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This article, titled "Spatio-temporal dynamic evolution characteristics and driving mechanism analysis of urban pseudo human settlements," explores the impact of pseudo human settlements on urban production, life, and ecology in China. The study analyzes the spatial and temporal evolution of pseudo human settlements in 14 cities in Liaoning Province from 2011 to 2020. The results show that the overall quality of pseudo human settlements has been on the rise, with a concentration of high-quality development centers and a pattern of high in the central part and low in the surrounding area. The study also identifies the emerging driving forces and leading factors behind the development of pseudo human settlements. This research aims to contribute to the field of human settlements geography and provide insights for the construction of human settlements in the future. [Extracted from the article]

The formation and preservation of cratons--the oldest parts of the continents, comprising over 60 per cent of the continental landmass--remains an enduring problem. Key to craton development is how and when the thick strong mantle roots that underlie these regions formed and evolved. Peridotite melting residues forming cratonic lithospheric roots mostly originated via relatively low-pressure melting and were subsequently transported to greater depth by thickening produced by lateral accretion and compression. The longest-lived cratons were assembled during Mesoarchean and Palaeoproterozoic times, creating the stable mantle roots 150 to 250 kilometres thick that are critical to preserving Earth's early continents and central to defining the cratons, although we extend the definition of cratons to include extensive regions of long-stable Mesoproterozoic crust also underpinned by thick lithospheric roots. The production of widespread thick and strong lithosphere via the process of orogenic thickening, possibly in several cycles, was fundamental to the eventual emergence of extensive continental landmasses--the cratons. Cratons are the oldest parts of the Earth's continents; this Review concludes that the production of widespread, thick and strong lithosphere via the process of orogenic thickening was fundamental to the eventual emergence of extensive continental landmasses., Author(s): D. Graham Pearson [sup.1] , James M. Scott [sup.2] , Jingao Liu [sup.3] , Andrew Schaeffer [sup.4] , Lawrence Hongliang Wang [sup.5] , Jeroen van Hunen [sup.6] , Kristoffer [...]

Slab width is a significant factor in controlling subduction zone dynamics, particularly the retreat velocities, which tend to decrease with wider slabs. However, observations of natural narrow subduction zones reveal no correlation between slab width and trench velocities. This suggests that other factors may exert a greater influence. In this study, we employ 3D numerical subduction models to systematically assess the impact of slab width, strength of slab coupling to the lateral plate (LP), and overriding plate (OP) thickness on trench kinematics and geometry. Our models focus on narrow slabs (400–1,200 km), and the results demonstrate that, in the case of narrow subduction zones, the slab width has little effect on trench migration rates and the viscous coupling at the lateral slab edge is only important for very narrow subduction zones (≤800 km). Conversely, the OP thickness emerges as a crucial factor, with increasing plate thickness leading to a strong decrease in trench velocities. These findings provide an explanation for the observed trench velocities in natural narrow subduction zones, where an inverse relationship with OP thickness is evident. Furthermore, our study reveals that not only slab width, but also the OP thickness and the slab coupling to the LP, significantly influence trench geometry. Strong lateral coupling promotes the formation of concave trench geometries, while thick overriding plates favor the development of "w"‐shaped geometries. Overall, a comprehensive understanding of subduction processes necessitates considering the interplay between slab width, OP thickness, and slab coupling to the LP. Plain Language Summary: Subduction zones are the main drivers of plate tectonics and control much of the seismic and volcanic activity on Earth. For that reason, subduction processes have been widely studied in the last few decades. Because of the limited amount of available data, one of the key techniques has been numerical modeling. Some earlier models have shown that the velocity of the trench (long region marking where subduction starts) is affected by the width of the subduction zone, but this is not observed for narrow subduction zones in nature. In this work, we model 3D narrow subduction systems and find that the thickness of the unsubducted overriding plate (OP) affects trench velocities much more than the width of the subduction zone: the thicker the plate, the slower the trench motion. Moreover, the thickness of the OP affects the shape that the trench develops during progressive subduction. Our models explain some key observations in Earth's narrow subduction zones and help to better understand subduction processes. Key Points: Three‐dimensional numerical models of narrow subduction zones including all plates are performedThe effect of slab width, overriding plate (OP) thickness and coupling of the slab with the lateral plate on trench kinematics is studiedOP thickness is the main factor affecting trench retreat velocities in narrow subduction zones [ABSTRACT FROM AUTHOR]

