Nonparametric Estimation to Reconstruct the Deformation History of an Active Fold in the Caspian Basin

Abstract

The complete shape of an active fold in the western margin of the South Caspian Basin is established with a seismic section from a post-stacked cube migrated in depth. The structure is an open anticline, which deforms a thick sequence (more than 7 km) of Late Miocene to Pliocene sediments: the productive series (PS 5.9–3.1 Ma). A major erosive unconformity separates the most recent sediments with onlap and draping geometries towards the anticline culmination. Deformation is reconstructed using the complete fit of numerous reflections by the nonparametric regression method. This fold is detached above a weak, shale-rich unit located between 8 and 10 km depth dipping gently towards the E (7$^{\circ }$). The fold geometry resembles a detachment fold, although it is reconstructed a long-lived history of basinward tilting accompanying sedimentation and folding, which diminished from 0.61 to 0.34$^{\circ }$/Ma during deposition of the PS. Fold growth started at 3.5 Ma within the upper PS with a shortening rate of 0.2 mm/year and coinciding with maximum sedimentation rates (3.24 mm/year). Folding continued up-to-Present under lower sedimentation rates (av. 0.66 ± 0.2 mm/year) and a shortening rate that increased slightly from 0.17 mm/year at 3.1 Ma to 0.29 mm/year during the last 1.6 Ma. The balance between sedimentation and shortening rates varies during folding, explaining the contrasting geometries observed in the growth sediments. It is also inferred that the possible occurrence of lateral flow of overpressured mud-rich sediments inflated the fold core during shortening, similarly to the upwelling process commonly described in salt-core anticlines.