Dike Intrusion Research Articles

Complex arrangement of early pyroclastic and intrusive rock records onset of intraplate Dunedin Volcano

Little is understood about the birth of composite volcanoes because products of early eruptive activity are buried by products of later eruptions. The oldest rocks of the Dunedin Volcano (active 16.0–11.0 Ma) are exposed at Otapahi (Allans Beach) and allow us to study the inception of a long-lived composite volcano. Seven rock units are visible at Otapahi, investigated with fieldwork and detailed componentry. A bedded deposit of phonolitic pumice clasts is vertically cross-cut by at least three diatreme deposits, as well as a compositionally diverse array of dikes emplaced penecontemporanously with the diatremes. The complicated cross cutting relationships among diatremes and different generations of dikes allow inference of the relative timing of events. Componentry revealed lithic clasts from the underlying Cenozoic sedimentary sequence and basement schist in the oldest diatreme, thus indicating it quarried to basement depths. A lack of coherent cognate igneous clasts in the oldest diatreme is also consistent with the earliest eruption in the area having formed the ~60 m thick bedded pumice deposit. Younger diatremes lack any identified lithic fragments from underlying strata but contain abundant basaltic clasts with compositions matching those of dikes injected penecontemporaneously. The increase of igneous clasts in the younger diatreme deposits, together with the decreasing number of cross-cutting dikes records a chaotic succession of diatreme formation and repeated dike intrusion. This interplay of exogeneous with endogenous processes built an early submarine eruptive centre, and initiated development of a long-lived intraplate composite volcano with activity that persisted across the next 5 million years.

Geochronological and Geochemical Constraints on the Petrogenesis of Lamprophyre from the Giant Weishan REE Deposit in China

The Weishan REE deposit is located in the southwest of the Luxi Terrane of the North China Craton (NCC), where a large number of lamprophyre dikes are spatially exposed with the deposit. Here, we report petrology, geochemistry and zircon U-Pb geochronology data for the lamprophyre of the Weishan REE deposit in order to develop constraints for the determination of the petrogenesis, magma source and evolution of the lamprophyre and the tectonic environment. LA-LCP-MS zircon U-Pb dating shows that the crystallization age of the lamprophyre is 125 ± 0.86 Ma. The geochemical data suggest that these lamprophyres have high levels of Al2O3, K2O, MgO and alkalis, moderate level of Na2O and low levels of SiO2, Fe2O3 and TiO2, and that they are enriched with LREEs (La, Ce) and LILEs (Rb, Ba) and depleted with regard to HREEs and HFSEs (Nb, Ta, Ti). They displayed negative εHf(t) values of −14.98 to −9.03, TDM1 ages of 1.1–1.4 Ga and TDM2 ages of 1.7–2.1 Ga, which suggest that the magma source originates from an enriched mantle. Low Rb/Sr and high Dy/Yb ratios suggest that the enriched mantle source was partially melted at the amphibole-bearing lherzolite garnet-facies. The high Ba/Th and Sr/Th ratios indicate that the enriched source was derived from subduction dehydration fluids of the oceanic crust. We propose that the mafic dike intrusions are consistent with an Early Cretaceous alkaline magma emplacement in an extensional setting, in which the magma was not contaminated by crustal material during its emplacement.

Inter-eruptive spatiotemporal variation of caldera contraction on Miyakejima volcano revealed by PALSAR and PALSAR-2 time-series data

Monitoring caldera deformation helps improve the understanding of pressure conditions within a volcanic conduit beneath the caldera. Miyakejima volcano in Japan experienced a caldera collapse associated with an eruption in 2000, which included dike intrusions. Although a previous InSAR study suggested deceleration of caldera subsidence in 2009, few observations on the caldera deformation have been reported. We applied SAR time-series analysis to Phased Array-type L-band Synthetic Aperture Radar (PALSAR) and PALSAR-2 data to investigate spatiotemporal variations of ground deformations at Miyakejima volcano during 2006–2011 and 2014–2019. The spatial characteristics of the caldera contraction transformed from a concentric pattern into a north-south asymmetric pattern in mid-2009, implying migration of a deformation peak from the central aspect to the southern edge of the caldera floor. These peaks of the caldera contraction are located at the initial position of the caldera collapse on the central caldera floor, and at an active fumarole on its southern edge. The asymmetric pattern of the caldera contraction continued during 2014–2019 based on PALSAR-2 data. Using the results of numerical modeling and earthquake distribution beneath the caldera, we interpreted that the continuous asymmetric caldera contraction was driven by brittle failures induced by continuous hydrothermal alterations. Although a previous study proposed deceleration of caldera subsidence in mid-2009, we believe that their findings revealed only a snapshot of the spatial variation of the caldera contraction detected in this study.

