| 1. |
- bazargan, mohsen, et al.
(author)
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Stresses and displacements in layered rocks induced by inclined (cone) sheets
- 2020
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In: Journal of Volcanology and Geothermal Research. - : Elsevier BV. - 0377-0273. ; :106965
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Journal article (peer-reviewed)abstract
- Currently, the sheet-intrusion paths and geometries, including the sheet opening/thickness as well as the depth to sheet tip, are commonly determined from geodetic surface data using elastic dislocation models. These models assume the volcanic zone/central volcano to be an elastic half-space of uniform mechanical properties. Field observations, however, show that volcanic zones/volcanoes are composed of numerous layers whose mechanical properties (primarily Young's modulus) vary widely. Here we provide new numerical models on the effects of a typical variation in Young's modulus in an active volcanic zone/central volcano on the internal and surface stresses and displacements induced by a sheet-intrusion whose tip is arrested at a depth below the surface of 100 m. The sheet has a dip dimension (height) of 2 km. It's opening (thickness) depends on the magmatic overpressure, sheet dimension and host-rock Young's modulus. For the values used here, sheet thickness would be in the range of 0.5–1.4 m, similar to commonly measured sheet thicknesses in the field. The only loading is internal magmatic overpressure in the sheet of 5 MPa. The modelled crustal segment/volcano consists of 5 layers, all with the same Poisson's ratio (0.25). Each of the 4 uppermost layers is 10 m thick. Layer 1 (the top or surface layer) has Young's modulus of 3 GPa, layer 2 a modulus of 20 GPa, layer 3 a modulus of 30 GPa, and layer or unit 5 a modulus of 40 GPa. We vary Young's modulus or stiffness of the fourth layer from 10 GPa to 0.01 GPa, while the dip of the sheet takes the following values: 30°, 45°, 60° (for an inclined sheet) and 90° (for a dike). The resulting displacement and stresses are highly asymmetric across the sheet tip (except for the dike), with the main surface stresses and displacements being above the dipping sheet and highest for the 30°-dipping sheet. For comparison, three elastic half-space models of the same sheet configuration and loading but uniform Young's modulus in each model (40GPa, 20GPa, and 10 GPa) all yield much higher surface stresses and displacements than any of the layered models. As the stiffness of layer 4 decreases the surface stresses gradually decrease while changes in vertical displacements are comparatively small but greater in horizontal displacements. In particular, as the stiffness of layer 4 decreases from 10 GPa to 0.01 GPa, for the 30°-dipping sheet, the maximum surface shear stress decreases from about 6.6 MPa to 2.2 MPa and the maximum tensile stress from about 6.9 MPa to about 2.3 MPa. Thus, even a single comparatively thin (10 m) soft layer close to the surface of a central volcano/volcanic zone (where such layers are almost universal) may cause a great change in the maximum sheet-induced stresses at the surface and, thereby, in any sheet-induced fracture pattern. Furthermore, the stress peaks in the layered models do not coincide with the displacement peaks; fracture formation is most likely at the location of the stress peaks. The results have important implications for the correct interpretation of geodetic data and fracturing during unrest periods with magma-chamber rupture and sheet injection.
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| 2. |
- Greiner, Sonja H. M., et al.
(author)
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Interaction between propagating basaltic dikes and pre-existing fractures : A case study in hyaloclastite from Dyrfjoll, Iceland
- 2023
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In: Journal of Volcanology and Geothermal Research. - : Elsevier. - 0377-0273 .- 1872-6097. ; 442
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Journal article (peer-reviewed)abstract
- Magma in the Earth's crust is commonly transported through dikes. Fractures and faults, which are common in the shallow crust, form structural weaknesses that can act as energy-efficient propagation pathways. Although examples of this are known from active and extinct volcanoes in varying host rocks, the conditions and mechanisms of how and when dikes are influenced by these structures are not yet fully understood. This study investigates how basaltic dikes propagating through hyaloclastite in the shallow crust interact with pre-existing fractures. Using virtual 3D-models from drone-based photogrammetry, we mapped basaltic dikes exposed in a caldera-filling hyaloclastite in the extinct Dyrfjoll volcano, NE-Iceland, to measure the orientations of fractures and dikes, and quantify their interactions. We observe 39 changes in strike among 45 dikes and found a strong control of the governing stress field on orientations and interactions. Three types of dike-fracture interaction were identified: (1) Dikes propagating along pre-existing fractures. This is most frequently observed for dikes following the tectonic stress field. (2) Dikes with an abrupt change in strike occurring near or at a crosscutting fracture, but without magma flow into the fracture. (3) Dikes arrested at a crosscutting fracture. Such dikes may develop offshoots near the dike tip, which may approach the fracture at different angles and be able to cut across. Understanding how dikes interact with pre-existing fractures in moderately fractured host rock such as hyalo-clastite is relevant for hazard assessment and monitoring of volcanically active areas.
