Comment on: “Exhumation of subcontinental mantle rocks: evidence from ultramafic-bearing clastic deposits nearby the Lherz peridotite body, French Pyrenees” by C. Clerc et al. (Bull. Soc. géol. Fr., 2012, 183, 5, 443–459)

Abstract

In their recently published paper, C. Clerc et al . [2012] describe the geological setting of the Etang de Lherz peridotite body and give a short review of the associated clastic deposits, proposing a detailed analysis of the breccias cropping out in a limited area east of the lherzolite massif. The cartographic, sedimentological and metamorphic evidence leads the authors to regard these breccias as a shallow marine to lacustrine equivalent of the Albo-Cenomanian “Flysch noir” of the Pyrenees. This interpretation backs up their previous attribution of the breccias to a thick submarine clastic formation, of Albo-Cenomanian age, which incorporates olistoliths derived from the famous Etang de Lherz lherzolite and its neighbouring bodies. Clerc et al. firstly present a somewhat theoretical discussion concerning a 3D reconstruction of the depositional environment of the Lherz breccias, along with a possible scenario for their origin and structural history, which then leads onto three possible scenarios for North-Pyrenean rifting. The authors use this approach to support their previous conclusions, i.e.: the heterometric peridotite fragments contained in the Lherz breccias (from sand-sized debris up to olistoliths) provide clear evidence for an Albo-Cenomanian subaqueous exhumation of the subcontinental mantle at the borders or on the floor of the North-Pyrenean Aulus basin. Our comment focuses on a number of geological details concerning the Lherz breccias and their local or regional context, which the authors fail to take into account, either in their present paper or in earlier studies. Indeed, these details are of crucial interest since they call into question the assumption of a sedimentary origin for the Lherz breccias and, consequently, the exhumation of mantle in the Aulus basin. This map covers an area of 100×200 m located on the western side of the middle part of the so called “Ravin de Paumeres” [or “Ravin de l’Homme-mort” …