persistent scatterer interferometric synthetic aperture radar for crustal deformation analysis with application to volcan alcedo galapagos 1 while conventional interferometric synthetic aperture radar insar is a very effective technique for measuring crustal deformation almost any interferogram includes large areas where the signals decorrelate and no measurement is possible persistent scatterer ps insar overcomes the decorrelation problem by identifying resolution elements whose echo is dominated by a single scatterer in a series of interferograms existing ps methods have been very successful in analysis of urban areas where stable angular structures produce efficient reflectors that dominate background scattering however man made structures are absent from most of the earth s surface furthermore existing methods identify ps pixels based on the similarity of their phase history to an assumed model for how deformation varies with time whereas characterizing the temporal pattern of deformation is commonly one of the aims of any deformation study we describe here a method for ps analysis stamps that uses spatial correlation of interferogram phase to find pixels with low phase variance in all terrains with or without buildings prior knowledge of temporal variations in the deformation rate is not required for their identification we apply stamps to volcan alcedo where conventional insar fails because of dense vegetation on the upper volcano flanks that causes most pixels to decorrelate with time we detect two sources of deformation the first we model as a contracting pipe like body which we interpret to be a crystallizing magma chamber the second is downward and lateral motion on the inner slopes of the caldera which we interpret as landsliding

deflection of dykes into sills at discontinuities and magma chamber formation abstract many rift zones contain composite volcanoes central volcanoes most of which are supplied with magma from shallow crustal magma chambers volcanotectonic studies of eroded rift zones such as in iceland as well as geophysical studies indicate that many shallow magma chambers evolve from sills here i provide field description of sills and extinct magma chambers in the quaternary and tertiary palaeorift zones of iceland focusing on the felsic pluton of slaufrudalur southeast iceland as a fine example of a magma chamber developed through the injection of sills in this paper i review analyse and develop further two related mechanisms for the deflection of dykes into sills and thus the potential initiation of a magma chamber namely a cook gordon debounding delamination and b stress barriers and propose a third mechanism c favourable material toughness ratios due to elastic mismatch difference in young s moduli or stiffnesses of layers in contact in the cook gordon mechanism a weak contact opens up as a result of dyke induced tensile stress this mechanism is likely to operate primarily at shallow depths as is supported by field observations and numerical models a stress barrier is a layer where the local stress is unfavourable to a particular type of rock fracture here a dyke field observations and numerical models show that on meeting a stress barrier a dyke either changes into a sill or becomes arrested the material toughness mechanism indicates that when the upper layer at a contact has the same or less stiffness than the lower layer hosting the dyke there is little tendency for the dyke to become deflected into the contact however when the upper layer is stiffer such as when a stiff basaltic lava flow is on the top of a soft pyroclastic layer the dyke tends to become deflected into the contact to form a sill field results suggest that all these mechanisms may cause dyke deflection into sills and may operate together particularly where the rock consists of alternating soft and stiff layers since the quaternary lava pile in iceland has many more soft hyaloclastite layers than the tertiary lava pile these mechanisms were probably very efficient in generating sills and by implication shallow magma chambers during the quaternary this may be one reason why in comparison with the tertiary lava pile the quaternary lava pile contains so many sills and extinct plutons and active shallow magma chambers

a volcano bursting at the seams inflation faulting and eruption at sierra negra volcano galapagos the results of geodetic monitoring since 2002 at sierra negra volcano in the galapagos islands show that the filling and pressurization of an 2 km deep sill eventually led to an eruption that began on 22 october 2005 continuous global positioning system cgps monitoring measured 2 m of accelerating inflation leading up to the eruption and contributed to nearly 5 m of total uplift since 1992 the largest precursory inflation ever recorded at a basaltic caldera this extraordinary uplift was accommodated in part by repeated trapdoor faulting and coseismic cgps data provide strong constraints for improved deformation models these results highlight the feedbacks between inflation faulting and eruption at a basaltic volcano and demonstrate that faulting above an intruding magma body can relieve accumulated strain and effectively postpone eruption

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

thermomechanical interactions between crustal magma chambers in complex tectonic environments insights from eastern turkey abstract crustal segments such as those in eastern turkey which host magma chambers volcanoes and fault zones experience complex stresses generated through interactions between regional tectonic loading magmatic pressure and geothermal dynamics here we model these competing loading mechanisms and examine their effects on generating stresses in a complex crustal segment which has been subjected to orogenesis and hosted voluminous magma discharge this simulated region hosts several volcanoes thermal fields and has experienced complex tectonic deformation which manifests as crustal faults we present a suite of purely thermal models to show the temperature distribution within the crust assuming an arrangement of crustal magma chambers and a realistic geothermal gradient we also present a suite of linear elastic mechanical models to investigate the effect of magma pressure and regional tectonic loading in the absence of temperature variations using different arrangements of magma chambers and faults finally we present coupled linear elastic thermomechanical models that highlight the influence of temperature on the distribution of both crustal stresses and deformation using the same complex geometries results show that thermal stresses generate two competing consequences 1 they increase the level of shear stress around the magma chambers potentially leading to fault nucleation or reactivation and 2 they partially act to suppress the level of tensile stress originally generated both by magma pressure and tectonic loading this implies that for any magmatic recharge event which increases the internal magma chamber pressure the contribution of temperature increase in the surrounding host rocks must also be taken into account when considering the distribution magnitude and type of stresses around magma chambers in such crustal environments

