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

successions of invertebrate microfossils and the late quaternary deglaciation of the central st lawrence lowland canada and united states abstract foraminiferal and ostracode assemblages in late glacial and early post glacial sediments of the central st lawrence lowland show that a proglacial lake glacial lake st lawrence preceded the champlain sea in some parts of the lowland and that the champlain sea was in contact with the laurentide ice sheet during deglaciation of other parts of the lowland glaciolacustrine and glaciomarine environments were followed by marine environments with salinity as high as 24 34 the higher salinity water in the champlain sea was part of a salt water wedge extending from the gulf of st lawrence into the central st lawrence lowland the beginning of the champlain sea is placed between 11 4 and 11 6 ka bp on the basis of radiocarbon ages for the arrival of the salt water wedge in the central st lawrence lowland the distribution of rhythmites associated with glacial lake st lawrence shows that subaqueous outwash fans in the ottawa area were deposited in glacial lake st lawrence rather than in the champlain sea

magma reservoir response to transient recharge events the case of santorini volcano greece igneous bodies are built incrementally by episodic magma injection events that vary in rate and duration such recharge events can either grow an upper crustal magma reservoir or trigger an eruption this bifurcation in behavior remains poorly constrained but is essential to volcanic hazard assessment here we use a numerical model that couples the thermal and mechanical processes in a magma reservoir to study the evolution of the santorini magmatic system greece over the past 20 k y our results constrain the recharge rate and duration that are necessary to trigger an eruption for the known long term average inflow rate of 10 3 km3 yr the size of the chamber and its exsolved volatile content are dominant controls on the critical recharge rate and duration that will trigger an eruption our model successfully reproduces the main features of the minoan eruption and nea kameni activity providing volume estimates for the active part of the current subvolcanic reservoir as well as information regarding the presence of exsolved volatiles thermomechanical models offer a new framework to integrate the historic eruption record with geodetic measurements and provide a context to understand the past present and future of active volcanic centers

thermal evolution of convergent plate margins thermal modeling and reevaluation of isotopic ar ages for blueschists in the franciscan complex of california modeling of heat flow through a wedge shaped accretionary complex emplaced beneath the overriding plate at a convergent plate margin provides insight into the tectonic significance of ar isotopic ages and metamorphic progressions across blueschist belts if convergence rates are maintained at rates of a few centimeters per year or more material recrystallized at depth where the accretionary wedge is narrow can be rapidly heated to high temperatures and then rapidly cooled to the low temperatures where ar loss by diffusion and recrystallization effectively ceases maximum temperatures are lower and the periods of heating and cooling are longer at shallower levels where the wedge is wider because of this difference in thermal history k ar and 40ar 39ar ages from blueschists recrystallized in the upper part of the accretionary wedge can be younger by several tens of million years than ages from material initially recrystallized at the same time but at greater depth application of this thermal modeling to the franciscan subduction complex of northern california indicates that the coherent blueschists in the eastern belt typically with 115 to 125 m y ar isotopic ages and the high grade blueschist blocks in the mud matrix melanges of the western portion of the central belt typically with 140 to 150 m y ar isotopic ages could both have formed during a single prolonged period of continuous convergence

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

mechanisms for transition in eruptive style at a monogenetic scoria cone revealed by microtextural analyses lathrop wells volcano nevada u s a explosive activity at lathrop wells volcano nevada u s a originated with weak strombolian ws eruptions along a short fissure and transitioned to violent strombolian vs activity from a central vent with lava effusion during both stages the cause for this transition is unknown it does not reflect a compositional change as evidenced by the consistent bulk geochemistry of all the eruptive products however comparison of agglutinate samples from the early ws events with samples of scoria from the later vs events reveal differences in the abundance and morphology of groundmass phases and variable textures in the rims of olivine phenocrysts scanning electron microscope sem examination of thin sections from the ws samples show euhedral magnetite microlites in the groundmass glass and olivine phenocrysts show symplectite lamellae in their rims secondary ion mass spectrometry sims depth profiles of these symplectites indicate they are diffusion controlled the calculated dfe mg allows an estimation of the oxygen fugacity fo2 and indicates an increased fo2 during eruption of the ws products conversely the vs samples show virtually no magnetite microlites in the groundmass glass a lack of symplectites in the olivines and a lower calculated fo2 these microtextural features suggest that the lathrop wells trachybasalt experienced increased oxidation during ws activity as magma ascended through the original fissure exsolved bubbles were concentrated in the wider part s the protoconduit and this bubble flux drove convective circulation that oxidized the magma through exposure to atmosphere and recirculation this oxidation resulted in groundmass crystallization of magnetite within the melt and formation of symplectites within the olivine phenocrysts bubble driven convection mixed magma vertically within the protoconduit keeping it fluid and driving strombolian bursts while microlite crystallization in narrower parts of the fissure helped to focus flow development of a central conduit increased the magma ascent velocity due to a greater product volume in the later eruptive stages and caused the shift in eruption intensity consequently variations in microtextures of the lathrop wells products reveal how a combination of fluid dynamic and crystallization processes in the ascending magma resulted in different styles of activity while the products maintained a consistent bulk composition

