ABSTRACT The Pliocene–Quaternary igneous record of the Tyrrhenian Sea area features a surprisingly large range of compositions from subalkaline to ultra-alkaline and from ultrabasic to acid. These rocks, emplaced within the basin and along its margins, are characterized by strongly SiO 2 -undersaturated and CaO-rich to strongly SiO 2 -oversaturated and peraluminous compositions, with sodic to ultrapotassic alkaline and tholeiitic to calc-alkaline and high-K calc-alkaline affinities. We focused on the different models proposed to explain the famous Roman Comagmatic Region, part of the Quaternary volcanism that spreads along the eastern side of the Tyrrhenian area, in the stretched part of the Apennines thrust-and-fold belt. We reviewed data and hypotheses proposed in the literature that infer active to fossil subduction up to models that exclude subduction entirely. Many field geology observations sustain the interpretation that the evolution of the Tyrrhenian-Apennine system was related to subduction of the western margin of Adria continental lithosphere after minor recycling of oceanic lithosphere. However, the lateral extent of the subducting slab in the last millions of years, when magmatism flared up, remains debatable. The igneous activity that developed in the last millions of years along the Tyrrhenian margin is here explained as originating from a subduction-modified mantle, regardless of whether the large-scale subduction system is still active.

It is widely assumed that the boundary layer above the core is the source of intraplate volcanoes such as Hawaii, Samoa, and Yellowstone, and that the sub-plate boundary layer at the top of the mantle is thin and entirely subsolidus. In fact, this upper layer is thicker and has higher expansivity, buoyancy, and insulating power than the lower one, and may have higher potential temperatures. The observed seismic structure of the low-velocity zone (LVZ) including attenuation, anisotropy, sharp boundaries, and a reduction of both compressional and shear moduli can be taken as strong evidence for the ubiquitous presence of melt in the upper mantle. If the LVZ contains as little as 1%–2% melt, then it is the most plausible and accessible source for midplate magmas; deeply rooted active upwellings are unnecessary. The upper boundary layer is also the most plausible source of ancient isotopic signatures of these magmas and their inclusions.

Near-vertical multiple ScS (S waves reflected at the core-mantle boundary) phases are among the cleanest seismic phases traveling over several thousand kilometers in the Earth's mantle and are useful for constraining the average attenuation and shear wave speed in the whole mantle. However, the available multiple ScS pairs are limited. We took advantage of the recent dramatic increase in the number of global broadband stations and made a thorough computer-assisted search for high-quality data of multiple ScS pairs. We could find 220 station-event pairs which provided us with robust local estimates of average Q (quality factor) and two-way shear wave travel times. With the assumption that geometric focusing caused by lateral velocity heterogeneity does not seriously affect the amplitude measurements, the Q values exhibit strong short-range lateral variations, with very high and very low Q regions adjacent to each other. The mantle beneath seismic station KIP (Hawaii) has normal Q and shear wave speed, which supports the result of earlier studies. The mantle beneath station AFI (Samoa Islands) has a very high Q , possibly larger than 1400, and the slowest shear wave speed. The stations on the upper plate of the Tonga and Japan subduction zones yield average to low Q values. In contrast, the stations on the trenchward side of the upper plate of some subduction zones, e.g., station LVC (Chile) and station PET (Kamchatka, Russia), indicate high Q values, larger than 1000. We found no obvious correlation between Q and shear wave speed, which suggests that different factors like temperature, composition, anisotropy, etc., are controlling these properties in the mantle of different tectonic environments.

In his pioneering 1961 paper on seismic anisotropy in a layered earth, Don L. Anderson (hereafter referred to as DLA) introduced a parameter often referred to in global seismology as η without providing any reasoning. This note hopes to clarify the significance of η in the context of the dependence of body wave velocities in a transversely isotropic system on the angle of incidence, and also its relation with the other well-known anisotropic parameters introduced by Leon Thomsen in 1986.