Many rifts are influenced by pre-existing structures and heterogeneities during their evolution, a process known as structural inheritance. During rift evolution, these heterogeneities may aid rift nucleation, growth, and segmentation of faults, encourage linkage of various segments, or even inhibit the formation of faults. Understanding how structural inheritance influences early rift evolution could be vital for evaluating seismic risk in tectonically active areas. The Shanxi Rift in the North of China is an active rift system believed to have formed along the trend of the Proterozoic Trans North China Orogen. However, the influence of these pre-existing structures on the present-day rift architecture is poorly known. Here we use tectonic geomorphological techniques, e.g., hypsometric integral (HI), channel steepness (ksn) and local relief to identify the impact of structural inheritance on the formation of the Shanxi Rift. We found that HI was less sensitive to lithology and may be more valuable in evaluating the tectonic signal. Based on their geomorphic expression we characterise the activity levels of active faults and found that activity is concentrated in the Rift Interaction Zones (RIZs) that formed in between the sub-basins. Furthermore, we found that many faults formed parallel to inherited structures. Based on these observations we propose a new model for the evolution of the Shanxi Rift where inherited structures play an important part in segmenting the rift. Geomorphic indices might prove useful in the study of the evolution of structural inheritance in other active rifts, such as the East African Rift. [ABSTRACT FROM AUTHOR]

The Cryogenian period (720–635 million years ago) in the Neoproterozoic era featured two phases of global or near-global ice cover termed "snowball Earth". Climate models of all kinds indicate that the inception of these phases must have occurred in the course of a self-amplifying ice–albedo feedback that forced the climate from a partially ice-covered to a snowball state within a few years or decades. The maximum concentration of atmospheric carbon dioxide (CO2) allowing such a drastic shift depends on the choice of model, the boundary conditions prescribed in the model, and the amount of climatic variability. Many previous studies reported values or ranges for this CO2 threshold but typically tested only a very few different boundary conditions or excluded variability due to volcanism. Here we present a comprehensive sensitivity study determining the CO2 thresholds in different scenarios for the Cryogenian continental configuration, orbital geometry, and short-term volcanic cooling effects in a consistent model framework using the climate model of intermediate complexity CLIMBER-3 α. The continental configurations comprise two palaeogeographic reconstructions for each of both snowball-Earth onsets as well as two idealised configurations with either uniformly dispersed continents or a single polar supercontinent. Orbital geometries are sampled as multiple different combinations of the parameters obliquity, eccentricity, and argument of perihelion. For volcanic eruptions, we differentiate between single globally homogeneous perturbations, single zonally resolved perturbations, and random sequences of globally homogeneous perturbations with realistic statistics. The CO2 threshold lies between 10 and 250 ppm for all simulations. While the thresholds for the idealised continental configurations differ by a factor of up to 19, the CO2 thresholds for the continental reconstructions differ by only 6 %–44 % relative to the lower thresholds. Changes in orbital geometry account for variations in the CO2 threshold of up to 30 % relative to the lowest threshold. The effects of volcanic perturbations largely depend on the orbital geometry and the corresponding structure of coexisting stable states. A very large peak reduction in net solar radiation of 20 or 30 W m -2 can shift the CO2 threshold by the same order of magnitude as or less than the orbital geometry. Exceptionally large eruptions of up to -40 W m -2 shift the threshold by up to 40 % for one orbital configuration. Eruptions near the Equator tend to, but do not always, cause larger shifts than eruptions at high latitudes. The effects of realistic eruption sequences are mostly determined by their largest events. In the presence of particularly intense small-magnitude volcanism, this effect can go beyond the ranges expected from single eruptions. [ABSTRACT FROM AUTHOR]

