Caldera Structure Research Articles

Morphometric analysis of the submarine arc volcano Monowai (Tofua–Kermadec Arc) to decipher tectono-magmatic interactions

Morphometric analysis of multibeam bathymetry and backscatter data is applied to Monowai, a submarine volcano of the active Tofua–Kermadec Arc to map and document the structure and evolution of the volcanic centre. Low rates of erosion and sedimentation, and pervasive tectonic and magmatic processes, allow quantification through detailed structural analysis and measurement of deformation. The Slope, Aspect, Curvature, Rugosity, and Hydrology (flow) tools of ArcGIS provide a robust structural interpretation and the development of a model of Monowai evolution.A nested caldera structure with a volume of ~31km3 and a stratovolcano of ~18km3 dominate the magmatic constructs. The outer caldera is elongate along 125°, and the inner caldera along 135°. Numerous parasitic cones and fissure ridges are also observed, oriented at 039° and 041°, respectively. Northeast trending faults (with a regional average strike of 031°) are widespread within this part of the backarc, forming a nascent rift graben to the west of the Monowai caldera complex. The distribution of throw varies spatially, reaching a maximum total along-rift of 320m and across rift of 120m, with greater throw values measured in the west.Elongation directions of the two nested calderas are near-perpendicular to the trends of faults and fissure ridges. The inner caldera is more orthogonal to the magmatic constructs (fissure ridges and aligned vent cones) and the outer caldera is approximately orthogonal to the regional fault fabric, suggesting a strong interaction between magmatic and tectonic processes, and the directions of the horizontal principal stress. We present a detailed morphometric analysis of these relationships and the data are used to interpret the spatial and temporal evolution of the tectono-magmatic system at Monowai, and classify the type of rifting as transtensional. Similar analysis is possible elsewhere in the Kermadec backarc and within other regions of submarine volcanism.

Caldera structure, amount of collapse, and erupted volumes: The case of Bolsena caldera, Italy

Calderas are common on volcanoes, but their structure is seldom visible. The 19-km-wide Bolsena caldera, Italy, formed between 0.6 and 0.2 Ma. The largely preserved structural rim and subsurface data make Bolsena ideal to investigate caldera structure in relation to the subsidence and erupted volume. In this paper, we use remote sensing, field analysis, and available subsurface data. At the surface, the caldera passes from a downsag (south rim) to a narrow and densely faulted area (north rim), with outer normal and inner reverse faults. The caldera structure on the widely faulted east rim appears to be scale dependent, with a staircase-like fault zone (larger scale), horst-and-graben–like structures (intermediate scale), and domino-like structures (smaller scale). Subsurface data indicate asymmetric collapse, with a northward increase in subsidence, ranging from diffuse (to the south) to focused (to the north) deformation at the surface. The collapse rate, constant between ca. 490 and 175 ka, was more than magma output between ca. 330 and 130 ka, highlighting significant (∼200 m) and prolonged (∼200 ka) posteruptive subsidence. As the nearby Latera caldera (west rim of Bolsena) was mostly active between ca. 265 and 160 ka, much of the subsidence at Bolsena may be related to this activity, suggesting a common magmatic reservoir. The subsidence-related structural variations along the caldera rim and the significant posteruptive subsidence found at Bolsena have not been found in other calderas.

Hydrologic, Magmatic, and Tectonic Controls on Hydrothermal Flow, Taupo Volcanic Zone, New Zealand: Implications for the Formation of Epithermal Vein Deposits