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

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

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

the late quaternary geology of the chalk river region ontario and quebec glacial ice covered the chalk river area through most of the wisconsin stage about 11 300 years ago an ice retreat was followed immediately by a short incursion of champlain sea waters which deposited at least 2 5 m of clay silt and sand a local readvance probably associated with the st narcisse event deposited till on the marine sediments following the final retreat of the ice from the area lacustrine and aeolian deposition occurred locally for a short time about 10 500 years ago the north bay drainage route opened greatly increasing the discharge of the ottawa river a faint terrace at a present elevation of 209 m probably formed at this time changes in the drainage routes of proglacial lakes and in the rate of ice retreat caused a general decrease in discharge rates and resulted in the formation of pronounced terraces now at 180 160 and 129 m and fainter terraces at 170 141 and 137 m by about 5000 years bp the north bay outlet closed and the river fell to approximately 111 m it

geochemistry of the active azufre planchon peteroa volcanic complex chile 35 15 s evidence for multiple sources and processes in a cordilleran arc magmatic system

from quiescence to unrest 20 years of satellite geodetic measurements at santorini volcano greece periods of unrest at caldera forming volcanic systems characterized by increased rates of seismicity and deformation are well documented some can be linked to eventual eruptive activity while others are followed by a return to quiescence here we use a 20 year record of interferometric synthetic aperture radar insar and gps measurements from santorini volcano to further our understanding of geodetic signals at a caldera forming volcano during the periods of both quiescence and unrest with measurements spanning a phase of quiescence and slow subsidence 1993 2010 followed by a phase of unrest january 2011 to april 2012 with caldera wide inflation and seismicity mean insar velocity maps from 1993 2010 indicate an average subsidence rate of 6 mm yr over the southern half of the intracaldera island nea kameni this subsidence can be accounted for by a combination of thermal contraction of the 1866 1870 lava flows and load induced relaxation of the substrate for the period of unrest we use a joint inversion technique to convert insar measurements from three separate satellite tracks and gps observations from 10 continuous sites into a time series of subsurface volume change the optimal location of the inflating source is consistent with previous studies situated north of nea kameni at a depth of 4 km however the time series reveals two distinct pressure pulses the first pulse corresponds to a volume change v within the shallow magma chamber of 11 56 0 14 106 m3 and the second pulse has a v of 9 73 0 10 106 m3 the relationship between the timing of these pulses and microseismicity observations suggests that these pulses may be driven by two separate batches of magma supplied to a shallow reservoir we find no evidence suggesting a change in source location between the two pulses the decline in the rates of volume change at the end of both pulses and the observed lag of the deformation signal behind cumulative seismicity suggest a viscoelastic response we use a simple model to show that two separate pulses of magma intruding into a shallow magma chamber surrounded by a viscoelastic shell can account for the observed temporal variation in cumulative volume change and seismicity throughout the period of unrest given the similarities between the geodetic signals observed here and at other systems this viscoelastic model has potential use for understanding behavior at other caldera systems

the link between volcanism and tectonics in the southern volcanic zone of the chilean andes a review abstract the nature of the interplay between tectonics and volcanism is a major question in continental margin tectonics the southern andes volcanic zone svz located at the obliquely convergent nazca south america plate margin between 33 s and 46 s offers a unique opportunity to address this question because of along strike changes in crustal thickness tectonic style and well constrained long term and short term kinematic history the complex interaction between tectonic and magmatic processes is evidenced by both the architecture and geochemical signature of volcanic systems main first order factors accounting for the along strike variations in the nature and composition of volcanism are crustal thickness and the existence of a major intra arc fault system the liquine ofqui fault zone lofz second order factors include the local nature of the volcanic arc basement two main categories of volcano tectonic associations have been identified according to the spatial distribution and internal organization of individual volcanoes and clusters of volcanoes with respect to both the overall strike of the volcanic arc and the first and second order active inactive basement faults kinematically coupled associations those that are directly related to the current dextral transpressional tectonic regime include ne trending volcanic alignments of stratovolcanoes and or monogenetic cones kinematically uncoupled associations in which coupling with the current regime is not required include stratovolcanoes built on top of ancient reverse and strike slip faults of the volcanic arc basement and monogenetic cones lying along master faults of the lofz whereas mostly primitive magmas are found in the first category more evolved terms can occur genetically associated with the second category we propose a simplified three dimensional model for the magma plumbing system along and across the svz that integrates kinematically coupled and kinematically uncoupled volcano tectonic associations within the framework of first and second order geological and structural constraints

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

rates of magma differentiation and emplacement in a ballooning pluton recorded by u pb tims tea adamello batholith italy abstract geochemical structural field and geochronological data have been used to arrive at very different models for the construction of upper crustal batholiths models for pulsed intrusion of small magma batches over long timescales 1 ma versus transfer of larger magma bodies on shorter timescales predict a different thermal metamorphic and rheological state of the crust highlighting the importance of robust time constraints this study focuses on a well characterized upper crustal intrusion the 15 km2 lago della vacca complex lvc adamello batholith n italy previous studies used structural and petrologic data to argue that the lvc was emplaced through pulsed magma injection and in situ expansion or ballooning of a short lived 105 yr magma chamber we test this model using a dense sampling strategy and high precision id tims u pb geochronology of zircon and titanite combined with hafnium isotope and trace element analyses of the same volume of dated mineral u pb tims tea these data show that the marginal mafic pulses of magma crystallized zircon with primitive hf isotopes and negligible eu anomalies during fractional crystallization and ascent through the crust on 10 30 ka timescales subsequent more felsic pulses yield individual zircon dates spanning as much as 200 ka within single handsamples and restrict the total construction time of the lvc to

