Magmatic Sills Research Articles

The importance of sill thickness and timing of sill emplacement on hydrocarbon maturation

Magmatic sills in sedimentary basins can be relatively easy to interpret on seismic data because they stand out due to their high acoustic impedance in relation to the sediments. However, determining sill thickness is challenging unless the sills are drilled. This study explores the influence of magmatic sill thickness and timing of emplacement on temperature and hydrocarbon maturation in sedimentary basins. A 230 km long 2D transect through the Vøring Basin in the Norwegian Sea is modeled and sill thickness and timing of emplacement are the only parameters varied. The transect holds ∼40 sills that intruded at ∼55 Ma with a temperature of 1000 °C in a shale-dominated sequence. Several sill thickness scenarios were tested, but results for 0 m (no sills), 50 m and 100 m are presented here. Furthermore, the 50 m and 100 m thick sills were tested to intrude as upper and lower clusters separated by time intervals of 10 and 100 kyr. To study the effect of sills below seismic resolution, 1D modeling of well 7316/5-1 in the Barents Sea was performed. This well contains 9 sills distributed in an upper and a lower group. Vitrinite reflectance data from the well allow studying the effect of thin sills on temperature and maturation. The results show a clear connection between sill thickness and the temperature and maturation history of the basin. The largest impact of sill intrusions is at 3–5 km depth. For sills intruding as clusters, there are regionally marginal differences related to timing of emplacement but local differences especially in the close vicinity of the sills occur. This work indicates that the interpretation of sill thickness has a substantial influence on the prediction of the maturation and petroleum potential of sedimentary basins holding magmatic intrusions. Sills below seismic resolution contribute to increased maturation in its surroundings. Differences in the timing of sill emplacement within 10–100 kyr show local effects that may influence the petroleum potential of source rocks between two sill complexes.

Magmatic-tectonic conditions for hydrothermal venting on an ultraslow-spread oceanic core complex

