Mantle geodynamic constraints
Published:January 01, 2008
The Archean mantle was probably warmer than the modern one. Continental plates underlain by such a warmer mantle would have experienced less subsidence than modern ones following extension because extension would have led to widespread melting of the underlying mantle and the generation of large volumes of mafic rock. A 200 °C increase in mantle temperature leads to the production of nearly 12 km of melt beneath a continental plate extended by a factor of 2, and the resulting thinned plate rides with its upper surface little below sea level. The thick, submarine, mafic-to-ultramafic volcanic successions on continental crust that characterize many Archean regions could therefore have resulted from extension of continental plates above warm mantle.
Long-term subsidence of passive margins is driven by thermal relaxation of the stretched continental plate (cf. McKenzie). With a warmer mantle, the relaxation is smaller. For a continental plate stretched by a factor of 2, underlain by a 200 °C warmer mantle than at present, the cooling-driven subsidence drops from 2.3 km to 1.1 km. The combined initial and thermal subsidence declines by more than 40%, and by even more than this if initial continental crustal thicknesses were lower. The greatly reduced subsidence results in a concomitant decline in accommodation space for passive-margin sediments and may explain the scarcity of passive-margin sequences in the Archean record.
The formation of diamonds in the Archean requires geotherms similar to modern ones, which in turn probably reflect the presence of cool mantle roots beneath the continents. Stretching of continents underlain by cool mantle roots would yield passive margins similar to modern ones. Thus, development of significant passive margins may have occurred only through rifting of continents underlain by cool mantle roots. Furthermore, the widespread subcontinental melting associated with rifting of continents devoid of roots may have been a significant contributor to development of the roots themselves.