Bernhard Steinberger

Long-Term Sea-Level Fluctuations Driven by Ocean Basin Dynamics

Earths long-term sea-level history is characterized by widespread continental flooding in the Cretaceous period (∼145 to 65 million years ago), followed by gradual regression of inland seas. However, published estimates of the Late Cretaceous sea-level high differ by half an order of magnitude, from ∼40 to ∼250 meters above the present level. The low estimate is based on the stratigraphy of the New Jersey margin. By assimilating marine geophysical data into reconstructions of ancient ocean basins, we model a Late Cretaceous sea level that is 170 (85 to 270) meters higher than it is today. We use a mantle convection model to suggest that New Jersey subsided by 105 to 180 meters in the past 70 million years because of North Americas westward passage over the subducted Farallon plate. This mechanism reconciles New Jersey margin–based sea-level estimates with ocean basin reconstructions.

Plumes in a convecting mantle: Models and observations for individual hotspots

The motion of hotspots and the deformation of their underlying plume conduits as calculated within models of global mantle flow are presented. A new list of 44 possible hotspots with associated tracks has been compiled. For all of them, calculations have been performed under consideration of individual age and anomalous mass flux for three different models of plume buoyancy and mantle flow. Plume source depth has usually been assumed to be the top of D”, but an alternative source depth at the 670-km discontinuity has also been considered. Using models of relative plate motions and boundaries, hotspot tracks on plates have been calculated and compared with age data, ocean floor topography, and distribution of volcanics on continents. Absolute plate motions have been redetermined under consideration of hotspot motion, using a new least squares method. For the Hawaiian and Yellowstone hotspots, source locations and hotspot motion have been computed for a total of up to 23 different models. The results show plume conduits being tilted, with source regions at the D” moving in the lowermost mantle flow, generally toward large-scale upwellings under southern Africa and the south central Pacific. Hotspot surface motion often represents the horizontal component of midmantle flow, which is frequently opposite to plate motion, toward ridges and away from subduction zones. In particular, almost all models tested predict southward motion of the Hawaii and Kerguelen hotspots and westward motion of the Iceland hotspot. For models including hotspot motion the agreement between calculated and observed hotspot tracks is frequently about as good as, or better than, for the fixed hotspot model, but sometimes fixed hotspots give the best fit. In some cases where the track ends at a subduction zone, e.g., for the Bowie hotspot, results can give indications about the otherwise unknown age of the hotspot. In other cases, especially for the Tahiti hotspot, results suggest an origin shallower than D”, and in yet other cases, particularly East Africa, the failure of the hotspot models used supports other evidence indicating the presence of comparatively broad upwellings rather than localized plumes.

Diamonds sampled by plumes from the core-mantle boundary

Diamonds are formed under high pressure more than 150 kilometres deep in the Earth’s mantle and are brought to the surface mainly by volcanic rocks called kimberlites. Several thousand kimberlites have been mapped on various scales, but it is the distribution of kimberlites in the very old cratons (stable areas of the continental lithosphere that are more than 2.5 billion years old and 300 kilometres thick or more) that have generated the most interest, because kimberlites from those areas are the major carriers of economically viable diamond resources. Kimberlites, which are themselves derived from depths of more than 150 kilometres, provide invaluable information on the composition of the deep subcontinental mantle lithosphere, and on melting and metasomatic processes at or near the interface with the underlying flowing mantle. Here we use plate reconstructions and tomographic images to show that the edges of the largest heterogeneities in the deepest mantle, stable for at least 200 million years and possibly for 540 million years, seem to have controlled the eruption of most Phanerozoic kimberlites. We infer that future exploration for kimberlites and their included diamonds should therefore be concentrated in continents with old cratons that once overlay these plume-generation zones at the core–mantle boundary.

