Dike Injection Research Articles

Generation of deposit-derived pyroclastic density currents by repeated crater rim failures at Stromboli Volcano (Italy)

The gravitational instability of hot material deposited during eruptive activity can lead to the formation of glowing avalanches, commonly known as deposit-derived pyroclastic density currents (PDCs). These currents can travel hundreds of metres to several kilometres from the source at exceptionally high temperatures, posing a catastrophic hazard to areas surrounding steep-slope volcanoes. The occurrence of deposit-derived PDCs is often associated with crater rim failure, which can be triggered by various factors such as magma thrust from dike injection, magma fingering, bulging or less commonly, powerful explosions. Here, the in-depth study of data from the multi-parametric monitoring network operating on Stromboli (Italy), including video surveillance, seismicity and ground deformation data, complemented by remote topographic sensing data, has facilitated the understanding of the events leading to the crater rim collapse on 9 October and 4 December 2022. The failures resulted in the remobilisation of 6.4 ± 1.0 × 103 m3 and 88.9 ± 26.7 × 103 m3 of material for the 9 October and the 4 December 2022, respectively, which propagated as PDCs along the NW side of the volcano and reached the sea in a few tens of seconds. These events were characterised by a preparatory phase marked by an increase in magmatic pressure in the preceding weeks, which correlated with an increase in the displacement rate of the volcano’s summit. There was also an escalation in explosive degassing, evidenced by spattering accompanied by seismic tremors in the hours before the collapse.These events have been interpreted as an initial increase in magma vesicularity, followed by the release of gas once percolation threshold was reached. The degassing process induced densification of the magma, resulting in increased thrust on the conduit walls due to increased magmastatic pressure. This phase coincided with crater rim collapse, often followed or accompanied by the onset of lava overflow phases. A mechanism similar to the one proposed may shed light on similar phenomena observed at other volcanoes. The analysis performed in this study highlights the need for a multi-parametric and multi-platform approach to fully understand such complex phenomena. By integrating different data sources, including seismic, deformation and remote sensing data, it is possible to identify the phenomena associated with the different phases leading to crater rim collapse and the subsequent development of deposit-derived PDCs.

Seismic Constraints on Hydrothermal Circulation and Magmato‐Tectonic Interactions Beneath Lucky Strike Volcano, Mid‐Atlantic Ridge

AbstractThe Lucky Strike volcano is the central edifice of the Lucky Strike segment on the slow spreading Mid‐Atlantic Ridge. The volcano summit hosts one of the largest known deep‐sea hydrothermal fields, and overlies an axial magma chamber (AMC) whose summit reflector lies 3–3.8 km beneath the seafloor. We present a 12‐year microearthquake catalog beneath the volcano, which we constructed using data collected from 2007 to 2019 as part of the EMSO‐Azores observatory. The catalog reveals continuous low magnitude seismicity (85% with ML < 0), focused mainly north‐northwest of the hydrothermal field and 0–2 km above the AMC reflector. Focal mechanisms estimated only from the 2010–2011 deployment show a mixture of thrust, normal and strike/slip faulting mechanisms. We propose that the observed microearthquake activity is due to thermal cracking, tectonic cracking, and possible small dike injections occurring at the base of hydrothermal downflow zones. We document a ∼800 m eastward shift of the seismicity sometime during a catalog gap from June 2013 to April 2015, accompanied by a change in the seismicity pattern from a patch just above the AMC to a more vertically aligned structure. We also identify several higher seismicity periods, often including relatively large magnitude events (ML > 0.8). We interpret the spatio‐temporal variations in seismicity patterns and the correlation between larger magnitude events and higher seismicity periods to indicate that the hydrothermal domain is a dynamic interface, where the variations are caused by changes in permeability and/or temperature induced by tectonic and small episodic melt injections.

Origin, dynamics, and chemical evolution of garnet-bearing leucogranitic magma, Eastern Desert of Egypt: Controls on the rare-metal enrichment in the A-type magmatism

