Abstract Marine carbonate carbon isotopes (δ13C), over orbital to multi‐million‐year timescales, provide critical insight into the carbon cycle connecting Earth's atmosphere, lithosphere, hydrosphere and biosphere. However, the influence of astronomical forcing on deep time carbon cycle dynamics remains poorly constrained. Here, we present a ∼8 Myr‐long astrochronology based on a carbonate δ13C record from a Late Jurassic succession in the Lower Saxony Basin, Germany, northwestern Tethys. Astronomical tuning and spectral analyses reveal dominant 405‐kyr cycles of the orbital eccentricity amplitude modulations, and 173‐kyr and 1.2 Myr cycles of the obliquity amplitude modulations. The new astrochronology precisely dates one of the Late Jurassic carbon isotope excursions (Middle Oxfordian Event) from 159.0 to 157.2 Ma. Notably, the subordinate, low‐amplitude positive δ13C excursions within the Event correlate with 405‐kyr orbital eccentricity minima and 173‐kyr obliquity maxima, whereas the negative shifts correspond to eccentricity maxima and obliquity minima. Our results demonstrate a binary carbon cycle response to orbital eccentricity and obliquity extremes in the northwestern Tethyan region. Low eccentricity likely maintains permanently wet seasons and high obliquity enhances monsoon‐induced precipitation. Both mechanisms accelerate hydrological dynamics and subsequently intensify continental weathering and 12C‐enriched organic carbon burial in marine sediments, leading to carbonate positive δ13C excursions. Conversely, high eccentricity generates strong dry‐wet seasonality and low obliquity weakens precipitation, both of which decelerate organic carbon burial and ultimately cause negative δ13C shifts. These findings advance our understanding of how astronomical forcing modulates the Late Jurassic marine carbon cycle in the northwestern Tethys.
Volcanism in continental rifts, rifted volcanic arcs, and back-arc basins is fundamentally coupled with crustal extension. However, the precise geometry and timing of the fault systems that facilitate magma transport and accommodate extension remain poorly constrained. The Christiana-Santorini-Kolumbo volcanic field lies within the Santorini-Amorgos Tectonic Zone, an actively extending back-arc rift where volcanism transitioned from dominantly effusive and moderately explosive andesitic activity (similar to 570-250 ka) to repeated caldera-forming silicic eruptions after similar to 250 ka. Here we integrate scientific drilling results from IODP Expedition 398 with a dense grid of high-resolution seismic profiles and identify a previously unrecognized rift structure, the Kolumbo Graben, bounded by the NE-SW-striking Kolumbo Fault and hosting Kolumbo Volcano and the Kolumbo Volcanic Chain. Subsidence and fault throw rates across the rift system accelerated between similar to 330 and similar to 160 ka, with the Kolumbo Fault accommodating similar to 220 m of displacement. This phase of accelerated extension preceded the emergence of Kolumbo and the onset of repeated caldera-forming silicic eruptions at Santorini. Volcanic edifices of the Kolumbo Volcanic Chain cluster within the graben interior rather than along the master fault, indicating that distributed intra-graben deformation and locally reduced horizontal stresses provided preferred pathways for magma ascent. Structural continuity further suggest that the Kolumbo Graben extends beneath Santorini. We conclude that accelerated rifting and strain localization exerted first-order control on magma ascent and the transition to highly explosive volcanism over the past similar to 250 kyr, while magma intrusion and associated thermal weakening likely provided a positive feedback that further localized deformation during periods of peak activity.
Information on the past of Earth's magnetic field can be retrieved from magnetic grains in rock samples. Micromagnetic Tomography (MMT) is a recently developed method that uses magnetic surface scans from a Quantum Diamond Microscope (QDM) combined with the position of magnetic grains in rock samples to calculate the magnetic moments of those grains. An important parameter for these calculations is the sample-to-sensor distance during the magnetic measurements. Until recently, sample placement in the QDM was typically done manually. To improve sample placement, we developed an automated sample placement system (JAMES). JAMES eases the sample placement process by enabling the positioning of the sample in the x and y direction, as well as positioning of the sample plan-parallel against the sensor or at a known distance from the sensor with a precision of 1 mu m. We evaluated the importance of a known sample-to-sensor distance for retrieving quantitative magnetic information with MMT. With a known distance, individual grain magnetic moments and summed intensities (summed magnetic moments of individual grains) can be accurately determined. The sample-to-sensor distance is less important for determining directions, when studying isolated grains with dipolar magnetic surface expressions. However, when grains do not meet those conditions, a known sample-to-sensor distance is required to determine accurate directions. JAMES is now a standard element in the MMT procedure, ensuring reliable and repeatable magnetic measurements with the QDM. This greatly improves the precision of the magnetic moment calculations in MMT and advances the extraction of magnetic information from rock samples.
