What tectonic structures delineate the split between the Australian and Indian tectonic plates? Growing up, when looking in textbooks and other references about plate tectonics, there was always the Indo-Australian (or Indian-Australian) plate, such as shown below:



Image source: The Geologic Story of Yosemite National Park (1987) by N. King Huber

But, recently it has been recognised that there are separate Indian and Australian plates, such as shown below:



Image source: An Introduction to
Plate Tectonics

This would make sense given the differences in the collisional history of the two landmasses.

What tectonic structures delineate the split between the Australian and Indian tectonic plates?
The structures are not mature or well defined and deformation was thought to be diffused, i.e., distributed over a large area. Earthquakes such as the M8.6 2012 Indian Ocean earthquake (largest strike-slip earthquake on record) appear to accommodate some, if not all, of the deformation. 

Seafloor geodesy is relatively a new field. Most studies are limited to areas near the coast, e.g., US (Cascadia), Japan etc.