Craton Edge Research Articles

Mantle structure and dynamics at the eastern boundary of the northern Cascadia backarc

The tectonics of southwestern Canada are dominated by the Cascadia subduction zone. The northern Cascadia backarc encompasses a > 400 km wide region of the Southern Canadian Cordillera. Geophysical observations, including seismic tomography and surface heat flow, show that the backarc is characterized by a hot, thin lithosphere (60–70 km). The eastern limit of the backarc approximately underlies the Rocky Mountain Trench, where there is an abrupt eastward increase in lithosphere thickness to the ∼250 km thick North American (Laurentian) Craton. Seismic tomography studies show that the transition in lithosphere thickness occurs over a horizontal distance of 50–100 km, resulting in a subvertical to west-dipping lithosphere step, with a dip angle of 75–90°. Using numerical models, we show that such a structure can be readily destabilised by internal buoyancy forces, edge-driven convection, and shearing by regional mantle flow. To maintain a subvertical step for > 50 Myr, the lowermost craton mantle lithosphere must be both dry and moderately chemically depleted. The observed westward dip may reflect partial lateral extrusion of the lowermost craton lithosphere, as well as shearing from west-directed mantle flow associated with the Cascadia subduction zone. The models also show that the backarc mantle must be relatively weak, such that vigorous convection maintains the hot, thin lithosphere. This also provides a mechanism to explain the observed lateral seismic gradient between the low-velocity backarc mantle and high-velocity craton. Our models demonstrate that the eastern limit of the Cascadia backarc is a region of active mantle flow, including possible slow deformation of the craton edge.

Zinc on the edge—isotopic and geophysical evidence that cratonic edges control world-class shale-hosted zinc-lead deposits

The North Australian Zinc Belt is the largest zinc-lead province in the world, containing three of the ten largest known individual deposits (HYC, Hilton-George Fisher, and Mount Isa). The Northern Cordillera in North America is the second largest zinc-lead province, containing a further two of the world’s top ten deposits (Red Dog and Howards Pass). Despite this world-class endowment, exploration in both mineral provinces during the past 2 decades has not been particularly successful, yielding only two significant discoveries (Teena, Australia, and Boundary, Canada). One of the most important aspects of exploration is to choose mineral provinces and districts within geological belts that have the greatest potential for discovery. Here, we present results from these two zinc belts that highlight previously unused datasets for area selection and targeting. Lead isotope mapping using analyses of mineralized material has identified gradients in μ (238U/204Pb) that coincide closely with many major deposits. Locations of these deposits also coincide with a gradient in the depth of the lithosphere-asthenosphere boundary determined from calibrated surface wave tomography models converted to temperature. Furthermore, gradients in upward-continued gravity anomalies and a step in Moho depth correspond to a pre-existing major crustal boundary in both zinc belts. A spatial association of deposits with a linear mid- to lower-crustal resistivity anomaly from magnetotelluric data is also observed in the North Australian Zinc Belt. The change from thicker to thinner lithosphere is interpreted to localize prospective basins for zinc-lead mineralization and to control the gradient in lead isotope and geophysical data. These data, when combined with data indicative of paleoenvironment and changes in plate motion at the time of mineralization, provide new exploration criteria that can be used to identify prospective mineralized basins and define the most favorable parts of these basins.

ADJUSTMENT OF THRUSTING STRUCTURE IN THE KOVYKTA-KHANDINSKAYA REFLECTED FOLDING ZONE

The Kovykta-Khandinskaya zone, which includes the Kovykta gas condensate field, is a giant located in the junction zone of the Angara-Lena stage with the craton edge – the Cis-Baikal trough. Here, according to the results of long-term geological exploration, the Verkhnelenskoe uplift was identified, the northern part of which is reflected on tectonic maps as the Kovykta ledge. At the same time, the geological model of the field today is based on the standard two-member model, which includes a slightly disturbed sedimentary cover and basement. However, new geophysical studies revealed that the sedimentary cover of the eastern part of the zone is intensely deformed and has a two-tiered nodular-thrust structure (the lower layer is autochthon, the upper layer is allochthon). The main object for gas exploration within the Kovykta gas condensate field is the Vendian formation. The middle (halogen-carbonate) section has been studied fragmentarily, mainly as an object of geological geohazards when drilling deep wells. The involvement of new data from 3D seismic and 3D transient electromagnetic methods made it possible to clarify the Kovykta gas condensate field tectonic structure to assess the potential of secondary carbonate reservoirs characterized by intense fluid (natural gas, brines) inflows.