Egypt hosts numerous rare-metal granites, i.e., highly evolved granites enriched in rare metals (Ta, Nb, Be, Sn, Zr, Th, and REE). However, the processes involved in the rare-metal enrichment are not fully understood. We present new data on the textural characteristics and chemical composition of rare-metal mineralization associated with microgranite dikes in the Ras Abdah area of the Egyptian Eastern Desert. These dikes are garnet-bearing leucogranites (GLG) composed of perthitic alkali-feldspars and quartz. When compared to other Egyptian A-type granites, microgranite dikes are alkaline rocks with particularly higher HREE contents. Zircon, huttonite, fergusonite (Y), and Fe-Ti-Zn oxides (magnetite, Zn-bearing ilmenite and pyrophanite) are largely associated with the altered domains, which are also enriched in Nb, Zr, Y, Ta, Th, and REE. However, similarities between the chondrite-normalized REE patterns of the altered and unaltered domains of the GLG dikes favor the hypothesis of a unique magmatic signature. Moreover, the chemical and textural features of rare-metal minerals indicate that the alteration of primary minerals was caused by deuteric fluids or aqueous residual melt exsolved from the parental granitic magma (autometasomatism). Garnet compositions are rich in the spessartine component (up to 84 %), which is typical of garnet in highly evolved pegmatitic rocks. Furthermore, garnet exhibits no major element zoning but shows chemical fluctuations in trace element concentrations, suggesting correspondingly abrupt changes in melt composition due to sequential magma pulsing. This magma emplacement may cause crystal nucleation and oscillatory crystallization followed by magmatic segregation. Overall, parental magma type, dike injection, and magmatic-hydrothermal processes all play a role in the unusual enrichments of rare metals.

The propagation and emplacement mechanisms of the Tenteniguada volcanic debris avalanche (Gran Canaria): Field evidence for brittle fault-accommodated spreading

The Tenteniguada volcanic debris avalanche deposit is located on the east of the island of Gran Canaria, Spain. Its internal structure is composed of a complex assemblage of extensional features and shearing structures including normal faults, horst and graben, brittle/ductile boudinage and clastic dike injections. Examination of these features in the field and evaluation of their distribution have allowed the generation of a new conceptual model for the transport and emplacement of this debris avalanche, and potentially others. In the majority of the deposit, the degree of disaggregation is low, with large portions of the original edifice preserved, although displaced by brittle deformation. Greater disaggregation is observed deeper and in the more distal section of the deposit. The findings suggest that the propagation of the volcanic debris avalanche was most likely facilitated by the normal fault-accommodated spreading and extension of the mass, with the majority of stress focused in fault zones. The greater disaggregation exhibited in the deeper and the more distal part of the deposit is likely to be due to greater stress accommodation from fault convergence and momentum transfer respectively. The abundance of competent lava lithologies and scarcity of weaker material that could be easily disaggregated is the most likely reason Tenteniguada did not fully evolve from a slide to a granular flow, and therefore generated a deposit which bears resemblance to non-volcanic blockslide deposits. Therefore, lithological properties are potentially a vital factor for the propagation mechanisms, distribution of stress and consequently the evolution of a debris avalanche from the initial collapse to its emplacement. The present study highlights the importance of dedicated field examinations of sedimentological, morphological, and structural features for providing constraints for models of debris avalanche propagation mechanisms and the factors dictating them.

Subsurface structure of magmatic segments during continental breakup: Perspectives from a gravity data analysis along the Main Ethiopian Rift

In magma-rich continental rifts, extension is commonly localized as dike injection in discrete magmatic segments which appear from surface geology to mimic the along-axis segmentation of ocean ridges. However, the subsurface morphology of these zones of localized magma injection is unclear, and whether and at what depth range they remain with discrete subvolcanic plumbing systems is not fully understood. In addition, the relationship between zones of magma injection and tectonic faulting is also debated. The Main Ethiopian Rift (MER) provides a unique opportunity to examine the spatial scale, locus, and subsurface geometries of magmatic localization in active continental rifts. We examined spatial variations in the structure of the crust in the MER using gravity data to better understand the subsurface pattern of magma injection. Results of our study reveal discrete Bouguer gravity highs in the centers of the distinct right-stepping magmatic segments of the MER (Aluto, Gedemsa, and Boseti), and interpreted as the locus of mafic intrusions. The gravity data show that the boundary between each magmatic segment is observed down to depths of ∼20 km, suggesting that magmatic segments have discrete subvolcanic plumbing systems at least down to the mid-crust. Additionally, incorporating the fault network shows that the densest faulting occurs at the tips of the zones of densest magma injection shown by Bouguer gravity highs. This is similar to the axial segmentation observed at slow and ultra-slow spreading ridges and suggests strong along-axis variations in deformation mechanism, with magma injection accommodating extension at the segment centers and faulting dominating towards the segment ends. In addition, rift margin volcanic zones (Siltie Debrezeit, Yerer Tulu, and Galema) are also inferred from bandpass gravity anomaly maps, supported by petrologic evidence, which implies that extension by dike injection has also occurred near the rift flanks.

