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IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS017 Oral Presentation 2379
Continental Elevation and Upper Crustal Radioactivity
Prof. David Chapman
Geology and Geophysics University of Utah IASPEI
Derrick Hasterok
Continental elevation is an accurate and easily observable dataset that may be used as a constraint on
geodynamic processes. Elevation contributions due to geodynamic processes are typically on the order
of 10s to 1-2000 meters, which can be easily masked due to the larger contributions of compostional
and thermal buoyancy. In order to observe the geodynamic contributions to elevation, the compositional
and thermal effects must be estimated and removed. Our ability to identify the geodynamic
contributions to elevation is made possible when uncertainties related to the compositional buoyancy
can be accurately determined. In this study, we make an isostatic correction for compositional
buoyancy, develop a general thermal isostaticmodel for the continents, and explore heat production as
a large potential contriutor to observed elevation. The elevation of 73 global tectonic provinces are
adjusted for the effect of compositional buoyancy by computing an isostatic adjustment relative to a
standard crustal section (average density of 2850 kg/m3 and a 39 km thick crust). The crustal thickness
and P-wave seismic velocity (VP) structure are determined for each province from seismic models;
densities are computed using an empirical relationship derived from laboratory pressure-temperaturevelocity-density
data. Uncertainties in the elevation adjustment are estimated using a Monte Carlo
analysis. Compositional elevation adjustments applied to a global set of tectonic provincesrange from ~-
1600 m in the Ukranian Shield to 2300 m in the Gulf of California. Estimated uncertainties range from
~200 m to >600 m. Thermal buoyancy is estimated by integrating the difference between a geotherm
derived from observed values of heat flow and a reference geotherm. The best fitting continental heat
flowelevation model has a reference heat flow of 43 mW/m2 at 0 km elevation and a 70:30 partitioning
of surface heat flow between reduced heat flow and upper crustal radioactive heat production. A set of
North American-only provinces show a 60:40 partitioning. Whereas raw elevations of continental
provinces show little correlation with heat flow, the compositionally adjusted elevations show a clear
trend with about 4 km difference between cold and hot provinces. A continental heat flow-elevation plot
is used to identify outliers in adjusted province elevations. A three provinces with anomalously high
upper crustal heat production, the Wopmay orogen and the North and South Australian cratons, fall >1
km below the thermal isostatic modeled elevation. These regions illustrate the importance of
constraining lithospheric heat production variations prior to estimating the geodynamic contributions to
elevation.
Keywords: lithosphere, elevation, heatflow