Surface‐Wave Tomography of the Northern Canadian Cordillera Using Earthquake Rayleigh Wave Group Velocities

AbstractWe measure fundamental‐mode Rayleigh wave group velocity dispersion from regional earthquakes and employ transdimensional Bayesian tomography to invert for azimuthally anisotropic group velocity maps at 10–60 s periods. Seismic azimuthal anisotropy within the crust and uppermost mantle shows fast‐axis alignment with principal northern Canadian Cordillera (NCC) tectonic structures suggesting large‐scale and coherent deformation across the crust and upper mantle. We extract group velocity dispersion curves at each grid point of the isotropic group velocity maps and invert them independently for VS as a function of depth in a probabilistic framework. At mid‐to‐lower crustal depths, our pseudo 3‐D VS model reveals an anomalously low VS structure (<3.8 km/s) across the NCC, which likely reflects elevated crustal temperatures that buoyantly support regional high elevations. Two high‐velocity regions (≥4.5 km/s) at uppermost mantle depths underpin the edges of the arcuate Mackenzie fold and thrust belt. We observe variations in the dip direction of the high‐velocity craton edge in the upper mantle, from subvertical in the northern NCC to westward in the southern NCC. We interpret a low‐velocity region (≤4.3 km/s) between the Tintina and Denali faults at uppermost mantle depths to reflect upwelling asthenosphere, which likely postdates 430 km of previously estimated Eocene lithosphere‐scale horizontal displacement along the Tintina fault. Our surface‐wave model resolves a high‐velocity region (≥4.4 km/s) dipping north‐northeast below the Wrangell volcanic field, which we interpret as a down‐going segment of the Pacific slab and may contribute to the observed volcanism in the area.

Linking Intraplate Volcanism to Lithospheric Structure and Asthenospheric Flow

AbstractSeveral of Earth's intraplate volcanic provinces are hard to reconcile with the mantle plume hypothesis. Instead, they exhibit characteristics that are more compatible with shallower processes that involve the interplay between uppermost mantle flow and the base of Earth's heterogeneous lithosphere. The mechanisms most commonly invoked are edge‐driven convection (EDC) and shear‐driven upwelling (SDU), both of which act to focus upwelling flow and the associated decompression melting adjacent to steps in lithospheric thickness. In this study, we undertake a systematic numerical investigation, in both 2‐D and 3‐D, to quantify the sensitivity of EDC, SDU, and the associated melting to key controlling parameters. Our simulations demonstrate that the spatio‐temporal characteristics of EDC are sensitive to the geometry and material properties of the lithospheric step, in addition to the magnitude and depth‐dependence of upper‐mantle viscosity. These simulations also indicate that asthenospheric shear can either enhance or reduce upwelling velocities and the associated melting, depending upon the magnitude and orientation of flow relative to the lithospheric step. When combined, such sensitivities explain why step changes in lithospheric thickness, which are common along cratonic edges and passive margins, only produce volcanism at isolated points in space and time. Our predicted trends of melt production suggest that, in the absence of potential interactions with mantle plumes, EDC and SDU are viable mechanisms only for Earth's shorter‐lived, lower‐volume intraplate volcanic provinces.

Tracking decratonization process along a cratonic edge through late Permian to late Triassic magmatic flare-up in northwestern Liaoning, North China Craton

Resolving magmatic tempos of multiple scales along convergent plate margins presents a pivotal task in any regional tectonic and supercontinental reconstructions, as is of paramount importance for the northern North China Craton (NCC) that converged with the Central Asian Orogenic belt (CAOB) during the Paleozoic to early Mesozoic. This study applies the integrated zircon U–Pb dating and geochemical tracing to late Permian to late Triassic intrusive flare-up in northwestern Liaoning, with three episodes of granitic suites deciphered. Their contrasting elemental and isotopic features called for three distinct crustal end-members involved in their formation: ancient amphibolitic protolith, Paleozoic-accreted mafic meta-igneous ingredients and relaminated juvenile intermediate arc plutons. Sequential crustal anatexis of these multiply juxtaposed protoliths in a felsic hot zone tends to yield successive parental magmas for forming three granite suites. While the late Permian (260–250 Ma) granites, with their felsic adakitic affinity and variably evolved isotopic compositions (87Sr/86Sri = 0.70637 to 0.70659, εNd(t) = −16.7 to −10.2, zircon εHf(t) = −18.1 to +1.1), are consistent with partial melts of major ancient crustal materials and minor newly-underplated mafic ingredients, the middle Triassic (242–240 Ma) granites show a more juvenile isotope signature (εNd(t) = −11.8 to −4.7, zircon εHf(t) = −14.7 to +4.1) and attest to distinctively higher input from newly-underplated mafic lower crust in their formation. By contrast, the late Triassic (227–223 Ma) granites exhibit an A-type magmatic affinity. Their radiogenic whole-rock Nd (εNd(t) = −3.7 to +1.1) and zircon Hf (εHf(t) = +0.7 to +12.0) isotopic values suggest magma derivation from high-temperature fusion of dominantly relaminated juvenile charnockitic protolith. What follows is one phase of shoshonitic mafic dykes that fingerprint a slab-melt metasomatized asthenosphere source with depleted isotopic compositions (87Sr/86Sri = 0.70374 to 0.70388, εNd(t) = +3.5 to +4.1). Synthesizing these events with the previously recognized ones leads to a first-order duration of ca. 260–218 Ma for the flare-up and its further differentiation into three secondary pulses including ca. 260–250 Ma appinite and granite, ca. 242–238 Ma ferroan granites and ca. 227–218 Ma A-type granites and mafic dykes. While such primary duration is typical of post- collisional/orogenic magmatic tempos of convergent continental margins, three secondary pulses encapsulate successive geodynamic processes from crustal thickening through orogenic collapse to lithospheric dripping in the aftermath of the Paleo-Asian oceanic closure. Featuring a coupled scenario of lithospheric thinning and crustal growth, the northwestern Liaoning case of cratonic-edge destruction serves as a prelude but also a contrast to the late Mesozoic large-scale decratonization across the eastern NCC possibly due to oceanic subduction of the paleo-Pacific plate.

