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Investigation of the Effect of Pre- and Syn-Rift Stratigraphy from the Propagation of Normal Faults: A Discrete Element Approach

Finch, Emma 1; Gawthorpe, Rob 1
1 SEAES, University of Manchester, Manchester, United Kingdom.

A discrete element technique has been developed to investigate the effect of cover strength on the propagation of blind normal faults of varying dip. It has illustrated that slip on a basement fault through a pre-rift sequence with no syn-rift sedimentation is initially observed through the growth of an upward-widening monocline in the cover where the width of the monocline increases as fault dip decreases. Increased slip results in the monocline being breached by a single, through-going structure, where strain localizes onto a single fault resulting in hangingwall synclines and footwall anticlines. Fault propagation and folding in the cover are strongly controlled by the strength of the overburden material. The inclusion of syn-tectonic sedimentation alters dramatically the response of the pre-rift material to fault propagation. The relative competence of the syn-rift material modifies deformation in the pre-rift, where a less competent syn-rift results in rapid propagation of the fault, whereas a more competent material rotates the deformation field into the footwall and results in greater failure of the pre-rift, with observable footwall thinning and hangingwall thickening of beds being replaced by shallow gradient beds dipping into the hangingwall.

The inclusion of less competent layers in the pre-rift at varying depth during constant syn-rift sedimentation results in the fault propagating further into the cover and the monocline being breached earlier. The depth of the weakness within the pre-rift also affects deformation where antithetic faults develop in the hangingwall when less competent layers are higher in the pre-rift. Replacing these weak layers with localized detachments narrows the deformation field ahead of the fault tip when it is low in the pre-rift and results in greater monoclinal folding in the hangingwall when it is high in the pre-rift.

The introduction of increased thicknesses of pre-rift with varying basement fault dip and material properties clearly demonstrates the importance of mechanical stratigraphy on fault propagation. The inclusion of weak layers causes deformation to localize and back-step into the footwall where faults develop along layer boundaries and are not necessarily linked to the basement fault at depth. The location of relatively less-competent layers, or detachments, within the pre-rift dominates subsequent failure in the cover sequences and controls fault evolution in both the pre- and syn-rift sediments.

 

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009