Oblique lineations in transpression; models utilizing anastomosing shear zone geometries.
Czeck, Dyanna M.; Hudleston, Peter J. Department of Geology and Geophysics, University of Minnesota, Minneapolis, MN.
Classical transpression theory predicts that stretching lineations should be either vertical or horizontal. In many natural settings, however, oblique lineations are observed, and new models are needed to account for such obliquity. Here, two geometrical models are presented that predict a range of lineation plunges based solely on geometrical constraints imposed by anastomosing shear zone patterns. The models are compared to a natural example from the Wabigoon-Quetico Subprovince boundary in the Archean Superior Province. One model includes shear localized along anastomosing shear zones surrounding rigid blocks in lozenge or diamond shapes. In this model, bulk transpression can create kinematics identical to classical transpression in the necks of the lozenges. Simultaneously, the lineation can be skewed from verticality along the sides of the lozenges. The second model is similar, but involves the development of local velocity gradients that effectively extrude material from the corners of the lozenges during transpression, creating a range of lineation plunges. In the first model, the lineations have preferred orientations along each bounding face of the lozenges, and the lozenges do not rotate. In the second model, the lineations show continuous variation with distance from the extrusion outlets, and the lozenges rotate. The Rainy Lake Seine River Fault Zone along the Wabigoon-Quetico subprovince boundary may provide a natural example of this type of behavior. Shear zones within this boundary surround less intensely strained blocks. A wide range of stretching lineation plunges from 0-180 degrees have been observed. Ubiquitous asymmetric indicators such as sigma and delta style deformed conglomerate clasts on the subhorizontal surface indicate noncoaxiality. The geometrical lozenge models differ from other transpression models in that they can explain a full range of lineation plunges from 0-180 degrees instead of just 0-90 degrees, and that they may be able to explain the orientation of asymmetric noncoaxiality indicators as observed at the Wabigoon-Quetico Subprovince boundary.
Return to publications.