In the previous post, we discussed how Occam’s Razor is of little use in some arguments, leading to the principle of least astonishment. But here GG would like to suggest that the shear immensity of geologic time means that Occam sometimes cuts us off from explanations we need to consider.
In this case, let’s talk Laramide. Orogeny, that is, the creation of the Southern Rocky Mountains between something like 75 and 45 million years ago. The prevailing explanation is that the subducting ocean floor only went down to about 100 km or so and turned flat, interacting with the continent in a way to make mountains far from the plate edge. It is a nice compact explanation.
The thing is, there are a lot of places where slabs today are flat and none of them produce anything of the scale of the Laramide Orogeny. Closest are the Sierras Pampeanas in Argentina, which are far closer to the trench than the Laramide ranges were, among other difficulties. Even looking over past orogenies yields few plausible rivals–maybe the Alice Springs orogeny in Australia, or if you push things hard, perhaps the Atlas ranges in northern Africa. Or, of course, the Ancestral Rockies in almost the same place as the Laramide. But these are just as cryptic and far less common than all the events that created the Appalachians, or the Urals, or the Caledonides, or the bulk of the Alpine-Himalayan system.
Perhaps, when we encounter oddities in the past, we need to recognize that something unusual happened, meaning that Occam’s bias for parsimony might in fact be precisely the wrong bias. For instance, somebody walks up and says they will flip a coin ten times and it will come up heads. He asks a passerby for a coin and then does as he says. Parsimony says this was luck, but perhaps a better explanation is that it is a trick either involving an accomplice or sleight-of-hand [scientists are suckers for sleight-of-hand, as the Amazing Randi often showed].
Given the number of times slabs probably have been flat and given the far rarer production of mountain ranges far from the trench, maybe our bias for parsimony should be relaxed–odd and unusual results might demand more than a single cause. Maybe things were a bit Rube Goldberg-ish for awhile. In a similar vein, some workers are arguing that the impact ending the Cretaceous was so effective not just because of its size but because of the sulfur-rich rocks it hit (this in part a response to the absence of other impacts in causing extinction events and other extinction events seemingly lacking a coincident impact). Arguably something like this has or will emerge in explaining how one branch of the great apes led to humans despite lots of earlier evolutions of animals failing to reach a similar end. We often focus on the positive outcome–the mountains made, the extinction that happened–and miss how often the simple explanation predicts something that didn’t happen (kind of like the old quip that the stock market predicted nine of the past five recessions). We don’t ask, why are there no mountains in Iowa, for instance; we ask, why are there mountains in Colorado? But perhaps we need to ask both.
Occam reminds us to be distrustful of overly-complex explanations, but maybe we need to be careful not to demand too much simplicity. All theories will conflict with some observations in some way; there are always strange things that happen that are coincidences or results of unrelated phenomena. This reality means that no theory will fit every possible observation; what’s more, we tend to accept more misfits for simpler theories (for instance, the half space cooling model for ocean floor topography is widely accepted despite all the oceanic plateaus and seamounts one has to ignore to get a decent fit). Given that, we should wield the Razor more carefully least we cut off our theoretical nose to spite our parsimonious face….
A favorite shortcut employed by many in trying to decide between hypotheses is to enlist Occam’s Razor–that the simplest explanation for something is most probably right. Now this has strength because humans are pretty good at rationalizing notions they put forward, adding in new ingredients to keep a favored explanation from collapse. But a theory that has probably passed its must-use-by date will have enough extra bells and whistles to discourage Rube Goldberg from trying to get it to work.
However, there is nothing that says Mother Nature had to be supremely parsimonious. In a complex system like Earth, there can be odd coincidences that are meaningless (like the Moon and Sun sharing the same apparent diameter from Earth’s surface) and outcomes that might be highly improbable (taking over 500 million years to get intelligence after making complex animals with hard parts seems like dawdling, especially when burning most of that time on dinosaurs). Even so, Occam can be a help if used with care.
But lots of times you can face competing hypotheses that lack Occam-style clues. For instance, which is simpler: that post-5 Ma erosion of the High Plains of the U.S. was caused by an eastward tilt, or that this was the product of a changing climate? Both are pretty easy to describe; both have issues. Yet many earth scientists feel pretty comfortable arguing that one is correct; what is the basis of such assurance?
