Inversion of accommodation zones in salt-bearing extensional systems:insights from analog modeling

This work uses sandbox analog models to analyze theformation and subsequent inversion of a decoupled extensional systemcomprised of two segmented half-grabens separated by a diffuse accommodationzone with thick early syn-rift salt. The segmented half-grabens strikeperpendicular to the direction of extension and subsequent shortening.Rifting first created a basement topography that was infilled by model salt,followed by a second phase of extension and sedimentation, followedafterwards by inversion. During the second phase of extension, syn-riftsyncline minibasins developed above the basement extensional system andextended beyond the confines of the fault blocks. Sedimentary downbuildingand extension initiated the migration of model salt to the basement highs,forming salt anticlines, reactive diapirs, and salt walls perpendicular tothe direction of extension, except for along the intervening accommodationzone where a slightly oblique salt anticline developed. Inversion resultedin decoupled cover and basement thrust systems. Thrusts in the cover systemnucleated along squeezed salt structures and along primary welds. Newprimary welds developed where the cover sequence touched down on basementthrust tips due to uplift, salt extrusion, and syn-contractionaldownbuilding caused by the loading of syn-contractional sedimentation. Modelgeometries reveal the control imposed by the basement configuration anddistribution of salt in the development of a thrust front from the inversionof a salt-bearing extensional system. In 3D, the interaction of saltmigrating from adjacent syn-rift basins can modify the expected saltstructure geometry, which may in turn influence the location and style ofthrust in the cover sequence upon inversion. Results are compared to theNorthern Lusitanian Basin, offshore Portugal, and the Isabena area of the South-Central Pyrenees, Spain.