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Finding Importance

In the memorial symposium for Peter Molnar, Phillip England suggested that Peters brilliance in many ways was recognizing the next important problem to address. And Phillip asked, how do you know? Is it internal, that you just know what is important? Do other people tell you? What criteria might you use? One possibility is that in opening up a line of research, many other follow you in and the research that follows is long and fruitful. In Peter’s case, it was (initially) continental tectonics. But just how you identify important problems is, though, itself a knotty problem.

Consider two problems: earthquake prediction and paleoelevation. Earthquake prediction has been the subject of decades of research, some of it very productive (being able to recover the history of earthquakes on many faults) and much of it not (as in, virtually every prediction actually put forward). Almost nobody would say that prediction is unimportant–provided it is successful. But how about if it isn’t possible? Is an insoluble problem an important one?

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Some Au Gravel Thoughts

So having survived a few too many vehicle adventures on the recent field forum, GG is trying to consolidate some thoughts. Many of which are still in flux, so this is a snapshot of thinking and not necessarily a final product…. Some of this is kind of trivial but some has some major implications. Others are rehashes of older thoughts that haven’t yet been banished.

First up, the age of the gravels and the meaning of Eocene zircon grains. Reexamining the detrital zircons from the Malakoff-Alpha system, it seems this was entirely an intra-Sierra drainage and the Eocene zircons are airfall. The still-unpublished bigger samples of Tye and Niemi at Malakoff seem to make this pretty clear as we are now seeing 8 and 10 Eocene zircons in two samples instead of the 1 or 2 from older work, and those two samples seem to have very distinct and separate peaks, which is probably hard to do with fluvial grains but perfectly understandable from airfall. Toss in the very robust results from Haskell Peak of a lot of Cretaceous grains no matter where you sample in the section and the previously drawn connection of Haskell Peak and its eastern tributaries to the Alpha-Malakoff drainage is clearly wrong.

So then we face the issue with the K-absent samples from Malakoff and the K-present samples from adjacent North Columbia. There are three options GG can see: they are different ages (North Columbia being older), Malakoff’s river did not flow into North Columbia’s, and Malakoff’s zircons were overwhelmed by whatever else was flowing into North Columbia. It seems hard to make these significantly different in age, in part because there is some overlap in elevation and in part because of one miserable zircon in the Cecil et al. measurement at North Columbia. While it is quite plausible that the lower parts of North Columbia are older than the accessible gravels at Malakoff, it does seem that the upper North Columbia and lower Malakoff gravels are nearly coeval.

Because there are so many gravels hanging out in this area, you can steer Malakoff’s channel around some, probably most plausible to the southwest towards Nevada City. But you have to cross the Blue Tent-Enterprise-Spring Creek exposures, and even if you get to the Manzanita channel, you still see that large change in zircons. So GG’s guess is that the most likely scenario is that Malakoff was a relatively minor stream while the main river that fed into North Columbia dominated the sediment volume. The absence of early Paleozoic Bowman Lake batholith zircons at North Columbia from existing samples would speak to the kind of dilution of the Malakoff material; presumably larger samples of North Columbia would yield a few of these zircons. Such samples might also yield better age control. Finding a downstream deposit with the early Pz grains could be quite helpful.

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Why so emotional?

GG’s spouse made an interesting observation the other day. She noted that a lot of problems in tectonics have a couple of features: they don’t seem to be getting solved, and they produce a lot of raised voices. In other words, lots of heat, little light. In contrast, she finds that the development of new techniques does not seem to produce the emotional outpourings seen in tectonics, so she has been more focused in that area. In pondering this, GG thinks it is in general about right. Why might this be?

Let’s start with the easier end, the development of new techniques. In seismology, for instance, we have seen the creation of ambient noise tomography. GG can still recall seeing one of the early posters at an AGU meeting and thinking, wow, pretty cool. In subsequent years different groups worked on improving and extending the technique. While they differed in some respects on the details of processing, these were never make-or-break disagreements. The technique has continued to be refined and applied widely.

Now some other fields might see a bit more controversy. The origination of U-Th/He dating of apatites had a lot of friction as there were disagreements about the physics of helium loss. And the use of clumped isotopes as a means of getting paleotemperatures and the oxygen isotope ratio of ancient waters has had a bumpy ride as the origin of the carbonates that are the source of measurements has proven to be a challenge. But in these cases too, while different groups emphasize different problems, they are seeking to overcome those problems and so these techniques are very much in the mainstream. No doubt somebody got hot under the collar once or twice about whether their carbonate was pedogenic or lacustrine, but that had a lot more to do with interpreting the measurement than making it.

