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The text is a summary of the GSA Connects 2023: Annual Convention of the Geological Society of America, held in Pittsburgh, USA. The convention brought together 4,463 Earth scientists from 45 countries, including professionals, early career professionals, and students. The event featured paper and poster presentations, technical sessions, invited talks, and exhibits showcasing instruments and capabilities in the field of Earth science. The convention focused on the theme of "Diverse Science for a Sustainable Future and Climate and Energy Transition." Additionally, the convention facilitated collaborations and partnerships between the Geological Society of America and other geological societies, including the Geological Society of India and the Geological Society of Nigeria. [Extracted from the article]

Earth's lithosphere is made of rheologically heterogeneous elements of a wide range of characteristic lengths. A micromechanics-based self-consistent MultiOrder Power-Law Approach is presented to account for lithospheric deformations and the accompanying multiscale fabric development. The approach is principally based on the extended Eshelby theory for the motion of a power-law viscous ellipsoid in a power-law viscous matrix and the idea of embedding inhomogeneities within inhomogeneities. The extended theory provides a general means for investigating deformation partitioning in heterogeneous rocks. The “inhomogeneities within inhomogeneities” method allows multi-hierarchical levels of flow field partitioning and hence multiscale deformations to be investigated. Partitioned flow fields are used to investigate fabric development. Being based fully on micromechanics, the approach generates model predictions of both kinematic quantities (strain, strain rates, and vorticity) and stress histories. The former can be directly compared with field and laboratory structural observations while the latter can help to understand the physics of natural deformations. The self-consistent and multiscale approach is applied to a natural example of the Cascade Lake shear zone in the east Sierra Nevada of California. The modeling shows that the fabrics are most consistent with a steeply-dipping transpression zone with a convergence angle of 20° and a strike-slip displacement about 26 km. Further, the strength evolution of the model zone confirms that a transpression zone is a weakening system with respect to the simple shearing component and a hardening one for the pure-shearing component. This is consistent with slip partitioning in obliquely convergent plate boundaries: boundary-normal convergence tends to spread over a broad area whereas boundary-parallel shear tends to localize in major strike-slip zones. [ABSTRACT FROM AUTHOR]

Thermodynamic calculations predict mineralogy from temperature and pressure and vice versa. Such calculations assume that stress is isotropic despite the fact that differential stresses prevail in Earth, resulting from large-scale tectonics and/or differences between fluid and rock pressures in porous rocks. New calculations show that differential stress can have significant effects on thresholds for metamorphic reactions, depending on the grain-scale reaction pathways. A differential stress may, depending on the reaction pathway, have an effect equivalent to a pressure difference on the order of (assemblage volume)/(reaction volume change) x (differential stress). The multiplying factor is typically 10 or more. For example, the onset of a garnet + clinopyroxene breakdown reaction may be offset, up or down, by the equivalent of 500 MPa in pressure for a 50 MPa differential stress. The effect is equivalent to a temperature difference on the order of (assemblage volume)/(reaction entropy change) x (differential stress). For example, the onset of muscovite + quartz breakdown may be offset, up or down, by the equivalent of 130 °C for a 50 MPa differential stress. Much of Earth is under differential stress, so the new calculations invite a reappraisal of metamorphic mineralogy and microstructure, indicating that new insights into stresses and fluid pressures on Earth can be gained. [ABSTRACT FROM AUTHOR]

The western margin of Sundaland is affected by two major tectonic events, the India-Eurasia collision and subduction along the Burmese-Andaman-Sunda Arc. Geotectonic analysis of the Burmese-Andaman-Sunda Arc areas reveals that it represents an arc setting exposed both in land and sea environment. Within this broad arc setting, there are individual segments that illustrate discernible trends in terms of structural, seismic and geological characteristics. The Equatorial Region around Sumatra has some peculiar geological/geophysical characteristics that make it a tectonic hotspot. Here a relationship between the active volcanism and the development of forearc basins is established. It is also proposed that the downgoing oceanic plate is more strongly coupled to the overlying plate where it is youngest (∼ 40 Ma), has the highest temperature and is topographically most elevated with highest seismic activity. In particular, an increase in convergence rate and presence of youngest oceanic crust appear to be the main controlling factor underpinning the tectonics and surge of recent seismic activity in Sumatra. [ABSTRACT FROM AUTHOR]

Abstract: The crust and upper mantle in mainland China were relatively densely probed with wide-angle seismic profiling since 1958, and the data have provided constraints on the amalgamation and lithosphere deformation of the continent. Based on the collection and digitization of crustal P-wave velocity models along related wide-angle seismic profiles, we construct several crustal transects across major tectonic units in mainland China. In our study, we analyzed the seismic activity, and seismic energy releases during 1970 and 2010 along them. We present seismogenic layer distribution and calculate the yield stress envelopes of the lithosphere along the transects, yielding a better understanding of the lithosphere rheology strength beneath mainland China. Our results demonstrate that the crustal thicknesses of different tectonic provinces are distinctively different in mainland China. The average crustal thickness is greater than 65km beneath the Tibetan Plateau, about 35km beneath South China, and about 36–38km beneath North China and Northeastern China. For the basins, the thickness is ~55km beneath Qaidam, ~50km beneath Tarim, ~40km beneath Sichuan and ~35km beneath Songliao. Our study also shows that the average seismic P-wave velocity is usually slower than the global average, equivalent with a more felsic composition of crust beneath the four tectonic blocks of mainland China resulting from the complex process of lithospheric evolution during Triassic and Cenozoic continent–continent and Mesozoic ocean–continent collisions. We identify characteristically different patterns of seismic activity distribution in different tectonic blocks, with bi-, or even tri-peak distribution of seismic concentration in South Tibet, which may suggest that crustal architecture and composition exert important control role in lithosphere deformation. The calculated yield stress envelopes of lithosphere in mainland China can be divided into three groups. The results indicate that the lithosphere rheology structure can be described by jelly sandwich model in eastern China, and crème brulee models with weak and strong lower crust corresponding to lithosphere beneath the western China and Kunlun orogenic belts, respectively. The spatial distribution of lithospheric rheology structure may provide important constraints on understanding of intra- or inter-plate deformation mechanism, and more studies are needed to further understand the tectonic process(es) accompanying different lithosphere rheology structures. [Copyright &y& Elsevier]

Abstract: Supracrustal tectonics and mantle flow interact to create Earth''s topography. While tectonics is associated with the isostatic components of topography, the deflections caused by mantle dynamics, or dynamic topography, represent the non-isostatic components. South America is an ideal natural laboratory to analyze these two contrasting components from the high Andes to the distal plains. Both regions are active and affected by complex geodynamic processes like the subduction of oceanic ridges, geometry and age of slabs, etc. These subducting anomalies affect not only the convergence dynamics and stresses along the entire margin, but also the distribution of mass anomalies in the mantle, which are the main cause of sublithospheric flow and dynamic topography. Here we revisited five examples from north to south, which demonstrate that, the Andes and the distal forelands have been uncompensated since the beginning of the Cenozoic and that additional forces, such as mantle downwellings and upwellings, are required to account for the observed topographies in basins and elevations. [Copyright &y& Elsevier]

Abstract: One of the most prominent geological structures in Borborema Province, northeast Brazil, is the Transbraziliano Lineament that crosscuts most of the South American Platform and was active at least until the Devonian. This continental structure is responsible for the formation of rift and pull-apart basins in Northeastern Brazil, most of which filled with volcanic and continental sedimentary rocks (Parente et al., 2004). In the region of Sobral, Ceará State, this same continental structure controlled the intrusion of the Meruoca pluton and the formation of the Jaibaras Basin, which is bounded by strike-slip shear zones. Hydrothermal alterations seem to have been pervasive in Meruoca, as indicated by disturbances in both the Rb–Sr and U–Pb systems (Sial et al., 1981; Fetter, 1999) and by the large dispersion of anisotropic magnetic susceptibility (AMS) (Archanjo et al., 2009). In this paper, we address the origin of the hydrothermal fluids that affected the borders of the Meruoca batholith and their relationship with the activity of the Transbraziliano Lineament. These fluids were responsible for carbonate veins and Fe–Cu mineral concentrations that are commonly found associated with hydrothermally altered breccias. The carbon and oxygen isotope composition of these carbonate veins suggest that they may be related to CO2-bearing mantle-derived fluids that were channelized by the Transbraziliano Lineament. Based on oxygen isotopes, we argue that Fe–Cu concentrations may have formed in isotope equilibrium with the rhyolitic rocks at temperatures between 500 and 560 °C. This scenario points to magmatism as the main process in the formation of these rocks. We also report a K–Ar age of 530 ± 12 Ma for muscovite associated with the last ductile event that affected the Sobral-Pedro II Shear Zone and a U–Pb age of 540.8 ± 5.1 Ma for the Meruoca pluton. We further suggest that this granite is a late-kinematic intrusion that is most likely associated with the Parapuí volcanic rocks of the Jaibaras basin (535.6 ± 8.5 Ma, Garcia et al., 2010). [Copyright &y& Elsevier]

