Plate Tectonic Forces Research Articles

Beyond ‘ever closer union’: retrospective on 30 years of union citizenship and what they tell us about the future

Abstract Binary descriptions have traditionally taken centre stage in many analyses of Union citizenship, signalling the assumption of an inbuilt dynamism towards a pre-determined objective. This contribution moves beyond the vision of constant betterment for Union citizenship and the European Union at large. For that purpose, it reconstructs the institutional practice, with its seemingly erratic combination of ‘progress’ and ‘standstill’, as normative fault lines. In geology, a fault line is a section of the earth’s surface where rock has been visibly displaced as a result of plate tectonic forces, which, in the domain of Union citizenship, correspond to several constitutional tensions pushing and pulling the case law in diverse directions. For many years, these tensions did not exercise a decisive influence; anyone could project their understanding onto the skeleton of abstract Treaty articles. This is no longer possible at a time when Union citizenship touches upon salient topics which define the democratic context. It is these constitutional tensions, such as disagreement over the degree of centralization, the value of (im)mobility, or the reconfiguration of membership, which sustain our hypothesis that Union citizenship is defined by postmodern ambiguity rather than the modernist ideal of irrefutable progress. An awareness of this change allows readers to chart their way through the institutional practice by defining the legal and normative parameters for future developments.

Stability and wave velocity of novel iron magnesium oxide (Mg, Fe)2O3.5 in the lower mantle

The mineral composition of the anomalous wave velocity zone in the lower mantle is indispensable to understanding seismic wave velocity anomalies and the genesis as well as driving forces of plate tectonics. We here systematically investigate the structural stability, density, elastic wave velocity and anisotropy of the wave velocity for (Mg, Fe)2O3.5, a new type of the low wave velocity mineral, in the 0–136 GPa range by means of first-principles calculations. Calculations show that (Mg, Fe)2O3.5 possesses high density, with a value higher than that in the Preliminary Reference Earth Model at the core-mantle boundary by up to 17.31 %, and its longitudinal and transverse wave velocities are reduced by about 9.23 % and 17.41 %, respectively. Moreover, the elastic wave velocity is characterized by anisotropy, and is consistent with the characteristics of the seismic wave velocity in ultra–low velocity Zone (ULVZ). Our study elucidates that (Mg, Fe)2O3.5 is a candidate mineral for the wave velocity anomalies in the ULVZ that survive in the deep mantle. Such findings may pave the way towards exploring the origin of the ULVZ from the viewpoint of chemical composition anomalies.

Estimating Plate Tectonic Forces at the New Zealand Plate Boundary Using Strain Rates Derived From a Kinematic Model of Fault Slip

AbstractWe construct a model of forces at the New Zealand plate boundary from dynamical thin‐sheet modeling. Stress magnitudes estimated are 10–50 MPa and effective viscosities are 0.5–5 × 1021 Pa s within actively deforming regions. Models that include only far‐field forces and forces from variations in topography and bathymetry can fit observations in most of the plate boundary, but basal tractions are required to fit extension in Havre Trough. For models that include nontopographic forces, we specify a rheology to acquire a unique solution for each of three rheologies: power‐law rheology with n = 3, power law with n = 5, and a pseudo‐plastic equal‐stress rheology. The inclusion of nontopographic forces allows these models to fit observations very well. We predict forces in Havre Trough equivalent to basal tractions of 7–10 MPa at 20‐km depth. Models with n = 3 and n = 5 require antiparallel forces on opposite sides of the plate boundary to drive deformation in South Island which would imply a plate boundary zone that is more localized at depth. The equal‐stress pseudo‐plastic model drives deformation with plate motion boundary conditions and localizes it with variations in effective viscosity. The effective rheology of South Island is most realistically modeled by an equal‐stress pseudo‐plastic rheology. The n = 3 or n = 5 power‐law rheology models require a highly localized boundary zone in the lower lithosphere, but a broader zone in the upper lithosphere.

