Plate tectonic motions are commonly considered to be driven by slab pull at subduction zones and ridge push at mid-ocean ridges, with motion punctuated by plumes of hot material rising from the lower mantle 1,2 . Within this model, the geometry and location of mid-ocean ridges are considered to be independent of lower-mantle dynamics, such as deeply sourced plumes that produce voluminous lava eruptions-termed large igneous provinces 2 . Here we use a global plate model 3 to reconstruct the locations of large igneous provinces relative to plumes and mid-ocean ridges at the time they formed. We find that large igneous provinces repeatedly formed at specific locations where mid-ocean ridges and plumes interact. We calculate how much mantle material was converted to oceanic lithosphere at the mid-ocean ridges and find that slowly migrating ridge systems that have been stabilized by upwelling plumes have extracted large volumes of material from the same part of the upper mantle over periods up to 180 million years. The geochemical signatures of mid-ocean ridge basalts and seismic tomographic data show that upper-mantle temperatures are elevated at significant distances from ridge-plume interactions, indicating a far-field, indirect influence of plume-ridge interactions on the upper-mantle structure. We conclude that strong feedbacks exist between the dynamics of slowly migrating ridges and deeply sourced plumes.It has long been recognized that mid-ocean ridges (MORs) migrate 4 relative to the deep underlying mantle 4,5 . Present-day MOR migration rates are known to be intimately related to seafloor morphology 6-8 and the physical state of the upper mantle 9,10 , with present-day migration rates correlating with asymmetric seafloor spreading 6 , ridge-crest lava generation 7 , melting asymmetry 9 , depth and geochemical discontinuity 10 , and ridge morphology 8 , but the influence of ancient ridge migration rates remains unknown.Plumes have long been associated with mid-ocean ridges 11 and ridge-plume interaction is known to cause deviation from expected MOR migration rates (assuming symmetric spreading without affecting overall surface motions), with slower MOR migration rates recorded at spreading systems experiencing ridge-plume interactions 12 . Reconstructing relative and absolute plate motions 3 for the past 140 Myr reveals that MOR systems have experienced a variety of contrasting behaviours-combinations of stationary to rapidly migrating MORs coupled with a wide range of seafloor spreading rates. Slowly migrating, fast-spreading MORs extract significant volumes of melt from the same source region, whereas rapidly migrating MORs extract relatively smaller volumes of melt across a broader source region.The effects of long-term variations in MOR migration and the resulting non-uniform sampling of the upper mantle have not previously been studied, although there is growing evidence supporting close linkages between slowly migrating MORs and plumes at the present-day, including MORB geochemistry and upper-mantle sei...

S U M M A R YThe lithospheric contribution to the geomagnetic field arises from magnetized rocks in a thin shell at the Earth's surface. The lithospheric field can be calculated as an integral of the distribution of magnetization using standard results from potential theory. Inversion of the magnetic field for the magnetization suffers from a fundamental non-uniqueness: many important distributions of magnetization yield no potential magnetic field outside the shell. We represent the vertically integrated magnetization (VIM) in terms of vector spherical harmonics that are new to geomagnetism. These vector functions are orthogonal and complete over the sphere: one subset (I) represents the part of the magnetization that produces a potential field outside the shell, the observed field; another subset (E) produces a potential field exclusively inside the shell; and a third, toroidal, subset (T ) produces no potential field at all. E and T together span the null space of the inverse problem for magnetization with perfect, complete data. We apply the theory to a recent global model of VIM, give an efficient algorithm for finding the lithospheric field, and show that our model of magnetization is dominated by E, the part producing a potential field inside the shell. This is largely because, to a first approximation, the model was formed by magnetizing a shell with a substantial uniform component by an potential field originating inside the shell. The null space for inversion of lithospheric magnetic anomaly data for VIM is therefore huge. It can be reduced if the magnetization is assumed to be induced by a known inducing field, but the null space for susceptibility is not so easily recovered.

No abstract

No abstract