factors that control the angle of shear bands in geodynamic numerical models of brittle deformation abstract numerical models of brittle deformation on geological timescales typically use a pressure dependent mohr coulomb or drucker prager plastic flow law to simulate plastic failure despite its widespread usage in geodynamic models of lithospheric deformation however certain aspects of such plasticity models remain poorly understood one of the most prominent questions in this respect is what are the factors that control the angle of the resulting shear bands recent theoretical work suggest that both roscoe 45 coulomb angles 45 2 where is the angle of internal friction and arthur angles 45 4 where is the dilation angle as well as all intermediate angles are possible published numerical models however show a large range of shear band angles with some codes favoring arthur angles whereas others yield coulomb angles in order to understand what causes the differences between the various numerical models here i perform systematic numerical simulations of shear localization around an inclusion of given length scale both numerical element type geometrical and rheological viscoplastic versus viscoelastoplastic effects are studied results indicate that the main factor controlling shear band angle is the non dimensional ratio between the length scale of the heterogeneity d and the size of the numerical mesh x coulomb angles are observed only in cases where the inclusion is resolved well d x 5 10 and in which it is located sufficiently far from the boundary of the box in most other cases either arthur or roscoe orientations are observed if heterogeneities are one element in size coulomb angles are thus unlikely to develop irrespective of the employed numerical resolution whereas differences in element types and rheology do have consequences for the maximum obtainable strain rates inside the shear bands they only have a minor effect on shear band angles shear bands initiated from random noise or from interactions of shear bands with model boundaries or other shear bands result in stress heterogeneities with dimensionless length scales d x 1 2 such shear bands are thus expected to form roscoe or arthur orientations consistent with the findings in previous numerical models

a finite amplitude necking model of rifting in brittle lithosphere we formulate a mechanical model describing the formation of rifts as finite amplitude necking of an elastic plastic layer overlying a fluid substrate a perfectly plastic rheology is a continuum description of faulting in rift zones two important aspects of rift evolution are illustrated by this model the evolution of the rift width as extension proceeds and the finite strain that occurs a region at yield initially develops with a width determined by the thickness of the brittle layer and the internal deformation within this yield zone is proportional to the topographic slope as extension proceeds the surface within the rift subsides and the width of the subsiding yield zone decreases at any stage of rifting material in regions just outside the yield zone is deformed but no longer deforming the width of these deformed regions increases with increasing extension vertical forces due to the mass deficit of the rift depression will flex the elastic layer outside the yield zone creating flanking uplifts the external force required to maintain active rifting increases with the amount of lithospheric stretching indicating that rifting is a quasi static stable process because the yield zone will revert to elastic behavior if the external force causing extension is removed the model predicts that the rift depression and flanking uplifts will be preserved after extension stops our simple mechanical model demonstrates the inherent relationship among graben formation lithospheric thinning and rift shoulder uplift in rift zones

a two phase model for compaction and damage 3 applications to shear localization and plate boundary formation a new two phase theory employing a nonequilibrium relation between interfacial surface energy pressure and viscous deformation bercovici et al this issue provides a model for damage void generation and microcracking and thus a continuum description of weakening failure and shear localization here we demonstrate applications of the theory to shear localization with simple shear flow calculations in which one phase the matrix representing for example silicate is much stronger more viscous than the other phase the fluid this calculation is motivated as a simple model of plate boundary formation in a shear zone even without shear the two phases eventually separate due to gradients in surface tension however the influence of shear on phase separation is manifest in several ways as shear velocity increases the separation rate of the phases increases demonstrating a basic feedback mechanism accumulation of the fluid phase causes focused weak zones on which shear concentrates causing more damage and void generation and thus greater accumulation of fluid beyond a critical shear velocity phase separation undergoes intense acceleration and focusing leading to a tear localization in which the porosity becomes nearly singular in space and grows rapidly like a tear or crack at an even higher value of shear velocity phase separation is inhibited such that shear localization gives way to defocusing of weak zones suggestive of uniform microcracking and failure throughout the layer our two phase damage theory thus predicts a wide variety of shear localization and failure behavior with a continuum model applications of the theory to various fields such as granular dynamics metallurgy and tectonic plate boundary formation are numerous

