Outcrop-scale tomography: insights into the 3D structure of multiphase boudins

Abstract

We analyze the 3D geomorphology of a boudinaged amphibolite layer encased in marble using meter-scale tomography by serial sectioning, high resolution imaging and 3D reconstruction of a 2 m³ block from a quarry in the high-grade core of the Naxos core complex. Local uncertainties in the model are resolved by dissolution of marble in selected slabs, yielding a large, highly accurate 3D model.Results show five generations of brittle extension of the amphibolite in ductile marble, although examination of the boudinaged layer on the outside of the block only suggested one generation. Most prominent are the rotated domino boudins, where part of the dilatancy between domino boudins can be shown to be due to later reactivation. Shear fractures between domino boudins have relatively large throw gradients, indicating ductile rupture. Layer-parallel stretch as measured by reconstruction of domino boudins decreases with amphibolite layer thickness although the layer has undergone the same bulk stretch everywhere. This suggests that with decreasing layer thickness deformation becomes delocalized, offering tantalizing glimpses of rheology under metamorphic conditions.