Constraints on Brittle Lithospheric and Viscous Asthenospheric Properties From Models of Extensional Faulting

The strength of lithosphere in extension is better known than for compression, because extensional pore pressures should remain sub-hydrostatic. Thus, we can relate laboratory measured rock properties to field observations of normal fault structures in a more direct way than we can for thrust systems. Faults have to be weaker than surrounding rock and we assume that faults weaken as a linear function of strain in numerical models of lithospheric deformation. The model fault patterns depend on average lithospheric thickness and strength as well as the rate and amount of strain weakening. Observation of normal faults with large offsets (i.e. comparable to lithospheric thickness) seems to require moderately slow loss of more than one third of the extensional layer strength during lithospheric stretching. The more than 5 km normal sense offset of parts of the Vema Transform Fault cutting thick, old lithosphere indicates that friction and not just cohesion has to be reduced on the fault. A fundamentally different constraint on brittle properties comes from the pattern of faults formed by lithospheric bending. Bathymetric and seismic data from the outer rise of subduction zone trenches indicates ``horst and graben'' faulting due to plate bending. Numerical models only give comparable bending fault patterns when a small fraction of lithospheric strength is lost very fast (i.e. for very small fault offset). In the last decade, laboratory and theoretical work on the drying effect of manlte melting and melt extraction have lead many to infer that the sub-ridge asthenospheric viscosity is 1021 Pas or greater. Analytic and numerical models show that for such a uniform asthenospheric viscosity the viscous flow stresses would rip apart reasonably thick lithosphere over a region ~100 km wide at fast spreading centers. The observation that most extension is concentrated in a narrow (less than 1 km wide) zone at fast spreading centers indicates that parts of the sub-ridge asthenosphere must have a viscosity less than about 1019 Pas.