Continental collision slowing due to viscous mantle.pdfVIP

LETTER doi:10.1038/nature10848 Continental collision slowing due to viscous mantle lithosphere rather than topography Marin Kristen Clark1 Because the inertia of tectonic plates is negligible, plate velocities result from the balance of forces acting at plate margins and along their base1. Observations of past plate motion derived frommarine magnetic anomalies provide evidence of how continental deforma- tionmay contribute to plate driving forces2–8. A decrease in conver- gence rate at the inception of continental collision is expected because of the greater buoyancy of continental than oceanic lithosphere2,3, but post-collisional rates are less well understood. Slowing of convergence has generally been attributed to the development of high topography that further resists convergent motion7–10; however, the role of deforming continental mantle lithosphere on plate motions has not previously been considered. Here I show that the rate of India’s penetration into Eurasia has decreased exponentially since their collision. The exponential decrease in convergence rate suggests that contractional strain across Tibet has been constant throughout the collision at a rate of 7.033 10216 s21, which matches the current rate. A constant bulk strain rate of the orogen suggests that convergent motion is resisted by constant average stress (constant force) applied to a relatively uniform layer or interface at depth. This finding follows new evidence that the mantle lithosphere beneath Tibet is intact11, which supports the interpretation that the long-term strain history of Tibet reflects deformation of the mantle lithosphere. Under con- ditions of constant stress and strength, the deforming continental lithosphere creates a type of viscous resistance that affects plate motion irrespective of how topography evolved. Marked slowing of the convergence rate of India with Eurasia deter- mined from plate circuit motions2,3 agrees broadly with the estimate of continental collision age from