More Googling reveals the following authors described as "upmarket" in newspapers/magazines/online sources: Ian McEwan; Patricia Cornwell; Charles Frazier; Zadie Smith; Umberto Eco; J.K. Rowling; Arthur Golden (Memoirs of a Geisha); Tracy Chevalier; Graham Swift; Martin Amis.
Quite a spectrum here. I think McEwan, Frazier and Golden are more literary than upmarket/mainstream, as is Updike. Amy Tan is definitely upmarket, as is John Irving. I don't see Rowling as "upmarket" at all. She's a genre writer who becomes extremely popular.
Rowling, I think, is described as "upmarket" in the UK because she uses long words. Although I appreciate what ORION is saying and think it's valid to some extent in the US, a lot of the UK sources describe male writers as well as female writers as "upmarket".
In my Googling, I came across a discussion of "upmarket historical fiction" on a librarians' newsletter:
Historical fiction was everywhere, but nowhere: It had become the genre that dared not speak its name. This has changed to some degree in the early twenty-first century. It has finally become fashionable again to talk about the historical novel in public. Publishers are actively promoting their books as “upmarket historical fiction”—novels, in other words, that one wouldn't be embarrassed to be caught reading on the subway.
Maybe the time is right for "upmarket" Westerns?
Interesting that the librarian omitted Sabatini, Costain, Shellabarger, and many others who wrote wonderful imnsho historicals and stimulated my love for historical reading and research.
So if I write a book with these "upmarket" traits and featuring a female MC, is it neither women's fiction or upmarket because I'm a guy?