The India-Asia collision continuously loads the Tibetan Plateau (TP), driving its Cenozoic eastward growth. This process has caused extensive crustal deformation and a diverse magmatic response. Here, we report on newly identified 35-6.5 Ma granitoids from Kangding area in southeastern (SE) TP. Based on elemental and isotopic data obtained from this study, including the first silicon isotope study of the Kangding Cenozoic granites, and data from the literature, the samples can be divided into two subgroups. Subgroup 1 (Sg1) has diagnostic geochemical characteristics (lower MgO, epsilon Nd(t), epsilon Hf(t)-zircon, and higher K2O/Na2O, sum of light rare earth elements, Th, Th/U, Rb/Sr, (87Sr/86Sr)i totally) compared with Sg2. Sg1 also shows a strong correlation (R 2 = 0.82) between delta 30Si and Al/Si, while Sg2 does not. The features of Sg1 are best explained by a significant contribution from metasediments. Based on the restricted ages of the Sg1 magmas, we identify a temporary shift in the melt source during the early Miocene (ca. 20-10 Ma). At that time, the source involved the addition of metasedimentary materials, in contrast to the sediment-poor source during the rest of the Cenozoic. Based on previous geophysical and regional deformation observations, the growth of the TP involved a gradual accentuation of imbrication structures. This was followed by the formation of a stress overload zone and the involvement of a crustal sedimentary source. Lastly, stress unloading formed an asymmetric topography across the SE plateau margin.
Rifting is a tectonic process that leads to extensive magmatic activity, continental breakup, and the formation of new oceanic crust. The interplay between rifting and dynamic mantle flow driven by thermal heterogeneity in the mantle along the rift-axis can influence magmatism and deformation beyond the rift zone. We constructed a series of three-dimensional geodynamic models to simulate mantle flow during the syn- and post-rift phases by integrating different initial temperature conditions for the extending rift zone and rift process zone (RPZ). We define the RPZ as the region beyond the rift-tip where strain-rates are elevated relative to the surroundings, enabling potential rift propagation. Our study demonstrated that rift-induced mantle flow, derived from a model with a higher initial temperature (+100K), channelized into the RPZ, reaching up to approximately 900 km beyond the rift-tip. Even after rift termination, the channelized mantle flow continued to transport melt and heat, maintaining a melt flux of approximately 6 & times; 1016 kg/Myr toward the cold and thick lithospheric mantle. Thermal instability induced by the channelized mantle caused localized lithospheric erosion and mantle upwelling. Our study demonstrates that the channelized flow provides new insights into geodynamic evolution, encompassing rift propagation and magma supply to intraplate volcanism. The persistent transport of heat and mass to the RPZ beyond the rift-tip, even after rift termination, can contribute to both accretion and erosion of cold continental roots.
Assessing seismic and tsunami hazards along coastlines requires understanding past earthquakes and their recurrence along active submarine faults. Subaqueous paleoseismology commonly relies on sediment cores and seismic reflection data, but these methods may be limited by local site conditions or data quality. To overcome these limitations, a new approach using submersible-based optical imagery can provide valuable insights into the paleoseismic history of submarine faults. In this study, we conducted a near-bottom geological survey using submersibles along the Roseau normal fault (Lesser Antilles, France). This fault can host a M7 event if broken entirely. In 2004, it generated the Mw6.3 Les Saintes earthquake that produced a prominent coseismic ribbon at its base. High-resolution submersible observations of the fault free face allowed mapping and characterizing detailed fault scarp morphologies, including new deformation markers such as abrasion bands, notches, roughness variations, dark bands, and uplifted sediments. These geomorphic markers form on the seafloor and are interpreted as records of fault scarp exhumation linked to changes in base level driven by sedimentation and tectonic activity. At one site, the observed morphology can be explained by three distinct earthquakes combined with episodes of rapid sedimentation. The penultimate earthquake produced a minimum vertical offset of 3 m, exceeding the 1.4 m slip associated with the 2004 event, indicating that it was at least as energetic. Sedimentation rates from nearby cores indicate that this penultimate event occurred much more recently than 2.8 kyr. These results demonstrate the value of submersible-based offshore fault studies for reconstructing seismic histories.
