Establishing genetic relationships between the Takidani pluton and two large silicic eruptions in the Northern Japan Alps

The Takidani pluton (1.1-1.6 Ma) represents a shallow magmatic reservoir at the base of an exhumed caldera floor. The deposits of two large caldera-forming eruptions including the Nyukawa Pyroclastic Flow Deposit (1.76 Ma; crystal-rich dacite) and the Chayano Tuff and Ebisutoge Pyroclastic Deposits (1.75 Ma; a sequence of crystal-poor rhyolite) are distributed concentrically around the pluton. We use major and trace element chemistry of whole-rock, glass and minerals to show (1) that the crystal-rich dacite (>400 km3 DRE; dense rock equivalent) is the erupted portion of a shallow mush zone constituting the Takidani pluton and (2) that the crystal-poor rhyolite (>100 km3 DRE) was extracted from a deeper part of this vertically extended magmatic plumbing system. Whole-rock geochemistry indicates that the Nyukawa and Takidani compositions were produced dominantly through crystal fractionation of amphibole, pyroxene and plagioclase in the mid-to-lower crust and subsequently emplaced in the upper crust prior to eruption and solidification, respectively. The crystal-poor Chayano-Ebisutoge rhyolite (>100 km3 DRE) is compositionally distinct from the Nyukawa and Takidani magmas and its generation is associated with a substantial contribution of crustal melts. Yet, plagioclase and orthopyroxene textures and chemistry provide strong evidence that the ascending rhyolite percolated through the upper Takidani-Nyukawa mush zone prior to eruption. Overgrowth of “rhyolitic plagioclase” on “xenocrystic dacitic plagioclase” typical of the Takidani-Nyukawa magmas indicates that the extraction and accumulation of the rhyolitic melts could have occurred in less than 10 kyr (i.e. time between eruptions) prior to eruption providing maximum timescales for pre-eruption storage. Overall, our findings show a progressive growth and thermal maturation of a vertically extended magmatic plumbing system over hundreds of thousands of years and imply that large volcanic eruptions can occur in relatively short succession without dramatic changes in the plumbing system, thus, complicating the identification of signs of an impending large eruption.

Pityusa Patera, Mars: Structural analyses suggest a mega-caldera above a magma chamber at the crust-mantle interface

Pityusa Patera is the southernmost of four paterae in the 1.2 × 106 km2 wrinkle-ridged plains-dominated Malea Planum region of Mars. Based on their texture, morphology, and uniqueness to Pityusa Patera, we interpret layered, folded massifs as pyroclastic deposits emplaced during patera formation as a collapse caldera. Such deposits would not be expected in a previously suggested scenario of patera formation by subsidence from lithospheric loading. Our structural measurements and modeling indicate that the folding and high relief of the massifs resulted from ~1.3%–6.9% of shortening, which we show to be a reasonable value for a central plug sagging down into an assumed piston-type caldera. According to a previously published axisymmetric finite-element model, the extent of shortening structures on a caldera floor relative to its total diameter is controlled by the roof depth of the collapsed magma chamber beneath it, which would imply Pityusa Patera formed above a chamber at 57.5–69 km depth. We interpret this value to indicate a magma chamber at the crust-mantle interface, which is in agreement with crust-penetrating ring fractures and mantle flows expected from the formation of the Hellas basin. As such, the folded massifs in Pityusa Patera, which are partially superposed by ca. 3.8 Ga wrinkle-ridged plains, should consist of primordial mantle material, a theory that might be assessed by future hyperspectral observations. In conclusion, we do not favor a formation by load-induced lithospheric subsidence but suggest Pityusa Patera to be one of the oldest extant volcanic landforms on Mars and one of the largest calderas in the solar system, which makes the folded, likely mantle-derived deposits on its floor a prime target for future exploration.

Microscopy Observation of Samosir Formation Paleosoil, Tuktuk Sidaong, North Sumatera, Indonesia.

Samosir is the islands that emerge and standing upon on Toba Caldera after it’s the last eruption at 74.000 years ago. Samosir Island known as the caldera floor that uplifts parallel with Toba’s caldera flooding. In this study, we have observed an outcrop in Tumutuk, Samosir Island that hypothesized as a lacustrine deposit, and we found a paleosoil layer that might give more answers about the geological process in this area at the past time. Based on this outcrop, we described it, followed to measure its stratigraphy section, and took representative samples from the paleosoil layer, then observed the samples under the stereo-microscope as polish rock section, in normal light & negative images. As the result we identify several features of paleosoil & its sedimentary grain that shown this paleosoil layer, two events of the volcanoclastic deposits flown, and exposed two-time, and forming soil, it may form in the shallow swamp in a lacustrine environment, coincide with caldera flooding and caldera floor uplift event.

