Saucer-shaped Sills Research Articles

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.

Three-dimensional geological modelling of igneous intrusions in LoopStructural v1.5.10

Abstract. Over the last 2 decades, there have been significant advances in the 3D modelling of geological structures via the incorporation of geological knowledge into the model algorithms. These methods take advantage of different structural data types and do not require manual processing, making them robust and objective. Igneous intrusions have received little attention in 3D modelling workflows, and there is no current method that ensures the reproduction of intrusion shapes comparable to those mapped in the field or in geophysical imagery. Intrusions are usually partly or totally covered, making the generation of realistic 3D models challenging without the modeller's intervention. In this contribution, we present a method to model igneous intrusions in 3D considering geometric constraints consistent with emplacement mechanisms. Contact data and inflation and propagation direction are used to constrain the geometry of the intrusion. Conceptual models of the intrusion contact are fitted to the data, providing a characterisation of the intrusion thickness and width. The method is tested using synthetic and real-world case studies, and the results indicate that the method can reproduce expected geometries without manual processing and with restricted datasets. A comparison with radial basis function (RBF) interpolation shows that our method can better reproduce complex geometries, such as saucer-shaped sill complexes.

An intrusive cryomagmatic origin for northern radial labyrinth terrains on Titan and implications for the presence of crustal clathrates

Titan's labyrinth terrains are an organic-rich, topographically elevated, highly dissected and puzzling geomorphic unit. How these features came to be composed of organics and remain elevated may hold clues about Titan's complicated history, and in particular the dynamics and composition of Titan's crust. One subtype of labyrinth terrains, the radially networked labyrinth terrains, is found in Titan's mid-latitudes. They are dome-shaped with radial drainage patterns and appear to be a clustering of uplifted, organic-rich dissected plateaux. We use scaling relationships to determine whether they formed as elevated surfaces that were uplifted by solid-state diapirs or cryomagmatic laccoliths at depth. Based on the large, variable spacing between features, we find it unlikely that they formed via density-driven diapirism. Instead, their dimensions suggest that they are cryomagmatic intrusions that formed near the most prominent rheological contrast in the ice shell, the brittle–ductile transition (BDT). At that location, we argue that intrusions spread horizontally and inflate, forming large cryomagmatic laccoliths (upturned, large saucer-shaped sills). The intrusions would flex the overlying lithosphere and surficial sedimentary layers (likely undifferentiated plains), resulting in prominent domed features that are susceptible to erosion and incision by methane rain and wind. This process then leads to the highly dissected, dome-shaped labyrinth terrains. To determine whether the laccoliths formed near Titan's BDT, we calculate lithospheric strength envelopes for a pure water ice shell with and without an insulating methane clathrate crust using two conductive heat flows: 4 and 7 mW/m2. The plausible range for the BDT for an ice shell with a 1 km thick methane clathrate crust is 12–34 km for the 4 mW/m2 heat flow. This agrees well with the expected intrusion depths of the laccoliths (21–28 km) associated with a cluster of radial labyrinths in Titan's northern mid-latitudes, as derived from a scaling relationship relating intrusion depth to dome width. We find that methane clathrate thicknesses of 2 km or greater result in a BDT that is generally too shallow (3–24 km) to match our observations. If we consider the higher heat flow, these BDTs are even shallower. Although methane clathrate is known to be stronger than ice, its low thermal conductivity significantly raises the underlying ice shell's temperature, which results in a significantly thinner lithosphere that is not able to support these large plateaux. We conclude that in the location of this radial labyrinth terrain cluster there may be intrusive cryomagmatic activity approximately 21 km deep within the water ice shell, with a putative surface methane clathrate crust, if it exists atop a conductive ice shell, that is constrained to be <2 km thick.

