gravity anomalies and crustal signature of volcano tectonic structures of pico island azores pico island is the youngest 300 ka island of the azores archipelago dominated on its western sector by the stratovolcano of pico mountain which rises about 3 5 km above the surrounding ocean floor the other two major morpho tectonic units of the island include the shield volcano of topo and the achada plateau a 29 km long volcanic ridge with a wnw ese trend a gravimetric study on pico island coupled with geological and volcanological data was aimed towards a better understanding of its deep volcano tectonic structures from a total of 407 stations a regional model of density contrasts was developed in order to reproduce the crustal structure of the island in addition a local high resolution model of the sub surface structure of the westernmost part of the island is presented both models were obtained by means of an inversion global adjustment on fixed density contrasts in order to determine the three dimensional geometry of the sources of the anomalous gravity field the most prominent anomalous gravity structure found in pico island is a body with positive density contrast located on the se slopes of topo volcano this anomaly extends to depth of approximately 8 km mean depth of 4 5 km and has a volume of 310 km 3 this anomaly maybe best interpreted by the presence of a major intrusive system beneath topo volcano and can be also correlated to gravitational collapses at the shield volcano a second body of positive density contrast which appears to extend to 4 5 km depth is located near the town of madalena this anomaly may be correlated to a high density anomaly recently detected in the ne sector of faial island smaller sized high density bodies at shallow depths of around 1 3 km are located beneath pico mountain volcano and all together define a semi circular arc with an average diameter of 8 km we propose that this arcuate structure might outline a ring fracture associated with the collapse of the central sector of pico mountain volcano early in its growth low density bodies are located on the nw flanks of pico mountain along a linear ene trending structure these bodies extend to depths of 4 km and have a total volume of 69 km 3 they are interpreted as being associated with an old extensional fracture zone previous studies suggested the existence of a shallow and large magma reservoir with volumes up to 1000 km 3 beneath pico mountain volcano in this study we have not found any evidence of the existence of such huge magma body on the contrary we propose that those discrete low density bodies correspond to complex intersections of different sets of tectonic lineaments along which magma tends to intrude as shallow 2 5 km and small 10 20 km 3 magma pockets c 2006 elsevier b v all rights reserved

tectonic framework of the azores triple junction the lack of accurate and detailed magnetic information has in the past limited the development of well constrained models for the plate tectonic evolution of the azores triple junction an aeromagnetic survey made possible by the existing airport facilities has long been desired as it can provide high quality magnetic data whose homogeneity and coherency is far better than those provided by classic marine surveys the results presented in this paper concern only a part of the aeromagnetic survey conducted by the portuguese instituto nacional de meteorologia e geofisica and already allow an improved definition of the basic tectonic boundaries at the central part of the azores plateau

the isostatic compensation of the azores plateau a 3d admittance and coherence analysis the compensation of the azores plateau is re examined by means of a 3d admittance study involving computations of the admittance between the bathymetry and 1 the geoid 2 the free air gravity and 3 the mantle bouguer anomalies the geoid to bathymetry relationship rather than being analysed in the space domain is studied in the spectral domain in this way the information related to the flexural response of the lithosphere is retained in the geoid to bathymetry signal and can be compared to that of the gravity to bathymetry we find that the anomalously shallow depths of the azores plateau are not due to dynamic forces sustained by mantle upwelling the plateau is supported by a thickened crust which mainly results from large volumes of accreted extrusives and consequent deflection of the underlying elastic plate both the free air gravity and the mantle bouguer admittance point to a flexural isostatic model with a moho depth of 12 km and an elastic thickness in the range of 3 6 km however the analysis of the coherency between bathymetry and the mantle bouguer anomaly indicates that buoyant material at the base of the crust partially accounts for the uplift of the plateau in our best fitting model the average elastic thickness is 4 km and the amplitude of the buoyant load at the moho is 1 3 of that of the surface volcanic load buoyancy forces at the moho may arise from underplated material resulting from subcrustal plutonism

crustal seismic velocity structure near faial and pico islands azores from local earthquake tomography the azores archipelago portugal is located on an oceanic plateau in a geodynamic environment prone to intense seismic and volcanic activity in order to investigate the crustal structure in this region we have conducted a local earthquake tomography study in the area of the islands of faial pico and s jorge using data recorded in july 1998 the july 9th 1998 earthquake near faial island triggered an aftershock sequence of thousands of events that lasted for several months and were recorded by a total of 14 stations located on the three islands surrounding the epicentral area in the upper crustal layers consistency is seen between the tomographic results and the islands surface volcanic units beneath the faial central volcano a low vp b6 0 km s anomaly roughly located at 3 7 km depth suggests a connection to the plumbing system possibly the presence of a magma chamber in ne faial a high vp n6 3 km s body was found located at mid lower crust most likely an intrusion of gabbroic composition that is bordered by the registered seismic activity its shape suggesting a tectonic controlled mechanism the relocated hypocenters together with the overall analysis of the tomographic model suggest a tectonic segmentation of faial island the crustal thickness under the islands volcanic buildings of the faial pico area was estimated at around 14 km 2007 elsevier b v all rights reserved

when did hominids first leave africa new high resolution magnetostratigraphy from the erk el ahmar formation israel new paleomagnetic results from the erk el ahmar formation israel resolve age ambiguities of one of the oldest hominid sites outside africa where ancient oldowan tools and artifacts were excavated we identified in the section the upper and lower boundaries of the olduvai subchron and we conclude that these sediments were deposited between ca 1 7 and 2 0 ma this result is consistent with the hypothesis that earliest hominid migrations from africa to eurasia during the early paleolithic traversed the levantine corridor

upper mantle structure beneath the azores hotspot from finite frequency seismic tomography the azores archipelago is a classic expression of a hotspot that has interacted with a mid ocean ridge avariety of geochemical studies and geophysical observations point to the influence of some sort of mantle plume on the formation of the azores plateau and the adjacent mid atlantic ridge mar but the mantle structure beneath the azores has not heretofore been imaged tomographically at a sufficiently fine resolution to resolve a narrow plume we present a p wave velocity model of the mantle beneath the hotspot derived from teleseismic body waves recorded by six broadband seismic stations on the azores islands including the global seismographic network station cmla three dimensional travel time sensitivity kernels are used to account for the effects of wavefront healing and scattering of seismic waves an inversion of 228 p wave travel time delays reveal a lowvelocity anomaly in the shallow mantle less than 200 km deep along the azores archipelago and beneath the azores plateau centered approximately at 38 5 n 28 5 w the low velocity anomaly extends northeastward and downward to connect to a plume like column of low velocity material from 250 km to at least 400 km depth centered northeast of terceira these results are consistent with other geophysical and geochemical observations and provide seismic evidence for a model of plume ridge interaction in which the plume conduit is deflected to the southwest in the shallow mantle by asthenospheric flow and plate motion and contributes hot mantle material and excess melt to the mar south of the azores

second order segmentation the relationship between volcanism and tectonism at the mar 38 n 35 40 n deep tow sidescan sonar data acquired along 240 km of the mid atlantic ridge mar between 35 40 n and 38 n have been combined with new bathymetric compilations and used to establish the recent tectonic and volcanic history of six second order segments and their bounding non transform offsets ntos the segments show a range of volcanic and tectonic types but in general the northernmost segments i e those with greater influence from the azorean hotspot are shallower and more volcanically robust than those to the south with hydrothermal activity in segment centres for example menez gwen and lucky strike nonetheless this generalisation requires some modification due to temporal variations in the balance between magmatic supply and tectonic dismemberment the ntos are broad right stepping discontinuities locally up to 25 km wide and accommodate offsets between 10 and 50 km the discontinuities are mostly sediment floored and link the spreading segment tips with a range of structures these include locally dense arrays of en echelon extensional normal faults short lengths of linear strike slip fault strands occasionally cutting basement blocks apparently stranded within the offsets basement blocks within the offsets are cross cut by complexes of intersecting faults suggesting that deformation is distributed across the zone the pervasive faulting taking place at the ntos favours fluid circulation and associated hydrothermal activity as at the rainbow site at 36 17 n

results from geodetic measurements in the western part of the african eurasian plate boundary abstract since 1988 the astronomical observatory university of porto and the institute for geodesy and navigation university of the federal armed forces munich have been involved in a common project to establish and maintain a high precision geodetic network in the area of the azores triple junction along the western boundary of the eurasian plate the network has stations on three of the main plates north american eurasian and african this is a long term project with planned re observations every three years first epoch measurements were done in 1988 using the global positioning system gps a second gps campaign was conducted in 1991 during which the network was enlarged to include more stations on the african plate in order to monitor also the azores gibraltar plate boundary further strength of the network was obtained by the establishment in 1992 of a gravimetric network in the azores archipelago the gravimetric network comprises two absolute stations and relative measurements on all the nine islands of the archipelago re observation campaigns with gps and gravity measurements took place in 1993 and 1994 this paper focuses on comparison of the gps results achieved in the different campaigns annual values for point displacement direction and magnitude are computed for some of the stations and analyzed in light of the results derived from geological and geophysical data interpretation our preliminary geodetic results show ocean spreading across the mid atlantic ridge at rates consistent with those determined from 3my averages nuvel 1 in addition there is marginal evidence for a leaky transform boundary transversing the central azores islands possible directions for future research on the dynamics of this plate boundary are also presented

the insular shelves of the faial pico ridge azores archipelago a morphological record of its evolution shelves surrounding reefless volcanic ocean islands are formed by surf erosion of their slopes during changing sea levels posterosional lava flows if abundant can cross the coastal cliffs and fill partially or completely the accommodation space left by erosion in this study multibeam bathymetry high resolution seismic reflection profiles and sediment samples are used to characterize the morphology of the insular shelves adjacent to pico island the data show offshore fresh lava flow morphologies as well as an irregular basement beneath shelf sedimentary bodies and reduced shelf width adjacent to older volcanic edifices in pico these observations suggest that these shelves have been significantly filled by volcanic progradation and can thus be classified as rejuvenated despite the general volcanic infilling of the shelves around pico most of their edges are below the depth of the last glacial maximum revealing that at least parts of the island have subsided after the shelves formed by surf erosion prograding lava deltas reached the shelf edge in some areas triggering small slope failures locally decreasing the shelf width and depth of their edges these areas can represent a significant risk for the local population hence their identification can be useful for hazard assessment and contribute to wiser land use planning shelf and subaerial geomorphology magnetic anomalies and crustal structure data of the two islands were also interpreted to reconstruct the long term combined onshore and offshore evolution of the faial pico ridge the subaerial emergence of this ridge is apparently older than previously thought i e before 850 ka

effects of ridge geometry on mantle dynamics in an oceanic triple junction region implications for the azores plateau abstract plate boundary geometry can affect the nature of magmatism along a mid ocean ridge the azores plateau is located in a complex geological setting that includes a triple junction tj an oblique and recently formed ultra slow spreading ridge a zone of diffuse seafloor deformation a major fracture zone and a postulated hotspot the precise character of the hotspot is somewhat debated as some lines of evidence indicate it may not be a classic deep seated plume however seismic and gravity data suggest plateau crustal thicknesses of 8 km or more implying some mechanism for excess melting to assess the role of ridge geometry in creating the azores plateau this study uses a finite element numerical model to isolate the effects of selected aspects of plate boundary configuration on mantle flow and melt production in a tj kinematically similar to the azores tj the model focuses on the slowest spreading ridge in the tj analogous to the terceira rift the effect of the varying ridge obliquity observed along the terceira rift is also assessed using an independent 1 d melting model in general relatively little melt production is predicted along the terceira rift analogue except for regions closest to the tj where the proximity of a faster spreading ridge increases temperatures within the melting zone in the 1 d melting model with mantle temperatures of 1350 c melt thicknesses of 2 km are calculated for the least oblique segments while more oblique segments produce little to no melt the presence of a long discontinuity simulating the gloria fz has little effect on mantle dynamics for axial distances

reconstructing the architectural evolution of volcanic islands from combined k ar morphologic tectonic and magnetic data the faial island example azores the morpho structural evolution of oceanic islands results from competition between volcano growth and partial destruction by mass wasting processes we present here a multi disciplinary study of the successive stages of development of faial azores during the last 1 myr using high resolution digital elevation model dem and new k ar tectonic and magnetic data we reconstruct the rapidly evolving topography at successive stages in response to complex interactions between volcanic construction and mass wasting including the development of a graben we show that 1 sub aerial evolution of the island first involved the rapid growth of a large elongated volcano at ca 0 85 ma followed by its partial destruction over half a million years 2 beginning about 360 ka a new small edifice grew on the ne of the island and was subsequently cut by normal faults responsible for initiation of the graben 3 after an apparent pause of ca 250 kyr the large central volcano cv developed on the western side of the island at ca 120 ka accumulating a thick pile of lava flows in less than 20 kyr which were partly channelized within the graben 4 the period between 120 ka and 40 ka is marked by widespread deformation at the island scale including westward propagation of faulting and associated erosion of the graben walls which produced sedimentary deposits subsequent growth of the cv at 40 ka was then constrained within the graben with lava flowing onto the sediments up to the eastern shore 5 the island evolution during the holocene involves basaltic volcanic activity along the main southern faults and pyroclastic eruptions associated with the formation of a caldera volcano tectonic depression we conclude that the whole evolution of faial island has been characterized by successive short volcanic pulses probably controlled by brief episodes of regional deformation each pulse has been separated by considerable periods of volcanic inactivity during which the faial graben gradually developed we propose that the volume loss associated with sudden magma extraction from a shallow reservoir in different episodes triggered incremental downward graben movement as observed historically when immediate vertical collapse of up to 2 m was observed along the western segments of the graben at the end of the capelinhos eruptive crises 1957 58

east pacific rise from siqueiros to orozco fracture zones along strike continuity of axial neovolcanic zone and structure and evolution of overlapping spreading centers we have used the sea beam multibeam echo sounding system to survey the east pacific rise epr from 8 20 n to 18 30 n obtaining complete coverage of the epr axial neovolcanic zone and all intervening transform faults here we focus on the epr neovolcanic zone and the definition of overlapping spreading centers ocs s between the orozco and siqueiros transform faults the neovolcanic zone is narrow 0 5 2 0 km and continuous along strike and occurs within and near a strike continuous axial summit graben the neovolcanic zone and summit graben reside along the crest of a volcanic axial high which is 2 10 km wide and which continues uninterrupted along strike for 40 140 km within 20 50 km of an intersection with a transform fault the axial neovolcanic zone narrows and deepens and within only 3 6 km of the intersection the neovolcanic zone turns sharply into the transform valley at seven locations between the siqueiros and orozco transform faults the axial neovolcanic zone is discontinuous and is laterally offset a short distance 1 15 km in contrast with a classic ridge transform ridge plate boundary however offset rise terminations overlap each other by a distance approximately 3 times greater than their offset they curve toward each other and often one merges into the other along strike separating the osc s is an elliptical overlap basin up to 600 m deep whose long axis is subparallel to the adjacent spreading centers the overlap basin is characterized by volcanic constructional edifices a key difference between these offsets and small transform faults in the atlantic is that there are no transform fault structures in the overlap basin which link the offset rise axes a continuous axial depth profile reveals a long wavelength 20 60 km undulation of the rise axis which may be associated with variations in the magmatic budget along strike the lowest points in the axial depth profile occur near transform faults and at osc s suggesting that these features are associated with along strike minima in the magmatic budget we suggest that major phases of volcanic activity propagate episodically along the axis away from centers of magmatic inflation and that osc s develop where magmatic pulses fail to meet due to misalignment of fracture systems sinuous bends in the rise axis or other heterogeneities transform faults fail to develop on fast spreading centers where the lateral offsets are small 15 km because the lithosphere is too thin and weak to maintain a classic rigid plate ridge transform fault pattern the osc geometry is unstable and evolves rapidly and the overlapping rises may propagate at rates faster than the local spreading rate one of the two osc s prevails while the other is abandoned while the locations of osc s may not be fixed relative to the rise axis they may tend to recur near the same place creating scars in older lithosphere which resemble fracture zones but whose fine scale structure is quite different there is excellent agreement between the observed structure of osc s and laboratory studies of osc s using wax models photoelastic studies of overlapping cracks and boundary element modeling of overlapping cracks seismic refraction and reflection near two osc s suggests that large offset osc s have separate magma chambers beneath the offset rise axes while small offset osc s may share a single axial magma chamber three dimensional inversion of magnetic anomalies suggests that significant petrologic anomalies may occur near some osc tips waxing and waning of overlapping magma chambers may give rise to the eruption of highly fractionated feti basalts and associated magnetic anomalies