Which brings us to tectonics. At times it just seems like tong wars erupt with regularity. In the 1980s there was the conflict between “pure shear” and “simple shear” interpretations of metamorphic core complexes that resulted in some fairly heated exchanges both in person and in print. The Baja-BC hypothesis has been around the block so many times it has worn a rut down so deep it isn’t clear anybody can escape it. It isn’t hard to find others (when did plate tectonics start? How high was the Sevier hinterland? Age of the uplift of the Rockies?). Why is this field so stalled out while producing so much controversy?

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The 85 Ma Trainwreck: The Sevier Waistland

No, that isn’t a typo, as we’ll see. In essence, the question to be considered here is, what might the crustal structure of the backarc/foreland looked like c. 85 Ma?

A trick that igneous petrologists sometimes like to use is to look at rare earth element distributions and see if elements that like to go into garnet are relatively rare in a granitic rock. Such an absence is then ascribed to the melt having been generated at a sufficiently high pressure that garnet was stable. Thus the absence of such elements would require a crust thick enough to have the high pressures necessary for the garnet. Related tricks look for plagioclase feldspar crystalizing; this phase is only present at shallower levels.

One such analysis was published by Economos et al. (2021) for some of the younger plutonic rocks in the Mojave. In general, they found that there was not a signal of plagioclase but there was a signal from the rare earths that garnet was stable where melts were originated. For the most part, that puts the origin of the melts below 35 km depth. An empirical calibration suggested by Profeta et al. (2015) was applied by Economos et al to their measurements, yielding Moho depths of ~50-80 km. While the most consistent results were from the younger Cadiz Valley batholith, a consistent range of La/Yb ratios suggested that there was no large change in crustal thickness back to 83 Ma. It seems likely then that the crust at 85 Ma was already very thick.

For the most part, this seems in reasonable agreement with the deformational story we’ve already looked at.

This might seem unexceptional. The Coney and Harms (1984) reconstruction of crustal thicknesses, while deeply flawed (it is a rather circular argument, which we could discuss some other time), does provide some context regionally and crustal thicknesses approaching 60 km were inferred from restoring Cenozoic deformation. More recent work by Long (2019) and Bahadori et al. (2018) tends to confirm pre-extensional crustal thicknesses in the Great Basin to be ~55 km, though these are mainly along the Sevier belt, much as Coney and Harms envisioned. These all tend to have some issues with magmatic additions and flow in the lower crust, so some caution is warranted.

Consider this cartoon of the situation c. 80 Ma:

Elements of the Cordillera c. 80 Ma, with restoration of post 36 Ma deformation (so 80-36 Ma deformation not accounted for)

Here is where the “waistland” business comes in. Something like 200 km of shortening was seen across the Sevier belt in northern Utah prior to 80 Ma (e.g., DeCelles, 2004). With the ending width of the fold and thrust belt being about 400 km (from the map above), that would be a thickening of about 50%, so if the average thickness was 55 km afterwards, it was about 37 km before.

Now there is not much reason to suspect a different shortening in the Mojave (DeCelles and Coogan, 2006, had over 300 km of total shortening in central Utah, and Levy and Christie-Blick, 1989, while having smaller shortening numbers, saw little variation from SE Idaho to southern Nevada). The hinterland in the Mojave can’t get too wide as the Colorado Plateau restores pretty closely to the arc; this is the “waist” of the Sevier belt alluded to before. If this hinterland was maybe 150 km after shortening (and quite plausibly after considerable early Cenozoic extension), then if it started with a 37 km thick crust, it could have reached thicknesses over 80 km with 200 km of shortening, in the ballpark from the Economos et al. analysis.

Farther to the southeast, we encounter an area that was thinned significantly in the Jurassic, with the effects of that still evident into the Cretaceous, when strata broadly correlative with the Mesa Verde group capped Paleozoic rocks along the Mogollon Rim. So while that hinterland could have been very thin, too, the crust was pretty thin to start with.

