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1.
Changing stresses in multi-stage caldera volcanoes were simulated in scaled analogue experiments aiming to reconstruct the mechanism(s) associated with caldera formation and the corresponding zones of structural weakness. We evaluate characteristic structures resulting from doming (chamber inflation), evacuation collapse (chamber deflation) and cyclic resurgence (inflation and deflation), and we analyse the consequential fault patterns and their statistical relationship to morphology and geometry. Doming results in radial fractures and subordinate concentric reverse faults which propagate divergently from the chamber upwards with increasing dilation. The structural dome so produced is characterised bysteepening in the periphery, whereas the broadening apex subsides. Pure evacuation causes the chamber roof to collapse along adjacent bell-shaped reverse faults. The distribution of concentric faults is influenced by the initial edifice morphology; steep and irregular initial flanks result in a tilted or chaotic caldera floor. The third set of experiments focused on the structural interaction of cyclic inflation and subsequent moderate deflation. Following doming, caldera subsidence produces concentric faults that characteristically crosscut radial cracks of the dome. The flanks of the edifice relax, resulting in discontinuous circumferential faults that outline a structural network of radial and concentric faults; the latter form locally uplifted and tiltedwedges (half-grabens) that grade into horst-and-graben structures. This superimposed fault pattern also extends inside the caldera. We suggest that major pressure deviations in magma chamber(s) are reflected in the fault arrangement dissecting the volcanoflanks and may be used as a first-order indication of the processes and mechanisms involved in caldera formation.  相似文献   

2.
Regional-scale faulting, particularly in strike-slip tectonic regimes, is a relatively poorly constrained factor in the formation of caldera volcanoes. To examine interactions between structures associated with regional-tectonic strike-slip deformation and volcano-tectonic caldera subsidence, we made scaled analogue models. Tabular (sill-like) inclusions of creamed honey in a sand/gypsum mix replicated shallow-level granitic magma chambers in the brittle upper crust. Lateral motion of a base plate sited below half the sand/gypsum pack allowed simulation of regional strike-slip deformation. Our experiments modelled: (1) strike-slip deformation of a homogeneous brittle medium; (2) strike-slip deformation of a brittle medium containing a passive magma reservoir; (3) caldera collapse into sill-like magma reservoirs without regional strike-slip deformation; and (4) caldera collapse into sill-like magma reservoirs after regional strike-slip deformation. Our results show that whilst the magma chamber shape principally influences the development and geometry of volcano-tectonic collapse structures, regional-tectonic strike-slip faults (Riedel shears and Y-shears) may affect a caldera’s structural evolution in two main ways. Firstly, regional strike-slip faults above the magma chamber may form a pre-collapse structural grain that is exploited and reactivated during subsidence. Our experiments show that such faults may preferentially reactivate where tangential to the collapse area and coincident with the chamber margins. In this case, volcano-tectonic extension in the caldera periphery tends to localise on regional-tectonic faults that lie just outside the chamber margins. In addition, volcano-tectonic reverse faults may link with and reactivate pre-collapse regional-tectonic faults that lie just inside the chamber margins. Secondly, where regional-tectonic strike-slip faults define corners in the magma chamber margin, they may halt the propagation of volcano-tectonic reverse faults. The experiments also highlight the potential difficulties in assessing the relative contributions of volcano-tectonic and regional-tectonic subsidence processes to the final caldera structure seen in the field. Disruption of the pre-collapse surface by regional-tectonic faulting was preserved during coherent volcano-tectonic subsidence to produce a caldera floor of differentially-subsided fault blocks. Without definitive evidence for syn-eruptive growth faulting, thickness changes in caldera fill across such regional-tectonic fault blocks in nature could be mistaken as evidence for piecemeal volcano-tectonic collapse.  相似文献   