Abstract: We present possible tectonic models for two crustal domains of the Proterozoic Eastern Ghats Belt, India based on recent petrological, geophysical and geochronological data. Although both the domains presently expose deep crustal sections, they evolved in two distinct time segments of the Precambrian through accretion–collision processes. This is why we argue that no unique model can explain the complexities of the belt. The southern part of the belt evolved through subduction-dominated accretionary processes encompassing India, east Antarctica, Australia and Laurentia as part of supercontinent Columbia during the span of ca. 1.90–1.60Ga. To the contrary, the central domain witnessed a prolonged accretion–collision history initiated at ca. 1.50Ga and culminated at ca. 0.90Ga with the formation of supercontinent Rodinia. The latter united cratonic India with east Antarctica as a separate continent Enderbia that existed until about ca. 0.50Ga. The pre-1.50Ga history of this domain is ambiguous at the present state of knowledge. The northern domain has a much younger (ca. 0.90–0.50Ga) tectonothermal history which is unrelated to either of the studied domains. The present models explain the reported petrotectonic processes including the ultra high temperature metamorphism in both the domains. The episodic growth of the Eastern Ghats Belt matches with Proterozoic supercontinent cycles. [Copyright &y& Elsevier]

Abstract: Plate Tectonics requires a specific range of thermal, fluid and compositional conditions before it will operate to mobilise planetary lithospheres. The response to interior heat dispersion ranges from mobile lids in constant motion able to generate zones of subduction and spreading (Plate Tectonics), through styles of Lid Tectonics expressed by stagnant lids punctured by volcanism, to lids alternating between static and mobile. The palaeomagnetic record through Earth history provides a test for tectonic style because a mobile Earth of multiple continents is recorded by diverse apparent polar wander paths, whilst Lid Tectonics is recorded by conformity to a single position. The former is difficult to isolate without extreme selection whereas the latter is a demanding requirement and easily recognised. In the event, the Precambrian palaeomagnetic database closely conforms to this latter property over very long periods of time (~2.7–2.2Ga, 1.5–1.3Ga and 0.75–0.6Ga); intervening intervals are characterised by focussed loops compatible with episodes of true polar wander stimulated by disturbances to the planetary figure. Because of this singular property, the Precambrian palaeomagnetic record is highly effective in showing that a dominant Lid Tectonics operated throughout most of Earth history. A continental lid comprising at least 60% of the present continental area and volume had achieved quasi-integrity by 2.7Ga. Reconfiguration of mantle and continental lid at ~2.2Ga correlates with isotopic signatures and the Great Oxygenation Event and is the closest analogy in Earth history to the resurfacing of Venus. Change from Lid Tectonics to Plate Tectonics is transitional and the geological record identifies incipient development of Plate Tectonics on an orogenic scale especially after 1.1Ga, but only following break-up of the continental lid (Palaeopangaea) in Ediacaran times beginning at ~0.6Ga has it become comprehensive in the style evident during the Phanerozoic Eon (<0.54Ga). [Copyright &y& Elsevier]

Abstract: Temporal trends in Earth''s near-surface mineralogy correlate with major events in Earth''s geochemical and tectonic history. New and published analyses of 422 molybdenite (MoS2) specimens from 135 localities with known ages from 2.91 billion years (Ga) to 6.3 million years (Ma) reveal two statistically significant trends. First, systematic increases in average and maximum trace concentrations of Re in molybdenite since 3.0Ga point to enhanced oxidative weathering by subsurface fluids. In addition, episodic molybdenum mineralization correlates with five intervals of supercontinent assembly from ∼2.7Ga (Kenorland) to 300Ma (Pangaea). [Copyright &y& Elsevier]

Abstract: The Amundsen Sea Embayment (ASE), with Pine Island Bay (PIB) in the eastern embayment, is a key location to understanding tectonic processes of the Pacific margin of West Antarctica. PIB has for a long time been suggested to contain the crustal boundary between the Thurston Island block and the Marie Byrd Land block. Plate tectonic reconstructions have shown that the initial rifting and breakup of New Zealand from West Antarctica occurred between Chatham Rise and the eastern Marie Byrd Land at the ASE. Recent concepts have discussed the possibility of PIB being the site of one of the eastern branches of the West Antarctic Rift System (WARS). About 30,000km of aeromagnetic data – collected opportunistically by ship-based helicopter flights – and tracks of ship-borne magnetics were recorded over the ASE shelf during two RV Polarstern expeditions in 2006 and 2010. Grid processing, Euler deconvolution and 2D modelling were applied for the analysis of magnetic anomaly patterns, identification of structural lineaments and characterisation of magnetic source bodies. The grid clearly outlines the boundary zone between the inner shelf with outcropping basement rocks and the sedimentary basins of the middle to outer shelf. Distinct zones of anomaly patterns and lineaments can be associated with at least three tectonic phases from (1) magmatic emplacement zones of Cretaceous rifting and breakup (100–85Ma), to (2) a southern distributed plate boundary zone of the Bellingshausen Plate (80–61Ma) and (3) activities of the WARS indicated by NNE–SSW trending lineaments (55–30Ma?). The analysis and interpretation are also used for constraining the directions of some of the flow paths of past grounded ice streams across the shelf. [Copyright &y& Elsevier]

Terra Nova, 25, 57-64, 2013 Abstract Characterizing the interaction of a fault with its surroundings is vital to fully understand the tectonic processes involved and predict future behaviour. Regional and local stress orientations affect different fracture length scales, manifested by numerous associated fault, fracture and crack structures. We use seismic anisotropy to constrain the dominant orientation of aligned rupture planes of various length scales. In particular, we study shear-wave splitting of regional seismic events in Trans-Alboran Shear Zone (TASZ), south-east Spain. The TASZ consists of three major left-lateral strike-slip faults and numerous secondary strike-slip and thrust faults. The observed orientations for S-waves vary from roughly N-S in the northern segment of TASZ, to E-W in the centre, to NNW-SSE and NNE-SSW in the south. We show that the strikes of fast polarizations reflect both structural and lithological differences, indicating complex interactions of principal and secondary faults within the crust to accommodating tectonic stresses. [ABSTRACT FROM AUTHOR]

Abstract: Geoid data are more sensitive to density distributions deep within the Earth, thus the data are useful for studying the internal processes of the Earth leading to formation of geological structures. In this paper, we present much improved version of high resolution (1′×1′) geoid anomaly map of the northern Indian Ocean generated from the altimeter data obtained from Geodetic Missions of GEOSAT and ERS-1 along with ERS-2, TOPEX/POSIDEON and JASON satellites. The geoid map of the Indian Ocean is dominated by a significant low of −106m south of Sri Lanka, named as the Indian Ocean Geoid Low (IOGL), whose origin is not clearly known yet. The residual geoid data are retrieved from the geoid data by removing the long-wavelength core–mantle density effects using recent spherical harmonic coefficients of Earth Gravity Model 2008 (EGM2008) up to degree and order 50 from the observed geoid data. The coefficients are smoothly rolled off between degrees 30–70 in order to avoid artifacts related to the sharp truncation at degree 50. With this process we observed significant improvement in the residual geoid data when compared to the previous low-spatial resolution maps. The previous version was superposed by systematic broad regional highs and lows (like checker board) with amplitude up to ±12m, though the trends of geoid in general match in both versions. These methodical artifacts in the previous version may have arisen due to the use of old Rapp’s geo-potential model coefficients, as well as sharp truncation of reference model at degree and order 50. Geoid anomalies are converted to free-air gravity anomalies and validated with cross-over corrected ship-borne gravity data of the Arabian Sea and Bay of Bengal. The present satellite derived gravity data matches well with the ship-borne data with Root Mean Square Error (RMSE) of 5.1–7.8mGal, and this is found to be within the error limits when compared with other globally available satellite data. Spectral analysis of ship-borne and satellite data suggested that the satellite gravity data have a resolution down to 16–18km. Further, the geoid, residual geoid and gravity anomalies are integrated with seismic data along two profiles in the Bay of Bengal and Arabian Sea, and inferences have been made in terms of density distributions at different depths. The new residual geoid anomaly map shows excellent correlation with regional tectonic features such as Sunda subduction zone, volcanic traces (Chagos–Laccadive, Ninetyeast and 85°E ridges) and mid-ocean ridge systems (Central Indian and Carlsberg ridges). [Copyright &y& Elsevier]