seismicity patterns during a period of inflation at sierra negra volcano galapagos ocean island chain basaltic shield volcanoes of the western galapagos islands are among the most rapidly deforming volcanoes worldwide but little was known of the internal structure and brittle deformation processes accompanying inflation and deflation cycles a 15 station broadband seismic array was deployed on and surrounding sierra negra volcano galapagos from july 2009 through june 2011 to characterize seismic strain patterns during an inter eruption inflation period and to evaluate single and layered magma chamber models for ocean island volcanoes we compare precise earthquake locations determined from a 3d velocity model and from a double difference cluster method using first motion of p arrivals we determine focal mechanisms for 8 of the largest earthquakes ml 1 5ml 1 5 located within the array most of the 2382 earthquakes detected by the array occurred beneath the broad 9 km wide sierra negra caldera at depths from surface to about 8 km below sea level although outside our array frequent and larger magnitude ml 3 4ml 3 4 earthquakes occurred at alcedo and fernandina volcano and in a spatial cluster beneath the shallow marine platform between fernandina and sierra negra volcanoes the time space relations and focal mechanism solutions from a 4 day long period of intense seismicity june 4 9 2010 along the southeastern flank of sierra negra suggests that the upward migrating earthquake swarm occurred during a small volume intrusion at depths 5 8 km subsurface but there was no detectable signal in insar data to further constrain geometry and volume focal mechanisms of earthquakes beneath the steep intra caldera faults and along the ring fault system are reverse and strike slip these new seismicity data integrated with tomographic geodetic and petrological models indicate a stratified magmatic plumbing system a shallow sill beneath the large caldera that is supplied by magma from a large volume deeper feeding system the large amplitude inter eruption inflation of the shallow sill beneath the sierra negra caldera is accompanied by only very small magnitude earthquakes although historical records indicate that larger magnitude earthquakes mw 6 occur during eruptions trapdoor faulting episodes without eruptions and large volume flank intrusions

calculation of dike trajectories from volcanic centers 1 patterns of dike swarms around volcanic centers or above mantle plumes are interpreted by a mechanical analysis of regional and dike induced stresses in which dike emplacement is controlled and guided by the stress state comparisons of dike patterns with patterns of principal stress trajectories caused by a source and a regional stress system are commonly used to infer paleostresses however dike trajectories are determined by a complex interaction between dike induced stresses and the source regional stress system we present numerical calculations based on a novel boundary integral formulation which examines the simultaneous effects of regional stresses magma pressure and dike injection on the local stress state around a continuously curving dike dike paths are calculated from the condition that dikes propagate by mode i failure our results suggest that the magnitude of the regional stresses would be 2 5 times higher than previous estimates based on principal stress trajectory analysis

thermal and mechanical aspects of magma emplacement in giant dike swarms we consider the thermal history and dynamics of magma emplacement in giant feeder dikes associated with continental flood basalts for driving pressure gradients inferred for giant dike swarms thicknesses of 10 m would enable dikes to transport magma laterally over the distances observed in the field up to thousands of kilometers without suffering thermal lock up using time dependent numerical solutions for the thermal evolution of a dike channel under laminar and turbulent flow conditions in the presence of phase transitions we investigate the possibility that the observed dike thicknesses of the order of 100 m result from thermal erosion of the country rocks during dike emplacement this implies that the observed range of dike widths in giant dike swarms may reflect variations in the source volume and not the excess magma pressure it is found that the total volume of intruded magma required to produce an order of magnitude increase in dike width via wall rock melting broadly agrees with the estimated volumes of individual flows in continental flood basalts the presence of chilled margins and apparently low crustal contamination characteristics of some giant dikes may be consistent with turbulent magma flow and extensive melt back during dike emplacement in this case measurements of the anisotropy of magnetic susceptibility most likely indicate magma flow directions during the final stages of dike intrusion shear stresses generated at the dike wall when the dike starts to freeze strongly decrease with increasing dike width which implies that thicker dikes may have less tendency to produce consistent fabric alignment our results suggest that if the dike was propagating downslope off a plume related topographic swell the mechanism responsible for flow termination could possibly have been related to underpressurization and collapse implosion of the shallow magma plumbing system feeding the intrusion radial dikes that erupted at the periphery of the topographic uplift might have increased rather than decreased extensional stresses in the crust within the topographic uplift upon their solidification

the may 2005 eruption of fernandina volcano galapagos the first circumferential dike intrusion observed by gps and insar the may 2005 eruption of fernandina volcano galapagos occurred along circumferential fissures parallel to the caldera rim and fed lava flows down the steep southwestern slope of the volcano for several weeks this was the first circumferential dike intrusion ever observed by both insar and gps measurements and thus provides an opportunity to determine the subsurface geometry of these enigmatic structures that are common on galapagos volcanoes but are rare elsewhere pre and post eruption ground deformation between 2002 and 2006 can be modeled by the inflation of two separate magma reservoirs beneath the caldera a shallow sill at 1 km depth and a deeper point source at 5 km depth and we infer that this system also existed at the time of the 2005 eruption the co eruption deformation is dominated by uplift near the 2005 eruptive fissures superimposed on a broad subsidence centered on the caldera modeling of the co eruption deformation was performed by including various combinations of planar dislocations to simulate the 2005 circumferential dike intrusion we found that a single planar dike could not match both the insar and gps data our best fit model includes three planar dikes connected along hinge lines to simulate a curved concave shell that is steeply dipping 45 60 toward the caldera at the surface and more gently dipping 12 14 at depth where it connects to the horizontal sub caldera sill the shallow sill is underlain by the deep point source the geometry of this modeled magmatic system is consistent with the petrology of fernandina lavas which suggest that circumferential eruptions tap the shallowest parts of the system whereas radial eruptions are fed from deeper levels the recent history of eruptions at fernandina is also consistent with the idea that circumferential and radial intrusions are sometimes in a stress feedback relationship and alternate in time with one another

slopes lava flow volumes and vent distributions on volcan fernandina galapagos islands digital elevation data from topsar an airborne synthetic aperture radar system that uses interferometry to derive topography are used to determine slope distributions and lava flow thicknesses on fernandina volcano galapagos islands four extracaldera slope regions are defined from the coast inland a coastal plain average slope 2 an apron 6 12 steep slopes 250 600 m high 20 43 and a 0 5 to 1 km wide summit platform 10 lava flows and vents are mapped using shuttle imaging radar c sir c spot landsat thematic mapper tm and air photos over 500 flows are identified and categorized as young intermediate or old based on albedo vegetation cover and margin preservation by area young flows constitute 55 of the island 34 are intermediate and 7 are old the aa pahoehoe ratio of the young flows is 85 15 whereas for the intermediate flows it is 58 42 of the 423 vents that were classified 236 are radial and 80 of these are in the apron 143 are arcuate and 94 of these are in the summit platform and 46 have transitional orientations and are about equally divided between the steep slopes and summit platform 95 of all vents are within 13 km of the caldera center topsar data allow flow volumes to be estimated and young flows range from 0 01 to 0 12 km3 with a total volume of 2 3 km3 by volume 91 of the young lava erupted from radial vents below the steep slopes many of which are concentrated within the se apron about 5 6 km from the caldera similar concentrations to the ne nw and sw consist of young and intermediate flows different proportions of lava flows and vents form the different slope regions the coastal plain averages 0 1 vents km2 and the slightly steeper apron averages 0 6 to 0 9 vents km2 increasing inland the summit platform averages 4 7 vents km2 and this concentration supports previously proposed mechanisms for producing higher elevations and steeper slopes in the central part of the volcano temporal changes in the plumbing system and or magma supply rate are suggested by the change in aa pahoehoe ratio it appears that in the past low effusion rate eruptions were more common perhaps from a filled caldera whereas more recently high effusion rate eruptions have dominated

using numerical modeling to explore the origin of intrusion patterns on fernandina volcano galapagos islands ecuador 1 using parameterized finite element models we investigate the emplacement of both radial and circumferential intrusions in the configuration observed at fernandina volcano in the galapagos islands when situated within the edifice at depths consistent with petrological and surface displacement data inflation of a mildly oblate magma reservoir to the point of rupture can initiate either radial or circumferential intrusions in response to minor volcanologically plausible variations in reservoir geometry i e aspect ratio in addition more oblate reservoirs inject lateral sills into an inflation derived stress field consistent with rotation about their propagation axis to form gently dipping radial dikes a mode of behavior recently inferred from insar data at fernandina all three styles of intrusion occur in near surface configurations consistent with field observations

magma reservoirs and neutral buoyancy zones on venus implications for the formation and evolution of volcanic landforms we examine the production of magma reservoirs and neutral buoyancy zones nbzs on venus and the implications of their development for the formation and evolution of volcanic landforms the high atmospheric pressure on venus reduces volatile exsolution and generally serves to inhibit the formation of nbzs and shallow magma reservoirs for a range of common terrestrial magma volatile contents magma ascending and erupting near or below mean planetary radius mpr should not stall at shallow magma reservoirs such eruptions would be characterized by relatively high total volumes and effusion rates for the same range of volatile contents at 2 km above mpr about half of the cases result in direct ascent of magma to the surface and half in the production of neutral buoyancy zones in general neutral buoyancy zones and shallow magma reservoirs begin to appear as gas content increases and are nominally shallower on venus than on earth for a fixed volatile content nbzs become deeper with increasing elevation over the range of elevations treated here 1 km to 4 4 km depths differ by a factor of 2 4 these analyses reveal several factors that may help to account for the low height of volcanoes on venus larger primary reservoirs cause the wide dispersal of conduits building edifices models of the position of the shallow nbz reservoir during edifice growth show that for earth the magma chamber center remains at a constant depth below the growing edifice summit thus keeping pace with the increasing elevation while on venus the chamber center becomes deeper relative to the summit of the growing edifice because of the major change in atmospheric pressure as a function of altitude therefore neutral buoyancy zones and magma reservoirs on venus will remain in the prevolcano substrate longer and in many cases may not emerge into the edifice at all the lower rate of vertical migration implies that magma reservoirs would tend to stabilize undergo greater lateral growth and become larger on venus than earth the proportion of the available magma going into production of the edifice relative to that intruded into the substrate is smaller on venus than earth large reservoirs would encourage multiple and more widely dispersed source vents and large volumes for individual eruptions in large reservoirs positively buoyant materials are likely to be produced from differentiation substrate remelting and volatile exsolution nonbuoyant materials exsolving volatiles in a shallow reservoir will need higher gas bubble concentrations to produce eruptions than on earth and when this gas enriched melt emerges at the surface it is more likely to retain its bubbles than to undergo explosive disruption due to the high surface atmospheric pressure therefore there is the potential for the production of a range of erupted lavas that have very high gas bubble concentrations leading to anomalous more viscous rheological properties inhibition of disruption of volatile rich magma for both basaltic and more evolved compositions can lead to the production of 1 more bubble rich vesicular flows characterized by higher viscosity and greater thicknesses the venus equivalent of effusions that would have lost much of their volatiles in the fire fountaining process on earth 2 high volume high discharge rate extremely gas rich basaltic effusions that increase viscosity considerably upon surface extrusion and produce domes whose volumes are considerably higher than their dense rock equivalent and 3 more evolved compositions that are gas rich but do not undergo disruption upon effusion and produce domes and flows of high viscosity the venus equivalent of terrestrial ignimbrites on the basis of these analyses we predict that intrusions and dike emplacement should be common particularly around both shallow and deep magma reservoirs and the tops of dikes should be manifested as radial fractures there should be a range of diameters of circular features associated with the presence of both shallow nbz related and deep plumerelated magma reservoirs large volume lava outpourings should be favored at lower elevations and shallow reservoirs and edifices should be more common at intermediate elevations at the highest elevations magma reservoirs are predicted to be large and relatively deep with low associated edifices direct ascent of magma where neutral buoyancy zones are unlikely low elevations and the propensity for relatively larger magma reservoirs on venus when they do form are both factors which will tend to favor high volume eruptions independent of variations in magma temperature or chemistry individual volcanic features should be interpreted in terms of regional geologic setting and the dynamics of elevation changes commonly associated with various scales of mantle processes and magma emplacement in order to assess global crustal growth processes on venus