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

thermomechanical behavior of large ash flow calderas the fundamental thermal and mechanical processes that occur within the ash flow caldera magma chamber systems remain largely enigmatic to date the only models of caldera collapse are simple mostly elastic or viscoelastic mechanical models that can predict some of the conditions preceding the collapse they cannot however predict the collapse itself because they are incapable either of reproducing the formation of faults or of accounting for the brittle ductile transitions and fault related thermal anomalies we have constructed analytical and numerical therrnomechanical that account for both elastic plastic ductile rheology and physical properties of the caldera rocks the overpressured magma is evacuated through a central vent and transforms into ash flow units deposited within the forming caldera brittle deformation faulting and subsequent collapse of the structure are reproduced the results show that in the absence of a regional stress field the collapse on both sides will occur only for aspect ratios i e caldera diameter to the depth of the magma chamber exceeding 5 and that internal embedded faults may also appear when the aspect ratio exceeds 10 thermal conductivity contrasts in ash flow calderas give rise to strong heat refraction that localizes deep seated thermal anomalies to the outer sides of the faults in the presence of regional extension the border faulting can be attenuated or disappear and the faults tend to localize around the central part of the chamber roof coupled therrnomechanical modeling suggests that the outer sides of the border faults have high trapping capabilities for hydrothermal fluids the geometry of the brittle ductile transition largely controls that of the fractured zones within and around the chamber roof thus justifying a new mechanical definition of magma chamber geometry

a tectonostratigraphic synthesis of the sub andean basins implications for the geotectonic segmentation of the andean belt the tectonic structural and depositional history of 19 sub andean foreland and eight southern caribbean basins has been synthesized and is summarized by regional geohistory charts these charts recapitulate the subsidence regimes recognized and have been used to evaluate correlations between genetically related stratigraphic sequences based on spatial and temporal changes in palaeo depositional setting the sub andean region can be subdivided longitudinally into several tectonostratigraphic domains the differential amount of subsidence between two adjacent tectonostratigraphic provinces or sub provinces relies on the presence of a transverse zone of structural accommodation the location of these transfer zones correlates with a number of known and several newly identified zones of intracontinental deformation combined with changes in the structural geometry of the andean fold and thrust belt and the relative dominance of basement fault systems along the length of the andean foreland it is suggested that the andean belt can be separated into five tectonic domains which are themselves bounded by transverse structural accommodation zones representing the multiphase reactivation of pre existing basement fault systems the influence of these broad zones of transcontinental deformation is most apparent during the triassic jurassic period suggesting that they were intimately associated with accommodating intraplate stresses during the breakup of the gondwana supercontinent

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

evolution of shallow and deep structures along the maipo tunuyan transect 33 40 s from the pacific coast to the andean foreland fil giambiagi laura beatriz consejo nacional de investigaciones cientificas y tecnicas cientifico tecnologico mendoza instituto argentino de nivologia glaciologia y ciencias ambientales argentina

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

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

mafic monogenetic vents at the descabezado grande volcanic field 35 5 s 70 8 w the northernmost evidence of regional primitive volcanism in the southern volcanic zone of chile in the andean southern volcanic zone svz the broad distribution of mafic compositions along the recent volcanic arc occurs mainly south of 37 s above a comparatively thin continental crust 35 km and mostly associated with the dextral strike slip regime of the liquine ofqui fault zone lofz north of 36 s mafic compositions are scarce this would be in part related to the effect resulting from protracted periods of trapping of less evolved ascending magmas beneath a thick meso cenozoic volcano sedimentary cover that lead to more evolved compositions in volcanic rocks erupted at the surface here we present whole rock and olivine mineral chemistry data for mafic rocks from four monogenetic vents developed above a svz segment of thick crust 45 km in the descabezado grande volcanic field 35 5 s whole rock chemistry mgo 8 wt and compositional variations in olivine 92 fo 88 and ni up to 3650 ppm indicate that some of the basaltic products erupted through these vents e g los hornitos monogenetic cones represent primitive arc magmas reaching high crustal levels the combined use of satellite images regional data analysis and field observations allow to recognize at least 38 mafic monogenetic volcanoes dispersed over an area of about 5000 km2 between 35 5 and 36 5 s a link between ancient structures inherited from pre andean tectonics and the emplacement and distribution of this mafic volcanism is suggested as a first order structural control that may explain the widespread occurrence of mafic volcanism in this andean arc segment with thick crust

crustal extensional faulting triggered by the 2010 chilean earthquake the pichilemu seismic sequence 1 we report a sequence of crustal quakes that began after the mw 8 8 thrust subduction maule earthquake that affected the central chile margin on 27 february 2010 this activity lasted by several months having the most important events on 11 march 2010 mw 6 9 and mw 7 0 with normal focal mechanisms seismicity shows a rupture oriented along a nw striking and sw dipping normal fault from the surface down to the interplate contact seismicity can be correlated with neotectonics extensional structures similarly oriented in the region which have coexisted with nne ssw reverse faults since the late pliocene even though both have older periods of activity since the paleozoic this crustal rupture would have been triggered by the high coulomb stress change produced by the maule earthquake enhanced by likely fluid presence along weakened zones of the forearc crust as evidenced by high vp vs ratio the occurrence of relevant neotectonic activity in coincidence with short term deformation suggests a relationship with long term tectonic features of this region which would have been acting as a barrier during the interseismic period increasing the strain accumulation and triggering contractional faulting in the crust as well as producing high slip patches during great subduction ruptures favoring triggering of crustal extensional faulting crustal faulting in pichilemu suggests that this kind of events should be considered in seismic hazard analysis despite the absence of historical crustal seismic activity before the maule earthquake