3.
Apoyo caldera, near Granada, Nicaragua, was formed by two phases of collapse following explosive eruptions of dacite pumice about 23,000 yr B.P. The caldera sits atop an older volcanic center consisting of lava flows, domes, and ignimbrite (ash-flow tuff). The earliest lavas erupted were compositionally homogeneous basalt flows, which were later intruded by small andesite and dacite flows along a well defined set of N—S-trending regional faults. Collapse of the roof of the magma chamber occurred along near-vertical ring faults during two widely separated eruptions. Field evidence suggests that the climactic eruption sequence opened with a powerful plinian blast, followed by eruption column collapse, which generated a complex sequence of pyroclastic surge and ignimbrite deposits and initiated caldera collapse. A period of quiescence was marked by the eruption of scoria-bearing tuff from the nearby Masaya caldera and the development of a soil horizon. Violent plinian eruptions then resumed from a vent located within the caldera. A second phase of caldera collapse followed, accompanied by the effusion of late-stage andesitic lavas, indicating the presence of an underlying zoned magma chamber. Detailed isopach and isopleth maps of the plinian deposits indicate moderate to great column heights and muzzle velocities compared to other eruptions of similar volume. Mapping of the Apoyo airfall and ignimbrite deposits gives a volume of 17.2 km3 within the 1-mm isopach. Crystal concentration studies show that the true erupted volume was 30.5 km3 (10.7 km3 Dense Rock Equivalent), approximately the volume necessary to fill the caldera. A vent area located in the northeast quadrant of the present caldera lake is deduced for all the silicic pyroclastic eruptions. This vent area is controlled by N—S-trending precaldera faults related to left-lateral motion along the adjacent volcanic segment break. Fractional crystallization of calc-alkaline basaltic magma was the primary differentiation process which led to the intermediate to silicic products erupted at Apoyo. Prior to caldera collapse, highly atypical tholeiitic magmas resembling low-K, high-Ca oceanic ridge basalts were erupted along tension faults peripheral to the magma chamber. The injection of tholeiitic magmas may have contributed to the paroxysmal caldera-forming eruptions.  相似文献   

4.
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.Editorial responsibility: J. Stix  相似文献   

5.
Collapse calderas have received considerable attention due to their link to Earth's ore deposits and geothermal energy resources, but also because of their tremendous destructive potential. Although calderas have been investigated through fieldwork, numerical models and experimental studies, some important aspects on their formation still remain poorly understood. One key issue concerns the volume of magmas involved in caldera-forming eruptions. We perform analogue experiments to correlate the structural evolution of a collapse with the erupted magma chamber volume fraction. The experimental device consists of a transparent box (60 × 60 × 40 cm) filled with dry quartz sand and a water-filled latex balloon as a magma chamber analogue. Evacuation of water through a pipe causes a progressive deflation of the balloon that leads to a collapse of the overlying structure. The experimental design allows to record the temporal evolution of the collapse and to track the evolution of fractures and faults. We study the appearance and development of specific brittle structures, such as surface fractures or internal reverse faults, and correlate each different structure with the corresponding removed magma chamber volume fraction. We also determine the critical conditions for caldera onset. Experimental results show that, at any stage of caldera developments, the experimental relationship between volume fraction and chamber roof aspect ratio fits a logarithmic curve. It implies that volume fractions required to trigger caldera collapse are lower for chambers with low aspect ratios (shallow and wide) than for chambers with high aspect ratios (deep and small). These results are in agreement with natural examples and previous theoretical studies.  相似文献   

6.
Structures at calderas may form as a result of precursory tumescence, subsidence due withdrawal of magmatic support, resurgence, and regional tectonism. Structural reactivation and overprinting are common. To explore which types of structures may derive directly from subsidence without other factors, evidence is reviewed from pits caused by the melting of buried ice blocks, mining subsidence, scaled subsidence models, and from over 50 calderas. This review suggests that complex patterns of peripheral deformation, with multiple ring and arcuate fractures both inside and outside caldera rims, topographic embayments, arcuate graben, and concentric zones of extension and compression may form as a direct result of subsidence and do not require a complex subsidence and inflation history. Downsag is a feature of many calderas and it does not indicate subsidence on an inward-dipping ring fault, as has been inferred previously. Where magmatic inflation is absent or slight, initial arcuate faults formed during collapse are likely to be multiple, and dip outwards to vertical. Associated downsag causes the peripheries of calderas undergo radial (centripetal) extension, and this accounts for some of the complex peripheral fractures, arcuate crevasses, graben, and some topographic moats. The structural boundary of a caldera, defined here as the outermost limits of subsidence and related deformation including downsag, commonly lies outside ring faults and outside the embayed topographic wall. It is likely to be funnel-shaped, i.e. inward-dipping, even though ring and arcuate fractures within it may dip outward. Inward-dipping arcuate normal faults at shallow levels and steep inward-dipping contacts between a caldera's fill and walls may both occur at a caldera that has initially subsided on outward-dipping ring faults. They arise due to peripheral surficial extension, gravitational spreading and scarp collapse. Topographic enlargement at some calderas and the formation of embayments may reflect general progressive downsag and localized downsag, respectively. These processes may occur in addition to surficial degradation of oversteep ring-fault scarps.  相似文献   

