As the Indian continental block ploughed northwards and approached Asia starting earliest Cenozoic, when did the sea between the two continents disappear. When did the two continents become one, when did the first "land bridges" between the two continents develop.
Last week's post about early Eocene insect fossils preserved in amber from the Cambay shale in western India prompted these questions. The insects show similarities to Cenozoic insect fauna from Europe, Asia and Australiasia.
Although similarities between insect fauna imply faunal exchanges between continents they may not necessarily indicate that the two continents were connected by land bridges. Insects can survive on rafts for long periods and favorable ocean currents and island chains in the ocean between the two continents may have provided sufficient stepping stones for insect trans-continental migration.
Two types of evidence speak more directly to the presence of land bridges. One is the first appearance of distinctly Indian mammals on the Asia plate and vice versa. Mammals would have needed a solid connection between the two continents to migrate.
Another type of direct evidence is the presence of terrestrial sediments in the zone of collision. As the two continents collided, there would have been the development of complex topographic in the zone of collision. High thrust mountains but also regions which sagged and became basins. Of interest are basins which were filled with lakes or became courses for rivers.
The presence of lacustrine or fluvial sediments resting on rocks of the Indian plate but containing grains which show the provenance or source to be rocks of the Asian plate is strong evidence that the two continents had sutured into one and rivers originating on one plate were transporting sediments and depositing them on the other, i.e. land bridges had formed between the two.
There are at least two other indirect ways of inferring land bridges. One is the reduction in plate velocity. The moving Indian plate had set a Cretaceous record for speed but abruptly slowed in the earlier Cenozoic. That implies that continental crust was meeting resistance from another continental block and the two continents were suturing.
The other indirect way is to look at the metamorphism of continental crust. As the continents collided, continental crust was transported to depth and transformed into a suite of metamorphic minerals at high temperatures and pressures. The timing of these metamorphic events speak to the coming together of continents.... so do the presence of intrusive granites which formed by melting of the continental crust during the collision.
We know from all these above types of evidence that the collisions began in early Eocene and it was not uniform and simultaneous across the present length of the Himalayas. The western edge of the Indian plate met first and the continents sutured eastwards through the Cenozoic.