Igneous Sills Research Articles

Mercury efficiently volatilized but not completely removed from sediments around igneous intrusions

The impact of large igneous province (LIP) volatile emissions from magmatic and thermogenic sources has been widely studied using sedimentary mercury (Hg). Igneous sill emplacement in sedimentary basins was a key component of several LIPs, generating environmentally significant amounts of thermogenic gases (e.g., CH4, CO2, Hg) and fluids by heating sedimentary rocks. Understanding these processes is key to realizing the promise of Hg in sedimentary archives to understand LIP impacts and track LIP carbon outputs. Published data show sedimentary rocks in contact aureoles of such LIP-related intrusions retain some Hg despite exposure temperatures above the stability limit of sedimentary Hg phases (>300 °C). We examine this unexpected Hg retention using Hg thermal desorption profiles (TDPs). The TDPs reveal a remarkable pattern in Hg speciation related to intrusions: with increasing aureole temperature, Hg release is progressively dominated by a low-temperature phase that could have formed only during or after cooling of the intrusion. We calculate a temperature-dependent Hg volatilization efficiency and show that 70%−100% of Hg was mobilized above 350 °C. Despite the efficient volatilization, only ∼50% of the volatilized Hg was removed from the studied aureole. Mercury recapture in and around the thermal aureole likely extends into the associated hydrothermal vent systems, reduces the Hg:C ratios of emitted thermogenic gases, and may lead to underestimation of Hg-based LIP thermogenic CO2 and CH4 emissions estimates and their environmental impact.

Magma solidification effects during sill emplacement: Insights from laboratory experiments

Igneous sills and interconnected sill complexes transport magma both vertically through the Earth's crust and laterally over potentially long distances. Although cooling and solidification of magma are acknowledged to play a major role in the propagation and emplacement of sills, their contributions to sill formation remain poorly understood. Here, the effects of solidification on sill propagation dynamics and the resulting intrusion morphologies are investigated using scaled laboratory experiments. Molten coconut oil (magma analogue), which solidifies during emplacement, is injected directly into the horizontal interface between two layers of a colder, layered, solid visco-elasto-plastic gel (Laponite RD®, host rock analogue) to facilitate sill formation. The injection temperature and volumetric flow rate of the coconut oil, and the temperature of the host material, are varied between experiments to control the relative degree of solidification. When solidification effects are relatively weak, corresponding to high injection temperatures, sill propagation is continuous and forms penny-shaped intrusions that later turn into saucer-shaped sills with marginal segmentation. Conversely, when solidification effects are intermediate to relatively strong, corresponding to lower injection temperatures, sills develop complex elongate morphologies that lengthen parallel to the long-axis of the magma flow direction. Such sills also form in a discontinuous manner and propagate in pulses by growth of discrete marginal lobes, representing periods of tip arrest due to freezing, followed by growth of new lobes at the sill margins. A striking morphological feature that occurs in experiments with intermediate to relatively strong solidification effects is the presence of internal flow channels within sills, which can be: (a) thermally controlled, long-lived channels in experiments with intermediate solidification effects; or (b) structurally controlled, randomly oriented short-lived channels in experiments with relatively strong solidification effects. Our experimental findings are consistent with field and seismic observations of sill geometries, and they highlight that the relative degree of solidification during magma emplacement controls both how sills propagate and their internal flow dynamics.

Spatial and Temporal Distribution of Igneous Sills in the Central Tarim Basin and Their Geological Implications

Interpretation of the seismic reflection profiles associated with borehole data from the petroleum industry offers a novel way to study sill emplacement in sedimentary basins. This study uses this approach to reveal the intrusive part of the Tarim Large Igneous Province (LIP) within the basin, which has not been systematically reported. A large number of igneous intrusions (sills) are identified in the sedimentary layers of the Central Tarim Basin. The burial depth of the sills is 6–8 km, and they are mainly located within the upper Ordovician strata. According to their seismic facies and drilling data, it is inferred that they are dolerite sills. Based on the uplift of the overlying strata above the intrusions, it is concluded that the sills were mainly formed during the depositional period of the middle Permian Kupukuziman Formation and Kaipailezike Formation (early stage), with a few formed during the depositional period of the upper Permian strata (late stage). It is likely that these two stages of sill intrusion correspond to the main basaltic eruptions within the basin and the mafic dike emplacement in the Bachu area of the Tarim LIP, respectively. The study suggests that that the dolerite sills reported in this study are also an important component of the Permian Tarim LIP.

