Superswells

The most pronounced features in the lower mantle revealed by seismic tomography are two extensive regions of low velocity beneath the south Atlantic and southern Africa, and the central and southwest Pacific (Plates 12.1, 12.2 between pp. 244 and 245). These correlate with anomalously high elevation of the Earth's surface in these areas. Indeed the width of the topographic swell in each case, several thousand kilometers, is so large that they have been termed superswells (McNutt & Judge, 1990; Nyblade & Robinson, 1994). This is in contrast to the topographic swells associated with hot spots that are typically less than 1000 kilometers across. However the elevated topography and bathymetry of superswells cannot be explained by anomalously high temperatures and/or low density rock types in the lithosphere and asthe-nosphere beneath these regions (Ritsema & van Heijst, 2000). The only plausible explanation is that they are dynamically supported by major upwellings of hot material in the lower mantle (Hager et al., 1985; Lithgow-Bertelloni & Silver, 1998). These hot, low velocity regions, defined by seismic tomography, appear to rise from the thermal boundary layer at the core-mantle boundary (Plate 12.2 between pp. 244 and 245) (Section 12.8.4).

Just as upwellings in the mantle produce regional uplift of the Earth's surface, downwellings produce regional subsidence (Gurnis, 2001). The most notable example of depressed crust at the present day is the Indonesian region. This is situated above anomalously high seismic velocities in the transition zone and upper part of the lower mantle (Plate 12.2b between pp. 244 and 245) that probably reflect a confluence of downgo-ing lithospheric slabs. Seismic tomography can only map regions of low and high velocity, and hence possible upwellings and downwellings in the mantle, at the present day. However, evidence from the geologic record for regional scale elevation and subsidence of the Earth's crust may indicate that a particular area has been underlain by a major mantle upwelling or deep subducting slabs in the past. Originally it was assumed that changes in sea level, causing major marine transgressions and regressions on continental crust, were synchronous worldwide, away from areas of active tectonism. However, as more data accumulated it became clear that this was not so, although an obvious explanation was lacking. It is now apparent that elevation and subsidence of the lithosphere associated with convection in the mantle, could provide an explanation for what were previously some very enigmatic observations.

Denver, Colorado in the central USA has an elevation of 1.6 km but is underlain by Cretaceous sediments typical of shallow water deposition. At that time the Farallon plate, the eastern flank of the East Pacific Rise in the northeast Pacific, was being subducted beneath western and central North America and is thought to have caused depression of the crust above it. With the progressive elimination of the East Pacific Rise in the northeast Pacific throughout the late Cenozoic, the Farallon plate has become detached and continues to sink eastwards, allowing the buoyancy of the crust of the western and central USA to reassert itself, thereby causing the uplift of the Colorado region. Van der Hilst et al. (1997), using seismic tomography, imaged the sinking Farallon plate 1600 km beneath the eastern USA. Similar anomalous vertical movements of parts of Australia since the early Cretaceous are thought to be due to the influence of downwellings created by subduction zones, initially to the east of Australia, and more recently to the north (Gurnis et al., 1998).

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