FIGURE 2.12 Interpretive line drawing from seismic reflection data. The upper crust where the Bagdad Reflection Sequence is observed has remained relatively intact throughout Tertiary deformation, limiting the location of significant deformation associated with detachments. From R. K. Litak and E. C. Hauser (1992), Bull. Geol. Soc. Am. 104, 1315-1325.

rifting, extensional events, thermal events, and volcanic activity. Therefore, detailed seismic studies of these features provide information not only on present characteristics but also on the geological and tectonic evolution of the continental crust.

Substantial amounts of new information are generated nearly continuously by the recording of earthquake-generated seismic signals on network and array stations. These data are being used to locate and study earthquake sources and to map crustal structure variations beneath the network of stations.

Higher-than-average velocities for seismic shear waves are observed in the upper mantle beneath continents to depths of at least 150 km. Though diminished in amplitude, the high velocities locally appear to extend down as far as 400 km, as in Canada's 2.7-billion-year-old Superior geological province. This evidence for continental roots suggests that the mineralogical constitution, and hence bulk chemical composition, of the upper mantle beneath ancient continental crust differs from that of the surrounding mantle. One explanation for the origin of this distinct mantle beneath continents is that it represents the residue left behind when partial melts were removed to form overlying crust. Melt removal leaves a residue that is less dense than the original material. Therefore, a melt-depleted mantle root could be buoyantly stable beneath a continent even though it might eventually cool to lower temperatures than the surrounding mantle. The presence of the anomalous mantle material may help to protect the overlying continental block from the effects of convection at greater depth. Indeed, old continental crust may owe its long survival in an otherwise very active and changing surface environment to the distinctive composition of its underlying mantle.

Direct samples of the subcontinental mantle reach the surface as fragments torn off conduit walls of certain types of explosive volcanic eruptions. Pressure- and temperature-dependent changes during eruption have left their signature in differences between certain minerals in these rocks. Recognition of the signatures allows the depth of origin and original temperature of these materials to be determined. Based on this type of information, a well-catalogued sample suite spanning a depth range to 200 km is available in many areas, particularly southern Africa. Results of the analyses of coexisting minerals in these xenoliths have provided estimates of temperature as a function of depth down to 150 to 200 km, giving a fossil geotherm for the date of the eruption, tens of hundreds of millions of years ago. This remarkable geophysical result from mineral analyses is a good example of the interdependence of different approaches to the earth sciences. Field and petrological studies identified the rocks as samples derived from the mantle, experimental calibrations and thermodynamic calculations defined the mineral compositions in terms of pressure and temperature, and refinement of the electron microprobe facilitated analyses of sufficient accuracy that the calibration could be applied to the rocks. These samples of subcontinental mantle are depleted of their easily meltable component but, curiously, are enriched in a number of trace elements that also would be expected to be depleted by melt removal. The pattern of trace element enrichment of subcontinental mantle mirrors that of the continental crust. This suggests that the subcontinental mantle may serve both as the ultimate source

The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement