RIDGE TORQUES AND CONTINENTAL COLLISION IN THE INDIAN-AUSTRALIAN PLATE

The excess gravitational potential energy of a collisional orogen that may be supported by focusing distributed ridge torques in a plate dep ends on the ratio l' of the effective lengths of ridge systems and col lisional fronts. In the Indian-Australian plate, the net torque due to the asymmetric distribution of mid-ocean ridges and young oceanic lit hosphere along the southern boundary is similar to 8.5 x 10(25) N . m and is capable of supporting an excess potential energy of the Himalay an collision zone of up to 7.5 x 10(12) J/m(2). The concept that colli sion in the Indian-Australian plate may be sustained by ridge torques suggests that collisional driving stresses may be sensitive to l'. In the Indian-Australian plate, l' changed significantly during progressi ve collision from similar to 55 to 45 Ma, during amalgamation of the I ndian and Australian plates at similar to 45 Ma, and during the develo pment of the New Guinea orogen through the late Oligiocene and Miocene . Such changes in l' may help explain the termination of spreading bet ween India and Australia following the onset of the Himalayan collisio n and the early Miocene normal faulting in the high Himalaya.