Research Article| July 26, 2017 Magmatic-tectonic conditions for hydrothermal venting on an ultraslow-spread oceanic core complex Jennifer L. Harding; Jennifer L. Harding 1Institute for Geophysics, Jackson School of Geosciences, University of Texas at Austin, J.J. Pickle Research Campus, 10100 Burnet Road, Building 196, Austin, Texas 78758, USA Search for other works by this author on: GSW Google Scholar Harm J.A. Van Avendonk; Harm J.A. Van Avendonk 1Institute for Geophysics, Jackson School of Geosciences, University of Texas at Austin, J.J. Pickle Research Campus, 10100 Burnet Road, Building 196, Austin, Texas 78758, USA Search for other works by this author on: GSW Google Scholar Nicholas W. Hayman; Nicholas W. Hayman 1Institute for Geophysics, Jackson School of Geosciences, University of Texas at Austin, J.J. Pickle Research Campus, 10100 Burnet Road, Building 196, Austin, Texas 78758, USA Search for other works by this author on: GSW Google Scholar Ingo Grevemeyer; Ingo Grevemeyer 2GEOMAR Helmholtz Centre of Ocean Research, Wischhofstr. 1-3, D-24148 Kiel, Germany Search for other works by this author on: GSW Google Scholar Christine Peirce; Christine Peirce 3Department of Earth Sciences, University of Durham, South Road, Durham DH1 3LE, UK Search for other works by this author on: GSW Google Scholar Anke Dannowski Anke Dannowski 2GEOMAR Helmholtz Centre of Ocean Research, Wischhofstr. 1-3, D-24148 Kiel, Germany Search for other works by this author on: GSW Google Scholar Author and Article Information Jennifer L. Harding 1Institute for Geophysics, Jackson School of Geosciences, University of Texas at Austin, J.J. Pickle Research Campus, 10100 Burnet Road, Building 196, Austin, Texas 78758, USA Harm J.A. Van Avendonk 1Institute for Geophysics, Jackson School of Geosciences, University of Texas at Austin, J.J. Pickle Research Campus, 10100 Burnet Road, Building 196, Austin, Texas 78758, USA Nicholas W. Hayman 1Institute for Geophysics, Jackson School of Geosciences, University of Texas at Austin, J.J. Pickle Research Campus, 10100 Burnet Road, Building 196, Austin, Texas 78758, USA Ingo Grevemeyer 2GEOMAR Helmholtz Centre of Ocean Research, Wischhofstr. 1-3, D-24148 Kiel, Germany Christine Peirce 3Department of Earth Sciences, University of Durham, South Road, Durham DH1 3LE, UK Anke Dannowski 2GEOMAR Helmholtz Centre of Ocean Research, Wischhofstr. 1-3, D-24148 Kiel, Germany Publisher: Geological Society of America Received: 10 Feb 2017 Revision Received: 21 Apr 2017 Accepted: 26 May 2017 First Online: 26 Jul 2017 Online Issn: 1943-2682 Print Issn: 0091-7613 © 2017 Geological Society of America Geology (2017) 45 (9): 839–842. https://doi.org/10.1130/G39045.1 Article history Received: 10 Feb 2017 Revision Received: 21 Apr 2017 Accepted: 26 May 2017 First Online: 26 Jul 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Jennifer L. Harding, Harm J.A. Van Avendonk, Nicholas W. Hayman, Ingo Grevemeyer, Christine Peirce, Anke Dannowski; Magmatic-tectonic conditions for hydrothermal venting on an ultraslow-spread oceanic core complex. Geology 2017;; 45 (9): 839–842. doi: https://doi.org/10.1130/G39045.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Hydrothermal venting, an important cooling mechanism of the Earth, supports a diverse array of seafloor and sub-seafloor ecosystems that are sustained by large thermal and chemical fluxes. Vents have been found along even the slowest and coldest spreading centers, calling into question the driving heat source for these vents. The ultraslow-spreading Mid-Cayman Spreading Center in the Caribbean Sea, which hosts the axial-flank Von Damm Vent Field (VDVF), provides an opportunity to probe the mechanisms for venting at ultraslow spreading rates. Using active-source seismic data from the 2015 CaySeis (Cayman Seismic) experiment, we determined the seismic velocities in the large massif beneath the VDVF. We propose that this massif was produced by a pulse of on-axis magmatism at ca.2 Ma, which was then followed by exhumation, cooling, and fracturing. A low seismic velocity anomaly 5 km below the VDVF is evidence for either a cracking front mining lithospheric heat or intrusive magmatic sills, both of which could drive ongoing deep hydrothermal fluid circulation. We conclude that the transient magmatism and variable crustal thickness at ultraslow-spreading centers create conditions for long-lived hydrothermal venting that may be widespread, and other VDVF-like vents may be common in these areas. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Crustal structure and evolution of the Arctic Caledonides: Results from controlled-source seismology

The continuation of the Caledonides into the Barents Sea has long been a subject of discussion, and two major orientations of the Caledonian deformation fronts have been suggested: NNW-SSE striking and NE-SW striking. A regional NW-SE oriented ocean bottom seismic profile across the western Barents Sea was acquired in 2014. In this paper we map the crust and upper mantle structure along this profile in order to discriminate between different interpretations of Caledonian structural trends and orientation of rift basins in the western Barents Sea. Modeling of P-wave travel times has been done using a ray-tracing method, and combined with gravity modeling. The results show high P-wave velocities (4km/s) close to the seafloor, as well as localized sub-horizontal high velocity zones (6.0km/s and 6.9km/s) at shallow depths which are interpreted as magmatic sills. Refractions from the top of the crystalline basement together with reflections from the Moho give basement velocities from 6.0km/s at the top to 6.7km/s at the base of the crust. P-wave travel time modeling of the OBS profile indicate an eastwards increase in velocities from 6.4km/s to 6.7km/s at the base of the crystalline crust, and the western part of the profile is characterized by a higher seismic reflectivity than the eastern part. This change in seismic character is consistent with observations from vintage reflection seismic data and is interpreted as a Caledonian suture extending through the Barents Sea, separating Barentsia and Baltica. Local deepening of Moho (from 27km to 33km depth) creates “root structures” that can be linked to the Caledonian compressional deformation or a suture zone imprinted in the lower crust. Our model supports a separate NE-SW Caledonian trend extending into the central Barents Sea, branching off from the northerly trending Svalbard Caledonides, implying the existence of Barentsia as an independent microcontinent between Laurentia and Baltica.