The missing craton edge: Crustal structure of the East European Craton beneath the Carpathian Orogen revealed by double-difference tomography

The Trans-European Suture Zone (TESZ) is the most important and extensive continental suture in Europe, marking the edge of the East European Craton (EEC), from the North Sea to the Black Sea. It corresponds to significant changes in surface geology and deep crustal structure, evident in seismic, gravitational and magnetic studies. However, the TESZ disappears beneath the Eastern Carpathians accretionary nappes and Neotethys ophiolites, thrusted over the subducted EEC passive margin in Romania that may have experienced progressive southward break-off generating post-collisional volcanism and anomalous seismicity in the Vrancea region of the South-East Carpathians. To illuminate the missing TESZ section and investigate the change in crustal properties from the Precambrian EEC across the collisional orogen and the impact of related volcanism, we determined a 3D seismic model of P and S wave velocities of the Eastern Carpathians in Romania. With the advent of new permanent broadband and short-period seismic stations of the Romanian Seismic Network, we were able to lower the earthquake magnitude detection threshold to 0.5 ML, largely expanding the earthquake database and create seismic images of the crust across the Carpathians. Using double-difference tomography and waveform cross-correlation differential times, we relocated local crustal earthquakes between 2010 and 2017 and jointly inverted for the 3D P and S -wave velocity structure down to the Moho discontinuity. Our study provides the highest resolution 3D crustal seismic model of this area to date and emphasizes the manifestation of surface tectonic boundaries at lower crustal depths. The TESZ is highlighted at mid-to lower-crustal depths beneath the Carpathian nappes, east of the post-collisional volcanic belt, as a gently-dipping transition from high Vp in the eastern footwall, the dipping Precambrian basement, to low Vp beneath the Carpathian Orogen to the west and a downward decrease in Vp/Vs. In parts of the volcanic region, Vp/Vs ratios increase suggesting the presence of fluids, mafic material and partial melts that may have altered the seismic structure of the TESZ. Relocated hypocenters are vertically distributed in the velocity transition zone from low to high especially beneath the youngest volcanoes in the south, consistent with a hypothetically active magmatic plumbing system. In the northern segment, where volcanoes are older, relatively high velocities are estimated and seismicity tends to cluster in the top 10 km, likely connected to a recently reactivated fault system. Seismicity in the foreland of the East Carpathians aligns along the NW-SE trending major crustal faults and continues beneath the thrust belt, suggesting the East European Craton basement extends beneath the orogen as far as the volcanic belt.

Passive seismic imaging of a craton edge – Central Australia

Passive seismic recordings of teleseismic P wave arrivals and their immediate coda along a dense profile of stations can be used to image the reflection structure beneath the profile. The process exploits the autocorrelation of the seismic signals that extracts the reflection response from the transmitted signals recorded at the surface, which can be migrated to provide a depth image. This common reflection point (CRP) imaging exploits the same portion of the seismic record used in receiver function studies, but is much less influenced by multiple problems in the presence of sediments. This style of passive seismic imaging is applied to a dense line of 68 stations at 3.5 km spacing at the northern edge of the Gawler Craton in South Australia in the MAL experiment. With nine months of passive seismic monitoring it has been possible to image fine-scale structural variations throughout the lithosphere. There is a strong variation in crustal thickness as the MAL profile crosses the inferred location of the craton edge under sedimentary cover. A transitional zone between crust and mantle can be identified that extends beneath the receiver function Moho, pinching out gently at the craton edge. The CRP imaging reveals distinct reflectivity in the lithospheric mantle with horizontal scales of 10–30 km, and vertical scales of no more than a few kilometres. The character of this reflectivity appears to change across the craton edge. There are indications of a semi-continuous mid-lithosphere discontinuity at around 90 km depth.