physical volcanology and structural development of sierra negra volcano isabela island galapagos archipelago sierra negra is the most voluminous of the western galapagos shields and exhibits many unusual morphologic features extensive lower flank apron steep upper flanks a broad summit plateau a large complex caldera ubiquitous occurrence of aa and radial and circumferential fissures a field based investigation of the volcano was conducted in order to establish the distribution and characteristics of eruptive zones and associated flow fields and to construct a volcano wide surface flow stratigraphy geochronologic data were used to determine the age of the units and combined with flow volume calculations to estimate both the historic and total volcano eruption rates ninety one kilometers of radial and circumferential fissures were mapped and grouped into four eruptive zones which form a volcano wide east northeast trending rift system that deflects around the caldera the eruptive zones are restricted spatially and temporally and document long term changes in the volcano s development long fluid lava flows are broadly distributed among five ages resulting in a subdued flank profile by the development of an extensive lower flank apron around the volcano ten historical eruptions have occurred with eruption rates of 44 to 248 m3 s and an average growth rate of 12 106 m3 yr age determinations of lava flows by 14c and cosmogenic 3he range from 400 to 6900 yr the total volcano growth rate is estimated to be 1 106 m3 yr these results confirm the young age of sierra negra lavas and the validity of the relative ages determined from stratigraphic study in total the volcano has undergone over 90 resurfacing in the past 4500 yr the summit of sierra negra is entirely occupied by a shallow elliptical 7 10 km caldera near vertical ring faults circumscribe the summit and expose 100 to 140 m of lava flows subparallel sets of caldera growth faults cut the western and southern caldera walls and are not associated with eruptive activity the inclination of remnant magnetism and the attitudes of the flows exposed in the caldera walls indicate that the lavas have not been appreciably deformed nor have they been intruded by dikes or sills the caldera interior is structurally complex and contains a 14 km long c shaped sinuous ridge composed of a complex set of normally faulted blocks with inward dipping 20 60 flow tops the sinuous ridge is interpreted to have formed gradually as the caldera floor pivoted in a trap door fashion jammed its western edge and then fractured in several places concomitant with the long term vertical and lateral growth of the volcano a regional tectonic stress field superimposed upon the galapagos platform appears to have exerted primary control on the location of sierra negra including the early development of the east northeast rift system intervolcano stresses have controlled the distribution of eruptive zones only where significant subaerial overlap of adjacent volcanoes occurs large volume high discharge rate eruptions from fissures along the east northeast rift system are responsible for the development of an extensive coastal apron flow stacking at mid flank elevations and generally subdued slopes in comparison to other western galapagos volcanoes the summit caldera and underlying magma chamber are subordinate in influencing the distribution of lower flank stress but dominate the development of the summit stress field and orientation of eruptive fissures the absence of summit deformation together with the ubiquitous occurrence of short stubby summit flows indicates that the steep upper flanks are constructional in origin

a new model for the growth of basaltic shields based on deformation of fernandina volcano galapagos islands article history space geodetic measurements of surface deformation produced by the most recent eruptions at fernandina the most frequently erupting volcano in the galapagos archipelago reveal that all have initiated with the intrusion of subhorizontal sills from a shallow magma reservoir this includes eruptions from fissures that are oriented both radially and circumferentially with respect to the summit caldera a synthetic aperture radar sar image acquired 1 2 h before the start of a radial fissure eruption in 2009 captures one of these sills in the midst of its propagation toward the surface galapagos eruptive fissures of all orientations have previously been presumed to be fed by vertical dikes and this assumption has guided models of the origin of the eruptive fissure geometry and overall development of the volcanoes our findings allow us to reinterpret the internal structure and evolution of galapagos volcanoes and of similar basaltic shields furthermore we note that stress changes generated by the emplacement of subhorizontal sills feeding one type of eruption may control the geometry of subsequent eruptive fissures specifically circumferential fissures tend to open within areas uplifted by sill intrusions that initiated previous radial fissure eruptions this mechanism provides a possible explanation for the pattern of eruptive fissures that characterizes all the western galapagos volcanoes as well as the alternation between radial and circumferential fissure eruptions at fernandina the same model suggests that the next eruption of fernandina will be from a circumferential fissure in the area uplifted by the 2009 sill intrusion just southwest of the caldera rim 2013 elsevier b v all rights reserved

