Syn-folding remagnetization events in the French-Belgium Variscan thrust front as markers of the fold-and-thrust belt kinematics

Abstract

Abstract New paleomagnetic studies have been carried out within the Ardennes segment of the N France - S Belgium Variscan fold-and-thrust belt to set constraints on the fold-thrust belt kinematics and reveal the casual relationships between vertical-axis rotations and major strike deviated zones localised along the general trend of the belt. Magnetite-bearing Devonian and Carboniferous limestones yielded two characteristic, secondary components of the natural remanent magnetization : a low temperature component recorded most probably during the late stages of folding and a high temperature component, acquired during incipient stages of deformation. Both post- and synfolding magnetizations were identified in the Lower Devonian hematite bearing sandstones. Ages of magnetization, inferred from the analysis of characteristic remanence inclinations compared to the reference curves for the stable parts of the Old Red Sandstones Continent (ORC), suggest the previous remagnetization event to be due to the burial of sedimentary rocks under the thick molassic foreland basin of Namurian-Westphalian age and the second to the final out-of-sequence activation of the thrust front in Stephanian times. Irrespective of the age of the magnetizations, orientations of paleomagnetic directions are dominantly governed by second-order structural trends. Clockwise rotations are observed in relatively narrow zones featuring deviated orientations of fold axes, other sites show paleomagnetic directions akin to those known from the ORC. We interpret this feature as a result of local transpressive deformations and related rotations, which occurred at lateral borders of propagating thrust-sheets. The latter deformation zones are suggested to be controlled by deep-seated discontinuities inherited from the Devonian Rheno-hercynian basin development. The Ardennes thrust belt was thus not rotated as a whole unit with respect to the ORC after the Namurian, preserving the initial orientation of the continental margin.