Basement-Cover Decoupling During the Inversion of a Hyperextended Basin: Insights From the Eastern Pyrenees

Abstract

Deformation processes related to early stages of collisional belts, especially the inversion of rifted systems remain poorly constrained, partly because evidence of these processes is usually obliterated during the subsequent collision. The Pyrenean belt resulting from the inversion of a Cretaceous hyperextended rifted margin associated with a high-temperature and low-pressure metamorphism in the Internal Metamorphic Zone (IMZ), is a good example for studying the early stage of orogenic deformation. This study is focused on the Eastern Pyrenees where the relation between inverted Mesozoic rifted basins and their basement are well-preserved. By using maximum temperatures (Tmax) estimated by the Raman Spectroscopy of Carbonaceous Materials geothermometer and structural data, we describe the spatial distribution of the various tectono-metamorphic units. Tmax recorded in the sedimentary cover exposed to the north and to the south of a Paleozoic basement block (Agly massif), exceed 550°C, while the Paleozoic metasediments and their autochthonous Mesozoic cover show Tmax < 350°C. The metamorphic sedimentary cover is affected by ductile deformation, while the basement is only affected by brittle deformation. Post-metamorphism breccias are observed between the basement and the metamorphic sedimentary cover, which are interpreted as a décollement level in the Upper Triassic evaporites. Unlike previous models suggesting that the basement block separated two metamorphic basins (Boucheville and Bas Agly) during rifting, we propose a large displacement of a single metamorphic basin by a large thrust above the basement block. This novel interpretation emphasizes the general allochthonous position of the former hyperextended rift basin (IMZ) thrusted along a decoupling layer.