Global distribution of sediment-hosted metals controlled by craton edge stability

Sustainable development and the transition to a clean-energy economy drives ever-increasing demand for base metals, substantially outstripping the discovery rate of new deposits and necessitating dramatic improvements in exploration success. Rifting of the continents has formed widespread sedimentary basins, some of which contain large quantities of copper, lead and zinc. Despite over a century of research, the geological structure responsible for the spatial distribution of such fertile regions remains enigmatic. Here, we use statistical tests to compare deposit locations with new maps of lithospheric thickness, which outline the base of tectonic plates. We find that 85% of sediment-hosted base metals, including all giant deposits (>10 megatonnes of metal), occur within 200 kilometres of the transition between thick and thin lithosphere. Rifting in this setting produces greater subsidence and lower basal heat flow, enlarging the depth extent of hydrothermal circulation available for forming giant deposits. Given that mineralization ages span the past two billion years, this observation implies long-term lithospheric edge stability and a genetic link between deep Earth processes and near-surface hydrothermal mineral systems. This discovery provides an unprecedented global framework for identifying fertile regions for targeted mineral exploration, reducing the search space for new deposits by two-thirds on this lithospheric thickness criterion alone. Major sediment-hosted base metal deposits are located within 200 km of the border between thick and thin lithosphere, according to statistical comparisons between global lithospheric thickness and known deposit locations.

Displaced cratonic mantle concentrates deep carbon during continental rifting.

Continental rifts are important sources of mantle carbon dioxide (CO2) emission into Earth's atmosphere1-3. Because deep carbon is stored for long periods in the lithospheric mantle4-6, rift CO2 flux depends on lithospheric processes that control melt and volatile transport1,3,7. The influence of compositional and thickness differences between Archaean and Proterozoic lithosphere on deep-carbon fluxes remains untested. Here we propose that displacement of carbon-enriched Tanzanian cratonic mantle concentrates deep carbon below parts of the East African Rift System. Sources and fluxes of CO2 and helium are examined over a 350-kilometre-long transect crossing the boundary between orogenic (Natron and Magadi basins) and cratonic (Balangida and Manyara basins) lithosphere from north to south. Areas of diffuse CO2 degassing exhibit increasing mantle CO2 flux and 3He/4He ratios as the rift transitions from Archaean (cratonic) to Proterozoic (orogenic) lithosphere. Active carbonatite magmatism also occurs near the craton edge. These data indicate that advection of the root of thick Archaean lithosphere laterally to the base of the much thinner adjacent Proterozoic lithosphere creates a zone of highly concentrated deep carbon. This mode of deep-carbon extraction may increase CO2 fluxes in some continental rifts, helping to control the production and location of carbonate-rich magmas.

Significance of hydrothermal reworking for REE mineralization associated with carbonatite: Constraints from in situ trace element and C-Sr isotope study of calcite and apatite from the Miaoya carbonatite complex (China)

A majority of carbonatite-related rare earth element (REE) deposits are found in cratonic margins and orogenic belts, and metasomatic/hydrothermal reworking is common in these deposits; however, the role of metasomatic processes involved in their formation remains unclear. Here, we present a comprehensive in situ chemical and isotopic (C-Sr) investigation of calcite and fluorapatite within the Miaoya carbonatite complex located in the South Qinling orogenic belt, with the aim to better define the role of late-stage metasomatic processes.Carbonatite at Miaoya commonly occurs as stocks and dykes intruded into associated syenite, and can be subdivided into equigranular (Type I) and inequigranular (Type II) calcite carbonatites. Calcite in Type I carbonatite is characterized by the highest Sr (up to ∼22,000 ppm) and REE (195–542 ppm) concentrations with slight LREE-enriched chondrite normalized patterns [(La/Yb)N = 2.1–5.2]. In situ C and Sr isotopic compositions of calcite in Type I carbonatite define a limited range (87Sr/86Sr = 0.70344–0.70365; δ13C = −7.1 to −4.2 ‰) that are consistent with a mantle origin. Calcite in Type II carbonatite has lower Sr (1708–16322 ppm) and REEs (67–311 ppm) and displays variable LREE-depleted chondrite normalized REE patterns [(La/Yb)N = 0.2–3.3; (La/Sm)N = 0.2–2.0]. In situ 87Sr/86Sr and d13C isotopic compositions of Type II calcite are highly variable and range from 0.70350 to 0.70524 and −7.0 to −2.2 ‰, respectively. Fluorapatite in Type I and Type II carbonatites is characterized by similar trace-element and isotopic compositions. Both types of fluorapatite display variable trace element concentrations, especially LREE contents, whereas they exhibit relatively consistent near-chondritic Y/Ho ratios. Fluorapatite is characterized by consistent Sr isotopic compositions with a corresponding average 87Sr/86Sr ratio of 0.70359, which suggests that fluorapatite remained relatively closed in relation to contamination. The combined geochemical and isotopic data for calcite and fluorapatite from the Miaoya complex suggest that carbonatite-exsolved fluids together with possible syenite assimilation during the Mesozoic metasomatism overprinted the original trace-element and isotopic signatures acquired in the early Paleozoic magmatism. Hydrothermal reworking resulted in dissolution-reprecipitation of calcite and fluorapatite, which served as the dominant source of REE mineralization during the much younger metasomatic activity. The results from this study also suggest that carbonatites located in orogenic belts and cratonic edges possess a great potential for forming economic REE deposits, especially those that have undergone late-stage metasomatic reworking.