What this suggests is that the Mojave was a special place before the POR schists were emplaced, and before a putative oceanic plateau was thrust under the area. It was unusual because of the preexisting structure of the North American plate. What is more, this unusually thick crust is a potent driver for a lot of the peculiarities we’ve already been examining.

Is there other evidence that might support this inference of a peculiarly thick crust? Well, maybe; for that, we’ll want to look at the sedimentary history of the region…which hopefully GG will finally get around to finishing up…

85 Ma Trainwreck pages

The 85 Ma Trainwreck: Crustal Architecture

At some point GG is going to actually have to commit to assembling all these thoughts into a real paper (or admit defeat in trying to understand this area). But for now, time to resume exploring some of the issues in the Mojave.

We’ll look over the details of sedimentation later on. Here the focus is briefly on just what sits under the region where schists are seen in fensters. Consider this map that we’ve used before:

Palinspastic location of POR-type schist exposures using McQuarrie and Wernicke (2005) reconstruction. Dates are range between maximum depositional age and oldest metamorphic age (usually hornblende Ar-Ar) from A. Chapman (2017), except for Salinian Schist (Ducea et al.), Plomosa Mountains (Seymour et al., 2018) and Cemetery Ridge (Jacobson et al., 2017). Red dots are intrusive rocks dated with U-Pb with ~2 m.y. of 75 Ma. Heavy line is trench position c. 36 Ma. Queried numbers reflect overlap of maximum depositional age and some metamorphic ages.

There are two main areas where these schists are exposed: the Rand Schist in the northern swath somewhere in the 77-85 Ma range, and the Pelona/Orocopia swath to the south that is younger. As the boundaries between these schists and overlying rocks are taken to be old megathrust zones (usually reactivated), the common interpretation is that the area under these belts is made up entirely of these POR schists. If you dig around a bit, you will find cross sections drawn to show that nearly the whole crust is made up of POR schists. So let’s ask a few questions: was there enough of this stuff to fill out these regions? And what can we say about what is there now?

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Is parsimony really a guiding principle of science?

Make things simple, but not simpler. Occam’s razor. Reductionist science lives on finding an underlying structure that accounts for the important differences in observations. If you can explain a bunch of observations with one rule, that beats having a special rule for each observation. But is this really a (or the) guiding principle of science?

Well, arguably the most parsimonious explanation for stuff is “God made it that way.” Why did we abandon such a universal explanation for everything? While today we look to science for explanations about why something happens (auroras, shooting stars, earthquakes, tsunamis), it feels like the origin of science was the more prosaic “what will happen if I do this?” Flinging things at enemies was a popular option in warfare for a long time, but the trial-and-error approach isn’t so wonderful if your enemy, seeing where you are firing from, is quicker to lob a shell at you more precisely. Recognizing that there are rules that are quite predictable gives you an edge–you can get things done more efficiently or even do things you previously couldn’t do at all. You don’t need to answer “why is there gravity” to be able to use a theory for it to do things like go to the moon.

So maybe science is being parsimonious while being able to predict things. Yet some theories look less than optimally parsimonious. The Standard Model for physics looks like something Rube Goldberg might have come up with. Is string theory really parsimonious? You get the feeling Occam’s Razor will draw blood on some pretty well established theories.

Earth science really slams into these problems. Say, you want a theory in how mountain ranges are created. You look today and see the Himalaya rising as India hits Asia. OK, maybe mountain ranges are made as two continents collide. Oh, but we have the Andes, too, and mountains in Alaska. Um, OK, well, mountains are made where two plates collide. OK, great. A fairly simple explanation that allows us to look for mountains. (We’ll put aside where plates collide and all we get are a few volcanoes).

That explain all mountains? It does seem helpful for the Appalachians and Urals and Alps. How about the Sierra Nevada? Assuming the young Sierra story holds water (it is argued), the range has largely risen up with plates not colliding. Seems trouble for our universal mountain-building theory. Or the ranges of the Basin and Range; why is all that going on? Sure seems distant from the plate boundary.

But then we have the Rockies about 1000km from the edge of a plate. Why are the Rockies there instead of where the plates were apparently colliding? Maybe a plate was scraping the bottom of North America. Maybe the Colorado Plateau was really strong. Maybe there was dynamic flow in the mantle. Maybe the Ancestral Rockies had set things up. How universal and parsimonious is our plates-colliding theory if we keep finding troublesome mountains?