Assessing the application of ultradeep azimuthal electromagnetic resistivity tools in detecting igneous formations ahead of the transmitter

The Faroe–Shetland basin (FSB) is considered challenging for oil and gas exploration due to its complex geological structure and limited exploration drilling activity. The costs of drilling in the basin are the most expensive within the United Kingdom Continental Shelf (UKCS) due to varying geological and engineering challenges. The prevalence of igneous sill complexes within the basin present drilling hazards that contribute to drilling nonproductive time (NPT) and are documented in multiple studies. Recent advancements in ultradeep azimuthal resistivity (UDAR) technology which utilizes the look-ahead portion of the electromagnetic (EM) signal presents an opportunity to assess its suitability as a derisking tool to ‘look-ahead’ and detect igneous intrusions in volcanic basins. This paper utilizes resistivity log data from Well 214/28-1, which encountered multiple igneous intrusions and is used to validate the ability of these tools to detect the intrusions. The EM look-ahead technology detected with good confidence the top of an igneous intrusion 72 ft (22 m) true vertical depth (TVD) ahead of the transmitter and once drilled, detect the base of the intrusion 54 ft (16 m) TVD ahead of transmitter. The detection of the intrusions prior to drilling and information on intrusion thickness can enable proactive drilling decisions to reduce nonproductive time.

Impact of permeability evolution in igneous sills on hydrothermal flow and hydrocarbon transport in volcanic sedimentary basins

Abstract. Sills emplaced in organic-rich sedimentary rocks trigger the generation and migration of hydrocarbons in volcanic sedimentary basins. Based on seismic and geological observations, numerical modeling studies of hydrothermal flow around sills show that thermogenic methane is channeled below the intrusion towards its tip, where hydrothermal vents nucleate and transport methane to the surface. However, these models typically assume impermeable sills and ignore potential effects of permeability evolution in cooling sills, e.g., due to fracturing. Here, we combine a geological field study of a volcanic basin (Neuquén Basin, Argentina) with a hybrid finite-element–finite-volume method (FEM–FVM) of numerical modeling of hydrothermal flow around a sill, including hydrocarbon generation and transport. Our field observations show widespread veins within sills composed of graphitized bitumen and cooling joints filled with solid bitumen or fluidized shale. Raman spectroscopy indicates graphitization at temperatures between 350 and 500 ∘C, suggesting fluid flow within the intrusions during cooling. This finding motivates our modeling setup, which investigates flow patterns around and through intrusions that become porous and permeable upon solidification. The results show three flow phases affecting the transport of hydrocarbons generated in the contact aureole: (1) contact-parallel flow toward the sill tip prior to solidification, (2) upon complete solidification, sudden vertical “flushing” of overpressured hydrocarbon-rich fluids from the lower contact aureole towards and into the hot sill along its entire length, and (3) stabilization of hydrocarbon distribution and fading hydrothermal flow. In low-permeability host rocks, hydraulic fracturing facilitates flow and hydrocarbon migration toward the sill by temporarily elevating porosity and permeability. Up to 7.5 % of the generated methane is exposed to temperatures >400 ∘C in the simulations and may thus be permanently stored as graphite in or near the sill. Porosity and permeability creation within cooling sills may impact hydrothermal flow, hydrocarbon transport, and venting in volcanic basins, as it considerably alters the fluid pressure configuration, provides vertical flow paths, and helps to dissipate overpressure below the sills.

The anomalous magmatism in the southern part of the Santos basin, and the non-continuous salt layer over Abimael Ridge

The magmatism in the Santos Basin is related to the following main events: (1) the pre-rift volcanic episode; (2) the syn-rift to late syn-rift episode; (3) the late Cretaceous episode; and (4) the Paleogene episode. This works focuses on the characterization of these events using seismic data and borehole interpretation in the southern part of the basin. The late Cretaceous magmatic episode is recorded in the shelf break region by 3D seismic data. A mini-basin with possible turbidite reservoirs was tested by an exploratory borehole that revealed several igneous features, including volcaniclastic sequences, sills, and volcanic plugs. Basinward, the evaporite layers are thicker and form salt walls above a detachment at the base of the salt reflector, which climbs up oceanward. The analysis of strong reflectors within the distal salt wall suggested three alternative interpretations, including igneous sills within the salt mass, anhydrite lenses, carbonate rafts, carbonate stringers, pre-salt microbialite buildups, and pre-salt volcanic rocks. The interpretation of the seismic and potential field data, constrained by the borehole results, indicates that the strong reflector corresponds to the top of the volcanic layers below the pre-salt microbialites.The anomalous magmatism and salt tectonics styles in the southern part of the Santos Basin are related to a swath of embryonic oceanic crust that was emplaced during the late Aptian to early Albian. The interpretation of a regional profile extending from the continental shelf to the oceanic crust provides evidence for the separation of the salt masses by a failed oceanic spreading center, which is corroborated by the gravity and magnetic models.