Deep mantle structure as a reference frame for movements in and on the Earth

Significance Since the Pangea supercontinent formed about 320 million years ago, plumes that sourced large igneous provinces and kimberlites have been derived from the edges of two stable thermochemical reservoirs at the core–mantle boundary. We test whether it is possible to maintain this remarkable surface-to-deep Earth correlation before Pangea through the development of a new plate reconstruction method and find that our reconstructions for the past 540 million years comply with known geological and tectonic constraints (opening and closure of oceans, mountain building, and more). These results have important implications for Earth history, including the style of mantle convection in the deep past and the long-term stability of mantle reservoirs. Earth’s residual geoid is dominated by a degree-2 mode, with elevated regions above large low shear-wave velocity provinces on the core–mantle boundary beneath Africa and the Pacific. The edges of these deep mantle bodies, when projected radially to the Earth’s surface, correlate with the reconstructed positions of large igneous provinces and kimberlites since Pangea formed about 320 million years ago. Using this surface-to-core–mantle boundary correlation to locate continents in longitude and a novel iterative approach for defining a paleomagnetic reference frame corrected for true polar wander, we have developed a model for absolute plate motion back to earliest Paleozoic time (540 Ma). For the Paleozoic, we have identified six phases of slow, oscillatory true polar wander during which the Earth’s axis of minimum moment of inertia was similar to that of Mesozoic times. The rates of Paleozoic true polar wander (<1°/My) are compatible with those in the Mesozoic, but absolute plate velocities are, on average, twice as high. Our reconstructions generate geologically plausible scenarios, with large igneous provinces and kimberlites sourced from the margins of the large low shear-wave velocity provinces, as in Mesozoic and Cenozoic times. This absolute kinematic model suggests that a degree-2 convection mode within the Earth’s mantle may have operated throughout the entire Phanerozoic.

On the role of slab pull in the Cenozoic motion of the Pacific plate

We analyze the role of slab pull acting on the Pacific plate during its early Tertiary change in motion. Slab pull forces are estimated by integrating the negative buoyancy of a 700 km long slab along a revised subduction boundary model adopting the Muller et al. (2008) seafloor age reconstructions. Our results indicate that torques predicted from a simple slab pull model match the Pacific plate Euler vectors during the Tertiary fairly well. The change of the Pacific motion at similar to 50-40 Ma appears to be driven by the onset of the Izu-Bonin-Mariana system and, soon afterwards, by the Tonga-Kermadec subduction zones.

Plate-tectonic reconstructions predict part of the Hawaiian hotspot track to be preserved in the Bering Sea

We use plate reconstructions to show that parts of the Hawaiian hotspot track of ca. 80– 90 Ma age could be preserved in the Bering Sea. Based on these reconstructions, the Hawaiian hotspot was beneath the Izanagi plate before ca. 83 Ma. Around that time, the part of the plate carrying the hotspot track was transferred to the Kula plate. After 75–80 Ma the Hawaiian hotspot underlay the Pacifi c plate. Circa 40–55 Ma, subduction initiated in the Aleutian Trench. Part of the Kula plate was attached to the North American plate and is preserved as the oceanic part of the Bering Sea. We show that for a number of different plate reconstructions and a variety of assumptions covering hotspot motion, part of the hotspot track should be preserved in the Bering Sea. The predicted age of the track depends on the age of Aleutian subduction initiation. We speculate that Bowers and Shirshov Ridges were formed by paleoHawaiian hotspot magmatism.

Continental crust beneath southeast Iceland

Significance The Iceland hotspot is widely thought to be the surface expression of a deep mantle plume from the core–mantle boundary that can be traced back in time at least 62 My. However, some lavas contain continental material, which has previously been proposed to have been recycled through the plume. Here, we argue that the plume split off a sliver of continent from Greenland in the Early Eocene. This sliver is now located beneath southeast Iceland where it locally contaminates some of the plume-derived magmas. The magmatic activity (0–16 Ma) in Iceland is linked to a deep mantle plume that has been active for the past 62 My. Icelandic and northeast Atlantic basalts contain variable proportions of two enriched components, interpreted as recycled oceanic crust supplied by the plume, and subcontinental lithospheric mantle derived from the nearby continental margins. A restricted area in southeast Iceland—and especially the Öræfajökull volcano—is characterized by a unique enriched-mantle component (EM2-like) with elevated 87Sr/86Sr and 207Pb/204Pb. Here, we demonstrate through modeling of Sr–Nd–Pb abundances and isotope ratios that the primitive Öræfajökull melts could have assimilated 2–6% of underlying continental crust before differentiating to more evolved melts. From inversion of gravity anomaly data (crustal thickness), analysis of regional magnetic data, and plate reconstructions, we propose that continental crust beneath southeast Iceland is part of ∼350-km-long and 70-km-wide extension of the Jan Mayen Microcontinent (JMM). The extended JMM was marginal to East Greenland but detached in the Early Eocene (between 52 and 47 Mya); by the Oligocene (27 Mya), all parts of the JMM permanently became part of the Eurasian plate following a westward ridge jump in the direction of the Iceland plume.

