Coalification history of the Stephanian Ciñera-Matallana pull-apart basin, NW Spain: Combining anisotropy of vitrinite reflectance and thermal modelling

• Published in: International journal of earth sciences : Geologische Rundschau 2004-02, Vol.93 (1), p.92-106
• Main Authors: Frings, Kai, Lutz, Rüdiger, de Wall, Helga, Warr, Laurence N.
• Format: Article
• Language: English
• Subjects: Anisotropy; Deformation; Heat flow; Overburden; Plate tectonics; Reflectance; Rock deformation; Rocks
• ISSN: 1437-3254; 1437-3262
• DOI: 10.1007/s00531-003-0370-7

The Stephanian Ciñera-Matallana Basin of NW Spain comprises 1,500 m of alluvial to lacustrine coal-bearing sediments, which were deposited in a late Variscan transtensional/transpressional pull-apart setting. The relationship between coalification pattern and rock deformation was evaluated by measurements of the anisotropy of vitrinite reflectance (AVR). The AVR ellipsoids reveal both pre-tectonic elements related to the bedding fabric and syn-tectonic elements related to folding, producing biaxial ellipsoid shapes with the maximum reflectance parallel to fold axes. The mean coalification gradient for the Stephanian succession is about 0.62 %Rr/km. Calculations of the mean palaeo-geothermal gradient are presented on the basis of three different empirical equations. A palaeo-geothermal gradient of 85 °C/km is considered the most realistic, with an overburden of about 1,000 m. 1-D numerical modelling of the burial history results in two possible scenarios, the most preferable involving a palaeo-heat flow of 150 mW/m^sup 2^ and an overburden of ca. 1,050 m. These results indicate that maximum coalification was related to a localised but high palaeo-heat flow/-geothermal gradient. The anisotropy of vitrinite reflectance highlights the interactive and transitional nature of sedimentary compaction and rock deformation on the maturation of organic material within strike-slip fault zones.[PUBLICATION ABSTRACT]