Preliminary assessment of the characteristics of Late Miocene - Quaternary intrusive and extrusive magmatism in the Tu Chinh - Vung May basin, southeastern continental shelf of Vietnam

Based on the seismic and well dataset provided by the Vietnam Petroleum Institute (VPI), the authors have mapped and described the characteristics of the distribution and morphology of magmatic bodies as well as relatively dated them in the Tu Chinh - Vung May basin and adjacent areas. To distinguish magmatic bodies from other amplitude anomalies such as gas zone or carbonate build-up/layers, multiple criteria were used such as cross-cutting relationship, associated deformation of surrounding strata, morphology and geological relationship between different magmatic bodies. Intrusive bodies are usually sheet-like or saucer-shaped sills that cross-cut strata and even deform overlying strata, while extrusive bodies are usually cone-shaped vents/volcanoes or extensive lava sheets that conform to strata. The magmatic bodies often distribute in clusters around one or more magmatic conduits. Middle Miocene and older syn-rift faults controlled the pathway of the conduits. Magmatic bodies are more abundant closer to the East Sea spreading margin. Late Miocene - Quaternary magmatism is widespread in the study area in particular, and in the East Sea and adjacent areas in general. These activities took place after rifting and oceanic crust formation had ended, which is characteristic of magma-poor margins.

Forced folding in the Kora Volcanic Complex, New Zealand: A case study with relevance to the production of hydrocarbons and geothermal energy

The intrusion of magma into relatively shallow strata often results in structural deformation. Forced folds are typically formed above intrusions so as to accommodate the addition of new volumes of magma into the sedimentary pile. This work uses a high-resolution 3D seismic volume from the offshore Taranaki Basin, New Zealand to investigate large forced folds formed in Upper Cretaceous-Paleocene sequences due to the intrusion of magmatic dikes and sills. These magmatic intrusions are related to the plumbing system of the Kora volcano. We apply a novel filtering technique to enhance the contrast between the dip-azimuth of seismic reflectors and that of any overprinting noise, preserving the lateral continuity of seismic events. Forced folds were generated by the intrusion of flat-laying saucer-shaped sills into shallow strata. These folds (Fold A and Fold B) have a domal geometry and a structural relief of 250−350 m, for a total area of 45 km2 and 35 km2, respectively. Out of the 18 interpreted sills, two sills (S1 and S4) contributed the most to the generation of the forced folds in the study area. The sill-fold relationships observed in this work suggest that sills with greater length-to-intrusion ratios deform the strata more effectively. However, it is also clear that forced folds are, in parts of the study area, larger than the intruded sills below. This hints at the presence of a larger volume of intruded magma than that imaged on seismic data where forced folds are significantly larger than the volume of the sills below. Such a character makes the establishment of fold-sill relationships crucial to the identification of magmatic bodies below forced folds - if one identifies a large forced fold on seismic profiles, but only a few small sills are imaged, then the mass of igneous rock below the fold is potentially larger than that imaged on seismic data. More importantly, forced folding in the Taranaki Basin is shown to have led to the development of four-way closures to form prolific traps for hydrocarbon and geothermal energy resources.

Analysis of vertical position relationships between igneous sills and an unconformity surface — Interpretation of seismic profiles from the Northern Tarim Basin, NW China

Sill emplacement mechanisms are very complex, diverse, and regional, and insights from sill reflections are helpful for understanding the emplacement process of magma in the Tarim Basin. This study takes advantage of high-quality 2D seismic data, which are rarely used to study sills in the Tarim Basin, to analyze the sills’ geometric characteristics, plan-view distributions, emplacement timing, and emplacement mechanisms with unconformity surfaces. In the seismic-reflection profiles of the middle-upper Ordovician in the North depression and the southern part of the Tabei uplift in the Tarim Basin, sills with strong positive polarity reflections appear, and they are closely distributed near the Tg52 unconformity surface, which represents the interface between Middle Ordovician limestone and Upper Ordovician mudstone. According to the vertical position of the sills relative to the unconformity, we can divide the sills into saucer-shaped or quasi-saucer-shaped sills above the unconformity surface, sill complexes and saucer-shaped sills on the unconformity surface, and saucer-shaped sills below the unconformity surface. Potential hydrothermal vents and peripheral faults associated with sill intrusion terminate upward in the Middle Permian strata, suggesting that these sills formed in the Middle Permian. Sills with inner flat sheets on the Tg52 unconformity surface formed when the magma ascended and encountered an abrupt change in the fracture toughness and tensile strength between the two adjacent host rock layers. The sills above and on the Tg52 unconformity surface overlap or are vertically linked; therefore, the sills above the Tg52 unconformity surface are the result of the continuous upward expansion of the sills on the unconformity surface, forming sill complexes. Our findings further confirm that unconformities are important interfaces that affect the emplacement of sills.

