Testing models for obliquely plunging lineations in transpression: a natural example and theoretical discussion

Theory predicts that stretching lineations in an ideal vertical transpressional zone should be either vertical or horizontal. Many field descriptions of transpressional zones, however, indicate a range of lineation orientations between these extremes. Several theoretical models have been developed to explain such departures from expected lineation orientation, and we discuss these in the context of a field example from the Archean Superior Province in the North American craton. Existing models are insufficient to explain obliquely plunging lineations in this example because: (1) obliquely plunging lineations cannot be accounted for by shear zone boundary effects imposed by a no-slip condition, (2) foliations and lineations vary independently, (3) the vorticity-normal section is subhorizontal, limiting possibilities for inclined simple shear, (4) high vorticity is needed for finite strains and lineations to match previously proposed triclinic models, but vorticity is relatively low, and (5) juxtaposed east and west plunging lineations are unlikely in the previously proposed triclinic models. Because existing theoretical models are not applicable to our field example, we contemplate a new model to explain obliquely plunging lineations within quasi homogeneous transpression.