Variscan Inheritance Induces Alpine Upper Crustal Delamination in East Spanish‐Portuguese Central System

Abstract

The Spanish‐Portuguese Central System (SPCS) is an Alpine Mountain range with crystalline basement characterized by a two‐layer rheological structure. This structure formed after primary (protolith) and secondary (tectonometamorphic) processes during the extensional collapse of the Variscan Orogen. The SPCS structure is usually controlled by foreland‐directed thrusts and strike‐slip faults. However, the eastern SPCS is dominated by NW‐directed, imbricate backthrusts and lacks the main thrust directed to the foreland basin located southeast of the mountain range (Madrid Cenozoic Basin). The SPCS exhibits a crustal root (>40 km depth) supporting SE‐directed crustal‐scale thrusting. Alpine backthrusts sole into an SE‐dipping décollement within the Variscan basement. Variscan extension‐related structures parallel the SE‐dipping geometry of Alpine backthrusts, so they provided favorably oriented rheological weaknesses to accommodate Alpine shortening. Backthrusts geometry, their hanging wall position within the fault that raised the SPCS and gravity modeling support an Alpine crustal delamination process. Tectonic wedging and delamination of the more competent basement occurred in the footwall of Variscan extensional faults (Daurius domain), which enforced the shearing off of a rheologically weaker upper layer of the crust, located in the hanging wall of the Variscan extensional faults (Arriaca domain) by inverting Variscan extensional faults. This led to NW‐directed incipient continental subduction of the weaker crust. Intraplate subduction and crustal delamination can be independent from lithosphere‐scale inheritance and be conditioned by structural inheritance in the overlying crust. Alpine shortening for the Cretaceous cover is around 17.7 km (10.5% shortening), and 11 km (7%) for the upper‐lower crust limit.

Mega‐Folding of a Basement During Incipient Intra‐Plate Continental Subduction (Alpine Central Iberia)

“…In a continental collision, this type of folds can also be produced under conditions of strong coupling along the plate boundary (Luth et al, 2010). The SCS and its prolongation to the NE into the study area has been recently imaged as a crustal-scale thrust that displaces the Moho and produces a crustal imbrication (Andrés et al, 2019(Andrés et al, , 2020 that mimics an incipient continental subduction zone (de Vicente et al, 2022). In this scenario, the mechanical coupling between "plates" (Daurius and Arriaca; two pieces of regular continental crust) is expected to be strong; therefore, the movement of the lower plate could have dragged the section of the upper plate closer to the "trench" down.…”

“…The pre-orogenic sedimentary cover is missing to the west of the SPCS up to the Lusitanian Basin in Portugal. The Triassic Keuper facies serve as a detachment level in the IC, whereas in the SPCS, the sedimentary cover is attached to the basement with no detachments (de Vicente et al, 2018(de Vicente et al, , 2022.…”

“…Reference points (e.g., RP‐1) are used to guide data presentation and interpretations in the main text. The map has been produced after our data and a compilation and synthesis of previous works (de Vicente et al., 2022; Fernández Rodríguez, 1991; González Lodeiro, 1980; Rubio Pascual, 2013).…”

“… (a) Cross‐section showing the structure of the basement (see trace in Figure 2; after Rubio Pascual, 2013; de Vicente et al., 2022). (b) Stereoplot showing the orientation of the bedding of the sedimentary cover (poles to S 0 ; Permian—Cenozoic strata) and of the axes of Alpine folds (measured directly in the field).…”

“…The Alpine Somosierra Pass Fault mimics the trace of the Berzosa Fault (Figure 2), indicating the Alpine reactivation of the uppermost structural sections of the latter. The set of NW‐directed (back)thrusts that cut across the Hiendelaencina Antiform branches off from an underlying crustal detachment that should extend beneath the entire basement of the eastern SPCS (de Vicente et al., 2022). The SE‐dipping geometry of those Alpine (back)thrusts fits the geometry of the Variscan Berzosa Fault at depth (Escuder Viruete et al., 1998; Hernaiz Huerta et al., 1996; Rubio Pascual et al., 2013), if slightly steeper.…”