Orthogonal extensional quartz veins in a famous 'Chocolate-Tablet Structure' from Almograve (SW Portugal), associated with early Variscan Orogeny

Orthogonal vein sets, orientated perpendicularly to the bedding, are present in fold-and-thrust belts; yet the date of their origin in relation to orogeny is ambiguous. This research aims to clarify the formation of perpendicular orthogonal vein sets from the iconic outcrops in northern Almograve in SW Portugal, referred to as “Chocolate-Tablet Structures,” which are influenced by the Variscan Orogeny. Establishing whether these vein sets developed earlier than to and/or during the folding associated with the main deformation (i.e., Variscan) requires many independent lines of evidence. Previous investigations, based on limited outcrops, indicate that these veins are vertical and parallel to the Variscan folded strata. We provide a comprehensive structural analysis using drone photogrammetry (with resolutions ranging from a few cm to m) of inaccessible sections of the coastline zone. This research has structurally studied a practically continuous and much longer section of the coast at Almograve and Zambujeira do Mar. Field observations and stereographic projections of several vein sets and the refolded host rock reveal a continuous perpendicular connection between two vein sets, both of which are also perpendicular to the bedding. A genetic relation to the Variscan folding is tempting, but our recent research challenges such prior findings. This study proposes that the perpendicular orthogonal vein sets are the result of hydraulic fracturing, formed during the early phase of the Variscan Orogeny, either via sedimentary loading (hydraulic fracturing) and simultaneously veining or through the stretching of the initial foreland basin due to forebulge-foredeep dynamics.