Geologic controls on development of high-flux hydrothermal conduits that promote epithermal ore formation are evaluated at large and small scales for geothermal systems of the Taupo Volcanic Zone, New Zealand. Most geothermal systems occur within a rifted volcanic arc (~150 km long) dominated by silicic volcanism, and they occur in association with major faults near caldera structures or within accommodation zones that transfer ex tension between rift segments . The geothermal systems are hosted in a thick sequence (1:>3 km) of young vol canic deposits that rest unconformably on weakly metamorphosed Mesozoic argillite and graywacke. Flow regimes and permeability controls in one extinct (Ohakuri) and six active (Broadlands-Ohaaki, Waiotapu, Roto kawa, Waimangu, Te Kopia, and Orakeikorako) geothermal systems show that in general, hydrothermal fluid flow is controlled by (1) heat from magmatic intrusions which drives convective circulation; (2) intergranular host-rock porosity and permeability; (3) fault-fracture network permeability produced by tectonism, volcanism, and/or diking; (4) pipelike vertical conduits produced by volcanic and hydrothermal eruptions; and (5) hydro thermal alteration and mineral deposition that may cause heterogeneity in the porosity and permeability of a fluid reservoir. Such controls influence fluid flow within three distinctive depth zones: (1) a feed zone (>2,000 m depth), (2) an epithermal mineralization zone ( 100 km 3 that encloses geothermal reservoirs and high-flux fluid conduits. Fracture-dominated flow becomes important with decreasing porosity induced by hydrothermal alteration. In the discharge zone, the re duction in confining pressure, combined with mineral deposition and alteration, hydrothermal eruptions, and interplay of hot and cold waters create complex, but strongly localized flow paths that feed hot springs. The permeability structure conducive to epithermal vein formation is analogous to a geothermal well: short in horizontal dimension (10s:100s m) but long in vertical dimension (>1,500 m) and possibly pipelike in shape. Episodic high-flux occurs over time scales of tens to thousands of years to accumulate sufficient amounts of gold and silver to form orebodies. During these episodes when faults and fractures are dilated, development of an upward-expanding column of boiling fluid promotes rapid ascent and high mass flow but also promotes silica and calcite precipitation, which can quickly reduce hydrothermal flow. Seismic activity and/or dike intru sion create and reactivate these high-flux pathways through extension and extensional shearing, caused by low differential stresses. The Taupo Volcanic Zone is highly prospective for epithermal-style mineralization, but the predominance of weak porous host rocks at shallow depths is prone to disseminated-style mineralization (e.g., Ohakuri). Structurally controlled mineralization forms in volcanic rocks where they have been embrittled by silicification through seismicity and fault displacement, caldera-forming eruptions, and dike intrusion.

In situ hydroclastic fragmentation of subaqueous ponded lavas; New Senator caldera, Abitibi greenstone belt, Quebec, Canada

Subaqueous ponded lavas have been recognized within the structure of the New Senator caldera in the Blake River Group of the Abitibi greenstone belt, Quebec, Canada. Sub-vertical to near-vertical dips of extrusive volcanic facies within the southern sector of the caldera permit the identification of the internal architecture of an Archean subaqueous volcanic complex. Detailed facies analyses of the dominantly mafic sequences have shown two localities with atypical primary volcanic structures and facies inconsistent with those observed at modern subaqueous mafic seamounts. Combined with the use of facies-specific geochemical analyses, a coherent model for the two localities accounting for all observed volcanic structures and features has been developed.Mapping completed at a scale of 1:100 at Localities 1 and 2 reveals a series of sub-parallel to parallel hyaloclastite-rich horizons separating aphanitic to medium grained ponded units. Several primary volcanic structures are observed within the hyaloclastite horizons and are identified as pillowed forms, cigar forms and v-shaped structures. Ponded units range in thickness from 2.5 to 5m, with the exception of one unit which is approximately 30m thick. In central portions, this thick unit is medium-grained and has a sub-ophitic texture. Units at Locality 1 strike approximately NW, while those at Locality 2 strike ENE. Several families of mafic dykes cross-cut Locality 1 and trend E-W and NNE. Both volcaniclastic and effusive facies have similar compositions, affinities and trace and rare earth element signatures. All facies have a tholeiitic affinity, range in composition from basalt to andesite and have MORB-type trace and rare earth element signatures. Locality 1 appears slightly more evolved than Locality 2, with later dykes and a late sill being the most evolved facies.Volcanic facies at Localities 1 and 2 are consistent with subaqueous ponded lavas. Bounding synvolcanic structures and volcanic facies validate this interpretation and indicate these ponded lavas are hosted within a large synvolcanic depression at the summit of a mafic shield complex. Primary volcanic structures within hyaloclastite horizons formed via in situ fragmentation when water entered the system as ponded lavas began to cool. Water entered the system through synvolcanic fractures and encountered semi-molten pockets of lava, leading to hydroclastic fragmentation of the lavas. V-shaped and cigar structures formed during more energetic events and changed laterally into pillowed forms as energy dissipated from the system.

