eruptive processes and caldera formation in a nested downsagcollapse caldera cerro panizos central andes mountains abstract the cerro panizos ignimbrite center in the central andes mountains produced two ignimbrite sheets and many lava flows the ignimbrite of quebrada cienago was erupted at 7 9 ma and effusive eruptions continued until the two cooling units of the crystal rich dacitic cerro panizos ignimbrite were emplaced at 6 7 ma the lower unit has no laterally continuous flow breaks and was erupted from a single vent or small cluster of vents with limited fluctuation in discharge lithic fragments reach significant concentrations 5 only in the uppermost two meters of this cooling unit where they document vent wall collapse or the opening of a new vent the upper cooling unit contains many flow units with variations in welding thicknesses and lithic fragment concentrations implying an unsteady eruption column the opening of many vents and probable caldera collapse triangulation of anisotropy of magnetic susceptibility flow direction measurements locate a single vent for the lower cooling unit whereas the upper cooling unit had many vents within the present dome cluster a 15 km diameter topographic depression marked by inward dips of 4 8 at the cooling unit contact is centered on the vent area of the lower cooling unit the depression is interpreted as a downsag caldera formed during emplacement of the lower cooling unit collapse began late in the eruption of the lower cooling unit and continued through the emplacement of the upper cooling unit resurgent magmatism occurred as lava flows that mundated the caldera area a ring of dacite domes erupted until at least 6 1 ma in the northern half of the downsag caldera traces the margin of a collapse caldera associated with the upper cooling unit maximum caldera subsidence 353 km 3 is not enough to account for the erupted volume 652 km 3 dre minimum the ash poor crystal rich nature up to 50 crystals in the pumice 75 in the matrix of the cerro panizos pyroclastic flows resulted in poor retention of gases and there is little evidence of fluidization the high crystal content however favored modified grain flow processes in which several particle support mechanisms combine with grain dispersive forces it is postulated that the flows were initially partially fluidized but rapidly lost their gases and grain flow processes grew in relative importance grain flow processes destroyed evidence of fluidization before deposition

a coupled sheet conduit mechanism for jokulhlaup propagation 1 the largest glacier outburst flood jokulhlaup ever recorded in iceland occurred in 1996 and came from subglacial lake grimsvotn in vatnajokull ice cap among other noteworthy features this flood was characterized by an unprecedentedly high lake level prior to flood initiation extremely rapid linear rise in lake discharge delay between the onset of lake drainage and floodwater arrival at the glacier terminus formation of short lived supraglacial fountains and initially unchannelized outbursts of floodwater at the terminus observations suggest that the 1996 flood propagation mechanism was fundamentally different than that of previously observed floods from grimsvotn we advance a new model whereby floodwater initially propagates in a turbulent subglacial sheet which feeds a nascent system of conduits this model is able to explain key observations made of the 1996 jokulhlaup and may shed light on other outburst floods that do not conform to the standard model

the geometry and emplacement of conical sandstone intrusions conical sandstone intrusions with a geometry comparable with that of many igneous sills have been identified using 3d seismic data from large areas of the north sea and faeroe shetland basins these intrusions are of reservoir scale ranging from 100 to 2000 m or so in diameter 50 300 m in height and 1 80 m in thickness aperture they are concentrated in specific stratigraphic intervals in the cenozoic fills of both basins two geometrical end members are recognised and defined here apical cones and flat based bowls the former consist of inward dipping conical inclined sheets meeting at a prominent apex and the latter of similarly dipping discordant margins climbing from the edges of a concordant sheet both end members are associated with domal folds that are interpreted as resulting from the hydraulic elevation of the overburden during intrusion and which are analogous to similar structures associated with bowl shaped igneous sills measurements of aperture versus distance exhibit systematic relationships with the structural relief of these folds offering a potentially predictive method for estimation of sandstone intrusion aperture and reservoir volume prior to drilling a growth model for these end member geometries is presented drawing on existing theory for igneous sill emplacement aperture versus distance plots x are used to illustrate two contrasting models for aperture inflation during propagation but these require much further data before any specific growth model can be adopted

the pattern of circumferential and radial eruptive fissures on the volcanoes of fernandina and isabela islands galapagos maps of the eruptive vents on the active shield volcanoes of fernandina and isabela islands galapagos made from aerial photographs display a distinctive pattern that consists of circumferential eruptive fissures around the summit calderas and radial fissures lower on the flanks on some volcano flanks either circumferential or radial eruptions have been dominant in recent time the location of circumferential vents outside the calderas is independent of caldera related normal faults the eruptive fissures are the surface expression of dike emplacement and the dike orientations are interpreted to be controlled by the state of stress in the volcano very few subaerial volcanoes display a pattern of fissures similar to that of the galapagos volcanoes some seamounts and shield volcanoes on mars morphologically resemble the galapagos volcanoes but more specific evidence is needed to determine if they also share common structure and eruptive style

lithic breccia and ignimbrite erupted during the collapse of crater lake caldera oregon abstract the climactic eruption of mount mazama 6845 y b p vented a total of 50 km3 of compositionally zoned rhyodacitic to basaltic magma from a a single vent as a plinian pumice fall deposit and the overlying wineglass welded tuff and b ring vents as ignimbrite and coignimbrite lithic breccia accompanying the collapse of crater lake caldera new field and grain size data for the ring vent products are presented in this report the coarse grained poorly bedded clast supported lithic breccia extends as far as 18 km from the caldera center like the associated ignimbrite the breccia is compositionally zoned both radially and vertically and silicic mixed and mafic types can be recognized based on the proportion of rhyodacitic pumice matrix fractions in silicic breccias are depleted of fines and are lithic and crystal enriched relative to silicic ignimbrite due to vigorous gas sorting during emplacement ignimbrite occurs as a proximal veneer deposit overlying the breccia a medial 8 to 25 km from the caldera center compositionally zoned valley fill as much as 110 m thick and an unzoned distal 20 km facies which extends as far as 55 km from the caldera breccia within 9 km of the caldera center is interpreted as a coignimbrite lag breccia formed within the deflation zone of the collapsing ring vent eruption columns expanded pyroclastic flows of the deflation zone were probably vertically graded in both size and concentration of blocks as recently postulated for some turbidity currents an inflection in the rate of falloff of lithic clast size within the lithic breccia at 9 km may mark the outer edge of the deflation zone or may be an artifact of incomplete exposure the onset of ring vent activity at mt mazama was accompanied by a marked increase in eruptive discharge pyroclastic flows were emplaced as a semicontinuous stream as few ignimbrite flow unit boundaries are evident as eruption from the ring vents progressed flow runout distance and the extent of breccia deposition decreased due to a greater internal flow friction and b decreasing eruption column heights effect b probably resulted from a progressive decrease in magmatic gas content and discharge rate waning discharge may have been promoted by the tapping of more viscous crystal rich magma collapse of conduit walls and declining caldera collapse rate

structure of la primavera caldera jalisco mexico deduced from gravity anomalies and drilling results abstract previous studies of la primavera caldera have mostly been based on surface geology and topography since 1980 many wells exploring for geothermal energy have reached depths of about 2 to 3 km at the center of the caldera the results of the drillings together with those of the gravity surveys provide information about the subsurface structure of the caldera and shed light on its formation the drilling results and gravity anomalies at la primavera caldera and san marcos located at about 40 km distance from the caldera suggest that regional gravity anomalies can be interpreted in terms of depths of the granitic basements the basement beneath la primavera caldera is about 3 km deep and consists of roughly the same horizon as that beneath san marcos the drilling results within the caldera reveal that the depth of the caldera fills ranges from 0 3 to 1 km at the drilling sites the andesite basement about 1 km deep remains approximately horizontal and the granitic basement has a depth of about 3 km the surface topographies such as the postcaldera domes scarcely disturb the subsurface strata the local gravity anomalies show two lows within the caldera reflecting the configuration of caldera bottom two funnel shaped depressions one of which corresponds to a vent of the tala tuff deduced from geological observations the mass deficiency within the caldera estimated from the gravity anomaly satisfies the general relationship that the mass deficiency is proportional to the caldera diameter cubed this means that caldera structure is three dimensional the larger the diameter the deeper the funnel shape at present this argument may be limited to funnel shaped calderas