Basaltic Plinian eruptions challenge our understanding of explosive volcanism. The 122 B.C. Plinian eruption of Etna ranks among the most powerful mafic explosive events known. Here, we combine volatile barometry of 122 B.C. from olivine-hosted melt and fluid inclusions with comparative data from the sub-Plinian Fall Stratified eruption at Etna (3930 BP) to assess the storage depths and degassing paths that govern explosive eruptive styles. Our results indicate that the 122 B.C. magma storage started deep (similar to 22 km) within Etna's plumbing system, had a complex ascent history, and was finally stored pre-eruptively at shallow levels (similar to 2-5 km) for a minimum of similar to 3 weeks, resulting in low equilibrated H2O (similar to 2 wt%) and CO2 (<= 500 ppm in bubble-free melt inclusions) contents. The Fall Stratified event, in contrast, records deeper storage (similar to 24-30 km), high magmatic volatile contents (CO2 concentrations up to 9600 ppm, H2O concentrations up to 6.3 wt%), and exceptionally rapid magma ascent rates (17.5 m/s). We propose that the 122 B.C. eruption likely resulted from multiple episodes of replenishment by basaltic magma with a much lower CO2 concentration (<4,500 ppm) than that of the Fall Stratified event. The eruption was probably triggered by an increase in magma effective viscosity due to extensive microlite crystallization at shallow levels, which caused a secondary vesiculation event. This mechanism contrasts with mantle-derived eruptions such as the Fall Stratified event, where deep volatile-exsolution, controlled by CO2, is thought to drive the eruption.
The seismic behavior of subduction megathrusts varies spatially and is influenced by the properties of subducting plates, including their sedimentary cover. Characterizing these subduction inputs is essential for understanding the mechanisms behind fault slip variability. The Hikurangi subduction margin exhibits well-documented variations in fault slip ranging from large earthquakes to slow slip events and aseismic creeping. The variations coincide with along-margin changes in convergence rate, trench sediment thickness, and accretionary wedge development. Here, we integrate regional seismic reflection profiles with drill cores from the International Ocean Discovery Program Expeditions 372 and 375, and with other seafloor samples. We find that the Hikurangi Trough hosts some of Earth's thickest trench sediments, at up to 10 km, including pelagic sediments and a >6 km thick siliciclastic trench wedge that is younger than previously thought. Seismic imaging reveals that the shallow megathrust depth increases by 3.5-4.5 km southward along the margin in parallel with increasing trench sediment thickness despite a "step up" of the fault's stratigraphic position to a different assemblage of host rocks. In contrast to the irregular northern megathrust associated with subducting seamounts and heterogeneous volcaniclastic and pelagic host rocks, the southern megathrust is smooth, hosted in calcareous pelagic rocks, and straddles inherited polygonal faults. These along-strike variations in physical properties and megathrust geometry correspond to observed changes in fault slip behavior. Our findings provide new insights into the factors controlling fault slip variability, with implications for seismic hazard assessment at subduction zones worldwide and for understanding subduction margin evolution.
We present a comprehensive multidisciplinary investigation of the Healy submarine volcano in the southern Kermadec arc, northeast of New Zealand. We show the first results of sidescan-sonar data collected at a submarine arc volcano by an Autonomous Underwater Vehicle (AUV). We derive a seafloor geological map using visual observations including video and still photographs as well as samples collected by the manned submersible Pisces V, and integrate them with high-resolution bathymetric data, AUV magnetics, and hydrothermal plume geochemistry, to model the complex interplay of magmatic and tectonic processes and their control on the evolution of seafloor hydrothermal activity. Our results show that recent basaltic dikes, possibly associated with the onset of back-arc extension, preferentially intrude along structurally weakened sectors of the older caldera rim, particularly its southeastern walls, exploiting inherited structures that facilitate the upwelling of magma and fluids. Hydrothermal activity exhibits a similar spatial-temporal transition, shifting from older venting associated with arc magmatism around the caldera rim to more recent spatially diffuse venting linked to the emplacement of basaltic dikes and extensional fracturing. This relative spatial shift in magma, vent, and fluid distribution reflects the progressive reorganization of magmatic and hydrothermal activity within an arc-backarc transitional setting, making Healy a relevant case study for examining relative feedback among magmatic, structural, and hydrothermal processes.
Monitoring data are critical for understanding volcanic unrest and eruption, but they often lack the ability to constrain the pre-eruptive magma processes. As such, an increasing number of studies couple monitoring data with petrological tools to obtain insights into the causes and durations of magmatic processes. The 2018-2020 eruption of the submarine Fani Maor & eacute; (FM) volcano, Comoros Archipelago, is an excellent location to correlate diffusion chronometry with real-time monitoring data. Zoned olivine in FM testify that deep basanitic magma interacted with a more evolved reservoir. Diffusion modeling in olivine shows an increase in the magma interaction times (0.5-3 to similar to 20 months) as the eruption progresses, implying that basanitic magma stalled near the more evolved reservoir. Additionally, two major intrusive events were identified-in March 2019 and March 2020. The first intrusion occurred as seismic and deformation signals decreased possibly due to enhanced mixing and unlocking of the more evolved mushy reservoir after intrusion. The second intrusion correlates with a change in the locations of seismic events before seismicity and deformation become minor, implying that this intrusion occurred before magma drainage and waning of the eruption. Lastly, by coupling diffusion geochronometry and geophysical monitoring, the magmatic reservoir at FM can be placed at a greater depth (similar to 30 km) than previously thought, which is consistent with seismic events. Hence, our integrated approach of petrologic tools and real-time monitoring aids in providing new insights into the magma storage locations and magma-mush interactions at submarine volcanos.
The cratonic lithospheric mantle records complex metasomatic processes and is frequently tapped by alkaline magmatism, offering a unique opportunity to trace the progressive evolution of the mantle. In the present contribution, we investigate a newly identified calc-alkaline lamprophyre field from the Neoarchean Jonnagiri Schist Belt, Eastern Dharwar Craton, southern India. Petrographic and mineralogical observations reveal reversely zoned amphibole phenocrysts, indicating magma mixing and recharge within a multi-level magma plumbing system. Amphibole cores, interpreted as antecrysts, crystallized in a deep-seated magma reservoir (similar to 20 km) and were later overgrown by rims equilibrated with a more mafic melt at shallower crustal levels (similar to 7 km), recording a dynamic history of magmatic evolution. 40Ar/39Ar age (2,200-2,400 Ma) of the lamprophyres cast a significantly distinct Paleoproterozoic calc-alkaline event that predates the widespread Mesoproterozoic (similar to 1,100 Ma) alkaline magmatism elsewhere in the craton. Whole-rock geochemistry characterized by negative Nb-Ta, Zr-Hf, and Ti anomalies in the spidergram, suggests derivation from a sub-continental lithospheric mantle source modified by slab-derived fluids during the Neoarchean subduction. The Th/Yb (5.3-2.5) and Nb/U (9.4-2.1) trace-element ratios, and the fractionated LREE/HREE pattern (LaN/YbN: 29-18) of the investigated lamprophyres are indistinguishable from those of lamprophyres from the Superior- and Yilgarn- Cratons. Near Chondritic epsilon Ndi signatures and enriched Archean mantle-like Sr isotopic signatures imply a short residence time of the generated magma after metasomatic modification. The geodynamic setup of the Jonnagiri schist belt reflects post-collisional reworking of the subduction-modified lithosphere, possibly triggered by the slab break-off and asthenospheric upwelling during the waning period of Neoarchean accretion. Partial melting of this enriched mantle during the Paleoproterozoic (similar to 2.3 Ga), likely related to Superia supercontinent break-up, contributed to the polychronous magmatic activity in this domain. The Jonnagiri lamprophyres record the prolonged influence of a subduction-enriched lithospheric domain tapped episodically during the Neoarchean to Mesoproterozoic.
Abstract Deep borehole observatories installed during IODP Expedition 375 at Sites U1518 and U1519 along the northern Hikurangi subduction margin offshore New Zealand provide new constraints on the thermal structure of the frontal prism, where frequent shallow slow slip events occur. Site U1518, located just landward of the trench, targets the frontal thrust near the deformation front, whereas Site U1519 lies farther landward on the hanging wall of the megathrust. We analyze temperature time series from 2020 together with thermal conductivity measurements from adjacent boreholes at each site to estimate heat flow. Mean heat flow is estimated to be 26.93 mW m−2 at Site U1518 and 22.84 mW m−2 at Site U1519. These values lie at the lower end of nearby shallow marine heat flow probe measurements, which also include substantially higher values. At both sites, the deep measurements are lower than estimates from advanced piston corer temperature tool data, and at U1519 they are lower than values inferred from bottom‐simulating reflectors. Because the deep observatories record long‐duration, equilibrated conditions within the forearc wedge, they more reliably capture the slab‐cooled, conductive background than shallow observations that can be influenced by fluid advection, sedimentation, bottom water variability, or uncertainty in estimating formation temperatures. These results provide improved constraints on the thermal state of the northern Hikurangi forearc and help distinguish the slab‐cooled conductive baseline from near‐surface effects.
