Shameless Self-promotion

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.

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