Exhumation of subcontinental mantle rocks: evidence from ultramafic-bearing clastic deposits nearby the Lherz peridotite body, French Pyrenees

In the eastern Pyrenees, the lherzolites always outcrop as restricted bodies scattered across Mesozoic sedimentary units along the North Pyrenean fault. In this article, we focus on the sedimentary processes associated with the tectonic exhumation of the mantle rocks during the Lower Cretaceous and we provide a new detailed analysis of the ultramafic-bearing sedimentary sequences associated with the Lherz body. The Lherz body, 1.5 km long only, belongs to a series of ultramafic bodies of restricted size (a few m to some hundred of m), occurring within sedimentary formations composed mostly of carbonate breccias originating from the reworking of Mesozoic platform limestones and dolomites. The carbonate clasts include marbles displaying different evolutions in relation with the Cretaceous thermal metamorphic event. The elastic formations also include numerous layers of polymictic breccias reworking ultramafic clasts. These layers are found far from any peridotitic body, implying that ultramafic clasts have been transported far away from their sources by sedimentary processes after their disaggregation on the seafloor. In addition, clasts of ultramafic-rich breccias and sandstones are also observed which indicates rapid sedimentation and diagenesis within a tectonically active environment. We discuss the significance of these deposits and their bearing on our understanding of the tectono-sedimentary processes associated with mantle exhumation in a context of development of narrow intracontinental basins suffering extreme crustal thinning.