Open System evolution of peralkaline trachyte and phonolite from the Suswa volcano, Kenya rift

Suswa is the southernmost volcanic center in the Central Kenya Peralkaline Province (CKPP) and represents the only salic center to have erupted significant volumes of peralkaline silica-undersaturated lavas and tuffs (trachyte, nepheline trachyte and phonolite). The eruptive products of Suswa can be clearly divided into two series, which correspond closely to the volcano's eruptive history. The earlier series (C1) includes lavas and tuffs that built the initial shield volcano (pre-caldera, unit S1) and erupted during the first caldera collapse (syn-caldera, units S2–S5); these rocks are dominated by peralkaline, silica-saturated to mildly under-saturated trachyte. The later series (C2) includes lavas and tuffs that erupted within the caldera structure following the initial collapse (post-caldera, units S6–S7) and during the creation of a second smaller, nested caldera and central “island block” (ring trench group, RTG, unit S8); these rocks are dominated by peralkaline phonolite. In this study, we combine mineralogical evidence with the results of major-element, trace-element, and thermodynamic modelling to propose a complex model for the origin of the Suswa volcano. From these results we conclude that C1 is the result of protracted fractional crystallization of a fairly “dry” alkali basalt (<1wt.% H2O) under relatively high pressure (400MPa) and low oxygen fugacity (FMQ to FMQ-1). Although C1 appears to be primarily the result of closed system processes, a variety of open system processes are responsible for C2. We propose that crystallization of C1 trachyte resulted in the formation of a syenitic residue, which was assimilated (Ma/Mc=0.1) during a later stage of recharge and differentiation of alkali basalt to produce post-caldera ne-trachyte. Post-caldera (S6-7) phonolites were in turn the result of fractional crystallization of this ne-trachyte. RTG phonolites, however, are the result of feldspar resorption prompted perhaps by magma recharge as evidenced by reverse zoning in alkali feldspar and linear compatible trace element patterns.

Rock magnetism and compositional investigation of Brown Tuffs deposits at Lipari and Vulcano (Aeolian Islands — Italy)

A rock-magnetic investigation was carried out on the nonwelded ash deposits of the Brown Tuffs (Aeolian Islands, southern Tyrrhenian Sea) to improve the stratigraphic correlation between the deposits cropping out on Lipari and Vulcano islands and locate their source area. The study was supplemented by petrographical and geochemical analyses on selected strata, with the intent to compare the Brown Tuffs to other rocks emplaced at Vulcano in the same time span. More than 30 levels were sampled in the intermediate (56 ± 4 ka > IBT > 21–22 ka) and upper (21–22 ka > UBT) parts of the Brown Tuffs sequences on the two islands. Their characteristic remanent magnetization (ChRM) directions were derived from stepwise thermal demagnetization, and the magnetic fabric from measurements of the anisotropy of magnetic susceptibility. The levels with indistinguishable ChRM directions were regarded as coeval and to form an individual stratigraphic unit. The units were referred to the Brown Tuffs sequence of Lucchi et al. (2008) on the grounds of their emplacement age, provided by comparison of their mean paleomagnetic direction with the paleosecular variation curves of the southern Tyrrhenian region, as well as the field constraints. The closer correlation between the sequences of Lipari and Vulcano contributes to a better understanding of the volcanic activity that produced the Brown Tuffs, and shows that most of the IBT and the oldest UBT levels were emplaced in a short time span, between ≈ 24 and 20–17 ka. The magnetic fabric is typically well developed, but at most sites the magnetic foliation is very close to horizontal and no imbrication is defined. The source area of the Brown Tuffs parent pyroclastic flows, as constrained from the intersection of the magnetic lineations, falls in the northeastern part of La Fossa Caldera structure. Although limited to major elements, compositional data provide further indication about the parent plumbing system and its behaviour. Magma batch(es) involved in the IBT eruptions have homogeneous features and underwent frequent refilling and tapping processes. Conversely, those involved in the early UBT eruptions are compositionally more variable. This suggests more complex evolution and plumbing system activity: the UBT eruptions represent either residual mafic magmas from the previous eruptions or the arrival of new, fresh shoshonitic magma in the system.