Stability of active mantle upwelling revealed by net characteristics of plate tectonics

Viscous convection within the mantle is linked to tectonic plate motions and deforms Earth’s surface across wide areas. Such close links between surface geology and deep mantle dynamics presumably operated throughout Earth’s history, but are difficult to investigate for past times because the history of mantle flow is poorly known. Here we show that the time dependence of global-scale mantle flow can be deduced from the net behaviour of surface plate motions. In particular, we tracked the geographic locations of net convergence and divergence for harmonic degrees 1 and 2 by computing the dipole and quadrupole moments of plate motions from tectonic reconstructions extended back to the early Mesozoic era. For present-day plate motions, we find dipole convergence in eastern Asia and quadrupole divergence in both central Africa and the central Pacific. These orientations are nearly identical to the dipole and quadrupole orientations of underlying mantle flow, which indicates that these ‘net characteristics’ of plate motions reveal deeper flow patterns. The positions of quadrupole divergence have not moved significantly during the past 250 million years, which suggests long-term stability of mantle upwelling beneath Africa and the Pacific Ocean. These upwelling locations are positioned above two compositionally and seismologically distinct regions of the lowermost mantle, which may organize global mantle flow as they remain stationary over geologic time.

A failure to reject: Testing the correlation between large igneous provinces and deep mantle structures with EDF statistics

Absolute reconstructions of large igneous provinces (LIPs) for the past 300 Ma reveal a remarkable spatial pattern suggesting that almost all LIPs have erupted over the margins of the two large-scale structures in the Earths lower mantle commonly referred to as the Large Low Shear-wave Velocity Provinces (LLSVPs). This correlation suggests that mantle plumes that have triggered LIP eruptions rose from the margins of LLSVPs, implying long-term stability of these structures and suggesting that they may be chemically distinct from the bulk of the mantle. Yet, some researchers consider the LLSVPs to be purely thermal upwellings, arguing that the observed distribution of LIPs can be explained by plumes randomly forming over the entire areas of LLSVPs. Here we examine the correlation between the LIPs and LLSVPs using nonparametric statistical tests, updated plate reconstructions, and a large number of alternative definitions of LLSVPs based on seismic tomography. We show that probability models assuming plume sources originating at the margins of LLSVPs adequately explain the observed distribution of reconstructed LIPs. In contrast, we find strong evidence against the models seeking to link LIPs with plumes randomly forming over the entire LLSVP areas. However, the hypothesis proposing that the correlation can be explained by plumes randomly forming over a larger area of slower-than-average shear wave velocities in the lowermost mantle cannot be ruled out formally. Our analysis suggests that there is no statistically sound reason for questioning the hypothesis that the LIPs correlate with the margins of LLSVP globally.

Survival of LLSVPs for billions of years in a vigorously convecting mantle: Replenishment and destruction of chemical anomaly

We study segregation of the subducted oceanic crust (OC) at the core-mantle boundary and its ability to accumulate and form large thermochemical piles (such as the seismically observed Large Low Shear Velocity Provinces (LLSVPs)). Our high-resolution numerical simulations of thermochemical mantle convection suggest that the longevity of LLSVPs for up to three billion years, and possibly longer, can be ensured by a balance in the rate of segregation of high-density OC material to the core-mantle boundary (CMB) and the rate of its entrainment away from the CMB by mantle upwellings. For a range of parameters tested in this study, a large-scale compositional anomaly forms at the CMB, similar in shape and size to the LLSVPs. Neutrally buoyant thermochemical piles formed by mechanical stirring—where thermally induced negative density anomaly is balanced by the presence of a fraction of dense anomalous material—best resemble the geometry of LLSVPs. Such neutrally buoyant piles tend to emerge and survive for at least 3 Gyr in simulations with quite different parameters. We conclude that for a plausible range of values of density anomaly of OC material in the lower mantle—it is likely that it segregates to the CMB, gets mechanically mixed with the ambient material, and forms neutrally buoyant large-scale compositional anomalies similar in shape to the LLSVPs.