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We perform inversions for the shear-wave velocity structure of the southeastern United States (SEUS) using Rayleigh-wave phase and amplitude data from the broadband stations of the South Eastern Suture of the Appalachian Margin Experiment (SESAME) and EarthScope Transportable Array (TA). Our tomographic images of shear-wave velocities in the upper mantle beneath the SEUS provide new constraints on the evolution of mantle lithosphere, both from the inheritance of structures from repeated Wilson cycles and from processes that have occurred while at a passive margin setting. Our images also allow us to correlate these structures to evidence of Eocene to recent tectonism observed at the surface. We find evidence for both inherited structures and more recently evolved structures, both of which bear some correlation to observations of ongoing tectonism. Our results suggest that lithospheric mantle continues to evolve while in a passive margin setting and that even relatively "stable" continental mantle lithosphere is subject to episodes of delamination, foundering, and erosion due to processes that are still not well understood. Our results provide structural constraints on the types of processes that may be ongoing and on possible explanations for the numerous observations of comparatively recent tectonic activity occurring along this passive margin setting. [ABSTRACT FROM AUTHOR]

The occurrence of numerous chromitite layers within the Rustenburg Layered Suite of the Bushveld Complex, South Africa, has been widely cited in models to explain the origin of the igneous layering. Most hypotheses are based around the principle of episodic replenishment of the magma chamber. Chromitite layers occur in both the Lower Critical Zone (LCZ), which is wholly ultramafic, and the Upper Critical Zone (UCZ), where they are part of repetitive units that include pyroxenite (± minor harzburgite), norite and anorthosite. The UCZ also reveals stringers of disseminated chromite, which, despite being only a few millimetres thick, are laterally very persistent. We investigate chromite stringers from the uppermost part of the UCZ at Winnaarshoek in the Eastern Limb of the intrusion, where they are preferentially located on contacts between layers of pyroxenite and anorthosite. Stringers overlain by anorthosite are of particular interest as they are located in the centre of units and are unlikely to have developed from replenishment by basal flows of magma. The uppermost of the two chromite stringers associated with the Merensky Reef is unusual as it is located wholly within a layer of pyroxenite, and does not demarcate a lithological contact. Stringers are categorized as Type III chromite to distinguish them from the thicker layers of chromitite, Type I (in the LCZ) and Type II (in the UCZ). The Cr-spinel in the stringers is characterized by relatively low Cr/Fe ratios and is associated with recrystallized, unusually calcic, plagioclase as the principal silicate phase. Accessory phases include rutile, corundum, zircon and baddeleyite, in addition to base-metal sulphides and PGM. The origin of Type III stringers is ascribed to replenishment by sheets of picritic magma injected, not as basal flows, but as sills into an earlier-formed crystalline substrate dominated by well-defined layers of norite and anorthosite. In the framework of this hypothesis, the units that characterize the UCZ are, therefore, not differentiation cycles: the ultramafic components crystallized from U-type (picritic) magmas and the norite–anorthosite from A-type (tholeiitic) magmas. The two components of the units were emplaced non-sequentially, but the ultramafic rocks (± chromitite layers and/or chromite stringers) still occur in stratigraphic sequence relative to each other. Type III stringers developed on either the lower or upper contacts of the picritic magma sheets, dependent on whether they were emplaced above or below a layer of anorthosite. Nucleation of Cr-spinel was triggered by contamination of the picritic magma by partial melting of the anorthosite. Melting of low-temperature oikocrysts of pyroxene and interstitial plagioclase produced a thin boundary layer of melt mush. This boundary layer achieved rapid saturation in Cr-spinel, in part owing to Cr2O3 released from the pyroxene oikocrysts. Heat was insufficient to melt the main framework of plagioclase crystals in the anorthosite or the noritic wall-rocks. The different occurrences of chromite in the Bushveld should not be aggregated into a single overarching hypothesis. Type III stringers are probably an entirely different phenomenon from the thicker layers and provide corroborating evidence of the sill hypothesis. [ABSTRACT FROM AUTHOR]

As one of the most geochemically unique drill cores recovered within the Yellowstone–Snake River Plain (YSRP) province, the Sugar City geothermal test well was drilled into intra-caldera rhyolite lavas and tuffs erupted during the middle to late Pliocene and the resurgent basaltic volcanism erupted during the Pleistocene. This sequence parallels the two main stages proposed for YSRP hotspot calderas: i.e. the eruption of several large-volume, ash-flow tuff sheets followed by caldera collapse, then cessation of major rhyolitic activity and gradual subsidence accompanied by filling and eventual burial of the caldera by basalt lava flows. We employ stratigraphic relationships, paleomagnetism, and major, trace element, and Sr–Nd isotope geochemistry to develop models for the origin of the basaltic and rhyolitic magmas within a geographical and temporal context. The basalts are characterized by distinct groupings based on depth and geochemistry and reflect the dominant compositions observed on the surface, e.g. Snake River olivine tholeiite (SROT) and evolved type (e.g. Craters of the Moon). We also observe contaminated basalts that interacted with rhyolite/granite. The basaltic magma formed by shallow partial melting in the plume channel carved into the lithosphere. The older rhyolites preserve the classical characteristics of A-type granites and display major element and trace element concentrations typical for Eastern SRP caldera centres and minimal stratigraphic variation. Multiple lines of evidence document extensive magmatic differentiation and coupled basalt–rhyolite interactions. We find that the most plausible origin for the rhyolites is via partial melting of a hybrid source, comprising Archean crustal components and younger juvenile mafic intrusions. Assimilation of hydrothermally altered material is also required for some eruptive units. The rhyolites did not evolve from residual magma left over from the climactic Kilgore eruption (4·0 Ma), but instead represent discrete magma generation events in the course of a few hundred thousand years between 4·0 to 3·8 Ma. Beginning at approximately 3.3 Ma, basalts were able to erupt through the solidified composite pluton that formed below the caldera. The transition from rhyolite to basalt is tied to the declining flux of basaltic magma as North America moved away from the Yellowstone hotspot core. [ABSTRACT FROM AUTHOR]

The “Yanshanian movement” (tectonism) was first recognized in the Western Hills of Beijing and represents the most intense Mesozoic tectonic event in eastern China. Although many researches have been conducted since its identification in 1927, the timing for initiation of the Yanshanian tectonism has not been well constrained. The Jurassic sedimentary succession in the Western Hills of Beijing consists of two volcanic units and interbedded coal-bearing strata, which is similar to those in other regions of the Yanshan Fold-and-Thrust Belt (YFTB). As the earliest Mesozoic volcanism in the Western Hills of Beijing, the Nandaling Formation contains the earliest volcanic and sedimentary records that are associated with the initiation of the Yanshanian tectonism. Previous researches considered the Nandaling volcanic rocks as Early Jurassic in age based on the floral fossil assemblage of Coniopteris–Phoenicopsis in the Yaopo Formation that conformably overlies the Nandaling Formation. In this study we perform a systematic geological survey and present the results of zircon U–Pb dating of the Nandaling volcanic rocks to demonstrate that both the Nandaling volcanic rocks and the overlying floral fossil assemblage are Middle Jurassic in age. LA–ICP–MS zircon U–Pb dating of the Nandaling volcanic rocks in the northern part of the basin yields eruption ages of 175 ± 1 to 168 ± 2 Ma. Dioritic porphyry sills are intruded concordantly into folded Mesoproterozoic–lower Paleozoic strata of the Qingbaikou anticline, and yield a zircon U–Pb age of 177 ± 1 Ma. These ages indicate that the initiation of the Yanshanian basins, and therefore the initiation of the Yanshanian tectonism, occurred no earlier than the Middle Jurassic. Our 40 Ar– 39 Ar dating of the Nandaling volcanic rocks yields younger ages of 138–151 Ma that show a southward-younging trend, indicating that the Nandaling volcanic rocks in the Western Hills of Beijing were overprinted by a tectono-thermal event that peaked during the Early Cretaceous. [ABSTRACT FROM AUTHOR]

