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?
So Paul Braterman was asking the other day about some advance knowledge of how the new infatuation with detrital zircons and HeFTy plots might be misread in some circles. This is really a question for a real card-carrying geochronologist, but GG will take a swing at it because he’s kind of worried a bit about this and is not staking out any particular terrain.
For the most part both of these are simple variants on classical age-dating techniques (uranium-lead dating and potassium-argon dating), and though rooted in geochronology, most of the applications are elsewhere (e.g., detrital zircons are mainly used as a means of identifying the sources (provenance) of clastic sedimentary rocks, while HeFTy (Helium-Fission Track analysis) is dominantly a means of assessing the thermal or unroofing history of some body of rock through application of multiple geochronometers). But they start pointing at things that could be misleading, so let’s look a bit more….
Not so long ago, you would get a date (one) for some igneous unit. And that was hard enough that you wouldn’t bother with two or three. Dates were so valuable one well-known scientist had an equally well-known safe to keep them in (we still live with a rule at GSA related to this fellow in that recording or photographing presentations is forbidden). Then there was recognition that some systems closed up shop at different temperatures than others. So maybe you’d see a U-Pb date and a K-Ar date. A few labs did this work, often under contract; you (the non-geochronologist) might wrap up a sample and send it on to be dated. Dates, while important, were just some numbers that were part of a geologic story.
Now, however, dates are everything (that, and chemical and isotropic analyses at the tiniest levels, which is a related outgrowth). It seems like more than half the talks at GSA involved dating detrital zircons, or dating zoning in zircons, or dating helium diffusing out of zircons. Dates are used to understand erosion, tectonics, stratigraphy, sedimentology, volcanology, paleoearthquakes, glacial action and more. Arguably this ability is utterly changing geomorphology and sedimentology and it seeps into other fields more slowly.
If you haven’t seen a pdf (probability distribution function, not portable document format) or a HEFTY thermal evolution-o-gram, you haven’t been in a geological talk in some time now.
And so it is about time for the revenge of the grumps. Not GG so much as others. For the broad application of these new techniques has excited most geologists, but history tells us that there will be a reckoning. As GG watched lots of folks who have not themselves sat in front of an LA-ICPMS machine in their lives display plot ofter plot of geochronology-derived stuff, you sense that something will come along to threaten this grand promise.
This has always been the way of new techniques. They appear, they are exciting and new, they are applied everywhere, and then discrepancies emerge. Look back in olden days and see how potassium-argon dating started; it took awhile for practitioners to recognize that sometimes crystals would lose argon and they got dates that were too young, or that certain materials would introduce an excess of argon from other minerals and a date would be too old. Some early results were discarded, the community identified situations when problems were likely to arise, and early over interpretations were scaled back.
There are hints of this already. Conflicts between U-Th/He dating and some classic geologic constraints hints at some problems in some places. Some work in the past few years indicated that fission-track thermal histories relying on track length distributions were dependent on specific laboratory practices that are not uniform. Puzzling results are emerging in some sedimentological studies where things that simply cannot be seem to be. On occasion, dates seem backwards, with younger dates from systems that should have closed well before materials yielding older dates.
None of this is really a worry. It is the shake-out that is needed. And as long as you keep in mind that there might be some landmines out there, the hazards are manageable. It is a kind of “trust, but verify” environment. But there will be reverses ahead, and some promising studies might turn out to be chimera. Don’t be surprised to see some papers saying that a certain technique is wrong when applied under certain conditions. But in the end, we will still come out with a host of tools well suited to consider geologic problems. The age of ages is upon us, like it or not.
Somehow that doesn’t sound good…but it helps to illustrate the problem with “the flat slab” in the western U.S. If you are interested in the emplacement of the Pelona/Rand/Orocopia Schists, your slab is shallow at shallow depths: basically, it eats into the crust. It certainly was NOT flat in the crust: there was a definite dip as these schists only go so far inland. You may not care if it is flat or shallowly dipping or anything else farther inland.
If you want your arc to die like the arcs in the Andes have died, you want your slab flat somewhere near 100 km depth. It doesn’t have to be unusually shallow near the trench, but you need it to go flat for some distance to prevent asthenosphere from creeping in and making volcanoes.
If you want a flat slab to make mountains far inland, the stakes get higher. The most common and physically defensible means of doing this is by imparting a basal shear stress to the continental lithosphere, which carries some consequences.
