Evolution and architecture of reactivated primary minibasins and salt weld systems: outcrop analogues from Sierra Mágina (Central Betic Cordillera, Southern Spain)

The Betic Cordillera is a collisional orogen developed during Cenozoic times by the tectonic inversion of a former Mesozoic hyperextended rift system. The region contains an extensive Upper Triassic salt unit that enabled the development of salt withdrawal minibasins and diapirs during rift, post-rift, and inversion stages. Diapir squeezing and the resulting salt extrusion during orogenesis culminated with the advance of a hundreds-of-kilometers-scale salt canopy in the frontal part of the cordillera. Even so, the primary minibasins are exceptionally well-preserved, providing a rare opportunity to analyze their tectono-stratigraphic architecture and the associated salt weld systems.

Based on geological maps, cross-section restoration, and outcrop observations, this communication provides an overview of the structural framework and the sedimentary infill of Sierra Mágina, located in the Central Betic Cordillera. It exposes a set of welded primary minibasins, making it possible to study in outcrop their evolution and their relation to the surrounding salt sheets. Six main asymmetric primary Jurassic to Cretaceous minibasins, developed above Upper Triassic salt, have been identified (south to north: Gargantón, Mata Bejid, Mágina, Almadén, Carluco, and Cuadros). They exhibit a synformal geometry, with sizes ranging from 5-50 km in length and 2.5-5 km in width. Strata steepen and thicken southwards, consisting of 2-3 km of Lower Jurassic carbonates for the southern minibasins (Gargantón, Mata Bejid, Mágina, and Almadén), and 1-2 km of Jurassic and Lower Cretaceous carbonates and pelagic facies for the northern minibasins (Carluco and Cuadros).

The primary minibasins were reactivated during contractional deformation, and the surrounding diapirs were squeezed, creating vertical welds. In the central parts of Sierra Mágina, vertical welds frequently include smears of salt and incorporate folded Lower-Middle Miocene detrital limestones and turbiditic sandstones, which reveal their contractional origin. Vertical welds transition into thrust-welds to the north and northwest, towards the foreland. To the south and southeast, the minibasins are surrounded by allochthonous salt. In the southernmost portion, the Gargantón minibasin exhibits a panel of vertical to overturned strata, with the lower boundary being concordant with the top salt. This extends for several hundreds of meters and displays a hook geometry, which is associated with flaring salt and likely played a significant role during salt sheet extrusion.

At the orogen scale, the minibasins in Sierra Mágina are thinner and have experienced greater reactivation than their equivalents in the main depocenter of the precursor rift system (approximately 5 km thick), currently buried beneath the salt canopy. Shortening was accommodated by salt expulsion and the stacking of minibasins, with moderate thrust-weld displacements of a few kilometers. We hypothesize that the smaller thickness and weaker mechanical behavior of the minibasins in Sierra Mágina favored salt expulsion and localized shortening during contractional deformation.

These outcomes enhance our understanding of the South-Iberian paleomargin's salt tectonic framework and provide new insights into the role of structural inheritance in the evolution of collisional orogens.