distribution of clay minerals in surface sediments from the eastern barents and south western kara seas surface samples from the eastern barents and south western kara seas have been analysed for clay mineralogy transport paths the role of regional sources and local bedrock outcrops and the influence of hydrodynamic and glacigenous processes for clay distribution on the shelves are discussed in relation to central arctic ocean deep sea and sea ice sediments franz josef land and novaya zemlya show significantly different clay mineral associations although smectite concentrations are fairly high franz josef land can be excluded as a source for central arctic sea ice sediments which are relatively rich in smectite in the kara sea smectite concentrations in coastal sediments surpass even the franz josef land concentrations the large cyclonic gyre in the eastern barents sea between novaya zemlya and franz josef land which serves as a mixing zone between arctic and north atlantic water is apparently reflected within the smectite distribution pattern with the exception of franz josef land the area of investigation is typically low in kaolinite in particular coastal areas and areas north of novaya zemlya influenced by the inflow of arctic waters show the lowest kaolinite concentrations a high kaolinite occurrence within the nansen basin is most probably related to franz josef land and emphasizes the importance of long range downslope transport of sediments across the continental slope the surface water circulation pattern in close interaction with local outcrops onshore novaya zemlya and locally restricted occurrences within the eastern barents sea significantly alter the illite dispersal pattern illite concentrations are lowest around franz josef land chlorite is generally low in the area of investigation submarine outcrops and important chlorite occurrences onshore novaya zemlya bias its distribution pattern

caldera formation in the rum central igneous complex scotland the northern marginal zone of the rum central igneous complex in nw scotland represents part of the early felsic phase of the volcano the marginal zone is a relic of the early caldera floor and the infilling of sedimentary and igneous rocks its formation has been explored through field examination of the ring fracture system of the complex and its pyroclastic and epiclastic intracaldera facies a sequence of magmatic tumescence and chamber growth caused initial doming followed by the formation of a collapse structure without accompanying volcanism this collapse structure circular in plan is akin in origin to a salt basin formed by crustal stretching above a rising diapir we call this the proto caldera collapse breccias which represent the slumping and sliding of megablocks blocks and boulders of the torridonian sandstones which form the walls of the basin were the original infilling logs of these deposits reveal considerable variation in thickness of the breccias from 80 170 m in the complex indicating an uneven floor to the proto caldera consistent with piecemeal collapse following accumulation of up to 70 m thickness of breccia thin interbedded rhyodacitic crystal tuffs 10 30 cm record the earliest eruptions of felsic magma in the caldera caldera formation was then interrupted by a period of quiescence recorded by the presence of an epiclastic sandstone of locally several metres thickness formed by washout of fines from the breccias subsequent resurgence created a fracture pattern characteristic of doming along which rhyodacite magma rose in dykes and erupted up to perhaps 10 km3 of rhyodacitic intracaldera ignimbrites this major eruption caused further incremental subsidence of the caldera floor into a now partly emptied magma chamber mafic inclusions in the ignimbrites point to the eruption being triggered by multiple injections of basic magma into a chamber occupied by felsic magma

estimation of magma depth for resurgent domes an experimental approach post collapse resurgence is a phenomenon affecting many calderas attributed to a renewed magma rise the process is still poorly understood and the associated source parameters remain poorly constrained a set of experiments has been conducted to gain insight into the structural evolution of caldera resurgent domes a sand plaster mixture was chosen as an analogue for the brittle pile of volcanic rocks and silicone putty simulates the ductile behavior of the intruding magma resurgence is driven by the vertical uplift of the silicone with variable shape and depth similarity conditions are achieved through eight dimensionless numbers which are of the same order of magnitude in both nature and experiments results show that extension due to doming is in many cases accommodated by one axial graben opposite master faults of this graben intersect at depth at the junction with the rising viscous silicone the simplicity of the geometry of the whole analogue system provides equations which allow the estimation of the silicone depth from surface parameters these equations are then used in some field examples to assess the magma depth beneath natural resurgent domes

glacial geomorphic features in the northern barents sea direct evidence for grounded ice and implications for the pattern of deglaciation and late glacial sedimentation

the late weichselian glacial maximum on western spitsbergen inferred from offshore sediment cores abstract the late weichselian glacial history of the continental shelf off western spitsbergen is discussed based on acoustic sub bottom records and sediment cores the outer part of isfjorden and the inner shelf to the west of this fjord are characterized by a thin veneer 10 20 m of glacigenic sediments and absence of ice marginal features towards the outer shelf the sediment thickness increases significantly and exceeds 500 m at the shelf edge possible moraine complexes were identified in this outer part sediment cores from the inner shelf sampled a firm diamicton interpreted as till beneath soft glaciomarine sediments radiocarbon dates on shells from the clay resting directly on the till suggest an age of around 12 500 yrs b p for the base of the marine sequence we argue that grounded ice covered the sites shortly before in contrast to suggestions that the fjords and coast were partly ice free during the late weischselian we conclude that the ice must have reached out onto the continental shelf

influence of pre existing volcanic edifice geometry on caldera formation 1 volcanic edifice construction at the earth s surface significantly modifies the stress field within the underlying crust with two main implications for caldera formation first tensile rupture at the earth s surface is favored at the periphery which enables ring fault formation second edifice formation amplifies the amount of pressure decrease occurring within a magma reservoir before the eruption stops taking into account both of these effects caldera formation can be initiated during a central eruption of a pre existing volcano even when assuming elastic behaviour for the surrounding crust providing the roof aspect ratio is small enough conditions for caldera formation by reservoir withdrawal can be reached whatever the reservoir shape is however ring fault initiation is easier for laterally elongated reservoirs

how the development of magma chambers affects collapse calderas insights from an overview here we review the studies of pluton emplacement and caldera collapse proposing a model linking the two processes the shallow rise of magma occurs by dyking and its emplacement occurs along major anisotropies many plutons are emplaced at subhorizontal discontinuities forming sills these will eventually grow forming laccoliths the most common mechanism to store shallow magma calderas are a surface expression of these reservoirs and can be approximated by a piston cylinder which sinks due to 1 underpressure in the reservoir producing outward dipping reverse faults 2 overpressure within a silllike chamber undergoing doming and 3 overpressure within a laccolith generating apical tensile stresses in the latter two cases inward dipping normal faults form we suggest that collapse calderas are the surface expression of pressure variations within laccoliths or tabular intrusions their geometric relationships depend on the shape and aspect ratio of the intrusion and its pressure conditions tabular intrusions generating ring faults mainly at their tips form calderas with a similar width to the intrusion laccoliths may generate ring faults along the intrusion roof forming calderas narrower than the intrusion the outward or inward dip of the caldera faults results from the underpressure or overpressure conditions within the

structural evolution of calderas insights from two dimensional discrete element simulations the manner in which calderas develop is a key question in volcanology and has important implications for associated volcanogenic risk and geothermal and ore exploitation to better understand the structural evolution of calderas i use a discrete element model of a frictional cover undergoing piston like subsidence at its base simulating magma chamber deflation and cover collapse these novel simulations capture not only the initiation of cal deras but also much of the complexity of faulting during their later development in all models both normal and reverse faults accommodate deeper subsidence at higher structural levels curved outward dipping reverse faults are consistently the first structures to develop subsequent caldera growth is mainly the result of movement on vertical or steeply inward dipping normal faults this may be followed by a later phase of lateral collapse on shallower normal faults the roof aspect ratio r cover thickness piston width is seen to be an important factor calderas with low aspect ratios r 1 exhibit a coherent central subsiding block while those with higher aspect ratios have a complexly faulted internal structure