You could have a subduction system with all these–but there arguably is no such example today (the inland mountains best hope are the Sierras Pampeanas, where the shallow part of the subduction system is pretty normal; if you want the erode the continental lithosphere, Alaska might be your best game).
Things could be pretty complicated. For instance, subducting some amazingly thick piece of ocean floor under the Mojave Desert makes a lot of sense–but such material will plummet into the deep mantle once the thick pile of basalts gets deep enough to become a thick pile of eclogite. The Mojave’s flat slab might become a steep slab not very much farther inland. It is even possible that such a scenario would generate a more long-lived flat slab, as it is possible that you have to be disconnected from the deeper parts of a subducting slab for a slab to become shallow and track along the base of the continent. So you might have a “flat slab” in the Mojave 10 or 20 million years before subduction becomes “flat” for purposes of places far inboard.
The point? Say what you mean; “flat slab” is too generic a term to be useful.
Certain knee-jerk phrases and assumptions just kind of get GG all grumpied-up. The two from today? “Nevadaplano” and “Laramide flat slab”.
Now GG is not in possession of God’s plan for the universe or operating a time machine with X-ray vision into the earth. In fact, those who are explicitly investigating such concepts are not the targets of this venom today. It is those who use these terms–or, probably more properly, use these memes–that drive GG to distraction.
Why? When Peter Bird spoke of a flat slab in his 1988 paper, it was crystal clear what he meant. There was no real ambiguity. His flat slab had a real purpose, it was a firm creation that could be encountered face-to-face (after a fashion) and be dealt with. If you spoke of Bird’s flat slab, you knew why it was there, how it hooked into everything, what it was and was not supposed to do. It was something you could–and many did–disprove. It was an honest to goodness hypothesis.
But the flat slab of the meeting talks is a nebulous invention designed to deflect attention. “The Laramide flat slab” could be almost anywhere in the western U.S. It could start back at 90 Ma. It might be lurking today under Mississippi or the Great Lakes or New Jersey [all such suggestions are indeed out there]–or under your bed calling you on the phone! [OK, that one isn’t in the literature]. It is, essentially, an invitation to suspend critical thought. The flat slab can move mantle lithosphere, hydrate crust hither and yon, it can depress the crust, or raise the crust, stop volcanoes or start them. It is all-powerful. Need something to happen? Invoke the flat slab and criticism is silenced.
The other boogeyman is of a different stripe. The “Nevadaplano” is one of those portmanteaus so easily rolled off the tongue that it was, from the moment of conception, a favorite in oral presentation. It was just too fun a phrase to pass up. While it lacks the powers of the flat slab, it, like many superheroes, has its own abilities: it flickers in existence between eastern Nevada, western Utah, eastern California and southern Arizona, appearing where needed just in the nick of time–whether that time be in the mists of the Cretaceous or the dying days of the Oligocene. Its partial namesake, the Altiplano, is known for being flat, a product of internal drainage, yet many (most?) incarnations of the Nevadaplano are externally drained. Nearly all the times speakers call upon the spirit of Nevadaplano, they really have no real need of it. They just need a highland in the right place at the right time–and there is good evidence for many of these highlands. They just don’t look or behave like the image projected by the Nevadaplano, and one speaker’s Nevadaplano would spit on another speaker’s. You really do wish that the spirit of Nevadaplano would object and not show its face in such instances.
Science is supposed to be a precise business. When we speak of the San Andreas Fault, the Navajo Sandstone or the Channeled Scablands, these are things that are well defined even if there are some blurry edges somewhere. Even multifaceted terms like “lithosphere” rarely convey different notions to different listeners within the context of a talk. But the flat slab and the Nevadaplano are, as usually used, lazy shortcuts designed to avoid grappling with a more complex world. They are oral mirages, temping visions made in one’s mind that cannot be examined too closely or compared with others. Simply enough, they are not science.
Geologists have for a long, long time been telling people not to build things in certain places. Barrier islands? They move and evolve, which means property comes and goes. Not good. Floodplains? They, um, get flooded. Landslides? Only if you want a mobile home with a mobile yard. Sometimes we get heard, but usually we don’t. And the more subtle stuff, like recognizing how paving large areas can make floods worse? Lots of luck there. Doesn’t matter if the communities are rich or poor, building in bad places seems a national habit.
