Archive | October 2017

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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Dating Stumbles

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….

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The Age of Ages

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.

How flat is your slab?

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.

Prisoners of the Meme

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.

Meeting Survival

Sorry to have left loyal readers in the lurch without a dose of grumpiness for awhile–been taking the show on the road the past couple of weeks.

Presently though at the Geological Society of America conference, which means it is time to try to get through one of these meetings without going either broke or insane.

Now, if you have a grant where you actually budgeted for the real cost of the meeting and you aren’t trying to stretch that money to cover two meetings and an extra week of field work, stop here.  This isn’t for you, you lucky dog.

The rest of us find ourselves in meeting hotels that are usually insanely expensive but, thanks to the group rate from the meeting organizers, are merely expensive. What is amazing is just how expensive they can be after you’ve already paid for the room: of course there is the honor bar (“honor”–yeah, right, that is why they installed a frigging weight sensor in the fridge to charge you if you lift anything out). Gotta love that $20 mini bottle of wine–nothing says desperate than drinking a small bottle of grossly overpriced hotel wine from the honor bar while alone. And there is the little place in the lobby to buy things at only slightly under honor bar prices, or the gift store with fine and tastefully lacquered beer steins with pictures of the host city on them for a mere $50. For parents, nothing says “I love you, my child” more than a $15 hotel gift store pencil case with the corporate logo on it (unless it is the little bouncy balls some of the booths in the expo give away for free).

But while the rest of the world has decided that free wifi and a complementary breakfast should be part of nearly every American hotel stay costing more than about $60, here in the land of the $250 room (if you are so lucky), there is not so much as complementary mint at the front desk (well, ok, GG sees a Dasani on the desk here with a note claiming its free–but you know what? Can you really trust that its free?). Unless the meeting has arranged free wifi, be ready to drop a sawbuck a day on an internet connection (or plan to frequent local coffee houses that will let you use their wifi for the cost of a cup of joe–sorry, in Seattle now, PUMPKIN SPICE LATTE), or join the ranks of your peers plopped on the edges of corridors in the meeting area like the street people cadging quarters outside the facility while the glom off the free meeting internet).

Heaven help you if you brought a car.  That goes double if you rented it.

Then there is the actual reason you are here: the meeting.  Younger attendees wonder why the graybeards they are trying to meet are so hard to find, and the answer is that they have strategies to avoid the meeting except near their invited talks or when there is an NSF program officer available to strong arm. They’ve learned that a four or five day meeting is enough to send the strongest to a week of recovery in a spa.

With all of that, here are GG’s suggestions for surviving a professional meeting with a minimum of financial and mental damage.

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Monolithic Logic

Two substantial rockfalls at the east end of El Capitan (near where Horsetail Falls sometimes appears) have resulted in one death and two injuries. Frankly with all the climbers and tourists it is kind of surprising that this is limit of the human toll. This corner of the face of El Capitan seems to have had less activity prior to this than some other nearby corners of Yosemite.  Things could be a lot worse: Stock and Uhrhammer (2010) dated the very large rock avalanche from the east face of El Capitan to about 3600 years ago (in red on map below excerpted from Wieczorek et al., 1999), and a couple other younger rockfalls have come off El Capitan in historic time (the orange areas on the map). From the photos out there, GG has guessed at the approximate location of the debris that came down this past week (added to map below; the rockfall source is on an essentially vertical rock face).

YoseRockfallMapYoseRockfallKey

Anyways, the intent here is not to consider the geology of this so much as a controversy that coverage of this event has sparked in some corners, namely, is El Capitan the “largest granite monolith” as termed by some reports?

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