Intraplate Setting Research Articles

Constraints on the origin of the cenozoic intraplate aldama volcanic field, tamaulipas, NE México

The Cenozoic Aldama Volcanic Field (8–0.24 Ma) is located on the southeastern margin of the Sierra de Tamaulipas and is the largest field of the Eastern Mexican Alkaline Province. It contains cinder cones, tuff rings, maars, and simple cones, as well as widespread lava flows. The volcanic rocks are geochemically characterized as alkali basalts, subalkaline basalts, trachybasalts (potassic and hawaiites), latites to trachytes, and grouped in six rock types according to their Mg-content and normative composition: (a) Alkaline near-primary rocks, (b) Alkaline basic rocks, (c) Alkaline intermediate rocks, (d) Subalkaline near-primary rocks, (e) Subalkaline basic rocks, and (f) Subalkaline intermediate rocks.Trace elements and isotopic ratios show that the source of the basaltic melts displays asthenospheric characteristics and stability conditions, indicating that the partial melting (<5%) occurred in lithospheric levels within the spinel stability depth. The coexistence in time and space of alkaline and subalkaline rocks could be explained by selective alkali metasomatism of a regional HFSE and REE-enriched OIB-like source. The magma generation occurred in a typical continental intraplate setting, associated with mantle disturbances produced by the eclogitization and detachment of the Hess conjugate subducted under NE Mexico. Some alkaline and subalkaline melts evolved to intermediate members by fractional crystallization.

Origin of carbonate melts in orogenic belts by anatexis of downthrust carbonate sediments

Carbonate melts produced by partial melting of the carbonated mantle or by liquid immiscibility from mantle-derived silicate melts provide insights into the deep carbon cycle. Sedimentary geochemical signatures in some of the carbonate melts have been attributed to subducted material within their mantle sources. However, subducted carbonates at slab tops may not melt but be transported to the deep mantle, where sedimentary signatures would be diluted by mantle rocks. Here, we report the intruded Cenozoic carbonate melt dykes from the Yanshan orogen – an intracontinental orogenic belt far from the subduction zone – which show strong limestone-like geochemical features. Xenocrysts from the deep crustal basement, together with Sr-Nd-C-O isotope variations, indicate assimilation of the deep continental crust accompanied by CO2 degassing. We suggested they were formed by melting of downthrust limestone strata in the Yanshan orogen, triggered by underplating with basaltic melts during the Cenozoic upwelling of asthenosphere below the area. A compilation of global Phanerozoic carbonate magmas shows that this model can apply to many carbonate melts in orogenic belts which show much more evolved Sr-Nd isotopes and heavier C-O isotopes than those in intraplate settings. Trace elements of these carbonate melts indicate shallow derivation with a negligible contribution of mantle rocks. Spatial-temporal distribution of the orogenic carbonate melts supports them being related to continental crustal collision rather than oceanic subduction. Our findings highlight the carbonate sediments deposited in the continental crust as an important source that may deeply downthrust in the orogenic belt and melt during the post-collision stage.

Seamount chains and hotspot tracks: Superficially similar, deeply different

Nearly 80% of the seafloor extension has not been covered by high-resolution bathymetry, impeding direct observation of seamounts. Nevertheless, lists of seamount location and height at a global scale have been produced using different techniques. In this work four of such databases (publicly available) are compared with each other to assess their differences. Results identify large differences among databases that could have exerted strong influences on models of seamount production and associated geodynamic processes. Despite those differences, it is shown that all databases allow the identification of seamount lines both along the present-day Mid Ocean Ridge (MOR) system and on intraplate settings. Notably, those seamount lines do not coincide with the so-called hotspot tracks that commonly were defined by selectively focusing attention on the larger seamounts. Examination of all the databases also shows that distinction based only on seamount size between seamounts produced at Mid-Ocean Ridge (MOR) environments from those associated with mantle-plum fed-hotspot activity has been overestimated. This, combined with the fact that most seamount lines defined by the available databases can be traced back to past locations of MOR indicates that most of the present-day intraplate linear arrays of seamounts, which include some large seamounts, were not produced by the action of underlying mantle anomalies envisaged in the form of mantle plumes. The evidence presented here calls for a reassessment of the form in which volcanic and tectonic activities are conceptually related to each other.