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

extent of azores plume influence on the mid atlantic ridge north of the hotspot a complete bathymetry and geophysical survey of the recent domain of the mid atlantic ridge north of the azores between 40 30 n and 45 n was achieved during the 1998 triatnord cruise of r v l9atalante interpretation of along track data and geochemical analyses of rock samples show that the surveyed section represents a peculiar section of the mid atlantic ridge this new data set allows us to tentatively place the northern limit of the influence of the azores hotspot on the accretion processes between lat 43 n and 44 n this limit is marked by a 1 wide transition zone this fits well with the asymmetry of the upper mantle thermal structure as previously inferred from seismic tomography and geoid models thus enhancing the links between shallow and deeper accretion processes in a hotspot ridge interaction context observations of along ridge variations of geochemical parameters at various scales shed light on the relations between mantle geochemical heterogeneities and the accretion processes along the ridge axis confirming the asymmetry in the along ridge horizontal extent of the azores plume

oxide minerals in a layered kimberlite carbonate sill from benfontein south africa the lower sill at benfontein south africa shows a high degree of magmatic sedimentation to kimberlite oxide carbonate and carbonate layers the iron titanium oxide minerals are similar in the carbonate rich and silicate rich layers and are represented by titaniferous mg al chromite mg al titanomagnetite magnesian ilmenite rutile and perovskite the spinel crystallization trend was toward enrichment in mg and ti and depletion in cr this trend is similar to that observed in many kimberlites the ilmenite has mg and cr contents within the range observed in kimberlites and lacks the mn enrichment observed in ilmenites from carbonatites perovskite in silicate rich and carbonate rich layers shows similar total ree contents and lree enrichment and lacks the remarkable nb enrichment observed in perovskite from carbonatites these new data on the iron titanium oxide minerals in the lower benfontein sill do not support a genetic relationship between kimberlites and carbonatites

strain accumulation in western united states this review is principally concerned with recent geodetic strain measure ments in western united states undertaken by the us geological survey as part of the earthquake studies program and as a consequence is heavily biased toward the author s own publications most of the publications reporting crustal strain measurements in western united states prior to about 1968 have been compiled in one volume national geodetic survey 1973 and more recent work with complete bibliographies is summarized in three successive quadrennial reports to the international union of geodesy and geophysics meade 1971 savage 1975 thatcher 1979b the following conventions are employed in this paper strain a dimensionless quantity is reported in units of 10 6 extension is taken as positive to distinguish between engineering and tensor shear strain we denote the former by y and quote the units as j lrad whereas the latter is denoted by e and given the units of j lstrain uncertainties in all cases are quoted as one standard deviation at the present time geodetic techniques furnish the only reliable measure of strain accumulation in the seismic regions of the world the superiority of geodetic techniques derives principally from the long baseline of the measurements difficulties in coupling fiducial marks to the earth have generally made short baseline measurements of strain accumulation questionable wyatt 1982 however long baselines introduce the dis advantage that the measured strain is not closely localized the current technique of measuring strain accumulation involves the

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

ground motion and tectonics in the terceira island tectonomagmatic interactions in an oceanic rift terceira rift azores triple junction abstract the interpretation of high resolution topography bathymetry gps and insar data and detailed structural geology indicate that 1 terceira developed at the intersection of two major volcano tectonic lineaments wnw ese local tr s direction and nnw sse submarine chain of volcanoes here firstly recognised and coined terceira seamount chain 2 terceira is affected by four main fault systems the ca n165 normal faults dipping to east and west mostly across the middle of the island the n140 normal faults mostly making up the lajes graben the n110 faults with oblique striations normal dextral making up the main volcanic lineament and the more subtle n70 the transform direction related to the nubia eurasia plate boundary seismicity gps data and faults displacing the topography indicate that all systems are active 3 the whole island is subsiding at a rate of ca 5 mm yr as attested by both gps and insar data which is exceptionally high for the azores islands common explanations like thermal contraction or bending of the lithosphere or magmatic processes or collapse of the island under its own weight likely cannot justify the observed subsidence rate the estimated average of tr s subsidence rate is also not enough therefore we conclude that the measured 5 mm yr can be a peak 4 the fault geometry and kinematics are consistent with the current direction of maximum extension in the azores ca n65 and the rotation of nubia relative to eurasia 5 given that the ne shoulder of the lajes graben is moving upwards at 5 mm yr and sits directly on the tr s ne shoulder we conclude that the tr s shoulder is moving up most likely as a result of the elastic rebound associated with rifting the elastic rebound in both ne and sw tr s shoulders is most likely responsible for the observed ridge morphology all along the tr

