Resurgent Dome Research Articles

The use and beauty of ultra-high-resolution seismic reflection imaging in Late Quaternary marine volcaniclastic settings, Napoli Bay, Italy

Very high-resolution, single channel (IKB-Seistec™) reflection profiles acquired offshore the Napoli Bay, complemented with geological and geophysical data from the literature, provide unprecedented, superb seismic imaging of the Latest Pleistocene-Holocene stratigraphic architecture of the submerged sectors Campi Flegrei and Somma-Vesuvius volcanic districts. Seismic profiles were calibrated by gravity core data and document a range of depositional systems, volcanic structures and hydrothermal features that evolved after the onset of the Last Glacial Maximum (ca. 18 ka BP) over the continental shelf on the Campania coastal zone.Seistec profiles from the Pozzuoli Bay yield high-resolution images of the shallow structure of the collapse caldera-ring fault - resurgent dome system associated with the eruption of the Neapolitan Yellow Tuff (NYT) (ca 15 ka BP) and support a working hypothesis to assess the timing and the styles of deformation of the NYT resurgent structure throughout the Latest Quaternary. Seismic images also revealed the nature of the fragile deformation of strata along the NYT ring fault system and the occurrence of hydrothermal fluids and volcanic/sub-volcanic intrusions ascending along the ring fault zone. Seismic data acquired over the continental shelf off the Somma-Vesuvius stratovolcano, display evidence of gravitational instability of sand wave deposits originated by the underwater modification of pyroclastic flows that entered the seawater after destroying the Roman city of Herculaneum during the 79 CE eruption of Vesuvius.At the Banco della Montagna, a hummocky seafloor knoll located between the Somma-Vesuvius and the Pozzuoli Bay, seismic profiles and gravity core data revealed the occurrence of a field of volcaniclastic diapirs formed by the dragging and rising up of unconsolidated pumice, as a consequence of fluid overpressure at depth associated with active degassing and fluid venting at the seafloor.

Bridging the Gap From Caldera Unrest to Resurgence

Calderas often inflate up to a few metres for weeks to years, which is an evidence of short-term unrest. Some calderas also show a larger uplift (up to a thousand metres), achieved on the longer-term (hundreds to thousands of years), testified by a resurgent dome or block. How the short-term inflation relates to long-term resurgence is still poorly understood, even though established views consider the two processes distinct. This study exploits the longer deformation time series now available for several calderas, as well as the better understanding of magmatic processes and their evolution, to try to bridge the gap between these two scales of uplift. Available data challenge established views, suggesting that resurgence, rather than being produced by constant or continuous uplift, is the net cumulated result of tens to thousands distinct episodes of inflation, even interrupted by deflation episodes, as observed on short-term unrest. These inflation episodes are ascribed to distinct pulses of shallow magma emplacement, with most of the magma remaining intruded, especially in felsic calderas. This supports an incremental growth of magmatic systems, consistently with what observed below resurgent calderas and what inferred for plutons. Comparing the uplift (as expression of the intrusive record) and eruptive histories or resurgent calderas opens new exciting research paths to understand the causal relationships between intruded and erupted magma at a given caldera, thus enhancing its long-term eruptive forecast.

The structural architecture of the Los Humeros volcanic complex and geothermal field

The Los Humeros Volcanic Complex (LHVC) is a large silicic caldera complex in the Trans-Mexican Volcanic Belt (TMVB), hosting a geothermal field currently in exploitation by the Comisión Federal de Electricidad (CFE) of Mexico, with an installed capacity of ca. 95 MW of electric power. Understanding the structural architecture of LHVC is important to get insights into the interplay between the volcano-tectonic setting and the characteristics of the geothermal resources in the area. The analysis of volcanotectonic interplay in LHVC benefits from the availability of subsurface data obtained during the exploration of the geothermal reservoir that allows the achievement of a 3D structural view of the volcano system. The LHVC thus represents an important natural laboratory for the development of general models of volcano-tectonic interaction in calderas. In this study, we discuss a structural model of LHVC based on morphostructural and field analysis, integrated with well logs, focal mechanism solutions and magnetotelluric imaging. The structural analysis suggests that inherited regional tectonic structures recognized in the basement played an important role in the evolution of the magma feeding system, caldera collapses and post-caldera deformations. These inherited weak planes have been reactivated by resurgence faults and post-caldera magma-driven hydrofractures under a local radial stress field generated by the shallow LHVC magmatic/hydrothermal system. The local stress field induced caldera resurgence and volcanotectonic faulting. The results of this study are important to better constrain the structural architecture of large caldera complexes. Also, our study is useful to understand the structure of the Los Humeros geothermal field and support the exploration of deeper Super-Hot Geothermal Systems (SHGSs) and engineering of Enhanced Geothermal Systems (EGSs) for electric power production in the LHVC and other active resurgent calderas.

