The excess gravitational potential energy of a collisional orogen that may be supported by focusing distributed ridge torques in a plate depends on the ratio l& of the effective lengths of ridge systems and collisional fronts. In the Indian-Australian plate, the net torque due to the asymmetric distribution of mid-ocean ridges and young oceanic lithosphere along the southern boundary is ∼8.5 × 1025 N . m and is capable of supporting an excess potential energy of the Himalayan collision zone of up to 7.5 × 1012 J/m2. The concept that collision 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 progressive collision from ∼55 to 45 Ma, during amalgamation of the Indian and Australian plates at ∼45 Ma, and during the development of the New Guinea orogen through the late Oligiocene and Miocene. Such changes in l may help explain the termination of spreading between India and Australia following the onset of the Himalayan collision and the early Miocene normal faulting in the high Himalaya.

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