Forced folding and complex overburden deformation associated with magmatic intrusion in the Vøring Basin, offshore Norway

In this study, three-dimensional seismic reflection and borehole data from the Vøring Basin, offshore Norway have been used to characterize a supra-sill related forced fold to understand its evolution and relevance in the context of regional tectonics. Magmatic sills were recognised to be positive high-amplitude anomalies with similar polarity to the seabed reflection. The seismic dataset reveals two groups of sills in the study area comprising interconnected sills beneath the regional forced fold, and those intruded into the overburden. Magmatic sills forming the interconnected sill complex are emplaced at a depth of about 5.5s TWTT below the modern seafloor. Aspect ratio (length/width), A for the sills ranges from 1.63–6.90. The regional forced fold is interpreted based on its bathymetric and seismic-stratigraphic expression on horizon H7, which is part of the Palaeocene to Eocene Tang Formation. Amplitude of the accommodation fold is about 780km2. Hydrothermal vent complexes and fluid-flow conduits in the study area develop above the sill edges and on the flanks of the interconnected sill complex extending from the lower part of the Tang Formation to the uppermost section of the Brygge Formation evidencing vertically focussed fluid flow in the study area. The overlying overburden is in turn deformed and structurally compartmentalized through forced folding and Late Cenozoic tectonics. We demonstrate that accommodation folding is formed in response to the emplacement of several interconnected sills during the opening of the Norwegian-Greenland Seas. Sill emplacement in the study area causes uplift of the Cretaceous to Palaeocene depocentre prior to further restructuration during Cenozoic tectonic inversion. Magmatic intrusions documented in this study have wider implications for understanding supra-sill deformations along volcanic margins with well-developed emplaced sills at depth and likewise hydrocarbon prospectivity in the study area.

Seismic imaging of magma sills beneath an ultramafic-hosted hydrothermal system

Hydrothermal circulation at mid-ocean ridge volcanic segments extracts heat from crustal magma bodies. However, the heat source driving hydrothermal circulation in ultramafic outcrops, where mantle rocks are exhumed in low-magma-supply environments, has remained enigmatic. Here we use a three-dimensional P -wave velocity model derived from active-source wide-angle refraction-reflection ocean bottom seismometer data and pre-stack depth-migrated images derived from multichannel seismic reflection data to investigate the internal structure of the Rainbow ultramafic massif, which is located in a non-transform discontinuity of the Mid-Atlantic Ridge. Seismic imaging reveals that the ultramafic rocks composing the Rainbow massif have been intruded by a large number of magmatic sills, distributed throughout the massif at depths of ∼2–10 km. These sills, which appear to be at varying stages of crystallization, can supply the heat needed to drive high-temperature hydrothermal circulation, and thus provide an explanation for the hydrothermal discharge observed in this ultramafic setting. Our results demonstrate that high-temperature hydrothermal systems can be driven by heat from deep-sourced magma even in exhumed ultramafic lithosphere with very low magma supply.

Waveform modeling of the seismic response of a mid-ocean ridge axial melt sill

Seismic reflections from axial magma lens (AML) are commonly observed along many mid-ocean ridges, and are thought to arise from the negative impedance contrast between a solid, high-speed lid and the underlying low-speed, molten or partially molten (mush) sill. The polarity of the AML reflection (P AML P) at vertical incidence and the amplitude vs offset (AVO) behavior of the AML reflections (e.g., P AML P and S-converted P AML S waves) are often used as a diagnostic tool for the nature of the low-speed sill. Time-domain finite difference calculations for two-dimensional laterally homogeneous models show some scenarios make the interpretation of melt content from partial-offset stacks of P- and S-waves difficult. Laterally heterogeneous model calculations indicate diffractions from the edges of the finite-width AML reducing the amplitude of the AML reflections. Rough seafloor and/or a rough AML surface can also greatly reduce the amplitude of peg-leg multiples because of scattering and destructive interference. Mid-crustal seismic reflection events are observed in the three-dimensional multi-channel seismic dataset acquired over the RIDGE-2000 Integrated Study Site at East Pacific Rise (EPR, cruise MGL0812). Modeling indicates that the mid-crustal seismic reflection reflections are unlikely to arise from peg-leg multiples of the AML reflections, P-to-S converted phases, or scattering due to rough topography, but could probably arise from deeper multiple magma sills. Our results support the identification of Marjanovic et al. (Nat Geosci 7(11):825–829, 2014) that a multi-level complex of melt lenses is present beneath the axis of the EPR.