The Pacific–Australian plate boundary in the South Island of New Zealand is a transpressive boundary through continental lithosphere consisting of multiple terranes that were amalgamated during previous periods of subduction and plate reorganization. The style and locus of deformation within the present-day plate boundary is controlled by the mechanical behaviour and distribution of these different lithospheric blocks. Geological studies are limited when it comes to illuminating lithospheric structure and rheology at depth. Imaging the 3-D seismic velocity and attenuation (1/ Q), with distributed local earthquakes, helps unravel regional structure and variations in strength, fractures and fluids. We determine the 3-D distribution of Qp and Qs, which show much more variation than seismic velocity (Vp), underlining the utility of Q (1/attenuation). The Haast schist belt, previously shown as ca. 25-km thick dry unit with moderate Vp and low Vp / Vs, is imaged with high Qs, and the highest Qs areas correlate with zones of higher grade schist. Below 25-km depth, the distribution of high Qp and Qs is markedly different from that of the overlying geological terranes. Both the strike and depth of the high Q regions indicate that they represent the subducted Hikurangi Plateau and its adjacent Cretaceous oceanic crust. The thickest part of the plateau, previously identified by Vp  > 8.5 km s−1 from seismic tomography and P -wave precursors and associated with an eclogite layer at the base of the plateau, also has the highest Q. This confirms that the strong plateau extends southwestwards as a narrow salient to the northern Fiordland subduction zone, where moderate- Q Eocene oceanic crust on the Australian plate is being subducted and bent to vertical. In the ductile crust, Q results suggest fluid saturation and elevated temperature conditions in the crustal root of the Southern Alps, and confirm that the shape of this crustal root is influenced by both the orientation and depth of the underlying plateau. Q also provides insight into the failed rifting that occurred in oceanic crust at the edges of the Hikurangi Plateau, with a region of relatively low Q at the on-land extension of the Bounty Trough and Canterbury Basin, at the narrowest part of the South Island. In the brittle crust above 10-km depth, low Q is related to regions of active recent seismicity that have high fracture density, with low Qs where fluids are present. In contrast, the locked Alpine Fault does not exhibit low Q in the brittle crust. [ABSTRACT FROM AUTHOR]

The SW Barents Sea is an epicontinental platform consisting of N- to NNE-oriented basins separated by basement highs. The basins were formed during four distinct rift-phases in the Carboniferous, Late Permian, Late Jurassic-Early Cretaceous, and Paleocene-Eocene. Progressive rifting culminated in continental breakup and seafloor spreading along the North Atlantic axis approximately at 55 Ma. We present tectonic and thermal models of basin evolution along two seismic profiles crossing the SW Barents Sea. The thermal and isostatic history of basins is constrained through time-forward basin modelling based on an automated inverse basin reconstruction approach. We estimate the effects of continental breakup and near-margin processes (magmatic underplating and sill intrusions) on the thermal history and kerogen maturity below the Vestbakken Volcanic Province. Basin models are calibrated against well data. The results imply that both breakup and underplating alter the thermal and isostatic history of sediments along the margin. The hydrocarbon potential of source rocks modelled along the margin suggests breakup has a permanent thermal effect on the present-day sediments promoted by lateral heat flow, while heat conduction by underplating is more diffuse, inhibited to some degree by deeply-buried, low-conductivity shales and limestones. [ABSTRACT FROM AUTHOR]

Abstract: The Andes of southern Patagonia experienced a Miocene shift towards faster and higher angle subduction followed by the approach and collision of the Chile oceanic ridge. We present a kinematic study characterizing palaeostress fields computed from brittle tectonics to better constrain upper crustal deformation during this complex scenario. Although previous studies already suggested variable kinematics, it is striking that in a long‐lasting subduction environment, the computed palaeostress tensors are mostly strike‐slip (55%), while 35% are extensional, and only 10% compressive which are concentrated along a main frontal thrust. Cross‐cutting relationships and synsedimentary deformation indicate that a long‐lived strike‐slip regime was punctuated by a lower Miocene extensional event in the foreland before the main compressional event. The results are discussed in contrasting geodynamic models of plate coupling/decoupling versus direction and rate of convergence of the subducting plate, to explain the main mechanisms that control back‐arc deformation. [ABSTRACT FROM AUTHOR]

The evolution of a rifted and sheared continental margin may control the formation of marginal ridges, which are prominent morphostructural features located at the extremities of fracture zones. We investigate the Ceará Terrace, a marginal ridge located at the western limit of the Romanche Fracture Zone in the Brazilian Equatorial Margin. Our data consist of 2000 km of 2D seismic lines parallel and orthogonal to the continental equatorial margin, as well as four exploratory wells. The Ceará Terrace has an evolution similar to that of the Ivory Coast-Ghana Ridge, the corresponding conjugate end of the Romanche Fracture Zone in the western African continental margin. The Ceará Terrace surface morphology displays an asymmetric ridge; the north slope is aligned to the Romanche Fracture Zone, related to a bathymetric step at ~ 850 m. The paleo relief represents a ridge consisting of a rift sequence (Lower Cretaceous) that is similar to the Ivory Coast-Ghana Ridge. This ridge is bounded by two half-grabens to the southeast associated with the reactivation of preexisting tectonic weakness zones related to the Precambrian Transbrasiliano Lineament. This structure was buried by a drift sequence, which comprises three sedimentary units: Unit 1, shale; unit 2, limestone that interfingers with shale and sandstone; and unit 3, limestone and shale. The fossil ridge is located near a seamount associated with important Oligocene volcanic units. Both conjugate marginal ridges were formed by a Late Albian to Cenomanian transpressional event (marked by folds and tectonic uplift) and flexural uplift due to erosion and thermal exchanges with oceanic spreading centers. Differences in the amplitude of uplift are related to the thermal influences of the distinct oceanic spreading centers. Furthermore, the results indicate that the studied area is still tectonically active. [ABSTRACT FROM AUTHOR]

The geodynamic regime of the early Earth remains elusive, with so far proposed hypotheses ranging from stagnant lid convection to rapid plate tectonics. Available geological data are severely limited for the first two billion years of the Earth's history, and this scarcity of relevant data is often compounded by the nonuniqueness of interpretation. Here we propose that the samarium–neodymium isotope evolution, which has been suggested to be consistent with stagnant lid convection in the early Earth, may be better understood as the result of rapid crustal growth and extensive crustal recycling. We delineate the permissible scenario of crustal evolution through geochemical box modeling with a Monte Carlo sampling of the model parameter space, and our results suggest that the net growth of continental crust was complete by the end of the Hadean and that the rate of crustal recycling could have been as high as 2 − 4 × 10 22 kg Gyr −1 at that time and has gradually decreased since then. Such crustal evolution yields a specific prediction for the present-day distribution of crustal formation ages, which is shown to be in remarkable agreement with a recent estimate based on the global compilation of zircon age data. The mode of subsolidus mantle convection after the putative magma ocean is probably plate tectonics, but its style could have been very different from that of contemporary plate tectonics, characterized by more voluminous magmatism and more destructive subduction. [ABSTRACT FROM AUTHOR]

During Late Cretaceous to Eocene times, a vast epicontinental sea existed in the western Tarim Basin of China, and its appearance, evolution, and demise played an important role in the paleogeographic and paleoclimatic history of central Asia. Sea-level changes in the Tarim Sea and its controlling factors have been poorly studied so far. Here we present a detailed stratigraphic and sedimentological analysis of six Upper Cretaceous to Eocene stratigraphic sections in the southwestern Tarim Basin. Seventeen lithofacies types were identified overall, based on which changes of paleowater-depth were reconstructed. Between the mid-Cretaceous (Cenomanian) and the late Eocene (Priabonian), the Tarim Sea experienced five transgressive-regressive cycles. Deposition took place in middle-ramp to inner-ramp settings during transgressions, and in coastal-plain, lacustrine, and braided-river environments during regressions. The sea retreated but did not withdraw completely during Late Cretaceous to early Paleocene regressive stages, whereas seaways disappeared completely from the studied area during late Paleocene to Eocene regressions. The Tarim Basin lay on a stable craton in the Late Cretaceous, when paleowater-depth was chiefly controlled by eustatic change. Tectonic subsidence started to increase at middle Paleocene time, simultaneously with the Selandian dry event in the Tarim Basin, when a thick gypsum unit was deposited during incipient development of a foreland basin. Such a middle Paleocene onset of tectonic activity, also indicated by discrepancies between the regional paleowater-depth curve and the global sea-level curve and by penecontemporaneous uplift of the West Kunlun range in northeastern Pamir, were most likely a far-field effect of the onset of the India-Asia continental collision to the south. [ABSTRACT FROM AUTHOR]

Tectonic stress is one of the most active deformation factors in the Earth’s crust. Therefore, determinations of its mode of action and form of conversion are critical for guiding research that evaluates preservation conditions in shale gas. Through a survey that targeted many structural features, such as vertical folds and conjugated joints, we investigated the revolution characteristics of the tectonic stress field, the mode of action and the sequence of tectonic stress in the Dingshan region of SE Sichuan by structural analysis methods. Meanwhile, based on a geologic analysis of tectonic stress mechanisms, the structural stress in the region using finite element numerical simulations is quantitatively analyzed. The results show that the region has experienced tectonic stress fields over four different periods and in four different orientations: NW-SE, nearly N-S, nearly E-W and NE-SW. The NW-SE force occurred earlier than that of the nearly N-S force, followed by nearly E-W compression, and finally N-E force. It is concluded that the structural framework of the study area was formed by the effects of the NE-SW compressional stress, which is still active today. The simulation demonstrated that overlapping parts of the intensive strain zone had a higher maximum principal stress, particularly in areas of energy concentration. As for mud and shale, which show excellent sealing capacity, the key to preserving shale gas is the accumulation of energy and a low degree of breakage. Considering the nip-out line of the Longmaxi Formation in the Dingshan region, the buried depth limit of shale-gas production (4500 m), and fault length, the preservation conditions of shale gas and identified favorable targets within the Longmaxi Formation is evaluated. [ABSTRACT FROM AUTHOR]