evolution of the caldera forming eruption at crater lake oregon indicated by component analysis of lithic fragments crater lake caldera 8 10 km formed 6845 years b p 14c age during the climactic eruption of the volcanic edifice known as mount mazama is intermediate in size between small calderas associated with central vent eruptions and large calderas that have ring fracture vent systems our quantitative study of lithic fragments in the ejecta confirms the existing model of changes in vent configuration during the climactic eruption of mount mazama initial activity was from a single vent that produced a rhyodacite pumice fall from a plinian column altered preexisting volcanic rocks are the predominant lithic type in the plinian deposit and their extensive hydrothermal alteration is considered as evidence of their relatively deep origin the wineglass welded tuff lies atop the plinian deposit and contains a higher proportion of fresh volcanic rocks suggesting enlargement of the single vent by slumping of its walls this same vent enlargement caused the plinian eruption column to collapse and feed valley hugging pyroclastic flows that deposited the wineglass welded tuff when enough material was erupted from the shallow magma chamber that its roof was no longer adequately supported mount mazama collapsed to form the caldera while highly energetic pyroclastic flows produced the climactic ignimbrite a lag breccia that represents the proximal facies of the compositionally zoned climactic ignimbrite lies atop the wineglass welded tuff and contains predominantly altered volcanic rocks of deeper origin accompanied by minor granitoids from the magma chamber walls azimuthal differences in lithic component proportions in the lag breccia correlate well with the geology of the caldera walls indicating that the climactic ignimbrite was ejected by multiple vents along a ring fracture system systematic lithic component changes within the lag breccia suggest different quarrying levels that reflect waxing and waning of the discharge rate during the caldera collapse phase of the climactic eruption our lithic component analysis demonstrates that calderas that may be too small to experience structural resurgence such as crater lake nevertheless may form by syneruptive subsidence along ring fractures

volcanic and hydrothermal processes in submarine calderas the kulo lasi example sw pacific abstract the study area is located at the transition between the northern end of the tonga trench and the north fiji fracture zone where tectonic movements are reputed to be the fastest in the world to the southeast of futuna island a broad area of volcanism occurs within a region characterized by a change in the tectonic fabric between a ne sw oriented volcanic graben and the n s oriented alofi ridge in 2010 the active volcano kulo lasi which represents the most recent volcanic episode in the futuna area was discovered in the center of this extensive volcanic zone kulo lasi is a 20 km diameter shield volcano that rises 400 m above the seafloor it is composed of basaltic to trachy andesitic lava with no obvious geochemical affinity with the tonga subduction that occurs 500 km to the east the central caldera is 5 km in diameter and 300 m deep and is located at a water depth of 1500 m diving operations with the submersible nautile and high resolution auv mapping have revealed the presence of numerous active and inactive hydrothermal fields on the floor and the walls of the caldera four tectono volcanic stages can be distinguished at kulo lasi caldera in stage 1 the shield volcano is built annular reverse faults develop at the summit and control circulation of water rock dominated hydrothermal fluids and high temperature alteration of rocks along the nascent normal faults mixing of hydrothermal fluids with seawater is favored along normal superficial faults leading to the formation of low temperature fe mn mineralization at the summit of the volcano during stage 2 the caldera collapse gradually revealing outcrops of the altered and mineralized zones formed during stage 1 as the magma chamber cools and collapses less heat is available as a result medium to low temperature

mud volcanism by repeated roof collapse 3d architecture and evolution of a mud volcano cluster offshore nigeria abstract using 3d seismic data we document the subsurface architecture of a cluster of 3 mud volcanoes located on the upper continental slope offshore nigeria in a background of gently dipping stratified sediments the subsurface of the cluster contains 8 seismic sequences made of one subcircular syncline 1 3 km in diameter and adjoining unstratified units synclines flatten out upward through intercalation of unstratified concave up lenses between stratified units and are eventually filled by a convex up unstratified lens onlapped by background sediments the lower boundary of each sequence is a 0 5 1 5 km deep bowl shaped unconformity across which high dip strata rest above truncated low dip series we interpret each seismic sequence as a mud volcano system mvs sourced from a connected region and evolving through episodes of roof collapse three are associated with the surface mud volcanoes and another 4 to fossil ones mud volcano systems are dynamic features that record the evolution of 1 the material extruded from one single connected source region depletion zone as it grows downwards and stepwise becomes depleted 2 the basal unconformity which joins strata originally separated by material evacuated in the remobilization process by analogy with salt tectonics we propose to call it a mud weld 3 the conduits and 4 the successive collapsed roofs of the depletion zone the evolution of each mud volcano system in this interpretation records progressive growth of the depletion zone by downbuilding punctuated by episodes of roof collapse and extrusion the geometry and evolution of this type of mvs can be interpreted in the light of analogue models of magmatic caldera development in early stages the thin roof experiences downsagging while remobilized material is extruded through an axial pipe like conduit in later stages the thickened roof undergoes piston like subsidence and mud extrusion occurs along the peripheral fault

ice melt collapse pits and associated features in the 1991 lahar deposits of volcan hudson chile criteria to distinguish eruption induced glacier melt in subaerial volcaniclastic sequences structures formed by ice blocks can provide information about a volcano s history of lahar generation by glacier melt at volcan hudson in chile catastrophic lahars were initiated by eruption induced melting of glacier ice in august and october 1991 they transported large ice blocks 50 km down the rio de los huemules valley to the sea large current crescents with lee side lenses were formed where ice blocks were deposited during waning stages of the flood when stranded blocks of ice melted they left cone shaped and ring shaped heaps of ice rafted debris on the sediment surface several hundred ice blocks were completely buried within the aggrading lahar sediment and when these melted circular collapse pits formed in the sediment collapse types included subsided coherent blocks of sediment bounded by an outward dipping ring fracture trapdoor structures with horseshoe shaped fractures downsag pits with centroclinal dips locally up to 60 pits with peripheral graben and crevasses piecemeal highly fragmented collapse structures and funnel shaped pits containing disaggregated sediment a sequence of progressive collapse is inferred in which initial downsag and subsidence on an outward dipping ring fracture produces a small diameter pit this is followed by widening of the pit by progressive development of concentric ring fractures and downsag outside the early formed pit and by collapse of overhanging pit walls to produce vertical to inward dipping walls and aprons of collapse debris on the pit floor the various structures have potential for preservation even in regions prone to high rainfall and flooding and they can be used to indicate that former lahars contained abundant blocks of ice

conceptual and numerical models of ring fault formation abstract most ring faults of collapse calderas are primarily shear fractures the initiation and development of which depends on the state of stress in the host rock the state of stress in a volcano is controlled by the loading conditions such as magma chamber geometry and pressure but also by the mechanical properties of its rock units and structures such as existing contacts faults and joints ring fault formation is thus essentially a problem in rock physics field observations show that some ring faults are dip slip whereas others are partly faults shear fractures and partly ring dykes extension fractures although slip on existing ring faults is much more common in basaltic edifices shield volcanoes than in true composite volcanoes in both types of volcanoes most caldera unrest periods do not result in ring fault slip here i present new conceptual and numerical models of caldera formation in volcanoes with shallow spherical circular or oblate ellipsoidal sill like magma chambers in the layered models the host rock above the chamber is composed of 30 comparatively thin layers with stiffnesses young s moduli alternating between 1 gpa and 100 gpa the chamber itself is located in a single thick layer the crustal segment hosting the chamber is either 20 km or 40 km wide but has a constant thickness of 20 km the loading conditions considered are 1 a crustal segment subject to 5 mpa tension 2 crustal segment subject to excess magmatic pressure of 10 mpa at the bottom doming of the volcanic field containing the chamber 3 a combination of tension and doming and 4 chamber subject to underpressure negative excess pressure of 5 mpa the main results are as follows 1 excess pressure and underpressure in a chamber normally favour dyke injection rather than ring fault formation 2 for doming or tension a spherical magma chamber favours dyke injection except when the layer hosting the chamber is very soft 10 gpa or one with recent dyke injections in which case the surface stress field favours ring fault formation 3 for a sill like chamber in a 20 km wide crustal segment a ring fault can be generated by either tension or tension and doming for a 40 km wide segment doming alone is sufficient to generate a ring fault 4 since individual layers in a volcano may develop different local stresses stress field homogenisation through all the layers between the chamber and the surface is a necessary condition for ring fault formation 5 because the mechanical properties of the layers that constitute basaltic edifices are more uniform than those that constitute true composite volcanoes it follows that stress field homogenisation and thus ring fault formation or slip is more commonly reached in basaltic edifices than in composite volcanoes 6 both for basaltic edifices and composite volcanoes the stress fields most likely to initiate ring faults are those generated around sill like chambers subject to tension doming or both

