Periodic instabilities during compression of the lithosphere: 2. Analogue experiments

We have modeled the behavior of the continental and oceanic lithospheres under compression, using materials with analogous properties in laboratory experiments, to study the development of lithospheric buckling. Periodic instabilities, which are a major deformation process during the compression of the lithosphere, have already been described by several authors using an analytical perturbation method. At small strains, analogue experiments corroborate most of the results obtained by the perturbation method: (1) the deformation modes (geometrical relationships of interfaces and related wavelengths) are mainly dependent on the spatial distribution of the brittle layer(s), and (2) the amplitude of buckling is an exponential function of the horizontal strain. Some departure from the perturbation method occurs when there are two instabilities growing concurrently. The breakdown of the exponential growth occurs for strains of about 5%, and is concomitant with the appearance of thrust faults. In experiments including one brittle layer, which model the compression of the oceanic lithosphere, faults are regularly located at the inflection points of the folds. In experiments including two brittle layers, which model continental lithosphere, faults form more complicated patterns with an asymmetrical deep thrust overlain by a fan‐shaped symmetrical thrust system in the upper brittle layer. Such fault geometries give some new highlights on typical compressive geological structures such as those encountered in Central Asia.