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| 3. |
- Heap, Michael J., et al.
(author)
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Imaging strain localisation in porous andesite using digital volume correlation
- 2020
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In: Journal of Volcanology and Geothermal Research. - : Elsevier BV. - 0377-0273. ; 404
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Journal article (peer-reviewed)abstract
- Strain localisation structures, such as shear fractures and compaction bands, are of importance due to their influence on permeability and therefore outgassing, a factor thought to influence eruptive style. In this study, we aim to develop a better understanding of strain localisation in porous volcanic rocks using X-ray tomographic images of samples of porous andesite (porosity = 0.26) acquired before and after deformation in the brittle and ductile regimes. These 3D images have been first analysed to provide 3D images of the porosity structure within the undeformed andesite, which consists of a large, well-connected porosity backbone alongside many smaller pores that are either isolated or connected to the porosity backbone by thin microstructural elements (e.g., microcracks). Following deformation, porosity profiles of the samples show localised dilation (porosity increase) and compaction (porosity reduction) within the samples deformed in the brittle and ductile regimes, respectively. Digital volume correlation (DVC) of the images before and after triaxial deformation was used to quantify the tensor strain fields, and the incremental divergence (volumetric strain) and curl (used as an indicator of shear strain) of the displacement fields were calculated from the DVC. These fields show that strain localisation in the sample deformed in the brittle regime manifested as a ~ 1 mm-wide, dilatational shear fracture oriented at an angle of 40–45° to the maximum principal stress. Pre- and post-deformation permeability measurements show that permeability of the sample deformed in the brittle regime increased from 3.9 × 10−12 to 4.9 × 10−12 m2, which is presumed to be related to the shear fracture. For the sample deformed in the ductile regime, strain localised into ~1 mm-thick, undulating compaction bands orientated sub-perpendicular to the maximum principal stress with little evidence of shear. Taken together, our data suggest that these bands formed during large stress drops seen in the mechanical data, within high-porosity zones within the sample, and within the large, well-connected porosity backbone. Pre- and post-deformation permeability measurements indicate that inelastic compaction decreased the permeability of the sample by a factor of ~3. The data of this study assist in the understanding of strain localisation in porous volcanic rocks, its influence on permeability (and therefore volcanic outgassing), and highlight an important role for DVC in studying strain localisation in volcanic materials.
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| 4. |
- Heap, Michael J., et al.
(author)
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The tensile strength of hydrothermally altered volcanic rocks
- 2022
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In: Journal of Volcanology and Geothermal Research. - : Elsevier. - 0377-0273 .- 1872-6097. ; 428
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Journal article (peer-reviewed)abstract
- The tensile strength of volcanic rocks is an important parameter for understanding and modelling a wide range of volcanic processes, and in the development of strategies designed to optimise energy production in volcanic geothermal reservoirs. However, despite the near-ubiquity of hydrothermal alteration at volcanic and geothermal systems, values of tensile strength for hydrothermally altered volcanic rocks are sparse. Here, we present an experimental study in which we measured the tensile strength of variably altered volcanic rocks. The alteration of these rocks, quantified as the weight percentage of secondary (alteration) minerals, varied from 6 to 62.8 wt%. Our data show that tensile strength decreases as a function of porosity, in agreement with previous studies, and as a function of alteration. We fit existing theoretical constitutive models to our data so that tensile strength can be estimated for a given porosity, and we provide a transformation of these models such that they are a function of alteration. However, because porosity and alteration influence each other, it is challenging to untangle their individual contributions to the measured reduction in tensile strength. Our new data and previously published data suggest that porosity exerts a first-order role on the tensile strength of volcanic rocks. Based on our data and observations, we also suggest that (1) alteration likely decreases tensile strength if associated with mineral dissolution, weak secondary minerals (such as clays), and an increase in microstructural heterogeneity and (2) alteration likely increases tensile strength if associated with pore- and crack-filling mineral precipitation. Therefore, we conclude that both alteration intensity and alteration type likely influence tensile strength. To highlight the implications of our findings, we provide discrete element method modelling which shows that, following the pressurisation of a dyke, the damage within weak hydrothermally altered host-rock is greater and more widespread than for strong hydrothermally altered host-rock. Because the rocks in volcanic and geothermal settings are likely to be altered, our results suggest that future modelling should consider the tensile strength of hydrothermally altered volcanic rocks.
