Superposition of tectono-thermal episodes in the southern Cantabrian Zone (foreland thrust and fold belt of the Iberian Variscides, NW Spain)

Abstract

The combined use of the illite “crystallinity” Kübler index (KI) and the conodont colour alteration index (CAI) has revealed the existence of three thermal episodes in an area affected by thin-skin tectonics, close to the internal zones of the Variscan orogen in NW Spain. In the southernmost part of the study area, the first episode gave rise to a regional syntectonic Variscan metamorphism. The associated deformation involves the development of a slaty cleavage, which is mainly recognized in Precambrian rocks. Towards the foreland, the syntectonic metamorphism disappears and only an incipient burial metamorphism, giving rise to anchizonal conditions in the basal part of the thrust units, is observed. Another metamorphic episode occurred close to the Carboniferous-Permian boundary in an extensional tectonic regime. This metamorphism is restricted to the northern part of the study area, where it reached anchizonal or epizonal conditions. It is associated with a subhorizontal or moderately north-dipping cleavage and can be considered as a late-Variscan episode. The last thermal episode occurred during the Permian. It was produced by heat flow due to hydrothermal fluids, whose migration was favoured by faults. The effects of this episode are irregularly distributed, and they are apparent in the unconformable Stephanian rocks in which anchizonal or epizonal conditions were reached. It is interpreted as a post-Variscan episode.

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

“…Estimated geothermal gradients of 52 °C/km (Botor 2012) are lower than those at other Stephanian basins (Mendez 1985;Frings et al 2004;Ayllon et al 2003) and in the Palaeozoic basement (c. 35 °C/km; Brime et al 2001). Aller et al (2005) argued that increased heat flow due to hydrothermal-fluid circulation was the last important thermal episode in the SCZ, facilitated by pathways along faults. As a consequence, the effects of this episode are irregularly distributed within the basins (Brime et al 2001;Aller et al 2005).…”

“…In the SCZ, the Stephanian coal-bearing basins are distributed along deep-reaching strike-slip faults formed as a consequence of transtensional movements that migrated westward with time during the late stages of the Variscan orogeny (Aller et al 2005;Colmenero et al 2008). As the Stephanian sedimentary rocks in the basins generally have a higher coal rank than older Palaeozoic sediments in the same region, it has been suggested that the local magmatism (sills and dykes, volcanism) increased thermal maturation (Aller et al 2005;Colmenero et al 2008), producing a coal rank of up to anthracite level (above 2.0 % R r ).…”

“…As the Stephanian sedimentary rocks in the basins generally have a higher coal rank than older Palaeozoic sediments in the same region, it has been suggested that the local magmatism (sills and dykes, volcanism) increased thermal maturation (Aller et al 2005;Colmenero et al 2008), producing a coal rank of up to anthracite level (above 2.0 % R r ). In the Cinera-Matallana Coalfield (14 km west of the Sabero Coalfield), where the coal ranks are higher (R r up to 2.6 %; R r gradients are also higher), the average palaeogeothermal gradient during the Early Permian has been calculated as 85 °C/km with a heat flow of 150 mW/m 2 (Frings et al 2004), although Mendez (1985) estimated a lower palaeogeothermal gradient of 60-70 °C/km, similar to other Stephanian coal basins across the Cantabrian Zone .…”

“…Aller et al (2005) argued that increased heat flow due to hydrothermal-fluid circulation was the last important thermal episode in the SCZ, facilitated by pathways along faults. As a consequence, the effects of this episode are irregularly distributed within the basins (Brime et al 2001;Aller et al 2005). Fluid inclusion results confirm that there was fracture-controlled fluid flow along thrusts and/or strike-slip faults (Gasparrini et al 2003(Gasparrini et al , 2006Ayllon et al 2003).…”

“…In the Permian period, localized increases in heat flow occurred along the fault areas due to the proximity of heat sources (magmatic intrusions and local hydrothermal flow related to extensional development) (Aller et al 2005). Hydrothermal events were more or less simultaneous with the increased heat flow related to the magmatism of the whole area (Bastida et al 1999).…”