Early Tertiary subduction zones and hot spots

Abstract

Hot spots and subduction zones may control the position of the earth's rotation axis. Paleomagnetic reconstructions in the hot spot framework require a relative motion of about 10° between the hot spot reference frame and the earth's spin axis since the early Tertiary. If the viscosity of the mantle permits, a changing distribution of mass anomalies can control the position of the earth's rotation axis as it tracks the maximum principal axis of the perturbations. The present plate geometry should give the best indication of what features are associated with the controlling density anomalies. For the present, geoid anomalies indicate that subducting slabs are major mass contributions; removing these effects, a simpler ‘residual’ geoid remains with highs strongly correlated with hot spot locations. Although neither the hot spots alone nor subducting slabs alone have maximum principal axes near the present pole, adding the contributions of the two gives a combined axis within a degree of the pole, suggesting that these two effects control the location of the spin axis. To establish whether changes in plate geometry could account for the observed polar motion, the locations of subduction zones are reconstructed in the hot spot framework for the early Tertiary, a time of significantly different plate motions. For this reconstruction, the combined maximum principal axis of the subductionhot spot systems is not at the present pole but about 10° away in a location similar to the pole position required by paleomagnetic studies. This suggests that changes in subduction geometry may cause a shift in the position of the rotation pole and, furthermore, that inertia tensor effects may be the link between plate velocities and the spin axis.