Variable modification of continental lithosphere during the Proterozoic Grenville orogeny: Evidence from teleseismic P-wave tomography

Cratons, the ancient cores of the continents, have survived thermal and mechanical erosion over multiple Wilson cycles, but the ability of their margins to withstand modification during continental convergence is debated. The Proterozoic Grenville orogeny operated for ≥300 Myr along the eastern edge of the proto-North American continent Laurentia, whose age varied north-to-south from ∼1.5−0.25 Gyr at the time of collision. The preserved Grenville Province, west of the Appalachian terranes, has remained largely tectonically quiescent since its formation. Thick, cool, mantle lithosphere that underlies these Proterozoic regions is typically identified by elevated seismic velocities but lithospheric modification by fluid/melt-derived metasomatic enrichment above a subduction zone, can lead to a reduction in VP with little effect on VS and density. Absolute P-wavespeed constraints are therefore a vital complement to existing S-wave tomographic models of North America to investigate craton edge modification mechanisms in the Grenville orogen.New P-wave tomographic imaging of the North American continent, which benefits from recent developments in arrival-time processing of regional network deployments from the Canadian shield, reveals along strike wavespeed variation in the Grenville orogen. In the north, high seismic wavespeeds (to depths of 250 km) extend eastwards, from the Archean core of North America to beneath the Canadian Grenville Province. In contrast, below the southern U.S., high lithospheric wavespeeds are restricted to west of the Grenville Province, in particular at depths less than 150 km. We argue that subduction-derived metasomatism beneath eastern Laurentia modified the southern Grenville, prior to thermal stabilization and perhaps mantle keel formation. Beneath the northern Grenville, the thick, depleted Laurentian lithosphere resisted extensive metasomatism. Along strike age differences in Grenvillian terranes and their resulting metasomatic modification histories suggests that at least 250 Myr is required for Proterozoic lithosphere to gain resistance to modification.

The impact of Aptian glacio‐eustasy on the stratigraphic architecture of the Athabasca Oil Sands, Alberta, Canada

AbstractHigh‐frequency incised valley systems and subaerial exposure surfaces are prevalent in Aptian‐aged units, having been documented in numerous basins around the world. In light of a growing body of palynological, geochemical and sedimentological evidence, a glacio‐eustatic mechanism for their generation has been suggested, driven by the waxing and waning of polar ice caps. The densely‐penetrated McMurray Formation of north‐eastern Alberta, Canada, provides a unique opportunity to test for an Aptian glacio‐eustatic signal within the Western Canadian Foreland Basin. This study combines detailed drill core description with wireline log‐based stratigraphic analysis to map and evaluate a series of drainage networks located along the cratonic edge of the Western Canadian Foreland Basin. Four stratigraphically distinct composite channel‐form bodies are documented across an area >14 000 km2. Composite channel‐form bodies are up to 50 km wide and locally exceed 70 m thick, subtending from upward‐coarsening parasequence sets 2 to 12 m thick; bounding marine flooding surfaces are mappable over distances exceeding 280 km along the north–south trending basin. Detailed internal fill characterization of the four composite channel‐form bodies suggests that three represent tidal–fluvial incised valley networks, and one represents a deltaic distributary system. At least six cycles of transgression and regression are apparent within the McMurray Formation stratigraphy. Recent biostratigraphic assessment has constrained deposition of the McMurray Formation between ca 125 Ma and 118 Ma, and periods on the order of 0·7 to 1·7 Ma are estimated for the mapped transgressive–regressive cycles. The high frequency nature of these Aptian sequences suggests that the sea‐level oscillations recorded in McMurray Formation strata are consistent with a glacio‐eustatic driver. Linking the internal stratigraphic architecture of the McMurray Formation with interpreted global cooling events and sea‐level drops provides the first evidence of glacio‐eustasy within Early Cretaceous strata of western Canada.