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

cooling dominated cracking in thermally stressed volcanic rocks most studies of thermally induced cracking in rocks have focused on the generation of cracks formed during heating and thermal expansion both the nature and the mechanism of crack formation during cooling are hypothesized to be different from those formed during heating we present in situ acoustic emission data recorded as a proxy for crack damage evolution in a series of heating and cooling experiments on samples of basalt and dacite results show that both the rate and the energy of acoustic emission are consistently much higher during cooling than during heating seismic velocity comparisons and crack morphology analysis of our heated and cooled samples support the contemporaneous acoustic emission data and also indicate that thermal cracking is largely isotropic these new data are important for assessing the contribution of cooling induced damage within volcanic structures and layers such as dikes sills and lava flows

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

localization of volcanic activity 2 effects of pre existing structure abstract the capture of vertically rising dikes by faults is examined with a combination of an analytic solution numerical simulations and field observations one of the numerical solutions and the field observations also provide insight into the intrusion of sills into the hanging wall of faults that are reactivated when lubricated by magma the approximate analytic solution indicates that dike capture by a fault will be limited primarily to steeply dipping faults capture will become easier meaning that it will be possible for faults with shallower dips as the intersection of the dike with the fault occurs at smaller depths capture will also be easier as it is harder for crack to grow in the hanging wall this may be interpreted in terms of a higher tensile strength or in terms of shorter pre existing cracks or higher fracture toughness numerical analysis using a discrete element code confirms this conclusion but also shows that the analytic solutions overestimate the difficulty of intruding a pre existing fault the maximum deviation occurring for faults dipping at 45 this finding that dike capture by faults should be restricted to high angle faults and shallow depths is supported by field observations of exhumed miocene basaltic volcanic terrain in central nevada usa differences in style seen in other regions are attributed to both the rock properties and the regional stress field the numerical solutions also indicate that synintrusive fault slip is likely to lead to formation of sills in the hanging wall at depths of a few hundred meters or less a finding that is also consistent with the field observations in nevada

comparative mass balance of volcanic edifices at the southern volcanic zone of the andes between 33 s and 46 s volcano edifice volume calculations are presented for 65 volcanoes of the 1400 km long chilean southern volcanic zone svz as a basic step in subduction zone mass budgets volume calculations are performed in a geographical information system that integrates digital elevation models based on of shuttle radar topography mission as well as aster gdem topographic data landsat satellite images and geological maps the method of volume calculation is straightforward for isolated morphologically well defined stratovolcanoes uncertainties increase for volcanic edifices that formed on pre existing rugged terrain for multi phase eruptive centers as well as for eroded edifices a revised segmentation of the arc is used to describe the spatial volume distribution of extruded magma along the svz and to discuss controlling tectonic factors peak volumes between arc and back arc are offset by 400 km the total volcanic extrusion is in the range of 10 13 km3 km ma major differences between the svz and the central american subduction system are notable with regard to volcano density and maximum volumes

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

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

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

natural equivalents of thermal gradient experiments abstract crystallization experiments using the intrinsic thermal gradient in 10 mm length capsules loaded in piston cylinder assemblies were used to investigate silicic magma crystallization the application of experimental results to natural environments requires the scaling of physical parameters of petrological interest therefore we propose here a comparative study between thermal gradients and numerical simulations of natural magma chambers we use the finite element method to calculate thermal profiles across a cooling silicic magma chamber these numerical profiles are compared with the intrinsic thermal structure of half inch piston cylinder assemblies at 500 mpa it is concluded that a set of varied magma chamber geometries and or distinct stages of their cooling history can approach the intrinsic thermal structure of laboratory experiments once the thermal properties for magma and its host rock are fixed the experimental numerical approach is mostly dependent on the volume and aspect ratio of the magma chamber our results indicate for instance that a 10 mm length capsule with a thermal gradient of 40 c mm from 1100 to 700 c may represent a 150 1100 m wide portion of a cooling magma chamber of 10 20 km diameter and 2 10 km height emplaced at a depth of 18 km additional possible scenarios are represented by larger magma chambers up to 30 km diameter in which the experimental thermal gradient can represent a 150 3700 m thin section of the large igneous bodies

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

the late jurassic to present evolution of the andean margin drivers and the geological record 1 uncommonly long lived subduction and variable plate geometry along the south american andean plate margin resulted in diverse relationships between magmatic flux and extensional and contractional deformation as recorded by the overriding continental plate convergence velocities absolute overriding plate velocities and subducting slab ages were resolved along the trench from 170 ma to the present using a recently developed kinematic global plate model to identify any relationship between subduction conditions deformation style and magmatic features in the overriding plate key correlations reflect the dependence of macroscopic crustal strain style on subduction mechanism and relative plate vectors extensional back arc basins involving mafic oceanic crust developed only when the overriding plate velocity of south america was directed away from the trench and the modeled age of the subducting slab was older than 50 myr the development of fold and thrust belts and uplift of major plateaus was accompanied by trench normal convergence velocities in excess of 4 cm yr parameters investigated in this study revealed no correlation with the timing of major magmatic events nor was any correlation observed with the structural style of fold and thrust belts