Allan R. Sanford (1927–2016)

Allan (Al) R. Sanford, professor of geophysics, emeritus, at New Mexico Tech (NMT) passed away quietly at his home in Woodinville, Washington, on 9 August 2016. He was 89. Al was a pioneer in the recording and interpretation of microearthquake seismograms. For decades, he was the leading authority concerning seismicity across New Mexico and within the Rio Grande rift (RGR). He published numerous papers with graduate students and colleagues on the unique, high‐amplitude reflected phases seen on Socorro‐area microearthquake records. Al and his students eventually attributed these phases to the presence of an extensive midcrustal magmatic sill now known as the Socorro Magma Body (SMB). Figure 1. Allan R. Sanford (1927–2016) Al was born in Pasadena, California. He attended Pomona College, but then entered the Navy to train as a radar specialist during the final 18 months of World War II. Back at Pomona following the war, he completed a B.S. degree in physics in 1949. He then spent time as the “computer” on a seismic crew prospecting for oil across the western United States. This experience led him to graduate school at California Institute of Technology where he gained knowledge in seismology while working for C. F. Richter and he completed his doctorate in geophysics under the guidance of C. H. Dix in 1958. Al was employed as a professor at NMT in Socorro from 1957 to 1997 where he taught courses in exploration geophysics and earthquake seismology, and supervised the research projects of about 50 masters students and about one dozen doctoral students. He joined Seismological Society of …

Three‐dimensional attenuation model of Sierra Negra Volcano, Galápagos Archipelago

AbstractThe shallow magma system beneath Sierra Negra was imaged using attenuation tomographic methods. The t∗ spectral decay method for P wave phases was used to highlight regions of high which suggest the presence of magma melt. High‐ anomalies ranging from 0.005 to 0.04 are concentrated below the caldera from 0.5 to 10.5 km depths. Attenuation is sensitive to temperature and fluid presence; thus, this high attenuation is interpreted as possible zones of magma accumulation. An imaged shallow body is consistent with geodetic studies on caldera deformation that modeled a magma sill or flattopped diapir of unknown thickness at ∼1 km depth below sea level.

Crustal structure of the Newer Volcanics Province, SE Australia, from ambient noise tomography

Intraplate volcanism is a widespread phenomenon, and is generally regarded to be independent of plate tectonics. The Newer Volcanics Province (NVP) of SE Australia represents the most recent (and arguably still active) intraplate volcanism on the Australian continent, and has been postulated to originate from the combined effect of localized shear flow and edge-driven convection. In this study, we adopt ambient noise tomography and Monte-Carlo inversion methods to construct the first local-scale 3D crustal Vs model of the NVP region with a resolution of ~35km. The model displays distinct crustal velocity features near the eastern and western margins of the NVP, which may point to the existence of a lithosphere-scale plumbing system for the migration of melt associated with the boundary between the Delamerian Orogen and the Lachlan Orogen, which underlies the NVP. In particular, exceptionally high velocities are observed in the middle crust of the Delamerian Orogen, and are best explained by buried magmatic arcs. This interpretation is consistent with a subduction-accretion origin for the Delamerian Orogen. Trans-lithospheric faults might have developed during the accretion processes, providing possible pathways for the migration of NVP magmas to the crust and surface. Our 3D model also images small localized velocity reductions in the lower crust at the region where the two distinct lithospheric units meet. The low velocity zone is spatially correlated with the top of a prominent lithosphere-scale low-resistivity zone (10–30Ωm), which we interpret to represent intruded magmatic sills with small proportions of melt remaining.

