Hmmm, it depends on your definition of 'rock', Alec. There were plenty of female artists who topped the charts before 1973, but I admit most of them tended to be ballads rather than rock songs. Helen Shapiro's 'Walking Back To Happiness' was fairly up-tempo though, as is These Boots Are Made For Walkin' (Nancy Sinatra), both 1960s no. 1 hits. There were plenty of no. 1 hits from Cilla, Dusty, et al in the 60s, too. And further back in the 50s we had Connie Francis, Kay Starr, Rosemary Clooney, etc. topping the charts. These women may not have topped the charts with 'rock' songs, but many of them recorded some fairly up-tempo stuff as well as the ballads, e.g. Lipstick on Your Collar, Stupid Cupid by Connie F.

It's what we said earlier, the female singers conformed to that style until, probably the late 1960s when the likes of Janis Joplin (and of course the soul singers) paved the way for females to branch out into more 'rocky' material.