The lateral distribution of strength within the crust is non-uniform, dictated by crustal lithology and the presence and distribution of heterogeneities within it. During continental extension, areas of crust with distinct lithological and rheological properties manifest strain differently, influencing the structural style, geometry, and evolution of the developing rift system. Here, we use 3D thermo-mechanical models of continental extension to explore how pre-rift upper-crustal strength variations influence rift physiography. We model a 500×500×100 km volume containing 125 km wide domains of mechanically "strong" and "weak" upper crust along with two reference domains, based upon geological observations of the Great South Basin, New Zealand, where extension occurs parallel to the boundaries between distinct geological terranes. Crustal strength is represented by varying the initial strength of 5 km 3 blocks. Extension is oriented parallel to the domain boundaries such that each domain is subject to the same 5 mm yr -1 extension rate. Our modelling results show that strain initially localises in the weak domain, with faults initially following the distribution of initial plastic strain before reorganising to produce a well-established network, all occurring in the initial 100 kyr. In contrast, little to no localisation occurs in the strong domain, which is characterised by uniform strain. We find that although faults in the weak domain are initially inhibited at the terrane boundaries, they eventually propagate through and "seed" faults in the relatively strong adjacent domains. We show characteristic structural styles associated with strong and weak crust and relate our observations to rift systems developed across laterally heterogeneous crust worldwide, such as the Great South Basin, New Zealand, and the Tanganyika Rift, East Africa. [ABSTRACT FROM AUTHOR]

This document is an editorial published in the journal Frontiers in Earth Science. It discusses the challenges in seismology related to understanding volcanic islands and their magmatic unrest. Volcanic islands are complex geological features that pose significant geohazards, including volcanic eruptions, earthquakes, landslides, and tsunamis. The editorial emphasizes the importance of seismic monitoring for detecting magmatic movements, issuing pre-eruptive warnings, and mitigating hazards and risks. The document also highlights the need to enhance seismic networks and generate 3D models to improve our understanding of the geodynamic processes driving magma ascent and volcanism. The editorial features four original research articles that explore dynamic subsurface changes, volcano-monitoring methodologies, and complex mantle structures beneath volcanic islands. These studies contribute to the ongoing efforts to understand and manage volcanic hazards and improve forecasting capabilities. [Extracted from the article]

The lateral distribution of strength within the crust is non-uniform, dictated by crustal lithology and the presence and distribution of heterogeneities within it. During continental extension, areas of crust with distinct lithological and rheological properties manifest strain differently, influencing the structural style, geometry and evolution of the developing rift system. Here, we use 3D thermo-mechanical models of continental extension to explore how pre-rift upper crustal strength variations influence rift physiography. We model a 500x500x100 km volume containing 125 km wide domains of mechanically 'Strong' and 'Weak' upper crust along with two reference domains, based upon geological observations of the Great South Basin, New Zealand, where extension occurs perpendicular to distinct geological terranes and parallel to terrane boundaries. Crustal strength is represented by varying the initial strength of 5 km³ blocks. Extension is oriented parallel to the domain boundaries such that each domain is subject to the same 5 mm/yr extension rate. Our modelling results show that strain initially localises in the Weak domain, with faults initially following the distribution of Initial Plastic Strain before reorganising to produce a well-established network, all occurring in the initial 100ky timestep. In contrast, little to no localisation occurs in the Strong domain, which is characterised by uniform strain. We find that although faults in the Weak domain are initially inhibited at the terrane boundaries, they eventually propagate through and 'seed' faults in the relatively stronger adjacent domains. We show characteristic structural styles associated with 'strong' and 'weak' crust and relate our observations to rift systems developed across laterally heterogeneous crust worldwide, such as the Great South Basin, NZ, and the Tanganyika rift, East Africa. [ABSTRACT FROM AUTHOR]

