Estimated obliquities between the horizontal traces of XY surfaces and foliation trajectories in strained orthogneiss
Abstract
The geometrical style of mineral foliation varies greatly within deformed metaplutonic masses, and the angle between foliation trajectories and horizontal traces of XY surfaces of total strain depends largely on this variation. In laminated orthogneiss derived from protoliths with isolated megacrysts or polycrystalline aggregates, the foliation trajectories can be markedly oblique to the traces of the XY surface. The obliquity angle (δ) depends on (1) the boundary migration of mineral aggregates, (2) the finite strain accumulated during the production of laminae and (3) the finite deformation imposed on laminated gneiss. Prediction of the obliquity angle is difficult because (1) and (2) are effectively unknown, and (3) varies greatly with position. By assuming that (i) all isolated aggregates behave as if they deform coaxially while being transformed into laminae and (ii) all laminae rotate as passive markers, we were able to calculate the obliquity angle for a range of laminaeproducing strains and a spectrum of imposed plane deformations. These results apply to rocks located on the symmetry planes of large folds and XZ sections of shear zones. Here the obliquity between foliation trajectory and the horizontal trace of XY plane is < 15°, for any amount of laminaedeforming strain, if the formation of laminae requires horizontal strain ratios of >2.4. Even if the laminaeproducing strain is as low as 2.0, δ < 15° provided that the ratio of laminaedeforming strain is < 1.8. Higher strain ratios may be ruled out if the laminae are effectively planar, on the scales of centimetres to metres, or where the short limbs of open buckles in gneissosity resemble conjugate kink bands or chevron folds.
 Publication:

Journal of Structural Geology
 Pub Date:
 January 1992
 DOI:
 10.1016/01918141(92)90140R
 Bibcode:
 1992JSG....14...11S