Recent basement fault reactivation and fluvial drainage modification in an intraplate setting, eastern Bundelkhand Craton, Madhya Pradesh, India

North-central India is underlain by three northeast-trending paleo-topographic ridges of Precambrian Indian basement, bounded by lithospheric-scale basement faults. The Pokhara Fault defines the eastern edge of the Faizabad paleo-topographic ridge, which exposes the Archean Bundelkhand Craton. Near Panna, Madhya Pradesh, the Pokhara Fault coincides with a monocline developed in the Proterozoic Vindhyan Supergroup rocks along the Bundelkhand cratonic margin. Fluvial morphology of basins along the monocline, as well as a deeply incised abandoned meander preserved where the Ken River flows through the monocline, are investigated using longitudinal stream profiles and other geomorphic data along the length of the monocline to assess potential linkages with active basement fault reactivation.Normalized steepness indices in analyzed channel profiles along the monocline increase significantly downstream, with some channels exhibiting convex, over-steepened segments. Normalized steepness indices, knickpoint elevations, and incision depths are on average much higher in streams along the southwest segment of the monocline compared to streams along the northeast segment. Most channel profiles exhibit a knickpoint with slope-break morphology. The obtained longitudinal channel profiles exhibit characteristics of a fluvial system in a transient state subjected to an increase in rock uplift that intensifies to the southwest over a reactivated basement fault. Using previously published relationships between erosion rates and channel steepness, we conservatively estimate the onset age of monocline growth to be Plio-Quaternary. Fluvial geomorphic features preserved near Panna also suggest that the Ken River is an antecedent river. The incised meander feature near the crest of the monocline appears to be the abandoned river valley of a former Ken River course that was orphaned during the evolution of the landscape by what is now the present-day Ken River.Our study supports growing evidence that Indian basement faults such as the Pokhara Fault are long-lived and have recorded multiple periods of reactivation. Recent fluvial modification in the Panna region demonstrates that Indian basement faults are also active on the south side of the Ganga Basin, far south of the Himalayan deformation front, and may be linked to intraplate seismicity and active tectonic modification of the landscape.

Wet mantle transition zone beneath the Indian shield: Constraints from P-to-S receiver functions analysis

Indian plate passed over four plumes on its way toward the Eurasian plate. These interactions with plumes certainly affected the upper mantle and crust of the Indian plate as manifested by its thinned lithosphere and relatively low shear velocities both in the crust and upper mantle, but the depth extent of these effects of plume-lithosphere interaction remains ambiguous. In this study, we investigate the mantle transition zone beneath the Indian shield using P receiver functions computed at 23 stations covering the entirety of the Indian shield to investigate the depth extent of the imprints of the plume-lithosphere interaction as well as to study the lateral variations of transition zone beneath a stable intraplate setting like the Indian shield. Our results show good agreement with the results of previous studies as well as with the tomographic models in terms of the average apparent depths of the 410 and 660 discontinuities and the transition zone thickness. However, unlike previous studies, we find compelling evidence of a persistent mid-transition zone discontinuity beneath all the stations and a low-velocity layer beneath some regions. We also investigated the frequency dependence of amplitudes of receiver functions and found that most of the stations showed a strong dependence of amplitudes on frequency. Based on the evidence from our investigation, we demarcate the regions potentially containing relatively more weight percentage of water inside the otherwise considered as a ‘dry’ mantle transition zone. These regions should be further investigated in detail by a dense seismic network and a realistic 3-D velocity model.

Impact of tectonic topographic rejuvenation in landscapes with high bedrock/duricrust strength: Insights from geomorphic evidence in a post-rifted region (SE Brazil)

The evolution of Cenozoic relief in post-rifted and post-orogenic landscapes reveals enigmatic and intricate dynamics. Unsteady conditions long after the cessation of ongoing tectonics have been reported in these landscapes with evidence for relief growth rather than the expected slow topographic decay. Various controlling factors (e.g., lithology and isostasy) have been invoked to explain their geomorphic evolution; however, topographic rejuvenation controls remain puzzling, particularly within intraplate landscapes. Here, we investigated topographic rejuvenation, exploring the interactions between the river long profiles of the drainage network, the broader landscape topography, and their collective relationships with the strength of the bedrock and duricrust soils that cap the land surface in the Southern Espinhaço Range of Brazil. This is a humid tropical post-orogenic and post-rifted landscape characterised by resistant bedrock and duricrusts, despite some rock-type strength variability. We show robust evidence for a first-order control exerted by the lithology/rock strength. However, results revealed an overall non-collinearity between the transformed river profiles. These profiles displayed knickpoints separating upstream low-relief uplands capped by Fe/Al duricrusts from an incised downstream transient landscape associated with uplift/base-level fall. Regionally, the river profiles display an asymmetric pattern of relief growth with an eastward gain in elevation and fault-controlled dismantling of duricrusted surface that cannot be explained exclusively by bedrock strength variations. This spatial configuration suggests that links exist between the internal and external factors that collectively control the landscape development. The interplay between faulting/dismantling of the duricrusts and river incision/topographic patterns provides compelling evidence for Pliocene tectonic/topographic rejuvenation. Our findings are consistent with a pattern of regional surface uplift related to combined flexural isostasy and far-field stress interactions. Collectively, our study demonstrates that lithology, as a first control factor on topographic rejuvenation, is not exclusive to post-orogenic landscapes with strongly heterogeneous bedrock. The bedrock/duricrust strength acts in combination with locally inherited discontinuities and large-scale tectonics/flexural isostasy. Such interactions add significant complexity to the evolution and rejuvenation of post-rifted landscapes, particularly within intraplate settings.