7.
 Diverse subsidence geometries and collapse processes for ash-flow calderas are inferred to reflect varying sizes, roof geometries, and depths of the source magma chambers, in combination with prior volcanic and regional tectonic influences. Based largely on a review of features at eroded pre-Quaternary calderas, a continuum of geometries and subsidence styles is inferred to exist, in both island-arc and continental settings, between small funnel calderas and larger plate (piston) subsidences bounded by arcuate faults. Within most ring-fault calderas, the subsided block is variably disrupted, due to differential movement during ash-flow eruptions and postcollapse magmatism, but highly chaotic piecemeal subsidence appears to be uncommon for large-diameter calderas. Small-scale downsag structures and accompanying extensional fractures develop along margins of most calderas during early stages of subsidence, but downsag is dominant only at calderas that have not subsided deeply. Calderas that are loci for multicyclic ash-flow eruption and subsidence cycles have the most complex internal structures. Large calderas have flared inner topographic walls due to landsliding of unstable slopes, and the resulting slide debris can constitute large proportions of caldera fill. Because the slide debris is concentrated near caldera walls, models from geophysical data can suggest a funnel geometry, even for large plate-subsidence calderas bounded by ring faults. Simple geometric models indicate that many large calderas have subsided 3–5 km, greater than the depth of most naturally exposed sections of intracaldera deposits. Many ring-fault plate-subsidence calderas and intrusive ring complexes have been recognized in the western U.S., Japan, and elsewhere, but no well-documented examples of exposed eroded calderas have large-scale funnel geometry or chaotically disrupted caldera floors. Reported ignimbrite "shields" in the central Andes, where large-volume ash-flows are inferred to have erupted without caldera collapse, seem alternatively interpretable as more conventional calderas that were filled to overflow by younger lavas and tuffs. Some exposed subcaldera intrusions provide insights concerning subsidence processes, but such intrusions may continue to evolve in volume, roof geometry, depth, and composition after formation of associated calderas. Received: 13 February 1997 / Accepted: 9 August 1997  相似文献   

8.
The Vatukoula caldera is semi-elliptical in shape with the long axis trending north-easterly and occupies about 14 square miles of an undulating topographical basin located near the central north coast of Viti Levu, the largest island of the Fiji Group. The caldera formed when Tertiary basalts collapsed after prolonged explosion from a central vent area. The ensuing subsidence, which appears to have been cyclic, was accompanied by the deposition of andesitic volcanic material to form 5,000 to 7,000 feet of rhythmic tuffs, breccias and agglomerates partly under lacustrine conditions. The peripheral basalts were shattered during the stages of collapse forming a ring fault zone around the caldera. The depositional and subsidence stages were followed by an intrusive augite andesitic one from which extensive cone sheets formed in the caldera rocks. Radial and tangential dykes formed around the caldera in the peripheral basalts. After a time interval, the comparatively shallow central depression of the caldera received biotite andesitic pyroclastics and flows. Biotite andesite dykes followed a similar structural pattern to the augite andesitic ones. Finally, plug like bodies of porphyrite and monzonite intruded into the highly fractured zones, particularly the ring fault zone in the peripheral basalts. An important younger structural development with economic significance was the formation of a north-westerly shear system across the caldera. Flatly dipping structures formed in the peripheral basalts from the resettling of major blocks around the caldera. After the monzonite intrusions, epithermal mineralisers were liberated with economic amounts of gold in the form of telluride and auriferous pyrite. The mineralisers favoured the north-westerly shear system and, in the peripheral basalts, the accompanying flatly dipping structures. Thermal spring activity appears to mark the last phase of volcanicity.  相似文献   

9.
Hakone caldera, now 8 by 12 km in diameter, was formed by collapse of a center of a volcano probably 2700 m high. The collapse took place at two separate periods each of which was followed by periods of deep denudation. The central part of the caldera has been covered by a thick pile of lavas of post-caldera cones and domes. For the purpose of finding thermal spring, drilling to depths of a few hundred to one thousand meters was carried out at various points within the caldera except for its central region. The study of the drill cores revealed that the average amount of subsidence at points 2 and 3 km away from the base of the present caldera wall is 600 m and 1200 m respectively, and probably more than 1800 m in the middle of the caldera. Within the caldera, the pre-caldera lavas and pyroclastic rocks are either lacking or much thinner than would be expected. It is concluded therefore that the present topographic depression of the caldera owed its origin to both subsidence and denudation. It is inferred that the subsidence took place along a complicated system of concentric faults combined with tilting of individual fault blocks toward the middle of the caldera. The magma reservoir into which the fault blocks sank probably had a shape of a cupola with a diameter comparable to or a little smaller than the diameter of the caldera.  相似文献   

10.
Ambrym Island has an unusually large, well-preserved basaltic caldera 13 km across. The caldera occurs in the central region of an early broad composite cone, which formed a north-south line with three smailer volcanoes. Alter the caldera was formed volcanism occurred within it and along fissure lines running nearly east-west. Two volcanic cones are active almost continuously and historic fissure cruptions have been recorded. The caldera formed by quiet subsidence, or by subsidence accompanied by eruption of scoria lappili similar to that erupted prior and subsequent to caldera formation. The collapse was at least 600 metres and radiocarbon dating suggests it took place less than 2000 years ago. The caldera is detined by gravity anomalies 10 to 14 milligals lower than those at its rim suggesting predominantly ash infilling. Aeromagnetic anomalies show a prominent. nearly east-west lineation, with normally magnetised bipole anomalies over the centre of the caldera and over fissure lines east of it. The source of the present volcanic activity is believed to be located along dyke fissures, with a perched magma chamber beneath the caldera. The geophysical evidence on Ambrym, together with that of regional east trending magnetic anomalies and recent bathymetric results, suggests that the volcanic activity is localised by the intersection of an east-west fracture zone with the axis of the New Hebrides island are.  相似文献   

11.
The Christmas Mountains caldera complex developed approximately 42 Ma ago over an elliptical (8×5 km) laccolithic dome that formed during emplacement of the caldera magma body. Rocks of the caldera complex consist of tuffs, lavas, and volcaniclastic deposits, divided into five sequences. Three of the sequences contain major ash-flow tuffs whose eruption led to collapse of four calderas, all 1–1.5 km in diameter, over the dome. The oldest caldera-related rocks are sparsely porphyritic, rhyolitic, air-fall and ash-flow tuffs that record formation and collapse of a Plinian-type eruption column. Eruption of these tuffs induced collapse of a wedge along the western margin of the dome. A second, more abundantly porphyritic tuff led to collapse of a second caldera that partly overlapped the first. The last major eruptions were abundantly porphyritic, peralkaline quartz-trachyte ash-flow tuffs that ponded within two calderas over the crest of the dome. The tuffs are interbedded with coarse breccias that resulted from failure of the caldera walls. The Christmas Mountains caldera complex and two similar structures in Trans-Pecos Texas constitute a newly recognized caldera type, here termed a laccocaldera. They differ from more conventional calderas by having developed over thin laccolithic magma chambers rather than more deep-seated bodies, by their extreme precaldera doming and by their small size. However, they are similar to other calderas in having initial Plinian-type air-fall eruption followed by column collapse and ash-flow generation, multiple cycles of eruption, contemporaneous eruption and collapse, apparent pistonlike subsidence of the calderas, and compositional zoning within the magma chamber. Laccocalderas could occur else-where, particularly in alkalic magma belts in areas of undeformed sedimentary rocks.  相似文献   

12.
Through examination of the vent region of Volcán Huaynaputina, Peru, we address why some major explosive eruptions do not produce an equivalent caldera at the eruption site. Here, in 1600, more than 11 km3 DRE (VEI 6) were erupted in three stages without developing a volumetrically equivalent caldera. Fieldwork and analysis of aerial photographs reveal evidence for cryptic collapse in the form of two small subsidence structures. The first is a small non-coherent collapse that is superimposed on a cored-out vent. This structure is delimited by a partial ring of steep faults estimated at 0.85 by 0.95 km. Collapse was non-coherent with an inwardly tilted terrace in the north and a southern sector broken up along a pre-existing local fault. Displacement was variable along this fault, but subsidence of approximately 70 m was found and caused the formation of restricted extensional gashes in the periphery. The second subsidence structure developed at the margin of a dome; the structure has a diameter of 0.56 km and crosscuts the non-coherent collapse structure. Subsidence of the dome occurred along a series of up to seven concentric listric faults that together accommodate approximately 14 m of subsidence. Both subsidence structures total 0.043 km3 in volume, and are much smaller than the 11 km3 of erupted magma. Crosscutting relationships show that subsidence occurred during stages II and III when ∼2 km3 was erupted and not during the main plinian eruption of stage I (8.8 km3). The mismatch in erupted volume vs. subsidence volume is the result of a complex plumbing system. The stage I magma that constitutes the bulk of the erupted volume is thought to originate from a ∼20-km-deep regional reservoir based on petrological constraints supported by seismic data. The underpressure resulting from the extraction of a relatively small fraction of magma from the deep reservoir was not sufficient enough to trigger collapse at the surface, but the eruption left a 0.56-km diameter cored-out vent in which a dome was emplaced at the end of stage II. Petrologic evidence suggests that the stage I magma interacted with and remobilized a shallow crystal mush (∼4–6 km) that erupted during stage II and III. As the crystal mush erupted from the shallow reservoir, depressurization led to incremental subsidence of the non-coherent collapse structure. As the stage III eruption waned, local pressure release caused subsidence of the dome. Our findings highlight the importance of a connected magma reservoir, the complexity of the plumbing system, and the pattern of underpressure in controlling the nature of collapse during explosive eruptions. Huaynaputina shows that some major explosive eruptions are not always associated with caldera collapse. Editorial responsibility: J Stix  相似文献   

13.
Collapse mechanism of the Paleogene Sakurae cauldron, SW Japan   总被引：1，自引：0，他引：1  
The Paleogene Sakurae cauldron of SW Japan is characterized by a nested structure with a polygonal outline (21×13 km2) including a circular collapsed part (5 km in diameter). Total thickness of the caldera infill amounts to 2,000 m. The lower member of the infill consists mainly of felsic crystal tuff and lesser intercalated andesitic lava flows, whereas the upper member is composed of high-grade ignimbrite capped with a large rhyolitic lava dome. These members represent the first and second stage eruptions, respectively. Faults bounding the cauldron rim comprise intersecting radial and concentric faults, producing the polygonal outline of this cauldron. The primary collapse of this cauldron thus occurred as a polygonal caldera basin where products of the first stage eruption accumulated. In contrast, the inner collapse part is defined by a ring fracture system. This sector subsided concurrently with accumulation of the high-grade ignimbrite of the second stage eruption. This inner circular collapse thus represents syn-eruptional subsidence concurrent with the climactic eruption. Magma drainage during the first stage probably induced outward-dipping ring fractures in the chamber roof. Opening of the ring fractures following subsidence of the central bell-jar block caused rapid evacuation of magma as voluminous pumice flows, even though magma pressure may have decreased to some degree.  相似文献   

14.
 A radar and gravity survey of the ice-filled caldera at Volcán Sollipulli, Chile, indicates that the intra-caldera ice has a thickness of up to 650 m in its central part and that the caldera harbours a minimum of 6 km3 of ice. Reconnaissance geological observations show that the volcano has erupted compositions ranging from olivine basalt to dacite and have identified five distinct volcanic units in the caldera walls. Pre- or syn-caldera collapse deposits (the Sharkfin pyroclastic unit) comprise a sequence which evolved from subglacial to subaerial facies. Post-caldera collapse products, which crop out along 17 of the 20 km length of the caldera wall, were erupted almost exclusively along the caldera margins in the presence of a large body of intra-caldera ice. The Alpehué crater, formed by an explosive eruption between 2960 and 2780 a. BP, in the southwest part of the caldera is shown to post date formation of the caldera. Sollipulli lacks voluminous silicic pyroclastic rocks associated with caldera formation and the collapse structure does not appear to be a consequence of a large-magnitude explosive eruption. Instead, lateral magma movement at depth resulting in emptying of the magma chamber may have generated the caldera. The radar and gravity data show that the central part of the caldera floor is flat but, within a few hundred metres of the caldera walls, the floor has a stepped topography with relatively low-density rock bodies beneath the ice in this region. This, coupled with the fact that most of the post-caldera eruptions have taken place along the caldera walls, implies that the caldera has been substantially modified by subglacial marginal eruptions. Sollipulli caldera has evolved from a collapse to a constructional feature with intra-caldera ice playing a major role. The post-caldera eruptions have resulted in an increase in height of the walls and concomitant deepening of the caldera with time. Received: 12 June 1995 / Accepted: 7 December 1995  相似文献   

15.
At Gross Brukkaros a central depression has developed within domed Nama Group sediments and has functioned as a local depocenter, with a primary fill deposited during the Cretaceous and a small secondary fill by alluvial fans during the Tertiary and Quaternary. The diameter of the entire structure is about 10 km and that of the central depression is about 3 km. Within this depocenter the sedimentary sequence consists mainly of debris-flow and mudflow deposits, with minor intercalations of fluviatile (braided channel) sediments and fossiliferous lacustrine deposits. The sedimentary system represents a set of coalesced subaerial fans which formed a fringing sedimentary apron along the margin of the depocenter. This sedimentary apron passed distally and centrally into a permanent lake, which was characterized by a fluctuating water level. Facies transitions observed are typical of those described from modern and ancient fan delta systems. Contact relationships show the Gross Brukkaros sediments to be about the same age (Upper Cretaceous) as the surrounding carbonatitic volcanism. An Upper Cretaceous age is also consistent with the plant fossil association recently recognized within the lacustrine beds of Gross Brukkaros. We attribute the genesis of the dome structure to the shallow intrusion of a laccolith-shaped, strongly alkaline to carbonatitic magma body. Subsequent depletion of the reservoir due to volcanic activity around and in(?) Gross Brukkaros led to subsidence resulting in the development of the Gross Brukkaros depocenter. Differences between Gross Brukkaros and the general caldera model consist of a radially oriented dike pattern and the formation of the caldera by downsagging rather than cauldron subsidence, as derived from the absence of ring faults and ring dikes. The first (radial dikes) may be attributed to comparatively strong initial doming; the latter (lack of ring faults) to the small size of the caldera, its incremental subsidence, and finally the sedimentary wall rocks instead of a rigid crystalline crust.  相似文献   

16.
The recently discovered La Pacana caldera, 60 × 35 km, is the largest caldera yet described in South America. This resurgent caldera of Pliocene age developed in a continental platemargin environment in a major province of ignimbrite volcanism in the Central Andes of northern Chile at about 23° S latitude. Collapse of La Pacana caldera was initiated by the eruption of about 900 km3 of the rhyodacitic Atana Ignimbrite. The Atana Ignimbrite was erupted from a composite ring fracture system and formed at least four major ash-flow tuff units that are separated locally by thin air-fall and surge deposits; all four sheets were emplaced in rapid succession about 4.1 ± 0.4 Ma ago. Caldera collapse was followed closely by resurgent doming of the caldera floor, accompanied by early postcaldera eruptions of dacitic to rhyolitic lava domes along the ring fractures. The resurgent dome is an elongated, asymmetrical uplift, 48.5 × 12 km, which is broken by a complex system of normal faults locally forming a narrow discontinuous apical graben. Later, postcaldera eruptions produced large andesitic and dacitic stratocones along the caldera margins and dacitic domes on the resurgent dome beginning about 3.5 Ma ago and persisting into the Quaternary. Hydrothermally altered rocks occur in the eroded cores of precaldera and postcaldera stratovolcanoes and along fractures in the resurgent dome, but no ore deposits are known. A few warm springs located in salars within the caldera moat appear to be vestiges of the caldera geothermal system.  相似文献   