Abstract: The strong interaction between the eastward flow escaping from Tibet and the rigid Sichuan Basin resulted in the rise of the Longmenshan. However, the detailed dynamics in the mantle remains controversial. In this study, the structure of the mantle transition zone (MTZ) beneath eastern Tibet and Sichuan Basin is investigated using 5080 receiver functions from 51 broadband stations. The depth of the 410km discontinuity is close to the global average, except for the Longmenshan where the 410 and 660-km discontinuities are found to be depressed by up to 10–25km and 5–10km, respectively. The observed simultaneous depressions of the 410 and 660-km discontinuities distributed along the LMS, together with proofs from tomography and regional tectonics, suggest that asthenospheric flow sinks into the MTZ, resulting in a high velocity zone, as well as variation in the MTZ thickness. The depressions are not from the traditional Clapeyron slopes or temperature variation. Also, the depression of the 410km discontinuity and the dehydration of wadsleyite are syngenetic, both of which originate from the dry mantle flow traveling across the old 410km interface. [Copyright &y& Elsevier]

Abstract: We present a 3D S-velocity model for the crust and upper mantle of the South China Sea and the surrounding regions, constrained from the analysis of over 12,000 of fundamental Rayleigh wave dispersion curves between 10s and 150s periods. The lateral resolution was found to vary from 2° to 4° with the increasing period over the study region. A robust scheme of Debayle and Sambridge allowed us to conduct the tomographic inversion efficiently for massive datasets. Group velocity maps varying with period show lateral heterogeneities, well related to the geological and tectonic features in the study region. The 3D S-velocity model was constructed from the 1D structure inversion of the tomographic group velocity dispersion curves at each node. The obtained average crustal structure is similar to the PREM model, while the average mantle velocity is typically lower than the global average. The complicated 3D structures reveal three prominent features correlated with geological divisions: sea basin regions, island and arc regions, and continental regions. The derived crustal and lithospheric thicknesses range from ∼15 to >50km and from ∼60 to >140km, respectively, with the thinnest in the South China Sea, the thickest in eastern Tibet and the Yangtze Block, and the medium in the South China Fold Belt, Indochina, and island arc regions. Our results further confirm that (1) a Mesozoic subduction zone, which is interpreted as the tectonic weak zone during the Paleogene, exists along the South China margin; (2) the influence of the Indochina extrusion along the Red River Fault is limited for the South China Sea region; (3) there is a slab remnant of the proto-South China Sea beneath Borneo. New findings suggest that the Mesozoic subduction zone should be built into any evolution model for the region, as well as the other two major tectonic boundaries of the Red River Fault and proto-South China Sea subduction zone. [Copyright &y& Elsevier]

Abstract: One of remarkable geological and geomorphological features in continental China is the presence of a north–south trending tectonic zone which separates the seismically active Tibetan Plateau to the west from the tectonically stable South China and Ordos blocks to the east. The zone also owns a name of “north–south seismic belt” as a large number of great historical earthquakes occur on it. The most recent one, the 2008 Wenchuan earthquake, attests its recent tectonic activity. In this review I take the Western Sichuan region as an example to probe the structural styles and kinematic pattern as well as deep geological process associated with tectonic deformation of the north–south trending zone. Through integrated studies on active faults, GPS crustal deformation, and geophysical structure, we show that deformation in the Western Sichuan is governed by interactions among three crustal blocks (Songpan, Chuandian, and South China) of distinctive rheological properties under the tectonic framework that eastward growth of the “soft” Eastern Tibet is blocked by the “hard” lithosphere of the South China block. The left-lateral Xianshuihe Fault continues to the north–south trending fault system without crustal shortening to form a bounding fault to limit the northern extend of the magnificent clockwise rotation of crustal material around the Eastern Himalaya Syntax. Upper crust of the three blocks is dominated by brittle deformation, whereas the ductile flow of lower crust would drag the brittle upper crustal blocks to move with respect to each other. The relative motions among the brittle upper crustal blocks cause strain accumulations among their bounding faults to generate large earthquakes. Deformation of the Western Sichuan region can thus be described in terms of combined model of rigid block movement and continuous deformation. We suggest this combined model can be applied to entire continental China. [Copyright &y& Elsevier]

Abstract: The development of the East African Rift System (EARS) caused by tectonic forcing during the past 20million years is believed to influence the regional climate in Africa. However, changes in the Earth''s orbital parameters also have an influence on climate on these timescales. To analyze the influence of both tectonic and orbital forcing, we applied the non-hydrostatic regional climate model COSMO-CLM (COSMO model in Climate Mode, CCLM). The regional simulations are driven by different global simulations performed with the coupled ocean–atmosphere general circulation model ECHO-G. To analyze the impact of tectonic forcing a different topography representing a possible past stage of the development of the EARS is applied in the model. The results indicate that tectonic forcing has a strong impact on precipitation in Africa caused by circulation changes. To analyze the impact of orbital forcing, which means the impact of changing insolation patterns, we chose configurations of the Earth''s orbit that result in significantly different patterns of insolation. One example is the orbital configuration of the last interglacial at 125,000years before present, when the seasonality of insolation in the Northern Hemisphere was greatly enhanced, whereas in the Southern Hemisphere it was substantially weakened compared to preindustrial conditions. The simulation of this time slice shows a strong impact of orbital forcing on precipitation in large parts of Africa, caused by altered moisture transport. The application of a biome model helps in analyzing the impact of the two forcing factors on vegetation for a more direct comparison with proxy data. [Copyright &y& Elsevier]

Abstract: Many orogens, including the Central Andes and Himalayas, are characterized by wide areas that have undergone upper crustal shortening and surface uplift. The behaviour of the deep lithosphere is poorly constrained, and in some mountain belts, lower crust and mantle lithosphere appear to have been removed through delamination during orogen development. Thermal–mechanical numerical models demonstrate that as crust thickens during shortening, the lowermost crust may undergo metamorphic eclogitization, which increases its density. Even a small density increase (7% or more) causes shortening to localize above the eclogitic crustal root, promoting the development of thick lithosphere in this area which is then prone to gravitational removal. Complete removal of orogen mantle lithosphere occurs if the eclogitized lower crust is weak enough to allow full detachment of negatively buoyant mantle lithosphere; this can occur even if the lower crust is less dense than the mantle. The onset of delamination may be determined by the hydration state of the lower crust, as the presence of water promotes eclogitization and significantly reduces rock strength. Two distinct styles of delamination are observed: (1) retreating delamination in which weak mantle lithosphere rolls back and peels away from the crust, producing a contemporaneous migration of crustal thickening, surface uplift and magmatism, and (2) stationary delamination in which strong lithosphere separates from the weak lower crust and slides into the deep mantle at a stationary detachment point, followed by widespread crustal deformation and magmatism. [Copyright &y& Elsevier]

Abstract: The surface expression of a lateral polarity change of continental mantle lithosphere subduction has been studied by using lithosphere-scale physical models. Key parameters investigated were: the degree of lateral coupling between adjacent domains of opposing subduction polarity, the width of the zone separating the domains, and the lithosphere geometry and rheology. The model results illustrate an asymmetric lithospheric structure induced by deformation of the downgoing plates, which have been separated by a narrow transition zone. A wide and symmetric orogenic wedge overlying a region of thickened mantle lithosphere and hampered subduction characterizes this transition zone. In addition, interaction between the neighboring subduction domains caused downbending of the upper plates and resulted in the lateral termination of crustal structures and lowering of surface topography. The lateral extent of interaction between the domains strongly depends on the degree of coupling between the domains, the rheology of the mantle lithosphere and the amount of bulk shortening. The modelling results have major implications on the interpretation of seismic and tomographic data from the European Alps in terms of the crust and lithosphere geometries. It appears that an observed lateral change of subduction polarity at mantle depth can explain the variations of wedge build-up between the Western/Central and Eastern Alps. [Copyright &y& Elsevier]

Abstract: Twenty-five new long-period magnetotelluric sites near 31.5°S were collected in a west–east profile. This profile and the previous one, aligned with and adjacent to the eastern end, have been merged to form a single profile of more than 700km long, extending from the Precordillera to the Chaco-Pampean Plain. The geotectonic scenario is characterized by a modern flat subduction zone of the Nazca plate located at a depth of around 120km and clearly defined by the distribution of earthquake hypocenters recorded by local and regional networks. A “bulge” shape at 68.5°W, with an anomalous dip to the west, is observed within this segment. The smooth slab deformation might result from the restriction on eastward motion due to the presence of an electrically resistive zone. The magnetotelluric model shows that this thick zone of increased resistivity is found from shallow crustal levels to upper mantle depths. The bulge geometry allows hot fluids and volatiles to rise from the deeper asthenospheric wedge, and reach the lower crust reducing its viscosity and letting it flow. The zones of low resistivity in the lower crust show spatial correlation with the areas of foreland deformation from Precordillera to the Sierras Pampeanas and may also suggest a ductile regime. Shear zones reactivated by Cenozoic faulting must necessarily have their roots in the levels of the ductile lower crust associated to conductive channels. The zone where the lower crust is closer to the surface coincides with the areas of greatest structural relief and erosion. The interface between the folded ductile lower crust and the brittle upper crust might act as the main level of décollement of the bordering structures between the Precordillera, Sierra de Pie de Palo and the Sierras Pampeanas. In addition, the geometry of the interface might be conditioning the vergence of those structures. [Copyright &y& Elsevier]