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

an alnoite in the sierras subandinas northern argentina a unique and hitherto unknown intrusion of micaceous ultrabasic rock similar to alnoite in the sierras subandinas zone of northern argentina is described the sheet like intrusion extends over at least 10 km and was emplaced into various sedimentary horizons during the early carboniferous the mineralogy of the rock consists of plates of phlogopite 35 mode that enclose serpentinized olivine 20 and minor ti magnetite 4 and apatite 1 other major minerals are diopsidic clinopyroxene 8 perovskite 10 and calcite 5 accessory andradite garnet is either ti rich red or ti poor orange the rock is mineralogically similar to dike material from the marathon area ontario and referred to as ultrabasic lamprophyre with alnoitic affinity the presence of a tinguaite peralkaline phonolite in the same general area of argentina plus the occurrence of other alkaline rocks to the north in bolivia may indicate the presence of an important alkalic province in this region

composition and emplacement of the benfontein kimberlite sill complex kimberley south africa textural petrographic and melt inclusion constraints abstract the benfontein kimberlite is a renowned example of a sill complex and provides an excellent opportunity to examine the emplacement and evolution of intrusive kimberlite magmas we have undertaken a detailed petrographic and melt inclusion study of the benfontein upper middle and lower sills these sills range in thickness from 0 25 to 5 m new perovskite and baddeleyite u pb dating produced ages of 85 7 4 4 ma and 86 5 2 6 ma respectively which are consistent with previous age determinations and indicate emplacement coeval with other kimberlites of the kimberley cluster the benfontein sills are characterised by large variations in texture e g layering and mineral modal abundance between different sill levels and within individual samples the lower sill is characterised by carbonate rich diapirs which intrude into oxide rich layers from underlying carbonate rich levels the general paucity of xenogenic mantle material in the benfontein sills is attributed to its separation from the host magma during flow differentiation during lateral spreading the low viscosity is likely responsible for non explosive emplacement of the benfontein sills while the rhythmic layering is attributed to multiple magma injections the benfontein sills are marked by the excellent preservation of olivine and groundmass mineralogy which is composed of monticellite spinel perovskite baddeleyite ilmenite apatite calcite dolomite along with secondary serpentine and glagolevite namg6 si3alo10 oh o 8 h2o this is the first time glagolevite is reported in kimberlites groundmass spinel exhibits atoll textures and is composed of a magnesian ulvospinel magnetite mum or chromite core surrounded by occasional pleonaste and a rim of mg al magnetite we suggest that pleonaste crystallised as a magmatic phase but was resorbed back into the residual host melt and or removed by alteration analyses of secondary inclusions in olivine and primary inclusions in monticellite spinel perovskite apatite and interstitial calcite are largely composed of ca mg carbonates and to a lesser extent alkali carbonates and other phases these inclusions probably represent the entrapment of variably differentiated parental kimberlite melts which became progressively more enriched in carbonate alkalis halogens and sulphur during crystal fractionation carbonate rich diapirs from the lower sill contain more exotic phase assemblages e g ba fe titanate barite ancylite pyrochlore which probably result from the extreme differentiation of residual kimberlite melts followed by physical separation and isolation from the parental carbonate rich magma it is likely that any alkali or halogen rich minerals crystallising in the groundmass were removed from the groundmass during syn post magmatic alteration or in the case of na remobilised to form secondary glagolevite the benfontein sill complex therefore provides a unique example of how the composition of kimberlites may be modified after magma emplacement in the upper crust