Origin and quantification of diffuse CO2 and H2S emissions at Crater Hills, Yellowstone National Park

We characterized volatile emissions based upon diffuse soil degassing measurements and fumarolic gas chemistry at Crater Hills, a thermally-altered area adjoining the Sour Creek resurgent dome that is located within the Yellowstone Caldera. The objective of this study was to investigate the source and flux of CO2 and H2S gases to improve our understanding of both the total emissions and origin of the spatial distribution. The total emission of CO2 estimated using the sequential Gaussian simulation method (sGs) was 66 to 109 t day−1 with 95% confidence, which is an underestimation due to the: (1) inability to measure a high flux area on a steep slope, and (2) absence of measurements from fumarole and hot pool emissions. Based on gas chemistry data obtained for a fumarole at Crater Hills in 2007, the proportion of CO2 calculated to be derived from magma would be at least 38%, but could be as high as 50%. The spatial distribution of prominent geothermal features with the highest gas flux are broadly consistent with the regional fault pattern and, therefore, likely reflect the pattern of blind faults and/or fractures covered by overlying alluvium. The estimated emission of H2S was 0.39 t day−1, based on the linear correlation between H2S and CO2. The heat output was also estimated to be ~35 MW with an average heat flux of ~100 W m−2 based upon CO2-H2O-heat relations.

Dynamics of caldera collapse during the Coranzulí eruption (6.6 Ma) (Central Andes, Argentina)

The Coranzulí caldera (23°00′ S – 66°15′ W) is one of the least known caldera complexes in the eastern part of the Argentinian Altiplano-Puna plateau (Central Andes). It lies at the intersection of N-S, NW-SE and NE-SW fault systems and was formed about 6.6 Ma ago; during the eruption, four main crystal-rich dacitic ignimbrites were emplaced in different directions around the caldera. Caldera collapse was not homogeneous, rather it occurred along different sectors of the ring fault as subsidence progressed. The location of co-ignimbrite lag breccias and the composition of the dominant lithic fragments within the different ignimbrite flow units reveal how the caldera collapse developed. According to the succession of deposits, in particular the lack of an initial fallout and the presence of co-ignimbrite lag breccias associated with the different ignimbrite units, we interpret this caldera-forming eruption as a pulsating boiling-over event in which the caldera collapse developed immediately after the onset of the eruption, favored by a transtensive tectonic system. Within the central part of the caldera, there is Cerro Coranzulí, a resurgent dome that exposes a thick intracaldera ignimbrite succession covered by ~ 100-m-thick dacite lava flows that erupted at the end of caldera formation.

Damage and Strain Localization Around a Pressurized Shallow‐Level Magma Reservoir

AbstractStructures developing above long‐term growing shallow‐level magma reservoirs, such as resurgent domes, may contain information on the reservoir itself. To understand the formation of such tectonic features, we have investigated the deformation process around a shallow pressurized magma reservoir embedded in a damaging elastic volcanic edifice. Our model allows evidencing the effect of the progressive damage in producing the fault pattern associated to tectonic surface deformation. Damage is first isotropic around the cavity and constitutes a damaged zone. Then the free‐surface effect appears, and an anisotropic shear strain develops from the boundary of the damaged zone; it localizes on reverse faults that propagate upward to the surface. When the surface deformation is sufficient, normal faulting appears. Finally, the complete structure shows an undeformed wedge above the damaged zone. This structure is similar to those found by analog modeling and from field geologic observations. From this model, we found a relation to estimate the reservoir radius and depth from the graben and dome widths. From limit analysis, we deduced an analytical expression of the magma reservoir pressure which provides a better understanding of the magma pressure buildup during doming. The dip of reverse faults limiting the dome can be inferred from the minimal pressure required to rupture the crust around the reservoir. Finally, the magma reservoir overpressure, the dip of the faults, the reservoir depth, and the damaged zone radius are inferred from three parameters: the ratio ρR computed from the dome and graben widths, the cohesion, and the friction angle.

Volcanic and hydrothermal processes in submarine calderas: The Kulo Lasi example (SW Pacific)