subaqueous calderas in the archean abitibi greenstone belt an overview and new ideas abstract the 300 700 km abitibi greenstone belt in canada contains numerous world class archean volcanogenic massive sulfide vms deposits yet documentation of archean subaqueous calderas hosting such deposits is lacking the modern sunrise deposit in the myojin knoll caldera of the izu bonin arc shows that submarine calderas are first order sites for vms the hunter mine and normetal calderas as well as the blake river megacaldera complex are reviewed with respect to geometry regional geology physical volcanology dyke emplacement and hydrothermal carbonate alteration these subaqueous calderas are placed into the geodynamic context of the abitibi greenstone belt the abitibi belt displays a complex history of oceanic arc formation evolution collision and fragmentation with numerous types of volcanic edifices and sedimentary basins controlled by oblique subduction and mantle plumes analogue models yielding new insights into the sequence of caldera forming events and their synvolcanic fault patterns are applied to explain archean examples underpressure experiments develop a consistent sequence of fault patterns that compare favourably with natural caldera events during subsidence two sets of faults propagate from the magma chamber margin 1 early reverse outward dipping faults and 2 normal inward dipping faults the largest massive sulfide deposits form preferentially at the caldera margin based on experimental studies and detailed facies mapping the hunter mine caldera is of the segmented piecemeal variety and the normetal caldera is akin to a piston structure the blake river megacaldera contains 1 the e w trending misema 2 the nw trending new senator and 3 the ne trending noranda calderas with the latter two being nested graben type calderas consistent with oblique archean subduction the poorly documented hydrothermal carbonate alteration is discussed and a new exploration model for calderas is presented the alteration study was evaluated at three levels microprobe analysis geochemical staining of drill core and whole rock analyses there were two principal stages of alteration related to vms formation an early silicification phase developed either at depth and or at the seawater interface whereby silicification was bedding parallel in fine grained volcaniclastic deposits near the water surface a pervasive and extensive semi conformable carbonate alteration zone with a discordant focused root zone along faults evolved subsequently three distinct carbonate pairings were observed 1 proximal siderite sideroplesite fe ankerite next to the vms deposit 2 an intermediate ankerite fe dolomite zone and 3 a distal calcite dolomite zone the carbonate phase is associated with the mineralizing event transitions between carbonate species are subtle and changes are indicated by mineral assemblage overlap the silica carbonate hydrothermal alteration pattern is a replacement product and is far more widespread than has been documented this alteration can be easily confused with primary carbonates and chemical precipitation deposits e g banded iron formation if detailed volcano sedimentary facies mapping with alteration has not been conducted and if outcrop is insufficient

calderas and caldera structures a review abstract calderas are important features in all volcanic environments and are commonly the sites of geothermal activity and mineralisation yet it is only in the last 25 years that a thorough three dimensional study of calderas has been carried out utilising studies of eroded calderas geophysical analysis of their structures and analogue modelling of caldera formation as more data has become available on calderas their individuality has become apparent a distinction between caldera caldera complex cauldron and ring structure is necessary and new definitions are given in this paper descriptions of calderas based on dominant composition of eruptives basaltic peralkaline andesitic dacitic rhyolitic can be used and characteristics of each broad group are given styles of eruption may be effusive or explosive with the former dominant in basaltic calderas and the latter dominant in andesitic dacitic rhyolitic and peralkaline calderas four end member collapse styles occur plate or piston piecemeal trapdoor and downsag but many calderas have multiple styles features of so called funnel and chaotic calderas proposed in the literature can be explained by other collapse styles and the terms are considered unnecessary ground deformation comprises subsidence or collapse essential characteristics of a caldera and uplifting doming and fracturing due to tumescence and or resurgence frequent but not essential collapse may occur on pre existing structures such as regional faults or on faults created during the caldera formation and the shape of the collapse area will be influenced by depth size and shape of the magma chamber the final morphology of a caldera will depend on how the caldera floor breaks up whether collapse takes place in one event or multiple events whether vertical movement is spasmodic or continuous throughout the eruptive sequence and whether blocks subside uniformly or chaotically at one or more collapse centres a meaningful description of any caldera should therefore include number of collapse events presence or absence of resurgence caldera floor coherency caldera floor collapse geometry and dominant composition of eruptives

reappraisal of los humeros volcanic complex by new u th zircon and 40ar 39ar dating implications for greater geothermal potential

outward dipping ring fault structure at rabaul caldera as shown by earthquake locations the locations of a large number of earthquakes recorded at rabaul caldera in papua new guinea from late 1983 to mid 1985 have produced a picture of this active caldera s structural boundary the earthquake epicenters form an elliptical annulus about 10 kilometers long by 4 kilometers wide centered in the southern part of the rabaul volcanic complex a set of events with well constrained depth determinations shows a ring fault structure that extends from the surface to a depth of about 4 kilometers and slopes steeply outward from the center of the caldera this is the first geophysical data set that clearly outlines the orientation of an active caldera s bounding faults this orientation however conflicts with the configuration of many other calderas and is not in keeping with currently preferred models of caldera formation

the hechiceros caldera a recently identified mid tertiary caldera in eastern chihuahua mexico geologic mapping of tertiary volcanic rocks in the sierra hechiceros 50 to 80 km south of big bend national park has delineated a 20 by 25 km mid tertiary caldera herein named the hechiceros caldera this caldera was formed by the eruption of the palomas tuff which has a minimum volume of approximately 170 km3 and is distributed radially around the caldera the caldera was filled with more than 500 meters of postcaldera rhyolitic lavas which overflowed the caldera margin and caused additional subsidence the rocks of the sierra hechiceros are subalkalic and include rhyolites and dacites the chemistry of these rocks is transitional between the calc alkalic province of the sierra madre occidental and the alkalic rocks of the eastern trans pecos magmatic province the potassium argon ages both the palomas tuff and the postcaldera el jobero rhyolite are about 29 0 6 ma

experimental studies of collapse calderas we present experiments to investigate the formation of calderas using inflatable balloons in a medium of fused alumina powder this system provides a model of a magma chamber in an homogeneous elastic media which scales to volcanic systems in terms of geometry and strength of the media three kinds of experiment were performed i the balloon was inflated and then deflated simulating pre emption tumescence and caldera collapse ii the balloon was deflated without prior inflation simulating caldera formation without pre eruption tumescence iii three balloons were inflated and deflated in a line simulating multiple magma chambers the influence of magma chamber shape was investigated using different shaped balloons balloon inflation formed a dome with a surface pattern of shallow polygonal fractures centrally and radial fractures peripherally uplift occurs along deep inward dipping concentric reverse faults deflation generated depressions with an outer set of concentric ring fractures surrounding a central funnel formed by sagging along numerous minor faults some of which formed during doming during collapse the concentric reverse faults were converted to normal faults and the radial normally faulted fractures were converted to reverse faults without prior tumescence an outer set of near vertical ring faults surround a flat central depression due to collapse of a coherent block an inner set of outward dipping reverse ring faults also develop the experimental caldera area increased with balloon size and increased as the balloon depth decreased for a given balloon volume and depth collapse area is significantly smaller if doming had occurred prior to collapse arcuate fractures can provide the pathways for magma ascent during pre emption tumescence smith bailey 1968 but the shallow radial and polygonal fracture systems may also influence vent location changes in fault geometry and sense of movement extension to compression and vice versa occur when subsidence initiates and can account for changes in vent location that occur in many eruptions multiple overlapping collapse calderas are associated with magma chamber migration whereas nested calderas can result from the activity of a single magma chamber

experiments on cauldron formation a polygonal cauldron and ring fractures abstract the sequence of formation of the motojuku type cauldron fujita et al 1970 is summarized as follows 1 doming due to ascent of a magma body 2 development of normal faults which produce a polygonal cauldron and 3 eruption of acidic to intermediate pyroclastic material model experiments on a scale of 1 200 000 reveal that ascent of the magma body which was imitated by a ball of hardened putty produced a polygonal cauldron composed of radial and concentrically arranged short fractures on the roof of a dome no ring fractures were formed this type of cauldron develops near the surface on the other hand emission of magma which was imitated by evaporation of a ball of dry ice in brittle powdered material caused ring faults dipping outward and a circular cauldron without up or downwarping this type of cauldron develops upward from the magma body the former model is equated to the motojuku type cauldron and the latter model to the other types of cauldrons with ring fractures

numerical and experimental models on the formation mechanism of collapse basins during the green tuff orogenesis of japan the geological model about volcanism of the green tuff geosyncline deduced from the field observations consists of the following processes 1 dome shaped uplift with a mean diameter of 30 km 2 collapse of the central part of the domes forming basins with a mean diameter of 10 km 3 volcanic activity inside the collapse basins it is considered that these consecutive processes resulted from the magmatic uplift from a deep part of the crust

reservoir architecture and diagenesis in downthrown fault block plays the lowlander prospect of block 14 20b witch ground graben outer moray firth uk north sea

the tavua caldera fiji a complex shoshonitic caldera formed by concurrent faulting and downsagging the shoshonitic tavua volcano in viti levu fiji developed from a submarine to a subaerial environment and comprises lava flows dominantly of absarokite composition with debris flows and conglomerates on its flanks the tavua caldera formed in the centre of the volcano within the caldera avalanche deposits are overlain by the turtle pool formation which consists of lacustrine sediments shoshonite lavas and hydrovolcanic pyroclastic lapilli tuffs subsequently the inner caldera formed in the centre of the tavua caldera this was infilled by the morrisons pool formation which comprises lavas and pyroclastic rocks of banakite composition with minor intercalated sediment thus the magmas became progressively more evolved in addition to normal movement along regional and concentric faults subsidence which exceeds 3 km was accomplished via substantial downsagging evidence for this is the consistent centripetal dip of caldera fill material which locally exceeds 50 in parts of the turtle pool formation centripetal dips are also recognized in pre caldera rocks immediately outside the tavua caldera subsidence was episodic caused by periodic non explosive withdrawal of underlying magma to erupt on the flanks and in part within the caldera downsagging and fault movement were responsible for approximately equal amounts of subsidence and their interaction produced a caldera with the shape of a stepped funnel

shishimuta caldera the buried source of the yabakei pyroclastic flow in the hohi volcanic zone japan 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

vent evolution and lag breccia formation during the cape riva eruption of santorini greece the 18 500 yr b p cape riva cr eruption of santorini vented several km3 or more of magma generating four eruption units a basal plinian fall deposit cr a and three pyroclastic flow deposits cr b to cr d upwards cr b and cr d are welded ignimbrites cr c consists predominantly of up to 25 m thick coarse lithic rich co ignimbrite lag breccias resulting from a climactic phase of the eruption the initial plinian phase occurred from a localized vent in n santorini and subsequent column collapse resulted in emplacement of cr b towards the end of cr b new conduits were activated and pyroclastic flows discharged from multiple vents to generate the lag breccias cr c cr d probably records a return to a localized vent as the eruption waned the eruption sampled a zoned magma chamber containing rhyodacite overlying andesite and leaks of these magmas were manifested as the skaros therasia lavas preceding the cr eruption plinian and initial ignimbrite stages occurred while the magma chamber was ove

submarine volcanic mounds in the pearl river mouth basin northern south china sea a number of enigmatic km scale mound structures have been discovered in the baiyun sag of the pearl river mouth basin using high resolution 2d and 3d seismic data combined with multi beam bathymetry based on detailed seismic characterization we interpret these as sill fed volcanic mounds associated structures include igneous sills trans tensional faults lava flows and compactional drape folds the igneous intrusions and associated mounds were emplaced in the late early miocene c 18 5 ma or shortly thereafter constrained by the onlap relationship between the mounds and their overburden the volcanic mounds are preferentially developed above or adjacent to basement highs indicating a structural control on the igneous plumbing system we propose that the fractures and tectonic faults above the raised basement follow pre existing zones of weakness and may have acted as feeding conduits for the magmatic material the discovery of the early miocene igneous province has important implications for the understanding of the evolution of the pearl river mouth basin and for regional hydrocarbon prospectivity c 2014 elsevier b v all rights reserved

caldera faults capture and deflect inclined sheets an alternative mechanism of ring dike formation the subsurface structures of caldera ring faults are often inferred from numerical and analog models as well as from geophysical studies all of these inferred structures need to be compared with actual ring faults so as to test the model implications here we present field evidence of magma channeling into a caldera ring fault as exhibited at hafnarfjall a deeply eroded and well exposed 5 ma extinct volcano in western iceland at the time of collapse caldera formation over 200 m of vertical displacement was accommodated along a ring fault which is exceptionally well exposed at a depth of approximately 1 2 km below the original surface of the volcano there are abrupt changes in the ring fault attitude with depth but its overall dip is steeply inward several inclined sheets within the caldera became arrested at the ring fault other sheets became deflected up along the fault to form a multiple ring dike we present numerical models showing stress fields that encourage sheet deflection into the subvertical ring fault our findings provide an alternative mechanical explanation for magma channeling along caldera ring faults which is a process likely to be fundamental in controlling the location of post caldera volcanism

incremental caldera collapse of suswa volcano gregory rift valley kenya suswa volcano located at 1 10 s 36 20 e is quaternary in age 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

geomorphology and structural development of the nested summit crater of lascar volcano studied with terrestrial laser scanner data and analogue modelling abstract many volcano summits host craters that are partially overlapping the formation of such nested craters has been commonly interpreted as vent migration here we present an additional mechanism that may explain the geometry of nested craters at volcanoes lascar volcano the most active volcano of the central volcanic zone in the chilean andes hosts ene wsw trending summit craters that are partially overlapping nested details on the evolution and interaction between the different craters remain unclear to create a robust dataset terrestrial laser scanner tls data were collected at the summit of lascar in 2013 the resulting topographic data set consisting of more than 15 million data points with centimetre sampling allows visualising almost the complete eastern edifice of the volcano s summit from the tls data a digital elevation model dem and a slope map were generated allowing us to create a lineament map and quantify the observed morphological and structural features to further improve our understanding of the processes responsible for the formation of the craters and geomorphology we designed sandbox analogue models results suggest that one of the craters is a parasite crater formed as a consequence of ongoing activity in the adjacent crater our data suggest that the nested craters have all been modified since the last major eruption in 1993 by near surface effects associated with cooling compaction and gravitational sliding of the crater floor infill as the active crater deepens the adjacent inactive crater extends and partially slumps towards the active one understanding the structural development of these nested craters is relevant for assessing potential future eruption sites thus making lascar a dynamic target for a detailed morphology study these findings may similarly be applied to other volcanoes where nested craters have developed

downsag and extension at calderas new perspectives on collapse geometries from ice melt mining and volcanic subsidence 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

an active ring fault detected at tendurek volcano by using insar 1 although ring faults are present at many ancient deeply eroded volcanoes they have been detected at only very few modern volcanic centers at the so far little studied tendurek volcano in eastern turkey we generated an ascending and a descending insar time series of its surface displacement field for the period from 2003 to 2010 we detected a large 105 km2 region that underwent subsidence at the rate of 1 cm yr during this period source modeling results show that the observed signal fits best to simulations of a near horizontal contracting sill located at around 4 5 km below the volcano summit intriguingly the residual displacement velocity field contains a steep gradient that systematically follows a system of arcuate fractures visible on the volcano s midflanks rapideye satellite optical images show that this fracture system has deflected holocene lava flows thus indicating its presence for at least several millennia we interpret the arcuate fracture system as the surface expression of an inherited ring fault that has been slowly reactivated during the detected recent subsidence these results show that volcano ring faults may not only slip rapidly during eruptive or intrusive phases but also slowly during dormant phases

structural eruptive and intrusive evolution of the grizzly peak caldera sawatch range colorado volcanic and shallow intrusive features of the deeply dissected 34 ma grizzly peak caldera in west central colorado record evolution of the magmatic center from pre caldera through post resurgent stages pre caldera dikes zones of hydrothermally altered rocks and lava flows formed along a circular swarm of cone sheet fractures around the site of the future caldera early largely rhyolitic uppercrustal magmatism culminated in the caldera forming eruption of the grizzly peak tuff intracaldera tuff is zoned from high silica rhyolite at the base to low silica rhyolite at the eroded top and further contains dacite to mafic latite pumice lumps in two heterogeneous tuff layers in the upper third of the preserved section half of the erupted tuff ponded in the 17 by 23 km 600 km 3 caldera filling the asymmetric depression to a compacted thickness locally 2 7 km including intercalated rock avalanche megabreccias shed from ring fault scarps the asymmetric caldera has an inner ring fracture zone that separates two structural segments that collapsed to different depths caldera fill buried collapse structures as they formed the inner ring fracture zone is a growth or syn depositional fault in the single cooling unit tuff welded tuff ring dikes are locally exposed at erosion levels below the caldera fill these dikes are remnants of fissure vents in the outer ring fracture zone following collapse the grizzly peak caldera was uplifted forming a complexly faulted resurgent dome resurgence resulted partly from emplacement of a composite granodiorite laccolith now exposed in the eroded core of the dome a belt of mafic latite to rhyolite porphyry dikes and small stocks formed across the center of the domed caldera during the waning of the magmatic center latite intrusions in this suite represent the penetration of relatively mafic magma to the surface following solidification of the felsic subcaldera batholith

