N.B.: This blog post contains several spoilers for Star Trek (2009).
In the standard interpretation of the most recent Star Trek movie—the interpretation endorsed by its writers—the destruction of the USS Kelvin at the hands of the Romulan time-traveler Nero creates an alternate timeline within which the events of the movie take place.
While this is certainly one way to interpret the film, I think an alternate explanation is more fitting with what’s depicted on screen while also leaving the future even more open for the writers.
Obviously, time travel’s appeared in many episodes and movies of the 40+ year run of Star Trek. As you might expect given the volume of material and the timeframe involved, time travel in the Star Trek universe rarely works the same way from episode to episode. In some cases, time travel mechanics flatly contradict each other.
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I think the variety of time-travel that best fits the observed facts is Paul Di Filippo's, from his novel Fuzzy Dice. Under his variety of the many-worlds interpretation, time always moves forward, and "time travel" is simply moving sideways into a parallel universe which is similar to your originating universe was in the past.