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?
Just what, if any, significance is there to the paleochannels from the Eocene on the west side of the Sierra Nevada? These have been held up as demonstrations of post-Eocene uplift of the range and demoted to insignificant artifacts of a landscape developed on metamorphic rock. Consider these conflicting statements from the abstracts of two recent papers:
Eocene paleochannels show lowest gradients parallel to the range axis, steepest ones perpendicular, and reaches with significant “uphill” gradients that rise in the paleo-downstream direction. Modern Sierran rivers lack this relationship. The azimuth-gradient relationships of paleochannels, especially the uphill gradients, require late Cenozoic tilting and uplift.-Wakabayashi, Geosphere, 2013
and the counterpoint:
The studies supporting recent tilting in the northern Sierra Nevada are inconclusive and rely on observations not unique to tectonic forcing. Indeed, much of the evidence based on the paleogradients of the Tertiary channels is consistent with an early trellis drainage network formed across alternating bands of resistant and weak lithologies. –Gabet, Am. J. Sci., 2014
Now to be transparent, GG has published the view that the drainages do support post-Eocene uplift, but that was then and this is now; given the work done in the past decade, reexamining this is worth some effort. (Hopefully sometime we’ll take a long look at the Gabet paper, which is a more comprehensive attempt to consider the surface geology of the Sierra).
That’s to let you know that GG is an author on this paper, but hopes you might take a look anyways. And yeah, it is kind of a grumpy old man paper, but hopefully justifiably so.
The paper in question is by Molnar, England and Jones and is titled “Mantle dynamics, isostasy, and the support of high terrain.” It is now in press at JGR, meaning that you can read it (if you have access) and cite it even before it is all prettified for official publication. Basically, it is suggesting some improved ways to better understand the physical processes elevating continents. The paper is kind of long and has no shortage of math, so here’s a really brief overview:
First off, we discuss the term “dynamic topography” and note it gets very confusing between papers. This is more or less a plea to be clear with terminology as it does matter: for instance, the force available to drive continental deformation differs quite a bit between thinning mantle lithosphere and elevating a region on upwelling asthenosphere, yet both have been termed “dynamic topography” in the literature.
Second is a discussion of the kinds of gravity anomalies you might expect from various dynamic scenarios. Flow that is separated from any sublithospheric density anomaly will make for a really large free-air (or isostatic) gravity anomaly if it produces any topography (something like 140 mGal/km of subaerial elevation). Flow above a density anomaly well below the lithosphere will also produce a large gravity anomaly, but not nearly as large because the gravity effect of the deep load will tend to offset that of the topography produced (something like 50 mGal/km of subaerial elevation). Free-air anomalies are also created by flexurally supported topography and variations of the density-depth function, but these should be isolatable from the sublithospheric signals (for instance, flexure should be at shorter wavelengths and density-depth variations usually won’t correlate with topography). Free-air anomalies should provide some upper bound on the magnitude of topography generated by flow and density anomalies below the lithosphere.
A third part notes the perils of trying to remove the lithosphere to get at topography generated from greater depths (we have explored that with a couple of specific western US papers here and here). Although the text is discussing the process of generating “residual topography”, the same issue exists if trying to correct the gravity field. Basically, our ability to (1) know crustal seismic structures and (2) interpret them in terms of density is still so poor (especially in very large parts of the globe) that trying to remove these effects is apt to simply map errors into any residual.
This leads to a final part, a few concrete examples using the free-air anomaly and the isostatic anomaly to estimate the upper bound on the contribution from the sublithospheric mantle. This analysis would suggest that sublithospheric loading is only generating up to about 100m-200m of topography in many places where far more has been inferred.
What’s the point? It isn’t to say that studying dynamic topography is pointless, it is to argue for identification of the physical process producing topography and to test that against the free-air or isostatic anomaly that we observe. The contribution of the sub-lithospheric mantle to surface topography is of significance in understanding continental deformation (for instance, GG included a hypothesis that essentially invokes a very special dynamic process to help create the Rockies in a paper a few years back). With more precise definitions and stronger and more robust tests, we can better learn about the peculiar ways the earth deforms.
As a reminder, arguably one of the most promising means of estimating paleoelevation (and one being used a lot) is to measure the isotopic composition of rainfall that has been stored in the rock record (in altered volcanic glasses, in bones and teeth, in clays, in soils, etc.). The idea is that as rain-bearing clouds rise up terrain, they rain on that terrain and the farther up they go, the more rain they have lost. Since heavy isotopes tend to rain out first, the ratio of heavy to light isotopes in water decreases. And if you look at river water or rainfall today, you broadly find that the most depleted water is falling in the highest areas–but it is a noisy record and so even if you were handed a modern water sample you might be hard-pressed to determine its elevation.
For the moment, let’s assume that the measurements of isotopic composition of paleo-rainwater is robust. Can we just use some regression of depletion vs. elevation to get paleoelevation? There are several who have argued no; at its heart, the basic problem is that it is not elevation that you are measuring but the amount of water that has been wrung out of the clouds. What else might control rainfall? These two articles point out two elements that are very challenging in the paleo-realm: air trajectories and rain nucleation. (We’ll leave out a lot of other issues for today).
Perhaps the most impressive aspect of the Sierra Nevada is its eastern front. A bold wall of rock challenges any who would cross the range; the southern part in particular is so daunting that Americans weren’t inclined to try to cross it until well after the Gold Rush was underway. No road crosses the range crest between Devil’s Postpile and Nine Mile Canyon Road to Kennedy Meadows (on the South Fork of the Kern). That eastern front is controlled by a system of normal faults, and as such they define the eastern edge of the Sierra.
For many years in talking about the uplift of the Sierra Nevada, GG has been asked from time to time about whether the Sierran uplift is a response to normal faulting on the east side. And the answer has been essentially no; this adds some height near the fault but isn’t the main player. Now a couple of papers have shown up in the last few years on this, and they proceed from some occasionally complex assumptions to investigate this. Frankly this was more work than was warranted; you can show pretty directly with little work that there is a firm upper bound on how much that faulting could create. What is more, what little is predicted also carries a prediction about the gravity field, one that is very much at odds with observation.