Research paper3D geometry of a shale-cored anticline in the western South Caspian Basin (offshore Azerbaijan)

- The studied anticline is detached from overpressured muds in the South Caspian Sea.
- Geometrical analysis evidences two folding episodes and one mud diapirism event.
- Folding is nucleated above a buried fault with normal and right-lateral components.
- Subsurface mud flow and basin tilting accompany the deposit of the Productive Series.
- Mud teardrop diapirs are isolated in fold culminations rooted by sub-vertical welds.

The internal structure of one of the common fold structures in the western margin of the South Caspian Basin (SCB) has been characterized in 3D using a depth-migrated seismic cube in offshore Azerbaijan. The fold corresponds to a NNW-SSE anticline with a basinward vergence; i.e., eastwards, because in this direction of the margin the SCB is floored by a probable oceanic crust. The anticline has two culminations cut by mud-diapirs and is bounded by two parallel rim synclines with contrasting sedimentary thickness. This anticline deforms congruently the thick Productive Series (PS; Messinian to Late Pliocene), whereas the most recent sequences (<3.1 Ma; e.g., Akchagyl and Apsheron units) onlap or thin towards the fold crest.We reconstruct the existence of two episodes of folding. During deposition of the uppermost PS sequences (ca. 3.5-3.4 to 3.1 Ma), fold uplift initiated with a significantly lower rate than sedimentation. During this epoch, folding was accompanied by basin tilting and by faulting in a basinward normal fault with a limited right lateral, strike-slip component. Motion along this fault zone promoted the downdip flow of a weak layer formed by fluid- and mud-rich sediments (Maykop Formation), which also migrated along strike to build-up the growing anticline. Henceforth, fold growth accelerated and sedimentary units like the Akchagyl (3.1-1.7 Ma) were deposited preferentially in the subsiding flanks. Seafloor upwarping due to folding conditioned the sediment transport, and large deltas adapted their prograding pattern to the growing anticline crest.This structure resembles a detachment fold with a leading, East-vergent forelimb. Nevertheless, the occurrence of progressive tilting accompanying sedimentation and folding, or the mud inflation of the fold core by deep flow parallel to the anticline axis, make this example in the SCB a special example of this fold type.