Tectonic stress controls saucer-shaped sill geometry and emplacement mechanism

AbstractSaucer-shaped sills are common in sedimentary basins worldwide. The saucer shape relates to asymmetric sill-tip stress distributions during intrusion caused by bending of the overburden. Most saucer-shaped sill models are constructed using a magma-analogue excess source pressure (Po) to drive host-rock failure, but without tectonic stress. Here we present axisymmetric finite-element simulations of radially propagating sills for a range of tectonic stress (σr) conditions, from horizontal tension (σr &amp;lt; 0) to horizontal compression (0 &amp;lt; σr). Response to σr falls into four regimes, based on sill geometry and failure mode of the host rock. The regimes are considered in terms of the ratio of tectonic stress versus magma source pressure R = σr/Po: (I) initially seeded sills transition to a dike during horizontal extension (R &amp;lt; 0); (II) with R increasing from 0 towards 1 (compressive σr), sill base length increases and sill incline decreases; (III) where 1 &amp;lt; R &amp;lt; 2, sill base length relatively decreases and sill incline increases; and (IV) where R &amp;gt; 2, sills grow as inclined sheets. Sills in regimes I–III grow dominantly by tensile failure of the host rock, whereas sills in regime IV grow by shear failure of the host rock. Varying σr achieves a range of sill geometries that match natural sill profiles. Tectonic stress therefore represents a primary control on saucer-shaped sill geometry and emplacement mechanism.

Cenozoic sill intrusion in the central and southern East China Sea Shelf Basin

Cenozoic igneous rocks dominated by basalts are exposed sporadically onshore East China, and they provide important information about the Cenozoic evolution of the region. Importantly, their geometries and distribution are still not well constrained in the 200–600 km wide offshore continental shelf region. Based on seismic profiles, this study presents the geometric characteristics of 133 igneous sills that are widely distributed in the central and southern regions of the East China Sea Shelf Basin (ECSSB). Sills in high-amplitude, continuous seismic reflections mostly occur in a time interval of 1.1–3.3 s in multiple seismic profiles, corresponding to a depth range of 1.1–4.8 km. They can be classified into five types: saucer-shaped sills, strata-concordant sills, laccoliths, inclined sheets, and hybrid sills. Of these, the saucer-shaped sills predominate. Strata deformation associated with these igneous sills indicate that sill intrusion has become intense since the Miocene, and it potentially affected petroleum systems in the ECSSB. Forced folds induced by the sills are prospective petroleum traps. We infer that sill intrusion in the ECSSB was due to magma upwelling caused by dehydration and/or small-scale convection in a large mantle wedge above the stagnant Pacific slab. The subduction of the Philippine Sea plate resulted in tectonic inversion in the ECSSB and extension in the Okinawa Trough since the Early Miocene triggered more magmatism in the tectonically thinned ECSSB. Thick sedimentary strata over a thin crust in the ECSSB may have induced more intrusive than extrusive Cenozoic magmatism.

Stratigraphy of Architectural Elements of a Buried Monogenetic Volcanic System

AbstractLarge volumes of magma emplaced and deposited within sedimentary basins can have an impact on the architecture and geological evolution of these basins. Over the last decade, continuous improvement in techniques such as seismic volcano-stratigraphy and 3D visualisation of igneous bodies has helped increase knowledge about the architecture of volcanic systems buried in sedimentary basins. Here, we present the complete architecture of the Maahunui Volcanic System (MVS), a middle Miocene monogenetic volcanic field now buried in the offshore Canterbury Basin, South Island of New Zealand. We show the location, geometry, size, and stratigraphic relationships between 25 main intrusive, extrusive and sedimentary architectural elements, in a comprehensive volcano-stratigraphic framework that explains the evolution of the MVS from emplacement to complete burial in the host sedimentary basin. Understanding the relationships between these diverse architectural elements allows us to reconstruct the complete architecture of the MVS, including its shallow (&lt;3 km) plumbing system, the morphology of the volcanoes, and their impact in the host sedimentary basin during their burial. The plumbing system of the MVS comprises saucer-shaped sills, dikes and sill swarms, minor stocks and laccoliths, and pre-eruptive strata deformed by intrusions. The eruptive and associated sedimentary architectural elements define the morphology of volcanoes in the MVS, which comprise deep-water equivalents of crater and cone-type volcanoes. After volcanism ceased, the process of degradation and burial of volcanic edifices formed sedimentary architectural elements such as inter-cone plains, epiclastic plumes, and canyons. Insights from the architecture of the MVS can be used to explore for natural resources such as hydrocarbons, geothermal energy and minerals in buried and active volcanic systems elsewhere.