Abstract: We determined a 3-D P-wave anisotropic tomography of the crust and uppermost mantle beneath North China Craton (NCC) using 107,976 P-wave arrival times from 16,073 local earthquakes recorded by 380 seismic stations. Our results show significant lateral heterogeneities beneath NCC. The lower crust and uppermost mantle beneath the North China Basin show widespread low-velocity anomalies which may reflect high-temperature materials caused by the late Mesozoic basaltic magmatism in the NCC. Low-velocity anomalies also exist beneath the Trans-North China Orogen, which may reflect asthenospheric upwelling since late Mesozoic. Large crustal earthquakes generally occurred in high-velocity zones in the upper to middle crust, while low-velocity and high-conductivity anomalies that may represent fluid-filled, fractured rock matrices exist in the lower crust to the uppermost mantle under the source zones of the large earthquakes. The crustal fluids may lead to the weakening of the seismogenic layer in the upper and middle crust and hence cause the large crustal earthquakes. The NW–SE P-wave fast velocity directions seem to be dominant in the uppermost mantle under the central parts of eastern NCC, suggesting that these mantle minerals were possibly regenerated but keep the original fossil anisotropy formed before the new lithospheric mantle was produced during the Mesozoic to Cenozoic. [Copyright &y& Elsevier]

Abstract: A broad-band seismic network of 28 three-component seismometers was deployed in southeastern Australia to examine variations in crustal thickness across the transition between Precambrian and Phanerozoic lithosphere. Receiver function observations and modelling of P-to-S conversions at the Mohorovičić discontinuity (Moho) have been employed to investigate: (i) the variations in the Moho depth across southeastern Australia, and (ii) the nature of the transition between crust and mantle. Data from temporary deployments were used together with data from the few permanent broad-band stations in the region. The extraction of P-receiver functions from high-quality seismic data recorded on these stations has enabled the determination of the crustal thickness across the region. The crustal thicknesses lie in the range 28–48km. The Moho depth is generally well correlated with the Earth surface elevation in the southeastern Australia. The Moho estimates from receiver functions are in good agreement with results from reflection profiling. The average crustal thickness is found to be around 39km beneath the Precambrian area in the west and even thicker beneath the Lachlan Orogen in the east (~43km). The average crustal thickness in between, beneath the Murray Basin is thinner ~32km. Interestingly, the crust in the Mount Gambier volcanic area is rather thick ~41km, suggesting that the limit between the Delamerian and western Lachlan orogens is located east of Mount Gambier. Our results favour a position for the Tasman Line generally consistent with the interpretation by Direen and Crawford (2003) and thus to the east of the location favoured by many authors. The broader crust–mantle transition and thicker crust beneath the Lachlan Orogen suggest the presence of magmatic underplating at the base of the lower crust. The intermediate nature of the crust–mantle transition also suggests magmatic underplating beneath the Gawler Craton and the Curnamona Province. [Copyright &y& Elsevier]

Abstract: This study presents the crustal shear wave velocity structure and radial anisotropy along two linear seismic arrays across the North China Craton (NCC) from ambient noise tomography. About a half to one year long ambient noise data from 87 stations were used for obtaining the inter-station surface wave empirical Green''s functions (EGFs) from cross-correlation. Rayleigh and Love dispersion curves within the period band 5–30s were measured from the EGFs of the vertical and transverse components, respectively. These dispersion data were then used to determine the crustal shear wave velocity structure (VSV and VSH) and radial anisotropy (2(VSH −VSV)/(VSH +VSV)) from point-wise linear inversion with constraints from receiver function analysis. Our results reveal substantial structural variations among different parts of the NCC. The Bohai Bay Basin in the eastern NCC is underlain by a thin crust (~30km) with relatively low velocities (particularly VSV) and large positive radial anisotropy in the middle to lower crust. Such a crustal structure is no longer of a cratonic type and may have resulted from the widespread tectonic extension and intensive magmatism in this region since late Mesozoic. Beneath the Ordos Basin in the western NCC, the crust is relatively thicker (≥40km) and well stratified, and presents a large-scale low velocity zone in the middle to lower crust and overall weak radial anisotropy except for a localized lower crust anomaly. The overall structural features of this region resemble those of typical Precambrian shields, in agreement with the long-term stability of the region. The crustal structure under the Trans North China Orogen (TNCO, central NCC) is more complicated and characterized by smaller scale velocity variations, strong positive radial anisotropy in the middle crust and rapid change to weak-to-negative anisotropy in the lower crust. These features may reflect complex deformations and crust–mantle interactions, probably associated with tectonic extension and magmatic underplating during the Mesozoic to Cenozoic evolution of the region. Our structural images in combination with previous seismic, geological and geochemical observations suggest that the Phanerozoic lithospheric reactivation and destruction processes may have affected the crust (especially the middle and lower crust) of the eastern NCC, and the effect probably extended to the TNCO, but may have minor influence on the crust of the western part of the craton. [Copyright &y& Elsevier]

Based on the combined analysis of the seismologic and other geophysical data (the gravity and magnetic fields and the acoustic basement topography), it was revealed that the largest earthquakes that occurred in the region of the Central Kuril Islands in 2006, 2007, and 2009 were related to the block structure of the Earth's crust. The aftershocks of the largest earthquakes recorded in 2006 and 2007 became foreshocks for the earthquakes of 2007 and 2009, respectively. In total, the earthquakes that occurred in this area in the period of 2006-2012 are closely related to the tectonic destruction zone existing there, clarify its position, and support the previous concept of the superimposed and cross cutting relationships of this zone with respect to the Kuril Island Arc. [ABSTRACT FROM AUTHOR]

Abstract: The Tibetan Plateau is the largest orogenic system on Earth, and has been influential in our understanding of how the continental lithosphere deforms. Beneath the plateau are some of the deepest () earthquakes observed within the continental lithosphere, which have been pivotal in ongoing debates about the rheology and behaviour of the continents. We present new observations of earthquake depths from the region, and use thermal models to suggest that all of them occur in material at temperatures of . Thermal modelling, combined with experimentally derived flow laws, suggests that if the Indian lower crust is anhydrous it will remain strong beneath the entire southern half of the Tibetan plateau, as is also suggested by dynamic models. In northwest Tibet, the strong underthrust Indian lower crust abuts the rigid Tarim Basin, and may be responsible for both the clockwise rotation of Tarim relative to stable Eurasia and the gradient of shortening along the Tien Shan. [Copyright &y& Elsevier]

Abstract: We quantify the effects of heterogeneous hydration in the incoming oceanic plate, rehydration in the subducting slab, and hydration in the lowermost mantle wedge on the pattern of H2O release from the slab in the forearc and arc regions of subduction zones. The release and uptake of H2O by dehydration and hydration reactions, respectively, are calculated using a subduction zone thermal model and thermodynamic calculations for the distribution of mineralogically bound H2O for idealized lithologies. We consider two end-member models for the initial hydration state of the incoming plate: (1) uniform hydration, in which H2O is stored homogeneously within each lithologic unit, and (2) localized hydration, in which H2O is stored at its saturation level in discrete zones. The former is commonly assumed in thermo-petrologic models, while the latter approximates the effect of localized hydration along faults in the oceanic plate inferred from geophysical observations. Our modeling results show that for the same bulk H2O content, localized hydration results in shallower H2O release compared to uniform hydration, and that the H2O flux off the subducting slab beneath the forearc and arc regions can be almost twice as large from a locally hydrated slab as from a uniformly hydrated slab. Rehydration can occur in the subducting slab and delays the liberation of H2O. The effect of pervasive rehydration in the slab on the pattern of fluid release in the forearc and arc regions is relatively small if the slab is uniformly hydrated, but it is large if the slab is locally hydrated. Hydration of the overlying mantle also delays the liberation of H2O, but the volume of H2O absorbed in the mantle is small compared to that released from the subducting slab, thereby implying that mantle wedge hydration has only a modest effect on the pattern of fluid release. [Copyright &y& Elsevier]

