The intriguing geometry of the Central Iberian Orocline: a combined paleomagnetic and structural approach
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The Variscan Belt in Iberia forms an “S” shape double orocline. The northern orocline, the Cantabrian Orocline (CO), is kinematically constrained as a secondary orocline and thought to be a thick-skinned feature. The geometry, kinematics and mechanism of formation of the southern orocline, the Central Iberian Orocline (CIO), are still unclear. Several geometries have been proposed for the CIO, most of them suggesting that the Morais allochthonous complex (NE Portugal) is in its core and that the axial trace is parallel to the fold axis of the last major Variscan deformation phase. The aim of this study is to get better constraints on the geometry and kinematics of the CIO, by combining paleomagnetic directions from the hypothesized hinge with structural analyses of an outcrop in the core of the orocline. Paleomagnetic directions indicate that no differential rotations occurred around the hypothesized hinge zone. The directions from syn-kinematic granites and the Cambrian Tamames limestone formation range from ENE to SE, while post-kinematic granites have directions pointing to the SE. The range of directions is interpreted as a remagnetization during rotation, which is happening because of the intrusion of the post-kinematic granites. This constrains the timing of rotation to the age of the post-kinematic granites, which is between 310 and 298 Ma. No differential rotations were observed around the hypothesized hinge. The ENE direction is similar to directions from the southern flank of the CO, while the SE direction represents the Permian direction and the end of rotation. Structural analyses indicate that three phases of deformation affected the studied outcrop. Deformation phases D1 and D3 are quasi-coaxial, whereas perpendicular directions would be expected in the hinge zone if the orocline is the result of buckling of an originally straight mountain belt. This implies that the orocline is not formed by buckling because of D3 deformation. The results from the paleomagnetic directions and the structural analysis are consistent and indicate that the CIO is not a secondary orocline with the Morais complex in its core. Instead, the studied area forms part of the southern limb of the CO. This means that the geometries proposed by previous authors are not compatible with the data presented in this study. Therefore, taking into account the new data from this study, three new possible geometries for the CIO are proposed.