Reconstruction of initial thickness and geometry of the Lower Palaeozoic strata in the Podlasie and Baltic basins, East European Craton

The aim of this study was to use the structural restoration technique to verify the correctness of the structural and palaeothickness maps created during the BLUE GAS Project. On the basis of well data as well as refined structural and palaeothickness maps of Cambrian, Ordovician, Silurian, Devonian, Carboniferous, Permian, Triassic, Jurassic and Cretaceous horizons, a cross-section running across the Baltic Basin, the Mazury High and the Podlasie Basin was created. During the restoration process, the effects of compaction and fault activity were removed sequentially. The amount of erosion was estimated on the basis of the corrected palaeothickness maps. The resulting restoration is geologically reasonable and therefore both the structural and palaeothickness maps should be regarded as reliable. The reconstruction also allowed reproduction of the initial geometry and thickness of the Cambrian–Devonian strata and the recognition of three main episodes in the evolution of the sedimentary cover of this part of the East European Craton. The first episode was related to the deposition of the Lower Palaeozoic (up to the Lower Devonian) sedimentary complex on the relatively flat surface of the East European Craton edge. During the second episode, lasting most probably to the Permian, the Baltic and Podlasie Basins subsided significantly. The amount of subsidence was much higher in the Podlasie Basin. The third episode is related to the deposition of the almost flat-lying Mesozoic–Cainozoic complex.

Седиментология и стратиграфическая корреляция вендских отложений на юго-западе Сибирской платформы: выдающийся вклад внешнего источника кластического материала в образование осадочных систем

The study focuses on the sedimentation and stratigraphy of a large Vendian (Ediacaran) sedimentary basin in the southwestern Siberian craton, with implications for the origin of its units and subunits. Sediments are been correlated along the craton edge and inward into the craton interior using stratigraphic sequences, including glacial and post-glacial deposits. Clastic complexes in the Vendian: upper Chapa, Taseeva, Oselok, and Baikal groups composed of fluvial deposits are of special importance for correlations. The structure of the continental Aysa Fm. of the Oselok Group in the Sayany area has been characterized in detail, based on universal classification of lithofacies and their sequences, and correlated with sediments in the craton interior. There are two Ediacaran subunits: a continental-marine complex below and a continental complex above. The continental-marine subunit in the subsided craton margins was deposited mainly upon postglacial plains (sandurs) and marginal sea shelves, with contributions from intracratonic clastic sources. The deposits of the continental subunit belong to centripetal fluvial systems directed from distant craton-margin mountain upland toward the epicontinental sea in the central Siberian craton. The continental subunit is composed of large sections of red and variegated fluvial deposits from a foreland basin which was accumulated mainly in foredeeps in the over-compensation setting and partly reached the slopes of forebulges. Big deep sand rivers with large channel bars were formed at early and late during orogenic activity, while perennial anastomosing and braided mud-sand rivers with levees originate during passive stages of orogeny. The largest progradation of fluvial systems onto the Siberian craton is marked by channel deposits of the lower members from the Nemchanka, Moshakovka, Aysa, Khuzhir and Kachergat fms., as well as by the Bokhan sandstone member of the Chora Fm.

Lithospheric structure of the southeastern margin of the Tibetan Plateau from Rayleigh wave tomography

AbstractLithospheric shear wave velocity beneath the southeastern margin of the Tibetan Plateau is obtained from Rayleigh wave tomography using earthquake data recorded by the temporary ChinArray and permanent China Digital Seismic Array. Fundamental mode Rayleigh wave phase velocities at periods of 20–100 s are determined and used to construct the 3‐D shear wave velocity model. Low‐velocity anomalies appear along or close to the major faults in the middle crust and become a broad zone in the lower crust, suggesting block extrusion in the shallow crust and diffuse deformation in the lower crust, both of which play important roles in accommodating the collision between the Indian and Eurasian plates. A vertical low‐velocity column beneath the Tengchong Volcano is observed, which could be caused by upwelling of warm mantle due to the lithosphere extension in the Thailand rift basin to the south or by fluid‐induced partial melting due to the subduction of the Burma slab. The western Yangtze Craton is characterized by low velocity in the crust and uppermost mantle above the fast mantle lithosphere, indicating possible thermal erosion at the western craton edge resulted from the extrusion of the Tibetan Plateau. A low‐velocity zone is imaged at the depths of 70–150 km beneath the eastern part of the Yangtze Craton, which could be caused by small‐scale mantle convection associated with the subduction of the Burma microplate and/or the opening of the South China Sea.