Subsurface characterization of Cenozoic igneous activity at Cerro Dragón area (Golfo San Jorge Basin, central Patagonia): Implications for basin evolution and hydrocarbon prospectivity

We use a 3D seismic dataset and wireline logs obtained during hydrocarbon exploration to describe the three-dimensional geometry and characteristics of mafic igneous rocks in the Cerro Dragón oilfield, Golfo San Jorge Basin, southern Argentina. Igneous intrusions occur within Upper Cretaceous and Paleocene sedimentary rocks at depths ranging from 2600 m to a ~200 m below of contemporaneous surface. Three distinctive types of shallow intrusions were recognized (layer-parallel, saucer-shaped, and transgressive sills). The largest sills are layer-parallel, some with estimated volumes up to 9,5 km3, whereas saucer-shaped sills and transgressive sills show widely variable intrusion geometries due to interaction with previous extensional tectonic structures and intrusion-related dikes. Sill intrusions were sourced either through vertical mafic dikes of NNW to NNE orientation, some up to 19 km in length and by sill-sill vertical connections. The presence of three imaged lava flows documents subaerial volcanic events coincident with the deposition of the Sarmiento Formation (late Eocene to early Miocene), whose trajectories were traced for 25–46 km of length.Cenozoic igneous intrusions produce both changes in the temperature history of the basin, and the remobilization of hosted hydrocarbon through the creation of supra-intrusion forced folds and new pathways generated during the igneous emplacement. Besides, sills and dikes act as barriers to both lateral and vertical fluid migration, and thus they need to be considered in petroleum prospect analysis.

Synrift magmatism in a Cenozoic rift basin from 3D seismic data, Wichianburi Sub-basin, Petchabun Basin, Thailand: part 2. How rift structure and stratigraphy modify intrusion morphology

The Latest Oligocene–Pleistocene basic sheet intrusions in the Late Oligocene–Pliocene Wichianburi Sub-basin (onshore Thailand) and Eocene intrusions of the Ceduna Sub-basin (offshore southern Australia) provide examples of igneous intrusion architecture in rift and post-rift basin settings respectively that have been imaged by 3D seismic reflection data. These examples indicate that rift-related intrusions are overall smaller, have a higher aspect ratio, and tend to be more planar or transgressive in a single direction than the saucer-shaped sills that dominate in the post-rift setting of the Ceduna Sub-basin. The long-axis trend and dip direction of most sills in the rift setting investigated tend to conform with rift structure (i.e. fault strike directions and bedding dip and strike directions), and give a strong orientation bias, whereas orientations in the post-rift basin are more varied. The few saucer-shaped sills that formed in the Wichianburi Sub-basin are asymmetric and incompletely developed. A third area, the Kora region of the Taranaki Basin, is a region where intrusions are related to a volcanic centre. This gives the intrusions a mixture of circumferential and radial intrusions related to magmatic processes (e.g. loading by the volcanic edifice, magma chamber pressure) and rift-related influences. Rift-related sills commonly form stacked arrays. Fundamental factors that affect the morphology of sills include magma composition, volume, reservoir pressure and the mechanical stratigraphy of the host rocks. In rifts extra complexities affecting some sills, or parts of sills, include rift structure (faults, rotated, folded bedding), basin size and morphology, stress distribution and sedimentary architecture. Supplementary material: Thumbnail time–structure maps of 107 sills from the Wichianburi Sub-basin mapped from 3D seismic reflection data are available at https://doi.org/10.6084/m9.figshare.c.4620101