Variations in eruption style during the 1931 A.D. eruption of Aniakchak volcano, Alaska

The 1931 A.D. eruption of Aniakchak volcano, Alaska, progressed from subplinian to effusive eruptive style and from trachydacite to basaltic andesite composition from multiple vent locations. Eyewitness accounts and new studies of deposit stratigraphy provide a combined narrative of eruptive events. Additional field, compositional, grain size, componentry, density, and grain morphology data document the influences on changing eruptive style as the eruption progressed. The eruption began on 1 May 1931 A.D. when a large subplinian eruption column produced vesicular juvenile-rich tephra. Subsequent activity was more intermittent, as magma interacted with groundwater and phreatomagmatic ash and lithic-rich tephra was dispersed up to 600 km downwind. Final erupted products were more mafic in composition and the eruption became more strombolian in style. Stratigraphic evidence suggests that two trachydacitic lava flows were erupted from separate but adjacent vents before the phreatomagmatic phase concluded and that basaltic andesite lava from a third vent began to effuse near the end of explosive activity. The estimated total bulk volume of the eruption is 0.9 km 3, which corresponds to approximately 0.3 km 3 of magma. Eruption style changes are interpreted as follows: (1) a decrease in magma supply rate caused the change from subplinian to phreatomagmatic eruption; (2) a subsequent change in magma composition caused the transition from phreatomagmatic to strombolian eruption style. Additionally, the explosion and effusion of a similar magma composition from three separate vents indicates how the pre-existing caldera structure controlled the pathway of shallow magma ascent, thus influencing eruption style.

Mechanical and geometric controls on the structural evolution of pit crater and caldera subsidence

[1] Pit craters and calderas are volcanic depressions produced by subsidence of a magma reservoir roof. To identify how geometric and mechanical factors may influence the structural evolution of this subsidence, we used two-dimensional distinct element method numerical models. The reservoir host rock was represented as an assemblage of bonded circular particles that interact according to elastic-frictional laws. Varying particle and bond properties produced a range of bulk material properties characteristic of natural rock masses. Fracturing results when bonds break, once their shear or tensile strength is exceeded. The magma reservoir was represented as a region of nonbonded low-friction particles. Withdrawal of magma was simulated by incrementally reducing the area of the reservoir particles. Resultant gravity-driven failure and subsidence of the reservoir roof were explicitly replicated. Interaction of the roof's strength, Young's modulus, thickness/diameter ratio (T/D), and the reservoir's shape yields a variety of model structures and subsidence styles. In conceptual terms, four end-member subsidence styles developed: (1) “central sagging” favored by low strength and low T/D; (2) “central snapping” favored by high strength, low T/D, and a sill-like reservoir shape; (3) “single central block” favored by low to intermediate strength, high Young's modulus, and intermediate T/D; and (4) “multiple central blocks” favored by high strength, low Young's modulus, and high T/D. Most model realizations incorporated some combination of each style, however. The models provide a geomechanical framework for understanding natural pit crater or caldera structures, as at Nindiri (Nicaragua), Fernandina (Galapagos), Dolomieu (La Reunion), and Miyakejima (Japan).

Geology and geochemistry of volcanic centers within the eastern half of the Sonoma volcanic field, northern San Francisco Bay region, California

Volcanic rocks in the Sonoma volcanic field in the northern California Coast Ranges contain heterogeneous assemblages of a variety of compositionally diverse volcanic rocks. We have used field mapping, new and existing age determinations, and 343 new major and trace element analyses of whole-rock samples from lavas and tuff to define for the first time volcanic source areas for many parts of the Sonoma volcanic field. Geophysical data and models have helped to define the thickness of the volcanic pile and the location of caldera structures. Volcanic rocks of the Sonoma volcanic field show a broad range in eruptive style that is spatially variable and specific to an individual eruptive center. Major, minor, and trace-element geochemical data for intracaldera and outflow tuffs and their distal fall equivalents suggest caldera-related sources for the Pinole and Lawlor Tuffs in southern Napa Valley and for the tuff of Franz Valley in northern Napa Valley. Stratigraphic correlations based on similarity in eruptive sequence and style coupled with geochemical data allow an estimate of 30 km of right-lateral offset across the West Napa-Carneros fault zones since ∼5 Ma. The volcanic fields in the California Coast Ranges north of San Francisco Bay are temporally and spatially associated with the northward migration of the Mendocino triple junction and the transition from subduction and associated arc volcanism to a slab window tectonic environment. Our geochemical analyses from the Sonoma volcanic field highlight the geochemical diversity of these volcanic rocks, allowing us to clearly distinguish these volcanic rocks from those of the roughly coeval ancestral Cascades magmatic arc to the west, and also to compare rocks of the Sonoma volcanic field to rocks from other slab window settings.