fluid pressure and effective stress in sandbox models abstract we have done a series of simple tests on sandpacks involving upward flow of compressed air through the pores and its effect on the yield strength the ultimate objective is to model deformation coupled with fluid flow in sedimentary basins for all tests we used a single batch of fontainebleau sand sieved to a grain size between 0 200 and 0 315 mm poured into a cylindrical container and then fluidized the density of this sand was 1 585 g cm3 irrespective of sand thickness the lithostatic pressure was proportional to the thickness of the sandpack a yield envelope was obtained by shearing the sandpack horizontally compressed air entered the base of the sandpack and flowed upwards through the pore spaces for 69 measurements on air flow without shearing a plot of discharge velocity versus gradient of fluid pressure is close to a straight line verifying darcy s law and yielding an intrinsic permeability of about 1 7 darcy for the sand for 72 tests on simultaneous shearing and fluid flow the estimated effective stress lithostatic stress minus estimated pore fluid pressure ranged from 0 to 1600 pa and the pore fluid ratio between air pressure and lithostatic pressure from 0 0 to 1 0 a plot of shear stress versus effective stress at failure is almost linear verifying terzaghi s principle of effective stress the best fit slope coefficient of internal friction is about 0 55 and the intercept on the shear stress axis cohesion is less than 85 pa the tests show that it is feasible to use compressed air within sandpacks as a means of modelling deformation coupled with fluid flow the next step will be to build sandbox models of layered sequences and to investigate detachments caused by abnormal fluid pressures

abnormal reverse faulting above a depleting reservoir subsurface deformation is observed during pumping of some hydrocarbon fields deformation features include subsidence centered on the field and subsidence related centripetal horizontal displacements and faulting focal mechanisms yield reverse movements on steeply dipping faults in our sand silicone analogue model the reservoir is represented by a latex balloon or by undercompacted ground sand deflation of the reservoir results in formation of steeply dipping reverse faults bounding a downward opened cone the cone moves downward to follow the reservoir contraction faults along the cone are straight beneath a thick reservoir cover and tend to curve upwards with decreasing cover our results similar to natural structures observed around magma chambers allow us to reinterpret paul segall9s numerical model of poroelastic stresses caused by changes in the distribution of pore fluids and draw a new pattern of active faults

recent glaciation at high elevations on arsia mons mars implications for the formation and evolution of large tropical mountain glaciers 1 the 166 000 km2 fan shaped deposit at arsia mons contains three characteristic facies ridged knobby and smooth which are interpreted as the depositional remains of a cold based glacier that was present on the west northwestern flanks in the late amazonian period of mars history here we consider several high elevation graben on the western flank of arsia mons that are interpreted as the source regions for late stage cold based glaciers that overflowed graben walls advanced tens to hundreds of kilometers downslope experienced subsequent retreat and left distinctive depositional features similar to those associated with cold based glaciers in the dry valleys of antarctica these new observations provide additional support for a cold based glacial interpretation of the tharsis montes fan shaped deposits morphological evidence suggests that the largest of these graben was the primary source region for the most proximal smooth facies lobes at arsia mons and map plane ice sheet reconstructions are consistent with these observations this relationship has significant implications for the relative ages of the individual facies within the fan shaped deposit including the possibility that the smooth facies represents several distinct phases of glaciation the high relief and large elevation differences between the upslope and downslope walls of these graben appear to be critical factors for ice accumulation and the generation collection of rockfall within glacial accumulation zones we suggest that rockfall was the dominant process responsible for debris cover formation on the glaciers sourced within these graben and consider the implications for the entire arsia fan shaped deposit new crater count data suggest that the fill material in the large arsia graben formed within the past 100 myr model age of 65 ma taken together these new observations provide additional evidence for several periods of late amazonian climate change on mars within the past few 100 myr resulting in episodes of tropical mountain glaciation finally mola topography reveals that several lobate features interpreted as remnant debris covered ice from the most recent phase of glaciation are presently hundreds of meters thick suggesting the possibility of long term near surface water ice survival in the equatorial regions of mars

local and regional controls on the lateral emplacement of the ben hiant dolerite intrusion ardnamurchan nw scotland abstract sub volcanic and related satellite intrusions record the delivery storage and accommodation of magma before eruption however when several volcanic centres are in close proximity the relationship between centre and satellites may be ambiguous here we examine the structure of the ben hiant dolerite satellite intrusion discuss its relationship with the ardnamurchan central complex located 2 km to the northwest and explore the possibility of a genetic connection with the next nearest centre the mull central complex 35 km to the southeast structural field observations and anisotropy of magnetic susceptibility ams fabric analyses reveal that the ben hiant dolerite was emplaced as a series of lobes in a sequence of stacked sheets the ams fabric data further indicate that the ben hiant dolerite intruded laterally and may have been sourced from either the ardnamurchan central complex or the mull central complex testing either model and discriminating between these two possible source reservoirs remains an outstanding challenge however the potential implications of the lateral movement of magma over many 10 s of kilometres between upper crustal magmatic centres suggests hidden complexities associated with volcanic plumbing systems that must be tested structurally as well as geochemically

the subsurface structure of the aira caldera and its vicinity in southern kyushu japan abstract southern kyushu japan includes a chain of large and small calderas and active volcanoes and the greatest part of it is covered with thick pyroclastic ejecta the regional and local structures of this area are discussed from the standpoint of physical volcanology with consideration of all available data the regional structure of this area is examined in the light of gravity and geomagnetic anomalies two layers of the earth s uppermost crust are defined by spectrum analysis of the gravity anomalies these two layers are identical with the two identified by seismicwave velocities the bouguer gravity anomalies are relatively high and rather monotonous over outcrops of the mesozoic basement and the granite but are relatively low and perturbed over calderas and caldera like structures two low gravity anomalies in kagoshima bay are remarkable one is circular with its center on the aira caldera the other is elongated between the satsuma and oosumi peninsulas the southern end of the latter anomaly is occupied by the ata caldera discussion of the gravity anomalies of the aira caldera suggests that the subsurface basement has a funnel shape and is overlain by fallback the sub bottom geology of the caldera suggests that it is formed by a few smaller depressions though the distribution of the overall gravity anomalies is parallel with its shape the southern part of kagoshima bay is characterized by a graben like topography and low gravity anomalies and moreover by several calderas the middle part between the aira and ata calderas may have a graben like structure a profile crossing the bay through sakurajima volcano is modeled on the basis of results from drilling and gravity surveys the basement has a graben like structure and is filled with coarse and low density deposits and the structure continues northwards to the aira caldera with a funnel shape a comparison of this area with the taupo rotorua depression in new zealand and lake toba in indonesia leads the authors to the conclusion that such major volcanic depressions may have been formed by amalgamation of a series of caldera like structures which were formed by multiple violent explosions accompanied by ejection of a tremendous amount of pyroclastic material

shallow bedrock geology of the olga basin area northwestern barents sea 1 the east west trending olga basin has possibly the largest post jurassic sedimentary rock thickness in the northwestern barents sea the thickness of these mainly shallow marine deposits exceeds 750 m in the study area and increases eastward the olga basin is characterized by relatively undisturbed layers which except for local areas rarely dip more than 3 degrees the storbanken area is heavily penetrated by normal faults which probably are related to the late cretaceous uplift in the region differential uplift along the flanks of the olga basin during the oxfordian hauterivian led to deposition of onlapping units in the central parts whereas erosion and limited deposition prevailed on the topographic highs the overlying barremian sequence is more uniform and is t least 350 m thick in the hammerfest basin in the southern barents sea corresponding barremian deposits are found the post jurassic strata contain sandstones of three different kinds subarkose sublitharenite litharenite and lithic arkose the barremian subarkoses sublitharenites have relatively high porosities and may be potential reservoir rocks the organic geochemistry analyses show somewhat higher hydrogen index values than those found in the corresponding sedimentary strata on spitsbergen however the strata are slightly less mature than the corresponding strata on spitsbergen