Continental collisions commonly involve highly curved passive plate margins, leading to diachronous continental subduction during trench rollback. Such systems may feature backarc extension and ophiolite obduction postdating initial collision. Modern examples include the Alboran and Banda arcs. Ancient systems include the Newfoundland and Norwegian Caledonides. While external forces or preexisting weaknesses are often invoked, we suggest that ophiolite obduction can equally be caused by internal stress buildup during collision. Here, we modeled collision with an irregular subducting continental margin in three-dimensional (3-D) thermo-mechanical models and used the generated stress field evolution to understand resulting geologic processes. Results show how tensional stresses are localized in the overriding plate during the diachronous onset of collision. These stresses thin the overriding plate and may open a back-arc spreading center. Collision along the entire trench follows rapidly, with inversion of this spreading center, ophiolite obduction, and compression in the overriding plate. The models show how subduction of an irregular continental margin can form a highly curved orogenic belt. With this mechanism, obduction of back-arc oceanic lithosphere naturally evolves from a given initial margin geometry during continental collision. [ABSTRACT FROM AUTHOR]

Pull-apart basins are structural features linked to the interactions between strike-slip and extensional tectonics. Their morphology and structural evolution are determined by factors such as extension rate, the basin length/width ratio, and changes in extension direction. In this work, we investigate the effect of a change in the plate motion direction on a pull-apart basin's structure, using analogue modelling experiments with a two-layer ductile-brittle configuration to simulate continental crust rheology. We initially impose orthogonal extension on an interconnected rift and strike-slip system to drive pull-apart development. Subsequently, we rotate the relative motion vector, imposing transtensional deformation and continuing with this new relative motion vector to the end of the experiment. To compare with natural examples, we analyse the model using seismic interpretation software, generating 3D fault structure and sedimentary thickness interpretations. Results show that the change in the direction of plate motion produces map-view sigmoidal oblique slip faults that become normal-slip when deformation adjusts to the new plate motion vector. Furthermore, sediment distribution is strongly influenced by the relative plate rotation, changing the locus of deposition inside the basin at each model stage. Finally, we compare our observations to seismic reflection images, sedimentary package thicknesses and fault interpretations from the Northern Gulf of California and find good agreement between model and nature. Similar fault arrays occur in the Bohai Basin in northern China, which suggests a rotational component in its evolution. More broadly, such similar structures could indicate a role for oblique extension and fault rotation in any pull-apart basin. [ABSTRACT FROM AUTHOR]

We present a high‐resolution 2‐D P‐wave velocity model from a 225‐km‐long active seismic profile, collected over ~60–75 Ma central Atlantic crust. The profile crosses five ridge segments separated by a transform and three nontransform offsets. All ridge discontinuities share similar primary characteristics, independent of the offset. We identify two types of crustal segment. The first displays a classic two‐layer velocity structure with a high gradient Layer 2 (~0.9 s−1) above a lower gradient Layer 3 (0.2 s−1). Here, PmP coincides with the 7.5 km s−1 contour, and velocity increases to >7.8 km s−1 within 1 km below. We interpret these segments as magmatically robust, with PmP representing a petrological boundary between crust and mantle. The second has a reduced contrast in velocity gradient between the upper and lower crust and PmP shallower than the 7.5 km s−1 contour. We interpret these segments as tectonically dominated, with PmP representing a serpentinized (alteration) front. While velocity‐depth profiles fit within previous envelopes for slow‐spreading crust, our results suggest that such generalizations give a misleading impression of uniformity. We estimate that the two crustal styles are present in equal proportions on the floor of the Atlantic. Within two tectonically dominated segments, we make the first wide‐angle seismic identifications of buried oceanic core complexes in mature (>20 Ma) Atlantic Ocean crust. They have a ~20‐km‐wide "domal" morphology with shallow basement and increased upper crustal velocities. We interpret their midcrustal seismic velocity inversions as alteration and rock‐type assemblage contrasts across crustal‐scale detachment faults. Key Points: We identify that magmatic and tectonic modes of slow Atlantic crustal accretion are distinct and equal, based on seismic characteristicsThe structure of crustal discontinuities is controlled by the accretion mode of neighboring segments, independent of ridge offset and orderTwo buried oceanic core complexes in mature Atlantic Ocean crust are identified, based on their dimensions and seismic structure [ABSTRACT FROM AUTHOR]