Arguably the most common discriminator used by earth scientists is the principle of least astonishment. What surprises you least feels, in an Occam kind of way, like the interpretation that is most likely. The problem is, we all are astonished differently.
If you are a sedimentologist, you might look at the problem of the High Plains as one of depositing the Ogallala Group in the Miocene as crucial. Could you possibly deposit something like that on a slope like that we have today? This seems so astonishing that if can’t be right; the original slope had to be lower.
But maybe you are a geophysicist looking at the ways to create a tilt about 5 million years ago over something like 1000 km. That looks really hard to do, especially if dynamic topography from flow under the lithosphere is ruled out. It would be astonishing if that happened; it must be that the grade was already there much longer ago.
Skepticism from both geoscientists is warranted; either of these seems really hard to do. Data is gathered by both sets of experts. Margaret McMillan and colleagues measure paleogradients in the Ogallala using a widely applied approach and find there must have been a lot of tilting. Will Levandowski and colleagues (including GG) look at geophysical measurements and find support for the elevations comes from within the crust, where changes over the past 5 million years seem exceptionally implausible.
Could these be resolved? Well, you could posit that prior to 5 Ma there was dynamic subsidence holding the western end down and once that was released, the crustal buoyancy expressed itself. But now Occam detectors are flashing red–this feels ad hoc. Of course, there could be mistakes in the measurements of paleogradient, or in relating seismic wavespeeds to densities–each side has poison darts to shoot at the other side.
What makes this frustrating is what makes this interesting. After all, in the end somebody will be astonished–the earth did something they didn’t expect.
And a funny thing, shoes flip feet in the Sierra, where those studying the sediments argue for no tilting despite deposition at an even steeper grade than modern-day Ogallala, while geophysicists feel they have good evidence for a very recent change in the buoyancy structure of the region.
Are you astonished yet?
It used to be when we thought what the southwestern U.S. looked like at 85 million years ago, back in the Cretaceous, we thought things were pretty simple. There was a nice volcanic arc running from the Sierra down through the Mojave into the (restored) Peninsular Ranges. To the east was a fold-and-thrust belt extending nicely from well up in Canada down to the Las Vegas area in continuous fashion before taking a left turn to angle more erratically into more complex geology across Arizona and New Mexico. East of that was the foreland, its western edge bowed down under the weight of those fold-thrust mountains and the torrent of sediment washed off those ranges toward the inland sea that stretched up from Texas towards the Arctic Ocean. The only real wrinkle in this had to do with where the troublesome exotic terranes now in British Columbia were at the time.
Now it feels like 85 Ma is instead a pivot for everything about the western U.S. A number of puzzling things seem to be going on around this time.
- Emplacement of the first of the subducted oceanic/forearc schists (the Catalina and San Emigdio Schists) appears to occur about 85-90 Ma [Jacobson et al., 2011]
- The Sierra Nevada sees the culmination of the most massive plutonic episode in its history just prior to a complete end to plutonism [e.g., Ducea, 2001]
- The Mojave Desert will continue to see both peraluminous and metaluminous magmatism for another 10 million years despite the inferred emplacement of most of the Rand Schist over that time. [e.g., Barth et al., 2013]
- Shortening in the hinterland appears to be a short-lived interval between extensional episodes [Wells et al., 2012]
- The Sevier fold-and-thrust belt in the vicinity of Las Vegas is dead or dying–some put its end before 90 Ma [e.g., Fleck et al., 1994]
- Perhaps a northwest-trending thrust belt has instead invaded the area about this time. [Pavlis et al., 2014]
- Canyons start to form in the southwesternmost Colorado Plateau [Flowers and Farley, 2012].
