Plate tectonics. look it up.
And why ist thou talking of the Argentine? From the article
The newly reported bone was found by co-author Tom Rich of Museum Victoria in Melbourne, Australia. It was uncovered in Dinosaur Cove in southeast Australia, where occasional fossils were being found.
Probably because of that song lyric, “do not cry for me southeast Australia”.
He mistakenly thought a T-Rex had previously been found in the southern hemisphere (Argentina), when in fact it was actually a dinosaur described only as "T-Rex like" that was found there. Of course many "T-Rex like" dinos have previously been found in the southern hemisphere, including Carcharodontosaurs (Africa), Giganotosaurus (South America) and Allosaurs (just about everywhere I think).
The title of thread is "First tyrannosaur fossil from Southern Hemisphere"