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The 2014–2015 CE rift event associated with the Bárðarbunga eruption at Holuhraun, Iceland, offers a unique opportunity to study the spatial and temporal evolution of a rift graben. We present the first four-dimensional (three-dimensional plus time) monitoring of the formation and evolution of a graben during active magma transport using a suite of digital elevation models spanning from shortly before the eruption throughout 6 months of magma transport and up to 4.5 years after the eruption. This multiscale data set enables investigations of how magma supply and eruption dynamics affect tectonic structures that feed eruptions. After formation (time scale of a few days), the graben is remarkably stable throughout the eruption and for years beyond. It is unaffected by large changes in eruptive activity and effusion and seismicity rates within the plumbing system. These data document that (1) there was no direct feedback between eruptive dynamics and graben topography, and (2) graben formation is near instantaneous on tectonic time scales. These results challenge the overarching role ascribed to magma transport in recent studies of tectonomagmatic relationships in rift events, favoring regional tectonics as the fundamental driving force.

Knowledge of the nature of the earliest metamorphism experienced by collisional orogenic belts is essential for reconstruction of tectonic processes that build high mountain chains and their environmental consequences. Understanding the metamorphic nature of Eohimalayan-phase orogeny of the Himalayan orogen, one of the typical examples of orogenic belts worldwide, could provide some important constraints to test different tectonic models (shallow continental subduction vs. slab breakoff) for the early phases of the development of large-scale orogenic belts. As exhumed middle- to lower-crustal rocks in the Kangmar gneiss dome, the garnet amphibolites with a protolith age of 176.4 ± 3.6 Ma experienced a phase of metamorphism at 47.2 ± 1.8 Ma with an increase in pressure as well as temperature from 3–5 kbar and 550–600 °C to over ~11 kbar and 650 °C. This suggests that the middle- to lower-crustal rocks experienced heating at least by ~50 °C while they underwent compression and thickening. Heat-flow estimation further demonstrates that the self-produced heat was high enough to achieve the observed pressure-temperature conditions recorded by the garnet amphibolite. Therefore, an additional heat supply is not required during early Eocene metamorphism. A breakoff of the leading part of the subducting Indian continental slab, if it occurred, should be younger than ca. 47 Ma.

Despite decades of research, the mechanisms and processes of subduction initiation remain obscure, including the tectonic settings where subduction initiation begins and how magmatism responds. The Cretaceous Mawgyi Volcanics represent the earliest volcanic succession in the Wuntho-Popa arc of western Myanmar. This volcanic unit consists of an exceptionally diverse range of contemporaneously magmatic compositions which are spatially juxtaposed. Our new geochemical data show that the Mawgyi Volcanics comprise massive mid-oceanic ridge basalt (MORB)-like lavas and dikes, and subordinate island arc tholeiite and calc-alkaline lavas. The Mawgyi MORB-like rocks exhibit flat rare earth elements (REEs) patterns and are depleted in REEs, high field strength elements (except for Th) and TiO2 concentrations relative to those of MORBs, resembling the Izu-Bonin-Mariana protoarc basalts. Our geochronological results indicate that the Mawgyi Volcanics formed between 105 and 93 Ma, coincident with formation of many Neotethyan supra-subduction zone ophiolites and intraoceanic arcs along orogenic strike in the eastern Mediterranean, Middle East, Pakistan, and Southeast Asia. Combined with its near-equatorial paleo-latitudes constrained by previous paleomagnetic data, the Wuntho-Popa arc is interpreted as a segment of the north-dipping trans-Neotethyan subduction system during the mid-Cretaceous. Importantly, our restoration with available data provides new evidence supporting the hypothesis of a mid-Cretaceous initiation of this >8000-km-long subduction system formed by inversion of the ∼E-W–trending Neotethyan oceanic spreading ridges, and that this was contemporaneous with the final breakup of Gondwana and an abrupt global plate reorganization.

The location of the West African craton (WAC) has been poorly constrained in the Paleoproterozoic–Mesoproterozoic supercontinent Nuna (also known as Columbia). Previous Nuna reconstruction models suggested that the WAC was connected to Amazonia in a way similar to their relative position in Gondwana. By an integrated paleomagnetic and geochronological study of the Proterozoic mafic dikes in the Anti-Atlas Belt, Morocco, we provide two reliable paleomagnetic poles to test this connection. Incorporating our new poles with quality-filtered poles from the neighboring cratons of the WAC, we propose an inverted WAC-Amazonia connection, with the northern WAC attached to northeastern Amazonia, as well as a refined configuration of Nuna. Global large igneous province records also conform to our new reconstruction. The inverted WAC-Amazonia connection suggests a substantial change in their relative orientation from Nuna to Gondwana, providing an additional example of large-magnitude cumulative azimuthal rotations between adjacent continental blocks over supercontinental cycles.

Forecasting heightened magmatic activity is key to assessing and mitigating global volcanic hazards, including eruptions from lateral rift zones at basaltic volcanoes. At Kı-lauea volcano, Hawai’i (United States), planar dikes intrude its east rift zone (ERZ) and repeatedly affect the same segments. Here we show that Kı-lauea’s upper and middle ERZ dikes in the last four decades intruded at regular intervals of ∼8 or ∼14 yr. Segments with shorter recurrence intervals are adjacent to faster-moving parts of the flank, and ∼1–5 MPa of tension accumulates from flank spreading in the time between dike events. Intrusion frequency was neither advanced nor delayed during magma supply variations, supporting the role of long-term flank motion on the timing of dike intrusions. Although fewer historical dikes have occurred near the 2018 CE eruption site in the lower ERZ and the adjacent slowly sliding lower eastern flank, similar tension accumulated between the 1955 and 2018 eruptions. Regular dike intrusion recurrence intervals indicate the importance of including both extrusive and (commonly neglected) intrusive activity in eruption hazard analyses.

Four areas (Loriu, Lojamei, Muranachok-Muruangapoi, Kamutile Hills) of well-developed Miocene-age dikes in the northern Kenya Rift (Turkana, Kenya) have been identified from fieldwork and satellite images; in total, >3500 dikes were mapped. Three areas display NNW-SSE– to N-S–oriented dike swarms, with straight, radial, and concentric patterns in zones

The Vredefort granophyre dikes have long been recognized as being derived from the now-eroded Vredefort melt sheet. One dike, in particular, the Daskop granophyre dike, is notable for a high abundance of lithic clasts derived from various stratigraphic levels. In this study, we mapped the distribution of the clasts throughout the continuously exposed section of the dike using field mapping and aerial drone photography and attempted to constrain the emplacement mechanisms of the dike. We found that the clasts are not homogeneously spread but instead are distributed between clast-rich zones, which have up to 50% by area clasts, and clast-poor zones, which have 0–10% by area clasts. We examined three models to explain this distribution: gravitational settling of clasts, thermally driven local assimilation of clasts, and mechanical sorting of clasts due to turbulent flow. Of the three models, the gravitational settling cannot be supported based on our field and geophysical data. The assimilation of clasts and turbulent flow of clasts, however, can both potentially result in inhomogeneous clast distribution. Zones of fully assimilated clasts and nonassimilated clasts can occur from spatial temperature differences of 100 °C. Mechanical sorting driven by a turbulent flow can also generate zones of inhomogeneous clast distribution. Both local assimilation and mechanical sorting due to turbulent flow likely contributed to the observed distribution of clasts.

The Nutzotin basin of eastern Alaska consists of Upper Jurassic through Lower Cretaceous siliciclastic sedimentary and volcanic rocks that depositionally overlie the inboard margin of Wrangellia, an accreted oceanic plateau. We present igneous geochronologic data from volcanic rocks and detrital geochronologic and paleontological data from nonmarine sedimentary strata that provide constraints on the timing of deposition and sediment provenance. We also report geochronologic data from a dike injected into the Totschunda fault zone, which provides constraints on the timing of intra–suture zone basinal deformation. The Beaver Lake formation is an important sedimentary succession in the northwestern Cordillera because it provides an exceptionally rare stratigraphic record of the transition from marine to nonmarine depositional conditions along the inboard margin of the Insular terranes during mid-Cretaceous time. Conglomerate, volcanic-lithic sandstone, and carbonaceous mudstone/shale accumulated in fluvial channel-bar complexes and vegetated overbank areas, as evidenced by lithofacies data, the terrestrial nature of recovered kerogen and palynomorph assemblages, and terrestrial macrofossil remains of ferns and conifers. Sediment was eroded mainly from proximal sources of upper Jurassic to lower Cretaceous igneous rocks, given the dominance of detrital zircon and amphibole grains of that age, plus conglomerate with chiefly volcanic and plutonic clasts. Deposition was occurring by ca. 117 Ma and ceased by ca. 98 Ma, judging from palynomorphs, the youngest detrital ages, and ages of crosscutting intrusions and underlying lavas of the Chisana Formation. Following deposition, the basin fill was deformed, partly eroded, and displaced laterally by dextral displacement along the Totschunda fault, which bisects the Nutzotin basin. The Totschunda fault initiated by ca. 114 Ma, as constrained by the injection of an alkali feldspar syenite dike into the Totschunda fault zone.These results support previous interpretations that upper Jurassic to lower Cretaceous strata in the Nutzotin basin accumulated along the inboard margin of Wrangellia in a marine basin that was deformed during mid-Cretaceous time. The shift to terrestrial sedimentation overlapped with crustal-scale intrabasinal deformation of Wrangellia, based on previous studies along the Lost Creek fault and our new data from the Totschunda fault. Together, the geologic evidence for shortening and terrestrial deposition is interpreted to reflect accretion/suturing of the Insular terranes against inboard terranes. Our results also constrain the age of previously reported dinosaur footprints to ca. 117 Ma to ca. 98 Ma, which represent the only dinosaur fossils reported from eastern Alaska.