The study area is located at the transition between the northern end of the Tonga Trench and the North Fiji fracture zone, where tectonic movements are reputed to be the fastest in the world. To the southeast of Futuna Island, a broad area of volcanism occurs within a region characterized by a change in the tectonic fabric between a NE-SW oriented volcanic graben and the N-S oriented Alofi ridge. In 2010, the active volcano Kulo Lasi, which represents the most recent volcanic episode in the Futuna area, was discovered in the center of this extensive volcanic zone. Kulo Lasi is a 20 km diameter shield volcano that rises 400 m above the seafloor. It is composed of basaltic to trachy-andesitic lava with no obvious geochemical affinity with the Tonga subduction that occurs 500 km to the east. The central caldera is 5 km in diameter and 300 m deep and is located at a water depth of 1500 m. Diving operations with the submersible Nautile and high-resolution AUV mapping, have revealed the presence of numerous active and inactive hydrothermal fields on the floor and the walls of the caldera. Four tectono-volcanic stages can be distinguished at Kulo Lasi caldera. In stage 1, the shield volcano is built. Annular reverse faults develop at the summit and control circulation of water/rock-dominated hydrothermal fluids and high-temperature alteration of rocks along the nascent normal faults. Mixing of hydrothermal fluids with seawater is favored along normal superficial faults, leading to the formation of low-temperature Fe/Mn mineralization at the summit of the volcano. During stage 2, the caldera collapse, gradually revealing outcrops of the altered and mineralized zones formed during Stage 1. As the magma chamber cools and collapses, less heat is available. As a result, medium to low-temperature (<100 °C) Fe/Si deposits form on the floor of the caldera. In stage 3, refilling and ascent of the magma chamber at depth promote the uplift of a central resurgent dome devoid of recent lava emission. The depth of the magma chamber under the dome is estimated to be 1.6 km. Annular dykes feeding the new lava flows are controlled by the deep reverse faults at the outer rim of the caldera floor. The eruptive events are accompanied by short-lived emission of magmatic fluids, rich in SO2, materialized by the presence of native sulfur depositions on the surface of the most recent lava flows. During stage 4 (present day), black smoker sulfide chimneys, controlled by the deep reverse faults, form at the surface of the most recent lava flows at the outer part of the caldera floor. The fluids emitted result from the mixing of a deep fluid of the water/rock reaction type and of shallow seawater superheated in contact with the hot dykes. Hydrogen is generated during reaction of seawater with the hot dikes and H2 is enriched in the mixed fluids. Active (43 °C) siliceous chimneys at the base of the caldera walls and low-temperature vents (6 °C) perched on the caldera walls suggests a decrease in the venting temperature at a distance from the floor of the caldera. In chimneys, the sphalerite/pyrrhotite/isocubanite/barite paragenesis, probably linked to the high H2 concentrations in the fluids, indicate unusual reducing conditions for a back-arc. The sulfide mineralization also has unusual trace element concentrations. The concentration in Pb and Ba appears characteristic of back-arc environments while the enrichment in Co, Se, and Sn is more common to mineralization associated with basalt in mature back-arcs and associated with ultramafic rocks on mid ocean ridges.

Contrasting perspectives on the Lava Creek Tuff eruption, Yellowstone, from new U–Pb and 40Ar/39Ar age determinations

The youngest major caldera-forming event at Yellowstone was the ~ 630-ka eruption of the Lava Creek Tuff. The tuff as mapped consists of two major ignimbrite packages (members A and B), linked to widespread coeval fall deposits and formation of the Yellowstone Caldera. Subsequent activity included emplacement of numerous rhyolite flows and domes, and development of two structurally resurgent domes (Mallard Lake and Sour Creek) that accommodate strain due to continual uplift/subsidence cycles. Uplifted lithologies previously mapped on and adjacent to Sour Creek dome were thought to include the ~ 2.08-Ma Huckleberry Ridge Tuff, cropping out beneath Lava Creek Tuff members A and B. Mapped outcrops of this Huckleberry Ridge Tuff material were sampled as welded ignimbrite (sample YR345) on Sour Creek dome, and at nearby Bog Creek as welded ignimbrite (YR311) underlain by an indurated lithic lag breccia containing blocks of another welded ignimbrite (YR324). Zircon near-rim U–Pb analyses from these samples yield weighted mean ages of 661 ± 13 ka (YR345: 95% confidence), 655 ± 11 ka (YR311), and 664 ± 15 ka (YR324) (combined weighted mean of 658.8 ± 6.6 ka). We also studied two samples of ignimbrite previously mapped as Huckleberry Ridge Tuff on the northeastern perimeter of the Yellowstone Caldera, ~ 12 km ENE of Sour Creek dome. Sanidines from these samples yield 40Ar/39Ar age estimates of 634.5 ± 6.8 ka (8YC-358) and 630.9 ± 4.1 ka (8YC-359). These age data show that all these units represent previously unrecognized parts of the Lava Creek Tuff and do not have any relationship to the Huckleberry Ridge Tuff. Our observations and data imply that the Lava Creek eruption was more complex than is currently assumed, incorporating two tuff units additional to those currently mapped, and which themselves are separated by a time break sufficient for cooling and some reworking. The presence of a lag breccia suggests that a source vent lay nearby (< ~ 3 km) for some of the tuffs and that the Yellowstone Caldera boundary in this area could be reconsidered.

Caldera resurgence driven by magma viscosity contrasts

Calderas are impressive volcanic depressions commonly produced by major eruptions. Equally impressive is the uplift of the caldera floor that may follow, dubbed caldera resurgence, resulting from magma accumulation and accompanied by minor eruptions. Why magma accumulates, driving resurgence instead of feeding large eruptions, is one of the least understood processes in volcanology. Here we use thermal and experimental models to define the conditions promoting resurgence. Thermal modelling suggests that a magma reservoir develops a growing transition zone with relatively low viscosity contrast with respect to any newly injected magma. Experiments show that this viscosity contrast provides a rheological barrier, impeding the propagation through dikes of the new injected magma, which stagnates and promotes resurgence. In explaining resurgence and its related features, we provide the theoretical background to account for the transition from magma eruption to accumulation, which is essential not only to develop resurgence, but also large magma reservoirs.

