Preliminary results from nonlinear global travel-time tomography

Recent global tomography models, constructed through linearized inversion, by van der Hilst et al. (Nature, 1997) and Zhou (JGR, 1996) show smaller scale mantle structures than had been imaged on a global scale before.
In a similar, but still linear approach we have been able to take their improvement a step further, mapping upper mantle structure with detail (60-100 km) directly comparable with that obtained in high resolution regional studies. This was achieved through the implementation of an accurate global data set and a model parameterization with cells of variable sizes.
To further improve global travel-time tomography we extend the method to nonlinear inversion. Up to now the nonlinear global tomography problem has always been linearized about the ray paths in the 1-D reference Earth model used. The bending of rays due to lateral heterogeneity has not been taken into account, but may be very important when imaging small-scale structure.
We will present results from the linear inversion (focussing on the behaviour of slabs in the transition zone, deep subduction, remnants of past subduction in the lower mantle and the deep roots of hotspots) and first results from nonlinear inversion.