Dynamic surface topography: A new interpretation based upon mantle flow models derived from seismic tomography

High-resolution seismic models of three-dimensional mantle heterogeneity are interpreted in terms of upper mantle thermal and compositional anomalies. These anomalies produce density perturbations that drive mantle flow and corresponding convection-related geophysical observables, such as the nonhydrostatic geoid, free-air gravity anomalies, and dynamic surface topography, and provide constraints on internal mantle density structure. The convection related observables are corrected for the isostatically compensated crustal heterogeneity and compared with those predicted by tomography-based mantle flow models. Occam inversions of the surface topography and gravity data provide inferences of the velocity–density scaling coefficients, which characterize mantle density anomalies below North America. The inferred density anomalies require simultaneous contributions from temperature and composition. The density and seismic shear velocity anomalies place constraints on the thermochemical structure of the mantle beneath the North American craton. Perturbations in the molar ratio of iron, R = XFe/(XFe + XMg), are used to quantify the compositional anomalies in terms of iron depletion in the sub-continental mantle. Estimates of the extent of basalt depletion in the tectosphere beneath North America are obtained. This depletion is interpreted to produce a local balance between positive chemical buoyancy and the negative thermal buoyancy that would otherwise be produced by the colder temperatures of the sub-cratonic mantle relative to its sub-oceanic counterpart.

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Similarities between the Madeira and Canary Hotspots Revealed by Seismic Anisotropy from Teleseismic and Local Shear-Wave Splitting with the SIGHT Project

10.5194/egusphere-egu21-10797 ◽

2021 ◽

Author(s):

David Schlaphorst ◽

Graça Silveira ◽

João Mata ◽

Frank Krüger ◽

Torsten Dahm ◽

...

Keyword(s):

Shear Wave ◽

Canary Islands ◽

Delay Time ◽

Seismic Anisotropy ◽

Vertical Component ◽

Mantle Flow ◽

Crust And Upper Mantle ◽

Length Scales ◽

Flow Models ◽

Using Data

The Madeira and Canary archipelagos, located in the eastern North Atlantic, are two of many examples of hotspot surface expressions, but a better understanding of the crust and upper mantle structure beneath these regions is needed to investigate their structure in more detail. With the study of seismic anisotropy, it is possible to assess the rheology and structure of asthenosphere and lithosphere that can reflect a combination of mantle and crustal contributions.Here, as part of the SIGHT project (SeIsmic and Geochemical constraints on the Madeira HoTspot), we present the first detailed study of seismic anisotropy beneath both archipelagos, using data collected from over 60 local three-component seismic land stations. Basing our observations on both teleseismic SKS and local S splitting, we are able to distinguish between multiple layers of anisotropy. We observe significant changes in delay time and fast shear-wave orientation patterns on short length-scales on the order of tens of kilometres beneath the western Canary Islands and Madeira Island. In contrast, the eastern Canary Islands and Porto Santo the pattern is much more uniform. The detected delay time increase and more complex orientation patterns beneath the western Canary Islands and Madeira can be attributed to mantle flow disturbed and diverted on small-length scales by a strong vertical component. This is a clear indication of the existence of a plume at each of those archipelagos, nowadays exerting a strong influence on the western and younger islands. We therefore conclude that a plume-like feature beneath Madeira exists in a similar way to the Canary Island hotspot and that regional mantle flow models for the region should be reassessed.This is a contribution to project SIGHT (Ref. PTDC/CTA-GEF/30264/2017). The authors would like to acknowledge the financial support FCT through project UIDB/50019/2020 &#8211; IDL.

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