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Waning buoyancy in the crustal roots of old mountains

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Abstract

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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References

  1. 1

    Jull, M. & Kelemen, P. B. On the conditions for lower crustal convective instability. J. Geophys. Res. 106, 6423–6446 (2001)

  2. 2

    Bousquet, R., Goffé, B., Henry, P., Le Pichon, X. & Chopin, C. Kinematic, thermal and petrological model of the Central Alps: Lepontine metamorphism in the upper crust and eclogitisation of the lower crust. Tectonophysics 273, 105–127 (1997)

  3. 3

    Carbonell, R. et al. Seismic wide-angle constraints on the crust of the southern Urals. J. Geophys. Res. 105, 13755–13777 (2000)

  4. 4

    Knapp, J. H. et al. Seismic reflection fabrics of continental collision and post-orogenic extension in the Middle Urals, central Russia. Tectonophysics 288, 115–126 (1998)

  5. 5

    Schueller, W., Morozov, I. B. & Smithson, S. B. Crustal and uppermost mantle velocity structure of Northern Eurasia along the Profile Quartz. Bull. Seismol. Soc. Am. 87, 414–426 (1997)

  6. 6

    Taylor, S. R. in Geophysical Framework of the Continental United States Memoir 172 (eds Pakiser, L. C. & Mooney, W. D.) 317–348 (Geological Society of America, Boulder, CO, 1989)

  7. 7

    Li, A., Fischer, K. M., van der Lee, S. & Wysession, M. E. Crust and upper mantle discontinuity structure beneath eastern North America. J. Geophys. Res. 107, 10.1029/2001JB000190 (2002)

  8. 8

    Belousov, V. V. et al. Structure of the crust and upper mantle of the [former] USSR. Int. Geol. Rev. 34, 345–444 (1992)

  9. 9

    Clitheroe, G., Gudmundsson, O. & Kennett, B. L. N. The crustal thickness of Australia. J. Geophys. Res. 105, 13697–13713 (2000)

  10. 10

    GGT/SVEKA Working Group, Korsmann, K., Korja, T., Pajunen, M. & Virransalo, P. The GGT/SVEKA transect: Structure and evolution of the continental crust in the Paleoproterozoic Svecofennian orogen in Finland. Int. Geol. Rev. 41, 287–333 (1999)

  11. 11

    Baird, D. J., Nelson, K. D., Knapp, J. H., Walters, J. J. & Brown, L. D. Crustal structure and evolution of the Trans-Hudson orogen: Results from seismic reflection profiling. Tectonics 15, 416–426 (1996)

  12. 12

    Nguuri, T. K. et al. Crustal structure beneath southern Africa and its implications for the formation and evolution of the Kaapvaal and Zimbabwe cratons. Geophys. Res. Lett. 28, 2501–2504 (2001)

  13. 13

    Rondenay, S., Bostock, M. G., Hearn, T. M., White, D. J. & Ellis, R. M. Lithospheric assembly and modification of the SE Canadian Shield: Abitibi-Grenville Teleseismic Experiment. J. Geophys. Res. 105, 13735–13754 (2000)

  14. 14

    Wissinger, E. S., Levander, A. & Christensen, N. I. Seismic images of crustal duplexing and continental subduction in the Brooks Range. J. Geophys. Res 102, 20847–20871 (1997)

  15. 15

    Vacher, P. & Souriau, A. A three-dimensional model of the Pyrenean deep structure based on gravity modelling, seismic images and petrological constraints. Geophys. J. Int. 145, 460–470 (2001)

  16. 16

    Fernández-Viejo, G. et al. Crustal transition between continental and oceanic domains along the North Iberian margin from wide angle seismic and gravity data. Geophys. Res. Lett. 25, 4249–4252 (1998)

  17. 17

    Musacchio, G., Zappone, A., Cassinis, R. & Scarascia, S. Petrographic interpretation of a complex seismic crust-mantle transition in the central-eastern Alps. Tectonophysics 294, 75–88 (1988)

  18. 18

    Marchant, R. H. & Stampfli, G. M. Subduction of continental crust in the Western Alps. Tectonophysics 269, 217–235 (1997)

  19. 19

    Chalot-Prat, F. & Girbacea, R. Partial delamination of continental mantle lithosphere, uplift-related crust-mantle decoupling, volcanism and basin formation: a new model for the Pliocene-Quaternary evolution of the southern East-Carpathians, Romania. Tectonophysics 327, 83–107 (2000)

  20. 20

    James, D. E. & Sacks, I. S. in Geology and Ore Deposits of the Central Andes Spec. Pub. 7 (ed. Skinner, B. J.) 1–25 (Society of Economic Geologists, Littleton, Colorado, 1999)

  21. 21

    Owens, T. J. & Zandt, G. Implications of crustal property variations for models of Tibetan plateau evolution. Nature 387, 37–43 (1997)

  22. 22

    Mahdi, H. & Pavlis, G. L. Velocity variations in the crust and upper mantle beneath the Tien Shan inferred from Rayleigh wave dispersion: Implications for tectonic and dynamic processes. J. Geophys. Res. 103, 2693–2703 (1998)

  23. 23

    Hacker, B. R. et al. Hot and dry deep crustal xenoliths from Tibet. Science 287, 2463–2466 (2000)

  24. 24

    Forsyth, D. W. Subsurface loading and estimates of the flexural rigidity of the continental lithosphere. J. Geophys. Res. 90, 12623–12632 (1985)

  25. 25

    Stewart, J. & Watts, A. B. Gravity anomalies and spatial variations of flexural rigidity at mountain ranges. J. Geophys. Res. 102, 5327–5352 (1997)

  26. 26

    McKenzie, D. & Fairhead, D. Estimates of the effective elastic thickness of the continental lithosphere from Bouguer and free air gravity anomalies. J. Geophys. Res. 102, 27523–27552 (1997)

  27. 27

    Djomani, Y. H. P., Fairhead, J. D. & Griffin, W. L. The flexural rigidity of Fennoscandia: reflection of the tectonothermal age of the lithospheric mantle. Earth Planet. Sci. Lett. 174, 139–154 (1999)

  28. 28

    Döring, J. & Götze, J.-J. The isostatic state of the southern Urals crust. Geol. Rundsch. 87, 500–510 (1999)

  29. 29

    Hittelman, A. M., Kinsfather, J. O. & Meyers, H. Geophysics of North America [CD-ROM] (National Geophysical Data Center, Boulder, CO, 1990)

  30. 30

    Rudnick, R. L. & Fountain, D. M. Nature and composition of the continental crust: a lower crustal perspective. Rev. Geophys. 33, 267–309 (1995)

  31. 31

    Christensen, N. I. & Mooney, W. D. Seismic velocity structure and composition of the continental crust: A global view. J. Geophys. Res. 100, 9761–9788 (1995)

  32. 32

    Hacker, B. R. in Subduction Top to Bottom Geophysical Monograph 96 (eds Pakiser, L. C. & Mooney, W. D.) 337–346 (American Geophysical Union, Washington DC, 1996)

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Acknowledgements

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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Competing interests

The author declares that she has no competing financial interests.

Correspondence to Karen M. Fischer.

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Further reading

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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