late jurassic bimodal magmatism in the northern sea floor remnant of the rocas verdes basin southern patagonian andes magmatic and detrital zircon ages from the rocas verdes basin a tectonically juxtaposed remnant of sea floor in the magallanes fold and thrust belt southern patagonia south america indicates that a rifting phase of the rocas verdes basin occurred between 152 and 142 ma and was accompanied by bimodal magmatism a dacite dyke cross cutting pillow basalt successions and a plagiogranite dyke in mixed mafic felsic terranes of the basal sarmiento ophiolite complex contain 150 ma zircon crystals indicating that mafic submarine volcanism had started prior to or during the late jurassic 10 15 ma earlier than previously thought the silicic pyroclastic rocks of the tobifera formation with two samples dated at 148 and 142 ma were heralded by synrift sedimentation along fault bounded grabens within palaeozoic metasediments no evidence for an active volcanic arc during the early formation of the rocas verdes basin was detected in detrital zircon grains of the lower sedimentary member of the tobifera formation a minimum of 25 ma of continuous sedimentation in the rocas verdes basin is suggested by detrital zircon grains in the upper member of the zapata formation the rocas verdes basin was rimmed on the western side by an incipient and subaerial magmatic arc only in its later evolution

have the southernmost andes been curved since late cretaceous time an analog test for the patagonian orocline the kinematic evolution of the enigmatic arc shaped southernmost andes of patagonia and tierra del fuego has been a subject of debate for most of the past century we compared the results from analog sandbox experiments with the tectonic evolution and actual configuration of the mountain chain in order to elucidate whether oroclinal bending took place during the tertiary or if the southernmost andes have been a curved orogen since at least late cretaceous time experiments simulating oroclinal rotation produced strong along strike variations in shortening and failed to account for structural data compiled from the fuegian andes results from experiments simulating an l shaped concave to foreland indenter were in agreement with the known tertiary structural evolution of the southernmost andes the diachronicity of principal shortening events previously recognized in patagonia and tierra del fuego could only be reproduced by moving the indenter in two successive orthogonal directions first approximately northward to form the fuegian fold and thrust belt and then approximately eastward to propagate thrusting in the patagonian andes this two phase evolution is consistent with a recorded change in the convergence direction of the farallon nazca plate that occurred at ca 27 ma

time dependent modeling of magma intrusion during the early stage of the 2000 miyakejima activity the high frequency gps analysis at miyakejima volcano captured rapid deformation following the early stage of miyakejima 2000 activity given a specified dike and spherical source geometry we solve the opening distribution on each sub dike that minimizes a linear combination of the norm of the weighted data residual and roughness of the opening distribution the time dependence of dike intrusion and spherical source deflation shows that magma migrated laterally from the southern region to the west coast although the depth of the maximum opening does not change with time a very shallow intrusion can be seen at the eastern part of the dike this may explain why cracks and graben structures developed on the west coast most of the magma migrated northwest with only a fraction coming to the surface in an eruption below sea level

change in seismic activity in the tokai region related to weakening and strengthening of the interplate coupling abstract we investigate the relationship between changes in seismicity and crustal deformations in the tokai region we describe how seismicity in the subducted slab increased remarkably in the fall of 2000 and decreased in the fall of 2001 while in contrast the crust seismicity decreased in the fall of 2000 and increased in the fall of 2001 we note that the trend of horizontal displacement at gps stations changed coincidentally and we propose interpreting the increase and decrease in seismic activities and the changes in crustal deformations in a unified way based on changes in the state of the interplate coupling i e the back slip rate was reduced in the fall of 2000 and was partially restored in the fall of 2001 we explain why reduction of the back slip rate increases seismogenic stress in the slab and decreases stress in the crust we also describe the substantial positive dilatation observed in the region around mt fuji in the fall of 2000 and suggest that the remarkable increase of low frequency earthquakes beneath mt fuji in october 2000 may have been caused by deceleration of the converging motion of the izu micro plate with the eurasian plate the decrease of the subduction velocity of the izu micro plate on the suruga trough in late 2000 would also have contributed to weakening of the interplate coupling beneath the tokai region since reduction of the relative velocity between overriding and subducting plates produces the same effect on the plate interface as a diminishing back slip rate however subduction of the izu micro plate on the suruga trough was accelerated in early 2003 which may have caused increases in both slab and crust seismicities in that period

absolute gravity change associated with magma mass movement in the conduit of asama volcano central japan revealed by physical modeling of hydrological gravity disturbances the gravity signal originating from magma mass movement in a volcanic conduit is retrieved from the hydrologically disturbed absolute gravity data obtained at asama volcano central japan in 2004 using a three dimensional hydrological model we improve the hydrological model of the previous study using realistic soil parameters and boundary conditions to better estimate the spatiotemporal land water distributions and the consequent hydrological gravity disturbances the newly estimated gravity disturbances agree with the absolute gravity values observed by fg5 gravimeters in 2004 2009 within about 2 6 gal by additionally accounting for the excess discharge of groundwater mass associated with a sloping impermeable surface below the discharge area after the gravity disturbance of 20 gal amplitude is subtracted from the absolute gravity data observed during the 2004 eruptive event the gravity residual of 5 gal amplitude shows a significant decrease in synchronization with eruptions because the ascending magma mass in the conduit affects the upward attraction force to the gravimeters installed on the flank of asama volcano the magma head altitude to which the residual gravity is converted assuming a homogeneous linear density in the conduit shows a comprehensive agreement of the time variation in the magma head with those in other volcanic observations such as gas emission rate and earthquake frequency by correcting the hydrological gravity disturbances using this hydrological model and simultaneously obtained meteorological data in real time spatiotemporal variations in the magma mass can be instantaneously monitored at asama volcano even before eruptions during future volcanic events

on the formation of eruption columns following explosive mixing of magma and surface water when magma vents into the sea or a crater lake the ensuing magma water interaction can affect the style of eruption dramatically if the mass of surface water incorporated into the erupting material is small 108 kg s the height progressively decreases with surface water content this occurs when the magma and surface water begin to constitute a significant fraction of the mass at the top of the column so that an increasing fraction of the initial magmatic thermal energy is converted to the surface water rather than the entrained air the transitions in eruption style which result from changes in the mass of surface water mixing with the magma may account for observations of both buoyant plumes and wet surge during the eruptions of taal in 1965 and miyake jima in 1983 and for the changes in the eruptive activity at surtsey in 1963 1964 as the access of seawater to the vent became more restricted we also present calculations which suggest that the accretionary lapilli which are often found in wet flow deposits may result from condensation of vapor in both the cold wet collapsing fountains and in the flows themselves

the april 2007 eruption and the dolomieu crater collapse two major events at piton de la fournaise la reunion island indian ocean abstract after 10 years of high activity and 24 eruptions piton de la fournaise pdf produced on april 2 to may 1 2007 one of its most voluminous and intense eruption since at least one century the eruption focused at 590 m elevation in the grand brule on the south east flank of piton de la fournaise 7 km away from summit it was located close to the southern cliff of the grand brule volcano tectonic collapse the rempart du tremblet and to the nearby village called le tremblet and it highly rattled and incommoded the inhabitants there eleven hours after the beginning of the eruption the lava flows reached the sea and created 0 45 km 2 of new land until the end of eruption in the night of april 5 the rock column beneath dolomieu crater started to collapse with a piston like mechanism into the magma chamber forming within less than 24 h the most recent and well documented caldera we report in this paper the chronology of these two related major events in the recent activity of pdf seen by the scientific networks of the volcanological observatory ovpf ipgp and field observations which allowed to back up the scenario of the caldera formation and to quantify these two exceptional episodes

continental rift to back arc basin jurassic cretaceous stratigraphical and structural evolution of the larsen basin antarctic peninsula the larsen basin developed in jurassic times as a result of continental rifting during the early stages of gondwana break up lower upper jurassic non marine sedimentary and volcanic rocks constitute a syn rift megasequence recording initial largely amagmatic extension and subsequent widespread extension related silicic volcanism a succeeding kimmeridgian early berriasian transgressive megasequence consisting largely of anoxic dysoxic hemipelagic mudstones is thought to have been deposited during a thermal subsidence phase when relative magmatic quiescence and peak jurassic eustatic sea levels served to maximize sediment starvation the fragmentary record for late berriasian barremian times suggests that a regressive megasequence may have developed in the earlier part of this period recording increased sediment yield to the larsen basin from the increasingly emergent antarctic peninsula arc subsequently strata in the southern but not the northern part of the basin underwent relatively intense eastward verging deformation possibly during the formation of a retro arc fold thrust belt where exposed the lower part of the succeeding aptian eocene megasequence consists of a deep marine clastic wedge deposited along the fault bounded western basin margin during a phase of arc uplift and related differential subsidence following partial basin inversion in late cretaceous times regression took place as reduced basinal subsidence rates allowed shallow marine facies to prograde basinward

the south patagonian batholith 150 my of granite magmatism on a plate margin a new database of 70 u pb zircon ages mostly determined by shrimp indicates that the south patagonian batholith resulted from the amalgamation of subduction related plutons from the late jurassic to the neogene construction of the batholith began with a voluminous previously undetected late jurassic bimodal body mainly composed of leucogranite with some gabbro emplaced along its present eastern margin within a restricted time span 157 to 145 ma this episode is at least in part coeval with voluminous rhyolitic ignimbrites of the tobifera formation deposited in the deep rocas verdes basin east of the batholith this was the last of several southwestward migrating silicic volcanic episodes in patagonia that commenced in an early jurassic extensional tectonic regime the quasi oceanic mafic floor of the basin was also contemporaneous with this late jurassic batholithic event as indicated by mutually cross cutting field relationships changes in subduction parameters then triggered the generation of earliest cretaceous plutons cretaceous 1 144 137 ma west of the late jurassic ones a westward shift that culminated at 136 127 ma cretaceous 2 along the present western margin of the batholith most mid to late cretaceous cretaceous 3 126 75 ma and paleogene 67 40 ma granitoids are represented by geographically restricted plutons mainly emplaced between the previously established margins of the batholith and mostly in the far south no associated volcanic rocks of similar age are known at present in this area during the final neogene stage of plutonism 25 15 ma a recurrence of coeval volcanism is recognized within and east of the batholith typical endt values for the granitoids vary from strongly negative 5 in the late jurassic to progressively higher values for cretaceous 1 4 cretaceous 2 0 7 cretaceous 3 2 and the paleogene 5 followed by lower and more variable ones in the neogene 1 to 5 these variations may reflect different modes of pluton emplacement large crustal magma chambers developed in the early stages late jurassic to cretaceous 1 leading to widespread emplacement of plutons with a crustal signature whereas the cretaceous 2 cretaceous 3 and palaeogene parts of the batholith resulted from incremental assembly of small plutons generated at greater depths and with higher endt this does not in itself justify the idea of a reduction in crustal character due to progressive exhaustion of fusible material in the crust through which the magmas passed

shallow magma accumulation at kīlauea volcano hawai i revealed by microgravity surveys for vertical displacement at the summit of about 1 3 m and 160 gal respectively the volume of magma lost based on grav ity data and assuming a reasonable magma density exceeded the volume of summit col lapse suggesting the creation of 40 90 10 6 abstract using microgravity data collected at k lauea volcano hawai i united states between november 1975 and january 2008 we document signifi cant mass increase beneath the east margin of halema uma u crater within k lauea s summit caldera surprisingly there was no sustained uplift accompanying the mass accumulation we propose that the positive grav ity residual in the absence of signifi cant uplift is indicative of magma accumulation in void space probably a network of interconnected cracks which may have been created when magma withdrew from the summit in response to the 29 november 1975 m 7 2 south fl ank earthquake subsequent refi lling documented by gravity represents a gradual recovery from that earthquake a new eruptive vent opened at the summit of k lauea in 2008 within a few hundred meters of the positive gravity residual maximum probably tapping the reservoir that had been accumulating magma since the 1975 earthquake