Comment on “Estimating the depth and evolution of intrusions at resurgent calderas: Los Humeros (Mexico)” by Urbani et al. (2020)

A multiple shallow-seated magmatic intrusion model has been proposed by Urbani et al. (2020) for the resurgence of the Los Potreros caldera floor, in the Los Humeros volcanic complex (LHVC). This model predicts (1) the occurrence of localized bulges in the otherwise undeformed caldera floor, and (2) that the faults corresponding to different bulges exhibit different spatial and temporal evolution. Published data and a morphological analysis show that these two conditions are not met at Los Potreros caldera. A geothermal well (H4), located at the youngest supposed bulge (Loma Blanca) for which Urbani et al. (2020) calculated an intrusion depth (425±170 m), does not show any thermal and lithological evidence of such a shallow-seated cryptodome. Finally, published stratigraphic data and radiometric dating disprove the proposed common genesis of Holocene resurgence faulting and viscous lavas extruded in the centre of the caldera. Even if recent shallow intrusions do exist in the area, published data indicate that the pressurization of the LHVC magmatic–hydrothermal system driving resurgence faulting occurs at greater depth. Thus, we suggest that the model and calculation proposed by Urbani et al. (2020) are unlikely to have any relevance to the location, age and emplacement depth of magma intrusions driving resurgence at the Los Potreros caldera.

Comment on Estimating the depth and evolution of intrusions at resurgent calderas: Los Humeros (Mexico) by Urbani et al. (2020)

A multiple shallow–seated magmatic intrusions model has been proposed by Urbani et al. (2020) for the resurgence of the Los Potreros caldera floor, in the Los Humeros Volcanic Complex. This model predicts (1) the occurrence of localized bulges in the otherwise undeformed caldera floor, and (2) that the faults corresponding to different bulges exhibit different spatial and temporal evolution. Published data and a morphological analysis show that these two conditions are not met at Los Potreros caldera. A geothermal well (H4), located at the youngest supposed bulge (Loma Blanca) for which Urbani et al. (2020) calculated an intrusion depth (425±170 m), doesn’t show any thermal and lithological evidence of such a shallow–seated cryptodome. Finally, published stratigraphic data and radiometric dating disprove the proposed common genesis of Holocene resurgence faulting and viscous lavas extruded in the centre of the caldera. Even if recent shallow intrusions may exist in the area, published data indicate that the pressurization of the LHVC magmatic/hydrothermal system driving resurgence faulting occurs at greater depth. Thus, we suggest that the model and calculation proposed by Urbani et al. (2020) are unlikely to have any relevance to the location, age and emplacement depth of magma intrusions driving resurgence at the Los Potreros caldera.

Comment on Estimating the depth and evolution of intrusions at resurgent calderas: Los Humeros (Mexico) by Urbani et al. (2020)

A multiple magmatic intrusions model has been proposed by Urbani et al. (2020) for the resurgence of the Los Potreros caldera floor, in the Los Humeros Volcanic Complex. This model predicts (1) the occurrence of few localized bulges in the otherwise not deformed caldera floor, and (2) that the faults corresponding to different bulges exhibit different spatial and temporal evolution. Already available field data from easily accessible outcrops and a simple morphological analysis show that these two conditions are not met at Los Potreros caldera. Also, a geothermal well (H4), located in the most recent supposed bulge for which Urbani et al. (2020) calculated an intrusion depth (Loma Blanca, intrusion depth of 425 ± 170 m), doesn't show any thermal and lithological evidence of such a shallow cryptodome. Finally, already published stratigraphic data and radiometric dating apparently disprove the proposed correlation between extruded viscous lavas and faulting. Thus, even if recent shallow intrusions may exist in the area, Urbani et al. (2020) fails to provide any useful information on their occurrence, location, age, emplacement depth, role in the resurgence of the Los Potreros caldera floor, and influence on the structure of the Los Humeros geothermal field.

Paleomagnetic observations from lake sediments on Samosir Island, Toba caldera, Indonesia, and its late Pleistocene resurgence

Approximately 74 ka, Toba caldera in Sumatra, Indonesia, erupted in one of the most catastrophic supereruptions in Earth's history. Resurgent uplift of the caldera floor raised Samosir Island 700 m above Lake Toba, exposing valuable lake sediments. To constrain sediment chronology, we collected 173 discrete paleomagnetic 8 cm3 cubes and 15 radiocarbon samples from six sections across the island. Bulk organic 14C ages provide an initial chronostratigraphic framework ranging from ~12 to 46 ka. Natural and laboratory magnetizations were studied using alternating field demagnetization. A generally well-defined primary magnetization is isolated using principal component analysis. Comparison of inclination, and to a lesser degree declination, across independently dated sections suggests paleomagnetic secular variation (PSV) is recorded. Average inclination of −6° is more negative than a geocentric axial dipole would predict, but consistent with an eastward extension of the negative inclination anomaly observed in the western equatorial Pacific. The 14C- and PSV-derived age model constrains resurgent uplift, confirming faster uplift rates to the east and slower rates to the west, while suggesting that fault blocks moved differentially from each other within a generally trapdoor-type configuration.