Abstract: Spectral analysis of the Indian Ocean geoid low provides depth to the large wavelength sources as ∼1300, ∼700 and 340km that are supported from the spectral analysis and the modeling of the corresponding large wavelength regional gravity anomaly with negative density contrasts at these levels. The three levels coincide with the sharp changes in the gradient of the seismic velocities related to the olivine–spinel transformation of successively increasing Fe/Mg ratio as depth increases, known as transition zones. The first two segments are supported from continuous wavelet transform analysis of the large wavelength component of the corresponding gravity field. The low density rocks in this section appear to be related to the subducted Indian/Tethyan lithosphere that roll back and drifted southwards after subduction as inferred from tomography experiments. The relatively short wavelength sources of the spectrum of the geoid data at depths of 162 and 85km suggest sources along the lithosphere – asthenosphere boundary (LAB) under the Indian continent and surrounding oceans, respectively. A low viscosity zone has been envisaged below 660km discontinuity that may correspond to the low density rocks in this section which is popularly referred to as graveyards of the subducted rocks under geoid lows. The subducted slab is hydrated due to dehydration from metamorphism that causes upwelling in the mid-to-upper mantle which is likely to set in baby plumes. Presence of fluid may transform part of olivine to serpentine further reducing the bulk density of rocks in this section. They would make this region buoyant that appears to be responsible for the Central Indian Ocean Deformation Zone with large scale folding, faulting, seismicity, and high heat flow. The same also appear to be responsible for the rapid drift of the Indian plate. Short wavelength component of the Indian Ocean geoid low reflects most of the surface/shallow tectonics of the region similar to the gravity anomaly providing an additional data set for this purpose. Kachchh and the Shillong plateau inspite of being in the intra plate region are highly seismogenic that compare almost to the activity along the plate boundaries. Tectonics and residual geoid anomalies of the Indian continent and adjoining regions suggest that the geoid highs of Kachchh and the Shillong plateau along the NW and the NE corners of India are connected to the geoid highs of the plate boundaries through several lineaments and faults that also show geoid highs and trends of seismic activity leading to plate boundaries. Geoid highs indicating high density mafic rocks suggest their connections/extensions to plate boundaries indicating them to be plausibly part of diffused plate boundaries. Besides, both these regions are affected by (i) prominent lithospheric flexure of the Indian plate (ii) large scale mafic intrusions and (iii) interaction of proterozoic mobile belts with effects of present day Himalayan orogeny that make them more vulnerable for seismic activity. [Copyright &y& Elsevier]

SUMMARY Receiver functions have been calculated from teleseismic events recorded by an array of seismometers deployed on an E-W transect between the coasts of central Norway and Sweden. Forward and inverse modelling and migration of the receiver functions yields models for the subsurface velocity structure along the profile which have the crust thickening from c. 32 km at the Norwegian coast to c. 43 km beneath the central Scandinavian mountain range and then remaining constant beneath Sweden. There is some evidence for a low-velocity layer in the upper 10 km of the crust beneath parts of Norway and western Sweden and good evidence for a high-velocity lower crust underlying much of Sweden which thins to the west beneath Norway. Inverting the seismic velocities to density results in a very good correspondence between calculated and observed gravity anomalies. The results of this study do not support the presence of a significant crustal root providing buoyant support for the mountain range. Low topography and thick crust beneath Sweden are maintained by the high-velocity, high-density lower crustal layer. The upper crustal low-velocity layer is consistent with models based on existing refraction profiling and known geology and physical properties of the crust. There is no direct correlation between properties of the crust and topography suggesting that recent epeirogenic uplift has not resulted from modification of the crust. [ABSTRACT FROM AUTHOR]

A number of Paleoproterozoic mafic dykes are reported to intrude volcano-sedimentary sequences of the Mahakoshal supracrustal belt. They are medium to coarse-grained and mostly trend in ENE-WSW to E-W. Petrographically they are metadolerite and metabasite. Geochemical compositions classify them as sub-alkaline basalts to andesites with high-iron tholeiitic nature. Both groups, i.e. metabasites and metadolerites, show distinct geochemical characteristics; high-field strength elements are relatively higher in metadolerites than metabasites. This suggests their derivation from different mantle melts. Chemistry does not support any possibility of crustal contamination. Trace element modeling advocates that metabasite dykes are derived from a melt originated through ∼20% melting of a depleted mantle source, whereas metadolerite dykes are probably derived from a tholeiitic magma generated through <10% melting of a enriched mantle source. Chemistry also reveals that the studied samples are derived from deep mantle sources. HFSE based discrimination diagrams suggest that metabasite dykes are emplaced in tectonic environment similar to the N-type mid-oceanic ridge basalts (N-MORB) and the metadolerite dykes exhibit tectonic setting observed for the within-plate basalts. These inferences show agreement with the available tectonic model presented for the Mahakoshal supracrustal belt. The Chitrangi region experienced N-MORB type mafic magmatism around 2.5 Ga (metabasite dykes) and within-plate mafic magmatism around 1.5-1.8 Ga (metadolerite dykes and probably other alkaline and carbonatite magmatic rocks). [ABSTRACT FROM AUTHOR]

Abstract: Melting of continental ice sheets and glaciers, changes in ocean circulation pattern and in sea level, variations of surface and ground water levels and river discharge, glacial-isostatic adjustment, mantle convection and tectonics, all this causes transport and (re-) distribution of mass inside the Earth and at its surface. Equipped with precise sensor systems, gravity field and altimeter satellites observe these mass-transport processes. During 2006–2012, the German Research Association DFG had established the SPP 1257, ‘Mass distribution and Mass Transport in the Earth System’ as a coordinated research programme to facilitate integrated analysis of these data, to improve our knowledge about several transport processes within the Earth system and to investigate their interactions. This special issue reports about the findings of the first 4years within the programme. [Copyright &y& Elsevier]

Abstract: The interpretation of a comprehensive two dimensional (2D) seismic reflection data set discloses several rifting events for a typical passive rifted margin. Two major phases have been identified: Rift I phase is in agreement with the initial rifting of Gondwana postulated for the early Jurassic. This rift resulted in break-up stage 1 governed by a north-eastern motion of Antarctica along the reactivated Pebane shear zone. Indications for lava flows were identified in the seismic profiles along the Offshore Zambezi Depression. They form a link between early rifting, the initial break-up and early sea-floor spreading. The nature of these lava flows may be associated with the emplacement of thick volcanic dykes during post-rift magmatism that occurred when the Antarctica Plate (with the Beira High) drifted dextrally from the west to east until the Mid-Jurassic times. It is more likely that the break-up in stage 1 resulted from high tension due to strain relief of Rift I phase. Above all, the extensional deformation occurred in a narrow-rift mode. Therefore, the V-shape of the Offshore Zambezi Depression suggests a possible rift-failure structure. This is best explained by the Reeves and de Wit model (2000). This model postulates that the motion of Antarctica changed to southward direction at around 170 Ma. Consequently a “rift jump” from the Offshore Zambezi Depression in the northwest to the south-eastern edge of the Beira High occurred, and Rift II phase may have started leading to the break-up in stage 2. During this stage (Rift II phase) the extension migrated towards the east, thus thinning the crust, and exhuming the sub-continental mantle in the continental–ocean transition zone. The Rift II phase shows a sequence of half-graben morphologies confining the syn-rift infill that is subdivided in three units: syn-rift I, syn rift-II and rift sag. All three units appear to have developed under minor extensional regimes in the crust evidenced by gently dipping, low-angle detachment faults. With respect to the presence of the two rift phases, it is deduced that break-up and sea-floor spreading are diachronous within Rift I and Rift II segments. [Copyright &y& Elsevier]

Abstract: This paper attempts to review the descriptions on the genetic series of neotectonic forms in Eurasia. Morphotectonically, the Eurasian continental block exhibits a radial-concentric pattern consisting of four kinds of tectonic units: platforms, rejuvenated and youthful mobile belts, and the continent-ocean transition zones. Vast areas of young and ancient platforms, such as Siberia, have experienced slow-rate Late-Cenozoic uplift and little interior deformation. The youthful orogenic belts are clustered into the giant Alpine-Himalayan megabelt. The rejuvenated mountain belts are characterized by a variety of structural-morphological types of orogens, such as domelike uplifts, block uplifts and intermountain basins. The continent-ocean transition zones in Eastern Asia, including marginal rifts and extensional basins, are resulted from interaction between the continental block and Pacific Ocean and Philippine Sea since the Late Cenozoic. One of the conspicuous features of Eurasia is that most areas lie in the largest geoid depression of the Earth, and the NS trending Uralian-Oman lineament expresses a break on the geoid slope, implying a relationship to deep structure, including density inhomogeneities, downward to the core-mantle interface. Besides, the Eurasian continent fully demonstrates morphotectonic and recent geodynamic features of the Northern Hemisphere of the Earth, just in contrast to that of the Southern Hemisphere. It is best to view the surface morphotectonics and deep structure of the Earth as a geodynamic ensemble which has spawned the large-scale geomorphic features on the surface. [Copyright &y& Elsevier]