Intrusive Seismic Swarms as Possible Precursors of Destructive Earthquakes on Mt. Etna’s Eastern Flank

The Timpe Fault System (TFS) represents the source of shallow earthquakes that strike numerous towns and villages on Mt. Etna’s eastern flank. In the last 40 years, three destructive seismic events reached I 0 = VIII EMS (heavily damaging) in 1984 (October 25), 2002 (October 29), and 2018 (December 26). These events followed a few days after the occurrence of strong seismic swarms and the sudden acceleration of the eastern flank seaward. The damaging seismic events in 2002 and 2018 were associated with dike intrusions and eruptions of the volcano; however, no eruptive activity was observed at the time of the 1984 earthquakes. In this study, we investigate seismic parameters for the 1984 sequence, in order to interpret the seismicity in terms of volcanic activity. Parameters such as localization, cumulative seismic moment, and hourly occurrence frequency of the 1984 seismic swarm have been analysed and shown to have typical values of Mt. Etna’s intrusive seismic swarms. This suggests that the 1984 episode may have been an aborted intrusive magma episode that triggered similar processes (long and powerful intrusions with acceleration of the eastern flank movement and destructive earthquakes), as in 2002 and 2018. These three episodes suggest that an evaluation of some seismic parameters during future intrusive swarms may furnish indications of a possible reactivation of the TFS.

Explosive Activity on Kīlauea's Lower East Rift Zone Fueled by a Volatile‐Rich, Dacitic Melt

AbstractMagmas with matrix glass compositions ranging from basalt to dacite erupted from a series of 24 fissures in the first 2 weeks of the 2018 Lower East Rift Zone (LERZ) eruption of Kīlauea Volcano. Eruption styles ranged from low spattering and fountaining to strombolian activity. Major element trajectories in matrix glasses and melt inclusions hosted by olivine, pyroxene and plagioclase are consistent with variable amounts of fractional crystallization, with incompatible elements (e.g., Cl, F, and H2O) becoming enriched by 4–5 times as melt MgO contents evolve from 6 to 0.5 wt%. The high viscosity and high H2O contents (∼2 wt%) of the dacitic melts erupting at Fissure 17 account for the explosive Strombolian behavior exhibited by this fissure, in contrast to the low fountaining and spattering observed at fissures erupting basaltic to basaltic‐andesite melts. Saturation pressures calculated from melt inclusion CO2‐H2O contents indicate that the magma reservoir(s) supplying these fissures was located at ∼2–3 km depth, which is in agreement with the depth of a dacitic magma body intercepted during drilling in 2005 (∼2.5 km) and a seismically imaged low Vp/Vs anomaly (∼2 km depth). Nb/Y ratios in erupted products are similar to lavas erupted between 1955 and 1960, indicating that melts were stored and underwent variable amounts of crystallization in the LERZ for >60 years before being remobilized by a dike intrusion in 2018. We demonstrate that extensive fractional crystallization generates viscous and volatile‐rich magma with potential for hazardous explosive eruptions, which may be lurking undetected at many ocean island volcanoes.

ГЕОХІМІЧНІ ОСОБЛИВОСТІ РУДНО-ПОРФІРОВОЇ МІНЕРАЛІЗАЦІЇ У МАГМАТИЧНІЙ СИСТЕМІ ГОШГАРЧАЙ (МУРОВДАРСЬКИЙ РАЙОН, АЗЕРБАЙДЖАНСЬКА ЧАСТИНА МАЛОГО КАВКАЗУ)

The geochemical features of intrusive rocks and the surrounding rocks associated with porphyry copper mineralization in the Goshgarchay magmatic system were studied. Major factors of concentration of copper and copper-porphyritic mineralization in rocks of gabbro-diorite-granodiorite formation has been revealed. It has been established that the structure of this ore region was formed as a result of successive alteration of some deformation stages accompanied by development of fractures in intrusive massive, by dike intrusions, locks displacements along faults and the fractures composed of various mineral associations. Main ore components were defined and relations hips among elements were considered. Geochemical associations allowing the localization of the development area of mineralization have been analyzed in various formation stages of porphyry copper system according to the results of cluster analysis. In this study, Cu, Mo, Pb, Zn, Ag, Ni, Co, Mn, Ti, Cr, V, and Sr both in the surrounding rocks and in ore-bearing intrusive rocks were considered as stable elements. This group of elements can be regarded as indicator elements in porphyry copper. The results of the claster analysis show that the elements which are less characteristical for hydrothermal solutions (Mn, Ti, Cr, V, Sr) keep their freedom in all wallrocks and the existence of their grouping is probably associated with their transportation process from silicate rocks with surrounding basic-average content. The elements such as Cu, Mo, Ag, Pb and Zn which form free group are connected with the generation from the magmatic source in the course of crystallization differentiation process and exposure of rocks to the hydrothermal activity which is considered the source of these elements.The identification of reasons of the association of V with Cu and Zn and Pb with Fe group elements in the intrusive facies rocks is characterized with its uncertainty. The analysis of the schedule made according to the results of Cu and basic ore components (Mo, Ag, As, Bi, Gd, Co, Cr, Se, Ge, Li, Nb and etc.) in the ores of Goshgarchay deposit show their dependence on one another equally. The amount of Cu in the ores have Ag, As, Bi, Cd, Ce, Ge, Sr, Mo, Sb, Th and Ti positive correlation relationship, but it has negative correlation relationship with Co, Cr, Cs, La, Li, Pb, Rb, Ni, Nb and Zn. Direct dependence is observed for ore mass between concentrations of Cu, Mo, Pb, Zn, Ag, Se, Te and other elements in the Goshgarchay porphyry copper deposit. Three various mineral associations differing from one another participate in stock work type copper-porphyry mineralization: 1) primary sulfide minerals; 2) oxidized ores; 3) primary sulfide minerals significantly enriched with products of hydrothermal ore carrying solutions.

Post-collisional orogen-parallel extension in the Trans-North China Orogen: Evidence from syn-kinematic pegmatite dikes

Orogen-parallel extension is common in collisional settings, but its cause is debated. The Paleoproterozoic Trans-North China Orogen (TNCO) provides insights into the process of orogen-parallel extension. We present structural and geochronological data from pegmatite dikes in the Xiaoqinling region of central China to constrain the time and nature of Paleoproterozoic extension in the TNCO. The pegmatite dikes strike roughly E–W, have various dip angles, and are either deformed or undeformed. A progression from ductile to brittle deformation is commonly observed in the deformed dikes, suggesting synchronous orogenic uplift probably driven by isostatic adjustments. The deformation structures demonstrate that the intrusion of the pegmatite dikes was associated with a single phase of regional extension, thus making them syn-kinematic. Both ductile fabrics and fault-slip data from the dikes indicate that N–S (orogen-parallel) extension was responsible for dike deformation. Zircon U–Pb ages for the syn-kinematic pegmatite dikes constrain the time of extension to between 1840 and 1781 Ma, suggesting a duration of ∼ 59 Myr for the orogen-parallel extension. These data suggest that the extension postdated the syn-collisional shortening in the TNCO, which occurred between 1863 and 1840 Ma. The orogen-parallel extension, the synchronous isostatic uplift, and the intense magmatism in the TNCO are consistent with a gravitational collapse model in a post-collisional setting.

Characterizing unrest: A retrospective look at 20 years of gas emissions and seismicity at Iliamna Volcano, Alaska

Episodes of unrest are not as well documented as eruptions at most volcanoes globally. Iliamna is an andesitic stratovolcano in the Cook Inlet of Alaska that has experienced several episodes of unrest. Unrest in 1996 was previously studied. Here we present data from a minor period of unrest between 2002 and 2006, and a more significant period in 2012. None of the episodes led to an eruption. A dike intrusion was suggested for the 1996 unrest based on increases in gas emissions and seismic analysis. The 2002–2006 period was characterized by a slight increase in the rate of seismicity to 13 events per day and was particularly notable due to an increase in deep long period (DLP) seismic events between 15 and 37 km that were not observed at other times. This period also included one airborne gas measurement with and elevated CO2/SO2 molar ratio (17). In 2012, Iliamna unrest was characterized by significantly elevated gas emissions (up to 582 t/d SO2 and 1385 t/d CO2) and up to 49 located earthquakes per day (M > 0), and was remarkably similar to the 1996 unrest. Differences in the observed evolution of the CO2/SO2 gas ratio in 2012 (2.2–4) compared to that in 1996 (up to 18) suggests that no new deep magma was involved in 2012, however this does not preclude the movement of a previously intruded magma. A months-long increase in the SO2/H2S molar ratio from 8 to 17 during the peak of the activity could reflect a temperature increase on the order of 10–30 °C of the emitted gas. Compared to pre-eruptive unrest at other Cook Inlet volcanoes, Iliamna unrest in 2012 differed in that gas emissions were < 1500 t/d and seismicity lacked a rapidly escalating sequence of earthquakes and volcanic tremor, which is normally observed in the hours to days before eruption. The observation of DLPs, the fact that Iliamna produces moderately elevated degassing over decadal timeframes, and the persistent dominance of SO2 over H2S, suggests that periodic input of fresh magma from the lower crust sustains the shallower magmatic system over time, which sets it apart from neighboring volcanoes in the Cook Inlet that show minimal activity between eruptions. Various scenarios could explain why Iliamna did not proceed to eruption in 2012. Finally, we present criteria by which monitoring data may suggest an increased likelihood of eruption at Iliamna in the future.