a mechanical model for deformation and earthquakes on strike slip faults a two dimensional model for stress accumulation and earthquake instability associated with strike slip faults is considered the model consists of an elastic lithosphere overlying a viscous asthenosphere and a fault of finite width with an upper brittle zone having an elastoplastic response and a lower ductile zone having an elastoviscoplastic response for the brittle or seismic zone the behavior of the fault material is assumed to be governed by a relation which involves strain hardening followed by a softening regime with strength increasing with depth for the fault material in the ductile or aseismic section the viscous effect is included through use of a nonlinear creep law and the strength is assumed to decrease with depth hence because of the lesser strength and the viscous effect continuous flow occurs at great depths causing stress accumulation at the upper portion of the fault and leading to failure at the bottom of the brittle zone the failure is initially due to localized strain softening but with further flow the material above the softened zone reaches its maximum strength and begins to soften this process accelerates and may result in an unstable upward rupture propagation

the mineral chemistry of new titanates from the jagersfontein kimberlite south africa implications for metasomatism in the upper mantle new members of the crichtonite mineral series are described in which k ba ca and ree are in significant concentrations 5 wt oxides filling the a formula position in am21o38 these phases are chromium 16 wt cr2o3 titanates 58 wt tio2 enriched in zro2 5 wt and constitute a mineral repository for refractory and large ion lithophile elements in the upper mantle the mineral senes coexists with mg cr ilmenite nb cr rutile and ca cr nbzr armalcolite that have equally unusual chemistries kimberlitic crichtonites are depleted in the intermediate lanthanides but highly enriched in lree and hree with chondrite normalized abundances of 103 to 105 crichtonite armalcolite and nb cr rutile occupy a compositional range in tio2 contents bridging the gap between ilmenite and rutile two minerals having a widespread distribution in kimberlites and mantle derived nodule suites in common with other associations and based on similarities in mineral chemistry it is concluded that these minerals formed at p 20 30 kb 900 1100 c by reaction of peridotite with metasomatizing fluids kimberlitic crichtonite may be expressed as spinel cr ferropseudobrookite and armalcolite is equivalent to cr geikielite rutile in the system femg tio2 cr2o3 this system contains a number of cr ti compounds not found as minerals but it is proposed that the ubiquitous occurrence of ilmenite intergrowths in kimberlitic rutile results from decomposition of high pressure pbo2 type crystallographic shear structures the new minerals have exotic chemistries and the high k affinities broaden the scope for the origin of alkalic rocks the generation of highly potassic magmas in the upper mantle and suggest that alkali metasomatism may be pervasive

volcano spacing and plate rigidity in plane stresses which accompany the flexural deformation of the lithosphere under the load adjacent volcanoes may govern the spacing of volcanoes in hotspot provinces specifically compressive stresses in the vicinity of a volcano prevent new upwelling in this area forcing a new volcano to develop at a minimum distance that is equal to the distance in which the radial stresses change from compressional to tensile the inflection point if a volcano is modeled as a point load on a thin elastic plate then the distance to the inflection point is proportional to the thickness of the plate to the power of 3 4 compilation of volcano spacing in seven volcanic groups in east africa and seven volcanic groups of oceanic hotspots shows significant correlation with the elastic thickness of the plate and matches the calculated distance to the inflection point in contrast volcano spacing in island arcs and over subduction zones is fairly uniform and is much larger than predicted by the distance to the inflection point reflecting differences in the geometry of the source and the upwelling areas

