Abstract:
This thesis studies the dynamics of mantle melting during continental breakups by
geophysical, geochemical, and numerical analyses. The first part focuses on the mantle
melting and crustal accretion processes during the formation of the Southeast Greenland
margin, on the basis of deep-crustal seismic data. A new seismic tomographic method is
developed to jointly invert refraction and reflection travel times for a compressional
velocity structure, and a long-wavelength structure with strong lateral heterogeneity is
successfully recovered, including 30- to 15-km-thick igneous crust within a 150-km-wide
continent-ocean transition zone. A nonlinear Monte Carlo analysis is also conducted to
establish the absolute uncertainty of model parameters. The derived crustal structure is first
used to resolve the origin of a margin gravity high, with new inversion schemes using both
seismic and gravity constraints. Density anomalies producing the gravity high seem to be
confined within the upper crust, not in the lower crust as suggested for other volcanic
margins. A new robust framework is then developed for the petrological interpretation of
the velocity structure of igneous crust, and the thick igneous crust formed at the continent-ocean
transition zone is suggested to have resulted from vigorous active upwelling of
mantle with only somewhat elevated potential temperature. In the second part, the nature
of mantle melting during the formation of the North Atlantic igneous province is studied
on the basis of the major element chemistry of erupted lavas. A new fractionation
correction scheme based on the Ni concentrations of mantle olivine is used to estimate
primary melt compositions, which suggest that this province is characterized by a large
degree of major element source heterogeneity. In the third part, the nature of preexisting
sublithospheric convection is investigated by a series of finite element analyses, because
the strength of such convection is important to define the "normal" state of mantle, the
understanding of which is essential to identify any anomalous behavior of mantle such as a
mantle plume. The results suggest that small-scale convection is likely in normal
asthenosphere, and that the upwelling velocity in such convection is on the order of 1-10
cm/yr.
