Neogene Shale-Detached Extension and Contraction Linked System in the Akal Fold-and-Thrust Belt, Se Mexico

Structures related to shale mobility represent an important challenge for proper interpretation in seismic datasets. 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 revealed the geometry of a gravity-driven extension-contraction structural system that developed in the Late Miocene to the Early Pliocene in the central portion of the AFTB. The gravity-driven system was triggered by increasing sedimentary load in the Miocene developing a set of listric faults and related growth strata. The contracting structures are 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 provided more information about the progression of the gravity-driven system. We suggest that mobility started when the Oligocene shale reached critical condition at gt;2-3 km burial depth. The coeval northeastward regional Chiapanecan shortening interfered with the last stage of the shale detachment (Early Pliocene) in the study area and masked the previous interpretation of the gravity-driven system.