| Popis: |
Kinematic analysis of asymmetric detachment folds from the Wildhorn nappe, central Helvetics, Switzerland, supports models of buckle fold formation in overthrust shear between nappe boundaries. The overall geometry shows a 2500 m thick multilayer of competent Jurassic limestones deflected into super- and subjacent less competent marls and shales by buckling above the basal decollement. Detailed investigation and measurement of the distribution, orientation, and relative timing of mesoscale structures, primarily dilatant veins and solution cleavage, allow documentation and quantification of the following kinematic development: (1) the nappe boundaries originated at a low angle (ca. 10°) to undeformed bedding; (2) local stress reorientation and buckling instabilities initiated symmetric buckle folds possibly characterized by tangential longitudinal strain; (3) the far-field stress directions imposed by the shear zone became dominant, and mesoscale structures developed asymmetrically on backlimbs and forelimbs; and (4) buckling ceased when fold interlimb angles reached 90–100°, and further shearing caused the fold limbs and axial planes to rotate into their current orientations. The total shear strain required is 2.9, equivalent to ca. 8 km of displacement of the overlying Cretaceous with respect to the base of the nappe. It is suggested that only an overthrust shear model can explain all the observations, and that other models, such as gravitational sliding and buttressing by normal faults, are incompatible with the data. |