Recycling of metal-fertilized lower continental crust: Origin of non-arc Au-rich porphyry deposits at cratonic edges

Recent studies argue that subduction-modified, Cu-fertilized lithosphere controls the formation of porphyry Cu deposits in orogenic belts. However, it is unclear if and how this fertilization process operates at cratonic edges, where numerous large non-arc Au-rich deposits form. Here we report data from lower crustal amphibolite and garnet amphibolite xenoliths hosted by Cenozoic stocks that are genetically related to the Beiya Au-rich porphyry deposits along the western margin of the Yangtze craton, China. These xenoliths are thought to represent cumulates or residuals of Neoproterozoic arc magmas ponding at the base of arc at the edge of the craton that subsequently underwent high-pressure metamorphism ca. 738 Ma. The amphibolite xenoliths are enriched in Cu (383–445 ppm) and Au (7–12 ppb), and a few garnet amphibolite xenoliths contain higher Au (6–16 ppb) with higher Au/Cu ratios (2 × 10 −4 to 8 × 10 −4 ) than normal continental crust. These data suggest that metal fertilization of the base of an old arc at the edge of the craton occurred in the Neoproterozoic via subduction modification, and has since been preserved. The whole-rock geochemical and zircon Hf isotopic data indicate that melting of the Neoproterozoic Cu-Au–fertilized low-crustal cumulates at 40–30 Ma provided the metal endowment for the Au-rich porphyry system at the cratonic edge. We therefore suggest that the reactivated cratonic edges, triggered by upwelling of asthenosphere, have the potential to host significant Au ore-forming systems, especially non-arc Au-rich porphyry deposits.

A complete Wilson Cycle recorded within the Riacho do Pontal Orogen, NE Brazil: Implications for the Neoproterozoic evolution of the Borborema Province at the heart of West Gondwana

The Riacho do Pontal Orogen (RPO) is part of the extensive Brasiliano/Pan-African orogenic system in northeastern South America (Borborema Province). This region is crucial for understanding the Neoproterozoic evolution of West Gondwana because it occupies a strategic position between three of its most important constituents, namely the São Francisco/Congo, Amazon and West Africa cratons. Based on lithostratigraphic, lithochemical, geochronological and isotopic data, a five-stage model for the geodynamic evolution of the RPO and adjacent areas in the Borborema Province is proposed: (1) rift phase – Development of a triple junction rift system at ca. 900–820Ma ago, probably related to the emplacement of a mantle plume and leading to intense mafic–ultramafic magmatism of the Brejo Seco layered intrusion (ca. 900Ma). This was followed by the development of a continental rift system testified by the Paulistana Complex metavolcanosedimentary sequence (ca. 882Ma); (2) drift phase – With continued lithospheric thinning, the rift system further evolved to a broad passive margin in the northern São Francisco craton edge, represented by the Barra Bonita Formation platformal sediments. The drift phase culminated in the development of true oceanic crust, represented by the Monte Orebe ophiolite, around 820–630Ma ago; (3) convergence phase – At circa 630–620Ma, onset of subduction in the Borborema Province caused the inversion of the basins, obduction of oceanic crust slices, and sedimentation of the Mandacaru Formation syn-orogenic greywackes; (4) collisional phase – Continental collision between the São Francisco craton (lower plate) and the Pernambuco-Alagoas block (upper plate) occurred around 620–590Ma, with the stacking of the Casa Nova nappes upon the lower plate, crustal thickening, deformation, metamorphism, melt generation and intrusion of the syn-collisional Rajada Suite two-mica granites; (5) lateral escape phase – The collisional stage was followed by a lateral escape tectonics stage around 590–530Ma, that generated the western branch of the E–W trending Pernambuco Shear Zone, which truncates the northern part of the orogen and displaces its counterparts in the Transversal Zone in a dextral-sense shear. This phase was accompanied by extensive alkaline magmatism in the form of the Serra da Aldeia/Caboclo suites. Thus, the RPO represents a complete plate tectonic cycle that spans the whole Neoproterozoic and preserve excellent time markers for both the break-up and dispersion of the Rodinia Supercontinent fragments and for the re-amalgamation of those fragments in West Gondwana. The interpretation of a complete Wilson Cycle in this area challenges current views of the Borborema Province as a coherent block since Paleoproterozoic times (part of the Atlantica Supercontinent); suggesting, instead, a dynamic setting where multiple plates interacted to form the heart of West Gondwana.