Investigating Strike‐Slip Faulting Parallel to the Icelandic Plate Boundary Using Boundary Element Models

AbstractMost faults in Iceland strike roughly parallel to the divergent plate boundary, a part of the North American‐Eurasian plate boundary, which would be expected to lead to primarily normal faulting. However, several studies have observed a significant component of rift‐parallel strike‐slip faulting in Iceland. To investigate these fault kinematics, we use the boundary element method to model fault slip and crustal stress patterns of the Icelandic tectonic system, including a spherical hotspot and uniaxial stress that represents rifting. On a network of faults, we estimate the slip required to relieve traction imposed by hotspot inflation and remote rifting stress and compare the model results with observed slip kinematics, crustal seismicity, and geodetic data. We note a good fit between model‐predicted and observed deformation metrics, with both indicating significant components of normal and strike‐slip faulting and consistency between recent data and longer‐term records of geologic fault slip. Possible stress permutations between steeply plunging σ1 and σ2 axes are common in our models, suggesting that localized stress perturbations may impact strike‐slip faulting. Some increases in model complexity, including older hotspot configurations and allowing fault opening to simulate dike intrusion, show improvement to model fit in select regions. This work provides new insight into the physical mechanisms driving faulting styles within Iceland away from the current active plate boundary, implying that a significant portion of observed strike‐slip faulting is likely caused by the combined effects of tectonic rifting, hotspot impacts, and mechanical interactions across the fault network.

Machine Learning Approaches to Identifying Changes in Eruptive State Using Multi‐Parameter Datasets From the 2006 Eruption of Augustine Volcano, Alaska

AbstractUnderstanding the timing of critical changes in volcanic systems, such as the beginning and end of eruptive behavior, is a key goal of volcanic monitoring. Traditional approaches to forecasting these changes have used models motivated by the underlying physics of eruption onset, which assume that geophysical precursors will consistently display similar patterns prior to transition in volcanic state. We present a machine learning classification approach for detecting significant changes in patterns of volcanic activity, potentially signaling transitions during the onset or end of volcanic activity, which does not require a model of the physical processes underlying critical changes. We apply novelty detection, where models are trained only on data prior to eruption, to the precursory unrest at Augustine Volcano, Alaska in 2005. This approach looks promising for geophysically monitored volcanic systems which have been in repose for some time, as no eruptive data is required for model training. We compare novelty detection results with multi‐class classification, where models are trained on examples of both non‐eruptive and eruptive data. We contextualize the results of these classification models using constraints from petrological, satellite and visual observations from the 2006 eruption of Augustine Volcano. The transition from non‐eruptive to eruptive behavior we identify in mid‐November 2005 is in agreement with previous estimates of the initiation of dike intrusion prior to the 2006 eruption. We find that models which include multiple types of data (seismic, deformation, and gas emissions) can better distinguish between non‐eruptive and eruptive data than models formulated on single data types.

New insights on faulting and intrusion processes during the June 2007, East Rift Zone eruption of Kīlauea volcano, Hawai'i

The East Rift Zone (ERZ) of Kīlauea Volcano, Hawai'i, represents one of the most volcanically active regions in the world. The 2007 Father's Day (FD) dike intrusion, eruption, and accompanying slow-slip event (SSE) has been previously modeled using geodetic data to constrain the geometry of the intrusion and the timing and magnitude of the SSE. Here, we perform inversions of three interferometric synthetic aperture radar (InSAR) datasets and a new intensity offset tracking dataset to assess the effect of integrating intensity cross-correlation offsets into inversion problems and explore additional potential models for the intrusion geometry of the FD event based on this additional data. The overall lowest misfit single Okada model for all datasets opens 2.3 m, strikes 73 degrees while dipping sub-vertically at 83 degrees, and extends approximately 2.9 km to the ENE and 2.4 km downdip. The differences are minor between complex en-echelon distributed Okada and decollement model of (Montgomery-Brown et al., 2010) or 3D-MBEM breaching models including multiple surface breaches and free-slipping decollement movement. Finally, we examine the static Coulomb stress changes for the proposed decollement fault created by our preferred model and a representative model of deep rift opening and find that deep rift zones dilation, not shallow ERZ intrusions, are likely modulating slip on the decollement.

Mapping Hydrothermal Alteration at the Fentale-Dofan Magmatic Segment of the Main Ethiopian Rift

The Main Ethiopian Rift (MER) is characterized by extensional tectonics and volcanism, associated with active hydrothermal systems, hydrothermal alteration and fumarolic deposits. The spatial distribution of these hydrothermal products and their link with faults and rock types provides important clues to what controls fluid flow in the subsurface. However, little is known about this in the East African Rift. We address this issue with a multidisciplinary approach in the Fentale-Dofan magmatic segment of the MER, an area characterized by intense volcanic and tectonic activity and a geothermal prospect. Primarily we conduct mapping of hydrothermal alteration and fumarolic deposits, and rock lithologies using a surface feature classification technique of multispectral satellite images. Then we interpret the map using a new database of faults and active hydrothermal manifestations such as hot-springs and fumaroles. We find that the surface hydrothermal alteration and deposits are mainly focused near Fentale and the Dofan Volcanic Complex (DVC). At DVC the hydrothermal products are focused on rhyolites on the western side of the volcano, in an area of intense NNE striking, rift parallel faults. At Fentale volcano the hydrothermal products are mainly associated with ignimbrite and show a circular pattern around the volcanic edifice, but also in places follow the NNE striking faults. At Fentale, the more complex association of hydrothermal products and active manifestations around the edge of the ignimbrite suggests formation contacts may also localize fluid flow in places. At both volcanoes the association between hydrothermal products with either the rhyolites and ignimbrites is likely due to them being relatively easily altered (in comparison to basalt), and also their brittle nature allows for fracturing through which localized fluid flow can occur (as opposed to the sediments). The general pattern of hydrothermal products suggests a stronger structural influence at the DVC with respect to Fentale. The presence of hydrothermal products and active hydrothermal manifestations, along with other lines of evidence such as locus of subsurface dike intrusion at the volcanic centres, suggest that discrete and localized magma reservoirs beneath Fentale and the DVC are the heat source for hydrothermal circulation. Our study also demonstrates that geology, including hydrothermal deposits, can be successfully mapped using automated remote sensing based classification.

Metamorphic stages in mountain belts during a Wilson cycle: A case study in the central Sanandaj–Sirjan zone (Zagros Mountains, Iran)