Evidence of Seismic-Related Liquefaction Processes within the Volcanic Record of the Campi Flegrei Caldera (Italy)

We report the occurrence of several sand liquefaction structures, such as sand dikes, in the stratigraphic record of the Campi Flegrei volcano, located both inside and outside the caldera. Five sites were analyzed within the caldera and two outside. The grain size analysis of the sand fillings indicates that these deposits are very fine-to-coarse sands generally poorly sorted. All of the granulometry curves fall within the field of the liquefiable, loose sediments. Frequently, dikes are characterized by two fillings: a rim showing poorly sorted finer sands and a core with extremely poorly sorted coarser sediments. We suggest that seismic-related liquefaction processes triggered the injection of these sand dikes during unrest episodes in the last 15 kyr. In particular, the sand dikes located outside the caldera, characterized by larger thicknesses and lengths, mark an important extensional episode, probably associated with the caldera formation during the Neapolitan Yellow Tuff eruption at 15 ka. Furthermore, liquefaction structures within the caldera are related to the seismic activity, probably occurring during the Agnano–Monte Spina caldera formation and the volcano-tectonic ground deformation, predating Epoch 3b (4.3 ka) and the Monte Nuovo (1538 CE) eruptions. This study highlights that these seismic-related liquefaction structures are common within the volcanic record of the Campi Flegrei, suggesting that the sand source can be both the widespread marine succession underlying the Epoch 3 deposits in the caldera central sector and the primary ash layers extensively present in the volcanic record.

Magma Chamber Formation by Dike Accretion and Crustal Melting: 2D Thermo‐Compositional Model With Emphasis on Eruptions and Implication for Zircon Records

AbstractWe present a 2D thermo‐elastic model of a magma body formation in a granitic crust by injection of rhyolitic or basaltic dikes and sills. Elastic analytical solutions enable the computation of rock displacement in response to magma intrusion and evacuation during volcanic eruptions. Phase diagrams for magma and rocks govern melting/crystallization behavior and temperature evolution in 16 million computational cells. Calculated temperature histories are used to predict zircon crystallization/dissolution, their ages, and isotopic ratios within individual batches of magma and rocks. Incremental dike injection naturally generates magma batches of melt that form kilometer‐wide clusters appearing in different parts of the domain with diverse shapes, horizontal and vertical interconnectivity. The volume of eruptive melt strongly depends on magma influx rates Q, the width of the injection region W, and eruptions. For example, rhyolitic dike injection with Q = 0.125 m3/s with W = 5 km during 100 ka generates ∼50 km3 of eruptive melt while no significant melt forms if W = 10 km. Injection of basaltic dikes into the granitic crust generates comparable amounts of rhyolitic melt from dike‐residual and country rocks. High injection intensities produce magma reservoirs capable of large eruptions, while repetitive eruptions lead to shrinkage of magma bodies. High heat input causes host rock zircons to lose a significant portion of their old cores. Magmatic zircons in the periphery form quickly due to rapid cooling of intruded dikes while in the central part crystals can grow during several hundreds of ka. The model predicts highly heterogeneous O and Hf isotopic ratios recorded in zircons.

Mapping and evaluating kinematics and the stress and strain field at active faults and fissures: a comparison between field and drone data at the NE rift, Mt Etna (Italy)

Abstract. We collected drone data to quantify the kinematics at extensional fractures and normal faults, integrated this information with seismological data to reconstruct the stress field, and critically compared the results with previous fieldwork to assess the best practice. As a key site, we analyzed a sector of the northeast rift of Mt Etna, an area affected by continuous ground deformation linked to gravity sliding of the volcano's eastern flank and dike injections. The studied sector is characterized also by the existence of eruptive craters and fissures and lava flows. This work shows that this rift segment is affected by a series of NNE- to NE-striking, parallel extensional fractures characterized by an opening mode along an average N105.7∘ vector. The stress field is characterized by a σHmin trending northwest–southeast. Normal faults strike parallel to the extensional fractures. The extensional strain obtained by cumulating the net offset at extensional fractures with the fault heave gives a stretching ratio of 1.003 in the northeastern part of the study area and 1.005 in the southwestern part. Given a maximum age of 1614 CE for the offset lavas, we obtained an extension rate of 1.9 cm yr−1 for the last 406 years. This value is consistent with the slip along the Pernicana Fault system, confirming that the NE rift structures accommodate the sliding of the eastern flank of the volcano.

