Lower Mantle Dynamics Perceived With 50 Years of Hindsight From Plate Tectonics

Abstract

Continually improving resolution of lower mantle structure and understanding of mantle dynamics suggest that, as in the plate-tectonics era, the upper and lower mantle may undergo little exchange. Although the phase transition at~660 km forms the sharpest seismologically mapped boundary within the mantle, another at 1,000 (±100) km might be more important for geodynamics. Contrasts in density and viscosity may limit penetration of upper and lower mantle through this boundary. Strong evidence, if still not definitive, calls for chemical heterogeneity of the lower mantle that manifests itself in a few energetic plumes rising from near the core mantle boundary and slowly evolving, larger piles or blobs of chemically distinct material. Those blobs or piles may deform little on billion-year timescales. Convection in the lower mantle might dictate some aspects of flow in the upper mantle, such as the positions of plumes and hot spots and the breakup of supercontinents, but its contribution to the speeds and directions that plates move might be negligible. The improved understanding of mantle convection has not overturned the simple, 50-year-old image that plates "drive themselves," with forces per unit length pushing them apart at spreading centers, excess mass in downgoing slabs pulling them down, and viscous drag on their bases (and tops at subduction zones) retarding movement. Remaining challenges include determining how convection in the lower mantle affects geologic history and using that history to constrain lower mantle dynamics.Plain Language Summary Fifty years ago, plate tectonics united many aspects of the surface geology of the Earth, but little connection to the lower mantle was seen. Today, most view plate tectonics as the relative movements of cold, top, stiff boundary layers of a convecting system that reaches to the core-mantle boundary and with aspects of the deep structure not foreseen decades ago. Large provinces in the deepest~1,000 km, in which P and S wave speeds are relatively low, not only seem to be chemically different from the neighboring mantle and from that at shallower depths, but their distribution also correlates with some aspects of the overlying surface geology, including the positions of major plumes rising from deep in the mantle and the positions of continents 100 to 200 Ma. These correlations imply a geodynamic connection between the lower mantle and the crust. Scaling laws derived from experiments in geophysical fluid mechanics suggest that the chemically distinct provinces may be relics from the earliest formation of the earth, but if not, they nevertheless have evolved slowly on the timescales of geologic eras. A concurrent emerging view of the lower mantle, however, also places increased emphasis on a boundary at~1,000 (±100) km depth, and this boundary might define a barrier to cold sinking slabs of lithosphere. A few isolated plumes of hot material that are also chemically different from most of the mantle penetrate this interface at 1,000 km, but it seems possible that this...

Coupled Evolution of Plate Tectonics and Basal Mantle Structure

“…Geoid anomalies are not computed here, because forward mantle flow models are known not reproduce the observed geoid well (e.g., Flament, 2019). Some slabs are tomographically imaged as stagnating, such as the Pacific slab under east Asia (C. Li et al., 2008), which is consistent with our model and previous similar models (Seton et al., 2015), although stagnating slabs are sometimes used as an argument in favor of layered convection (e.g., Molnar, 2019). Nevertheless, intraplate ocean island basalts often have a primitive composition (e.g., Mukhopadhyay & Parai, 2019), suggesting the existence of chemically different reservoirs in the mantle, which is discrepant with the idea of efficient convective mixing across the whole mantle.…”

Vestiges of the Pre-Caledonian Passive Margin of Baltica in the Scandinavian Caledonides: Overview, Revisions and Control on the Structure of the Mountain Belt

“…Examples from the present Atlantic Ocean include the Paraná-Etendeka LIP of the South Atlantic, the Central Atlantic Magmatic Provinces (CAMP) and the North Atlantic Igneous Province (NALIP). Most geodynamicists, supported by petrological evidence, agree that the basalt-dominated magmatism in LIPs is triggered by the arrival of mantle plumes producing excess base lithosphere temperatures of up to 200 • C and causing large-scale melting of the upper mantle [29][30][31][32][33]. These observations can be used to develop plate tectonic reconstructions with respect to the mantle throughout the Phanerozoic [34,35] and now into the Neoproterozoic [29].…”