volcanic signatures in time gravity variations during the volcanic unrest on el hierro canary islands gravity changes occurring during the initial stage of the 2011 2012 el hierro submarine eruption are interpreted in terms of the preeruptive signatures during the episode of unrest continuous gravity measurements were made at two sites on the island using the relative spring gravimeter lacoste and romberg gphone 054 on 15 september 2011 an observed gravity decrease of 45 mu gal associated with the southward migration of seismic epicenters is consistent with a lateral magma migration that occurred beneath the volcanic edifice an apparently clear precursor of the eruption that took place 25 days later on 10 october 2011 high frequency gravity signals also appeared on 6 11 october 2011 pointing to an occurring interaction between a magmatic intrusion and the ocean floor these important gravity changes with amplitudes varying from 10 to 90 mu gal during the first 3 days following the onset of the eruption are consistent with the northward migration of the eruptive focus along an active eruptive fissure an apparent correlation of gravity variations with body tide vertical strain was also noted which could indicate that concurrent tidal triggering occurred during the initial stage of the eruption

geodetic determination of relative plate motion and crustal deformation across the scotia south america plate boundary in eastern tierra del fuego 1 global positioning system gps measurements provide the first direct measurement of plate motion and crustal deformation across the scotia south america transform plate boundary in tierra del fuego this plate boundary accommodates a part of the overall motion between south america and antarctica the subaerial section of the plate boundary in tierra del fuego about 160 km in length is modeled as a two dimensional strike slip plate boundary with east west strike along the magallanes fagnano fault system the principal fault of this portion of the plate boundary relative plate motion is left lateral strike slip on a vertical fault at 6 6 1 3 mm year based on an assumed locking depth of 15 km the site velocities on the scotia plate side are faster than the relative velocity by an additional 1 2 mm yr suggesting there may be a wider region of diffuse left lateral deformation in southern patagonia the north south components of the velocities however do not support the existence of active large scale transpression or transtension between the south america and scotia plates along this section of the plate boundary

late cretaceous early eocene counterclockwise rotation of the fueguian andes and evolution of the patagonia antarctic peninsula system abstract the southernmost andes of patagonia and tierra del fuego present a prominent arc shaped structure the patagonian bend whether the bending is a primary curvature or an orocline is still matter of controversy new paleomagnetic data have been obtained south of the beagle channel in 39 out of 61 sites they have been drilled in late jurassic and early cretaceous sediments and interbedded volcanics and in mid cretaceous to eocene intrusives of the fuegian batholith the anisotropy of magnetic susceptibility was measured at each site and the influence of magnetic fabric on the characteristic remanent magnetizations chrm in plutonic rocks was corrected using inverse tensors of anisotropy of remanent magnetizations normal polarity secondary magnetizations with west directed declination were obtained in the sediments and they did not pass the fold test these characteristic directions are similar to those recorded by mid cretaceous intrusives suggesting a remagnetization event during the normal cretaceous superchron and describe a large 90 counterclockwise rotation late cretaceous to eocene rocks of the fueguian batholith record decreasing counterclockwise rotations of 45 to 30 these paleomagnetic results are interpreted as evidence of a large counterclockwise rotation of the fueguian andes related to the closure of the rocas verdes basin and the formation of the darwin cordillera during the late cretaceous and paleocene the tectonic evolution of the patagonian bend can thus be described as the formation of a progressive arc from an oroclinal stage during the closure of the rocas verdes basin to a mainly primary arc during the final stages of deformation of the magallanes fold and thrust belt plate reconstructions show that the antarctic peninsula would have formed a continuous margin with patagonia between the early cretaceous and the eocene and acted as a non rotational rigid block facilitating the development of the patagonian bend

variability of the gravity to height ratio due to surface loads we study the ratio between the gravity variation and vertical displacement on the surface of a self gravitating earth model when a surface load is applied we adopt a theoretical and numerical point of view excluding any observations first we investigate the spectral behavior of the ratio of the harmonic components of the gravity variation and vertical displacement then we model the gravity to height ratio for different surface loads continental hydrology atmospheric pressure ocean tides using outputs of global numerical models in order to relate the predicted spatial values to theoretical mean values deduced from the spectral domain for locations inside loaded areas the ratio is highly variable because of the newtonian attraction of the local masses and depends on the size of the load for the hydrological loading soil moisture and snow the mean ratio over the continents is 0 87 gal mm 1 but increases with decreasing size of the river basins for the atmospheric loading assuming an inverted barometer response of the ocean the ratio is positive with larger values for high latitudes 0 49 gal mm 1 particularly on the coasts than for lower latitudes 0 30 gal mm 1 the ratio however is much less variable outside the loaded areas in desert areas such as the sahara and arabia its mean value is 0 28 gal mm 1 for the ocean tidal loading we find a mean ratio of 0 26 gal mm 1 over the continents for the diurnal tidal waves both results are close to the theoretical mean value of 0 26 gal mm 1 combining elastic and remote attraction contributions

scotia sea regional tectonic evolution implications for mantle flow and palaeocirculation abstract the scotia sea and surrounding scotia arc have evolved over the past 40 ma by extension behind an east migrating subduction zone at the boundary between the south american sam and antarctic ant plates the considerable data set now available regional geology and geophysics earthquake seismology satellite altimetry global plate analyses suggest why east migrating subduction began what has been the driving force that has sustained it and what other processes have controlled the mode of back arc extension in the scotia sea a suite of six reconstructions has been developed based on this data set the reconstruction to 40 ma creates a compact cuspate continental connection between south america and the antarctic peninsula at the subducting pacific margin with fragments now dispersed around the scotia arc occupying positions within it compatible with their known geology the driving force has been subduction of south american ocean floor which began as a result of southward migration of the pole of south american antarctic plate rotation and a key modulator of back arc extension has been collision of ridge crest sections of the south american antarctic plate boundary with the east advancing trench cenozoic regional tectonic evolution has two other likely consequences which greatly increase its importance firstly this region saw the tectonic disruption of the final barrier to complete circum antarctic deep water flow that may have had a profound effect on palaeoclimate secondly it is possible that the rapid roll back of the hinge of subduction is related to shallow eastward flow in the sub lithospheric mantle both of these consequences are explored the reconstructions show that rapid roll back of the subduction hinge averaging 50 mm a over the last 40 ma with respect to the south american plate has been a feature of all of scotia sea evolution and provide a history of motion of several oceanic microplates most of which are now welded together within the scotia sea this will guide the location of seismometers and or dredge hauls to test the hypothesis of shallow mantle flow and help interpret the results the reconstructions also allow an assessment of the creation of deep water pathways that would have permitted the development of the present day antarctic circumpolar current acc an early miocene onset within the period 22 17 ma seems likely for the acc depending on the structure and palaeo elevation of davis bank and aurora bank sections of the north scotia ridge however the study shows there was a delay of one or more million years between initial provision of a deep water pathway and the major mid miocene change in global climate involving the general level of antarctic glaciation that may have been related if these changes were related then the delay suggests that other factors possibly rough elevated ocean floor but also non tectonic factors such as atmospheric co2 were important in determining palaeoclimate

scotia arc kinematics from gps geodesy 1 gps crustal velocity data from the scotia and south sandwich plates transform azimuths spreading data and an updated earthquake slip vector catalog provide the first scotia and south sandwich plate euler vector estimates not dependent on closure as the gps data tie them to the global plate circuit neither the gps data which sample limited portions of the plates nor the geologic data which are not tied to the global spreading circuit are sufficient individually to define the euler vectors as scotia plate gps measurements do not sample the stable plate interior plate boundary deformation field modeling is necessary for euler vector estimation our south america antarctic and scotia south sandwich euler pole estimates agree with previous estimates from either gps or geologic data our south america scotia euler vector however is significantly different and near the south america antarctic euler vector producing an approximately coaxial motion of scotia between south america and antarctica

magnetic fabrics and compositional evidence for the construction of the caleu pluton by multiple injections coastal range of central chile the caleu pluton which is located in the coastal range of central chile consists of three n s elongated lithologic zones which define an across pluton compositional variation characterized by a westward increase in sio2 content the gabbro diorite gdz tonalite tz and granodiorite gz zones a total of 547 cores were drilled at 68 sites within the different zones to carry out magnetic fabric and paleomagnetic studies the highest values of magnetic susceptibility and remanent magnetization are found in the gdz 0 04 0 1 si whereas the lowest values are observed in the gz 0 01 0 02 si this is consistent with the significant decrease in magnetite content from the gdz to the gz the dips of the magnetic foliation planes are steep within gdz and tz and shallower dips are observed in gz magnetic foliations along some borders of the pluton are parallel to the corresponding contacts the parallelism observed between the magnetic foliations and the contacts between zones suggests that these zones have recorded the same mechanism of emplacement at the magmatic state the peculiar foliation pattern observed in the central parts of tz and gz is attributed to a lobe like structure that was acquired during the late stages of emplacement and inflation the distinct compositional characteristics of these three zones were not acquired in situ rather they were emplaced successively from mafic to felsic from a subjacent stratified sill like magma reservoir which was filled after tilting of the host volcanic succession the rapid upward transport of these melts of different compositions up to a shallow depth is attested by both the equilibration pressure of late magmatic hornblende 200 mpa and the pressure of early magmatic epidote crystallization 500 600 mpa we conclude that space was created by lateral flow propagation in which the intrusion of later pulses pushed earlier ones away from the feeder system followed by roof lifting and possibly floor depression

localized deformation at miyakejima volcano based on jers 1 radar interferometry 1992 1998 1 stacked radar interferograms at miyakejima volcano japan between 1992 and 1998 showed two localized significantly deforming areas with a magnitude of 4 6 mm yr in the radar line of sight one area is close to the 1983 eruption vent and using a simple closed analytical formulation the deformation is interpreted as due to a thermoelastic contraction of a formerly intruded magma the other one detected in the previous caldera is explained by a depressurization source whose depth 500 m coincides with that of a low resistivity zone interpreted as a hydrothermal reservoir also our computed co2 flux is consistent with in situ measurement data the reservoir volume is significantly less than the collapsed volume in 2000 and its depth is much shallower than the void detected prior to the collapse hence the depressurization would not directly induce the caldera collapse in 2000

post tectonic uplift of the cordillera darwin orogenic core complex evidence from fission track geochronology and closing temperature time relationships the cordillera darwin orogenic core complex is a structural and topographic culmination in the southern andes which underwent mid cretaceous penetrative deformation and metamorphism associated with the deformation and incipient obduction of a partly ophiolitic back are basin rocas verdes block along its southern boundary whereas structural evidence suggests uplift of the rocas verdes terrane relative to the cordillera darwin during this event metamorphic assemblages reflect a greater net uplift of the opposite sense new fission track ages together with existing k ar and rb sr ages were used to estimate cooling and uplift histories and their relationships to foreland sedimentation and present day topographic relief the results suggest that whereas initiation of uplift in the cordillera darwin and rocas verdes blocks may have accompanied mid cretaceous deformation the major rapid uplift around 0 5 1 5 mm y occurred after the deformation also uplift occurred earlier in the rocas verdes block than in the cordillera darwin block the latter having experienced a greater net uplift since the mid cretaceous the major post tectonic uplift of the cordillera darwin occurred at a slower rate 0 05 0 20 mm y and coincides with the deposition of thick flysch and conglomerates in the foreland basin

the onset of the permo carboniferous glaciation reconciling global stratigraphic evidence with biogenic apatite δ18o records in the late visean the 18 o values of phosphatic microfossils recovered from nw ireland are used to determine the timing and magnitude of cooling associated with the onset of the carboniferous glaciation microfossil fish 18 o apatite demonstrates a 2 4 v smow shift which once corrected for 18 o seawater changes owing to evolving ice volumes equates to an approximate 4 5 c reduction in equatorial sea surface temperature between the earliest asbian and the mid brigantian late visean both conodont and microfossil fish 18 o apatite indicate stabilization of an icehouse climate during the brigantian and into the serpukhovian substantial late visean cooling identified herein is in good agreement with global glacioeustatic records supplementary material further information on the palaeogeography lithostratigraphy and palaeoenvironments of the sections examined as well as the sample composition and analytical methods of oxygen isotope analyses is available at http www geolsoc org uk sup18511

