Constraints on shear velocity in the cratonic upper mantle from Rayleigh wave phase velocity

Abstract

The standard model of the thermal and chemical structure of cratons has been scrutinized in recent years as additional data have been collected. Recent seismological and petrological studies indicate that the notion of cratonic lithosphere as a thick thermal boundary layer with a very depleted and dehydrated composition may be overly simplified and does not explain all aspects of the seismological and petrological observations. We designed a simple forward-modeling experiment to identify a suite of one-dimensional shear-velocity profiles that are representative of the cratonic upper mantle. The mean and standard deviation of phase velocities for Rayleigh waves that travel paths primarily over cratons were calculated from a global data set in the period range 40 to 300 seconds. One-dimensional Earth models were calculated using two approaches, and the predicted Rayleigh wave phase velocity for each model was compared to the observed range to identify models that provide a satisfactory fit to the observations. With the first approach, shear velocity was predicted from cratonic geotherms that were calculated for a range of surface heat-flow and crustal-thickness values. With the second approach, profiles of shear velocity were generated using random perturbations to 1-D global Earth model STW105. In total 26250 geothermv generated and 80,000 randomly generated 1-D Earth models were generated for comparison to the observations. The results show that cratonic shear velocity is higher than STW105 to depths greater than 200 km. The majority of randomly generated models contain increasing velocity with depth to 250-300 km while the majority of geotherm-generated models contain a low velocity layer in the depth range 100-150 km.vi

Shear‐Wave Velocity Structure of Southern Africa's Lithosphere: Variations in the Thickness and Composition of Cratons and Their Effect on Topography

“…Plotting the mean models of the posterior distributions in Figure , we note, first, the very high velocities at 100–150 km depths, up to 4.7–4.8 in all subregions. These values fall near the highest‐velocity values within the range reported for cratons globally (Hirsch et al, ; Lebedev et al, ; Pedersen et al, ; Schaeffer & Lebedev, ). Another robust feature in all regions is the positive velocity gradient ( V s increase with depth) from the Moho to ∼100–150 km depth.…”

Seismic evidence for depth-dependent metasomatism in cratons

“…However, the vertical seismic velocity profiles predicted from such thermal models plus xenolith-derived lherzolitic to dunitic compositions, are systematically different from those inferred from seismic data [Bruneton et al, 2004;Hieronymus and Goes, 2010;Hirsch et al, 2015;. Such forward models predict maximum velocities directly below the Moho where the mantle is coldest, and a downward decrease in velocity (especially V S ) as temperature increases towards the base of the mantle lithosphere [Bruneton et al, 2004;Hieronymus and Goes, 2010;Hirsch et al, 2015]. By contrast, average 1-D velocities determined by modeling or inverting data tend to be relatively constant with depth or display non-monotonic trends [Bruneton et al, 2004;Fishwick and Rawlinson, 2012;Hieronymus and Goes, 2010;Lebedev et al, 2009].…”

“…Indeed, the first-order seismic velocity differences between cratonic and off-cratonic lithosphere can be explained by the differences in temperatures predicted from modeling surface heat flow [Goes and Van der Lee, 2002;Röhm et al, 2000;Shapiro and Ritzwoller, 2004]. However, the vertical seismic velocity profiles predicted from such thermal models plus xenolith-derived lherzolitic to dunitic compositions, are systematically different from those inferred from seismic data [Bruneton et al, 2004;Hieronymus and Goes, 2010;Hirsch et al, 2015;. Such forward models predict maximum velocities directly below the Moho where the mantle is coldest, and a downward decrease in velocity (especially V S ) as temperature increases towards the base of the mantle lithosphere [Bruneton et al, 2004;Hieronymus and Goes, 2010;Hirsch et al, 2015].…”

“…A range of previous studies have inverted seismic data or mapped tomographic seismic structures to lithospheric temperatures and sometimes major-element composition below the cratons [e.g. Afonso et al, 2016;Cammarano and Guerri, 2017;Hirsch et al, 2015;Khan et al, 2013;Lévy and Jaupart, 2011;Priestley and McKenzie, 2013;Shapiro and Ritzwoller, 2004] . In our study, we analyze Rayleigh-wave dispersion data.…”

Multidisciplinary Constraints on the Abundance of Diamond and Eclogite in the Cratonic Lithosphere

“…This approach will underestimate the isotropic shear velocity V siso . On the other hand, studies that take into account radial anisotropy, and base their modeling on profiles of V siso , have emphasized that the high shear velocity structure beneath cratons cannot be matched solely by peridotite in the depth range ~100–170 km (e.g., Hirsch et al, ). Meanwhile, in some cratons (e.g., Kaapvaal craton, South Africa: Jones et al, ; and Dharwar craton, India: Maurya et al, ), V s is known to be comparatively low, but these cratons are small—such that “pure path” estimations of velocities (source‐station paths contained entirely within the craton region) are more difficult to obtain, especially at the long periods sensitive to the deeper parts of the lithosphere.…”