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

40ar 39ar evidence for early deglaciation of the central chilean andes spatial variability of deglaciation and warming since the last glacial maximum is critical for relating hemispheric or regional climate forcing mechanisms to global orbitally induced insolation changes radiocarbon ages of moraines ice cores and deep sea sediments from the southern hemisphere suggest that initial warming and glacial recession began before 23 000 years ago but that in latitudinal belts of concentrated precipitation glaciers readvanced 18 000 and 13 000 years ago 40 ar 39 ar dating of glaciated and unglaciated lava flows 2200 m in elevation at laguna del maule central chilean andes implies that at 36 s the southern andean ice cap retreated between 25 600 1 200 and 23 300 600 years ago and did not expand afterwards these ages are reconciled if increased insolation ablated sea ice cover and mountain glaciers in the southern hemisphere 5 000 years before the northern and if subsequently laguna del maule lay north of the westerly winds delivering moisture to more southerly 39 52 s portions of the ice cap

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

magmatic tectonic effects of high thermal regime at the site of active ridge subduction the chile triple junction model abstract high thermal gradients are expected to be found at sites of subduction of very young oceanic lithosphere and more particularly at ridge trench trench rtt triple junctions where active oceanic spreading ridges enter a subduction zone active tectonics associated with the emplacement of two main types of volcanic products 1 morb type magmas and 2 calc alkaline acidic magmas in the forearc also characterize these plate junction domains in this context morb type magmas are generally thought to derive from the buried active spreading center subducted at shallow depths whereas the origin of calc alkaline acidic magmas is more problematic one of the best constrained examples of ridge trench interaction is the chile triple junction ctj located southwest of the south american plate at 46 12 s where the active chile spreading center enters the subduction zone in this area there is a clear correlation between the emplacement of magmatic products and the migration of the triple junction along the active margin the ctj lava population is bimodal with mafic to intermediate lavas 48 56 sio 2 and acidic lavas ranging from dacites to rhyolites 66 73 sio 2 previous models have shown that partial melting of oceanic crust plus 10 20 of sediments leaving an amphibole and plagioclase rich residue is the only process that may account for the genesis of acidic magmas due to special plate geometry in the ctj area a given section of the margin may be successively affected by the passage of several ridge segments we emphasize that repeated passages will lead to the development of very high thermal gradients allowing melting of rocks of oceanic origin at temperatures of 800 900 c and low pressures corresponding to depths of 10 20 km depth only in addition the structure of the ctj forearc domain is dominated by horizontal displacements and tilting of crustal blocks along a network of strike slip faults the occurrence of such a deformed domain implies that an important tectonic coupling may exist between the upper and the lower plates leading to the partitioning of the continental lithosphere and to the tectonic underplating of very young oceanic lithosphere below the continental wedge we assume that in the case of the ctj the uncommon situation of three successive ridge segments entering the trench at 2 3 ma intervals only resulted in a strong and finally long lived thermal anomaly this anomaly caused remelting of underplated portions of very young still hot oceanic lithosphere only particular geometrical rtt configurations are able to produce such features these include linear continental margin short ridge segments slightly oblique to the trench and short transform faults finally the ctj example shows that a possible scenario for the origin of calc alkaline acidic rocks in the near trench region involves coeval tectonic coupling and repeated passage of thermal anomalies due to successive subduction of short ridge segments therefore the local abundance of calc alkaline acidic rocks associated with morb type lavas in ancient series could be the tracer of plate tectonic configurations involving the subduction of short ridge segments in a relatively short duration

tectonic controls on the karliova triple junction turkey implications for tectonic inversion and the initiation of volcanism abstract few places on earth are tectonically as active as the karliova region of eastern turkey which comprises a triple junction ktj triple junctions result in complex kinematic and mechanical interactions within the lithosphere generating tectonic inversions and uplift extensive seismicity and volcanism here we present new data and summarize existing data on the tectonic evolution of the ktj in eastern turkey over the past 6 ma in particular we present a kinematic model for the ktj and the surrounding area as well as new structural maps the deformation or strain rate has varied over this 6 million year period the maximum strain rate occurred between 6 ma and 3 ma a period that coincides with the initiation of activity in varto volcano we suggest that increased strain rate and the initiation of activity at the varto volcano may be tectonically related subsequent to its formation the varto volcano was dissected by active faults associated with the varto fault zone including reverse normal and strike slip faults during the past 3 ma however the ktj area was deformed dominantly through dextral crustal movements associated to right lateral faults this deformation resulted in the development of a ne sw trending extensional transtensional regime together with a complementary nw se trending contractional regime in the past 6 ma the east end of the ktj has been subjected to incremental deformation this deformation has resulted in many episodes of faulting during i ongoing shortening phases driven by a regional scale thrust tectonic regime and ii local scale transtensional phases caused by westward extrusion

monogenetic volcanism personal views and discussion monogenetic volcanism produces small volume volcanoes with a wide range of eruptive styles lithological features and geomorphic architectures they are classified as spatter cones scoria or cinder cones tuff rings maars maar diatremes and tuff cones based on the magma water ratio dominant eruption styles and their typical surface morphotypes the common interplay between internal such as the physical chemical characteristics of magma and external parameters such as groundwater flow substrate characteristics or topography plays an important role in creating small volume volcanoes with diverse architectures which can give the impression of complexity and of similarities to large volume polygenetic volcanoes in spite of this volcanic facies complexity we defend the term monogenetic volcano and highlight the term s value especially to express volcano morphotypes this study defines a monogenetic volcano a volcanic edifice with a small cumulative volume typically 1 km3 that has been built up by one continuous or many discontinuous small eruptions fed from one or multiple magma batches this definition provides a reasonable explanation of the recently recognized chemical diversities of this type of volcanism

