Yvette H. Poudjom Djomani

The density structure of subcontinental lithosphere through time

Abstract This study uses information on composition, thermal state and petrological thickness to calculate the densities of different types of subcontinental lithospheric mantle (SCLM). Data from mantle-derived peridotite xenoliths and garnet–xenocryst suites document a secular evolution in the composition of SCLM: the mean composition of newly formed SCLM has become progressively less depleted, in terms of Al, Ca, mg# and Fe/Al, from Archean, through Proterozoic to Phanerozoic time. Thermobarometric analyses of xenolith and xenocryst suites worldwide show that the mean lithospheric palaeogeotherms rise from low values (corresponding to surface heat flows of 35–40 mW/m 2 ) beneath Archean terranes, to higher values (>50 mW/m 2 ) beneath regions with Phanerozoic crust. The typical thickness of the lithosphere (defined as a chemical boundary layer), ranges from about 250 to 180 km, 180–150 km and 140–60 km for Archean, Proterozoic and Phanerozoic terranes respectively. The depth of this lithosphere–asthenosphere boundary corresponds to a temperature of 1250–1300°C. Using the estimated compositions, average mineral compositions and experimental data on the densities of mineral end-members (tables 1 and 2), we calculate mean densities at 20°C for Primitive Mantle (3.39 Mg m −3 ) and for SCLM of Archean (3.31±.016 Mg m −3 ), Proterozoic (3.35±0.02 Mg m −3 ) and Phanerozoic (3.36±0.02 Mg m −3 ) age. Curves of density and cumulative density versus depth, which take into account variations in geotherm with tectonothermal age, have been constructed for each age type of lithospheric section to assess the buoyancy of these columns relative to the asthenosphere, modelled as a Primitive Mantle composition. The density curves show that Archean SCLM is significantly buoyant relative to the asthenosphere at depths greater than about 60 km. Proterozoic sections deeper than about 100 km thick also are significantly buoyant. The buoyancy of Archean and Proterozoic SCLM sections, combined with their refractory composition, leads to high viscosities and explains the longevity and stability of old SCLM. Replacement of Archean lithosphere, as beneath the present-day eastern Sino–Korean craton, probably involves mechanical dispersal by rifting, accompanied by the rise of hot, fertile asthenospheric material. Fertile Phanerozoic lithosphere is buoyant when the geotherm is sufficiently high, as in many Cenozoic volcanic provinces. However, as the geothermal gradient relaxes toward a stable conductive profile, Phanerozoic SCLM sections thinner than about 100 km become denser than the asthenosphere, and hence gravitationally unstable. This could help to induce delamination of the SCLM and upwelling of asthenospheric material, beginning a new cycle. The tectonic consequences of such lithosphere replacement would include uplift and magmatism, and basin formation during subsequent thermal relaxation.

The flexural rigidity of Fennoscandia: reflection of the tectonothermal age of the lithospheric mantle

Abstract Estimates of effective elastic thickness ( Te ), a measure of the strength of the plate, have shown that older and cool cratonic regions are characterised by a stronger lithosphere (higher Te values) than areas which have been tectonically reworked. We use geophysical data and information from xenoliths in volcanic rocks from Fennoscandia and show that the strength of the lithosphere reflects the tectonothermal age of the lithospheric mantle. Fennoscandia is made up of the Baltic shield (1.9–3.0 Ga) and the Caledonian Orogen of Norway and northern Sweden (400–500 Ma). The shield is characterised by an old, thick and cold lithosphere in contrast to the Caledonides where the lithosphere is hotter and thinner. Our geophysical analysis results show a regional variation in elastic plate thickness from 8 km to 70 km, or equivalently, a flexural rigidity between 0.4×10 22 and 3×10 24 Nm. These results suggest that the lithosphere is strongest in the relatively stable Archaean Province, weaker in the regions characterised by Proterozoic crustal formation, and lowest in the tectonically reworked and deformed Caledonian belt. Within the study area, there is a direct correlation between lithosphere strength, the age of the last major tectonothermal event registered in the crust and lithospheric mantle composition. These broad correlations reflect thinner and more fertile lithosphere, and higher geothermal gradients, beneath regions of progressively younger crust.