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The Sierra See-Saw

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.

How did this differ from the Amos et al. work a few years earlier that assigned all that uplift in the Sierra to unloading of the Central Valley?   Read More…

My Science Crimes

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).

Kaikoura A Year Later

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.

KaikouraFaults-Kaiser

Faults ruptures in the Kaikoura earthquake, from Kaiser et al, SRL, 2017.

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…

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?

Read More…

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.

A (nearly) forgotten constraint

Over the past decade or so, a fairly common event has been the publication of a paper taking on a piece of evidence for large magnitude extension in the Basin and Range.  For the most part, this has been going after the various individual constraints used to make Wernicke et al.’s (1988) estimate of 247 ± 56 km of WNW-oriented extension. For instance, there are papers attacking the displacement on (or existence of) the Mormon Peak detachment, on the reconstructions of the fold-and-thrust belt across the Death Valley region, and on the age and amount of extension across Panamint Valley. The net result some would infer is that extension in the Basin and Range here was actually a lot less than 247 km (yes, GG has been told “a lot less”).

Rather than wade into these multiple controversies, GG would prefer to step back and ask, are there other ways of coming at this? There are a couple of approaches.  For instance, one can try to improve the somewhat circular logic of Coney and Harms (1984) and try to use the modern and estimates of the pre-extensional thickness of the crust to get at total extension.  A problem is that the crust has been an open system and constraining the amount of magmatic additions limits this approach.

The other approach comes from a rather unexpected quarter: plate tectonics.  Or more precisely, plate tectonic reconstructions, and is has maybe been overshadowed by these other arguments. It is rather clever, but to see its power, we have to take a moment to understand what is going on.

Read More…