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No. 35: Sep-Oct 1984

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Confusing Seismic Data From The Deep Continental Crust

Seismic exploration of the deep continental crust seems to indicate that huge sheets of crystalline rock have been pushed over sedimentary strata. The crystalline sheets, perhaps kilometers in thickness, were forcibly shoved hundreds of kilometers over sedimentary deposits during continental collisions -- so the theory goes. One such crystalline sheet is under the Southern Appalachians. Seismic data say it is about 10 kilometers thick and was pushed westward some 225 kilometers. If it seems intuitively impossible for such a thin sheet to remain intact during 225kilometers of shoving over other rocks, consider a similar sheet in the Basin and Range province of Utah. This sheet was pulled down an inclined fault without coming apart!

These sliding sheets with remarkable structural integrities are required to explain what geophysicists see in the seismic reflections; namely, transparent zones of crystalline rock sitting on top of rocks that return strong reflections typical of layered sedimentary strata. However, one such situation in Arizona was explored with a drill bit. When the upper crystalline layer was penetrated, the drill found only more crystalline rocks, nothing sedimentary. In fact, the crystalline rock was not layered and was homogeneous. Thus, the source of the misleading seismic reflections is unknown.

(Kerr, Richard A.; "Continental Drilling Heading Deeper," Science, 224:1418, 1984. Also: Anonymous; "Probing the Deep Con-tinental Crust," Science, 225:492, 1984)

Comment. So-called "low-angle thrust faulting is often invoked to explain situations where large areas of older sedi-mentary rocks overlie younger rocks. More on these thrust faults may be found under ESR3 in Inner Earth. For ordering information, visit: here.

From Science Frontiers #35, SEP-OCT 1984. � 1984-2000 William R. Corliss