the evolution of santa maria island in the context of the azores triple junction santa maria is the oldest island in the azores formerly belonging to the eurasia plate and currently the only one sitting on the nubia plate thus sharing a geodynamic evolution with the azores triple junction it is therefore important to evaluate the effects of active tectonics on the evolution of santa maria for example on its vertical movements we present new stratigraphic geomorphologic structural and geochronological data from santa maria which shed further light on how a volcanic ocean island evolves in a tectonically active setting santa maria island started with a first shield volcano old volcanic complex that emerged ca 6 0 ma ago and was active until ca 5 3 ma the short time span between the first and second shield volcanoes ca 0 3 ma and the preservation of only the western flank of this first shield volcano indicate an initial flank collapse at ca 5 ma the collapse scar was covered by an eastwards dipping sedimentary complex intermediate sedimentary complex with a likely tsunami deposit at the base a second shield volcano young volcanic complex rapidly grew on these sediments from 4 8 to 3 8 ma and the island subsided by more than 100 m at 3 7 ma a second flank collapse occurred as inferred from the missing summit and eastern flank volcanism then resumed 3 6 to 2 8 ma giving rise to strombolian cones lying unconformably on the collapse scar and conformable parasitic cones on the unaffected flank submarine lavas occurring at up to 200 m altitude with a youngest age of 3 0 ma indicate major uplift of the island since at most that time here we interpret uplift as the result of rift flank uplift on the southern shoulder of the nearby terceira rift in the last ca 1 5 ma

three dimensional inversion of the magnetic field over the easter nazca propagating rift near 25 s 112 25 w the easter microplate boundary configuration is being reorganized by rift propagation a sea beam survey of the easter nazca spreading center which forms the eastern boundary of the microplate has revealed the presence of a young propagating rift growing northward naar and hey 1986 the tip of the propagating rift is associated with a high amplitude positive magnetic anomaly we have performed a three dimensional inversion of the magnetic field over the propagating rift tip area the magnetization solution suggests that the western and eastern pseudofaults strike 014 and 338 respectively and converge near the rift tip these orientations yield a propagation to spreading rate ratio of 1 5 slightly higher than the estimate of naar and hey 1986 using the revised estimate of the full spreading rate along the easter nazca spreading center near 25 s 80 mm yr d f naar and r n hey unpublished manuscript 1989 we obtain a propagation rate of 120 mm yr within 27 30 km of the rift tip the propagating rift curves by about 15 to the east toward the failing rift probably as a result of the interaction between the two offset spreading centers as at the galapagos propagating rift rift propagation appears to be a very orderly process along the easter nazca spreading center the magnetization distribution that we obtain exhibits a high at the propagating rift tip at other large ridge axis discontinuities similar magnetization highs have been interpreted as being the result of the eruption of highly differentiated basalts enriched in iron the origin of the high magnetization zone in the case of the easter nazca propagating rift near 25 s may be more complex preliminary rock magnetic measurements of basalts recovered in the vicinity of the propagating rift confirm the presence of highly magnetized basalts but suggest that the relationship between high magnetization intensities and high fe content is not straightforward

overlapping spreading centres 3 d inversion of the magnetic field at 9 03 n on the east pacific rise summary overlapping spreading centres oscs represent a new type of plate boundary interaction in which en echelon rise segments overlap significantly and are not joined by a transform fault a three dimensional fourier inversion of the magnetic field was performed on an overlapping spreading centre to remove the effects of topography and ridge orientation a magnetic high exists at the tip of one of the two ridge segments forward modelling suggests that the anomalous magnetic field cannot be attributed to the effects of topography alone the inversion reveals the existence of a magnetization high at the tip of the eastern spreading centre maximum magnetization values are consistent with ones obtained in other high amplitude zones in the pacific as well as with the measured magnetization of samples dredged in the same areas we suggest that the magnetization high over the eastern ridge tip of the 9 03 n osc is associated with highly evolved basalts enriched in iron and titanium such enrichment may be caused by enhanced crystal fractionation within an axial magma chamber which is intermittent and occasionally freezes as the eastern spreading axis propagates into older lithosphere

relationships between tectonics and magmatism in a transtensive transform setting an example from faial island azores portugal the relationships between tectonics and hotspot related magmatism in transform transtensive settings are poorly known the azores archipelago lying where the transform plate boundary between the nubian and eurasian plates meets the mid atlantic ridge is a rare site to investigate these relationships the distinct tectono magmatic features of faial make it the ideal island to focus the study here we analyze the relationships between tectonics and magmatism using remote sensing field and paleomagnetic analyses dominant wnw ese trending lineaments correspond to major faults with transtensive dextral motion and ne sw opening probably related to the nearby transtensive terceira rift most 60 dikes vent elongations and alignments are parallel to this system the lavas forming the basement of the distinctive wnw ese trending pedro miguel graben strike parallel to the graben axis dipping outward with a tilt consistent with that of paleomagnetic data these lavas are dissected by the wnw ese trending transtensive faults therefore the graben consists of outward tilted fault bounded blocks forming two opposite verging dominoes its estimated stretching factor 1 35 and mean extension rate between 3 4 and 8 2 mm yr are similar or slightly larger than those of terceira rift to the east part of the graben extension may be magma induced via diking we suggest that 1 faial along with the nearby pico island is a major locus of extension within the azores directly above the imaged hotspot and 2 the faial pico magmatic segment constitutes the offset westward magmatic continuation of the terceira rift the segment opening highlights the importance of extensional strain along magmatic transtensive plate boundaries

reply to the comment by quartau and mitchell on reconstructing the architectural evolution of volcanic islands from combined k ar morphologic tectonic and magnetic data the faial island example azores j volcanol geotherm res 241 242 39 48 by hildenbrand et al 2012

the role of lithospheric processes on the development of linear volcanic ridges in the azores abstract linear volcanic ridges lvrs are widespread along the azores plateau and are often used as a tectonic marker of the surface stress field nevertheless the mechanisms that drive the emplacement and development of these structures are not well established and they have been attributed to the plateau diffuse deformation off rift extension or the result of the interaction between a hotspot and the brittle lithosphere this study hypothesizes that linear volcanic ridges are the result of magma emplacement into pre existing damaged lithosphere using a 3d finite element representation of the brittle lithosphere and underlying ductile mantle and assuming that the deformation is driven by plate boundary forces applied at the edges as describe by global plate kinematic models the brittle layer is described by an elastoplastic rheology with progressive damage where fractures are assumed to be analogous to localized shear bands the ductile mantle underneath is modeled as a viscoelastic layer that exerts a shear drag at the base of the brittle layer the modeling shows that lithospheric processes alone can justify the spatial distribution of linear volcanic ridges and even the development of the faial ridge the factors controlling the fracturing pattern are the plate geometry and velocity boundary conditions the shearing introduced at the east azores fracture zone gloria fault limit and the interaction between the viscous mantle and the spatially varying brittle plate thickness along the terceira rift the predicted fractures match the orientation of the lvrs in the second n135 n140 and third n150 to n s sectors and provide an explanation for the arcuate shape of the rift itself the brittle plate thickness variations are crucial for the development of the more recent lvrs which are predicted to occur along the faial ridge in the best fit model the top mantle viscosity is 1 10 22 pa s at 5 15 km depth and the present day fracture network takes 3 ma to develop

condor seamount azores ne atlantic a morpho tectonic interpretation abstract high resolution datasets collected by multibeam and acoustic backscatter surveys were used to produce fine scale seafloor nature and morpho tectonic interpretations of the condor seamount condor constitutes an elongated volcanic ridge that extends for 39 km and rises more than 1800 m from the surrounding seafloor constructive morphologies include i linear eruptive centres ii volcanic cones with or without summit depressions iii lava flows and iv hummocky sectors eruptive type is interpreted to vary with depth on the deeper seamount extremities the predominance of highly acoustically backscattering volcanic cones and hummocky terrain is interpreted to result from effusive eruptions not yet covered by sediment deposits in contrast the smoother relief of the central seamount flanks is interpreted as draping and infilling of the underlying effusive relief by i primary volcaniclastic deposits produced by explosive eruptions on the shallowest parts of the ridge together with ii secondary volcanigenic sediments resulting from truncation of the seamount top by swell erosion and iii sediments resulting from biogenic production a set of wnw ese to nw se trending volcano tectonic structures are shown to control most of the fissural volcanism that formed the ridge a network of nnw sse trending faults is identified on the sea floor around condor but they show little relation with the distribution of volcanic edifices or with post emplacement dismantling of the seamount these fault sets are related to the transtensional regime acting on the azorean segment of the eurasia nubia plate boundary erosional features include i palaeo wave cut platforms on the seamount summit ii landslide scars produced by lateral collapses of the ne and sw facing flanks iii gullies and turbidity current channels and iv mass wasting deposits iceberg drag and bump marks are also identified on the seamount upper flanks representing the first reference to such features in the azores and an additional low latitude record given the lack of major erosional and tectonic dismantling condor is suggested to be a relatively young seamount a revised factoring of eustatic erosional and isostatic processes does not exclude that the summit may have been eroded as late as the last glacial maximum

magma evolution of the sete cidades volcano sao miguel azores the sete cidades volcano sao miguel azores is situated at the eastern end of the ultraslow spreading terceira rift axis the volcano comprises several dominantly basaltic pre caldera eruptions a trachytic caldera forming stage and a post caldera stage consisting of alternating trachytic and basaltic eruptions the post caldera flank lavas are more primitive 5 wt mgo than the pre caldera lavas implying extended fractional crystallization and longer crustal residence times for the pre caldera shield building lavas thermobarometric estimates show that the ascending alkali basaltic magmas stagnated and crystallized at the crust mantle boundary similar to 15 km depth whereas the more evolved magmas mainly fractionated in the upper crust similar to 3 km depth the caldera forming eruption was triggered by a basaltic injection into a shallow trachytic magma chamber lavas from all stages follow a single continuous liquid line of descent from alkali basalt to trachyte although slight differences in incompatible element e g ba nb la nb and sr isotope ratios imply some heterogeneity of the mantle source major and trace element data suggest similar partial melting processes throughout the evolution of the volcano slight geochemical differences between post and pre caldera stage lavas from the sete cidades volcanic system indicate a variation in the mantle source composition with time the oxygen fugacity increased from the pre caldera to the post caldera stage lavas probably as a result of the assimilation of crustal rocks this is supported by the presence of crustal xenoliths in the lavas of the flank vents the lavas from the sete cidades volcano generally have low sr isotope ratios however rocks from one post caldera vent on the western flank indicate mixing with magmas resembling the lavas from the neighbouring agua de pau volcano having higher sr isotope ratios the different magma sources at sete cidades and the adjacent agua de pau volcano imply that despite their close proximity there is only limited interaction between them

asymmetric geophysical signatures in the greenland norwegian and southern labrador seas and the eurasia basin a thorough examination of geophysical data from the greenland norwegian sea eurasia basin and southern labrador sea shows significant asymmetry of several parameters basement topography adjusted for sediment loading free air gravity anomaly spreading half rate and seismicity with respect to crustal age 1 1 average zero age depth 0 57 m y b p depth of highest rift mountain summits and depth to magnetic basement 10 30 km from axis of mohns and knipovich ridges is less on the north american plate flanks the zero age depth asymmetry is 400 500 m for the eurasia basin 0 57 m y b p and for mohns ridge 57 22 m y b p and 150 200 m for younger mohns ridge crust 22 0 m y b p and for the extinct aegir ridge 57 27 m y b p there is little or no asymmetry in the labrador sea except near the extinct rift valley where the east flank is 150 300 m shallower magnetic depth to source computations provide an independent confirmation of basement asymmetry the belts 10 30 km from the axis of mohns and knipovich ridges are 100 150 m shallower on the west flank of these ridges the shallower ridge flank is topographically rougher so that average rift mountain summits are 300 m shallower on the west flanks of the mohns knipovich ridges a larger asymmetry than for average zero age depth the amount of topographic asymmetry is greatest near the mohns knipovich bend asymmetry appears to be greatest for ridges oriented normal to the spreading direction and less for oblique spreading 2 2 free air gravity anomaly asymmetries of 5 to 20 mgal sign indicates west flank is more positive are associated with topographic asymmetry at least within 10 15 m y of the axis of mohns and knipovich ridges gravity is reduced on the older flanks west of the extinct mid labrador ridge and east of mohns ridge asymmetric crustal layer thicknesses or densities provide one possible explanation although deep seated sources e g mantle convection unrelated to the crust cannot be excluded 3 3 spreading half rate was about 5 15 lower on the north american plate flanks of mohns ridge 57 35 m y and in the eurasia basin 0 57 m y thus the fast spreading flank tends to produce deeper smoother crust however topographic asymmetry cannot relate only to spreading rate asymmetry since for the young mohns ridge crust 9 m y b p faster spreading and higher topography are both associated with the west flank 4 4 mid plate seismicity is higher on the eurasia eastern flank of mohns and knipovich ridge but this effect may be unrelated to the other three the fluid dynamical model of stein et al correctly explains the sense of spreading rate asymmetry the north american plate moving faster over mantle is growing more slowly however the other asymmetries and their causal relationships remain theoretically unexplained

geological mapping of the menez gwen segment at 37 50 n on the mid atlantic ridge implications for accretion mechanisms and associated hydrothermal activity at slow spreading mid ocean ridges abstract slow spreading mid ocean ridges have the potential to form large seafloor massive sulphide sms deposits current exploration for sms deposits commonly targets associated active hydrothermal venting on the ridge axis which makes the discovery of inactive vent sites and sms deposits in the off axis regions unlikely geological maps of the seafloor which help understand the timing and location of sms formation usually focus on individual hydrothermal vent sites and their immediate surroundings and are often too small to aid in sms exploration this study uses ship based multibeam echosounder mbes data and a systematic classification scheme to produce a segment scale geological map when combined with spreading rate this allows us to not only reconstruct the segment s spreading history but also reveals important processes that localize hydrothermal venting geological mapping around two known hydrothermal vent sites on the menez gwen segment at 37 50 n on the slow spreading mid atlantic ridge showed that hydrothermal venting accompanies the tectonic break up of a large cooling magmatic body venting is focussed by faulting and resulting permeability changes the large magmatic body is associated with an axial volcano that formed as a last stage of a period with intense magmatic accretion such magmatic accretion periods occur every 300 to 500 ka at the menez gwen segment with increasing intensity over the past 3 5 ma years the most recent most intense magmatic period appears to be a regional phenomenon also affecting the neighbouring lucky strike and rifted hills segments understanding the accretional setting and the spatial and temporal constraints of hydrothermal venting enables us to develop criteria in mbes data to aid exploration for inactive sms deposits

a tectonic model for ridge transform ridge plate boundaries implications for the structure of oceanic lithosphere abstract the first order geologic and morphologic relationships at along and proximal to ridge transform ridge plate boundaries are used to construct an empirical and speculative tectonic model the distinctive but variable morphotectonic fabric and crustal structure of the transform domain are the product of a tectonic continuum that ranges from very slow rates of strain associated with very thick edges of lithosphere slowly slipping ridge transfrom ridge plate boundaries to extremely high rates of strain associated with very thin edges of lithosphere fast slipping ridge transform ridge plate boundaries the geometry of a ridge transform intersection necessitates the juxtaposition of a relatively cold thick edge of lithosphere against the truncated end of an accreting plate boundary the cold face of lithosphere cools the adjacent wedge of asthenosphere rising beneath the axis of accretion and restricts the amount of partial melting thus attenuating the volume of basaltic melt segregated from the asthenosphere per unit time the shallow level manifestation of this cold edge effect is a thinner oceanic crust at depth the evolving lithosphere thickens rapidly as the cold edge is approached creating changes in the properties of the young lithosphere and forming a weld of upper mantle material against the cold edge of truncating lithosphere the ridge transform weld of upper mantle material creates a shear couple in the lithosphere underlying the intersection resulting in the progressive reorientation of the maximum tensile stress from normal to the ridge axis at some distance from the ridge transform intersection to an oblique angle near the boundary the brittle carapace of oceanic crust that overlies the mantle weld will deform accordingly with the development of oblique dip slip faults the model predicts that the geologic expressions of this cold boundary effect will become more dramatic with increasing thickness of the truncating edge at very slow rates of accretion 100 km juxtapose thick 30 50 km edges of lithosphere against the accreting plate boundary all but nullifying the processes that lead to the emplacement of normal oceanic lithosphere in this environment a relatively strong mantle weld will form at the ridge transform intersection and the thick edges of opposing lithosphere along the length of the transform will tend to confine the strike slip tectonism to a relatively narrow and temporally stable principal transform displacement zone in contrast at very fast rates of accretion large offset transforms 100 km place relatively thin edges of lithosphere

surface displacement field at terceira island deduced from repeated gps measurements abstract we present the results of six gps global positioning system campaigns held in terceira island azores between 1999 and 2006 for a group of 17 stations to estimate the surface displacement field induced by the eurasian nubian interplate motion and their relationship with the rifting along terceira rift and volcanic processes at the island scale we show that the whole island mainly behaves like a rigid body which absolute motion is closer to eurasia the magnitude of the computed intra island horizontal deformation field difference between the individual site velocities and the mean rigid body motion is still below the observation accuracy and so cannot be directly related with local processes we also show that the island is globally subsiding with an average rate close to 1 mm yr as it moves to the ne away from the terceira rift mostly attached to the eurasian plate the high subsidence rate can be the result of the volcanic processes that shaped the serreta ridge nw of terceira

interaction between the mid atlantic ridge and the azores hot spot during the last 85 myr emplacement and rifting of the hot spot derived plateaus 1 multiple and single beam bathymetric data are compiled over the azores plateau to produce a 1 km 1 km grid between latitudes 32 n and 49 n and longitudes 22 w and 43 w mantle bouguer anomalies are then calculated from this grid and the satellite derived gravity these grids provide new insights on the temporal and spatial variations of melt supply to the ridge axis the elevated seafloor of the azores plateau is interpreted as resulting from the interaction of a mantle plume with the mid atlantic ridge mar the presence of a large region of elevated seafloor associated with a thick crust between the great meteor seamounts and the azores platform on the africa plate and less developed conjugate structures on the north america plate favors genetic relations between these hot spot derived structures this suggests that a ridge hot spot interaction has occurred in this region since 85 ma this interaction migrated northward along the ridge axis as a result of the sse absolute motion of the africa plate following a direction grossly parallel to the orientation of the mar kinematic reconstructions from chron 13 35 ma to the present allow a proposal that the formation of the azores plateau began around 20 ma and ended around 7 ma a sharp bathymetric step is associated with the beginning of important melt supply around 20 ma the excess of melt production is controlled by the interaction of the ridge and hot spot melting zones the geometry and distribution of the smaller scale features on the plateau record episodic variations of the hot spot melt production the periodicity of these variations is about 3 5 myr following the rapid decrease of widespread volcanism the plateau was subsequently rifted from north to south by the mid atlantic ridge since 7 ma this rifting begins when the mar melting zone is progressively shifted away from the 200 km plume thermal anomaly these results bear important consequences on the motion of the africa plate relative to the azores hot spot they also provide an explanation to the asymmetric geochemical signature of the azores hot spot along the mid atlantic ridge

lucky strike seamount implications for the emplacement and rifting of segment centered volcanoes at slow spreading mid ocean ridges the history of emplacement tectonic evolution and dismemberment of a central volcano within the rift valley of the slow spreading mid atlantic ridge at the lucky strike segment is deduced using near bottom sidescan sonar imagery and visual observations volcano emplacement is rapid 1 myr associated with focused eruptions and with effusion rates feeding lava flows that bury tectonic features developed prior to and during volcano construction this volcanic phase likely requires efficient melt pooling and a long lived crustal magma chamber as a melt source a reduction in melt supply triggers formation of an axial graben rifting the central volcano and the onset of seafloor spreading may eventually split it at lucky strike this results in two modes of crustal construction eruptions and tectonic activity focus at a narrow graben that bisects the central volcano and contains the youngest lava flows accumulating a thick layer of extrusives away from the volcano summit deformation and volcanic emplacement is distributed throughout the rift valley floor lacking a clear locus of accretion and deformation volcanic emplacement on the rift floor is characterized by axial volcanic ridges fed by dikes that propagate from the central axial magma chamber the mode of rapid volcano construction and subsequent rifting observed at the lucky strike seamount is common at other central volcanoes along the global mid ocean ridge system

recent geodetic results in the azores triple junction region gps global positioning system observations started to be carried out in the azores region under the scope of the tango transatlantic network for geodesy and oceanography project in 1988 the measurements carried out between 1993 and 2000 five campaigns on nine gps sites one per island were reprocessed using two state of the art software packages different methodologies were applied to compute each campaign solution and the derived velocity field the velocity fields including the motions of two permanent stations recently installed in the azores were computed within the most recent geodetic reference frame itrf2000 international terrestrial reference frame solution 2000 they are compared with the motions of the stable rigid tectonic plates using as reference deos2k a global tectonic model developed using geodetic data the relative motions between the western and central groups of islands yield to evaluate the opening rate of the mid atlantic ridge boundary between the north american plate and the eurasian and african plates concerning the boundary between the eurasian and african plates the motion of the tango sites in the central and eastern groups clearly identifies the transition pattern between those two plates two of the sites are considered to be located in the stable part of these plates whereas the remaining five are within the deformation region of the eurasia africa boundary the conclusions are analyzed in view of the different deformation models derived from geodynamic or geophysical data that have been proposed for the region

palaeomagnetic study of a subaerial volcanic ridge sao jorge island azores for the past 1 3 myr evidence for the cobb mountain subchron volcano flank instability and tectonomagmatic implications we present a palaeomagnetic study on 38 lava flows and 20 dykes encompassing the past 1 3 myr on s jorge island azores archipelagonorth atlantic ocean the sections sampled in the southeastern and central western parts of the island record reversed and normal polarities respectively they indicate a mean palaeomagnetic pole 81 3 degrees n 160 7 degrees e k 33 and a95 3 4 degrees with a latitude shallower than that expected from geocentric axial dipole assumption suggesting an effect of non dipolar components of the earth magnetic field virtual geomagnetic poles of eight flows and two dykes closely follow the contemporaneous records of the cobb mountain subchron odp dsdp programs and constrain the age transition from reversed to normal polarity at ca 1 207 0 017 ma volcano flank instabilities probably related to dyke emplacement along an nnwsse direction led to southwestward tilting of the lava pile towards the sea two spatially and temporally distinct dyke systems have been recognized on the island the eastern is dominated by nnwsse trending dykes emplaced before the end of the matuyama chron whereas in the central western parts the eruptive fissures oriented wnwese controlled the westward growth of the s jorge island during the brunhes chron both directions are consistent with the present day regional stress conditions deduced from plate kinematics and tectonomorphology and suggest the emplacement of dykes along pre existing fractures the distinct timing and location of each dyke system likely results from a slight shift of the magmatic source

historical tsunami in the azores archipelago portugal abstract because of its exposed northern mid atlantic location morphology and plate tectonics setting the azores archipelago is highly vulnerable to tsunami hazards associated with landslides and seismic or volcanic triggers local or distal critical examination of available data written accounts and geologic evidence indicates that since the settlement of the archipelago in the 15th century at least 23 tsunami have struck azorean coastal zones most of the recorded tsunami are generated by earthquakes the highest known run up 11 15 m was recorded on 1 november 1755 at terceira island corresponding to an event of intensity vii viii damaging heavily damaging on the papadopolous imamura scale to date eruptive activity while relatively frequent in the azores does not appear to have generated destructive tsunami however this apparent paucity of volcanogenic tsunami in the historical record may be misleading because of limited instrumental and documentary data and small source volumes released during historical eruptions the latter are in contrast with the geological record of massive pyroclastic flows and caldera explosions with potential to generate high magnitude tsunami predating settlement in addition limited evidence suggests that submarine landslides from unstable volcano flanks may have also triggered some damaging tsunamigenic floods that perhaps were erroneously attributed to intense storms the lack of destructive tsunami since the mid 18th century has led to governmental complacency and public disinterest in the azores as demonstrated by the fact that existing emergency regulations concerning seismic events in the azores autonomous region make no mention of tsunami and their attendant hazards we suspect that the coastal fringe of the azores may well preserve a sedimentary record of some past tsunamigenic flooding events geological field studies must be accelerated to expand the existing database to include prehistoric events information essential for more precisely estimating the average tsunami recurrence rate for the azores over a longer period a present day occurrence of a moderate to intense tsunami i e the size of the 1755 event would produce societal disruption and economic loss orders of magnitudes greater than those of previous events in azorean history to reduce risk from future tsunami comprehensive assessment of tsunami hazards and the preparation of hazards zonation maps are needed to guide governmental decisions on issues of prudent land use planning public education and emergency management

40ar 39ar constraints on the temporal evolution of graciosa island azores portugal lava flows spanning the eruptive record of graciosa island azores archipelago and a gabbro xenolith were dated by 40ar 39ar in order to constrain the pleistocene and holocene volcanic evolution of the island the results range from 1 05 ma to 3 9 ka whereas prior published k ar and 14c ages range from 620 to 2 ka the formation of the serra das fontes shield volcano started at minimum 1 05 ma and the magmatic system was active for ca 600 ky as suggested by the formation of the gabbro xenolith by magmatic differentiation evolved magmas making up the serra das fontes serra branca composite volcano were generated at ca 450 ka after a period of ca 110 ky of volcanic inactivity and erosion of volcanic edifices volcanism was reactivated with the formation of the vitoria unit nw platform later the development of the vulcao central unit started with the formation of monogenetic cones located to the south of the serra das fontes serra branca vitoria unit this volcanism became progressively more evolved and was concentrated in a main eruptive center forming the vulcao central stratovolcano with an age older than 50 ka the caldera related to this stratovolcano is older than 47 ka and was followed by effusion of basaltic magmas into the caldera resulting in the formation of a lava lake which ultimately spilled over the caldera rim at ca 11 ka the most recent eruptions on graciosa formed two small pyroclastic cones within the caldera and the pico do timao cone within the vitoria unit at ca 3 9 ka

famous and amar segments on the mid atlantic ridge ubiquitous hydrothermal mn ch4 δ3he signals along the rift valley walls and rift offsets dynamic hydrocast experiments enabled mn tdm ch4 concentrations and 3he ratio to be recorded through vertical cross sections of hydrothermal plumes along the famous segment and the southern part of the amar segment on the mid atlantic ridge between 36 n and 37 n mn ch4 and 3he figures all along both segments are well above the seawater background in the open ocean they are interpreted to be the result of time integrated hydrothermal discharges dispersed and mixed in a closed basin delineated by the rift valley and the segment ends hydrothermal activity along the famous and amar segments appears to be similar a comparison of the residence times of the three tracers from the dispersed time integrated signals is proposed although the background values in these closed basin are high some proximal and or large hydrothermal inputs overprinted on the general time integrated plume can be detected i e the rainbow site south of amar based on the depth and the location of plumes hydrothermal activity is not by far limited to the neo volcanic inner floor of the valley and should involve the walls and complex offsets of the rift valley considering the mn and ch4 concentrations in these plumes two types of ocean mantle interaction may be represented hot focused discharges on ultramafic exposures rainbow site and low temperature diffuse serpentinisation

distributed deformation close to the azores triple point abstract terceira rift and the northern and southern branches of the mid atlantic ridge mar form a triple junction close to 39 n known as the azores triple junction new swath bathymetric data are used to investigate the surface expression of faulting close to the triple junction by the systematic mapping of mar generated abyssal hills it is shown that close to the geometrical intersection between the three spreading axes there exists no single transform fault connecting terceira rift to the mar but a distributed tectonic deformation area characterized by mesoscale brittle deformation close to the surface covering approximately 90 km by 100 km and almost no volcanism which links terceira rift to the mar accommodating the relative displacement of the three plates close to the geometrically triple point magnetic chrons are used to compute the spatial variation of spreading velocity at the mid atlantic ridge and confirm the above interpretation they show a progressive increase of spreading velocity along a single mar segment between pure nubian at 38 30 n and pure eurasian at 39 25 n without the development of a transform fault that would integrate the eurasia nubia plate boundary the comparison of similar triple junctions where a slower axis joins two faster ridges shows that the slower arm does not reach the triple point and that there is always a finite triple junction area highly tectonized in which size is dependent on the angle between the two faster arms and consequently on the relative spreading velocity of the slower arm

insight on the crustal stress state in faial and pico islands azores from analysis of aftershocks of the 1998 earthquake the seismicity in the azores plateau can be of volcanic or tectonic origin the latter usually more destructive especially in the central group of islands in this study we aimed to obtain crustal stress indicators from seismic data collected in july 1998 following the july 9th m5 8 earthquake that struck near faial and pico islands the maximum horizontal stress shmax azimuth was obtained from two methods for the seismogenic volume the shmax was obtained from single and composite focal mechanisms whereas the state of stress beneath the seismic network was derived from shear wave splitting analysis of local earthquakes the composite focal mechanisms were obtained from a waveform clustering method applied to identify identical earthquakes the seismic anisotropy analysis could be applied to only six stations the polarization of the first s arrival being stable and with a secondary perpendicular pulse arriving 0 6 0 24 s later the focal mechanisms provided a nw se shmax with azimuth range of 96 114 and an associated transition of normal fault to strike slip dominant regime consistent with the estimated regional field of 110 120 and coherent with movement along three fault systems inland the shmax azimuths present greater variability in faial the stations located north of the caldeira present a stable azimuth of 151 153 and are consistent with the effect of a nnw sse fault system present in the islands rotating to 50 77 in the eastern sector of the pedro miguel graben probably due to a complex interaction of the wsw ene dominant fault system with old eroded volcanic complexes that segmented the island into several blocks in pico island the shmax direction azimuth ranges from 93 to 135 in a radial pattern around the island s main stratovolcano overlapping the regional stress field

reappraisal of the arabia india somalia triple junction kinematics abstract we propose alternative kinematics for the arabia india somalia triple junction based on a re interpretation of seismological and magnetic data the new triple junction of the ridge ridge ridge type is located at the bend of the sheba ridge in the eastern gulf of aden at 14 5 n and 56 4 e the owen fracture zone arabia india boundary is connected to the sheba ridge by an ultra slow divergent boundary trending n80 e 10 marked by diffuse seismicity the location of the arabia india rotation pole is constrained at 14 1 n and 71 2 e by fitting the active part of the owen fracture zone with a small circle the finite kinematics of the triple junction is inferred from the present day kinematics since the inception of the accretion 15 18 ma ago the sheba ridge has probably receded 300 km at the expense of the carlsberg ridge which propagated northwestward in the gulf of aden while an ultra slow divergent plate boundary developed between the arabian and indian plates the overall geometry of the new triple junction is very similar to that of the azores triple junction

kinematics of the north american african plate boundary between 28 and 29 n during the last 10 ma evolution of the axial geometry and spreading rate and direction abstract a high resolution survey sara segmentation ancienne de la ride atlantique collected sea beam bathymetry magnetic gravity and seismic reflection data on the flanks of the mid atlantic ridge between 28 and 29 n this survey was designed to complement a portion of an on axis study carried out between the atlantis and kane transform faults which revealed the configuration of the spreading axis the sara survey extends this coverage to an age of approximately 10 ma on either flank a sinuous isochron pattern was established from anomaly identification made using a two dimensional model the plate tectonic reconstruction based solely on this isochron pattern provides an accurate kinematic description of the relative motion between the african and north american plates within the zone our results confirmed by data from the famous area and a flowline comparison with the trend of the kane fracture zone show that a change in axial orientation spreading direction and spreading rate occurred at approximately anomaly 4 time 7 01 ma this was coincident with the appearance of a new axial segment all the axial segments observed have changed length continously during the last 10 ma by advancing or retreating at one or both ends in any combination so as to leave non parallel trails on the flanks that do not align with either the spreading direction or with the absolute plate motion factors contributing to the sinuous magnetic anomaly pattern are discussed

