The North American upper mantle: Density, composition, and evolution

Abstract

[1] The upper mantle of North America has been well studied using various seismic methods. Here we investigate the density structure of the North American (NA) upper mantle based on the integrative use of the gravity field and seismic data. The basis of our study is the removal of the gravitational effect of the crust to determine the mantle gravity anomalies. The effect of the crust is removed in three steps by subtracting the gravitational contributions of (1) topography and bathymetry, (2) low-density sedimentary accumulations, and (3) the three-dimensional density structure of the crystalline crust as determined by seismic observations. Information regarding sedimentary accumulations, including thickness and density, are taken from published maps and summaries of borehole measurements of densities; the seismic structure of the crust is based on a recent compilation, with layer densities estimated from P-wave velocities. The resultant mantle gravity anomaly map shows a pronounced negative anomaly (−50 to −400 mGal) beneath western North America and the adjacent oceanic region and positive anomalies (+50 to +350 mGal) east of the NA Cordillera. This pattern reflects the well-known division of North America into the stable eastern region and the tectonically active western region. The close correlation of large-scale features of the mantle anomaly map with those of the topographic map indicates that a significant amount of the topographic uplift in western NA is due to buoyancy in the hot upper mantle, a conclusion supported by previous investigations. To separate the contributions of mantle temperature anomalies from mantle compositional anomalies, we apply an additional correction to the mantle anomaly map for the thermal structure of the uppermost mantle. The thermal model is based on the conversion of seismic shear-wave velocities to temperature and is consistent with mantle temperatures that are independently estimated from heat flow and heat production data. The thermally corrected mantle density map reveals density anomalies that are chiefly due to compositional variations. These compositional density anomalies cause gravitational anomalies that reach ∼250 mGal. A pronounced negative anomaly (−50 to −200 mGal) is found over the Canadian shield, which is consistent with chemical depletion and a corresponding low density of the lithospheric mantle, also referred to as the mantle tectosphere. The strongest positive anomaly is coincident with the Gulf of Mexico and indicates a positive density anomaly in the upper mantle, possibly an eclogite layer that has caused subsidence in the Gulf. Two linear positive anomalies are also seen south of 40°N: one with a NE-SW trend in the eastern United States, roughly coincident with the Grenville-Appalachians, and a second with a NW-SE trend beneath the states of Texas, New Mexico, and Colorado. These anomalies are interpreted as being due to (1) the presence of remnants of an oceanic slab in the upper mantle beneath the Grenville-Appalachian suture and (2) mantle thick...

Mantle origin of the Emeishan large igneous province (South China) from the analysis of residual gravity anomalies

“…For example, if we rely on seismic profiles recorded before the 1990s, the uncertainty in the Moho depth could be up to 10% (4-5 km) (Mooney and Kaban, 2010;Mooney et al, 1998). However, when we consider a region sufficiently explored by more recent seismic profiles, the uncertainty can be reduced to 5% (2 km).…”

“…Therefore, although the topography and free-air effects have been removed from the Bouguer gravity, in order to isolate the gravity response of Emeishan LIP we have to remove other gravitational effects, in particular those caused by the sedimentary cover, crystalline basement, the undulation of the upper crust, Moho and the lithosphere. This sequential procedure, referred to as stripping, was first described by Hammer (1963) and later developed by other authors (Bielik, 1988;Bielik et al, 2013a, b;Mooney and Kaban, 2010). Low-density sediments result in a negative gravity anomaly relative to the crystalline crust.…”

“…It is generally assumed that any change affecting the horizontality of the homogeneous reference density model would lead to a change in the residual gravity anomaly (Mooney and Kaban, 2010), whereas the gravity from a uniform horizontal layer with invariable density is a constant. Our reference model (Fig.…”

“…Our reference model (Fig. 2) corresponds to a continental crust with flat topography, and consisting of a 15-km-thick upper crust with a density of 2.7 g/cm 3 (Mooney and Kaban, 2010), and a 25-kmthick lower crust with a density of 2.94 g/cm 3 Mooney and Kaban, 2010). The average density of the mantle lithosphere is set equal to 3.33 g/cm 3 , while that of the asthenosphere is 3.31 g/cm 3 (Burov, 2010).…”

“…Globally, there has been an increasing body of evidence for the existence of deep mantle plumes, particularly on the basis of the results supplied by seismic tomography (Goes et al, 1999;Lei and Zhao, 2006;Lü et al, 2013;Montelli et al, 2004Montelli et al, , 2006Rhodes and Davies, 2001;Ritsema and Allen, 2003;Romanowicz and Gung, 2002;Zhao, 2001). In this paper we assess the mantle plume model for the Emeishan LIP based on a detailed modeling of gravity data, because neither the seismic P-wave nor S-wave velocities by themselves provide sufficient information about the density structure of the lithosphere (Mooney and Kaban, 2010). The cratonic roots, which show positive S-wave velocities, offer a clear example of the ambiguity of the seismic models for determining lithospheric density.…”