There are things that puzzle GG and this is a partial list that will be updated from time to time.
- When did the High Plains become high?
- Why did the High Plains become high?
- What exactly did happen in the Mojave Desert c. 75 Ma?
- Why did the Sierra Nevada arc shut down?
- Related, why did it shut down from north to south?
- If the northern Sierra only went up prior to the Eocene, why are modern rivers cutting well below those deposits?
- If the northern Sierra have gone up significantly since the early Miocene, why do climate proxies disagree?
- If the northern Sierra only went up prior to the Eocene, then how did one form of support (crustal root?) get replaced by another (buoyant mantle)?
- What caused the Ancestral Rockies to rise up where they did?
- Why did the large thrust on the south side of the Uncompahgre Plateau fail to reactivate in the Laramide?
- How much of the Laramide foreland (basement-cored) uplifts reflect some different source of stress and how much do they reflect stresses from the Sevier belt driving shortening?
- It seems likely there was serious slip partitioning in the Sevier; where is it?
- There are some LANFs that seem to have really formed in the brittle crust at low angles. How?
Five years ago GG pointed to a paper threatening the cherished assumption in petrology that the pressure recorded by minerals is equal to the overburden pressure. GG has never been comfortable with that assumption, and missed (until now) a paper that is far more comprehensive in its impact. And frankly, it is so blazingly obvious that GG is embarrassed that this has been under the radar for so long. The paper is Yamato, P., and Brun, J.P., 2017, Metamorphic record of catastrophic pressure drops in subduction zones: Nature Geoscience, v. 10, p. 46–50, doi: 10.1038/ngeo2852. The killer money figure is this:
All the dots in the top panel are peak pressures reported in the literature versus the subsequent nearly isothermal pressure drop also reported, where the circled points actually have that second pressure separately measured. The first thing is that this linear array makes no sense: it would almost require that rocks go down on a spring: the farther down they go, the more rapidly they bounce back up. You’d think some rocks would just stay down there and heat up and that the subsequent rise could well be independent of the journey down. The second part is that this linear array makes perfect sense if you are looking at the difference between the pressure when the rocks are on horizontal compression versus horizontal extension, which is what the bottom panel is illustrating. In essence, if the vertical normal stress is constant (σv), then at failure in compression it would be σ3 but in extension σ1. With pressure being an average of the stresses, you then get a massive pressure drop, greater if the rock is in the brittle regime (∆PFRIC) than the ductile regime (∆PDUC). The authors estimate these curves as shown by the solid lines in the top panel and it sure seems like the simplest explanation for these massive decompression events is simply that the stress field changed.
How this changes a tectonic interpretation of the geobarometry is illustrated in their Figure 4:
The black line in the lefthand graph is what has typically been interpreted to date; in the righthand graph they correct for compressional and extensional stresses. Instead of a rock blasting its way to the surface and then stopping, in the right hand panel the rock goes down and then comes back up with the vertical axis now being the lithostatic pressure.
Now this isn’t without a pile of caveats and potential flaws. First, at these depths there is no reason for the principal normal stresses to be aligned with the Earth’s surface, so this is a worst case scenario. Second, it is a bit of a surprise that the points going all the way to over 4 GPa, seem to be in the brittle field. GG suspects that many of these rocks exhibit ductile features that would seem to contradict the inference of being in the brittle field. Third, a change in the stress field of this magnitude is pretty daunting and poses a challenge to the geodynamics community: how can stresses change that much? But if the rocks are sitting at roughly the same depth and temperature for a significant time, this might not be anything like the problem of near isothermal decompression, which does have some severe time constraints. But regardless of the challenges, frankly this makes way more sense than rocks just springing back up to some level and sitting there.
There is a follow-up paper that more fully develops some formalisms for investigating this effect in general: Bauville, A., and Yamato, P., 2021, Pressure-to-Depth Conversion Models for Metamorphic Rocks: Derivation and Applications: Geochemistry, Geophysics, Geosystems, v. 22, article e2020GC009280, doi: 10.1029/2020GC009280.
