High Old Times, the Plains Edition
One of the big challenges in tectonics (the study of mountain building, more or less) is figuring out what elevations were in the geologic past. This is highly entertaining when contemplating the High Plains. The reason is that this region sat near sea level for millions of years (prior to about 65 million years ago) before rising up to about 2 km above sea level at the western edge of the plains. This rise was unaccompanied by any surgical faulting or volcanism, which is even more peculiar. Some have argued that the uplift is relatively recent and some that it occurred tens of millions of years ago.
A new paper by Majie Fan and others in Geology seems to suggest that this elevation was acquired in the Eocene (prior to about 35 million years ago), saying in their abstract “…our work suggests that along our transect, the central Rockies and adjacent Great Plains underwent uplift during the late Eocene, and have not undergone any large-magnitude (>~500 m) uplift since that time. ” GG kind of likes this inference, but does this paper really support this conclusion?
The new observations here are of the isotopic ratios of hydrogen in some shards of volcanic glass from sedimentary rocks in Wyoming and western Nebraska. These ratios are thought to have reflected the ratios in rainfall; these in turn reflect the original water source and the amount of rainout before these rainclouds reached this area (when you generate rain, the heavier isotopes rain out first). This in turn is thought to reflect topography as rainout is often driven by storms moving upslope and cooling. And basically what is seen is that the difference in the isotopic ratio between the mountains in Wyoming and the plains in Nebraska hasn’t changed in the past 35 million years. During this time, the amount of heavy hydrogen (deuterium) in both places gradually increased. And so if precipitation patterns haven’t changed, you’d think the relative elevations would be the same.
That last “if” is really quite questionable, which is maybe a discussion for another day (the meteorology behind these variations is a bugaboo common to many modern studies). But even if we accept this, why would this mean that the region reached its modern elevations some 35 million years ago? The logic isn’t entirely clear to GG, but seems to be that the difference in hydrogen isotopes between the mountains and western plains is so high that you need a rain shadow effect like the modern one, and the authors seem to presume that the rain shadow effect scales with the height of the mountains. (The reason they think this happened just before 35 Ma is because of older work by some of these authors arguing that the basins near these mountains were near sea level nearly 50 million years ago).
Does GG buy this? Well, it would be nice to have this settled, but there is work suggesting that the rain shadow effect is not this straightforward. And there is that bugaboo of changing climate through this time, both in a global sense (the Hadley cell might have extended to higher latitude) and locally (the monsoonal effect of high elevation in the western U.S. might have varied with time). So although this work provides a few more data points with which to try to nail the history of the High Plains, it isn’t the final word on this topic.