Magnetotelluric imaging of the resurgent caldera on the island of Ischia (southern Italy): inferences for its structure and activity

The island of Ischia (located in the Bay of Naples, Italy) represents a peculiar case of a well-exposed caldera that has experienced a large (>800 m) and rapid resurgence, accompanied by volcanic activity. What drives the resurgence of calderas is a crucial issue to investigate, because this process is associated with potential eruptions and high risk to people living within and around such large active volcanic systems. To improve the knowledge of volcano-tectonic processes affecting the caldera of Ischia, electromagnetic imaging of the structures associated with its resurgence was performed and integrated with available geological information. A magnetotelluric (MT) survey of the island was carried out along two main profiles through the central-western sector, providing an electrical resistivity map to a depth of 3 km. These resistivity cross sections allowed us to identify the presence of a very shallow magmatic intrusion, possibly a laccolith, at a depth of about 1 km, which was responsible for both the resurgence and the volcanic activity. Furthermore, the tectonic structures bordering the resurgent area and the occurrence of a large thermal anomaly in the western sector of the caldera also provided a signature in the resistivity cross sections, with the magma intrusion producing advection of hot fluids with high geothermal gradients (>150 °C km−1) in the southern and western sectors. All of these data are fundamental for the assessment of the island’s volcano-tectonic dynamics and their associated hazards. The structure and activity of the island have been controlled by the process of resurgence associated with the arrival of new magma and the progressive intrusion of a laccolith at a shallow depth. The reactivation of such a shallow system may imply imminent eruption which would pose a major volcanic hazard.

Experimental and petrological constraints on long-term magma dynamics and post-climactic eruptions at the Cerro Galán caldera system, NW Argentina

Cerro Galán in NW Argentina records >3.5Myr of magmatic evolution of a major resurgent caldera complex. Beginning at 5.72Ma, nine rhyodacitic ignimbrites (68–71wt% SiO2) with a combined minimum volume of >1200km3 (Dense Rock Equivalent; DRE) have been erupted. The youngest of those ignimbrites is the eponymous, geochemically homogenous, caldera-forming 2.08±0.02Ma Cerro Galán Ignimbrite (CGI; >630km3 DRE). Following this climactic supereruption, structural and magmatic resurgence led to the formation of a resurgent dome and post-climactic lava domes and their associated pyroclastic deposits. A clear transition from amphibole to sanidine-bearing magmas occurred during the evolution of Cerro Galán and is inferred to represent a shallowing of the magma system. We test this hypothesis here using experimental phase equilibria.We conducted a series of phase equilibria experiments on the post-climactic dome lithologies under H2O-saturated conditions using cold seal Waspaloy pressure vessels with an intrinsic log fO2 of NNO+1±0.5 across a temperature-pressure range of 750–900°C and 50–200MPa (PH2O=Ptotal), respectively. Petrologic and geochemical analysis of the post-climactic lithologies shows that the natural phase assemblage (plagioclase+quartz+biotite+sanidine+Fe-Ti oxides±apatite±zircon) is stable at <50MPa (PH2O) and 805–815°C. Applying experimental results to the CGI pumice, which has the same phenocryst phase assemblage and modal abundance, whole rock and phenocryst chemistry, and overlapping temperature and fO2 as the post-climactic deposits, suggests that these pre-eruptive conditions (PH2O<50MPa) are relevant for the magmas that sourced the climactic CGI supereruption as well.Amphibole in the early Cerro Galán ignimbrites (Toconquis Group; 5.72–4.51Ma, and the Cueva Negra Ignimbrite, 3.77±0.08Ma) records crystallization across a range of pressures (500 to 200±60MPa). In the interval between the eruption of the Cueva Negra ignimbrite and the CGI (2.08±0.02Ma) the complex magma system shallowed and stalled at low pressures (<100MPa), resulting in a more simple magma reservoir configuration represented by a large-volume, geochemically homogenous magma body. The shallowing of the Cerro Galán magma system during this time explains the marked transition from amphibole to sanidine-bearing magmas and seems to characterize many large silicic caldera-forming magma systems that erupt over million year timescales to generate long-lived volcanic complexes.The post-climactic history of Cerro Galán is informed through a detailed investigation of the textural differences among the post-climactic dome lithologies, and a comparison of those textures with previously published decompression experiments. These suggest that the highly vesiculated, pumiceous clasts with rare microlites represent magma stored within the core of the lava dome that decompressed relatively rapidly (0.003–0.0003MPas−1) and evolved via closed system degassing. Resulting over-pressure of the dome may have triggered superficial explosion. In contrast, dense clasts with abundant crystalline silica precipitates represent more typical dome-forming magmas that decompressed more slowly (<0.00005MPas−1), evolved via open system degassing, and form the outer carapace of a lava dome. Integrating decompression histories with results from new phase equilibria experiments suggests that during post-climactic volcanic activity at Cerro Galán, remnant CGI dome-forming magmas ascended from the shallow magma reservoir (<4km) to motivate resurgent uplift and erupt as lava domes either explosively as vesiculated clasts or effusively as dense clasts that make up the outer structure of lava domes.