state of stress before and after the 1994 northridge earthquake the state of tectonic stress in the epicentral area of the 17 january 1994 northridge earthquake mw 6 7 is investigated by applying a stress inversion method to p wave polarity data from earthquakes in northridge from july 1981 to january 1994 and from the northridge aftershocks during january 1994 to december 1995 a 3 d crustal model is used to trace the rays taking off from the hypocenter which reduced the effects of large structural heterogeneities on the determination of the stress tensor we found significant temporal changes of stress orientations induced by the northridge earthquake the principal pressure p axis is oriented n32 e from 1981 to june 1992 and n30 e from 28 june 1992 to 16 january 1994 suggesting that the stress field in northridge was not affected by the 1992 landers earthquake during two weeks following the northridge mainshock the p axis is oriented n13 e which is a significant 17 change from that before the earthquake n30 e between february 1994 and august 1995 the p axis orientation changes from n18 e to n26 e and finally ends up at n34 by the end of 1995 which is close to that before the northridge earthquake these results suggest that the stresses rotated coseismically then rotated more slowly back to their original orientation the aftershocks caused by the mainshock changed the stress distribution in the crust which showed up as a regional stress change the stress recovery appears to have completed within two years after the mainshock which is very short compared to the time scale of the earthquake cycle

near ridge seamount chains in the northeastern pacific ocean high resolution bathymetry and side scan data of the vance president jackson and taney near ridge seamount chains in the northeast pacific were collected with a hull mounted 30 khz sonar the central volcanoes in each chain consist of truncated cone shaped volcanoes with steep sides and nearly flat tops several areas are characterized by frequent small eruptions that result in disorganized volcanic regions with numerous small cones and volcanic ridges but no organized truncated conical structure several volcanoes are crosscut by ridge parallel faults showing that they formed within 30 40 km of the ridge axis where ridge parallel faulting is still active magmas that built the volcanoes were probably transported through the crust along active ridge parallel faults the volcanoes range in volume from 11 to 187 km3 and most have one or more multiple craters and calderas that modify their summits and flanks the craters 1 km diameter range from small pit craters to calderas as large as 6 5 8 5 km although most are 2 4 km across crosscutting relationships commonly show a sequence of calderas stepping toward the ridge axis the calderas overlie crustal magma chambers at least as large as those that underlie kilauea and mauna loa volcanoes in hawaii perhaps 4 5 km in diameter and 1 3 km below the surface the nearly flat tops of many of the volcanoes have remnants of centrally located summit shields suggesting that their flat tops did not form from eruptions along circumferential ring faults but instead form by filling and overflowing of earlier large calderas the lavas retain their primitive character by residing in such chambers for only short time periods prior to eruption stored magmas are withdrawn probably as dikes intruded into the adjacent ocean crust along active ridge parallel faults triggering caldera collapse or solidified before the next batch of magma is intruded into the volcano probably 1000 10 000 years later the chains are oriented parallel to subaxial asthenospheric flow rather than absolute or relative plate motion vectors the subaxial asthenospheric flow model yields rates of volcanic migration of 3 4 3 3 and 5 9 cm yr 1 for the vance president jackson and taney seamounts respectively the modeled lifespans of the individual volcanoes in the three chains vary from 75 to 95 kyr these lifespans coupled with the geologic observations based on the bathymetry allow us to construct models of magma supply through time for the volcanoes in the three chains

effects of crustal scale mechanical layering on magma chamber failure and magma propagation within the venusian lithosphere understanding the connection between shallow subsurface magmatism and related surface expressions provides first order insight into the volcanic and tectonic processes that shape a planet s evolution when assessing the role of flexure previous investigations assumed homogeneous host rock but planetary lithospheres typically include crust and mantle material and the mechanical response of a layered lithosphere subjected to flexure may influence both shallow magma reservoir failure and intrusion propagation to assess the formation of giant radial dike systems such as those observed on venus we create axisymmetric elastic finite element models of a spherical reservoir centered at the contact between stiff dense mantle overlain by softer lighter crust we analyze magma chamber stability overpressure at rupture and resulting intrusion types for three distinct environments lithostatic upward flexure and downward flexure in the lithostatic case reservoir failure at the crust mantle contact favors lateral sill injection in the flexure cases we observe that failure location depends upon the crust lithosphere thickness ratio and at times will favor radial dike intrusion specifically upward flexure can promote the formation of giant radiating dike swarms a scenario consistent with a plume derived origin our results present a mechanical explanation for giant radial dike swarm formation showing that both the stability of magma chambers on venus and the type of intrusions that form are influenced by lithospheric layering furthermore where dike swarms occur our approach provides a powerful new way to constrain local crust mantle layering characteristics within the lithosphere at the time the swarm was forming