interaction of ascending magma with pre existing crustal fractures in monogenetic basaltic volcanism an experimental approach 1 magma transport through dikes is a major component of the development of monogenetic volcanic fields these volcanic fields are characterized by numerous volcanic centers each typically resulting from a single eruption therefore magma must be transported from source to surface at different places which raises the question of the relative importance of 1 the self propagation of magma through pristine rock and 2 the control exerted by pre existing fractures to address this issue we have carried out a series of analogue experiments to constrain the interaction of a propagating dike through a medium with pre existing fractures the experiments involved the injection of air into an elastic gelatin solid which was previously cut into its upper part to simulate pre existing fractures the volume of the dikes their distance from the fractures and the ambient stress field were systematically varied to assess their influence on potential dike fracture interactions the results show that distance and angle between dikes and fractures influence these interactions and the dike trajectory dike geometry and dynamics are also affected by both the presence of the fractures and the dike volume dikes propagating in between fractures tend to decelerate in nature interactions are expected for dikes and fractures separated by less than about 200m and dikes with a volume less than about 10 2 km 3 would experience a velocity decrease these results highlight the influence of pre existing fractures on the mechanics and dynamics of dikes these heterogeneities must be considered when studying the transport of magmas within the crust

anatomy of the andean subduction zone three dimensional density model upgraded and compared against global scale models summary we present an upgraded version of a previously published 3 d density model of the andean subduction zone between 18 s and 45 s this model consists of 3 d bodies of constant density which geometry is constrained by independent seismic data and is triangulated from vertical cross sections these bodies define the first order morphology and internal structure of the subducted nazca slab and south american plate the new version of the density model results after forward modelling the bouguer anomaly as computed from the most recent version of the earth gravitational model egm2008 the 3 d density model incorporates new seismic information to better constrain the geometry of the subducted slab and continental moho cmh and has a trench parallel resolution doubling the resolution of the previous model as an example of the potential utility of our model we compare the geometry of the subducted slab and cmh against the corresponding global models slab1 0 and crust2 0 respectively this exercise demonstrates that although global models provide a good first order representation of the slab and upper plate crustal geometries they show large discrepancies up to 40 km with our upgraded model for some well constrained areas the geometries of the slab lithosphere asthenosphere boundary below the continent cmh and intracrustal density discontinuity that we present here as supporting information can be used to study andean geodynamic processes from a wide range of quantitative approaches

on the longevity of large upper crustal silicic magma reservoirs understanding the processes involved in the formation and maturation of upper crustal magma reservoirs ultimately sourcing the largest volcanic eruptions on earth is one of the most fundamental aspects of volcanology while such reservoirs are known to assemble incrementally over extended periods of time debate persists regarding the time scales of melt preservation in the cold upper crust if rapid cooling individually freezes incoming replenishing intrusions accumulations of eruptible magma are impossible precluding the construction of voluminous volcanic reservoirs for all but the highest magma emplacement rates recent numerical thermal models have been used to assess the viability of upper crustal silicic magma survival and have suggested that supervolcanic reservoirs must form on geologically short time scales with anomalously high injection rates and subsist only ephemerally making their long term evolution less predictable motivated by geological observations suggesting the contrary we have improved upon these models by incorporating two fundamental features of natural systems not previously considered 1 a nonlinear crystallization temperature relationship adapted for upper crustal silicic magmas and 2 a temperature dependent thermal conductivity we demonstrate that the incorporation of both of these properties can allow an upper crustal reservoir to remain above its solidus for hundreds of thousands of years when fed by magma fluxes typical of large magmatic provinces while the crystallization temperature path plays the most significant role in maintaining a large pool of eruptible magma the incorporation of temperature dependent thermal properties significantly extends the lifetime of such reservoirs furthermore while deeper emplacement levels e g 10 km depth can both extend magma survival and increase melt availability we show that a 5 km depth can also provide an adequate magma storage environment these results provide strong support for long lived upper crustal mushes as a staging ground for accumulation of highly eruptible crystal poor silicic magmas and further assert the evolutionary link between volcanic and plutonic systems