new thermochronometric constraints on the rapid palaeogene exhumation of the cordillera darwin complex and related thrust sheets in the fuegian andes thermal modelling of new fission track and u th sm he data from the fuegian andes reveals rapid cooling 12 48 c ma 1 during the middle and late eocene followed by slow cooling 1 5 c ma 1 to the recent we interpret the rapid cooling as a result of exhumation from contractional uplift within the crystalline interior of the orogen this interpretation is consistent with independent evidence of eocene shortening flexural subsidence and provenance changes in the study area and is approximately coeval with marine geochemical evidence of the onset of drake passage opening in light of the palaeogene history of nazca south american plate convergence and the differences in shortening magnitudes and exhumation histories between the fuegian and patagonian andes our data support eocene development of the patagonian orocline which also provides a plausible explanation for early opening of drake passage

intermittent inflations recorded by broadband seismometers prior to caldera formation at miyake jima volcano in 2000 abstract very long period vlp pulses with widths of 20 s on velocity seismograms were observed during volcanic activity at miyake jima volcano japan in 2000 the vlp events occurred repeatedly during a few days prior to caldera formation and essentially vanished following the onset of caldera collapse waveform inversions of the pulse like signals point to a source offset 3 5 km beneath and 1 km south of the summit a candidate for the source mechanism is the inflation of an elliptical cylinder with axis tilted 20 30 from vertical and major axis of the elliptical cross section oriented northeast southwest the inferred mechanism appears consistent with a step like pressurization of a magma reservoir impacted by a falling rock mass in response to gravitational instability the repeated occurrences of the rock collapses lead to the caldera formation at miyake jima

structural tectonic and glaciological controls on the evolution of fjord landscapes abstract the fjord landscape of south america stretching 1500 km between golfo corcovado 43 s and tierra del fuego 56 s is the largest continuous fjord landscape on earth this paper presents the results of new structural geological and geomorphological mapping of this landscape using optical satellite images and digital elevation models first order geological structures are represented by strike slip faults forming lineaments up to hundreds of kilometres long the strike slip faulting has been active since late cretaceous times and is responsible for the presence of a conspicuous structural cleavage visible as lineaments up to 10 km long a detailed analysis of these second order lineaments from digital image data was carried out in three sectors in sector 1 located northwest of the north patagonian icefield there are three distinct mean orientations characterized by a main nearly orogen parallel orientation az 145 and two orogen oblique secondary orientations az 20 and az 65 in sector 2 located west of the south patagonian icefield there are also three separate mean orientations with most of the lineaments concentrated between azimuths 0 and 80 mean at 36 and two other orogen oblique means at azimuth 122 and 163 in sector 3 around the cordillera darwin there is a single main orogen parallel mean at 100 115 in all three sectors mapped fjord orientations bear a striking similarity to the structural data with fjords orientated preferentially in the same direction as structural lineaments we infer that successive glaciations followed the same ice discharge routes widening and deepening pre existing geological structures at the expense of the surrounding terrain to create the fjord landscape this study has broader implications for ice sheet reconstructions and landscape evolution beneath ice sheets because we demonstrate that the primary control on fjord development in glaciated areas is geological and not glaciological

chronology and products of the 2000 eruption of miyakejima volcano japan lateral migration of magma away from miyakejima volcanic island japan generated summit subsidence associated with summit explosions in the summer of 2000 an earthquake swarm beneath miyakejima began on the evening of 26 june 2000 followed by a submarine eruption the next morning strong seismic activity continued under the sea from beneath the coast of miyakejima to a few tens of kilometers northwest of the island summit eruptive event began with subsidence of the summit on 8 july and both explosions and subsidence continued intermittently through july and august the most intense eruptive event occurred on 18 august and was vulcanian to subplinian in type ash lofted into the stratosphere fell over the entire island and abundant volcanic bombs were erupted at this time another large explosion took place on 29 august this generated a low temperature pyroclastic surge which covered a residential area on the northern coast of the island the total volume of tephra erupted was 9 3 106 m3 dre much smaller than the volume of the resulting caldera 6 108 m3 migration of magma away from miyakejima was associated with crustal extension northwest of miyakejima and coincident shrinkage of miyakejima island itself during july august 2000 this magma migration probably caused stoping of roof rock into the magma reservoir generating subsurface cavities filled with hydrothermal fluid and or magmatic foam and formation of a caldera oyama caldera at the summit interaction of hydrothermal fluid with ascending magma drove a series of phreatic to phreatomagmatic eruptions it is likely that new magma was supplied to the reservoir from the bottom during waning stage of magma s migration resulting in explosive discharge on 18 august the 18 august event and phreatic explosions on 29 august produced a conduit system that allowed abundant so2 emission as high as 460 kg s 1 after the major eruptive events were over at the time of writing inhabitants of the island about 3 000 have been evacuated from miyakejima for more than 3 years

gravity change caused by a fissure eruption a simple relation holds between the gravity and elevation changes associated with a fissure eruption if combined with inversion schemes it enables us to estimate the location and geometry of the subsurface fractured area we applied the technique to the 1986 eruption of the izu oshima volcano japan where a textbook like fissure eruption was observed

revised tectonic implications for the magnetic anomalies of the western weddell sea abstract ten years after the usac u s a rgentina c hile project which was the most comprehensive aeromagnetic effort in the antarctic peninsula and surrounding ocean basins questions remain regarding the kinematics of the early opening history of the weddell sea key elements in this complex issue are a better resolution of the magnetic sequence in the western part of the weddell sea and merging the usac data set with the other magnetic data sets in the region for this purpose we reprocessed the usac data set using a continuation between arbitrary surfaces and equivalent magnetic sources the equivalent sources are located at a smooth crustal surface derived from the existing bathymetry topography and depths estimated by magnetic inversions the most critical area processed was the transition between the high altitude survey over the antarctic peninsula and the low altitude survey over the weddell sea that required downward continuation to equalize the distance to the magnetic source this procedure was performed with eigenvalue analysis to stabilize the equivalent magnetic source inversion the enhancement of the mesozoic sequence permits refining the interpretation of the seafloor spreading anomalies in particular the change in shape and wavelength of an elongated positive in the central weddell sea suggests that it was formed during the cretaceous normal polarity interval the older lineations in the southwestern weddell sea are tentatively attributed to susceptibility contrasts modeled as fracture zones numerical experimentation to adjust synthetic isochrons to seafloor spreading lineations and flow lines to fracture zones yields stage poles for the opening of the weddell sea since 160 ma to anomaly 34 time the corresponding reconstructions look reasonable within the known constraints for the motions of the antarctic and south america plates however closure is not attained between 160 and 118 ma if independent published east antarctica africa south america rotations are considered the lack of closure may be overcome by considering relative motion between the antarctic peninsula and east antarctica until 118 ma time an important component of convergence

spatio temporal changes in seismic velocity associated with the 2000 activity of miyakejima volcano as inferred from cross correlation analyses of ambient noise abstract miyakejima volcano japan showed strong and interesting volcanic activities in 2000 with the occurrence of dike intrusions caldera formation and a large amount of gas emission to understand temporal changes of the volcanic structure associated with this activity we apply ambient noise correlation analyses to the seismic records of five seismic stations at miyakejima volcano from july 1999 to december 2002 we calculate cross correlation functions ccfs for eight available station pairs at frequency bands of 0 4 0 8 hz and 0 8 1 6 hz and retrieve the rayleigh waves propagating with a group velocity of about 1 0 km s comparing ccfs obtained for each day with the reference ccfs that are calculated from a few months of data we estimate seismic velocity changes before and after the 2000 activity results at the two frequency bands show seismic velocity increase and decrease shallow structure at the volcanic edifice on the flanks experienced a seismic velocity increase up to 3 3 on the other hand the regions located close to the collapsed caldera show a seismic velocity decrease down to 2 3 to understand the mechanisms that may introduce these seismic velocity increase and decrease we examine several possible mechanisms of the velocity changes stress changes due to volcanic pressure sources and caldera formation and topographic changes by using the deflation sources previously determined from gps data we calculate the stresses at depths of 0 km and 1 km at the volcano the results suggest that the seismic velocity increases observed at seismic paths on the volcanic flank are explained by the compression caused by the deflation sources associated with magma activity in 2000 stress sensitivities of the seismic velocity changes are estimated to be 1 5 10 2 to 2 2 10 2 mpa 1 and 4 6 10 3 to 2 4 10 2 mpa 1 at frequency bands of 0 4 0 8 hz and 0 8 1 6 hz respectively which are in the range of the previously reported results our three dimensional numerical calculations on seismic wave propagation show that the seismic velocity decreases observed at seismic paths crossing the collapsed caldera can be explained by topographic changes introduced by a caldera we further suggest that the seismic velocity decreases detected around the regions close to the caldera may originate from the dilated regions cause by formation of the caldera

magma plumbing system of the 2000 eruption of miyakejima volcano japan during the 2000 eruption at miyakejima volcano two magmas with different compositions erupted successively from different craters magma erupted as spatter from the submarine craters on 27 june is aphyric basaltic andesite 5 vol phenocrysts 51 4 52 2 wt sio2 whereas magma issued as volcanic bombs from the summit caldera on 18 august is plagioclase phyric basalt 20 vol phenocrysts 50 8 51 3 wt sio2 the submarine spatter contains two types of crystal clots a type and a type andesitic type the phenocryst assemblages plagioclase pyroxenes and magnetite and compositions of clinopyroxene in these clots are nearly the same but only a type clots contain ca poor plagioclase an 70 we consider that the a type clots could have crystallized from a more differentiated andesitic magma than the a type clots because feo mgo is not strongly influenced during shallow andesitic differentiation the summit bombs contain only b type basaltic type crystal clots of ca rich plagioclase olivine and clinopyroxene the a type and b type clots have often coexisted in miyakejima lavas of the period 1469 1983 suggesting that the magma storage system consists of independent batches of andesitic and basaltic magmas according to the temporal variations of mineral compositions in crystal clots the andesitic magma became less evolved and the basaltic magma more evolved over the past 500 years we conclude that gradually differentiating basaltic magma has been repeatedly injected into the shallower andesitic magma over this period causing the andesitic magma to become less evolved with time the mineral chemistries in crystal clots of the submarine spatter and 18 august summit bombs of the 2000 eruption fall on the evolution trends of the a type and b type clots respectively suggesting that the shallow andesitic and deeper basaltic magmas existing since 1469 had successively erupted from different craters the 2000 summit collapse occurred due to drainage of the andesitic magma from the shallower chamber as the collapse occurred it may have caused disruption of crustal cumulates which then contaminated the ascending deeper basalt thus porphyritic basaltic magma could erupt alone without mixing with the andesitic magma from the summit caldera the historical magma plumbing system of miyakejima was probably destroyed during the 2000 eruption and a new one may now form

crustal deformation caused by magma migration in the northern izu islands japan intense crustal activity including earthquake swarms eruptions and a caldera formation in the northern izu islands started on june 26 2000 accompanied with large crustal deformation permanent gps data reveals the spatial pattern and time evolution of ground deformation the observations reveal shrinking and subsidence of miyakejima and extension between kouzushima and niijima we constructed a source model to explain the observed displacements during the period between june 26 and the end of august the model consists of a deflation source 0 12km beneath miyakejima tensile faults 1 04km located between miyakejima and kouzushima and several shear faults mass balance considerations suggest that a large amount of magma migrated 30km from miyakejima toward kouzushima

along strike variability of back arc basin collapse and the initiation of sedimentation in the magallanes foreland basin southernmost andes 53 54 5 s 1 the patagonian andes record the cretaceous demise of the quasi oceanic rocas verdes back arc basin and formation of the magallanes foreland basin for 500 km along the strike of the mountains this tectonic transition is marked by a sandstone mudstone package that records the beginning of turbiditic sand deposition and fan growth sandstone modal analyses and u pb detrital zircon spectra show changes in rock composition and provenance across the transition on a basin wide scale indicating it has tectonic significance and is related to orogenic uplift and the progressive evolution of the andean fold thrust belt spatial variations in transition zone characteristics indicate the foreland basin s central and southern sectors were fed by different sources and probably record separate fans at bahia brookes on tierra del fuego foreland basin sedimentation began at least after 88 89 ma and possibly after 85 ma several million years after it did 700 km away at the northern end of the basin this event coincided with increased arc volcanism and the partial obduction of the basaltic rocas verdes basin floor onto continental crust by 81 80 ma conglomerate deposition and increased compositional and provenance complexity including the abundance of metamorphic lithic fragments indicate that the obducted basaltic floor first became emergent and was eroding the results suggest that the beginning of turbidite sedimentation in the magallanes foreland basin and the progressive incorporation and exhumation of deeply buried rocks in the andean fold thrust belt occurred later in southern patagonia than in the north by a few million years