numerical modelling of triple junction tectonics at karliova eastern turkey with implications for regional magma transport few places on earth are as tectonically active as the karliova region of eastern turkey in this region complex interactions between the arabian eurasian and anatolian plates occur at the karliova triple junction ktj the relationship between tectonics and magma propagation in triple junction tectonic settings is poorly understood here we present new field and numerical results on the mechanism of magma propagation at the ktj we explore the effects of crustal heterogeneity and anisotropy in particular the geometry and mechanical properties of many faults and layers on magma propagation paths under a variety of tectonic loadings we propose that two major volcanic centres in the area the turnadag volcano and the varto caldera are both fed by comparatively shallow magma chambers at depths of about 8 km which in turn are fed by a single much larger and deeper reservoir at about 15 18 km depth by contrast the nearby ozenc volcanic area is fed directly by the deeper reservoir we present a series of two dimensional and three dimensional numerical models showing that the present tectonic stresses encourage magma chamber rupture and dyke injection the results show that inversion tectonics encourages the formation of magma paths as potential feeder dykes our three dimensional models allow us to explore the local stresses induced by complex loading conditions at the karliova triple junction using an approach that can in future be applied to other similar tectonic regions the numerical results indicate a great complexity in the potential magma dyke paths resulting from local stresses generated by interaction between mechanical layers major faults and magma chambers in particular the results suggest three main controls on magma path formation and eventual eruptions at ktj 1 the geometry and attitude of the associated faults 2 the heterogeneity and anisotropy of the crust and 3 mechanical stress interactions between deep and shallow magma chambers

radial dike formation on venus insights from models of uplift flexure and magmatism abstract giant radiating dike swarms on venus are geomorphic manifestations of deep seated volcano tectonic processes through an initial series of experiments we assess if these features can be explained as a product of the relationship between magmatism and the flexural stress state caused by lithospheric uplift due to a rising mantle diapir plume or underplating processes using gravitationally loaded axisymmetric elastic finite element models magma chambers situated in the upper extensional lithosphere fail at or near the crest redirecting magma vertically into radially aligned dikes but these are expected to feed surface eruptions conditions promoting lateral propagation of dikes do not occur magma chambers located in the lower compressional lithosphere are predicted to generate horizontal intrusions sills upon midsection failure when both uplift from basal loading and magmatic surface loading from edifice growth are introduced in sequence at discrete times however conditions that promote shallow lateral propagation of magma within radial dikes are created 1 as magma from shallow reservoirs ascends and erupts surface loading volcanic edifices can form 2 as the vertical loads increase flexing the lithosphere downward high compressional stresses produced within the edifice create a shallow stress barrier that makes it increasingly difficult for magma to complete the process of ascent and eruption and then 3 stresses acting on dikes ascending from a reservoir toward the surface will thus increasingly favor fracture along their lateral margins redirecting magma through radial dikes toward distal regions at shallow depth our results are consistent with observed patterns of laterally extensive dike systems at volcanic centers such as mbokomu mons and becuma mons the models thus provide a robust mechanical explanation that for the first time links lithosphere flexure and reservoir pressurization to the ascent and lateral spreading of magma through radial dike systems on venus thereby providing key new spatial and temporal insights into an important magmatic process operating on multiple terrestrial planets

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

state of stress faulting and eruption characteristics of large volcanoes on mars the formation of a large volcano loads the underlying lithospheric plate and can lead to lithospheric flexure and faulting in turn lithospheric deformation affects the stress field beneath and within the volcanic edifice and can influence magma transport modeling the interaction of these processes is crucial to an understanding of the history of eruption characteristics and tectonic deformation of large volcanoes we develop models of time dependent stress and deformation for the tharsis volcanoes on mars by means of a finite element code we calculate stresses and displacements due to a volcano shaped load emplaced on an elastic plate overlying a viscoelastic mantle models variously incorporate growth of the volcanic load with time and a detachment between volcano and lithosphere the models illustrate the manner in which time dependent stresses induced by lithospheric plate flexure beneath the volcanic load may affect eruption histories and the derived stress fields can be related to tectonic features on and surrounding martian volcanoes as a result of flexure there are three regions where stresses become sufficiently large to cause failure by faulting according to the mohr coulomb criterion at the surface of the plate just outward of the volcano near the base of the elastic lithosphere beneath the center of the volcano and on the upper flanks of the volcano early in its growth history normal faulting is the dominant mode of failure predicted for the first region consistent with circumferential graben observed around the tharsis montes and with the scarp at the base of olympus mons interpreted as a large offset listric normal fault normal faulting mostly radially oriented is predicted for the second region failure in the third region is predicted to consist of thrust faulting circumferentially oriented on the upper and middle flanks and radially oriented on the lower flanks in models simulating a growing volcano this portion of the edifice is subsequently covered by later units which exhibit lower stresses and are not predicted to fail this volume of early failure remains the most highly stressed area in the edifice concentric terraces interpreted by some workers as thrust faults on the upper flanks of olympus mons may correspond to the predicted circumferential thrust features if the most recent increments of volcano growth were relatively large or in the presence of local material property or stress field variations for volcanoes detached from the plate predicted failure in the edifice takes the form of radial normal faulting near the volcano base the addition of a local extensional stress arising from the regional topographic slope yields a pattern of predicted faulting which closely matches that observed on the tharsis montes including the development of radial rifts on the lower volcano flanks to the northeast and southwest and the asymmetric formation of circumferential flank graben this stress state is also consistent with an interpretation of the aureole deposits of olympus mons as the result of gravity sliding along a basal detachment our models also suggest an explanation for the lack of strike slip features predicted by previously published flexural models around the tharsis volcanoes for a given load increment the first mode of near surface failure for most of the area immediately outward of the load is circumferential normal faulting and graben formation as the volcano grows and the flexural response to the increasing load proceeds the predicted failure mode in a portion of this annular region surrounding the volcano changes to strike slip faulting because normal faulting has been predicted to have taken place earlier however it is likely that release of later stresses will occur by reactivation and growth of these normal faults and graben rather than by the formation of new strike slip faults