Geological Complications and Environmental Hazards of the Cement Raw Materials Quarry Sites in Yemen

Extensive field and site investigations were conducted to assess and evaluate the situation in the presently exploited quarries of cement raw materials in Yemen. These quarries have several geological complications represented mainly by high elevations, steep slopes, rugged topography, heterogeneity in bed thickness, lithologic composition and quality, presence of igneous sills and dykes, intensive fracturing and jointing and abundance of karstification features. Moreover, the processes of quarrying and related operations have several negative environmental impacts the most important of which are soil failure, overburden and land sliding, toppling and rock falls (which result in considerable mass wasting) and emission of dust and noise. Generally, quarrying operations are more hazardous in quarries of gypsum and basement rocks than in those of carbonates and volcanics. Furthermore, the quarries have problems related to the conservation of natural resources. These are represented primarily by the excavation of the valuable agricultural and reclaimed lands and improper exploitation of the cement raw materials and the fresh and underground waters the reserves of which decreased drastically. To deal with the above-mentioned problems, a number of recommendations are outlined. They comprise mitigation measures which must be strictly implemented. Also, it is of almost importance to conduct prospecting for new occurrences of the presently exploited cement raw materials and their possible substitutes as well as additional groundwater resources.

Post-salt magmatism in the Campos Basin, offshore SE Brazil: style, distribution, and relationship to salt tectonics

Substantial magmatism occurred during the development of the marginal basins of the central and southern South Atlantic. Situated on the Brazilian side of the central segment, the Campos and Santos basin represent a transitional margin, located between the magma-rich margin in the north and the magma-poor margin in the south. In addition to magmatism associated with the initiation of continental rifting, the southeast Brazilian basins experienced three main magmatic events; one during the early post-rift (i.e., sag stage) and two during the late post-rift (i.e., passive margin stage), both occurring after deposition of a thick, late Aptian salt layer. The products of post-salt, intrusive and extrusive magmatism in the southern Campos Basin are imaged in seismic reflection data and have been directly penetrated by boreholes, yet they remain poorly understood in terms of their style, detailed geometry, and distribution. Furthermore, the temporal, spatial and possibly genetic relationships between these post-salt igneous products and salt-tectonics are largely unknown. Documenting these possible salt-magma interactions is important to understanding the tectono-stratigraphic evolution of Campos Basin and other salt-bearing passive margin basins subject to magmatic activity. We use 2D and 3D seismic reflection and borehole data from the southeastern Campos Basin, offshore Brazil to map and characterize post-salt igneous sills and volcanoes, and to understand the interaction between salt tectonics and igneous products. We identified 99 sills emplaced within uppermost Aptian-to-Maastrichtian strata, 83 emplaced above the salt and 16 intra-salt, and 15 volcanoes within Upper Cretaceous-to-lower Paleogene strata, associated with a >310 km2 lava field. We also identify four, subsalt-sourced, vent-like structures, which vary in age from Paleogene to Late Cretaceous. Salt and overburden structures define a domain of extensional deformation on a base-salt basement high, and a domain of contractional deformation in the adjacent basement low. The relationship between igneous intrusions and salt tectonics is complex, i.e., in areas where they are common, sills appear to have locally restricted salt flow and the growth of large salt structures, whereas in other cases, salt structures and overlying faults appear to have partially acted as pathways to intruding magma, allowing it to ascend to relatively shallow depths. Extrusive igneous products exclusively developed after the main phase of salt movement. However, salt distribution influenced the location of volcanoes, given the latter developed where salt is welded or very thin. Salt flow and thickness also controlled the distribution of lava flow products, which accumulated in structural lows between large salt tectonic structures. Our work indicates a complex relationship between salt tectonics and magmatic emplacement that impact the post-rift evolution of Campos Basin.

