ABSTRACT The geodynamic origin of melting anomalies found at the surface, often referred to as “hotspots,” is classically attributed to a mantle plume process. The distribution of hotspots along mid-ocean-ridge spreading systems around the globe, however, questions the universal validity of this concept. Here, the preferential association of hotspots with slow- to intermediate-spreading centers and not fast-spreading centers, an observation contrary to the expected effect of ridge suction forces on upwelling mantle plumes, is explained by a new mechanism for producing melting anomalies at shallow (<2.3 GPa) depths. By combining the effects of both chemical and thermal density changes during partial melting of the mantle (using appropriate latent heat and depth-dependent thermal expansivity parameters), we find that mantle residues experience an overall instantaneous increase in density when melting occurs at <2.3 GPa. This controversial finding is due to thermal contraction of material during melting, which outweighs the chemical buoyancy due to melting at shallow pressures (where thermal expansivities are highest). These dense mantle residues are likely to locally sink beneath spreading centers if ridge suction forces are modest, thus driving an increase in the flow of fertile mantle through the melting window and increasing magmatic production. This leads us to question our understanding of sub–spreading center dynamics, where we now suggest a portion of locally inverted mantle flow results in hotspots. Such inverted flow presents an alternative mechanism to upwelling hot mantle plumes for the generation of excess melt at near-ridge hotspots, i.e., dense downwelling of mantle residue locally increasing the flow of fertile mantle through the melting window. Near-ridge hotspots, therefore, may not require the elevated temperatures commonly invoked to account for excess melting. The proposed mechanism also satisfies counterintuitive observations of ridge-bound hotspots at slow- to intermediate-spreading centers, yet not at fast-spreading centers, where large dynamic ridge suction forces likely overwhelm density-driven downwelling. The lack of observations of such downwelling in numerical modeling studies to date reflects the generally high chemical depletion buoyancy and/or low thermal expansivity parameter values employed in simulations, which we find to be unrepresentative for melting at <2.3 GPa. We therefore invite future studies to review the values used for parameters affecting density changes during melting (e.g., depletion buoyancy, latent heat of melting, specific heat capacity, thermal expansivity), which quite literally have the potential to turn our understanding of mantle dynamics upside down.

ABSTRACT Under fast-moving oceanic plates, the asthenosphere seismic B″ region becomes isolated from the convecting mantle by plate drag and acts as an advecting layer, which can serve as a long-lived source for intraplate volcanism. Geochemical enrichment of B″ can occur via infiltration by melts generated from the breakdown of serpentinite at ~200 km depth in subducting slabs. Ocean-island chains arise when melts generated within metasomatized B″ by shear melting and localized convection are released along lithospheric fractures controlled by the stress field of the plate. Intersection of metasomatized B″ with ocean-ridge systems produces oceanic plateaus. A strong anisotropy anomaly (V SH /V SV >1) at depths of ~150 km in the Pacific asthenosphere marks a metasomatized B″ domain that originated in the western paleo-Pacific basin in the Carboniferous, and that is now associated with Hawaiian volcanism. Metasomatized B″ can be trapped beneath orogenic belts during continental aggregation and tapped by edge-driven convection upon rifting to produce the correlation between intraplate volcanism and the fabric of sutures in opening ocean basins such as the Atlantic Ocean basin.

Shallow-marine carbonate facies from volcanic-arc settings provide an important, but commonly overlooked, record of relative sea-level change, differential subsidence-uplift, paleoclimate trends, and other environmental changes. Carbonate strata are thin where volcanic eruptions are frequent and voluminous, unless shallow, bathy-metric highs persist for long periods of time and volcaniclastic sediment and erupted materials are trapped in adjacent depocenters. Carbonate platforms and reefs can attain significant thickness, however, if subsidence continues after volcanic activity ceases or the volcanic front migrates. The areal extent of shallow-marine carbonate sedimentation is likewise affected by differential tectonic subsidence, although carbonate platforms are most laterally extensive during transgressive to highstand conditions and when arc depocenters are filled with sediment. Tectonic controls on shallow-marine carbonate sedimentation in arc depocenters include (1) coseismic fault displacements and associated surface deformation; (2) long-wavelength tectonic subsidence related to dynamic mantle flow, flexure, lithospheric thinning, and thermal subsidence; and (3) large-scale plate deformation related to local conditions of subduction. Depositional controls on carbonate sedimentation in arc depocenters include (1) the frequency, volume, and style of volcanic eruptions; (2) accumulation rates for siliciclastic-volcaniclastic sediment; (3) the frequency, volume, and dispersal paths of erupted material; (4) (paleo)wind direction, which influences both carbonate facies development directly and indirectly by controlling the dispersal of volcanic ash and other pyroclastic sediment, which can bury carbonate-producing organisms; (5) the frequency and intensity of tsunami events; and (6) volcanically or seismically triggered mass-wasting events, which can erode or bury carbonate strata. Regarding platform morphologies in arc-related settings, (1) fringing reefs or barrier reef systems with lagoons may develop around volcanic edifices throughout the long-term evolution of volcanic arcs; (2) local reefs and mounds may build on intrabasinal, fault-bounded highs within underfilled forearc, intra-arc, and backarc basins; (3) isolated platforms with variable platform margin-to-basin transitions are common in “underfilled” and tectonically active depocenters; and (4) broad ramps and rimmed carbonate shelves are typically found in tectonically mature and sediment-filled depocenters.