features caused by ground ice growth and decay in late pleistocene fluvial deposits paris basin france last glacial fluvial sequences in the paris basin show laminated lacustrine deposits osl and radiocarbon dated to between 24 6 and 16 6 ka in one site and overlying alluvial sandy gravel a thermokarst origin of the lakes is supported by abundant traces of ground ice particularly ice wedge pseudomorphs beneath the lacustrine layers and synsedimentary deformation caused by thaw settlement the features include brittle deformation normal and reverse faults resulting from ground subsidence owing to ice melting and ductile deformations caused by slumping of the sediments heaved by the growth of ice cored mounds these correspond to lithalsas or lithalsa plateaus and or to open system pingos at least two generations of thermokarst are recorded and may reflect the millennial climate variability typical of the last glacial the structures studied in quarries are associated with an undulating topography visible in 5 m dems and a spotted pattern in aerial photographs the search for similar patterns in the paris basin indicates that many other potential thermokarst sites exist in the last glacial terrace fy of rivers located north of 48 n when they cross the lower cretaceous sands and marls in some sites the presence of organic poor fine grained deposits presumably of lacustrine origin was confirmed by borehole data the site distribution coincides broadly with that already known for ice wedge pseudomorphs this study provides new evidence of permafrost induced ground deformations in france and strongly suggests that thermokarst played a significant and probably largely underestimated role in the genesis of late pleistocene landscapes

potential field and bathymetric investigation of the monowai volcanic centre kermadec arc implications for caldera formation and volcanic evolution the dynamics of caldera formation are affected by internal factors such as magma chamber structure and properties and regional tectonic stresses understanding their interrelationship is important for assessing volcanic hazard and reconstructing the evolution of a volcano the monowai volcanic centre located at the tonga kermadec arc is a basaltic andesitic submarine stratovolcano and caldera recent surveys have shown that it is active but little is known about its structure and evolution here we present a combined analysis of swath bathymetry and potential field data from monowai acquired during cruise so215 on the r v sonne in 2011 april june the monowai caldera is associated with a 20 25 mgal bouguer gravity anomaly high and a broad positive magnetic anomaly short wavelength magnetic anomalies of up to 1400 and 800 nt are observed along the caldera rim and on the summit of monowai cone inversion of the bouguer gravity anomaly data shows that the caldera high is caused by a buried dense body with density contrast of up to 450 kg m 3 with respect to the surrounding arc crust the body has a volume of 250 km3 and is divided into a main unit extending from 3 to 6 km depth and a shallower ring structure that underlies the rim of monowai caldera we estimate the density of the body to be 2650 2850 kg m 3 suggesting a mafic composition the ring structure is interpreted as set of ring dykes and the main unit as a solidified or partly solidified magma chamber system the monowai pluton the observed magnetic anomalies are consistent with pluton intrusion within the bruhnes magnetic polarity analysis of the swath bathymetry data shows that monowai is located in a 20 km wide graben part of an en echelon left stepping horst and graben system that spans the length of the arc and backarc the monowai caldera is elongated perpendicular to the direction of rifting suggesting that it is affected by the regional stress regime geomorphological analysis suggests that the caldera was created by repeated collapse of a single caldera structure and that its complex shape and multiple ring faults can be attributed to prolonged activity and multiple cycles of inflation and collapse we propose a multiphase evolution of the monowai volcanic centre comprising initial magma emplacement and crustal inflation magma chamber evacuation and caldera collapse caldera resurgence and mass waisting southward migration of the eruptive vent and formation of monowai cone it remains unclear whether the current eruption at the monowai cone is fed directly from a small underlying magma chamber or laterally from the monowai pluton observation of radial fissure ridges on its flanks suggests that a shallow magma body may have been recently emplaced beneath the summit causing inflation and the formation of extensional cracks the summit appears unstable and may be prone to lateral or vertical collapse therefore the region remains a potential source of regional volcanic and tsunami hazard

formation and development of normal fault calderas and the initiation of large explosive eruptions the ring fractures that form most collapse calderas are steeply inward dipping shear fractures i e normal faults at the surface of the volcano within which the caldera fault forms the tensile and shear stresses that generate the normal fault caldera must peak at a certain radial distance from the surface point above the center of the source magma chamber of the volcano numerical results indicate that normal fault calderas may initiate as a result of doming of an area containing a shallow sill like magma chamber provided that the area of doming is much larger than the cross sectional area of the chamber and that the internal excess pressure in the chamber is smaller than that responsible for doming this model is supported by the observation that many caldera collapses are preceded by a long period of doming over an area much larger than that of the subsequently formed caldera when the caldera fault does not slip eruptions from calderas are normally small nearly all large explosive eruptions however are associated with slip on caldera faults during dip slip on and doming of a normal fault caldera the vertical stress on part of the underlying chamber suddenly decreases this may lead to explosive bubble growth in this part of the magma chamber provided its magma is gas rich this bubble growth can generate an excess fluid pressure that is sufficiently high to drive a large fraction of the magma out of the chamber during an explosive eruption

the lateral growth and coalesence of magma systems thermal and mechanical models of magma reservoir growth need to be reconciled with deformation patterns and structural relationships observed at active magma systems geophysical observations provi

discussion on palaeoseismic structures in quaternary sediments related to an assumed fault zone north of the permian peissen gnutz salt structure nw germany neotectonic activity and earthquakes from the saalian to the holocene grube 2019 abstract we discuss the significance of deformation structures in quaternary sediments observed by grube 2019 in the peissen quarries nw germany in light of the geological context evidence for polygonal patterns visible in aerial images in the study area shows that the wedge structures interpreted by grube 2019 as earthquake induced sand blows may rather correspond to thermal contraction cracks filled with aeolian sand in a permafrost environment in the study sites brittle deformations caused by i the rise of a salt diapir ii salt dissolution iii the development of pleistocene permafrost and iv possibly water circulation under pressure in the scandinavian ice sheet margin may have coexisted we support the idea that while the morphology of deformation generally makes it possible to determine the stress state to which the sediments have been subjected and the quantity of water available in the system at the time of deformation the nature of the factors causing the stresses remains difficult to identify in the end we highlight other useful criteria that should be privileged for palaeoseismic research in such complex geological settings

glaciomarine sedimentation in epicontinental seas exemplified by the northern barents sea abstract sediments on high arctic shelves result from modern processes and the effect of former glaciations based on data from the northern barents sea an area with input from large and numerous surging glaciers we define two principal zones with different environmental regimes and corresponding sedimentary facies 1 a glacier proximal zone influenced by grounding line processes and the immediately adjacent areas affected by glacial sediment input and 2 a glacier distal sea ice and current controlled zone which also includes a wide sediment starved region dominated by biogenic carbonate accumulation characteristic of the glacier proximal zone are glacial surges which affect sedimentation rates and leave a diagnostic pattern of sea floor morphologies extensive ice gouging causes a homogeneous sediment texture in the glacier distal zone fine grained mud supplied from sea ice and infrequent coarser material deposited from icebergs is reworked by modern oceanographic processes on shallow banks in 30 50 m of water carbonates accumulate from a prolific bottom fauna formed in response to extensive reworking and nutrient supply

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 space problem of caldera resurgence an example from ischia island italy a space problem can arise in a resurgent caldera when the resurgent block is non cylindrical such as for example when it is bounded by inward or outward dipping faults ischia caldera italy is an excellent case study because it is well exposed and resurgence is ongoing on the western and eastern flanks of the ischia resurgent horst uplift occurred along nnw striking normal faults with inclination from sub vertical to vertical 85 the geometry of these faults suggests negligible extension within the horst along the northern flank uplift was accomplished by ene striking normal faults that dip 60 85 outward a few bear striae which indicate almost pure dip slip the southern flank of the horst is a monocline trending ene associated with vertical faults in a nnw sse section the resulting resurgent horst has a wedge shape with an upward apex the uplift of this wedge can be accommodated by contemporaneous regional extension along ne to east west striking normal faults whose motions create space for resurgence without deformation of the caldera floor similar interaction with regional tectonics could exist in other calderas such as yellowstone usa in an extensional setting los azufres mexico in a transtensional regime and chalupas ecuador in a transpressional one at other calderas resurgence was accommodated by caldera floor arching as at valles usa or by shortening deformations between the caldera rim and the uplifting block as at latera italy

