Transtension and transpression in an oblique subduction setting: evolution of the Bahia Basin, Colombian Caribbean margin

Abstract

The Bahia Basin is located in the NW corner of South America where a complex history of subduction, accretion and transpression is on-going since Cretaceous times. The Bahia Basin is located just offshore from major strike-slip fault systems that affect northern Colombia and lies behind the toe of the modern accretionary prism, where the Caribbean Plate is being subducted obliquely beneath South America. This study uses a high quality 3D-PSTM seismic volume, regional 2D seismic reflection lines, and well data, to investigate the basin formation and evolution in an oblique subduction setting. Standard 3D seismic mapping of key seismic stratigraphic surfaces (regional unconformities) and faults is used to identify the basin architecture and geological framework. The main surface mapped in the area is the upper Miocene to lower Pliocene unconformity, which highlights the presence of a young, deep, narrow basin trending NE-SW, bounded to the NW by a major fault: the Bahia Fault. This fault shows an initial phase of normal displacement with later inversion along its segments. To understand the geological evolution between pre- and post- late Miocene times, structures below and above the unconformity were mapped. In addition, time-thickness maps were calculated to assess the formation and migration of depocentres, as well as the growth of compression-related structures. The interpretation of the regional grid of 2D seismic reflection lines, extending from the toe of the accretionary prism to the shelf, document the structural styles of the area. These are the South Caribbean accretionary prism, an extensional/strike-slip zone to the rear of the prism and the mobile shale prevalent throughout the area. Detailed interpretation of 3D seismic reflection data demonstrates that prior to the late Miocene the Bahia Basin formed as a transtensional basin within a right-lateral shear zone. During this opening stage, areas within the basin acted as independent blocks bounded by strike-slip faults. These blocks were internally deformed by small extensional faults, whose geometry and distribution suggest clockwise, vertical-axis block rotation within the shear zone. Subsequently, the closing stage of the basin during Plio-Pleistocene times is discussed. The main structural style in this phase of basin evolution is basin inversion together with concomitant extension, all suggesting a transpressional setting. Moreover, this also suggests the occurrence of a complex interaction with major-onshore strike-slip fault systems, where the relative expulsion of the Maracaibo Block might induce the local onset of transpression in the Bahia Basin during Pliocene times, resulting in basin inversion, and ultimate closure of the basin. This study concludes that the formation and evolution of the Bahia Basin can be understood in the context of oblique subduction, where localised shale tectonics has also influenced the basin evolution, and extends evidence for strike-slip deformation along the southern boundary of the Caribbean plate further westward than previously documented.