The 3D propagation of décollement in the Jura

Summary The sequence of instabilizations leading to the formation of shallow nappes may be illustrated by the kinematics of the embryonic Jura Nappe. On the first order arc, due to regional boundary conditions, are superposed second and third order arcs that developed around more local irregularities in the distribution of strength. Successive instabilization of the Rheintal part of the Jura Décollement Nappe generally proceeds from S to N. Location of ramps where décollement climbs to the surface is largely influenced by pre-existing Rhinegraben structures. These also convey a certain anisotropy to the sedimentary cover which seems to be responsible, at least partly, for a different aspect of sinistral and dextral shear zones. Whereas sinistral shears are usually along discrete faults, mostly reactivated Rhinegraben faults, the dextral shears commonly are diffuse zones. Arcuate elements, convex to the N, connect the sinistral and dextral shear zones frontally. Regionally, the shear zones accommodate the divergence of movement, with lateral elongation, within the overall Jura Arc. More locally, they bound arcuate segments of ever smaller dimensions, and also seem to ease diverging movements in these smaller arcs.