Polymetamorphic units are important constituents of continent–continent collisional orogens, and rift metamorphic assemblages are often overprinted by subsequent metamorphism during subduction and collision. This study reports the metamorphic conditions and evolution of the Dorud–Azna metamorphic units in the central part of the Sanandaj–Sirjan zone (SSZ), Iran. Here, new geothermobarometry results are integrated with 40Ar/39Ar mineral and Th–U–Pb monazite and thorite ages to provide new insight of polyphase metamorphism in the two different basement units of the SSZ, the lower Galeh-Doz orthogneiss and higher Amphibolite-Metagabbro units. In the Amphibolite-Metagabbro unit, staurolite micaschist underwent a prograde P–T evolution from 640 ± 20 °C/6.2 ± 0.8 kbar in garnet cores (M1) to 680 ± 20 °C/7.2 ± 1.0 kbar in garnet rims (M2). Three Th–U–Pb monazite ages of 306 ± 5 Ma, 322 ± 28 Ma and 336 ± 39 Ma from the garnet-micaschists testify the Carboniferous age of M1 metamorphism. In the same unit, the metagabbro records P–T conditions of 4.0 ± 0.8 kbar and 580 ± 50 °C in the (magmatic) amphibole core (Late Carboniferous intrusion) to 7.5 ± 0.7 kbar and 700 ± 20 °C in the amphibole rim indicating a prograde P–T path during subsequent burial (M1). New 40Ar/39Ar dating of white mica from the staurolite micaschist yielded a staircase pattern ranging from 36 ± 12 Ma to 170 ± 2 Ma. This implies polymetamorphism with a minimum Late Jurassic cooling age through the Ar retention temperature of ca. 425 ± 25 °C after M2 metamorphism and a Paleogene low-grade metamorphic overprint (M3), while 40Ar/39Ar white mica dating of garnet micaschist yielded a plateau age of 137.84 ± 0.65 Ma. We therefore interpret the amphibolite-grade metamorphism M2 to have predated 170 Ma and is likely between 180 and 200 Ma. Furthermore, it is overprinted at about 36 Ma under retrogressive low-grade M3 metamorphism (at temperatures of ~350–240 °C) during final shortening and exhumation. In the underlying Galeh-Doz unit, the Panafrican granitic orthogneiss intruded at P–T conditions of 3.2 ± 4 kbar and 700 ± 20 °C, then it was metamorphosed and deformed at 600 ± 50 °C and 2.0 ± 0.8 kbar (metamorphic stage M1) prior to Late Carboniferous intrusion of mafic dikes. 40Ar/39Ar dating of amphibole from the Galeh-Doz orthogneiss gave plateau-like steps between 260 and 270 Ma, representing the age of cooling through ca. 500 °C after the M1 metamorphic event. Interestingly, the results of this study demonstrate polyphase metamorphic histories in both the Galeh-Doz orthogneiss and Amphibolite-Metagabbro units at different P–T conditions and final thick-skinned Paleogene emplacement of these units over the underlying low-grade metamorphic June Complex. Our findings suggest that both units are affected by high-T/low-P Late Carboniferous orogenic metamorphism along with the bimodal magmatism, as result of rifting. We propose that the Early Jurassic amphibolite-grade M2 metamorphism of the SSZ is correlated with the initial subduction of the Neotethyan Ocean. Eventually, the investigated units reflect various stages of a Wilson cycle, from rifting to initiation of the subduction in final plate collision.

What Triggers Caldera Ring‐Fault Subsidence at Ambrym Volcano? Insights From the 2015 Dike Intrusion and Eruption

AbstractSurface deformation accompanying dike intrusions is dominated by uplift and horizontal motion directly related to the intrusions. In some cases, it includes subsidence due to associated magma reservoir deflation. When reservoir deflation is large enough, it can form, or reactivate preexisting, caldera ring‐faults. Ring‐fault reactivation, however, is rarely observed during moderate‐sized eruptions. On February 21, 2015 at Ambrym volcano in Vanuatu, a basaltic dike intrusion produced more than 1 m of coeruptive uplift, as measured by InSAR, synthetic aperture radar correlation, and Multiple Aperture Interferometry. Here, we show that an average of ∼40 cm of slip occurred on a normal caldera ring‐fault during this moderate‐sized (VEI &lt; 3) event, which intruded a volume of ∼24 × 106 m3 and erupted ∼9.3 × 106 m3 of lava (DRE). Using the 3D Mixed Boundary Element Method, we explore the stress change imposed by the opening dike and the depressurizing reservoir on a passive, frictionless fault. Normal fault slip is promoted when stress is transferred from a depressurizing reservoir beneath one of Ambrym's main craters. After estimating magma compressibility, we provide an upper bound on the critical fraction (f = 7%) of magma extracted from the reservoir to trigger fault slip. We infer that broad basaltic calderas may form in part by hundreds of subsidence episodes no greater than a few meters, as a result of magma extraction from the reservoir during moderate‐sized dike intrusions.

Hydroacoustic Observations of Two Contrasted Seismic Swarms along the Southwest Indian Ridge in 2018

In 2018, two earthquake swarms occurred along spreading ridge segments of the ultra-slow Southwest Indian Ridge (SWIR). The first swarm was located at the spreading-ridge intersection with the Novara Fracture Zone, comprising 231 events (ISC catalogue) and spanning over 6 days (10 July to 15 July). The second swarm was more of a cluster of events focusing near a discontinuity, 220 km west of the Rodrigues Triple Junction, composed of 92 events and spanning over 31 days (27 September to 27 October). We examined these two swarms using hydroacoustic records from the OHASISBIO network with seven to nine autonomous hydrophones moored on either side of the SWIR. We detected 1109 hydroacoustic events spanning over 13 days (6 July to 18 July) in the first swarm and 4880 events spanning over 33 days in the second swarm (25 September to 27 October). The number of events per day was larger, and the hydroacoustic magnitude (source level) was, on average, smaller during the second swarm than the first. The spatio-temporal distribution of events from both swarms indicates a magmatic origin initiated by dike intrusions and followed by a readjustment of stresses in the surrounding crust.