gross brukkaros namibia an enigmatic crater fill reinterpreted as due to cretaceous caldera evolution 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

graben structure in the las canadas edifice tenerife canary islands implications for active degassing and insights on the caldera formation abstract a graben structure has been identified at the western area of the las canadas caldera wall here referred as the los azulejos graben this graben is 1 km wide and is bounded by two major normal faults trending ne sw the los azulejos fault and the ucanca fault the graben was active for at least 0 5 ma from the end of the ucanca fm to the end of the guajara fm and before the collapse of the las canadas edifice that formed the western caldera a right lateral transtension regime operated in the graben as suggested by small fault orientations and kinematics the prolongation of the ne rift zone of tenerife to the canadas edifice is the most likely volcano tectonic scenario for the graben in this context inflation of phonolitic shallow magma chambers may have produced reverse faults and reactivation of normal faults an intense and widespread hydrothermal alteration here called azulejos type occurred mainly before the graben formation while a fault related hydrothermal alteration occurred during and after the graben diffuse carbon dioxide and hydrogen degassing in and around the las canadas caldera show relatively enriched values along a ne sw trend suggesting that faults in the los azulejos graben act as a pathway for deep seated gases to the surface diffuse degassing and hydrothermalism indicate that the graben area has been a zone of intense fluid circulation during the evolution of the las canadas edifice

snake river sr type volcanism at the yellowstone hotspot track distinctive products from unusual high temperature silicic super eruptions a new category of large scale volcanism here termed snake river sr type volcanism is defined with reference to a distinctive volcanic facies association displayed by miocene rocks in the central snake river plain area of southern idaho and northern nevada usa the facies association contrasts with those typical of silicic volcanism elsewhere and records unusual voluminous and particularly environmentally devastating styles of eruption that remain poorly understood it includes 1 large volume lithic poor rhyolitic ignimbrites with scarce pumice lapilli 2 extensive parallel laminated medium to coarse grained ashfall deposits with large cuspate shards crystals and a paucity of pumice lapilli many are fused to black vitrophyre 3 unusually extensive large volume rhyolite lavas 4 unusually intense welding rheomorphism and widespread development of lava like facies in the ignimbrites 5 extensive fines rich ash deposits with abundant ash aggregates pellets and accretionary lapilli 6 the ashfall layers and ignimbrites contain abundant clasts of dense obsidian and vitrophyre 7 a bimodal association between the rhyolitic rocks and numerous coalescing low profile basalt lava shields and 8 widespread evidence of emplacement in lacustrine alluvial environments as revealed by intercalated lake sediments ignimbrite peperites rhyolitic and basaltic hyaloclastites basalt pillow lava deltas rhyolitic and basaltic phreatomagmatic tuffs alluvial sands and palaeosols many rhyolitic eruptions were high mass flux large volume and explosive vei 6 8 and involved h2o poor low 18o metaluminous rhyolite magmas with unusually low viscosities partly due to high magmatic temperatures 900 1 050 c sr type volcanism contrasts with silicic volcanism at many other volcanic fields where the fall deposits are typically plinian with pumice lapilli the ignimbrites are low to medium grade non welded to eutaxitic with abundant pumice lapilli or fiamme and the rhyolite extrusions are small volume silicic domes and coulees sr type volcanism seems to have occurred at numerous times in earth history because elements of the facies association occur within some other volcanic fields including trans pecos texas etendeka parana lebombo the english lake district the proterozoic keewanawan volcanics of minnesota and the yardea dacite of australia

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

caldera development during the minoan eruption thira cyclades greece the well known caldera of thira santorini greece was not formed during a single eruption but is composed of two overlapping calderas superimposed upon a complex volcanic field that developed along a ne trending line of vents before the minoan eruption of 1400 b c thira consisted of three lava shields in the northern half of the island and a flooded depression surrounded by tuff deposits in the southern half andesitic lavas formed the overlapping shields of the north and were contemporaneous with and in many places interbedded with the southern tuff deposits although there appears to be little difference between the composition of magmas erupted differences in eruption style indicate that most of the activity in the northern half of the volcanic field was subaerial producing lava flows whereas in the south eruptions within a flooded depression produced a sequence of mostly phreatomagmatic tuffs many of these tuffs are plastered onto the walls of what appears to have been an older caldera most probably associated with an eruption of rhyodiacitic tephra 100 000 years ago the minoan eruption of about 1400 b c had four distinct phases each reflecting a different vent geometry and eruption mechanism the minoan activity was preceded by minormore eruptions of fine ash 1 the eruption began with a plinian phase from subaerial vent s located on the easternmost of the lave shields 2 vent s grew toward the sw into the flooded depression subsequent activity deposited large scale base surge deposits during vent widening by phreatomagnetic activity 3 the third eruptive phase was also phreatomagmatic and produced 60 of the volume of the minoan tuff this activity was nearly continuous and formed a large featureless tuff ring with poorly defined bedding less

explosive rhyolitic volcanism in the jemez mountains vent locations caldera development and relation to regional structure the jemez volcanic field straddles the western margin of the rio grande rift where the rift is intersected by the jemez lineament in north central new mexico the field has a record of volcanism extending back to before 13 ma initial basaltic activity was related to active rifting with minor rhyolitic eruptions occurring along n s rift bounding faults between 10 and 7 ma voluminous andesitic volcanism took place in the central jemez mountains area overwhelming contemporaneous basaltic and rhyolitic magmatism an apparent tectonic lull took place from 7 to 4 ma accompanied by lower eruption rates during this interval dacitic magmas were erupted to form the tschicoma volcanic center but mafic and rhyolitic volcanism virtually ceased since 4 ma accompanying resumption of rifting a growing silicic magma system has been present under the central part of the jemez mountains ultimately evolving to the magma body that produced the voluminous rhyolitic bandelier tuffs explosive rhyolitic eruptions from this large magma body have occurred many times since 3 to 4 ma early eruptions 3 6 2 8 ma produced high silica rhyolite ignimbrites restricted to the southwest part of the jemez mountains formation of these units may have been accompanied by caldera collapse these events were followed by the two caldera forming bandelier tuff ignimbrite eruptions 1 45 and 1 12 ma post caldera explosive and effusive rhyolite eruptions have also tapped the magma body from vents generally located along ring fractures after both bandelier events vent and caldera locations for the rhyolitic eruptions during 3 4 ma have been inferred from grain size characteristics dispersal patterns and facies variations in the plinian deposits and ignimbrites lithic breccia zones of the pre bandelier ignimbrites indicate possible caldera sources in the southwest part of the present valles caldera during the eruption of both bandelier tuffs initial plinian falls and early pyroclastic flows emanated from vents centrally located in the jemez mountains in the lower bandelier tuff eruption a transition to ring fracture vents occurred before the emission of later pyroclastic flows but there is no strong evidence to suggest such a transition occurred during the upper bandelier eruption calderas associated with the lower and upper bandelier tuffs toledo and valles respectively are almost identical in location as are the plinian vent sites for these two large eruptions the toledo embayment northeast of the valles caldera contains lava domes from up to 3 6 ma and may be a caldera or crater associated with early explosive dacitic volcanism in the tschicoma volcanic center post 1 4 ma lava domes also fill this depression the main volcanic features of the jemez mountains field including the valles caldera complex eruption vents and the apical graben of the post valles redondo resurgent block appear to be aligned along the ne sw trending jemez fault zone this zone the local expression of a precambrian basement feature the jemez lineament has exerted strong control on the location and style of eruptions from the bandelier rhyolitic magma system

