Large-scale flat-lying isoclinal folding in extending lithosphere: Santa María de la Alameda dome (Central Iberian Massif, Spain)

Abstract

The exhumation mechanisms of deep-seated continental crust can be constrained by analyzing the structural and metamorphic imprints left in lithological ensembles. The Santa María de la Alameda dome formed during the collision of Gondwana and Laurussia in late Paleozoic time and is located in the Central Iberian Zone of the Iberian Massif (Spain). Rocks of the dome are part of the autochthonous Gondwanan sections of the Variscan belt, and they occur in the Variscan hinterland. The lithostratigraphy of the dome consists of metasedimentary rocks alternating with orthogneiss massifs showing irregular and sinuous structure. The metamorphic record indicates peak pressures indicative of lower-crust depths followed by isothermal decompression to middle-upper-crust levels. Exhumation resulted in the exposure of different crustal levels (represented by subsolidus vs. supersolidus mineral assemblages). The exhumation was accompanied by initial layer-parallel stretching and subsequent large-scale isoclinal folding developed in a heterogeneous, fl at-lying shear zone with top-to-the-SE kinematics. SE-directed shearing and lateral extensional fl ow occurred in response to thermomechanical disequilibrium of previously thickened orogenic crust, probably assisted by coeval accretion of tectonic slices and lithospheric bending about a vertical axis. Positive feedback among partial melting, exhumation, and crustal attenuation resulted in the formation of a NE-SW-trending, migmatite-cored dome, and in refolding of early isoclinal folds and an associated axial surface regional foliation. The dome formed beneath a set of extensional detachments and was reshaped by WNW-ESE upright folds during later convergent deformation. The latter event brought in further instabilities throughout the belt, triggering in this region the development of a late extensional detachment under low-grade metamorphic conditions (top-to-the-S kinematics). The development of a regional train of fl at-lying isoclinal folds is presented here as the macrostructural expression of the combination of vertical and lateral extensional fl ow, both of which are particularly common in orogens worldwide.

Tectonic evolution of Variscan Iberia: Gondwana–Laurussia collision revisited

“…The extensional detachments separating these two structural levels of the crust are characterized by low-grade phyllonites and mylonites (Escuder Viruete et al, 1994, 1998Díez Balda et al, 1995). Extensional ductile flow gave way to shearing and stretching of syn-kinematic granitoids (Díaz-Alvarado et al, 2012) as well as a profound distortion of previous structures via flattening (Díez Fernández et al, 2013b), sometimes resulting in large-scale isoclinal folding (Arango et al, 2013). Crustal attenuation varies depending on previous thickness, reaching decompression of the infrastructure up to the andalusite stability field in many cases.…”

From subduction to collision in the Parautochthon and autochthon of the NW Variscan Iberian Massif

“…Stretching 115 lineations and asymmetric microfolds in this foliation indicate a top-to-the-SE shear-sense during C2 (L C2 ) and probably E1, while upper crustal sections (here represented by the GTMZ) show top-to-the-NW shear-sense (L E1 ). Similar E1 extensional shear zones are widespread represented along the most internal zones of the CIZ (Doblas et al, 1994;Escuder Viruete et al, 1994, 1998Díez Balda et al, 1995;Arango et al, 2013;Díez Fernández et al, 2013;Rubio Pascual et al, 2013. From the metamorphic geology point of view, the materials of the CIZ cropping out from 120 immediately around the GTMZ to the central areas of the Iberian Massif are characterized by the onset of thick and complete Barrovian sequences, which are variably overprinted by the HT-LP metamorphism (Martínez Catalán et al, 2014).…”

The enigmatic curvature of Central Iberia and its puzzling kinematics

“…Palero-Fernández et al, 2015;Dias et al, 2016), or alternatively the interference between C1, C3 and the E2 structures ( Fig. 2A; Arango et al, 2013;Rubio Pascual et al 2013).…”