“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.
So in the previous two installments, we reviewed ideas for how the High Plains got so high and some of the observations out there that bear on this question. Beyond satisfying some curiosity, what does this do for earth science? Why pay money to do this?
Let’s consider three outcomes: that the High Plains gained their elevation by the end of the Laramide orogeny (say, 40 Ma), that they gained their elevation after the deposition of the Ogallala Group (say about 5 Ma), and that they were high, went down, and rose again. Read More…
So earlier we saw that there are a number of different ideas for how the High Plains got high, but what matters are the observations. The oldest of these–the classical reason for saying the Rockies are young–is that the Miocene Ogallala Group/Formation has been deeply incised and removed entirely from large areas like the Denver basin. The classical interpretation of a switch from deposition to erosion is uplift and tilting, but another possibility is that the changes in climate going into the Pleistocene changed the ability of rivers to incise. This has opened the door to multiple lines of evidence. Below is a stab at trying to get a handle on some of this literature (happy to hear what GG missed in the comments). [Note: this is subject to updating]. Read More…
No, not high in that sense…high like “Mile High City”. This still is a problem GG is interested in and so for grins let’s quickly review the main ideas GG has seen with their pros and cons. The candidates are thickening the crust mechanically or by piling on sediment, thinning the mantle lithosphere, dynamic topography, hydrating the mantle or the crust, depleting the lithosphere, and emplacing depleted lithosphere. Whew! GG’s hot takes on these below the fold… Read More…
Spent many hours in November sitting in on sessions and perusing posters at the Geological Society of America annual meeting; one goal was to see what’s up with the evolution of elevation of the U.S. Cordillera.
First a quick recap. There are two camps, more or less, on each side of the Cordillera. The old mountains camp on both sides points mainly to oxygen and hydrogen isotope variations in proxies for precipitation. There are also attempts to retrodeform the lithosphere resulting in thick crust and high elevations. The dominant counterargument is that the paleometeorology used to interpret the isotopic values is flawed. On the young mountain side, classical geologic observations are invoked, including apparent tilting of river channels and the recent incision events in many places. The counterargument to this is that the appearance of a tilted channel may be biased by the depositional environment and that changes in climate can drive incision as easily as uplift. In between in some ways are geophysical observations of the lithosphere; recent changes in the lithosphere seem likely in much of the region, supporting younger mountains, but seem older east of the Southern Rockies.
Well, a meeting in Indianapolis isn’t one to bring out all the western geologists (next year’s meeting in Phoenix is a whole different matter), but a couple of things popped up. Did anything look to change the landscape, either by opening up new vistas or overturning old results? Not that GG discerned. Below are some notes probably only of interest to the most interested….
GG has been piddling along though the Sierra (ostensibly to give a campfire talk in Mineral King) and in doing so stared a bit longer at a recent paper on the age of a pediment in the Sierran foothills by Sousa et al. in Geosphere in 2017. In a way this is a callback to concepts from far back in the geologic literature, namely the significance of an “Eocene erosion surface.”
Here, to be brief, low-temperature thermochronology from a low-elevation pediment in the western foothills of the Sierra yields very old ages–in fact, overlapping with the emplacement of plutons in the Sierran crest [this was not a unique observation; Cecil et al., 2006, had a pretty old point in their collection]. Sousa and coauthors model these data and get a cooling to surface conditions by about 40 Ma. Because these pediments abut noticeable topography, this means there was at least that much local relief in the ancient Sierra. While the pediments had been noticed by others, many suspected a far more recent age.
In some ways, this is old news. The Eocene sediments in the northern Sierra have long made clear the presence of significant local relief, and many workers had inferred that such relief was probably higher in the southern Sierra (e.g., Wakabayashi and Sawyer, 2001). But the southern Sierra lacked the Eocene sediments necessary to know what the Eocene landscape might have looked like, so this paper opens up a new window for us.
Where does this lead us? Kind of down a rabbit hole only to come up with no strong and useful statement–though perhaps future work could nail things down. This is more a personal attempt to try and grasp what is going on, so profound errors might exist and insights are few. So, proceed at your own risk….
