I believe the Himalayas were originally the seabeds of fresh volcanic material which filled the original hole. The older stable shield areas around them, collapsing back onto themselves to retain an equilibrium while the sphere beneath them increased ( isostatic rebound: 'pt' ), pushed them upwards, buckling them up into the huge fold mountain range.

This collapsing of the shields creates compressional areas within overall extensional plates, as can be seen along the fault lines of the Middle East and the Mediterranean.

Note: In conventional Plate-tectonics it is believed that these compressional fault lines exist between areas of the crust that have been moving towards each other, or at the points where huge ancient seafloors existed, now long subsumed back into the mantle by subduction. Traditionally, India is believed to have collided with Asia. But again, this is partly based on the reconstruction of the continents on a constant diameter.