Generation of continental intraplate alkaline basalts by edge-driven convection: Insights from the Cenozoic basalts beyond the Big Mantle Wedge

Continental intraplate basalts are widespread across Central-East Asia, and their melting mechanisms are poorly known. Herein, we present integrated studies of petrology, elemental-isotope systematics, and thermodynamic modeling of Cenozoic Liangcheng basalts, a representative volcanic field of the vast intracontinental basaltic magmatic province covering Central-East Asia, with the aim of constraining the source characteristics and melting dynamics in this intraplate setting. These basalts have moderate-to-low silica (45.2 to 49.0 wt%), high Fe2O3T (10.0 to 12.2 wt%), and alkali (Na2O + K2O, 4.27 to 7.38 wt%) contents with ocean-island basalt (OIB) -like trace-element patterns and moderately depleted to slightly enriched Sr–Nd–Pb–Hf isotopes (87Sr/86Sr = 0.703924–0.705176, 143Nd/144Nd = 0.512540–0.512861, 206Pb/204Pb = 17.2752–17.9180, 207Pb/204Pb = 15.4860–15.7383, 208Pb/204Pb = 37.6995–38.3599, and 176Hf/177Hf = 0.282852–0.282999). The major elements (e.g., high Fe/Mn, low CaO) and olivine chemistry (e.g., high Ni, Zn/Fe, and low Mn/Zn) favor derivation from a silica-deficient pyroxenite-bearing source, while the trace-element and isotope systematics suggest the involvement of recycled components including oceanic slab and sediment (both terrigenous and pelagic). A grid search with a thermodynamic melting model using incompatible trace elements was carried out to impose quantitative constraints on the mantle source. The modeling results show that the source of the Liangcheng basalt contains 80% primitive mantle peridotite and 20% silica-deficient pyroxenite. This exercise and geobarometric calculation using major elements suggest that melting occurs at a potential temperature of 1380 °C underneath a thinned continental lithosphere with a basal pressure of ∼2GPa, consistent with the source characteristics and melting conditions estimated for many other volcanic fields in the vast Central-East Asia magmatic province. By combining these results with the regional geology and tectonic history, we can suggest that these basalts were formed by decompression melting owing to mantle convection driven by lithospheric thickness variations (edge-driven convection). We suggest that such a melting scenario is ubiquitous beneath Central-East Asia and may have played a vital role in the formation of widespread intraplate continental alkaline basalts.

Earthquake-induced landslides in Norway

Abstract. Norway is located in an intraplate setting with low-to-moderate seismicity. The mountainous landscape leads to a high level of landside activity throughout the country. Earthquake-induced landslides (EQILs) are common in seismically active areas, but there are only a few studies of EQILs in intraplate regions. We systematically analyse all earthquakes in Norway with magnitudes ≥ 4.5 in the time period 1800–2021 CE. For each event we search for reports of EQILs in the available macroseismic data and in the Norwegian landslide database. We furthermore consider precipitation data from the Norwegian Centre for Climate Services to evaluate the role of precipitation in the triggering of the identified potential EQILs. Through this approach, we identify 22 EQILs that have been triggered by eight earthquakes in the magnitude range 4.5–5.9. The events are widely distributed in northern and southern Norway. The maximum landslide distance limits and landslide-affected areas are much larger than those found in empirical studies of global datasets and are in agreement with data from other intraplate regions. For four of the earthquakes, it seems that landslide triggering was due to a combined effect of precipitation and earthquake ground shaking. Our observations confirm that intraplate earthquakes have the potential to trigger EQILs over large distances, most likely due to the low ground motion attenuation in such regions. Slope susceptibility seems to be another important factor in the triggering. Our conclusions demonstrate the importance of considering EQIL potential in earthquake risk management in intraplate regions.