Sill complexes in the Karoo LIP: Emplacement controls and regional implications

Field and sub-surface data from the Victoria West sill complex in the Karoo Large Igneous Province (ca. 180 Ma) of South Africa are used to constrain the emplacement controls of the regional-scale sill complexes in the central Karoo basin. Cross-cutting relationships point to the presence of five distinct and successively emplaced saucer-shaped sills. Growth of the sill complex was achieved through magmatic underaccretion of magma batches below earlier sills and associated uplift of the overlying strata. The magmatic underaccretion suggests that earlier sills were fully crystallized during the emplacement of later magma pulses and that the rigid (high E) dolerites, in particular, acted as stress barriers that impeded further upward propagation of steep feeder sheets. The resulting nested structure of sills-in-sills within a confined area of less than 2000 km2 also suggests the reutilization of the same or similar feeder system even after full crystallization thereof.The emplacement controls of sills in the central Karoo through stress barriers implies that sill emplacement occurred under very low deviatoric stresses or in a mildly compressional stress regime prior to the break-up of Gondwana. The swap from earlier (184-180 Ma), mainly sill complexes to later (182-174 Ma) dykes and dyke swarms is indicative of a switch in the stress field during the early stages of Gondwana break-up. We speculate that loading, thermal subsidence and lithospheric flexure associated with the emplacement of the earlier, stacked and voluminous sill complexes in the Karoo basins may have determined the formation of the large Karoo dyke swarms, particularly when coinciding with deeper crustal structures. The original and inherited basin geometry and lithospheric structure is pivotal in the development of later Karoo magmatism.

Structural and lithological controls on the architecture of igneous intrusions: examples from the NW Australian Shelf

Rift-related magmatism resulting in widespread igneous intrusions has been documented in various basins, including the Faroe Shetland Basin (UK), the Voring and Møre basins (Norway), and along the NW Shelf of Australia. Seismic mapping, combined with fieldwork, has resulted in greater understanding of subsurface intrusive plumbing systems but knowledge of emplacement style and the mechanisms by which intrusions propagate is limited. The interpretation of a 3D seismic dataset from the Exmouth Sub-basin, NW Shelf of Australia, has identified numerous igneous intrusions where a close relationship between intrusions and normal faults is observed. These faults influence intrusion morphology but also form pathways by which intrusions have propagated up through the basin stratigraphy. The steep nature of the faults has resulted in the intrusions exploiting them and thus manifesting as fault-concordant, inclined dykes; whereas in the deeper parts of the basin, intrusions that have not propagated up faults typically have saucer-shaped sill morphologies. This transition in the morphology of intrusions related to fault interaction also highlights how dykes observed in outcrop may link with sills in the subsurface. Our interpretation of the seismic data also reveals subsurface examples of bifurcating intrusions with numerous splays, which have previously only been studied in outcrop. Supplementary material: Figures showing uninterpreted seismic lines are available at https://doi.org/10.6084/m9.figshare.c.4395974

Coulomb failure of Earth's brittle crust controls growth, emplacement and shapes of igneous sills, saucer-shaped sills and laccoliths

Tabular intrusions are common features in the Earth's brittle crust. They exhibit a broad variety of shapes, ranging from thin sheet intrusions (sills, saucer-shaped sills, cone sheets), to more massive intrusions (domed and punched laccoliths, stocks). Such a diversity of intrusion shapes reflects different emplacement mechanisms caused by contrasting host rock and magma rheologies. Most current models of tabular intrusion emplacement assume that the host rock behaves purely elastically, whereas numerous observations show that shear failure plays a major role. In this study, we investigate the effects of the host rock's Coulomb properties on magma emplacement by integrating (1) laboratory models using dry Coulomb granular model hosts of variable strength (cohesion) and (2) limit analysis numerical models. Our results show that both sheet and massive tabular intrusions initiate as a sill, which triggers shear failure of its overburden along an inclined shear damage zone at a critical sill radius, which depends on the emplacement depth and the overburden's cohesion. Two scenarios are then possible: (1) if the cohesion of the overburden is significant, opening of a planar fracture along the precursory weakened shear damage zones to accommodate magma flow, leads to the formation of inclined sheets, or (2) if the cohesion of the overburden is negligible, the sill inflates and lifts up the overburden, which is dissected by several faults that control the growth of a massive intrusion. Finally, we derive a theoretical scaling that predicts the thickness-to-radius aspect ratios of the laboratory sheet intrusions. This theoretical prediction shows how sheet intrusion morphologies are controlled by a mechanical equilibrium between the flowing viscous magma and Coulomb shear failure of the overburden. Our study suggests that the emplacement of sheet and massive tabular intrusions are parts of the same mechanical regime, in which the Coulomb behavior of the Earth's brittle crust plays an essential role.

The elusive feeders of the Karoo Large Igneous Province and their structural controls

Dolerite sills in the central parts of the Karoo large igneous province on the Kalahari Craton in South Africa record the emplacement of magic magmas over 350,000 km2, but the feeders to the voluminous magmatism have yet to be identified. Extensive exploration and mining data across a 7400 km2 area and to a depth of 2.5 km in the northern Karoo Basin provide unprecedented access to the subsurface geometry of dolerite sills and their relationship to underlying basement strata. The data show saucer-shaped sills in the Karoo Supergroup with distinctly funnel-shaped geometries along their base that are seated within basement rocks. The funnel-shaped dolerite structures are interpreted to represent the feeders to the overlying regional-scale sill-saucer complexes. Four distinct saucer geometries can be identified (1) cone-shaped saucers that extend outwards from a single funnel-shaped point (Type A), (2) elongated saucers that dip towards multiple aligned funnel-shaped points (Type B), (3) the more commonly described elongated saucers with flat inner sills (Type C) and (4) elongated saucers with a combination of Type B and C geometries (Type D). Type A and C saucer geometries and point-like contacts are consistent with analogue models of saucers for respective pipe-like and linear feeders. However, the multiple pipe-like feeders associated with Type B and combination thereof with sheet-like feeders (Type D) represent saucer-feeder relationships thus far not modelled. Notably, the lower dolerite funnels are commonly rooted in older faults and/or dykes within the basement rocks. This spatial correlation emphasizes the significance of pre-existing basement structures in the Kalahari craton for the long-range transfer of the magmas through the crust. The distributed occurrence of numerous magma feeders across much of the craton is more consistent with thermal incubation of the sub-lithospheric mantle beneath Gondwana than the impact of a mantle plume.

Sill geometry and emplacement controlled by a major disconformity in the Tarim Basin, China

Igneous sills are widely distributed in sedimentary basins worldwide. Sill emplacement has significant impacts on structural and thermal evolutions of sedimentary basins and also provides important insights into volcanic and sub-volcanic processes. Dyke deflection and sill formation are largely controlled by mechanical layering of the host rock, and a number of mechanisms have been proposed. However, most models of sill formation are based much on numerical and analogue modeling. For many natural sills, the key mechanism governing the dyke deflection and sill formation remains to be discussed and, in some cases, controversial. To better understand the control of mechanical layering on sill formation and test the existing models, we study detailed geometries of igneous sills from the central part of the Tarim Basin, China, based on seismic reflection data and borehole data. Nineteen igneous sills which are expressed as packages of high-amplitude reflections are observed, and they were all emplaced in the Upper Ordovician strata at current burial depths of about 5–8 km. The sills can be classified into three geometric types: type 1, saucer-shaped sills; type 2, strata-concordant sills; and type 3, hybrid sills, which have the characteristics of both type 1 and 2. The geometry of the saucer-shaped sills is typical and comprises three distinct parts: a flat inner sill, inclined sheets and, in many cases, flat outer sills. All the bases of the saucer-shaped sills coincide with the disconformity between the Middle and Upper Ordovician strata (the M disconformity), indicating that the disconformity controlled the sill geometry and emplacement depth in the basin. The M disconformity is characterized by a lithological change from underlying limestones to overlying mudstones. We suggest that among the three main mechanisms for the deflection of dykes into sills, namely Cook–Gordon debonding, stress barriers and elastic mismatch, the Cook–Gordon debonding is the dominant mechanism in the study area. This is a mechanism by which a weak contact opens up ahead of a vertically propagating dyke, allowing the dyke to be deflected into a sill when the dyke meets the open contact. This study indicates that the control of mechanical layering of the host rock on sill formation is largely depends on the host-rock lithologies and confirms again that the saucer shape is the fundamental sill geometry under the control of layering.