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

ti phlogopites of the shaw s cove minette a comparison with micas of other lamprophyres potassic rocks kimberlites and mantle xenoliths the two generations of micas in the shaw s cove minette mica potash feldspar lamprophyre of northern new brunswick canada are tio2 rich phlogopites with variable but low silica and potash contents low soda and efectively all their aluminum in tetrahedral sites a majority of the micas have an excess of y cations and a deficiency of x cations there are positive correlations between excess y ions and number of al ions in z sites between k2o and sio2 contents and between k2o and volatile free totals yet the micas optically appear fresh do not respond to hydration or glycolation and x ray diffraction patterns show none of the peaks of talc montmorillonites vermiculite or 7 or l4 a chlorites this suggests that some normally octahedrally coordinated cations and water or hydronium ions are located in the interlayer x sites phenocryst micas are sharply zoned cores are paler colored and have high mg atomic ratio mg mg fe 2 0 810 88 cr2o3 i 0 88 wt vo and nio i 0 08 wt but relatively low tio2 2 93 wt and negligible bao phenocryst rims and groundmass micas are darker reddish brown and have lower mg 0 68 0 79 nil crzor and nio but high tio2 x 5 40 wt and bao x 0 38 wt micas in minettes from other localities 62 analyses are frequently richer in silica and potash than the shaw s cove samples but otherwise are generally chemically comparable chemical zoning or bimodalism is nearly universal in minette micas though the boundary between high zg low tioz phenocryst and low mghigh tioz groundmass groups varies from province to province and even from minette to minette within a swarm comparison of analyses of minette micas with 353 analyses of micas from other mafic largely potassic rocks indicates that micas chemically indistinguishable from those of minettes can be found within some other lamprophyres ultra potassic rocks kimberlites and high pressure xenoliths in alkaline volcanic rocks and kimberlites though each ofthese groups also contains micas unlike those in minettes those kimberlite phlogopites that match minette micas are predominantly secondary rims of phenocrysts or megacrysts or groundmass crystals mainly type ii but some are unzoned pre fluidization phenocrysts while primary primary metasomatic and uaruo suite phlogopites of mantle xenoliths are unlike minette micas higher mg and sioz lower tio2 and alzor some of the secondary metasomatic phlogopites in sheared garnetiferous mantle xenoliths are wholly minettelike the chemical identity of minette mica phenocryst cores and some phenocrysts of diverse mantle derived rocks implies crystallization under similar conditions by analogy with the chemistry of phlogopites produced experimentally in potassic systems at high pressures and studies of equilibration conditions of natural phlogopites it is concluded that phenocrystic phlogopites in minettes may form at temperatures and pressures up to at least 1250 c and 40 kbar and probably under o conditions between the nno and hm buffers higher o than for primary primary metasomatic and uanto suite micas in kimberlites and their included xenoliths

a morphometric analysis of the submarine volcanic ridge south east of pico island azores a region of crustal extension the azores plateau contains excellent examples of submarine volcanic edifices constructed over a wide range of ocean depths along the pico ridge using bathymetric data and towed ocean bottom instrument tobi side scan sonar imagery we measured the dimensions diameter height slopes shape and texture of these volcanic edifices to further understanding of the geometric development of a submarine ridge our analysis and interpretation of the measurement and texture data suggest the following 1 the various edifice types do not correlate with depth ranges suggesting that ambient water pressure is not a controlling factor in configuration of the edifice formed 2 the cones have a mean diameter of 948 m height of 152 m and slope of 15 6 and are peaked rather than displaying flat topped summits and 3 while hummocky textured cones also occur smooth textured cones predominate the cones seem to develop preferentially outwards then upwards with only a weak correlation between diameter and height suggesting that the cone population does not evolve self similarly although smooth and hummocky cone populations are not statistically different in mean slope angle and eruption depth the hummocky cones have a significantly greater mean diameter than the smooth cones we suggest that hummocky textured cones probably involving eruption of pillow lavas are formed after voluminous smooth textured flows nearest neighbour analysis suggests that submarine cones are distributed randomly whereas subaerial cones are not we interpret this finding to suggest subaerial cones being masked by over covering flows which tend to flow further than submarine lava flows furthermore the spatial distribution of cones away from the ridge centre is not easily explained by a magma supply fed via lava tubes from central eruptions because of a lack of consistent pathway down gradient fissure vents seem to be important in determining construction of the ridge and result in linear arrangements of edifices and cone elongation consistent with the regional tectonic trend of this part of the azores plateau cones form as each feeding dyke intrusion cools and the eruption becomes localized along point source vents along the fissure the ridge seems to be predominantly formed from fissure eruptions along the ridge axis with subordinate transport of lava down its flanks