Intraplate lithospheric extension revealed by seismic reflection profiling of South China

How lithospheric extension evolves in an intraplate setting remains uncertain due to the lack of reliable constraints on the lithospheric architecture. Here we present seismic reflection data across the Cretaceous extensional system of South China. Our results show that extension in magma-poor conditions was accommodated by localized normal faulting in the upper crust and distributed ductile stretching in the lower crust, followed by localized crustal necking and Moho uplift associated with mantle shear-zone formation. These vertically spaced crustal and mantle features appear to be kinematically linked as follows. First, lower-crustal stretching was accompanied by normal faulting and localized exhumation of extensional domes along low-angle detachments in the upper crust. Second, sub-horizontal lower-crustal stretching tends to compensate for upper-crustal heterogeneous thinning and distribute crustal strain evenly, enabling the crust to maintain an overall smooth and flat Moho. Third, mantle shear zones likely affected lithospheric extension by controlling localized Moho uplift and crustal necking. Our compilation of seismic observations suggests that the extensional modes vary laterally from magma-poor to magma-rich conditions, reflected in increased crustal melting, decreased crust-mantle decoupling, and the replacement of a two-layer (high-strength vs. low-strength) lithospheric mantle by a single-layer, low-strength lithospheric mantle. These findings reveal a first-order configuration of depth-dependent extension over ∼800 km, with vertical and lateral variations as a function of lithospheric strength, rheology, and temperature. This extension mechanism provides a basis for assessing modes of lithospheric extension in other tectonic settings.

Dominance of felsic continental crust on Earth after 3 billion years ago is recorded by vanadium isotopes

The transition from mafic to felsic upper continental crust (UCC) is crucial to habitability of Earth, and may be related to the onset of plate tectonics. Thus, defining when this crustal transition occurred has great significance for the evolution of Earth and its inhabitants. We demonstrate that V isotope ratios (reported as δ51V) provide insights into this transition because they correlate positively with SiO2 and negatively with MgO contents during igneous differentiation in both subduction zones and intraplate settings. Because δ51V is not affected by chemical weathering and fluid-rock interactions, δ51V of the fine-grained matrix of Archean to Paleozoic (3 to 0.3 Ga) glacial diamictite composites, which sample the UCC at the time of glaciation, reflect the chemical composition of the UCC through time. The δ51V values of glacial diamictites systematically increase with time, indicating a dominantly mafic UCC at ~3 Ga; the UCC was dominated by felsic rocks only after 3 Ga, coinciding with widespread continental emergence and many independent estimates for the onset of plate tectonics.

The Origin and Occurrence of Natural Hydrogen

Hydrogen is an attractive, clean, sustainable energy source primarily produced via industry. At present, most reviews on hydrogen mainly focus on the preparation and storage of hydrogen, while the development and utilization of natural hydrogen will greatly reduce its cost. Natural hydrogen has been discovered in many geological environments. Therefore, based on extensive literature research, in this study, the distribution and sources of natural hydrogen were systematically sorted, and the identification method and occurrence state of natural hydrogen were examined and summarized. The results of this research show that hydrogen has been discovered in oceanic spreading centers, transform faults, passive margins, convergent margins, and intraplate settings. The primary sources of the hydrogen include alterations in Fe(II)-containing rocks, the radiolysis of water, degassed magma, and the reaction of water- and silica-containing rocks during the mechanical fracturing. Hydrogen can appear in free gas, it can be adsorbed and trapped in inclusions. Currently, natural hydrogen exploration is in its infancy. This systematic review helps to understand the origin, distribution, and occurrence pattern of natural hydrogen. In addition, it facilitates the exploration and development of natural hydrogen deposits, thus enabling the production of low-cost hydrogen.

