I think that a natural marriage can not be dissolved by any civil power. Here’s what Pie XI wrote, in Casti Connubii:
«Therefore although the sacramental element may be absent from a marriage as is the case among unbelievers, still in such a marriage, inasmuch as it is a true marriage there must remain and indeed there does remain that perpetual bond which by divine right is so bound up with matrimony from its first institution that it is not subject to any civil power. And so, whatever marriage is said to be contracted, either it is so contracted that it is really a true marriage, in which case it carries with it that enduring bond which by divine right is inherent in every true marriage; or it is thought to be contracted without that perpetual bond, and in that case there is no marriage, but an illicit union opposed of its very nature to the divine law, which therefore cannot be entered into or maintained."
«And if this stability seems to be open to exception, however rare the exception may be, as in the case of certain natural marriages between unbelievers, or amongst Christians in the case of those marriages which though valid have not been consummated, that exception does not depend on the will of men nor on that of any merely human power, but on divine law, of which the only guardian and interpreter is the Church of Christ.»