<blockquote><font class="small">In reply to:</font><hr /><p>There is no way these circles, whatever they are, have anything to do with ancient meteorite impacts because plate tectonic movements have completely shuffled the organisation of the continents.<p><hr /></p></p></blockquote>I've learned to stay away from absolutes.<br /><br />Plate tectonics moves <i>plates</i> but the continent riding on top goes through a different sort of morphological shift, like a newsprint image on Silly Putty as you shift and stretch it. Here's a mechanism by which these <i>could</i> be records of impacts from the last phase of planet building. The crust would have been really pliable, and the planet scarcely differentiated. So a really large impactor alters the crust, the upper mantle, the lower mantle, the mantle/core interface... in other words, all the convective layers of rock. All of the later, surface events, the continent building events, occur on top of this. But it's such a profound alteration of the underlying processes, that some sort of measurable phenomenon exists to this day.<br /><br />As a hypothesis.<br /><br />PS thanks for the geology map. There are many of these, and some show the boundaries are coincident with the arcs.