Very high geothermal gradient during mantle exhumation recorded in mylonitic marbles and carbonate breccias from a Mesozoic Pyrenean palaeomargin (Lherz area, North Pyrenean Zone, France)

摘要

Although they are famous among Earth scientists, the Lherz peridotites are exposedrnwithin geological formations of the North Pyrenean Zone (NPZ) still lacking detailedrninvestigations. Our study focuses on the metasediments of the Aulus basin hosting thernLherz peridotite body and associated ultramafic fragments of smaller size. The new datarnset comprises of structural analysis and detailed geological mapping of the massivernMesozoic marbles that form the prerift sequence typical of the NPZ and of thernultramafic-rich clastic breccia formations surrounding the peridotite bodies. Thernmassive marbles display an evolution from hot and ductile to cold and brittlerndeformation, indicative of an exhumation process ending with the sedimentaryrnreworking of both the deformed Mesozoic metasediments and the exhumed ultramaficrnrocks. Crystal Preferred Orientations (CPO) measured in the marbles support arndeformation mechanism by dislocation creep of calcite, which is dominant betweenrn400 8C and 600 8C; these deformation temperatures are within the range determinedrnearlier by Clerc et al. (2015), using RSCM (Raman Spectroscopy of CarbonaceousrnMaterial) geothermometry. As a consequence, we better describe the transition fromrnductile to brittle deformation in the prerift marbles and clarify the origin of the syn-riftrnbreccias. Due to continuous exhumation along detachments’ faults, the brecciatedrnmetamorphic carbonates of the prerift NPZ sedimentary cover were passively upliftedrntowards shallower levels and progressively unroofed, while transported passively on thernback of the exhumed ultramafic footwall. These results are consistent with the recentrninterpretations of the North Pyrenean peridotites as remnants of subcontinental mantlernrocks exhumed within the pre-Pyrenean rift system. We emphasize the importance ofrntectonic decoupling between the Mesozoic sedimentary cover and the Palaeozoicrnbasement, which leads to the juxtaposition of metamorphosed and deformed Mesozoic sediments directly on top of exhumed mantle rocks. We favor a model of tectonicrndenudation of the peridotites below prerift sediments metamorphosed during thernextension of the basin floor under high temperatures and in a thermal regimerncharacterized by a very high gradient.