anatomy and global context of the andes main geologic features and the andean orogenic cycle the andes make up the largest orogenic system developed by subduction of oceanic crust along a continental margin subduction began soon after the breakup of rodinia in late proterozoic times and since that time it has been intermittently active up to the present the evolution of the pacifi c margin of south america during the paleozoic occurred in the following stages 1 initial proterozoic rifting followed by subduction and fi nal re amalgamation of the margin in early cambrian times as depicted by the puncoviscana and tucavaca basins and related granitoids in southern bolivia and northern argentina 2 a later phase of rifting in the middle cambrian and subsequent collisions in middle ordovician times of parautochthonous terranes derived from gondwana such as paracas arequipa and antofalla and exotic terranes originating in laurentia such as cuyania chilenia and chibcha 3 fi nal permian collision between south america and north america to form pangea during the alleghanides orogeny leaving behind rifted pieces of laurentia as the tahami and tahuin terranes in the northern andes and other poorly known orthogneisses in the cordillera real of ecuador in the late permian early triassic and 4 amalgamation of the mejillonia and patagonia terranes in early permian times representing the last convergence episodes recorded in the margin during the gondwanides orogeny these rifting episodes and subsequent collisions along the continental margin were the result of changes of the absolute motion of gondwana related to global plate reorganizations during proterozoic to paleozoic times generalized rifting during pangea breakup in the triassic concentrated extension in the hanging wall of the sutures that amalgamated the paleozoic terranes the opening of the indian ocean in early jurassic times was associated with a new phase of subduction along the continental margin the northeastward absolute motion of western gondwana produced a negative trench roll back velocity that controlled subduction under an extensional regime until late early cretaceous times the northern andes of venezuela colombia and ecuador record a series of collisions of island arcs and oceanic plateaus from the early cretaceous to the middle miocene as a result of interaction with the caribbean plate the remaining central and southern andes record periods of orogenesis and mountain building alternating with periods of quiescence and absence of deformation as recorded in parts of the oligocene based on the generalized occurrence of fl at slab subduction episodes through time as recorded in most ramos v a 2009 anatomy and global context of the andes main geologic features and the andean orogenic cycle in kay s m ramos v a and dickinson w r eds backbone of the americas shallow subduction plateau uplift and ridge and terrane collision geological society of america memoir 204 p 31 65 doi 10 1130 2009 1204 02 for permission to copy contact editing geosociety org 2009 the geological society of america all rights reserved

curie point depth map of turkey summary a curie point isotherm map of the whole of turkey has been prepared from aeromagnetic data by means of a spectral analysis technique the most characteristic signature of this map is that the shallow depths generally correlated with the young volcanic rocks and the thinned crustal thicknesses in the west reveal geothermal fields and several hot springs the orogenic pontide belt of the black sea region in the north the western taurus belt in the south and some high plateaus in eastern turkey show that the deepest curie isotherm depths range between 20 and 29 km depth contour lines elongating in an almost e w direction are due to the overlapping percentage of adjacent data blocks of 50 per cent in the e w direction and to magnetic anomalies trending in the e w direction curie isotherm depths are in the range 6 to 10 km in the aegean region and are characterized by e w and ne sw trending graben structures as expected since the region has crustal thinning and many hot springs and geothermal fields these imply the existence of shallow heat sources that could be utilized for electricity heating of towns and cities greenhouse and health purposes

neogene patagonian plateau lavas continental magmas associated with ridge collision at the chile triple junction extensive neogene patagonian plateau lavas 46 5 to 49 5 s southeast of the modern chile triple junction can be related to opening of asthenospheric slab windows associated with collisions of chile rise segments with the chile trench at 12 ma and 6 ma support comes from 26 new total fusion whole rock 40ar 39ar ages and geochemical data from back arc plateau lavas in most localities plateau lava sequences consist of voluminous tholeiitic main plateau flows overlain by less voluminous 2 to 5 million year younger alkalic postplateau flows northeast of where the ridge collided at 12 ma most lavas are syncollisional or postcollisional in age with eruptions of both sequences migrating northeastward at 50 to 70 km ma plateau lavas have ages from 12 to 7 ma in the western back arc and from 5 to 2 ma farther to the northeast trace element and isotopic data indicate main plateau lavas formed as larger percentage melts of a garnet bearing oceanic island basalt oib like mantle than postplateau lavas the highest percentage melts erupted in the western and central plateaus in a migrating slab window model main plateau lavas can be explained as melts that formed as upwelling subslab asthenosphere which flowed around the trailing edge of the descending nazca plate and then interacted with subduction altered asthenospheric wedge and continental lithosphere alkaline postplateau lavas can be explained as melts generated by weaker upwelling of subslab asthenosphere through the open slab window thermal problems of high pressure melt generation of anhydrous mantle can be explained by volatiles h2o and co2 introduced by the subduction process into slab window source region s an oib like rather than a mid ocean ridge basalt morb like source region and the lack of magmatism northeast of where ridge collision occurred at 13 to 14 ma can be explained by entrainment of weak plume s or regional variations in an ambient oib like asthenosphere

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

eruptive stratigraphy of the tatara san pedro complex 36 s southern volcanic zone chilean andes reconstruction method and implications for magma evolution at long lived arc volcanic centers

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

time scales of magmatic processes from modeling the zoning patterns of crystals the advent of polarized light microscopy in the middle of the 19th century allowed mineralogists and petrologists interested in igneous rocks to recognize the widespread occurrence of fine scale heterogeneities in the optical properties of minerals e g see young 2003 for details the interpretation of mineral zoning patterns as archives of magmatic processes has been with us for some time e g larsen et al 1938 tomkeieff 1939 the development of the electron microprobe in the 1960 s allowed mineral zoning profiles to be quantitatively analyzed and modeled e g bottinga et al 1966 moore and evans 1967 enhanced textural observations permitted recognition of many kinds of detailed structures during the growth and dissolution of magmatic minerals fig 1 e g anderson 1983 pearce and kolisnik 1990 and these processes were also explored using experimental and numerical models e g albarede and bottinga 1972 lofgren 1972 kirkpatrick et al 1976 loomis 1982 major element zoning patterns are routinely measured and with the arrival of the ion microprobe the identification of heterogeneous distribution of trace elements opened the window for more realistic and sophisticated scenarios and models fig 1 e g kohn et al 1989 blundy and shimizu 1991 singer et al 1995 we now measure abundances of naturally occurring isotopes at the scale of tens of micrometers e g davidson et al 2007a ramos and tepley 2008 and this allows in situ dating of crystals e g cooper and reid 2008 figure 1 a nomarski dic image of a plagioclase crystal from a gabbroic xenolith of volcan san pedro costa et al 2002 the crystal shows a well defined core and a rim separated by an oscillatory zoned interval ion microprobe analysis pits and the trace of the electron microprobe traverse between the pits are evident b major