Estimating the location of geologic and tectonic features on a subducting plate is important for interpreting their spatial relationships with other observables including seismicity, seismic velocity and attenuation anomalies, and the location of ore deposits and arc volcanism in the over-riding plate. Here we present two methods for estimating the location of predictable features such as seamounts, ridges and fracture zones on the slab. One uses kinematic reconstructions of plate motions, and the other uses multidimensional scaling to flatten the slab onto the surface of the Earth. We demonstrate the methods using synthetic examples and also using the test case of fracture zones entering the Lesser Antilles subduction zone. The two methods produce results that are in good agreement with each other in both the synthetic and real examples. In the Lesser Antilles, the subducted fracture zones trend northwards of the surface projections. The two methods begin to diverge in regions where the multidimensional scaling method has its greatest likely error. Wider application of these methods may help to establish spatial correlations globally. [ABSTRACT FROM AUTHOR]

In the present-day Earth, some subducting plates (slabs) are flattening above the upper–lower mantle boundary at ~670 km depth, whereas others go through, indicating a mode between layered and whole-mantle convection. Previous models predicted that in a few hundred degree hotter early Earth, convection was likely more layered due to dominant slab stagnation. In self-consistent numerical models where slabs have a plate-like rheology, strong slabs and mobile plate boundaries favour stagnation for old and penetration for young slabs, as observed today. Here we show that such models predict slabs would have penetrated into the lower mantle more easily in a hotter Earth, when a weaker asthenosphere and decreased plate density and strength resulted in subduction almost without trench retreat. Thus, heat and material transport in the Earth’s mantle was more (rather than less) efficient in the past, which better matches the thermal evolution of the Earth. [ABSTRACT FROM AUTHOR]

Earth has likely cooled by several hundred degrees over its history, which has probably affected subduction dynamics and associated magmatism. Today, the process of compositional buoyancy driven upwelling, and subsequent underplating, of subducted materials (commonly referred to as “relamination”) is thought to play a role in the formation of continental crust. Given that Archean continental crust formation is best explained by the involvement of mafic material, we investigate the feasibility of mafic crust relamination under a wide range of conditions applicable to modern and early Earth subduction zones, to assess if such a process might have been viable in an early Earth setting. Our numerical parametric study illustrates that the hotter, thicker-crust conditions of the early Earth favour the upward relamination of mafic subducting crust. The amount of relaminating subducting crust is observed to vary significantly, with subduction convergence rate having the strongest control on the volume of relaminated material. Indeed, removal of the entire mafic crust from the subducting slab is possible for slow subduction (∼2 cm/yr) under Archean conditions. We also observe great variability in the depth at which this separation occurs (80–120 km), with events corresponding to shallower detachment being more voluminous, and that relaminating material has to remain metastably buoyant until this separation depth, which is supported by geological, geophysical and geodynamical observations. Furthermore, this relamination behaviour is commonly episodic with a typical repeat time of approximately 10 Myrs, similar to timescales of episodicity observed in the Archean rock record. We demonstrate that this relamination process can result in the heating of considerable quantities of mafic material (to temperatures in excess of 900 °C), which is then emplaced below the over-riding lithosphere. As such, our results have implications for Archean subduction zone magmatism, for continental crust formation in the early Earth, and provide a novel explanation for the secular evolution of continental crust. [ABSTRACT FROM AUTHOR]

Abstract: Survival of thick cratonic roots in a vigorously convecting mantle system for billions of years has long been studied by the geodynamical community. High strength of the cratonic root is generally considered to be the most important factor, but the role of lithospheric mantle depletion and dehydration in this strengthening is still debated. Geodynamical models often argue for a significant strength or buoyancy contrast between cratonic and non-cratonic mantle lithosphere, induced by mantle depletion and dehydration. But recent laboratory experiments argue for only a modest effect of dehydration strengthening. Can we reconcile laboratory experiments and geodynamical models? We perform and discuss new numerical models to investigate craton stability and longevity with different composition-dependent rheology and buoyancy. Our results show that highly viscous and possibly buoyant cratonic root is essential to stabilise a geometry in which thick cratonic lithosphere and thinner non-cratonic lithosphere coexist for billions of years. Using non-Newtonian rheology, a modest strengthening factor of can protect compositionally buoyant cratonic roots from erosion by mantle convection for over billions of years. A larger strengthening factor ( ) can maintain long term craton stability even with little or no intrinsic buoyancy. Such composition-dependent rheology is comparable to the laboratory experiments. This implies that a strict isopycnic state of cratonic lithosphere may not be necessary for the preservation of a cratonic root, provided a sufficient level of compositional strengthening is present. [Copyright &y& Elsevier]