evolution of the olympus mons caldera mars synoptic images of the martian volcano olympus mons are of a quality and quantity that are unique for mars and somewhat surprisingly are appreciably better than image data that exist for many volcanoes on earth useful information about the evolution of shield volcanoes on earth can thus be derived from the investigation of this extraterrestrial example we have used shadow length measurements and photoclinometrically derived profiles to supplement and refine the topographic map of the olympus mons caldera as much as 2 5 km of collapse took place within the 80 65 km diameter caldera and the elevation of the caldera rim varies by almost 2 0 km low around the oldest collapse events high around the youngest an eight stage evolutionary sequence for the caldera of olympus mons is identified which shows that caldera subsidence was a longterm process rather than the near instantaneous event that has been interpreted from comparable terrestrial examples tectonic features on the caldera floor indicate a transition from an extensional environment graben formation around the perimeter of the caldera to compression ridge formation towards the caldera center this transition from a compressional to extensional environment is surprisingly sudden occurs at a radial distance of 17 km from the caldera center and is import because it can be used to infer that the magma chamber was relatively shallow thought to be at a depth of 16 km beneath the caldera floor zuber and mouginis mark 1990 ample evidence is also found within the olympus mons caldera for solidified lava lakes more than 30 km in width and for the localzed overturning and or withdrawal of lava within these lakes

fracture pattern analysis as a tool for constraining the interaction between regional and diapir related stress fields poza de la sal diapir basque pyrenees spain during diapir evolution the local stress field results from the interaction between diapirrelated and remote regional stress fields the fracture pattern hosted in the overburden can register this evolving interaction providing significant insights for the unravelling of the diapir kinematics in this work we present the example of the poza de la sal diapir which is a salt diapir that pierces the synorogenic deposits of the basque pyrenees the integrated analysis of mesostructural data which includes field collected faults joints and fracture traces digitalized on high resolution orthophotos allowed us to establish an evolutionary model for the stress field around diapirs the presented data support the idea that during diapir evolution stress fields evolve from mostly regional related to mostly diapir related contrasting with other geological data seismic data geological map and cross section we observe that different methods provide complementary information coherent with the proposed evolutionary model of the poza de la sal diapir the study of fracture development associated with the growth of salt diapirs is of interest for both academic and industrial purposes in fact fractures play an important role in fluid flow and its accumulation in salt diapir related reservoirs johnson bredeson 1971 koestler ehrmann 1987 davison et al 2000a b on the other hand the analysis of fracture patterns is useful to constrain the host structure evolution thorbjornsen dunne 1997 tavani et al 2008 and accordingly can provide useful information for understanding diapir kinematics alsop 1996 stewart 2006 yin groshong 2007 yin et al 2009 the fracture pattern associated with salt diapirs and in general with doming structures is characterized by two classes of elements radial and concentric fig 1a b e g parker mcdowell 1955 withjack 1979 squyres et al 1992 branney 1995 malthe sorenssen et al 1999 rowan et al 1999 2003 davison et al 2000a b freed et al 2001 stewart 2006 this pattern is documented in simple structures where the remote stress field is negligible in relation to the diapir related stress fields when such a condition is not verified the interaction of these two stress fields leads to the development of more complex patterns ode 1957 withjack scheiner 1982 studies of interacting regional and punctual stress fields e g ode 1957 indicate that the influence of diapir induced stress progressively reduces away from the dome i e the punctual structure as a consequence far from the diapir the fracture pattern responds to the remote stress field and approaching the diapir the fracture pattern tends to be radial and concentric fig 1c although these patterns have been observed by several authors in different structures withjack scheiner 1982 mege masson 1996 ernst et al 2003 there are no detailed field studies focusing on this topic in this work we present a simple 2d theoretical stress field model where the stress tensor resulting from the interaction between remote and diapirrelated stress fields is assumed to be for a fixed distance from the diapir centre a 2 2 matrix provided by the sum of the horizontal components of the two stress tensors the eigenvectors associated with this matrix are assumed to be the principal axes of the local stress field the working hypothesis is that if the stress field changes the azimuth of fractures joints developed under these stress fields also change by understanding interactions between diapir related and regional stress fields we can predict the azimuth of the developed fractures to test the validity of this theoretical model we have applied it to a well exposed salt diapir located at the western pyrenees poza de la sal diapir this is a near circular diapir with a mainly carbonate overburden affected by a complex pattern of fractures results of this comparison indicate that despite its simplicity the provided stress model is a useful tool for understanding the kinematics of salt diapirs accordingly in this paper we provide from alsop g i archer s g hartley a j grant n t hodgkinson r eds 2012 salt tectonics sediments and prospectivity geological society london special publications 363 521 532 http dx doi org 10 1144 sp363 25 the geological society of london 2012 publishing disclaimer www geolsoc org uk pub ethics guidelines to obtain a fracture pattern using the theoretical model presented to complement other techniques of investigation such as field work or 3d seismic interpretations

asymmetric multiple block collapse at rotorua caldera taupo volcanic zone new zealand calderas worldwide have been classified according to their dominant collapse styles although there is a good deal of speculation about the processes involved recent laboratory experiments have tried to constrain these processes by modelling magma withdrawal and observing the effects on overlying materials however many other factors also contribute to final caldera morphology rotorua caldera formed during the eruption of the mamaku ignimbrite collapse structure and evolution of rotorua caldera is interpreted based its geophysical response geology and geomorphology and the stratigraphy of the mamaku ignimbrite rotorua caldera is situated at the edge of the extensional taupo volcanic zone in which major faults strike ne sw a second less dominant fault set strikes nw se these two fault sets have a strong influence on the morphology of rotorua caldera no one style of collapse can be applied to rotorua caldera it was formed during a single eruption but subsided as many blocks and shows features of trapdoor piecemeal and downsag types of collapse here rotorua caldera is described according to its composition activity and geometry as a rhyolitic single event asymmetric multiple block single locus collapse structure the mamaku ignimbrite is the only ignimbrite to have erupted from rotorua caldera extracaldera thickness of the mamaku ignimbrite is up to 145 m whereas inside the caldera it may be greater than 1 km thick the mamaku ignimbrite can be separated into a basal tephra sequence and main ignimbrite sequence the main ignimbrite sequence contains no observable flow unit boundaries but can be split into lower middle and upper parts lmi mmi umi respectively based on crystal content welding jointing devitrification and vapour phase alteration juvenile clasts within the ignimbrite comprise three consanguineous silicic pumice types and andesitic fragments only the most evolved pumice type occurs in the basal tephra sequence all three pumice types occur together throughout the main ignimbrite sequence whereas the andesitic fragments are only present in umi lithic lag breccias in umi indicate a late stage of caldera collapse concentration of lithic fragments increases towards the middle of the ignimbrite and may also reflect increased subsidence rate during an earlier stage collapse of rotorua caldera is thought to have occurred throughout the eruption of the main ignimbrite sequence of the mamaku ignimbrite allowing simultaneous eruption of all the different pumice types and causing the abrupt transition from deposition of the basal tephra sequence to the main ignimbrite sequence

formation of caldera periphery faults an experimental study 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

sedimentary and volcano tectonic processes in the british paleocene igneous province a review research on the british paleocene igneous province bpip has historically focused on the emplacement chemistry and chronology of its elaborate central intrusive complexes and lava fields however the bpip has also been dramatically shaped by numerous erosion sedimentation and volcano tectonic events the significance of which becomes ever clearer as localities in the bpip are re investigated and our understanding of volcano sedimentary processes advances the resultant deposits provide important palaeo environmental palaeo geographical and stratigraphical information and highlight the wide range of processes and events that occur in ancient volcanic settings such as the bpip in this paper we review the sedimentary and volcano tectonic processes that can be distinguished in the bpip and conceptualize them within a generalized framework model we identify and describe the sedimentary responses to four broadly chronological stages in the history of the bpip volcanoes 1 the development of the lava fields 2 early intrusion induced uplift 3 caldera collapse and 4 post volcano denudation and exhumation of central complexes we highlight and illustrate the range of sedimentary processes that were active in the bpip these operated on and helped shape a dynamic landscape of uplands and lowlands of alluvial fans braided rivers lakes and swamps and of volcanoes torn apart by catastrophic mass wasting events and or caldera collapse

