Horizontal/vertical displacements and their rates

This chapter widens our scope towards translation and rotation. These displacements are critical in tectonic studies along with distortion, which has so far been the main subject of this book. Note that a translation needs a fixed reference to be defined. Consider, for example, the glide of the lithosphere over the asthenosphere with respect to the mid-Atlantic ridge. On a spherical surface, a translation is also a rotation, where the horizontal displacement of a plate is governed by a rotation axis (or pole) and an angular velocity. Vertical displacements of the Earth’s surface are one order of magnitude less (on the order of mm per year) than horizontal displacements. Lots of studies, performed in the frame of a discipline called neotectonics, attempt to determine uplift rates through different techniques, such as fission track and radioactive decay. They will be briefly presented in this chapter. The main reference for vertical displacements is the sea level, which itself is mobile through time. This mobility is obviously sensitive when processes such as erosion or post-glacial rebound are considered, but much less significant when exhumation of deep-seated rocks may reach kilometres during an orogeny.