Density, temperature, and composition of the North American lithosphere—New insights from a joint analysis of seismic, gravity, and mineral physics data: 2. Thermal and compositional model of the upper mantle

Abstract

Temperature and compositional variations of the North American (NA) lithospheric mantle are estimated using a new inversion technique introduced in Part 1, which allows us to jointly interpret seismic tomography and gravity data, taking into account depletion of the lithospheric mantle beneath the cratonic regions. The technique is tested using two tomography models (NA07 and SL2013sv) and different lithospheric density models. The first density model (Model I) reproduces the typical compositionally stratified lithospheric mantle, which is consistent with xenolith samples from the central Slave craton, while the second one (Model II) is based on the direct inversion of the residual gravity and residual topography. The results obtained, both in terms of temperature and composition, are more strongly influenced by the input models derived from seismic tomography, rather than by the choice of lithospheric density Model I versus Model II. The final temperatures estimated in the Archean lithospheric root are up to 150 C higher than in the initial thermal models obtained using a laterally and vertically uniform ''fertile'' compositional model and are in agreement with temperatures derived from xenolith data. Therefore, the effect of the compositional variations cannot be neglected when temperatures of the cratonic lithospheric mantle are estimated. Strong negative compositional density anomalies (<20.03 g/cm 3 ), corresponding to Mg # (100 3 Mg/ (Mg 1 Fe)) >92, characterize the lithospheric mantle of the northwestern part of the Superior craton and the central part of the Slave and Churchill craton, according to both tomographic models. The largest discrepancies between the results based on different tomography models are observed in the Proterozoic regions, such as the Trans Hudson Orogen (THO), Rocky Mountains, and Colorado Plateau, which appear weakly depleted (>20.025 g/cm 3 corresponding to Mg # 91) when model NA07 is used, or locally characterized by high-density bodies when model SL2013sv is used. The former results are in agreement with those based on the interpretation of xenolith data. The high-density bodies might be interpreted as fragments of subducted slabs or of the advection of the lithospheric mantle induced from the eastward-directed flat slab subduction. The selection of a seismic tomography model plays a significant role when estimating lithospheric density, temperature, and compositional heterogeneity. The consideration of the results of more than one model gives a more complete picture of the possible compositional variations within the NA lithospheric mantle.

Density structure and isostasy of the lithosphere in Egypt and their relation to seismicity

“…The initial 3-D density model of the upper mantle is created based on available seismic models. The velocity-todensity conversion factor is computed based on mineral physics relations (e.g., Tesauro et al, 2014).…”

“…A complete description of this technique can be found in Tesauro et al (2014) and Kaban et al (2016a). Three slices of density variations for the depths 100, 150, and 200 km are shown in Fig.…”

“…The density of the crustal layers corresponds to the estimates of Christensen and Mooney (1995). The mantle densities are determined as the global averages estimated based on the seismic velocities provided by the model of Schaeffer and Lebedev (2013), assuming a "fertile" composition of the upper mantle material (Tesauro et al, 2014). The parameters of this model are the same as those used in studies of other regions and on larger scales (e.g., Kaban et al, 2016), which provides the possibility for direct comparison of the results.…”

“…However, they cannot provide a complete image of the structure of the crust and upper mantle. For example, seismic velocities in the upper mantle are more sensitive to temperature than to compositional variations (e.g., Tesauro et al, 2014); therefore, tomography images primarily reflect temperature variations. A clear example is represented by high-density eclogitic rocks, which are characterized by seismic velocities that are close to normal upper mantle conditions and therefore are almost invisible in seismic models (Krystopowicz and Currie, 2013).…”

3D density model of the upper mantle of Asia based on inversion of gravity and seismic tomography data

“…SL2013sv provides variations of vertically polarized share‐wave velocities ( V s ) for the upper mantle and transition zone. For conversion of seismic velocity into density, we used the same method as that previously employed for North America [ Kaban et al ., ; Tesauro et al ., ] and the Middle East [ Kaban et al ., ]. The initial temperatures and densities were computed globally in the uppermost mantle up to a depth of 325 km by using the approach of Stixrude and Lithgow‐ Bertelloni [].…”