Determination of early fold axis orientations in multiply deformed rocks using porphyroblast inclusion trails

Porphyroblast rotation: eppur si muove*?

“…Possible natural examples of such a situation have been reported by Fyson (1975Fyson ( , 1980. The local occurrence of such 'protected' porphyroblasts does not imply, however, that rotation of equidimensional porphyroblasts does not occur, although this is claimed by Bell (1985, 1986), Bell & Johnson (1989, Bell & Cuff (1989), Steinhardt (1989), Johnson (1990a, b) and Hayward (1990). Flow partitioning as described above is commonly referred to in the literature as 'deformation partitioning'; although this is not strictly correct we will adhere to this use here in order to avoid confusion.…”

“…Statement (1)-porphyroblasts do not rotate because they are mechanically fixed in the rock by strain shadows (Bell, 1985(Bell, , 1986Bell eta/., 1986;Bell & Johnson, 1989;Bell & Cuff, 1989;Steinhardt, 1989;Johnson, 1990a, b;Hayward, 1990) Rigid objects in a linear or non-linear viscous fluid will rotate with respect to the kinematic frame of the flow if the fluid deforms by a homogeneous non-coaxial flow type such as simple shear (Jeffery, 1922;Bretherton, 1962;Ghosh & Ramberg, 1976;Gierszewski & Chaffey, 1977). The angular velocity of the object depends on the vorticity of the bulk flow and, for non-spherical objects, on object orientation and aspect ratio.…”

“…, 1986;Bell &Johnson, 1989Steinhardt, 1989;Johnson, 1990a, b; Hayward, 1990) One of the main advances in the study of porphyroblasts in the last decade has been the discovery by Fyson (1975Fyson ( , 1980, Bell (1985) and Bell et al (1986) (1980). These structures are interpreted by Bell (1985Bell ( , 1986 to form by flow partitioning around a stationary porphyroblast, as indicated; (b) millipede structures can also form by passive deformation of a foliation around a rigid object in absence of flow partitioning of the type envisaged by Bell (1985Bell ( , 1986; three possible settings are shown, two of which are situations where the porphyroblast rotates with respect to the kinematic frame.…”

“…Besides the fact that presently published evidence for non-rotation of porphyroblasts is dubious, the papers dealing with the subject (Fyson, 1975(Fyson, , 1980Bell, 1985;Steinhardt, 1989;Bell & Johnson, 1989Johnson, 1990a, b;Hayward, 1990) commonly apply unsound argumentation, based on statements which cannot be tested. On the one hand, the authors claim that porphyroblasts do not rotate, since Si in porphyroblasts has the same orientation over large areas, in spite of intense later deformation (Fyson, 1975(Fyson, , 1980Steinhardt, 1989;Bell & Johnson, 1989Johnson, 1990a, b;Hayward, 1990).…”

“…1986;Bell et al, 1986;Bell & Johnson, 1989Steinhardt, 1989;Johnson, 1990a, b;Hayward, 1990). These papers have made the useful contribution of calling attention to the fact that many apparently rotational fabrics can also be explained as non-rotational.…”