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

magmatic processes under quizapu volcano chile identified from geochemical and textural studies quizapu is part of a linear system of active volcanos in central chile the volcanic petrology and geology have been used to infer the plumbing system beneath the volcano the 1846 1847 eruption 5 km3 started with small flows of dacite then changed to a range of andesite dacite compositions and finally terminated with large flows of dacite andesitic enclaves 10 occur in some of these flows activity restarted explosively in 1932 4 km3 dre with an initial andesite dacite ash followed by uniform dacite ash and then a terminal andesite ash all samples including the enclaves have chemical compositions that lie on an almost perfect mixing line with a few exceptions the abundant plagioclase macrocrysts in the matrix were divided into five petrographic classes on the basis of colour in cold cathode cathodoluminescence images and zonation in visible light all populations of macrocrysts have csds characteristic of coarsening although they differ in detail two classes can be ascribed to growth in andesite and dacite magmas but the three other classes are associated with particular magma batches a model is developed which started with ponding of andesite magma in the crust this differentiated to produce a dacite magma most of which probably solidified to make a granodiorite batholith activation of a n s fault enabled volcanism andesite magma traversed the dacite filled chamber heating and raising it up into storage areas hosted by the fault where it mixed to form a homogeneous magma a short time before the 1846 1847 eruption more andesite magma was injected into the shallow part of the system where it mingled with existing mixed magmas the first magma to be erupted from quizapu was a dacite but soon other storage areas along the fault started to feed the system first mixed magmas then back to dacites the eruption then terminated until 1932 when renewed injection of andesite into the system created a conduit that tapped an undegassed dacite chamber and resulted in a strong explosive eruption the whole story is one of continual andesite magmatism modulated by storage degassing and mixing

volcanic subsidence triggered by the 2011 tohoku earthquake in japan the 2011 tohoku earthquake caused high levels of crustal deformation in japan analysis of satellite radar and gps data show that the earthquake caused nearby volcanic regions to subside instantaneously creating elliptical depressions that are parallel to the direction of quake induced crustal extension

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

volatile sulphur and chlorine major and trace element geochemistry of mafic to intermediate tephras from the chilean southern volcanic zone 33 43 s here we present the first systematic investigation of volatile geochemistry along the southern volcanic zone svz of chile holocene olivine hosted melt inclusions in the most mafic tephras sampled from 16 volcanoes along the volcanic front of the svz between 33 s and 43 s were analysed for pre eruptive sulphur chlorine and major element contents these results are combined with trace element compositions of the host whole rocks the highest fractionation corrected gas contents occur in the least degassed melt inclusions from small monogenetic cones of los hornitos cabeza de vaca and apagado from both the transitional and the southern central svz reaching 3 000 g g s and 1 400 g g cl while the lowest abundances of 1 100 g g s and 600 g g cl were found in the central svz at volcan lonquimay volcan llaima and volcan villarrica chlorine co varies with trace element indicators for the degree of melting and or source enrichment such that the lowest cl contents are found in high degree melts from the most depleted mantle sources the size of the volcanic edifices correlates inversely with cl abundances in the melt this could reflect more extensive degassing during ascent through the complex magma plumbing systems beneath the stratovolcanoes or greater dilution during larger degrees of melting of more depleted sources or a combination of these factors compared to other subduction zones the svz melt inclusions exhibit cl and s abundances in the same range as most of those from the central american and those from the marianas arcs

east anatolian plateau constructed over a continental basement no evidence for the east anatolian accretionary complex the east anatolian plateau turkey is extensively covered by neogene to quaternary volcanic sedimentary rocks and is characterized by an attenuated lithospheric mantle its pre neogene basement is commonly considered to consist entirely of late cretaceous to oligocene oceanic accretionary complexes formed at the junction of several continental blocks here we report on three main exposures of the pre neogene basement in this region the exposed areas consist mainly of amphibolite to granulite facies metamorphic rocks including marble amphibolite metapelite metagranite and metaquartzite an upper amphibolite to granulitefacies domain is equilibrated at similar to 0 7 gpa and similar to 800 degrees c at 83 2 ma 2 sigma u pb dating of magmatic zircons from the metagranite yielded a late ordovician early silurian protolith age 444 9 ma 2 sigma the detrital zircons from one metaquartzite point to a neoproterozoic early paleozoic provenance ophiolitic rocks tectonically sit on the metamorphic rocks both the metamorphic and ophiolitic rocks are in turn unconformably covered by lower maastrichtian clastic rocks and reefal limestones suggesting that the whole exhumation process and juxtaposition with the ophiolitic rocks had occurred by the early maastrichtian several lines of evidence such as 1 the absence of any indication of a former high pressure metamorphism in the metamorphic rocks 2 the allochthonous nature of the ophiolitic rocks 3 the presence of metagranite with a late ordovician early silurian protolith age and 4 the neoproterozoic early paleozoic provenance of detrital zircons in the metaquartzite in contrast to the dominance of late paleozoic mesozoic crystalline rocks in the adjacent continental blocks indicate a substantial component of continental basement beneath the neogene to quaternary cover thus the loss of the lithospheric mantle probably resulted from lithospheric foundering processes beneath the plateau rather than just slab steepening and break off