Extensional perpendicular orthogonal quartz veins related to early Variscan Orogeny

Orthogonal sets of veins perpendicular to the bedding occur in fold-and-thrust belts; however, the timing of their formation remains unclear in relation to orogeny. This study aims to explain the development of perpendicular orthogonal vein sets from the iconic outcrops at northern Almograve in southwestern Portugal also known as “Chocolate-Tablet Structures”, which are affected the Variscan Orogeny. Determining whether these vein sets formed pre- and/or syn-folding of the main deformation (i.e., Variscan) requires multiple independent lines of evidence. Based on a limited number of outcrops, previous studies suggest that these veins are nearly vertical and horizontal to the Variscan folded strata. Here, we present a detailed structural study including drone photogrammetry (a few cm to m resolution) of stretches of the coastal section that are hard to reach. A nearly continuous and much longer stretch of the coast near Almograve and Zambujeira do Mar has been structurally analysed in this study. Field observations and stereographic projections of different sets of veins and the refolded host rock show a very consistent perpendicular relationship between two sets of veins, both perpendicular to the bedding as well. A genetic relation to the Variscan folding is tempting, but our new data question such previous interpretations. This study proposes that the perpendicular orthogonal vein sets are results of hydraulic fracturing and have formed during an early stage of the Variscan Orogeny, either through sedimentary loading (hydraulic fracturing) and simultaneous veining or through forebulge-foredeep stretching of the initial foreland basin.