In a weird way, earth science almost moves in the opposite direction of, say, particle physics. The physicists are looking for the one equation to rule them all; earth scientists are teasing out all the different ways Earth can do something. Parsimony in earth science is almost backwards from the way a lot of folks regard Occam’s Razor. We will hone an explanation to its bare essentials and then compare with all the examples we have. The ones it explains we can set aside. The ones it cannot we go on to investigate. There are two possibilities: our original explanation was wrong and focused on immaterial aspects, or there is more than one way to achieve some outcome. The great challenge in all this is to somehow sidestep the features that are not important while really nailing the ones that matter.

Consider the Rockies again. A fairly likely candidate for the same process is in South America, the Sierras Pampeanas. A paper some time ago pointed out that the geometry of these ranges (length and width) looked to be about the same as in the Rockies, and the bounding faults are reverse or thrust faults in both places. Is this then the key element that provides the insight into the origin of the Rockies? Some think so, but GG (and some others) have argued this is simply what happens when you squish an area in a continental interior with a thin cover of sedimentary rocks. Kind of like that you can’t really tell if a nail was driven by a hammer or a nailgun; the different tools can make the same outcome. GG argues that it is the source of the compressional stress that we care about and that important differences between the Sierras Pampeanas and the Rockies cannot be dismissed. Which is really right? With so few possible candidates, it is hard to tell. Occam’s Razor has little effect when your choices are so few and potential confounding features are so widespread.

Parsimony is an important tool, but not really the be-all and end-all some make it out to be. There is a temptation to force discrepant cases into a theory’s box when you value parsimony over all. Sometimes it is the right call, sometimes not. Relying on Occam to answer the question can be a big mistake.

My Wonder List

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

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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 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…

Continental Push-ups

“What goes up, must come down”

Does this apply to the rocky world beneath our feet? For most of the time we’ve had geology as a field of inquiry, the main route envisioned for elevated areas to descend back to near sea level was erosion. So something would create mountains and then erosion would level them down. What went up would, eventually, come back down.

But that simple conceptualization is coming up against observations suggesting that this can’t quite be the whole story. Much work (mostly from low-temperature thermochronology) has shown that, for instance, the Canadian Shield (which is most of eastern Canada) has been covered by sediments and then stripped of sediments, probably a few times.  All without making mountains in the region. Now some of this could reflect oceans rising and falling, but it seems like you do need more.

The current favorite in the community is dynamic topography, which in this case is mainly subsidence as a dense oceanic slab is somewhere under the continent followed by recovery when it is gone. It has the advantage that over long times (say, a few hundred million years) it should average out to zero, which is about what the shield rocks are telling us. And it should have the really long wavelengths appropriate for wide continental interiors.

Are there alternatives? Well, possibly, though they aren’t in the literature to any degree and so are untested.  One possibility is that there is a continental equivalent to the flattening of the age-depth curve seen in the oceans.  This flattening (meaning that very old ocean floor isn’t as deep as simple cooling models would predict) has been attributed to the lithosphere thickening enough to become unstable, so the bottom part drips off, raising the sea floor and starting a new round of subsidence. While the bulk of continental lithosphere appears to be stabilized by being poor in iron, the bottom might well be just as unstable as in the oceans.  So maybe the continents rise and fall as the bottom falls off from time to time. But would this be in sync across a continent? Which brings up the question of just how synchronous are these subsidence and emergence episodes?  If they are highly diachronous, that could be a problem for a dynamic origin while consistent with a deblobbing one.

An even more remote possibility is change in the crust itself over time.  GG and colleagues have speculated that the High Plains have risen up as water interacted with the lower crust and removed dense garnet.  Pair this with Karen Fischer’s proposal that garnet slowly grows into the lower crust with time and you might have a see-saw of garnet in, then garnet out. This might be temporally asymmetric, with fairly rapid uplifts compared with the far slower pace of the reestablishment of garnet. Do we have the resolution to test for that?

This will be interesting to watch simply because we’ve ignored this part of the continents for a long time. At the moment, the community focus solely on dynamic topography is kind of making for self-fulfilling prophecies (can you tune your convection model to reproduce the uplift/erosion/subsidence/deposition signal which is, itself, usually pretty vague). So bringing some of the alternatives in out of the cold to compete with dynamic topography would seem to be a good way of focusing effort on the observations that will most distinguish between mechanisms.