Analysis of Variscan dynamics; early bending of the Cantabria–Asturias Arc, northern Spain

Calcite twinning analysis in the Cantabria–Asturias Arc (CAA) of northern Spain provides a basis for evaluating conditions of Variscan stress and constrains the arc’s structural evolution. Twinning typically occurs during earliest layer-parallel shortening, offering the ability to define early conditions of regional stress. Results from the Somiedo–Correcilla region are of two kinds: early maximum compressive stress oriented layer-parallel and at high angles to bedding strike (D1σ1) and later twin producing compression oriented sub-parallel to strike (D2σ1). When all D1 compressions are rotated into a uniform east–west reference orientation, a quite linear, north–south trending fold–thrust belt results showing a slight deflection of the southern zone to the south–southeast. North–south-directed D2σ1 compression was recorded prior to bending of the belt. Calcite twinning data elucidate earliest structural conditions that could not be obtained by other means, whereas the kinematics of arc tightening during D2 is constrained by paleomagnetism. A large and perhaps protracted D2σ1 is suggested by our results, as manifested by approximately 50% arc tightening prior to acquisition of paleomagnetic remagnetizations throughout the CAA. Early east–west compression (D1σ1) likely resulted from the Ebro–Aquitaine massif docking to Laurussia whereas the north-directed collision of Africa (D2σ1) produced clockwise bending in the northern zone, radial folding in the hinge, and rotation of thrusts in the southern zone.