Gas desorption index of drill cuttings affected by magmatic sills for predicting outbursts in coal seams

Magma intrusion events accelerate the thermal evolution of coal seams, which could change their adsorption-desorption characteristics and increase the outburst risk. To study the effects on the coal seam desorption index of drill cuttings under the magmatic sill, we compared and analyzed multiple physical parameter variations and desorption characteristics of the no. 10 coal seam in the normal and thermal evolution zone in the Haizi and Yangliu mines under a thick sill. Lab simulation, theoretical analysis, and field validation were used for determination of the coal seam gas desorption index. The results indicated that the metamorphic grade of the no. 10 coal seam is increased under the sill, and the coal sample desorption rate and desorption quantity were enhanced by the sill’s heat effect, which could also make the coal seam gas desorption index of the drill cutting indexes Δh2 and K1 based on desorption characteristics increase, in addition to increasing the coal risk. The influence on gas desorption indexes Δh2 and K1 could be reflected by the value D which shows a proportional trend; the effect on the coal seam of the Haizi coal mine with a 120-m-thick sill is stronger than in the Yangliu mine with approximately 70 m. In addition, with the increase in the thickness of the sill, the gas desorption index becomes larger; the Δh2 of the no. 10 coal seam could have higher reliability than K1 in the normal and thermal evolution area. And the linear relationship between Δh2 and K1 will not change under different situations.

Imaging before an unimaginable eruption

Volcanology![Figure][1] Lake Toba, northern SumatraCREDIT: BARRY KUSUMA/GETTY IMAGES Volcanic super-eruptions eject thousands of times the volume of the largest documented eruptions over human history. Several “supervolcanoes” capable of this type of unimaginable devastation dot the surface of Earth today. Jaxybulatov et al. use seismic background noise to estimate the size and maturity of one of the world's largest volcanic reservoirs, the Toba caldera in northern Sumatra. Magma storage under the caldera occurs slowly over time, in the form of horizontal layers of magma injected into the crust. These magmatic sills are documented to 20 kilometers below the surface, but more sills could exist even deeper. The characterization of magma storage in large volcanic systems may help us to prepare for future volcanic super-eruptions. Science , this issue p. [617][2] [1]: pending:yes [2]: /lookup/volpage/346/617?iss=6209

Off-rift volcanism in rift zones determined by crustal unloading

When continents are stretched over a long period of time, deep elongated rift valleys form at Earth’s surface and zones of ponded magma, centred beneath the rift, form at the crust–mantle boundary1, 2. Ascending magma sometimes erupts within the rift valley3, 4 or, counterintuitively, at volcanic fields away from the rift valley that are offset by tens of kilometres from the source of magma at depth5, 6, 7, 8. The controls on the distribution of this off-rift volcanism are unclear. Here we use a numerical model of magmatic dyke propagation during rifting to investigate why some dykes reach the surface outside the rift valley, whereas others are confined to the valley. We find that the location of magmatism is governed by the competition between tectonic stretching and gravitational unloading pressure, caused by crustal thinning and faulting along the rift borders. When gravitational unloading dominates over tectonic stretching forces, dykes ascending from the ponded magma are steered towards the rift sides, eventually causing off-rift eruptions. Our model also predicts the formation of stacked magma sills in the lower crust above the magma-ponding zone, as well as the along-rift propagation of shallow dykes during rifting events, consistent with observations of magmatism and volcanism in rift zones globally. We conclude that rift topography-induced stress changes provide a fundamental control on the transfer of magma from depth to the surface.

