Experimental modeling of the microscopic and mesoscopic structures of upper mantle diapirs
Kunze, Florence Raffaele
Lallemant, Hans G. Avé
Master of Arts
Samples of Mt. Addie dunite have been deformed in a rotational system where the principal stresses were not parallel to the final principal strains. Three types of experiments were performed: 1) extrusion tests (dunite was extruded through a hollow carbide cylinder), 2) indentation tests (a solid carbide cylinder was pushed into the dunite) , and 3) "mylonite" tests (compression of a composite cylindrical sample that consists of a thin slice of dunite oriented at 45° to the sample axis between two websterite pieces. All samples have been subjected to axial compression at 5 - 1 kb confining pressure over a large range of temperatures, strain rates, and strains. At low temperatures and high strain rates little or no recrystallization occurred; the most common slip system is (1) [lOC^. At higher temperatures and lower strain rates recrystallization is the predominant flow mechanism. There is some indication from these experiments that the preferred orientation developed in the deformed original grains is dependent on strain. The pattern of the preferred orientation developed in the syntectonically recrystallized grains appears to be dependent on stress. The velocity gradient does not appear to effect the preferred orientation of the indicatrix axes. Extrusion experiments using low viscosity materials were performed to model kinematically the mesoscopic structures found in diapir walls of salt and some ophiolite complexes. The materials were extruded through an elongate slot into an open-ended chimney. Spines, separated by sear zones, and isoclinal folds with vertical fold axes developed; they compare well with structures of salt diapirs. Rotation of vertical structures into a horizontal orientation occurs when the material overflows out of the chimney. Such rotations have been recognized in shallow mushrooming diapirs; they can be explained by a divergent flow regime.