The Bathurst Island archipelago in the central part of the Canadian Arctic Islands, includes Bathurst Island proper and six other significant islands to north and west. This is the site of two intersecting fold belts: the northerly-trending Cornwallis Fold Belt and the westerly-trending Parry Islands Fold Belt Significant components of the report area include Precambrian seismic basement, the Franklinian shelf (Cambrian to Devonian), a Middle and Upper Devonian clastic wedge, and outliers of the Sverdrup Basin (Carboniferous-Cretaceous). Total thickness of the Cambrian to Devonian succession is 7.6–8.8 km in the island group. Oldest identified strata are evaporites of the Middle Ordovician Bay Fiord Formation. These form a ductile decollement zone that at a depth of 4100–5500 m everywhere underlies thrust-folds. Above this are shelf carbonates, deep water mudrocks, and shallow to deep water strata associated with tectonic activity through the upper Silurian to Lower Devonian Cornwallis Fold Belt. The end of uplift-related deposition is marked by a transition to basin fill mudrocks, succeeded in the later Devonian by shelf-deltaic and fluvial sandstones and other clastic rocks. Structures of Cornwallis Fold Belt feature north and in part west-striking thrust panels and evidence of unroofing in early Ludlow to mid-Emsian time. Also included are westerly-transported thrusts kinematically linked to easterly- striking wrench faults that are presumably reactivated as thrusts and forming large interference structures during development of Parry Islands Fold Belt deformation in the Late Devonian to early Carboniferous. Parry Islands Fold Belt features upright surface folds, subsurface thrusts, ductile deformation and detachment in Ordovician salt and Devonian shale. Structures include northerly- and southerly-transported thrusts, zig-zag structures, pop-up and pop-down structures. [ABSTRACT FROM AUTHOR]

The opening and pre-opening history of the Amerasia Basin is poorly constrained. Key tectonic events have been interpreted from outcrop data in adjacent sedimentary basins or from geophysical data along its continental margins. However, due to its location in a polar region, data are scarce and the existing models of tectonic evolution are controversial. The northernmost exposed adjacent basin is the Mississippian to Late Cretaceous Sverdrup Basin, the original structural framework of which is unclear because it was overprinted by the latest Cretaceous to Paleogene Eurekan Orogeny. This paper presents a restored structural cross-section of the eastern half of the Sverdrup Basin in order to remove deformation attributed to Eurekan shortening and identify previous Mississipian to Late Cretaceous structures. Our results reveal that the evolution of the Sverdrup Basin during the Mesozoic was dominated by the rising of salt diapirs in the basin centre and by Triassic to Jurassic extensional fault development at the basin’s margins. Diapirs were squeezed during Eurekan tectonic inversion whilst extensional faults formed thrusts. The discovery of Triassic fault activity is a new finding in the Sverdrup Basin and forms an extra element to be considered in tectonic models for the opening of the Amerasia Basin. [ABSTRACT FROM AUTHOR]

The structure of the Fuegian Andes central belt is characterized by a first phase of peak metamorphism and ductile deformation, followed by a brittle-ductile thrusting phase including juxtaposition of different (first phase) structural levels; both related to the closure and inversion of the Late Jurassic-Early Cretaceous Rocas Verdes basin. The second phase involved thrust sheets of pre-Jurassic basement, as well as Upper Jurassic and Lower Cretaceous units from the volcanic-sedimentary fill of the basin. Rock exposures in the Parque Nacional Tierra del Fuego reveal a diversity of metamorphic mineral assemblages, dynamic recrystallization grades and associated structures, evidencing a variety of protoliths and positions in the crust during their orogenic evolution. Among the units present in this sector, the Lapataia Formation portrays the higher metamorphic grade reported in the Argentine side of the Fuegian Andes, and since no precise radiometric ages have been established to date, its stratigraphic position remains a matter of debate: the discussion being whether it belongs to the pre-Jurassic basement, or the Upper Jurassic volcanic/volcaniclastic initial fill of the Rocas Verdes basin. The mapping and petrographic/microstructural study of the Lapataia Formation and those of undoubtedly Mesozoic age, allow to characterize the former as a group of rocks with great lithological affinity with the Upper Jurassic metamorphic rocks found elsewhere in the central belt of the Fuegian Andes. The main differences in metamorphic grade are indebted to its deformation at deeper crustal levels, but during the same stages than the Mesozoic rocks. Accordingly, we interpret the regional structure to be associated with the stacking of thrust sheets from different structural levels through the emplacement of a duplex system during the growth of the Fuegian Andes. [ABSTRACT FROM AUTHOR]

This article reports new zircon laser ablation-multicollector-inductively coupled plasma-mass spectrometry U-Pb and Hf isotope, whole-rock major and trace element, and Sr-Nd isotope data for mineralized and barren intrusions associated with the Duolong porphyry-epithermal copper-(gold) deposit (DPCD, a mining camp containing several individual deposits) in the western Qiangtang Terrane (QT), central Tibet. These data are used to further our understanding of the geological evolution of this region. The mineralized and barren DPCD intrusions are typical I-type granitoids that were synchronously emplaced at ca. 112.6-125.9 Ma. These igneous rocks show arc affinities that are characterized by enrichments in the light rare earth elements (LaN/YbN = 4.08-15.23) and the light ion lithophile elements (Rb, Th, U, K, and Pb), and depletions in the high field strength elements (Nb, Ta, and Ti). They have 87Sr/86Sr(i) values of 0.7046-0.7079, Nd(t) values of -6.0 to +1.1, and two-stage Nd model ages of ca. 823-1410 Ma. Zircons from these intrusive rocks have variable but generally positive εHf(t) values (-2.7 to +13.7) and relatively young zircon Hf crustal model ages of 335-1351 Ma. Combining these data with geochemical data reported in recent studies, we infer that the mineralized and barren DPCD intrusions formed in a continental marginal arc setting and likely originated from a common parental magma that was result of magma mixing of juvenile crust-derived basaltic melts and old lower crust-derived melts. The formation of the DPCD intrusions indicates that the Bangongco-Nujiang oceanic lithosphere was still undergoing northward subduction beneath the western QT at ca. 112.6-125.9 Ma, suggesting in turn that the oceanic basin have not closed completely during the Early Cretaceous. These new data also indicate that the processes that occur during the subduction of oceanic crust in continental marginal arc settings produce and preserve juvenile crustal material, leading to net continental crust vertical growth and thickening. [ABSTRACT FROM AUTHOR]

The northern margin of the North China Craton (NCC) was an active convergent margin during Palaeozoic and preserves important imprints of magmatic and metasomatic processes associated with oceanic plate subduction. Here, we investigate the mafic-ultramafic rocks in the Xiahabaqin-Sandaogou complexes from the northern NCC including pyroxenite, hornblendites, hornblende gabbro, and their rodingitized counterparts within a serpentinite domain. We present petrological, zircon U-Pb geochronological, and geochemical data to constrain the nature and timing of the magmatic and metasomatic processes in the subduction zone mantle wedge. The rock suites investigated in this study are characterized by low contents of SiO2, Na2O, and K2O, with high CaO, FeO, Fe2O3, and MgO. The rodingitized rocks show markedly high CaO and lower MgO compared to their ultramafic protolith, suggesting extensive post-magmatic infiltration of Ca-rich, Si-poor fluids derived by serpentinization of mantle peridotite. The enrichment of large ion lithophile and light rare earth elements such as Ba, Sr, K, La, and Ce with relative depletion of high field strength elements like Nb, Ta, Zr, and Hf in the ultramafic rocks collectively suggest metasomatism of a fore-arc mantle wedge by fluids released through dehydration of subducted oceanic slab and subduction-derived sediments. Dehydration and decarbonation leading to metasomatic fluid influx and serpentinization of mantle wedge peridotite account for the enriched geochemical signatures for the rodingitized rocks. The zircon grains in these rocks show textures indicating magmatic crystallization followed by fluid-controlled dissolution-precipitation. Magmatic zircons from altered pyroxenite, hornblendite, and rodingitized pyroxenite in Xiahabaqin yield protolith crystallization ages peaks at 396 Ma and 392 Ma and metasomatic grains show ages of 386 Ma, 378 Ma, and 348 Ma. The zircons from hornblendite and basaltic trachyandesite indicate protolith emplacement during 402-388 Ma. Metasomatic zircon grains from rodingitized hornblende gabbro in Sandaogou complex show a wide range of ages as 412 Ma, 398 Ma, 383 Ma, and 380 Ma. The common magmatic zircon ages peaks at 398-388 Ma in most of the rocks suggest a similar time for magma crystallization in the Xiahabaqin and Baiqi during Middle Devonian. Subsequently, repeated pulses fluids and melts resulted in metasomatic reactions in mantle wedge until early Permian. The Lu-Hf analysis of the zircon grains from these rocks display markedly negative εHf(t) values ranging from −22.4 to −7.7, suggesting magma derivation from an enriched, hydrated lithospheric mantle through fluid-rock interaction and mantle wedge metasomatism. Rodingitization processes are associated with exhumation of ultramafic mantle wedge rocks within a serpentinized subduction channel close to the subducted slab in response to slab roll back in a long-lasting subduction regime. This study offers insights into magmatic and metasomatic processes of ultramafic rocks in the fore-arc mantle wedge which were exhumed and accreted to an active continental margin during the southward subduction of the Palaeo-Asian oceanic lithosphere beneath the NCC. [ABSTRACT FROM AUTHOR]