Seismic wave propagation around subsurface igneous sill complexes

Imaging both within and beneath subsurface igneous sill complexes is a seismic exploration challenge. A significant aspect of this challenge is due to a lack of understanding of the interaction between the heterogeneous geological structures and the seismic wavefield, which includes the seismic response to sub-resolution ‘thin’ sills. This study aims to provide some insight into the effect subsurface sills have on the observed seismic wavefield. This is achieved through high-resolution full-waveform elastic seismic modelling, using a realistic geological model developed from interpreted seismic data and statistics of sills from wireline logs and conceptual understanding from sill complexes in outcrop. We find that little energy penetrates through the sill complex to a target reflector below the sill complex, which is consistent with real-world observations. This is due to a number of factors, including energy lost to strong internal multiples (stratigraphic filtering), converted modes and leaky guided waves within the sills. These processes remove energy from the primary transmitted wavefront, contributing to degraded seismic imaging. Whilst these are all fundamental physical limitations that cannot be overcome, further work should focus on the processing of seismic data to ensure that these aspects of the seismic wave-field around sill complexes are optimally treated within processing workflows.

Carbon released by sill intrusion into young sediments measured through scientific drilling

AbstractThe intrusion of igneous sills into organic-rich sediments accompanies the emplacement of igneous provinces, continental rifting, and sedimented seafloor spreading. Heat from intruding sills in these settings alters sedimentary organic carbon, releasing methane and other gasses. Recent studies hypothesize that carbon released by this mechanism impacts global climate, particularly during large igneous province emplacements. However, the direct impacts of sill intrusion, including carbon release, remain insufficiently quantified. Here, we present results from International Ocean Discovery Program (IODP) Expedition 385 comparing drill-core and wireline measurements from correlative sedimentary strata at adjacent sites cored in Guaymas Basin, Gulf of California, one altered by a recently intruded sill and one unaffected. We estimate 3.30 Mt of carbon were released due to this sill intrusion, representing an order of magnitude less carbon than inferences from outcrops and modeling would predict. This attenuated carbon release can be attributed to shallow intrusion and the high heat capacity of young, high-porosity sediments. Shallow intrusion also impacts sub-seafloor carbon cycling by disrupting advective fluxes, and it compacts underlying sediments, increasing potential carbon release in response to subsequent intrusions.

Off-sequence plume magmatism near Ninetyeast Ridge in the Indian Ocean: evidence for extensive lateral flow of the Kerguelen plume

One of the fundamental questions about Earth processes that is still outstanding is how far a moving plate can drag plume material at its base after its initial impact with the plume. The IODP-362 Expedition recovered igneous samples from drilled sites near Ninetyeast Ridge (NER) in the Indian Ocean. A detailed geochemical and isotopic (Sr–Nd) investigation of these samples found that the basement basalts of the core are typical N-MORB tholeiites, but the igneous sills intruding the upper layer of sediments are highly alkaline, with a composition equivalent to the Kerguelen plume magma. Based on biostratigraphy, the minimum age of the alkaline samples was calculated as c. 58 Ma, which is younger than the adjacent NER crust ( c. 82–78 Ma). The existence of such young plume magma amidst the older blocks of the NER is unusual and contrary to its southwards-younging age pattern. We propose that the fast-moving Indian plate dragged a considerable amount of Kerguelen plume material underneath the Indian Ocean lithosphere northwards during the Cretaceous–Paleocene, covering a longitudinal distance of c. 2220 km. The reactivation of deep fractures triggered decompression melting of the underlying plume material and emplaced this as magmatic sills and lava flows near the NER at c. 58 Ma.

Unlocking the Hydrocarbon Potential of the Mannar Basin (Sri Lanka) Based on New Data and New Ideas

The Mannar Basin is a failed rift basin located between Sri Lanka and India. Massive flood basalts have been deposited during latest Cretaceous/earliest Paleocene magmatic activity that is also represented by numerous igneous sill intrusions. The two discoveries are associated with this magmatism: Barracuda and Dorado, both discovered in 2011, have been drilled in the northern part. One of the key challenges for hydrocarbon exploration in volcanic provinces is sub-basalt imaging due to the velocity contrast between basalts and siliciclastics. To unlock hidden play types and identify geological features within Mesozoic intervals we utilized both infill 2D seismic data acquired in 2018 and vintage 2D seismic data reprocessed in 2021. These provide new insights for hydrocarbon exploration and the tectono-stratigraphic evolution of Sri Lanka’s offshore basins. We integrated these new study results into a regional 2D petroleum system modelling study. Modelling results indicate hydrocarbon potential for several play types: Cretaceous reef, turbidite and fan plays. Since the tectonic evolution of the Mannar Basin is still a matter of debate, especially related to timing and duration of extension and Sri Lanka’s position during Gondwana breakup, we created different heat flow scenarios to validate the impact on the hydrocarbon prospectivity.

