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754A.B. Weil et al. / Journal of Structural Geology 22 (2000) 735±756with superimposed F 3 folding about variably plungingeast±west axes (Figs. 8 and 16). Regionally, this superpositionis caused by a change in the overall shorteningdirection from east±west in the Carboniferous tonorth±south in the Permian (in present-day coordinates).In the La Queta, La Vega de los Viejos andLagos de Valle (Van der Voo et al., 1997) synclines, F 3fold axes change from intermediately plunging in thenorth to (sub)vertical in the south. In these three structures,F 3 fold axes all lie approximately in east±westaxial surfaces (Fig. 16b). The en eÂ chelon arrangementof the three synclines (Figs. 1 and 16a), and the similarityof their F 3 geometry, suggests that F 3 foldingsystematically a€ected entire thrust units within theCAA. Originally broad synclines and tight anticlinesthat formed by F 1 and F 2 folding were transformedinto large sinuous basins and tightly buckled anticlinesduring F 3 folding (Fig. 16a and c). This type of con-®guration, which is seen throughout the fold-andthrustprovince of the CAA, requires originally opencylindrical folds with variable wavelengths and amplitudes(Ramsay, 1960, 1962; Julivert and Marcos,1973). Such a geometry allows later F 3 folding torefold the limbs of these early structures (F 1 +F 2 )around variably plunging axes (i.e. the La Queta Syncline),causing oroclinal bending with an overall clockwiserotation in the north and a counterclockwiserotation in the south.Our work further shows that fault reactivation duringF 3 folding caused thrust-sheets to rotate and F 1 and F 2folds to buckle. As a consequence, there is increasedthrust stacking and shortening during F 3 , which producedsteepened bedding, thrust propagation and arctightening (oroclinal bending). The Proza Anticline ofthe inner Sobia thrust unit is a good example of anearly F 1 +F 2 structure that has been breached, thrustand tightened during F 3 deformation (Fig. 1). Bothstructural trend and magnetic declination have experiencedup to 908 of relative rotation during F 3 folding.According to the paleomagnetic analysis presentedhere, an originally linear north±south anticline wasformed during F 1 +F 2 deformation, which was laterfolded by F 3 deformation. This oroclinal style of rotationresulted in a radial set of east±west-trending F 3folds, and, due to space problems in the hinge area,thrust initiation and propagation in the southern halfof the structure. Thus, the curvature observed today inthis structure is primarily due to the interference of foldgenerations (F 1 +F 2 and F 3 ), and not a primary featurecreated during thrust emplacement, as suggested byJulivert and Marcos (1973), Julivert and Arboleya(1984) and PeÂ rez-EstauÂ n et al. (1988). Their scenariosleave the CAA's hinge zone with signi®cant curvatureprior to F 3 deformation, whereas the paleomagneticdata suggest that the hinge zone of the CAA wasessentially linear after F 1 +F 2 folding and thrusting.6. ConclusionsWe ®nd that on the regional scale, the kinematics ofthe Cantabria±Asturias Arc is more complicated thanindicated by earlier paleomagnetic studies (Bonhommetet al., 1981; Perroud, 1982, 1986; Hirt et al., 1992;PareÂ s et al., 1994; Stewart, 1995). The suggestion oflate tightening (F 3 ) about vertical axes is too simplisticto account for the observed local structural perturbations,which include rotations, tilts, and combinationsof the two. Our observations are also notconsistent with the idea that the CAA's curvature isthe result of continuous rotation of individual thrustsheets about pivotal rotation axes during initial thrusting(PeÂ rez-EstauÂ n et al., 1988). The early phase of deformationcan be characterized by two foldinggenerations (F 1 and F 2 ) based on remagnetizationevents. During F 3 folding the early F 1 and F 2 structuralfabric caused F 3 fold axes to take on variableorientations that resulted in complex geometries withinand between individual structural domains.The mechanism for producing the curvature we seetoday in the CAA (oroclinal bending) was largely anearly Permian north±south shortening event (F 3 ),which produced both vertical-axis rotations and tilts oforiginal F 1 and F 2 structures in the hinge zone of theCAA. This multiphase deformation history ultimatelyresulted in tightening of the belt from its pre-F 2 linearcon®guration (Fig. 16). On a broader scale, the changein regional shortening direction from east±west duringF 1 to north±south during F 3 suggests that late deformationrequired regional compression (or transpression)of Iberia between a northern Laurasian block and asouthern Gondwana block. The origin of this change intectonic regime may re¯ect late transpressional activityduring ®nal Pangea amalgamation, possibly related to aproposed continent-scale mega-shear in the North Atlanticregion (Arthaud and Matte, 1977; Ponce de Leon andChoukroune, 1980; Burg et al., 1981; Matte, 1986).AcknowledgementsThis work was supported by the National ScienceFoundation, Division of Earth Sciences, grants EAR9705755 and 9508316. The Scott Turner Fund, Departmentof Geological Sciences, University of Michiganprovided summer ®eld support in 1996 and 1997. Wethank B. Luyendyk and A. PeÂ rez-EstauÂ n for theirreviews.ReferencesAller, J., Gallastegui, J., 1995. Analysis of kilometric scale superposedfolding in the Central Coal Basin (Cantabrian Zone, NWSpain). Journal of Structural Geology 17, 961±969.
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