Return of the Young Sierra
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…
Compare the above figure with the Hammond et al. map with the rates estimated at individual GPS stations:
What stands out is the small number of stations in the Sierra (the dense concentration of points near 38N 119W is the Long Valley caldera). The points with the highest uplift rates are near the west side of the range, most consistent with the elastic unloading hypothesis. If the foothillsare going up at more than 1 mm/yr today, the relatively low elevations in these areas would only require these rates continue for a few hundred thousand years, so we are not in the geologic ballpark the way it seemed from the first map. There are some additional concerns. The processing used to estimate the uplift rates essentially assumes that seasonal rates don’t alias into the long term rates. The duration of time used varies considerably between stations in this image, so if rates have any temporal variation, that is muddled in this image. And that initial “money image” resulted from a number of processing steps (median filters and kriging-like interpolation) that might tend to gloss over problems with individual points. So while it is encouraging that the GPS might be showing up ongoing uplift of the range, this doesn’t feel like the final word.
A second piece of work was presented at the Fall American Geophysical Union meeting by Scott McCoy and colleagues. Here the intent was in part to see how stable the drainage networks in the Sierra are. North of 37°N they find that the drainage divides appear to be unstable, with evidence of stream capture and asymmetry across many divides (this is following approaches described in a Science paper by some of these authors). In contrast, to the south they find that drainage divides appear to be very stable but large knickpoints exist in these drainage basins, indicating a transient response. They tried some synthetic tests to better understand their observations. In one case, they created a tilted planar landscape and then allow rainfall on it to create river systems and see how that evolves. The parameter χ they use should reflect a steady-state elevation of a drainage network as a function of distance upstream from base level. In their model, χ initially varies substantially but over time it eventually collapses into a pretty uniform variations with distance from a river’s mouth. The modern northern Sierra resembles that case over the time period when the drainages are evolving. They thus interpret the northern Sierran drainages as reflecting an ongoing adjustment as rivers cut into the broad plain created in the Miocene and Pliocene by deposition of volcanic, volcaniclastic and fluvial sediments across most of the western Sierra.
The story in the south is a bit different. Another synthetic test takes an already fully evolved stream network and tilts it. In this case the value of χ as a function of distance along the stream changes, with the newer values of χ appearing behind a knickpoint migrating up all the drainages in parallel. This most resembles the southern Sierra case and suggests that drainages in the southern Sierra are mature but have been rejuvenated by uplift.
This is promising and carries things in a different direction than the discussion by Gabet we’ve examined parts of here and here and here. But the work is in an early stage and there are two distinctly different ways of interpreting this. In the first, uplift could be occurring all along the range, but the continued reorganization of the northern rivers hides the migration of a knickpoint. When asked, McCoy seemed to indicate this was possible. In the other, the knickpoints migrating up the southern drainages might reflect a climatic change, which was not one of the synthetic examples presented at the meeting; if this is possible, then Sierran elevations could be old as far as the geomorphology of the drainages is concerned. And of course there are intermediate interpretations where uplift in the south is not matched in the north.
So maybe we are no closer to figuring out the age of they high elevations of the Sierra. But these two lines of inquiry look promising. There is still more cleanup needed on the Sierran GPS points, and the McCoy et al. study probably will need a few more synthetic tests to see if the ambiguities mentioned above can be resolved.