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Waning buoyancy in the crustal roots of old mountains


When mountains form through the collision of lithospheric plates, uplift of the Earth's surface is accompanied by thickening of the crust, and the buoyancy of these deep crustal roots (relative to the surrounding mantle) is thought to contribute to the support of mountain topography. Once active tectonism ceases, continuing erosion will progressively wear away surface relief. Here I provide new constraints on how crustal roots respond to erosional unloading over very long timescales. In old collisional mountain belts, ratios of surface relief to the thickness of the underlying crustal root are observed to be smaller than in young mountains. On the basis of gravity data, this trend is best explained by a decrease in the buoyancy of the crustal root with greater age since the most recent mountain-building episode—which is consistent with metamorphic reactions1,2 produced by long-term cooling. An approximate balance between mountain and root mass anomalies suggests that the continental lithosphere remains weak enough to permit exhumation of crustal roots in response to surface erosion for hundreds of millions of years. The amount of such uplift, however, appears to be significantly reduced by progressive loss of root buoyancy.

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Figure 1: Comparison of surface topography to crustal root thickness, crustal root buoyancy, and crustal root temperature for young and old collisional mountain belts.
Figure 2: Schematic view of two hypotheses of how R ( = h/m) may decrease from young (upper) to old (lower) orogens.
Figure 3: Observed topography and observed and predicted gravity profiles across four mountain belts of increasing thermotectonic age.


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I thank S. Zaranek for the finite difference cooling calculations, S. Grand for the global shear-wave velocity model, G. Abers for his gravity code, and D. Forsyth, D. Scheirer and Y. Liang for discussions. This research was supported by the NSF Geophysics Program.

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Correspondence to Karen M. Fischer.

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Fischer, K. Waning buoyancy in the crustal roots of old mountains. Nature 417, 933–936 (2002).

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