THE MORPHOLOGY OF CLEAVAGE DOMES AND THE KINEMATIC SIGNIFICANCE OF METAMORPHIC FOLIATIONS

Most discussions of the kinematic significance of metamorphic foliations have, because of their mathematical simplicity, focused homogeneous, circulation preserving isochoric deformations. However a number of other, equally tractable, 2D analytical "creeping flow" models are available for conditions that may model a number of important geologic situations. One noteworthy example is a solution for viscous flow towards a stagnation point (x=0) on plane solid boundary: v_y=-k y² v_x=2k x y which may approximate the displacement of the surrounding matrix during the growth of a porphyroblasts. Cleavage domes, a nested arrangement of spherical foliation caps attached and concave to a planar face of a porphyroblastic crystal are generally considered to result from such displacement growth. The amplitude of a dome is typically less than one quarter of its diameter. In synkinematic examples the dome foliation is, except for the concentration of graphite, texturally identical to the matrix foliation. Some insight into the kinematic significance of the domal foliation may be obtained by comparing its morphology to the spatial variation of kinematic variables. The orientations of the principal axes of finite strain, the instantaneous stretching axes (ISA), "the fabric attractor" (FA), the eigenvectors of the velocity and acceleration gradient tensors relative to the porphyroblast, and the directions of minimum stretch and spin rate of change are all clearly unrelated to the foliation. The eigenvectors of the velocity gradient tensor relative to the ISA directions approximate the orientation of the dome foliation in the region y > |x|/2, but are imaginary and thus undefined elsewhere. The orientation of the material plane relative to which the matrix norm of the time derivative of the stretching tensor is minimized is the only kinematic parameter that seems to match the morphology of the dome foliation.