gravity height correlations for unrest at calderas abstract calderas represent the sites of the world s most serious volcanic hazards although eruptions are not frequent at such structures on the scale of human lifetimes there are nevertheless often physical changes at calderas that are measurable over periods of years or decades such calderas are said to be in a state of unrest and it is by studying the nature of this unrest that we may begin to understand the dynamics of eruption precursors here we review combined gravity and elevation data from several restless calderas and present new data on their characteristic signatures during periods of inflation and deflation we find that unless the bouguer gravity anomaly at a caldera is extremely small the free air gradient used to correct gravity data for observed elevation changes must be the measured or calculated gradient and not the theoretical gradient use of which may introduce significant errors in general there are two models that fit most of the available data the first involves a mogi type point source and the second is a bouguer type infinite horizontal plane source the density of the deforming material usually a magma chamber is calculated from the gravity and ground deformation data and the best fitting model is to a first approximation the one producing the most realistic density no realistic density is obtained where there are real density changes or where the data do not fit the point source or slab model we find that a point source model fits most of the available data and that most data are for periods of caldera inflation the limited examples of deflation from large silicic calderas indicate that the amount of mass loss or magma drainage is usually much less than the mass gain during the preceding magma intrusion in contrast deflationary events at basaltic calderas formed in extensional tectonic environments are associated with more significant mass loss as magma is injected into the associated fissure swarms

orogenesis at the southern tip of the americas the structural evolution of the cordillera darwin metamorphic complex southernmost chile abstract new detailed lithologic and structural data are presented from three separately mapped areas along the southern boundary of the cordillera darwin metamorphic complex of southernmost chile cordillera darwin is a unique uplift because it exposes the highest grade rocks in the andes south of peru and averages 1 km higher in elevation than adjacent areas the structural data indicate that cordillera darwin experienced mid late cretaceous trans pressional deformation with a partitioned strike slip component localized along the beagle channel that forms the southern boundary to the range foliation lineation and fold axis trends indicate ne sw directed contraction and nw se strike slip shearing present directions during progressive d1 d2 andean deformation d2 deformation is marked by outcrop to 10 km scale south southwest vergent folds late cretaceous tertiary brittle ductile and brittle left lateral strike slip faults and shear zones crosscut all d1 d2 structures although limited structural evidence for extensional tectonics was documented in this study apparent normal offsets across both arms of the beagle channel and previously documented field evidence for extension from other areas in cordillera darwin suggest that transtensional displacements also may have occurred in southern cordillera darwin during the late cretaceous early tertiary cordillera darwin s position within the evolving patagonian orocline adjacent to an evolving mesozoic cenozoic left lateral transform boundary between the south american and antarctic plates and later the south american and scotia plates necessitates consideration of the possible effects of regional counterclockwise rotation on development of structures regional counterclockwise rotation of cordillera darwin may have controlled the temporal and spatial transition of deformational regimes within cordillera darwin exhumation of the metamorphic core of cordillera darwin during the late cretaceous early tertiary is interpreted to have been due to rapid uplift and normal rates of erosion rapid uplift may have been due to migration of contraction to northern cordillera darwin and crustal relaxation in southern cordillera darwin as contractional structures rotated into parallelism with 1 and either ceased to be active or were reactivated as strike slip or oblique slip faults rapid isostatic rebound of cordillera darwin may have also been a response to aborted continental subduction of southern cordillera darwin and detachment of a subducted mafic root during mid cretaceous closure of the rocas verdes marginal basin

transition from back arc to foreland basin development in the southernmost andes stratigraphic record from the ultima esperanza district chile during the mesozoic evolution of the southernmost andes back arc basin formation in an extensional setting was followed by foreland basin development in a compressional setting a stratigraphic record of this tectonic transition is described from jurassic and cretaceous rock units exposed in the patagonian fold thrust belt at 51 s latitude initiation of an extensional deep marine trough in late jurassic time is documented by interstratified rhyolites and marine mudstones in the tobifera formation development of the early cretaceous rocas verdes back arc basin is recorded by zapata formation submarine slope deposits which draped the west facing passively subsiding cratonic margin of the basin the onset of andean compressional orogenesis and creation of the magallanes foreland basin is marked by the influx of coarse grained sandstone turbidites of the albian cenomanian punta barrosa formation during late cretaceous time the foreland basin configuration consisted of a narrow foredeep trough bounded by a gently sloping foreland ramp on the craton to the east this geometry reflects greater flexural subsidence of the stretched and thermally weakened passive back arc basin margin beneath the load of the obducted rocas verdes basin floor changes in depositional regime and sediment dispersal patterns in the latest cretaceous together with an eastward shift in the magallanes basin depocenter record cratonward migration of deformation in the patagonian fold thrust belt loading of the thicker cratonic lithosphere caused subsidence of the foreland ramp to form a wide flexural basin

an 40ar 39ar geochronological and electron microprobe investigation of an archean pyroxenite and its bearing on ancient atmospheric compositions a detailed 40ar 39ar step heating geochronological study of pyroxene mineral separates from the munro warden ultramafic sill in the archean abitibi greenstone belt of the canadian superior province yields a precise primary age of 2703 11 ma 1 this is in excellent agreement with previous u pb zircon dates of 2700 2712 ma for the enclosing felsic volcanics and a sm nd age of 2622 60 ma for the sill electron microprobe analyses coupled with the argon evolution patterns demonstrate that deuteric amphibole is responsible for this primary age whereas the volumetrically dominant diopside shows anomalously young ages an isotope correlation plot reveals two components of contaminating argon with differing 40ar 36ar ratios in distinctly different sites one component trapped during the deuteric formation of amphibole has an 40ar 36ar ratio that is significantly less 258 3 than the present day atmospheric value of 295 5 this may be an indication of the atmospheric ratio 2 7 ga ago the other comp

first detection of absolute gravity change caused by earthquake unequivocal coseismic gravity change was detected for the first time by absolute gravimetry the measurements were done just one day before and only 7 days after a m6 1 earthquake m6 1 the absolute gravity change was 6 microgal significantly larger than the observational error of about 1 microgal of particular interest is that the observed spatial gravity variations agree remarkably well with those expected from an elastic dislocation model constructed on displacement data alone the result strongly encourages us to perform a simultaneous inversion of both gravity and displacements to have better insight of earthquakes

combining relative and absolute gravity measurements to enhance volcano monitoring to achieve a balance between uncertainty and efficiency in gravity measurements we have investigated the applicability of combined measurements of absolute and relative gravity as a hybrid method for volcano monitoring between 2007 and 2009 three hybrid gravity surveys were conducted at mt etna volcano in june 2007 july 2008 and july 2009 absolute gravity data were collected with two absolute gravimeters which represent the state of the art in recent advances in ballistic gravimeter technology 1 the commercial instrument fg5 238 and 2 the prototype instrument imgc 02 we carried out several field surveys and confirmed that both the absolute gravimeters can still achieve a 10 gal or better uncertainty even when they are operated in severe environmental conditions the use of absolute gravimeters in a field survey of the summit area of mt etna is unprecedented the annual changes of the gravity measured over 2007 2008 and 2008 2009 provide unequivocal evidence that during the 2007 2009 period two main phenomena of subsurface mass redistribution occurred in distinct sectors of the volcano accompanying different eruptive episodes from 2007 to 2008 a gravity change of 60 gal was concentrated around the north east rift this coincided with a zone affected by strong extensional tectonics and hence might have been related to the opening of new voids between 2008 and 2009 a north south elongate feature with a maximum gravity change of 80 gal was identified in the summit craters area this is interpreted to indicate recharge of a deep intermediate magma storage zone which could have occurred when the 2008 2009 eruption was still ongoing

effects of tectonic deformation on the remanent magnetization of rocks paleomagnetic investigations have usually been impossible in deformed rocks because of the effects of distortion on the remanent magnetization vector deformation produces a magnetic anisotropy in rocks which can deflect the remanence away from the minimum susceptibility axis however the assumption that the intersection points of great circles joining these two directions represent an improved estimate of the undisturbed remanence is invalid because the amount of the deflection of each remanence is not really known anisotropy of magnetic susceptibility ams provides a quantitative basis for strain determination the principal axes of the magnetic anisotropy ellipsoid have the same directions as those of the strain ellipsoid a linear relationship between the normalized principal susceptibility differences and the logarithmic strains has been found in several rock types if the correlation method used is valid ams observations can be used to establish the magnitudes and directions of the principal strains strain modifies bedding planes and realigns the grains that carry magnetic remanence compensation for deformation requires application of a modified bedding tilt correction and correction of the remanent vector for distortional strain the effects of strain on bedding can be corrected using passive marker theory attempts to compensate for distortional strain by treating the remanent vector as a passive marker have proved inconclusive during deformation linear and planar elements deflect away from the axis of maximum shortening the passive marker correction shifts the remanent vectors toward the local shortening axis for the site and reduces the directional scatter tests of the passive marker method in strongly deformed rocks resulted in a remanence distribution dominated by the shortening axis directions the passive marker model becomes inapplicable when the deformation results in recrystallization of the rock matrix

3d integrated geophysical modeling for the 2008 magma intrusion at etna constraints on rheology and dike overpressure abstract we present a 3d numerical model based on finite element method fem to jointly evaluate geophysical changes caused by dislocation and overpressure sources in volcanic areas a coupled numerical problem was solved to estimate ground deformation gravity and magnetic changes produced by stress redistribution accompanying magma migration within the volcano edifice we successfully applied the integrated numerical procedure to image the magmatic intrusion occurring in the northern flank of etna during the onset of the 2008 eruption a multi layered crustal structure of the volcano constrained by geological models and geophysical data was considered geodetic and gravity data provide information on the strain field while piezomagnetic changes give constraints on the stress field therefore the integrated modeling gives insights on mt etna rheology and dike overpressure involved in the magma propagation and improves understanding of dike emplacement in the northern sector of the volcano our fem based approach improves the reliability of model based inference of geophysical parameters obtained during monitoring of the onset of etna lateral intrusions that can prelude to an impending eruption

magallanes fagnano continental transform fault tierra del fuego southernmost south america 1 multichannel seismic reflection profiles gravity measurements and bathymetric soundings in conjunction with field geological reconnaissance and remote sensing images reveal with unprecedented detail the morphostructure of a major segment of the south america scotia plate boundary in the tierra del fuego region this segment known as the magallanes fagnano fault system is a continental transform margin arranged in an en echelon geometry along which prominent asymmetric basins were developed data acquired off the atlantic coast of isla grande the main island of tierra del fuego in its central and eastern part and in the central and western magallanes strait image the surface and subsurface structure of the transform fault and its associated basins the magallanes fagnano fault system is composed of distinct tectonic lineaments that are segments of the transform system and are represented by mostly near vertical faults in the atlantic sector the fault system trends broadly n70 e and seems to be composed by a single master fault along which a highly asymmetric basin has formed at around 63 w the fault terminates by splaying into secondary normal faults that dissipate the horizontal displacement along the system in the central eastern part of isla grande the fault segments have been principally identified from analyses of remote sensing images on the basis of their morphological expression these segments are located within river valleys and are generally associated with localized gravity minima lago fagnano a 105 km long e w trending depression is a large mostly asymmetric pull apart basin developed within the principal displacement zone of the magallanes fagnano fault system restraining bends and overlapping step over geometry characterize the central part of the magallanes strait along the western part of the fault system in the vicinity of the pacific entrance of the magallanes strait asymmetric sedimentary basins have also developed the sedimentary architecture of the basins formed within the principal displacement zone of the fault in which the thick end of the depositional wedge abuts the transform segment suggest simultaneous strike slip motion and transform normal extension a common feature found in other continental transtensional environments strike slip faulting in the tierra del fuego region is also documented along other prominent lineaments which parallel the magallanes fagnano fault system along at least two of these lineaments characterized by a remarkable morphological expression widespread quaternary activity occurs the present day motion between the south america and scotia plates is slow 5 mm yr also the modern seismicity monitored in the tierra del fuego region is low individual events 3 5 in magnitude the low seismicity may be explained by the slow relative motion between plates and may be further affected by slip partitioning along the different segments which make up the magallanes fagnano fault array and along the subsidiary wrench lineaments that traverse the region

comparison of 40ar 39ar conventional and laser dating of biotites from the north tregor batholith bulk samples and single grains of biotites from the north tregor batholite armorican massif france were analysed by the 40ar39ar method apparently pure single biotite grains analysed by the step heating procedure with the laser probe displayed plateau ages or slightly disturbed age spectra one strongly chloritized single biotite grain gave a very disturbed age spectrum with younger ages the bulk samples of biotites of the same rocks displayed disturbed age spectra characterized by younger low temperature ages and a hump at intermediate temperature steps these disturbances are explained by 39ar recoil from the biotite into the chlorite which is clearly evidenced by hrtem observations and not by excess 40ar the more concordant results obtained on single biotites are probably due to a more careful selection of the minerals in most cases we obtain good concordance of the ages 600 610 ma measured on the apparently pure single biotite grains and the high temperature ages of both the bulk samples of amphiboles and biotites from the same rocks

cenozoic crustal thickening wrenching and rifting in the foothills of the southernmost andes abstract the southernmost andes form an orocline between the patagonian cordillera trending n s and the fueguian cordilleras trending e w on the foreland side is the magellan basin the area has a history of paleozoic compression triassic to early cretaceous rifting and late cretaceous to quaternary compression in response to changing plate tectonics major structures of late cretaceous and cenozoic age vary along the strike in the patagonian cordillera and foothills folds and thrusts trend nnw slightly oblique to the orogen whereas strike slip faults are parallel to the orogen and right lateral in the fueguian cordilleras and foothills folds and thrusts trend ese slightly oblique to the orogen whereas strike slip faults are parallel to the orogen and left lateral in the axial zone of the magellan basin folds and thrusts are parallel to the orogen and rifts are sub perpendicular to it to a first approximation the pattern of structures has mirror symmetry about the axis of the magellan basin in detail however wrenching appears to be more prevalent in the fueguian cordillera and foothills than it is in the patagonian cordillera and foothills minor faults of cenozoic age are common in the foothills from a kinematic analysis of fault slip data 1 shortening and stretching directions are mostly sub horizontal 2 shortening directions vary in trend from ene in the patagonian foothills to ne in the fueguian foothills and 3 stretching directions are sub parallel to traces of major thrusts in the fueguian cordillera and foothills strike slip faulting is prevalent in the patagonian foothills crustal thickening is prevalent over strike slip faulting the kinematics reflect a combination of thrusting and wrenching and they are consistent with the major structures to investigate the origin of the cenozoic structures we used analogue models on a fully lithospheric scale where an oceanic plate subducted beneath a continental corner the corner was an area of transition from frontal subduction to transcurrent motion the boundary conditions may not have been fully realistic but the experiments did account for the major elements of the structural pattern in southernmost south america including rifts that are perpendicular to the orogen and counterclockwise block rotations

triggering basaltic volcanic eruptions by bubble melt separation understanding the processes by which volcanic eruptions are triggered is crucial for volcanic hazard prediction and assessment in intermediate and silicic magmas where water is the dominant volatile phase1 2 second boiling provides an effective eruption trigger1 as the magma cools and crystallizes volatiles are increasingly concentrated in the residual melt where they eventually become saturated and are exsolved thereby raising the magma pressure to the point of eruption1 2 in contrast in basaltic magma chambers in which carbon dioxide is the dominant volatile phase the effect of second boiling is to decrease the chamber pressure but basaltic magmas are also relatively inviscid so volatile bubbles can separate from the turbulently convecting melt to produce a foam layer at the chamber roof3 4 decompression of the rising bubbles can increase the chamber pressure and so trigger an eruption5 6 here we model the complex competition between the processes of magma cooling and bubble ascent we find that in basalt of very low viscosity 1 10pas bubble separation dominates and can increase the reservoir pressure by as much as 10 mpa sufficient to trigger an eruption7 on timescales of 1 10 years comparable to the duration of eruptive episodes at kilauea volcano hawaii4 8

tectonics of the antarctic scotia plate boundary near elephant and clarence islands west antarctica over 5000 km of new bathymetric data collected from near the northern antarctic peninsula 60 s 63 5 s latitude 53 5 w 63 w longitude show the morphology of an irregular segment of the antarctic scotia plate boundary and nearby shetland microplate the irregular plate boundary is formed by an oblique intersection 70 of the sinistral transpressional shackleton fracture zone sfz and the sinistral transtensional south scotia ridge transform ssr near elephant ei and clarence ci islands mapped boundaries of the shetland microplate include the south shetland trench and the volcanic rift axis of bransfield strait marginal basin bathymetric data single channel seismic reflection profiles and geosat ers 1 free air gravity data show a southeast trending fault zone on the northeast side of a prominent ridge in the sfz the fault zone is defined by scarps that affect ocean floor sediments fault bounded subbasins rotated sedimentary layers angular unconformities linear gravity trends and transtensional followed by contractional deformation southeast of a termination of the sfz ridge at the south shetland trench the fault zone subdivides into segments displaying steep scarps up to 23 and canyons on the northeast margin of the ei platform these features become east west trending nearer to the western ssr south of the islands southwest trending extensional or transtensional fault zones disrupt the bransfield strait volcanic rift axis these data suggest that 1 recent 4 ma changes in the configuration of the antarctic plate near the antarctic peninsula caused a segment of the sfz transform to adjust to a more stable rectilinear geometry with the ssr transform and 2 diffuse transtension resulting from current antarctic scotia relative motion is dissecting the shetland microplate near ei and ci and transferring slivers of the scotia plate onto the antarctic plate

hydrological modeling of groundwater disturbances to observed gravity theory and application to asama volcano central japan 1 the gravity disturbance caused by groundwater is derived based on hydrological physics by solving nonlinear hydrological diffusion equations for three dimensional and temporal groundwater distributions the gravity disturbance is then estimated by the spatial integral of the groundwater distributions this approach aims to resolve the problems of previous methods for the correction of groundwater disturbances in gravity data such as instrumental drift of relative gravimeters empirical estimation of gravity assuming a linear gravity response to precipitation and the use of lower dimensional water transfer models the disturbance estimated using the proposed model is consistent with the observed gravity change at asama volcano in central japan during the rainy summer of 2006 the model reproduces the rapid increase and subsequent gradual decrease in gravity following rainfall events the water mass within 150 m of the gravimeter is shown to dominate the observed gravity change during precipitation it is also demonstrated that the use of adequately representative soil parameters is essential in order to accurately estimate the groundwater distributions and consequent gravity variations this study shows that correcting for hydrological disturbances requires a more sophisticated model of water movements particularly during heavy rainfall

spatiotemporal gravity variations to look deep into the southern flank of etna volcano 1 a 14 year long microgravity data set october 1994 to september 2007 collected along a 24 km east west trending profile of 19 stations was analyzed to detect underground mass redistributions related to the volcanic activity involving the southern flank of mt etna italy a multiresolution wavelet analysis was applied to separate the volcano related anomalies from the unwanted components the residual image having both spatial extension and temporal duration evidenced two complete gravity increase decrease cycles mainly affecting the central and eastern stations of the profile the first gravity increase early 1995 to the end of 1996 and decrease end of 1996 to late in 1998 cycle reached a maximum amplitude of approximately 90 gal the second gravity increase mid 1999 to mid 2000 and decrease mid 2000 to early 2004 cycle attained an amplitude of about 80 gal after about 5 years of a persistent negative gravity anomaly a new semicycle started at the end of 2006 and continued during the last survey carried out in september 2007 the density changes modeled over time since 1994 using a quadratic programming algorithm are mainly located at a depth of 2 4 km bsl in a region recognized to be a preferential pathway of magma rising and an intermediate zone of magma storage withdrawal the computed positive mass variations of about 105 109 kg were interpreted as magma accumulation while negative mass changes of about 120 109 kg were associated with either magma drainage or opening of new voids by tectonic stresses within a source volume where tensional earthquakes occurred

mechanism of explosive eruptions of kilauea volcano hawaii a small explosive eruption of kilauea volcano hawaii occurred in may 1924 the eruption was preceded by rapid draining of a lava lake and transfer of a large volume of magma from the summit reservoir to the east rift zone this lowered the magma column which reduced hydrostatic pressure beneath halemaumau and allowed groundwater to flow rapidly into areas of hot rock producing a phreatic eruption a comparison with other events at kilauea shows that the transfer of a large volume of magma out of the summit reservoir is not sufficient to produce a phreatic eruption for example the volume transferred at the beginning of explosive activity in may 1924 was less than the volumes transferred in march 1955 and january february 1960 when no explosive activity occurred likewise draining of a lava lake and deepening of the floor of halemaumau which occurred in may 1922 and august 1923 were not sufficient to produce explosive activity a phreatic eruption of kilauea requires both the transfer of a large volume of magma from the summit reservoir and the rapid removal of magma from near the surface where the surrounding rocks have been heated to a sufficient temperature to produce steam explosions when suddenly contacted by groundwater

constraints on density changes in the funnel shaped caldera inferred from gravity monitoring of the lusi mud eruption abstract from its inception may 29 2006 the lusi mud eruption has continuously erupted fluids boiling mud and clasts through large active vents approximately 100 m in diameter in 2016 we conducted a dynamic gravity survey dg using a network built over four locations and two continuous gravity monitoring cg experiments to monitor the eruption activity the cg was done for 8 days from june 2nd to june 10th and for 9 days from august 20th to august 29th 2016 atmospheric pressure and atmospheric temperature variations were recorded during each experiment to constrain potential environmental effects and earth and oceanic tide effects were removed from gravity signals cg and dg atmospheric pressure effects were removed from cg gravity signals at the station closest to the hydrothermal pond the dg survey results show a gravity increase of 0 009 mgal month 1 which we interpret as the growth of the mud edifice in the central area cg monitoring shows that gravity variations occur at a period of 12 13 h with amplitudes reaching up to 0 020 mgal we interpret this as relating to density variation of the rising mud mixture fluids coarse mud clasts the observed 12 13 h period variations appear to indicate that tides may have some control on the density change of rising mud mixture by triggering the release of gas trapped at depth our 3d gravity results around the lusi vents show that density variations range from 100 kg m 3 to 775 kg m 3 similarly vent diameters better constrain density contrasts occurring within the caldera zone which is more likely to range between 400 and 450 kg m 3 and is equivalent respectively to 27 and 31 of gas ratio change over time

late cretaceous to paleocene oroclinal bending in the central pontides turkey 1 the turkish pontide fold and thrust belt formed since the paleozoic and is an important element in the africa eurasia convergence and the resulting closure of the neo tethys ocean it has a peculiar arc shaped geometry in its central part along the black sea coast which may have resulted from oroclinal bending we have determined the vertical axis rotation history of this area using paleomagnetism on cretaceous to eocene rocks from 47 sites and critically analyzed previously published data we applied the same reliability criteria to all data our results show that late cretaceous sites have clockwise and counterclockwise rotations perpendicular to the structural trend in the central pontides in the eastern pontides they show only local rotations paleocene to eocene rocks in the central and eastern pontides show no rotation we conclude that the central pontide northward arc shaped geometry results from oroclinal bending in latest cretaceous to earliest paleocene times the timing and scale of geological processes that occurred in the region make it likely that orocline formation resulted from neo tethys closure between the pontides and the metamorphic promontory of the anatolide tauride block earlier studies on the southerly located cankiri basin reveal that clockwise and counterclockwise rotations occurred in eocene oligocene times this implies that the entire region underwent continuous deformation from late cretaceous to eocene caused by convergence of the pontides and the anatolide tauride block with a southward moving deformation front deformation was first localized in the northern part of the central pontides until the paleocene resulting in oroclinal bending and from at least eocene times it shifted toward the cankiri basin region

