Detailed nature of the 660 km region of the mantle from global receiver function data

We use a global data set of receiver functions to study the range of observations for the region at the base of the mantle transition zone, around the 660 km discontinuity. Avariety of signals has already been observed in regional studies; here we characterize the breadth of such observations on a global scale. While the 410 km discontinuity is usually found to be simple in shape, the 660 km discontinuity often shows broad or asymmetric peaks and sometimes multiple signals. Unlike previous studies on the subject, we show that complexity of structure is not limited to subduction zones. Reflectivity synthetics are used to show that the observed signals can be generated by structures with combinations of sharp discontinuities and velocity gradients around 660 km depth. Mineral physics suggests that at 410 km depth only the olivine phase transition is important. In contrast, at 660 km depth both the olivine and nonolivine transitions generate multiple discontinuities and gradients, which could be visible to Pds signals. Seismic observations of a simple 410 km signal and a complex 660 km signal might therefore be expected from the predictions of mineral physics. However, a mantle of uniform composition fails to explain the variety of signals seen in our data set, requiring laterally varying chemical composition at the base of the mantle transition zone.