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Extensional sedimentary basins form a lateral heterogeneity in the lithosphere that may localise deformation if the lithosphere is contracted. Shortening of sedimentary basins may lead to uplift of basin fill sediments and reverse reactivation of normal faults in a process which is referred to as basin inversion. Observations show that not only young, and weak, basins are inverted, but also older, and stronger, basins still localise shortening a long time after the thermal signature from extension has dissipated. The strength of the basin area relative to its surroundings mainly determines how easily shortening may localise in the basin. Our aim is to evaluate the mechanical and thermal parameters that control the ease of inversion of extensional sedimentary basins. We focus on basins which form by continental extension and which do not reach the full rifting stage before they are mildly inverted.
We use a two-dimensional thermo-mechanical model to study extensional basin formation, post-rift cooling and subsequent shortening. Extension of our models leads to the formation of a sedimentary basin with a generally symmetric geometry. This basin is easily inverted after a short post-rift cooling time, while long cooling and strengthening of the lithosphere tends to suppress basin inversion. Localisation of shortening in the model basin is promoted by: (1) strain-weakening of shear zones in the extension phase, which helps their reactivation in contraction, (2) geometrical irregularities created in extension which focus later deformation, (3) surface erosion of uplifted basin fill, (4) lower density sediments which reduce the basin strength, and (5) sediment blanketing which may keep the basin warm, and thus weak, after a long post-rift cooling phase.
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