Now this paper dealt with high pressure-low temperature rocks typically associated with subduction zones, and this strongly suggests that inferences of continental rocks going to 100 km depths are mistaken. But there are a whole bunch of rather similar looking curves that are not quite as dramatic but similarly difficult to understand without this mechanism. GG is referring to the widespread evidence for massive decompression of lower crustal rocks seen the Sevier hinterland of Nevada, Utah and southeastern California. (For instance, can work outward from the overview of Hodges and Walker, GSA Bull., 1992). This has long been a major mystery as shallow level extensional structures are largely missing. Many workers have noted that Miocene and younger Basin and Range extension has led to very deep basins being created, but equivalent Cretaceous and early Tertiary sedimentary piles are rare.
This brings us to a second paper that considers this problem in the metamorphic rocks of eastern Nevada: Zuza, A.V., Thorman, C.H., Henry, C.D., Levy, D.A., Dee, S., Long, S.P., Sandberg, C.A., and Soignard, E., 2020, Pulsed Mesozoic Deformation in the Cordilleran Hinterland and Evolution of the Nevadaplano: Insights from the Pequop Mountains, NE Nevada: Lithosphere, v. 2020, Article ID 8850336, doi: 10.2113/2020/8850336. On the basis of geologic mapping and new geochronological data, these workers conclude that both Cretaceous thickening and decompression are less significant in this area, possibly indicating that the geobarometry in the nearby Ruby and East Humboldt mountains has been affected by overpressure issues like that considered above. And when you toss in structural evidence in other core complexes for changes between shortening and extension (e.g., Wells, M.L., Hoisch, T.D., Cruz-Uribe, A.M., and Vervoort, J.D., 2012, Geodynamics of synconvergent extension and tectonic mode switching: Constraints from the Sevier-Laramide orogen: Tectonics, v. 31, TC1002, doi: 10.1029/2011TC002913) it seems that much of the geobarometry in the western U.S. is due for reexamination.
Overall, this feels like a liberation of sorts. The decompression problems had produced some imaginative solutions that might no longer be necessary (e.g., Wernicke, B.P., and Getty, S.R., 1997, Intracrustal subduction and gravity currents in the deep crust: Sm-Nd, Ar-Ar, and thermobarometric constraints from the Skagit Gneiss Complex, Washington: Geological Society of America Bulletin, v. 109, p. 1149–1166.). The next few years might see wholesale revision of what was going on in the Sevier hinterland.
A discussion with colleagues brought up an interesting question: What examples do we have from the geological record of a broad elevated ramp like we have today in the High Plains of the United States? The answer is somewhat unclear as it depends on the cause of the uplift.
Some proposals mean that the lithosphere is permanently changed. In this case, assuming isostasy over the long haul, areas about 1500m above sea level will eventually end up near sea level–but to do that you have to strip off about 7 times that 1500m, meaning that about 10 km of the upper crust erodes off. That would take away pretty much all the sedimentary rock in the region and eat into the crystalline rock underneath. In the end, the area might resemble the Canadian Shield, a vast expanse of middle crust sitting at the surface. So could places with exposures of such perviously deep rocks be the products of whatever created the High Plains?
Alternatively, the modern topography is ephemeral, perhaps a product of dynamic topography or a thermal rejuvenation of the continental interior. In this case, it is a bit of a race between erosion and subsidence. The faster the area subsides, the greater the record that is preserved for the distant future. Our modern uplift would be an unconformity between the material still remaining and the overriding sediments. One wonders if some of the unconformities out there might reflect a similarly broad and extensive uplift.
