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The active mine of Loulé is a privileged place to analyze salt deformation structures in a special moist free environment.
The Loulé salt wall formed at the intersection of two major faults, the Loulé-S. Brás and Loulé Faults. The salt mine gallery orientation system (E-W and N-S) is parallel to the strike and dip directions of the salt wall, respectively, providing excellent exposures for structural analysis. Structural mapping revealed the existence of three phases of deformation: i) D1 - a phase of horizontal movement during which low angle thrusts and recumbent folds formed, ii) D2 - a phase of diapiric ascent that generated upright sheath folds and, iii) D3n - various phases of compression of the diapir, during which the antiform sheath folds were sheared out and later, a suite of ductile to brittle thrusts were formed. The salt is cross cut by basic dykes of Upper Cretaceous age, according to Martins (1991).
The first and second phases of deformation (D1 and D2) occurred during the main phases of extensional deformation of the basin. The formation of D1 structures was probably caused by southerly slip of the sedimentary cover, triggered by crustal extension and block tilting at a time when both the salt layer and overlying sediments were not thick enough to cause a density inversion between the salt and sedimentary cover. Generally, a minimum of 600 m thick sedimentary cover is needed to cause a density inversion, according to the sedimentary record this situation should have happened during Middle Jurassic times.
Structures that suggest fluid migration inside the bulk of the diapir are also present, such as, i) metric scale, tension gashes, ii) reactivation of shears as channels for fluids and iii) blow out structures (auto-explosion of salt during mechanical excavation), probably related to accumulated stress and/or degasification.
A pervasive network of anastomosed shears form three-dimensional cells that contain the synform sheath folds. These shears suffered simple and pure shearing during various compressive events (they contain centimetre scale imbrications, SC fabrics and are boudinaged in their strike and dip directions). They also show the following features: a) they are made up of a concentration of insoluble materials (clays and chlorite) that occur interbedded with the primary salt, b) they display axial symmetry with respect to an internal layer of recrystallized halite (Fig. 5) and c) they bound only upright synform sheath folds.
These features are interpreted as follows: a) the concentration of primary insoluble materials is the residual material that resulted from dissolution of primary salt rock, b) the axial symmetry and recrystallized halite suggest that the shears were used as planar fluid channels, the recrystallized halite being the oversaturated residue of the paleofluid and, c) the lack of antiforms and the fact that the shears separate synforms suggests that the antiforms were tectonically eroded along the shears.
The shears are interpreted as: i) the first structures that formed within the salt diapir due to a regional tectonic compressive regime at a time when the salt wall probably had already extruded to the surface, and ii) fluid paths, which at least accounted for the tectonic erosion of the volume of the antiformal salt folds.
The above mentioned basic dykes of Upper Cretaceous age (probably Campanian, according to Martins, 1991) cross cut D2 sheath folds in the salt as well as the shears along which the antiformal sheath folds were sheared out. These dykes are folded and boudinaged, from which it can be extrapolated that they post-date the ascent of the salt and the above described first compressive event but predate later compressional episodes.
Compressive events that post-date the intrusion of the dykes generated a series of thrusts that range from ductile to brittle. The most brittle thrusts are the late, which indicates that either the strain rate had increased or the region had been considerably exhumed. Orientation of i) salt thrusts, ii) striae in interbedded non-evaporite sediments, iii) linear fabrics in tectonic recrystallized salt and iv) chocolate tablet structure in basic dykes, suggest a N-S oriented compression.
TERRINHA, P.A. (1998) – Structural geology and tectonic evolution of the Algarve basin, South Portugal. Tese doutoramento, Univ. Londres. 425 pp. (inédito).
TERRINHA, P.A.G.,, COWARD M.P.; RIBEIRO, A. (1990) - Salt tectonics in the Algarve Basin: the Loulé diapir, Comunicações dos Serviços Geológicos de Portugal, 76: 33 – 40.
TERRINHA, P., RIBEIRO, C., KULLBERG, J. C., ROCHA, R.; RIBEIRO, A. (2002). Compression episodes during rifting and faunal isolation in the Algarve Basins, SW Iberia. Journal of Geology, 110: 101 - 113.
MARTINS, L.M. (1991) - Actividade ígnea mesozóica em Portugal (contribuição petrológica e geoquímica), PhD thesis Univ. Lisbon, 418 pp. (unpublished).