Kutcharo is the largest Quaternary caldera in Japan and one of the youngest worldwide. It is subcircular, with a diameter of 20-26 km, and was formed by repeated rhyolitic explosive eruptions from 340 to 35 ka. Kutcharo has a previously unrecognized resurgent dome (10-12 km across, 360 m high) that was produced by reinflation of the underlying magma chamber between 28 and 23 ka. The resurgent dome consists of uplifted intracaldera lacustrine deposits and overlying dacite lavas and is transected by an L-shaped graben 3 km wide. A rhyolite lava and 11 rhyolite lava domes occur around the resurgent dome and within the graben. The architecture of Kutcharo caldera closely resembles that of well-understood resurgent calderas such as Valles caldera and Long Valley caldera. Geochronological data indicate that caldera resurgence occurred within ~10 k.y. after caldera collapse, i.e., much faster than other calderas where the timing of resurgence is known. Kutcharo has shown signs of unrest (earthquakes and ground deformation) since A.D. 1938. Seismic, geodetic, geothermal, and resistivity data all suggest that magma occurs at depth beneath the resurgent dome. Kutcharo is located at the intersection of two regional faults that accommodate the ongoing movement of the Kuril forearc sliver, caused by oblique subduction of the Pacific plate. The structure and activity of the resurgent dome are linked to this tectonic setting. Kutcharo has the potential for future volcanic eruptions. Our new view of the architecture of the Kutcharo caldera provides invaluable information for hazard assessment of resurgent calderas. [ABSTRACT FROM AUTHOR]

Initiation of subduction is rarely encountered in modern tectonic environments due to its ephemeral and destructive nature. We report the geological and geophysical evidence indicating a transitional phase from buckling to embryonic subduction along the eastern Korean margin. The transition appears to be caused by compressional reactivation of the strike-slip boundary between the continental (Korean Peninsula) and oceanic (Ulleung Basin) crusts since the Early Pliocene. Evidence for compressional reactivation includes (1) a west-dipping major thrust and coincident crustal buckling of the Ulleung Basin; (2) an east-west structural asymmetry inferred from the gravity anomaly and P-wave tomography; and (3) ongoing crustal uplift and high-angle faults along the eastern Korean margin. The juxtaposition of underthrusting and buckling of the crust in the Ulleung Basin, and its associated ubiquitous reverse faulting on the eastern Korean margin, imply the potential development of a new subduction system along the western margin of the East Sea (Japan Sea). We propose that the East Sea comprises two incipient subduction margins (i.e., the Korean and Japanese sides), which are now competing to reach a self-sustaining subduction stage. [ABSTRACT FROM AUTHOR]

In this study, we documented a case involving large-scale macroscopic anomalies in the Xichang area, southwestern Sichuan Province, China, from May to June of 2002, after which no major earthquake occurred. During our field survey in 2002, we found that the timing of the high-frequency occurrence of groundwater anomalies was in good agreement with those of animal anomalies. Spatially, the groundwater and animal anomalies were distributed along the Anninghe-Zemuhe fault zone. Furthermore, the groundwater level was elevated in the northwest part of the Zemuhe fault and depressed in the southeast part of the Zemuhe fault zone, with a border somewhere between Puge and Ningnan Counties. Combined with microscopic groundwater, geodetic and seismic activity data, we infer that the anomalies in the Xichang area were the result of increasing tectonic activity in the Sichuan-Yunnan block. In addition, groundwater data may be used as a good indicator of tectonic activity. This case tells us that there is no direct relationship between an earthquake and these anomalies. In most cases, the vast majority of the anomalies, including microscopic and macroscopic anomalies, are caused by tectonic activity. That is, these anomalies could occur under the effects of tectonic activity, but they do not necessarily relate to the occurrence of earthquakes. [ABSTRACT FROM AUTHOR]

The detailed stage of engineering-geological and hydrogeological survey for the motorway R2 Rožňava - Jablonov nad Turňou assessed engineering-geological, geotechnical and hydrogeological conditions along the Soroška tunnel route. The roles of the geological survey consisted of the verification of geology, partitioning and characteristics of the rock massif. The tunnel tubes were divided into quasihomogeneous blocks, carried out together with categorization of the rock mass in the RMR (Rock Mass Rating) and QTS (Classification of rock and semi-rock formations for underground constructions) classification systems. Geological survey has contributed to the geological knowledge by the finding that the Szin Beds manifest metamorphic overprint typical for Turnaicum, as well as strong tectonic overprint. Over the Szin Beds in the length 1.725-2.3 km of the tunnel route (from the western portal) the bed of Gutenstein limestones/dolomites of Silicicum was north-vergently tectonically displaced. Moreover, the Szin Beds along the Soroška tunnel route are tectonically reduced along an E-W to ESE-WNW trending fault zone with steep dip to N, displacing the Szin Beds with Gutenstein limestones/dolomites in their tectonic overlier on the Steinalm and Wetterstein limestone massif, as well as smaller body of Reifling/pseudoreifling limestones, all belonging to Silicicum. Results of communication tracking tests showed common direction of the groundwater currents in karstic-fissure environment towards the Eveteš groundwater source and the Hrušovská jaskyňa cave at the Hrušov village, as well as connections of the tunnel section with the springs in the Krásnohorska Dlhá Lúka area. [ABSTRACT FROM AUTHOR]

Situated between the South Tianshan suture zone to the west and the Solonker suture zone to the east, the Yagan and Zhusileng-Hangwula arcs (YZHAs) in western Inner Mongolia in China occupy a critical place to investigate the tectonic history of the middle segment of the southern Central Asian Orogenic Belt (CAOB). In this work, field-based petrological studies and zircon U-Pb dating results reveal several episodes of granitic magmatism from 400 to 230 Ma in the YZHAs. Whole-rock geochemical and zircon Lu-Hf isotopic data indicate that all the 400-230 Ma granitoids underwent intensive fractional crystallization and were generated by magma mixing involving different proportions of mantle- and crust-derived materials. The ∼400 Ma monzogranites show (high-K) calc-alkaline affinities, akin to S-type granitoids. They were most likely generated in a postcollisional setting, corresponding to the assembly of the YZHAs before the Early Devonian. The 298-290 Ma granitoids belong to transitional I/S-type to A-type, whereas the 280-277 Ma granitoids are typical I-type. These Permian granitoids show increasingly evolved zircon εHf(t) values and formed from crust-mantle magma mixing, suggesting an advancing subduction setting. The ∼230 Ma monzogranites exhibiting fairly positive zircon εHf(t) values (+6.26 to +10.49) and high contents of mafic compositions and transition elements probably formed in a postcollisional setting after the assembly of the YZHAs and the Alxa Terrane. We infer that the final assembly of the middle segment of the southern CAOB probably occurred in the Early-Middle Permian. [ABSTRACT FROM AUTHOR]