EL DISTRITO DE ORO EL BRONCE Y SU RELACION CON LA CALDERA MORRO HEDIONDO, REGION DE VALPARAISO, CHILE

RESUMEN:The El Bronce gold district is located in Central Chile (32°11'S-70°56'W), and contains epithermal polimetallic (Au, Ag, Cu, Pb, Zn) vein deposits. The district has been mined since the end of the 18th century and its estimated production up to the present is more than 4 x 105 oz Au and 106 oz Ag, besides an important volume of copper. The geological framework of the district is represented by Cretaceous volcanic and intrusive rocks. The oldest stratigraphic units are the Cerro Morado Formation (late Lower Cretaceous) and Las Chilcas Formation (late Lower Cretaceous-Upper Cretaceous (?)), formed by andesitie breccias, agglomerates and lava flows, with volcaniclastic interbeddings. An Upper Cretaceous eruptive centre unconformably overlies these units cutting the volcanic sequence. The centre is defined by the presence of a caldera structure (Morro Hediondo Caldera), of a 7 km radius semicircular section. Dacitic lapilli tuffs (K-Ar: 86±3 Ma), andesitic lavas (K-Ar: 82-80 Ma), and breccias, assigned to the Lo Valle Formation, are the rocks associated with this centre. Two groups of intrusive rocks are recognized in the area. The western group is the oldest one, and is composed of quartz monzodiorites belonging to a batholith dated (K-Ar) as 134 - 86 Ma, which extends between 31° and 32°S. The eastern group is younger and is composed by dioritie (andesitic) to granodioritic (dacitic) stocks, dikes and sills, among which the outstanding components are the Porfido Petorea (K-Ar: 86±3 Ma) and a Ring Dike (K-Ar: 80-79 Ma), which define the outer border of the above mentioned caldera. Extensive hydrothermal alteration zones considered as genetically associated with the intrusives, according to their spatial distribution and K-Ar data (109±4; 82±9; 81±14 Ma) also crop out in the district. The rocks of the area are cut by a number of mainly NW striking faults and dikes, which played an important role in the control of mineralization. Some of these structures show a radial or concentric pattern to me Hediondo Caldera. About 90 ore bodies, mainly polimetallie veins, some copper veins, and one copper breccia pipe (Dulcinea Mine) have been identified. The most important and productive ore deposit is the El Bronce vein, emplaced in a 7 km long, NS-N20°E striking fault. Different lenticular, structurally controlled ore shoots are observed along the fault, ranging from 100-600 m in length, 200-400 m in depth, and 1-20 m in width. The mineralogy consists of pyrite, sphalerite, chalcopyrite, galena, tennantite-tetrahedrite, bornite, arsenopyrite, pyrrhotite, hematite, and magnetite in that order of abundance. Gangue minerals are quartz (massive, microcrystalline, amethyst, and crystalline-transparent), ankerite, siderite, and barite. These minerals occur as massive concentrations, disseminated grains and in stockworks. A barren andesitic dike is also emplaced along the mineralized structure. A few meters thick, hydrothermally altered (sericite, clays, chlorite and carbonates) halo surrounds the vein. According to paragenetic studies, mineralization would have been produced in three stages, gold and silver being deposited during the two earlier ones, associated with quartz, pyrite, sphalerite and chalcopyrite. Fluid inclusion studies show homogenization temperatures ranging between 150° and 349°C, and salinities (weight %NaCI) from 1 to 10, while sulfide isotope analyses yielded values of 34S between +0,5 and +2,3. The above data support the hypothesis that the ore was formed at a depth of less than 1,000 m below paleosurface, due to a mixing process between hydrothemal magmatic ore fluids and meteoric ground waters. Keywords: Paragenesis, Hidrothermal alteration, El Bronce district, Epithermal ores, Gold, Morro Hediondo caldera, K-Ar Dating, Fluid inclusions, Upper Cretaceous, Valparaiso Region, Chile.

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.

