstone Hot Spot Track. The next closest OIB sources are probably Galapagos, Hawaii or Iceland depending on where the plates were at the time. I suspect Iceland as NA is moving SW and the Farallon/Kula/Resurrection/Pacific plate system is moving North but I digress point is mantle plumes are the only known primary source for OIB melts and the YHS is the only candidate around.

[@BudKnocka]

@@Dragrath1 what about pre Siletzia accretion? The sediments wouldve been NA Craton that would be subducting…as Siletzia accretes the rivers reversed course to flow east so those sediments exhumed filled the trench in front of Siletzia prior to being subducted…Siletzia having that 20-25mile thickness on top of oceanic plate thickness of 12 miles so a total thickness of almost 40 miles coming into contact with Craton crust of 120 miles…Siletzia at 30% of that total makes sense for it to break off the slab making a anticline and a syncline on the mantle pre accretion…forcing the plate material up then down into the trench shifting a shallow subduction into a steeper angle as the craton and Siletzia got closer forcing the break on the ocean in between them…appears to me that situation would be expressed with the explosion of the challis magmas where the wave sought to return up higher in the crust…then again in Montana…the tillamook grays river basalts are waves of magmas that got reflected back from the accretion…which probably started the plate subduction over again kicking up the start of the cascades…

[@williewilson8244]

Could these Magmas be apart of the Original Rocky Mountains (Proper) 150Ma the very basement of that Western Range made its way into the Idaho Baffelft and into these Challest Magmas? Could these Challest Magmas be the semi vertical dikes fissers of a Plug in the Yellowstone Hotspot?

[@susanwymer6912]

I so enjoyed this intellectual challenge for me and discussion. Are slab breaks/rollbacks more common than we know? Perhaps it is a necessary process to make room for more subducting plates or the LIPS????

[@hjumper8238]

How to parse the principle of "It's Complicated!" EXCITING TIMES! Thank you, Nick, Jeff, et al. Hmm, I wonder if Karin has what the slabs looked like during this era that helps to contribute to the efforts to put viable definitions to help in this? At the least, is there any evidence of present plates that may support this endeavor?

[@2whostruckjohn]

So that southern BC spreading ridge around ~50 mya is also the split between Siletzia and Yakutat? Yakutat takes an express ride north on the Queen Charlotte fault to Alaska while Resurrection plate disappears under BC, leaving Kula plate off BC's shore in the 40s mya?

[@2whostruckjohn]

Also, I thought Camp and Wells (2021) argue for the Tillamook magmas to be part of the Yellowstone hot spot track rather than a slab window, at least at the southern edge of Siletzia.

[@johnnash5118]

@6:28 The Farallon plate is on the wrong side (@South) of its derived spreading ridge for your diagram to work; no matter how you angle it, the Farallon can't move North unless you ignore the physics behind plate formation, conveyance and termination. Plates cannot convey contrary to their formation perpendicular to their spreading ridge. Your arrows pointing in oblique angles from and even toward their spreading ridge is impossible. I think what you're confusing is vector angle (of NA conveyance/Farallon conveyance relationship) -v- plate conveyance direction. Vector arrows should be associated at the trench, not the spreading ridge. Imho, a workable diagram would include red arrows correctly pointed perpendicular away from the yellow spreading ridge, with a blue R vector arrow pointing NE @the trench and a blue F vector arrow pointing no more than East @the trench, which depends on the two rates of convergence and NA counter-clockwise rotation rate.

[@skyecooleyartwork]

Is there a rule of thumb for making the call on mantle-melt fingerprint - a minimum proportion of samples with Ta, Nb troughs necessary? Also, Klondike Mtn volcanics are discussed in USGS Bulletin 1216 (Muessig, 1967).

[@justmenotyou3151]

That was kinda an abrupt end.

[@lucquenneville875]

Sorry Nick, I don't understand why the dates challis Magmas sites are from 46 (west) to 52 (east)... If the phenomena down under is stationnary and the american plate is moving east to west, should the date be reversed? 52 (west) to 46 (east)?

[@donnparis137]

To be continued!

[@skyecooleyartwork]

What happens in Montana may not be related to the slab window, 'cause all we know is ROCK and ROLL, baby!

[@Kyle-b5x2k]

The Tillamook volcanics was related to the Yellowstone hot spot in one paper (Ray Wells?). Jeff is relating it to rollback? Given the clockwise rotation it sure 'fits' with the YHS. Could be two (or more ) things happening at once but if it follows Jeff's idea - where's the hot spot and what is it doing at this time?

[@ArthurDearinger]

REMEMBER! I WILL SPELL 'REMECER' TO HELP YOU TO REMEMBER. THE SQUARE ROOT OF 45, IS 2025!