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

the geological history of nili patera mars nili patera is a 50 km diameter caldera at the center of the syrtis major planum volcanic province the caldera is unique among martian volcanic terrains in hosting i evidence of both effusive and explosive volcanism ii hydrothermal silica and iii compositional diversity from olivine rich basalts to silica enriched units we have produced a new geological map using three mosaicked 18 m pixel context camera digital elevation models supplemented by compact remote imaging spectrometer for mars hyperspectral data the map contextualizes these discoveries formulating a stratigraphy in which nili patera formed by trapdoor collapse into a volcanotectonic depression the distinctive bright floor of nili patera formed either as part of a felsic pluton exposed during caldera formation or as remnants of welded ignimbrite s associated with caldera formation both scenarios deriving from melting in the noachian highland basement after caldera collapse there were five magmatic episodes 1 a basaltic unit in the caldera s north 2 a silica enriched unit and the associated nili tholus cone 3 an intrusive event forming a 300 m high elliptical dome 4 an extrusive basaltic unit emplaced from small cones in the east and 5 an extreme olivine bearing unit formed on the western caldera ring fault the mapping together with evidence for hydrated materials implies magmatic interaction with subsurface volatiles this in an area of elevated geothermal gradient presents a possible habitable environment sampled by the hydrothermal deposits additionally similarities to other highland volcanoes imply similar mechanisms and thus astrobiological potential within those edifices

influence of lithologic erodibility on alluvial fan area western white mountains california and nevada differences in lithologic erodibility and sediment storage within a drainage basin affect the relationship between alluvial fan area and drainage basin area along the western white mountains large fans are produced by basins underlain by resistant rocks which have steep and narrow trunk stream canyons with little sediment in storage small fans are produced by basins composed of erodible lithologic units which have wider valley floors lower valley side slopes and considerably more sediment stored along trunk stream canyons than is the case in basins underlain by resistant rocks

the eurasian ice sheet formation and collapse resulting from natural atmospheric co2 concentration variations finite element model simulations of ice sheet dynamics are conducted to investigate the role of natural atmospheric co2 variations obtained from ice core bubbles in creating climatic conditions necessary to glaciate the eurasian arctic effects of co2 variations are sufficient to support extensive marine ice sheet and ice shelf development at the last glacial maximum and to deglaciate the region during the holocene from approximately 14 ka to 9 ka the ice sheet coverage simulated for the last glacial maximum agrees with evidence for extensive ice sheet grounding over the shallow continental shelves of the barents kara laptev and east siberian seas an ice shelf also forms over the arctic ocean and greenland and norwegian seas

multiple caldera collapses inferred from the shallow electrical resistivity signature of the las canadas caldera tenerife canary islands the las canadas caldera of tenerife lcc is a well exposed caldera depression filled with pyroclastic deposits and lava flows from the active teide pico viejo complex tpvc the caldera s origin is controversial as both the formation by huge lateral flank collapse s and multiple vertical collapses have been proposed although vertical collapses may have facilitated lateral slope failures and thus jointly contribute to the exposed morphology their joint contribution has not been clearly demonstrated using results from 185 audiomagnetotelluric amt soundings carried out between 2004 and 2006 inside the lcc our study provides consistent geophysical constraints in favour of multiple vertical caldera collapse one dimensional modelling reveals a conductive layer at shallow depth 30 1000 m presumably resulting from hydrothermal alteration and weathering underlying the infilling resistive top layer we present the resistivity distribution of both layers resistivity images the topography of the conductive layer across the lcc as well as a cross section in order to highlight the caldera s evolution including the distribution of earlier volcanic edifices the amt phase anisotropy reveals the structural and radial characteristics of the lcc

an archean megacaldera complex the blake river group abitibi greenstone belt abstract the archean blake river group of the southern abitibi greenstone belt is defined as a subaqueous megacaldera compelling evidence include 1 a radial and concentric organization of synvolcanic mafic to intermediate dykes 2 an overall dome geometry defined by the volcanic strata 3 a peripheral distribution of subaqueous volcaniclastic units 4 a zonal distribution of carbonate alteration and 5 a distinct annular synvolcanic inner and outer ring fault pattern three caldera forming events have been identified 1 the early misema caldera 2 the new senator caldera and 3 the well known noranda caldera the multi vent brg mafic volcanic complex developed on a monotonous sequence of tholeiitic basalts forming a submarine plain and experienced a first major collapse that created the giant misema caldera 80 km in diameter an endogenic dyke swarm intruded the synvolcanic fractures and an underlying magma chamber developed major volcaniclastic units were generated by local volcanic centers and summit calderas formed along the outer and inner ring faults this fault system was used as a conduit for co2 rich hydrothermal activity renewed volcanic activity was associated with a resurgent central dome inside the misema caldera this second collapse event created the 35 km by 14 km nw trending new senator caldera this caldera was produced by multi step sagging after the underlying magmatic chamber migrated to the se and formed the flavrian powel plutons the final collapse resulted in the formation of the noranda caldera that generated a well developed 070 trending fracture pattern associated with several vms deposits the multiple caldera setting provides an effective model to explain the presence of vms mineralization along the synvolcanic fractures associated with the three collapse episodes however the importance of the new senator caldera fracture pattern is emphasized because of its role in the positioning of the giant horne mine deposit

late quaternary sedimentation and glacial history of the western svalbard continental margin abstract glacigenic sediments recovered in shallow cores from the western svalbard continental slope are subdivided into five facies associations based on grain size sedimentary structure mineralogy petrography and geochemistry two diamicton facies are recognised one of which is interpreted as hemipelagic mud with variable amounts of ice rafted debris ird and the other as a product of mass movement the diamictons are associated with melting of icebergs during glacial melt events and debris flow deposition on the submarine fans during peak glaciation respectively laminated to layered mud and turbidites seem to be closely related to eustatic fall in sea level and erosion of banks located on the continental shelf resulting in accumulation of fine grained organic rich deposits and thin silt and sand layers on the continental slope middle weichselian was characterised by several phases of extensive iceberg production and input of ird the first phase 60 55 ka is correlated with a glacier advance on svalbard but the following phases 54 44 ka can not be correlated with glacier advances on land this demonstrates that there may have been more glaciations than recorded on svalbard and that ice sheets at high northern latitudes fluctuated more frequently than previously assumed the composition of the ird deposited after 54 ka suggests that the ice sheets during these advances were located in the eastern svalbard barents sea area the late weichselian growth of the svalbard barents sea ice sheet occurred probably in two steps nucleation of a continental based ice sheet between 27 and 22 5 ka followed by a rapid advance to the shelf edge when the ice margin reached the soft clay rich sediments on the continental shelf eustatic lowering of the sea level during the initial phase led to bank exposure and erosion bringing large quantities of fine grained organic rich sediments to the deep sea during peak glaciation glaciers provided a localised supply of sediments to the shelf troughs and the upper slope which were redistributed by debris flows the marine based svalbard barents sea ice sheet started to retreat around 14 5 ka through massive iceberg discharge the retreat was interrupted by a short lived advance accompanied by erosion of the shelf banks and redistribution of organic rich sediments to the continental slope the final ice recession started around 12 ka and ended close to 9 ka when the fjords of svalbard were essentially ice free large amounts of meltwater released during the removal of the svalbard barents sea ice sheet cooled down the surface waters of the norwegian greenland sea and formed a widespread sea ice cover between 12 and 8 ka input of terrigenous material was greatly reduced during the early part of holocene the present interglacial is characterised by erosion of the shelf and upper slope by bottom currents leaving a prominent lag deposit

