Valles Marineris, as 'the scar of the original impact' is more specifically the canyon pulled open by the volcanic activity around it which result from the the collapsing of mountain ranges formed at the original entry point of Mars, which corresponds to the Himalayas on this Earth ( entry point being the point which corresponds to the hole in my original shield reconstruction. see: 'Shield Re-Assembly' ).

As part of its contraction and post global warming activity its surface has undergone millions of years of global super-storms, which will have eradicated any surface evidence of its former, now defunct, plate boundaries. ( see: 'Global Warming 20: plate boundaries' )

The absence of any significant martian magnetic field is also indicative of the slowing, or ceased, internal convection activity, which simultaneously drives the continent movement and generates magnetism.