SUMMARY One of the outstanding problems in modern geodynamics is the development of thermal convection models that are consistent with the present-day flow dynamics in the Earth's mantle, in accord with seismic tomographic images of 3-D Earth structure, and that are also capable of providing a time-dependent evolution of the mantle thermal structure that is as 'realistic' (Earth-like) as possible. A successful realization of this objective would provide a realistic model of 3-D mantle convection that has optimal consistency with a wide suite of seismic, geodynamic and mineral physical constraints on mantle structure and thermodynamic properties. To address this challenge, we have constructed a time-dependent, compressible convection model in 3-D spherical geometry that is consistent with tomography-based instantaneous flow dynamics, using an updated and revised pseudo-spectral numerical method. The novel feature of our numerical solutions is that the equations of conservation of mass and momentum are solved only once in terms of spectral Green's functions. We initially focus on the theory and numerical methods employed to solve the equation of thermal energy conservation using the Green's function solutions for the equation of motion, with special attention placed on the numerical accuracy and stability of the convection solutions. A particular concern is the verification of the global energy balance in the dissipative, compressible-mantle formulation we adopt. Such validation is essential because we then present geodynamically constrained convection solutions over billion-year timescales, starting from present-day seismically constrained thermal images of the mantle. The use of geodynamically constrained spectral Green's functions facilitates the modelling of the dynamic impact on the mantle evolution of: (1) depth-dependent thermal conductivity profiles, (2) extreme variations of viscosity over depth and (3) different surface boundary conditions, in this case mobile surface plates and a rigid surface. The thermal interpretation of seismic tomography models does not provide a radial profile of the horizontally averaged temperature (i.e. the geotherm) in the mantle. One important goal of this study is to obtain a steady-state geotherm with boundary layers which satisfies energy balance of the system and provides the starting point for more realistic numerical simulations of the Earth's evolution. We obtain surface heat flux in the range of Earth-like values : 37 TW for a rigid surface and 44 TW for a surface with tectonic plates coupled to the mantle flow. Also, our convection simulations deliver CMB heat flux that is on the high end of previously estimated values, namely 13 TW and 20 TW, for rigid and plate-like surface boundary conditions, respectively. We finally employ these two end-member surface boundary conditions to explore the very-long-time scale evolution of convection over billion-year time windows. These billion-year-scale simulations will allow us to determine the extent to which a 'memory' of the starting tomography-based thermal structure is preserved and hence to explore the longevity of the structures in the present-day mantle. The two surface boundary conditions, along with the geodynamically inferred radial viscosity profiles, yield steady-state convective flows that are dominated by long wavelengths throughout the lower mantle. The rigid-surface condition yields a spectrum of mantle heterogeneity dominated by spherical harmonic degree 3 and 4, and the plate-like surface condition yields a pattern dominated by degree 1. Our exploration of the time-dependence of the spatial heterogeneity shows that, for both types of surface boundary condition, deep-mantle hot upwellings resolved in the present-day tomography model are durable and stable features. These deeply rooted mantle plumes show remarkable longevity over very long geological time spans, mainly owing to the geodynamically inferred high viscosity in the lower mantle. [ABSTRACT FROM AUTHOR]

Abstract: Cenozoic Mg-rich potassic rocks (MPRs; with MgO⩾6wt.%, K2O/Na2O⩾1) that are widespread in the Tibetan Plateau are commonly explained as the products of partial melting of subcontinental lithospheric mantle (SCLM) sources. This is because the MPR have characteristic geochemical features, including high contents of K2O and other large ion lithophile elements, coupled with depletion in the high field strength elements, suggesting an enriched lithospheric mantle source metasomatized by earlier subduction events. In this paper, we report new data together with a synthesis of the Tibetan MPR. Relative to MPR from northern Tibet (i.e., the Songpan–Ganzi and Qiangtang blocks), MPR from southern Tibet (i.e., the Lhasa block) have obviously higher SiO2, K2O, Rb, Th and Zr contents, and higher K2O/Na2O (⩾2) and Rb/Sr (>0.2) ratios, coupled with lower Al2O3, CaO, Na2O, Sr and Ba/Rb (<12). In addition, MPR from southern Tibet show distinctly lower 143Nd/144Nd(i) and higher 87Sr/86Sr(i) isotopic ratios than those of the north. The geochemical variations suggest that MPR from southern Tibet have mantle source compositions and mineral phases different from those from northern Tibet, reflecting the heterogeneity in the SCLM beneath the plateau. The presence of such mantle heterogeneity, combined with the spatial and temporal changes reported in other types of Cenozoic lavas on Tibet, implies that the Tibetan Plateau experienced multiple-stage tectonic events and heterogeneous uplift processes at the surface. Although existing models such as convective removal and oblique subduction can explain some of the geological phenomena related to Tibetan formation, we suggest the integrated effect of multiple tectonic events to explain the spatial and temporal distribution of MPR and other Cenozoic lavas and associated topographic uplift. [Copyright &y& Elsevier]

Abstract: The Qinghai-Tibet Plateau, composed of several continental slivers within the eastern Tethyan domain, is one of the pivotal sites to examine to better understand the theory of plate tectonics and the orogenic evolution on Earth. This plateau is generally inferred to be a collage of several continental blocks that rifted from Gondwanaland and subsequently accreted to the Asian continent. However, recent recognition of over twenty ophiolite mélange zones and their associated island arcs indicates that the traditional model of tectonic evolution requires revision. Based on 177 recently finished 1:250,000 scale geological maps and related studies, we summarize the main tectonic context of the Qinghai-Tibet Plateau and propose a new integrated model to account for the new findings. The complex orogen of the immense Qinghai-Tibet Plateau, consisting of multiple island arc-basin systems that developed at different stages while surrounded by the North China, Yangtze, Tarim, and Indian plates, is emphasized. The entire orogen, surrounded by suture zones that mark the locations of oceanic closure, is investigated by examining (I) the first-order tectonic units and ophiolitic mélanges (including arc–arc/continent collision zones) and (II) their internally enclosed blocks as the second-order tectonic units. Therefore, the Qinghai-Tibet Plateau is divided into three major orogenic systems, namely, from northeast to southwest, the Early Paleozoic Qinling–Qilianshan–Kunlunshan (Qin–Qi–Kun), the Late Paleozoic–Triassic Qiangtang–Sanjiang, and the Late Paleozoic to Cenozoic Gangdese–Himalaya orogenic systems, which are separated by the Kangxiwa–Muzitagh–Maqin–Mianxian and the Bangong–Shuanghu–Changning–Menglian sutures, respectively. We propose that the formation and evolution of the Qinghai-Tibet Plateau to have been intrinsically related to those of the eastern Tethys, recorded by the Longmu Co-Shuanghu ophiolite mélange zone, the Southern Qiangtang Paleozoic accretionary arc-basin system, the Bangong–Nujiang suture zone, and their associated, composite island arc-basin systems. The present-day Bangong–Shuanghu–Changning–Menglian suture system marks the final closure of the Tethyan Ocean. The Yarlung Zangbo Ocean opened as a back-arc basin in response to the southward subduction of the Tethyan Ocean lithosphere in the Middle Triassic and closed as a result of the India–Asia collision at the end of Cretaceous, followed by the northward indention of the Indian plate that resulted in significant intra-continental deformation and plateau uplift in the Cenozoic. [Copyright &y& Elsevier]

Abstract: Impact crater formation is a highly dynamic and complex geological phenomenon. Methods of structural geology are capable of giving insights into deformation processes that occur during cratering. While most deformation observed in craters exhibits a generally radial symmetry, recent discoveries of non-radial structural elements in the central uplifts of terrestrial craters have led to the suggestion that obliquely impacting projectiles can cause non-radial deformation in the central subsurface area of the crater. The lack of ejecta blankets as accepted indicators of an oblique impact on Earth make it difficult to correlate the observed non-radial structures with the impact direction. Therefore, in this study three martian complex impact craters were selected that exhibit an oblique ejecta blanket and layered bedrock in the central uplift. The central uplifts were structurally mapped using High Resolution Imaging Science Experiment (HiRISE) imagery and additional high-resolution digital terrain models (DTMs). Structural data were evaluated and show a correlation between the orientation of at least two structural criteria and the direction of impact: (1) The strike of upturned bedrock layers is on average perpendicular to the impact trajectory and (2) the majority of faults show a preferred trend parallel to the impact trajectory. A fourth martian crater was examined as a counterexample, which had a radial ejecta blanket and thus no indication of an oblique trajectory. Strike in this crater showed no preferred orientation. These results will help to solidify the suggested correlation between non-radial structural features in impact craters and the horizontal component of momentum transferred from an obliquely impacting projectile to the target. [Copyright &y& Elsevier]

SUMMARY Since 1977 we have developed statistical short- and long-term earthquake forecasts to predict earthquake rate per unit area, time and magnitude. The forecasts are based on smoothed maps of past seismicity and assume spatial and temporal clustering. Our new program forecasts earthquakes on a 0.1° grid for a global region 90N-90S latitude. We use the PDE catalogue that reports many smaller quakes ( M≥ 5.0). For the long-term forecast we test two types of smoothing kernels based on the power law and on the spherical Fisher distribution. The effective width of these kernels is much larger than the cell size, thus the map discretization effects are insignificant. We employ adaptive kernel smoothing which improves our forecast both in seismically quiet and active areas. Our forecasts can be tested within a relatively short-time period since smaller events occur with greater frequency. The forecast efficiency can be measured by likelihood scores expressed as the average probability gains per earthquake compared to spatially or temporally uniform Poisson distribution. Another method uses the error diagram to display the forecasted point density and the point events. Our short-term forecasts also assume temporal clustering described by a variant of Omori's law. Like the long-term forecast, the short-term version is expressed as a rate density in location, magnitude and time. Any forecast with a given lower magnitude threshold can be recalculated, using the tapered Gutenberg-Richter relation, to larger earthquakes with the maximum (corner) magnitude determined for appropriate tectonic zones. [ABSTRACT FROM AUTHOR]