Late Quaternary–Recent intraplate topographic uplift of the St. Francois Mountains (Ozark Plateau), Missouri (USA), driven by differential density and erosion

AbstractThe origin of relatively high topography in intraplate settings is elusive. The St. Francois Mountains, a tectonically inactive region with high relief, a Mesoproterozoic granite and rhyolite core, and Paleozoic sedimentary flanks, provide insights into the evolution of intraplate landscapes. We determined 14 in situ10Be exposure ages, 23 10Be catchment-wide denudation rates (CWDRs), and geomorphic indices using a geographic information system, and we modeled surface uplift using an isostatic-flexure model. CWDRs varied from ~5 to 10 ± 1.7 mm/k.y. and averaged 1.5 times greater in streams draining granite and rhyolite than in those draining flanking sedimentary rocks. In situ results from bedrock strath terraces gave Quaternary incision ages and incision rates that were tenfold higher than CWDRs. Major knickpoints (locally called shut-ins), all at 200 ± 70 m elevation, are located at contacts between the igneous and sedimentary rocks. Our exposure and denudation results inform isostatic-flexural modeling, which together support the conclusion that late Quaternary–Recent surface uplift driven by differential density and erosion focused in the igneous core is creating significant topography and relief in this intraplate setting.

Age and petrogenesis of Ni-Cu-(PGE) sulfide-bearing gabbroic intrusions in the Lausitz Block, northern Bohemian Massif (Germany/Czech Republic)

The Lausitz Block, located in the northernmost part of the Bohemian Massif, hosts a large number of dike- to stock-shaped gabbroic intrusions that mainly comprise brown hornblende-poor (Group I; i.e. olivine gabbronorite, olivine gabbro, gabbro and diorite) and subordinately brown hornblende-rich lithologies (Group II; i.e. olivine-hornblende gabbro and hornblende gabbro). Several of these intrusions host small-scaled magmatic Ni-Cu-(PGE) sulfide accumulations. The intrusions are part of interconnected mafic–(ultramafic) plumbing systems that intruded Cadomian granodiorites of Lausitz Block in the Middle to Late Devonian during the early stages of the Variscan Orogeny. The previously inferred Devonian age of the intrusions is refined by biotite Ar-Ar dating that yield ages between 389.1 ± 3.9 Ma and 372.2 ± 3.7 Ma (2σ). Group I and Group II lithologies differ in their mineralogical and geochemical composition. Compared to the Group I lithologies those of Group II are characterized by higher modal contents of primary brown hornblende, Fe-Ti oxides and apatite, by Ti- and Al-enriched clinopyroxene and by lower contents of SiO2 and increased contents of TiO2, P2O5, LILE, HFSE and LREE. The differences suggest at least two different magmatic series where Group I rocks are linked to tholeiitic basaltic magmas with low to moderate Ti and volatile contents, whereas Group II rocks are derived from Ti- and volatile-enriched alkaline basaltic magmas. The magmas experienced clinopyroxene fractionation during their crustal ascent and storage, but were only minor affected by crustal contamination (< 5%) according to Sr-Nd-Pb isotope systematics. The tholeiitic and alkaline basaltic magmatism could be explained by a tectonic environment between a subduction and intraplate setting. Group II rocks have an intraplate chemical signature and their parental magmas are clearly originated from a deeper-seated mantle source with melting of garnet peridotite, likely associated with an asthenospheric upwelling. By contrast, precursor magmas of Group I rocks are originated from partial melting of a less-enriched source, presumably spinel peridotite, from a subduction-related environment. Trace elements systematics suggest that both groups at least partly resulted from an interaction (mixing) of the alkaline and tholeiitic magmas. Partial melting estimates for both groups vary between 5 and 20%. The proposed tectonic setting could have been related to a subduction slab retreat within the framework of the Variscan orogeny, which led to an upwelling of the asthenospheric mantle. Cu/Zr ratios <1 of gabbroic rocks from several intrusions suggest a previous segregation of magmatic sulfides in other sections of the magmatic plumbing system and give rise for a vertical and lateral Ni-Cu exploration potential.

The isotopic origin of Lord Howe Island reveals secondary mantle plume twinning in the Tasman Sea