Thermal effects of magmatic sills on coal seam metamorphism and gas occurrence

Igneous intrusions in coal seams are found in 80 % of coal mines in the Huaibei coalfield, China, and coal and gas outburst accidents have occurred 11 times under a 120-m-thick sill in the Haizi mining field. The magma’s heat had a significant controlling effect on coal seam gas occurrence. Based on theoretical analysis, experimental tests and site validation, we analyzed the temperature distribution following magma intrusion into coal measure strata and the variations in multiple physical parameters and adsorption/desorption characteristics between the underlying coal seams beneath the sill in the Haizi mining field and coal seams uninfluenced by magma intrusion in the adjacent Linhuan mining field. The research results show that the main factors controlling the temperature distribution of the magma and surrounding rocks in the cooling process include the cooling time and the thickness and initial temperature of the magmatic rock. As the distance from sill increases, the critical effective temperature and the duration of sustained high temperatures decrease. The sill in the Haizi mining field significantly promoted coal seam secondary hydrocarbon generation in the thermally affected area, which generated approximately 340 m3/t of hydrocarbon. In the magma-affected area, the metamorphic grade, micropore volume, amount of gas adsorption, initial speed of gas desorption, and amount of desorption all increase. Fluid entrapment by sills usually causes the gas pressure and gas content of the underlying coal seams to increase. As a result, the outburst risks from coal seams increases as well.

Contact metamorphism of organic-rich mudstones and carbon release around a magmatic sill in the Basque-Cantabrian Basin, western Pyrenees

Recent studies have documented the expulsion of methane and oil to the Albian paleoseabed in the Basque-Cantabrian Basin. They interpret that hydrocarbon generation and expulsion were triggered by seismically recorded magmatic intrusions which metamorphosed organic-rich host sediments. An outcrop within the basin was selected to investigate organic matter evolution and sediment degassing due to an igneous body. This intrusion is a 5m thick Late Albian basaltic sill that intruded mudstones of the Black Flysch Group, near Gorliz (north Iberia). Vitrinite and bitumen reflectance profiles and metamorphic mineral distribution in the overburden indicate that the sill produced a thermal effect that increases toward the intrusion, defining a 2m thick (minimum) contact aureole.Geochemical profiles of TOC, S1, S2, HI and PC show a gradual decrease toward the sill indicating organic carbon loss and increase in the thermal maturity of the organic matter in the same direction. Concordantly, gas chromatograms show a loss of n-alkanes and a predominance of the shorter chain length homologues adjacent to the sill. Tmax and PI (S1/S1+S2) values increase toward the sill which suggests an increase in the thermal stress and in the extent of kerogen pyrolysis, respectively.Organic carbon loss in the aureole was the result of carbon devolatilization and formation of CO2 and CH4 gases. The newly formed CO2 reacted with pore waters to precipitate 13C depleted carbonate minerals in the aureole and in sill fractures. CH4 escaped from the aureole via hydrofractures to the paleoseabed, where methane-derived authigenic carbonates were formed.

Density and lithospheric thickness of the Marius Hills shield volcano on the Moon

Marius Hills is a large volcanic complex on the Oceans Procellarum of the lunar nearside. Numerous volcanic features, including domes, cones, and rilles occur in this region. Due to limitations in lunar remote sensing data, most previous studies on this region focused on its morphological and geochemical properties but little was known about the subsurface and interior structures of this volcanic complex. Knowledge of the local crustal density is meaningful to both determine the composition for this volcanic complex and understand the crustal evolutionary history for this region. Constraining the lithospheric thickness in this region is helpful to estimate the heat flux at the time of volcanic loading, which is a crucial parameter to study the thermal evolutionary history of the nearside mantle. Here, we applied a localized gravity and topography admittance analysis for the Marius Hills region to constraint its crustal density and lithospheric thickness. The gravity filed is modeled using a thin elastic lithspheric model that considers both surface and subsurface loads. Localized admittance and correlations spectra are used to constrain these modeled parameters. The best-fit crustal density in our model is ~3040 kg m-3, which is much higher than the average lunar crustal density as 2550 kg m-3, indicating that magma chambers or sills has intruded to the shallow crust and/or that intrusive magma has filled up the porous subsurface crust. The total volume of basalts emplaced in the Marius Hills region is ~2.9×104 km3, suggesting that Marius Hills is a major volcanic center in the Oceanus Procellarum. The best-fit lithospheric thickness of this region is constrained to be as small as ~4 km, indicating that a significant amount of heat production elements have concentrated in this region during the formation of the volcanic complex. This result is consistent with the previous spectral studies that heat production elements (such as thorium) are more abundant in the Oceanus Procellarum.

