continental drifts did happen. it has caused some really puzzling occurences in the animal world. we have seals in inland seas 1000 kms from their cousins. but i thinkif man had been around he would have been in animal skins during that time.
there was one geographical explanation to this historical question
during the later ice ages however the sea was diferent. though during some ice ages the tropics were not actually covered by ice , many of those lands that are today seperated by sea could be reached by sea bed which was dry. whn the ice started thawing much of this land was covered by water. it has happened before and will happen again say geologists. so srilanka south india and a huge part of bay of bengal and indian ocean would have formed a land mass which went down after the ice age was over.but a sangam on that gondwona land i dont think really happened.
In my opinion, there was no such thing as a "Tamil Sangam" in Madurai or in ancient Tamilnadu. Because, none of the anthologies were composed ( meaning collecting individual songs, collating, editing etc., ) by the Pandian kings. The anthologies were collected and collated by the Cera kings.
There is no record ( inscription or otherwise ) of these sangams excepting for those in commentories of poetry, which is a reminsience of the "Thiramila Sangam" founded by a Jain monk at Madurai during 3-6th century.
I will change my mind, if somebody proves otherwise.