Northwest Zhejiang Province (NWZJ) is located in the southeastern Lower Yangtze River Belt, southeastern China. Here we document the occurrence of both magnesian (149-131 Ma) and ferroan (162-121 Ma) granitoids in NWZJ. The magnesian granitoids are calc-alkalic peraluminous in composition, with a wide range of SiO2 (58-72 wt%) contents. They have high K2O/Na2O, Sr/Y, and (La/Yb)N ratios, with insignificant Eu anomalies, whereas the calc-alkalic peraluminous ferroan granites have high SiO2 (76-77 wt%) contents, Fe indices (FeO*/(FeO*+MgO)), and Ga/Al ratios. The ferroan granites also have low Ce/Pb and Nb/U ratios, with strong Ba, Sr, and Eu negative anomalies. Most of the rocks have similar zircon Lu-Hf isotopes (εHf(t)=−6.0 to −0.7). However, rocks from two ferroan granitic bodies (Huangshitan and Jiuligang) have more depleted Hf isotopes, with εHf(t) ranging from −1.9 to 5.9. The whole-rock Nd isotopes of the ferroan granites (εNd(t)=−6.5 to −3.2) are slightly more depleted than those of magnesian granitoids (εNd(t)=−8.8 to −5.1). In addition, all ferroan granites show similar and high present-day whole-rock Pb isotopic ratios (18.3-18.8 for 206Pb/204Pb, 15.6-15.7 for 207Pb/204Pb, and 38.5-39.0 for 208Pb/204Pb). On the basis of published data and our new results, we propose that the magnesian granitoids were generated by partial melting of lower-crustal materials, whereas the ferroan granites were derived from a similar source but some more-depleted materials were added into their source after ∼135 Ma. The water contents of the magma may have played an important role in determining the different geochemical affinities of the felsic magmatism. The felsic magmatism occurred under an extensional setting during the period 162-121 Ma. The extension of the lithosphere was further enhanced and followed by upwelling of asthenospheric mantle after ∼135 Ma. This study suggests that a change in the tectonic regime occurred at ∼135 Ma in NWZJ, which may have been triggered by the rollback of the subducted Paleo-Pacific Plate. [ABSTRACT FROM AUTHOR]

On 28 May 2016 at 23:54 (UTC), an M w 5.4 earthquake occurred in Mihoub village, Algeria, 60 km southeast of Algiers. This earthquake was the largest event in a sequence recorded from 10 April to 15 July 2016. In addition to the permanent national network, a temporary network was installed in the epicentral region after this shock. Recorded event locations allow us to give a general overview of the sequence and reveal the existence of two main fault segments. The first segment, on which the first event in the sequence was located, is near-vertical and trends E–W. The second fault plane, on which the largest event of the sequence was located, dips to the southeast and strikes NE–SW. A total of 46 well-constrained focal mechanisms were calculated. The events located on the E–W-striking fault segment show mainly right-lateral strike-slip (strike N70°E, dip 77° to the SSE, rake 150°). The events located on the NE–SW-striking segment show mainly reverse faulting (strike N60°E, dip 70° to the SE, rake 130°). We calculated the static stress change caused by the first event (M d 4.9) of the sequence; the result shows that the fault plane of the largest event in the sequence (M w 5.4) and most of the aftershocks occurred within an area of increased Coulomb stress. Moreover, using the focal mechanisms calculated in this work, we estimated the orientations of the main axes of the local stress tensor ellipsoid. The results confirm previous findings that the general stress field in this area shows orientations aligned NNW–SSE to NW–SE. The 2016 Mihoub earthquake sequence study thus improves our understanding of seismic hazard in north-central Algeria. [ABSTRACT FROM AUTHOR]

A key issue in understanding the physics of deep (15–50 km) slow-slip events (D-SSE) at plate convergent margins is how their initially unstable motion becomes stabilized. Here we address this issue by quantifying a rate-strengthening mechanism using a viscoplastic shear-zone model inspired by recent advances in field observations and laboratory experiments. The well-established segmentation of slip modes in the downdip direction of a subduction shear zone allows discretization of an interseismic forearc system into the (1) frontal segment bounded by an interseismically locked megathrust, (2) middle segment bounded by episodically locked and unlocked viscoplastic shear zone, and (3) interior segment that slips freely. The three segments are assumed to be linked laterally by two springs that tighten with time, and the increasing elastic stress due to spring tightening eventually leads to plastic failure and initial viscous shear. This simplification leads to seven key model parameters that dictate a wide range of mechanical behaviors of an idealized convergent margin. Specifically, the viscoplastic rheology requires the initially unstable sliding to be terminated nearly instantaneously at a characteristic velocity , which is followed by stable sliding (i.e., slow-slip). The characteristic velocity, which is on the order of <10 −7 m/s for the convergent margins examined in this study, depends on the (1) effective coefficient of friction, (2) thickness, (3) depth, and (4) viscosity of the viscoplastic shear zone. As viscosity decreases exponentially with temperature, our model predicts faster slow-slip rates, shorter slow-slip durations, more frequent slow-slip occurrences, and larger slow-slip magnitudes at warmer convergent margins. [ABSTRACT FROM AUTHOR]

The Xiangshan mafic-ultramafic complex is one of the major Early Permian mafic-ultramafic intrusions in eastern Tianshan (Xinjiang, NW China), and consists of two major intrusive phases. The first intrusive phase is mainly gabbroic rocks hosting ilmenite mineralization, while the second intrusive phase is mainly lherzoilite associated with Ni-Cu sulfide mineralization. The Xiangshan ilmenite orebodies hosted in the Fe-Ti oxide-bearing gabbro occur along the contact between hornblende gabbros and leucogabbros. The hornblende gabbros and Fe-Ti oxide rich gabbros at Xiangshan are newly dated to be Early Permian (280.1 and 279.2 Ma, respectively). Major and trace element compositions of zircons and whole rocks from Xiangshan hornblende gabbro and Fe-Ti oxide gabbro have been measured by in situ excimer laser ablation ICP-MS. Zircon Ce4+/Ce3+ ratios based on lattice-strain model and Ti-in-zircon temperatures of hornblende gabbro and Fe-Ti oxide gabbro of the Xiangshan complex are calculated to evaluate the physicochemical variations during the ilmenite mineralization. Whole-rock geochemistry and zircon trace element geochemistry suggest that Fe-Ti oxide gabbros were formed from a basaltic parent magma which had undergone a transfromation from being H2O-rich to H2O-poor. During the magmatic evolution, primitive, H2O-poor basaltic melts may have been replenished into the system, increasing its solidus temperature and decreasing its oxygen fugacity and H2O contents. This may have supperessed the Ti-rich poikilitic hornblende fractionation and promoted the plagioclase fractionation, which consequently concentrated the ore-forming components in the residual melts and generated the ilmenite mineralization. [ABSTRACT FROM AUTHOR]

Stream profile analysis of the lower Kern River and its tributaries help to constrain the landscape response to late Cenozoic tectonics in the southern Sierra Nevada of California. In this study, we identify two relict landscapes that have been offset from the Kern Plateau by periodic displacement along the southern Sierra Nevada fault system starting ca. 20 Ma. These remnants provide context from which to evaluate existing models of rock uplift and exhumation in the region. Reconstructed channel profiles on the relict landscapes indicate a slow incision rate of ~0.07 mm/yr throughout most of the Miocene. An increase in incision ca. 6 Ma resulted in the formation of the Kern Canyon. This increase in incision likely occurred in response to east-west delamination of a lithospheric root beneath the southern Sierra Nevada and is reflected in a pulse of vertical incision (***0.11 mm/yr) that has propagated upstream at a rate of ~7.3 mm/yr to its current position near Isabella Lake. Another two pulses of rapid incision were likely generated by increased rock uplift associated with the northward migration of the delamination hinge after 1 Ma. Incision rates of 0.58-1.2 mm/yr have propagated up the lower Kern River resulting in formation of the Kern River gorge. These new constraints on the incision history of the lower Kern River provide further corroborating evidence for existing models of late Cenozoic mantle delamination and associated epeirogenic processes that have helped shape the landscape of the southern Sierra Nevada. [ABSTRACT FROM AUTHOR]

The tectogenesis of the Atlantic Ocean segments is complicated by the axial difference in spreading half-velocities, which causes additional shear displacements between the lithospheric blocks along the transform faults. The intensity of these processes and density of the fault zones iis related to the presence of “cold” sublithospheric lenses along the MAR at a depth of 500 km. [ABSTRACT FROM AUTHOR]

The East African Rift System (EARS) is thought to be an intra-continental ridge that meets the Red Sea and the Gulf of Aden at the Ethiopian Afar as the failed arm of the Afar triple junction. The geodynamics of EARS is still unclear even though several models have been proposed. One model proposes that the EARS developed in a local tensile stress field derived from far-field loads because of the pushing of oceanic ridges. Alternatively, some scientists suggest that the formation of the EARS can be explained by upwelling mantle plumes beneath the lithospheric weak zone (e.g., the Pan-African suture zone). In our study, a shell model is established to consider the Earth’s spherical curvature, the lithospheric heterogeneity of the African continent, and the coupling between the mantle plumes and the mid-ocean ridge. The results are calculated via the finite element method using ANSYS software and fit the geological evidence well. To discuss the effects of the different rock mechanical parameters and the boundary conditions, four comparative models are established with different parameters or boundary conditions. Model I ignores the heterogeneity of the African continent, Model II ignores mid-ocean spreading, Model III ignores the upwelling mantle plumes, and Model IV ignores both the heterogeneity of the African continent and the upwelling mantle plumes. Compared to these models is the original model that shows the best-fit results; this model indicates that the coupling of the upwelling mantle plumes and the mid-ocean ridge spreading causes the initial lithospheric breakup in Afar and East Africa. The extension direction and the separation of the EARS around the Tanzanian craton are attributed to the heterogeneity of the East African basement. [ABSTRACT FROM AUTHOR]