Mechanical Imaging of a Volcano Plumbing System From GNSS Unsupervised Modeling

AbstractIdentification of internal structures in an active volcano is mandatory to quantify the physical processes preceding eruptions. We propose a fully unsupervised Bayesian inversion method that uses the point compound dislocation model as a complex source of deformation, to dynamically identify the substructures activated during magma migration. We applied this method at Piton de la Fournaise. Using 7‐day moving trends of Global Navigation Satellite System (GNSS) data preceding the June 2014 eruption, we compute a total of 15 inversion models of 2.5 million forward problems each, without a priori information. Obtained source shapes (dikes, prolate ellipsoids, or pipes) show magma migration from 7–8 km depth to the surface, drawing a mechanical “tomography” of the magma pathway. Our results also suggest source geometries compatible with observed eruptive fissures and seismicity distribution. In case of finite magma volume involved in final dike injection, source volume estimates from this method allow forecasting volumes of erupted lava.

A Joint Inversion for Three‐dimensional P and S Wave Velocity Structure and Earthquake Locations Beneath Axial Seamount

AbstractAxial Seamount is a prominent volcano located at the intersection of the Juan de Fuca Ridge and the Cobb‐Eickelberg hot spot in the northeast Pacific Ocean that has erupted in 1998, 2011, and 2015. The 2015 eruption was recorded by a seven‐station seismic network in the southern part of the summit caldera that forms part of the Ocean Observatories Initiative Cabled Array. We utilize a data set of ~3,900 well‐recorded earthquakes from January 2015 to February 2017 and a three‐dimensional P wave velocity model obtained previously from active source data to conduct a joint inversion for three‐dimensional P and S wave velocities and hypocentral parameters. The resulting velocity models are used to relocate >76,000 earthquakes with ≥10 arrival times. The velocity models reveal a low‐velocity anomaly in the center of the southern caldera at depths less than ~2 km, which corresponds to the top of the magma chamber and is interpreted as a region that is intensely fractured by the cyclical deformation of the caldera. High velocities around the caldera rim are likely due to consolidated undeformed lava flows. Low VP/VS in the southern caldera is consistent with the presence of hydrothermal vapor. Low S wave velocities and high VP/VS in the northern caldera may indicate a region dominated by thin cracks caused by dike injection. The relocated earthquakes delineate outward‐dipping ring faults more clearly than previous studies and image a subvertical inward‐dipping fault within the network that connects to the east caldera wall and eruptive fissures.

Dike-induced stresses and displacements in layered volcanic zones

During a volcanic unrest period with dike injection, one of the main scientific tasks is to assess the geometry and the propagation path of the dike and, in particular, the likelihood of the dike reaching the surface to erupt. Currently, the dike path and geometry (including depth and opening/aperture) are both partly determined from geodetic surface data using mostly dislocation models that assume the volcanic zone/volcano to be an elastic half space of uniform mechanical properties. By contrast, field observations of volcanic zones/volcanoes (active and extinct) show that they are composed of numerous layers whose mechanical properties (primarily Young's modulus) vary widely and whose contacts commonly arrest dikes. Here we provide field observations and numerical models on the effects of a typical variation in Young's modulus in an active volcanic zone on the internal and surface stresses and displacements induced by a dike whose tip is arrested at 0.5 km depth below the surface of the volcanic zone. Above the layer or unit hosting the dike are four layers of equal thickness. We vary the Young's modulus or stiffness of the fourth layer (the one adjacent to the layer or unit hosting the dike) from 10 GPa to 0.01 GPa, while all the other layers/units maintain their Young's moduli in the model runs. The results show that as the fourth layer becomes more compliant or soft (0.1–0.01 GPa) dike-induced stresses and displacements (lateral and vertical) above the layer, including those at the surface, become suppressed but the stresses and displacements of the layer/unit hosting the dike increase and their peaks do not coincide in location or magnitude with those of the other layers. Thus, the dike-induced internal deformation of the volcanic zone increases as the fourth layer becomes softer. Also, the tensile-and-shear stress peaks at the surface occur at locations widely different from those of maximum surface uplift. More specifically, for a comparatively stiff fourth layer (1–10 GPa), the surface tensile and shear stresses peak at lateral distances of 0.5–0.7 km from the projection of the dike to the surface. (Essentially no tensile/shear stresses reach the surface when the fourth layer is as soft as 0.1–0.01 GPa, so that there are no stress peaks). By contrast the maximum surface displacements (uplift) peak at lateral distances of 2.8–3.3 km from the dike projection to the surface. If tension fractures or faults – in particular the boundary faults of a graben – are induced by the dike, they should form at the tensile/shear stress peaks and not, as is commonly suggested, at the location of the surface displacement peaks. Our results thus suggest that any dike-induced graben is likely to be of a width about twice the depth to the tip/top of the arrested dike. The results demonstrate that elastic half-space models overestimate the dike-induced surface stresses, and thus the depth to the tip/top of the associated dike. In particular, the models presented here indicate that, for typical dikes little or no dike-induced surface deformation would be expected until the dike tip propagates to depths below the surface of less than a kilometre.