Gravity structure of Akan composite caldera, eastern Hokkaido, Japan: Application of lake water corrections

Abstract Akan volcano, eastern Hokkaido, Japan, is characterized by a rectangular-shaped caldera (Akan caldera: 24 km by 13 km) with a complex history of caldera-forming eruptions during the Quaternary. A new Bouguer anomaly map of the caldera is presented on the basis of a gravity survey around Akan volcano. As part of and in addition to this survey, we applied gravimetry over the frozen caldera lake including lake water corrections. The Bouguer map shows the distribution of at least three sub-circular minima indicative of multiple depressions inside the caldera. Lake water corrections, performed by a numerical integration method using rectangular prisms, sharpen edges of the sub-circular minima. This gravity feature is consistent with geological investigations suggesting that caldera-forming eruptions of Akan volcano occurred from at least three different sources. It is concluded that Akan caldera can be described as a composite caldera with three major depressed segments.

Stress fields controlling the formation of nested and overlapping calderas: Implications for the understanding of caldera unrest

Commonly, the formation of a collapse caldera does not necessarily imply the end of the volcanic activity in the area. In many cases, successive calderas may form close to the previous collapse depression or intersecting it leading to overlapping collapse structures. Occasionally, subsequent caldera collapses may take place at the interior of the first caldera creating nested collapse structures. During the last years several authors have investigated numerically how the stress field around magma chambers may favour the formation of collapse calderas assuming that the host rock surrounding the magmatic reservoir behaves as a linear elastic homogeneous medium. The numerical models presented in this work study how a caldera collapse may modify the stress field of a volcanic area and hence the conditions for the formation of future collapse calderas. Our models take into account the effect of the collapse structure considering it as a mechanical discontinuity. We also investigate the mechanical influence of the intra- and extra-caldera deposits on the formation of new calderas. All the numerical models are two-dimensional assuming plane strain and considering that the surrounding crust behaves as a linear homogeneous elastic material. The computational domain corresponds to a cross-section of the upper crust (50 × 25 km) and magma chambers are modelled as sill-like cavities located at a certain depth below the Earth's surface. The existing collapse caldera depression is 8 km wide and 2.75 km deep, however we consider the caldera infill (i.e. intra-caldera material) to be 1.25 or 1.75 km thick. We assume as loading conditions an underpressure of 10 MPa imposed at the chamber walls, that is, negative excess pressure in the chamber. In some numerical runs we have considered the existence of a previous ring fault by introducing a thin and elongate vertical weak zone at the caldera margins. We find that the stress field around shallow-level magma chambers favouring the formation of either nested or overlapping caldera structures depend on: 1) the dimensions of the previous caldera collapse structure, 2) the size of the new magma chamber susceptible to generate the subsequent caldera-forming event, and 3) the mechanical properties of the surrounding host rock and the syn- and post-collapse materials. Moreover, results obtained in this paper may be relevant to the interpretation of unrest episodes in active caldera systems. Our numerical results confirm that the formation of multiple calderas (nested, overlapping, or separate) imply the development of new shallow magma chambers different from the one that lead to the first collapse caldera. In consequence, interpretation of magmatic unrest episodes in active collapse calderas should not be based on the assumption that the responsible for the current unrest is the same system that originated the caldera.

Anisotropy of magnetic susceptibility analysis of the Cantera Ignimbrite, San Luis Potosi, México: flow source recognition

Abstract Anisotropy of magnetic susceptibility (AMS) was selected as the key technique to find the source of the widespread Cantera Ignimbrite and to seek its possible relationship with the San Luis Potosí Caldera. Eighteen sites (372 specimens from 155 cores) from the Oligocene Cantera Ignimbrite were sampled. AMS was measured on a KLY2 Kappabridge. AMS data were processed with Anisoft 3 software using Jelinek statistics as well as ‘SpheriStat’ principal components and density distribution. Mean susceptibilities range from 290 to 5026 × 10-6 SI (average = 2526 × 10-6 SI). The anisotropy degree (P j) ranges from 1.005 to 1.055, with only one site displaying a value of 1.134 (P j average = 1.031). AMS ellipsoid shapes are mostly oblate, with the T-factor ranging from 0.843 to 0.144 (T average = 0.529), although one site is mainly prolate (T = -0.005), and three additional sites have an important proportion of prolate specimens. Magnetic fabrics of most sites shows k3 axes around nearly circular distributions and k 1-k 2 axes around elongated-girdle distributions defining sub-horizontal foliation planes; exceptions to this are related to sites with a significant percentage of prolate specimens. Flow directions inferred from AMS analysis indicate several ignimbrite sources located along selected NW-SE linear features (faults and fractures such as El Potosino Fault) as well as along the rim of the caldera structure. The geometry of volcanic outcrops, the NW-SE faulting-fracture system, as well as the AMS results suggest that this is a caldera structure resembling the trapdoor-type (Lipman, 1997).