Tectonic position, structure, and Holocene activity of the Hofsjökull volcanic system, central Iceland

The Hofsjökull volcanic system is located at the northern border of the Hreppar microplate on the Mid-Atlantic plate boundary in Iceland, between the more active Western and Eastern volcanic zones. In this study, fractures, and faults within the fissure swarms of the volcanic system were mapped and the throw and orientation of faults measured. This was done using both aerial photographs and ArcticDEM digital elevation models, as well as during fieldwork. The Hofsjökull volcanic system contains 3–4 fissure swarms extending northwards or southwards from the glacially covered Hofsjökull central volcano. Although these fissure swarms have been active during the Holocene, no clear sink holes were found along the faults, suggesting that they have been filled by sediments. This indicates that the fissure swarms have been less active than fissure swarms in other branches of the plate boundary in Iceland where GPS geodetic measurements show current spreading. Unbroken hyaloclastite covering a fault in the northern Hofsjökull fissure swarm suggests that this part of the northern Hofsjökull fissure swarm has not been active since the earliest part of Holocene, or during the latest stage of glaciation in the area. Still, the fault scarps in the northern Hofsjökull fissure swarm are rather sharp, indicating little erosion by glaciers. This may suggest increased activity in the Hofsjökull fissure swarm during the end of the last glaciation or at the beginning of the Holocene, which is in line with other studies showing increased magmatic activity in Iceland during that period. Fractured Holocene lava flows in the southern and western Hofsjökull fissure swarms indicate that they have been active during the Holocene. The Kerlingarfjöll rhyolitic massif is located south of the Hofsjökull central volcano. The southern fissure swarms are located both east and west of Kerlingarfjöll, but no clear indications are found of the fissure swarm in the rhyolitic massif itself. This may occur as dike intrusions (which likely form the faults) are prevented from penetrating the rhyolite due to density differences, and/or due to the topographic high of Kerlingarfjöll.

Stacked Magma Lenses Beneath Mid‐Ocean Ridges: Insights From New Seismic Observations and Synthesis With Prior Geophysical and Geologic Findings

AbstractRecent multi‐channel seismic studies of fast spreading and hot‐spot influenced mid‐ocean ridges reveal magma bodies located beneath the mid‐crustal Axial Magma Lens (AML), embedded within the underlying crustal mush zone. We here present new seismic images from the Juan de Fuca Ridge that show reflections interpreted to be from vertically stacked magma lenses in a number of locations beneath this intermediate‐spreading ridge. The brightest reflections are beneath Northern Symmetric segment, from ∼46°42′‐52′N and Split Seamount, where a small magma body at local Moho depths is also detected, inferred to be a source reservoir for the stacked magma lenses in the crust above. The imaged magma bodies are sub‐horizontal, extend continuously for along‐axis lengths of ∼1–8 km, with the shallowest located at depths of ∼100–1,200 m below the AML, and are similar to sub‐AML bodies found at the East Pacific Rise. At both ridges, stacked sill‐like lenses are detected beneath only a small fraction of the ridge length examined and are inferred to mark local sites of higher melt flux and active replenishment from depth. The imaged magma lenses are focused in the upper part of the lower crust, which coincides with the most melt rich part of the crystal mush zone detected in other geophysical studies and where sub‐vertical fabrics are observed in geologic exposures of oceanic crust. We infer that the multi‐level magma accumulations are ephemeral and may result from porous flow and mush compaction, and that they can be tapped and drained during dike intrusion and eruption events.

Effects of igneous intrusions on diagenesis and reservoir quality of sandstone in the Songliao Basin, China

An exceptional example for evaluating effects of igneous intrusions on diagenesis and reservoir quality of sandstone has been found in the Qianjiadian area, Songliao Basin. Sandstone of the Upper Crataceous Yaojia Formation near diabase dikes is characterized by kaolinite, illite-smectit mixed-layer minerals, ankerite, scheelite, sphalerite, galena, chalcopyrite, brannerite, and dissolution of quartz, alkali feldspar, plagioclase and siderite. Diagenesis related to the intrusion of diabase dikes has gone through two stages. The early stage with a temperature and salinity of 175–225 °C and 12.0–18.0 wt% NaCl equiv respectively is featured by kaolinite and dissolution of quartz, alkali feldspar, plagioclase and siderite, whereas the late stage with a temperature and salinity of 125–150 °C and 8.0–10.0 wt% NaCl equiv respectively is distinguished by ankerite, scheelite, sphalerite, galena, chalcopyrite, brannerite and dissolution of quartz. Values of the plane porosity, pore diameter, porosity and permeability covary with the distance between sandstone and dike contacts: the values rapidly decrease when the distance decreases while the values gradually increase when the distance increases. Correspondingly, the extent to which the reservoir quality was reduced is far smaller than that to which the reservoir quality was improved. Two-stage hydrothermal fluids have been aroused by the diabase dikes: acidic in early stage and alkalic in late stage, and affecting range of the former is wider than that of the later. These hydrothermal fluids lead to development of secondary pores in the sandstone.

