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Other Analyses |
Department of Geology, Science Laboratories, South Road, Durham
Seismic observations show that young fold mountains are typically associated with thickening of the crust, and normal P-wave velocity in the top-most mantle. Gravity observations show that fold mountains are in approximate regional isostatic equilibrium. The gravity and seismic observations together show that the density in the upper mantle beneath the mountains is not strongly anomalous. In these fundamental respects fold mountains differ greatly from ocean ridges and plateau uplifts (such as western U.S.A.). Beneath ocean ridges and plateau uplifts the crust is not thickened but the upper mantle has anomalously low density and seismic velocity.
The structure of the crust and upper mantle beneath mountain ranges can be explained by crustal shortening amounting to about half the width of the range; on this hypothesis the primary cause of the mountain range is horizontal compression. Alternatively, the root may have been added to the crust by differentiation from the mantle, the mountain range being primarily formed by vertical movement The crustal-shortening hypothesis has been greatly strengthened by the near-proof that large horizontal displacements related to continental drift have affected the ocean crust, and by the increasing realization that mantle convection is a mechanically valid hypothesis. Further indirect support for crustal shortening comes from energy considerations—for instance the gravitational energy associated with dissipation of a root by vertical uplift appears to be inadequate to explain the present-day energy release by earthquakes.
