the ocean continent boundary off the western continental margin of iberia i crustal structure at 40 30 n summary the western continental margin of the iberian peninsula has the characteristics of a rifted non volcanic margin with half graben and tilted fault blocks seen in several places the ocean continent boundary ocb is therefore expected to be where thinned continental crust and oceanic crust are juxtaposed as at many similar margins worldwide it is particularly useful to locate the ocb off western iberia in order to constrain the pre rift fit of north america to iberia and by implication the shape of the proto bay of biscay the fit is only marginally constrained by sea floor spreading magnetic anomalies because anomaly 34 is believed to be far to the west of the ocb and it is even possible that all older oceanic crust was created during the cretaceous constant polarity interval the best way to distinguish oceanic crust from thinned continental crust appears to be the crustal seismic velocity structure therefore in 1986 a series of seismic refraction lines was shot parallel to and normal to the continental margin these lines enabled us to bracket the location of the ocb a further constraint on the location was obtained by modelling an east west magnetic profile which included the enigmatic j anomaly this anomaly can be explained as either just pre anomaly mo or as part of the cretaceous constant polarity interval depending on whether spreading began about 127 or after 118 myr ago respectively the evidence favours the former explanation lastly the depth to acoustic basement was contoured from a compilation of seismic reflection profiles this indicated a new fracture zone at 41 15 n which offsets the ocb a few key reflection profiles also suggest that the ocb can be identified by an abrupt landward step down in acoustic basement we conclude that the ocb in the eastern iberia abyssal plain lies between 12 10 and 12 30 w and has a trend just east of north this westerly location is consistent with recent estimates of the location of the ocb off the grand banks but brings into question the proposed location at about 11 w of the ocb in the tagus abyssal plain

crustal structure of the labrador sea conjugate margin and implications for the formation of nonvolcanic continental margins wide angle seismic studies have determined the detailed velocity structure along a 350 km long profile across the labrador margin combination of this model with a previously published cross section for the southwestern greenland margin constitutes the first combined conjugate margin study based on seismic velocity structure the results indicate three distinct zones across the labrador margin similar to the structure of the conjugate greenland margin zone 1 represents 27 to 30 km thick continental crust thinning gradually seaward over 100 km distance farther seaward zone 2 is 70 80 km wide characterized by a distinct lower crust 4 5 km thick in which velocity increases with depth from 6 4 to 7 7 km s interpretation for this lower crustal block favors an origin by serpentinized peridotite rather than by magmatic under plating zone 3 represents two layered normal oceanic crust the cross sections from both margins are reconstructed to an early drift stage at chron 27 this demonstrates that the serpentinites in zone 2 are symmetrically distributed between previous identifications of chrons 31 and 33 on both margins zone 1 shows a marked asymmetry with a gradual thinning of continental crust off labrador contrasted with a rapid thinning off greenland the abundant serpentinization of upper mantle peridotite in zone 2 and the asymmetric shape of zone 1 are both probably related to a very slow rate of continental rifting which produced little if any melt

influence of offset weak zones on the development of rift basins activation and abandonment during continental extension and breakup 1 we use numerical modelling to investigate reactivation of inherited offset weak zones owz in continental crust and mantle weak zones mwz to form offset rift basins during continental rifting and breakup offset rift basins are basins that are set off offset from the main rift locus of breakup weak zones embedded in a stiff layer are preferentially and rapidly reactivated whereas the same zones are either ignored or slowly reactivated when embedded in pliable layers here stiff implies a nonlinear flow law with a high stress exponent n 10 000 a plastic material and pliable means a low stress exponent n 2 5 as in ductile power law creep of rocks whether offset rift basins form during rifting of a composite lithosphere i e comprising stiff and pliable layers depends on the competition between necking instabilities that develop at the weak zones in the stiff layers and the coupling between the stiff and pliable layers stiff cratonic lithosphere results in early localization of the deformation at the mwz rapid necking and breakup without developing offset rift basins in contrast warm pliable lithosphere develops significant offset basins and has protracted rifting because the mwz is now embedded in a pliable layer we also investigate the influence of owz dip sedimentation and the sensitivity of reactivation to the distance from owz to the mwz and to the size of the mwz a tectonic rifting styles diagram is used to show that the model results agree with natural examples

the structure of archean ketilidian crust along the continental shelf of southwestern greenland from a seismic refraction profile the velocity structure of the continental crust on the outer shelf of southwestern greenland is determined from dense wide angle reflection refraction data obtained with large air gun sources and ocean bottom seismometers along a 230 km seismic line this line crosses the geological boundary between the archean block and the ketilidian mobile belt although the data have high noise levels p and s wave arrivals from within the upper intermediate and lower crust and at the moho boundary can be consistently identified and correlated with one dimensional wkbj synthetic seismograms in the archean p and s wave velocities in the upper crust are 6 0 and 3 4 km s while in the intermediate crust they are 6 4 and 3 6 km s these velocities match for the upper crust a quartz feldspar gneiss composition and for the intermediate crust an amphibolitized pyroxene granulite in the ketilidian mobile belt p and s wave velocities are 5 6 and 3 3 km s for the upper crust and 6 3 and 3 6 km s for the intermediate

crustal taper and topography at passive continental margins terra nova 23 349 361 2011 abstract the long term evolution of kilometre s high seaward facing escarpments at passive margins is linked directly to the crustal thinning gradient the development of post rift faults in the onshore parts of the margin and the associated distribution of drainage patterns landscape types and sediment dispersal patterns reflect this linkage for the seismically well imaged norwegian margin as well as for a number of passive margins worldwide we identify a scaling relationship that correlates escarpment height with the distance to the highly extended distal margin or more specifically the taper break the highest escarpments and the most asymmetric margin topography are found inboard of sharply tapering crystalline crust independent of margin age conversely escarpments are lower where the taper is gentle thus the topography of passive margins does not primarily reflect age magmatism climate or mantle convection but more probably the response to loading of the laterally variable fault controlled architecture of the crystalline crust

source amplitudes of volcano seismic signals determined by the amplitude source location method as a quantitative measure of event size article i nfo the amplitude source location asl method which uses high frequency amplitudes under the assumption of isotropic s wave radiation has been shown to be useful for locating the sources of various types of volcano seismic signals we tested the asl method by using synthetic seismograms and examined the source amplitudes determined by this method for various types of volcano seismic signals observed at different volcanoes our synthetic tests indicated that although asl results are not strongly influenced by velocity structure and noise they do depend on site amplification factors at individual stations we first applied the asl method to volcano tectonic vt earthquakes at taal volcano philippines our asl results for the largest vt earthquake showed that a frequency range of 7 12 hz and a q value of 50 were appropriate for the source location determination using these values we systematically estimated source locations and amplitudes of vt earthquakes at taal we next applied the asl method to long period events at cotopaxi volcano and to explosions at tungurahua volcano in ecuador we proposed a practical approach to minimize the effects of site amplifications among different volcano seismic networks and compared the source amplitudes of these various volcano seismic events with their seismic magnitudes we found a proportional relation between seismic magnitude and the logarithm of the source amplitude the asl method can be used to de termine source locations of small events for which onset measurements are difficult and thus can estimate the sizes of events over a wider range of sizes compared with conventional hypocenter determination ap proaches previously there has been no parameter widely used to quantify the sources of volcano seismic signals this study showed that the source amplitude determined by the asl method may be a useful quan titative measure of volcano seismic event size

shallow s wave attenuation and actively degassing magma beneath taal volcano philippines taal volcano philippines is one of the world s most dangerous volcanoes given its history of explosive eruptions and its close proximity to populated areas a real time broadband seismic network was recently deployed and has detected volcano tectonic events beneath taal our source location analysis of these volcano tectonic events using onset arrival times and high frequency seismic amplitudes points to the existence of a region of strong attenuation near the ground surface beneath the east flank of volcano island in taal lake this region is beneath the active fumarolic area and above sources of pressure contributing inflation and deflation and it coincides with a region of high electrical conductivity the high attenuation region matches that inferred from an active seismic survey conducted at taal in 1993 these features strongly suggest that the high attenuation region represents an actively degassing magma body near the surface that has existed for more than 20 years

high seismic velocities associated with reflections from within the lower oceanic crust near the continental margin of eastern canada abstract a deep seismic reflection profile on the continental rise off eastern canada shows strong dipping reflections apparently from within lower oceanic crust over a region of limited lateral extent the origin of these and similar reflections described elsewhere is uncertain this paper reports new seismic refraction results from the region of these reflections they appear to be associated with a layer of p wave velocity of about 7 4 km s 1 or higher while the surrounding crust has a velocity 7 km s 1 the moho with velocity 8 1 km s 1 lies just below the reflectors and the total crustal thickness is around 8 km the crust is interpreted to be oceanic rather than continental and the high velocities of the dipping reflectors are comparable to those of the mafic ultramafic crust mantle transition zone in the bay of islands ophiolite complex

combined electromagnetic geochemical and thermal surveys of taal volcano philippines during the period 2005 2006 since 1572 33 phreatic to phreatomagmatic eruptions have occurred on taal volcano philippines some of them causing several hundred casualties considering the time delay between two consecutive eruptions there is an 88 probability that taal volcano should have already erupted since 1992 several phases of seismic activity have been recorded accompanied by ground deformation opening of fissures and surface activity the volcanic activity of taal appears to be controlled by dike injections and magma supply buffered by a hydrothermal system that releases fluids and heat through boiling and subsequent steaming in early 2005 a multidisciplinary project was launched for studying the hydrothermal activity to map the hydrothermal system combined surveys were carried out to investigate self potential total magnetic field ground temperature and carbon dioxide soil degassing along with satellite thermal imaging of the main crater lake the elevated temperatures and high concentrations of carbon dioxide as well as electromagnetic anomalies indicate large scale hydrothermal degassing this process is enhanced along the tectonic features e g crater rim and faults of the volcano while active fissures opened along the e w northern flank during the 1992 1994 seismic activity heat and fluids from the hydrothermal system are essentially released in the northern part of the crater which is bounded to the south by a suspected nw se fault along which seismicity seems to take place and dikes are thought to be intruded during the january 2005 surveys a new seismic crisis started and the felt earthquakes prompted spontaneous evacuation of hundreds of inhabitants living on the volcano repeated surveys show changes of self potential total magnetic field and ground temperature with time without any noticeable spatial enlargement these observations suggest that the northern flank located between the crater rim and the 1992 1994 fissures is connected with a deep thermal source in main crater and is reactivated during seismic crises this sector could be subjected to flank failure

the structure evolution and symmetry of the magma poor rifted margins of the north and central atlantic a synthesis abstract magma poor rifted margins consistently show extreme crustal thinning accompanied by normal faulting the serpentinization of the mantle beneath crust thinned to less than 8 2 km and the unroofing of a broad zone of mantle within the continent ocean transition accompanied by the development of detachment and other large offset faults these observations are the logical result of the progressive extension of cool lithosphere away from thermal anomalies such as plumes although the paucity of magmatism may be explained by depth dependent extension of the lithosphere and by pre depleted sub crustal lithosphere rifting above initially cool sub lithospheric mantle also may explain the subsidence deficit observed at some margins synrift subsidence is buffered by the simultaneous influx of warmer oceanic asthenosphere as it extends thins and cools ductile creeping layers in the mid and deep crust become progressively more brittle resulting in increased coupling between the upper and lower crust and eventually the embrittlement of the entire crust faults cutting from the surface across the moho bringing water into the mantle and causing its serpentinization increased coupling and the development of serpentine detachments predict the development of late stage asymmetry once the entire crust is brittle such detachments are imaged on some margins and inferred on others analysis of the crustal structure across conjugate margins shows that these are approximately symmetric until this late stage when they become markedly asymmetric similar analyses show that depth dependent stretching of the crust is insufficient to explain the discrepancy between the amount of the visible extension along faults and the amount of crustal thinning instead this extension discrepancy may be related to the complex evolution of brittle deformation through multiple phases and styles of faulting related to the changes in the rheological character and strength of the lithosphere as it is thinned complex polyphase faulting continues after complete crustal separation resulting in the exhumation of broad expanses of peridotitic basement the top of which is everywhere marked by an exhumed slip surface similar to the corrugated surface observed at mid ocean ridges the similarity in processes between mantle unroofing and seafloor spreading makes the distinction between the cot and true oceanic crust difficult and possibly moot

contrasting styles of rifting models and examples from the eastern canadian margin we present the results of a dynamical model of lithospheric rifting and rupture which show that a wide range of crustal thinning patterns across rifted passive margins can be produced by varying the steady state geotherm lithospheric composition dry versus wet materials and strain rate the basic mechanism of continental rupture is assumed to be passive rifting and necking we use a numerical thermomechanical model of lithosphere extension based on a finite element approach when plasticity is significant i e at lower temperatures or for harder materials deformation is unstable and thinning takes place abruptly over a narrow area conversely a progressive thinning across the margin is observed when creep is dominant i e in warm or ductile conditions cooling and associated hardening of the thinned area can occur during extension and cause the locus of extension to migrate laterally in these circumstances rupture is likely to take place asymmetrically along one edge of the thinned area producing a narrow margin and a very wide conjugate the eastern margins of canada and their conjugates across the north atlantic provide examples which cover this range of theoretical profiles the crustal thinning patterns inferred from deep seismic data and the duration of rifting compare well with model results we discuss also the constraints that these geodynamical models provide on such current issues as the seismic reflectivity of the lower crust or the location of the ocean continent boundary in wide areas supposedly underlain by 5 km thin continental crust

seismic amplitude ratio analysis of the 2014 2015 bar arbunga holuhraun dike propagation and eruption magma is transported in brittle rock through dikes and sills this movement may be accompanied by the release of seismic energy that can be tracked from the earth s surface locating dikes and deciphering their dynamics is therefore of prime importance in understanding and potentially forecasting volcanic eruptions the seismic amplitude ratio analysis sara method aims to track melt propagation using the amplitudes recorded across a seismic network without picking the arrival times of individual earthquake phases this study validates this methodology by comparing sara locations filtered between 2 and 16hz with the earthquake locations same frequency band recorded during the 2014 2015 bar arbunga holuhraun dike intrusion and eruption in iceland integrating both approaches also provides the opportunity to investigate the spatiotemporal characteristics of magma migration during the dike intrusion and ensuing eruption during the intrusion sara locations correspond remarkably well to the locations of earthquakes several exceptions are however observed 1 a low frequency signal was possibly associated with a subglacial eruption on 23 august 2 a systematic retreat of the seismicity was also observed to the back of each active segment during stalled phases and was associated with a larger spatial extent of the seismic energy source this behavior may be controlled by the dike s shape and or by dike inflation 3 during the eruption sara locations consistently focused at the eruptive site 4 tremor rich signal close to ice cauldrons occurred on 3 september this study demonstrates the power of the sara methodology provided robust site amplification quality factors and seismic velocities are available plain language summary locating earthquakes usually implies picking phase arrivals p and s waves another technique called seismic amplitude ratio analysis sara was recently introduced to locate them only by using the amplitude recorded at different pairs of seismic stations however this technique was never proven to be true this study shows that the earthquake locations derived by sara compares remarkably well with the locations of 30 000 seismic events triggered when magma migrated in the icelandic crust prior to the 2014 2015 holuhraun eruption but the results also provide new insight into the magma dynamics that led to the largest eruption of the last two centuries in europe we show that ground vibration was continuously triggered during the 2week period preceding the eruption when magma forced its way toward the eruption site but also during the eruption itself several intriguing features were observed including low frequency vibrations possibly associated with eruption below the ice or large patches of seismic activity when the magma stopped propagating toward the eruption site this methodology performs very well provided some parameters are available and allows to gain insights into the complex dynamics associated with magma movements

crustal structure of a nonvolcanic rifted margin east of newfoundland a detailed seismic refraction study was undertaken of the rifted continental margin southeast of the grand banks of newfoundland along a preexisting deep multichannel seismic reflection profile the primary objectives were to determine the topography of moho across the margin to detect any evidence for crustal underplating and to determine the position of the continent ocean boundary data were obtained from nine ocean bottom seismometers deployed along the profile with an array of six 1000 inch3 air guns as the source two additional refraction lines were shot parallel to structure over the continental slope the resultant velocity model is well constrained and consistent with the observed gravity the crust thins abruptly beneath the continental slope to a major structural transition identified as the continent ocean boundary there is no significant magmatic underplating of the thinned continental crust seaward of the transition the seismic velocity structure is characterized by a 50 km wide zone of intermediate velocity 7 2 7 6 km s beyond this is a thin 3 5 km crust with low velocity 4 5 5 km s and high gradient over mantle with a velocity of 7 7 km s increasing to 8 0 km s the zone of intermediate velocity is interpreted to be highly serpentinized mantle material beneath a very thin crust suggesting little or no magmatic activity during and immediately after rifting this interpretation is consistent with results from adjacent and conjugate margins

tomographic inversion of p wave velocity and q structures beneath the kirishima volcanic complex southern japan based on finite difference calculations of complex traveltimes summary we estimate the p wave velocity and attenuation structures beneath the kirishima volcanic complex southern japan by inverting the complex traveltimes arrival times and pulse widths of waveform data obtained during an active seismic experiment conducted in 1994 in this experiment six 200 250 kg shots were recorded at 163 temporary seismic stations deployed on the volcanic complex we use first arrival times for the shots which were hand measured interactively the waveform data are fourier transformed into the frequency domain and analysed using a new method based on autoregressive modelling of complex decaying oscillations in the frequency domain to determine pulse widths for the first arrival phases a non linear inversion method is used to invert 893 first arrival times and 325 pulse widths to estimate the velocity and attenuation structures of the volcanic complex wavefronts for the inversion are calculated with a finite difference method based on the eikonal equation which is well suited to estimating the complex traveltimes for the structures of the kirishima volcano complex where large structural heterogeneities are expected the attenuation structure is derived using ray paths derived from the velocity structure we obtain 3 d velocity and attenuation structures down to 1 5 and 0 5 km below sea level respectively high velocity pipe like structures with correspondingly low attenuation are found under the summit craters these pipe like structures are interpreted as remnant conduits of solidified magma no evidence of a shallow magma chamber is visible in the tomographic images

crustal structure of the goban spur rifted continental margin ne atlantic summary we present results that constrain the crustal structure across a typical non volcanic rifted continental margin the goban spur in the ne atlantic traveltime and amplitude modelling of wide angle ocean bottom seismograph profiles shows that the continental crust thins in a seaward direction to about 7 km before it breaks to allow a new oceanic spereading centre to develop the crustal velocity structure the stretching factors and the density distribution inferred from gravity anomalies are consistent with bulk pure shear stretching for the development of the rifted continental margin the seismic velocity structure of the thinned continental crust suggests that thinning was accompanied by only limited igneous intrusion and extrusion in the upper crust the adjacent oceanic crust is abnormally thin 5 4km this is explained most readily by accretion at a very slow spreading centre approx 11 mm yr full rate although additional cooling by conductive heat loss from the parent mantle as it rose slowly beneath the stretched and thinned continental lithosphere together with minor igneous intrusion into and surface flows onto the adjacent continental crust may also have reduced the oceanic igneous thickness immediately adjacent to the continent ocean transition

seismic image of undercrusted serpentinite beneath a rifted margin during the continental rifting the upper mantle was unroofed and the mantle rocks were transformed into serpentinite at the ocean continent transition of the west galicia margin spain the serpentinite layer several km thick extends probably eastwards beneath the highly thinned continental crust of the margin the serpentinite layer was recently imaged by seismic reflection it is discontinuously and deeply layered as serpentinized peridotite can have densities and seismic velocities comparable to those of the lower continental crust we suggest that undercrusting by serpentinite can play a part in building the lower seismic crust in highly stretched continental rifted areas

thin crust at the western iberia ocean continent transition and ophiolites western iberia is bounded by a nonvolcanic rifted continental margin made up of three apparently independent segments the age of breakup decreases from south to north seismic refraction and reflection profiles and magnetic and gravity data from each segment show a consistent pattern of geophysical observations across the ocean continent transition oct zone which is a few tens of kilometers wide we emphasize here the discovery of thin 2 4 km oceanic crust underlain by 7 6 km s 1 material within the oct the available evidence favors the suggestion that the 7 6 km s 1 layer is serpentinized peridotite and that the thin oceanic crust is primarily the result of a poor magma supply for a few million years immediately after continental breakup this thin crust may be the source of some ophiolites which exhibit thin crustal sections and continental margin affinities

relative importance of strain rate and rheology for the mode of continental extension summary both extension rate and rheology have been shown to influence the style of continental rifting so far their effect on the dynamics of extension has been discussed separately in the literature leading to diverging conclusions the rate of extension mostly affects the dynamics of rifting through the occurrence of syn rift cooling at low extension rates this cooling results in an increase of strength if the lithosphere has a temperature dependent rheology england 1983 first suggested that this effect will limit the amount of extension that an area may undergo kusznir park 1987 argued more generally that it will induce a widening of the rift and concluded that extension rate is the major control on wide versus narrow rifting later studies bassi 1991 bassi keen potter 1993 however have demonstrated the importance of rheological layering within the lithosphere for the mode of continental extension by showing that a plasticity dominated rheology will lead to rapid and narrow necking while a creep dominated rheology results in wider rifts and margins gravitational stresses arising from density anomalies due to lithosphere thinning also affect the style of rifting buck 1991 in this paper we examine the combined effect of these different parameters in order to reconcile earlier studies and assess the importance of syn rift cooling for the process of lithospheric necking the approach introduced in a previous study bassi 1991 uses a finite element technique to simulate extension of a vertical cross section of the lithosphere the effect of cooling on the necking pattern depends strongly on the rheology of the thinned lithosphere when it starts to cool if heat diffusion always induces strengthening and delays rupture in time it only modifies significantly the geometry of the rift when the upper mantle is weak and viscous in this case the locus of maximum strain rate migrates laterally once the central area starts to cool and harden as hypothesized by kusznir park 1987 eventually strain concentrates at the transition between deformed and undeformed lithosphere we predict that this pattern will induce asymmetric breakup and produce a very narrow margin and a wide uniformly thinned conjugate margin in all other cases i e when the upper mantle is initially in the low plasticity regime or if it reaches the yield stress during extension the rate of extension appears to have little effect on the rifting pattern

a synthesis and review of historical eruptions at taal volcano southern luzon philippines abstract the philippines is an area of persistent volcanism being located in one of the most tectonically active regions in the world taal volcano in southern luzon is the second most frequently erupting volcano of the 24 active volcanoes in the philippines a comprehensive and critical review of published and unpublished references describing the 33 known historical eruptions of taal may provide answers to knowledge gaps on past eruptive behavior processes and products that could be utilized for hazard and risk assessment of future eruptions data on the prehistoric eruptions and evolution of taal caldera and subsequent deposits are limited only four caldera forming events were identified based on four mapped ignimbrite deposits from oldest to youngest these are the silicic alitagtag ali and caloocan cal pumice flow deposits the dacitic sambong ignimbrite sam and the basaltic andesitic taal scoria flow renamed scoria pyroclastic flow sfl except for sfl with 14c dating yielding 5380 70 to 6830 80 ky there are no age constraints or estimates of extent for the three older deposits a comprehensive review of the historical eruptions of taal volcano is the central element of this paper and includes all eruptions from ad1572 the first known historic event to ad1977 eruption styles and the interplay between processes and products for each eruption are reinterpreted based on the narrative descriptions from all available accounts a change of classification of eruption styles and eruptive products is undertaken for some events at least nine reported eruptions were deemed uncertain including the ad1605 ad1611 event more likely seismic swarms the ad1634 ad1635 and ad1645 may simply be solfataric or hydrothermal activity events and the ad1790 ad1825 ad1842 ad1873 and ad1903 events that were listed in recent published and unpublished documents but do not provide any details to describe and confirm the eruptions except for listing a default vei of 2 pyroclastic density currents brought devastating impacts to the communities around taal during the ad1749 ad1754 ad1911 and ad1965 eruptions and remain the biggest threat in the case of renewed volcanic activity significant implications for aviation are implied by the narrative of tephra fall dispersal towards manila the central gateway of international aviation operation in the philippines during the ad1754 eruptions the dispersal of tephra in the event of an explosive eruption at taal towards metro manila would have catastrophic effects to transport utilities and business activity potentially generating enormous economic losses hazards from earthquake events associated with future volcanic activity may also have localized impacts occurrences of liquefaction phenomena as a consequence of severe ground shaking are interpreted during the ad1749 ad1754 and ad1911 eruptions more work needs to be done to develop a comprehensive understanding of the hazards and risks associated with an explosive eruption at taal volcano especially related to the older quaternary caldera forming eruptions that produced large volume pyroclastic deposits that are extensively distributed and exposed we acknowledge that there may be additional prehistoric eruptions where the eruptive products have not been preserved recognized or reported events that cannot be verified or do not have sufficient details to confirm the eruption have been downgraded to uncertain eruptions that are confirmed with identified dispersal and emplacement of tephra fall and other eruptive deposits as interpreted from narrated records could provide crucial information that may be utilized in hazard assessment

the geodynamic province of transitional lithosphere adjacent to magma poor continental margins two different types of transitional lithosphere have been documented along magma poor rifted margins one consists of apparently subcontinental mantle that has been exhumed brit tlely deformed and serpentinized during late stages of rifting a second is thinned 10 km con tinental crust which in some cases is known to have been supported near sea level at least early in the rift history and thus is interpreted to reflect depth dependent extension in both cases it is typically assumed that oceanic crust forms at the time that the brittle continental crust is breached or soon thereafter and thus that transitional lithosphere has relatively limited width here three representa tive cases of transitional lithosphere are examined one in the newfoundland iberia rift and one at goban spur both exhumed mantle and one off the angola congo margin thin continental crust flanked seaward by apparently exhumed lower continental crust exhumed mantle considering the geological and geophysical evidence it appears that depth dependent extension riftward flow of weak lower continental crust and or upper mantle may be a common phenomenon on magma poor margins and that this can result in a much broader zone of transitional lithosphere than has hitherto been assumed transitional lithosphere in this wide zone may consist of subcontinental mantle lower continental crust or some combination thereof depending on the strength profile of the pre rift continental lithosphere transitional lithosphere ceases to be emplaced i e final break up occurs only when emplacement of heat and melt from the rising asthenosphere becomes domi nant over lateral flow of the weak lower lithosphere this model implies a two stage break up first the rupture of the brittle continental crust and second the eventual separation of the ductile sub continental lithosphere which is coincident with emplacement of normal oceanic crust well defined magnetic anomalies can form in transitional lithosphere that consists of highly serpenti nized exhumed mantle and such anomalies therefore are not diagnostic of oceanic crust where present the anomalies can be helpful in interpreting and dating the rifting history

tectonic evolution of southern baffin bay and davis strait results from a seismic refraction transect between canada and greenland 1 wide angle reflection refraction seismic data were acquired on a 450 km long transect in southern baffin bay extending from baffin island to greenland dense air gun shots were recorded on 22 ocean bottom seismometers apwave velocity model was developed from forward and inverse modeling of the observed travel times beneath the baffin island shelf a three layered continental crust is observed with velocities of 5 5 to 6 9 km s typical for transform margins there is a sharp transition between continental and oceanic crust off baffin island 7 km thick oceanic crust is interpreted to lie in a major transform fault identified on the gravity map beneath the deep baffin bay basin 9 km thick oceanic crust is encountered but thins to 6 km within an assumed fracture zone the thicker than normal oceanic crust indicates an ample magma supply possibly related to melt extracted from a mantle plume seaward of the greenland continental crust 20 km thick igneous crust 6 3 to 7 3 km s is encountered in a 25 km wide zone interpreted as a leaky transform fault that can be correlated southward through davis strait the igneous crust is bounded by a 20 km wide basin to the west underlain by 4 km thick crust of unknown affinity this structure is probably associated with transform movements a high velocity lower crustal layer 7 1 km s of 8 km thickness is indicated beneath the greenland crust and can be correlated into the adjacent thick igneous crust both the thick igneous and greenland crust are covered by up to 4 km of paleogene volcanics 5 2 to 5 7 km s

internal structure of a spreading segment of mesozoic oceanic crust we define the crustal architecture and internal structure of a 70 km long segment of mesozoic oceanic crust combined reflection and wide angle refraction profiles show the relations between reflectivity and seismic velocity within the crust the crust thins by 4 km from the segment center to the bounding fracture zones a 2 km thick lens at the base of the crust in the segment center exhibits enhanced reflectivity in the otherwise mainly opaque lower crust which coincides with abnormally high seismic velocities of 7 5 km s we attribute these characteristics to the presence of a melt injection region the reflection profiles in the 4 km spaced grid show that there are two distinct approximately orthogonal sets of dipping reflectors within the crust ridge normal reflectors dip at typically 25 in both directions the majority dip west toward the spreading center and their common association with basement steps suggests that they represent normal faults they terminate above or near the base of the crust ridge parallel reflectors are shallower dipping averaging 15 and planar many are restricted to the middle crust and we speculate that they represent magmatic rather than tectonic features a prominent dipping reflector beneath a fracture zone cuts at 20 into the upper mantle similar reflectors are found elsewhere beneath fracture zones in the north atlantic and may be caused by thermal stresses generated as the lithosphere cooled and moved away from the spreading axis

dynamics of active magmatic and hydrothermal systems at taal volcano philippines from continuous gps measurements 1 a dense network of continuous single and dual frequency gps receivers at taal volcano philippines reveals four major stages of volcanogenic deformation deflation from installation in june 1998 to december 1998 inflation from january to march 1999 deflation from april 1999 to february 2000 and inflation from february to november 2000 the largest displacements occurred during the february november 2000 period of inflation which was characterized by 120 mm of uplift of the center of volcano island relative to the northern caldera rim at average rates up to 216 mm yr deformation sources were modeled using a constrained least squares inversion algorithm the source of 1999 deflation and inflation in 2000 were modeled as contractional and dilatational mogi point sources centered at 4 2 and 5 2 km depth respectively beneath volcano island the locations of the inflationary and deflationary sources are indistinguishable within the 95 confidence estimates modeling using a running 4 month time window from june 1999 to march 2001 reveals little evidence for source migration we suggest that the two periods of inflation observed at taal result from episodic intrusions of magma into a shallow reservoir centered beneath volcano island subsequent deflation may result from exsolution of magmatic fluids and or gases into an overlying unconfined hydrothermal system

factors influencing magmatism during continental breakup new insights from a wide angle seismic experiment across the conjugate seychelles indian margins we present a model of the northern seychelles continental margin derived from controlled source wide angle seismic traveltime inversion and teleseismic receiver functions this margin has been widely cited as a classic example of rifting in association with a continental flood basalt province the deccan traps however we do not find the typical set of geophysical characteristics reported at other margins linked to continental flood basalts such as those of the north atlantic the oceanic crust formed immediately after breakup and throughout the first 3 ma of seafloor spreading is just 5 2 km thick less than half that typically seen at other volcanic margins the continent ocean transition zone is narrow and while two packages of seaward dipping reflectors are imaged within this transition they are weakly developed beneath the thinned continental crust there is an approximately 4 km thick layer of high velocity material 7 5 7 8 km s that we interpret as mafic material intruded and underplating the lower crust however we believe that this underplating most likely happened prior to the breakup overall the observations show that the rifting of india from the seychelles was characterized by modest magmatism the spatial extent of the deccan flood basalt province is therefore smaller than previously thought we speculate that either the lateral flow of deccan related hot material beneath the breakup region was hampered perhaps as the rifted margins did not intersect the center of the deccan source or there was incomplete melt extraction from the wide melting region that formed between the rapidly diverging plates if the latter explanation is correct then the rate of plate separation as indicated by the initial seafloor spreading rate is more important in controlling the volume of magmatism generated during continental rifting than has been previously recognized

the seismic structure of thinned continental crust in the northern bay of biscay summary the stretching and thinning of the continental crust which occurs during the formation of passive continental margins may cause important changes in the velocity structure of such crust further crust attenuated to a few kilometres thickness can be found underlying oceanic water depths this paper poses the question of whether thinned continental crust can be distinguished seismically from normal oceanic crust of about the same thickness a single seismic refraction line shot over thinned continental crust as part of the north biscay margin transect in 1979 was studied in detail tau p inversion suggested that there are differences between oceanic and continental crust in the lower crustal structure this was confirmed when synthetic seismograms were calculated the thinned continental crust p 7 0 exhibits a two gradient structure in the non sedimentary crust with velocities between 5 9 and 7 4 km s l an upper 0 8 s l layer overlies a 0 4 s l layer no layer comparable to oceanic layer 3 was detected the uppermost mantle also contains a low velocity zone

deep seismic reflection data across the conjugate margins of the labrador sea a deep seismic reflection transect of the conjugate margins of the labrador sea is described which represents one of the few data sets of this kind a characteristic reflectivity is ascribed to a 120 km wide ribbon of very thin crust that may be either thinned continental crust which has perhaps been intruded or oceanic crust perhaps modified by the proximity of the continent most of the major changes in crustal thickness and in the subsidence and sedimentation patterns on the margins occur landward of these transitional zones which are found on both margins an interpretation of these regions as continental in origin is compatible with other seismic observations on the west greenland margin but does not match the magnetic anomaly interpretation which requires the transitional crust to be oceanic in origin models that satisfy the gravity anomalies and the subsidence history have been used to assist in interpreting the seismic data the subsidence models include the effects of decompression meltin

three dimensional p and s wave velocity structure of redoubt volcano alaska the three dimensional p and s wave structure of redoubt volcano alaska and the underlying crust to depths of 7 8 km is determined from 6219 p wave and 4008 s wave first arrival times recorded by a 30 station seismograph network deployed on and around the volcano first arrival times are calculated using a finite difference technique which allows for flexible parameterization of the slowness model and easy inclusion of topography and source receiver geometry the three dimensional p wave velocity structure and hypocenters are determined simultaneously while the three dimensional s wave velocity model is determined using the relocated seismicity and an initial s wave velocity model derived from the p wave velocity model assuming an average vp vs ratio of 1 78 convergence is steady with approximately 73 and 52 reduction in p and s wave arrival time rms respectively after 10 iterations the most prominent feature observed in the three dimensional velocity models derived for both p and s waves is a relative low velocity near vertical pipelike structure approximately 1 km in diameter that extends from 1 to 6 km beneath sea level this feature aligns axially with the bulk of seismicity and is interpreted as a highly fractured and altered zone encompassing a magma conduit the velocity structure beneath the north flank of the volcano between depths of 1 and 6 km is characterized by large lateral velocity variations high velocities within this region are interpreted as remnant dikes and sills and low velocities as regions along which magma migrates no large low velocity body suggestive of a magma chamber is resolved in the upper 7 8 km of the crust

very low frequency resistivity self potential and ground temperature surveys on taal volcano philippines implications for future activity abstract taal volcano is one of the most dangerous volcanoes in the philippines thirty three eruptions have occurred through historical time with several exhibiting cataclysmic phases most recent eruptions are confined to volcano island located within the prehistoric taal collapse caldera that is now filled by taal lake the last eruptive activity from 1965 to 1977 took place from mt tabaro about 2 km to the southwest of the main crater center since this time episodes of seismic activity ground deformation gas release surface fissuring fumarole activity and temperature changes are recorded periodically around the main crater but no major eruption has occurred this period of quiescence is the third longest period without eruptive activity since 1572 in march 2010 a campaign based on very low frequency vlf resistivity surveys together with repeated surveys of self potential ground temperature and fissure activity was intensified and the results compared to a large scale electrical resistivity tomography experiment this work fortunately occurred before within and after a new seismovolcanic crisis from late april 2010 to march 2011 the joint analysis of these new data together with results from previous magnetotelluric soundings allows a better description of the electrical resistivity and crustal structure beneath the main crater down to a depth of several kilometers no indication of growth of the two geothermal areas located on both sides of the northern crater rim was apparent from 2005 to march 2010 these areas appear controlled by active fissures opened during the 1992 and 1994 crises that dip downward towards the core of the hydrothermal system located at about 2 5 km depth beneath the crater older mineralized fissures at lower elevations to the north of the geothermal areas also dip downward under the crater repeated self potential and ground temperature surveys completed between 2005 and 2015 show new geothermal and hydrothermal activity in the areas of these older mineralized fissures that occurred during the april 2010 to march 2011 seismovolcanic crisis this dramatically extends the geothermal activity further to the north on the volcano the occurrence of these newly activated fissures after a long period of quiescence and indications of inflation in mid 2010 under the north rim of the main crater suggests that new eruptive activity near the north rim of the crater could occur in the future

western davis strait a volcanic transform margin with petroliferous features abstract we present a compilation of the western davis strait region offshore southeastern baffin island nunavut canada with new subsurface geological structural details and observations regarding past hydrocarbon occurrences from scientific and commercial exploration this consists of seismic mapping with archival data correlated with a filtered marine bouguer anomaly gravity compilation and magnetic data sets covering northern saglek basin the western part of the lady franklin basin and the ungava fault zone from south of baffin bay a regional seismic horizon for mapping basin architecture comes from the top of the paleogene volcanic syn magmatic zone where pervasive volcanic flows and intrusions are intermingled with the sedimentary section the seismic depth map to the top of the regional volcanic seismic horizon shows pre rift sedimentary basins having maximum depths of approximately 4 5 km flanking the shallower ungava fault zone correlation of bouguer anomaly gravity and magnetic data interpretations with the seismic mapping indicate that over some areas true crystalline basement is deeper than can be determined by reflection seismic as the base of the syn magmatic section is not resolvable for seismic mapping extensive paleogene mafic intrusives and extrusive basalts dominate the architecture of this volcanic rifted margin as seen by the dominant high amplitude magnetic anomalies associated with many deeper structures the over 250 km long davis strait fault is recognised as the key structural element of the ungava fault zone ufz and valley complex the adjoining davis strait high maps on seismic as a continuous structure running the length of davis strait plunging southwards to a bouguer gravity high feature that terminates the western end of the extinct eocene spreading zone south of davis strait this horn shaped structural feature establishes the presence of an accommodation zone with multiple faults and localised thrust faulting as required to fit the complex mechanics for strike slip motion that occurs along the south end of this transform fault system the revised marine bouguer anomaly gravity data set also defines two new near shore grabens east of cumberland sound the southern tariut basin and the northern imaqpik basin each extending over 100 km in length with sediment thicknesses of at least 4 km these pre rift basins most likely originate from the paleozoic based on seafloor dredge and nearby shallow drill cores from previous studies the hydrocarbon charge of a previously unexplained petroliferous shallow marine drill core that is adjacent the eastern edge of the tariut basin is attributed to paleozoic source rocks that have undergone enhanced thermal maturation from sill intrusion associated with rifting the imaqpik basin shows strong evidence of a hydrocarbon system from the proximity of clustered interpreted sea surface oil slicks mapped from satellite radar images and a local zone of anomalously high dissolved methane measured within the water column that originates from the seafloor immediately east of cape dyer at the northern limit of that basin the overlying cape dyer flood basalt field extends from limited onshore exposures into the offshore and maps with magnetic data over an area of approximately 13 000 km2 this extrusive feature and it s implicit underlying intrusive components are likely the unconventional heat source for the anomalous enhanced thermal maturation of paleozoic to paleogene source rocks in these pre rift basins that host this previously unrecognised petroleum system

conjugate margins of canada and europe results from deep reflection profiling deep seismic reflection profiles have been collected across the conjugate margins of the north atlantic ocean the eastern north atlantic margin is traversed in the goban spur region and the western north atlantic margin is crossed in the vicinity of flemish cap in the grand banks region these seismic profiles allow us to examine the deep structures and mode of extension in crust that was once contiguous the flemish cap and goban spur margins have different structural styles thick and relatively unfaulted crust is present on the flemish cap margin west of the continent ocean boundary whereas the goban spur margin exhibits a zone of extensional faulting and thinned continental crust the restored rift zone displays overall symmetry in which a bridge of thin crust 15 km thick about 60 km wide joins the two crustal blocks of normal continental thickness 28 km the configuration of the moho the geometry and distribution of the lower crustal reflections and the overall symmetry of the rift zone favor pure shear stretching in the lower lithosphere although either pure or simple shear could accommodate crustal extension

the ocean continent boundary off the western continental margin of iberia ii crustal structure in the tagus abyssal plain summary an 80 km long reversed seismic refraction line line 5 was shot over the tagus abyssal plain off portugal the main p wave reflected and refracted phases were modelled both for traveltime and amplitude the resulting p wave velocity depth model has the following features a an extremely thin crust of about 2 km b the absence of oceanic layer 3 and c very low upper mantle velocities between 7 6 and 7 9 km s this very unusual seismic velocity crustal structure is quite unlike thinned continental crust but is remarkably similar to the seismic crustal structures found at atlantic fracture zones and in particular to the structures found in profiles shot along the transform valley and near ridge transform intersections a magnetic anomaly chart seems to allow the possibility of several fracture zones one of which could intersect the centre of line 5 as an alternative to the fracture zone hypothesis we show that if the oceancontinent transition in the tagus abyssal plain is located at about 11 30 w in a symmetric position with respect to the ocean continent transition in the conjugate south newfoundland basin then magnetic anomalies can be modelled simply by assuming sea floor spreading west of 11 45 w at 10 mm yr beginning at m11 time 133 myr bp and blocks of rifted continental crust to the east the location of the proposed ocean continent transition in the tagus abyssal plain is marked by a well defined n s linear magnetic anomaly which is adjacent to the oldest sea floor spreading block east of the proposed ocean continent transition there is an increase in the depth to basement similar to that found east of the ocean continent transition in the iberia abyssal plain and elsewhere this model also allows us to explain why purdy s 1975 seismic refraction line a ar in the tagus abyssal plain cannot be interpreted as a conventional reversed pair because most of line a was shot over the ocean continent transition zone and most of line ar over thinned continental crust remarkably similar velocity depth structures to that under line 5 are found close to the ocean continent transition zone off the whole of western iberia in areas which show no clear evidence of fracture zones therefore it appears more likely that the seismic structure of line 5 is due to its proximity to the ocean continent transition than to a local association with a fracture zone and further that its structure is typical of this transition off the western margin of iberia we also suspect that the low upper mantle velocities associated with the ocean continent transition indicate the widespread occurrence of serpentinized peridotite

evolution of nonvolcanic rifted margins new results from the conjugate margins of the labrador sea the crust across each margin of the labrador sea a conjugate rift margin pair consists of three well defined zones thinned continental crust 70 80 km wide transitional crust and oceanic crust the transition zone is characterized by a low velocity upper crust 4 5 km s underlain by a 6 4 7 7 km s layer we propose that the lower layer is serpentinized upper mantle it is less likely to be gabbroic igneous lower crust the low velocity upper crust may be either an oceanic basaltic layer or very thin 2 km continental crust results from this first combined seismic reflection and refraction study of a conjugate margin pair support very slow initial continental rifting 1 mm yr on average resulting in an asymmetric crustal thinning profile with breakup occurring against the west greenland shelf this asymmetry may be an important feature of rifting at nonvolcanic margins and may be related to strain hardening of the cooling and thinning lithosphere or to small scale convection in the lower lithosphere and asthenosphere at the sides of the rift the thin crust in the transition zone implies that the amount of melt generated in the upwelling mantle is negligible consistent with slow spreading or rifting the transition from this slow formational period to normal oceanic accretion processes is marked by a sharp decrease in basement depth signaling an increase to normal oceanic crustal thickness

geochemical evidence of magma intrusion inferred from diffuse co 2 emissions and fumarole plume chemistry the 2010 2011 volcanic unrest at taal volcano philippines we report herein significant increases in the diffuse co2 emission through the lake and soils of the taal volcano main crater philippines during a volcano seismic unrest period in april 2010 june 2011 monitoring of co2 emissions from the taal main crater lake and soils in the fumarole areas started in 2008 with a collaborative project between iter and phivolcs in august 2010 the total diffuse co2 emission from the taal main crater lake increased almost three times from a previously measured baseline emission value 1 000 t day 1 to 2 716 54 t day 1 this increase occurred during anomalous seismic activity which started in april 2010 the highest co2 emission value in the crater lake was measured in march 2011 at 4 670 159 t day 1 when the volcano was still showing signs of unrest significant changes in the co2 so2 co2 h2o and so2 h2s ratios in the fumarole gases suggest the occurrence of a magmatic injection several weeks before the onset of the volcano seismic unrest period these observed changes in the co2 emission rate and the fumarole gas composition as the result of magmatic activity were likely caused by magma moving from a deep reservoir to a shallow reservoir

the moho boundary above upper mantle peridotites or lower crustal eclogites a global review and new interpretations for passive margins abstract we have performed a global study of 2d crustal scale wide angle profiles across passive margins with regard to local elevations in the moho which could possibly be interpreted as indicative of lower crustal eclogites a total of 16 candidates have been found mainly in the north atlantic and around australia these cases make up c 6 only of the total profile length studied confirming the interpretation of the moho generally representing the top of the mantle the interpreted candidates for lower crustal eclogites indicate that there may be a link between eclogite bodies and continental suture zones in the barents sea off mid norway in the newfoundland appalachians and in the yilgarn craton western australia it is also possible that there is a genetic link between the formation of caledonian eclogites and the jan mayen fracture zone which is the only major fracture zone in the north atlantic several of the inferred lower crustal eclogite bodies are located close to lines of major changes in strain such as coastlines and shelf edges indicating that lower crustal eclogite bodies may be important in guiding the evolution of basin architecture interpreting the moho beneath the bedout high on the northwest shelf of australia as the top of a body of lower crustal eclogites may imply that the northern termination of the lambert shelf represents a paleo suture zone and that the western termination of the broome platform acted as a major transfer zone the significant increase in crustal thickness implied by the eclogite model has important implications for estimates of stretching history subsidence and hydrocarbon maturation modelling

kinematic evolution of the southern north atlantic implications for the formation of hyperextended rift systems we focus on the southern north atlantic rifted margins to investigate the partitioning and propagation of deformation in hyperextended rift systems using plate kinematic modeling the kinematic evolution of this area is well determined by oceanic magnetic anomalies after the cretaceous normal polarity superchron however the rift and early seafloor spreading evolution 200 83 ma remains highly disputed due to contentious interpretations of the j magnetic anomaly on the iberia newfoundland conjugate margins recent studies highlight that the j anomaly is probably polygenic related to polyphased magmatic events and therefore does not correspond to an isochron we present a new palinspastic restoration without using the j magnetic anomaly as the chron m0 we combine 3 d gravity inversion results with local structural stratigraphic and geochronological constraints on the rift deformation history the restoration of the southern north atlantic itself is not the primary aim of the study but rather is used as a method to investigate the spatiotemporal evolution of hyperextended rift systems we include continental microblocks that enable the partitioning of the deformation between different rift segments which is of particular importance for the evolution of the iberia eurasia plate boundary our modeling highlights the following 1 the segmentation of the iberia newfoundland rift system during continental crust thinning 2 the northward v shape propagation of mantle exhumation and seafloor spreading 3 the complex partitioning of deformation along the iberia eurasia plate boundary and 4 a three plate propagation model which implies transtension

characterization of scattered seismic wavefields simulated in heterogeneous media with topography 1 we performed numerical simulations of seismic waveforms with frequencies up to 10 hz in heterogeneous media with topography to investigate the effects of topography and structural heterogeneity on seismic scattering we used the simulated waveforms to test the source location method assuming isotropic radiation of s waves for long period events and tremor at volcanoes the assumption of isotropic radiation has been shown previously to be valid in a high frequency band because of the path effect caused by the scattering of seismic waves our simulation results showed that distortion of the wavefields increased as the correlation distance of structural heterogeneity a decreased as the frequency increased and as the travel distance increased topography alone considerably distorted the wavefields however we found that strong scattering due to topography was suppressed if the correlation distance of structural heterogeneity was longer than the s wave wavelength isotropic radiation of s waves by scattering due to topography was not achieved in our simulations our results indicated that scattering due to structural heterogeneity becomes stronger than that due to topography at ka 1 and a 50 m where k is the wave number of the s waves this suggests that strong short scale structural heterogeneity is required to achieve isotropic radiation of s waves although we could not reproduce isotropic radiation our results support the validity of the assumption of isotropic radiation in the source location method

