HR: 16:30h
AN: T34A-03 INVITED     [Abstracts]
TI: Depth of the Lithosphere-Asthenosphere Transition and Related Viscosity Structure
AU: * Faul, U
EM: uli.faul@anu.edu.au
AF: Australian National University, Research School of Earth Sciences, Canberra, ACT 0200 Australia
AU: Jackson, I
EM: ian.jackson@anu.edu.au
AF: Australian National University, Research School of Earth Sciences, Canberra, ACT 0200 Australia
AB: The lithosphere - asthenosphere transition can be defined in a number of different ways, for example rheologically, thermally or chemically. A thermal definition of the lithosphere - asthenosphere transition can be the depth at which the geotherm approaches the adiabat to within 10 K. With our recent experimental measurements of shear modulus and attenuation as a function of temperature and grain size (Faul and Jackson, EPSL, 05) seismological models of the upper mantle can be used to constrain temperatures. Shear velocities calculated for geotherms of conductively cooling oceanic lithosphere compare well with seismological models of the Pacific. In particular, the age-dependent deepening and lessening of the low velocity zone is reproduced, implying that the low velocity zone can be explained by temperature effects alone. However, an important aspect is that the velocity minimum occurs at sub-adiabatic temperatures, i.e. is part of the (thermally defined) lithosphere. The low velocity zone is due to the transition from the relatively steep temperature gradients of the conductively cooling lid to the much smaller adiabatic temperature gradients in the asthenosphere where pressure effects dominate. The magnitude of the velocity minimum depends on the depth at which the geotherm joins the adiabat. The shallow depth for young oceanic lithosphere results in a pronounced velocity minimum, whereas for archean cratons the very minor or absent velocity minimum implies a thermal boundary layer that is > 300 km thick. Similarly, viscosities calculated for deformation of olivine (e.g. Hirth and Kohlstedt, Geophys. Monogr., 03) are affected by the trade-offs between temperature and pressure effects, such that the viscosity minimum is near the velocity minimum. Young oceanic lithosphere has a relatively pronounced, shallow viscosity minimum, whereas for archean cratons the viscosity minimum is significantly deeper and less pronounced.
DE: 3902 Creep and deformation
DE: 3909 Elasticity and anelasticity
DE: 5144 Wave attenuation
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
DE: 8160 Rheology: general (1236, 8032)
SC: Tectonophysics [T]
MN: Fall Meeting 2005