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My Wonder List

There are things that puzzle GG and this is a partial list that will be updated from time to time.

  • When did the High Plains become high?
  • Why did the High Plains become high?
  • What exactly did happen in the Mojave Desert c. 75 Ma?
  • Why did the Sierra Nevada arc shut down?
  • Related, why did it shut down from north to south?
  • If the northern Sierra only went up prior to the Eocene, why are modern rivers cutting well below those deposits?
  • If the northern Sierra have gone up significantly since the early Miocene, why do climate proxies disagree?
  • If the northern Sierra only went up prior to the Eocene, then how did one form of support (crustal root?) get replaced by another (buoyant mantle)?
  • What caused the Ancestral Rockies to rise up where they did?
  • Why did the large thrust on the south side of the Uncompahgre Plateau fail to reactivate in the Laramide?
  • How much of the Laramide foreland (basement-cored) uplifts reflect some different source of stress and how much do they reflect stresses from the Sevier belt driving shortening?
  • It seems likely there was serious slip partitioning in the Sevier; where is it?
  • There are some LANFs that seem to have really formed in the brittle crust at low angles. How?

Depressurizing Geobarometry

Five years ago GG pointed to a paper threatening the cherished assumption in petrology that the pressure recorded by minerals is equal to the overburden pressure. GG has never been comfortable with that assumption, and missed (until now) a paper that is far more comprehensive in its impact. And frankly, it is so blazingly obvious that GG is embarrassed that this has been under the radar for so long. The paper is Yamato, P., and Brun, J.P., 2017, Metamorphic record of catastrophic pressure drops in subduction zones: Nature Geoscience, v. 10, p. 46–50, doi: 10.1038/ngeo2852. The killer money figure is this:

All the dots in the top panel are peak pressures reported in the literature versus the subsequent nearly isothermal pressure drop also reported, where the circled points actually have that second pressure separately measured. The first thing is that this linear array makes no sense: it would almost require that rocks go down on a spring: the farther down they go, the more rapidly they bounce back up. You’d think some rocks would just stay down there and heat up and that the subsequent rise could well be independent of the journey down. The second part is that this linear array makes perfect sense if you are looking at the difference between the pressure when the rocks are on horizontal compression versus horizontal extension, which is what the bottom panel is illustrating. In essence, if the vertical normal stress is constant (σv), then at failure in compression it would be σ3 but in extension σ1. With pressure being an average of the stresses, you then get a massive pressure drop, greater if the rock is in the brittle regime (∆PFRIC) than the ductile regime (∆PDUC). The authors estimate these curves as shown by the solid lines in the top panel and it sure seems like the simplest explanation for these massive decompression events is simply that the stress field changed.

How this changes a tectonic interpretation of the geobarometry is illustrated in their Figure 4:

The black line in the lefthand graph is what has typically been interpreted to date; in the righthand graph they correct for compressional and extensional stresses. Instead of a rock blasting its way to the surface and then stopping, in the right hand panel the rock goes down and then comes back up with the vertical axis now being the lithostatic pressure.

Now this isn’t without a pile of caveats and potential flaws. First, at these depths there is no reason for the principal normal stresses to be aligned with the Earth’s surface, so this is a worst case scenario. Second, it is a bit of a surprise that the points going all the way to over 4 GPa, seem to be in the brittle field. GG suspects that many of these rocks exhibit ductile features that would seem to contradict the inference of being in the brittle field. Third, a change in the stress field of this magnitude is pretty daunting and poses a challenge to the geodynamics community: how can stresses change that much? But if the rocks are sitting at roughly the same depth and temperature for a significant time, this might not be anything like the problem of near isothermal decompression, which does have some severe time constraints. But regardless of the challenges, frankly this makes way more sense than rocks just springing back up to some level and sitting there.

There is a follow-up paper that more fully develops some formalisms for investigating this effect in general: Bauville, A., and Yamato, P., 2021, Pressure-to-Depth Conversion Models for Metamorphic Rocks: Derivation and Applications: Geochemistry, Geophysics, Geosystems, v. 22, article e2020GC009280, doi: 10.1029/2020GC009280.

Now this paper dealt with high pressure-low temperature rocks typically associated with subduction zones, and this strongly suggests that inferences of continental rocks going to 100 km depths are mistaken. But there are a whole bunch of rather similar looking curves that are not quite as dramatic but similarly difficult to understand without this mechanism. GG is referring to the widespread evidence for massive decompression of lower crustal rocks seen the Sevier hinterland of Nevada, Utah and southeastern California. (For instance, can work outward from the overview of Hodges and Walker, GSA Bull., 1992). This has long been a major mystery as shallow level extensional structures are largely missing. Many workers have noted that Miocene and younger Basin and Range extension has led to very deep basins being created, but equivalent Cretaceous and early Tertiary sedimentary piles are rare.

This brings us to a second paper that considers this problem in the metamorphic rocks of eastern Nevada: Zuza, A.V., Thorman, C.H., Henry, C.D., Levy, D.A., Dee, S., Long, S.P., Sandberg, C.A., and Soignard, E., 2020, Pulsed Mesozoic Deformation in the Cordilleran Hinterland and Evolution of the Nevadaplano: Insights from the Pequop Mountains, NE Nevada: Lithosphere, v. 2020, Article ID 8850336, doi: 10.2113/2020/8850336. On the basis of geologic mapping and new geochronological data, these workers conclude that both Cretaceous thickening and decompression are less significant in this area, possibly indicating that the geobarometry in the nearby Ruby and East Humboldt mountains has been affected by overpressure issues like that considered above. And when you toss in structural evidence in other core complexes for changes between shortening and extension (e.g., Wells, M.L., Hoisch, T.D., Cruz-Uribe, A.M., and Vervoort, J.D., 2012, Geodynamics of synconvergent extension and tectonic mode switching: Constraints from the Sevier-Laramide orogen: Tectonics, v. 31, TC1002, doi: 10.1029/2011TC002913) it seems that much of the geobarometry in the western U.S. is due for reexamination.

Overall, this feels like a liberation of sorts. The decompression problems had produced some imaginative solutions that might no longer be necessary (e.g., Wernicke, B.P., and Getty, S.R., 1997, Intracrustal subduction and gravity currents in the deep crust: Sm-Nd, Ar-Ar, and thermobarometric constraints from the Skagit Gneiss Complex, Washington: Geological Society of America Bulletin, v. 109, p. 1149–1166.). The next few years might see wholesale revision of what was going on in the Sevier hinterland.

The Curious History of Counties

Apparently some folks are thinking that Biden lost the election because he won so few counties. Which, of course, is silly because counties don’t elect presidents in any way shape or form. And the Census Bureau pointed out awhile back that most Americans live in just a few counties. But just how is it that the counties look this way? A trip through the Atlas of Historical County Boundaries can provide some insight.

GG has an exercise he’s used in a class to examine how counties have changed in western states over time. It is interesting to compare, say, Kansas and Colorado. In Kansas, the size of counties is unimodal: they are all about the same size. Pretty much as soon as an area was occupied by settlers, they broke a previously large county into smaller ones. The result, early on, was that counties tended to have the same population (with the exception of Kansas City, Kansas). Whether the county size was dictated by how far away a county seat could be considered accessible or by a count of people per county, the effect was pretty obvious: Kansas counties were all about the same by the early twentieth century. As of 1960 (and essentially the same as 1880), Kansas counties had an average area of 784 ± 218 (1 sigma) square miles.

Since then, of course, western Kansas was decimated by the Dust Bowl and has further suffered by the consolidation of farms into megafarms. Aside from communities along the main highway (I-70), most have plenty of empty store fronts. Meanwhile, the cities in Kansas have grown, so the population per county has become bimodal despite the uniform size of the counties.

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Commencement 2020

SO a couple years back, GG made up a fake commencement address for scientists, thinking he was safely insulated from such tasks. Apparently these are desperate times as he was asked to give the address to his department’s virtual commencement. (Why we blew the chance to get some big name to Skype in for cheap remains a mystery).

Anyways, the old draft wasn’t really going to work given our situation, and so GG went a different direction when push came to shove. What do you say when lives have been so disrupted? Here for your amusement is the address as written (note it was prerecorded so campus could put in subtitles)…

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Panoramania 5: Bernese Alps

A panorama from the ridgeline north of Grosse Scheidegg near Grindelwald, Switzerland

Will make a zoomable version elsewhere before too long. The huge pyramid in the center of the view initially is the north face of the Wetterhorn (the visible summit might be the Scheideggwetterhorn). Grosse Scheidegg sits at the base of the little green triangle directly below the Wetterhorn. To the right, the next major ridgeline is the north ridge of the Schreckhorn (one visible summit is Mättenberg, and Kleines Schreckhorn is just visible over a notch on the flank of Wetterhorn), and then the next ridge down is capped by the Eiger, which is partially shrouded by clouds. Grindelwald sits in the valley below the Eiger. The next major summit to the left of the Wetterhorn is the Wellhorn. Far down valley to the left (east) are a pair of prominent summits, Wendenstöke and Titlis, which is above Engelberg. In the opposite direction from the Wetterhorn rises the bare summit of Schwarzhorn, part of the ridgeline that towers over Lake Brienz to the north.

Geologically, this region encompasses the contrast between the high crystalline rocks of the Air massif to the south and the sedimentary rocks of the valley and ridges to the north. This juxtaposition reflects the emplacement of the Helvetic nappes. The front of the ranges seen here are dominantly limestones of those nappes with some of the interior peaks being granites and gneisses of the Aar massif.

Panoramania 3: Pinchot Pass

The view from Pinchot Pass in the Sierra Nevada (experiment with 360 view).

A more directly interactive version (zoomable) is here.

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The Plains Fate

A discussion with colleagues brought up an interesting question: What examples do we have from the geological record of a broad elevated ramp like we have today in the High Plains of the United States? The answer is somewhat unclear as it depends on the cause of the uplift.

Some proposals mean that the lithosphere is permanently changed. In this case, assuming isostasy over the long haul, areas about 1500m above sea level will eventually end up near sea level–but to do that you have to strip off about 7 times that 1500m, meaning that about 10 km of the upper crust erodes off. That would take away pretty much all the sedimentary rock in the region and eat into the crystalline rock underneath.  In the end, the area might resemble the Canadian Shield, a vast expanse of middle crust sitting at the surface. So could places with exposures of such perviously deep rocks be the products of whatever created the High Plains?

Alternatively, the modern topography is ephemeral, perhaps a product of dynamic topography or a thermal rejuvenation of the continental interior. In this case, it is a bit of a race between erosion and subsidence. The faster the area subsides, the greater the record that is preserved for the distant future. Our modern uplift would be an unconformity between the material still remaining and the overriding sediments. One wonders if some of the unconformities out there might reflect a similarly broad and extensive uplift.

Maybe High Plains-type uplifts are somewhat more common in geologic history than we would guess. It could be one of those things that you have to believe before you can see it…

Which Question to Answer?

There is a survey circulating within AGU asking for suggestions for the most important questions or challenges facing geoscience.  This is kind of a regular thing (NSF gathers meetings around similar questions), but GG wonders if this is a productive exercise.

First, most important to whom? If we are talking the public at large, then you are almost certainly talking about geohazards from climate change to hurricanes to tsunamis to earthquakes to landslides.  Better predicting or mitigating these hazards are probably the things that society most wants.  Close behind are some more traditional concerns like locating mineral deposits.

If these are the class of most important problems we should pursue, then it might well make sense to encourage scientists to focus on these. And for what it is worth, there is a lot of effort directed toward these ends.

But are these the most important questions at a more abstract level?  A lot of the work on hazards isn’t addressing more general principles, it is applying specialized knowledge to particular situations. The basic physics of most landslides has been well known for a long time.  The conditions that produce tsunamis are pretty well understood as well. So maybe there are things we really have no solid grasp of that might be worth getting at.

When you shift to this style of questioning, things necessarily break apart by discipline or study topic.  Who is to say that determining the presence or absence of century-scale atmospheric oscillations is more or less important than resolving the physical state and composition of material near the core-mantle boundary? Is learning when the Andes rose up more important than when the Tibetan Plateau went up? Or the Rocky Mountains?  GG is at a loss; he makes his own calls, of course, but of the numerous issues in earth science that remain unclear, how would you choose a subset that really are the  “most important”? And having confronted that ambiguity, what do you gain from answering the question?

Keeping in mind that abstract or non-directed science is funded because it produces unexpected insights that can be of great but unanticipated utility, how do you pick winners? GG is of a mind that trying to get some community to settle on a set of questions is probably not the most effective way of getting really juicy new knowledge.  Having everybody pile on, say, calculating dynamic topography would probably produce far more chaos than insight while starving other experiments that might be just as valuable. And yet Congress might bridle at giving out money without some kind of master goal (perhaps this is why NASA has been rather successful in its probe initiatives: saying we are going to look for life on Mars or on Europa or Titan sounds sexy even if the probes also get to do a lot of other, less sexy, things).

If we sidestep Congress wanting some clear mileposts, what might be the most effective way to get somewhere?  Probably a good way is in fact how many NSF programs work at present: on a case-by-case basis, proposal by proposal. If some proposal comes in that has nothing to do with the community’s wish list of problems but is well thought out and makes a good case that its problem is significant, why should it be rejected in favor of some crank-turning me-too middling thing that is pointed at that wish list? GG would say it shouldn’t.  Committees are notorious for compromised and pasteurized repackaging of some advocates’ favorites (the old saw of a camel being a horse made by a committee comes to mind). So maybe we should bypass the group-think in making target lists and just try to follow the problems that really engage us. Some of us will choose well, which is the best we can hope for.

So, for instance, GG phrases his interests in the western U.S. as stating that this orogen is the largest non-collisional orogen on Earth.  It is arguably the most poorly understood feature of its size. Does this make studies of this more important than, say, untangling the slip history of major faults in Southern California? Not necessarily–but it is better than saying that this research addresses point 1(b) section 4 of some summary document.

Slab Tectonics?

So Howard Lee over at Ars Technica took a swing at how our understanding of global tectonics has been changing over the past 40 or 50 years and wrote a lengthy article on it.   It is full of quotes and assertions that really don’t hang together very well, making a certain geophysicist kind of grumpy.  It doesn’t seem that any of the scientists quoted were really saying anything wrong, but the assembly in the article, which doesn’t seem to recognize the discrepancies nor fully master the techniques being used, can lead to a sense of “WTF?”

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The 85 Ma Trainwreck: Mojave Mojo

One of the strange things about the 85 Ma trainwreck (which we have discussed components of here and here and here and here) is the central place that the Mojave Desert seems to play.  For the most part we might look to the 2003 paper by Jason Saleeby for elevating the Mojave to star status, tapping it as the entry point of an oceanic plateau (specifically the hypothesized conjugate to the Shatsky Rise now in the northwest Pacific) that he proposed as the main actor in driving the Laramide Orogeny.

SaleebyNarrowSlab

Saleeby (2003) vision of the key elements in the Laramide orogeny, with the shallowly subducting part of the slab corresponding to the highly disrupted geology of the Mojave Desert with the slab then proceeding to the north-northeast into Wyoming.

So what is really peculiar about the Mojave c. 75-85 Ma?  After working through parts of the literature and getting some education at GSA this past week, GG would suggest these are possible candidates:

  • Emplacement of subducted sediments against the middle/lower crust after experiencing greenschist grade metamorphism.
  • Extension that permitted this juxtaposition to rise to the surface, either shortly after (as in the Salinian Schist) or much later (as the Orocopia Schist)
  • An extraordinarily unusual pattern of late Cretaceous magmatism (83-72 Ma) including metaluminous plutons that lack the temporal eastward shift seen elsewhere

There are two pertinent questions for these: (1) how localized are these things really, and (2) do these point towards an impacting plateau and a broader Laramide implication?

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