It has been known for over 50 years that seismic anisotropy must be included in a realistic analysis of most seismic data. The evidence for this consists of the observed dependency in many contexts (reviewed briefly here) of seismic velocity upon angle of propagation and upon angle of S-wave polarization. Despite this well-established understanding, many current investigations continue to employ less realistic isotropic assumptions. One result is the appearance of artifacts which can be interpreted in terms of details of Earth structure rather than of the restrictive assumptions in the analysis. The reason for this neglect of anisotropy is presumably the greater algebraic complexity and the larger number of free parameters of anisotropic seismics. However, the seismic anisotropy in the Earth is usually weak, and the equations for weak anisotropy are only marginally more complex than for isotropy. Further, the additional parameters are commonly required to describe the data. Moreover, the parameters of weak anisotropy defined below (combinations of the anisotropic elastic moduli) are less subject to compounding of uncertainty and to spatial resolution issues than are the individual anisotropic moduli themselves. Hence inversions should seek to fit data with these parameters, rather than with those individual moduli. We briefly review the theory for weak anisotropy and present new equations for the weakly anisotropic velocities of surface waves. The analysis offers new insights on some well-known results found by previous investigations, for example the “Rayleigh wave–Love wave inconsistency”, including the facts that Raleigh wave velocities depend not only on the horizontal SV velocity but also on the anisotropy, and Love wave velocities depend not only on the horizontal SH velocity but also on the anisotropy.

The role of decoupling in the low-velocity zone is crucial for understanding plate tectonics and mantle convection. Mantle convection models fail to integrate plate kinematics and thermodynamics of the mantle. In a first gross estimate, we computed at >300 km 3 /yr the volume of the plates lost along subduction zones. Mass balance predicts that slabs are compensated by broad passive upwellings beneath oceans and continents, passively emerging at oceanic ridges and backarc basins. These may correspond to the broad low-wavespeed regions found in the upper mantle by tomography. However, west-directed slabs enter the mantle more than three times faster (~232 km 3 /yr) than in the opposite east- or northeast-directed subduction zones (~74 km 3 /yr). This difference is consistent with the westward drift of the outer shell relative to the underlying mantle, which accounts for the steep dip of west-directed slabs, the asymmetry between flanks of oceanic ridges, and the directions of ridge migration. The larger recycling volumes along west-directed subduction zones imply asymmetric cooling of the underlying mantle and that there is an “easterly” directed component of the upwelling replacement mantle. In this model, mantle convection is tuned by polarized decoupling of the advecting and shearing upper boundary layer. Return mantle flow can result from passive volume balance rather than only by thermal buoyancy-driven upwelling.

We present here the first geochemical data from adakitic rocks from an extensional system—the U.S. East Coast rifted margin. Adakitic magmas are high-K melts that have been petrogenetically interpreted to be partial melts of subducting slab and/or lower crustal lithologies in delamination events. The adakitic rocks presented here are from a small volcanic region in the Valley and Ridge province in Virginia and were probably emplaced around the time of continent rupture and Central Atlantic magmatic province activity. They are bimodal in character (high Si and low Si) and have the typical high- and low-Si adakitic geochemical characteristics such as high K 2 O (up to 9.88 wt%) abundances, steep rare earth element patterns, and significantly high Sr (2473 ppm) and relatively low Rb (35 ppm) contents for high-Si adakitic rocks. The petrogenetic relation of these melts to partial melting of metagabbroic rocks (high-Si adakites) and interaction of these melts with ambient peridotite (low-Si adakites) suggests that the geodynamic process for the formation of the studied Jurassic central Virginia igneous rock succession is delamination of mantle lithosphere and lower crust below the volcanic rifted margin. We present with geodynamic models that negatively buoyant mantle lithosphere instabilities developed below this passive margin during continent rupture. After foundering, warm asthenosphere welled up and heated the lower crust of the East Coast margin. This lithosphere was interspersed in our study area with fragmented hydrated metamorphic mafic to ultramafic lithologies. In situ and/or dripping melting of such meta-igneous rocks reproduces the observed geochemistry of the studied high-Si adakitic rocks. Further recycling processes within the convecting mantle of delaminated floating fertile meta-igneous rock packages could be responsible for Atlantic melting anomalies such as the Azores or Bermuda.

