Well, the geneticists and Beringia-crossing folks seem to have gotten together.http://www.sciencedaily.com/releases/2009/01/090108121618.htm
After the Last Glacial Maximum some 15,000 to 17,000 years ago, one group entered North America from Beringia following the ice-free Pacific coastline, while another traversed an open land corridor between two ice sheets to arrive directly into the region east of the Rocky Mountains. (Beringia is the landmass that connected northeast Siberia to Alaska during the last ice age.)
Hi min -
If you go looking for Dr Firestone's team's AGU presentation from 2006, you'll see the peppered mammoth tusks from a massive iron asteroid impact around 32,400 BCE. That impact may have ended the C mt DNA haplogroup's migration to the Americas via Berringia.
That impact along with another massive iron impact in eastern Siberia around 24,300 BCE, probably mark the separation of the North American C and A mt DNA haplogroups, with A arriving after 24,300 BCE.
At least that's my current best guess. Your own results may differ.
Man and Impact in the Americas