Print this pageKinematic Characteristics of Reactivated Versus Newly Formed Strike-Slip Zones in Frontal Fold-Thrust Belts: Examples from the Alps
SCHOENBORN, GREG
Strike-slip faults and transverse zones are widespread in frontal fold-thrust belts where they accommodate lateral strains. The existence, nature and orientation of reactivated structures plays a key role in the development of such belts.
In the Jura mountains and the eastern Southern Alps, orientations of strike-slip faults that developed contemporaneously to thrusting swing around with the bend of the arc. They redistribute shortening from one side to the other in a smooth way, lateral movements are nearly absent. Pairs of conjugate faults form symmetric triangular indenters, the transpression along the borders of the indenter is concentrated in narrow fault zones or single faults.
Observations near plutons and carbonate platforms in the Southern Alps suggest that such rigid obstacles tend to give raise to broad and symmetric indenters. Subsequently formed thrust fronts curve around them. Lateral movements are restricted and geometries of strike-slip zones are simple.
Early Mesozoic rifting lead to important normal faults in the thick passive margin sequence of the Lombardian Alps. During Alpine thrusting, reactivation of these faults lead to asymmetric, triangular indenters with severe deviation of local transport directions. They caused lateral escape to both sides with extension in front of the indenter, rotation of blocks around vertical axes with important and complex lateral movements (transtensive and transpressive, partly strain partitioning), and reactivation of other inherited faults during the rotation. The most complex geometries are observed near the intersection of reactivated faults forming acute salients. Similar characteristics are observed in an area deformed by Mesozoic rifting and two superposed phases with strongly oblique orientations: the most complex strike-slip zones develop where two intersecting inherited structures are reactivated.