The microstructure of nonpolar heteroepitaxial wurtzite films (GaN and ZnO-based) is dominated by the presence of planar basal stacking faults (BSFs). In this paper, transmission electron microscopy studies of both GaN and ZnO nonpolar films oriented either (11-20) or (1-100) and grown on sapphire substrates, permit to propose and evaluate different mechanisms of BSFs formation. The main mechanism of formation of BSFs results from a Volmer Weber growth mode. The first stage of the growth is a 3D nucleation. The 3D nuclei are relaxed at least along one in-plane orientation. BSFs are created in the coalescence boundaries in order to compensate translations between neighbouring islands. BSFs are well adapted to compensate in-plane translations in the case of nonpolar films. In fact, their plane is perpendicular to the substrate surface and this orientation is similar to the orientation of the coalescence boundaries. Moreover, their displacement vector has a component parallel to the translation between islands and their formation energy is low. On the other hand, in the case of the polar growth, BSFs are not adapted to compensate in-plane translation as their plane is perpendicular to the coalescence boundaries. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4768686]