PERIODIC INSTABILITIES DURING COMPRESSION OF THE LITHOSPHERE .2. ANALOG EXPERIMENTS

We have modeled the behavior of the continental and oceanic lithospher es under compression, using materials with analogous properties in lab oratory 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 se veral authors using an analytical perturbation method. At small strain s, analogue experiments corroborate most of the results obtained by th e perturbation method: (1) the deformation modes (geometrical relation ships of interfaces and related wavelengths) are mainly dependent on t he 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 inst abilities growing concurrently. The breakdown of the exponential growt h occurs for strains of about 5%, and is concomitant with the appearan ce of thrust faults. In experiments including one brittle layer, which model the compression of the oceanic lithosphere, faults are regularl y located at the inflection points of the folds. In experiments includ ing two brittle layers, which model continental lithosphere, faults fo rm 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.