Maybe High Plains-type uplifts are somewhat more common in geologic history than we would guess. It could be one of those things that you have to believe before you can see it…
“What goes up, must come down”
Does this apply to the rocky world beneath our feet? For most of the time we’ve had geology as a field of inquiry, the main route envisioned for elevated areas to descend back to near sea level was erosion. So something would create mountains and then erosion would level them down. What went up would, eventually, come back down.
But that simple conceptualization is coming up against observations suggesting that this can’t quite be the whole story. Much work (mostly from low-temperature thermochronology) has shown that, for instance, the Canadian Shield (which is most of eastern Canada) has been covered by sediments and then stripped of sediments, probably a few times. All without making mountains in the region. Now some of this could reflect oceans rising and falling, but it seems like you do need more.
The current favorite in the community is dynamic topography, which in this case is mainly subsidence as a dense oceanic slab is somewhere under the continent followed by recovery when it is gone. It has the advantage that over long times (say, a few hundred million years) it should average out to zero, which is about what the shield rocks are telling us. And it should have the really long wavelengths appropriate for wide continental interiors.
Are there alternatives? Well, possibly, though they aren’t in the literature to any degree and so are untested. One possibility is that there is a continental equivalent to the flattening of the age-depth curve seen in the oceans. This flattening (meaning that very old ocean floor isn’t as deep as simple cooling models would predict) has been attributed to the lithosphere thickening enough to become unstable, so the bottom part drips off, raising the sea floor and starting a new round of subsidence. While the bulk of continental lithosphere appears to be stabilized by being poor in iron, the bottom might well be just as unstable as in the oceans. So maybe the continents rise and fall as the bottom falls off from time to time. But would this be in sync across a continent? Which brings up the question of just how synchronous are these subsidence and emergence episodes? If they are highly diachronous, that could be a problem for a dynamic origin while consistent with a deblobbing one.
An even more remote possibility is change in the crust itself over time. GG and colleagues have speculated that the High Plains have risen up as water interacted with the lower crust and removed dense garnet. Pair this with Karen Fischer’s proposal that garnet slowly grows into the lower crust with time and you might have a see-saw of garnet in, then garnet out. This might be temporally asymmetric, with fairly rapid uplifts compared with the far slower pace of the reestablishment of garnet. Do we have the resolution to test for that?
This will be interesting to watch simply because we’ve ignored this part of the continents for a long time. At the moment, the community focus solely on dynamic topography is kind of making for self-fulfilling prophecies (can you tune your convection model to reproduce the uplift/erosion/subsidence/deposition signal which is, itself, usually pretty vague). So bringing some of the alternatives in out of the cold to compete with dynamic topography would seem to be a good way of focusing effort on the observations that will most distinguish between mechanisms.
One of the most popular explanations for the High Plains is that they were dragged upward by a buoyant body, probably in the upper mantle under the Rio Grande Rift. This is arguably the only late Miocene to Pliocene event one could plausibly associate with post-Ogallala Formation tilting. GG has tended to be dismissive of this but hasn’t been through the math. Now there must be a simple analysis somewhere in the literature, but GG isn’t seeing it, so let’s make a simple model and see what it takes to make it work. We’ll assume a north-south trending horizontal cylinder with some density contrast under an elastic plate represents the source of uplift (although many folks like a “broken” plate, the physics of such a boundary are inappropriate here). We’ll place the cylinder at a depth z and calculate the uplift and the gravity anomaly from this body. We’ll tweak these until we can fit the observations.
Now we have a little difficulty in that the modern topography is due to more than just the Rift: the sub-Ogallala unconformity reveals rather clearly that there were east-flowing streams when deposition began, meaning that topography back then was tilted to the east, though that potentially was very close in time to deposition. So that topography was presumably compensated by some mechanism that might be well distributed (e.g., variation in crustal thickness). Since the free-air anomaly across the Plains is near 0, the Bouguer anomaly for local compensation of topography should be 0.112 mGal/meter. We’ll just add that to our theoretical models as needed.
