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Esla Unit thrust system

Spain

In the foreground ridge, Cambrian dolostones of the Esla Nappe (right) rest over Devonian limestones (left) along an overturned thrust. In the distance, folded unconformable succession.

In the foreground ridge, Cambrian dolostones of the Esla Nappe (right) rest over Devonian limestones (left) along an overturned thrust. In the distance, folded unconformable succession.

Geological Period

Upper Devonian to Carboniferous (Pennsylvanian)

Main geological interest

Tectonics
Stratigraphy and sedimentology

Location

Cantabrian Mountains, León Province, Spain
42°52’26”N, 005°09’50”W

In the foreground ridge, Cambrian dolostones of the Esla Nappe (right) rest over Devonian limestones (left) along an overturned thrust. In the distance, folded unconformable succession.

An exceptional natural laboratory for the study of the evolution of foreland fold and thrust belts in the Cantabrian Zone.

The Esla Unit has attracted the attention of numerous international research groups since the 1930s owing to its variety of thin-skinned deformation-related structures, its outstanding examples of post-emplacement deformation of thrust sheets, its fault rock assemblages in basal nappe shear zones, the complete Palaeozoic record of its sedimentary succession and, in addition, its excellent outcropping conditions. The unit has proven to be a key area in the understanding of the geological evolution of the Cantabrian Zone and the Ibero-Armorican Arc. The area has contributed to the educational training of numerous generations of geologists from several European geoscience institutions.

Primajas Duplex, formed by horsts with Cambrian carbonates of the Láncara Formation and shales of the Oville Formation. Succession is overturned, younging towards the valley.

Primajas Duplex, formed by horses with Cambrian carbonates of the Láncara Formation and shales of the Oville Formation. Succession is overturned, younging towards the valley.

  • Geological description

The Esla Unit is located in the SE Cantabrian Zone, the external foreland fold and thrust belt of the Variscan Orogen (NW Iberian Massif). It is formed by a sedimentary succession that displays an almost complete record of the Palaeozoic Era, starting in lower Cambrian strata and ending in Pennsylvanian synorogenic deposits (Comte, 1959). The unit, structured into three nappes and three duplexes of kilometric scale that accommodate a total displacement of 92 kilometers, was emplaced during the Moscovian towards the NE in present-day coordinates (Alonso, 1987). The unit contains outstanding examples of thrust-related fault rocks (Arboleya, 1989; de Paz-Álvarez et al., 2021) and structures, including fault-bend folds, frontal and lateral culmination walls formed in response to the geometry of deeper thrust surfaces, and duplexes (Alonso, 1987).
Post-nappe emplacement deformation in the Esla Unit resulted in the rotation of its western area around a horizontal axis, generating a cartographic pattern that shows zenithal and cross-sectional views of the unit that allow a three-dimensional observation of the structures. Late deformation also resulted in fold interference patterns and a variety of geometrical relations between reactivated folds and unconformity surfaces truncating them which depend on the folding mechanism active during reactivation (Alonso, 1989).

  • Scientific research and tradition

The Esla Unit is at the forefront of studies regarding the stratigraphy and deformation of the Cantabrian Zone. With modern studies starting in the 1930s (e.g. Comte, 1959), it still constitutes an active research area and an outstanding laboratory visited by numerous European geology departments for field courses.

Geological map of the Esla Unit (modified from Merino-Tomé et al., 2014). Variable fold orientations result from nappe frontal and lateral structures and post-emplacement deformation.

Geological map of the Esla Unit (modified from Merino-Tomé et al., 2014). Variable fold orientations result from nappe frontal and lateral structures and post-emplacement deformation.

  • Reference

Alonso, J.L. (1987) ‘Sequences of thrusts and displacement transfer in the superposed duplexes of the Esla Nappe Region (cantabrian zone, nw spain)’, Journal of Structural Geology, 9(8), pp. 969–983. Available at: https://doi.org/10.1016/0191-8141(87)90005-8.

Alonso, J.L. (1989) ‘Fold reactivation involving angular unconformable sequences: theoretical analysis and natural examples from the Cantabrian Zone (Northwest Spain)’, Tectonophysics, 170(1), pp. 57–77. Available at: https://doi.org/10.1016/0040-1951(89)90103-0.

Arboleya, M.L. (1989) ‘Fault Rocks of the Esla Thrust (Cantabrian Mountains, N Spain) an Example of Foliatec Cataclasites’, Annales Tectonicae, III(2), pp. 99–109.

Comte, P. (1959) Recherches sur les terrains anciens de la Cordillère cantabrique. Madrid: Tip.-Lit. Coullaut (Memorias del Instituto Geológico y Minero de España, 60).

IGME (2014) ‘Spanish digital Geological Map. Cantabrian Zone’. Madrid (GEODE). Available at: http://info.igme.es/cartografiadigital/geologica/geodezona.aspx?Id=Z1000.

de Paz-Álvarez, M.I., Llana-Fúnez, S. and Alonso, J.L. (2021) ‘Intrusion fracturing and quartz sand-rich injections in thrust-related fault rocks within the basal shear zone of the Esla Nappe (Cantabrian Zone, NW Iberia)’, Journal of Structural Geology, 142, p. 104230. Available at: https://doi.org/10.1016/j.jsg.2020.104230.

  • Author(s)

Manuel Ignacio de Paz Álvarez.
Departamento de Geología, Universidad de Oviedo, C/ Jesús Arias de Velasco s/n, 33005 Oviedo, Spain.

Juan Luis Alonso.
Departamento de Geología, Universidad de Oviedo, C/ Jesús Arias de Velasco s/n, 33005 Oviedo, Spain.

Sergio Llana Fúnez.
Departamento de Geología, Universidad de Oviedo, C/ Jesús Arias de Velasco s/n, 33005 Oviedo, Spain.

María Luisa Arboleya.
Departament de Geologia, Universitat Autònoma de Barcelona, 08193, Bellaterra, Barcelona, Spain.

Thomas G. Blenkinsop.
School of Earth and Environmental Sciences, Cardiff University, Main Building, Park Place, CF10 3AT, Cardiff, UK.

Ernest Rutter.
Department of Earth and Environmental Sciences, University of Manchester, 5 Manchester M13 9PL, UK.

David Buchs.
School of Earth & Environmental Sciences, Cardiff University, Cardiff, CF10 3AT. UK.

Lesley Cherns.
School of Earth and Environmental Sciences, Cardiff University, Main Building, Park Place, CF10 3AT, Cardiff, UK.

Adriana Georgina Flórez Rodríguez.
Departamento de Geología, Universidad de Oviedo, C/ Jesús Arias de Velasco s/n, 33005 Oviedo, Spain.

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