The Canary Islands hotspot consists of seven volcanic islands, mainly of Neogene age, rooted on oceanic Jurassic lithosphere. Its complex structure and geodynamic setting have led to different hypotheses about its origin and evolution, which is still a matter of a vivid debate. In addition to the classic mantle plume hypothesis, a mechanism of small-scale mantle convection at the edge of cratons (Edge Driven Convection, EDC) has been proposed due to the close proximity of the archipelago to the NW edge of the NW African Craton. A combination of mantle plume upwelling and EDC has also been hypothesized. In this study we evaluate these hypotheses quantitatively by means of numerical two-dimensional thermo-mechanical models. We find that models assuming only EDC require sharp edges of the craton and predict too narrow areas of partial melting. Models where the ascent of an upper-mantle plume is forced result in an asymmetric mantle flow pattern due to the interplay between the plume and the strongly heterogeneous lithosphere. The resulting thermal anomaly in the asthenosphere migrates laterally, in agreement with the overall westward decrease of the age of the islands. We suggest that laterally moving plumes related to strong lithospheric heterogeneities could explain the observed discrepancies between geochronologically estimated hotspot rates and plate velocities for many hotspots. • Model edge-driven convection with and without an upper mantle plume. • Edge-driven convection inconsistent with magmatism and thermal anomaly distribution. • Models account for the interplay between plume upwelling and heterogeneous lithosphere. • Combined models are consistent with the westward migration of the age of the islands. • Lithospheric edges cause discrepancies between hotspots rates and plate velocities. [ABSTRACT FROM AUTHOR]

Terra Nova, 25, 130-136, 2013 Abstract High-Al2O3 tonalite, trondhjemite and granodiorite (TTG) magmas characterise felsic Archaean crust, yet are uncommon in the post-Archaean rock record. Consequently, understanding the petrogenesis of these rocks provides valuable insights into early Earth processes. Fluid-absent slab melting represents the dominant hypothesis for the origin of these rocks; however, the absence of voluminous magmas of intermediate composition formed concurrently with these TTGs is incompatible with expectations of slab water loss prior to slab melting. This study demonstrates that for reasonable Archaean mantle temperatures, slab-derived water is captured by an anatectic zone near the slab surface, which melts via reactions that consume quartz, clinopyroxene and water to produce high-Al2O3 Archaean trondhjemite. Late in the Archaean, the mantle cooled sufficiently to prevent wet melting of the slab, allowing slab water to migrate into the wedge and produce intermediate composition magmatism, which has since been associated with subduction zones. [ABSTRACT FROM AUTHOR]

Comparison of modeling results with observed subsidence patterns from the West Siberian Basin provides new insight into the origin of the Siberian Traps, and constrains the temperature, size, and depth of an impacting mantle plume head during and after the eruption of the Siberian Traps at the Permian-Triassic boundary (250 Ma). We compare subsidence patterns from one-dimensional conductive heat flow models to observed subsidence from backstripping studies of wells in the basin. This results in a best-fit scenario with a 50-km-thick initial plume head with a temperature of 1500°C situated 50 km below the surface, and an initial regional crustal thickness of 34 km, in agreement with published values. Backstripping and modeling results agree very well, including a 60-90 m.y. delay between the rifting phase and the first regional sedimentation. Regional subsidence patterns indicate that the plume head was present across a minimum area of ~2.5 x 106 km². These results re-emphasize the viability of a mantle plume origin for the Siberian Traps, provide important constraints on the dynamics of mantle plume heads, and suggest a thermal control for the subsidence of the West Siberian Basin. [ABSTRACT FROM AUTHOR]

