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Strength of the lithosphere: Constraints imposed by laboratory experiments. David Kohlstedt Brian Evans Stephen Mackwell. POINT OF THE PAPER its all about lithospheric strength profiles. where do they come from (a brief review)
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Strength of the lithosphere: Constraints imposed by laboratory experiments David Kohlstedt Brian Evans Stephen Mackwell
POINT OF THE PAPERits all about lithospheric strength profiles • where do they come from (a brief review) • Constraints for the constitutive equations that describe deformation of the lithosphere • Some limitations and applications of strength profiles to understanding the dynamic behavior of the lithosphere
Experimental observations suggest that frictional strength increases linearly with increasing pressure and is insensitive to temperature and different materials (e.g. Byerlee’s rule). Plastic strength decreases with temperature and is insensitive to pressure (Dislocation creep is generally thought to be the dominant mechanism governing the plastic strength at pressures >10 Mpa) Frictional strength is generally used instead of fracture strength because we assume that the brittle crust is fractured, jointed and faulted. The strength profiles are calculated from constitutive equations for which the variables are constrained by conducting laboratory experiments (at high strain rates and temperatures) and extrapolating to nature. Where do they come from?
Constitutive laws and the strength curves Wet quartz Dry Olivine
Additional evidence from seismicity Chen and Molnar [1983] discussed the presence of a weak, aseismic middle-lower crust sandwiched between the seismic upper crust and seismic uppermost mantle [jelly sandwich], showing that the lithospheric strength profiles predicted by extrapolating experimental data may be valid
Some Implications of these lithospheric strength profiles Continents • The strongest part of the lithosphere may be the upper mantle which deforms by ductile flow • In this case the upper crust would be coupled to the strong upper mantle by the weak ductile lower crust thus large scale continental deformation should best be described as a viscous continuum where oblique convergence results in strain partitioning [Molnar 1992]. • Also the presence of this weak layer above the MOHO may allow the lithosphere to delaminate at the MOHO (lithospheric thrust nappes and detachment faults) • With enough gravitational potential energy (crustal thickening) the “jelly” could also flow laterally from between the “bread slices”….channel flow!
Fluids • Water makes quite a difference • In lab experiments for upper crustal conditions the presence of water can reduce the coefficient of friction by 30% (i.e. weaker faults) • For plastic deformation of quartz water is very important. Strain rate increases with increasing water fugacity. (i.e. wet quartz is a lot weaker than dry quartz)
Wang et al 1994 show that, at high temperatures and stresses lower than 300 Mpa, quartzite can deform by Newtonian dislocation suggesting that the lower crust may flow like a Newtonian viscous fluid. • So wet quartz might be weaker than we thought at lower crustal levels (as much as 2 orders of magnitude weaker)
Fluids continued • We can see from the extrapolated experimental data that wet olivine is significantly weaker than dry olivine • A wet upper mantle may not be strong enough to influence deformation in the upper crust.
Fluids continued • Also more recent data from Mackwell et al [1998] suggests that dry diabase or wet granulite in the lower crust could be quite strong relative to wet upper mantle olivine Jackson [2002]
Experiments on single phases • The plastic strength curves are extrapolated from experiments done one single phase aggregates (olivine and quartz). • However, the lithosphere is probably not composed of single phases. • It turns out to be very difficult to deal with polymineralic rock in the lab and so finite element modeling techniques will have to be used.
Other problems Laboratory strain rates are between 5 and 10 orders of magnitude higher than those found under normal geologic conditions. Experimental temperatures are commonly much higher than those appropriate for the lithosphere Extrapolating from laboratory conditions to nature require the accurate determination of activation energies (Q) and that no changes in deformation mechanism occurs.
Friction v.s. Fracture We see in the field that faults, fractures and joints heal or are filled with precipitate…in some cases fault zone rocks are stronger than the surrounding material. Should we use fracture strength (Mohr coulomb criterion) then instead of frictional strength (Byerlee’s rule)? Would it make that much of a difference?
How certain can we be about the focal depths of Chen and Molnar’s earthquakes? Jackson [2002, GSA Today article] argues for a very different continental strength profile based on recalculating Chen and Molnar’s focal depths and getting no mantle earthquakes.