the continent ocean boundary south of flemish cap constraints from seismic refraction and gravity a seismic refraction survey providing deep crustal structural information on the continent ocean boundary south of flemish cap on the east coast of canada was carried out using large air gun sources and ocean bottom seismometers the seismic refraction results and gravity modelling suggest that thinned continental crust extends 25 km seaward of the shelf break the transition from continental to oceanic crust with a main crustal layer p wave velocity of 7 3 km s extends seaward over 100 km to the south one refraction profile with thin 4 km oceanic crust was probably shot on or very near the trace of a fracture zone previous plate reconstructions have suggested that cretaceous age sea floor spreading south of flemish cap occurred as a series of short spreading segments offset by transform fauits or by asymmetric rifting between iberia and flemish cap this study suggests that an oblique shear margin may have formed south of flemish cap possibly as a result of transcurrent motion between flemish ca

undercrusting by serpentinite beneath rifted margins the west galicia passive margin spain is bounded by a continuous belt of serpentinized peridotite here we present the results of petrological studies of the peridotite 39ar 40ar dating of high temperature ductile shearing of a dioritic dyke crossing the peridotite and regional seismic stratigraphy which demonstrate that the uplift ductile shearing and pervasive serpentinization of mantle rocks occurred at early cretaceous time during the rifting stage of the margin the serpentinite layer resting on fresh peridotite is probably several kilometres thick at present seismic data and gravity data cannot distinguish serpentinite from crustal material but our results from the galicia margin indicate that the seismic crust beneath rifted margin may include mantle material transformed into serpentinite by syn rift hydrothermal activity in some cases this undercrusting process might explain the young age and mobility of the moho beneath stretched continental basement

theoretical basis of the amplitude source location method for volcano seismic signals the source location method using high frequency seismic amplitudes based on the assumption of isotropic radiation of s waves has been used successfully to locate seismic events and tremor at volcanoes this amplitude source location asl method overcomes the limitations of traditional hypocenter determination methods that use onset arrival times and has great potential to improve volcano seismic monitoring and the investigation of source processes of volcano seismic signals however theoretical justification of the basic assumption used in the asl method has not been provided in previous studies in this study we tested the asl method by using seismic waveforms simulated with a finite difference method in realistic heterogeneous volcanic structures with intrinsic attenuation and topography our results showed that asl determinations were successful when using waveforms characterized by multiple scattering that can be approximated by the diffusion model we found that the energy solution of the diffusion model provided an amplitude distance relation that is similar to that used in the asl method our results suggest that the asl method is applicable to high frequency seismograms in highly heterogeneous media with transport mean free paths of 103 m or smaller and strong intrinsic attenuation these medium parameters are consistent with those estimated at various volcanoes our study validates the basic assumption of the asl method and justifies its use to locate the sources of high frequency seismic signals observed at volcanoes

a large hydrothermal reservoir beneath taal volcano philippines revealed by magnetotelluric resistivity survey 2d resistivity modeling taal volcano located in the southwestern part of luzon island philippines has frequently experienced catastrophic eruptions from both the main crater on volcano island and flank eruptions these eruptions have been magmatic phreatomagmatic and hydrothermal with the latter implying the existence of a large scale hydrothermal system beneath the volcano we conducted an electrical resistivity survey using the magnetotelluric method in order to identify the location and geometry of the hydrothermal reservoir and sealing cap rock two dimensional inversion using the observed data indicates four similar resistivity sections the structure at shallow depths corresponds to volcanic deposits and an aquifer below 1 km the structure features a relatively resistive zone beneath the main crater surrounded by a conductive shell we interpreted these to be a large hydrothermal reservoir with an impermeable cap rock sealing it recent ground deformation detected by gps measurements suggests that the hydrothermal reservoir is active the interpreted cap rock thins just beneath the main crater and could easily be destroyed by an imbalance in the hydrothermal system we conclude that this hydrothermal reservoir plays a significant role in driving catastrophic eruptions that begin with a hydrothermal explosion at the main crater

calibrating the amplitude source location asl method by using active seismic sources an example from te maari volcano tongariro national park new zealand a frequently applied amplitude source location asl method is here calibrated and optimized by using active seismic sources located at the surface of a dry stream channel the asl produced location discrepancies larger than 1 0 km laterally and 500 m in depth by using independently determined velocity model attenuation and site amplification factors afs sensitivity tests for asl input parameters show that attenuation and velocity have moderate influence on the location but are easy to independently constrain afs are shown to strongly influence the location and their application may introduce substantial location uncertainties model uncertainties are accommodated with either lateral or depth changes depending on the input parameters station corrections and the source station geometry

detachment faulting and the evolution of passive continental margins major detachment faults play a key role in the lithospheric extension process in the basin and range province and may also be important in other continental extension terranes such detachment faulting leads to an inherent asymmetry of extensional structure and of uplift subsidence patterns detachment models developed for the formation of metamorphic core complexes can also be applied to the formation of passive continental margins we therefore suggest the existence of upper plate and lower plate passive margins these give rise to a complementary asymmetry of opposing margins after continental breakup transfer faults offset marginal features and allow margins to switch from upper plate to lower plate characteristics along strike

new evidence on the structure of the labrador sea greenland continental margin a 275 km section of a regional seismic reflection profile from the greenland side of the labrador sea has been reprocessed and interpreted three zones of different structural style have been identified the innermost of which is block faulted magnetic modelling based on the seismic line shows that sea floor spreading anomalies are confined to the outermost zone and the oldest identified anomaly is 27n all attempts to model the area landward of anomaly 27n as a series of remanent magnetizations of alternating polarity failed however a model in which the innermost zone is interpreted as extended and block faulted continental crust and the intermediate zone as continental crust intruded and overlain by reversely magnetized igneous material fits the magnetic and seismic data it is concluded that at least in the northern labrador sea sea floor spreading started in palaeocene times chron 27 and large areas formerly thought to be underlain by oceanic crust should now be considered to be continental one plate kinematic consequence may be a reduced need to postulate large strike slip movements along nares strait

early tertiary basalts from the labrador sea floor and davis strait region fine to medium grained phyric and aphyric basalt samples from odp leg 105 site 647a in the labrador sea show little evidence of alteration chemically these rocks are low potassium 0 01 0 09 wt k 2 o olivine to quartz normative tholeiites that compare closely with the very depleted terrestrial paleocene volcanic rocks in the davis strait region of baffin island and west greenland however differences exist in the sr nd isotope systematics of the two suites the labrador sea samples have e nd values 9 3 indicative of a more depleted source and are higher in 87 sr 86 sr 0 7040 relative to the davis strait basalts e nd 2 54 to 8 97 mean 87 sr 86 sr 0 7034 the higher 87 sr 86 sr in the labrador sea samples may reflect seawater exchange despite no petrographic evidence for significant alteration the labrador sea and early davis strait basalts may have been derived from a similar depleted mantle source composition however the later davis strait magmas were generated from a different mantle none of the baffin island west greenland or labrador sea samples show unequivocal geochemical evidence for contamination with continental crust

fragility and an extremely low shear modulus of high porosity silicic magma abstract the rheology and strength of bubbly magma govern eruption dynamics by determining the possibility of fragmentation of ascending magmas they are also required parameters for understanding seismic monitoring we measured the rheology and strength of high porosity rhyolitic magma at 500 950 c the measured shear modulus and strength are several orders of magnitude lower than bubble free rhyolite melt implying that high porosity magma cannot avoid fracturing during magma ascent the occurrence of fractures is observed in the low temperature magma 800 c in this temperature range the measured attenuation is low that is the elastic energy originated by deformations avoids attenuation and is stored in the bubbly magma until released by fracturing q 1 the newly found porosity dependent strength based on our measurements comprehensively explains three different fragmentation criteria that have been previously proposed independently our measurements also show that the shear modulus becomes lower by increasing porosity which can slow the shear wave velocity these results suggest that knowing the attenuation of the seismic wave is useful to evaluate magma temperature and the possibility of a fragmentation event that may determine subsequent volcanic activities

deep structure of the west african continental margin congo zaire angola between 5 s and 8 s from reflection refraction seismics and gravity data summary the deep structure of the west african continental margin between 5 s and 8 s was investigated using vertical reflection and wide angle reflection refraction techniques during the zaiango project a joint programme conducted in 2000 april by ifremer and totalfinaelf to penetrate below the salt layer a non conventional low frequency seismic source was used in the single bubble mode together with ocean bottom instruments hydrophones and seismometers and a 4 5 km long streamer that recorded multichannel seismic reflection mcs the data show that the continental crust thins abruptly over a lateral distance of less than 50 km from 30 km thick below the continental platform based on gravity data to less than 4 km thick below the lower congo basin that formed prior to the aptian salt deposition this subsalt sedimentary basin 180 km wide 4 km thick with velocities varying from 4 7 km s 1 to 5 8 km s 1 at the bottom is located between the foot of the continental slope and the oceanic domain it is underlain by crust of an intermediary or transitional type between continental crust and what can be recognized as oceanic crust in the transitional zone a crustal upper layer is present below the pre salt sedimentary basin 3 to 7 km thick with velocities increasing from 5 8 km s 1 at the top to 6 8 km s 1 at the bottom of the layer this layer appears to thin regularly from 6 7 km thick below the depocentre of the pre salt sedimentary basin to 3 4 km thick below the western termination of the basin below this upper crustal layer an anomalous velocity layer 7 2 to 7 8 km s 1 is documented below the eastern side of the basin where the crustal thinning is at a maximum the origin of this layer is unknown several arguments like rifting duration between 15 ma and 30 ma or the absence of seaward dipping reflectors precludes the hypothesis of underplated mantle material but other hypotheses such as serpentinized material or high grade metamorphic crustal rocks or a mixture of mafic and ultramafic crustal rocks are plausible near the ocean termination of the basin the transitional zone is bounded to the west by a basement ridge that is clearly documented on two profiles 7 11 and 14 having a dense ocean bottom seismometer hydrophone obs obh spacing on these profiles an anomalous velocity layer is present in the westernmost part of the transitional zone below the basement ridge and in the oceanic domain this layer absent on profile 3 may be related either to oceanization and slow seafloor spreading processes or to a consequence of the rifting process

