Analogue Models of Basin Inversion by Transpression: Role of Structural Heterogeneity

William Sassi, Jean-Paul Callot, Luca Mattioni, and Narjes Kallel
Institut Français du Pétrole, Rueil Malmaison, France

The process of fault generation and reactivation within a crustal-scale rift basin inverted by transpression is studied in a series of analogue experiments using sand and silicone. Each structural scenario is a three steps story with first extension of a sandpack resting above a basal silicone layer, then sedimentation with silicone and sand layers, and lastly transpression. The interbedded silicone layer introduces a potential décollement between the pre- and the post-rift sediments. After sedimentation, deformation by transpression was applied for different velocity vectors, with progressive increase of shearing. The occurrence of pre-existing grabens leads in general to a two phases structure generation during transpression: (1) creation of reverse faults parallel to the rift-bounding faults; (2) reactivation of old thrust as sinistral oblique slip structures at more advanced stages of deformation. Strike-slip faults develop at more advanced stages of transpression, with an orientation that is consistent with the far field applied strain. The general sequence of fault generation and reactivation suggests a temporal change in the stress regime which may result from the permutation of the minimum and the intermediate principal stress axes as well as from a progressive rotation of the axis of the maximum compressive stress. The last parameter that influence the stress regime partitioning is the topography of the interbedded ductile layer, which introduces a mechanical decoupling between the post rift cover and the stretched basin. In summary, this work suggests that, when a pre-existing graben is present, there is in time a succession of two distinct "tectonic phases", whereas without any rift, the resulting fault pattern correspond to a single stress regime and tectonic phase. A comparison with the modelling of a natural example of forced fold (Split Mountain, Utah) will be proposed.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005