The effective elastic thickness ( T e ) is an important parameter that characterizes the long-term strength of the lithosphere. Estimates of T e at subduction zones have important tectonic and geodynamic implications, providing constraints for the strength of the oceanic lithosphere at a short-term scale. We estimated T e values in several subduction zones worldwide by using models including both surface and subsurface loads from the analysis of free-air gravity anomaly and bathymetric data, together with a moving window admittance technique (MWAT). Tests with synthetic gravity and bathymetry data show that this method is a reliable way to recover T e of oceanic lithosphere. Our results show that there is a noticeable reduction in the effective elastic thickness of the subducting plate from the outer rise to the trench axis for most studied subduction zones, suggesting plate weakening at the trench-outer rise of the subduction zones. These subduction zones have T e range of 6–60 km, corresponding to a wide range of isotherms from 200 to 800 °C. Different trenches show distinct patterns. The Caribbean, Kuril-Japan, Mariana and Tonga subduction zones show predominantly high T e . By contrast, the Middle America and Java subduction zones have a much lower T e . The Peru-Chile, Aleutian and Philippine subduction zones show considerable scatter. The large variation of the isotherm for different trenches does not show clear relationship with plate weakening at the outer rise. [ABSTRACT FROM AUTHOR]

This paper analyzes differences in the history of hydrocarbon (HC) generation by the rocks of the Bremer 1-6 formations in adjacent areas of the nonvolcanic passive continental margins of Australia and Antarctica. The problem is examined by the example of the numerical reconstruction of the burial and thermal history of two sedimentary sequences of approximately equal thicknesses: the section of well 19-2012 in the Bremer sub-basin of the southwestern margin of Australia and the section of pseudowell 2 in the adjacent area of the passive margin of Antarctica on seismic profile 5909 across the Mawson Sea. The asymmetry of Gondwana rifting in the region of interest resulted in asymmetry in the tectonic structure and development of adjacent areas of passive margins and, as a consequence, significantly different histories of HC generation by the rocks of the Bremer 1-6 formations in these areas. Modeling indicates that the rocks of the Bremer 1 and 2 formations are mainly gas-prone in the Bremer basin and can become oil-prone in the Mawson Sea region of the Antarctic margin. In contrast, according to modeling, the rocks of the Bremer 4 and 5 formations generate a minor amount of HC in the well 19-2012 area of the Bremer sub-basin and considerable amounts of heavy and light oil in the adjacent Antarctic margin area at pseudowell 2 in the Mawson Sea. [ABSTRACT FROM AUTHOR]

Abstract: A review of geological and geophysical observations points towards a worldwide kinematic change at around 6 Ma. The synchronicity of many manifestations (tectonics, magmatism, kinematics, ecological events, among others) at ~6 Ma, similar to those recognized from time to time on the geological time‐scale, argues for a global geodynamic event that has led to many regional consequences on Earth's surface. In particular, we propose that this global event was the main trigger for the three fold increase in sediment deposits in the world ocean over the last ~5 Ma, but also for the onset of the Messinian Salinity Crisis in the Mediterranean area, one of the most severe ecological crises in the Earth's history. We suggest this Messinian revolution to be the last occurrence of cyclic successions of global events. [ABSTRACT FROM AUTHOR]

The changes of the tectonic boundary locations between the stable and unstable shelf on different version of the tectonic maps of Iraq is discussed. It is found that the location of the boundary varies due to philosophy and the type of the geological parameters. It is used the stratigraphic, geophysical, structural, geochemical, hydrological and others information in delineating the boundary. The deep crustal structures obtained by using gravity data show that the boundary shifted toward the west by about 100 km in southern Iraq. According to the review, and when taking in consideration all the geological and geophysical parameters from the lower crust to the earth surface, it is believed that the boundary is not a line but it is a tectonic crustal zone. This zone represents a transition zone between the main two regions of the platform in Iraq, and it is segmented to many subzones by transverse faults. In addition, it is believed that the data of the parameters of shallow geology confirm the eastern side of the main zone, while the lower crustal structures confirm the western side of the main zone. [ABSTRACT FROM AUTHOR]

Rift related magmatism during Permian time in the northern margin of Indian plate is represented by basic dykes in several Himalayan terranes including north western Pakistan. The field relations, mineralogy and whole rock geochemistry of these basic dykes reveal significant textural, mineralogical and chemical variation between two major types (a) dolerite and (b) amphibolite. Intra-plate tectonic settings for both rock types have been interpreted on the basis of low Zr/Nb ratios (< 10), K/Ba ratios (20-40) and Hf-Ta-Th and FeO-MgO-Al2O3 discrimination diagrams. The compositional zoning in plagioclase and clinopyroxene, variation in olivine compositions and major elements oxide trends indicate a vital role of fractional crystallization in the evolution of dolerites, which also show depletion in rare earth elements (REEs) and other incompatible elements compared to the amphibolites. The equilibrium partial melting models from primitive mantle using Dy/Yb, La/Yb, Sm/Yb and La/Sm ratios show that amphibolite formed by smaller degrees (< 5%) of partial melting than the dolerites (< 10%). The trace elements ratios suggest the origination of dolerites from the subcontinental lithospheric mantle with some crustal contamination. This is consistent with a petrogenetic relationship with Panjal trap magmatism, reported from Kashmir and other parts of north western India. The amphibolites, in contrast, show affinity towards Ocean Island basalts (OIB) with a relatively deep asthenospheric mantle source and minimal crustal contribution and are geochemically similar to the High-Ti mafic dykes of southern Qiangtang, Tibet. These similarities combined with Permian tectonic restoration of Gondwana indicate the coeval origin for both dykes from distinct mantle source during continental rifting related to formation of the Neotethys Ocean. [ABSTRACT FROM AUTHOR]

As the largest Cenozoic terrestrial intermountain basin on the Tibetan Plateau, the Qaidam Basin is an ideal setting to understand the coupled controls of tectonics and climate on hydrological evolution. In this study, we used 47,846 data of carbonate and chloride contents from 146 boreholes to reconstruct the Neogene-Quaternary basin-wide hydrological evolution of the Qaidam Basin. Our results show that during the early Miocene (22–15 Ma), the palaeolake in the Qaidam Basin was mainly situated in the southwestern part of the basin, and its water was mostly brackish. From then on, this palaeolake progressively migrated southeastward, and its salinity increased from late Miocene saline water to Quaternary brines. This generally increasing trend of the water palaeosalinity during the late Cenozoic corresponded with regional and global climate changes at that time, suggesting the dominance of climatic control. However, the paces of the salinity increase from sediments in front of the three basin-bounding ranges were not the same, indicating that extra tectonic controls occurred. Sediments in front of the Eastern Kunlun Shan to the southwest and the Altyn Shan to the northwest showed an abrupt, dramatic increase in salinity at ~15 Ma and ~8 Ma, respectively; sediments in front of the Qilian Shan to the northeast showed steady increase without prominent, abrupt changes, indicating the occurrence of asynchronous tectonic controls from the basin-bounding ranges. The late Miocene depocentre migration was synchronous with the hydrological changes in front of the Altyn Shan, while the more significant migration during the Quaternary was consistent with the pulsing, intense extrabasinal and intrabasinal tectonic movements along the Tibetan Plateau. [ABSTRACT FROM AUTHOR]

