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We study the distribution of nearby thick disc and halo stars in subspacesdefined by their characteristic orbital parameters. Our aim is to establish theorigin of the structure reported in particular, in the $R_{\rm max}-z_{\rmmax}$ space. To this end, we compute the orbital parameters and frequencies ofstars for a generic and for a St\"ackel Milky Way potential. We find that forboth the thick disc and halo populations very similar prominent substructuresare apparent for the generic Galactic potential, while no substructure is seenfor the St\"ackel model. This indicates that the origin of these features isnot merger-related, but due to non-integrability of the generic potential. Thisconclusion is strengthened by our frequency analysis of the orbits of stars,which reveals the presence of prominent resonances, with $\sim 30\%$ of thehalo stars associated to resonance families. In fact, the stars in resonancesdefine the substructures seen in the spaces of characteristic orbitalparameters. Furthermore, we find that some stars in our sample and in debrisstreams are on the same resonance as the Sagittarius dwarf, suggesting thissystem has also influenced the distribution of stars in the Galactic thick discand halo components. Our study constitutes a step towards disentangling theimprint of merger debris from substructures driven by internal dynamics. Giventheir prominence, these resonant-driven overdensities could potentially beuseful to constrain the exact form of the Galactic potential.

Frequencies, chaos and resonances: a study of orbital parameters of nearby thick disc and halo stars