tectonic record magmatic history and hydrothermal alteration in the hercynian guerande leucogranite armorican massif france abstract the guerande peraluminous leucogranite was emplaced at the end of the carboniferous in the southern part of the armorican massif at the scale of the intrusion this granite displays structural heterogeneities with a weak deformation in the southwestern part whereas the northwestern part is marked by the occurrence of s c and mylonitic extensional fabrics quartz veins and pegmatite dykes orientations as well as lineations directions in the granite and its country rocks demonstrate both e w and n s stretching therefore during its emplacement in an extensional tectonic regime the syntectonic guerande granite has probably experienced some partitioning of the deformation the southwestern part is characterized by a muscovite biotite assemblage the presence of restites and migmatitic enclaves and a low abundance of quartz veins compared to pegmatite dykes in contrast the northwestern part is characterized by a muscovite tourmaline assemblage evidence of albitization and gresenization and a larger amount of quartz veins the southwestern part is thus interpreted as the feeding zone of the intrusion whereas the northwestern part corresponds to its apical zone the granite samples display continuous compositional evolutions in the range of 69 8 75 3 wt sio 2 high initial 87 sr 86 sr ratios and low e nd t values suggest that the peraluminous guerande granite a cnk 1 1 was formed by partial melting of metasedimentary formations magmatic evolution was controlled primarily by fractional crystallization of k feldspar biotite and plagioclase an20 the samples from the apical zone show evidence of secondary muscovitization they are also characterized by a high content in incompatible elements such as cs and sn as well as low nb ta and k rb ratios the apical zone of the guerande granite underwent a pervasive hydrothermal alteration during or soon after its emplacement u th pb dating on zircon and monazite revealed that the guerande granite was emplaced 309 7 1 3 ma ago and that a late magmatic activity synchronous with hydrothermal circulation occurred at ca 303 ma these new structural petrological and geochronological data presented for the guerande leucogranite highlight the interplay between the emplacement in an extensional tectonic regime magmatic differentiation and hydrothermal alteration and provide a general background for the understanding of the processes controlling some mineralization in the western european hercynian belt

crustal growth during back arc closure cretaceous exhumation history of cordillera darwin southern patagonia the cordillera darwin metamorphic complex is unique in the andes in exposing kyanite staurolite schist north of the beagle channel in southern patagonia garnet in amphibolite facies pelitic schists from bahia pia has patchy textures whereby some grains consist of clear grossular rich garnet with fine grained s1 inclusion trails truncated by regions of turbid spessartine pyrope rich garnet with biotite muscovite plagioclase and quartz inclusions micron scale aqueous inclusions in turbid garnet are consistent with recrystallization facilitated by fluid ingress s2 inclusion trails indicate this was broadly contemporary with the growth of kyanite and staurolite in the matrix pseudosection modelling in na2o cao k2o feo mgo al2o3 sio2 h2o tio2 fe2o3 nckfmashto is used to infer a p t path dominated by decompression from 12 to 9 kbar at t 620 c coupled with garnet mode decreasing from 5 to 1 u th pb in situ dating of s2 monazite indicates that staurolite and kyanite growth and thus exhumation was underway before 72 6 1 1 ma contact aureoles developed adjacent to late granite intrusions include sillimanite bearing migmatites formed at p 6 kbar after 72 ma metamorphism of southern cordillera darwin induced by continental underthrusting beneath the arc related to closure of the rocas verdes back arc basin was terminated by thrusting controlled exhumation with the rocks at p 9 kbar by c 73 ma and 6 kbar by c 70 ma

magma movements in etna volcano associated with the major 1991 1993 lava eruption evidence from gravity and deformation the 1991 1993 eruption was probably the largest on mt etna for 300 years since then the volcano has entered an unusually quiescent period a comprehensive record of gravity and ground deformation changes presented here bracket this eruption and give valuable insight into magma movements before during and after the eruption the gravity and deformation changes observed before the eruption 1990 1991 record the intrusion of magma into the summit feeder and the sse trending fracture system which had recently been active in 1978 1979 1983 and 1989 creating the feeder dyke for the 1991 1993 eruption in the summit region gravity changes between 1992 and 1993 spanning the end of the eruption reflect the withdrawal of magma from the conduit followed more recently 1993 1994 by the re filling of magma in the conduit up to pre eruption levels in contrast in the vicinity of the fracture zone gravity has remained at the 1991 1992 level indicating that no withdrawal has occurred here rather magma has solidified in the fracture system and sealed it such that the 1993 1994 increase in magma level in the conduit was not accompanied by further intrusion into the flanks mass calculations suggest that a volume of at least 107 m3 of magma has solidified within the southeastern flank of the volcano

very long period seismic signals observed before the caldera formation with the 2000 miyake jima volcanic activity japan 1 very long period seismic signals whose waveform consists of an initial impulsive signal and a later oscillatory wave with 0 2 or 0 4 hz in dominant frequency were observed before the caldera formation in the 2000 activity of miyake jima volcano japan the results of waveform inversion show that the initial and later parts can be explained by a northward single force of 1 5 108 n working at a depth of 2 km beneath the summit and a moment tensor solution at a depth of 5 km below and 2 km southwest of the summit with 1012 nm respectively a clear positive correlation of the amplitudes between the two sources strongly suggests that the shallow single force triggers the deeper moment source in spite of the several km distance between the two sources to analyze the source time functions of the moment tensor that do not always oscillate in phase we introduce a new method of moment tensor diagonalization which is performed in the frequency domain according to the analysis the two principal components have similar amplitudes and are greater than the third principal component suggesting an axially symmetric oscillation one of the possible systems is a combination of two cracks intersecting perpendicularly our interpretation is that the single force was generated when magma containing rock blocks suddenly began to move in a choked subsurface magma path and the resultant pressure waves propagated and excited a resonance oscillation of the two cracks

magma dynamics and collapse mechanisms during four historic caldera forming events 1 four historic caldera forming events were studied to understand the relationship of magma withdrawal processes and caldera subsidence mechanisms two calderas are silicic katmai in 1912 and pinatubo in 1991 and two are basaltic fernandina in 1968 and miyakejima in 2000 all events have sufficient geophysical geologic and petrologic data with which to examine and model magma withdrawal and caldera collapse the data reveal that the magmas erupted at katmai and pinatubo were in a bubbly state in the reservoir immediately before and during caldera collapse the bubbly magma allowed for its efficient extraction from the reservoir causing significant underpressures to develop rapidly particularly in the case of katmai where the erupted rhyolite was voluminous and nearly aphyric and has very low viscosity the rapidly developing underpressures at katmai and pinatubo caused sudden en masse caldera collapse halfway through the climactic eruptions thereby liberating large amounts of seismic energy at fernandina and miyakejima by contrast caldera collapse was initiated early and continued for an extended period of time from weeks to months consisting of a series of discrete subsidence events manifested by large earthquakes at fernandina and by very long period vlp signals at miyakejima systematic changes in earthquake magnitudes and quiescent intervals at both volcanoes reveal changes in friction as collapse took place during extended time intervals

geothermal reservoir monitoring with a combination of absolute and relative gravimetry microgravity monitoring is a valuable tool for mapping the redistribution of subsurface mass and for assessing changes in fluid recharge from reservoir boundaries associated with geothermal exploitation to further the development of a high precision absolute relative hybrid gravity measurement technique we conducted measurements using an absolute gravimeter in two geothermal fields in japan the absolute gravity measurements were performed in the central production areas to directly measure gravity changes caused by fluid withdrawal we succeeded in measuring long term trends within an accuracy of a few microgals in the okuaizu and ogiri fields which have been producing electricity for several years absolute measurements in the center of the field provide reliable and local reference datum anchor points for more widely distributed relative gravity measurements in the ogiri field we carried out time lapse hybrid measurements with this combination of absolute and relative gravimetry and delineated the

continental underthrusting and obduction during the cretaceous closure of the rocas verdes rift basin cordillera darwin patagonian andes 1 the patagonian andes record a period of cretaceous neogene orogenesis that began with the compressional inversion of a late jurassic rift called the rocas verdes basin detrital zircon ages from sediment that filled the southern part of the basin provide a maximum depositional age of 148 ma suggesting that the basin opened approximately simultaneously along its length during the late jurassic structural data and u pb isotopic ages on zircon from granite plutons near the beagle channel 55 s show that basin inversion involved two stages of shortening separated by tens of millions of years an initial stage created a small 60 km wide thrust wedge that placed the basaltic floor of the rocas verdes basin on top of adjacent continental crust prior to 86 ma structures and metamorphic mineral assemblages preserved in an exhumed middle to lower crustal shear zone in cordillera darwin suggest that this obduction was accompanied by south directed subduction of the basaltic crust and underthrusting of continental crust to depths of 35 km beneath a coeval volcanic arc a subsequent stage of out of sequence thrusting culminating in the paleogene shortened basement and upper jurassic igneous rock in the internal part of the belt by at least 50 km forming a bivergent thrust wedge this latter period coincided with the exhumation of rocks in cordillera darwin and expansion of the fold thrust belt into the magallanes foreland basin this orogen provides an important example of how orogenesis initiated and led to continental underthrusting and obduction of basaltic crust during closure of a quasi oceanic rift basin

spatiotemporal gravity changes at miyakejima volcano japan caldera collapse explosive eruptions and magma movement 1 calderas are ubiquitous topographic features of volcanos yet caldera formation itself has not been recorded intensively by modern measurement techniques here we report the spatiotemporal gravity changes before and after caldera collapse at the miyakejima volcano japan in 2000 a gravity decrease of as much as 145 gal 1 gal 10 8 m s2 at the summit area since june 1998 was detected 2 days prior to the collapse interpreted as reflecting the formation of a large void beneath the volcano gravity changes detected after the initiation of collapse can mostly be corrected by the effect of collapsed topography from which a rapid rate of collapse of more than 1 6 107 m3 d can be inferred correcting for the effect of topography change we identified a temporal decrease in gravity from the middle of july to late august despite ground subsidence the gravity decrease is interpreted as a reduction of the density in a cylindrical conduit attributed to water inflow from an ambient aquifer that also promoted intensive magma water interaction and subsequent explosive eruptions from september to at least november 2000 gravity values at all sites increased significantly by a degree that cannot be explained by ground displacement alone we interpret this temporal evolution as primarily due to magma ascent and refilling of the conduit

cretaceous and early tertiary paleomagnetic results from the antarctic peninsula paleomagnetic results are presented for early tertiary and cretaceous igneous and volcanic units from the northern antarctic peninsula when combined with previously published data they permit the construction of a polar wander path that is very similar to one derived for the east antarctic craton by rotating tertiary and cretaceous poles from india and australia into the antarctic frame of reference when considered in this context the paleomagnetic data from the orville coast which was previously interpreted as evidence in favor of oroclinal bending or relative rotation of the southern antarctic peninsula with respect to the northern part may be considered as an in situ resulte the implications of the new data are as follows 1 the antarctic peninsula down to the orville coast has been a part of the antarctic plate since 100 m y ago 2 movement of the antarctic peninsula required to avoid overlap in reconstructions of gondwanaland must have occurred before 100 m y ago

