Numerical models of the inversion of half‐graben basins

Abstract

We investigate the dynamic evolution of fold and thrust structures which form by compression and inversion of a sequence of half‐graben basins. The choice for a half‐graben geometry is motivated by seismic studies and reconstructions of preinversion geometry of inverted regions, which show that rifting often leads to a series of half‐grabens. To examine the deformational structures which result from basin inversion, we use a two‐dimensional, viscous‐plastic numerical model and start our experiments from a preexisting extensional geometry. We find that synrift and postrift sediments are uplifted in the initial stages of basin inversion. This uplift is accompanied by rotation of the basement blocks beneath the basins. In the postrift sequence new shear zones form which are a continuation of basin‐bounding faults. With continuing shortening, further inversion is more difficult owing to relative strengthening of the half‐graben region. Significant surface erosion facilitates inversion. Back thrusts mainly develop in association with listric basin‐bounding faults and less for planar (domino array) faults. Weak sediments (such as salt or shales) at the base of the basins promote the development of basement shortcut faults. The presence of a postrift decollement layer tends to decouple deformation of the postrift sediments from the material below it. In our model, preexisting weak basin‐bounding shear zones are a requirement for substantial amounts of basin inversion to occur. Our numerical model results display many first order characteristics of examples from nature and analogue studies.