Sediment buoyancy controls the effective slab pull force and deviatoric stress along trenches: Insights from 3D free-subduction model

In the complicated force equilibrium of subduction dynamics, the slab pull resulting from the negative buoyancy of the lithosphere is considered the primary driving force of plate tectonics. In contrast, the isostatic restoring force due to lighter materials covering the oceanic crust (e.g., sediment) has been recognized as a relatively minor contributor. However, as the restoring force counteracts the gravitational pull, the resultant interaction has the potential to control the force balance governing the subduction system. Here, we conducted a series of three-dimensional viscoelastic-free subduction simulations with varying sediment distributions, thicknesses, and densities. We found that a higher restoring force suppressed the trench retreat and decreased the trench velocity. The trench curvatures increased with a high differential restoring force along the trench strike over time. Our results consistently showed a negative correlation between the effective slab pull and the isostatic restoring forces. Moreover, the magnitude and distribution of the isostatic restoring force along the trench strike determined the deviatoric stress level and location of the stress concentration within the subducting slab. We also found that locally abundant sediment margins may lead to a high deviatoric stress in the trench, which is associated with the occurrence of large earthquakes. We argue that sediment buoyancy can significantly affect the subduction kinematics and dynamics to a greater degree than previously suggested. Our findings provide fresh insights into the development of a global predicted velocity model, that reflects the effect of the isostatic restoring force, and provides a better understanding of subduction earthquakes.

Late Eocene subduction initiation of the Indian Ocean in the North Sulawesi Arc, Indonesia, induced by abrupt Australian plate acceleration

Subduction of oceanic lithosphere is the main driving force of plate tectonics on Earth, yet, how subduction began (i.e. subduction initiation; SI) remains enigmatic, due to limited SI examples. Here, we report new whole-rock geochemical and zircon U-Pb-Hf isotopic data of arc magmatism from the North Sulawesi Arc, Indonesia, to constrain the petrogenesis, magmatic tempo, and tectonic setting, as well as the timing and trigger of SI. Zircon U-Pb dating yielded crystallization ages of ~29.7–16.7 Ma for parental magmas, which are calc-alkaline I-type in nature and show typical subduction-related signatures. Geochemical data and high positive zircon ε Hf (t) values indicate magma generation by partial melting of preexisting mafic basement rocks. Our new results, together with literature data, allow the identification of an Indian SI in the Late Eocene, as evidenced by the last vanishing of BAB-type mafic basement rocks at ~37 Ma and the first appearance of calc-alkaline felsic arc magmatism at ~30 Ma in the North Sulawesi Arc, as well as ~36–32 Ma ophiolite and ~34–29 Ma metamorphic sole in the nearby East Sulawesi Ophiolite. Such a temporal duration (maximum of ~7 Ma) from SI to typical arc magmatism is identical to that (~8–7 Ma) of the Izu-Bonin-Mariana system. Given the contemporaneousness of a sudden increase (from ~4–5 to ~8–9 cm/y) in northward drifting rate of the Australian plate, we propose that Late Eocene Indian SI was probably linked to the sharp Australian plate acceleration. Such plate reorganization may have played a significant role in driving SI in Earth history. • Calc-alkaline I-type felsic rocks in the North Sulawesi Arc developed at ~30–16 Ma. • Arc magmatism formed by melting of preexisting mafic rocks during Indian subduction. • Indian SI occurred in the Late Eocene, driven by Australian plate acceleration.

Yellowstone Plume Drives Neogene North American Plate Motion Change

AbstractPlate motion changes provide powerful constrains on plate tectonic forces. North America is ideal to explore these forces. We use recently published high‐temporal resolution plate reconstructions of North America, which reveal a velocity slow‐down at 17 Ma, roughly coeval with the eruption of the Yellowstone plume, together with a stratigraphic analysis of hiatus surfaces across the continent, which provides proxy information for paleotopography. Using a simple Couette/Poiseuille flow models we estimate asthenosphere flow beneath North America and its impact on Neogene plate motion. We find that North America's Neogene plate motion change can be explained by Poiseuille flow in the asthenosphere generated upon the arrival of the Yellowstone plume and that the flow length‐scale matches the extent of hiatus surfaces. While plume driven upper mantle flow constitutes a geodynamically viable model to explain North America's Neogene plate motion change it provides an intrinsic link between vertical and horizontal plate motions.

