Pavel V. Doubrovine

Greater India Basin hypothesis and a two-stage Cenozoic collision between India and Asia

Cenozoic convergence between the Indian and Asian plates produced the archetypical continental collision zone comprising the Himalaya mountain belt and the Tibetan Plateau. How and where India–Asia convergence was accommodated after collision at or before 52 Ma remains a long-standing controversy. Since 52 Ma, the two plates have converged up to 3,600 ± 35 km, yet the upper crustal shortening documented from the geological record of Asia and the Himalaya is up to approximately 2,350-km less. Here we show that the discrepancy between the convergence and the shortening can be explained by subduction of highly extended continental and oceanic Indian lithosphere within the Himalaya between approximately 50 and 25 Ma. Paleomagnetic data show that this extended continental and oceanic “Greater India” promontory resulted from 2,675 ± 700 km of North–South extension between 120 and 70 Ma, accommodated between the Tibetan Himalaya and cratonic India. We suggest that the approximately 50 Ma “India”–Asia collision was a collision of a Tibetan-Himalayan microcontinent with Asia, followed by subduction of the largely oceanic Greater India Basin along a subduction zone at the location of the Greater Himalaya. The “hard” India–Asia collision with thicker and contiguous Indian continental lithosphere occurred around 25–20 Ma. This hard collision is coincident with far-field deformation in central Asia and rapid exhumation of Greater Himalaya crystalline rocks, and may be linked to intensification of the Asian monsoon system. This two-stage collision between India and Asia is also reflected in the deep mantle remnants of subduction imaged with seismic tomography.

Geodynamo, Solar Wind, and Magnetopause 3.4 to 3.45 Billion Years Ago

Early Origin of Earths Magnetic Field Earths magnetic field protects us from stellar winds and radiation from the Sun. Understanding when, during the Earths formation, the large-scale magnetic field was established is important because it impacts understanding of the young Earths atmosphere and exosphere. By analyzing ancient silicate crystals, Tarduno et al. (p. 1238; see the Perspective by Jardine) demonstrate that the Earths magnetic field existed 3.4 to 3.45 billion years ago, pushing back the oldest record of geomagnetic field strength by 200 million years. This result combined with estimates of the conditions within the solar wind at that time implies that the size of the paleomagnetosphere was about half of that typical today, but with an auroral oval of about three times the area. The smaller magnetosphere and larger auroral oval would have promoted loss of volatiles and water from the early atmosphere. Analysis of ancient silicate crystals indicates that Earth’s magnetic field existed 3.40 to 3.45 billion years ago. Stellar wind standoff by a planetary magnetic field prevents atmospheric erosion and water loss. Although the early Earth retained its water and atmosphere, and thus evolved as a habitable planet, little is known about Earth’s magnetic field strength during that time. We report paleointensity results from single silicate crystals bearing magnetic inclusions that record a geodynamo 3.4 to 3.45 billion years ago. The measured field strength is ~50 to 70% that of the present-day field. When combined with a greater Paleoarchean solar wind pressure, the paleofield strength data suggest steady-state magnetopause standoff distances of ≤5 Earth radii, similar to values observed during recent coronal mass ejection events. The data also suggest lower-latitude aurora and increases in polar cap area, as well as heating, expansion, and volatile loss from the exosphere that would have affected long-term atmospheric composition.

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.

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.

Global correlation of lower mantle structure and past subduction

Abstract Advances in global seismic tomography have increasingly motivated identification of subducted lithosphere in Earths deep mantle, creating novel opportunities to link plate tectonics and mantle evolution. Chief among those is the quest for a robust subduction reference frame, wherein the mantle assemblage of subducted lithosphere is used to reconstruct past surface tectonics in an absolute framework anchored in the deep Earth. However, the associations heretofore drawn between lower mantle structure and past subduction have been qualitative and conflicting, so the very assumption of a correlation has yet to be quantitatively corroborated. Here we show that a significant, time‐depth progressive correlation can be drawn between reconstructed subduction zones of the last 130 Myr and positive S wave velocity anomalies at 600–2300 km depth, but that further correlation between greater times and depths is not presently demonstrable. This correlation suggests that lower mantle slab sinking rates average between 1.1 and 1.9 cm yr−1.

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.

Pacific plate motion change caused the Hawaiian-Emperor Bend

A conspicuous 60° bend of the Hawaiian-Emperor Chain in the north-western Pacific Ocean has variously been interpreted as the result of an abrupt Pacific plate motion change in the Eocene (∼47 Ma), a rapid southward drift of the Hawaiian hotspot before the formation of the bend, or a combination of these two causes. Palaeomagnetic data from the Emperor Seamounts prove ambiguous for constraining the Hawaiian hotspot drift, but mantle flow modelling suggests that the hotspot drifted 4–9° south between 80 and 47 Ma. Here we demonstrate that southward hotspot drift cannot be a sole or dominant mechanism for formation of the Hawaiian-Emperor Bend (HEB). While southward hotspot drift has resulted in more northerly positions of the Emperor Seamounts as they are observed today, formation of the HEB cannot be explained without invoking a prominent change in the direction of Pacific plate motion around 47 Ma.

Intraoceanic subduction spanned the Pacific in the Late Cretaceous–Paleocene

Intraoceanic subduction drove both the Pacific plate’s ~80- to 47-Ma northward motion and its redirection at ~47 Ma. The notorious ~60° bend separating the Hawaiian and Emperor chains marked a prominent change in the motion of the Pacific plate at ~47 Ma (million years ago), but the origin of that change remains an outstanding controversy that bears on the nature of major plate reorganizations. Lesser known but equally significant is a conundrum posed by the pre-bend (~80 to 47 Ma) motion of the Pacific plate, which, according to conventional plate models, was directed toward a fast-spreading ridge, in contradiction to tectonic forcing expectations. Using constraints provided by seismic tomography, paleomagnetism, and continental margin geology, we demonstrate that two intraoceanic subduction zones spanned the width of the North Pacific Ocean in Late Cretaceous through Paleocene time, and we present a simple plate tectonic model that explains how those intraoceanic subduction zones shaped the ~80 to 47 Ma kinematic history of the Pacific realm and drove a major plate reorganization.

Continental Drift (Paleomagnetism)

Paleomagnetism is the study of the Earth’s ancient magnetic field through the record of remanent magnetism preserved in rocks. The directions of remanent magnetization are used to deduce the position of the Earth’s magnetic pole relative to the study location at the time when this magnetization was acquired. By studying magnetizations of varying age from a single lithospheric plate, one can construct a path of apparent polar wandering (APWP) that tracks the motion of that plate relative to the geographic pole. A well-defined APWP can serve as a geochronological tool, i.e., for dating magnetizations of unknown age through a comparison of their directions with those expected from the reference APWP. Paleomagnetism can be used to date any geologic event that engenders the acquisition of remanent magnetization, including formation of igneous and sedimentary rocks, deposition of ore minerals, episodes of deformation, and other remagnetization processes.