COAXIAL PROGRESSIVE DEFORMATION TEXTURES IN EXTRUSIVE AND SHALLOW INTRUSIVE ROCKS, SOUTHWEST JAPAN

Fine-grained rocks from an andesite dyke, a dacite dome and a basalt f low contain microshear zones overprinting an aligned microlitic ground mass. In each case, the shear zones comprise conjugate sets bisected b y the plane of crystal alignment. This texture indicates coaxial (or p ure shear) flow in contrast to simple shear which is generally conside red to be the dominant kinematic flow type in extrusive and shallow in trusive rocks. This is interpreted as resulting from a change from non -coaxial to coaxial flow, coinciding with increasing viscosity during emplacement. This suggests that deformation of extrusive and shallow i ntrusive rocks in the near-solid state during the latter stages of emp lacement may be more important in the development of aligned rock text ures than fluid-state flow of the magma as is commonly envisaged.