We present paleostress analyses of dike swarms intruded during the subduction initiation in the northern Oman Ophiolite to understand the tectonomagmatic environment. Five swarms of subparallel dikes extending WNW-ESE are 1–5 km in width and are spaced every 5 km N-S. Each swarm has a core of 100% sheeted dikes 1–2 km in width, which emanated from the dunite-wherlite-clinopyroxenite-gabbronorite-diorite-tonalite complexes below and intruded through V1 and into V2 extrusive rocks. Individual dike strikes are varied but generally subparallel to the overall trend of the swarm. Paleostress analyses indicate subvertical σ1, ∼σ2, and subhorizontal σ3 with high magma pressures, resulted in the mutually intrusive, extensional shear dikes and abrupt changes in dike strike at high angles. These occurrences suggest intrusions under a more compressive environment compared to the extensional stress field that formed the N-S–striking sheeted dikes of V1 spreading stage. Most E-W–striking dikes possess both boninitic and tholeiitic geochemistry. The latter resemble the V1 flows and dikes with affinities of mid-ocean ridge basalt. Some tholeiitic dikes strike N-S, which are mutually intrusive to E-W–striking dikes. Tholeiitic dikes are more intensely altered than boninite, suggesting their older ages. Conversion of the stress field from a N-S–running spreading axis to inextensional E-W–running rift zones associated with the change in magma geochemistry agree with the relatively compressive V2 arc above a forced subduction zone, which originated from intraoceanic thrusting caused by the clockwise rotation of a microplate including the future northern ophiolite.

The Kahiltna assemblage in the western Alaska Range consists of deformed Upper Jurassic and Cretaceous clastic strata that lie between the Alexander-Wrangellia-Peninsular terrane to the south and the Farewell and other pericratonic terranes to the north. Differences in detrital zircon populations and sandstone petrography allow geographic separation of the strata into two different successions, each consisting of multiple units, or petrofacies, with distinct provenance and lithologic characteristics. The northwestern succession was largely derived from older, inboard pericratonic terranes and correlates along strike to the southwest with the Kuskokwim Group. The southeastern succession is characterized by volcanic and plutonic rock detritus derived from Late Jurassic igneous rocks of the Alexander-Wrangellia-Peninsular terrane and mid- to Late Cretaceous arc-related igneous rocks and is part of a longer belt to the southwest and northeast, here named the Koksetna-Clearwater belt. The two successions remained separate depositional systems until the Late Cretaceous, when the northwestern succession overlapped the southeastern succession at ca. 81 Ma. They were deformed together ca. 80 Ma by southeast-verging fold-and-thrust–style deformation interpreted to represent final accretion of the Alexander-Wrangellia-Peninsular terrane along the southern Alaska margin. We interpret the tectonic evolution of the Kahiltna successions as a progression from forearc sedimentation and accretion in a south-facing continental magmatic arc to arrival and partial underthrusting of the back-arc flank of an active, south-facing island-arc system (Alexander-Wrangellia-Peninsular terrane). A modern analogue is the ongoing collision and partial underthrusting of the Izu-Bonin-Marianas island arc beneath the Japan Trench–Nankai Trough on the east side of central Japan.