The lack of high heatflow values at hotspots has been interpreted as showing that the mechanism forming the associated swells is not reheating of the lower half of oceanic lithosphere. Alternatively, it has recently been proposed that the hotspot surface heatflow signature is obscured by fluid circulation. We re-examine closely spaced heatflow measurements near the Hawaii, Réunion, Crozet, Cape Verde, and Bermuda hotspots. We conclude that hydrothermal circulation may redistribute heat near the swell axes, but it does not mask a large and spatially broad heatflow anomaly. There may, however, be heatflow perturbations associated with the cooling of igneous intrusions emplaced during hotspot formation. Although such effects may raise heatflow at a few sites, the small heatflow anomalies indicate that the mechanisms producing hotspots do not significantly perturb the thermal state of the lithosphere.

A conceptual shift is overdue in geodynamics. Popular models that present plate tectonics as being driven by bottom-heated whole-mantle convection, with or without plumes, are based on obsolete assumptions, are contradicted by much evidence, and fail to account for observed plate interactions. Subduction-hinge rollback is the key to viable mechanisms. The Pacific spreads rapidly yet shrinks by rollback, whereas the subduction-free Atlantic widens by slow mid-ocean spreading. These and other first-order features of global tectonics cannot be explained by conventional models. The behavior of arcs and the common presence of forearc basins on the uncrumpled thin leading edges of advancing arcs and continents are among features indicating that subduction provides the primary drive for both upper and lower plates. Subduction rights the density inversion that is produced when asthenosphere is cooled to oceanic lithosphere: plate tectonics is driven by top-down cooling but is enabled by heat. Slabs sink more steeply than they dip and, if old and dense, are plated down on the 660 km discontinuity. Broadside-sinking slabs push all sublithosphere oceanic upper mantle inward, forcing rapid spreading in shrinking oceans. Down-plated slabs are overpassed by advancing arcs and plates, and thus transferred to enlarging oceans and backarc basins. Plate motions make sense in terms of this subduction drive in a global framework in which the ridge-bounded Antarctic plate is fixed: most subduction hinges roll back in that frame, plates move toward subduction zones, and ridges migrate to tap fresh asthenosphere. This self-organizing kinematic system is driven from the top. Slabs probably do not subduct into, nor do plumes rise to the upper mantle from, the sluggish deep mantle.

The magmatic and tectonic processes of the pre–2.5 Ga hot, young Earth differed profoundly from those of the modern planet. The ancient rocks differ strikingly in individual and collective composition, occurrence, association, and structure from modern rocks. Widespread forcing of Archean geology into plate-tectonic frameworks reflects unwarranted faith in uniformitarianism and in inappropriate chemical discriminants, and disregard for the lack of features that characterize plate interactions. Archean crust records extreme and prolonged internal mobility and was far too weak and mobile to behave as rigid plates, required, by definition, for plate tectonics. None of the geologic indicators of subduction, arc magmatism, and continental sundering, separation, and convergence have been documented. No Archean oceanic crust or mantle has been recognized, and the only known basement to supra-crustal rocks, including the thick basalts, high-Mg basalts, and ultramafic lavas that typify greenstone successions, consists of tonalite-trondhjemite-granodiorite (TTG) migmatites and gneisses. A thick global melabasaltic protocrust likely formed by ca. 4.45 Ga, and from it TTG suites were extracted by partial melting over the next 2 b.y. Delamination of the increasingly dense restitic protocrust enabled rise of lighter and hotter depleted mantle and hence more melting. The oldest known crustal materials are zircons, which scatter in age back to 4.4 Ga and are recycled in migmatites whose final crystallization was after 3.8 Ga, and in ancient sediments. Earth may have had a dense greenhouse atmosphere, not a hydrosphere, before 3.6 Ga, for the oldest proved supracrustal rocks are of that age, and older felsic crust may have been too hot to permit rise of dense melts. Rigid plates of lithosphere did not stabilize until a billion years after that and then were mostly small and local. Dense lavas erupted atop mobile felsic crust after 3.6 Ga produced a density inversion that was partly righted by sinking of the volcanic rocks and rising of the subjacent TTG. In some places, the early dense rocks retained cohesion and sank as synclinal keels between rising domiform diapiric batholiths. In others, the early dense rocks sank deep into mobile TTG crust, and only later in Archean time was the felsic substrate strong enough to enable dome-and-keel style. The TTG substrate rose slowly, with variable amounts of partial melting to generate more-fractionated melts and with additions of new TTG from the underlying protocrust, for hundreds of millions of years. The mantle beneath preserved cratons generated ultramafic melts that required a temperature ∼300°C hotter than modern asthenosphere ca. 3.5 Ga. Severe and prolonged lateral deformation was superimposed on large parts of some cratons during the era of volcanism and diapirism, obscuring dome-and-keel geology over broad tracts. Lower crust was at high temperature for prolonged periods and flowed pervasively, coupled discontinuously to the upper crust to produce lateral deformation therein. Rifting, separation, rotation, and collision of internally more rigid lithosphere fragments began ca. 2.1 Ga, but may have been dominantly intracontinental deformation, quite distinct from modern plate tectonics. The products of this regime differ greatly from those of Phanerozoic plate tectonics, and reflect a transitional era of erratically stiffening lithosphere. An early-depleted upper mantle has been progressively re-enriched, by delamination and subduction of crustal materials, while new “juvenile” crust derived from it has become progressively more depleted, during Pro-terozoic and Phanerozoic time.