SUMMARY This study provides new constraints on the upper-mantle structure from western Greece to central Anatolia using seismic data of permanent broad-band networks recently installed in Greece and Turkey and from a two-year temporary array (SIMBAAD experiment). We used ∼200 seismic events recorded at 146 broad-band stations with a typical interstation distance of 60-100 km across the study area. The high-resolution 3-D shear wave velocity model of the mantle is obtained by inversion of fundamental-mode Rayleigh wave phase velocity maps for periods between 20 and 195 s. The tomography is based on ray tracing in heterogeneous media taking into account external propagation effects. The horizontal resolution is approximately 100 km, however small heterogeneities may suffer from some horizontal smearing and damping. The vertical resolution is approximately 100 km. The vertical smoothing is necessary to avoid unresolved spurious shear wave velocity oscillations in the upper mantle. The errors on shear wave velocities in our 3-D model (0.02-0.1 km s−1) are significantly smaller than the amplitude of Vs variations (0.3-0.5 km s−1). In spite of the vertical and horizontal smoothing, our model shows details in the upper-mantle structure never reached at regional scale in the area. The overall structure is characterized by a low-velocity zone (80-200 km depth) reflecting a slow and warm asthenosphere underlying a thin lithosphere. The southwesternmost termination of the low-velocity anomaly corresponds to the northward dipping Hellenic slab. The detailed shear velocity structure of the upper mantle beneath Anatolia appears to be far more geometrically complex than revealed in previous tomographic studies of the area. At depths larger than or equal to 160 km, velocities are overall high beneath Anatolia, partly due to the presence of dipping high-velocity anomalies which we tentatively interpret as remnant slabs. The southernmost high-velocity anomaly beneath Anatolia is separated from the eastern edge of the Hellenic slab by a major low-velocity anomaly which we interpret as the trace of asthenospheric mantle material rising inside a vertical slab tear beneath southwestern Anatolia. [ABSTRACT FROM AUTHOR]

We report on a series of analogue modeling experiments that study the oroclinal buckling process as a thick-skinned process involving the entire lithosphere. The results obtained in the experiments suggest that, during oroclinal buckling, extension in the outer arc and significant shortening in the inner arc are produced by tangential longitudinal strain as the main mechanism of deformation. The models also reveal that the mantle lithosphere thickens in different noncylindrical ways depending on the initial lithospheric mantle thickness: from almost recumbent to folding with subvertical axial planes for the thinnest to the thickest mantle lithosphere, respectively. The results provide useful insights into thick-skinned orocline buckling as it is interpreted to have happened in the Iberian-Armorican Arc. [ABSTRACT FROM AUTHOR]

SUMMARY In this paper, we propose a new model of the crustal structure and seismotectonics for central Sicily (southern Italy) through the analysis of the depth distribution and kinematics of the instrumental seismicity, occurring during the period from 1983 to 2010, and its comparison with individual geological structures that may be active in the area. The analysed data set consists of 392 earthquakes with local magnitudes ranging from 1.0 to 4.7. We defined a new, detailed 1-D velocity model to relocate the earthquakes that occurred in central Sicily, and we calculated a Moho depth of 37 km and a mean VP/ VS ratio of 1.73. The relocated seismic events are clustered mainly in the area north of Caltanissetta (e.g. Mainland Sicily) and in the northeastern sector (Madonie Mountains) of the study area; only minor and greatly dispersed seismicity is located in the western sector, near Belice, and along the southern coast, between Gela and Sciacca. The relocated hypocentral distribution depicts a bimodal pattern: 50 per cent of the events occur within the upper crust at depths less than ∼16 km, 40 per cent of the events occur within the middle and depth crust, at depths between 16 and 32 km, and the remaining 10 per cent occur at subcrustal depths. The energy release pattern shows a similar depth distribution. On the basis of the kinematic analysis of 38 newly computed focal plane solutions, two major geographically distinct seismotectonic domains are distinguished: the Madonie Mountain domain, with prevalent extensional and extensional-oblique kinematics associated with upper crust Late Pliocene-Quaternary faulting, and the Mainland Sicily domain, with prevalent compressional and compressional-oblique kinematics associated with thrust faulting, at mid to deep crust depth, along the north-dipping Sicilian Basal Thrust (SBT). The stress inversion of the Mainland Sicily focal solutions integrated with neighbouring mechanisms available in the literature highlights a regional homogeneous compressional tensor, with a subhorizontal NNW-SSE-striking σ1 axis. In addition, on the basis of geodetic data, the Mainland Sicily domain may be attributed to the SSE-ward thrusting of the Mainland Sicily block along the SBT plane. Seismogenic shearing along the SBT at mid-crustal depths was responsible for the unexpected Belice 1968 earthquake ( Mw 6.1), with evident implications in terms of hazard assessment. [ABSTRACT FROM AUTHOR]

SUMMARY We discuss a recent publication by Tedla et al., which presents a crustal thickness map of Africa determined by Euler deconvolution of gravity. In our comment, we first outline the limitations of the data set used by the authors and the deleterious effects of working with free air gravity. Next, we discuss the grievous parameter choices (grid interval, Euler window size, structural index) and incorrect assumptions made by the authors and their serious effects on the results. Finally, we demonstrate with two examples why their results are not helpful in understanding crustal thickness and consequently the tectonic history. This is not specific to Africa, but also true for the application to other parts of the Earth. [ABSTRACT FROM AUTHOR]

SUMMARY This study images upper-mantle structure beneath different tectonic and geomorphological provinces in southern Scandinavia by P-wave traveltime tomography based on teleseismic events. We present results using integrated data from several individual projects (CALAS, MAGNUS, SCANLIPS, CENMOVE and Tor) with a total of 202 temporary seismological stations deployed in southern Norway, southern Sweden, Denmark and the northernmost part of Germany. These stations, together with 18 permanent stations, yield a high density data coverage and enable presentation of the first high resolution 3D seismic velocity model for the upper mantle for this region, which includes the entire northern part of the prominent Tornquist Zone and the Southern Scandes Mountains. P-wave arrival time residuals of up to ±1 s are observed indicating large seismic velocity contrasts at depths. Relative regional as well as absolute global tomographic inversion is carried out and consistently show upper-mantle velocity variations relative to the ak135 global reference model of up to ±2-3 per cent corresponding to P-wave velocity differences of 0.4-0.5 km s-1 from depths of about 100 km to more than 300 km. High upper-mantle velocities are observed to great depth to the east in Baltic Shield areas of southwestern Sweden suggesting the existence of a deep lithosphere keel. Lower velocities are found to the west and southwest beneath the Danish and North German sedimentary basins and in most of southern Norway. A well defined, generally narrow and deep boundary is observed between areas of contrasting upper-mantle seismic velocity. In the southern part of the study area, this boundary is localized along and east of the Sorgenfrei-Tornquist Zone. It seems to follow the eastern boundary of a zone of significant Late Carboniferous-Permian volcanic activity from southwestern Sweden to the Oslo Graben area. To the north, it crosses shield units, Caledonides as well as areas of high topography. Supported by independent results of surface wave studies, we interpret this velocity boundary as a first order lithosphere boundary representing the southwestern edge of thick shield lithosphere. In basin areas to the southwest, low upper-mantle velocities are associated with asthenosphere beneath thinned lithosphere and velocity contrasts are likely to arise mainly from temperature differences. To the north structural and geodynamic relations are more complex and both temperature and compositional differences may play a part. Reduced upper-mantle velocity beneath southern Norway also seems, despite relatively low heat flow, to be associated with areas of thinned lithosphere, pointing towards increased temperatures and reduced density in the upper mantle. This feature extends over large areas and seems not directly correlated to the shorter wavelength high topography of the Scandes Mountains, but may contribute with some isostatic buoyancy on a regional scale. For this northern area, there is no obvious geodynamic explanation to reduced upper-mantle velocity. A number of candidates are available including deep transient thermal influence from basin areas to the southwest. [ABSTRACT FROM AUTHOR]

