Tomographic Stratigraphy: a Possible New Framework for Mantle Dynamics

Abstract:

Stratification of the Earth's interior into spherically symmetric shells with boundaries that correspond to changes in composition or mineralogical phase is well established and represents a static view. The analysis of long wavelength features of recent 3-D mantle seismic velocity models suggests a somewhat different dynamic stratification, with rheological boundaries that do not always coincide with the known mineralogical transitions. This is reflected in different patterns of the power spectrum, radial correlation functions and overall distribution of degree 2 and 3 anomalies, which dominate the mantle below 300 km depth.

The outermost shell (Moho to ~250km depth), i.e. "heterosphere", contains surface tectonics, with cooling plates and cratons, and is characterized by strongest lateral heterogeneity. Its lower boundary is defined by a decrease in power by an order of magnitude.

The next shell ( 300 ~ 1000 km depth), i.e. "extended transition zone" (ETZ), includes the 400 and 660 km discontinuities. Many slabs stagnate above 660 km, but some flatten above 1000 km. While there is no sharp global seismic discontinuity at this depth, the long wavelength structure in the ETZ is strongly decorrelated from that in the rest of the lower mantle.

Striking features in the bulk of the lower mantle below the ETZ are: 1) long wavelength structure dominated by harmonic degrees 2 and 3 whose expression is strongest at the base of the mantle (the so-called LLSVPs); 2) embedded within them, about 20 broad low velocity plumes, whose vertical orientation indicates the absence of mantle "wind". Notably, some of them are deflected horizontally around 1000 km depth or give rise to thinner conduits in the ETZ, suggesting contrasting rheology above and below that depth. The steep heterogeneity gradient in the last 500 km of the mantle indicates the presence of a boundary layer with possibly distinct chemical composition.

The “tomographic stratigraphy” presented here may be a useful framework for achieving progress in understanding whole mantle dynamics. In particular, the ~1000 km transition may be a key feature separating relatively vigorous flow in the ETZ from very sluggish motion below. The seismic tomography snapshot thus captures the history of subduction in the last 30 Ma in the ETZ, and below it, the integrated flow for at least 200 Ma.