Inversion of extensional sedimentary basins: A numerical evaluation of the localisation of shortening

Geological observations show that extensional sedimentary basins may be inverted by shortening long after they formed. This is counterintuitive because, unlike young basins, they are no longer underlain by a warm weak mantle. We examine processes that control whether extensional sedimentary basins are inverted in shortening using a numerical approach to model the formation of a continental extensional basin, postrift cooling, and the response to imposed shortening. Extension of our models leads to the formation of a symmetric sedimentary basin. Shortening inverts the basin by uplift of sediments and reverse reactivation of normal shear zones. We find that localisation of shortening in extensional domains is promoted by (1) a short postrift phase that allows the basin to warm up while preventing the upper mantle from cooling too much, (2) strain weakening of shear zones in extension, which helps their reactivation in contraction, (3) synrift sediments that are mechanically weak and have a low thermal conductivity, and (4) irregularities in crust and Moho geometry which were created in extension and which help focus deformation during inversion. Once basin uplift starts, surface erosion strongly promotes further localisation of shortening. We show that old and thermally strong basins can still be inverted if they are associated with weak shear zones, weak sediments with a low thermal conductivity, and/or crustal mechanical irregularities.