Cordilleran Contradictions, 2018 edition
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….
Brian Yanites tried to bring quantitative geomorphology to the party, trying to use side drainages as a means of isolating contributors to incision like variations in rock type. Applying this to central Idaho tributaries to the Salmon River led to the inference that stranded low-relief uplands are higher and higher to the south, reflecting uplift associated with the passage of the Yellowstone hotspot since c. 10 Ma. Parts of this felt like deja vu as projecting profiles from these upland surfaces was a game played long ago by Francois Matthes in Yosemite–work largely discarded in that area after Clyde Wahrhaftig argued that such projections were internally inconsistent. A more recent attempt using stream-power relations was advanced by Marin Clark and colleagues a few years back and was strongly criticized by Manny Gabet a few years later. Is this latest work so different? Didn’t seem so to GG, but it is quite possible some subtleties went past him.
Jordan Anderson and coauthors reexamined the geology of the paleo-Salt River Canyon in central Arizona. Most of the presentation recapitulated the previously documented reversal of the stream from flowing west-to-east to its modern east-to-west trend. Aside from renewing the observation that topography reversed as normal faulting deformed the Basin and Range, they infer a higher erosion rate in the modern headwaters than in the Basin and Range to the west; they associate the higher rate with an uplift of the Plateau by 300m over the past 3 million years. This is precisely the kind of argument discounted by those who would argue that a changing climate had led to increased incision, so this isn’t apt to move the needle towards young regional uplift.
Lu Zhu (in a talk GG missed due to classroom demands) showed how the drainage systems in Colorado changed in the middle Tertiary from detrital zircons in sediments in southern Colorado. There is probably something here of use in getting at paleoelevations, but it is mainly telling of intra-orogen topography more than the overall elevation of the region.
A fun juxtaposition were a couple of papers on paleoelevation, and while neither directly dealt with the western U.S., these show how difficult paleoclimate proxies can be. The first presentation by Mark Brandon showed a spatially high-resolution climate model that tracks the isotopic evolution of precipitation. He argues that the far lower resolution of GCMs makes them unhelpful in doing paleoaltimetric studies. This approach, though, sacrifices the broader context of a GCM in favor of investigating how precipitation’s isotopes get changed in mountain ranges. He supported his model with data from the Andes.
The next paper (Farnsworth et al.) was presented by Paul Valdes, who, while agreeing that GCMs are best suited for big regions, argued that the GCM approach better captures the physics of precipitation generating mechanisms. He showed synthetic data for different scenarios of Tibet at 45 Ma. Scatter plots of elevation vs. isotropic depletion were pretty scattered, and generating a map of paleoelevation from the synthetic data using a simple d18O-elevation relationship was rather hilariously off the mark. So rather than the systematic relationship of fractionation with elevation found in the previous presentation, this talk showed rather complicated relationships. How much of this was the difference between the simple zonal flow of the Andes used as a control by Brandon vs. the more complex monsoon-driven systems of Tibet remains unclear, but probably underscores the need to have a grasp of the broader climate system before using precipitation-based proxies for topography.
Most directly relevant in the western U.S. was Elizabeth Cassel’s talk earlier in that session; this focused on dD records in volcanic glasses in the Tertiary across the Cordillera; she spent some time justifying these as useful proxies, arguing these systems close in 10,000 year or less–but there is 20-30 ppt variation within Mazama ash [GG’s notes indicate this was real scatter, but don’t recall if this was plotted against modern elevation or water dD values]. Much of this has been previously published or reconfirms earlier estimates from other proxies: they find eastward-decreasing dD ratios across the Sierra, then rather stable values east into Utah. Newer results seem to be very low values found in the Absarokas in Wyoming and evidence of multiple moisture sources in the area of foreland uplifts.
A different perspective was Katie Snell’s talk pointing out the possibility of aliasing of climate signals into various proxy measurements. This has been a big issue with clumped isotope work; in this case, the illustration was the ability of different proxies to recover climatic optima in the early Eocene. Is it possible that something similar is going on with other proxies like volcanic glass? Frankly, it doesn’t seem too likely but it is a question worth pondering.
So along these lines we have another proxy to elevation presentation, this a poster by Lu Zhu et al. with the dD volcanic glass approach on a transect from Texas to the Southern Rockies from 35 Ma to the present. (Yes, same Zhu as above). In some ways this is an exact mirror image of what Cassel’s work has seen: in essence, the values look a lot like modern values. The abstract claims a change in the low-altitude values in Texas, but from the plots displayed it looked like all values shifted together over time by 10-20ppt, out of a difference of ~80 ppt). The plots on the poster didn’t show any decrease in dD above about 2000m elevation. Much as the dD work from Cassel showed, this tended to suggest old high elevations.
A more classical analysis was that presented by Lon Abbott and others, which looked at the poorly studied West Elk Mountains. This area experienced considerable volcanism like the San Juans to the south, but volcanics have been stripped from the West Elks, exposing intrusives as young as 12 Ma. Apatite U-Th/He ages confirmed an episode of fairly rapid erosion in the 13-7 Ma time frame, which is pretty awkward for everybody–a bit on the old side for the young uplift crowd, and pretty late if you want to do this with a changing climate (like Eocene-Oligocene). Most probably this has a lot to do with the focused igneous activity, and it further highlights the variability of rock uplift in the Rockies, though the exact connection to surface uplift remains vague.
So nothing really solved here, which is hardly a shock. In general, patterns of varying depletion of heavy isotopes of water is reconfirmed while the significance of such variations remains in some doubt. Local focused erosion producing certain rock uplift and plausible surface uplift is found in a couple of places where magmatism was pretty pronounced.
Oddly, something covered even more at this meeting were flat slabs, which maybe we’ll review later….