Neogene shale-detached extension and contraction linked system in the Akal Fold and Thrust Belt, Southeast Mexico

Abstract

Structures related to shale mobility represent an important challenge for proper interpretation in seismic data sets. The large number of seismic sections obtained in recent times of the Akal Fold and Thrust Belt (AFTB) makes this province a natural laboratory for studying shale-related structural styles. The interpretation of two orthogonal 2D seismic sections reveals the geometry of a gravity-driven extension-contraction structural system that developed in the central portion of the AFTB during the Late Miocene to the Early Pliocene. The gravity-driven system was triggered by increasing the sedimentary load in the Miocene and developed a set of listric faults and related growth strata. The contracting structures are the in-sequence fold thrust that evolved from shale-cored detachment anticlines. An Oligocene sequence of soft, weak shale with alternating, rigid layers of sandstone and limestone and a low-resistivity signature acts as the detachment of the gravity-driven system. The progressive sequential restoration of a structural cross section provides more information about the progression of the gravity-driven system. We suggest that mobility started when the Oligocene shale reached a critical condition at >2–3 km burial depth. The coeval northeastward regional Chiapanecan shortening interferes with the last stage of the shale detachment (the Early Pliocene) in the study area and masks the previous interpretation of the gravity-driven system.