Pan‐American Symposium on the Upper Mantle

Abstract

The Pan‐American Symposium on the Upper Mantle, held in Mexico City from March 18 to 21, 1968, was attended by 27 participants from countries south of Mexico, by 22 from countries north of Mexico, and by 3 from Europe. A significant number of registrants and other guests attended from the host country. Forty scientific papers were presented in a program formally subdivided into collections of offerings pertinent to the geophysics of North America, Mexico and Central America, South America, and problems of general geophysical interest.Because of the large number of papers presented, it is impossible to review all of them thoroughly. Only the highlights will be reviewed here. Much of the excitement at the Symposium was generated by information contributed to the geophysics of the Americas by recent results concerning global tectonics. These results are derived from direct observations of sea‐floor spreading and the relative motion (or drift) of large blocks of matter of the surface of the Earth relative to one another. This point of view was elaborated by McKenzie in describing the ‘paving block’ theory of the motion of parts of the surface of the Earth relative to one another. The theory of McKenzie and Parker indicates that a surface area is being created at oceanic ridges, is disappearing into the oceanic trenches, and is neither created nor destroyed along transform faults; earthquake motions along the ridges are tensile, under the trenches are mainly compressional, and along the transform faults are of the strike‐slip type. A global map of earthquake epicenters thus illuminates large plates or ‘paving block,’ each bounded by all three types of fault. The paving blocks move more or less as rigid bodies over the surface of the Earth. Where only two blocks are in contact, the relative displacements are in the same direction over their entire common boundary. The surface of the Earth can be described by at least six major blocks, all shifting about relative to one another.