Paleomagnetic and structural analyses of the Western European Variscan Belt (WEVB) suggest that the most viable kinematic model for Variscan deformation in northern Iberia is oroclinal bending of an originally linear belt in a two-stage tectonic history. This history represents two regional compression phases (E-W in the Late Carboniferous and N-S in the Permian, both in present-day coordinates), which resulted in the refolding (about steeply plunging axes) of initially north-south trending thrusts and folds in the hinge zone, and oroclinal tightening due to vertical axis rotation of the belt's limbs. However, the orocline model has yet to be critically tested in the WEVB's core. This study reports new paleomagnetic, rock magnetic, and structural data from the inner core of the WEVB in order to test opposing kinematic models for the well-documented fault and fold interference structures formed by late stage Variscan deformation and to better understand the overall development of the WEVB arc. Map-scale structural features within the WEVB core have a highly sinuous geometry characterized by transverse and thrust-parallel fold systems formed by fault bend folding over footwall ramps. The intersections of these two fold systems produce steeply plunging interference folds, which are best exposed in the Ponga thrust unit. A total of 67 paleomagnetic sites were collected in the Barcaliente Formation of the Ponga Unit, with emphasis placed on detailed spatial coverage of individual structural domains. A multicomponent paleomagnetic remanence was measured, which is interpreted to be composed of two components; a low unblocking temperature recent viscous magnetization and a high unblocking temperature ancient magnetization that was acquired in the latest Stephanian to Early Permian after initial D1 thrusting and folding bur prior to major secondary rotation. Rock magnetic experiments show that the characteristic remanence magnetization is carried by secondary authigenic pseudosingle-domain magnetite. These paleomagnetic results are used to determine the kinematics and geometry of postmagnetization deformation by restoring in situ magnetic vectors back to a defined reference direction. On the basis of the restored Ponga Unit geometry, the steeply plunging interference folds found in the WEVB core are best described by a combination of secondary buckling of frontal-ramp-parallel hanging wall anticlines and modification of D1 lateral/oblique ramps as frontal ramps during late stage north-south shortening. This combination of structural modification of the inner core of the WEVB was necessary to accommodate oroclinal bending during the final amalgamation of Pangea in the late Paleozoic.
Copyright 2006 American Geophysical Union. To view the published open abstract, go to http://dx.doi.org and enter the DOI, 10.1029/2005TC001861.
Weil, A. B. (2006) Kinematics of orocline tightening in the core of an arc: Paleomagnetic analysis of the Ponga Unit, Cantabrian Arc, northern Spain, Tectonics, 25, TC3012.