The consequences of collision

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India's collision was associated with dramatic deceleration from about 15 cm/yr to ~5 cm/yr. The sorts of speeds that operated prior to collision are consistent with plates which have significant lengths of ridges and slabs working together to produce large driving torques, without the resistance of collision. In the modern day these plates include the Nazca and Pacific plates. Such plates require some sort of "governor" to balance the driving torques, which presumably is achieved as a velocity dependent resistance from the passage across the deeper mantle.

Palaemagnetic coinstraints on posiion and velocity of India from Mesozoic times after Patriat and Acache (1984) (Needs Macromedia Flash plugin to view)

It seems to me that the dramatic deceleration during collision implies the likely detachment of the Tethyan slab beneath southern Asia from the trailing Indian plate. If so, we are faced with the question of what sustained the collision?

The deceleration of the Indian plate effectively ceased the divergence between the Indian and Australian plates, locking the strange spreading centres in the vicinity of the Ninety East Ridge and resulting in the amalgamated Indo-Australian plate. From the Australian perspective this amalgamation resulted in a change from a more-or-less symmetric 'ridge-bound' geometry to a less symmetric geometry. In the modern earth the two largely ridge-bound plates (Antarctica and Africa) are much slower moving (and charcaterised by extensional intraplate stress regimes) than the large asymmetric plates (which tend to be characterized by compressional intraplate stress regimes. It does not seem surprising that this transition was accompanied by a dramatic increase in the southern ocean spreading rates. I expect it was characterized by change in the intraplate stress regime.

NNE-trending Eocene basaltic dyke approximately perpendicular to the present day SHmax orientation, Flinders, Victoria, implying a fundamental change in the stress regime in SE Australia during the Tertiary.

It is clear from the modeling of the intraplate stress regime that significant collision-related topography can be supported in an asymmetric plate like the Indo-Australian plate where the length of collision is much smaller than the length of the ridge.

Predicted intraplate stress field modelled as a balance of ridge-torques and continental collision (Coblentz et al, 198)

This raises the possibility that the ongoing collision required a union of the Indo-Australian plate as illustrated below in a schematic evolution of the length of ridge and collision in the Indian and Indo-Australian Plates.

Schematic illustration of the dynamics of the Indian plate in terms of l_MOR : length of ridge, and l_c : length of collision (Sandiford et a, 1995), Contours reflect the excess potential energy of the collisional orogen sustainable by ridges torques.

and further imputes significant long-wavelength (i.e. plate-scale) correlations should exist in the tectonic response of plates.

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