Early Carboniferous tholeiitic dikes in the New Carlisle area of Quebec, Canada, are abnormally enriched in high field strength elements (HFSEs), including rare earth elements. The enrichment is systematic and was apparently caused by an enhanced incompatibility of HFSEs during a prolonged episode of crystal fractionation. As a result, HFSE concentrations are up to one order of magnitude higher than those of typical mafic rocks. Very high F and Cl contents suggest that halogen complexing was largely responsible for the trace-element enrichment. The high halogen contents are in part accounted for by a subcontinental lithospheric mantle source that had previously been enriched in these elements by prolonged subduction. Additional Cl enrichment is interpreted to have occurred in a magma chamber that developed within porous and brine-rich country rocks of the upper crust. This conclusion is supported by the observation that HFSE-enriched mafic plutons in the same magmatic province occur in nonmetamorphosed upper-crustal rocks, suggesting high buoyancy and therefore high temperatures. Such evidence for high heat in the late Paleozoic magmatic system of eastern Canada corroborates previous studies suggesting that the transtensional basin in which it evolved was overriding a mantle plume at the time. In the case of the New Carlisle dikes, which are more than twice as enriched in incompatible trace elements as slightly older mafic rocks of the same magmatic system, the regional paleoenvironmental setting suggests that the associated upper-crustal magma chamber may have evolved in rocks with saltier pore water due to long-lasting evaporitic conditions at the surface.

The unique, giant, rare earth element (REE) deposit at Bayan Obo, northern China, is the world’s largest REE deposit. It is geologically complex, and its genesis is still debated. Here, we report in situ Th-Pb dating and Nd isotope ratios for monazite and Sr isotope ratios for dolomite and apatite from fresh drill cores. The measured monazite ages (361–913 Ma) and previously reported whole-rock Sm-Nd data show a linear relationship with the initial Nd isotope ratio, suggesting a single-stage evolution from a Sm-Nd source that was formed before 913 Ma. All monazites show consistent to those of the adjacent 1.3 Ga carbonatite and mafic dikes. The primary dolomite and apatite show lower than the recrystallized dolomite (0.7038–0.7097). The REE ores at Bayan Obo are interpreted to have originally formed as products of ca. 1.3 Ga carbonatitic magmatism and to have undergone subsequent thermal perturbations induced by Sr-rich, but REE-poor, metamorphic fluids derived from nearby sedimentary rocks.eNd(1.3Ga) values (0.3 ± 0.6) close87Sr/86Sr ratios (0.7024–0.7030)

Dike swarm emplacement accommodates extension during rifting and large igneous province (LIP) formation, with ancient dike swarms serving to localize strain during later tectonic events. Deciphering three-dimensional (3-D) dike swarm geometry is critical to accurately calculating magma volumes and magma-assisted crustal extension, allowing syn-emplacement mantle and tectonic processes to be interrogated. It is also important for quantifying the influence of ancient dike swarms on post-emplacement faulting. However, the essentially 2-D nature of Earth's surface, combined with the difficulties in imaging subvertical dikes in seismic reflection data and the relatively low resolution of geophysical data in areas of active diking, means our understanding of dike swarm geometry at depth is limited. We examine an ~25-km-wide, >100-km-long, west-southwest–trending dike swarm imaged, due to post-emplacement rotation to shallower dips, in high-quality 2-D and 3-D seismic reflection data offshore southern Norway. Tuned reflection packages correspond to thin (

Comparisons of pre-earthquake and post-earthquake microfossils in tidal sequences are accurate means to measure coastal subsidence during past subduction earthquakes, but the amount of subsidence is uncertain, because the response times of fossil taxa to coseismic relative sea-level (RSL) rise are unknown. We measured the response of diatoms and foraminifera to restoration of a salt marsh in southern Oregon, USA. Tidal flooding following dike removal caused an RSL rise of ∼1 m, as might occur by coseismic subsidence during momentum magnitude (M w ) 8.1–8.8 earthquakes on this section of the Cascadia subduction zone. Less than two weeks after dike removal, diatoms colonized low marsh and tidal flats in large numbers, showing that they can record seismically induced subsidence soon after earthquakes. In contrast, low-marsh foraminifera took at least 11 months to appear in sizeable numbers. Where subsidence measured with diatoms and foraminifera differs, their different response times may provide an estimate of postseismic vertical deformation in the months following past megathrust earthquakes.

Laramide structures that are exposed along much of the Laramide porphyry copper province of southeastern Arizona have been dismembered and tilted by superimposed Cenozoic normal faults, such that the overall style of shortening (e.g., thin-skinned, low-angle thrusts versus basement-cored, moderate-angle reverse faults) is unclear pending a compelling reconstruction of superimposed extension. This study integrates new geologic mapping in the Romero Wash–Tecolote Ranch area with earlier work and utilizes palinspastic reconstructions of structures related to extension and shortening to determine the geometry of structures in this segment of the Laramide orogenic belt and their relationship to local products of magmatism and hydrothermal alteration. Geologic mapping indicates that multiple generations of Cenozoic normal faults can be identified through crosscutting relationships. Near both ends of the study area, the oldest synextensional strata dip vertically, indicating that the area has been tilted 90° eastward during extension. Structural reconstructions of normal faults reveal two reverse faults within the area, the Romero Wash fault and the Tecolote fault. Once restored, both faults originally dipped in opposite directions at moderate angles and demonstrate clear evidence for associated fault-propagation folds. As a result of 90°E tilting by Cenozoic normal faulting, the Romero Wash fault became an overturned reverse fault, whereas the Tecolote fault became an apparent normal fault. Constraints from the surface geology and forward models indicate that the Tecolote fault has ∼2.8 km of displacement on a fault that had an initial dip of 55°E, whereas the Romero Wash fault has ∼0.75 km of displacement on a fault that had an initial dip of 58°W. Considering their relative displacements and orientations, the Romero Wash fault is interpreted to be a backthrust to the Tecolote fault. These results are consistent with the interpretation that Laramide shortening in southeastern Arizona was dominated by basement-cored uplifts. Based on reconstruction of a cross section, the amount of Laramide shortening ( e = Δ l / l i ) in the study area was 1.8 km or 20%, and the total amount of subsequent Cenozoic extension ( e = Δ l / l i ) was 14.6 km or 200%. Two stocks of granodiorite, with associated porphyry dikes and related hydrothermal alteration, occur within the study area. Sericitic and ­propylitic ­alteration and sparse quartz veins are present without observed potassic altera­tion. A barren porphyry dike (65.9 ± 0.7 Ma, U-Pb zircon) cuts the Romero Wash fault, limiting the upper age of the fault. Similar dikes nearby are sericiti­cally altered, suggesting that alteration postdates shortening, similar to interpretations reached at the nearby porphyry copper systems of Ray, Resolution, and Kelvin-Riverside.

Exhumed magma conduits provide important evidence of the development and evolution of subvolcanic plumbing systems. We use a 5−14-m-thick flow-banded rhyolite dike in Arran (Scotland) to present the first reconstruction of the directions and styles of initial propagation and subsequent magma flow, based on mesoscale kinematic indicators. The dike has concave-inward dike-margin segments with plumose-like structures that record vertical and horizontal propagation of lobes, which inflated and linked to form a through-going sheet. Devitrified rhyolite zones at the dike margins show gentle to open folds. In contrast, glassy central parts of the dike are flow laminated and preserve folded and refolded isoclinal, curvilinear folds and sheath folds that record sustained progressive deformation. The inner interface between the glassy and devitrified facies is abrupt and marked by elongation lineations and mullions. In the dike center, fold axes plunge 27°NE along the dike, and parallel to elongation lineations. Combined with shear sense indicators (σ- and δ-objects, sheared vesicles, and asymmetric folds), these features indicate that magma flow was obliquely upward, to the southwest, and locally ≤60° to the propagation direction of the dike. The distribution of structures within the rhyolite indicates local accretion of the (now) devitrified material to the margins, with localization of flow into the center of the dike. We find that the initial magma flow direction was controlled by fracture propagation and interaction, with the subsequent flow record controlled by accretion and flow localization in the conduit. This study demonstrates that analysis of mesoscopic structural and kinematic features (several of which have not previously been reported from dikes) is a powerful tool that can be used to reconstruct the complex evolution of conduit initiation and magma flow processes.

Ultraslow-spreading ridges are a novel class of spreading centers symbolized by amagmatic crustal accretion, exposing vast amounts of mantle-derived peridotites on the seafloor. However, distinct magmatic centers with high topographies and thick crusts are also observed within the deep axial valleys. This suggests that despite the low overall melt supply, the magmatic process interacting with the tectonic process should play an important role in crustal accretion; however, this has been obscured due to the lack of seismic images of magma chambers. Using a combination of seismic tomography and full waveform inversion of ocean bottom seismometer data from the Southwest Indian Ridge at 50°28′E, we report the presence of a large low-velocity anomaly (LVA) ∼4–9 km below the seafloor, representing an axial magma chamber (AMC) in the lower crust. This suggests that the 9.5-km-thick crust here is mainly formed by a magmatic process. The LVA is overlain by a high-velocity layer, possibly forming the roof of the AMC and defining the base of hydrothermal circulation. The steep velocity gradient just below the high-velocity layer is explained by the ponding of magma at the top of the AMC; this could provide the overpressure for lateral dike propagation along the ridge axis, leading to a complex interaction between magma emplacement, tectonic, and hydrothermal processes, and creating a diversity of seafloor morphology and extremely heterogeneous crust.