La influencia de los modelos cinemáticos en el análisis de tectonitas

Abstract

The most common procedure when analyzing ductilely deformed rocks (tectonites and mylonites) is to study the section nor- mal to the tectonic foliation and parallel to the tectonic lineation, which is generally assumed to contain the X (long) and Z (short) axes of the finite strain ellipsoid. This method relies on the assumptions, arisen from the application of the simple shear kinematic model, that (1) deformation is plane strain and (2) the rotational component of deformation is restricted to the XZ plane, which is thus considered as the section with maximum asymmetry (VNS, Vorticity Normal Section). However, more recent kinematic models assuming 3D deformation and several natural examples suggest that the orientation of the VNS and that of the finite strain ellipsoid are independent (Díaz-Azpiroz et al., 2019). Both are controlled by the kinematics of deformation derived from the boundary conditions (essentially, the angular relationship between the shear zone boundaries and the vector describing the relative movement between blocks) but, assuming steady-state deformation, the orientation of the VNS remains constant with time in relation to a fixed reference frame at the boundaries of the shear zone, whereas the finite strain ellipsoid (the XYZ axes and thus the resulting tectonic fabric) rotates its orientation with strain accumulation. Therefore, if we suspect that the deformation under analysis could be nonplanar, we should avoid the routine analysis of only the XZ section. Instead, in these cases, finding the orientation of the VNS via independent methods should be itself part of the kinematic analysis.