Rapid resurgence of the subglacial Bárdarbunga caldera following collapse in 2014-2015, quantified with repeated Bouguer gravity surveys

<p>The 65 km<sup>2</sup> Bárdarbunga caldera is located in the NW part of the Vatnajökull glacier in central Iceland.  The caldera floor lies under 500-800 m thick ice and the rims are fully subglacial as well.  The caldera subsided by 65 m during the Bárdarbunga-Holuhraun eruption in 2014-2015, when about 2 km<sup>3</sup> of magma drained out from a magma reservoir at ~10 km depth leading to the largest eruption in Iceland since Laki in 1783.  Deformation surveys outside the caldera have indicated inflation since soon after the end of the eruption in February 2015 and seismicity has been elevated.  The extensive ice cover precludes conventional microgravity surveys or detailed surveys of caldera floor elevation.  However, we have studied gravity changes by comparing results of repeated Bouguer anomaly surveys.  We perform a full Bouguer correction using detailed DEMs of both the ice surface and the ice-radar-derived bedrock.  Ice surface changes are also mapped, allowing the removal of effects on gravity by ice mass changes.  Possible sources of significant anomalies are either changes in bedrock elevation between surveys, other more deep-seated mass changes beneath the volcano, or changes in the water table and pore pressure.  Surveys were carried out using a Scintrex CG-5 in 2015, 2016, 2018 and 2019, with measurements done at 25-50 locations each time.  As no benchmarks exist on the ice the spatial difference in station location of 10-20 m exists between survey years.   However, post-processing provides kinematic GPS position and elevation accuracy better than 0.1 m. Analysis of the data and error sources indicate an accuracy in estimates of changes of 50-100 µGal. The results obtained indicate change with an amplitude of a few hundred µGals; over the four years between 2015-2019 a clear Bouguer anomaly increase is recorded over the caldera relative to the surrounding area. Sharp gradients in the gravity difference near the caldera boundary point to a shallow source, consistent with the gravity signal arising from or near the ice-bedrock boundary.  This indicates fast resurgence at Bárdarbunga since 2015. The elevation of bed reflections delineated from radio echo sounding profiles (~2 MHz), measured within the caldera in June 2015 and accurately repeated in June 2019, further supports this.  The suggested deformation mechanisms can be compared to geodetic observations outside the caldera for further evaluation. If all the signal is interpreted in terms of magma movements, a rise of the caldera floor by several meters and the inflow of 0.2-0.3 km<sup>3</sup> of new magma is inferred.</p>

Doming and faulting processes driving ground deformation at Campi Flegrei caldera (southern Italy): a modeling for the last 6 ka

<p>We investigated the major episodes of dome growth in the Campi Flegrei caldera occurred during the last period of large eruptive activity (Epoch 3, between 5.5 and 3.5 ka), and in the historical time. The first doming event occurred at the start of Epoch 3  where the caldera floor raised for at least 100 m. Following the Plinian eruption of Agnano-Monte Spina (AMS, 4.55 ka), a new uplift phase occurred with the set up of several lava domes (e.g., Olibano, Accademia and Solfatara cryptodome), the Averno-Solfatara  (AVS, 4.3 ka) and Astroni (AST, 4.2 ka) eruptions. This unrest episode was accompanied by severe and widespread faulting and fracturing well recorded in the stratigraphic record (Vitale et al., 2019). Finally, the last episodes of doming occurred before the eruption of Monte Nuovo volcano (MN, 1538 CE) and in the last century (1950-1985 CE). The 1538 CE uplift reached a maximum vertical displacement of ca. 15 m, whereas the 1950-1985 events reached a total dislocation of ca. 4 m. In order to study the former ground deformation pattern, we reconstructed the top surface of the La Starza succession, the latter formed by marine-transitional sediments deposited between 15 and 5.5 ka deposited in large part of the caldera floor. We used information from onland well-logs and seismic profiles in the Gulf of Pozzuoli. The same approach was used for the top surface of the younger marine succession, called Pozzuoli Unit (PU) (Isaia et al., 2019), emplaced following the AMS eruption and predating the AVS eruption. Subtracting the historical deformation pattern and considering the sea-level change in that time frame, we observe that the center of vertical deformation was located, for both Top Starza and Top PU surfaces, close to the Cigliano vent, and therefore not coinciding with the 1538 CE and recent deformation center, both defined by the same deformation center located close to the town of Pozzuoli. The resulting surfaces well mark local deformations related to the activity of major faults and the minor caldera formed following the AMS Plinian eruption. The restoring of the deformation of major faults with the Okada’s fault model has furnished useful information about the amount of displacement and rates of the faults' activity in the last ca. 6 ka.</p><p>Isaia, R., Vitale, S., Marturano, A., Aiello, G., Barra, D., Ciarcia, S., Iannuzzi, E., Tramparulo, F.D.A., 2019. High-resolution geological investigations to reconstruct the long-term ground movements in the last 15 kyr at Campi Flegrei caldera (southern Italy). Journal of Volcanology and Geothermal Research, 385, 143-158. doi: 10.1016/j.jvolgeores.2019.07.012</p><p>Vitale, S., Isaia, R., Ciarcia, S., Di Giuseppe, M. G., Iannuzzi, E., Prinzi, E. P., Tramparulo, F.D’A., Troiano, A. 2019. Seismically induced soft‐sediment deformation phenomena during the volcano‐tectonic activity of Campi Flegrei caldera (southern Italy) in the last 15 kyr. Tectonics, 38(6), 1999-2018.</p>