Another look at the 2009 seismic activity, Harrat Lunayyir, Saudi Arabia

This study aims to recognize the influence of source- and path-effects on seismic spectra using the waveform data from the 2009 seismic activity, Harrat Lunayyir (HL), eastern flank of the Red Sea coastline, Saudi Arabia. This seismic activity is considered as a series of continuously evolving events reaching at some stage the maximum on 19 May 2009, with the largest shock of Mw 5.4, and then decayed. A total number of 218 events of magnitudes ML ≥ 2.0 were analyzed, which include the largest shock, 101 and 116 seismic events representing the seismic events within the early and decay stages, respectively. The data analysis using Frequency-time analysis revealed diversity patterns of frequency contents along the recorded seismic stations. Moreover, the modified WADATI diagram showed VP/VS variations of 1.7 ± 0.003 and 1.72 ± 0.002 before and after the occurrence of the largest-sized event among the HL seismic activity, respectively. Using the grid-search method to recover the space parameters representing the average source spectrum of each earthquake over the recorded seismic stations, estimates of source sizes and stress drops imply variations in self-similarity scaling relationship among the HL seismic activity. According to the results obtained from the current analyses, we propose a scenario began with an emplacement of seismic activity due to the local stress accumulation imports from an upward dike injection that is blocked by cooled and hardened magmatic materials in the upper crust from the earlier intrusions. The stress accumulation continued and was then released, at some stage, by the event having the maximum magnitude followed by seismic activities released due to stress relaxations.

The Table, a flat-topped volcano in southern British Columbia: Revisited

More than 65 years ago, W.H. Mathews published a seminal paper in American Journal of Science describing the Table, an iconic, flat-topped volcano in southern British Columbia, Canada. The Table is recognized as the type-example of a glaciovolcanic lava-dominated tuya. We use field mapping, 3-D photogrammetry, lithofacies identification and 40Ar/39Ar geochronology to re-examine the emplacement origins of this unique volcanic edifice. The Table is a steep-sided, flat-topped mass of dense, porphyritic andesite preserving a near-original outer surface of hackly fractured aphanitic material. The age of the edifice is 100 ± 12 ka. A new model for the emplacement involves dike injection and endogenous inflation within an overlying ice mass that is estimated to have been ∼250 to 350 m thick. The model assumes that the edifice expanded outwardly as an elliptical cylinder, endogenously inflating and melting an equal volume of ice. The thermal exchange between the expanding massif and the melting ice operates across a subvertical, well-drained, enveloping carapace of quench breccia. For effusion rates of 0.5 to 2 m3 s−1, marginal heat fluxes are 43 to 186 W m−2, implying low overall heat transfer coefficients (0.04 to ∼0.18 W m−2 K−1). The total volume of ice melted serves as a calorimeter and constrains the average thickness (∼3 m) and properties (thermal conductivity; 0.1 – 0.4 W m−1 K−1) of the breccia carapace. The heat transfer efficiency of intermediate lava-dominated tuyas is significantly lower (∼2 %) than reported for other glaciovolcanic systems (commonly ∼45 – 77 %) but similar to that found for block-lava flows and lava tubes.

Thermal model of successive dike injections and implications for the development of intraplate volcanoes