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| 5. |
- Heap, Michael J., et al.
(author)
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The tensile strength of volcanic rocks : Experiments and models
- 2021
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In: Journal of Volcanology and Geothermal Research. - : Elsevier. - 0377-0273 .- 1872-6097. ; 418
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Journal article (peer-reviewed)abstract
- The tensile strength of volcanic rock exerts control over several key volcanic processes, including fragmentation and magma chamber rupture. Despite its importance, there is a paucity of laboratory data for the tensile strength of volcanic rocks, leading to an incomplete understanding of the influence of microstructural parameters, such as pore size and shape (factors that vary widely for volcanic rocks), on their tensile strength. To circumvent problems associated with the variability of natural samples, we provide here a systematic study in which we use elastic damage mechanics code "Rock Failure Process Analysis" to perform numerical experiments to better understand the influence of porosity, pore diameter, pore aspect ratio, and pore orientation on the tensile strength of volcanic rocks. We find that porosity and pore diameter exert a first-order control on the tensile strength of volcanic rocks, and that pore aspect ratio and orientation also influence tensile strength. Tensile strength is reduced by up to a factor of two as porosity is increased from 0.05 to 0.35 or as pore diameter is increased from 1 to 2 mm. Small, but systematic, reductions in tensile strength are observed as the angle between the loading direction and the major axis of an elliptical pore is increased from 0 to 90 degrees. The influence of pore aspect ratio (the ratio of the minor to major axis of an ellipse) depends on the pore angle: when the pore angle is 0 degrees, a decrease in pore aspect ratio, from 1 (a circle) to 0.2, increases the tensile strength, whereas the same decrease in pore aspect ratio does not substantially change the tensile strength when the pore angle is 90 degrees. These latter numerical experiments show that the tensile strength of volcanic rocks can be anisotropic. Our numerical data are in broad agreement with new and compiled experimental data for the tensile strength of volcanic rocks. One of the goals of this contribution is to provide better constrained constitutive models for the tensile strength of volcanic rocks for use in volcano modelling. To this end, we present a series of theoretical and semi-empirical constitutive models that can be used to determine the tensile strength of volcanic rocks, and highlight how tensile strength estimations can influence predictions of magma overpressures and assessments of the volume and radius of a magma chamber. (c) 2021 Elsevier B.V. All rights reserved.
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| 6. |
- Heap, Michael J., et al.
(author)
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The thermal properties of porous andesite
- 2020
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In: Journal of Volcanology and Geothermal Research. - : ELSEVIER. - 0377-0273 .- 1872-6097. ; 398
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Journal article (peer-reviewed)abstract
- The thermal properties of volcanic rocks are crucial to accurately model heat transfer in volcanoes and in geothermal systems located within volcanic deposits. Here we provide laboratory measurements of thermal conductivity and thermal diffusivity for variably porous andesites from Mt. Ruapehu (New Zealand) and variably altered basaltic-andesites from Merapi volcano (Indonesia) measured at ambient laboratory pressure and temperature using the transient hot-strip method. The specific heat capacity of each sample was then calculated using these measured values and the bulk sample density. Thermal conductivity and thermal diffusivity decrease as a function of increasing porosity, but specific heat capacity does not vary systematically with porosity. For a given porosity, saturation with water increases thermal conductivity and specific heat capacity, but decreases thermal diffusivity. Measurements on samples from Merapi volcano show that, compared to the unaltered samples from Mt. Ruapehu, hydrothermal alteration deceases thermal conductivity and thermal diffusivity, and increases specific heat capacity. We use an effective medium approach to parameterise these data, showing that when the porosity and pore-fluid properties are scaled for, the measured values agree well with theoretical predictions. We find that despite the microstructural complexity of the studied andesites, porosity is the principal parameter dictating their thermal properties. To understand whether the measured changes in thermal properties are sufficient to influence natural processes, we model heat transfer from magma to the surrounding host-rock by solving Fick's second law cast in 1D Cartesian (dyke geometry) and cylindrical (conduit geometry) coordinates. We provide models for different host-rock porosities (0-0.6), different initial magmatic temperatures (800-1200 degrees C), and different levels of host-rock alteration. Our modelling shows how the cooling of a dyke and conduit is slowed by a higher host-rock porosity and by increased hydrothermal alteration. The thermal properties provided herein can help improve modelling designed to inform on volcanic and geothermal processes. (C) 2020 Elsevier B.V. All rights reserved.
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| 7. |
- Juncu, Daniel, et al.