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

evidence for full glacial flow and retreat of the late weichselian ice sheet from the waters around kong karls land eastern svalbard abstract marine geophysical and geological data from the seafloor surrounding kong karls land in eastern svalbard are used to reconstruct late weichselian full glacial flow dynamics and retreat history of the barents sea ice sheet bsis grounded ice extended over the entire area during the last glacial maximum lgm and produced streamlined sedimentary landforms in the broad bathymetric troughs that flank the kong karls land archipelago the landforms were produced in subglacial till as a result of subglacial processes at the base of the ice sheet drumlins and hill hole pairs confirm that regional ice flow was towards the east northeast through the troughs based on the absence of ice margin recessional features deglaciation in olga strait erik eriksen strait and the unnamed deep northeast of kong karls land appears to have been rapid in the deeper outer parts of the troughs in contrast in the shallower parts of the troughs ice recession was slower and minor readvances still stands of the ice margin resulted in the formation of recessional moraines during deglaciation temporary calving bays formed in the deeper parts of the troughs and calved icebergs were evacuated away from the ice margin through the troughs grounding zone features formed in olga strait indicate that retreat here was gradual and punctuated by longer still stands the transition from a grounded ice sheet to ice proximal settings is marked locally by a laminated mud sequence deposited from meltwater plumes from a nearby ice margin the presence of meltwater derived facies suggests that melting may have also been a significant ice loss mechanism during retreat in a broader context this study is one of the first investigations of the seafloor east of svalbard providing evidence that ice drained towards the east northeast during full glacial conditions ice from this part of the bsis was an important contributor to the palaeo ice stream in the large franz victoria trough during the lgm

relative scale and the strength and deformability of rock masses abstract the strength and deformation of rocks depend strongly on the degree of fracturing which can be assessed in the field and related systematically to these properties appropriate mohr envelopes obtained from the rock mass rating rmr classification system and the hoek brown criterion for outcrops and other large scale exposures of fractured rocks show that rock mass cohesive strength tensile strength and unconfined compressive strength can be reduced by as much as a factor often relative to values for the unfractured material the rock mass deformation modulus is also reduced relative to young s modulus a cook book example illustrates the use of rmr in field applications the smaller values of rock mass strength and deformability imply that there is a particular scale of observation whose identification is critical to applying laboratory measurements and associated failure criteria to geologic structures

shear wave splitting vp vs and gps during a time of enhanced activity at aso caldera kyushu 1 we performed shear wave splitting analyses and calculated vp vs ratios on local earthquakes recorded at seven stations around aso volcano japan between 2001 and 2008 the mean vp vs of 1 61 0 01 from all stations is among the lowest average values found anywhere in the world a temporal change of shear wave splitting fast polarization is observed at one station additionally spatial variations of shear wave splitting delay times and vp vs ratios show a correlation at three stations we suggest that stress controlled anisotropy dominates at these stations gps baselines and 2d strain reveal extension in the caldera in 2003 and compression before and after 2003 strain shortening axes indicate either a variation in regional stress or a local stress source that overprints the regional field in 2003 baseline changes correlate with periods of increased low frequency seismic activity located deeper than 15 km compaction and degassing of a previously reported sill at approximately 15 km depth could lead to constantly high pressure of volcanic gases over a wide area and explain the generally low vp vs ratios and the observed compression in the caldera we propose that a small volume of magma intruded into the sill in 2003 caused extension at the surface and triggered ash eruptions between 2003 and 2005 associated localized migration of gases in overpressured cracks with aspect ratios potentially 0 5 in the upper crust with porosities 0 1 could be the source of changes in seismic anisotropy and explain the slight variations observed in vp vs

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

experimental and analytic modeling of piercement structures 1 piercement structures such as mud volcanoes hydrothermal vent complexes pockmarks and kimberlite pipes form during the release of pressurized fluids the goal of this work is to predict under which conditions piercement structures form from the insights gained by sand box experiments injecting compressed air through an inlet of width w at the base of a bed of glass beads of height h at an imposed critical velocity vf a fluidized zone consisting of a diverging cone like structure formed with morphological similarities to those observed in nature dimensional analysis showed that vf is correlated to the ratio of h over w in addition we derived an analytical model for vf which is compared to the experimental data the model consists of a force balance between the weight and the seepage forces imparted to the bed by the flowing gas the analytic model reproduces the observed correlation between vf and h w although a slight underestimate was obtained the results suggest that the gas particle seepage force is the main triggering factor for fluidization and that the commonly used proxy which the fluid pressure must equal or exceed the lithostatic weight needs to be reconsidered by combining the experiments and the model we derived critical pressure estimates which were employed to a variety of geological environments comparing the estimated and measured pressures prior to the lusi mud volcano shows that the presented model overestimates the critical pressures the model paves the way for further investigations of the critical conditions for fluidization in earth systems

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

quantifying the origin and geometry of circular sag structures in northern fort worth basin texas paleocave collapse pull apart fault systems or hydrothermal alteration three dimensional seismic data reveal numerous subcircular sag structures in the northern fort worth basin the structures are defined by concentric faults that extend vertically upward 760 1060 m 2500 3500 ft from the lower ordovician ellenburger group the largest structures remained active into the lower desmoinesian strawn group criteria are outlined for defining seismically resolvable sag structures and a detailed quantitative analysis of the geometries of these circular features was undertaken results are compared and contrasted with reviews of subsurface collapse mechanisms and strike slip processes that are known to produce subsurface circular to subcircular sag geometries in plan view in this manner we develop several constraints for differentiating collapse related sag structures from strike slip related sag structures qualitative analyses indicate that the geometries observed are strongly analogous to subsurface collapse features where material is removed at depth to create a void into which the overburden subsequently sags and collapses quantitative analyses support the formation of these features by incremental collapse and suprastratal deformation above a linked system of coalesced collapsed paleocaves within the ellenburger group observations and criteria presented herein provide valuable information in defining seismically resolvable collapse features worldwide and help distinguish sag features of collapse affinity from those of other origins

tectonically controlled piecemeal caldera collapse a case study of glencoe volcano scotland caldera collapse at glencoe scotland was incremental and involved complex movements of numerous fault blocks before formation of the ring fault for which the volcano is renowned intracaldera depocenters had the form of grabens or half grabens locally with downsag and bounding monoclinal flexures and these changed form and location for each of the five major ignimbrite eruptions that represent the early volcanic history this piecemeal caldera collapse is in contrast to previous interpretations in which all subsidence was thought to have involved a coherent crustal block moving on the ring fault collapse and magmatic plumbing were profoundly influenced by preexisting tectonic faults trending northwest and northeast a dominant northwest trending graben controlled the general location and form of major caldera depocenters and repeatedly channeled a major river through glencoe the main graben was transected orthogonally by two cross grabens each of the first three caldera eruption cycles involved initial phreatomagmatic explosivity which built tuff cones followed by magmatic fountaining that produced lava like silicic ignimbrites the three lava like ignimbrites total more than 300 m thick two later eruptions produced eutaxitic silicic ignimbrites together more than 300 m thick and contemporaneous progressive downsag was associated with the formation of extensional fractures crevasses hundreds of meters deep sills up to 100 m thick of mingled andesite and rhyolite between the intracaldera ignimbrites were accommodated by increments of subsidence unconformities and sedimentary layers within the stratigraphic succession record fluvial erosion and abrupt switches to alluvial and or lacustrine deposition between each ignimbrite eruption the changes in drainage and sedimentation and the development of coarse debris avalanche breccias reflect tectonic faulting during the periods between major eruptions the duration of the period including the five caldera forming eruptions was probably 0 5 m y and the magnitude of the tectonic faulting that occurred prior to and during the caldera developments 0 5 km m y normal displacement is similar to that of the most actively subsiding sedimentary basins glencoe shows that the piecemeal nature of calderas may be plainly evident only in deeply dissected systems which with knowledge that caldera volcanoes commonly overlie faults suggests that piecemeal calderas may be more common than previously recognized the research also shows that the emplacement of granites s l was episodic rather than synchronous throughout the magmatic province and it suggests that numerous plutons in the province e g in donegal ireland may have formed at sites of central volcanoes that are no longer preserved

