Large scale structure of the eclogite-blueschist of New Caledonia

Abstract

This paper considers the large-scale geometry of the northern part of the eclogite-blueschist belt of New Caledonia, based largely on classical-style structural analysis, but in consideration of modern concepts regarding crustal extension, exhumation of high-pressure rocks and uplift. Shear zones played an important role in the evolution of the region. Early shear zones and high pressure metamorphism appear to be associated with overthrusting of an ultramafic sheet. Middle-stage shear zones are associated with large-scale continental extension, during which the high-pressure rocks were exhumed. The extended crust was subsequently folded during renewed compression, producing an orogen-scale antiform throughout the high-pressure belt that folds all previously formed structures. Late stage shear zones formed when the orogen was once again thrown into extension. Relatively youthful normal faults caused late block-faulting, uplifting a regional peneplain and producing the present geomorphology. Previously published interpretations of the structure and geometry of the belt have suggested that the high-pressure rocks are found in the core of a regionally developed antiform, interpreted as a metamorphic core complex. Our structural mapping shows that allochthonous slices of high-pressure rocks are draped over a younger (unrelated) foliation antiform. Lower-grade (retrogressed equivalent) rocks are found within the core of the antiform. Thus the metamorphic core complex model is rejected.