Cyclic sedimentation has varied at several timescales and this variability has been geologically well documented at Milankovitch timescales, controlled in part by climatically (insolation) driven sea-level changes. At the longer (tens of Myr) timescales connection between astronomical parameters and sedimentation via cyclic solar-system motions within the Milky Way has also been proposed, but this hypothesis remains controversial because of the lack of long geological records. In addition, the absence of a meaningful physical mechanism that could explain the connection between climate and astronomy at these longer timescales led to the more plausible explanation of plate motions as the main driver of climate and sedimentation through changes in ocean and continent mass distribution on Earth. Here we statistically show a prominent and persistent ~36 Myr sedimentary cyclicity superimposed on two megacycles (~250 Myr) in a relatively well-constrained sea-level (SL) record of the past 542 Myr (Phanerozoic eon). We also show two other significant ~9.3 and ~91 Myr periodicities, but with lower amplitudes. The ~9.3 Myr cyclicity was previously attributed to long-period Milankovitch band based on the Cenozoic record. However, the ~91 Myr cyclicity has never been observed before in the geologic record. The ~250 Myr cyclicity was attributed to the Wilson tectonic (supercontinent) cycle. The ~36 Myr periodicity, also detected for the first time in SL record, has previously been ascribed either to tectonics or to astronomical cyclicity. Given the possible link between amplitudes of the ~36 and ~250 Myr cyclicities in SL record and the potential that these periodicities fall into the frequency band of solar system motions, we suggest an astronomical origin, and model these periodicities as originating from the path of the solar system in the Milky Way as vertical and radial periods that modulate the flux of cosmic rays on Earth. Our finding of the ~36 Myr SL cyclicity lends credibility to the existing hypothesis about the imprint of solar-system vertical period on the geological record. The ~250 Myr megacycles are tentatively attributed to a radial period. However, tectonic causal mechanisms remain equally plausible. The potential existence of a correlation between the modeled astronomical signal and the geological record may offer an indirect proxy to understand the structure and history of the Milky Way by providing a 542 Myr long record of the path of the Sun in our Galaxy. [ABSTRACT FROM AUTHOR]

The article discusses the concept of global tectonics and what some of the mobilist mechanisms of unknown magnitude can do and cannot do. It notes that many advocates of the mobilist model feel that some of the fundamental mechanisms or hypotheses of the model are still in the process of transmutation. It adds that the accredited value of mid-oceanic ridge push is only around ten percent of the shear strength that offers frictional resistance at the base of the oceanic crust.

The article examines the continuing problems facing the near hegemonic paradigm of plate tectonics. Topics discussed include the concept involving the removal of protons that resulted in the water having a molecular structure like ice that was electrically negative, the role of artesian water layer in increasing the local gravity field at the surface, causing calculated depths, and study of small vortices observed in flowing streams and rivers.

Under the aegis of plate tectonics, plates are an integral part. Earth's crust is divided into many of them, not the least of which are the Southern, or Antarctic, and African plates. Plates are surrounded by divergent and convergent margins, each being defined by earthquakes. For these two plates, that idea does not hold true. Each is surrounded by divergent margins with very little linear distance in convergent margins, especially the Southern plate. Each continental piece was separated from the parent continental crust long before the formation of intervening spreading centers, or divergent margins. The spread between Antarctica and the Kerguelen Plateau is a case in point. Crustal attenuation to the point that the crust becomes so thin that fractures form could be an answer. The midocean ridges are the result of those fractures being an avenue for leaking excess magma from the asthenosphere. [ABSTRACT FROM AUTHOR]

Remote-sensing data from altimetry and gravity satellite missions combined with seismic information have been used to investigate the Earth’s interior, particularly focusing on the lithospheric structure. In this study, we use the subglacial bedrock relief BEDMAP2, the global gravitational model GOCO05S, and the ETOPO1 topographic/bathymetric data, together with a newly developed (continental-scale) seismic crustal model for Antarctica to compile the free-air, Bouguer, and mantle gravity maps over this continent and surrounding oceanic areas. We then use these gravity maps to interpret the Antarctic crustal and uppermost mantle structure. We demonstrate that most of the gravity features seen in gravity maps could be explained by known lithospheric structures. The Bouguer gravity map reveals a contrast between the oceanic and continental crust which marks the extension of the Antarctic continental margins. The isostatic signature in this gravity map confirms deep and compact orogenic roots under the Gamburtsev Subglacial Mountains and more complex orogenic structures under Dronning Maud Land in East Antarctica. Whereas the Bouguer gravity map exhibits features which are closely spatially correlated with the crustal thickness, the mantle gravity map reveals mainly the gravitational signature of the uppermost mantle, which is superposed over a weaker (long-wavelength) signature of density heterogeneities distributed deeper in the mantle. In contrast to a relatively complex and segmented uppermost mantle structure of West Antarctica, the mantle gravity map confirmed a more uniform structure of the East Antarctic Craton. The most pronounced features in this gravity map are divergent tectonic margins along mid-oceanic ridges and continental rifts. Gravity lows at these locations indicate that a broad region of the West Antarctic Rift System continuously extends between the Atlantic-Indian and Pacific-Antarctic mid-oceanic ridges and it is possibly formed by two major fault segments. Gravity lows over the Transantarctic Mountains confirms their non-collisional origin. Additionally, more localized gravity lows closely coincide with known locations of hotspots and volcanic regions (Marie Byrd Land, Balleny Islands, Mt. Erebus). Gravity lows also suggest a possible hotspot under the South Orkney Islands. However, this finding has to be further verified. [ABSTRACT FROM AUTHOR]

The lithosphere-asthenosphere boundary (LAB) is the seismic discontinuity with negative velocity contrasts in the upper mantle. Seismic detections on the LAB are of great significance in understanding the plate tectonics, mantle convection and lithospheric evolution. In this paper, we study the LAB in the Izu-Bonin subduction zone using four deep earthquakes recorded by the permanent and temporary seismic networks of the USArray. The LAB is clearly revealed with sP precursors (sdP) through the linear slant stacking. As illustrated by reflected points of the identified sdP phases, the depth of LAB beneath the Izu-Bonin Arc (IBA) is about 65 km with a range of 60-68 km. The identified sdP phases with opposite polarities relative to sP phases have the average relative amplitude of 0.21, which means a 3.7% velocity drop and implies partial melting in the asthenosphere. On the basis of the crustal age data, the lithosphere beneath the IBA is located at the ~ 1100 °C isotherm calculated with the GDH1 model. Compared to tectonically stable areas, such as the West Philippine Basin (WPB) and Parece Vela Basin (PVB) in the Philippine Sea, the lithosphere beneath the Izu-Bonin area shows the obvious lithospheric thinning. According to the geodynamic and petrological studies, the oceanic lithospheric thinning phenomenon can be attributed to the strong erosion of the small-scale convection in the mantle wedge enriched in volatiles and melts. [ABSTRACT FROM AUTHOR]

The gravity and magnetic maps of the area between Adana-Kahramanmaras-Hatay provinces were produced from a compilation of data gathered during the period between 1973 and 1989. Reduced to the pole (RTP) and pseudo-gravity transformation (PGT) methods were applied to the magnetic data, while derivative ratio (DR) processing was applied to both gravity and magnetic data, respectively. Bouguer, RTP and PGT maps show the image of a buried structure corresponding to ophiolites under undifferentiated Quaternary deposits in the Adana depression and Iskenderun Gulf. DR maps show two important faults which reflect the tectonic framework in the study area: (1) the Karatas-Osmaniye Fault extending from Osmaniye to Karatas in the south between Adana and Iskenderun depressions and (2) Amanos Fault (southern part of East Anatolian Fault) in the Hatay region running southward from Turkoglu to Amik Basin along Amanos Mountain forming the actual plate boundary between the Anatolian block (part of Eurasian plate) and Arabian plate. [ABSTRACT FROM AUTHOR]

Ganymede’s bright terrain formed during a near-global resurfacing event (or events) that produced both heavily tectonized and relatively smooth terrains. The mechanism(s) by which resurfacing occurred on Ganymede (e.g., cryovolcanic or tectonic), and the relationship between the older, dark and the younger, bright terrain are fundamental to understanding the geological evolution of the satellite. Using a two-dimensional numerical model of lithospheric extension that has previously been used to successfully simulate surface deformation consistent with grooved terrain morphologies, we investigate whether large-amplitude preexisting topography can be resurfaced (erased) by extension (i.e., tectonic resurfacing). Using synthetically produced initial topography, we show that when the total relief of the initial topography is larger than 25–50 m, periodic groove-like structures fail to form. Instead, extension is localized in a few individual, isolated troughs. These results pose a challenge to the tectonic resurfacing hypothesis. We further investigate the effects of preexisting topography by performing suites of simulations initialized with topography derived from digital terrain models of Ganymede’s surface. These include dark terrain, fresh (relatively deep) impact craters, smooth bright terrain, and a viscously relaxed impact crater. The simulations using dark terrain and fresh impact craters are consistent with our simulations using synthetic topography: periodic groove-like deformation fails to form. In contrast, when simulations were initialized with bright smooth terrain topography, groove-like deformation results from a wide variety of heat flow and surface temperature conditions. Similarly, when a viscously relaxed impact crater was used, groove-like structures were able to form during extension. These results suggest that tectonic resurfacing may require that the amplitude of the initial topography be reduced before extension begins. We emphasize that viscous relaxation may be the key to enabling tectonic resurfacing, as the heat fluxes associated with groove terrain formation are also capable of reducing crater topography through viscous relaxation. For long-wavelength topography (large craters) viscous relaxation is unavoidable. We propose that the resurfacing of Ganymede occurred through a combination of viscous relaxation, tectonic resurfacing, cryovolcanism and, at least in a few cases, band formation. Variations in heat flow and strain magnitudes across Ganymede likely produced the complex variety of terrain types currently observed. [ABSTRACT FROM AUTHOR]

