THE KINEMATICS OF BELT BENDING: INSIGHTS FROM PALEOMAGNETISM AND MODELING

The kinematic evolution of arcs has long been debated in the last decades. It is still an open question whether curved fronts arise from local perturbation of stress directions or from bending of originally rectilinear structures. Till now paleomagnetism has been used in field analyses to detect vertical axis rotations in curved belts, but magnetic mineralogy problems, magnetic overprint, and the poor knowledge of local and regional tectonic settings rarely allowed to fully unraveling the arc formation process.

Paleomagnetism of physical models could overcome these difficulties as all the physical parameters controlling the system are imposed and fully controlled. Some experiments were performed to model the formation of arcuate fronts due to lateral termination of thrusting and crustal obstacles impeding the forward propagation of the belt.

The experiments were performed in sandbox using sand mixed with hematite powder. The models were magnetized before deformation. After deformation the models were sampled using 1x1-cm Plexiglas cylinders and their remanent magnetization was measured by a cryogenic magnetometer. The samples were magnetically cleaned by alternating field up to the intensity of 0.1 T, and the remanence was measured after each demagnetization step.

We tested the formation of arcs due to: 1) lateral termination of thrust fronts; 2) presence of obstacles oblique to the shortening direction; 3) occurrence of two obstacles located at the lateral end points of the advancing curved belt.

The results from modeling have been compared to field examples from some curved belts.