Gravitational slope-deformation of a resurgent caldera: New insights from the mechanical behaviour of Mt. Nuovo tuffs (Ischia Island, Italy)

Ischia Island (Italy) is an impressive example of the rare phenomenon of caldera resurgence. The emplacement and replenishment of magmas at shallow depth resulted in a vertical uplift of about 900m, concentrated in the western portion of Mt. Epomeo (789m a.s.l.). As a consequence of this uplift, the island has experienced several slope instabilities at different scales since the Holocene, from shallow mass movements to large rock and debris avalanches. These mass wasting events, which mobilised large volumes of greenish alkali-trachytic tuff (the Mt. Epomeo Green Tuff, MEGT), were strictly related to volcano-tectonic activity and the interaction between the volcanic slopes and the hydrothermal system beneath the island. Deep-Seated Gravitational Slope Deformation (DSGSD) at Mt. Nuovo, located adjacent to densely populated coastal villages, is an ongoing process that covers an area of 1.6km2. The Mt. Nuovo DSGSD involves a rock mass volume of 190Mm3 and is accommodated by a main shear zone and a series of sub-vertical fault zones associated with high-angle joint sets. To improve our understanding of this gravity-induced process, we performed a physical (porosity and permeability) and mechanical (uniaxial and triaxial deformation experiments) characterisation of two ignimbrite deposits - both from the MEGT - that form a significant component of the NW sector of Mt. Epomeo. The main conclusions drawn from our experiments are twofold. First, the presence of water dramatically reduces the strength of the tuffs, suggesting that the movement of fluids within the hydrothermal system could greatly impact slope stability. Second, the transition from brittle (dilatant) to ductile (compactant) behaviour in the tuffs of the MEGT occurs at a very low effective pressure, analogous to a depth of a couple of hundred metres, and that this transition is likely moved closer to the surface in the presence of water. We hypothesise that compactant (porosity decreasing) behaviour at the base of the layer could therefore facilitate slope instability. Although our results show that transient exposure to 300°C does not influence the short-term strength of the tuff, we speculate that the high in-situ temperature could increase the efficiency of brittle and compactant creep and therefore increase the rate of slope deformation. Taken together, our experimental data highlight a potentially important role for the hydrothermal system (that reaches a minimum depth of ~1km) in dictating the DSGSD at Mt. Nuovo. An understanding of this deformation process is not only important for the proximal coastal villages, at risk of engulfment by a large debris avalanche, but also for the towns and cities along the coast of the Gulf of Naples that are at risk to a secondary consequence of such an avalanche - a tsunami wave.

Fluid-driven uplift at Long Valley Caldera, California: Geologic perspectives

Since persistent seismicity began in the Sierra Nevada adjacent to Long Valley caldera in 1978–1980, intracaldera unrest has been marked by (1) episodes of uplift totaling ~83cm, centered on the middle Pleistocene resurgent dome, and (2) recurrent earthquake swarms along a 12-km-long segment of the caldera's ring-fault zone that is contiguous with both the dome and the Sierran seismogenic domain. Others have attributed the recent unrest to magmatic intrusion(s), but it is argued here that evidence for new magma is lacking and that ongoing uplift and ring-fault-zone seismicity are both promoted by ascent of aqueous fluid released by second boiling of the residue of the enormous Pleistocene rhyolitic reservoir terminally crystallizing at depths ≥10km. For 2Myr, eruptive vent clusters migrated southwestward from Glass Mountain to Mammoth Mountain. There has been no eruption on the resurgent dome since 500ka, and since 230ka volcanism has been restricted to the caldera's west moat and contiguous Sierran terrain, both outside the structural caldera. High-temperature hydrothermal activity in the central caldera waned after ~300ka, cooling the Pleistocene rhyolitic focus to the extent that drilling on the resurgent dome found mid-caldera temperature to be only 100°C and isothermal at depths of 2–3km. Beneath most of the resurgent dome, there is little seismicity at any depth, no emission of magmatic CO2 or other magmatic gases, no elevated 3He/4He ratios, and only normal to below-normal heat flow. Most of the 75-km-long ring-fault zone is likewise aseismic, excepting only the 12-km segment contiguous with the extracaldera seismogenic domain in the Sierra. Since 1980, the Sierran seismicity has released 3.6 times more cumulative seismic energy than have intracaldera earthquakes. The caldera seismicity is not driven by stresses associated with the adjacent uplift but, instead, by the extracaldera tectonic stressfield. Sierran seismicity activated the directly contiguous south-moat segment of the ring-fault zone, which had originated in the caldera-forming eruption at 767ka and everywhere else remains sealed. Hypocenter relocation studies of 1000s of earthquakes along the seismic segment have resolved recurrent upward-migrating swarms within networks of cryptic faults, apparently triggered by rapidly ascending pulses of high-pressure low-viscosity aqueous fluid. Entering the brittle crust at depths of 8–10km, such fluid is just what should be expected from second boiling of the late-stage CO2-poor rhyolitic residue. The fluid provides the pressure source above the apex of the crystallizing caldera-wide pluton and then escapes laterally to the newly reactivated southern segment of the ring-fault zone, its only available permeable pathway, where it mediates the ongoing south-moat seismicity.

Reconnaissance survey of the Duolun ring structure in Inner Mongolia: Not an impact structure

AbstractThe Duolun basin, which is located in Inner Mongolia, China, has been proposed to be an impact structure with an apparent rim diameter of about 70, or even 170 km. The designation as an impact structure was based on its nearly circular topography, consisting of an annular moat that surrounds an inner hummocky region, and the widespread occurrences of various igneous rocks, polymict breccias, and deformed crustal rocks. Critical shock metamorphic evidence is not available to support the impact hypothesis. We conducted two independent reconnaissance field surveys to this area and studied the lithology both within and outside of the ring structure. We collected samples from all lithologies that might contain evidence of shock metamorphism as suggested by their locations, especially those sharing similar appearances with impact breccias, suevites, impact melt rocks, and shatter cones. Field investigation, together with thin‐section examination, discovered that the suspected impact melt rocks are actually Early Cretaceous and Late Jurassic lava flows and pyroclastic deposits of rhyolitic to trachytic compositions, and the interpreted impact glass is typical volcanic glass. Petrographic analyses of all the samples reveal no indications for shock metamorphic overprint. All these lines of evidence suggest that the Duolun basin was not formed through impact cratering. The structural deformation and spatial distribution pattern of the igneous rocks suggest that the Duolun basin is most likely a Jurassic–Cretaceous complex rhyolite caldera system that has been partly filled with sediments forming an annular basin, followed by resurgent doming of the central area.

Early postcaldera rhyolite and structural resurgence at Long Valley Caldera, California

After the 767-ka caldera-forming eruption of 650km3 of rhyolite magma as the Bishop Tuff, 90–100km3 of similar rhyolite erupted in the west-central part of Long Valley caldera in as many as 40 batches spread over the 110,000-year interval from ~750ka to ~640ka. Centrally, this Early Rhyolite (ER) is as thick as 622m, but it spread radially to cover much of the caldera floor, where half its area is now concealed by post-ER sediments and lavas. At least 75% of the ER is aphyric rhyolite tuff. Drillholes encountered ~22 (altered) ER lava flows intercalated in the pyroclastic pile, and another 11units of (largely fresh) ER lava are exposed on the caldera's resurgent dome and at Lookout Mountain. Exposed units have been distinguished, mapped, studied petrographically and chemically, and radioisotopically dated; each is described in detail. Their phenocryst contents range from 0 to 2.5wt%. All the phyric units have plagioclase, orthopyroxene, and ilmenite; most have biotite and rare tiny magnetite, and a few contain rare zircon. The compositional range of fresh obsidians is narrow—74.3–75.0% SiO2, 1.21–1.37% FeO*, and 5.12–5.26% K2O, but wider variations in Ti, Ba, Sr, and Zr permit distinction of individual units and eruptive groups. The limited chemical and petrographic variability shown by so many ER batches released episodically for ~110,000years suggests a thermally buffered and well-stirred reservoir.The ER central area, where ER eruptions had taken place, was uplifted ~400m to form a structural dome ~10km in diameter. Most of the inflation is attributable to 10 sills of ER that intrude the Bishop Tuff beneath the uplift, but other processes potentially contributing to resurgence are also considered. As shown by erratics of Mesozoic rocks ice-rafted from the Sierra Nevada and dropped on ER lavas, much of the ER had erupted early enough and at low enough elevation to be inundated by the intracaldera lake and was only later lifted by the resurgence that also raised clusters of the erratics hundreds of meters higher than any shoreline. Most of the uplift was over by ~570ka, but dome-crossing faults that exhibit normal throw of 10–30m cut lavas as young as 175–125ka.For most elements, chemical ranges of the ER lie within those of the zoned Bishop Tuff, which had erupted earlier from the same place. Only Ba, Zr, Hf, and Eu/Eu* extend to ranges outside those of the Bishop Tuff, nominally to less evolved compositions. Initial 87Sr/86Sr values of ER are likewise within the range of the Bishop Tuff, but ER ratios of 143Nd/144Nd and 206Pb/204Pb extend beyond those of the Bishop Tuff to values slightly more influenced by upper-crustal contributions. FeTi-oxide geothermometry yields 752°–844°C for ER, compared to 700°–820°C for the Bishop Tuff. ER fO2 values are 0.5–1.0 log units more reduced than those of the T–fO2 array of the Bishop Tuff. The postcaldera reduction may reflect reaction with graphite from the black lithics of Paleozoic graphitic metapelite so abundant in the Bishop Tuff. Much of the pumice emplaced during the later half of the Bishop Tuff eruption has 10–25wt% phenocrysts, dominantly quartz and sanidine, but the 100km3 of ER has only 0–2.5wt% and completely lacks quartz and sanidine. Postcaldera processes, including mixing, volatile ascent, and crystal resorption, as well as potential contaminants and magmatic inputs, are all considered.

Co-Evolution of Volcanic and Lacustrine Systems In Pleistocene Long Valley Caldera, California, U.S.A.

Abstract Sedimentary records from intracaldera lakes document unique interactions between volcanic and sedimentary processes and hold valuable information about the geologic evolution of the associated volcanic systems. Here, we present a comprehensive analysis of the lacustrine sedimentary package within Long Valley Caldera, California, which provides new constraints on the history of the Long Valley system as well as new insights into the development of intracaldera sedimentary systems in general. After the eruption of the Bishop Tuff (760 ka), the resulting caldera was partially filled by a large lake, within which volcaniclastic, chemical, and biogenic sediments were deposited. Contrary to previous interpretations, Long Valley Lake was initially evaporative and did not become relatively fresh and deep until later in its history. This observation, coupled with paleoshorelines documented on the resurgent dome, suggests an extended timeframe for resurgent dome uplift. Near-shore volcaniclastic sedimentation in Long Valley Lake was predominantly episodic and was concentrated in an extensive Gilbert-type delta system that built outwards into the lake from intracaldera rhyolite highlands. Deposition in distal environments was dominated by pelagic sedimentation that included accumulation of diatomite and marl with ostracod shell beds. The sedimentary package is herein assigned to two coeval formations that exhibit fundamentally different characteristics and modes of deposition. The Sagehen Formation comprises the proximal volcaniclastic succession, which was influenced by ongoing deformation and volcanism during the evolution of the Long Valley system. The Benton Crossing Formation comprises the distal pelagic sedimentary package, which appears to offer a unique paleoclimatic record within a well-constrained chronostratigraphic framework. Together these formations support models of intracaldera sedimentation in which event-driven deposition and diatomite accumulation are dominant sedimentary processes, although they also illustrate the importance of delta development and catchment-area evolution in focusing sedimentation and controlling the architecture of the deposits. The Long Valley intracaldera sedimentary package highlights the utility of lacustrine sediments in understanding caldera evolution.

The Campi Flegrei caldera (Italy): Formation and evolution in interplay with sea-level variations since the Campanian Ignimbrite eruption at 39 ka

To date, the origin of the Campi Flegrei caldera is still under debate and may be related to (1) a single caldera collapse associated with the Neapolitan Yellow Tuff (NYT) eruption, (2) two subsequent caldera collapses associated with the NYT and the preceding Campanian Ignimbrite (CI) eruptions forming a nested-caldera complex, or (3) not related to a caldera collapse after all. Here, we study the submerged portion of the caldera, which has favored a marine depositional setting and, thus, represents an ideal location for the reconstruction of its formation history, utilizing multichannel seismic data. Volcanic deposits and edifices were seismically distinguished from sedimentary successions, and the stratigraphy could be refined and extended back to the Campanian Ignimbrite eruption at ~39ka.High-resolution multichannel reflection seismic data revealed the existence of a nested-caldera complex formed during the CI eruption at ~39ka and the more recent NYT eruption at ~15ka. A ring-fault bounding an inner caldera collapse structure was clearly imaged. It appears that this inner ring-fault was initially activated during the CI caldera collapse and later reactivated during the NYT caldera collapse with different amounts of subsidence. The NYT caldera probably formed during an asymmetrical collapse with a maximum subsidence of ~75m in the offshore portion. The vertical displacement related to the CI caldera collapse may be significantly larger.The submerged caldera depression accommodates post-eruption sediments. Within this high-resolution archive, two major unconformities developed at ~8.6ka and 5ka, when resurgence-related uplift exceeded the rate of sea-level rise concurrent with the emersion of the La Starza terrace. A previously unknown post-collapse submarine volcanic mound located between Nisida Island and Nisida Bank probably formed between 4.8 and 3.7ka. Also, the Penta Palummo Bank appears to be constructed of at least two monogenetic volcanic edifices, the Penta Palummo volcano formed at ~100ka and a younger mushroom-shaped unit formed between 39 and 15ka. The main outcome of this study is a conceptual evolutionary model, providing novel insights on the formation of the Campi Flegrei nested caldera in the course of two large-scale eruptions (CI and NYT) with associated caldera collapses along mutual (i.e. reactivated) faults and subsequent caldera resurgence. As both the genesis and subsurface architecture of the Campi Flegrei caldera are still hotly debated topics in literature, our discoveries can be regarded as a substantial advancement in the understanding of the Campi Flegrei volcanic area.

Meteorite Impact Origin of Yellowstone Hotspot

Origin of the Yellowstone hotspot & Columbia River Basalts has remained uncertain until now. Here, we present evidence of meteorite impact origin. The hotspot is shallow, only 200 km deep, invalidating a theory of mantle plume origin. The hotspot track runs from the Yellowstone National Park in NW Wyoming to the volcanic Modoc Plateau in NE California. We present evidence of apparent remnants of an impact crater existing in the Modoc, a large multi-ring structure at least 160 km diameter. Much of the complex crater has become obliterated by later Cascadia and Sierra orogenies. The crater has a tall 4,100 foot central uplift cone, locally known as Chalk Mountain, consisting of diatomaceous earth, presumably the rebound cone of a meteorite impact. This falsifies a theoretical prohibition of cosmic impact volcanism. Based on recent insights into explosive volcanism a plausible mechanism is proposed for how meteorite impacts can lead to resurgent calderas of the Yellowstone type, & of apparent LIP origin from thin crust ET impacts, invalidating theoretical constructs of mantle plumes. We mention chaotic terrains, including plutons, arising antipodal to impacts.

Constraints on the source of resurgent doming inferred from analogue and numerical modeling — Implications on the current feeding system of the Yenkahe dome–Yasur volcano complex (Vanuatu)

Resurgence, defined as the post-collapse long-term uplift of a caldera floor, is commonly attributed to a renewed rise of magma. The Yenkahe dome (Vanuatu) exhibits a common morphology – elongated with a graben on top – among resurgent domes, and is also one of the most active structures of the kind. In this study, we performed a joint analysis based on analogue and finite element numerical models to (1) constrain the width and depth of the long-term deformation intrusive source of the Yenkahe dome and (2) discuss the close association between the Yenkahe dome and the active Yasur cone. We consider the resurgent deformation at the surface to be driven by the uplift of a magma reservoir roof in depth. As the edifice deformation response depends on the medium and the source properties, the mechanical behavior of the upper crust and the nature of the source are modeled using two very different sets of hypotheses. Analogue modeling uses silicone putty, an analogue for a large viscous magma body, intruding a sand–plaster mixture reproducing a Mohr–Coulomb behavior for the crust. Numerical models consider the vertical displacement of a rigid indenter, allowing the conservation of a flat-shaped roof, into an elastoplastic material. Numerical and analogue models show different resurgent dome structures at depth but similar dome and graben morphologies in the surface. Inverse faults – or equivalent shearing zones – delimiting the dome provide an explanation for the confined nature of resurgent doming and the persistent volcanic activity on the dome border represented by the Yasur volcano. Analogue and numerical models together provide an estimation range of 1–1.8km for the intrusive deformation source depth, and 1.3–2km for its width. The proposed association between the Yenkahe dome and the Yasur volcano is compatible with such a shallow depth of the magma reservoir, and argues for a discontinuous resurgence process.

Valles Caldera: Preserving the “Yellowstone of the Southwest”

Valles Caldera: Preserving the “Yellowstone of the Southwest” Jeffrey M. Widener (bio) Located northwest of Santa Fe, New Mexico, are the majestic Jemez Mountains—a volcanic mountain range that began forming around 20 million years ago (figure 1) (Wolff et al. 2005). This range sits atop the western edge of the Rio Grande rift, an area of extreme volcanic activity, and is part of the Jemez lineament stretching from southeast Arizona to southern Colorado (Heiken et al. 1990). Recurrent tectonic activity formed and re-formed the Jemez Mountains into a range of lava domes, eroded cones, and incised canyons. Of particular interest to researchers are the eruptions of two small vents around 1.5 million years ago. These explosions left the surface unsupported and, as a result, the Toledo Caldera formed. Around 300,000 years later, another eruption, which was about 100 times stronger than the 1980 Mount St. Helens explosion in Washington, demolished the majority of the Toledo Caldera but formed a new one, the Valles Caldera, which measures almost 15 miles in diameter (Wolff and Gardner 1995; Valles Caldera Trust [VCT] 2003). Click for larger view View full resolution Figure 1. The Valles Caldera National Preserve is located in the volcanic Jemez Mountain range (in the distance)—taken from Santa Fe, NM (photo by author). [End Page 583] After a collapse that forms a caldera, resurgent domes can begin to form if uplift occurs owing to pressure pushing molten rock in the magma chamber below the caldera upward. In the Valles Caldera, this started over 70,000 years ago (Martin 2003, 6). Redondo Peak, at 11,254 feet, is the Valles Caldera National Preserve’s only resurgent dome. This resurgence drained the lakes, created rivers, and formed the Cañon de San Diego, where the Jemez settled in the 1700s. Geographers, geologists, and other researchers recognize the Valles Caldera as an exceptional example of an exposed caldera formation, and, indeed, some call it the “Yellowstone of the Southwest” (VCT 2003, 18; Solomon 2004). Like Yellowstone, it is a place worthy of preservation for its beautiful landscapes. It is also of great importance culturally and ecologically to several indigenous groups, particularly the Towa-speaking Jemez Pueblo. The government continues to seek ways to best preserve such areas for posterity. Over the past few decades, United States (US) government officials have tried using new methods of acquiring and paying for publicly owned land. In 1996, Congress placed the Presidio, a historic military base in San Francisco, under the direction of the Presidio Trust—the first charter land management program. Congress implemented this trust not only to “preserve the Presidio’s natural, scenic, cultural, and recreational resources” but also to help it pay for its own upkeep (Presidio Trust [PT] 2010; Fairfax, Gwin, and Huntsinger 2004). Thus, they established a difference between setting aside public land to be government supported and placing public land under a trust that would become self-supporting. In this case, the Presidio Trust manages 80 percent and the National Park Service (NPS) oversees 20 percent (PT 2010). The Valles Caldera National Preserve, established in 2000, is the second site placed in a trust program—the Valles Caldera Trust—but this region has a historical cultural geography much richer than and different from that of the Presidio (figure 2). While several authors have described the history and the problems associated with the land management practices of the NPS and the United States Department of Agriculture Forest Service (USFS) (Foresta 1984; Dilsaver 1994; Spence 1999; Burnham 2000; Williams 2006; Sellers 2009; Runte 2010), only a handful of scholars have researched the land management system of public land trusts (Benton 1998; Fairfax, Gwin, and Huntsinger 2004; Little, Berrens, and Champ 2005; DeBuys and Usner 2006; Sullivan 2008). On the surface, public land trusts are similar to the NPS and USFS models, but the unique trust model focuses more on actual land use and management for sustainability and resilience than the more familiar NPS [End Page 584] Click for larger view View full resolution Figure 2. The Valles Caldera National Preserve (cartography by author). and USFS models do. Both land management models, however, have a number of important roles in...