Maybe that is changing.
Even as the national media seems to just be noting that flood insurance is encouraging building in vulnerable spots, Politico has a big story on Louisiana’s program to consider how some communities will be forced to move and how to prepare to absorb that exodus as it occurs. For the Grumpy Geophysicist, this is a moment of actual hope, a ray of sunshine in the currently clouded over world of using science to guide public policy. [If you want more darkness, consider that politicians are rewarded for disaster relief and not disaster preparedness.]
The basic point is that people don’t like getting hammered by really bad weather (you know, like floods). And so they leave–and this isn’t typically a slow migration but instead a real wave of refugees from hurricanes or floods or other such unpleasantries. They don’t often go really far away, so neighboring communities suddenly are flooded with people. There are two main forks to preparing for this: one is to try and get the vulnerable communities to start to think about how they will evolve in the face of the next storm, and the other is for those neighboring communities to prepare for the eventual migration of their neighbors. The state is actively trying to do this kind of work.
While there are uncertainties in our future, there are a few things that will happen. There will be sea level rise. There will be bigger rainfall events. These are both so clearly tied to the basic physics of increasing CO2 in the atmosphere that there really is no avoiding them; the best we can do now on that side of the ledger is to try and keep the magnitudes lower than they might otherwise be (and some areas also see land subsidence, which is unrelated to global warming but also causes problems). So we need to prepare, which means surrendering land we cannot defend and defending land we dare not surrender.
That Louisiana is starting to consider this landscape triage may just mean we’ve moved off the “we will rebuild it” mantra of the past century. As the article makes clear, this won’t be easy–but it should be much better than letting the chaos of the next disaster drive change.
For those of us in earth science, this past week has highlighted an awful lot of potential “told you so” moments. Like how warming climate and a warming ocean will lead to higher precipitation events. Like how you really do need to plan for floods. And we just missed hearing more about the barrier island/marsh protection talking point. And almost at the same time we’ve been greeted with ever more evidence that the Trump administration has little or no use for scientific input–not even choosing to ignore it, they seem more eager to simply not have any scientific input at all. Just as it is ever clearer that we are facing real decisions in trying to prepare for a warmer world, we seem the have a government yelling “la la LA LA” with its fingers in its ears.
But that isn’t the point here today.
One aspect of the tragedy in Houston is that the absence of any sensible planning has led to more flooding (the worst example might well be letting houses be built within the basin and below the spillway elevation of flood control dams); this is exacerbated by the combination of government subsidized flood insurance and the out-of-date or inadequate flood zone maps. Of course some now point to the zoning-free and laissez-faire approach to building in Texas as the bargain they made with the devil, implying that other places where strict zoning has been enforced will be safer.
If GG has noticed one thing about strict zoning (and Boulder has a pretty heavy hand on building), it is that it is rarely used to prevent building in stupid places–it is mainly used to keep people from building on land other people enjoy as it is. Some years ago when Colorado Springs was approached by a developer who wanted to build houses on an active landslide, the city council had to look away from the evidence they were given in order to approve this ongoing disaster. You can find similar stories elsewhere. Yes, fear of flooding is brought up when a new development is proposed…but mainly as part of the larger arsenal serving Fort NIMBY (sometimes there is a legitimate fear, but sometimes it is greatly exaggerated). California has the Alquist-Priolo act to prevent construction near active faults, but it only moves buildings 50 feet from an active fault. Direct destruction of a building by a fault being directly under it is one of the least likely modes of destruction (even some dams do OK on faults: the Upper Crystal Springs dam survived having several feet of offset in the 1906 earthquake). Earthquakes do most damage by shaking weak soils: recall the Marina District in San Francisco, far from surface faulting, where shaking from the Loma Prieta earthquake damaged dozens of structures. What strict zoning clearly does is raise housing prices.
The main exceptions to non-use of zoning as a disaster preventative is in the wake of disasters. Even then, the most common refrain after a disaster is “we’re going to rebuild and bring it back better than before.” After a tornado, this makes sense. After a flood, whether storm surge or heavy rain? Not so much. The harder statement? “We learned a lesson and we aren’t going to make that mistake again.” It is very hard to say, but if we are going to avoid paying to rebuild over and over again in increasingly vulnerable places, risking the lives of inhabitants in the meantime, it’s time to start saying it and then walking the walk.