Loulé: a shunted diapir affected by Late Cretaceous dyke intrusions and Alpine inversion, Algarve Basin, Portugal

Abstract

The Loulé Diapir is located in the Algarve Basin (Portugal), and is a shunted diapir, which has been highly deformed by South-vergent thrusting and folding during the Alpine orogeny. Halokinesis initiated during Jurassic rifting and the diapir was intensely squeezed during the Alpine collision. It is mainly composed of well-bedded halite with variable crystal size and colour and with occasional fine-grained clastic beds reaching up to 2 m in thickness. The pervasive bedding acts as an excellent marker of vertical displacements in the salt. Both vertical sheath-like folds associated with the salt ascent and recumbent folding indicating sub-horizontal shearing, can be observed.

Before Alpine inversion the diapir was injected by Campanian lamprophyre-basanites dykes which have been subsequently deformed during the Alpine inversion. The dykes were hydraulically fractured with ‘jigsaw- boudinage’ structures developed where halite flowed into the gaps. Occasionally fibrous salt is observed filling the resulting veins indicating repeated crack and seal.

Large pockets of CO₂ gas are trapped in porous ‘pop-corn’ salt, which are located along zones of coarser pure white halite crystals. Often when these zones are mined sudden gas release causes blow out holes that can reach up to 10 m in diameter producing spectacular conchoidal fracturing parallel to the cavity walls. Highs strain rates coupled with high fluid (CO₂ and H20) pressures have caused late brittle faults in the mine.

The margins of the salt diapir have also been mined and collapsed brecciated gypsum/anhydrite cap rocks are well preserved. The breccia is believed to be caused by underground cavern collapse of a dissolution-residue cap rock. There is still evidence of enhanced water percolation along the diapir walls with salt efflorescent encrustations in the gallery walls.

This is the only salt mine in Alpine belt of Southern Iberia, and it provides a rare opportunity to examine the effects of intense shortening on a diapir. This talk will illustrate the shortening effects with spectacular examples of all these deformation features.

AAPG Datapages/Search and Discovery Article #90226 © 2015 European Regional Conference and Exhibition, Lisbon, Portugal, May 18-19, 2015