The closure of the Paleo-Tethys Ocean was one of the most important global geological events which formed the framework of the East Asian continent during the early Mesozoic. The Longmu Co-Shuanghu suture zone is considered to record closure of the main basin of the Paleo-Tethys Ocean. However, the closure processes remain unclear, which crucially impedes our understanding of the tectonic evolution of the Tethyan oceanic system. In this study, we investigated newly discovered Triassic-Jurassic igneous rocks, including diorite, rhyolite, andesite, and dacite, along the Longmu Co-Shuanghu suture zone. The Middle Triassic diorite (ca. 242 Ma) has a geochemical affinity with andesite and is characterized by high Mg# (53.9-60.9) and variable zircon epsilon Hf(t) (-17.8 to +9.0) and whole-rocks epsilon Nd(t) (+1.25 to +0.87) values. These features indicate that the diorite was generated by interactions between melts of terrigenous sediment and the mantle in an oceanic subduction setting. Late Triassic rhyolite (ca. 213 Ma) has geochemical characteristics typical of A2-type granite, with zircon epsilon Hf(t) (-9.7 to +11.6) and whole-rock epsilon Nd(t) (-0.11 to -0.03) values. The Late Triassic rhyolites, and Early Jurassic andesites (epsilon Nd(t) = -0.59 to -0.43, epsilon Nd(t) = -10.45 to -9.66) are attributed to partial melting of crustal material, combined with ca. 40%-60% mantle-derived components introduced into the magma source. In contrast, the Jurassic dacites show limited mantle contribution (ca. 10%-20%). Based on data for igneous rocks from the North Qiangtang area, a Late Triassic continental-crust thinning event occurred at ca. 218 Ma. This event is roughly coeval with regional tectono-sedimentary evolution in the north-central Tibetan Plateau. Closure of the Longmu Co-Shuanghu Ocean involved subduction (242-230 Ma), syn-collision (230-218 Ma), and post-collision (218-188 Ma) stages. The comparable closure histories of the Longmu Co-Shuanghu and Changning-Menglian Oceans indicate a unified closure history of the Paleo-Tethys Ocean and further suggest that it may have closed nearly synchronously across East Asia.
Seafloor basalt samples recovered from the Caroline Basin (CB), the Parece Vela Basin (PVB), and the Western Philippine Sea Plate (WPSP) are analyzed for whole-rock major and trace elements (including H2O), Sr-Nd-Pb-Hf isotopes and 40Ar-39Ar age-dating. Two geochemically distinct mantle domains (I and II) are identified for the first time in these marginal basins and their spatio-temporal distribution and origin are discussed. The Mantle Domain I (PVB and Ayu Trough basalts) has normal mantle-like H2O/Ce ratios and TiO2/V ratios, whereas the Mantle Domain II (WPSP and CB basalts, and Mariana fore-arc basalts) has distinctly lower TiO2/V, distinctly more radiogenic Sr and Pb isotopes and higher H2O/Ce ratios. The normal mantle-like H2O/Ce of Mantle Domain I indicates negligible influence from subducted components. We provide evidences that the Mantle Domain II extends to the Pacific large low shear velocity province (LLSVP) based plate tectonic reconstruction, and such an Indian-type mantle ubiquitously exists beneath the Mesozoic Pacific Plate. Our geochemistry results indicate that the giant mantle plumes in LLSVP have played a key role in transporting lower mantle depleted and H2O-rich components to the upper mantle.