- In contrast, the simple foredeep geometry of sediment accumulation in the foreland begins to broaden out, suggesting a “dynamic subsidence”. [Liu and Nummedal, 2004]
By 75 Ma, it is really clear that things have gone super strange. The Sierran arc is dead with no real replacement. Magmatism at that latitude is limited to smaller intrusions in the Colorado Mineral Belt and some peraluminous (two-mica) granites in Nevada, eastern California and parts of Arizona. In contrast, the Mojave continues to see plutonic activity with both peraluminous and metaluminous magmatism, seeming to be the southwestern end of a long linear belt of unusual magmatism. The Sevier hinterland has several examples of significant decompression accompanied by more limited evidence of extension. The foredeeps in front of the fold-thrust belt seem to have ceased to accumulate much sediment, which is now captured in broader basins farther east. Laramide uplifts are certainly underway.
Although 75 Ma is often taken to be the start of the Laramide orogeny [Dickinson et al., 1988], it increasingly seems like the orogeny’s origins lurk in the preceding 10 million years. What exactly was going on?
Some of the things we don’t really have a solid handle on:
- Where was the Insular Superterrane? [We might have a more decent view of the Intermontane Superterrane due to work in Idaho over the past decade].
- What really was the plate geometry? The Kula plate dates back to about this time; were there other plates to the east of the Kula-Farallon-Pacific triple junction?
- Do we have a good handle on where plutonism was active in the Mojave/Sonoran deserts?
- How much of the Cretaceous decompression events are from extension vs. some kind of intra-crustal pseudo-convection?
- How could the southern Nevada part of the fold-thrust belt be inactive?
- How can schists be emplaced against the middle continental crust immediately below fresh plutonic rocks?
The Laramide Orogeny made North America look the way it does today; this time period holds the keys to understanding how it could happen. Some way-out ideas are kicking around [e.g., Hildebrand, 2009, 2013; Sigloch and Mihalynuk, 2013], though some are moderating a bit [Sigloch and Mihalynuk, 2017]. Conversely, most earth scientists have gravitated to some flavor of subduction of an oceanic plateau as the cause of all the misadventures near the start of the Laramide orogeny. That all of these have pretty substantial flaws shows that the community is struggling to understand just what was going on at the start of the Laramide. Hopefully over the next few months we can explore some of these topics in some detail…
Having just remembered the 1906 San Francisco Earthquake brings to mind Harry Fielding Reid’s model of elastic rebound for earthquakes developed from observations of that 1906 quake. The idea that the earth’s surface was slowly moving in opposite directions across a fault over a long time period, straining the rocks near the fault until a critical point was reached when the strained rocks would cause the fault to rupture, allowing each side of the fault to “catch up” with the more distant parts of the earth’s surface farther away.
Much later, when plate tectonics was developed, earth scientists could tell what the average velocity of plates were over a couple million years from analysis of magnetic anomalies on the seafloor. When space-based geodesy came along, first with VLBI and then with GPS, geodesists found that the plates were moving today at a rate equal to that seen over millions of years. It seemed as though the earth ran at a smooth and even pace.
The combination of ideas would suggest that one hope expressed about a hundred years ago was that faults would be triggered like clockwork. Every so many years, termed the recurrence interval, a fault would rupture with what would be called a characteristic earthquake. Ideally you could then predict the next earthquake if you knew when the last couple had happened.
This ideal view of the earthquake world has gradually unravelled, with a couple of observations in the past decade indicating that there really is something more variable in how geologic strain is created than the elastic rebound model and smooth plate motions would have suggested.
It seems like the Fall AGU meeting brings some new wrinkle to the GPS measurements in the Sierra. In the past we’ve seen suggestions that the Sierra were going up tectonically, then that they were going up because of water removal from the Central Valley, then they were still going up even with water removal in the Central Valley, and now we have the Sierra going up because of water removal in the Sierra itself. This latest missive is from Don Argus and several colleagues at JPL deserves a look; their paper on this was published about at the same time in the Journal of Geophysical Research (though that paper doesn’t have the coda from the AGU talk about the loss of elevation in the wet winter of 2016-2017)
Basically they wrote that during the California drought from 2011 to 2015 that the Sierra lost 48 km³ of water and so rose at least 17 mm from that loss while also rising an additional 5 mm from water loss in the adjacent Central Valley and then might have risen no more than 2 mm from tectonics for a total elevation gain of 24 mm (or just about an inch). That is a lot of uplift for a few years. This interpretation means that the Sierra actually stores a lot more water within its granites than is typically thought to be the case, which aligns with earlier work by Argus and colleagues.
