Rheology in Crust and MantleSt. Mackwell, Bayreuth
All tectonic processes within the interior of the Earth involve movement of solid or molten material. Convection in the mantle, driven by the thermal gradient between the core and lithosphere, occurs by solid-state deformation of the rocks and minerals that comprise the upper/lower mantle and the transition zone. Plate tectonics, as manifest by mountain building, earthquakes, volcanoes etc., occurs by plastic or brittle deformation of the rocks and minerals that comprise the oceanic and continental lithosphere. Temperature, pressure and rate of deformation to a large extent define the nature of deformation for most minerals and rocks in the interior of the Earth. However, the chemical environment (notably water and oxygen fugacity, activity of silica, etc.) may also have a significant influence. Thus, insight into the mechanisms by which rocks and minerals deform under conditions that approximate those in the Earth is critical to our understanding of the processes that shape our planet.
Deformation in the shallow crust (down to perhaps 10-15 km) is dominated by brittle processes, predominantly sliding on pre-existing faults. This behavior can be described approximately by a law of frictional sliding (Byerlee´s law), which predicts that the strength of rock will increase with overburden pressure (depth), but will be largely independent of temperature or rock type. The presence of fluid pressure will tend to decrease the strength of the fault. At greater depths, the increasing temperature will promote plastic deformation processes, resulting in a depth range in which both plastic and brittle deformation processes will be active. There will be a corresponding broadening of the zone of deformation. It is within this depth range that the highest moment release might be anticipated from intraplate earthquakes. Deformation here is incompletely understood and cannot be easily modeled from experiment.
At still greater depths, the higher temperatures will result in deformation that is dominated by fully plastic processes. However, during periods of high rates of deformation, such as associated with earthquakes, semi-plastic deformation behavior will penetrate deeper in the crust. Plastic deformation can generally be described by an Arrhenius law, with a strong dependence on temperature and only a relatively weak dependence on pressure. In cooler parts of the crust or mantle, deformation may occur by the glide of dislocations within the mineral grains that comprise the rock; in this regime (often referred to as low-temperature plasticity) the behavior of the rock is strongly dependent on the differential stress. Higher temperatures will promote diffusion within the mineral grains, allowing climb as well as glide of the dislocations, and resulting in a somewhat weaker dependence on stress (so-called power-law creep). At sufficiently low rates of deformation and/or small grain sizes, diffusional creep processes, where deformation results from diffusion of atomic species along the grain boundaries, may become dominant. The presence of a free fluid phase will also tend to promote diffusional creep processes (pressure-solution creep). For all these plastic deformation processes, elements in the chemical environment (water or oxygen fugacity, buffering of constituent phases, etc.) may also have a strong effect on the mechanical behavior.
Within the continental crust, deformation in the plastic and semi-plastic regimes will be vertically, laterally and temporally quite heterogeneous, depending on the mineralogy, presence or absence of fluids, and episodicity of straining. Much of the strain, even in the deeper crust and uppermost mantle, is likely to be focused in plastic shear zones, rather than distributed throughout the entire rock mass. Such zones are also likely to act as conduits for any fluid rising (continually or, more likely, episodically) from greater depth. While dry rocks in the deepest crust show strengths that are comparable to mantle rocks, wet crustal rocks may be significantly weaker, resulting in decoupling of the crust from the mantle lithosphere. Normally sub-continental mantle is sufficiently hot that deformation will always occur in the plastic regime.
The oceanic crust away from the ridge axis is sufficiently thin and cool that deformation should always occur within the brittle regime. Brittle and semi-plastic behavior is also expected to penetrate into the sub-oceanic upper mantle, and the overall strength of the oceanic lithosphere will be dominated by the mantle component.
This simplified description of deformation within the lithosphere is often illustrated through the use of a strength envelope (Christmas tree) diagram, which plots the rock strength as a function of depth for a constant rate of deformation. Such illustrations are highly useful in providing a clear visual display of predicted behavior, but are rather limited in the assumption of constant rate of homogeneous deformation. It is also unusual that we know enough about the mineralogy, fluid activity, and strain distribution at depth to utilize such diagrams in a more quantitative way. On the other hand, when we have almost no constraints on such parameters, such as for the lithospheres of other terrestrial or icy planetary bodies, such diagrams can be highly useful as a first order model of lithospheric deformation.
In this presentation, I will summarize our understanding of deformation processes in the major minerals and rocks of the Earth´s upper mantle and crust, focusing on how information from experiments can be used to understand earth dynamics. Examples will be given that highlight the utility and limitations of the strength envelope concept, and indicate where there is a present critical need for new experimental data. I will also discuss recent improvements in experimental apparatus and design that allow new insight into deformation processes occurring within zones of high shear, which may be the dominant locations for creep in the deep crust and uppermost mantle. I will also briefly describe current attempts to determine the mechanical behavior of mantle phases that are stable only under very high-pressure conditions. Such work has illuminated much about deformation mechanisms in the various high-pressure minerals under a range of laboratory conditions, but has provided few real constraints on absolute deformation rates (viscosities) in the Earth´s interior. Partly this reflects our limited knowledge of the chemistry of the deep Earth, and partly it reflects the inherent difficulty in performing meaningful experiments under the appropriate conditions.