Temperatures in the root zones of volcanoes play a critical role in the development and persistence of shallow-level magmatic reservoirs in the crust. Here, we present a 1D thermal model allowing evaluation of the thermal impact of magma travelling in conduits to the surface on the root zone of a volcano. This thermal model has been developed to better understand the formation of a vertical intrusion located in the root zone of a dismembered Miocene volcano on Fuerteventura, Canary Archipelago. This intrusion, named PX1, constitutes an almost pure amalgamation of dikes of either clinopyroxenitic or gabbroic composition. Both types of dikes display cumulate textures and are interpreted as resulting from the protracted crystallization of a mafic magma. The formation of clinopyroxenitic, in contrast to gabbroic dikes, requires that the residual melt was extracted at high temperature (>1050°) to avoid plagioclase crystallization.Simulations of multiple dike injections show that the temperature in the root zone increases significantly with the addition of dikes, but the maximum temperature reached in the system depends on the duration of magma flow in the conduits and the time interval between dike injections (i.e., repose period). Active flow is the critical parameter that distinguishes instantaneous dike injection from a magmatic conduit. Without significant magma flow (>1 month), high-temperature conditions (>1000 °C) cannot be maintained in the pluton unless dikes are very thick and the repose period is extremely small. On the other hand, magma flow times of one to several months, combined with short time intervals between dike injections (<25 years), which are conditions comparable to those recorded for historical eruptions of oceanic island volcanoes, allow the production and preservation of temperatures above the plagioclase liquidus for significant durations, as required to generate clinopyroxenitic dikes such as those observed in the PX1 pluton.Persistent high temperature in the vicinity of magma conduits limits the differentiation of melts in transit to the surface, providing a potential explanation for why lavas of mafic to intermediate composition predominate in intraplate volcanoes such as Fuerteventura or Fogo Island (Cape Verde Archipelago). In extreme cases, when temperatures over 1000–1050 °C in the central part of the feeding zone are maintained for years, the remaining magma in the conduit does not solidify but is preserved in a mushy state. New pulses of magma would not be able to cross this zone but would rather amalgamate in the incipient magma reservoir. The present model differs from previous models of sill intrusion in that magmas do not need to pond at depth to create a reservoir but merely supply heat while travelling to the surface. Depending on the time interval between dike injections and the duration of magma flow through the crust, magma rising in vertical conduits could directly feed the volcanic edifice or could lead to the formation of magma reservoirs. This process may explain why some volcanoes erupt mafic or differentiated magmas during distinct periods of activity.

Late-stage intrusive activity at Olympus Mons, Mars: Summit inflation and giant dike formation

By mapping the distribution of 351 lava flows at the summit area of Olympus Mons volcano on Mars, and correlating these flows with the current topography from the Mars Orbiter Laser Altimeter (MOLA), we have identified numerous flows which appear to have moved uphill. This disparity is most clearly seen to the south of the caldera rim, where the elevation increases by >200 m along the apparent path of the flow. Additional present day topographic anomalies have been identified, including the tilting down towards the north of the floors of Apollo and Hermes Paterae within the caldera, and an elevation difference of >400 m between the northern and southern portions of the floor of Zeus Patera. We conclude that inflation of the southern flank after the eruption of the youngest lava flows is the most plausible explanation, which implies that intrusive activity at Olympus Mons continued towards the present beyond the age of the youngest paterae ∼200–300 Myr (Neukum et al., 2004; Robbins et al., 2011). We propose that intrusion of lateral dikes to radial distances >2000 km is linked to the formation of the individual paterae at Olympus Mons. Two specific dikes to the SE of the volcano are inferred to have volumes of ∼4400 km3 and ∼6100 km3, greater than the volumes of individual calderas and implying triggering of both caldera collapse and lateral dike injection by the arrival of large inputs of magma from the mantle. A comparable disparity between lava flow direction and current topography, together with a tilted part of the caldera floor, has been identified at Ascraeus Mons.

High-Grade Gold Deposition and Collapse Breccia Formation, Cortez Hills Carlin-Type Gold Deposit, Nevada, USA

