Comment on: “Identification of an underfilled foreland basin system in the Upper Devonian of the Central Pyrenees: implications for the Hercynian orogeny” by Souquet et al., International Journal of Earth Sciences 92:316–337 (2003)
In an interesting contribution to the geological development of the central Axial Zone of the Pyrenees, Souquet et al. (2003) propose a number of new ideas on the local tectonics. That the existence of an early Late Devonian (Frasnian) foreland basin system within the Hercynian Axial Zone of the Central Pyrenees, developed in front of a southward progressing thrust system, has important implications for Hercynian tectonics. We can not comment on the sedimentology, but we would like to remark on the postulated tectonic history based on our own structural studies of the adjacent Garonne dome, also known as the Boss st dome (Mezger and Passchier 2003). Souquet et al. (2003) describe in detail Frasnian turbiditic foredeep successions that overlie unconformably Silurian-Early–Middle Devonian sediments within the Aran syncline. Silurian black slate structurally overlies Cambro–Ordovician slate and schist and upper Ordovician limestone of the Garonne dome to the west. Erosional truncations and progressive unconformities on the northern limb of the Aran syncline are viewed as evidence for continuous subsidence and southward tilting of the basin with synchronous uplift and rotation of the northern limb during Frasnian time. Progression of the Boss st thrust fault system and a developing anticline to the north, the Garonne dome, is thought to control the development of the Aran syncline. Souquet et al. (their Figs. 3 and 15; 2003) interpret the Garonne dome as a large duplex structure with two horses. Roof thrusts are located in the rheologically weak Silurian black slate, while the Cambro–Ordovician basement hosts floor thrusts that connect to basement thrusts extrapolated from the geological interpretations of the nearby ECORS-Pyrenees seismic profile (Mu oz 1992; Roure et al. 1989). These basement thrusts are interpreted as initial Hercynian midcrustal thrusts resulting from Late Devonian north-directed subduction (Souquet et al. 2003). Souquet et al. (2003) conclude that the Garonne dome is an early Hercynian structure (early Late Devonian, ca. 380 Ma), and extend this interpretation to other structural and gneiss domes in the Axial Zone, e.g. the Aston and HospitalÞt Massifs. This interpretation is in conflict with previous tectonic models that assign the development of structural domes to the latest Carboniferous (ca. 300 Ma; Carreras and Capella 1994; van den Eeckhout and Zwart 1988; Vissers 1992). Results of our structural and metamorphic studies of the Garonne dome are summarized in a map showing the foliation trace and metamorphic zones (Fig. 1).