Do smoothly curving, spiral-shaped inclusion trails signify porphyroblast rotation? Comment and reply

Article Abstract:

A comment and reply on smoothly curving, spiral-shaped inclusion trails as evidence for porphyroblast rotation is presented. The comment emphasizes the lack of evidence of the theory that there is porphyroblast rotation and that sense-of-shear using curving inclusion trails may explain the phenomenon. The response contends that the fluid nature of the deformation of rock as presupposed by the comment is not proven. There is no actual evidence of the proposed model of the phenomenon. Rather, actual simulations of rock deformation suggest that there is relative rotation in rocks.

Geology, Structural, Structural geology, Rocks, Igneous, Xenoliths

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Comment

Article Abstract:

The contention of Bell et. al. that porphyroblasts are fixed with respect to geographical coordinates is more kinematic than dynamic, and challenges conventional shear research. A change in external fabric orientation can be considered rotation depending upon one's reference frame. In a viscous medium, rigid elliptical objects have been shown to rotate, with the exception of some orientations. Tests have shown that porphyroblasts can and do rotate.

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Reply

Article Abstract:

Current models that simulate rock deformation are inadequate and thus do not provide conclusive proof that rigid objects rotate during noncoaxial deformation. The variation of inclusion trails around a fold may be due to porphyroblastic growth after folding. By cutting spatially oriented sections, one can also check whether matrix stretching lineation is related to the foliation intersection axis within porphyroblasts.

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