Density of the continental roots: compositional and thermal contributions

Abstract

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. ß

The Kalahari Epeirogeny and climate change: differentiating cause and effect from core to space

“…Like most other Archean continental blocks, this craton (which in aerial extent and lithospheric depth dominates all other younger tectonic domains) is underlain by thick lithospheric mantle that is chemically relatively depleted and buoyant compared to younger surrounding continental lithosphere (Jordan 1975;Vinik et al, 1996;Parman et al, 2004;James et al, 2004;Niu et al, 2004). The chemical characteristic of this thick mantle lithosphere has been pivotal in buffering the tectonic and thermal history of southern Africa as a whole (Doucouré and de Wit, 2003a;Kaban et al, 2003;Micheat et al, 2007, and references therein). Nevertheless, episodically secondary (metasomatic) processes have significantly affected the chemistry, mineralogy, and density of the Kaapvaal cratonic lithosphere to varying degrees subsequent to its initial formation more than 3.2 Ga (de Wit et al, 1992;le Roex et al, 2003;Bell and Moore, 2004;Becker and le Roex, 2004;Harris et al, 2004;James et al, 2004;Shirey et al, 2005;Michaut et al, 2007).…”

“…Nevertheless, episodically secondary (metasomatic) processes have significantly affected the chemistry, mineralogy, and density of the Kaapvaal cratonic lithosphere to varying degrees subsequent to its initial formation more than 3.2 Ga (de Wit et al, 1992;le Roex et al, 2003;Bell and Moore, 2004;Becker and le Roex, 2004;Harris et al, 2004;James et al, 2004;Shirey et al, 2005;Michaut et al, 2007). These have likely influenced at least second order epeirogenic features (Doucoure and de Wit, 2003a;Kaban et al, 2003).…”

The Sub-Crustal Stress Field in the Taiwan Region

“…Different estimates were given in earlier studies. Zhong (1997) reported on long wavelengths larger than 800 km, Sjöberg (1998) Kaban et al 2003Kaban et al , 2004. The tectonic plate collision in our study area could be the main reason for uncertainties in the gravimetrically determined Moho depths.…”

Effect of Crustal Density Structures on GOCE Gravity Gradient Observables

“…There are several different geophysical phenomena (such as glacial isostatic adjustment, plate flexure, crustal rigidity, plate motion, mantle convection/thermal compensation) which can also be interpreted by means of gravity field analysis. For more information we refer readers to studies by Kaban et al (1999Kaban et al ( , 2003Kaban et al ( , 2004, Watts (2001, p. 114), Braitenberg et al (2006), Wienecke et al (2007), Tenzer et al (2009aTenzer et al ( , 2012b, Bagherbandi and Sjöberg (2012), Negretti et al (2012), and others.…”