Thermal infrared image analysis of a quiescent cone on Piton de la Fournaise volcano: Evidence of convective air flow within an unconsolidated soil

We report on the detection of air convection with infrared thermal images for two quasi-circular craters, 20 m and 40 m wide, forming the volcanically inactive cone of Formica Leo (Reunion Island). The thermal images have been acquired from an infrared camera at regular time intervals during a complete diurnal cycle. During the night and at dawn, we observe that the rims are warmer than the centers of the craters. The conductivity contrast of the highly porous soils filling the craters and their 30° slopes are unable to explain the systematic temperature drop from rim to centers. We suggest that this signal could be attributed to air convection with gas entering the highly permeable soil at the center of each crater, then flowing upslope along the bottom of the soil layer, before exiting it along the crater rims. To quantify this process, we present a two-dimensional numerical modelling of air convection in a sloped volcanic soil with a surface temperature evolving between day and night. This convection depends on a unique dimensionless equivalent Rayleigh number Ra eq which is the product of the standard Rayleigh number with the volumetric heat capacity ratio of the air and the soil. The convective flow is unsteady: during some periods, the convective flow is entirely confined within the soil, and at other times air enters the crater at its center and exits it at the rim crests. When Ra eq = 6000, a value likely compatible with the soil permeability and the geothermal heat flux, a very strong transient cold air plume occasionally develops along the center of the crater. The interval of time between two plumes only depends on the thermal fluctuations within the top boundary layer of the convective cell, and thus is not contrasted by the diurnal cycle. The detachment of a cold plume can occur at any time, after few days of quiescence, and lasts several hours. During the whole convective cycle, the rim to center temperature drop persists and has an amplitude and a shape having an excellent agreement with that found in the IR-images. This work constitutes a preliminary step to explore the deep thermal structure of the active caldera of Bory-Dolomieu and could help to improve the understanding of volcanic hazards of the Reunion volcano.

Pucarilla-Cerro Tipillas volcanic complex: the oldest recognized caldera in the southeastern portion of central volcanic zone of Central Andes?

We recognize the most eastern and oldest collapse caldera structure in the southern portion of the Central Volcanic Zone of the Andes. A description of Middle-Upper Miocene successions related to explosive- effusive events is presented. The location of this centre close to Cerro Galn Caldera attests a recurrence in the volcanism between 12 and 2 Ma in this portion of the Altiplano - Puna Plateau.

Rhyolitic calderas and centers clustered within the active andesitic belt of Ecuador's Eastern Cordillera

In the Ecuadorian volcanic arc a cluster of scattered rhyolitic and dacitic centers within the mainly andesitic Eastern Cordillera includes large caldera structures (Chalupas, Chacana, Cosanga) as well as smaller edifices, built upon the Paleozoic-Mesozoic metamorphic basement. At the Chacana caldera magmatism dates from 2.7 Ma to historic times. These centers erupted enormous ash flows and thick pumice lapilli falls that covered the InterAndean Valley near Quito. The role of the 50-70 km-thick crust with a notable negative gravity anomaly appears to be related to the generation of this highly silicic magmatism occurring along the crest of the Andes in the NVZ.