Calculations of mantle convection generally use constant rates of internal heating and time-invariant core-mantle boundary temperature. In contrast, parameterized convection calculations, sometimes called thermal history calculations, allow these properties to vary with time but only provide a single average temperature for the entire mantle. Here I consider three-dimensional spherical convection calculations that run for the age of the Earth with heat-producing elements that decrease with time, a cooling core boundary condition, and a mobile lid. The calculations begin with a moderately hot initial temperature, consistent with a relatively short accretion time for the formation of the planet. I find that the choice of a mobile or stagnant lid has the most significant effect on the average temperature as a function of time in the models. However, the choice of mobile versus stagnant lid has less of an effect on the distribution of hot and cold anomalies within the mantle, or planform. I find the same low-degree (one upwelling or two upwelling) temperature structures in the mobile-lid calculations that have previously been found in stagnant-lid calculations. While having less of an effect on the mean mantle temperature, the viscosity of the asthenosphere has a profound effect on the pattern of temperature anomalies, even in the deep mantle. If the asthenosphere is weaker than the upper mantle by more than an order of magnitude, then the low-degree (one or two giant upwellings) pattern of temperature anomalies results. If the asthenosphere is less than an order of magnitude weaker than the upper mantle, then the pattern of temperature anomalies has narrow cylindrical upwellings and cold downgoing sheets. The low-degree pattern of temperature anomalies is more consistent with the plate model than the plume model.

Several workers have suggested that the locations of melting anomalies (hotspots) and the original locations of large igneous provinces (LIPs) and kimberlite pipes lie preferentially above the margins of two large low-shear-velocity provinces, or LLSVPs, near the bottom of the mantle, and that the geographical correlations have high confidence levels (>99.9999%). They conclude that the LLSVP margins are “plume-generation zones”, and that deep-mantle plumes cause hotspots, LIPs, and kimberlites. This conclusion raises questions about what physical processes could be responsible, because, for example, the LLSVPs are apparently dense and not abnormally hot. The supposed LIP-hotspot-LLSVP correlations probably are examples of the “hindsight heresy”, of performing a statistical test using the same data sample that led to the initial formulation of a hypothesis. In this process, an analyst will consider and reject many competing hypotheses, but will not adjust statistical assessments correspondingly. Furthermore, an analyst will test extreme deviations of the data, but not take this fact into account. “Hindsight heresy” errors are particularly problematical in Earth science, where it commonly is impossible to conduct controlled experiments. For random locations on the globe, the number of points within a specified distance of a given curve follows a cumulative binomial distribution. We use this fact to test the statistical significance of the observed hotspot-LLSVP correlation using several hotspot catalogs and mantle models. The results indicate that the actual confidence levels of the correlations are two or three orders of magnitude smaller than claimed. The tests also show that hotspots correlate well with presumably shallowly rooted features such as spreading plate boundaries. Nevertheless, the correlations are significant at confidence levels in excess of 99%. But this is confidence that the null hypothesis of random coincidence is wrong. It is not confidence about what hypothesis is correct. The correlations probably are symptoms of as-yet-unidentified processes. These observations emphasize the importance of the distinction between correlation and causation, and underline the necessity of taking geological factors into account. Consideration of the kimberlite data set in the context of geological setting, for example, suggests that the apparent association with the LLSVP margins results from the fact that the Kaapvaal craton, the site of most of the kimberlites considered, lies in southern Africa, and that kimberlite eruptions are sensitive primarily to stress in the lithosphere.

Popular models of slow unidirectional evolution of each planet are based on dogmatized 1970s–1980s speculations that Earth has a perpetually hot core that drives narrow vertical plumes of unfractionated mantle which produce volcanoes, propel lithosphere plates, and are compensated by subduction. Long-lasting hot cores, plumes, and minimal fractionation were dogmatized also for Venus and Mars, by analogy, but with a different stagnant-lid conjecture, rather by disrupted-lid plate tectonics, for each. Physics, empirical data, and planetary imagery disprove all three mutually incompatible models. Radiogenic heat, ~5× greater than now, forced synaccretionary magma-ocean fractionation of each planet before 4.5 Ga. This produced thick mafic protocrusts, concentrated radioactivity at shallow depths, and permanently depleted lower mantles. On Earth, the protocrust lay directly above refractory dunite, in turn above denser fractionates. The shallow concentrations of radioactivity allowed deep interiors to cool quickly. Venus and Mars have never since had hot cores or asthenospheres, and their “volcanoes” and other features popularly attributed to plumes are products of bolide impacts on internally inactive planets. Only Earth had enough radioactivity to remain warmer and to generate partial melts from protocrust to make Archean, and possibly Hadean, felsic crust. Dense garnet-rich residues of protocrust delaminated, sank through the low-density dunite, and began upper-mantle re-enrichment. Archean cratons stabilized where sinking of residua left derivative felsic crust directly upon sterile buoyant dunite. Where some protocrust remained, Proterozoic crustal activity ensued. This was mostly in the form of basin filling atop Archean felsic crust, commonly followed by radioactive heating, partial melting of basement plus fill, and structural inversion. Top-down enrichment of the upper mantle by evolving processes reached the critical level needed for plate tectonics only ca. 0.6 Ga. Plate motions are driven by subduction, which rights the density inversion due to top-down cooling of asthenosphere to lithosphere. Circulation is closed within the upper mantle. Primary fractionation was hot and dry. The inner planets may have received most of their water in a barrage of icy bolides, centered ca. 4.1 or 4.0 Ga, best dated on Mars and Venus but in accord with terrestrial and, possibly, lunar data. Earth's new water may have enabled formation of Archean tonalite-trondhjemite-granodiorite from protocrust. Increasing downward cycling of volatiles into Earth's upper mantle ever since has been essential for continuing tectonism and magmatism.