New sequence stratigraphic framework on a ‘passive’ margin: implications for the post-break-up depositional environment and onset of glaciomarine conditions in NE Greenland

Detailed sequence stratigraphic frameworks have been produced for most of the Cenozoic deposits around the North Atlantic and Greenland Sea. However, the sequence stratigraphic evolution of the NE Greenland shelf, a significant part of the Atlantic Ocean margin, has so far never been interpreted. This study provides, for the first time, a detailed sequence stratigraphic interpretation of the NE Greenland shelf since the late Eocene. The tectonic evolution, eustatic sea-level change and the impact of glaciations are integrated into a consistent geological history. Based on the identification of pronounced unconformities and their correlative conformities, ten stratigraphic sequences are identified. The onset of deposition is defined by a pronounced erosional event, where late Paleocene tectonic activity caused mass wasting of the uplifted footwall of the Danmarkshavn Ridge along a c. 200 km section of the ridge. A second pronounced unconformity developed across the margin during the mid-Miocene, which caused forced regressive submarine fans to prograde across the margin. The following tectonic quiet period is overlain by glaciomarine deposits, with evidence of ice deformation reaching the present day shelf break. Unknown super-regional plate tectonic forces are suggested as a driver of sequence formation based on correlation with the Canadian Arctic.

Slender salamanders (genus Batrachoseps) reveal Southern California to be a center for the diversification, persistence, and introduction of salamander lineages.

BackgroundThe southern California biodiversity hotspot has had a complex geological history, with both plate tectonic forces and sea level changes repeatedly reconfiguring the region, and likely driving both lineage splittings and extinctions. Here we investigate patterns of genetic divergence in two species of slender salamanders (Plethodontidae: Batrachoseps) in this region. The complex geological history in combination with several organismal traits led us to predict that these species harbor multiple ancient mitochondrial lineages endemic to southern California. These species belong to a clade characterized by fine-scale mitochondrial structure, which has been shown to track ancient splits. Both focal species, Batrachoseps major and B. nigriventris, are relatively widely distributed in southern California, and estimated to have persisted there across millions of years. Recently several extralimital populations of Batrachoseps were found in the San Joaquin Valley of California, a former desert area that has been extensively modified for agriculture. The origins of these populations are unknown, but based on morphology, they are hypothesized to result from human-mediated introductions of B. major.MethodsWe sequenced the mitochondrial gene cytochrome b from a geographically comprehensive sampling of the mitochondrial lineages of B. major and B. nigriventris that are endemic to southern California. We used phylogenetic analyses to characterize phylogeographic structure and identify mitochondrial contact zones. We also included the San Joaquin Valley samples to test whether they resulted from introductions. We used a bootstrap resampling approach to compare the strength of isolation-by-distance in both Batrachoseps species and four other salamander species with which they co-occur in southern California.ResultsThe northern lineage of B. major harbors at least eight deeply differentiated, geographically cohesive mitochondrial subclades. We identify geographic contact between many of these mtDNA lineages and some biogeographic features that are concordant with lineage boundaries. Batrachoseps nigriventris also has multiple deeply differentiated clades within the region. Comparative analyses highlight the smaller spatial scales over which mitochondrial divergence accumulates in Batrachoseps relative to most other salamander species in southern California. The extralimital populations of Batrachoseps from the San Joaquin Valley are assigned to B. major and are shown to result from at least two independent introductions from different source populations. We also suggest that B. major on Catalina Island, where it is considered native, may be the result of an introduction. Some of the same traits that facilitate the build-up of deep phylogeographic structure in Batrachoseps likely also contribute to its propensity for introductions, and we anticipate that additional introduced populations will be discovered.

A Role for Subducted Oceanic Crust in Generating the Depleted Mid‐Ocean Ridge Basalt Mantle

AbstractThe composition of Earth's mantle, continental crust, and oceanic crust continuously evolve in response to the dynamic forces of plate tectonics and mantle convection. The classical view of terrestrial geochemistry, where mid‐ocean ridges sample mantle previously depleted by continental crust extraction, broadly explains the composition of the oceanic and continental crust but is potentially inconsistent with observed slab subduction to the lower mantle and oceanic crust accumulation in the deep mantle. We develop a box model to explore the key processes controlling crust‐mantle chemical evolution. The model mimics behaviors observed in thermochemical convection simulations including subducted oceanic crust separating and accumulating in the deep mantle. We demonstrate that oceanic crust accumulation strongly depletes the mantle independently of continental crust extraction. Slab stalling depths and continental crust recycling rates also affect the extent and location of mantle depletion. We constrain model regimes that reproduce oceanic and continental crust compositions using Markov chain Monte Carlo sampling. Some regimes deplete the lower mantle more than the upper mantle, contradicting the assumption of a more enriched lower mantle. All regimes require oceanic crust accumulation in the mantle. Though a small fraction of the mantle mass, accumulated oceanic crust can sequester trace element budgets exceeding the continental crust, depleting the mantle more than continental crust extraction alone. Oceanic crust accumulation may therefore be as important as continental crust extraction in depleting the mantle, contradicting the paradigmatic complementarity of depleted mantle and continental crust. Instead, depleted mantle is complementary to continental crust plus sequestered oceanic crust.

