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…
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?
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.
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.
Last fall GG’s Western U.S. Tectonics class took on trying to evaluate the status quo challenging hypothesis of Robert Hildebrand that the western part of the U.S. (west of central Utah, roughly) was a separate ribbon continent, Rubia, prior to colliding with North America in the early Tertiary, creating the Rocky Mountains. (That status quo holds that the far west was gradually assembled from the latest Paleozoic going on to the Miocene, with an arc being present on the edge of North America from the Permian to the late Cretaceous and again in much of the Tertiary). As Hildebrand’s argument was wide ranging and published as two lengthy GSA Special Papers (457 and 495), it isn’t a casual affair to consider the question of whether Hildebrand has caught western geologists in a huge misinterpretation or not. Many workers, content with their personal knowledge, have not peered into this abyss, so the class set out to take a swing at this. Basically, has Hildebrand identified observations inconsistent with our current interpretation of the geology? And are observables more consistent with Rubia than the standard model? A “yes” to the first might show that Hildebrand has put his finger on a problem even if the answer to the second is a “no”.
The class broke the hypothesis into these elements:
- North America was subducted under Rubia in the late Cretaceous
- Mesozoic and late Paleozoic magmatism, widespread in Rubia, never extended into “true” North America
- The magmatic volumes at the end of the Cretaceous in the western arcs are far too voluminous to have been produced by subduction of oceanic lithosphere
- Much of the classic late Precambrian – Paleozoic Cordilleran miogeocline is exotic to North America (i.e., is Rubia)
- Deformation from accretionary events is limited to Rubia.
- Mesozoic thin-skinned thrusts contain too much shortening to be limited to North America and are far greater than found in backarcs of typical continental arcs
- Magmatism and uplift in the latest Cretaceous and early Tertiary was produced by the oceanic part of the subjected North American plate falling off.
You can go and read the individual assessments made by class members to particular parts of this analysis, but a summary is below.
Traditionally, the Laramide Orogeny starts around 75 million years ago. Probably most geoscientists would agree with the overall analysis of Dickinson et al. (1988), which is mainly based on sedimentary rocks preserved from that time. So their criteria were that marine sedimentation (diagonal hatch) had ended prior to the Laramide, individual basins shifted from sharing facies with adjacent areas (black square) to having distinctly thicker deposits (circle) and coarse clastic detritus derived from nearby uplifts (black triangle) as the Laramide started:
It would be hard to argue that the Laramide Orogeny started later than the kinds of dates that Dickinson et al. proposed–but could it be earlier? If you had shallow sea floor covered in muds and parts started to rise up, might the muds simply get entrained in the existing current systems and be scoured down, creating a lacuna that, later on, would erased by even deeper erosion? In other words, is it possible that there was early deformation that wasn’t vigorous enough to overcome the broad subsidence of the region and so failed to produce positive topography? And if so, would subsurface loads have started to create local depocenters that perhaps have escaped recognition?