The problem is that we don’t know how much topography we want to ascribe to the late Cenozoic Rift: one extreme view (seemingly that of Eaton, 1986, 1987, 2008) is that things were pretty flat in prior to the Rift on an east-west profile, with major rivers going more or less directly to the coast to the south-southeast; another that there was some gradient, though much lower than today (e.g., McMillan et al., 2002). Let’s tackle both and see what we get. In both cases we will focus on the topography east from about 105°W and we’ll place the cylinder at 106°W, under the axis of the Rift.
So Howard Lee over at Ars Technica took a swing at how our understanding of global tectonics has been changing over the past 40 or 50 years and wrote a lengthy article on it. It is full of quotes and assertions that really don’t hang together very well, making a certain geophysicist kind of grumpy. It doesn’t seem that any of the scientists quoted were really saying anything wrong, but the assembly in the article, which doesn’t seem to recognize the discrepancies nor fully master the techniques being used, can lead to a sense of “WTF?”
One of the strange things about the 85 Ma trainwreck (which we have discussed components of here and here and here and here) is the central place that the Mojave Desert seems to play. For the most part we might look to the 2003 paper by Jason Saleeby for elevating the Mojave to star status, tapping it as the entry point of an oceanic plateau (specifically the hypothesized conjugate to the Shatsky Rise now in the northwest Pacific) that he proposed as the main actor in driving the Laramide Orogeny.
So what is really peculiar about the Mojave c. 75-85 Ma? After working through parts of the literature and getting some education at GSA this past week, GG would suggest these are possible candidates:
- Emplacement of subducted sediments against the middle/lower crust after experiencing greenschist grade metamorphism.
- Extension that permitted this juxtaposition to rise to the surface, either shortly after (as in the Salinian Schist) or much later (as the Orocopia Schist)
- An extraordinarily unusual pattern of late Cretaceous magmatism (83-72 Ma) including metaluminous plutons that lack the temporal eastward shift seen elsewhere
There are two pertinent questions for these: (1) how localized are these things really, and (2) do these point towards an impacting plateau and a broader Laramide implication?
In previous entries, we’ve examined the emplacement of oceanic/forearc affinity (POR) schists and the igneous activity from a similar timeframe. Here we will consider what was deforming when and how. There are four main pieces to this puzzle: the termination of ongoing thrusting of the Sevier, Eastern Sierra belts, and Maria belts, the emergence of thrusting in southeastern Arizona and New Mexico, the appearance of extensional faulting, and the beginnings of Laramide shortening in the Colorado Plateau and Southern Rockies.
The outline version is that thrusting in the north-south trending Sevier foreland fold-and-thrust belt shutdown by about 90 Ma in southern Nevada but continued for another 30+ million years farther north. Northwest-trending retroarc thrusts probably continued to be active in southeastern California until 80 Ma and possibly 75 Ma. Rock uplift and extensional shear zones between ~75 and ~65 Ma in several localities may reflect extension of the crust in the Sevier hinterland, but some kind of intra-continental convection is hard to rule out (e.g., Wernicke and Getty, 1997). Closer to the coast, right-lateral strike-slip deformation in the dying Sierran arc reflects some obliquity to convergence at the plate margin. As time passes, Laramide-style basement-cored uplifts begin to emerge, perhaps including structures very close to the Sevier thrust front in the Kingman arch and associated uplift of the southwestern Colorado Plateau. Thrusting appears to have accompanied magmatism in expanding eastward across southern Arizona.
There is a lot here and yet GG is confident he’s missed some important papers–feel free to point some out in the comments.
Update 7/14/19. Things are steadily quieting down in this area, though there are still a lot of small (M<2.5) quakes just west of the rhyolite domes. This spot and the area near Little Cactus Flat to the north remain the most active areas outside of the original ruptures.