Subduction drives plate tectonics and builds continental crust, and as such is one of the most important processes for shaping the present-day Earth. Here we review both theory and observations for the viability and style of Archean subduction. High Archean mantle temperature gave low mantle viscosity and affected plate strength and plate buoyancy. This resulted in slower or intermittent subduction, either of which resulted in Earth cooling profiles that fit available data. Some geological observations are interpreted as subduction related, including an 'arc' signature in various igneous rocks (suggesting burial of surface material to depths of 50-100 km), structural thrust belts and dipping seismic reflectors, and high-pressure-low-temperature and low-pressure-high-temperature paired metamorphic belts. Combined geodynamical and geochemical evidence suggests that subduction operated in the Archean, although not, as often assumed, as shallow flat subduction. Instead, subduction was more episodic in nature, with more intermittent plate motion than in the Phanerozoic. [ABSTRACT FROM AUTHOR]

By combining geochemical data and geodynamical models, evidence is provided to address the existence and style of Archaean plate tectonics, a topic of vigorous debate for decades. Using careful analyses of lithostratigraphic Archaean assemblages and numerical model results, we illustrate that a short-term episodic style of subduction was a viable style of tectonics in the early Earth. Modeling results show how, due to the low strength of slabs in a hotter Earth, frequent slab break-off events prevented a modern-style long-lived subduction system, and resulted in frequent cessation and re-initiation of the subduction process on a typical time scale of a few million years. Results fit with geochemical observations that suggest frequent alternation of arc-style and non-arc-style volcanism on a similarly short time scale. Such tectonics could provide the link between early pre-plate tectonic style of tectonics (or stagnant-lid convection) and modern-style plate tectonics, in which short-term episodes of proto-subduction evolved over time into a longer-term, more successful style of plate tectonics as mantle temperature decayed. [ABSTRACT FROM AUTHOR]

Abstract: Conditions and dynamics of subduction–collision and subsequent 3-D slab break-off and slab tear propagation are quantified, for the first time, using fully dynamic numerical models. Model results indicate that collision after the subduction of old, strong subducting oceanic slab leads to slab break-off at 20–25Myr after the onset of continental collision, and subsequently a slab tear migrates more or less horizontally through the slab with a propagation speed of 100–150mm/yr. In contrast, young, weak oceanic slabs show the first break-off already 10Myr after continental collision, and can experience tear migration rates up to 800mm/yr. Slab strength plays a more important role in the timing of slab break-off and the speed of a propagating slab tear than (negative) slab buoyancy does. Slab break-off is viable even for slabs that are supported by the viscosity jump and phase change between the upper and lower mantle. The density of the oceanic slab and the subducting continental block is important for the amount of continental subduction and the depth of slab break-off. A 40-km thick continental crust can be buried to depths greater than 200km, although this maximum depth is significantly less for younger or very weak slabs, or thicker continental crust. Slab break-off typically starts at a depth of 300km, mostly independent of mantle rheology, but, like continental crustal burial, can be shallower for young or buoyant plates. Our 3-D models illustrate how, due to the difference in necking in 2-D and 3-D, break-off has an intrinsic small preference to start as a slab window within the slab''s interior, rather than as a slab tear at the slab edge. However, any significant asymmetry in the collision setting, e.g. earlier collision at one end of the subduction zone, would override this, and leads to slab tearing starting near one edge of the slab. These results put important new constraints on the dynamics of the collision and subsequent slab break-off for modern collision belts. For a proposed timing of the initial Arabia–Eurasia collision at 35Ma, break-off of the ~200-Myr-old Neo-Tethys slab is unlikely to have occurred before 15–10Ma. Furthermore, our results illustrate that shallow, early break-off of weak slabs provides a viable explanation for the absence of blueschists and ultra-high pressure metamorphism in the Precambrian geological record. [Copyright &y& Elsevier]

