Irina M. Artemieva

Thermal thickness and evolution of Precambrian lithosphere: A global study

The thermal thickness of Precambrian lithosphere is modeled and compared with estimates from seismic tomography and xenolith data. We use the steady state thermal conductivity equation with the same geothermal constraints for all of the Precambrian cratons (except Antarctica) to calculate the temperature distribution in the stable continental lithosphere. The modeling is based on the global compilation of heat flow data by Pollack et al. [1993] and more recent data. The depth distribution of heat-producing elements is estimated using regional models for ∼300 blocks with sizes varying from 1°×1° to about 5°×5° in latitude and longitude and is constrained by laboratory, seismic and petrologic data and, where applicable, empirical heat flow/heat production relationships. Maps of the lateral temperature distribution at depths 50, 100, and 150 km are presented for all continents except Antarctica. The thermal thickness of the lithosphere is calculated assuming a conductive layer overlying the mantle with an adiabat of 1300°C. The Archean and early Proterozoic lithosphere is found to have two typical thicknesses, 200–220 km and 300–350 km. In general, thin (∼220 km) roots are found for Archean and early Proterozoic cratons in the Southern Hemisphere (South Africa, Western Australia, South America, and India) and thicker (>300 km) roots are found in the Northern Hemisphere (Baltic Shield, Siberian Platform, West Africa, and possibly the Canadian Shield). We find that the thickness of continental lithosphere generally decreases with age from >200 km beneath Archean cratons to intermediate values of 200±50 km in early Proterozoic lithosphere, to about 140±50 km in middle and late Proterozoic cratons. Using known crustal thickness, our calculated geotherms, and assuming that isostatic balance is achieved at the base of the lithosphere, we find that Archean and early Proterozoic mantle lithosphere is 1.5% less dense (chemically depleted) than the underlying asthenosphere, while middle and late Proterozoic subcrustal lithosphere should be depleted by ∼0.6–0.7%. Our results suggest three contrasting stages of lithosphere formation at the following ages: >2.5 Ga, 2.5–1.8 Ga, and <1.8 Ga. Ages of komatiites, greenstone belts, and giant dike swarms broadly define similar stages and apparently reflect secular changes in mantle temperature and, possibly, convection patterns.

Density of the continental roots: compositional and thermal contributions

The origin and evolution of cratonic roots has been debated for many years. Precambrian cratons are underlain by cold lithospheric roots that are chemically depleted. Thermal and petrologic data indicate that Archean roots are colder and more chemically depleted than Proterozoic roots. This observation has led to the hypothesis that the degree of depletion in a lithospheric root depends mostly on its age. Here we test this hypothesis using gravity, thermal, petrologic, and seismic data to quantify differences in the density of cratonic roots globally. In the first step in our analysis we use a global crustal model to remove the crustal contribution to the observed gravity. The result is the mantle gravity anomaly field, which varies over cratonic areas from 3100 to +100 mGal. Positive mantle gravity anomalies are observed for cratons in the northern hemisphere: the Baltic shield, East European Platform, and the Siberian Platform. Negative anomalies are observed over cratons in the southern hemisphere: Western Australia, South America, the Indian shield, and Southern Africa. This indicates that there are significant differences in the density of cratonic roots, even for those of similar age. Root density depends on temperature and chemical depletion. In order to separate these effects we apply a lithospheric temperature correction using thermal estimates from a combination of geothermal modeling and global seismic tomography models. Gravity anomalies induced by temperature variations in the uppermost mantle range from 3200 to +300 mGal, with the strongest negative anomalies associated with mid-ocean ridges and the strongest positive anomalies associated with cratons. After correcting for thermal effects, we obtain a map of density variations due to lithospheric compositional variations. These maps indicate that the average density decrease due to the chemical depletion within cratonic roots varies from 1.1% to 1.5%, assuming the chemical boundary layer has the same thickness as the thermal boundary layer. The maximal values of the density drop are in the range 1.7^2.5%, and correspond to the Archean portion of each craton. Temperatures within cratonic roots vary strongly, and our analysis indicates that density variations in the roots due to temperature are larger than the variations due to chemical differences. ; 2003 Elsevier Science B.V. All rights reserved.

Deep Europe today: Geophysical synthesis of the upper mantle structure and lithospheric processes over 3.5 Ga

abstract We present a summary of geophysical models of the subcrustal lithosphere of Europe. This includes the results from seismic (reflection and refraction profiles, P- and S-wave tomography, mantle anisotropy), gravity, thermal, electromagnetic, elastic and petro-logical studies of the lithospheric mantle. We discuss major tectonic processes as reflected in the lithospheric structure of Europe, from Precambrian terrane accretion and subduction to Phanerozoic rifting, volcanism, subduction and continent-continent collision. The differences in the lithospheric structure of Precambrian and Phanerozoic Europe, as illustrated by a comparative analysis of different geophysical data, are shown to have both a compositional and a thermal origin. We propose an integrated model of physical properties of the European subcrustal lithosphere, with emphasis on the depth intervals around 150 and 250 km. At these depths, seismic velocity models, constrained by body- and surface-wave continent-scale tomography, are compared with mantle temperatures and mantle gravity anomalies. This comparison provides a framework for discussion of the physical or chemical origin of the major lithospheric anomalies and their relation to large-scale tectonic processes, which have formed the present lithosphere of Europe.

