HR: 15:25h
AN: V33D-08    [Abstracts]
TI: Thermal Evolution of Cratonic Roots
AU: Jaupart, C
EM: cj@ccr.jussieu.fr
AF: Institut de Physique du Globe de Paris, 4 place Jussieu, Paris, 75252, France
AU: * Michaut, C
EM: chloe.michaut@yale.edu
AF: Department of Geology and Geophysics, Yale University, 210 Whitney Avenue, New Haven, CT 06520, United States
AU: Mareschal, J
EM: mareschal.jean-claude@uqam.ca
AF: GEOTOP-UQAM-McGill, University of Quebec at Montreal, POB 8888, sta "downtown", Montreal, QC H3C3P8, Canada
AB: Thermal models for the stabilization of the cratonic lithosphere are constrained by present heat flow and heat production data from Archean Provinces. Archean provinces are presently characterized by low heat flow, with an average of 41 mWm^-2 less than the global continental average (56 mWm^-2). The range of regionally averaged heat flow values in Archean Provinces (18-54 mWm^-2) is narrower than in Proterozoic and Paleozoic terranes. However, at the end of the Archean, when crustal heat production was double the present, surface heat flow varied over a range (~45-90 mWm^-2) at least as wide as that presently observed in Paleozoic Provinces. The high crustal heat production during the Archean is not sufficient to account for elevated lower crustal temperatures without some additional heat input or without the crust being thicker or the vertical distribution of radio-elements being different from today's. Lithospheric heat production is a key variable in determining thermal conditions that permit stabilization of the crust and the preservation of a thick cratonic root. Stability of the crust and the cratonic root requires strong differentiation in the distribution of the radiogenic elements. Even for values of the surface heat flow higher than average in cratons, the crust can be stabilized before 2.5Ga if it is very differentiated and the radioelements are confined to shallow layers. Prior to differentiation, the lowermost crust could be near the solidus for present surface heat flow 40-45 mWm^-2. Present heat production in the mantle root is constrained by the estimates of the mantle heat flow. Further constraints can be obtained by modeling the past thermal regime of the root when heat production was higher. If heat generation is high and/or if the root is thick, the lower lithosphere has cooled more rapidly than the convecting mantle, which implies that the temperature gradient was inverted at the base of the lithosphere and a weak mechanical layer in the middle of the root, precluding the survival of the root. A low temperature gradient at the base of the root leads to the development of convective instabilities and possible removal of the lowermost part. The large time-scale of diffusive heat transport implies that the lithospheric mantle can remain thermally decoupled from the crust for as long as 1 Gyr. Temperatures in the lithospheric mantle may rise above their initial values inherited from the process of root formation due to in-situ radiogenic heat production.
DE: 8103 Continental cratons
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
DE: 8130 Heat generation and transport
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting