KINEMATIC CONSTRAINS ON LITHOSPHERIC-SCALE OROCLINAL BENDING OF THE IBERO-ARMORICAN ARC ALONG THE NORTHERN MARGIN OF GONDWANA: A PALEOMAGNETIC AND STRUCTURAL SYNTHESIS

The Paleozoic Variscan orogeny was a large-scale collisional event that involved amalgamation of multiple continents and micro-continents. Existing data from the western Variscan orogen, suggests oroclinal bending of an originally near-linear convergent margin during the last stages of Variscan deformation in the late Paleozoic. Closure of the Rheic Ocean resulted in E-W shortening (in present-day coordinates) in the Carboniferous, producing a near linear N-S trending, east verging belt. Subsequent N-S shortening near the Carb-Permian boundary resulted in oroclinal bending. This late-stage orogenic event remains an enigmatic part of final Pangaea amalgamation.

The present-day arc curvature of the western Variscan has inspired many tectonic models explaining how the orogen was created, with little agreement between models. While there is general consensus that two separate phases of deformation occurred, various models have considered that curvature was caused by: dextral transpression around a Gondwana indentor; strike-slip wrench tectonics; or a change in tectonic transport direction due to a changing stress field. More recent models explain the curvature as an orocline, with potentially two opposite-facing bends, caused by secondary rotations. Deciphering the kinematic history of curved orogens is difficult. To classify an orogen as an orocline, two deformation phases are required: an initial compressive phase that forms a relatively linear belt with little to no rotation, and a second phase that causes vertical-axis rotation of the orogenic limbs. Historically the most robust technique to accurately quantify vertical axis-rotation in curved orogens is paleomagnetic analysis, but recently other types of data, including fracture, paleo-current and calcite twin data, have been used to corroborate secondary bending. A review of existing and new Variscan data from Iberia will be presented that argues in support of secondary buckling of an originally linear orogenic system.

Together, these data constrain oroclinal bending of the IAA to have occurred during the latest Carboniferous over about a 10 million year time window, which agrees well with recent geodynamical models and structural data that relate oroclinal bending with lithospheric delamination in the Variscan.