The Cortez Hills breccia zone, a Nevada Carlin-type gold deposit located in the Cortez district on the Battle Mountain-Eureka trend, formed within a high-grade polylithic breccia, and grades at the center of the breccia zone are locally in excess of 1 oz/ton Au. Sample transects from low grade or below detection (<5 ppb) to high grade were collected and analyzed using petrography, X-ray diffraction (XRD), scanning electron microscopy (SEM), electron probe microanalysis (EPMA), and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analyses to correlate changes in ore pyrite geochemistry and alteration mineralogy with grade and to relate ore deposition to breccia formation. Ore-related minerals, textures, and analyses are consistent with fluid-rock reaction and replacement at relatively low hydrothermal temperatures (<~260°C) during a single heating and cooling event and over a relatively short time frame, coincident in time and space with regional magmatism. Prior to Carlin mineralization, Roberts Mountains, Wenban, and Horse Canyon Formations were affected by burial diagenesis and low-grade contact metasomatism and metamorphism, producing host rocks composed of calcite, dolomite, and quartz with generally minor dickite, pyrite, chalcopyrite, sphalerite, clinochlore, tremolite, and organic material, and minor to trace K-feldspar, muscovite, and biotite. Acidic ore fluids sulfidized Fe in carbonate rock to form rims of Au- and trace element-rich pyrite on earlier formed pyrite or produced small, generally <10- μ m, spheres of fuzzy ore pyrite with no apparent core. Trace elements in ore-stage pyrite are dominated by As with strongly correlated Hg, Tl, and Cu, all greater than Au; lesser and variable Sb; and minor to undetected Ag, Bi, Ti, W, and Te. Ore fluids also variably dissolved calcite and dolomite and replaced carbonate minerals with jasperoid or produced minor pore space. Ore pyrite and jasperoid after calcite are enclosed in illite that replaced dickite, clinochlore, and tremolite and filled minor open space, primarily between ~227° and 277°C for most analyzed samples based on illite chemistry. As hydrothermal collapse began, ore pyrite chemistry transitioned to an evolved ore-stage chemistry with abundant As, low to undetected Au, lesser Pb + Cu > Sb, and diminished Hg and Tl compared to the ore stage. Evolved ore-stage fluids coincidently precipitated sulfosalt minerals—initially, unusual Cu- and Hg-rich aktashite (Cu6Hg3As4S12) and Tl- and Hg-rich christite (TlHgAsS3) and, later, realgar as Cu and Zn in hydrothermal fluids—and chalcopyrite and sphalerite in the host rocks, respectively, were consumed. Calcite-only veins indicate termination of the hydrothermal system. Small fragmented clasts of realgar with chemically distinct Au-bearing, evolved ore-stage pyrite rims found in brecciated rock along a faulted dike-Wenban contact provide the first evidence in a Carlin-type gold deposit of precipitation of an Au-bearing pyrite rim on a late ore-stage mineral. These rare rims likely formed in response to fault movement along the dike-Wenban contact or by dike injection that allowed deep, Au-bearing fluids to access a higher and cooler part of the system where late ore-stage minerals had begun to form. These rims further highlight the normal textures in Carlin-type gold deposits where ore-stage pyrite was overgrown by late ore-stage sulfosalt minerals, demonstrating a single hydrothermal event followed by a single cooling event. This paragenesis is consistent with previously modeled deposit formation time frames of ~45,000 to 15,000 years. Two styles of breccia were identified within the deposit. Replacement, matrix-supported breccia, in which an illite-rich, punky matrix encloses Au-bearing pyrite, jasperoid, and variable relict calcite, formed during intense ore-stage alteration and mineralization. The fine-grained ore and alteration replacement minerals did not require significant open space to form. Later evolved and late ore-stage sulfosalt minerals and calcite that are visible at the hand-sample scale provide the earliest evidence for the development of significant open space. Hydrothermal collapse allowed the incursion of cooler meteoric water into the system, which—along with the retrograde solubility of calcite—provided coupled mechanisms driving significant calcite dissolution, collapse brecciation, and formation of the Cortez Hills breccia zone. Dissolution and collapse brecciation increasingly concentrated ore-stage pyrite as the hydrothermal system transitioned to the late ore stage, contributing to high Au grades.

Thermochronometric evidence for diffuse extension and two-phase rifting within the Central Arabian Margin of the Red Sea Rift

Numerical time-temperature models derived from a 2D network of apatite and zircon (U-Th)/He ages reveal a three-stage thermotectonic history for the central Arabian rift flank (CARF) of the Red Sea Rift (RSR) system. The pre-rift Arabian-Nubian Shield existed as part of a passive Paleo-Tethyan margin until a widespread tectonic event ca. 350 Ma exhumed the proto-CARF to mid-to-upper crustal structural levels. After remaining thermally stable through the Mesozoic, the first phase of RSR extension began with a distinct rift pulse ca. 23 Ma when fault blocks across a 150 km-wide area were exhumed along a diffuse set of rift-parallel faults from an average pre-rift flank depth of 1.7 ± 0.8 km. This rift onset age is mirrored in thermochronometric and sequence stratigraphic analyses elsewhere along the Red Sea Nubian and Arabian margins, confirming that rifting occurred concomitantly along the full Red Sea-Gulf of Suez rift system. Diffuse lithospheric extension lasted for 8 million years before a second rift pulse ca. 15 Ma, coincident with regional stress realignment, transferred active faulting basinward towards the modern RSR axial trough. CARF time-temperature models indicate that the prevalent rift style during both RSR extensional phases was one of localized, structurally controlled block faulting and contemporaneous dike injection, not wholesale rift flank uplift.