Reconstructing seafloor age distributions in lost ocean basins

Reconstructions of past seafloor age make it possible to quantify how plate tectonic forces, surface heat flow, ocean basin volume and global sea level have varied through geological time. However, past ocean basins that have now been subducted cannot be uniquely reconstructed, and a significant challenge is how to explore a wide range of possible reconstructions. Here, we investigate possible distributions of seafloor ages from the late Paleozoic to present using published full-plate reconstructions and a new, efficient seafloor age reconstruction workflow, all developed using the open-source software GPlates. We test alternative reconstruction models and examine the influence of assumed spreading rates within the Panthalassa Ocean on the reconstructed history of mean seafloor age, oceanic heat flow, and the contribution of ocean basin volume to global sea level. The reconstructions suggest variations in mean seafloor age of ~15 Myr during the late Paleozoic, similar to the amplitude of variations previously proposed for the Cretaceous to present. Our reconstructed oceanic age-area distributions are broadly compatible with a scenario in which the long-period fluctuations in global sea level since the late Paleozoic are largely driven by changes in mean seafloor age. Previous suggestions of a constant rate of seafloor production through time can be modelled using our workflow, but require that oceanic plates in the Paleozoic move slower than continents based on current reconstructions of continental motion, which is difficult to reconcile with geodynamic studies.

Plate convergence in the Indo-Pacific region

The Indo-Pacific convergence region is the best target to solve the two remaining challenges of the plate tectonics theory, i.e., subduction initiation and the driving force of plate tectonics. Recent studies proposed that the Izu-Bonin subduction initiation belongs to spontaneous initiation, which implies that it started from extension, followed by low angle subduction. Numerical geodynamic modeling suggests that the initiation of plate subduction likely occurred along a transform fault, which put the young spreading ridge in direct contact with old oceanic crust. This, however, does not explain the simultaneous subduction initiation in the west Pacific region in the Cenozoic. Namely, the subduction initiations in the Izu-Bonin- Mariana, the Aleutian, and the Tonga-Kermadec trenches are associated with oceanic crusts of different ages, yet they occurred at roughly the same time, suggesting that they were all triggered by a major change in the Pacific plate. Moreover, low angle subduction induces compression rather than extension, which requires external compression forces. Given that the famous Hawaiian-Emperor bending occurred roughly at the same time with the onset of westward subductions in the west Pacific, we propose that these Cenozoic subductions were initiated by the steering of the Pacific plate, which are classified as induced initiation. Induced subduction initiation usually occurs in young ocean basins, forming single-track subduction. The closures of Neo-Tethys Oceans were likely triggered by plumes in the south, forming northward subductions. Interestingly, the Indian plate kept on moving northward more than 50 Ma after the collision between the Indian and Eurasian continents and the break-off of the subducted oceanic slab attached to it. This strongly suggests that slab pull is not the main driving force of plate tectonics, whereas slab sliding is.

Contemporary stress and strain field in the Mediterranean from stress inversion of focal mechanisms and GPS data

Mapping the contemporary stress field of the Mediterranean provides fundamental insights on the complexity of plate tectonic forces throughout the region and at different depths. Despite increased data availability and methodological improvements, most recent comprehensive stress field characterization across the entire Mediterranean dates back to 1995. To extend the regional stress information, we use all earthquake focal mechanisms compiled in the World Stress Map database release 2016 for a formal stress inversion. Our main goals are to (1) improve the resolution of the stress field orientation, (2) evaluate the performance of the recently refined stress inversion methodology in a tectonically complex region, (3) test the hypothesis of a depth-dependent stress orientation heterogeneity, and (4) compare different types of stress and strain observations from surface using the GPS-derived information on the strain rate tensor down to the seismogenic crust using summation of normalized seismic potencies. The obtained stress orientations generally capture the main seismotectonic features, including tectonically complex settings such as the Alpine Orogeny or the Ionian Sea. The orientation of the maximum horizontal stress SHmax tends to be uniform with depth within uncertainties while larger stress heterogeneity (quantified by means of the focal mechanism diversity and misfit angles) is found between 5 and 14 km. Both, the orientation of the largest horizontal shortening axis of the strain field from potency tensors, and horizontal strain rate tensor from GPS data are generally sub-parallel to SHmax orientation, indicating a linear stress/strain relationship and that the orientations of the co-seismic release and interseismic strain accumulation are generally consistent.