17.
The Las Cañadas caldera of Tenerife (LCC) is a well exposed caldera depression filled with pyroclastic deposits and lava flows from the active Teide–Pico Viejo complex (TPVC). The caldera's origin is controversial as both the formation by huge lateral flank collapse(s) and multiple vertical collapses have been proposed. Although vertical collapses may have facilitated lateral slope failures and thus jointly contribute to the exposed morphology, their joint contribution has not been clearly demonstrated. Using results from 185 audiomagnetotelluric (AMT) soundings carried out between 2004 and 2006 inside the LCC, our study provides consistent geophysical constraints in favour of multiple vertical caldera collapse. One-dimensional modelling reveals a conductive layer at shallow depth (30–1000 m), presumably resulting from hydrothermal alteration and weathering, underlying the infilling resistive top layer. We present the resistivity distribution of both layers (resistivity images), the topography of the conductive layer across the LCC, as well as a cross-section in order to highlight the caldera's evolution, including the distribution of earlier volcanic edifices. The AMT phase anisotropy reveals the structural and radial characteristics of the LCC.  相似文献   

18.
Drill-hole, geochronologic, and gravity data identify the buried Shishimuta caldera beneath post-caldera lava domes and lacustrine deposits in the center of the Hohi volcanic zone. The caldera is the source of the Yabakei pyroclastic flow, which erupted 1.0 Ma ago with a bulk volume of 110 km3. The caldera is a breccia-filled funnel-shaped depression 8 km wide and > 3 km deep with a V-shaped negative Bouguer gravity anomaly up to 36 mgal. Neither ring vents nor resurgence was recognized; instead, post-caldera monogenetic volcanism in an extensional setting dominated the area. The andesitic breccia has a relatively low density and fills the caldera; it possibly formed by fragmentation of disrupted roof rock during the violent Yabakei eruption and related collapse. Fewer normal faults and shallow microearthquakes occur inside the caldera than around it, possibly because rocks beneath the caldera are structurally incoherent. A profile of Shishimuta caldera may be more elongated vertically, and have a more intensely fractured zone, than that of a Valles-type caldera.  相似文献   

19.
Caldera morphology on the six historically active shield volcanoes that comprise Isabela and Fernandina islands, the two westernmost islands in the Galapagos archipelago, is linked to the dynamics of magma supply to, and withdrawal from, the magma chamber beneath each volcano. Caldera size (e.g., volumes 2–9 times that of the caldera of Kilauea, Hawai'i), the absence of well-developed rift zones and the inability to sustain prolonged low-volumetric-flow-rate flank eruptions suggest that magma storage occurs predominantly within centrally located chambers (at the expense of storage within the flanks). The calderas play an important role in the formation of distinctive arcuate fissures in the central part of the volcano: repeated inward collapse of the caldera walls along with floor subsidence provide mechanisms for sustaining radially oriented least-compressive stresses that favor the formation of arcuate fissures within 1–2 km outboard of the caldera rim. Variations in caldera shape, depth-to-diameter ratio, intra-caldera bench location and the extent of talus slope development provide insight into the most recent events of caldera modification, which may be modulated by the episodic supply of magma to each volcano. A lack of correlation between the volume of the single historical collapse event and its associated volume of erupted lava precludes a model of caldera formation linked directly to magma withdrawal. Rather, caldera collapse is probably the result of accumulated loss from the central storage system without sufficient recharge and (as has been suggested for Kilauea) may be aided by the downward drag of dense cumulates and intrusives.  相似文献   

20.
The ring fractures that form most collapse calderas are steeply inward-dipping shear fractures, i.e., normal faults. At the surface of the volcano within which the caldera fault forms, the tensile and shear stresses that generate the normal-fault caldera must peak at a certain radial distance from the surface point above the center of the source magma chamber of the volcano. Numerical results indicate that normal-fault calderas may initiate as a result of doming of an area containing a shallow sill-like magma chamber, provided that the area of doming is much larger than the cross-sectional area of the chamber and that the internal excess pressure in the chamber is smaller than that responsible for doming. This model is supported by the observation that many caldera collapses are preceded by a long period of doming over an area much larger than that of the subsequently formed caldera. When the caldera fault does not slip, eruptions from calderas are normally small. Nearly all large explosive eruptions, however, are associated with slip on caldera faults. During dip slip on, and doming of, a normal-fault caldera, the vertical stress on part of the underlying chamber suddenly decreases. This may lead to explosive bubble growth in this part of the magma chamber, provided its magma is gas rich. This bubble growth can generate an excess fluid pressure that is sufficiently high to drive a large fraction of the magma out of the chamber during an explosive eruption. Received: 2 January 1997 / Accepted: 22 April 1998  相似文献   

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