structural and metamorphic evidence of local extension along the vivero fault coeval with bulk crustal shortening in the variscan chain nw spain abstract the vivero fault is a w dipping n s striking ductile shear zone separating the ollo de sapo antiform in its western hangingwall and the lugo dome in its eastern footwall two stages of deformation f 1 and f 2 produced nearly coaxial folds with sub horizontal axes a crenulation cleavage s 2 transposes an older s 1 three sets of shear bands in the hangingwall define a pervasive fabric consistent with an e w bulk shortening perpendicular to a composite s 1 2 foliation and nne stretching parallel to l 2 the vivero fault zone is marked by a mylonitic foliation with a steeply nw plunging stretching lineation and extensional crenulation cleavage ecc indicating normal slip in the vicinity of the fault sub horizontal nne trending f 3 folds with a crenulation cleavage s 3 deform earlier formed fabrics including a mylonitic foliation pressure temperature conditions obtained from mineral assemblages on both sides of the vivero fault yield a minimum throw of 5 5 km andalusite bearing pelite in the hangingwall was infolded by an f 2 synform into the kyanite field at 450 500 c the eastern edge of these rocks was later accreted to the footwall and heated to andalusite staurolite conditions at 600 c slip on the vivero and valdovino faults is kinematically related east west shortening during f 2 involved folding and sinistral strike slip on the valdovino fault which induced local extension along the newly generated vivero fault synkinematic emplacement of granitoids along the vivero fault is favoured by extension coeval slip on both faults took place during the later stages of f 2 folding geometrical constraints caused northwards escape of the crustal block bounded by the valdovino and vivero faults recorded by nne stretching defined by l 2

the azores triple junction evolution since 10 ma from an aeromagnetic survey of the mid atlantic ridge abstract in the past two decades several models have been presented to describe the evolution and the present structure of the azores triple junction these models were mainly based on morphological analysis of sea bottom topography sparse magnetic profiling sidescan sonar surveying over the plateaus and global plate kinematic considerations for the north atlantic in this paper we follow a different approach from a detailed aeromagnetic survey covering both sides of the mid atlantic ridge between 37 n and 40 30 n the magnetic anomalies up to anomaly 5 are accurately identified allowing careful modelling of the kinematics of this region for the past 10 ma and thus establishing a coherent framework for the design of geophysical models for the azores triple junction the analysis of magnetic anomalies and the use of fourier domain inversion techniques show that the ridge is made up of six segments each one varying in length from 50 to 60 km the more continuous section of the ridge can be defined from the first four northern segments although the north azores fracture zone right offsets the ridge at 39 30 n 29 40 w the fifth and sixth segments are respectively right offset by the acor fracture zone at 38 23 n 30 15 w and by the princess alice fracture zone at 38 00 n 30 50 w anomaly identifications using a two dimensional model and plate tectonics reconstruction techniques allowed the calculation of rotation pole parameters the results thus obtained reveal that at least between anomalies 5 and 3 10 3 85 ma the azores displayed an independent motion relative to the neighbouring plates and after anomaly 2a 2 45 ma the azores moved attached to the eurasian plate the triple junction azores north america africa or eurasia north america africa moved northward from 38 00 n 30 50 w to 38 20 n 30 15 w between 4 and 3a and probably to 38 50 n 30 00 w at anomaly 2a time a detailed reconstruction model is presented

gps and tectonic evidence for a diffuse plate boundary at the azores triple junction abstract we use gps bathymetric structural and seismic data to define the pattern of present deformation along the northern half of the azores plateau where the nubia eurasia plate boundary terminates at the axis of the mid atlantic ridge mar new and existing campaign gps velocities from the azores islands reveal extension oblique to a series of en echelon volcanic ridges occupied by terceira s jorge pico and faial islands in a frame of reference defined by 69 continuous gps stations on the eurasia plate terceira island moves 2 1 mm yr away from eurasia consistent with the island s location within the terceira rift and plate boundary structure the volcanic ridges south of the terceira rift move toward wsw at progressively faster rates reaching a maximum of 3 5 0 5 mm yr 2 for the pico faial volcanic ridge the hypothesis that the terceira rift accommodates all nubia eurasia plate motion is rejected at high confidence level based on the motions of sites on s jorge island just west of terceira rift all of the islands move relative to the nubia plate with pico island exhibiting the slowest motion only 1 0 5 mm yr 2 detailed bathymetry from the interior of the hypothesized azores microplate reveals faults that crosscut young mar seafloor fabric these observations and the gps evidence for distributed deformation described above argue against the existence of a rigid or semi rigid azores microplate and instead suggest that nubia eurasia plate motion is accommodated by extension across a 140 km wide zone east of the mar axis most likely bounded to the north by the northern shoulder of the terceira rift the mar spreading rate along the western end of the azores deformation zone 38 5 n 39 5 n is intermediate between the eurasia north america rate measured at 39 5 n and the nubia north america rate measured at 38 5 n consistent with the joint conclusions that the nubia eurasia boundary is broad where it intersects the mar and the azores triple junction is diffuse rather than discrete

analysis of geometry of volcanoes and faults in terceira island azores evidence for reactivation tectonics at the eur afr plate boundary in the azores triple junction abstract the late pliocene to quaternary 5 ma volcanism in the central and eastern azores archipelago is related to the eurasia africa divergence but a clear deformation pattern has not yet been established at this location this work focuses on the contribution of synthetic aperture radar sar scenes and digital elevation models dem complemented with geophysical geodetic and morpho structural data to establish the geometric relationships between volcanic edifices and tectonic structures in the central and eastern azores archipelago bathymetric data were also used to extend field observations to the significant submarine area of the azores plateau strikes of extension fractures directly observed or inferred from elongated volcanic vents or linear volcanic clusters in terceira island indicate that volcanism is mainly controlled by regional extension as given by nuvel 1a plate motion model additionally other directions were also detected for extension fractures around the santa barbara volcano defining a radial pattern and in the central part of the island exhibiting an s shape pattern although most of the volcanic vents are controlled by extension fractures some seem to be controlled by faulting such as the case of the ones rooted in releasing bends along strike slip or oblique slip faults in the central part of the island concerning the azores plateau most of the structures have directions that do not directly fit with present day direction of relative motion n70 between eurasia and africa directions ranging from n110 to n125 found mainly along the terceira rift are interpreted as ancient transform directions reactivating as transtensional fault zones due to the present day plate motion n s directions are also visible in the plateau being interpreted as former middle oceanic rift faults reactivated as left lateral fault zones these results contrast with the volcanic expression in other hotspot dominated oceanic islands such as the canaries or the hawaii islands chain probably due to the tectonic complexity promoted by the azores triple junction instability through time

persististrombus coronatus mollusca strombidae in the lower pliocene of santa maria island azores ne atlantic paleoecology paleoclimatology and paleobiogeographic implications abstract the family strombidae is one of the twenty three families and subfamilies of gastropods associated with tropical environmental conditions and therefore useful as a biogeographical and paleoclimatic proxy today the strombid genus persististrombus is represented in the ne atlantic by a single species restricted to the tropical mauritanian senegalese province this work reports the occurrence of persististrombus coronatus from the lower pliocene of santa maria island in the azores archipelago based on this occurrence and on the mio pliocene fossil record of the ne atlantic oceanic islands paleoclimatological considerations are discussed which allow for the first time to include the azores and the other macaronesian islands in a wider context of the ne atlantic paleobiogeographical molluscan provinces late miocene to present day molluscan biogeographic units ranging in latitude from 60 n down to 17 s are here defined and the boundaries of the proposed climatic zones are outlined we suggest that during the upper miocene lower pliocene the paleoclimate at santa maria island was drastically different from that seen at those latitudes today with mean annual sea surface temperatures ssts about 3 7 c to 6 3 c higher than the present day 20 6 c and with mean monthly ssts ranging from 20 c to 28 c with six months with mean ssts over 24 c conditions typical of a tropical setting

