It seems that Nantucket 1998 A.D. and Nantucket 1250 B.C. have changed places. However, the history (and prehistory) that generated Nantucket 1998 A.D. was one in which Nantucket 1250 B.C. was not replaced by Nantucket 1998 A.D. Thus, there are at least two timelines:
in timeline 1, Nantucket 1250 B.C. remains in place and, three thousand, two hundred and forty eight years later, has gradually metamorphosed into Nantucket 1998 A.D., which disappears;
in timeline 2, Nantucket 1998 A.D., having disappeared from timeline 1, appears in 1250 B.C., displacing Nantucket 1250 B.C. - three thousand, two hundred and forty eight years later, the temporally displaced Nantucket 1988 A.D. will have gradually metamorphosed into a Nantucket different from either Nantucket 1250 B.C. or Nantucket 1998 A.D.
So where does Nantucket 1250 B.C. go? To timeline 1? That seems to me like going in the wrong direction. The disappeared Nantucket 1998 A.D. could have left a vacuum. Nantucket 1250 B.C. could have gone to timeline 3, there to occupy the same space as whatever Nantucket exists in the equivalent of 1998 A.D. in that timeline, thus causing an explosion. I don't know, though.
To summarize theories of time travel:
-copied from here.1. The Logical Impossibility of Causality Violation in a Single Continuous Timeline
2. The Immutability of Past Events
3. A Single Discontinuous Timeline
4. Divergent Timelines
5. Disappearing Timelines
6. Parallel Timelines
7. Successive Timelines
8. A Mutable Timeline
9. Unwinding a Timeline
In theory 7, Successive Timelines, timeline 3 would be identical with timeline 2 except for any changes made by time travelers. Thus, the displaced Nantucket 1250 B.C. would occupy the same space as whatever Nantucket had existed in the equivalent of 1998 A.D. in timeline 2.