Jura Tectonics as a Décollement
For many years the structure of the Jura Mountains was interpreted as a decollement whose origin was related to the Alps; in recent years, however, this mode of origin has been questioned. Most of the alternative explanations recognize a decollement to some extent, but attribute it to movement of the basement beneath. Surface and subsurface data are here reviewed to show that the Jura deformation was produced in a gliding sheet, in which the forces of gravity and inertia were generated within the total moving mass. Features of the folded Jura which support the decollement hypothesis are: (1) Nowhere are rocks older than Middle Triassic exposed, which strongly suggests that the folding does not extend to the older rocks. (2) Subsurface data in the Lons-le-Saunier region clearly show that the external border of the Jura has moved northwestward over the eastern margin of the Bresse Basin. (3) Lower Jurassic rocks rest on Upper Jurassic along a horizontal fault 1234 m deep in the Risoux well near the middle of the Jura. (4) The tabular areas, with their absence of folds, are expectable in a decollement. (5) High-angle tear faults, interpreted as not extending into the basement, are normal features of a decollement sheet. A continuous decollement around the southwestern end of the Swiss Plain can reasonably be inferred, connecting the internal Jura, the Saleve, and the Subalpine folds as part of the decollement mass. Elsewhere, the internal border of decollement extends southeastward into the Molasse basin for an unknown distance and probably underlies the entire basin; if it does, a causal relation to the Alps is indicated.