Local displacement of diapir contacts and its importance to pluton emplacement study

The extent to which mid-crustal plutons have grown by diapirism cannot be deduced from the horizontal shortening of ductilely deformed host rocks and associated displacement of lateral intrusive contacts. By contrast, the vertical shortening above plutons and upward displacement of their roof contacts may provide a good measure of the diapiric component of pluton emplacement. This is related to the vertical bulk translation of buoyant material in immature diapirs and associated escape flow of overlying rocks. ngth change of a vertical material line situated above the crest of an immature upright diapir corresponds to the upward displacement of the local contact. The length change of horizonal material lines situated at lateral contacts of model diapirs. however, differs markedly from the contact displacement. This is related to the absence of horizontal symmetry planes in the finite-strain field about upright diapirs of any shape, and the importance of vertical shear at the lateral contacts. st-rock strain held of a hypothetical pluton grown in situ by spherical expansion (‘ballooning’) has an infinite number of symmetry planes. The strain held is characterized by large volume losses, but the length change of all radial lines corresponds to the local magnitude of contact displacement. This precludes the possibility of differentiating, in ductile host rocks above plutons. between vertical diapirism and spherical expansion—unless gauges of volume change are available.