Cenozoic Plate-Driving Forces

Figure 1. A comparison of observed Cenozoic plate motions (left) with the plate motions predicted by the plate-driving model (see how mantle slabs drive plate motions). Here arrow lengths and colors represent the plate velocity relative to the average plate speed for each time period. Note that the acceleration of the Pacific Plate (the large plate in the center that changes color from yellow/green to red/orange) is well predicted, as are the rapid speeds of the smaller Indian and Farallon plates early in the Cenozoic.

Figure 2. A comparison of the ratio of subducting to non-subducting plate speeds for slab suction only (blue) and upper mantle slab pull and lower mantle slab suction acting together (red). The combined model provides a good fit to the observed acceleration of subducting plate speeds (black).

Figure 3. Subduction parameters as a function of time through the Cenozoic. The average age of subducting material (magenta) has increased during the Cenozoic, which increased the total mass of slabs (green). This led to a net increase in the average slab pull force on plates, which explains the observed increase in subducting plates velocities.

Figure 4. The magnitude of the slab pull force on plates, expressed as a fraction of the total forces on plates. Because the weight of slabs has increased during the Cenozoic, the slab pull force has increased from about 40% of the forces on slabs early in the Cenozoic, to about 60% today.

Temporal Evolution of Plate-Driving Forces During the Cenozoic

C.P. Conrad and C. Lithgow-Bertelloni, "The temporal evolution of plate driving forces: Importance of "slab suction" versus "slab pull" during the Cenozoic," Journal of Geophysical Research, 109, B10407, doi:10.1029/2004JB002991, 2004. [abstract] [online version] [reprint]

Using our new model for how mantle slabs drive plate motions, we predicted plate motions for different time periods throughout the Cenozoic (Fig. 1, right column). In general, the magnitudes and directions of plate motions are well predicted (Fig. 1) by the combined model of slab suction from lower mantle slabs and slab pull from upper mantle slabs. In particular, we reproduce the Cenozoic trend of Pacific plate acceleration relative to the other plates.

Early in the Cenozoic, the Pacific moved only slightly faster than the other plates, as shown by its yellow and green arrows in Figs. 1c, 1d, and 1e. During the second half of the Cenozoic, the speed of the Pacific plate increased relative to the other plates, as shown by its orange and red arrows (Fig. 1a and 1b). In fact, the average speed of subducting plates has increased from about twice that of non-subducting plates early in the Cenozoic to about 4 times that of non-subducting plates recently (black line, Fig. 2). The combined model of slab pull from upper mantle slabs and slab suction from lower mantle slabs (red line, Fig. 2) predicts this trend nicely. By contrast, if upper mantle slabs are detached from the surface plates and operate in the slab suction mode, this speedup of subducting plates is not predicted (blue line, Fig. 2).

The Pacific plate accelerated during the Cenozoic because the average age of subducting oceanic lithosphere increased during this time period (magenta line, Fig. 3). This increasing age led to thicker and more massive slabs in the upper mantle (green line, Fig. 3), which generated a larger slab pull force (red line, Fig. 3). Because the slab pull force efficiently drives plates toward subduction zones, its increased magnitude led to the Cenozoic acceleration of the Pacific plate. During the Cenozoic, the slab pull force increased its contributed to the total forces on plates from about 40% early in the Cenozoic to about 60% today (Fig. 4). This increase in the importance of the slab pull force explains, for the first time, a temporal change in observed plate motions based on the force that drive plate motions.