Anatomy of intrusion related forced fold in the offshore Otway Basin, SE Australia

Magmatic sills emplaced at shallow-levels are commonly associated with bending and uplift of the overburden. Such phenomena result into domed-shaped structures, known as forced folds whose structural architecture is chiefly controlled by the geometry of underlying intrusions. Several models for host-rock deformation have been proposed and are based on elastic bending-related uplift of the associated overburden. However, the importance of inelastic compaction of overburden for accommodating intruded magma volume has remained underestimated. In this study, we document a forced fold developed above magmatic sills intruded at shallow levels into the sedimentary layers of the offshore Otway Basin, southeastern Australia, from high-quality 3D reflection seismic data. The intruded sills S1 and S2 have an average thickness of 81 m and 223 m, and its emplacement uplifted the paleo-seabed to an average amplitude of 93 m. The intruded sill S2 possess larger contribution in bending the overburden, however, the discrepancy (∼65%) between the sill thickness and vertical uplift observed at the paleo-surface, indicates the occurrence of localized host rock deformation caused by pore space collapse and fluidization of shallow buried consolidated sediments. In addition, we assume that the imaged igneous sill may include flakes of sedimentary rocks, which caused an overestimation of the sill thickness, using the widely accepted seismic velocity for mafic intrusions. Our observations imply that porosity reduction due to compaction of both proximal syn-emplacement and post-emplacement sediments, could play a vital role in accommodating this intrusion within the associated stratigraphic succession. Thus, our study provides critical insights into the compaction-related mechanism involved in overburden deformation as a result of sill intrusion.

Effects of ultrathin igneous sill intrusion on the petrology, pore structure and ad/desorption properties of high volatile bituminous coal: Implications for the coal and gas outburst prevention

As the one of the most catastrophic mining hazards, coal and gas outburst seriously threatens the mining safety of collieries. Previous studies indicate that lots of outbursts are closely related to igneous intrusion. Thus, to further understand the influence of igneous intrusion on coal’s physical properties and outburst risk (especially the impacts from ultrathin igneous sill intrusion, which is hard to be detected underground), the combined analyses of petrographic, proximate, pore structure and methane ad/desorption are applied. Results indicate that with the approaching to the ultrathin igneous sill, physical properties of coal change significantly. Due to thermal evolution effect from intrusion, metamorphic grade of coal increases from the high volatile bituminous coal (Ro = 0.6949) of unaffected normal region to the low volatile bituminous coal (Ro = 1.6684), and the altered range of thermal evolution zone is larger than 3 times of the sill thickness but of which the range of strong thermal evolution zone only reaches 0.8 times of the sill thickness. Within the thermal evolution zone: moisture content of coal decrease while the ash content increase when approaching to the sill; micropore volume and specific surface area of the coal are nearly unchanged in the weak thermal evolution zone but reduced in the strong thermal evolution zone, however, methane adsorption capacities of the coal are generally enhanced when comparing to the normal region sample; the transition pores and mesopores pore of the sample close to distal boundary of thermal evolution zone is seriously suppressed (which are 49.21–68.32 % and 20.14–63.31 % of the normal sample, respectively). By comprehensively considering the impact of igneous intrusion on coal, coal locates near the distal boundary of strong thermal evolution zone (about 0.5 times of sill thickness away from the intrusion boundary) seems to have the most serious threat of outburst risk. Additionally, the moisture, ash contents and the gas desorption property of coal has the feasibility to be used as the signs for the hard-detectable small-scale igneous intrusion ahead during roadway tunneling process. Meanwhile, due to the correlation between igneous intrusion and outburst, which can also be regarded as the early warning indicator for outburst prevention.