time space variation of volcano seismic events at la fossa vulcano aeolian islands italy new insights into seismic sources in a hydrothermal system we investigated the relationship between volcano seismic events recorded at la fossa crater of vulcano aeolian islands italy during 2004 2006 and the dynamics of the hydrothermal system during the period of study three episodes of increasing numbers of volcano seismic events took place at the same time as geothermal and geochemical anomalies were observed these geothermal and geochemical anomalies have been interpreted as resulting from an increasing deep magmatic component of the hydrothermal fluids three classes of seismic events long period high frequency and monochromatic events characterised by different spectral content and various similarity of the waveforms have been recognised these events clustered mainly below la fossa crater area at depths of 0 5 1 1 km b s l were space distributed according to the classes based on their features we can infer that such events at vulcano are related to two different source mechanisms 1 fracturing processes of rocks and 2 resonance of cracks or conduits filled with hydrothermal fluid in the light of these source mechanisms the increase in the number of events at the same time as geochemical and geothermal anomalies were observed was interpreted as the result of an increasing magmatic component of the hydrothermal fluids implying an increase of their flux indeed such variation caused an increase of both the pore pressure within the rocks of the volcanic system and the amount of ascending fluids increased pore pressures gave rise to fracturing processes while the increased fluid flux favoured resonance and vibration processes in cracks and conduits finally a gradual temporal variation of the waveform of the hybrid events one of the subclasses of long period events was observed likely caused by heating and drying of the hydrothermal system

broadband seismic monitoring of active volcanoes using deterministic and stochastic approaches 1 we systematically used two approaches to analyze broadband seismic signals for monitoring active volcanoes one is waveform inversion of very long period vlp signals assuming possible source mechanisms the other is a source location method of long period lp events and tremor using their amplitudes the deterministic approach of the waveform inversion is useful to constrain the source mechanism and location but is basically only applicable to vlp signals with periods longer than a few seconds the source location method assumes isotropic radiation of s waves and uses seismic amplitudes corrected for site amplifications this simple approach provides reasonable source locations for various seismic signals such as a vlp event accompanying lp signals an explosion event and tremor associated with lahars and pyroclastic flows observed at five or fewer stations our results indicate that a frequency band of about 5 12 hz and a q factor of about 60 are appropriate for the determination of the source locations in this frequency band the assumption of isotropic radiation may become valid because of the path effect caused by the scattering of seismic waves the source location method may be categorized as a stochastic approach based on the nature of scattering waves systematic use of these two approaches provides a way to better utilize broadband seismic signals observed at a limited number of stations for improved monitoring of active volcanoes

fylla bank structure and evolution of a normal to shear rifted margin in the northern labrador sea summary cenozoic seafloor spreading between greenland and north america is generally considered a major right stepping ridge transform ridge system between nw se trending spreading ridge segments in the labrador sea and baffin bay the ridges were linked by n s nne ssw trending transform motions in the davis strait in particular expressed by the 1000 km long ungava fault zone fylla bank part of the southern west greenland continental margin is located in the northernmost labrador sea at the transition between the normal and shear rifting regimes of the labrador sea and davis strait as such the bank may be compared with the demerara plateau part of the french guinea northeast brazil continental margin seismic reflection interpretations presented in this study show that fylla bank is situated above an extensive basin complex herein referred to as fylla structural complex which contains an up to 5 km thick cretaceous cenozoic sedimentary succession above an inferred pre cretaceous basement seismic mapping of basement structures show that the complex is dominated by nnw nw striking rift basins in its southern part and nne striking rift basins in its northern part the rift basins are interpreted to be the result of an initial late early cretaceous rift phase which mainly resulted in the formation of the nnw nw striking structures and a subsequent early campanian rift phase mainly resulting in the formation of large nne striking rotated fault blocks resumed rifting in the early cenozoic deepened the nne striking rift basins the nne oriented structures have previously been interpreted to initiate during the latest cretaceous however this study suggests that they initiated transfer faults already during the late early cretaceous rift phase and possibly correlate with along strike discontinuities in oceanic crust in the labrador sea to define margin segmentation in southern west greenland including the borders of fylla bank a structural kinematic model presented here thus suggests that the cretaceous cenozoic poly phase rifting to some extent was controlled by pre existing crustal fabric combined with an interpreted interplay between normal stresses in the labrador sea and oblique shear stresses in the davis strait this resulted in a very complex structural tectonic evolution and the formation of several distinct structural styles the seismic interpretations are supported by maps of the moho topography and crustal thickness which were compiled from results of pseudo 3 d gravity modelling the maps show minimum crustal thicknesses 11 km and maximum moho uplifts in areas where the nnw nw and nne striking structures interact moreover a strong correlation is found between moho topography crustal attenuation rift enforced thermal uplift and erosion and post rift subsidence in the area this is interpreted to be a result of thermally controlled basin dynamics

formation of non volcanic passive margins a composite model applies to the conjugate galicia and southeastern flemish cap margins one of the most controversial issues in geology today is the way the lithosphere extends the galicia margin is a particularly good place to adress this problem as it is fairly sediment starved has clear upper crustal structure well imaged by a variety of seismic reflection profiles and has well control to calibrate seismic data observed subsidence is compared with extension of the uppermost crust of the entire margin as computed from the geometry of tilted fault blocks imaged on a series of parallel seismic lines the pure shear or stretching model clearly accounts for the extension of the upper continental margin but not for the lower margin where the s reflector zone a prominent zone of intra crustal reflectors is present i suggest that the s reflector zone is a detachment zone accomodating decreasing displacement towards the continent and that this detachment zone active since the beginning of rifting outcrops on the canadian side and corresponds to the steep southeastern flemish cap continental slope pure shear is the mechanism which affects the whole lithosphere but in the upper part of the crust the rifting mechanism is locally simple shear where the detachment zone is active

source process of long period seismic events at taal volcano philippines vapor transportation and condensation in a shallow hydrothermal fissure 1 we analyzed observations of a swarm of more than 40 000 long period lp seismic events at taal volcano philippines in 2010 2011 the event waveforms are strongly correlated to each other consistent with a fixed source location and begin with a dilatational first motion they have a peak frequency around 0 8 hz and a quality factor q of 6 waveform inversion of the events pointed to a tensile crack source dipping 30 60 at a shallow 100 200 m depth a simulation using a fluid filled crack model indicated that the complex frequencies of the waveforms are explained by the fundamental longitudinal mode resonance of a vapor filled crack 188 m long a satellite thermal infrared image acquired during the swarm period suggests that the lp events were not accompanied by surface gas releases we considered a vapor transportation model in which vapor exsolved from magma and rose in a fissure extending to the lp source this model yielded estimates that 105 107 m3 of magma was involved in the lp swarm and that the temperature of vapor in the lp source crack was around 600 k we modeled a triggering mechanism of the crack resonance based on sudden condensation of vapor at the crack tip in a cold aquifer this model explained observed characteristics of the events including the dilatational first motion the total volumetric change and the fixed source location

structure of archean crust and passive margin of southwest greenland from seismic wide angle data to investigate the structure and composition of the archean crust beneath the shelf of southwest greenland parallel to the passive margin we conducted an onshore offshore seismic wide angle experiment in 1989 one of our goals was to obtain a velocity depth profile along the thinned continental or transitional crust to place constraints on the magmatic rifting history in the northern labrador sea seismic recording instruments at four locations along the coast recorded energy from an air gun array up to 400 km distance due to the dense shot spacing of 100 to 150 m we were able to identify continuous reflected and refracted phases from the upper to lower crust results from a one dimensional extremal inversion indicated a high velocity zone hvz with p wave velocities of more than 7 2 km s at the bottom of the transitional crust combined two dimensional travel time forward modeling and inversion produced a velocity depth model that contains midcrustal and lower crustal discontinuities between 12 and 25 km depth and a moho dipping northward at 30 to 42 km depth an average crustal velocity of about 6 6 km s indicates a high portion of mafic components in the crust the lowermost crust consists of a hvz with a maximum thickness of 8 km and temperature corrected velocities of 7 5 to 7 85 km s the hvz may be explained by an underplated igneous complex often observed under passive margins worldwide metamorphism to garnet pyroxene granulite during postrift isobaric cooling of the 30 to 40 km deep thinned continental crust could explain the velocities that are higher than on other margins a large volume of magmatic material must have been added to the crust but did not produce extrusions magmatic underplating of the thinned continental or transitional crust of sw greenland is likely associated with a possible hot spot magmatism in the davis strait and baffin bay region magmatism associated with continental rifting was found to reach farther south along the northern and central part of the archean block than previously thought

uniform sense normal simple shear of the continental lithosphere geophysical studies suggest that the thin crust characteristic of the basin and range province extends eastward beneath the west margin of the colorado plateau and rocky mountain regions in arizona and utah zones perhaps over 100 km wide may be defined bounded on the west by the east limit of upper crustal normal faults that account for more that 10 extension and on the east by the east limit of thinning beneath the colorado plateau a discrepancy exists within these zones between the negligible extension measurable in the upper crust and the substantial extension apparent from crustal thinning assuming the discrepant zone crust was as thick as or thicker than the colorado plateau rocky mountain crust prior to extensional tectonism if various theories appealing to crustal erosion are dismissed mass balance problems evident in the discrepant zones are most easily resolved by down to the east normal simple shear of the crust moving lower and middle crustal rocks that initially were within the zones up and to the west to where they now are locally exposed in the basin and range province west of the discrepant zones in both arizona and utah east directed extensional allochthons with large displacement are exposed these geophysical and geological observations complement one another if it is accepted that the entire crust in both arizona and utah failed during extension on gently east dipping east directed low angle normal faults and shear zones over a region several hundred kilometres wide large scale uniform sense normal simple shear of the crust suggests the entire lithosphere may do the same such a hypothesis predicts major lithospheric thinning without crustal thinning will occur in plateau areas in the direction of crustal shear in the case of the arizona utah and red sea extensional systems and possibly the death valley extensional terrain a broad topographic arch typically 1500 2000 m higher than the extended terrain is present suggesting lithospheric thinning in areas predicted by the hypothesis