Cenozoic tectonic evolution of southeastern Thailand derived from low-temperature thermochronology

Low-temperature thermochronological techniques, specifically apatite (U–Th)/He and apatite fission-track dating, were used to reconstruct the thermal history of southeastern Thailand. This area is intersected by vast and complex fault networks related to the Cenozoic Mae Ping and Three Pagodas Faults. These were identified from satellite imagery and confirmed by field observations. New apatite fission-track and apatite (U–Th)/He data were collected from crystalline basement blocks within these fault networks. Ages obtained range from 48 to 24 Ma, with most of the samples clustering between 36 and 24 Ma. Thermal history modelling indicates late Eocene–Oligocene exhumation of the exposed granitic and metamorphic basement rocks in southeastern Thailand. Exhumation was regional and was contemporaneous with sinistral fault activity during the late Eocene–early Oligocene along the Mae Ping Fault and Three Pagodas Fault. Moreover, this exhumation occurred coevally with a synrift phase of intracontinental offshore rift basin and half-graben basin development in the eastern Gulf of Thailand. The phase of exhumation ended in the early Miocene, as a result of the changing plate-tectonic forces along the complex plate boundaries of Sundaland. Supplementary material: AFT data, radial plots and the detailed procedure of thermal history modelling are available at https://doi.org/10.6084/m9.figshare.c.4633064

The Magma Engine and the driving force of plate tectonics

The true driving force behind drifts in plate tectonics is still a topic open for discussion. Currently, slab pull is taken as the dominant driving force. From the energy perspective, heat from the Earth’s interior is the main source of power maintaining plate tectonics. The organized release of heat is the key to transforming static energy into kinetic energy. Here we propose a Magma Engine model. According to this model, heat is changed to potential energy through magmatism, and this consequently drives movements in plate tectonics through gravity. New oceanic crust is formed continuously at the mid-Ocean ridges, and then cools down gradually. The newly formed oceanic crust is lighter and thinner than the older ones, resulting in tilted plates sitting on the asthenosphere mantle. Given that the mantle has a high viscosity of 1019−1021 in the asthenosphere, and even higher values further down, the oceanic plate would lie on a big slope, with a height difference of around 80 kilometers. The highest plate sliding forces reach 1.4 ´ 1014 N/m, which is an order of magnitude larger than the estimated value of slab pull. It is therefore the primary driving force of plate tectonics. Mantle plume is another type of Magma Engine. Large plume heads may elevate the overlying continental crust in kilometer scales, due to high temperatures and the bouncy of large amounts of plume magma. Such uplifting may lead to the overlying plate sliding away from the center of the plume. This may initiate plate subduction along weaker belts in case the plume is big enough and the overlying plate is strong enough. This is likely the main mechanism that initiated plate tectonics in the early history of the Earth. Under the plate tectonic system, the plume head plays a major role in the opening of new ocean basins, acting as an “igniter” of mid-ocean Magma Engines. There are two types of subduction initiations, spontaneous and induced (or forced). Spontaneous subduction initiation usually occurs in old ocean basins, forming double-track subductions on either side of the ocean basin. In contrast, induced subduction initiation usually occurs in young ocean basins, forming single-track subduction. The closures of Neo-Tethys Oceans were likely associated with induced subduction initiation, which always forms northward subductions. The Magma Engine works mainly on plates that are directly connected to spreading ridges and/or mantle plumes. For others, the energy for plate movement and deformation comes from plate interactions.