Abstract: P-to-s receiver functions mapped to depth through P and S body-wave tomography models image continuous 410 and 660km discontinuities beneath the area covered by USArray prior to the year 2011. Mean depths to the 410 and 660km discontinuities of 410 and 656km imply a mantle transition zone that is about 4km thicker than the global average and hence has a slightly cooler mean temperature and/or enhanced water content. Compared to the mean 660 depth beneath this ~2000km wide area, the 660 beneath the Yellowstone hotspot is deflected upward by 12–18km over an area about 200km wide. This is the most anomalous shallowing of the 660 imaged and its horizontal extent is similar to the area where P and S tomography image low-velocity mantle extending from the top of the transition zone to about 900km depth. Together, these results indicate a high-temperature, plume-like upwelling extending across the 660. The depth of 410km discontinuity beneath the Yellowstone region is within 5km of the mean depth implying that the plume is vertically heterogeneous and possibly discontinuous. Tomography indicates a similar vertically heterogeneous thermal plume. The irregular plume structure may be intrinsic to the dynamics of upwelling through the transition zone, or distortion may be caused by subduction-induced mantle flow. Topography of the 410 and 660 confirms that subducted slabs beneath the western U.S. are highly segmented, as inferred from recent tomography studies. We find no evidence of regionally pervasive velocity discontinuities between 750 and 1400km depth. The plume''s depth of origin within the lower mantle remains uncertain. [Copyright &y& Elsevier]

Seismic tomographic data showing the mantle structure of the Ethiopian-Afar superplume and various segments of the Alpine-Himalayan Orogenic Belt and their relationships with the adjacent megastructures of the Earth are presented. These data and their correlation with the geological evidence lead to the conclusion that lateral flows of mantle material are crucial for the evolution of the Tethys and its closure in the Cenozoic with transformation into an orogenic belt. The lateral flow of hot upper mantle asthenospheric matter spreading from the stationary superplume extending in the meridional direction (in present-day coordinates) was responsible for the accretion of the fragments torn away from Gondwana to Eurasia and for the development of subduction at the northeastern flank of the Tethys. The characteristic upper mantle structure of cold slabs passing into nearly horizontal lenses with elevated seismic wave velocity in the lowermost upper mantle is currently retained in the Indonesian segment of the orogenic belt. In the northwestern segments of this belt, a hot asthenospheric flow reached its northern margin after closure of the Tethys and onset of collision, having reworked the former structure of the upper mantle and enriched it in aqueous fluids. The effect of this active asthenosphere on the lithosphere gave rise to intense Late Cenozoic deformation, magmatism, and eventually resulted in mountain building. [ABSTRACT FROM AUTHOR]

Formed during late Proterozoic - early Paleozoic, the St. Lawrence rift system of eastern North America hosts an active seismic zone, also displaying evidence of subsequent tectonic reactivation in Paleozoic and Mesozoic times. However, lack of a detailed reconstruction of the basement geometry and limited geochronological constraints limit our understanding of its structural evolution. Late Silurian - Early Devonian fault reactivation is demonstrated here in typical fault sites of the St. Lawrence rift system by combining structural observations of basement structures and a mineralogical, morphological, and K-Ar isotopic study of clay-rich fault material (<0.4 µm et 1-2 µm). The K-Ar data of clay-rich size fractions from gouges of varied rock types from Saint-Laurent and Montmorency faults define two isochron ages at 436 ± 45 and 406 ± 22 Ma that are within analytical uncertainty and give an average at 421 ± 15 Ma. However, these two faulting episodes could also picture a single long-lasting phase of foreland subsidence of the Appalachian orogen starting during the Late Ordovician - Early Silurian deformation in the external Humber zone and continuing during Late Silurian - Early Devonian subsidence related to a normal faulting. This interpretation is based on a different mineral composition of the younger size fractions that consist of smectite-enriched clay minerals and could, therefore, correspond to the end of a reactivation event that was episodically active between 436 ± 45 and 406 ± 22 Ma. The faults were selectively reactivated depending on their location relative to pre-existing metamorphic and tectonic fabrics of the Grenvillian basement. The identified Late Silurian - Early Devonian reactivation of the Saint-Laurent and Montmorency faults was contemporaneous with the subsidence of the adjacent Appalachian foreland, resulting in SE-dipping faults in the crystalline basement of the St. Lawrence rift system and NW-dipping faults in the Appalachian cover. [ABSTRACT FROM AUTHOR]

SUMMARY Geological evidence indicates large-volume lithospheric drip instabilities form under regions of continental lithosphere. The size of these drips suggests that either the base of the lithosphere is volumetrically increased or the highly viscous upper lithosphere participates in the drip. Previous theoretical models using simple rheologies have been unable to produce unforced large-volume drip instabilities. Thus large drip instabilities are typically induced by thickening or destabilizing the mobile base of the lithosphere through tectonic forcing, such as convergent thickening or dense magmatic emplacement following extension. Here, we propose a theory of lithospheric drip instabilities that may arise naturally from the thermal and rheological structure of Earth, independent of specific tectonic forcings. Using damage physics relevant for Earth, we find a large portion of the lithosphere may be mobilized and entrained into growing drip instabilities. For a critical amount of damage, the growth is accelerated sufficiently that large-volume drip instabilities may form within geologically feasible time-frames. We therefore suggest large-volume lithospheric drip instabilities may arise independently of tectonic settings through damage-assisted mobilization and entrainment of the highly viscous lithosphere. [ABSTRACT FROM AUTHOR]

SUMMARY Shear wave splitting is perhaps the most unambiguous signature of the effect of anisotropic materials on the propagation of seismic waves. It has been used extensively to study anisotropy in the Earth, at global scales from the inner core to the tectonics of the uppermost mantle and crust, and at smaller scales for imaging deformation in hydrocarbon reservoirs. Well-established techniques exist for measuring shear wave splitting in a single (three-component) seismogram and more recently these have been extended to treat shear wave splitting in a tomographic fashion: determining non-uniform anisotropic models using large data sets of splitting measurements. Here, I propose an extension to a recent shear wave splitting tomography methodology which incorporates the data analysis into the inversion itself. This methodology uses a non-linear neighbourhood algorithm inversion to explore the parameter space defined by an anisotropic model consisting of a number of uniform domains. Each candidate model is assessed by applying the splitting it predicts to the entire data set. This approach is computationally expensive, but is highly amenable to parallelization. I apply the methodology to three simple synthetic cases to demonstrate the utility of the method. Finally, I apply the approach to the problem of inferring two-layer anisotropy from SKS splitting, which is a commonly attempted problem in global seismology. This uses data from the seismic station EKTN, where two-layer splitting has been previously inferred. This highlights some of the inherent trade-offs with such studies, and emphasizes the need to incorporate extra information to resolve these. This method is applicable to shear wave anisotropy analysis in a broad range of settings from global to reservoir scale. [ABSTRACT FROM AUTHOR]

Abstract: The disappearances of mafic shoshonitic and ultrapotassic magma prior to Late Oligocene in Western Anatolia post-collisional tectonic settings, and the sudden appearance of Early-Middle Miocene potassic lavas with orogenic geochemical signatures, indicate a striking change of mantle sources during the Early-Middle Miocene period, and require a special explanation. In this regard, the Simav (Kütahya) region of Western Anatolia represents a critical area, where the Early-Middle Miocene mafic potassic (shoshonite, absarokite, ultrapotassic) and high-K calc-alkaline (andesite, dacite-rhyolite, granite) series rocks overlap in the extensional geotectonic setting in a back-arc position. The appraisal of petrological data obtained from Simav igneous complex indicates that there is a remarkable geochemical and isotopic similarity (e.g., negative Eu anomalies; Nb–Ta depletions; high Sr, low Nd and variable Pb isotope compositions) between coevally generated mafic potassic and high-K calc-alkaline magma series. The near primitive mafic potassic (MHKS) lavas with high Sr isotope compositions require a heterogeneous mantle source contaminated with crustal materials. Dragged and delaminated crustal components, caused by shallow continental subduction and the late arrived subducted terrigenous sediments from the Aegean trench are likely candidate sources of continental materials incorporated into the mantle source of the Simav mafic potassic (MHKS) magmas. The nature of these components also played a significant role in the compositional variations of Simav mafic series rocks. The Simav mafic potassic (MHKS) magmas were derived from a crust-contaminated, subduction-modified (metasomatized) EM-II type mantle source, interacting with influxed asthenosphere in a back-arc mantle wedge, whereas mixing of lower crustal silicic melts with underplated potassic mafic magmas resulted in coeval high-K calc-alkaline rocks, matched by the extent of crustal contamination observed in the more evolved silicic rocks. The petrogenesis of the Simav magmatism was triggered by multiple driving forces with kinematic linkage: such as asthenospheric mantle flows, trench retreat, shallow continental subduction, regional extensional uplifting (e.g., Menderes Massif), and concomitant extension and delaminations of subducted (accreted) crust and mantle lithosphere. Considering the Late Tertiary geodynamic picture of the Western Anatolia back-arc extensional province, the initiation of post-collisional potassic and ultrapotassic magma pulses, as a tectonomagmatic precursor, provide evidence for (i) the timing of last stage of regional uplifting (e.g., Menderes Massif) and onset of extensional basin formations in different periods, and (ii) rapid tectonic transitions. [Copyright &y& Elsevier]