the magallanes and deseado fault zones major segments of the south american scotia transform plate boundary in southernmost south america tierra del fuego the 600 km long magallanes fault zone and the newly discovered 100 km long deseado fault zone represent large but little understood segments of the south american scotia sam sco transform on the south american continent 50 56 s latitude kinematic analyses of fault populations near mount hope on tierra del fuego indicate that these fault zones have accommodated sinistral strike slip motion with a probable component of normal slip on vertical subvertical faults during cenozoic time regional geological data and overprinting relationships between the strike slip oblique slip faults and mid cretaceous to oligocene contractional structures suggest that sinistral motion in the mount hope segments of the two fault zones occurred significantly since 60 ma and dominantly since 30 ma with the most recent episode of activity occurring during the quaternary results from mapping show 20 25 km of cumulative sinistral separation of a thrust contact across a well exposed segment of the magallanes fault zone north and east of mount hope although the exact amount of displacement accommodated by this segment of the magallanes fault zone remains uncertain the 20 25 km of cumulative sinistral separation combined with an estimate of 3 km of vertical separation is consistent with small magnitudes of displacement of the order of tens of kilometers this result apparently contrasts with predictions based on plate reconstructions that large amounts of strike slip displacement 200 km have been accommodated internally within southernmost south america since early cenozoic time a low magnitude of displacement along the mount hope segment of the magallanes fault zone suggests that 1 strike slip motion has been partitioned across the continent since about 30 ma and or 2 the mount hope segment of the magallanes fault zone represents a young part of the modern plate boundary that migrated to its present location during the last few million years

new global marine gravity model from cryosat 2 and jason 1 reveals buried tectonic structure gravity models are powerful tools for mapping tectonic structures especially in the deep ocean basins where the topography remains unmapped by ships or is buried by thick sediment we combined new radar altimeter measurements from satellites cryosat 2 and jason 1 with existing data to construct a global marine gravity model that is two times more accurate than previous models we found an extinct spreading ridge in the gulf of mexico a major propagating rift in the south atlantic ocean abyssal hill fabric on slow spreading ridges and thousands of previously uncharted seamounts these discoveries allow us to understand regional tectonic processes and highlight the importance of satellite derived gravity models as one of the primary tools for the investigation of remote ocean basins

relationship between uplift of the metamorphic core of the southernmost andes and shortening in the magallanes foreland fold and thrust belt tierra del fuego chile rocks containing upper amphibolite facies assemblages reflecting mesozoic cenozoic metamorphism form the 5000 km2 core of the southernmost andes in cordillera darwin approximately 55 s latitude results from mapping along a 50 km continuous transect across the southernmost andes on the main island of tierra del fuego show structural relationships between the late cretaceous to early tertiary uplift of the metamorphic core in northern cordillera darwin and mid cretaceous to paleogene shortening in the adjacent magallanes foreland fold and thrust belt new data indicate that pre late jurassic basement schists exposed along the northeastern edge of cordillera darwin were uplifted along a discrete brittle continent vergent thrust fault this thrust overlies a series of thin skinned thrusts and back thrusts that affect mesozoic tertiary rocks of the magallanes foreland basin the thrust that uplifts basement schists is interpreted to have formed out of sequence with respect to the thin skinned structures because it placed the schists over previously shortened lower cretaceous sedimentary strata of the magallanes basin the geometry of thrusts observed across the transect and their inferred sequence of deformation indicate a thick skinned contractional mechanism for uplift of the pre late jurassic schists of northern cordillera darwin relative to the structural level of metamorphic basement in the magallanes foreland the relative time of out of sequence thrusting and basement uplift inferred from thrust geometries in this paper is consistent with the latest cretaceous to early tertiary episode of rapid exhumation and cooling of the metamorphic core of the southern andean orogen determined in previous structural metamorphic and geochronological studies

small basins in the scotia sea the eocene drake passage gateway after isostatic correction for their sedimentary fill the depths of two small oceanic basins in the southern scotia sea suggest that both started to open in mid to late eocene times models of the short magnetic anomaly profiles across the basins provide support for these determinations the location of the basins adjacent to the present day drake passage marine gateway and the timing of continental stretching leading up to their opening during the ypresian early eocene onset of global cooling mean that their importance is potentially far greater than their small size implies extension in the region of the two basins would have opened drake passage to shallow or intermediate depth oceanic circulation between the pacific and atlantic oceans for the first time this coincided with a reorganisation of vertical mixing patterns in the global ocean a shift in the site of carbon sequestration from coal swamps and peatlands to ocean sediments and the onset of a long decline in atmospheric carbon dioxide concentrations cenozoic global cooling may therefore have begun as a result of the shallow opening of drake passage

gravity and potential changes due to shear and tensile faults in a half space analytic expressions derived from a simplified model are invaluable because they often provide deep insight into geophysical phenomena such is the case with potential and gravity changes here we present expressions of potential and gravity changes caused by faulting on a finite rectangular plane buried in a homogeneous half space the expressions can be used to evaluate coseismic changes in surface gravity and geoid height observed gravimetric data combined with our formulae thus provide constraints on the fault geometry and magnitudes of dislocation

first archaeomagnetic field intensity data from ethiopia africa 1615 12 ad abstract first archaeointensity determinations have been obtained from ethiopia seven bricks 34 specimens from the dabsan archaeological remains were subjected to archaeointensity determination by means of classical thellier thellier experiment including tests for magnetic anisotropy and magnetic cooling rate dependency the age of the dabsan palace is well controlled by historical information between 1603 when land grants were conceded to the jesuits and the catholicism was established as the official religion in ethiopia and the age of the palace foundation in 1626 27 successful archaeointensity determinations were obtained in 27 specimens from five individual bricks revealing an average field value of 33 5 1 1 t which is 11 26 lower than expected values from global geomagnetic models based on historical and archaeomagnetic data global models for 1615 ad predict a low in central southern africa related to past location of the present southern atlantic anomaly saa our results suggest that the field intensity in central africa may have been slightly lower than global model predictions this would indicate that the low could be probably more extended towards central eastern africa or more intense than previously considered further data from this region are especially welcome to delineate the evolution of the saa

magma chamber volume changes associated with ring fault initiation using a finite sphere model application to the aira caldera japan abstract we have derived an analytical solution expressing the magma chamber volume change required for ring fault caldera initiation in terms of a finite sphere model we consider the conditions by which magma chamber contraction volume reduction could contribute to the initiation of a ring fault at the surface above the chamber using the elastic plastic theory of host rock behaviour the model does not consider underpressure pressure less than lithostatic in the chamber but only magma chamber volume reduction or shrinkage the model is heuristic in the sense that we do not explore the detailed mechanical condition for the volume reduction apart from its being related to magma flow out of the chamber but rather focus on the consequences of the reduction for ring fault initiation and dip the solution obtained for the finite sphere model consists of a leading term that is proportional to the cube of the magma chamber depth and a second term that is proportional to the cube of the magma chamber radius the second term reflects explicitly the effects of a finite source radius whereas the first term corresponds to a solution for a point source model this follows because in a point source model the magma chamber radius is zero so that the second term drops out the equation that plays the key role in obtaining this solution is a linear function relating the radius of the caldera to the magma chamber depth the proportionality coefficient of this equation is dimensionless and depends not only on the elastic constants of the medium but also on the depth d and radius a of the magma chamber given by e a d in addition because this coefficient controls a geometric parameter related to the overall form of the ring fault our results indicate that the ring fault shape and size is decided by the magma chamber depth and size as well as by the host rock elastic properties and angle of internal friction the results indicate that the associated ring fault is inward dipping and that the magma chamber radius can be estimated from the fault radius and dip we applied the analytical solution to the ring fault of the aira caldera in southern kyushu japan the calculations indicate that the aira ring fault could have been initiated at the surface as a result of 2 8 reduction in the volume of a magma chamber with a radius of 4 7 km and at a depth of 9 1 km these results agree with independent information on the aira magma chamber we conclude that the present solution yields useful first order information on magma chambers associated with ring faults

new paleomagnetic data from the antarctic peninsula and their tectonic implications new paleomagnetic data presented here further constrain the relative motion of the antarctic peninsula relative to east antarctica during the mesozoic development of the southern ocean basins the antarctic peninsula ap is one of four crustal blocks that define west antarctica the others being the ellsworth whitmore mountains ewm thurston island eights coast ti and marie byrd land mbl a jurassic pole 155ma 124 e 64 s a95 7 1 n 10 vgps was obtained from the ap block which suggests that the ap block rotated clockwise between 175 and 155 ma due to significant early opening in the weddell sea basin a new early cretaceous paleomagnetic pole 182 e 74s a95 5 9 n 6 site mean vgps indicates that the ap block was in or near to its present day position with respect to east antarctica by 130 ma between 155 and 130 ma counterclockwise rotation of the ap ti blocks together with the southward motion of east gondwanaland probably resulted in subduction of weddell sea ocean floor beneath the southern ap block and initiated the palmer land deformational event a 130 ma pole from the ti block requires clockwise rotation of the ti and possibly the ewm blocks between 130 and 110 ma producing sinsistral strike slip motion between the ewm block and east antarctica and dextral transpressional motion between the ti ewm blocks and the ap block new ap block 110 ma and 85 ma poles from this study 199 e 74 s a95 6 9 n 13 vgps 152 e 86 s a95 7 5 n 6 vgps respectively are similar to equivalent age poles from east antarctica and suggest little or no relative motion between the antarctic peninsula and east antarctica northern and southern antarctic peninsula mid cretaceous poles are very much alike suggesting that the s shape of the antarctic peninsula is not due to oroclinal bending since 110 ma

paleointensity of the geomagnetic field recovered on archaeomagnetic sites from france abstract twenty new paleointensity determinations have been obtained from archaeomagnetic sites from france time intervals investigated range from 65 to 380 ad and from 1370 to 1700 ad two hundred forty two samples of tiles and bricks were treated with the classical thellier method for paleointensity determination rock magnetic studies identify magnetite substituted maghemite and hematite as the main magnetic minerals as is usual with archaeomagnetic samples especially tiles magnetic anisotropy was observed in all samples the effect of thermoremanent magnetization trm anisotropy upon the paleointensity values was corrected after the determination of the tensor of trm anisotropy in each sample these determinations allow corrections of the field strength obtained which are usually lower than 20 the effect of the cooling rate on the trm intensity acquired was also determined and seems relatively low on our samples most frequently eight to 10 samples per site were used to calculate the mean intensity standard deviations are typically lower than 7 of the average value during the roman period small variations of the field intensity in paris are observed between 62 and 70 t while during the more recent period the intensity of the field decreases from 62 to 52 t an analysis of the paleointensity data available for the last 2000 years obtained from sites in western europe was performed using a weighing factor which takes into account the number and type of the samples studied as well as the technique used for the paleointensity determination this weighing factor clearly shows that most of the existing data might not be reliable the analysis performed indicates that more data is needed especially from the time interval 500 to 1400 ad before it would be possible to determine a reference curve of the time variations of the archaeomagnetic intensity of the field in western europe

tectonic reconstructions for paleobathymetry in drake passage abstract a minimum complexity tectonic reconstruction based on published and new basin opening models depicts how the scotia sea grew by cenozoic plate divergence dismembering a jurassic sheared margin of gondwana part of the jurassic early cretaceous ocean that accreted to this margin forms the core of the central scotia plate the arc plate above a trench at the eastern end of the scotia sea which migrated east away from the antarctic and south american plates a sequence of extensional basins opened on the western edge of the central scotia plate at 50 30 ma decoupled from the south american plate to the northwest by slow motion on a long transform fault succeeding the basins seafloor spreading started around 30 ma on the west scotia ridge which propagated northwards in the 23 17 ma period and ceased to operate at 6 ma the circuits of plate motions inside and outside the scotia arc are joined via rotations that describe antarctic central scotia plate motion in powell basin until 20 ma and along the south scotia ridge thereafter the modelled relative motion at the northern edge of the scotia sea is thus constrained only by the plate circuit but nonetheless resembles that known coarsely from the geological record of tierra del fuego a paleobathymetric interpretation of nine time slices in the model shows drake passage developing as an intermediate depth oceanographic gateway at 50 30 ma with deep flow possible afterwards initially this deep flow would have been made tortuous by numerous intermediate and shallow barriers a frontal pattern resembling that in the modern scotia sea would have awaited the clearance of significant barriers by continuing seafloor spreading in the scotia sea at 18 5 ma at shag rocks passage and after 10 ma southeast of south georgia

