A Geophysical Perspective on Outstanding Questions in Ocean Mantle Dynamics

James B. Gaherty
School of Earth and Atmospheric Sciences, Georgia Institute of Technology

Plate tectonics and associated deformation at Earth's surface are the manifestation of 
thermal convection in the mantle. The oceanic domain is a logical focus for studies of 
mantle dynamic processes because fundamental components of convection can have 
simple tectonic manifestations beneath the oceans, and the signatures of dynamic 
processes are far less likely to be overprinted here than continental settings. In recent 
years, models have been developed to describe a number of important dynamic processes 
beneath the oceans, including focused upwelling and swell formation at hotspots, 
hotspot-ridge interaction, small-scale convection beneath the plates, mantle flow and melt 
migration beneath spreading centers and propagating rifts, and slab-induced flow in back-
arc and fore-arc environments.   Furthermore, new hypotheses have been postulated 
regarding mechanical and chemical heterogeneity within the mantle, including the 
distribution and dynamic importance of volatiles.

From a geophysical perspective, the principle constraints on these models come from two 
classes of observations.  The first are marine geophysical surveys, which provide detailed 
images of (primarily) crustal structure over length scales of order 1-100 km in discrete 
regions.  The second are land-based seismic observations, which provide evenly 
distributed images of upper-mantle structure with horizontal length scales of 1000+ km, 
except in limited regions near sources and/or receivers.  As a result, major gaps remain in 
our constraints on upper-mantle structure and dynamics.  Most critically, little is known 
about mantle structural variations with scale lengths of 100-1000 km, a range that 
encompasses most or all of the dynamic phenomena noted above.  (This length scale is 
well defined in satellite-derived data such as the geoid, but interpretation of these data in 
terms of mantle processes is indirect.)  Furthermore, the distribution of high-resolution 
studies is strongly biased to a few geographic regions, leaving much of the oceanic 
mantle largely unsampled at length scales < 1000 km.  As demonstrated by several recent 
regional broadband seismic experiments, the new generation of ocean-bottom seismic 
instrumentation provides a means to close these gaps in length scale and sampling and 
shed new light on the dynamics of the oceanic mantle.