The North American Upper Mantle: Density, Composition, and Evolution
The North American Upper Mantle: Density, Composition, and Evolution
Credit:
Walter D. Mooney • U.S. Geological Survey, Menlo Park; Misha Kaban • GeoForschungsZentrum (GFZ), Potsdam, Germany/IRIS
Description
Mantle gravity anomaly after removing the effect of lateral variations in mantle temperature. We calculated the gravity expression of that portion of the Farallon slab below 660 km assuming a positive density contrast of 0.025 g/cc. The resultant anomaly is in excellent geographic agreement with our anomaly located in the Gulf of Mexico.
The upper mantle of North America (NA) has been well studied using various seismic methods. Here we investigate the density structure of the NA upper mantle based on the Bouguer gravity anomaly map. The basis of our study is the removal of the gravitational effect of the crust to determine the mantle gravity field. The effect of the crust is removed in three steps by subtracting the gravitational contributions of: (1) topography; (2) low-density sedimentary accumulations; and (3) the 3D density structure of the crystalline crust. Topographic data are taken from a standard data base; information regarding sedimentary accumulations, including thickness and density, are taken from published maps and summaries of borehole measurements of densities; the structure of the crust is from a recent compilation, with layer densities estimated from compressional wave velocities. The resultant mantle gravity anomaly map shows a negative anomaly (-50 to –400 mgal) beneath western NA and the adjacent oceanic region, and positive anomalies (+50 to +350 mgal) to the east of the NA Cordillera. This pattern reflects the well-known division of NA into the stable eastern portion and the tectonically active western portion. In order to separate the contributions of thermal expansion from mantle composition, we apply an additional correction for the thermal structure of the uppermost mantle. The thermally-corrected map reveals mantle density anomalies that are solely due to compositional variations. These anomalies have a magnitude of +250 to –250 mgal. The upper mantle beneath the Canadian shield exhibits a negative anomaly (-50 to –200 mgal) that is consistent with chemical depletion that results in a mantle composition with lower density, also referred to as the mantle tectosphere. The strongest positive anomaly is co-incident with the Gulf of Mexico, and indicates a positive density anomaly in the upper mantle. Two linear positive anomalies are also seen: one with a NE-SW trend in the eastern USA roughly coincident with the Appalachians, and a second with a NW-SE trend beneath the states of Texas, New Mexico, and Colorado. These anomalies are interpreted as due either to: (1) the presence of the Farallon slab at a depth below 300 km; or (2) mantle density anomalies associated with the accretion of 1.8-0.8 Ga Proterozoic terrains along the southern margin of Laurentia. Based on these results, the evolution of the NA upper mantle is depicted in a series of cartoons that display the primary processes that have formed and modified the crust and lithospheric upper mantle.
Photographer / Contributor: Walter D. Mooney • U.S. Geological Survey, Menlo Park; Misha Kaban • GeoForschungsZentrum (GFZ), Potsdam, Germany
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