The occurrence of km-scale exhumations during syn- and post-rift stages has been documented along Atlantic continental margins, which are also characterised by basins undergoing substantial subsidence. The relationship between the exhuming and subsiding domains is poorly understood. In this study, we reconstruct the evolution of a 50 km long transect across the Moroccan rifted margin from the western Anti-Atlas to the Atlantic basin offshore the city of Sidi Ifni. Low-temperature thermochronology data from the Sidi Ifni area document a ca. 8 km exhumation between the Permian and the Early/Middle Jurassic. The related erosion fed sediments to the subsiding Mesozoic basin to the NW. Basement rocks along the transect were subsequently buried by 1–2 km between the Late Jurassic and the Early Cretaceous. From late Early/Late Cretaceous onwards, rocks present along the transect were exhumed to their present-day position. [ABSTRACT FROM AUTHOR]

The Longyao ground fissure (LGF) is the longest and most active among more than 1000 ground fissures on the North China Plain. There have been many studies on the formation mechanism of the LGF, due to its scientific importance and its potential for damage to the environment. These studies have been based on both regional tectonic analysis and numerical simulations. In order to provide a better understanding of the formation mechanism, the deformation of the crack and its surrounding environment should be taken into consideration. In this paper, PS-InSAR technology was employed to assess the ground deformation of LGF and its surrounding area, using L-band ALOS-1 PALSAR images from 2007 to 2011. The characteristics of ground deformation, relationships between fissure activity and surrounding faults and groundwater exploitation were analyzed. This study shows that the north side of Longyao fault (LF) is uplifting while the south side is subsiding. This provides the tectonic conditions responsible for the activity of the ground fissure. Local groundwater exploitation also plays an important role in the development of ground fissures. InSAR observations were modeled to infer the loading depth (-2.8 km) and the slip rate (31.1 mm/yr) of LF. [ABSTRACT FROM AUTHOR]

The geotectonic position of the Pyrenees mountain massif in the Alpine-Indonesian mobile belt is considered. The geological data testify to the formation of the structure of the Pyrenees in the setting of a subhorizontal compression perpendicular to the ridge. The commonly accepted interpretation considers this compression in the context of plate tectonic notions related to the collision between the Iberian and Eurasian lithospheric plates resulting from the convergence of the Eurasian and African plates. However, this interpretation is challenged by the the geodetic and seismological measurements. The GPS measurements suggest a certain cross-strike spreading rather than shortening of the Earth’s crust; the focal mechanisms of the earthquakes indicate the predominance of a subhorizontal extension perpendicular to the strike of the Pyrenees mountain range. The processes of the gravitational collapse of the mountain chain during the isostatic upwelling of the orogenic crust are considered as the most probable cause of this spreading by a number of the authors. [ABSTRACT FROM AUTHOR]

The paper presents the results from a study of original and published data on the chemical composition and age of mantle peridotites from Sakhalin Island ophiolites. The material and genetic proximity of peridotites from the Berezovsky and Shelting plutons, on the one hand, and mélange zone serpentinites, on the other, have been established. In composition and Fe2O3 and MgO variations, Sakhalin peridotites differ radically from those of the Northeast Asia ophiolite complexes (Krasnogorsky Massif, Karaginsky Island Massif, etc.), which are fragments of Pacific Plate mantle. Conversely, Sakhalin peridotites have a subcontinental genesis and are compositionally close to xenoliths of lherzolites from Hankai Microcontinent mantle (southern Sikhote-Alin). The rythmics of alternation in compression and expansion at the margin of the Asian continent in the last 180 million years, caused by cyclical changes in the Pacific spreading rate, have been considered. According to data obtained by U-Pb dating of zircons, the formation of the Berezovsky Massif took place 169-154 Ma ago during Jurassic expansion of the continental margin. Matching age and composition data demonstrate that the Sakhalin ophiolites formed within the marginal sea basin during riftinduced destruction of the periphery of the Hankai Craton. The assumed tectonic setting was close to that reconstructed for the Jurassic Josephine ophiolites of the California margin of the North American continent. The continental genesis of the studied ophiolites agrees with the age and tectonic mode of ophiolite formation for Sakhalin Island. [ABSTRACT FROM AUTHOR]

During breakup of the continental lithosphere, partial or complete separation of small continental blocks from the mainland frequently occurs, leading to the formation of microcontinents or partially separated submerged plateaus that advance toward the ocean, as well as to emergent nonvolcanic islands. The article reviews the geodynamic settings in which isolated blocks of continental crust can form. Depending on the thermomechanical conditions of continental rifting, such blocks may be preserved as emergent islands or as submerged blocks of continental crust. [ABSTRACT FROM AUTHOR]

A change in tectonics affects erosion rates across a mountain belt, leading to a period of non-steady sediment flux delivery downstream. The nonlinear relationship between tectonics and transient sediment delivery from an erosional catchment makes extraction of paleo-tectonic signals from stratigraphy challenging. We use a numerical landscape evolution model to explore how sediment flux from an erosional watershed responds to non-steady rock uplift. We focus on the time lag between the onset of a rock uplift change and the onset of a corresponding change in the sediment flux and the magnitude of the sediment flux relative to the steady rate. We observe that (1) sediment flux does not always record changes in the rock uplift rate when the duration of a rock uplift interval is less than 25% of landscape response time, or time for a landscape to transition from one steady state to another after a perturbation; (2) sediment flux response to variable rock uplift is positively correlated with the duration of rock uplift intervals; and (3) a nonlinear response between erosion rates and tectonic perturbations can result in increasing sediment flux through time even after rock uplift rate decreases. How quickly the sediment flux signal responds to a perturbation depends on how close the landscape was to steady state before the perturbation. These results illustrate conditions under which tectonic signals have the potential to be stored in the stratigraphic record or lost in an erosional system, and the importance of network dynamics for understanding signal propagation. [ABSTRACT FROM AUTHOR]

The buckling phenomenon of a flat or spherical shell lithosphere (tectonic plate) has been investigated in previous research. However, these studies do not give information regarding the curvature effect in the buckling phenomenon. Kondo applied Riemannian geometry to the yielding or buckling of curved materials. When the Riemannian manifold (Vn dimensional manifold) with a nonzero Euler-Schouten curvature tensor is manifested in the enveloping manifold (Euclid space: Vm dimensional manifold), the included Riemannian manifold (dimension Vn) protrudes into the enveloping manifold (dimension Vm). The curvature effect for the buckling phenomenon of materials can be formulated by a force-balance equation from mechanics and the Euler-Schouten curvature tensor from differential geometry. In this paper, using the Euler-Schouten curvature tensor from differential geometry, the authors derive a formulation for the buckling phenomenon with the curvature effect for a spherical shell lithosphere as a buckling equation with high-order strain for lithosphere deformation. [ABSTRACT FROM AUTHOR]

The western margin of the Yangtze Block is characterized by numerous felsic intrusions and many mafic-ultramafic plutons that record the lithospheric thermal state of South China during the Neoproterozoic. Numerous ~800 Ma mafic dikes, intruded into the Neoproterozoic Dengxiangying Group, consist of fine- to medium-grained diabase and have variable SiO2 (42.80-57.20 wt%), MgO (5.16-13.15 wt%), and total alkalis (0.05-4.41 wt%). They show light rare earth element-enriched patterns with negative to slightly positive Eu anomalies (Eu=Eu*/ p 0:67-1:09) and display enrichment of Pb and depletion of Nb, Ta, and Sr in the primitive mantle-normalized trace-element diagram. Rocks from the Dengxiangying mafic dikes have variable initial 87Sr/86Sr ratios (0.652431-0.730039) but relatively constant εNd values (22.5 to 12.6) and Pb isotope ratios (206Pb=204Pb p 17:53-17:88, 207Pb=204Pb p 15:60-15:63, 208Pb=204Pb p 37:66-38:38). These geochemical features suggest that the mafic dikes were derived from a lithospheric mantle enriched by slab-derived materials. PRIMELT3 modeling for the Neoproterozoic mafic rocks in the western margin of the Yangtze Block reveals that their primary magmas were in equilibrium with harzburgite and spinel peridotite with potential temperatures of 1377°-1498°C. They have high primary magmatic water contents (1900-72,000 ppm) that are similar to those of other arc basalts. These lines of evidences confirm that the Neoproterozoic mafic and ultramafic rocks were formed in an active continental setting. [ABSTRACT FROM AUTHOR]