This study presents data on geographically small-scale patterns of nitrogen (N) isotope signals (delta 15N) within the northern South China Sea (SCS) imprinted on planktic foraminifers (PF). PF from net tows on the shelf, continental slope, and in pelagic waters from summer 2019 were analyzed for delta 15N. PF tissue delta 15N was observed to diverge from the subsurface nitrate supply. The divergences were shared by PF species at a given station and depth, and differed among regions, indicating an environmental (as opposed to physiological-biochemical) driver. Moreover, the divergences were consistent with N cycle processes occurring in the respective oceanographic environments. In the oligotrophic open northern SCS, the PF tissue delta 15N of shallow-dwelling species was 1-2.4 parts per thousand lower than local subsurface nitrate delta 15N. This likely reflects the documented tendency for upper ocean N pools to decrease in delta 15N under intense euphotic zone N recycling. There may have also been low-delta 15N inputs from N2 fixation and/or atmospheric N deposition. On the continental slope, in a mixing-associated productivity plume, PF tissue delta 15N was higher and varied around the subsurface nitrate supply, reflecting the ecosystem's consumption of nitrate entrained from the subsurface. Our data highlight the sensitivity of PF tissue delta 15N to prevailing N cycling processes. Existing sediment data indicate that delta 15N of PF microfossils dominantly reflects the delta 15N of nitrate consumed in surface waters, but upper ocean studies such as ours show effects from seasonal and shorter-duration processes, that may lead to second-order controls on sedimentary PF delta 15N.
Abstract The origin of orthogonally spreading ridge segments separated by oceanic transform faults, versus obliquely spreading ridge segments without transform faults (TFs), is a long‐standing enigma of plate tectonics. We address this problem using three‐dimensional (3D) geodynamic models that simulate axial magmatic intrusions along two ridge segments, initially oriented orthogonally to seafloor spreading and the intervening TF. The intrusion zones dynamically relocate to local maxima in the depth‐averaged horizontal “effective stress.” Both 3D numerical models, which simulate a viscoelastic–plastic rheology as well as 2D thin elastic plate models predict shear stress on the TF to induce an asymmetry in lithospheric tension, causing the ridge segment ends to migrate toward each other and eventually form an obliquely spreading ridge. Orthogonal spreading with a TF is promoted by weak TFs, rapid lithospheric thickening beneath the plate boundary zone, higher rates of magmatically accommodated spreading, and intrusions that are confined to a narrow zone beneath the ridge axis. These conditions are observed or expected at slow and faster spreading rates, where orthogonal spreading occurs. In contrast, oblique spreading is promoted by strong TFs, slow lithospheric thickening beneath the plate boundary, lower rates of magmatically accommodated extension, and a broader width about the ridge axis where intrusions occur. These conditions are predicted to promote the observed oblique spreading in typical slow and ultra‐slow spreading environments. At the atypical, hotspot‐influenced Reykjanes Ridge, oblique spreading is inferred to be promoted by a broad subaxial zone of magma intrusion caused by anomalously high magma supply.
Abstract Fission track (FT) dating is a well‐established thermochronological technique widely used to reconstruct rocks' thermal histories. Over the past two decades, development of Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA‐ICPMS) has significantly advanced the application of FT dating by enabling rapid measurements of uranium concentrations and other compositions. Despite the advantages, current LA‐ICPMS FT methods often produce overdispersed age data sets. Consequently, interpretation typically relies on the central age model of Galbraith and Laslett (1993), 10.1016/1359‐0189(93)90185‐c, which calculates a weighted mean and a dispersion parameter based on the log‐normal distribution of individual grain ages. However, a synthesis of published data reveals that LA‐ICPMS methods tend to yield systematically older central ages compared to the pooled mean (area‐weighted) of multiple grains. Furthermore, the central ages of reference materials frequently exceed the known true values derived from other independent geochronometers. We found that the overdispersion and central‐age overestimation primarily results from underestimation of age uncertainties at the single‐grain level, especially for either low‐uranium grains or those with abundant spontaneous tracks, both of which tend to yield relatively older ages. Underestimated age uncertainties for such grains would result in overestimation of their weights, thereby leading to higher central ages. To address this issue, we introduce a practical age calculation method tailored to LA‐ICPMS FT data. Validated by a large multi‐laboratory data set, the new method produces pooled ages nearly identical to the conventional approaches, while significantly reducing intra‐sample dispersion and the offsets of central ages from both the pooled (for unknowns) and reference values (for reference materials).