Abstract: Amplitudes of azimuthal seismic anisotropy below the Pacific plate are observed to decrease with lithospheric age, whereas an increase is expected due to ongoing shearing between the plate and the underlying mantle. We illustrate that the convective disturbance of this laminar flow field by sub-lithospheric small-scale convection (SSC) provides an explanation for this anisotropy reduction. By combining numerical flow models with forward seismic anisotropy calculations we show that small-scale convection can disturb the anisotropy pattern significantly. When such disturbed signal is smoothed over >~500 km length scale (the approximate present-day seismic resolution for azimuthal anisotropy within the Pacific lithosphere and upper mantle), our results show a significant reduction in anisotropy amplitudes of about a factor two, similar to observed reductions. Perturbations of the fast polarization direction, however, remain relatively small (<~20°). This suggests that, despite the influence of any SSC, observed long-wavelength LPO orientations still represent the large-scale shearing between the lithosphere and the underlying mantle. For higher resolution LPO data, SSC can significantly complicate the interpretation of sub-lithospheric mantle flow from azimuthal seismic anisotropy data. [Copyright &y& Elsevier]

Abstract: The mechanical behavior of slabs in the mantle transition zone is a poorly understood phenomenon. To obtain a better understanding of the stress and deformation pattern inside subducting slabs, we performed 2D numerical simulations of the subduction process in a purely viscous, non-linear rheological model including diffusion creep, dislocation creep and a stress limiting mechanism. The model calculations are used to investigate the effects of yield stress, plate age and surface boundary condition (free-slip versus given plate velocity) on the stress development in and around a subducting slab and on the deformation of the subducting plate in the transition zone. The deformation and morphology of subducted slabs may differ between free-slip and kinematic boundary condition models in some cases, especially for young and relatively weak slabs. Even though kinematically driven subduction morphology may look realistic in most cases, our results show that stress distributions can differ significantly if the discrepancy between free-slip and kinematic subduction velocity is large, and in some model cases even influence the ability of the plate to penetrate into the lower mantle. Stress distributions such as the ones presented in this study can be a valuable tool in understanding the complex and enigmatic interaction between subducting slabs and the transition zone. [Copyright &y& Elsevier]

Abstract: Laboratory studies on deformation of olivine in response to applied stress suggest two distinct deformation mechanisms in the earth''s upper mantle: diffusion creep through diffusion of atoms along grain boundaries and dislocation creep by slipping along crystallographic glide planes. Each mechanism has very different and important consequences on the dynamical evolution of the mantle and the development of mantle fabric. Due to the lack of in situ observations, it is unclear which deformation mechanism dominates in the upper mantle, although observed seismic anisotropy in the upper mantle suggests the presence of dislocation creep. We examined the thermo-mechanical erosion of the lithosphere by thermal boundary layer instabilities in 3-D dynamical models. This study demonstrates that the seismically derived thermal structure of the Pacific lithosphere and upper mantle imposes an important constraint on the upper mantle deformation mechanism. The predominant deformation mechanism in the upper mantle is dislocation creep, consistent with observed seismic anisotropy. The acceptable activation energy range of 360–540 kJ/mol is consistent with, although at the lower end of, those determined from laboratory studies. [Copyright &y& Elsevier]

Shallow flat subduction is a relatively common feature at present-day subduction zones. Several mechanisms to explain this feature have been proposed, and can be subdivided into three groups: overthrusting of the subducting plate, subduction of a plume-generated oceanic plateau, and slab suction forces. We developed a numerical model to investigate these mechanisms and tested it through a comparison of the model results with the observations of the Peru flat slab where all three mechanisms seem to be contributing. The ratio of contributions of overthrusting continent to plateau subduction is in the range of 1:1 to 1:2, and the role of plate suction forces is likely to be significant. By applying the overthrusting continent and plateau subduction mechanisms separately, we were able to determine the most important model parameters for each of the mechanisms. Overthrusting easily results in flat subduction, and the flat slab length is primarily a function of slab age, overriding plate motion and mantle viscosity. An oceanic plateau is much less likely to cause flat subduction, and favorable conditions for flat subduction include a young slab age, long-lived plateau buoyancy after subduction, a strong mantle, and addition of slab suction forces that are large enough to further reduce the subduction dip angle, once the plateau initiates this flattening. Furthermore, we found that even though today flat subduction can be explained with the dominant model parameters within a reasonable range, for a slightly hotter, younger Earth, these flat subduction conditions are much less favorable, and so this style of subduction was probably not present in the past. This contradicts earlier predictions that flat subduction was a more wide-spread phenomenon in the early stages of plate tectonics in a younger earth. [Copyright &y& Elsevier]