Subduction beneath Laurentia modified the eastern North American cratonic edge: Evidence from P wave and S wave tomography

AbstractThe cratonic cores of the continents are remarkably stable and long‐lived features. Their ability to resist destructive tectonic processes is associated with their thick (∼250 km), cold, chemically depleted, buoyant lithospheric keels that isolate the cratons from the convecting mantle. The formation mechanism and tectonic stability of cratonic keels remains under debate. To address this issue, we use P wave and S wave relative arrival‐time tomography to constrain upper mantle structure beneath southeast Canada and the northeast USA, a region spanning three quarters of Earth's geological history. Our models show three distinct, broad zones: Seismic wave speeds increase systematically from the Phanerozoic coastal domains, through the Proterozoic Grenville Province, and to the Archean Superior craton in central Québec. We also recover the NW‐SE trending track of the Great Meteor hot spot that crosscuts the major tectonic domains. The decrease in seismic wave speed from Archean to Proterozoic domains across the Grenville Front is consistent with predictions from models of two‐stage keel formation, supporting the idea that keel growth may not have been restricted to Archean times. However, while crustal structure studies suggest that Archean Superior material underlies Grenvillian age rocks up to ∼300 km SE of the Grenville Front, our tomographic models show a near‐vertical boundary in mantle wave speed directly beneath the Grenville Front. We interpret this as evidence for subduction‐driven metasomatic enrichment of the Laurentian cratonic margin, prior to keel stabilization. Variable chemical depletion levels across Archean‐Proterozoic boundaries worldwide may thus be better explained by metasomatic enrichment than inherently less depleted Proterozoic composition at formation.

Melt evolution beneath a rifted craton edge: 40Ar/39Ar geochronology and Sr–Nd–Hf–Pb isotope systematics of primitive alkaline basalts and lamprophyres from the SW Baltic Shield

A new high-precision 40Ar/39Ar anorthoclase feldspar age of 176.7±0.5Ma (2-sigma) reveals that small-volume alkaline basaltic magmatism occurred at the rifted SW margin of the Baltic Shield in Scania (southern Sweden), at a time of global plate reorganization associated with the inception of Pangea supercontinent break-up. Our combined elemental and Sr–Nd–Hf–Pb isotope dataset for representative basanite and nephelinite samples (>8wt.% MgO) from 16 subvolcanic necks of the 30 by 40km large Jurassic volcanic field suggests magma derivation from a moderately depleted mantle source (87Sr/86Sri=0.7034–0.7048; εNdi=+4.4 to +5.2; εHfi=+4.7 to +8.1; 206Pb/204Pbi=18.8–19.5). The mafic alkaline melts segregated from mixed peridotite–pyroxenite mantle with a potential temperature of ∼1400°C at 2.7–4.2GPa (∼90–120km depths), which places ultimate melt generation within the convecting upper mantle, provided that the lithosphere–asthenosphere boundary beneath the southern Baltic Shield margin was at ⩽100km depth during Mesozoic–Cenozoic rifting. Isotopic shifts and incompatible element enrichment relative to Depleted Mantle reflect involvement of at least 20% recycled oceanic lithosphere component (i.e., pyroxenite) with some minor continent-derived sediment during partial melting of well-stirred convecting upper mantle peridotite. Although pargasitic amphibole-rich metasomatized lithospheric mantle is excluded as the main source of the Jurassic magmas from Scania, hydrous ultramafic veins (i.e., hornblendite) may have caused subtle modifications to the compositions of passing sublithospheric melts. For example, modeling suggests that the more radiogenic Hf (εHfi=+6.3 to +8.1) and Pb (206Pb/204Pbi=18.9–19.5) isotopic compositions of the more sodic and H2O-rich nephelinites, compared with relatively homogenous basanites (εHfi=+4.7 to +6.1; 206Pb/204Pbi=18.8–18.9), originate from minor interactions between rising asthenospheric melts and amphibole-rich metasomatic components. The metasomatic components were likely introduced to the lithospheric mantle beneath the southern Baltic Shield margin during extensive Permo-Carboniferous magmatic activity, a scenario that is supported by the geochemical and isotope compositions of ca. 286Ma lamprophyres from Scania (87Sr/86Sri=0.7040–0.7054; εNdi=+2.0 to +3.1; εHfi=+6.1 to +9.0; 206Pb/204Pbi=17.8–18.2).Strong variations in lithosphere thickness and thermal structure across the southern Baltic Shield margin may have caused transient small-scale mantle convection. This resulted in relatively fast and focused upwellings and lateral flow beneath the thinned lithosphere, where mafic alkaline magmas formed by low degrees of decompression melting of sublithospheric mantle. Such a geodynamic scenario would allow for enriched recycled components with low melting points to be preferentially sampled from the more depleted and refractory convecting upper mantle when channeled along a destabilizing craton edge. Similar to the ‘lid effect’ in oceanic island volcanic provinces, lithospheric architecture may exert strong control on the mantle melting regime, and thus offer a simple explanation for the geochemical resemblance of continental and oceanic intraplate mafic alkaline magmas of high Na/K affinity.