The Impact of Complex Volcanic Plumbing on the Nature of Seismicity in the Developing Magmatic Natron Rift, Tanzania

Constraining the architecture of complex 3D volcanic plumbing systems within active rifts, and their impact on rift processes, is critical for examining the interplay between faulting, magmatism and magmatic fluids in developing rift segments. The Natron basin of the East African Rift System provides an ideal location to study these processes, owing to its recent magmatic-tectonic activity and ongoing active carbonatite volcanism at Oldoinyo Lengai. Here, we report seismicity and fault plane solutions from a 10 month-long temporary seismic network spanning Oldoinyo Lengai, Naibor Soito volcanic field and Gelai volcano. We locate 6,827 earthquakes with ML−0.85 to 3.6, which are related to previous and ongoing magmatic and volcanic activity in the region, as well as regional tectonic extension. We observe seismicity down to ∼17 km depth north and south of Oldoinyo Lengai and shallow seismicity (3–10 km) beneath Gelai, including two swarms. The deepest seismicity (∼down to 20 km) occurs above a previously imaged magma body below Naibor Soito. These seismicity patterns reveal a detailed image of a complex volcanic plumbing system, supporting potential lateral and vertical connections between shallow- and deep-seated magmas, where fluid and melt transport to the surface is facilitated by intrusion of dikes and sills. Focal mechanisms vary spatially. T-axis trends reveal dominantly WNW-ESE extension near Gelai, while strike-slip mechanisms and a radial trend in P-axes are observed in the vicinity of Oldoinyo Lengai. These data support local variations in the state of stress, resulting from a combination of volcanic edifice loading and magma-driven stress changes imposed on a regional extensional stress field. Our results indicate that the southern Natron basin is a segmented rift system, in which fluids preferentially percolate vertically and laterally in a region where strain transfers from a border fault to a developing magmatic rift segment.

Dike Inflation Process Beneath Sakurajima Volcano, Japan, During the Earthquake Swarm of August 15, 2015

Magma intrusion usually causes seismicity and deformation in the surrounding rock and often leads to eruptions. A swarm of volcano-tectonic (VT) earthquakes associated with rapid dike intrusion in hours occurred beneath Sakurajima volcano on August 15, 2015. We determined the hypocenters and focal mechanisms of the VT earthquake swarm. The distributions of pressure (P)- and tension (T)-axes of the azimuths of the mechanisms are also obtained. The results indicate spatiotemporal changes of the distributions of the hypocenters and P- and T-axes. The hypocenters are distributed at depths of 0.3–1 km and 7:00–10:30 JST, and are located at depths of 0.3–3 km and 10:30–12:00 during which the seismic activity is the largest. At 12:00–24:00, the hypocenters are distributed in shallow and deep clusters at depths of 0.2–1 km and 1.5–3.5 km, respectively. The dike induced rapid ground deformation and is located between the shallow and deep clusters. The strike and opening directions of the dike are parallel to the NE–SW and NW–SE directions, respectively, corresponding to the regional maximum and minimum compression stress. The T-axes of the shallow cluster are distributed parallel to the opening direction of the dike. The P-axes of the deep cluster exhibit a pattern corresponding to the NE–SW direction and the T-axes are distributed in the NW–SE direction. In contrast, a 90° rotated pattern of strike-slip faulting is also observed at the deep cluster at 12:00–24:00, where the P-axes are distributed in the NW–SE direction and the T-axes are distributed in the NE–SW direction. This reflects the change in the stress field due to the dike inflation during the earthquake generation, and indicates that the alteration of stress in the vicinity of the dike due to the dike inflation and VT earthquakes are induced by the differential stress exceeding the brittle fracture strength of the rock. Future seismic and deformation observations in volcanoes will verify whether the spatiotemporal changes of the hypocenters and focal mechanism shown by this study are unique features of rapid dike intrusion.