Here's a very interesting document from NASA describing the
Transoceanic Abort Landing (TAL) sites for the Shuttle. It was a big effort preparing all of the emergency landing sites for each Shuttle launch.
A TAL could be declared at T+2:30. But neither a Return To Launch Site (RTLS) or TAL could begin until the SRB's were jettisoned.
This chart shows the abort conditions for loss of one or more SSMEs.
A RTLS abort procedure was interesting since it could require a hair-raising maneuver called a Powered Pitch Around (PPA). A PPA involved pitching the Orbiter 180deg at an altitude of >400kft so that the SSME thrust could be used to propel it back to the launch site for landing.
Here is an image of the Shuttle's abort mode selector switch:
