Age of emplacement of the schistes lustres nappe, Alpine Corsica

Abstract

Structural studies in the Schistes lustrés nappe west of Bastia, Corsica, demonstrate the existence of a tectonic mélange in which km-scale blocks and smaller lozenges of basement granite gneiss, thick-layered marble and dismembered Mesozoic ophiolite are enveloped in a matrix of calc-schist and blueschist. The main ( S 1) foliation is developed in both block and matrix and is concordant with lithologie contacts. Blueschist facies metamorphism was syn-kinematic with the main foliation. The S 1 in the Schistes lustrés was refolded about ENE-WSW trending, tight similar and monoclinal fold axes ( F 2). These second folds verge to the southeast and show km-scale axial culminations and depressions that are reflected by topography and residual Bouguer gravity anomalies. Parautochthonous Hercynian basement (Tenda-Corte complex) beneath the western edge of the Schistes lustrés nappe contains a mylonitic foliation which is concordant with the main foliation in the Schistes lustrés. The intensity of deformation in the basement decreases away from this contact and undeformed granites are found 3 km to the west. Whole rock samples of the deformed basement immediately beneath the Schistes lustrés yield an Rb-Sr isochron diagram ( n = 4) which has an age of 105 ± 8 Ma (1σ) and initial 87Sr 86Sr ratio of 0.7228 ± 0.0005 (1σ). This result is more precise than our preliminary age and initial ratio estimate of 98 ± 14 and 0.7296 ± 0.0068, respectively (Cohen et al., 1979). It is similar to a recently published mid-Cretaceous (90 Ma) 40Ar- 39Ar age from glaucophane mineral separates. We interpret this date as the age of a metamorphic overprint related to the emplacement of the Schistes lustrés nappe and associated ophiolites, the formation of the main foliation and blueschist facies metamorphism. These results indicate that the mid-Cretaceous blueschist facies metamorphism documented in the Western Alps formerly extended farther south of its present terminus. The data are consistent with mid-Cretaceous obduction of Tethyan oceanic crust onto the present-day eastern continental margin of Corsica. We postulate that during Eocene—early Oligocene time a polarity flip occurred outboard of the obducted crust and a new, southfacing subduction zone developed. This change in polarity was responsible for the development of southeast-vergent second folds and for the resetting of 40Ar− 39Ar and K-Ar geochronologic clocks described in the literature.