It seems a bit odd, but yesterday had, on average, the coldest high temperature here in Boulder of any day of the year. Coming all of 11 days after the winter solstice, this seemed rather quick to GG. After all, shouldn’t there be more thermal inertia in the system? This got GG to wondering about these things, which led to an inability to locate this information trivially. So a few quick numbers lifted from Intellicast’s archive, which is clearly very smoothed…(except for Boulder, which is from NOAA’s ESRL page–Denver is from Intellicast for comparison)
|Place||Date Lowest High||Date Highest High|
|Boulder, CO (40N)||1 January (41)||17 July* (87)|
|Denver, CO (39.7N)||5 January (46)||21 July (89)|
|New York City (40.7N)||19 January (36)||24 July (83)|
|St. Louis, MO (38.6N)||12 January (37)||22 July (90)|
|Los Angeles (34N)||7 January (68)||8 August (85)|
|San Francisco (37.8N)||2 January (57)||28 September (72)|
|Phoenix, AZ (33.5N)||29 December (66)||12 July (107)|
(*-but several almost as hot days are later in the month)
There is in fact quite a range. Phoenix wins as the place which comes closest to echoing sunlight, telling us that part of the equation is humidity. Boulder and Denver are a close second, which isn’t too surprising given that the altitude limits thermal blankets and the absolute humidity is pretty low. But some of the rest are a bit surprising…
Well, time to catch up on the evolution of the Sierra Nevada. Although a large collection of paleoaltimetry papers has bolstered a case for the elevations in the Sierra having been created by the Eocene (most based on Rayleigh distillation of precipitation), a couple of other recent works, one geodetic and the other geomorphic, seem to indicate that Sierran topography has grown over the last few million years.
First up is an update on vertical GPS velocities in California and Nevada by Hammond et al. in the Journal of Geophysical Research. They find “…the Sierra Nevada is the most rapid and extensive uplift feature in the western United States, rising up to 2 mm/yr along most of the range….Uplift patterns are consistent with groundwater extraction and concomitant elastic bedrock uplift, plus slower background tectonic uplift.” This in some ways is trimming the sails a bit on the earlier Amos et al. paper in Nature; as we previously discussed this wasn’t entirely unexpected. Their money figure would be this:
The red blob in most of eastern California is the Sierra Nevada. For most of the range, the pink colors correspond to uplift rates of 0.5-1.0 mm/yr. The presence of the pink/red colors in the central to northern Sierra, where there are no blue colors to the west, would indicate uplift is not being caused by groundwater withdrawal to the west (which is the cause of most of the dark blue south of 38°N and was the focus of the Amos et al. paper). Given the these rates would produce the modern mean elevation of the Sierra in under 6 million years, this would seem to strongly support the young Sierran story and be broadly consistent with the geologic story of a young uplift caused by removal of a dense root.
But, hmm, let’s look more closely…
Sometimes you can say something that proves to be true but illustrate it poorly enough that readers don’t believe you.
Case in point: effect of basement lithologies on the grade of rivers (in this case, for how we interpret paleoriver systems). Manny Gabet (among others) has suggested that this causes the azimuthal variation in grade of Eocene paleochannels, and he illustrated this with the example shown below:
Now one thing here is the distance axis on the three plots: is it measured along the channel, or airline? One might think along the channel. But in any event, look at the distance from B to C on the map and then on the plots. Airline it eyeballs to about 6 km on the map, but only 4 km on the plots. It is even worse if you measure along the river. So this quick eyeball reality check would make many readers pause and question the conclusion here.
So GG here has carried this slightly further, Read More…
A recent paper by Mix et al. seeks to further bolster the story about the Sierra Nevada having already reached essentially modern elevations back in the Eocene. Examining the paper made GG want to play with a few things, and in the end the feeling here is that the new data (oxygen isotopes) don’t really help the story. However reconsidering the whole of this dataset brings up questions about just what is being measured.
OK, first off, the paper appears to have two main goals: first, to show that temperatures were never so high as to have disturbed the ∂D (deuterium-hydrogen) measurements originally put forward by Mulch et al., and second to show that the oxygen isotope ratios support the original inference of near-modern elevations of this region.
The temperature results, which originate in differing fractionation coefficients for hydrogen and oxygen when making kaolinite, produce a very curious pattern:
(Note that “upriver” is distance from shoreline in the original paper, which turns out to be measured along paleorivers). Basically, if you take the temperatures at face value, it would seem that temperatures increased as you went upstream–that higher areas were hotter. Perhaps as curious, the spread of temperatures at a single site seems to be quite large. Although these results were used to argue that the hydrogen results had not been contaminated, the authors declined to interpret these temperatures as reflecting the local climate for several reasons, the most interesting being “uncertainty in the kaolinite- water fractionation at low temperatures (see Sheppard and Gilg, 1996) is likely greater than the resolution necessary for temperature-based paleoaltimetry reconstructions, at least across this modest climatic gradient.” One might take that to mean that the temperatures have no meaning at all, yet the mean temperature of all these is taken to be a significant piece of evidence supporting the Eocene origin of these isotopic patterns. This just feels like a bit of situational ethics–the temperatures are meaningful when they support your hypothesis (no problems with ∂D, matches expected Eocene temperatures) and not when they don’t (higher elevations seem to be warmer).
In playing with plotting, made this plot, the significance of which (if any) remains unclear to GG, being a grumpy geophysicist and not a grumpy geochemist:
Again, at least at face value, this is backwards: more depleted (more negative) ∂D values should be colder; if the temperature estimates were wholly random, you might not expect the rather noticeable correlation. But maybe this makes sense, just seemed strange to GG.
OK, but what about supporting the isotopic gradient story?