Thermochemical convective instabilities in the mantle, often referred to as mantle plumes, cause mantle melting that give rise to ocean island basalts (OIB) in intraplate settings. The array of radiogenic isotope signatures in global intraplate OIB indicate that although plumes are individual entities, they share enriched components that resemble various parts of subducted crust and a hypothetical mantle matrix termed “focal zone” (FOZO). Each plume is expected to rise with an individual buoyancy flux, thereby producing variable volumes of melting with some OIB forming subaerial islands, whereas others produce submarine volcanoes.Here, we report the first radiogenic isotope data (Sr, Nd, Pb and Hf) for lavas of Lord Howe Island (LHI) in the Tasman Sea, the most prominent subaerial expression of the hypothesised Lord Howe mantle plume. Major element data are consistent with a moderate degree of partial melting, and heavy rare earth element depletion indicates melting occurred in the presence of garnet, consistent with other global plume lavas. Radiogenic SrNd isotopic data are similar to those defining the relatively primitive FOZO component with no clear enriched mantle affinity. The nearby Tasmantid Seamounts are also sourced from a mantle plume, and have similar SrNd character. However, combined 208Pb/204Pb and 207Pb/204Pb vs 206Pb/204Pb ratios of LHI lavas are inconsistent with a FOZO-type source. Instead, Pb isotopes overlap with typical enriched mantle 1 (EM1) lavas (e.g., Pitcairn, Tristan da Cunha), trending slightly higher than the Northern Hemisphere Reference Line (NHRL). Hafnium isotopes follow trends observed in lavas of the archetypal EM1 Pitcairn-Gambier islands, and are partially decoupled from 143Nd/144Nd; this decoupling is rare among OIB. The combined element-isotope data indicate that LHI forms the most recent expression of a mantle plume track in the Tasman Sea. Albeit unique in its 208Pb*/206Pb* isotope code among global mantle plumes, the resemblance of LHI lavas with the spatially related and contemporaneous Taupo seamount of the parallel Tasmantid plume track in the Tasman sea supports a genetic relation of both plumes. The proposed common origin of the two parallel plume tracks requires a larger plume underpinning the Tasman sea with individual plume fingers rising towards the surface, possibly in a triple plume network considering the on-shore Cosgrove track in eastern Australia. The subaerial expression of LHI contrasts the submarine Tasmantid Seamounts; this expression is only possible due to the continental ribbon that forms the island's foundation, suggesting secondary plume fingers display similar low buoyancy fluxes. Plain language summaryLord Howe Island is predominantly composed of basaltic rocks, which are created when mantle underneath the crust melts and the subsequent magma rises to the surface and erupts out of a volcano. The chemical composition of these volcanic rocks can provide us with valuable information about geologic processes. Moreover, we can compare the composition of islands like Lord Howe to better investigate and understand volcanic networks that exist deep within the Earth. The composition of Lord Howe Island basalts was investigated for the first time using specialised instruments that can separate individual elements into different isotopes. The ratios of these different isotopes reflects the composition of the mantle source which can be enriched by the presence of subducted crustal rocks, oceanic rocks and sediments. Our data indicate the distinctive reservoir that Lord Howe Island samples were derived from is the same reservoir as that of the nearby Taupo Seamount volcano. We infer from this that a piece of ancient crust is stuck in the mantle, and the two volcanoes are sampling it. This study supports a theory of a large, linked volcanic network under the Tasman Sea, and adds to the growing body of research investigating ‘double-chain’ volcanism in the Pacific and Atlantic oceans.

Comparison of Earthquake Clusters in a Stable Continental Region: A Case Study from Nordland, Northern Norway

Abstract Nordland, Northern Norway, is a seismically active stable continental region. Along its coast, clusters of small earthquakes controlled by local extensional stresses are observed. In this study, we present a comparison between two adjacent seismically active areas along the Nordland coast: Jektvik and Rana, which have distinct spatiotemporal patterns. The seismicity in Jektvik, which presents a swarm-like behavior, shows outward activity progression from its center hinting at triggering between earthquake clusters. In contrast, the seismicity in Rana, where swarms are also observed, does not exhibit such pattern. Earthquakes in the Rana cluster occur within isolated spots and show repeating earthquake behavior. Singular spectrum analysis shows that seismicity in Jektvik has a dominant annual periodicity and is modulated by hydrological load, which is also observable on Global Navigation Satellite Systems stations. Although hydrological load changes also affect Rana, its seismicity does not exhibit an annual periodicity. We hypothesize that the Jektvik seismicity occurs within a fluid-rich fracture system that is affected by hydrological modulation, whereas the Rana seismicity occurs within fault irregularities, which accumulate stress and rupture repeatedly. This study presents a case where adjacent areas within an intraplate setting can have significantly different seismogenic behaviors.