Shallow magma diversions during explosive diatreme-forming eruptions

The diversion of magma is an important mechanism that may lead to the relocation of a volcanic vent. Magma diversion is known to occur during explosive volcanic eruptions generating subterranean excavation and remobilization of country and volcanic rocks. However, feedbacks between explosive crater formation and intrusion processes have not been considered previously, despite their importance for understanding evolving hazards during volcanic eruptions. Here, we apply numerical modeling to test the impacts of excavation and subsequent infilling of diatreme structures on stress states and intrusion geometries during the formation of maar–diatreme complexes. Explosive excavation and infilling of diatremes affects local stress states which inhibits magma ascent and drives lateral diversion at various depths, which are expected to promote intra-diatreme explosions, host rock mixing, and vent migration. Our models demonstrate novel mechanisms explaining the generation of saucer-shaped sills, linked with magma diversion and enhanced intra-diatreme explosive fragmentation during maar-diatreme volcanism. Similar mechanisms will occur at other volcanic vents producing crater-forming eruptions.

Unravelling intrusion-induced forced fold kinematics and ground deformation using 3D seismic reflection data

Sills emplaced at shallow-levels are commonly accommodated by overburden uplift, producing forced folds. We examine ancient forced folds developed above saucer-shaped sills using 3D seismic reflection data from the Canterbury Basin, offshore SE New Zealand. Seismic-stratigraphic relationships indicate sill emplacement occurred incrementally over ~31 Myr between the Oligocene (~35–32 Ma) and Early Pliocene (~5–4 Ma). Two folds display flat-topped geometries and amplitudes that decrease upwards, conforming to expected models of forced fold growth. Conversely, two folds display amplitudes that locally increase upwards, coincident with a transition from flat-topped to dome-shaped morphologies and an across-fold thickening of strata. We suggest these discrepancies between observed and expected forced fold geometry reflect uplift and subsidence cycles driven by sill inflation and deflation. Unravelling these forced fold kinematic histories shows complex intrusion geometries can produce relatively simple ground deformation patterns, with magma transgression corresponds to localisation of uplift.

Structure and dynamics of surface uplift induced by incremental sill emplacement

Shallow-level sill emplacement can uplift Earth’s surface via forced folding, providing insight into the location and size of potential volcanic eruptions. Linking the structure and dynamics of ground deformation to sill intrusion is thus critical in volcanic hazard assessment. This is challenging, however, because (1) active intrusions cannot be directly observed, meaning that we rely on transient host-rock deformation patterns to model their structure; and (2) where ancient sill-fold structure can be observed, magmatism and deformation has long since ceased. To address this problem, we combine structural and dynamic analyses of the Alu dome, Ethiopia, a 3.5-km-long, 346-m-high, elliptical dome of outward-dipping, tilted lava flows cross-cut by a series of normal faults. Vents distributed around Alu feed lava flows of different ages that radiate out from or deflect around its periphery. These observations, coupled with the absence of bounding faults or a central vent, imply that Alu is not a horst or a volcano, as previously thought, but is instead a forced fold. Interferometric synthetic aperture radar data captured a dynamic growth phase of Alu during a nearby eruption in A.D. 2008, with periods of uplift and subsidence previously attributed to intrusion of a tabular sill at 1 km depth. To localize volcanism beyond its periphery, we contend that Alu is the first forced fold to be recognized to be developing above an incrementally emplaced saucer-shaped sill, as opposed to a tabular sill or laccolith.

Formation of lenticulae on Europa by saucer-shaped sills

Europa's surface contains numerous quasi-elliptical features called pits, domes, spots and small chaos. We propose that these features, collectively referred to as lenticulae, are the surface expression of saucer-shaped sills of liquid water in Europa's ice shell. In particular, the inclined sheets of water that surround a horizontal inner sill limit the lateral extent of intrusion, setting the lateral dimension of lenticulae. Furthermore, the inclined sheets disrupt the ice above the intrusion allowing the inner sill to thicken to produce the observed relief of lenticulae and to fracture the crust to form small chaos. Scaling relationships between sill depth and lateral extent imply that the hypothesized intrusions are, or were, 1–5 km below the surface. Liquid water is predicted to exist presently under pits and for a finite time under chaos and domes.