Persistent basement wrenching as controlling mechanism of Variscan thin-skinned thrusting and sedimentation, Cantabrian Mountains Spain

In this classical area of thin-skinned tectonics current models of the complex fold and thrust belt seem to be approaching a successful synthesis without the necessity for extreme regional bending to account for the characteristic horse-shoe form of the orogen. Fundamental wrench fault zones (e.g., the León and Sabero-Gordón lineaments) whose influence is recorded throughout the Palaeozoic stratigraphic history have also played an important role in the sedimentary and structural events of the Variscan cycle. Strike-slip motion not only interfered with thrusting far into Stephanian times, but also effectively controlled molasse fanglomerate sedimentation, rendered traceable by multiple clast sources, depocentre migration, fan skewing and progressive unconformities. From the surficial pattern of sedimentation and structure a left-stepping pull-apart basement structure is deduced. It is held responsible for block tilting opposite to the thrusting, modifying the backfolding of the thrust sheets and generating the concurrent surficial collapse of its fanglomerate cover. It is considered that the persistence of strike-slip motion throughout the Palaeozoic justifies the conclusion that deeper crustal events of this type may form the source of the thrusting and induce the variable stress orientations in the upper crust to implement the complex near-surface deformation. The proposed basement configuration fits well into the plate tectonic concept of a Palaeo-Africo-Iberian promontory to account for the initiation of the Ibero-Armorican arc.