seismicity source mechanisms and tectonics of the azores gibraltar plate boundary abstract a study and partial revision of the seismicity and source mechanisms of earthquakes are presented for the oceanic plate boundary of eurasia and africa from the azores islands to the strait of gibraltar focal mechanisms of 36 earthquakes wave form modelling of 4 and seismic moments and dimensions of 28 have been calculated these results indicate that the boundary is divided into three sections with different characteristics the azores the central section and the east section the azores section has high seismicity of moderate magnitude and a low slip rate 0 76 cm yr the predominant mechanism is normal or transform faulting with horizontal tensions on average in a n25 e direction in the central section large earthquakes occur with strike slip right lateral motion along faults in an e w direction there is a high slip rate 3 39 cm yr the east section is of more complex nature large earthquakes also occur but the predominant mechanism is reverse faulting with a horizontal pressure axis in the n30 w direction resulting in underthrusting of africa from the slip vectors the pole of rotation for the african plate is located at 28 2 n 21 1 w nearer to the boundary than previous locations close to the continent there is a continuation in the ocean crust of some structural features of the iberian peninsula

the terceira rift as hyper slow hotspot dominated oblique spreading axis a comparison with other slow spreading plate boundaries abstract we suggest the 550 km long terceira rift tr azores plateau is the world s slowest spreading hyper slow 4 mm a plate separation 2 3 3 8 mm a perpendicular to oblique axial segments organized accreting plate boundary in its slightly sinuous ca 300 km radius of curvature axial trace its oblique spreading angles ca 40 65 and in frequency and first motions of earthquakes the tr resembles better known ultra or super slow spreading ridges e g gakkel and southwest indian ridges interpreted simply as volcanically unfilled rift valley segments the inter island basins e g the 3200 m deep hirondelle basin are slightly wider 30 60 km but not significantly deeper 1000 2200 m than the mid atlantic ridge mar median valley 20 28 mm a 10 n 53 n however along axis segmentation wavelengths ca 100 km are double those along the central mar but make tr comparable to the ultra slow 15 16 mm a southwest indian and gakkel 7 13 mm a ridges if this segmentation wavelength reflects rayleigh taylor instabilities the viscosity contrast between the overlying axial lithosphere and the partial melt zones is about an order of magnitude greater at ca 4 16 mm a than at 20 30 mm a the tr differs dramatically from ultra slow ridges only in the large amplitude of along strike topography 2000 4000 m 4200 m total variation owing perhaps to a copious melt flux from the azores hotspot combined with a spreading rate determined greater axial flexural strength and plate thickness and slower export of volcanics from the rift axis the probable tr youth ca 1 ma requiring less than 4 km new oceanic crust suggests lack of steady state spreading conditions which may explain the published gravity evidence against tr spreading absolute plate motions support the creation of the azores plateau by successive ne jumps of the rift axis to maintain its position over a fixed hotspot

the volcanotectonic evolution of flores island azores portugal abstract from the study and interpretation of the volcanic products and structures of flores island we infer that its volcanic history was dominated by two major periods 1 proto insular volcanism which includes all the submarine and emergent activities and 2 insular volcanism consisting exclusively of subaerial eruptions the first period includes two phases 1 the oldest 2 2 to 1 5 ma of shallow submarine volcanism 2 the youngest 1 0 to 0 75 ma includes emergent volcanism throughout the second period three volcanic stages are recognized 1 the first one 0 7 to 0 5 ma includes the most voluminous volcanism balanced between effusive and explosive events 2 an intermediate stage 0 4 to 0 2 ma that involves a larger number of small scale feeder centres with effusive eruptions prevailing 3 the third stage is the latest volcanic activity of the island 0 003 ma with strombolian and subsequent phreatic and phreatomagmatic activity centred at four volcanic vents from 1 0 ma to the present the volcanotectonic development of flores island also reflects the operation of two major tectonic processes 1 a marked volcanotectonic uplift during the first stage 1 0 to 0 55 ma and 2 subsidence particularly intensive in the island s central area which led to the subsequent formation of two and perhaps three large calderas during the period 0 55 to 0 4 ma

surveying the flanks of the mid atlantic ridge the atlantis basin north atlantic ocean 36 n new seismic reflection and okean 9 5 khz sidescan sonar data were used to characterise the seismic stratigraphy and structure of the region south of the azores islands north atlantic ocean the new data provide constraints on the tectono sedimentary evolution of deep ocean basins located on the flanks of slow spreading ridges south of the azores thick sedimentary sequences 1 0 s two way travel time correlated with units in the madeira abyssal plain odp sites 950 952 overlay an irregular basement generated by a combination of volcanic and tectonic processes three main cenozoic depositional events are recognised and signed by distinct seismic megasequences the moderate deformation of the sedimentary units and the structural fabric of the underlying oceanic crust shown on seismic and sidescan data suggest the existence of a complex tectonic setting south of the azores illustrating this complex setting is the change from a basement with prominent abyssal hills west of 30 w to a segmented oceanic crust denoting an extensional transtensional tectonic regime east of the latter meridian

the 1980 1997 and 1998 azores earthquakes and some seismo tectonic implications abstract we have studied the focal mechanisms of the 1980 1997 and 1998 earthquakes in the azores region from body wave inversion of digital gdsn global digital seismograph network and broadband data for the 1980 and 1998 shocks we have obtained strike slip faulting with the rupture process made up of two sub events in both shocks with total scalar seismic moments of 1 9 1019 nm mw 6 8 and 1 4 1018 nm mw 6 0 respectively for the 1997 shock we have obtained a normal faulting mechanism with the rupture process made up of three sub events with a total scalar seismic moment of 7 7 1017 nm mw 5 9 a common characteristic of these three earthquakes was the shallow focal depth less than 10 km in agreement with the oceanic type crust from the directivity function of rayleigh lr waves we have identified the nw se plane as the rupture plane for the 1980 and 1998 earthquakes with the rupture propagating to the se slow rupture velocity about of 1 5 km s has been estimated from directivity function for the 1980 and 1998 earthquakes from spectral analysis and body wave inversion fault dimensions stress drop and average slip have been estimated focal mechanisms of the three earthquakes we have studied together with focal mechanisms obtained by other authors have been used in order to obtain a seismotectonic model for the azores region we have found different types of behaviour present along the region it can be divided into two zones zone i from 30 w to 27 w zone ii from 27 w to 23 w with a change in the seismicity and stress direction from zone i in zone i the total seismic moment tensor obtained corresponded to left lateral strike slip faulting with horizontal pressure and tension axes in the e w and n s directions respectively in zone ii the total seismic moment tensor corresponded to normal faulting with a horizontal tension axis trending ne sw normal to the terceira ridge the stress pattern for the whole region corresponds to horizontal extension with an average seismic slip rate of 4 4 mm yr

reevaluation of magnetic chrons in the north atlantic between 35 n and 47 n implications for the formation of the azores triple junction and associated plateau 1 in this paper we present a new magnetic compilation for an area of the north atlantic located between 35 n and 47 n and up to anomaly 33r we also present a strategy to pick magnetic isochrones and compute finite rotation poles this technique is based on a continuous reduction to the pole technique and some basic assumptions regarding the direction of the remanent magnetization vector a cost function that measures the misfit between interpreted and rotated isochrones and the systematic exploitation of the parameter space is used to compute the best set of finite eulerian rotations for the chrons 5 6 6c 11 12 13 18 20 21 22 23 24 25 26 28 29 30 32 33 and 33r this set of chrons and poles is used to discuss the evolution of the north atlantic close to iberia and in particular the onset and early development of the azores triple junction area we show that the relative motion between the eurasian and the african plates can be coherently described in terms of rigid plate kinematics respecting both the anomalies shapes and the precise location of the main structural elements of the area the pico fracture zone the east azores fracture zone and the gloria fault we distinguish from the magnetic point of view two different areas of the azores plateau the south azores domain where almost undisturbed nnw magnetic lineations can be found and the azores domain close to the topographic highs and with no systematic magnetic stripping with the exception of a few recent lineations probably matuyama and brunhes we present an approximate reconstruction of the plate configuration after chron 18 to conclude that the attachment of iberia to eurasia was younger than previously thought lower miocene triggering the formation of the azores domain in which stretching took place essentially in the last 20 ma at an average rate of 3 8 mm a and progressively attaching the south azores domain to the african plate by a northward progression of the triple junction