Update 7/11/19. While the number of quakes in this area is declining, there was a M4.3 that also had a large non-double couple mechanism–according to Caltech. The USGS-NEIC also estimated a solution and got something much more like regular fault slip. Which indicates that getting mechanisms for very shallow M4s can be tweaky. While more action is now farther north, those events look more fault like–though those mechanisms are also from NEIC, so could be NEIC’s procedures tend towards double-couple solutions more than CIT’s. And as an aside, it is a bit surprising how little activity has been at Mammoth–it is an area that has had seismicity triggered by surface waves in the past, but has remained fairly quiet this go round.
Original post: One thing GG has kind of been looking for is whether the M7.1 Ridgecrest event is triggering things near the Coso volcanic field. And it seems there is something worth being concerned about going on.
Seismicity in this area is traditionally shallow, meaning above 5 km depth (Monastero et al., 2005). The tight cluster of orange dots include 2 M4+ earthquakes. This area is at the west edge of a seismic discontinuity at about 5 km depth inferred to represent the top of a magma chamber (Wilson et al., 2003). While there has certainly been seismicity in this region before, given the proximity to fairly recent volcanic activity, one has to wonder if there is magma on the move. Supporting that are the focal mechanisms for the two M4 earthquakes, both of which have substantial non-double couple components (indeed, the mechanism for one looks very much like a diking event). Given that all these events are being located in the top 2 km (probably relative to sea level, so top 3 km of crust), this could get pretty interesting pretty fast.
As background, the central core of the Coso volcanic field are silica-rich rhyolites that appear as blister-like bodies in the image above. Surrounding this core area that overlies the seismically inferred magma body are basaltic eruptions (like Red Cone, in lower left corner). The troubling seismicity is directly on the road into the geothermal area from Coso Junction to the west.
An overview of the M7.1 with the first InSAR image of the 7.1 rupture is at Temblor.com. This also discusses seismicity in this area, but with less consideration of volcanic activity.
All the attention on the Ridgecrest earthquakes has returned the Eastern California Shear Zone and its rather more obscure relative, the Walker Lane. Among the news articles out there is a pointer back to an article on the notion that the plate boundary will shift in to these fault systems sometime in the geologic future. This then results in western Nevada eventually facing an ocean–or, more extravagantly, Salt Lake City as a coastal town. So let’s clarify our terms, understand when and why these faults have emerged, and then set out to consider what it takes to move a chunk of continent onto an oceanic plate.
First off, there are two named parts to any potential new plate boundary (well, actually, they are already taking up about a quarter of the plate motion): the Eastern California Shear Zone and the Walker Lane. The Eastern California Shear Zone is a well-defined feature in the Mojave Desert south of the Garlock fault and named recently in 1990 by Dokka and Travis. The Walker Lane, in contrast, has had multiple incarnations: it was originally defined by Locke et al. (1940) on the basis of more confused topography along a swath of ground running from Las Vegas through the Walker Lake area to north of Reno. At times it was thought to be as old as Jurassic. John Stewart redefined it in a 1988 book chapter, and subsequent workers have now generally drawn the Walker Lane as kind of hugging the east side of the Sierra out about 150-200 km into the Basin and Range, removing the Las Vegas Valley Shear Zone from being within the Walker Lane and, generally, dating the strike-slip motion within the last 9 million years, with ~4 Ma seemingly a good guess (Andrew and Walker, 2009).
The northern edge is more of a mystery and the names for it more inconsistent. The Central Nevada Seismic Zone takes off from the Walker Lane to the northeast as a more dominantly normal fault system. Strike-slip faulting cuts into the Sierra on a Northern California Fault Zone (Wesnousky, 2005) and might connect to the coast (e.g., Unruh et al., 2003) or towards the Cascades arc (e.g., Waldien et al., 2019), and some have carried deformation northward through central and eastern Oregon and Washington (e.g., Pezzopane and Weldon, 1993). If we really are making a new transform boundary, its northern extent sure isn’t obvious.
So why is strike-slip faulting a relatively late arrival in this area where deformation extends back 15 or 25 million years?