(author)
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Injection-induced surface deformation and seismicity at the Hellisheidi geothermal field, Iceland
- 2020
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In: Journal of Volcanology and Geothermal Research. - : Elsevier BV. - 0377-0273 .- 1872-6097. ; 391
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Journal article (peer-reviewed)abstract
- Induced seismicity is often associated with fluid injection but only rarely linked to surface deformation. At the Hellisheidi geothermal power plant in south-west Iceland we observe up to 2 cm of surface displacements during 2011–2012, indicating expansion of the crust. The displacements occurred at the same time as a strong increase in seismicity was detected and coincide with the initial phase of geothermal wastewater reinjection at Hellisheidi. Reinjection started on September 1, 2011 with a flow rate of around 500 kg/s. Micro-seismicity increased immediately in the area north of the injection sites, with the largest seismic events in the sequence being two M4 earthquakes on October 15, 2011. Semi-continuous GPS sites installed on October 15 and 17, and on November 2, 2011 reveal a transient signal which indicates that most of the deformation occurred in the first months after the start of the injection. The surface deformation is evident in ascending TerraSAR-X data covering June 2011 to May 2012 as well. We use an inverse modeling approach and simulate both the InSAR and GPS data to find the most plausible cause of the deformation signal, investigating how surface deformation, seismicity and fluid injection may be connected to each other. We argue that fluid injection caused an increase in pore pressure which resulted in increased seismicity and fault slip. Both pore pressure increase and fault slip contribute to the surface deformation.
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| 8. |
- Jägerup, S. B., et al.
(author)
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Silicic frothy xenoliths (xeno-pumice) in recent volcanics from Gran Canaria, Canary Islands
- 2023
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In: Journal of Volcanology and Geothermal Research. - : Elsevier BV. - 0377-0273 .- 1872-6097. ; 440
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Journal article (peer-reviewed)abstract
- The Quaternary small-volume alkaline magmatic episode on Gran Canaria erupted dominantly basanite and nephelinite lavas and scoria deposits that contain a range of mantle and crustal xenoliths. These xenoliths comprise peridotite nodules, partially melted plutonic and volcanic rock fragments, and a group of light colored, felsic, and commonly frothy quartz-bearing rock fragments (xeno-pumice) that show evidence for intense interaction with their host magmas. Here we study a selection of these felsic and, in part, glassy and vesicular xenoliths from North and North-East Gran Canaria, with the aim to unravel their ultimate origin and learn more about magma storage and ascent within and below the island. Inspection of textures, mineral assemblages and glass compositions reveal one group of felsic xenoliths with fresh to partly altered igneous phenocryst assemblages and relict magmatic textures in addition to 818O values of 3.6 to 6.6%o. This group is interpreted to be of igneous origin. A second group of frothy felsic xenoliths displays mineralogy and textural characteristics more similar to sedimentary rocks with frequent occurrence of quartz, a mineral usually not present as phenocrysts in magmatic rocks from the Canary Islands. This second group displays relatively high 818O values (8.1 to 16.8%o), more typical for sedimentary lithologies, and is thus interpreted to represent material derived from the extensive pre-island sedimentary part of the ocean crust. The investigated xenoliths from North Gran Canaria thus provide a snapshot of pre-island sedimentary geology as well as the island's "magmatic" interior. These new data help constrain the available subsurface compositional variations within and below the Canary Islands and will hence be useful in interpreting magma evolution trends and magma storage levels.
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| 9. |
- Kim, Doyeon, et al.
(author)
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Magma "bright spots" mapped beneath Krafla, Iceland, using RVSP imaging of reflected waves from microearthquakes
- 2020
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In: Journal of Volcanology and Geothermal Research. - : ELSEVIER. - 0377-0273 .- 1872-6097. ; 391
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Journal article (peer-reviewed)abstract
- The geometry and distribution of magma in the crust remain controversial topics with recent studies questioning the role of large magma chambers. In this investigation, high-resolution 3D reflection images of crustal discontinuities beneath the Krafla geothermal field in northern Iceland were generated by applying Vertical Seismic Profiling (VSP) techniques adapted from reflection seismology to microearthquake data. Exceptionally large amplitude reflections (bright spots) at a depth of 2.1 km correlate with rhyolitic magma encountered in the IDDP-1 borehole. Although similarly bright reflectors at about 4 km correspond in depth to the top of an inferred magma chamber from previous seismic studies, the scattered reflectivity that persists beneath this deeper reflector argues for a distributed magma system rather than a large feeder chamber.
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| 10. |
- Pfeffer, M. A., et al.
(author)
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SO 2 emission rates and incorporation into the air pollution dispersion forecast during the 2021 eruption of Fagradalsfjall, Iceland
- 2024
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In: Journal of Volcanology and Geothermal Research. - 0377-0273. ; 449
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Journal article (peer-reviewed)abstract
- During the low-effusion rate Fagradalsfjall eruption (19 March – 18 September 2021), the emission of sulfur dioxide (SO2) was frequently measured using ground-based UV spectrometers. The total SO2 emitted during the entire eruption was 970 ± 540 kt, which is only about 6% of the SO2 emitted during the similar length Holuhraun eruption (2014–2015). The eruption was divided into five phases based on visual observations, including the number of active vents and the occurrence of lava fountaining. The SO2 emission rate ranged from 44 ± 19 kg/s in Phase 2 to 85 ± 29 kg/s in Phase 5, with an average of 64 ± 34 kg/s for the entire eruption. There was notable variability in SO2 on short timescales, with measurements on 11 August 2021 ranging from 17 to 78 kg/s. SO2 flux measurements were made using scanning DOAS instruments located at different distances from and orientations relative to the eruption site augmented by traverses. Four hundred and forty-four scan and traverse measurements met quality criteria and were used, along with plume height and meteorological data, to calculate SO2 fluxes while accounting for wind-related uncertainties. A tendency for stronger SO2 flux concurrent with higher amplitude seismic tremor and the occurrence of lava fountaining was observed during Phases 4 and 5 which were characterized by intermittent crater activity including observable effusion of lava and gas release interspersed with long repose times. This tendency was used to refine the calculation of the amount of SO2 emitted during variably vigorous activity. The continuous seismic tremor time series was used to quantify how long during these eruption phases strong/weak activity was exhibited to improve the calculated SO2 flux during these Phases. The total SO2 emissions derived from field measurements align closely with results obtained by combining melt inclusion and groundmass glass analyses with lava effusion rate measurements (910 ± 230 kt SO2). Specifically, utilizing the maximum S content found in evolved melt inclusions and the least remaining S content in accompanying quenched groundmasses provides an identical result between field measurements and the petrological calculations. This suggests that the maximum SO2 release calculated from petrological estimates should be preferentially used to initialize gas dispersion models for basaltic eruptions when other measurements are lacking. During the eruption, the CALPUFF dispersion model was used to forecast ground-level exposure to SO2. The SO2 emission rates measured by DOAS were used as input for the dispersion model, with updates made when a significant change was measured. A detailed analysis of one mid-distance station over the entire eruption shows that the model performed very well at predicting the presence of volcanic SO2 when it was measured. However, it frequently predicted the presence of SO2 that was not measured and the concentrations forecasted had no correlation with the concentrations measured. Various approaches to improve the model forecast were tested, including updating plume height and SO2 flux source terms based on measurements. These approaches did not unambiguously improve the model performance but suggest that improvements might be achieved in more-polluted conditions.
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| 11. |
- Riley, Teal R., et al.
(author)
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Cretaceous arc volcanism of Palmer Land, Antarctic Peninsula : Zircon U-Pb geochronology, geochemistry, distribution and field relationships
- 2020
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In: Journal of Volcanology and Geothermal Research. - : Elsevier BV. - 0377-0273 .- 1872-6097. ; 401
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Journal article (peer-reviewed)abstract
- The Cretaceous marks an episode of enhanced magmatism, sedimentation and tectonic processes along the entire proto-Pacific convergent margin of West Gondwana. Cretaceous magmatism across the Antarctic Peninsula is dominated by the intrusive Lassiter Coast intrusive suite which developed as episodic ‘flare-up’ events during the mid-Cretaceous, at a time of increased convergence. Volcanic rocks of this age are poorly defined, as a consequence of limited field observations and an absence of accurate geochronology. Recent field mapping, combined with unpublished field observations has identified a region >10,000 km2 of dominantly subaerial rhyolitic pyroclastic and epiclastic successions from northern Palmer Land of the Antarctic Peninsula. Volcanic successions up to 1500 m in thickness consist of dominantly silicic ignimbrites, lavas, heterolithic breccias and lahar deposits, fed by caldera-forming eruptions. The volcanic rocks of this region were widely considered to be Early Cretaceous in age based on field relationships and early geochronology. New U-Pb zircon ages identify three distinct volcanic episodes during the Late Cretaceous/Early Cenozoic at ~108 Ma, ~93 Ma and ~64 Ma. Lu-Hf data indicate significant proportions of ancient crust in the petrogenesis of the volcanic rocks and the slightly negative εHf values are consistent with a mid-position (~120 km) within the magmatic arc, relative to more juvenile compositions close to the trench.
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