Ediacaran to Cambrian tectonomagmatic events in the Southern Dom Feliciano Belt, Uruguay: From a plate margin to an intraplate setting and the assembly of SW Gondwana

We present new zircon U-Pb geochronological and Hf-O isotope data as well as whole rock geochemical and Sr-Nd-Pb isotope geochemical data of Ediacaran magmatic rocks from the Cuchilla Dionisio (CDT), Nico Pérez (NPT) and Tandilia (TT) terranes in Uruguay. Three different rock suites are recognized in the NPT. The Group 1 rocks are high Ba-Sr granodiorite, monzonite and syenite that formed from a subduction-related mafic source between 610 ± 3 and 593 ± 5 Ma, most likely in an extensional setting. The Group 2 plutons, syenite, formed from a mafic source between 573 ± 8 and 563 ± 7 Ma and were emplaced in an intraplate setting at the extended eastern Rio de la Plata cratonic margin. The Group 3 rocks as well as the La Paz Granite in the TT are intraplate granites that formed by crustal melting and/or by the incorporation of crustal melts at c. 557 ± 4 Ma. The Early Ediacaran Valdivia Pluton in the CDT is a subduction-related granite that formed from a mafic melt at an active continental margin. The 571 ± 3 and 566 ± 6 Ma subduction-related CDT José Ignacio and Florencia granites were emplaced at or close to an active margin. They, like the Valdivia Granite provide only little evidence of incorporating crustal rocks and/or melts. The coeval Maldonado and Rocha granites in the CDT are subduction-related but were emplaced in a distal position to the arc. The Early Cambrian Santa Teresa Granite is a collision-related pluton that formed during the final assembly of SW Gondwana at 532 ± 6 Ma. It is evident that the Ediacaran to Early Cambrian igneous rocks in the southern Dom Feliciano Belt in Uruguay originated from dissimilar source regions and were emplaced in different tectonic settings at different times. The data indicate a transition from Early Ediacaran subduction-related, arc-type magmatism to intraplate and, finally to Late Ediacaran/Early Cambrian collision-related igneous activity in the Uruguayan sector of the southern Dom Feliciano Belt.

Multi-method dating constrains the diversification of early eukaryotes in the Proterozoic Mbuji-Mayi Supergroup of the D.R.Congo and the geological evolution of the Congo Basin

We present a petrological and geochronological study of the Proterozoic Mbuji-Mayi sedimentary Supergroup (Sankuru-Mbuji-Mayi-Lomami-Lovoy basin, Democratic Republic of the Congo) and of the overlying subvolcanic doleritic rocks. Dating this Supergroup in Central Africa is crucial to reconstruct the geological history and evolution of the Congo Basin and because it contains a large diversity of organic-walled microfossils including early eukaryotes. For this study, we use the Re–Os dating method on kerogen from shales from the top of the lower Group BI (which contains the microfossils) and Ar–Ar, Sm–Nd and U–Pb methods on the dolerites emplaced near the top of the succession, defining the end of the sedimentation in the basin. The 187Re/187Os dating yield an age of 1041 ± 58 Ma. For the dolerites, 40Ar/39Ar dating provide a similar minimum age from 1006 ± 13 to 1009 ± 32 Ma in the whole basin. 147Sm/143Nd dating confirms a less precise but overlapping age of 926 ± 170 Ma for sample from the Western part of the basin. Some 40Ar/39Ar dating and U–Pb dating provide also inherited Archean ages. The weighted average age for igneous samples from the basin is 1006 ± 12 Ma. This confirms that the upper Group BII was deposited from 1030 to 1040 to 1006 Ma, consistent with previous U–Pb dating on diagenetic minerals and with biostratigraphy. This new dating provides constraints on the late Mesoproterozoic – early Neoproterozoic diversification of early eukaryotes in Central Africa.We also compare the petrology, geochemistry and geochronology of dolerites from different locations of the basin. Our new data evidence a common origin for all the subvolcanic rocks, emplaced in an intraplate setting and intruding the Archean basement, and indirectly date the end of the extensional stage that initiated the development of the Congo Basin.

Origin and geodynamic implications of basaltic rocks intercalated with Miocene turbidites around the İskenderun Basin (Eastern Mediterranean / Turkey)

