Abstract

The Loulé salt diapir grew through the Jurassic and Early Cretaceous, when tight subvertical curtain folds were developed. It was then squeezed during the Alpine orogeny in the Oligocene and Miocene. This produced an intense array of brittle faults, veins and brittle–ductile shear zones through most of the diapir. The diapir exhibits more localized shear and brittle structures than any other previously described diapir. The most unusual deformation structures are ‘streaky shear vein’ zones (a new term), with an average spacing of 1 – 5 m, and usually less than 30 cm wide. They consist of a central coarse-grained halite vein surrounded by parallel zones of wispy discontinuous shale streaks and recrystallized halite. A new mechanism is proposed for the development of these zones, where repeated pulses of fluid flow have caused extensive halite recrystallization and streaking-out of unlithified mud intercalated with crystalline halite. This explains the formation of wide zones of well-aligned mud streaks (now lithified to shale), but with only small amounts of simple shear offset observed across these zones (<0.2 m). The lower tips of the streaky shear vein zones exhibit splaying mud injection veins, which indicate that fluid overpressures preserved liquid mud from salt deposition at 200 Ma to the Alpine collision at 20 Ma.

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