The Effects of Earthquakes and Fluids on the Metamorphism of the Lower Continental Crust

AbstractRock rheology and density have first‐order effects on the lithosphere's response to plate tectonic forces at plate boundaries. Changes in these rock properties are controlled by metamorphic transformation processes that are critically dependent on the presence of fluids. At the onset of a continental collision, the lower crust is in most cases dry and strong. However, if exposed to internally produced or externally supplied fluids, the thickened crust will react and be converted into a mechanically weaker lithology by fluid‐driven metamorphic reactions. Fluid introduction is often associated with deep crustal earthquakes. Microstructural evidence, suggest that in strong highly stressed rocks, seismic slip may be initiated by brittle deformation and that wall‐rock damage caused by dynamic ruptures plays a very important role in allowing fluids to enter into contact with dry and highly reactive lower crustal rocks. The resulting metamorphism produces weaker rocks which subsequently deform by viscous creep. Volumes of weak rocks contained in a highly stressed environment of strong rocks may experience significant excursions toward higher pressure without any associated burial. Slow and highly localized creep processes in a velocity strengthening regime may produce mylonitic shear zones along faults initially characterized by earthquake‐generated frictional melting and wall rock damage. However, stress pulses from earthquakes in the shallower brittle regime may kick start new episodes of seismic slip at velocity weakening conditions. These processes indicate that the evolution of the lower crust during continental collisions is controlled by the transient interplay between brittle deformation, fluid‐rock interactions, and creep flow.

Major Strike-Slip Faults Identified Using Satellite Data in Central Borneo, SE Asia

We use freely available Google satellite data, instrumental seismicity, fault plane solutions, and previously mapped structural and geological maps to identify new fault zones in central Borneo. We have mapped a number of ~NW-SE trending dextral strike-slip faults and ~NE-SW to ~N-S trending sinistral strike-slip fault zones. The geomorphic expression of faulting is shown by the well-developed triangular facets, fault rupture scarps, truncated sedimentary beds, topographic breaks, displaced ridges, deflected streams, faulted Plio-Pleistocene volcanic deposits, and back-tilted Holocene to Recent sedimentary deposits. Some of the mapped faults are actively growing, and show text-book examples of dextral and sinistral offset, which ranges from ~450 m to tens of km. The dextral strike-slip fault systems are clearly developed in the central and eastern portions of Borneo where they cut through the folded sedimentary sequences for >220 km. The ~NE-SW to ~N-S trending sinistral strike-slip faults are dominantly developed in the eastern portion of central Borneo for >230 km. The geomorphic expression of faulting is clear and the fault scarps are ~SE facing for the sinistral fault system, and ~NE facing for the dextral fault system. The age of the faulting is constrained by the cross-cutting relationship where the fault cuts through Plio-Pleistocene volcanic deposits for >30 km, which suggests a neotectonic nature of faulting. The strike-slip fault systems that we have mapped here provide the first geomorphic evidence of large-scale strike-slip faulting in Borneo and suggest the presence of a major sinistral strike-slip fault that runs for >900 km through the center of Borneo, and forms a backbone onto which most of the mapped structures root. The mapped structures clearly suggest that plate tectonic forces dominantly control the geological structures that we have mapped and support the regional oblique convergence that is oblique with respect to the major trend of the Crocker Range, which forms the spine of the Borneo Island.

Post-ridge-subduction acceleration of the Indian plate induced by slab rollback

The driving forces of plate motion, especially that of its sudden change over time, has long been debated. During the closure of an old ocean, the subduction process of the mid-ocean ridge provides valuable clues to quantitative evaluation of the driving forces of plate tectonics. Here we show that the drifting rates of the Indian plate were correlated with a Late Cretaceous adakitic event hosting abundant adakites and adakitic charnockites in the Gangdese belt, southern Tibetan Plateau. While adakites form through slab melting, the ultra-high temperatures and dry nature of charnockites indicate major disturbance of the hot asthenosphere. Temporally, the oldest adakite corresponds to the initiation of the ridge subduction, whereas the youngest adakitic charnockite marks the onset of post-ridge-subduction slab rollback (steepening). Geodynamic modeling suggests that the initiation of the ridge subduction was facilitated by the Morondova mantle plume, corresponding to the lowest drifting rate of the Indian plate. Our analyses further show that the post-ridge-subduction slab rollback pushed the asthenospheric mantle backward, meanwhile it dramatically reduced the ridge-arc interaction force, leading to the first abrupt acceleration of the Indian plate. Slab rollback contributed ∼3.5 cm/yr but lasted for only ∼5 Ma, while slab pull, ridge push together with plume contributed ∼5 cm/yr to the acceleration of the Indian plate. Our study, therefore, provides evidence for a new type of driving forces of Indian plate acceleration during the Late Cretaceous Neotethys ridge subduction.

