| Author||J. Rupke|
|Title||The Esla Nappe, Cantabrian Mountains (Spain)|
|Journal||Leidse Geologische Mededelingen|
|Abstract||In the southern slopes of the Cantabrian mountains (prov. León NW Spain) a miogeosynclinal and non-metamorphic series, 2—3 km thick, of Precambrian to Carboniferous age has been studied. Four main stratigraphic and tectonic units have been recognized: 1. Northern fracture zone of Las Salas, 2. Autochthone of Valdoré, 3. Esla nappe, 4. Western-Bernesga thrust structures.|
Expression of the Caledonian orogeny is very vague. The rocks have been subjected to tectonic forces during the Hercynian and Alpine orogenies.
Epeirogenic movements during the Devonian (Bretonic phase of Stille) preceded large scale folding and thrusting during the early Westphalian (Sudetic phase). During this time the rocks of the Esla nappe have travelled a distance of 15—20 km to the north and northeast.
It is suggested, that folding and thrusting happened simultaneously in different parts of the area. Further it is shown that basement configuration as expressed in facies boundaries played an important role putting limits to the rather thin thrust sheet during its movement. Fundamental weakness zones border the thrusted area.
The Asturian phase of Stille may be held responsible for a great amount of refolding of the previously- formed thrust structures.
To the north of the thrusting boundary i.e. fracture zone of Las Salas otherwise León line of de Sitter (1962 b), Westphalian deposits are found resting unconformably on rocks, that are represented in the nappe. So in the north and in front of the thrusts deposition went on during „middle” and upper Carboniferous times.
Stephanian coal bearing rocks in the northern fracture zone are unconformably resting on both the Westphalian and the Older Palaeozoic thrusted series. Likewise Stephanian rocks of the Sabero basin in the south fill a depression in the nappe. This depression also occurs on a fundamental zone of weakness, the Sabero-Gordon line.
From several locations it is inferred, that the tectonic forces, which folded the Stephanian rocks severely, left the older Palaeozoic, Sudetic and Asturian folded,rocks practically unaltered.
The southern border zone is seen as an Alpine flexure zone; in places the Cretaceous steeply covers the previously mentioned series.
Morphogenetic uplift of the chain most probably is accounted for by the Pyrenean phase. The Tertiary conglomerates of the Duero basin are to be derived from this uplift.
It is held, that none of the mentioned unconformable rocks have covered the older Palaeozoic thrust series as full and uninterrupted blankets.
The basin configuration of the Cambrian as described by Lotze 1961 is supported by stratigraphic and tectonic observations in the area. Thus Lotze’s Cambro-Ordovician geosyncline may have been tectonised as late as the Devonian—Lower Carboniferous. De Sitter’s view, that the thrustsheets contained in the Leonides moved from south (center of preceding basin) to north is confirmed by stratigraphic and tectonic evidence. In the east-west striking part of the Asturian—Cantabrian chain only the miogeosynclinal part of the greater subsidence is disclosed to our inquiry, the orthogeosynclinal development was not uplifted.
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