Anatomy of a fluid pipe in the Norway Basin: Initiation, propagation and 3D shape

An exploration 3D seismic data set from the Gjallar Ridge off mid-Norway images a giant fluid seep structure, 3×5km wide, which connects to late Palaeocene magmatic sills at depth. Two of the pipes that have developed as hydrothermal vents reach all the way to the modern seafloor implying that they either were active much longer than the original hydrothermal activity or have been reactivated. We combine detailed seismic analysis of the northern pipe and sandbox modeling to constrain pipe initiation and propagation. Although both the seismic data and the sandbox models suggest that fluids at depth are focused through a vertical conduit, sandbox models show that fluids ascend and reach a critical depth migration where focused migration abruptly transforms into distributed fluid flow through unconsolidated sediments. This indicates that at this level the sediments are intensely deformed during pipe propagation, creating a V-shaped structure, i.e. an inverted cone at depth and a positive relief anomaly, 5 to 10m high, at the seafloor, which is clearly identified on 3D seismic data. Comparison of the geometries observed in sandbox modeling with the seismically observed geometries of the Giant Gjallar Vent suggests that the Giant Gjallar Vent may be a proto-fluid seep at an early stage of its development, preceding the future collapse of the structure forming a seafloor depression. Our results imply that the Gjallar Giant Vent can be used as a window into the geological processes active in the deep parts of the Vøring Basin.

Influence of different boiling points of pore water around an igneous sill on the thermal evolution of the contact aureole

A low-permeability magmatic sill is capable of acting as cap rock to isolate the connection of pore fluids between overlying and underlying host rocks and consequently elevates the fluid pressure of the underlying host rocks towards the lithostatic pressure. For shallow, buried igneous sills, this will enhance the boiling point ( T boil ) of and even cause the supercritical state of the pore water in the underlying host rocks during cooling of magma. By using an isolated diabase sill in Huimin Sag, Bohai Bay Basin, China as an example, this study presents an investigation on the effect of the different T boil (300 °C vs. 350 °C) or states (volatilization vs. the supercritical state) of pore water around the sill on the thermal evolution of the adjacent host rocks based on heat transfer models. Our results indicate: 1) Ignoring the difference in T boil between the underlying and overlying host rocks can cause the maximum deviation of ~ 20 °C in the predicted peak temperature ( T peak ) and of ~ 0.4% in the predicted vitrinite reflectance (VR r ) if the T boil below the sill is higher than that above the sill under real geological conditions; 2) Assuming the supercritical pressure for the pore water below the sill leads to the maximum deviation of more than 1.1% in VR r and of at least 50 °C in T peak , relative to the situation assuming the same T boil for all the host rocks; 3) The observation and analysis based on drill cores show that the T peak profile of the host rocks should be asymmetric to the horizontal central line of the sill. The thickness of the inner thermal alteration zone (hornfels zone) is somewhat larger below the sill than above the sill, whereas the outer thermal alteration zone (carbargilite zone) below the sill is obviously thicker than that above the sill. Only the predicted T peak profile by the model assuming the supercritical pressure for the pore water below the sill can match with such observation. This demonstrates that the assumption of the different states of pore water represents natural conditions. ► Ignoring the difference in T boil around a sill causes minor model deviation. ► Ignoring supercritical pressure below a sill causes obvious model deviation. ► Supercritical state causes much thicker thermal alteration zone below a sill. ► Core analysis shows capability of sills as cap rock to elevate pore pressure.

Reconciling Cretaceous paleomagnetic and marine magnetic data for Iberia: New Iberian paleomagnetic poles

The Cretaceous paleogeography and the kinematic evolution of the Iberian plate are poorly constrained. Especially problematic is to reconcile Iberian paleomagnetic data with paleomagnetic data of the neighboring plates and with Euler poles derived from seafloor magnetic anomalies. The first limitation arises from the Cretaceous Normal Polarity Superchron where paleogeographic reconstruction using marine magnetic anomalies is handicapped. The second arises from the paucity of reliable paleomagnetic poles with satisfactory statistical criteria and age. In order to address these shortcomings and provide new high quality paleomagnetic poles for Iberia, we conducted a detailed rock magnetic and paleomagnetic study of two Cretaceous magmatic sills, the Paço de Ilhas (PI) and Foz da Fonte (FF) sills, from the Lusitanian Basin, Portugal, recently dated at about 88 and 94 Ma, respectively. Our results show that the magnetic mineralogy of the sills is primary, i.e., acquired during magma cooling, and essentially represented by titanomagnetite. The corresponding paleomagnetic poles match the synthetic APWP from the African plate at 80 and 100 Ma. On the basis of a rigorous selection of Iberian Cretaceous poles, we then calculated mean paleomagnetic poles for different time intervals and found that Iberian paleomagnetic data fit well the global APWP between 70 and 120 Ma, but move far away from the APWP at pre‐rift times. Our approach shows that new and better constrained paleomagnetic poles can aide in solving part of the contradiction between Iberian and African APWPs.

The Yermak Plateau in the Arctic Ocean in the light of reflection seismic data-implication for its tectonic and sedimentary evolution

SUMMARY The separation of Northeast Greenland and Svalbard is the result of large-scale strike slip movements during Cenozoic times. Geological evidence for these movements can be found onshore both on North Greenland and Svalbard. However, the role of the submarine Yermak Plateau (YP) in this process is unclear. The compilation of available multichannel reflection and wide-angle seismic data give new insights into the sedimentary and crustal structure and evolution of the plateau. The flat surface of the present-day plateau is a quite young feature. Up to 2 km of Cenozoic sediments cover a rough basement, which show similarities to the rough topography and strike of geological structures of Spitsbergen Island. In some basins more than 4 km of sedimentary rocks could be mapped. The most pronounced structure is the Sverdrup Bank, which appears to be part of a larger crustal block. P-wave velocities of about 4.5 km s–1 derived from sonobuoy data indicate that its uppermost part is most probably composed of sedimentary or volcanic rocks. We have made a correlation of previously defined seismic units across the YP to outline the history of sediment deposition in the area. The existing graben structures on the plateau might have provided early shallow pathways for water exchange between the Arctic and the Atlantic Oceans. A chaotic sedimentary apron east of the Sverdrup Bank and bright reflections near the Mosby Seamount interpreted as magmatic sills suggest tectonic and magmatic events during the Miocene.

Palaeomagnetic results from Upper Triassic red-beds and CAMP lavas of the Argana Basin, Morocco

Abstract The continental Argana Basin of Morocco is the trans-Atlantic counterpart of the extensively studied Fundy, Hartford and Newark basins in north-eastern America, that have provided the astrochronologically tuned geomagnetic polarity timescale (GPTS) for the late Triassic and earliest Jurassic. The Argana red-bed successions also show astronomically driven time control, which allowed trans-Atlantic correlations and revealed that the interval towards volcanism of the Central Atlantic Magmatic Province (CAMP) is without any significant hiatuses. Here, we present palaeomagnetic results from the cyclically bedded upper Triassic red-beds and the intercalated volcanics associated with CAMP. Our composite Argana section comprises an interval of 3.5–4.0 Ma, but its magnetostratigraphic pattern does not allow a straightforward correlation to the Newark GPTS. The continental red-bed deposits of the Bigoudine Formation demonstrate a dominant magnetic overprint that could only be removed at temperatures above 600 °C. We suggest that this overprint could have been caused by a period of (Jurassic, c . 170 Ma) magmatism that caused pervasive overprinting of the Triassic palaeomagnetic signal. Correlations between the sections in the Tazantoute region are not straightforward, hampered by the presence of a magmatic sill. The CAMP lava sequences of Tazantoute are all of normal polarity and record secular variation in a manner that agrees with short-lived pulses of CAMP activity in Morocco. Our results indicate that the sedimentary successions of the Argana Basin have the potential to evaluate the Newark GPTS, but that detailed palaeomagnetic analyses of more suitable sections with long(er) cyclostratigraphic records are required.