Weighing Gravity
OK, GG has found something scientific to grump about instead of the increasing “Perils of Pauline” stuff going on politically. And it is, just how do you use gravity to understand the support of mountains? (This is going to get long…you’ve been warned). So look for the terms used, the history of gravity in the Sierra, what isostatic gravity is trying to tell you (and what it isn’t) and what a purely gravity-focused study might look like.
Terms (quickly). There are three main gravity anomalies out there. Free air is great in some places, but not mountains, so let’s not worry about it. Bouguer anomaly (and we’ll use that as meaning the complete Bouguer anomaly) removes effects like distance from the center of the Earth and the attraction of rock above sea level. The isostatic anomaly is a less standardized but often assumes a reference crust 30 km thick with a density of 2.67 g/cc above sea level that is in turn above a Moho with a 0.35 g/cc density contrast. This means the crust should be 30 km + (2.67/0.35)*elevation. The gravity from that root is then subtracted from the Bouguer anomaly.
History. The case under study is, to nobody’s surprise, the Sierra Nevada. For many years, the story there was that the Sierra were supported by an Airy crustal root and this was supported by gravity measurements. To be most charitable, sort of true. When Andrew Lawson wrote his 1936 paper on the “Sierra Nevada in the Light of Isostasy“, there was precisely one measurement of gravity in the range. So his hypothesis was lacking any real geophysical support. Which led Lawson to get his colleague Perry Byerly involved. Byerly was a seismologist and so he sought seismological evidence…which he found in late P-wave arrivals at stations on the east side of the Sierra. This, he inferred, meant there was a very thick crust–an Airy-type root–under the high part of the Sierra. By 1961, some seven seismological studies had confirmed that there should be a thick crust under the Sierra. This was the situation when Howard Oliver and colleagues tried to interpret a new dataset of >1000 gravity measurements, far above the 20 stations available prior to this work.
Read More…The Broken Plate fallacy
OK, this is kind of classic grumpy-ism. When people go to model deformation at scales of 10s to 100-200 km, they will often use elastic plate theory. This gives us things like foredeeps and forebulges and the topography of oceanic trenches. It has its issues (as a purely elastic solution, it should recover when the loading is removed…which isn’t what we see a lot of the time), but it is pretty helpful.
Well, until somebody says “oh, there is a big fault zone, so that must be breaking the elastic plate, so let’s use a broken plate model!” And, you know, many times this is utter silliness and reflects ignorance of just what the assumptions of a “broken plate” really amount to.
What is the assumption? It is that there is no moment transmitted at the broken end of the plate. But when you dig in, you find this is actually saying that no in-plate normal stresses beyond ambient pressure can be transmitted, which is quite a different issue.
Read More…Outsider/Insider
GG is polishing off a paper on a place where he has gathered precisely zero new observations. Does that make him an evil parachuting scientist, leaping in to interpret others’ data before zooming off to some other locale?
Um, er, maybe?
Read More…Reading A Section
or, “you point to this section a lot, but I don’t think it means what you say it means.”
The section in question today is a north-south profile drawn by Jason Saleeby in his paper arguing that the Shatsky conjugate came through the Mojave Desert…
What does this show? Saleeby wrote that “geophysical data indicate that the base of the batholithic crust and its underlying mantle lithosphere have been tectonically removed and replaced by the Rand schist.” Many people have given GG the impression that this diagram shows that the lower crust has been removed tectonically and therefore requires the collision of an oceanic plateau.
It shows no such thing.
Read More…The 85 Ma Trainwreck: Sedimentation
It’s been a long time, but GG needs to try to wrap all this up and do…something with it. So let’s plow a bit farther and look at what sedimentary rocks might tell us.
In previous entries, we’ve looked at the emplacement of oceanic-affinity schists, the evolution of magmatism and the changes in deformation in the broader Mojave region as well as considered which peculiarities of the Mojave are truly special. Here we will consider what significant patterns exist in the sediments that accumulated in and around this time.
Overall there are two foci of astonishment present. One is that there is very little in the way of sediment preserved in the greater Mojave area; this is surprising because of widespread evidence of major extension. Where did those extensional basins go? The other is far to the northeast, in the foreland, where sedimentation rapidly increased well away from the Sevier thrust front but died off farther to the south. These changes cannot be from loading effects at the surface but must reflect some degree of subsurface loading. A third, and seemingly underappreciated, aspect of accumulation of sedimentary material is that the Franciscan complex covers a large amount of ground north of the Mojave but virtually none south of the Mojave. Maybe this is hiding in the continental margin offshore, but the difference seems noteworthy.
For now, we’ll leave the more distant foreland deposits (which were largely discussed in Jones et al., 2011) and instead focus on the forearc and the immediate foreland.
The forearc is surprisingly well preserved, all things considered, but there are suggestions of hiatuses and discontinuities in sediment source that are likely clues to unusual deformational events. GG here is largely relying on an analysis by Sharman et al. (2015).