space geodetic evidence for multiple magma reservoirs and subvolcanic lateral intrusions at fernandina volcano galapagos islands 1 using interferometric synthetic aperture radar insar measurements of the surface deformation at fernandina volcano galapagos ecuador acquired between january 2003 and september 2010 we study the structure and the dynamics of the shallow magmatic system of the volcano through the analysis of spatial and temporal variations of the measured line of sight displacement we identify multiple sources of deformation beneath the summit and the southern flank at least two sources are considered to represent permanent zones of magma storage given their persistent or recurrent activity elastic deformation models indicate the presence of a flat topped magma reservoir at 1 1 km below sea level and an oblate spheroid cavity at 4 9 km b s l the two reservoirs are hydraulically connected this inferred structure of the shallow storage system is in agreement with previous geodetic studies and previous petrological analysis of both subaerial and submarine lavas the almost eight year long observation interval provides for the first time geodetic evidence for two subvolcanic lateral intrusions from the central storage system in december 2006 and august 2007 subvolcanic lateral intrusions could provide the explanation for enigmatic volcanic events at fernandina such as the rapid uplift at punta espinoza in 1927 and the 1968 caldera collapse without significant eruption

discrimination of fault planes from auxiliary planes based on simultaneous determination of stress tensor and a large number of fault plane solutions it is well known that there is a large difference in focal mechanism solutions of events even if they occur within a certain small area considering earthquake generating stress to be uniform in a studied area we developed a new method for simultaneously determining the stress tensor and the orientation of fault planes for many events by the use of polarity data of p waves first the number of inconsistent stations is calculated for various values of three parameters in the focal mechanism solution with intervals of 10 this calculation is made for all the events used then four parameters defining the stress tensor are determined by a grid search using the data calculated above with the assumption that the slip direction in the fault plane should be parallel to the direction where the shear stress on the plane becomes maximum a numerical test is made by applying the present method to an artificial data set composed of 25 events with 25 25 readings it is found that a stable solution of the stress tensor is determined by the inversion and about 60 of fault planes are distinguished from auxiliary planes there are no events whose auxiliary planes are incorrectly determined as fault planes this method is used to estimate the earthquake generating stress in the aftershock area of the 1984 western nagano prefecture earthquake central japan the results obtained show that the maximum principal stress is horizontal in the direction of n80 w s80 e there are strike slip events having fault planes almost perpendicular to the fault plane of the main shock

dynamics of dikes versus cone sheets in volcanic systems igneous sheet intrusions of various shapes such as dikes and cone sheets coexist as parts of complex volcanic plumbing systems likely fed by common sources how they form is fundamental regarding volcanic hazards yet no dynamic model simulates and predicts satisfactorily their diversity here we present scaled laboratory experiments that reproduced dikes and cone sheets under controlled conditions our models show that their formation is governed by a dimensionless ratio 1 which describes the geometry of the magma source and a dynamic dimensionless ratio 2 which compares the viscous stresses in the flowing magma to the host rock strength plotting our experiments against these two numbers results in a phase diagram evidencing a dike and a cone sheet field separated by a sharp transition that fits a power law this result shows that dikes and cone sheets correspond to distinct physical regimes of magma emplacement in the crust for a given host rock strength cone sheets preferentially form when the source is shallow relative to its lateral extent orwhen the magma influx velocity or viscosity is high conversely dikes form when the source is deep compared to its size or when magma influx rate or viscosity is low both dikes and cone sheets may form fromthe same source the shift fromone regime to the other being then controlled by magma dynamics i e different values of 2 the extrapolated empirical dike to cone sheet transition is in good agreement with the occurrence of dikes and cone sheets in various natural volcanic settings

consequences of volcano sector collapse on magmatic storage zones insights from numerical modeling abstract major volcano flank collapses strongly affect the underlying magmatic plumbing system here we consider the magma storage zone as a liquid pocket embedded in an elastic medium and we perform numerical simulations in two dimensional axisymmetric geometry as well as in three dimensions in order to evaluate the consequences of a major collapse event we quantify the pressure decrease induced within and around a magma reservoir by a volcano flank collapse this pressure reduction is expected to favor replenishment with less evolved magma from deeper sources we also estimate the impact of the magma pressure decrease together with the stress field variations around the reservoir on the eruptive event associated with the edifice failure we show that for a given magma reservoir geometry the collapse of a large strato volcano tends to reduce the volume of the simultaneous eruption destabilization of large edifices may even suppress magma emission resulting in phreatic eruptions instead this effect is greater for shallow reservoirs and is more pronounced for spherical reservoirs than for vertically elongated ones it is reduced for compressible magmas containing a large amount of volatiles over a longer time scale the modification of pressure conditions for dyke initiation at the chamber wall may also explain an increase in eruption rate as well as an apparent change of magma storage location

constraints on magma chamber geometry at sierra negra volcano galapagos islands based on insar observations abstract we investigate the problem of estimating magma chamber geometry using insar observations of sierra negra volcano galapagos ascending and descending interferograms are combined to determine vertical and one horizontal component of displacement the ratio of maximum horizontal to vertical displacement suggests a sill like source spherical or stock like bodies are inconsistent with the data we estimate the geometry of the sill assuming a horizontal uniformly pressurized crack with unknown periphery and depth the sill is discretized into small elements that are either open and are subjected to the pressure boundary condition or remain closed we find the best fitting sill to be located beneath sierra negra s inner caldera at a depth of about 2 km using boundary element calculations we show that any magma chamber with a flat top coincident with the sill model fits the data equally well the data are insensitive to the sides and bottom of the magma chamber

depths of magma chambers at three volcanic provinces in the karliova region of eastern turkey the size of a volcanic eruption and thus the associated potential hazards depends partly on the depth geometry and size of the source magma chamber to estimate magma chamber depths and sizes we apply a newly developed analytical method based on the aspect ratio length thickness of dikes to three volcanoes in the karliova region of eastern turkey namely turnadag varto and ozenc the results indicate that the depths of the source chambers are between 2 and 4 km at turnadag 2 to 5 km at varto both of which are located in transtensional tectonic regimes but from 22 to 27 km at ozenc which is located in a convergent tectonic regime a similar reservoir depth at ozenc is indicated by seismic tomography and this data also suggests that the reservoir is laterally continuous for more than 40 km the large volume of ignimbrites 40 km3 associated with varto a collapse caldera indicates that caldera subsidence may have maintained the excess magmatic pressure through tectonic forcing in the chamber over a longer time than during normal pyroclastic eruptions the dike aspect ratios further indicate magmatic overpressures of 13 21 mpa for varto 13 17 mpa for turnadag and 26 31 mpa for ozenc the combined results from seismic tomography analytical models and magma compositions indicate that both turnadag and varto volcanoes which are typical stratovolcanoes composed of mostly intermediate and more rarely acidic magmas were fed by two very shallow and comparatively small magma chambers 2 5 km depth whereas less evolved magmas were erupted from ozenc which hosts predominantly basaltic and intermediate lavas and dikes that were fed by a deep reservoir at 22 27 km depth our tomographic models show that none of the volcanoes are located directly over the center of a deep magma reservoir our data also indicates that the magma in the reservoir has migrated between 34 and 40 km in a right lateral motion to the east below varto and turnadag respectively and 23 km in a left lateral motion to the west at ozenc over the past 3 ma this lateral propagation of magma can be explained by tectonic escape of the anatolian block to the west through the northern anatolian fault and the varto fault zone over the last 6 ma

fernandina volcano s evolved well mixed basalts mineralogical and petrological constraints on the nature of the galapagos plume fernandina volcano the most active of galapagos volcanoes and the one most directly overlying the galapagos hotspot is one of a small number of active plume related oceanic island volcanoes with a well documented recent eruptive record whole rock and glass analyses of lava and tephra from all 12 known eruptions from 1958 to the present show them to be evolved mg mg mg fe2 of 0 47 0 56 plagioclase phyric clinopyroxene and olivine bearing tholeiites thermodynamic modeling indicates that this phase assemblage erupted at 1100 1130 c and is stable only at low pressures 0 1 0 2 gpa thereby representing crystallization within a shallow magma chamber underlying the well defined 850 m deep 5 by 6 5 km caldera these tholeiites are remarkably homogenous as is shown by narrow ranges in zr y 5 7 6 3 la sm n 1 4 1 7 and la yb n 3 8 4 3 prehistoric fernandina lavas selected for petrographic and geographic variation over the entire subaerial volcano are of remarkably similar composition e g zr y is 5 2 6 1 la sm n is 1 5 1 6 and la yb n is 3 4 4 1 with a single example of a more primitive lava having an mg of 0 63 with 8 5 feo at 8 mgo cr spinel and glass melt inclusions in plagioclase together with zoned olivine give evidence for a range of parental melts in the recent shallow magmatic system with mg as high as 0 62 extensive shallow magma mixing is required to explain these variations in mineral and inclusion chemistry indicating more thermal variation within the shallow magmatic system than is implied by the relatively monotonous whole rock compositions apparently fernandina s shallow magma chamber has physically blocked passage of more primitive magmas to the surface and homogenized the erupted lavas by extensive mixing small variations in lava isotopic and compatible element composition combined with the need to resupply melt to maintain thermal balance indicate that replenishment of the magma chamber is semicontinuous with resupply occurring of the order of a decade or less plagioclase dominated evolved fernandina lavas are in sharp contrast to more primitive olivine dominated hawaiian lavas the total lack of primitive melts in the shallow fernandina magmatic system implies that substantial fractionation and heat loss occurred during melt transportation from the mantle source to the edifice magma chamber despite the fact that these melts were transported through relatively warm young and thin lithosphere adjacent to the cocos nazca ridge the diffuse nature of galapagos volcanism with 20th century eruptions from eight distinct volcanoes over a 20 000 km2 area further supports this interpretation and suggests that the galapagos plume is relatively diffuse weak and significantly less thermally intense than the vigorous hawaiian plume it may in fact be waning

deformation associated with gypsum karst in the salt river escarpment northeastern alberta an escarpment formed by the carbonate keg river and evaporitic chinchaga formations of middle devonian age extends southwards from the slave river 30 km west of fort smith northwest territories newly described folds in the bedrock are due to hydration of the anhydrite to gypsum in the chinchaga formation underlying the escarpment local groundwater flow has also dissolved subsurface cavities in the chinchaga formation as these openings grew the carbonates of the keg river formation subsided along stepped normal faults depressions within the fault blocks occurred where subsidence was concentrated along joints