volcanic geology of tyrrhena patera mars tyrrhena patera is a large low relief volcano located in the southern cratered highlands of mars on a proposed ring fracture of the hellas basin the volcano is composed of four or possibly five principal units the oldest and most areally extensive of which are suggested to be ash on the basis of their morphology and erosional characteristics lava flows are observed in a flank flow unit extending from the central caldera to the southwest and apparently cap the summit region models derived from the study of terrestrial pyroclastic flows have been applied to the deposits at tyrrhena patera and indicate that the distribution and lengths of the observed deposits are consistent with an origin by the emplacement of gravity driven ash flows generated by hydromagmatic or magmatic explosive eruptions the hydromagmatic case requires considerable quantities of water 7 5 1016 kg however at volumetric water flow rates determined by other investigators 105 m3 s and calculated from the regional slopes in the hesperia planum region 106 m3 s the amounts necessary could be supplied relatively rapidly in geologic time tyrrhena patera represents the earliest central vent volcanism identified on mars and may reflect a transition from the flood style eruptions which dominated early martian history a hydromagmatic origin for tyrrhena patera is in agreement with suggested climatic changes on mars and may explain the absence of this style of volcanism in later eras alternatively a magmatic origin for tyrrhena patera implies an evolution in the types of magmas produced on mars from hesperian to amazonian time during which the prominent shields in the tharsis region formed

a volcanotectonic survey of ascraeus mons mars 1 ascraeus mons is one of the largest volcanoes on mars it is replete with well preserved features that can be used to understand its volcanotectonic evolution previous studies of this volcano focused on specific features and were limited by the quality and coverage of contemporary data our objective is to review and enhance the existing developmental model for ascraeus by considering all endogenic surface features on the volcano we surveyed the volcano s caldera complex flank terraces pit structures sinuous rilles arcuate grabens and small vents we report the spatial and temporal distributions of these features appraise their proposed formation mechanisms in light of our mapping results and propose a detailed geological history for ascraeus mons an initial shield building phase was followed by the formation of a summit caldera complex and small parasitic cones while compression due to flexure of the supporting basement led to extensive terracing of the shield flanks an eruptive hiatus followed ending with the construction of expansive rift aprons to the northeast and southwest against later extensive flank resurfacing in the late amazonian continued flexure formed arcuate grabens concentric to the edifice localized eruption and surface flow of a fluid agent lava and or water from within the volcano then produced a population of rilles on the lower flanks finally in a change of flank tectonic regime from compression to extension pit crater chains and troughs developed on the main shield and rift aprons eventually coalescing to form large embayments at the northeast and southwest base of the volcano

caldera subsidence in areas of variable topographic relief results from analogue modeling calderas form in volcanic areas commonly associated with topographic relief pre existing topography plays an important role in the style of the caldera subsidence topography increases the load and affects the principle stress trajectories located between the roof of the magma chamber and the surface the morphology internal structure and temporal evolution of calderas are therefore sensitive to the local topography we carried out scaled analogue experiments to investigate the effect of pre existing topographic relief on caldera subsidence by modeling the presence of stratocones and plateaus of variable mass diameter and positions prior to collapse we induced collapse in sandbox experiments by withdrawing water from a rubber bladder to simulate caldera collapse into a large shallow reservoir deformation was first manifested by sagging of the sand at the surface followed by the upward propagation of a set of subsidence controlling faults from the bladder in experiments with no topography these faults usually reached the surface near the centre of the cylinder as evacuation and incremental growth progressed a second outer set of subsidence controlling faults developed this outer set dominated the subsidence by increasing the topographic load by 6 the subsidence efficiency increased producing calderas up to approximately 20 deeper the inner set of subsidence controlling faults steepened and controlled most of the collapse by contrast to the experiments without topography adding load also reduced outward growth of peripheral sagging and development of extensional faults leading to a diameter of sagging 20 smaller than with no topography a comparative analysis of our results with mount mazama at crater lake oregon and glass mountain at long valley california supports the interpretation that the addition of topographic load modifies the evolution of the major caldera faults

the cretaceous messum igneous complex s w etendeka namibia reinterpretation in terms of a downsag cauldron subsidence model abstract the messum igneous complex mic lies within the ene trending zone of lower cretaceous 132 ma damaraland intrusive complexes in namibia intruded into pan african damara basement it is defined by a roughly circular structure 18 km in diameter the bounding ring fault exposed along the eastern sector encircling messum are the volcanic sequences of the goboboseb mountains comprising a lower basalt series tafelkop and tafelberg types followed with intervening basalts by four voluminous quartz latite ql eruptions goboboseb and springbok ql units inferred stages of development are a an initial very broad basaltic lava shield comprising the tafelberg and tafelkop basalts and messum crater basalts mcb possibly ponded in near vent lava lakes embedded within the lower basaltic sequence is a localised rhyolite dominated eruptive centre ca 5 km in diameter interpreted as a funnel caldera located towards the centre of the mic b downsagging extending northwards from messum following the goboboseb ql eruptions 2300 km3 ponding of overlying basaltic units c climactic springbok ql eruption 6300 km3 producing further downsag together with the inward radial dip of all volcanic units towards the mic ring fault initiation d cauldron subsidence emplacement of a granitoid suite forming the mic moat area between the ring fault and the core region e intrusion of gabbroic cone sheets into incompletely solidified granitic melts within the southeastern moat resulting hybridisation and magma mingling produced extensive development of heterogeneous granitoid and hybrid dioritic lithologies f cone sheet intrusions of the eastern gabbros into more highly solidified granitoids of the southeastern moat g intrusion of thick 1 2 km western gabbro cone sheets exhibiting local fine scale layering into solidified granitoids mainly within the western moat minor late stage granitic intrusions h 2 3 ma quiescent period followed by quartz and ne syenite intrusions and finally basanite dykes emplaced within the mic core accompanying differential uplift of the core uplift resurgence within the mic has accompanied intrusion of the moat granitoids and mafic cone sheets thereby juxtaposing volcanic and intrusive sequences phases of both subsidence and uplift have characterised the mic the nw scotland tertiary central igneous complexes and messum show evidence of a number of parallel developments but also important differences the mic differs markedly from caldera systems within the western usa and circum pacific messum is therefore suggested to represent a distinct class of intrusive extrusive central complex although probably common in large igneous provinces

folded cauldrons of the early proterozoic labine group northwestern canadian shield great bear magmatic zone 1 875 1 84 ga is a linear belt 100 km wide by 800 km long of little metamorphosed volcanic rocks and allied plutons of calc alkaline affinity the entire zone was folded shortly after magmatism and sections many kilometers thick through several cauldron complexes are exposed eruptions of rhyolite led to collapse of black bear cauldron in which 1 5 km of tuff ponded the cauldron then became the locus of fluvial and lacustrine sedimentation followed by augite plagioclase porphyritic andesite volcanism both of which also accumulated within the cauldron ash flow eruptions of dacite caused collapse of clut cauldron which was accompanied by landsliding and avalanching of the cauldron walls abundant andesitic and intrusive debris is intercalated with the propylitized intracauldron facies tuff adjacent to the walls and some blocks are as large as 1 km across clut cauldron also became the site for fluvio lacustrine sedimentation after collapse and was likely resurgent resurgence was probably related to the emplacement of a quartz monzonite pluton which occupies the core of the cauldron cornwall cauldron which is exposed entirely in cross section contains 1 2 km of intracauldron facies tuff overlain by a complex of dacite lava flows the cauldron was intruded by a granodiorite monzogranite pluton probably responsible for resurgence data from the area suggest the following 1 the similarity of early proterozoic cauldrons to cenozoic examples indicates that cauldron collapse is a process that has occurred since 1 9 ga 2 batholiths beneath ash flow tuff fields are probably composite bodies made up of many individual plutons 3 most of the plutons are sheetlike in cross section and 4 cauldron collapse begins as soon as ash flow eruptions and relief on the cauldron margin becomes large early in the subsidence history

formation of collapse calderas collapse calderas are common in many volcanic regions but the mechanics of their formation is not well understood i propose that collapse calderas may form as a result of the growth of associated shallow magma chambers excess magmatic pressure in a shallow chamber gives rise to tensile stresses at the surface but the location and magnitude of the maximum tensile stress depend on the shape of and depth d to the top of the chamber this model indicates that penny shaped chambers i e sill like chambers of circular horizontal cross sections are most likely to initiate collapse calderas for such a chamber with horizontal radius a the higher the a d radius depth ratio the greater the probability of forming a collapse caldera

