HR: 1340h
AN: V43B-1365    [Abstracts]
TI: Does the mantle control the maximum thickness of cratons?
AU: * Cooper, C M
EM: cooper@dtm.ciw.edu
AF: Carnegie Institute of Washington Department of Terrestrial Magnetism now at National Science Foundation, 5241 Broad Branch Road N.W., Washington, DC 20001, United States
AU: Conrad, C P
AF: Department of Earth and Planetary Sciences Johns Hopkins University, Johns Hopkins University, Baltimore, MD 21218, United States
AB: The stability and longevity of cratons depends on their ability to resist deforming forces induced by the flowing and evolving mantle. Previous studies point to the combination of buoyancy, viscosity and finite strength to provide the cratons with sufficient strength to maintain coherency atop a convecting mantle. This combination of parameters determines a minimum thickness that cratons must exceed in order to maintain stability and longevity. However, is there also a maximum thickness above which a craton is rendered unstable? More specifically, is there a critical craton thickness that would enhance its susceptibility to destruction by basal tractions that will shear and thin the craton? To address this question we employ an analytical approach relating the viscosity structure of the craton to the velocity of mantle flow within the asthenosphere. If the craton°s viscosity structure is determined by the interplay between a chemical and thermal boundary layer, then there exists a buffer zone with temperature dependent viscosity between the rigid chemical layer and the convecting mantle. This thermal boundary layer protects the chemically-distinct craton from the asthenospheric shear, but it cannot do so if it is too weak or too thin. Furthermore, if the net thickness of the chemical and thermal boundary layer increases, then the mantle-induced basal shear tractions on the combined structure will increase. This negative feedback prevents the cratonic lithosphere from growing thicker than some maximum value that depends on the viscosity structure of the thermal boundary layer. We present initial estimates of the maximum thickness of the thermal buffer zone, which in turn controls the maximum thickness of cratonic chemical lithosphere and is required to maintain craton stability in the face of destabilizing mantle flow. This could be a potential explanation for the geochemical observation that cratonic xenoliths arise from depths no greater than 250 km.
DE: 8103 Continental cratons
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
DE: 8168 Stresses: general
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting