Caldera Ring Fault Research Articles

Caldera formation and varied eruption styles on North Pacific seamounts: the clastic lithofacies record

Detailed examination of volcaniclastic and sedimentary rocks collected from the Taney (30 Ma), President Jackson (4 Ma), Vance (3 Ma) and Alarcon (2 Ma) near-ridge seamount chains of the North Pacific reveals seven clastic lithofacies that record various modes of eruption, magma fragmentation, and particle dispersal. Lithofacies are distinguished by differences in lithology, bedding habit, compositional heterogeneity, and relationship to volcanic landforms. Breccia lithofacies were produced through mechanical fragmentation during caldera collapse (polymict) or effusive eruptions onto steep slopes (monomict). Rare globular lapilli mudstone lithofacies contain clasts with morphologies formed by magma-sediment mingling processes (peperite). Seamount summit pyroclastic deposits include proximal lapilli tuff with vesicular pyroclasts, and more distal limu o Pele tuff lithofacies. Much finer-grained hydrothermal mudstone/tuff lithofacies occurs around caldera rims and contains greenschist minerals, hydrothermal clays and basaltic ash that record subsurface phreatomagmatic fragmentation processes. Very fine-grained ash is transported to distal regions by oceanic currents and hydrothermal plumes, and is a component of the regional pelagic sediment budget. Pyroclastic lithofacies only occur on seamount summits suggesting formation during the latter stages of seamount evolution. As a seamount drifts away from an adjacent ridge axis and associated heat source, its magma supply is reduced allowing for magmatic gas buildup and mild explosive eruptions. During this stage, the diminished melt supply under the seamount is unable to fully compensate for extension along the ridge axis and vertical seamount growth. Lateral intrusion into spreading-related structures in this stage causes magma withdrawal and caldera formation. Formation of caldera ring faults also promotes seawater ingress into subseafloor hydrothermal cells, which interact with magma conduits causing phreatomagmatic fragmentation.

Insights into caldera and regional structures and magma body distribution from lava domes at Rotorua Caldera, New Zealand

Lava domes emplaced after caldera-forming eruptions are often the only clue to the structure of the caldera and to the regional structure at the time of caldera formation. This is because the deposits from the subsequent eruptions commonly bury subsidence-controlling caldera faults and regional cross faults within calderas. Other calderas and lava domes in the Taupo Volcanic Zone (TVZ), New Zealand, have been previously reported to show strong regional control. At Rotorua Caldera there is a general lack of surface expression of faults, but rhyolite lava domes are well exposed and aligned with regional structures that have been mapped outside of the caldera. Our summary of published data and new observations allows these lava domes to be subdivided into three groups based upon their petrology, geochemistry, relative age and spatial distribution: (1) the Kaikaitāhuna Group of pre-collapse (451ka), crystal-rich domes at the caldera boundary, (2) the Utuhina Group of crystal-poor, high alkali, post-collapse domes (c. 200ka) surrounding the locus of collapse and directly related to the earlier (240ka) Mamaku ignimbrite eruption, and (3) The Ruamata Group of crystal-rich, more calcic, less devitrified post-collapse (60–36ka) domes further out from the collapse locus. Mapping of the morphology and internal structures of the Utuhina Group domes shows an ENE–WSW elongation direction and vent alignment, which is parallel to and down strike of (a) the extensional rift orientation independently mapped either side of the caldera, (b) the local caldera morphology and (c) structures mapped at nearby Okataina Caldera Complex. A second NW–SE trending orientation in the caldera morphology is parallel to a pre-collapse basement fabric, and has partially controlled the locations of the domes. A third orientation is hinted at through consistent N–S striking flow bands within Ngongotaha Dome, which are parallel with a gravity anomaly. We suggest that this orientation is a N–S section of a caldera ring fault that links regional structures and was utilised during caldera collapse. A subsequent dike intrusion also used this structure, erupting a fissure-style dome that contains radiating flow bands which mimic the orientation of the feeder dike. However, the structure, morphology and location of post-collapse lava domes generally echo the unburied regional structures outside the caldera indicating that dike intrusion also occurred along regional structures. We suggest that the regional structures at Rotorua Caldera, which are similar to those at Okataina Caldera Complex, show that the kink in the orientation of the rift across the TVZ to the SE of Rotorua was active to at least 240ka, and possibly earlier. This kink is significant due to its proximity and influence on both Rotorua and Okataina caldera systems and their associated geothermal fields. We conclude that internal structures, such as flow bands and morphology of lava domes are direct consequences of the shape and location of the feeder dike, which in turn is controlled by the structure of the caldera and region. We also suggest that lava dome eruptions at Rotorua were from a series of small, related nestled magma bodies. A key outcome of this study is the reconstruction of the wider structure of the caldera and surrounding structure based upon the localised internal features, distribution, and morphology of related lava domes.

Crustal Magnetization of Brothers Volcano, New Zealand, Measured by Autonomous Underwater Vehicles: Geophysical Expression of a Submarine Hydrothermal System

Near-bottom magnetic anomaly data have been acquired using autonomous underwater vehicles at Brothers volcano, southern Kermadec arc, New Zealand. Crustal magnetization for the study area was obtained by inverting the magnetic data and shows a strong correlation between areas of low magnetization and four hydrothermal fields, one of which was unknown prior to our surveys. The magnetization pattern is consistent with a model of discrete, individual zones of fluid upflow focused along caldera ring faults that provide preferred pathways for the ascent of the hydrothermal fluids. Differences in the amplitude of the magnetization over the vent fields appear to correlate with age and temperature variations of the hydrothermal fields.

An investigation of caldera-forming magma chambers using the timing of ignimbrite eruptions and pluton emplacement at the Mt. Aetna caldera complex

The temporal and chemical relationships of volcanic and plutonic rocks exposed at the Mt. Aetna caldera complex, central Colorado, provide insight into the emplacement history of pre-, syn-, and postcaldera plutons and the origin of caldera-related silicic magmas. 40Ar/39Ar sanidine ages indicate the rhyolitic Wall Mountain Tuff erupted at 37.3Ma. LA-ICP-MS U/Pb zircon ages of the compositionally zoned Mt. Princeton batholith, which has been interpreted by several previous studies to be the nonerupted, less-fractionated residuum of the Wall Mountain Tuff magma chamber, indicate that it was emplaced between 35.9 and 35.2Ma during postcaldera magmatism. Nested within the Mt. Princeton batholith is the Mt. Aetna caldera. The dacitic Badger Creek Tuff erupted at 34.3Ma during Mt. Aetna caldera collapse. The Badger Creek Tuff is deposited on some of the Mt. Princeton intrusive units, demonstrating that the Wall Mountain Tuff caldera was obliterated during exhumation and erosion prior to 34.3Ma. Shortly after the Mt. Aetna caldera collapse, the intracaldera ignimbrite and caldera ring faults were intruded by magma that is compositionally and temporally similar to the Badger Creek Tuff, suggesting that these intrusions represent a nonerupted, geochemically equivalent portion of the magma chamber. Emplacement of postcaldera luecogranites at ~31Ma caused localized thermal resetting of biotite and K-feldspar in older, adjacent intrusions. Most samples contain antecrystic zircon. Antecrystic zircon populations range from 38.8 to 33.6Ma, indicating that open system magmatic processes operated throughout the history of the caldera complex. The temporal, chemical, and spatial relationships of plutonic and volcanic rocks at the Mt. Aetna caldera complex indicate that the majority of the exposed plutons were emplaced during pre- and/or postcaldera magmatism. None of the exposed plutons represent the less-fractionated, mafic residuum of either caldera-forming magma ignimbrite. This suggests that either the mafic roots of each caldera-forming magma chamber are not exposed or upper crustal in situ fractionation did not generate the caldera-forming silicic magmas.

Insights into the kinematics of a volcanic caldera drop: Probabilistic finite-source inversion of the 1996 Bárdarbunga, Iceland, earthquake

The accurate modelling and prediction of volcanic eruptions depends critically on information concerning the interaction between the caldera and the underlying magma chamber. Knowledge concerning the kinematics of a caldera in the course of an eruption is therefore essential. Here we provide detailed seismological constraints on the kinematics of a volcanic caldera drop and the geometry of a caldera ring fault. For this we performed a finite-source inversion of the September 29, 1996 Bárdarbunga, Iceland, earthquake that was caused by caldera subsidence. Our methodology is based on spectral element simulations of seismic wave propagation through a realistic model of the Icelandic crust and upper mantle. A particularly robust feature is the initiation of the rupture in the north-western part of the ring fault that is about 10 km in diameter. From there it spread to the other fault segments within about 3 s. Without invoking super-shear propagation sensu stricto, we can explain this unusually fast rupture propagation by the triggering of fault segments through P waves that propagated across the caldera. Our results favour outward-dipping fault segments in the western half of the ring fault, while the eastern half is preferentially inward-dipping. This variability may reflect structural heterogeneities or an irregular magma chamber geometry. The individual segments of the caldera ring fault radiated approximately equal amounts of energy. This indicates that the caldera dropped coherently as one single block. The work presented here is intended to aid in the design of realistic models of magma chamber and caldera dynamics.

InSAR Permanent Scatterer analysis reveals fault re-activation during inflation and deflation episodes at Campi Flegrei caldera

Permanent Scatterers Synthetic Aperture Radar Interferometry (PSInSAR) and Global Position System (GPS) are applied to investigate the most recent surface deformation of the Campi Flegrei caldera. The PSInSAR analysis, based on SAR data acquired by ERS-1/2 sensors during the 1992–2001 time interval and by the Radarsat sensor during 2003–2007, identifies displacement patterns over wide areas with high spatial resolution. GPS data acquired by the Neapolitan Volcanic Continuous GPS network provide detailed ground velocity information of specific sites. The satellite-derived data allow us to characterize the deformation pattern that affected the Campi Flegrei caldera during two recent subsidence (1992–1999) and uplift (2005–2006) phases. PSInSAR results show the re-activation of the caldera ring-faults, intra-caldera faults, and eruptive fissures. We discuss the results in the light of the available volcanological, structural and geophysical data and propose a relationship between the structures activated during the recent unrest episodes and those responsible for the recent (<3.8–4ka) volcanism. The combined interpretation of the collected data show that (a) the caldera consists of two sectors separated by a N–S striking faulting zone and (b) the intra-caldera NW–SE faults and eruptive fissures in the central-eastern sector re-activated during the studied unrest episodes and represent possible pathways for the ascent of magma and/or gas to the surface. In this sector, maximum horizontal strain, recent volcanism (3.8–4ka), active degassing and seismicity concentrate. The fault re-activation is related to the dynamics of the caldera and not to tectonic stress. The deformation fields of the uplift and subsidence episodes are consistent with hydrothermal processes and degassing from a magmatic reservoir that is significantly smaller than the large (∼40km3) magma chamber responsible for the caldera formation. We provide evidence that the monitoring of the horizontal and vertical components of deformation improves the identification of active, aseismic faults. Accordingly, we suggest that future ground deformation models should include the re-activation of the detected structures.

Propagation, linkage, and interaction of caldera ring-faults: comparison between analogue experiments and caldera collapse at Miyakejima, Japan, in 2000

The formation of ring faults yields important implications for understanding the structural and dynamic evolution of collapse calderas and potentially associated ash-flow eruptions. Caldera collapse occurred in 2000 at Miyakejima Island (Japan) in response to a lateral intrusion. Based on geophysical data it is inferred that a set of caldera ring faults was propagating upward. To understand the kinematics of ring-fault propagation, linkage, and interaction, we describe new laboratory sand-box experiments that were analyzed through Digital Image Correlation (DIC) and post-processed using 2D strain analysis. The results help us gain a better understanding of the processes occurring during caldera subsidence at Miyakejima. We show that magma chamber evacuation induces strain localization at the lateral chamber margin in the form of a set of reverse faults that sequentially develops and propagates upwards. Then a set of normal faults initiates from tension fractures at the surface, propagating downwards to link with the reverse faults at depth. With increasing amounts of subsidence, interaction between the reverse- and normal-fault segments results in a deactivation of the reverse faults, while displacement becomes focused on the outer normal faults. Modeling results show that the area of faulting and collapse migrates successively outward, as peak displacement transfers from the inner ring faults to later developed outer ring faults. The final structural architecture of the faults bounding the subsiding piston-like block is hence a consequence of the amount of subsidence, in agreement with other caldera structures observed in nature. The experimental simulations provide an analogy to the observations and seismic records of caldera collapse at Miyakejima volcano, but are also applicable to caldera collapse in general.

A preliminary study of older hot spring alteration in Sevenmile Hole, Grand Canyon of the Yellowstone River, Yellowstone Caldera, Wyoming

Erosion in the Grand Canyon of the Yellowstone River, Yellowstone Caldera (640 ka), Wyoming, has exposed a cross section of older hydrothermal alteration in the canyon walls. The altered outcrops of the post-collapse tuff of Sulphur Creek (480 ka) extend from the canyon rim to more than 300 m beneath it. The hydrothermal minerals are zoned, with an advanced argillic alteration consisting of an association of quartz (opal) + kaolinite ± alunite ± dickite, and an argillic or potassic alteration association with quartz + illite ± adularia. Disseminated fine-grained pyrite or marcasite is ubiquitous in both alteration types. These alteration associations are characteristic products of shallow volcanic epithermal environments. The contact between the two alteration types is about 100 m beneath the rim. By analogy to other active geothermal systems including active hydrothermal springs in the Yellowstone Caldera, the transition from kaolinite to illite occurred at temperatures in the range 150 to 170 °C. An 40Ar/ 39Ar age on alunite of 154,000 ± 16,000 years suggests that hydrothermal activity has been ongoing since at least that time. A northwest-trending linear array of extinct and active hot spring centers in the Sevenmile Hole area implies a deeper structural control for the upflowing hydrothermal fluids. We interpret this deeper structure to be the Yellowstone Caldera ring fault that is covered by the younger tuff of Sulphur Creek. The Sevenmile Hole altered area lies at the eastern end of a band of hydrothermal centers that may mark the buried extension of the Yellowstone Caldera ring fault across the northern part of the Caldera.

Elliptical calderas in active tectonic settings: an experimental approach

Caldera volcanoes form due to collapse of a magma chamber roof into the underlying magma chamber. Many field, theoretical and experimental studies have postulated that calderas are delimited by reverse ring faults and are surrounded by peripheral concentric normal faults. In the simplest theoretical scenario, circular magma chambers produce circular calderas. Many calderas, however, are elliptical in shape, particularly those in extensional and compressive tectonic settings. Several factors may explain elliptical calderas. The first is the presence of an elliptical magma chamber, established by, for instance, preferential intrusion along pre-existing basement structures or differential spalling of the magma chamber walls. The second is the overlap (nesting) of several discrete calderas to form a single, larger elliptical structure. The third is asymmetric subsidence. The fourth is variable pre-collapse topography. A fifth possible factor is distortion of the caldera faults by the regional stress field during caldera formation. A sixth factor is the post-collapse distortion of the caldera structure due to continued regional deformation. To better understand relationships between caldera surface expression, reservoir geometry and regional tectonic stresses, we conducted scaled analogue experiments. These experiments examined the impact of regional stress and associated structures on calderas formed during evacuation of reservoirs (circular rubber balloons) of known dimensions and depths. The results show that, in principle, calderas produced in compression/extension experiments are elongated parallel to the direction of minimum horizontal compressive stress, despite the chamber beneath being circular in plan view. As a consequence, model ring fault orientation varied from steeply dipping where striking perpendicular to the minimum horizontal regional compressive stress, to shallower dips where striking parallel to the minimum horizontal regional compressive stress. This leads us to suggest that the influence of a regional stress field on caldera fault orientation during and/or after caldera formation may be significant in the development of elliptical calderas. In addition, such variation of caldera ring fault dip from steep to relatively shallow could influence location and behaviour of ring fissure eruptions.

Large-scale failures on domes and stratocones situated on caldera ring faults: sand-box modeling of natural examples from Kamchatka, Russia

Edifices of stratocones and domes are often situated eccentrically above shallow silicic magma reservoirs. Evacuation of such reservoirs forms collapse calderas commonly surrounded by remnants of one or several volcanic cones that appear variously affected and destabilized. We studied morphologies of six calderas in Kamchatka, Russia, with diameters of 4 to 12 km. Edifices affected by caldera subsidence have residual heights of 250–800 m, and typical amphitheater-like depressions opening toward the calderas. The amphitheaters closely resemble horseshoe-shaped craters formed by large-scale flank failures of volcanoes with development of debris avalanches. Where caldera boundaries intersect such cones, the caldera margins have notable outward embayments. We therefore hypothesize that in the process of caldera formation, these eccentrically situated edifices were partly displaced and destabilized, causing large-scale landslides. The landslide masses are then transformed into debris avalanches and emplaced inside the developing caldera basins. To test this hypothesis, we carried out sand-box analogue experiments, in which caldera formation (modeled by evacuation of a rubber balloon) was simulated. The deformation of volcanic cones was studied by placing sand-cones in the vicinity of the expected “caldera” rim. At the initial stage of the modeled subsidence, the propagating ring fault of the caldera bifurcates within the affected cone into two faults, the outermost of which is notably curved outward off the caldera center. The two faults dissect the cone into three parts: (1) a stable outer part, (2) a highly unstable and subsiding intracaldera part, and (3) a subsiding graben structure between parts (1) and (2). Further progression of the caldera subsidence is likely to cause failure of parts (2) and (3) with failed material sliding into the caldera basin and with formation of an amphitheater-like depression oriented toward the developing caldera. The mass of material which is liable to slide into the caldera basin, and the shape of the resulted amphitheater are a function of the relative position of the caldera ring fault and the base of the cone. A cone situated mostly outside the ring fault is affected to a minor degree by caldera subsidence and collapses with formation of a narrow amphitheater deeply incised into the cone, having a small opening angle. Accordingly, the caldera exhibits a prominent outward embayment. By contrast, collapse of a cone initially situated mostly inside the caldera results in a broad amphitheater with a large opening angle, i.e. the embayment of the caldera rim is negligible. The relationships between the relative position of an edifice above the caldera fault and the opening angle of the formed amphitheater are similar for the modeled and the natural cases of caldera/cone interactions. Thus, our experiments support the hypothesis that volcanic edifices affected by caldera subsidence can experience large-scale failures with formation of indicative amphitheaters oriented toward the caldera basins. More generally, the scalloped appearance of boundaries of calderas in contact with pre-caldera topographic highs can be explained by the gravitational influence of topography on the process of caldera formation.

Pre-eruptive magma mixing in ash-flow deposits of the Tertiary Rum Igneous Centre, Scotland

The Northern Marginal Zone of the Rum Igneous Centre is a remnant of an early caldera and its infill. It is composed of intra-caldera breccias and various small-volume pyroclastic deposits, overlain by prominent rhyodacite ash-flow sheets of up to 100 m thickness. The ash-flows were fed from a feeder system near the caldera ring-fault, and intrusive rhyodacite can locally be seen grading into extrusive deposits. A variety of features suggest that the ash-flows were erupted from a magma chamber that contemporaneously hosted felsic and mafic magmas: (i) chilled basaltic inclusions in rhyodacite; (ii) formerly glassy basaltic to andesitic enclaves with fluid-fluid relationships; (iii) feldspars with thick reaction rims enclosed in the basaltic to andesitic inclusions, yet with cores chemically resembling those of the rhyodacite: (iv) trace element compositions of the rhyodacite and the mafic enclaves form a mixing line between the end-member rhyodacite and basalt compositions. Additionally, textural and chemical features in the rhyodacite feldspar phenocrysts are consistent with magma mixing; (v) feldspars with resorption embayments cutting through internal zonation of the crystals; (vi) complexly zoned crystals with sieve-textured zones that are overgrown with euhedral zones; (vii) oscillatory zonation of feldspar phenocrysts in the rhyodacite, showing sharp increases in anorthite (ΔAn ≥ 10%) followed by gradual decrease in An-content (ΔAn ≤ 4%). This evidence points to eruption of ash-flows from a felsic magma chamber that was periodically replenished by injection of mafic magma. Diffusional mixing between the two magmas was permitted by temperature and compositional differences, but was slow due to the contrast in viscosities and densities. The Fe–Ti–P-enriched basic magma that replenished the chamber was degassing on entering the lower temperature environment and soon equilibrated thermally, followed by chemical exchange between the two end-member magmas. This process formed hybrid andesite enclaves enriched in trace elements beyond that caused by simple mixing, implying trace element diffusion in addition to bulk mixing. Eruption was caused by replenishment with, and degassing of, the basic magma and the chamber partially evacuated while the process of hybridisation was underway. The erupted products record magma mixing by chamber replenishment, blending of two magmas and elemental exchange in the magma chamber, and also physical mingling in the eruptive conduit.

A method for determining significant structures in a cloud of earthquakes

The interpretation of a cloud of earthquake hypocenters in terms of causative structures is not a simple task. Locations are subject to uncertainties, which will not be the same for every earthquake. The data should therefore not be interpreted simply by inspection, which is difficult in the case of three‐dimensional data anyway. Instead, we propose using the location uncertainties as a guide in processing the data. Earthquake locations are moved inside their uncertainty or confidence ellipsoids until a simplified picture of the earthquake cloud is obtained, which can then be interpreted in terms of some simplified structure such as faults. The aim of the approach is to give the simplest possible structure that is consistent with all the location and confidence ellipsoid data. The method is applied to three synthetic sets of data. These illustrate the potential and limitations of the method. Application to a real earthquake data set from Rabaul Caldera in Papua New Guinea gives an image of the caldera ring fault that suggests departures from the simple ring‐fault structure previously assumed. Sensitivity analysis on the Rabaul data shows that the method is not unduly sensitive to the assumptions that have to be made in applying it.

Incremental caldera collapse of Suswa volcano, Gregory Rift Valley, Kenya

Suswa volcano, located at 1°10′S, 36°20′E, is Quaternary in age (&lt;0.4 Ma), dominantly trachytic-phonolitic in composition, and has two calderas. Regional extension was a fundamental control on caldera collapse, providing pathways for the siting, drainage and recharge of magma chambers. Caldera I collapse was associated with magmatic overpressure from volatile exsolution, magma-water interaction, influx of denser magma and magma drainage at depth. Trachybasalt ash, trachyte globular-ash ignimbrites, trachyte pumice lapilli air-fall tuffs and carbonate-trachyte ignimbrites characterize the initial subsidence. Air-fall tuffs, erupted during caldera collapse at Longonot, are interbedded, suggesting a regional collapse event. Incremental, but dominantly Valles-type, collapse continued with the eruption of trachyte agglutinate flows from concentric ring-fractures outside the caldera ring-fault (Ring-Feeder Zone) and trachyte pumice lapilli air-fall tuffs from west caldera I. Following caldera I collapse, phonolite lava flows were erupted from the caldera floor. Centrally-erupted phonolite lava flows led to the construction of Ol Doinyo Onyoke lava cone. A pit-crater on the cone was a precursor to the collapse of caldera II, both of which were generated entirely by magma withdrawal. Regional decompression caused ring-fault bounded, block-resurgence of the caldera floor

An oxygen isotope study of hydrothermal alteration in the Lake City caldera, San Juan Mountains, Colorado

Abstract A 23-m.y.-old, fossil meteoric-hydrothermal system in the Lake City caldera (11 × 14 km) has been mapped out by measuring δ 18O values of 300 rock and mineral samples. δ 18O varies systematically throughout the caldera, reaching values as low as −2. Great topographic relief, regional tilting, and variable degrees of erosion within the caldera all combine to give us a very complete section through the hydrothermal system, from the surface down to a depth of more than 2000 m. The initial δ 18O value of the caldera-fill Sunshine Peak Tuff was very uniform (+7.2 ± 0.1), making it easy to determine the exact amount of 18O depletion experienced by each sample during hydrothermal alteration. Also, we have excellent stratigraphic control on depths beneath the mid-Tertiary surface, quantitative information on mineralogical alteration products, and accurate data on the shape of the central resurgent intrusion, which was the principal ‘heat engine’ that drove the hydrothermal circulation. Major conclusions are: (1) Although pristine mid-Tertiary meteoric waters in this area had δ 18 O ⋍ −14 , these fluids were 18O-shifted upward to about δ18O = −8 to −5 prior to entering the shallow convective system associated with the resurgent intrusive rocks. Although there was undoubtedly radial inflow toward the caldera from all directions, the highly fractured Eureka Graben, southwest of the caldera, was probably the principal source of recharge groundwater for the Lake City system. (2) Fluid flow within the caldera was dominated by three major categories of permeable zones: the porous megabreccia units (which dip outward from the resurgent dome), vertical fractures and faults related to resurgence, and the caldera ring fault itself. All of these zones exhibit marked 18O depletions, and they are also typically intensely mineralogically altered. (3) The resurgent intrusive stock and its contact metamorphic aureole of hornfels both experienced water/rock ratios lower than the permeable zones; however, they have similarly low δ 18O values because they were altered at higher temperatures. (4) Throughout the caldera, the δ 18O of Sunshine Peak Tuff decreases with increasing depth (about 6 per mil/km), indicative of a shallow thermal gradient, typical of a convective hydrothermal system. The near-surface portion of this gradient was controlled by the temperature drop associated with boiling in the uprising fluid. (5) Deeply circulating meteoric water rose along permeable ring fractures 3 to 5 km beneath the mid-Tertiary surface. These fluids were drawn into the shallow convective system through the lower, porous, megabreccia units. Near the resurgent intrusions, fluid flow was again directed upward where resurgence-related, near-vertical fractures intersect the megabreccia units.

Downsag calderas, ring faults, caldera sizes, and incremental caldera growth

Not all calderas conform to the currently favored model, in which a cylindrical block subsides as in cauldrons of deeply eroded volcanoes. Some calderas are downsagged structures, a fact deduced mainly from the backtilting of ignimbrite sheets assumed originally to have possessed an outwardly directed slope. Few calderas have postcaldera vent rings; postcaldera vents more commonly lie on a transverse line, occupy a central position, or are scattered, indicating that any possible caldera ring faults were not sufficiently significant lines of weakness to guide uprising magmas. About as many volcanoes have vent rings outside the caldera as inside, and vent rings are known also where caldera faults are lacking. These and other lines of evidence suggest that vent rings on intermediate or silicic volcanoes are more likely to be situated on cone sheet‐type fractures, caused by upwardly directed magmatic pressure and related to updoming, than on ring faults related to caldera subsidence. The prevailing view of caldera formation is a catastrophic one, but some calderas grow incrementally in response to a succession of moderatesized eruptions. On some volcanoes having a small caldera, the present caldera may be merely the youngest of a succession generated by relatively small‐magnitude events. Attention is drawn to the fact that cauldrons and ring‐dike complexes are a feature of intracontinental cratonic volcanoes. It is significant that the anomalously large Tertiary calderas of the Basin and Range Province, which conform more closely to the currently favored caldera model than do the calderas in epicontinental settings, also cut Precambrian crust.

Peralkaline ash flow tuffs and calderas of the McDermitt Volcanic Field, southeast Oregon and north central Nevada

The McDermitt volcanic field covers an area of 20,000 km2 in southeastern Oregon and northwestern Nevada and consists of seven large‐volume ash flow sheets that vented from 16.1 to 15 Ma ago. The volcanic field is characterized by peralkaline, high‐silica rhyolite, and all but one of the sheets are comendites. The ash flow field overlies a 1.5‐km‐thick section of Miocene Steens volcanic series and Steens Basalt, indicating a minimum age of 16.1 Ma for the Steens Basalt. The Orevada rift, a 200‐km‐long NNW‐striking structure, developed just prior to eruption of the mafic lavas. Each ash flow sheet resulted in the formation of a large collapse caldera along the Orevada rift. The center of the volcanic field is dominated by four overlapping and nested calderas; satellitic to these are three additional calderas. The areas adjacent to the incipient calderas subsided during ash flow eruption. Thickening of the ash flow sheets, monoclinal warping outside the caldera ring fault, and tilting in toward the caldera of blocks bounded by curvilinear faults all indicate regional subsidence prior to caldera collapse. The McDermitt caldera complex is highly mineralized; it contains ore deposits of Hg, Sb, Cs, Li, and U. The peralkaline tuffs have high contents of these elements and were the source rocks from which metals were leached by hydrothermal systems developed during the last stage of caldera‐related volcanism.