In keeping with this end-of-the-year theme of what GG is doing wrong, some “crimes against science,” which, as Bob Sharp defined them years ago, was doing some work of interest to the broader community and then not publishing it. (Thankfully, these aren’t the more serious offenses in the expanded criminal ledger GG proposed awhile back).
Now this isn’t an uncommon occurrence: students graduate with thesis chapters not quite ready for publication and discover that life beyond grad school doesn’t provide rewards for getting that stuff into journals. Some other times things just pile up enough that a paper isn’t completed when everything is handy, and it just gets harder to return to as time goes on.
So, in case anybody out there would benefit from some of this stuff, feel free to nudge GG to take some time and share, either informally or by actually publishing some of this. And if nobody seems interested, well, then maybe not much of a criminal act :-). Most of these are in some kind of manuscript form (there is other stuff that didn’t even get that far).
- Geologic map of the Alexander Hills and eastern China Lake basin. Yes, GG mapped while in grad school and actually handed over a copy of his map to Lauren Wright long ago, who included some of it in a never-published update to the SW Tecopa quad (now would be Tecopa 7.5″ quad map). A lot of cool stuff–probably the eastern end of the early Garlock Fault interacting with some low-angle, basin-bottom faults and a pre-China Lake basin history not evident in published maps.
- Seismicity of the Hansel Valley region. GG feel really bad about this, as there were a lot of coauthors on the 1983 experiment, which was one of the densest deployments of seismometers in an extending area. The results are in GG’s PhD thesis but still might merit publication as the data indicates how a low-angle normal fault might interact with ongoing seismic deformation.
- Magnetostratigraphy and some additional paleomag in the Lake Mead region. A collaborator dropped out and so the baton was dropped after a single paper. Some of the data is visible here.
- Paleomagnetic measurements in monoclines of the Colorado Plateau. Joya Tetreault’s thesis has this; substantial vertical-axis rotations exist in some folds (the Grand Hogback being the most dramatic), though the sampling is far less than ideal and some structures seem to make little sense.
- Paleomag and micropolar analysis of seismicity in the Coalinga area. Also part of Tetreault’s thesis. The micropolar work seemed to capture the bending component of folding in the seismicity while the paleomag suggested San Andreas-parallel shear within the fold limbs.
- Earthquakes in the southern Sierra located with the 1988 experiment. Jason Edwards, a CU BA graduate, did some of this work which was never carried farther. It seemed there were events under one of the Recent cinder cones in the s Golden Trout field as well as some deep events in the westernmost foothills of the southern Sierra.
- Geophysics of Panamint Valley and the Ivanpah Valley areas. These were datasets collected by the MIT Geophysics Course in 1987 and 1983, respectively. Both valley present a major challenge because a large basement gravity gradient exists across these valleys, complicating interpretation.
This is all in addition to various half-done projects still seeming to be active as well as datasets that never were fully exploited (for instance, data from a mixed broadband/short period array at Harrisburg Flat in Death Valley plus some more scattered instruments near Dante’s View, or our inability to get anything sensible out of array recordings of deep local events under the northern end of New Zealand’s South Island).
A year ago GG posted on the Kaikoura M7.8 earthquake with the title “Single quake slip partitioning”. With a year past, it seems a quick look at the literature that has appeared is in order. Was this diagnosis correct? In some work, it seems the answer is yes; in others, it seems no.
The most comprehensive overview is probably a paper by Kaiser et al. in Seismological Research Letters. This paper summarizes geologic, seismologic, geodetic, and engineering observations from this quake. They note that 13 separate mapped faults all ruptured together, more than was anticipated prior to the quake. It took about two minutes for things to unwind from south to north along this collection of faults, with substantial step-overs was one strand to the next. Most of the energy released came in two distinct jumps, one 20 seconds into the quake, the next about 70 seconds in.
But as to GG’s hypothesis of slip-partitioning during the quake, the interpretation of the slip history from high-frequency seismic data is no; the faulting was dominantly strike-slip to oblique-slip on land, though the authors do note a period during the rupture when they don’t really locate the source of seismic energy very well.
A second paper comes at this from a different angle. Read More…