The building blocks of igneous sheet intrusions: Insights from 3-D seismic reflection data

Abstract The propagating margins of igneous sills (and other sheet intrusions) may divide into laterally and/or vertically separated sections, which later inflate and coalesce. These components elongate parallel to and thus record the magma flow direction, and they can form either due to fracture segmentation (i.e., “segments”) or brittle and/or non-brittle deformation of the host rock (i.e., “magma fingers”). Seismic reflection data can image entire sills or sill-complexes in 3-D, and their resolution is often sufficient to allow us to identify these distinct elongate components and thereby map magma flow patterns over entire intrusion networks. However, seismic resolution is limited, so we typically cannot discern the centimeter- to meter-scale host rock deformation structures that would allow the origin of these components to be interpreted. Here, we introduce a new term that defines the components (i.e., “elements”) of sheet-like igneous intrusions without linking their description to emplacement mechanisms. Using 3-D seismic reflection data from offshore NW Australia, we quantify the 3-D geometry of these elements and their connectors within two sills and discuss how their shape may relate to emplacement processes. Based on seismic attribute analyses and our measurements of their 3-D geometry, we conclude that the mapped elements likely formed through non-elastic-brittle and/or non-brittle deformation ahead of the advancing sill tip, which implies they are magma fingers. We show that thickness varies across sills, and across distinct elements, which we infer to represent flow localization and subsequent thickening of restricted areas. The quantification of element geometries is useful for comparisons between different subsurface and field-based data sets that span a range of host rock types and tectonic settings. This, in turn, facilitates the testing of magma emplacement mechanisms and predictions from numerical and physical analogue experiments.

Post-rift magmatism and hydrothermal activity in the central offshore Otway Basin and implications for igneous plumbing systems

Spatially and volumetrically extensive igneous rocks are exposed onshore in southeastern Australia and provide important information about the Cenozoic evolution of the region. However, the morphology and distribution of igneous rocks are still not well constrained in the ~70 km wide continental shelf of the central Otway Basin. Within this region, we present seismo-geomorphological characteristics of 30 volcanoes and 27 hydrothermal vents, as well as several lava flows and magmatic sills, based on 2D and 3D seismic reflection data. Hydrothermal vents formed mainly during the mid-Eocene, while igneous complexes developed between the mid-Eocene to late Miocene, both post-dating continental break-up in the basin. The distribution and morphology of igneous rocks and hydrothermal vents is influenced by rift-related faults, as most of these features are linked to the underlying faults by near-vertical zones of disruption and are elongated in an NW-SE direction that parallels the primary fault trend. Magma for volcanoes and heat sources for hydrothermal vents within the study area are likely supplied by deep-sourced dykes, as magmatic sills only have a scattered distribution in this region. This implies a dyke-dominated plumbing system for magmatism in the Otway Basin. In comparison, the nearby Bight Basin, which has a thin basement but a relatively thick sedimentary sequence, displays magma propogation more prone to forming igneous sills. There are significant lithological and tectonic similarities between the Otway and Bight basins; thus, we propose that a relatively thin sedimentary sequence over a thick basement in the Otway Basin produced sufficient magma pressure for a predominantly dyke-dictated igneous plumbing system. This work highlights the critical role of basin structures, such as the thickness of basement and overlying strata, in the control of igneous plumbing styles and the distribution of post-rift igneous complexes along magma-poor continental margins. Our work, therefore, aids the estimation of different magmatic components within the sedimentary basins and facilitates the understanding of their diverse impacts on the exploration of frontier basins. This result also provides some constraints on the prediction of possible future eruption centres within active volcanic fields.

Widespread hydrothermal vents and associated volcanism record prolonged Cenozoic magmatism in the South China Sea

Abstract The continental margin of the northern South China Sea is considered to be a magma-poor rifted margin. This work uses new seismic, bathymetric, gravity, and magnetic data to reveal how extensively magmatic processes have reshaped the latter continental margin. Widespread hydrothermal vent complexes and magmatic edifices such as volcanoes, igneous sills, lava flows, and associated domes are confirmed in the broader area of the northern South China Sea. Newly identified hydrothermal vents have crater- and mound-shaped surface expressions, and occur chiefly above igneous sills and volcanic edifices. Detailed stratigraphic analyses of volcanoes and hydrothermal vents suggest that magmatic activity took place in discrete phases between the early Miocene and the Quaternary. Importantly, the occurrence of hydrothermal vents close to the present seafloor, when accompanied by shallow igneous sills, suggest that fluid seepage is still active, well after main phases of volcanism previously documented in the literature. After combining geophysical and geochemical data, this study postulates that the extensive post-rift magmatism in the northern South China Sea is linked to the effect of a mantle plume over a long time interval. We propose that prolonged magmatism resulted in contact metamorphism in carbon-rich sediments, producing large amounts of hydrothermal fluid along the northern South China Sea. Similar processes are expected in parts of magma-poor margins in association with CO2/CH4 and heat flow release into sea water and underlying strata.