stress fields generating ring faults in volcanoes ring faults in volcanoes have been recognized for a long time but their mechanics of formation is still poorly understood while the subsidence on a ring fault during a large eruption from the associated chamber is easily understood the initiation of the fault itself has been difficult to explain for a ring fault to form the tensile and shear stresses at the surface of the volcano must peak at a certain radial distance from the surface point above the center of the chamber empirical evidence however shows that during most periods of unrest in active volcanoes the stress field is not favorable for the initiation of ring faults we made several tens of boundary element models of magma chambers of various geometries and subject to different loading conditions the results indicate that overpressure or underpressure in the chamber as the only loading is unlikely to initiate ring faults for a spherical chamber subject to horizontal extension or doming and a sill like chamber subject to horizontal extension the tensile and shear stresses at the surface of the volcano peak at a certain radial distance from the surface point above the center of the chamber then however the maximum stresses occur at the boundary of the chamber itself which would normally lead to sheet injection rather than to ring fault formation by contrast a sill like magma chamber subject to doming gives rise to a stress field suitable for the initiation of a ring fault

mechanical modeling of circumferential and radial dike intrusion on galapagos volcanoes a distinctive and unusual pattern of eruptive fissures is observed on the active volcanoes of the galapagos islands reflecting circumferential dike intrusion near the calderas and radial dike intrusion on the volcano flanks elastic finite element models were used to investigate how a stress field could be produced and maintained to promote both circumferential and radial dike emplacement modeling results show that magma reservoirs of galapagos volcanoes are probably diapiric because this shape promotes both circumferential and radial intrusions but magma pressure alone cannot create the observed pattern of dikes loading by volcano growth and magma reservoir pressure could produce a stress field of suitable orientation but insufficient magnitude the intrusion of circumferential dikes could alter the stress field in a way that promotes future radial diking and vice versa faulting or slumping within the calderas or on the volcano flanks in response to repeated intrusions could also create a stress field conducive to continued intrusion

significance of the 1982 2014 campi flegrei seismicity preexisting structures hydrothermal processes and hazard assessment we find that the double difference relocated seismicity which occurred over the last 30 years at campi flegrei was triggered by the uprising of fluids preferentially concentrated along onshore and offshore nw striking preexisting caldera faults focal volumes of the 2005 2014 seismicity do not overlap that of the 1982 1984 period when a major uplift of 1 8 m occurred in the central sector of the caldera this indicates a transition from an elastic to a plastic behavior due to fluid saturation and heating of the rocks in the hydrothermal reservoir the 2012 2014 deeper earthquakes are located in a low vp vs zone at the western boundary of the hydrothermal reservoir where a volume increase from a magmatic body at 3 5 km depth has been recognized the progressive rheological change from elastic to plastic in the upper 4 km of the crust implies that a slow upward migration of magma may not necessarily be preceded by earthquakes or swarms

late weichselian deglacial history of the svyataya saint anna trough northern kara sea arctic russia abstract marine sediment core and seismic records from the svyataya saint anna trough provide new insight into the distribution of late weichselian glacial coverage ice retreat pattern and post glacial environments in the northern barents and kara seas these records indicate that the saint anna trough was filled with grounded glacier ice which likely reached the shelf edge during the late weichselian maximum several radiocarbon dates suggest early deglaciation of the deep axial part of the trough prior to 13 3 ka two sandy beds in the deglacial section of the cores imply distinct pulses of iceberg calving and or melting which were probably associated with stepwise retreat of the ice margin morainic ridges and glacial sole markings in the western part of the saint anna trough indicate that the northern central barents sea was a site of a large ice mass during deglaciation a smaller ice cap is inferred for the northern kara plateau at later stages ice retreat on the western flank of the trough was directed towards franz josef land and was presumably facilitated by a separation of the barents sea and novaya zemlya ice domes deglaciation of the saint anna trough was completed by ca 10 ka high post glacial sediment fluxes between 10 and 8 ka were probably related to sea floor coastal erosion and or siberian river discharge during the rising sea level

the episodic rise net growing rate and kinematics of radial faults of the salinas de oro diapir using paleoseismological techniques ne spain salt upwelling versus karstic subsidence abstract this paper explores the use of trenching and paleoseismological techniques to determine salt flow rates into a salt diapir for the first time in the literature the salinas de oro diapir located in the northern spanish pyrenees is an oval shaped triassic salt stock that extends vertically for 7 km down to the paleozoic basement salt dissolution subsidence and diapir growth are coeval active processes karstification is responsible for the development of large sinkholes a thick caprock and the monocline folding and ring faulting of the cretaceous and early tertiary limestone rim the evaporite karstic aquifer discharges high concentration water of up to 137 g l of total dissolved solids and a conductivity over 200 ms cm into the salado creek which drains the diapir top the salinity monitoring of this drainage provides a maximum karstic surface lowering rate of around 2 8 mm yr salt upwelling has caused the 150 m uplift of the annular limestone escarpment and the development of 3000 m long radial grabens with up to 90 m of vertical offset that disrupt drainages displace quaternary deposits and overprint concentric faults the 260 cm drag folding of lacustrine facies exposed in a 42 m long and 6 5 m deep trench due to the creep motion of the western radial fault of azanza graben yielded a minimum short term uplift slip rate of 1 75 mm yr for the last 1485 years and provides a minimum salt supply rate of over 5 mm yr considering karstification lowering this value that is several hundred times higher than average evidence the discontinuous growth of salinas de oro diapir

micromorphology of diamicton affected by iceberg keel scouring scoresby sund east greenland this research was primarily funded by the university of brighton s rising stars initiative and the school of environment and technology research investment fund with supplementary funds awarded by the quaternary research association the funding sources had no involvement in study design in collection analysis and interpretation of data in the writing of the article and in the decision to submit the article for publication

experimental study of caldera formation scaled experiments have been carried out on caldera collapse mechanisms using silicone as analogue magma and dry sand as analogue rock experiments were carried out in two and three dimensions using a range of roof aspect ratios thickness width 0 2 to 4 5 appropriate for caldera collapse they reveal a general mechanism of collapse only weakly dependent on the shape of the reservoir for low roof aspect ratios 1 subsidence starts by flexure of the roof and the formation of outward dipping reverse ring faults which in turn trigger formation of peripheral inward dipping normal ring faults the subsidence always occurs asymmetrically in cross section the reverse faults delimit a coherent piston bounded on each side by an annular zone of inwardly tilted strata located between the reverse and normal ring fault sets the surface depression consists of a nondeformed area piston surrounded by an annular extensional zone tilted strata for high aspect ratios 1 multiple reverse faults break up the roof into large pieces and subsidence occurred as a series of nested wedges 2 d or cones 3 d the extensional zone dominates the surface depression in the case where preexisting regional faults do not play a major role the collapse mechanics of calderas probably depends strongly on the roof aspect ratio calderas with low roof aspect ratios are predicted to collapse as coherent pistons along reverse faults the annular extensional zone might be the source of the large landslides that generate intracaldera megabreccias collapse into magma reservoirs with high roof aspect ratios may be the origin of some funnel calderas where explosive reaming is not dominant

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

the role of extensional structures on experimental calderas and resurgence the structure and shape of collapses and resurgences is often controlled by pre existing discontinuities such as normal faults in rift zones in order to study the role of extensional structures on collapse and resurgence we used analogue models dry sand simulated the brittle crust silicone located at the base of the sand pack simulated magma in the experiments regional extension pre dated collapse or resurgence forming normal faults in a graben like structure the graben was filled with additional sand simulating post rift deposits a piston then moved the silicone downward or upward inducing collapse or resurgence within the previously deformed sand the collapses showed an ellipticity length of minor axis length of major axis between 0 8 and 0 9 with the major axis parallel to the extension direction the partial reactivation of the pre existing normal faults was observed during the development of the caldera reverse faults which conversely to what was expected from experiments without pre existing extension became partly inward dipping resurgence showed an elongation of the uplifted part with the main axis perpendicular to the extension direction at depth pre existing normal faults were partly reactivated by the reverse faults formed during resurgence these locally became outward dipping normal faults a total reactivation of pre existing faults was also observed during resurgence the experiments suggest that the observed elongation of calderas and resurgences is the result of the reactivation of pre existing structures during differential uplift such a reactivation is mainly related to the loss in the coefficient of friction of the sand the results suggest that elliptic calderas and resurgences in nature may develop even from circular magma chambers