magmatic fissures and their systems in iceland a tectonomagmatic model 1 volcanic systems in iceland comprise a central volcano linked to magmatic fissures whose subsurface architecture remains enigmatic the surface dimensions and trends of active fissures show their spatial relation with fault systems structure of the fremri namur and hengill volcanic systems was analyzed the linear vent arrays marking recent fissures are tightly bound by vertical segmented faults while normal faults accommodating extension are common on the flanks of the fissure swarm evidence of normal frictional slip on the fissure bounded faults suggests that failure occurred in response to subsidence with subsequent dilation this is supported by inward tilting of strata adjacent to faults compatible with a downsag phase we propose that magmatic fissures have vertical feeders with lateral offshoots extending along the rift zone their inflation deflation during an eruptive cycle causes subsidence such magmatically generated faults can be subsequently modified by tectonic extension

evidence of incremental growth in the vulsinian calderas central italy abstract a comparative work on the vulsini calderas has been carried out fundamentally from volcanological field study drilling and gravity anomalies data on the basis of a detailed stratigraphy four volcanic complexes have been recognized in the vulsini volcanic district from the oldest to the youngest paleo bolsena bolsena montefiascone and latera the latter two complexes having a contemporaneous evolution in their activity each complex gave rise to several little calderas caused by piecemeal collapses due to moderate size explosive eruptions or coherent subsidence of a superficial piston like block nevertheless the extensional volcano tectonic regime affecting this area could have accounted for as much of the total subsidence as the caldera forming eruptions bolsena and latera calderas respectively 16 km and 8 km in average diameter represent the main depressions of the vulsini district and their topographic features are still well preserved several collapses forming the main poligenetic calderas of bolsena and latera are masked at present by the bolsena lake the younger volcanic covering the lacustrine deposits and erosion in a minor amount only the little montefiascone and vepe calderas which represent the youngest collapses of the vulsini area have not been masked bolsena caldera could be considered a depression caused mostly by a subsidence hinged to the southwest by down sagging and by collapses which occurred after the most important explosive eruptions of the paleo bolsena bolsena and montefiascone volcanic complexes meanwhile latera caldera is mostly due to a piecemeal collapse linked to several moderate size ignimbrite units which were emplaced throughout the explosive activity of the latera complex therefore the vulsinian calderas did not form by catastrophic eruptions because several and frequent moderate size eruptions downsagging and tectonic hinged subsidence concur indicating an incremental growth

tectonic influences in piecemeal caldera collapse at glencoe volcano scotland glencoe volcano is renowned as the archetype for caldera collapse that results from piston like subsidence of a coherent crustal block on a ring fault however the caldera floor rocks and the geometry of seven intracaldera units that record large volume explosive eruptions show that the collapse involved incremental and haphazard subsidence of numerous crustal blocks before development of the ring fault some subsidence involved flexure downsag with associated development of extensional crevasses hundreds of metres deep a system of orthogonal faults and related grabens records the influence of two intersecting basement discontinuities one of which is inferred to have linked with the active great glen fault substantial extensional and or transtensional faulting with distinct related sedimentary responses occurred without eruptions the tectonic framework substantially controlled the magmatic plumbing and the locations of vents caldera depocentres and through going rivers the related sediments influenced contrasting eruption styles e g transitions between phreatomagmatic and magmatic and the shallow level emplacement of sills

deformation of the wineglass welded tuff and the timing of caldera collapse at crater lake oregon abstract four types of deformation occur in the wineglass welded tuff on the northeast caldera rim of crater lake a vertical tension fractures b ooze outs of fiamme c squeeze outs of fiamme and d horizontal pull apart structures the three types of plastic deformation b d developed in the lower part of the wineglass welded tuff where degree of welding and density are maximum deformation originated from concentric normal faulting and landsliding as the caldera collapsed the degree of deformation of the wineglass welded tuff increases toward the northeast part of the caldera where plastic deformation occurred more easily because of a higher emplacement temperature probably due to proximity to the vent the probable glass transition temperature of the wineglass welded tuff suggests that its emplacement temperature was 750 c where the tuff is densely welded calculation of the conductive cooling history of the wineglass welded tuff and the preclimactic cleetwood lava flow under assumptions of a initially isothermal sheet and uniform properties suggests that a caldera collapse occurred a maximum of 9 days after emplacement of the wineglass welded tuff and that b the period between effusion of the cleetwood lava flow and onset of the climactic eruption was

calderas of the sierra madre occidental volcanic field western mexico approximately 296 000 km2 of western mexico is covered by a volcanic sequence that has an average thickness of 1 km and is mostly ash flow tuff although some 350 calderas are suggested by this tremendous volume only a few have been located or described in the state of durango the only well documented examples are in the 30 ma old chupaderos caldera complex near durango city although the complex is mostly covered by quaternary basalt and sediment the exposed portion reveals nested calderas that produced two major rhyolitic ash flow tuffs a third major tuff appears to have been erupted from the complex but no caldera has yet been found the caldera formed by the first eruption contains a resurgent dome and a thick sequence of moat sedimentary rocks and lava flows that are hosts for significant iron and tin deposits other possible durango calderas exist at el salto west of durango city at temoaya south of durango city and in the sierra de la silla to the north calderas in chihuahua have beep described from mountain ranges in the central part of the state and from the sierra madre occidental to the west a composite resurgent caldefa 35 ma old and approximately 22 km in diameter has been de scribed from the sierra pastorias immediately south of chihuahua city the main erupted unit is well exposed in the caldera s resurgent dome and a second tuff found outside the caldera was emplaced from a trap door structure nested in the caldera s northwest corner the area also contains a younger smaller resurgentiy domed caldera other possible calderas include a relatively large 35 km in diameter structure in the sierra del nido 75 km north of chihuahua city and a poorly defined structure related to a sequence f older 45 ma silicic volcanic rocks at cerro jesus maria 25 km northwest of chihuahua city calderas in the sierra madre plateau have been described from the tomochic and ocampo areas of western chihuahua the tomochic area records volcanic activity from 34 5 to 28 9 ma and contains at least three calderas these include the tomochic caldera a strikingly well preserved resurgent caldera approximately 20 by 25 km in diameter the north structural margin of the tomochic caldera cuts the intracaldera facies of an ash flow tuff related to the older las varas caldera epithermal precious metal fissure veins are located where the tomochic structural margin cuts this older unit a third structure the corallitos caldera is located only a few kilometers to the northeast reconnaissance studies in far western chihuahua indicate the existence of a large 40 km resurgent caldera centered on the village of ocampo landsat imagery shows a well developed arcuate drainage pattern and maps of the area show a distribution of lithologies that is consistent with the caldera interpretation gold and silver deposits hosted almost exclusively in precaldera rocks are best developed where the older rocks are cut by faults of the caldera s ring fracture zone and resurgent dome in general the calderas thus far mapped in western mexico follow the valles type resurgent caldera model variations however of the ideal caldera cycle are common as are repetitions of the cycle to form caldera complexes all of the better known calderas have associated mineral deposits although they may be best developed in precaldera rocks

the volcanic history of volcan alcedo galapagos archipelago a case study of rhyolitic oceanic volcanism volcan alcedo is one of the seven western galapagos shields and is the only active galapagos volcano known to have erupted rhyolite as well as basalt the volcano stands 4 km above the sea floor and has a subaerial volume of 200 km3 nearly all of which is basalt as volcan alcedo grew it built an elongate domal shield which was partly truncated during repeated caldera collapse and partial filling episodes an outward dipping sequence of basalt flows at least 250 m thick forms the steepest to 33 flanks of the volcano and is not tilted thus a constructional origin for the steep upper flanks is favored about 1 km3 of rhyolite erupted late in the volcano s history from at least three vents and in 2 5 episodes the most explosive of these produced a tephra blanket that covers the eastern half of the volcano homogeneous rhyolitic pumice is overlain by dacite rhyolite commingled pumice with no stratigraphic break the tephra is notable for its low density and coarse grain size the calculated height of the eruption plume is 23 30 km and the intensity is estimated to have been 1 2x108 kg s rhyolitic lavas vented from the floor of the caldera and from fissures along the rim overlie the tephra of the plinian phase the age of the rhyolitic eruptions is 120 ka on the basis of k ar ages between ten and 20 basaltic lava flows are younger than the rhyolites recent faulting resulted in a moat around part of the caldera floor alcedo most resently erupted sometime between 1946 and 1960 from its southern flank alcedo maintains an active transient hydrothermal system acoustic and seismic activity in 1991 is attributed to the disruption of the hydrothermal system by a regional scale earthquake