Seismic anisotropy is an efficient way to investigate the deformation field within the upper mantle. In the framework of rigid tectonic plates, we make use of recent tomographic models of azimuthal anisotropy to derive the best rotation pole of the Pacific plate in the uppermost 200 km of the mantle. It is found to be in good agreement with current plate motion (NUVEL1, HS3, and NNR). However, when dividing the Pacific plate into two subplates separated by what we refer to as the megagash, an east-west low-velocity and low-anisotropy band extending across the Pacific plate from Samoa-Tonga to the Easter–Juan Fernández Islands, the rotation pole of northern Pacific is still in agreement with current plate motion but not the rotation pole of the southern part of the Pacific, far away from the “classical” rotation pole of the Pacific plate. This result suggests a differential motion between the North and South Pacific and an ongoing reorganization of plates in the Pacific Ocean. The megagash might be a future plate boundary between the North and South Pacific plates, associated with the intense volcanism along this band.

The subsidence of an aging seafloor starts to slow down at ~70 m.y. old with respect to that expected from simple half-space cooling, and this phenomenon has long been known as seafloor flattening. The flattening signal remains even after removal of the influence of the emplacement of hotspot islands and oceanic plateaus. The combination of small-scale convection and radiogenic heating has been suggested as a mechanism to explain seafloor flattening, and this study explores the possibility of using the magnitude of seafloor flattening to constrain the amount of radiogenic heating in the convecting mantle. By comparison of properly scaled geodynamic expectations with the observed age-depth relation of the normal seafloor, the mantle heat production is estimated to be ~12 ± 3 TW, which supports geochemistry-based estimates. A widely held notion that small-scale convection enhances cooling, thus being unable to explain seafloor flattening, is suggested to be incorrect. The ability to accurately interpret the age-depth relation of seafloor based on the thermal budget of Earth has an important bearing on the future theoretical study of early Earth evolution.

The average depth and heat flow of oceanic lithosphere as functions of age are well described by cooling plate models in which old lithosphere approaches an asymptotic thermal structure, causing average depth and heat flow to flatten. However, some areas are significantly shallower or deeper than the global average for their age. One possibility is that the deviations reflect variations in lithospheric temperature structure. Another is that the deviations reflect processes including excess volcanism or dynamic effects of mantle flow. The first hypothesis assumes that the average flattening reflects thermal perturbations to halfspace cooling, so the temperature structures of areas that are unusually deep for their age reflect continued halfspace cooling and thus should have lower heat flow. Although this hypothesis predicts lower heat flow at deeper sites in old lithosphere, the deep sites are divided approximately evenly between ones with high and low heat flow. Instead, the anomalously deep sites occur primarily at passive continental margins, perhaps because of dynamic topography due to sublithospheric mantle processes, and in only a few cases thinner crust formed at slow spreading rates immediately after rifting. Similarly, preferentially high heat flow is essentially not observed at anomalously shallow sites, primarily on hotspot swells, indicating that the swells do not result from hotspots significantly reheating the lithosphere. Thus, in general, neither shallow nor deep areas reflect primarily perturbed lithospheric thermal structure. Hence a plate model is more useful than a halfspace model in describing how ocean depth and heat flow vary with lithospheric age, and excluding the vast majority of the seafloor while ascribing significance to the small fraction matching the halfspace model is pointless.