a method for determining significant structures in a cloud of earthquakes the interpretation of a cloud of earthquake hypocenters in terms of causative structures is not a simple task locations are subject to uncertainties which will not be the same for every earthquake the data should therefore not be interpreted simply by inspection which is difficult in the case of three dimensional data anyway instead we propose using the location uncertainties as a guide in processing the data earthquake locations are moved inside their uncertainty or confidence ellipsoids until a simplified picture of the earthquake cloud is obtained which can then be interpreted in terms of some simplified structure such as faults the aim of the approach is to give the simplest possible structure that is consistent with all the location and confidence ellipsoid data the method is applied to three synthetic sets of data these illustrate the potential and limitations of the method application to a real earthquake data set from rabaul caldera in papua new guinea gives an image of the caldera ring fault that suggests departures from the simple ring fault structure previously assumed sensitivity analysis on the rabaul data shows that the method is not unduly sensitive to the assumptions that have to be made in applying it

iceberg scouring in scoresby sund and on the east greenland continental shelf modern and relict scours produced by iceberg keels have been reported from a number of high latitude continental shelves in the northern hemisphere and antarctica but observations are largely absent from the east greenland shelf about 18 km3 yr 1 of icebergs are calved into the scoresby sund fjord system east greenland these icebergs have submarine keels with drafts up to 550 m modern iceberg contacts with the sea floor are inferred from calculated keel depths and stationary icebergs identified on sequential landsat imagery acoustic profiles parasound 3 5 khz of the sea floor show that scouring is a significant process in water depths 550 m iceberg scouring intensity varies inversely with water depth the most intense scouring occurs at depths of 300 400 m iceberg scours were observed on over 25 000 km2 of the east greenland shelf at 69 72 n and 75 n scours are being actively produced on the modern shelf but relict scours probably dating back to the late weichselian are also present the rate of iceberg production from greenland ice sheet outlet glaciers and iceberg drift tracks on the shelf suggests that iceberg scouring is important over a significant proportion of the 500 000 km2 area above the shelf break around greenland

sediment distribution and sea floor morphology of storbanken implications for the glacial history of the northern barents sea acoustical sparker 3 5 khz and side scan sonar and sedimentological data from a local study on storbanken in the northern barents sea support the concept of a late weichselian ice sheet covering most of the barents sea during a major halt in the retreat of the ice sheet locally thicker 38 m accumulations of ice proximal glaciomarine sediments were deposited after which rapid retreat took place sea floor morphology indicates that the retreat across storbanken most likely took place without surging or climatically controlled oscillations intense iceberg ploughing characterizes the sea floor down to water depths of 210 220 m most of this is relict but occasional gouges in the shallowest regions may have had a more recent origin the apparent lower limit of ploughing interpreted in terms of relative sea level indicates a rather moderate isostatic depression this probably resulted from a thin ice sheet fed from several ice source areas depth of the plough marks is largely dependent on thicknes

the barents sea ice sheet a model of its growth and decay during the last ice maximum on the basis of geomorphological and sedimentological data we believe that the entire barents sea was covered by grounded ice during the last glacial maximum 14c dates on shells embedded in tills suggest marine conditions in the barents sea as late as 22 ka bp and models of the deglaciation history based on uplift data from the northern norwegian coast suggest that significant parts of the barents sea ice sheet calved off as early as 15 ka bp the growth of the ice sheet is related to glacioeustatic fall and the exposure of shallow banks in the central barents sea where ice caps may develop and expand to finally coalesce with the expanding ice masses from svalbard and fennoscandia the outlined model for growth and decay of the barents sea ice sheet suggests a system which developed and existed under periods of maximum climatic deterioration and where its growth and decay were strongly related to the fall and rise of sea level

glacial geology of outer bjornoyrenna southwestern barents sea the upper cenozoic glacigenic succession in outer bjornoyrenna bear island trough southwestern barents sea is investigated utilizing data from six shallow boreholes 143 m below sea bed and seismic profiling overdeepened troughs in the lowermost part of a large plio pleistocene wedge at the shelf margin are interpreted as glacial erosion forms this suggests that the overlying succession which basinward attains a thickness of several km was deposited during a period with glaciations large scale possible sand ridges in the stratigraphic lower part of this succession are confined to two north south trending zones parallel to the present shelf break they probably formed in a continental shelf coastal environment with exceptionally high sediment supply and fluctuating relative sea level related to glaciations a core from a channel fill in the lower part of the succession consists of possibly glaciofluvial gravelly sand intercalated with till most of the other cores are dominated by middle and upper pleistocene muddy massive partly fissile diamictons interpreted to be mainly tills undrained shear strength of these cores ranges from about 20 kn m 2 to more than 300 kn m 2 and both underconsolidated intervals compared to the present burial depth are found an underconsolidated interval comprising eemian interglacial biourbated marine sediments overlain by laminated weichselian glaciomarine sediments on the outher shelf is covered by 50 100 m of muddy diamicton this diamicton is suggested to be mainly till deposited by a late weichselian ice sheet which was grounded to more than 500 m below present sea level earlier weichselian ice expansions did not reach the outer shelf based on the stratigraphy in the shallow cores and seismic interpretation we suggest that there have been five major expansions of grounded ice to the shelf break since ca 440 ka the present study suggests that glacial erosion was a major process in the late cenozoic denudation of the barents sea and in large areas perhaps as much as 200 250 m of preexisting strata could have been eroded by glaciers since about 440 ka

the svalbard barents sea ice sheet historical current and future perspectives abstract the history of research on the late quaternary svalbard barents sea ice sheet mirrors the developments of ideas and the shifts of paradigms in glacial theory over the past 150 years since the onset of scientific research there in the early 19th century svalbard has been a natural laboratory where ideas and concepts have been tested and played an important but rarely acknowledged role in the break through of the ice age theory in the 1870 s the history of how the scientific perception of the svalbard barents sea ice sheet developed in the mid 20th century also tells a story of how a combination of fairly scattered and often contradictory observational data and through both deductive and inductive reasoning could outline a major ice sheet that had left but few tangible fingerprints since the 1980 s with increased terrestrial stratigraphical data ever more marine geological evidence and better chronological control of glacial events our perception of the svalbard barents sea ice sheet has changed the first reconstructions depicted it as a static concentric single domed ice sheet with ice flowing from an ice divide over the central northern barents sea that expanded and declined in response to large scale late quaternary climate fluctuations and which was more or less in tune with other major northern hemisphere ice sheets we now increasingly perceive it as a very dynamic multidomed ice sheet controlled by climate fluctuations relative sea level change as well as subglacial topography substrate properties and basal temperature in this respect the svalbard barents sea ice sheet will increasingly hold the key for understanding the dynamics and processes of how marine based ice sheets build up and decay

pressure evolution during explosive caldera forming eruptions abstract caldera forming eruptions of silicic magmas result from a complex coupling of the mechanics and fluid dynamics of the associated magma chamber field studies of caldera forming eruption products suggest that great pressure variations occur inside the magma chamber and associated conduits during these eruptions pressure evolution during explosive caldera forming eruptions is investigated through a simple model that describes the first order quantitative behaviour of the chamber we consider a piston like model that assumes a coherent block subsiding along circular sub vertical ring faults into the magma chamber this subsidence occurs after significant decompression of the chamber by an initial central vent eruption we assume that the initial pressure distribution in the chamber is magmastatic once collapse has begun the chamber roof is supported by the magma so that magma pressure at the chamber roof increases to lithostatic we suggest that pressure variations during caldera forming eruptions are mainly controlled by variations in magma volatile content regardless of what induces the formation of ring faults the model suggests that the occurrence of explosive caldera forming events depends on the strength of the chamber walls and the depth water content and aspect ratio of the magma chamber no significant differences exist between model results for a cylindrical or a more realistic elliptical magma chamber geometry of comparable aspect ratio assuming a constant strength of the host rock the mass fraction of magma that must be erupted during the central vent phase in order to trigger caldera collapse ranges for deep gas poor chambers from a few percent up to 40 for shallow gas rich chambers the model suggests that zoned chambers tend to collapse earlier than homogeneous chambers dike shaped chambers will erupt less magma than sill like chambers before caldera collapse initiates although dike like geometries are not associated with stress fields appropriate to create ring faults the model suggests that once initiated caldera collapse will tend to force out most or all of the volatile rich magma from the chamber for volatile rich magma chambers the total volume of erupted magma during caldera forming event is of the same order as the chamber volume the model also explains the variation in the erupted mass during the different phases of explosive caldera forming eruptions and is in good agreement with natural examples

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

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

pits rifts and slumps the summit structure of piton de la fournaise a clear model of structures and associated stress fields of a volcano can provide a framework in which to study and monitor activity we propose a volcano tectonic model for the dynamics of the summit of piton de la fournaise la reunion island indian ocean the summit contains two main pit crater structures dolomieu and bory two active rift zones and a slumping eastern sector all of which contribute to the actual fracture system dolomieu has developed over 100 years by sudden large collapse events and subsequent smaller drops that include terrace formation small intra pit collapse scars and eruptive fissures are located along the southern floor of dolomieu the western pit wall of dolomieu has a superficial inward dipping normal fault boundary connected to a deeper ring fault system outside dolomieu an oval extension zone containing sub parallel pit related fractures extends to a maximum distance of 225 m from the pit at the summit the main trend for eruptive fissures is n80 normal to the north south rift zone the terraced structure of dolomieu has been reproduced by analogue models with a roof to width ratio of approximately 1 suggesting an original magma chamber depth of about 1 km such a chamber may continue to act as a storage location today the east flank has a convex concave profile and is bounded by strike slip fractures that define a gravity slump this zone is bound to the north by strike slip fractures that may delineate a shear zone the southern reciprocal shear zone is probably marked by an alignment of large scoria cones and is hidden by recent aa lavas the slump head intersects dolomieu pit and may slide on a hydrothermally altered layer known to be located at a depth of around 300 m our model has the summit activity controlled by the pit crater collapse structure not the rifts the rifts become important on the mid flanks of the cone away from pit related fractures on the east flank the superficial structures are controlled by the slump we suggest that during pit subsidence intra pit eruptions may occur during tumescence however the pit system may become blocked and a flank eruption is more likely intrusions along the rift may cause deformation that subsequently increases the slump s potential to deform conversely slumping may influence the east flank stress distribution and locally control intrusion direction these predictions can be tested with monitoring data to validate the model and eventually improve monitoring

flank tectonics of martian volcanoes on the flanks of olympus mons is a series of terraces concentrically distributed around the caldera their morphology and location suggest that they could be thrust faults caused by compressional failure of the cone in an attempt to understand the mechanism of faulting and the possible influences of the interior structure of olympus mons we have constructed a numerical model for elastic stresses within a martian volcano in the absence of internal pressurization the middle slopes of the cone are subjected to compressional stress appropriate to the formation of thrust faults these stresses for olympus mons are 250 mpa if a vacant magma chamber is contained within the cone the region of maximum compressional stress is extended toward the base of the cone if the magma chamber is pressurized extensional stresses occur at the summit and on the upper slopes of the cone for a filled but unpressurized magma chamber the observed positions of the faults agree well with the calculated region of high compressional stress three other volcanoes on mars ascraeus mons arsia mons and pavonis mons possess similar terraces extending our analysis to other martian volcanoes we find that only these three and olympus mons have flank stresses that exceed the compressional failure strength of basalt lending support to the view that the terraces on all four are thrust faults

the geometry of volcano flank terraces on mars abstract flank terraces are subtle expansive structures on the slopes of many large martian shield volcanoes several terrace formation hypotheses including self loading lithospheric flexure magma chamber tumescence volcano spreading and shallow gravitational slumping have been suggested terraces are not readily visible on photogeological data consequently terrace geometry has not yet been comprehensively described terrace provenance therefore is poorly understood we used three dimensional mars orbiter laser altimeter mola data to characterise the geometry of these elusive structures with a view to understanding better the role that flank terraces play in the tectonic evolution of volcanoes on mars terraces have a broad convex upward profile in section and a systematic fish scale imbricate stacking pattern in plan they are visible at all elevations on at least nine disparate martian volcanoes terrace like features also occur on three shield volcanoes on earth an observation not recorded before analysis of a suite of morphometric parameters for flank terraces showed that they are scale invariant with similar proportions to thrust faults on earth we compared predicted formation geometries to our terrace observations and found that only lithospheric flexure can fully account for the morphology distribution and timing of terraces as a volcano flexes into the lithosphere beneath it its upper surface will experience a net reduction in area resulting in the formation of outward verging thrusts we conclude therefore that flank terraces are fundamental volcanotectonic structures that they are the surface expressions of thrust faults probably formed by lithospheric flexure and that they are not restricted to mars

the geological evolution of atai mons venus a volcano corona hybrid atai mons venus is classified as a large volcano yet displays characteristics that are typical of coronae coronae are widespread volcano tectonic features that are apparently unique to venus detailed analysis of atai mons using photogeological and geophysical techniques illustrates that it has experienced a complex history and may still be active we show that processes found at both coronae and large volcanoes have occurred throughout the history of atai mons with no definite temporal preference it does not have an evolution that can be described simply as the evolution of one feature into another these results suggest that large volcanoes and coronae are more intimately linked than previously thought and that detailed mapping and gravity surveys are reliable ways of establishing the evolution of hybrid features

ibn batutah a possible simple impact structure in southeastern libya a remote sensing study abstract the ibn batutah feature is a circular structure centered on 21 34 10 n and 20 50 15 e and located in the prefecture of the hamadat ibn batutah plateau southeastern libya it is a singular geological structure formed in nubian sandstone of most likely early cretaceous age remote sensing investigations based on landsat etm dual band l and c and dual polarization hh and hv radar sir c and srtm data reveal a morphological resemblance of the ibn batuta structure to other confirmed terrestrial simple impact structures the structure is in the form of a circular basin 2 5 km in diameter and 25 m deep surrounded by a low relief rim that rises to within 5 m of the surrounding terrains over much of its extent no endogenic geological processes such as magmatism diapirism karst dissolution and glacial or fluvial erosion can decisively explain the formation of the ibn batutah structure within a large area of flat lying mesozoic sandstones thus this paper proposes that the ibn batutah structure represents a simple impact structure verification of this hypothesis will require collection and analyses of rock samples from in and around the structure although the area is currently almost inaccessible for environmental and security reasons

mechanism of caldera collapse and resurgence observations from the northern part of the kumano acidic rocks kii peninsula southwest japan abstract the northern part of the kumano acidic rocks on the kii peninsula southwest japan formed during miocene felsic igneous activity that involved caldera forming eruptions and subsequent intrusion of a massive sheetlike body of granite porphyry into intracaldera deposits detailed mapping has revealed that the northern part of the kumano acidic rocks has two separate collapse structures that show the structural margin of a caldera one is an arcuate fault zone characterized by several normal faults with gentle dips pyroclastic deposits characterized by the dominance of crystal rich welded ash vary in thickness across the fault zone the ash beds contain numerous boulders on the inner side of the collapse structure representing caldera collapse breccias sourced from the fault scarp during the caldera forming eruption the other is a 1 3 km wide ring dike of granite porphyry that encloses the prevolcanic sedimentary rocks and the pyroclastic deposits and is interpreted as a magma conduit along a subsurface circular fracture the caldera collapse area enclosed by these two structures is elliptical and about 20 30 km in size our observations suggest a coherent piston type collapse for the northern part of the caldera while asymmetric subsidence along the fault zone resulted in trapdoor or piecemeal subsidence in the southern part the ring dike is connected to the sheetlike body of granite porphyry we suggest that this sheetlike intrusion is a laccolith and may have caused resurgent doming of the caldera floor