Effects of axially variable diking rates on faulting at slow spreading mid-ocean ridges

Magma supply for dike injection can be highly variable within a segment of a slow-spreading mid-ocean ridge but the tectonic impact of this variability is not fully understood. Here, we use three-dimensional numerical models to quantify the effects of variable diking rates on the faulting mode at a 20 km-long slow spreading ridge segment. In addition to end-member faulting modes in which long-lived detachment faults or short-lived normal faults form along the whole segment, we newly identify a transitional mode in which a detachment and a short-lived normal fault form simultaneously but in respective domains separated by a transfer fault. Different faulting modes can be better correlated with the average dike intrusion rate, rather than the highest or lowest rate along the segment. Along-axis stress coupling tends to homogenize fault offset along the segment, inhibiting the domination of a particular faulting mode associated with an extreme local diking rate. This homogenizing effect explains why detachment faults can sometimes form even in the regions previously considered as unfavorable. Our results further suggest that a long (>15 km) and continuous detachment, partially overlain by younger faults, can create an oceanic core complex when faults weaken fast and diking rate is low. When faults weaken slow and diking rate is moderate, however, faulting occurs in the transitional mode, producing a detachment over only a part of the segment length.

On the mechanisms governing dike arrest: Insight from the 2000 Miyakejima dike injection

Magma stored beneath volcanoes is sometimes transported out of the magma chambers by means of laterally propagating dikes, which can lead to fissure eruptions if they intersect the Earth's surface. The driving force for lateral dike propagation can be a lateral tectonic stress gradient, the stress gradient due to the topographic loads, the overpressure of the magma chamber, or a combination of those forces.The 2000 dike intrusion at Miyakejima volcano, Izu arc, Japan, propagated laterally for about 30 km and stopped in correspondence of a strike-slip system, sub-perpendicular to the dike plane. Then the dike continued to inflate, without further propagation. Abundant seismicity was produced, including five M>6 earthquakes, one of which occurred on the pre-existing fault system close to the tip of the dike, at approximately the time of arrest. It has been proposed that the main cause for the dike arrest was the fault-induced stress.Here we use a boundary element numerical approach to study the interplay between a propagating dike and a pre-stressed strike-slip fault and check the relative role played by dike–fault interaction and topographic loading in arresting the Miyakejima dike. We calibrate the model parameters according to previous estimates of dike opening and fault displacement based on crustal deformation observations. By computing the energy released during the propagation, our model indicates whether the dike will stop at a given location. We find that the stress gradient induced by the topography is needed for an opening distribution along the dike consistent with the observed seismicity, but it cannot explain its arrest at the prescribed location. On the other hand, the interaction of dike with the fault explains the arrest but not the opening distribution. The joint effect of the topographic load and the stress interaction with strike-slip fault is consistent with the observations, provided the pre-existing fault system is pre-loaded with a significant stress, released gradually during the dike–fault interplay.Our results reveal how the mechanical interaction between dikes and faults may affect the propagation of magmatic intrusions in general. This has implications for our understanding of the geometrical arrangement of rift segments and transform faults in Mid Ocean Ridges, and for the interplay between dikes and dike-induced graben systems.

청송 북동부 유문암질 암맥군의 패턴과 성인

청송 북동부에서 중태산과 갈평지 관입체는 영양소분지에서 각각 가송동층과 도계동층을 관입하여 각각 병반과 암주로 산출된다. 그리고 청송 암맥군은 도계동층을 관입하며 갈평지 암주에 매우 밀접하게 조합되어있다. 이 암맥군은 방사상으로 갈평지에 수렴되고 전체 윤곽이 타원형 분포를 나타낸다. 또한 암맥들은 유상엽리, 구과상, 석정상 구조를 나타내는 유문암맥들이며 하나의 단일 관입상을 나타낸다. 암맥들의 방향성은 관입체에서 응력상태에 의하여 지배되는 것으로 해석할 수 있다. 즉 암맥들은 일반적으로 갈평지 암주의 관입에 의해 퇴적암류의 도움작용 동안에 최고 주응력축에 직각으로 발생하는 방사상 단열에 따르는 수직 인장절리를 따라 주입해 방사상 패턴을 나타낸다. 이 패턴은 본역에서 이 암맥들이 대부분 방사상 절리들 속으로 주입을 충분히 설명해줄 수 있다. Jungtaesan and Galpyeongji intrusions in the northeastern Cheongsong occur as laccolith and stock which intrude Gasongdong Formation and Dogyedong Formation, respectively. Cheongsong dike swarm, intruding the Dogyedong Formation, is closely associated with this stock. The dike swarm is more radial to focus into Galpyeongji and its outline is oval. The dikes of the dike swarm are only rhyolite dikes with flow banded, spherulitic and rare stony structures, and represents a single intrusive phase of magma. It can be interpretated that orientation of the dikes is controlled by stress states. Therefore, the dikes display a radial pattern through occupying vertical joints that have been generally attributed to radial fractures formed during doming of the sedimentary rocks by the intrusion of the Galpyeongji stock. The dike pattern could sufficiently account for dike injections into these joints.