Subduction initiation in nature and models: A review

How new subduction zones form is an emerging field of scientific research with important implications for our understanding of lithospheric strength, the driving force of plate tectonics, and Earth's tectonic history. We are making good progress towards understanding how new subduction zones form by combining field studies to identify candidates and reconstruct their timing and magmatic evolution and undertaking numerical modeling (informed by rheological constraints) to test hypotheses. Here, we review the state of the art by combining and comparing results coming from natural observations and numerical models of SI. Two modes of subduction initiation (SI) can be identified in both nature and models, spontaneous and induced. Induced SI occurs when pre-existing plate convergence causes a new subduction zone to form whereas spontaneous SI occurs without pre-existing plate motion when large lateral density contrasts occur across profound lithospheric weaknesses of various origin. We have good natural examples of 3 modes of subduction initiation, one type by induced nucleation of a subduction zone (polarity reversal) and two types of spontaneous nucleation of a subduction zone (transform collapse and plumehead margin collapse). In contrast, two proposed types of subduction initiation are not well supported by natural observations: (induced) transference and (spontaneous) passive margin collapse. Further work is therefore needed to expand on and understand the implications of these observations. Our future advancing understanding of SI will come from better geologic insights, laboratory experiments, and numerical modeling, and with improving communications between these communities.

Anisotropic Horizontal Thermal Contraction of Young Oceanic Lithosphere Inferred From Stress Release Due To Oceanic Intraplate Earthquakes

How freely the oceanic lithosphere contracts horizontally due to thermal contraction is important information, because it reflects the boundary condition of the oceanic lithosphere, which includes information regarding the magnitude of driving/resisting forces of plate tectonics. We investigated the horizontal thermal contraction of young oceanic lithosphere using an analysis of the intraplate stress release due to oceanic intraplate earthquakes (OCEQs) and numerical simulations. The stress release due to OCEQs in young oceanic lithosphere (5–15 Ma) shows significant differences between the spreading directional component and the ridge‐parallel component. The extensional stress release of the ridge‐parallel component is 6 times as large as that of the spreading directional component, while the compressional stress release of the ridge‐parallel component is one seventh that of the spreading directional component. We conducted a numerical simulation of the thermal stress evolution of the oceanic lithosphere to investigate how the difference in the horizontal contraction rates between the spreading direction and the ridge‐parallel direction can explain the observed anisotropic stress release. The result indicates that young oceanic lithosphere (5–15 Ma) barely contracts in the ridge‐parallel direction (only 0–30% of the spreading directional contraction rate), while it contracts freely in the spreading direction due to the weakness of the oceanic ridge strength and the low‐viscosity asthenosphere. From the results, we constrained the magnitude of the basal traction working on the bottom of the oceanic lithosphere to be smaller than 0.44 MPa.

The space geodesy revolution for plate tectonics and earthquake studies

Even if plate tectonics was a truly significant unifying theory that began to give sense to a series of different geological observations, plate motions were considered scattered. The advent of space geodesy confirmed and greatly refined the models of plate kinematics, allowing also to study the motion of the lithosphere with respect to the inner layers of the Earth. Switching from the no net rotation to the hotspot reference frames, plate motion assumes coherence with a mean global westward drift of the lithosphere with respect to the mantle. However, the driving forces of plate tectonics are still under investigation and space geodesy may provide fundamental tools to develop comprehensive models which can take into account the contributes of mantle density gradients and astronomical forces, as Earth’s rotation and tides. This paper is dedicated to Prof. Michele Caputo who was a pioneer in measuring the misalignment of the tidal bulge with respect to the Earth–Moon gravitational alignment. The misplaced mass in excess may account for the westerly directed torque of the lithosphere relative to the mantle. Local GPS networks and satellite observations are providing new insights on plate boundary tectonics and allow unravelling the evolution of the interplay between the shallow brittle upper crust and the underlying visco-plastic lower crust, which is deforming in a steady state regime without releasing relevant seismic waves. Along active tectonic areas, zones marked by low strain rates are suitable to store larger energy, subsequently dissipated during the coseismic stage. GPS and InSAR observations have widely increased the capability to monitor the spatial and temporal variations of deformation. The coseismic deformation pattern suggests a different mechanism of energy store and release, mainly gravitational in extensional tectonic settings and essentially elastic in strike-slip and contractional tectonic settings. The increasing details provided by future dense and low-cost geodetic networks will allow to detect reliable deformation transients and new insight on seismic precursors.