Mantle velocities, high

Tomographic results. Upper-mantle velocity structure (James et al. 20016) was determined by tomographic techniques based on the analysis of delay times from teleseismic broadband waveform data. Relative arrival times of phases P, PKPdf, S and SKS were retrieved via a multichannel cross-correlation procedure using all possible pairs of waveforms (VanDecar Crosson 1990). This procedure produces highly accurate delay times, with typical standard errors for the... 

Mudllows/dehris-llows that prograde downhill at high velocities, destroying obstacles in their path and depositing thick mantles of spoil on the surface Rill-erosion of spoil surface by surface runoff, especially where spoil is low permeability/rainfall is intense... 

The very high apparent seismic wave velocities of about 9 km/sec obtained for the layer below the crust imply that the lunar mantle is differentiated and its composition varies with depth. 

Other consequences of thermally perturbed phase relations in mantle peridotite (Figure 2) are also supported by seismological observations. These include anticorrelation of transition zone thickness and transition zone delay times, whereby positive delays in travel times, which imply slow velocities (and therefore high temperatures), are observed to correlate with negative changes in (thinning of) transition zone thickness (Gu and Dziewonski, 2002 Lebedev et al., 2002). 

Densities and bulk sound velocities in the lower mantle are also consistent with the high-pressure mineralogy of a bulk composition approximating an upper-mantle peridotite, such as pyrolite. Seismic constraints provide no support for iron enrichment of the lower mantle relative to such an upper mantle. Silica enrichment of the lower... 

Karato S.-I. and Jung H. (1998) Water, partial melting and the origin of the seismic low velocity and high attenuation zone in the upper mantle. Earth Planet. Sci. Lett. 157, 193-207. 

Tomographic results show that high-velocity mantle roots extend locally to depths of at least 250 km beneath undisturbed Archaean craton, with no comparable root structures beneath post-Archaean terranes. Neither body- nor surface-wave analyses have yet produced evidence of a low-velocity zone beneath the cratonic keel (Ritsema van Heijst 2000 Freybourger et al. 2001 ... 

---