The enigmatic ascent of Ca-sulphate rocks from a deep dense source layer: evidences of hydration diapirism in the Lesina Marina area (Apulia, southern Italy)

Abstract

In the Lesina Marina village area, the cropping out gypsum rocks, that rose up from the deep and thick Upper Trias Burano Fm anhydrite layer, have been analyzed to investigate the trigger mechanism for their ascent. In this regard, we focus on the anhydrite-to-gypsum transformation starting from the deep source layer. Indeed, petrographic observations of the widespread corroded anhydrite crystals embedded in these gypsum rocks revealed unequivocal evidences of the anhydrite-to-gypsum transformation. In addition, and referring to gypsum, microstructural features indicate that ductile deformation mechanisms initially operated under higher temperature conditions. This temperature should be close to 107 °C, namely the value of the upper temperature limit of stability of gypsum from Lesina Marina, above which it starts to transform and dehydrate, as revealed by microthermometry heating/dehydration experiments combined with micro-Raman analyses. All these evidences are in favor of the dramatic density decrease and volume increase due to anhydrite-to-gypsum transformation by hydration in the deep source layer; these variations of physical conditions, triggered by hydration, promoted diapirism of the gypsum mass, in other words “hydration diapirism”. As revealed by seismic lines interpretation, the diapirism, which gave rise to the Lesina diapir, occurred during Plio-Pleistocene and was genetically related to the Lesina graben-type structure. Hydration at depth was favored by the downward circulation of water-rich fluids channeled in faults, and the gypsum mass used the weakened zone of the southern fault to pierce the overlying Mesozoic and Tertiary sediments.