distortion of the apparent s wave radiation pattern in the high frequency wavefield tottori ken seibu japan earthquake of 2000 summary the distortion properties of the apparent s wave radiation pattern in the high frequency seismic wavefield of over f 2 hz is investigated using a large number of waveform records of the main shock and 29 aftershocks of the tottori ken seibu japan magnitude mw 6 6 earthquake in 2000 the dense seismic records from the kik net strong motion network show a clear four lobe pattern in the apparent s wave radiation pattern in the low frequency wavefield of f 2 hz and shows an almost isotropic distribution in all directions as the frequency increases above 5 hz the distortion of the apparent s wave radiation pattern in the high frequency wavefield increases as travel distance increases therefore the path effect caused by the scattering of seismic waves due to small scale heterogeneities in the crust is a major cause of distortion of the radiation pattern this hypothesis is examined by a 2 d finite difference method simulation of seismic waves in heterogeneous structure models the results of simulations clearly demonstrate the collapse of the s wave front due to seismic wave scattering in the heterogeneous structure by comparing the observed wavefield and the results of simulations using different sorts of stochastic heterogeneous models the most preferable model that can explain the observation is described by a von karman autocorrelation function with correlation distance of a 3 5 km order of 0 5 and rms value of e 0 07 however our simple stochastic random heterogeneity model proposed herein somewhat overestimates the scattering of low frequency signals below 2 hz

electrical resistivity tomography study of taal volcano hydrothermal system philippines taal volcano 311 m in altitude is located in the philippines 14 n 121 e and since 1572 has erupted 33 times causing more than 2 000 casualties during the most violent eruptions in march 2010 the shallow structures in areas where present day surface activity takes place were investigated by dc resistivity surveys electrical resistivity tomography ert lines were performed above the two identified hydrothermal areas located on the northern flank of the volcano and in the main crater respectively due to rough topography deep valleys and dense vegetation most measurements were collected using a remote method based on a laboratory made equipment this allowed retrieval of information down to a depth of 250 m erts results detail the outlines of the two geothermal fields defined by previous self potential co2 soil degassing ground temperature and magnetic mapping harada et al japan acad sci 81 261 266 2005 zlotnicki et al bull volcanol 71 29 49 2009a phys chem earth 34 294 408 2009b hydrothermal fluids originate mainly from inside the northern part of the main crater at a depth greater than the bottom of the crater lake and flow upward to the ground surface furthermore water from the main crater lake infiltrates inside the surrounding geological formations the hydrothermal fluids outlined by gas releases and high temperatures cross the crater rim and interact with the northern geothermal field located outside the main crater

mechanisms of unrest and eruption at persistently restless volcanoes insights from the 2015 eruption of telica volcano nicaragua many of earth s volcanoes experience well defined states of quiescence and unrest with unrest occasionally culminating in eruption some volcanoes however experience an unusually protracted i e decades long period of noneruptive unrest and are thus categorized as persistently restless volcanoes prvs the processes that drive persistently restless volcanism are poorly understood as our knowledge of prvs is currently based on a small number of case studies here we examine multidisciplinary observations of the 2015 eruptive episode at telica volcano nicaragua in the context of its long term behavior we suggest that the latter phases of the 2015 eruption were ultimately driven by destabilization of its shallow magma reservoir based on previous geodetic seismic studies of telica geirsson et al 2014 https doi org 10 1016 j jvolgeores 2013 11 009 rodgers et al 2013 https doi org 10 1016 j jvolgeores 2013 08 010 and 2015 https doi org 10 1016 j jvolgeores 2014 11 012 and on multiparameter observations at telica over a 7 year period we propose that three distinct states of unrest occur at telica over decadal timescales a stable open state involving steady conduit convection and two distinct unstable states that may lead to eruptions in the weak sealing state phreatic explosions result from steady conduit convection underlying a weak seal in the destabilized state destabilization of the top of the convecting magma in the conduit leads to rapid accumulation of high pressures leading to strong impulsive phreatomagmatic explosions our observations and interpretations suggest that continuous seismic ground based deformation gas emission and thermal monitoring and interpretation of these data within a paradigm of sustained conduit convection modulated by episodes of sealing and destabilization of shallow magma reservoirs may allow robust forecasting of eruption potential energy and duration at telica and similar prvs worldwide

variation with spreading rate of oceanic crustal thickness and geochemistry abstract away from the influence of fracture zones hotspots and marginal basins oceanic crust exhibits remarkably uniform crustal thickness rare earth element concentrations and bulk composition at all but the most slowly spreading ridges at full spreading rates below 15 mm a however all these observed parameters show marked and abrupt changes the seismically determined crustal thicknesses decrease sharply from their average of 7 1 km as do melt thicknesses inferred from rare earth element inversions of basalts bulk element compositions of basalts show an increase in the percentage of na2o and decreases in the feo content and cao al2o3 ratio as the full spreading rate drops below 15 mm a we present a model of the melting beneath oceanic spreading centres which reproduces the observed variations with spreading rate of crustal thickness and the main geochemical changes observed in basalts changes in the volume and composition of melt generated at very slow spreading rates are caused by conductive heat loss from the mantle welling up beneath rifts we model the observed behaviour with normal mantle potential temperatures of 1300 20 c greater crustal thicknesses are generated at the north atlantic spreading centre adjacent to the iceland mantle plume suggesting mantle temperatures about 80 c hotter than normal in this area

the continent ocean crustal transition across the southwest greenland margin this paper examines the complete crustal transition across the nonvolcanic southwest greenland continental margin of the labrador sea using wide angle and coincident vertical incidence seismic profiles six ocean bottom seismometers and a sonobuoy record p and s wave first and multiple arrivals from the crust and upper mantle which are analyzed by two dimensional dynamic ray tracing and one dimensional reflectivity modeling the resulting seismic velocity model requires that the preexisting 30 km thick continental crust is thinned abruptly to 3 km across the continental slope primarily by removal of the lower crust farther seaward the crust thickens to 6 km primarily through the addition of a high velocity 7 0 7 6 km s layer in the lower crust this lower crustal layer is 4 5 km thick extends for a horizontal distance of 80 km and is interpreted as partially serpentinized upper mantle it is overlain by a low velocity 4 0 5 0 km s upper layer which is interpreted as highly fractured upper continental crust our model suggests that seafloor spreading did not start until chrons 27 28 13 ma younger than previously suggested this interpretation is supported by two dimensional modeling of gravity and magnetic data along the refraction line our results are consistent with a simple shear mechanism for the initial rifting with the sw greenland margin as the upper plate however a full characterization of the rifting mechanism must await comparison with a seismic model for the conjugate margin east of labrador

late cretaceous tertiary sediments offshore central west greenland lithostratigraphy sedimentary evolution and petroleum potential five dry exploratory wells were drilled through upper cretaceous and tertiary sediments on the west greenland shelf in 1976 and 1977 two of these entered precambrian basement two bottomed in paleocene or upper cretaceous basalt and one in campanian mudstone on the basis of samples and logs supplied to the geological survey of greenland the sedimentary sequence has been divided into seven new formations the campanian narssarmiut formation consisting of coarse basement wash and black mudstone the campanian to eocene ikermiut formation consisting of marine organic rich mudstone the upper paleocene to eocene hellefisk formation comprising shallow marine to paralic sandstone and mudstone the eocene nukik formation consisting of turbiditic sandstone and mudstone the eocene to oligocene kang miut formation of shelf to shallow marine clean and argillaceous sandstone the oligocene to neogene manitsoq formation consisting of coarse paralic to fan delta sandstone and the neogene ataneq formation con

joint inversion of gravity and magnetic anomalies of eastern canada the power spectra and degree correlation of the surface topography and free air gravity anomalies of eastern canada show that the gravity anomalies are subdivided into three parts the short wavelength components 30 170 km shorter than 30 km are not well resolved largely arise from density perturbations in the crust and to a lesser extent from the surface topography and moho undulation whereas the contribution of intracrustal sources to the intermediate wavelength components 170 385 km is comparable with that of the topography the long wavelength components 385 1536 km are overcompensated at the moho we present a crustal model for the intermediate and long wavelength components which takes into account the surface topography density perturbations in the crust and moho undulation with a certain degree of isostatic compensation the general characteristics of this model resemble the crustal structure revealed from seismic measurements the reduced to pole magnetic anomalies of eastern canada sh

shallow resistivity structure of asama volcano and its implications for magma ascent process in the 2004 eruption asama volcano is an active volcano with many historical records of vulcanian eruptions its most recent eruptions occurred in 2004 at the summit crater in this paper we argue the resistivity structure shallower than 3 km obtained by a dense magnetotelluric survey the magnetotelluric data were obtained at 74 measurement sites mainly along the four survey lines across the volcano the resistivity profiles obtained by two dimensional inversions are characterized by a resistive surface layer and an underlying conductive layer the dominant feature of the profiles is the existence of resistive bodies at a depth range of a few hundred meters to a few kilometers surrounded by a highly conductive region considering that the location of resistive bodies correspond to the old eruption centers one corresponds to the 24 ka collapse caldera and the other to the 21 ka lava dome the resistive bodies imply zones of old and solidified intrusive magma with low porosity because geothermal activities exist near the resistive bodies the enclosing highly conductive regions are interpreted as a hydrothermal system driven by the heat from the old solidified magma beneath the resistive body under the collapsed caldera intrusion of magma is inferred from the studies of volcano tectonic earthquakes and continuous global positioning system gps observation implying the structural control of magma activity within the volcano in this study we propose that the magma ascent was impeded by the old and solidified remnant magma and partly migrated horizontally to the east and finally ascended to the summit resulting in the 2004 eruptions