Demographic model for admixed African Americans I am fairly new to genomics and population genetics. And I am reading this article 'Demographic history and rare allele sharing among human populations':

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3142009/pdf/pnas.201019276.pdf

In this article they propose a demographic model in Figure 4 on page 11986. However, I am confused how does African Americans fit in this demographic model that are known to have both African and European ancestry. And if I am to consider African American population than what is the split time between the two populations, time they merge into admixed population and the admixed proportion. 
My apologies if it's a really basic question but, any insights would be appreciated.
Just to further my answer, the problem with molecular dating within a species (I don't do humans [excepting immunology], but it's the same theory), is it requires an assumption of clonality (bifurcating trees). Mendelian species recombine towards panmixia, which will make any attempt to date via de novo point mutation, appear far younger than they really are, or for panmixia, incalculable (it breaks the central assumption). Dating through admixture requires a lot of extra data and/or assumptions for example the generation time,  putative admixture per generation, and shifts in population dynamics. Maybe you can calculate some of those requirements but  alot will remain assumptions. 

I have not read the paper you suggested, because I'm not involved in human genetics [outside immunology], but it is possible the authors may (and I stress may) have simplified an assumption that could be difficult to estimate. It is not clear what the objective of the paper was.

Quantitative analysis is extremely important in modern population genetics/ evolutionary theory because historically it was overlooked and resulted in lots of hypothese which were difficult to test.