Tectono-thermal evolution in a region with thin-skinned tectonics: the western nappes in the Cantabrian Zone (Variscan belt of NW Spain)

Abstract

The palaeotemperature distribution in the transition from diagenesis to metamorphism in the western nappes of the Cantabrian Zone (Somiedo, La Sobia and Aramo Units) are analysed by conodont colour alteration index (CAI) and illite crystallinity (IC). Structural and stratigraphic control in distribution of CAI and IC values is observed. Both CAI and IC value distributions show that anchizonal conditions are reached in the lower part of the Somiedo Unit. A disruption of the thermal trend by basal thrusts is evidenced by CAI and IC values. There is an apparent discrepancy between the IC and CAI values in Carboniferous rocks of the Aramo Unit; the IC has mainly anchizonal values, whereas the CAI has diagenetic values. Discrepant IC values are explained as a feature inherited from the source area. In the Carboniferous rocks of the La Sobia Unit, both IC and CAI indicate diagenetic conditions. The anchimetamorphism predated completion of emplacement of the major nappes; it probably developed previously and/or during the early stages of motion of the units. Temperature probably decreased when the metamorphosed zones of the sheets rose along ramps and were intensely eroded. In the context of the Iberian Variscan belt, influence of tectonic factors on the metamorphism is greater in the internal parts, where the strain and cleavage are always present, than in the external parts (Cantabrian Zone), where brittle deformation and rock translation are dominant, with an increasing role of the burial on the metamorphism.

Thermal history of the Sabero Coalfield (Southern Cantabrian Zone, NW Spain) as revealed by apatite fission track analyses from tonstein horizons: implications for timing of coalification

“…3) clearly indicated that the assumption about the latest Carboniferous-Permian heating allowed the best-fit calibration in all modelled samples (P37, P60, and P89). Previous studies applying different methods (clay minerals, fluid inclusions, vitrinite reflectance) in SCZ and the Sabero Coalfield have also led to similar conclusions (Bastida et al 1999;Ayllon et al 2003;Ayllon 2003;Frings et al 2004;Aller et al 2005;Colmenero et al 2008;Botor et al 2009;Botor 2012). Heating to temperatures of at least 110 °C caused the complete resetting of AFT ages.…”

“…The syn-orogenic Carboniferous pile has been interpreted as a foreland basin sequence (Julivert 1971;Marcos and Pulgar 1982). Both successions were folded and thrust between Westphalian B and Stephanian times (Julivert 1971;Pérez-Estaún et al 1988;Bastida et al 1999). The main period of the Variscan deformation ended around 300 Ma, based on recent palaeomagnetic data that record the oroclinal bending of the Variscan Ibero-Armorican Arc ( Van der Voo et al 1997;Weil et al 2001Weil et al , 2010.…”

“…Hydrothermal events were more or less simultaneous with the increased heat flow related to the magmatism of the whole area (Bastida et al 1999). This magmatism could have promoted hydrothermal-fluid migration, especially in the areas of deep-seated faults along the Sabero-Gordon line (Gómez-Fernández et al 2000).…”

“…These basins are considered to have initially formed as small pullapart basins along strike-slip fault systems under a Late Carboniferous syn-orogenic transtensional regime (Julivert 1971;Reading 1975;Heward and Reading 1980;Wagner and Winkler-Prins 1985;Alonso et al 1995;García-López et al 1997, 2007Bastida et al 1999). Many of these basins in the Southern Cantabrian Zone (SCZ) were structurally controlled by a major E-W trending Variscan strikeslip fault system, named the Sabero-Gordon Line (Julivert 1971;Reading 1975;Heward and Reading 1980;Alonso et al 1995;García-López et al 1997, 2007Bastida et al 1999). Transpressional faulting and folding during the later Abstract Apatite fission track (AFT) central ages from Carboniferous (Stephanian) tonsteins of the Sabero Coalfield, NW Spain, range from 140.8 ± 7.5 to 65.8 ± 8.1 Ma (Cretaceous), with mean c-axis projected track length values ranging from 12.5 to 13.4 μm.…”

“…Geological data combined with thermal models of AFT data indicate that Stephanian strata reached the maximum palaeotemperatures in the Permian period, which was therefore the major time of the coalification processes. The Permian magmatic activity was responsible for a high heat flow, which, with the added effect of sedimentary burial, could account for 1 3 stages of the Variscan orogeny at the end of the Carboniferous to the early Permian resulted in basin closure and inversion (Julivert 1971;Alonso et al 1995;García-López et al 1997, 2007Bastida et al 1999). This study examines the thermal history of one of these small basins (~12-15 km long and 3-4 km wide) located along the eastern end of the Sabero-Gordon fault line (Fig.…”