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Cordilleran Contradictions, 2018 edition

Spent many hours in November sitting in on sessions and perusing posters at the Geological Society of America annual meeting; one goal was to see what’s up with the evolution of elevation of the U.S. Cordillera.

First a quick recap. There are two camps, more or less, on each side of the Cordillera.  The old mountains camp on both sides points mainly to oxygen and hydrogen isotope variations in proxies for precipitation. There are also attempts to retrodeform the lithosphere resulting in thick crust and high elevations. The dominant counterargument is that the paleometeorology used to interpret the isotopic values is flawed. On the young mountain side, classical geologic observations are invoked, including apparent tilting of river channels and the recent incision events in many places. The counterargument to this is that the appearance of a tilted channel may be biased by the depositional environment and that changes in climate can drive incision as easily as uplift. In between in some ways are geophysical observations of the lithosphere; recent changes in the lithosphere seem likely in much of the region, supporting younger mountains, but seem older east of the Southern Rockies.

Well, a meeting in Indianapolis isn’t one to bring out all the western geologists (next year’s meeting in Phoenix is a whole different matter), but a couple of things popped up. Did anything look to change the landscape, either by opening up new vistas or overturning old results? Not that GG discerned.  Below are some notes probably only of interest to the most interested….

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Occam’s Cut

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….

The 85 Ma Trainwreck: Introduction

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…

Oops update

Update 7/29/18: The corrected figures are now officially online.

GG asked readers whether or not an error in a figure drafted 8 years ago should be corrected and the answer was a resounding yes.  So the figures in question have been redrafted and will go to the journal shortly.  If you are curious, the “dynamic” version (uses Flash) can be found here.

To be clear, the intent is simply to remove a mistake in the past and not to update the figure to reflect how it might get drawn today. So if you find things you don’t agree with that reflect scholarship since 2010 or so, don’t expect a correction, but if there are other mistakes substantial enough that somebody might misinterpret things, let GG know.

Retraction Watch was apparently amused at the notion of polling the web to decide whether or not to update the figure and so ran a story on this episode. When the journal puts out the correction, a link will get posted here.

Oops

Well, GG has from time to time pointed out mistakes in graphics in papers, so it is only fair that he share a mistake in his graphics pointed out by others. In Jones et al. (Geosphere, 2011), several panels of Figure 2 place the frontal thrust faults of the fold-thrust belt in the wrong place in Montana (to be clear, this is the upper left part of the full figure seen at right or below):

LaramideMistake  LaramideOrigFig2e

The black line with barbs was to represent the eastern limit of the fold-thrust belt, but GG apparently mistook a high-angle Laramide-style fault for a low-angle thrust connection up to the Helena Salient.  The proper line would be close to the red line in the figure above.

So a question to any readers is, do you think this merits a published correction?

[Updated 10:30 AM MDT 4/7 to show the full extent of the figure; poll added 12:06 pm MDT 4/7]

The Schist-Melt Conundrum

One of the things that has gotten increasingly glaringly obvious is that there is a big problem lurking in the Mojave Desert. The problem, most simply, is that the dates of plutons in the Mojave overlap rather severely with the dates of emplacement of schists in the lower crust. Dating activity reported at the GSA meeting this past week included a bunch of 72-85 Ma intrusive rocks, mostly metaluminous plutonic rocks that seem likely to have had a mantle melt as a primary source, sitting above the area where the Rand Schist was supposedly being emplaced.  Just as bad for mega-flat slab advocates, the extent of these 70-something plutonic rocks is now extensive enough that it seems awfully hard to sneak in a big flat slab through the Mojave–and even if you do, it is coming in way too late to be starting the Laramide orogeny, which was already chunking along at this point.

So two questions: what is going on in the Mojave? and what are the broader implications inland?

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