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Crustal Roots of Old Mountains

Waning buoyancy in the crustal roots of old mountains: When mountains form by 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. Seismic constraints on the thickness of crustal roots, combined with gravity data, suggest that root buoyancy systematically decreases with time.

Once active tectonism ceases, continuing erosion will progressively wear away surface relief. This study examines how crustal roots respond to erosional unloading over very long timescales. In old collisional mountain belts, surface relief relative to the magnitude of the underlying crustal root is observed to be smaller than in young mountains. Based on gravity data, this trend is best explained by a decrease in the buoyancy of the crustal root with greater thermo-tectonic age. Such an increase in crustal root density is consistent with metamorphic reactions produced by longterm cooling. Observed average root densities are in general comparable to the densities required to isostatically compensate surface relief, suggesting that the continental lithosphere remains weak enough to permit exhumation of crustal roots in response to erosion of surface topography for hundreds of millions of years. However, the amount of such uplift appears to be significantly reduced by progressive loss of root buoyancy.

The figure to the right shows a comparison of surface topography to crustal root thickness, crustal root buoyancy, and crustal root temperature for young and old collisional mountain belts.

Related Publications:
French, S. W., K. M. Fischer, E. M. Syracuse, and M. E. Wysession, Crustal structure beneath the Florida-to-Edmonton broadband seismometer array, Geophys. Res. Lett., 36, L08309, doi:10.1029/2008GL036331, 2009.

Fischer, K. M., Waning buoyancy in the crustal roots of old mountains, Nature, 417, 933-936, 2002.

Brown faculty collaborators:


Other project collaborators:

French, S. W., K. M. Fischer, E. M. Syracuse, M. E. Wysession,

a) Ratios (R) of mountain surface relief to crustal root thickness as a function of the time since collision ceased; b) R values as a function of time since the last major thermo-tectonic event in the region; c) Differences between crustal root and mantle density that best fit observed Bouguer gravity anomalies, assuming the seismically constrained shape of the crustal root and allowing a single value for upper crust density to vary; d) Temperature at the Moho for a 45 km thick crust from analytical and finite difference cooling models.  These calculations show that significant cooling is possible over 200-300 My timescales.

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