So what do the sediments say? In the Great Valley of California, the detrital zircon population of the forearc looks awfully close to the age population of Sierra rocks in the latest Cretaceous into the Eocene. The same thing is true in the south by the Peninsular Ranges. What goes on in the middle? As Sharman et al. point out, after resembling the areas to the north and south prior to ~75 Ma, you get an awful lot of Precambrian grains, and that population in the Maastrichtian and Paleocene looks a lot like the eastern Mojave and Mogollon highlands. When did this happen? The plots in Sharman et al. are a bit hard to use to really look at this in detail, but digging into the supplementary data, we can make this plot, where % grains > 300 Ma is a proxy for the pC grains coming through:
Now a lot of these points represent one or two grains, but that only adds clutter near the bottom. While there is a real hint of something going on as long ago as 85 Ma, clearly things change dramatically about 72 Ma. As this has to follow the creation of an elevated area, we have to have some highland by 72 Ma east of the late Cretaceous arc (which, in the Mojave, still seemed to be active).
What does this represent? Given the distribution of the Precambrian grains as examined by Sharman et al., the source almost has to be the eastern Mojave and/or the Mogollon highland of western Arizona. But how did those come to be present? Well, it could be incision through arc cover rocks into local Precambrian bedrock. It could be expansion of the drainage area into the backarc where these rocks are found. It could represent exposure of footwall basement rocks in some large normal faults. Regardless, this is profoundly different than what was going on to the north and south. So certainly by 72 Ma and quite probably earlier (c. 85 Ma) the regular order of structure along the arc was broken. What is more, this happened in a place where magmatic activity was continuing; by this time the Sierra was dead and the magmatism to the south had already started migrating well inland.
The other point worth noting is that the Salinian forearc rocks are missing in the 100-86 Ma timeframe; subsequent deposition is described by these authors as on to deeply eroded arc rocks. Just exactly what this unconformity represents (and exactly where this occurred relative to North America) isn’t entirely clear.
Let’s look to the other side of the orogen. For a long time we’ve known of erosion from the Mogollon highland from the Eocene Rim Gravels and the Music Mountain formation, but most of that is younger than our interests here. Clues that this was a longer lived activity started to show up, and Davis et al. (2010) noted that the Paleogene detrital zircons found at the south edge of the Uinta Basin in Utah looked a lot like the zircons in the Late Cretaceous Maria Basin of southeastern California. This led them to propose a “California River” that ran from southeastern California to the north-northeast, revising ideas on the sediment dispersal in the Laramide orogen.
But that is still younger than our interests here, but there has been followup. Two recent papers push the California River back into the Cretaceous: St. Pierre and Johnson (2022) and Wersen and Johnson (2023). This work more directly addresses how far transverse drainage were coming off the Sevier belt and added the use of lead isotopes in feldspar. At this time in the Cretaceous the Western Interior Seaway reached far to the west, so these sediments in this earlier California River didn’t make it to the Uinta Basin but instead were dropped in the Straight Cliffs Formation. They point back to the same source area near the McCoy Mountains, but now back in the 80-90 Ma range.
While there is some controversy on this California River (a PhD thesis by Winn (2020) argues that these sediments look more like material in the San Juan Basin than the Uinta Basin), new work in the more proximal area makes it clearer that the uplifted area in the eastern Mojave was generating sediment in the Late Cretaceous. The 2016 paper by Hill et al. dates a limestone within the Music Mountain Formation, which fills some rugged relief in the western Grand Canyon area, to be 64 Ma. This overturned longstanding biostrat interpretation of the unit to be Paleocene. This requires the incision of that topography to be Late Cretaceous or even earlier, consistent with the notion that sediment derived from the Mogollon highlands was heading north in the Cretaceous. The coarse sediments within the Music Mountain formation clearly are from the south to southwest.
This area (described by Dickinson et al., 2012, as a “paleotopographical syntaxis”) also provided sediment to the east. Lawton et al. (2009) placed the headwaters of rivers traversing northern Mexico as in this same area as means of getting 1.4 Ga and 1.8 Ga zircons into the sediments:
So the last place to look is underneath: how do the POR schists compare to these foredeep rocks? Although Sharman et al. updated this somewhat, the main presentation is in Jacobson et al., 2011. given our attention to the >300 Ma zircons above, it is interesting that virtually all of the POR sites have a fair amount of old zircon:
What is a bit striking here are the values from the Rand Schist. As that supposedly was emplaced by 75 Ma (middle Campanian), this feels like a bit of a timeline problem, for not only are the deposits as young as only 80 Ma, you also need time to stuff these underneath and begin to cool them. So either there is a different source of zircon for these protoliths or there is a timing issue. This is far less severe for the other schists as we had a lot of pC before their creation…except we also got a ton of Jurassic zircon. The proportions are not the same.