the curved magallanes fold and thrust belt tectonic insights from a paleomagnetic and anisotropy of magnetic susceptibility study the magallanes fold and thrust belt ftb presents a large scale curvature from n s oriented structures north of 52 s to nearly e w in tierra del fuego island we present a paleomagnetic and anisotropy of magnetic susceptibility ams study from 85 sites sampled in cretaceous to miocene marine sediments magnetic susceptibility is lower than 0 0005 si for 76 sites and mainly controlled by paramagnetic minerals ams results indicate that the sedimentary fabric is preserved in the undeformed areas of tierra del fuego and the more external thrust sheets units where an incipient lineation due to layer parallel shortening is recorded prolate ams ellipsoids indicating a significant tectonic imprint in the ams fabric are observed in the internal units of the belt ams results show a good correlation between the orientation of the magnetic lineation and the fold axes however in peninsula brunswick the ams lineations are at 20 counterclockwise to the strike of the fold axes pretectonic stable characteristic remanent magnetizations chrm were determined in seven sites a counterclockwise rotation 21 2 9 2 is documented by chrm data from four sites near the hinge of the belt in peninsula brunswick and near canal whiteside while there is no evidence of rotation near the nearly e w oriented vicuna thrust within tierra del fuego the curved shape of the cenozoic magallanes ftb is not related to vertical axis rotation and thus the magallanes ftb can be considered as a primary arc

crystallization of accessory phases in magmas by local saturation adjacent to phenocrysts abstract accessory minerals commonly occur attached to or included in the major crystalline phases of felsic and some intermediate igneous rocks apatite is particularly common as inclusions but fe ti oxides pyrrhotite zircon monazite chevkinite and xenotime are also known from silicic rocks accessories may nucleate near the host crystal liquid interface as a result of local saturation owing to formation of a differentiated chemical boundary layer in which accessory mineral solubility would be lower than in the surrounding liquid differentiation of this boundary layer would be greatest adjacent to ferromagnesian phenocrysts especially fe ti oxides it is with oxides that accessories are most commonly associated in rocks a boundary layer may develop if the crystal grows more rapidly than diffusion can transport incorporated and rejected elements to and from the phenocryst diffusion must dominate over convection as a mode of mass transfer near the advancing crystal liquid interface in order for a boundary layer to exist accumulation of essential structural constituent elements of accessory minerals owing to their slow diffusion in evolved silicate melt also may force local saturation but this is not a process that applies to all cases local saturation is an attractive mechanism for enhancing fractionation during crystallization differentiation if accessory minerals attached to or included in phenocrysts formed because of local saturation their host phenocrysts must have grown rapidly when accessories nucleated in comparison to lifetimes of magma reservoirs some inconsistencies remain in a local saturation origin for accessory phases that cannot be evaluated without additional information

subsurface cyclic magma sill expansions in the 2000 miyakejima volcano eruption possibility of two phase flow oscillation 1 step like tilt changes were observed at intervals of dozens of hours during the caldera formation stage occurred during the 2000 miyakejima volcano eruption our analysis suggests that these tilt steps were caused by the cyclic expansion of a subsurface sill like magma plumbing system trending from se to nw of the volcano in the same period a caldera gradually grew at the summit area to a very large 1 6 km in diameter and 450 m in depth the source mechanism of these tilt steps is fairly well explained by a two phase flow instability called a pressure drop oscillation this model indicates that the gas phase capacity in the upper part of the conduit acts as a pump that controls the flux of two phase magma in the sill like reservoir caldera formation as well as magma uplift and descent in the conduit may reflect temporal changes in the intervals between tilt steps which are themselves proportional to the volume of this gas capacity

the patagonian batholith s of tierra del fuego chile timing and tectonic implications reconnaissance field studies on the patagonian batholith exposed s of tierra del fuego together with 49 new k ar and rb sr mineral dates render new insights into two main aspects of the late mesozoic geology the geochronology and composition of the batholith and time relations between the emplacement of the plutons and deformation the envelope of the batholith in this area is mainly composed of late jurassic early cretaceous sedimentary and volcanic rocks formed in an island arc marginal basin system three main plutonic groups have been recognized which have yielded mineral dates interpreted as crystallization ages ranging from 141 to 34 ma the earliest is a gabbroic assemblage including gabbros diorites and hornblendites exposed as relicts within later granitoids and with dates ranging from 141 to 103 ma the canal beagle plutonic group is composed of granitoids characterized by a penetrative syn magmatic foliation and with dates of 113 to 81 ma the seno ano nuevo plutonic group is formed of granitoids with dates of 60 to 34 ma and is mainly exposed in the outer islands indicating a local oceanwards shift of plutonism with time lower cretaceous island arc volcanism was contemporaneous with the intrusion of the gabbroic assemblage and in part at least of the canal beagle plutonic group folding of the marginal basin infill which has been related to the closure of the marginal basin occurred from about 100 to 93 ma ago the gabbroic assemblage was apparently mainly pre tectonic whereas the seno ano nuevo plutonic group was clearly intruded after this deformational event a post tectonic or possibly late tectonic emplacement for the youngest plutons of the canal beagle plutonic group and a pre or syn tectonic intrusion for the oldest is inferred

constraining south atlantic growth with seafloor spreading data recent models of south atlantic opening history focus on early plate divergence by incorporating intracontinental deformation which is poorly constrained aiming to avoid the uncertainties in this approach we model the entire divergence history with a joint inversion for seafloor spreading data for this history the pre campanian motion parameters are the first to feature formal uncertainty estimates we date the onset of spreading at 138 ma with movement along intracontinental accommodation zones leading to the assembly of south america by 123 ma and africa by 106 ma part of the ridge in the agulhas basin jumped westward soon afterward toward the bouvet plume initiating the motion of a short lived malvinas plate the ne georgia and maud rises and agulhas plateau formed as a large igneous province over the plume farther north part of the ridge jumped eastward toward the tristan plume around 94 93 ma but seems not to have resulted in independent plate motion our results show that the south atlantic grew by diachronous breakup of continents on just two plates cretaceous intracontinental deformation in south america and africa can be interpreted in terms of the accommodation of stress associated with northward propagation of this process the pattern of accommodation is usually envisaged as focusing all of the strain in narrow belts with our rotations a commonly used set of such belts accounts instead for just 42 67 of the implied total strain we suggest that the remainder was accommodated at all scales within the continental interiors and the extended continental margins

very long period seismic signals and caldera formation at miyake island japan over a period of roughly 40 days starting on 8 july 2000 a caldera structure 1 7 kilometers in diameter developed by means of gradual depression and expansion of the summit crater at miyake island japan at the same time very long period vlp seismic signals were observed once or twice a day source mechanism analyses of the vlp signals show that the moment tensor solutions are smooth step functions over a time scale of 50 seconds with dominant volumetric change components we developed a model to explain the caldera and the vlp signals in which a vertical piston of solid materials in the conduit is intermittently sucked into the magma chamber by lateral magma outflow this model offers potential for making quantitative estimations of the characteristic physical properties of magma systems

trm deviations in anisotropic assemblages of multidomain magnetite anisotropic assemblages of multidomain magnetite particles develop an anisotropy of magnetic susceptibility ams which in turn induces deviations of thermo remanent magnetization trm from the field direction from the theories of multidomain trm acquisition it is shown that the trm anisotropy tensor has its eigenvalue ratios ti related to the principal weak field susceptibility ratios pi by the order of magnitude ti p2i this relation has been experimentally verified on two sets of highly anisotropic rock samples the exponent has been determined to be 1 94 in the samples from a peruvian gabbro and 1 81 in those from the granite of flamanville nw france accounting for experimental difficulties in determining the trm anisotropy tensors these exponents are judged to agree well with the expected one it is therefore stressed that ams measurements provide a good means of evaluating the magnetic field direction from deviated trm directions providing magnetic carriers are mainly multidomain magnetites

response to the comment by w h schwarz et al on joint determination of 40k decay constants and 40ar 40k for the fish canyon sanidine standard and improved accuracy for 40ar 39ar geochronology by p r renne et al 2010

paleomagnetism and tectonics of the south shetland islands and the northern antarctic peninsula new paleomagnetic data presented here from 61 sites in paleozoic mesozoic and cenozoic igneous and sedimentary rocks from the antarctic peninsula and the south shetland islands constrain the relative motion of the antarctic peninsula since the mid cretaceous and allow the quantification of tectonic rotation between the different blocks recognized within the area paleozoic and jurassic results failed the fold test and suggest an important remagnetization in the area the similarity between these results and those obtained from cretaceous intrusives indicates a mid cretaceous age for the remagnetization the paleopoles obtained for the different blocks and for different ages suggest that there is no relative rotation among them these combined results allow us to obtain a cretaceous 90 ma and paleocene 60 ma paleopole these paleopoles document a very low apparent polar wandering of the antarctic peninsula for the last 100 ma

40ar 39ar geochronology and p t t paths from the cordillera darwin metamorphic complex tierra del fuego chile ar 39ar data collected from hornblende muscovite biotite and k feldspar constrain the p t t history of the cordillera darwin metamorphic complex tierra del fuego chile these data show two periods of rapid cooling the first between c 500 and c 325 c at rates 225 c ma and the second between c 250 and c 200 c for high t cooling 40ar 39ar ages are spatially disparate and depend on metamorphic grade rocks that record deeper and hotter peak metamorphic conditions have younger 40ar 39ar ages sillimanite and kyanite grade rocks in the south central part of the complex cooled latest 40ar 39ar hbl 73 77 ma ms 67 70 ma bt 68 ma and oldest kfs 65 ma thermobarometry and p t path studies of these rocks indicate that maximum burial of 26 30 km at 575 625 c may have been followed by as much as 10 km of exhumation with heating of 25 50 c staurolite grade rocks have intermediate 40ar 39ar ages hbl 84 86 ma ms 71 ma bt 72 75 ma and oldest kfs 80 ma thermobarometry on these rocks indicates maximum burial of 19 26 km at temperatures of 550 580 c garnet grade rocks have the oldest ages ms 72 ma and oldest kfs 91 ma peak p t conditions were 525 550 c and 5 7 kbar regional metamorphic temperatures for greenschist facies rocks south of the beagle channel did not exceed c 300 325 c from 110ma to the present although the rocks are only 2 km from kyanite bearing rocks to the north one dimensional thermal models allow limits to be placed on exhumation rates assuming a stable geothermal gradient of 20 25 c km i the maximum exhumation rate for the st grade rocks is c 2 5 mm yr i whereas the minimum exhumation rate for the ky sil grade rocks is c 1 0 mm yr uniform exhumation rates cannot explain the disparity in cooling histories for rocks at different grades and so early differential exhumation is inferred to have occurred petrological and geochronological comparisons with other metamorphic complexes suggest that single exhumation events typically remove less than c 20 km of overburden this behaviour can be explained in terms of a continental deformation model in which brittle extensional faults in the upper crust are rooted to shallowly dipping ductile shear zones or regions of homogeneous thinning at mid to deep crustal levels the p t r data from cordillera darwin 1 are best explained by a wedge extrusion model in which extensional exhumation in the southern rear of the complex was coeval with thrusting in the north along the margin of the complex and into the magallanes sedimentary basin 2 suggest that differential exhumation occurred initially with st grade rocks exhuming faster than ky sil grade rocks and 3 show variations in cooling rate through time that correlate both with local deformation events and with changes in plate motions and interactions

tarawera 1886 new zealand a basaltic plinian fissure eruption abstract new zealand s biggest and most destructive volcanic eruption of historical times was that of tarawera in 1886 the resulting scoria fall has a dispersal very similar in extent to that of the vesuvius a d 79 pumice fall and is one of the few known examples of a basaltic deposit of plinian type a new estimate of the volume 2 km 3 is significantly greater than previous estimates the basalt came mainly from a 7 km length of fissure and emission and exit velocity were fairly uniform along at least 4 km of it this is one of the few documented examples of a plinian eruption from a fissure vent primary welding of the scoria fall resulted where the accumulation rate exceeded about 250 mm min 1 a model of the eruption dynamics is proposed which leads to an estimate of 28 km for the height of the eruption cloud and implies a magma volatile fraction of 1 5 3 violent phreatic explosions occurred in the southwestern extension of the fissure across the rotomahana geothermal field and it is thought that some of the water responsible for the power of the plinian eruption came from this source though its amount was not sufficient to turn the eruption into a phreatoplinian one