The role of the temperature and stress fields for evolution of asthenospheric layers

English Abstract:

The boundary between the lithosphere and the asthenosphere (LAB) is usually defined by a difference in response to stress. The reology of rocks depends on many factors but the basic differences of lithosphere and asthenosphere properties could be explained as a result of the temperature and pressure. The effective viscosity is proportional to C exp(A q), where q is the ratio (melting temperature/temperature), C and A are positive constants. The mantle is not molten, so q>1. In asthenosphere q is close to 1 and effective viscosity is low (e.g. 10¹⁸ Pa s).

In upper part of lithosphere q is high, but the temperature gradient in the lithosphere is high. Below the lithosphere, the gradient is low and the melting temperature is increasing with depth faster than true temperature. Hence, q and the viscosity reach minimum value below LAB and are increasing with depth. It is a typical situation..

Another important factor determining rheological properties is a stress tensor T. Generally viscosity is proportional to the power of the invariant of the stress tensor: I(T)^(1-n) where n is probably in the range from 3 to 5.

We investigate the processes of formation and evolution of low viscosity layers (“asthenospheric layers”) in the upper mantle. The time scale of the temperature changes is of the order of 10 Myr. The characteristic time of stress changes could be much shorter depending on tectonic processes. Eventually processes of formation and vanishing of low viscosity layers is very dynamical. In a relatively short time (below 1 Myr) the pattern the viscosity distribution and velocity gradient could change substantially.

Using results from deep seismic sounding and surface wave tomography we have found that below some regions there are structures in the mantle that could be a forming/vanishing low viscosity layers. Reflectors in the lower lithosphere are observed beneath Trans-European Suture Zone between Precambrian and Palaeozoic platforms. In a thick Baltic shield lithosphere (200 km or more) low velocity zones and seismic reflectors are observed in the depth range 60-100 km, which could be interpreted as mechanical low Vp velocity zones, in contrast to thermal velocity zone in deeper asthenosphere.

Acknowledgments: This work was partially supported by the National Science Centre (grant 2011/01/B/ST10/06653).