Fault inversion contributes to ground deformation above inflating igneous sills

Magma emplacement is commonly accommodated by uplift of the overburden and free surface. By assuming this deformation is purely elastic, we can invert the shape and kinematics of ground deformation to model the geometry and dynamics of underlying intrusions. However, magma emplacement can be accommodated by viscoelastic and/or inelastic processes. We use 3D seismic reflection data to reconstruct how elastic bending and inelastic processes accommodated emplacement of a Late Jurassic sill offshore NW Australia. We restore syn-emplacement ground deformation and compare its relief to sill thickness, showing that: (i) where they are equal, elastic bending accommodated intrusion; but (ii) where sill thickness is greater, inversion of a pre-existing fault and overburden compaction contributed to magma accommodation. Our results support work showing inelastic processes can suppress ground deformation, and demonstrate magmatism can modify fault displacements. Reflection seismology is thus powerful tool for unravelling links between magma emplacement, ground deformation, and faulting.

Key Strata Inducing Dynamic Disasters Based on Energy Condition: Criterion and Application

The thick and hard rock strata (THRS) exist widely in coal measure strata, which control the movements of overlying rock strata in stopes. When THRS break, great energy is released, which could aggravate the risks of coal and gas outburst, rock burst, and other dynamic disasters. Therefore, the foundation and key of preventing dynamic disasters are to distinguish the THRS that could induce coal-rock dynamic disasters and to analyze the laws of rock stratum breaking and energy releasing. The paper proposed the theoretical calculation methods of the energy accumulation and attenuation of rock breaking which is greatly affected by the hanging length of rock strata and the spreading distance. One or more roof strata that play a leading role in inducing dynamic disasters of the underlying coal mass are defined as the key disaster-inducing strata (KDIS). The disaster-inducing coefficient (DIC) is defined and used as the criterion of KDIS. The greater the source energy, the shorter the spreading distance, and the smaller the attenuation coefficient are, the easier the roof strata are to become KDIS. The disaster-inducing ability of the main THRS was analyzed, and the igneous sill was judged as KDIS, taking the Yangliu Coal Mine as project background. The breaking laws of the igneous sill were obtained by the methods of UDEC numerical simulation and microseismic monitoring, which verified the criterion of KDIS.

Attribute-assisted characterization of basement faulting and the associated sedimentary sequence deformation in north-central Oklahoma

Patterns of recent seismogenic fault reactivation in the granitic basement of north-central Oklahoma necessitate an understanding of the structural characteristics of the inherited basement-rooted faults. Here, we focus on the Nemaha Uplift & Fault Zone (NFZ) and the surrounding areas, within which we analyze the top-basement and intrabasement structures in eight poststack time-migrated 3D seismic reflection data sets. Overall, our results reveal 115 fault traces at the top of the Precambrian basement with sub-vertical dips, and dominant trends of west-northwest–east-southeast, northeast–southwest, and north–south. We observe that proximal to the NFZ, faults dominantly strike north–south, are fewer (<10), and have the lowest areal density and intensity, while displaying the largest maximum vertical separation. However, farther away (>30 km) from the NFZ, faults exhibit predominantly northeast–southwest trends, fault areal density and intensity increases, and maximum vertical separation decreases steadily. Of the analyzed faults, approximately 49% are confined to the basement (intrabasement), ~28% terminate within the Arbuckle Group, and approximately 23% transect units above the Arbuckle Group. These observations suggest that (1) proximal to the NFZ, deformation is dominantly accommodated along a few but longer fault segments, most of the mapped faults cut into the sedimentary rocks, and most of the through-going faults propagate farther up-section above the Arbuckle Group; and (2) with distance away from the NFZ, deformation is diffuse and distributed across relatively shorter fault segments, and most basement faults do not extend into the sedimentary cover. The existence of through-going faults suggests the potential for spatially pervasive fluid movement along faults. Further, observations reveal pervasive, subhorizontal intrabasement reflectors (igneous sills) that terminate at the basement-sediment interface. Results have direct implications for wastewater injection and seismicity in north-central Oklahoma and southern Kansas. Additionally, they provide insight into the characteristics of basement-rooted structures around the NFZ region and suggest a means by which to characterize basement structures where seismic data are available.