We report a detailed investigation and the geodynamic implications of a basaltic lava flow that is intercalated with the early–middle Miocene turbiditic sedimentary rocks (Karataş Formation) that are exposed near the İskenderun Basin in southern Turkey. In order to reveal the age and origin of the basaltic rocks, we systematically sampled clastic samples from measured sedimentary sections, which include the basaltic lava flow, from around Ceyhan (east of Adana city). Forty different species belonging to 14 calcareous nannoplankton genera were identified in the samples, representing the Langhian–Tortonian (middle–late Miocene) time interval. Considering the stratigraphic distribution of the calcareous nannoplankton species in the section, the age of the basaltic lava flow is determined to be Serravallian (middle Miocene). The basaltic rocks experienced only minor magmatic evolution and have a restricted compositional range. Analysed in situ major element contents (from electron microprobe analysis, EMPA) for minerals (plagioclase, olivine and clinopyroxene) hosted by the late Miocene basaltic rocks show that they originated in an intraplate setting. Geochemical analysis indicates that olivine have Fo (forsterite) compositions between 57.81% and 83.83%. The EMPA contents (primarily based on pyroxenes) suggests that the late Miocene volcanism was related to the Pleistocene Delihalil–Turunçlu basaltic volcanism. The studied late Miocene volcanism could have originated in two possible tectono-magmatic environments: 1) one involving collisional tectonics (e.g., folding) combined with a change from regional compression to strike-slip; or 2) an intraplate setting. We propose that the tectonic setting of the late Miocene basaltic lava flow was similar to that of the extensive Pleistocene Delihalil volcanism in the same region. This would extend the onset of the regional basaltic volcanism back to the middle–late Miocene, rather than being restricted to the Pleistocene–Quaternary.

Transtensional tectonics during the Gondwana breakup in northeastern Brazil: Early Cretaceous paleostress inversion in the Araripe Basin

Intraplate deformation during the Mesozoic breakup of West Gondwana predominantly occurred along major ancient lithospheric discontinuities. The Precambrian Patos Shear Zone (PASZ) is an inherited lithospheric discontinuity in the interior of northeastern Brazil which is closely linked to the evolution of the Araripe Basin. In this study, unlike previous interpretations of purely extensional tectonics, we conducted stratigraphic and structural field mapping as well as paleostress reconstruction to demonstrate the role of intraplate transtensional tectonics during the Araripe Basin evolution. Limited to the north by the PASZ and associated with sinistral reactivation, a set of normal to oblique NE–SW faults constituting a transtensional horsetail structure generated the initial basin geometry. Stratigraphic records of this rift phase comprise the Jurassic to Early Cretaceous Brejo Santo, Missão Velha, and Abaiara formations. Paleostress reconstruction showed a NE–SW/subhorizontal σ1 axis, a vertical σ2 axis, and a NW–SE/subhorizontal σ3 axis, resulting in a sinistral transtensional tectonic regime, which was mainly governed by intraplate processes that reflected the opening of the Brazilian East Margin. The subsequent Aptian post-rift early phase was characterized by NE–SW reverse faults and a succession of normal to oblique normal faults striking NW–SE to WNW–ESE. This fault arrangement generated a new NW–SE transtensional basin, which controlled the Barbalha Formation and modified and partially preserved the previous NE-SW transtensional basin. Paleostress reconstruction showed a NW–SE/subhorizontal σ1 axis and NE–SW/subhorizontal σ3 axis for strike-slip faults but NE–SW extension for normal faults. This is related to a second dextral reactivation of the PASZ following propagation of intraplate stress during the opening of the Brazilian Equatorial Margin. In summary, tectonic evolution of the Araripe Basin was strongly influenced by stress propagation from the Gondwana breakup into intraplate settings, with two distinct reactivations of the PASZ pre-existing basement structures which results in a paleostress inversion during basin evolution.

Early Au-rich sulfide liquid saturation explains the low Au endowment of continental intraplate alkaline magmas

Abstract Alkaline magmatic systems formed in intraplate settings have a low Au endowment, contrasting with the high Au fertility of alkaline magmatic systems in arc and rift settings, which host numerous Au-rich porphyries and Cu-Au and Au-Ag epithermal deposits. Among other factors, the Au fertility of a magmatic system is determined by the Au concentration of the ore-forming magma, which is strongly controlled by the amount and chemistry of magmatic sulfides that fractionated during magma differentiation. To better explore the factors controlling the low Au endowment of alkaline magmatic systems formed in intraplate settings, we studied magmatic sulfide and silicate melt inclusions hosted in various phenocrysts from the Mont-Dore Massif (France). The magmatic system was saturated with Cu-poor, Au-rich sulfide liquid during its differentiation from basanite to trachyandesite, leading to a strong depletion of both Cu and Au in the residual melt. This presumably reduced the capacity of the magmatic system to form Au-rich magmatic-hydrothermal deposits. Such evolution contrasts with previous studies conducted in sulfide-saturated magmatic systems formed in arc settings, where an Au-poor monosulfide solid solution was the main sulfide phase to crystallize, only marginally affecting the Au budget of those systems. We conclude that the observed saturation of Au-rich sulfide liquid during evolution of the studied volcanic products could explain the low Au endowment of intraplate alkaline magmatic systems.