The middle Cretaceous granitoid plutons of the Shalair Valley, situated in northeastern Iraq, constitute a principal magmatic component of the Sanandaj-Sirjan Zone within the northwestern segment of the Zagros Orogenic Belt. Among these plutons, the equigranular Aulan body (AG) and the porphyritic Laladar body (LG) were crystallized at 111.0 +/- 1.5 Ma and 112.4 +/- 2.1 Ma, respectively, as determined by zircon U-Pb dating. These bodies are metaluminous, calc-alkaline I-type granitoids. The AG rocks show positive epsilon Nd(t) values (+2.1 to +2.6) with identical Nd model ages of 0.78-1.27 Ga, whereas the epsilon Nd(t) values of the LG rocks range between +0.3 and +4.1 with Nd model ages of 0.87-1.5 Ga. These isotopic signatures collectively suggest derivation from a younger, juvenile crustal source for both granitoid bodies. Integration of the present results with previous studies on the middle Cretaceous granitoids of the Shalair Valley indicates that these intrusions were broadly synchronous and syngenetic. Moreover, Sr-Nd isotopic data reveal a distinct compositional contrast between the northern and southern limbs of the Shalair Valley volcanic arc-related anticline, with the northern intrusions incorporating a higher proportion of crustal material during magma evolution. Taken together, the geochemical and isotopic evidence substantiates the interpretation that the Shalair Valley granitoids were emplaced through fractional crystallization (FC) processes coupled with crustal assimilation, within a subduction-governed, mature Andean-type continental arc that developed along the northwestern Zagros Orogenic belt during the middle Cretaceous.
The Cretaceous-Eocene island arc of Hispaniola is currently shortened between the Bahamas carbonate platform to the north and the thickened crust of the Caribbean Large Igneous Province (CLIP) to the south. Within this transpressional setting, the 15-22-km-thick, similar to 100-km-wide Beata Ridge (BR), the thickest portion of the CLIP, acts as a mechanically strong, tectonic indenter controlling on upper-plate deformation in Hispaniola. Integration of marine seismic, multibeam bathymetry, gravity and magnetic anomalies, and onshore geological data reveals a similar to 140-km-wide counterclockwise rotation belt in central Hispaniola and coeval clockwise rotations in the offshore San Pedro Basin (SPB). The tectonostratigraphic evolution of the SPB provides a continuous Cenozoic record of indentation-related deformation, documenting the transition from a Late Cretaceous back-arc basin to a Late Eocene-present forearc basin associated with the Peralta-Muertos accretionary prism. Seismic stratigraphic analysis identifies key indicators of indentation, including rapid subsidence, angular unconformities, fault reactivation, and progressive rotation of Miocene-Pliocene depocenters, which together constrain the timing and landward propagation of deformation. Our results suggest that indentation began earlier than previously proposed, likely in the middle Miocene, and evolved progressively over time. The observed deformation patterns are consistent with models of narrow (100-400 km) arc-ridge indentation systems, where the indenter width controls the curvature and extent of upper-plate deformation. These findings demonstrate that the Beata Ridge governs the kinematic evolution of the Hispaniola forearc system and provide a framework for understanding deformation in confined arc-ridge collision zones worldwide.
Accurate identification of rammed-earth remains is essential for understanding early social complexity in Neolithic China. However, scientific methods for identifying rammed-earth materials in Chinese prehistoric archeology remain limited, and identification still rely largely on field observations. Here, we apply anisotropy of magnetic susceptibility (AMS) analysis to Neolithic rammed-earth remains (walls and platform foundations), cultural-layer deposits, and natural sediments from the Jiaojia and Chengziya sites at the lower Yellow River region. The results show that rammed earth mainly features well-clustered K min axes and stable or tilted magnetic foliation planes, reflecting strain-induced particle reorientation during repeated ramming. In most rammed-earth samples, K max and K int axes are commonly scattered or partly mixed within the foliation plane. Wall II, however, records a mixed foliation-lineation fabric with a near-horizontal K max lineation, probably related to horizontal compression or lateral constraint during wall construction and possible later repair. These rammed-earth fabrics are clearly distinct from the depositional fabrics of natural sediments and the randomized fabrics of cultural-layer deposits. Rock magnetic results show no major differences in magnetic mineral assemblages among the samples, suggesting that mineralogical variation is unlikely to be the primary cause of the AMS contrasts. These contrasts are therefore more plausibly linked to differences in magnetic-particle orientation and spatial organization associated with ramming, natural deposition, and anthropogenic disturbance. Together, these results demonstrate that AMS can complement traditional archeological approaches and provide a sensitive method for identifying Chinese Neolithic rammed earth. Future studies should test this method using more sites and experimentally compacted soils.
