Geodynamic constraints on the nature of the asthenosphere

Trustees Of Boston University, Boston

Investigators

Abstract

The Earth's rigid outer shell (the lithosphere) is broken into discrete units known as tectonic plates. These plates move about on the surface of the Earth in response to convection in the Earth's deep interior (the mantle). The motions of the plates ultimately give rise to phenomena such as earthquakes at the Earth's surface. The movement of the plates is made possible in part by a layer of relatively weak, easily deformable rock, known as the asthenosphere, which underlies the plates. While we have some appreciation of the important role the asthenosphere plays in accommodating the motion of tectonic plates, we do not yet understand what makes the rocks within the asthenosphere weak. A number of factors can influence the strength of rock, including its temperature, its composition (in particular the presence of volatile elements like water), and the presence of small amounts of magma within the rock. We will use computer simulations of convection in the asthenosphere beneath oceanic plates to determine which of these three mechanisms is responsible for the low viscosities in the asthenosphere. Distinguishing between these different mechanisms will improve our understanding of how and why plate tectonics developed on Earth but not on other terrestrial planets in the solar system such as Mars and Venus. It will also contribute to our understanding of how the Earth has evolved through time. The oceanic upper mantle is characterized by low seismic velocities, high seismic attenuation and high electrical conductivity at depths of ~80-200km. This corresponds roughly with the layer of relatively low viscosity within the upper mantle known as the asthenosphere. The close spatial correlation between these features suggests that a single mechanism may be responsible for all of them, and three competing explanations have been hypothesized: 1) variations in the physical properties of dry, melt-free peridotite with temperature and pressure; 2) the presence of volatiles (chiefly water); and 3) the presence of small degrees of partial melt. To date, geophysical observations alone have been unable to definitively identify which of these hypotheses is correct. However, it may be possible to gain further insight by considering their geodynamic implications. In particular, each proposed mechanism invokes a characteristic distribution of both volatiles and melts, which themselves influence the viscosity and density of the mantle, in addition to its seismic and electrical properties. Differences in viscosity and density structure will lead to differences in the development of convective instabilities at the base of the lithosphere, and thereby heat flow, bathymetry and seismic structure. We propose to test the three hypotheses regarding the nature of the asthenospheric mantle by developing regional-scale 3-D numerical models of mantle flow beneath oceanic plates that explicitly incorporate the effects of volatiles and melting. Results from the numerical experiments will be compared to a variety of geophysical and seismic observations to constrain the viability of each of the hypothesized mechanisms. In particular, model predicted temperature, composition and porosity will be mapped to seismic velocity and attenuation using empirical relationships derived from laboratory measurements, allowing direct comparison between the numerical experiments and global and regional seismic models. Ultimately the successful hypothesis will be identified as the one that best matches the constraints.