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Geology in “The High Sierra: A Love Story”

OK, no way for a grumpy geophysicist who has also written a book on the Sierra to dodge this volume. But, wow, just exactly what is this? GG sees four main threads: an autobiography, a set of mini-biographies, a trail/travel/gear guide, and musings on geology. A miscellany, as one reviewer suggested. These are scrambled together in relatively short chapters. In a way, this echoes the multiple threads running through Ministry for the Future, so perhaps this reflects the evolution of Robinson’s writing style overall. Most reviewers seem to like this approach; GG is no savant on the subject, so moves on to things he does have some understanding of.

This might be one of the more popular books to consider geology from the viewpoint of a non-geologist since John McPhee’s series that was eventually published as Annals of the Former World. Sadly, while McPhee somehow navigated the nuances of geologic terminology fairly successfully, Robinson struggles when he ventures in that direction. For GG this was kind of off-putting as so much of the remainder of the book was really interesting. (GG was a bit surprised that Robinson loved Jim Moore’s book enough to call it out in the text; while it is a good book, GG’s review was a bit less flattering, and the subsequent quarter century has seen some relevant work done on the range).

Before diving into the geology in the volume, it is worth contemplating Robinson’s fascination with placenames. Several chapters are devoted to names that should be removed, names that are good, etc. At first this puzzled GG, but then the realization hit that place names are a bit of a crutch here. If you don’t know Dusy Basin from a washbasin, a lot of this book will leave you either searching for an atlas or just letting all those names wash over you. But if you know the range, these names will evoke mental pictures and a framework of the travels the author is presenting, and it is hard not to think that this was in fact true for the author as well. Certainly many of the stories would be just as interesting without the place names, but Robinson is pretty consistent in putting those names in. In fact, you could almost wish for an index to be able to find specific descriptions of certain places. Descriptions of a couple of the travels in some areas GG isn’t so familiar with were frustrating in trying to mentally assemble the landscape from a gazetteer’s worth of names, but areas where he was familiar the text seemed to flow smoothly. Does that make it a book really only for Sierra-philes? Whatever the answer, it does make clear why place names are so central to the book.

Anyways, let’s start with the geological low points and move upward from there.

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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.

Is Science Intrinsically Colonial?

GG is not a particularly deep thinker, but some aspects of where science seems to be heading has gotten his attention. Consider for starters this portion of the geologic map of the Fredonia quad, USGS Scientific Investigation Map 3035:

Kind of interesting that there is a big blank area corresponding to the Kaibab Indian Reservation. The explanation in the accompanying pamphlet is a little unusual:

Field work on the Kaibab-Paiute Indian Reservation was conducted from 2002 to 2005 with permission from the Kaibab-Paiute Tribal Government of that administration and permission was granted to publish a geologic map of 4 quadrangles online (Billingsley and others, 2004). The Kaibab-Paiute Tribal government of 2006 to 2008 requested that all geologic information within the Kaibab-Paiute Indian Reservation not be published as part of the Fredonia 30 ́ x 60 ́ quadrangle (this publication).

At first blush, you might think “Hurrah! Native peoples can now control information about their own lands.” But what is this telling us about science in general?

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Gravel Grumpiness

So GG is a co-leader of the 2022 Thompson Field Forum in the Sierra Nevada next June and this has him refocused on evidence related to Eocene(?) gravels and topography. So let’s review how these have been interpreted, some issues we’d like to see solved, and some speculation from a somewhat different angle. We’re going deep into the weeds, so hang on…

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In Need of Hypotheses

GG wrote something about this awhile back, but it feels worthy of a revisit. Just why is it that geologists still like this “multiple working hypotheses” ideas?

What reminded GG of this was reading Naomi Oreskes’s book on the rejection of continental drift (or Amazon link). In there, it sort of seems as though multiple working hypotheses comes across as something of an excuse used by twentieth century American geoscientists to dance past the evidence for continental drift. It kind of comes across as a dated approach for pre-quantitative science. GG would argue that in studying complex phenomena that it is an important tool–one perhaps worthy of keeping in mind in dealing with the current pandemic.

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Oops, we did it again…

A story in the LA Times cuts close to home for the grumpy geophysicist. It describes scientists as vandals. Not a good look. And it is kind of hard to argue with that characterization. It cuts close to home because GG has done similar work and has worked in (or had students work in) the labs that have stumbled.

No doubt you are expecting a story of bad science, but it is really what happens when an old “anything goes” ethos hits a more nuanced reality. In this case, two prominent paleomagnetists were caught drilling holes in rocks right near petroglyphs in separate incidents. (“Prominent” you say? Well, one got an award from each of the American Geophysical Union and the Geological Society of America; the other was a president of GSA, among other laurels). The main case discussed came from Caltech some four years ago, where a class was taken to an outcrop near Bishop California to drill out samples. Without having obtained permission. Off a road that is mainly used to visit petroglyphs. A few feet from a petroglyph. Frankly, it is almost impossible to imagine how this came to pass (did nobody in the class notice where they were?), but a person who passes through to watch for vandalism spotted them and reported them. The other was a year later in a less prominent locale in Nevada with a UT Dallas investigator. Both resulted in five figure fines that their universities covered, not to mention this bad press.

Now GG may one day hear from the individuals involved their side of the story (and indeed one is to write a journal article as part of his reparation), but GG is willing to speculate on what went wrong so, you know, he and others might not screw up the same way. “How could you possibly think it a good idea to drill holes next to sacred petroglyphs?” Well, that was not the question at hand, not that this excuses what happened.

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