Investigations of deep resistivity structures at the Wairakei geothermal field

The Wairakei geothermal field was the proving ground for the use of electrical resistivity methods for geothermal exploration. At this site it was first demonstrated that a large contrast in resistivity existed between geothermal ground and the cold surroundings. Within the top 500 m of the geothermal field, low-resistivity (5–10 Ωm) reflects the effects of both the hot saline water in the pore spaces and the conductive rock-matrix. The first surveys at Wairakei used a Wenner array (a ∼550 m) to measure the resistivity values along tracks throughout the field; contour maps of the resistivities were used to estimate the lateral extent of the geothermal waters at a few hundred metres depth. In the late 1960s the Wenner array was superseded by the Schlumberger array (AB/2 = 500 m and 1000 m), which enabled deeper penetration and better definition of the extent of the geothermal waters. These early surveys showed that the bounds of the geothermal waters were often sharp, leading to the concept that a ‘resistivity boundary’ could be defined for New Zealand's liquid-dominated geothermal fields. As new methods of measuring electrical structure with greater precision became available, Wairakei was often chosen as the testing ground. In 1982, a deep penetrating tensor bipole–dipole survey was made over the northern part of the field to provide close-spaced data across the boundary, leading to a better definition of the boundary and its change with depth. A similar survey over the eastern edge of the field revealed a very sharp boundary with an apparent correlation between its location and a deeper caldera structure. The first application of tensor time-domain resistivity (tensor LOTEM) was made across the north-western edge of Wairakei in 1997 giving both a detailed image of the boundary and showing that it was possible to trace geological structures within the geothermal field. The most commonly used method for resistivity surveying in the 21st century is magnetotellurics (MT), which determines the resistivity of the ground from the currents induced in the earth by natural variations of the earth's magnetic field. While MT has not yet been applied within the Wairakei field, the method has been used to investigate the setting of the geothermal systems of the Taupo Volcanic Zone. These data reveal key elements of the deep source of heat, including the presence of a small fraction of connected molten rock at a depth of about 10 km and the possible pooling of magma at about 15 km.

Merging active and passive data sets in traveltime tomography: The case study of Campi Flegrei caldera (Southern Italy)

ABSTRACTWe propose a strategy for merging both active and passive data sets in linearized tomographic inversion. We illustrate this in the reconstruction of 3D images of a complex volcanic structure, the Campi Flegrei caldera, located in the vicinity of the city of Naples, southern Italy. The caldera is occasionally the site of significant unrests characterized by large ground uplifts and seismicity. The P and S velocity models of the caldera structure are obtained by a tomographic inversion based on travel times recorded during two distinct experiments. The first data set is composed of 606 earthquakes recorded in 1984 and the second set is composed of recordings for 1528 shots produced during the SERAPIS experiment in 2001. The tomographic inversion is performed using an improved method based on an accurate finite‐difference traveltime computation and a simultaneous inversion of both velocity models and earthquake locations. In order to determine the adequate inversion parameters and relative data weighting factors, we perform massive synthetic simulations allowing one to merge the two types of data optimally. The proper merging provides high resolution velocity models, which allow one to reliably retrieve velocity anomalies over a large part of the tomography area. The obtained images confirm the presence of a high P velocity ring in the southern part of the bay of Pozzuoli and extends its trace inland as compared to previous results. This annular anomaly represents the buried trace of the rim of the Campi Flegrei caldera. Its shape at 1.5 km depth is in good agreement with the location of hydrothermalized lava inferred by gravimetric data modelling. The Vp/Vs model confirms the presence of two characteristic features. At about 1 km depth a very high Vp/Vs anomaly is observed below the town of Pozzuoli and is interpreted as due to the presence of rocks that contain fluids in the liquid phase. A low Vp/Vs body extending at about 3–4 km depth below a large part of the caldera is interpreted as the top of formations that are enriched in gas under supercritical conditions.

The structure and formation of mud volcano summit calderas

Circular depressions bounded by inward-dipping faults found at the upper terminations of large mud volcano systems (&gt;500 m diameter) are termed ‘mud volcano summit calderas’. From new mapping and comparison with previously identified examples we describe a series of common structural and morphological features found at a number of calderas and propose a mechanism for caldera formation. A typical example consists of concentric deformational and volcanic zones including an outermost topographic rim, inward-dipping circular fault system, ‘moat’ and raised central ‘pedestal’ of freshly extruded mud volcanic sediment. This distinctive ‘moat and pedestal’ morphology can be explained in terms of the quantity and rheology of extruded mud, and appears characteristic of calderas from the South Caspian Basin and elsewhere. The association of fresh mud volcanic deposits with the calderas in combination with extensive literature on caldera modelling leads us to conclude that the most likely causal mechanism is subsidence as a result of evacuation of fluids and sediment from shallow structural levels during eruptions. Summit calderas mapped in this study characterize dormant periods of mud volcanism in subaerial and submarine settings, and appear to be a common structural element in the extrusive domain of mud volcano systems.