But when Jacobson et al. plot up all their forearc samples of specified age ranges (similar to the dataset in the 2015 paper above), they seem to get a decent match between the schists of different ages and the zircon populations:
Comparisons with other late K-early T sections farther inland do not look remotely like these distributions. While there are clearly issues here, the schists look an awful lot like the forearc sediments still out there.
So what is the big finding here? The thing striking GG is that region where the Sevier belt had died was high enough for canyons to form in the western Colorado Plateau and for sediment to head out from this area in all directions from sometime in the late Cretaceous into the Paleocene. That is quite a trick. Either we are selling short some other exposures of Precambrian basement or this area stuck up high enough to be getting eroded pretty seriously for a long time. The question is why…
Previous entries in this series:
- Introduction
- Schists
- Igneous Activity
- Deformation
- Mojave Mojo
- Crustal structure
- Sedimentation (this page)
- Mojave Waistland
- Igneous markers
The Heroic Age of Tectonics is over…
…so where to now?
One hundred years ago we didn’t know the age of the Earth (although radioactivity had freed many from assuming that Lord Kelvin was correct). Ideas about continents moving about were hotly debated and, in broad areas, dismissed. Geophysicists denied that low-angle thrust faults could exist. Only the very barest of information about the oceanic crust was available. The concept of an asthenosphere was less than 10 years old, and its implications remained to be fully recognized. We didn’t know that the Earth had a solid inner core.
Fifty years ago, though, a lot had changed. We had a solid estimate of the Earth’s age. Plate tectonics had swept the geophysical community and was rapidly changing the interpretations of geologists. We not only knew there was a solid inner core, but we knew that something fishy was going on at the core-mantle boundary. We had a solid first-order understanding of why major mountain belts were where they are, and why they were seismically active (or relatively inactive). While isostasy was widely accepted, we were seeing that you needed more than varying thickness of crust to explain variations in elevation. Foredeeps before mountain belts were directly understood using elastic plate theory.
The last 50 years?
Read More…Why study the Sierra…
GG has spent a lot of time deciphering the Sierra Nevada’s tectonic and geophysical history. A question we all should face from time to time is, why study this topic? There are tons and tons of geological questions we could ask and loads of places that have received a lot less attention. Maybe it is time to move on (and maybe that time was many years ago).
On one hand, a plausible history of the Sierra is that the formation of its massive batholith during the Cretaceous is the whole story. Since then all that has happened is that the range was erosionally unroofed in the latest Cretaceous and earliest Tertiary and, much like what is seen in the Rockies as erosion removed all the easy stuff on top, erosion greatly slowed as rivers got into the more resistant plutonic rocks. All that has happened since has been a slow lowering of the range through erosion, with a geomorphic rejuvenation from a changing climate interacting with varying sediment loads from the east and a relatively brief return of a volcanic arc in the north. Nothing to see here tectonically at all. A lot of the evidence for this proposal comes from paleoaltimetry.
And if that is right, GG has been barking up the wrong tree for a long time. And so rather than start with GG’s usual logic for studying the Sierra, let’s come at it from this angle.
Read More…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.
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.
A Rift Origin for the Plains?
One of the most popular explanations for the High Plains is that they were dragged upward by a buoyant body, probably in the upper mantle under the Rio Grande Rift. This is arguably the only late Miocene to Pliocene event one could plausibly associate with post-Ogallala Formation tilting. GG has tended to be dismissive of this but hasn’t been through the math. Now there must be a simple analysis somewhere in the literature, but GG isn’t seeing it, so let’s make a simple model and see what it takes to make it work. We’ll assume a north-south trending horizontal cylinder with some density contrast under an elastic plate represents the source of uplift (although many folks like a “broken” plate, the physics of such a boundary are inappropriate here). We’ll place the cylinder at a depth z and calculate the uplift and the gravity anomaly from this body. We’ll tweak these until we can fit the observations.
Now we have a little difficulty in that the modern topography is due to more than just the Rift: the sub-Ogallala unconformity reveals rather clearly that there were east-flowing streams when deposition began, meaning that topography back then was tilted to the east, though that potentially was very close in time to deposition. So that topography was presumably compensated by some mechanism that might be well distributed (e.g., variation in crustal thickness). Since the free-air anomaly across the Plains is near 0, the Bouguer anomaly for local compensation of topography should be 0.112 mGal/meter. We’ll just add that to our theoretical models as needed.
The problem is that we don’t know how much topography we want to ascribe to the late Cenozoic Rift: one extreme view (seemingly that of Eaton, 1986, 1987, 2008) is that things were pretty flat in prior to the Rift on an east-west profile, with major rivers going more or less directly to the coast to the south-southeast; another that there was some gradient, though much lower than today (e.g., McMillan et al., 2002). Let’s tackle both and see what we get. In both cases we will focus on the topography east from about 105°W and we’ll place the cylinder at 106°W, under the axis of the Rift.
The Grumpy Geophysicist 
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