Lithospheric structure, composition, and thermal regime of the East European Craton: implications for the subsidence of the Russian platform

A new mechanism for Paleozoic subsidence of the Russian, or East European, platform is suggested, since a model of lithosphere tilting during the Uralian subduction does not explain the post-Uralian sedimentation record. Alternatively, I propose that the Proterozoic and Paleozoic rifting (when a platform-scale Central Russia rift system and a set of Paleozoic rifts were formed) modified the structure and composition of cratonic lithosphere, and these tectono-magmatic events are responsible for the post-Uralian subsidence of the Russian platform. To support this hypothesis, (a) the thermal regime and the thickness of the lithosphere are analyzed, and (b) lithospheric density variations of non-thermal origin are calculated from free-board constraints. The results indicate that Proterozoic and Paleozoic rifting had different effects on the lithospheric structure and composition. (1) Proterozoic rifting is not reflected in the present thermal regime and did not cause significant lithosphere thinning (most of the Russian platform has lithospheric thickness of 150^180 km and the lithosphere of the NE Baltic Shield is 250^300 km thick). Paleozoic rifting resulted in pronounced lithospheric thinning (to 120^140 km) in the southern parts of the Russian platform. (2) Lithospheric density anomalies suggest that Proterozoic^Paleozoic rifting played an important role in the platform subsidence. The lithospheric mantle of the Archean^early Proterozoic part of the Baltic Shield is V1.4 6 0.2% less dense than the typical Phanerozoic upper mantle. However, the density deficit in the subcrustal lithosphere of most of the Russian platform is only about (0.4^0.8) 6 0.2% and decreases southwards to V0%. Increased densities (likely associated with low depletion values) in the Russian platform suggest strong metasomatism of the cratonic lithosphere during rifting events, which led to its subsidence. It is proposed that only the lower part of the cratonic lithosphere was metasomatized as a result of Proterozoic rifting; the boundary between a depleted upper and more fertile lower layers can be at ca. 90^150 km depth and can produce a seismic pattern similar to the top of a seismic low-velocity zone. Paleozoic rifting has modified the entire lithospheric column and the regions affected are still subsiding. Published by Elsevier B.V.

Processes of lithosphere evolution: new evidence on the structure of the continental crust and uppermost mantle

We discuss the structure of the continental lithosphere, its physical properties, and the mechanisms that formed and modified it since the early Archean. The structure of the upper mantle and the crust is derived primarily from global and regional seismic tomographystudies ofEurasia andfrom global andregional dataonseismic anisotropy. Thesedata asdocumentedin the papersof this special issue of Tectonophysics are used to illustrate the role of different tectonic processes in the lithospheric evolution since Archean to present. These include, but are not limited to, cratonization, terrane accretion and collision, continental rifting (both passiveandactive),subduction,andlithosphericbasalerosionduetoarelativemotionofcratonickeelsandtheconvectivemantle. D 2002 Elsevier Science B.V. All rights reserved.

Influence of volatiles in the upper mantle on the dynamics of thermal thinning of the lithosphere

Areas of Cenozoic tectonic and magmatic activity (including high plateaus and continental rifts) are characterized by anomalous low-velocity and low-density zones in the upper mantle. Petrochemical studies of Cenozoic volcanism have revealed such changes of magma composition with time that prove a consecutive uplift of magma sources. Thus, it is supposed that the process of tectonic rejuvenation is caused by an ascent of anomalous mantle to the base of the lithosphere, resulting in partial melting of lithospheric material, its consecutive replacement by material from the anomalous mantle, lithosphere thinning, and, hence, isostatic uplift of lithospheric blocks. A model of thermal thinning of the lithosphere that is specified by a 1-D heat-conductivity problem for the lithosphere with a moving lower boundary, is proposed as a model of Cenozoic tectonic activation. The presence of H20 and CO2 fluids in the upper mantle is taken into account. Numerical modelling of the process has revealed that the composition of the upper mantle and fluid phase has a strong influence on the dynamics of the process. The presence of volatiles in the upper mantle leads to the appearence of maxima and minima on the solidus curves. Lamination of fusible and refractory layers in the upper mantle may lead to a sharp change in lithosphere-thinning velocities and, hence, to a discrete character of surface vertical motions. The thickness of the lithosphere in a new equilibrium position is calculated for a different composition of the upper mantle and different values of heat flow supplied to the base of the lithosphere; the results show that for the models that seem best to fit present knowledge of the upper mantle composition, melting of the lower crust may take place only in the later stages of the process, when the lithosphere-anomalous mantle boundary approaches its new equilibrium position.

A lithospheric perspective on structure and evolution of Precambrian cratons

Keywords Craton, Pre-Cambrian, Lithosphere, age, evolution, tectonics, plate tectonics, Geophysical observations