40ar 39ar ages and paleomagnetism of sao miguel lavas azores we present new 40 ar 39 ar ages and paleomagnetic data for sao miguel island azores paleomagnetic samples were obtained for 34 flows and one dike successful mean paleomagnetic directions were obtained for 28 of these 35 sites 40 ar 39 ar age determinations on 12 flows from the nordeste complex were attempted successfully ages obtained are between 0 78 ma and 0 88 ma in contrast to published k ar ages of 1 ma to 4 ma our radiometric ages are consistent with the reverse polarity paleomagnetic field directions and indicate that the entire exposed part of the nordeste complex is of a late matuyama age the duration of volcanism across sao miguel is significantly less than previously believed which has important implications for regional melt generation processes and temporal sampling of the geomagnetic field observed stable isotope and trace element trends across the island can be explained at least in part by communication between different magma source regions at depth the 40 ar 39 ar ages indicate that our normal polarity paleomagnetic data sample at least 0 1 myr 0 0 1 ma and up to 0 78 myr 0 0 78 ma of paleosecular variation and our reverse polarity data sample approximately 0 1 myr 0 78 0 88 ma of paleosecular variation our results demonstrate that precise radiometric dating of numerous flows sampled is essential to accurate inferences of long term geomagnetic field behavior negative inclination anomalies are observed for both the normal and reverse polarity time averaged field within the data uncertainties normal and reverse polarity field directions are antipodal but the reverse polarity field shows a significant deviation from a geocentric axial dipole direction 1998 elsevier science b v all rights reserved

constitutive behavior and stability of frictional sliding of granite an understanding of the frictional sliding on faults that can lead to earthquakes requires a knowledge of both constitutive behavior of the sliding surfaces and its mechanical interaction with the loading system we have determined the constitutive parameters for frictional sliding of initially bare surfaces of westerly granite using a recently developed high pressure rotary shear apparatus that allows long distances of sliding and therefore a greater assurance of attaining steady state behavior from experiments conducted at room temperature and normal stresses of 27 84 mpa several important results have been found 1 a gouge layer 100 to 200 m thick was developed from the initially bare rock surfaces after 18 to 70 mm of sliding 2 the steady state frictional resistance attained after about 10 mm of sliding is proportional to the negative of the logarithm of the sliding velocity 3 abrup changes in the velocity of sliding result in initial changes in the frictional resistance which have the same sign as the velocity change and are followed by a gradual decay to a new steady state value over a characteristic distance of sliding this velocity weakening behavior is essentially identical with that found by several previous workers on the same material at lower normal stress 4 our results are well described by a two state variable constitutive law the values of the constitutive parameters are quite similar to those found previously at low normal stress but the characteristic distance is about an order of magnitude smaller than that found at 10 mpa normal stress with thicker layers of coarser gouge 5 we have approximated our results with a one state variable constitutive law and compared the results with the predictions of existing nonlinear stability analysis in addition we have extended the stability analysis to systems possessing two state variables with such formulations good agreement is found between the experimentally observed and theoretically predicted transitions between stable and unstable sliding these results allow a better understanding of the instabilities that lead to earthquakes

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

partition coefficients for ree between garnets and liquids implications of non henry s law behaviour for models of basalt origin and evolution partition coefficients for the rare earth elements ree ce sm and tm between coexisting garnets and hydrous liquids have been determined at high pressure and temperatures 30 kbar and 1300 and 1500 c two synthetic systems were studied mg3al2si3o12 h2o and ca3al2si3o12 h2o in addition to a natural pyrope bearing system deviations from henry s law behaviour occur at geologically relevant ree concentrations at concentrations 3 ppm ce 12 ppm sm 80 ppm tm in pyrope and 100 ppm ce 250 ppm sm 1000 ppm tm in grossular at 30 kbar and 1300 c dgarnet liquidree increases as the ree concentration in the garnet decreases at higher concentrations dree is constant dgrossular liquidree also constant when the garnet contains less than about 2 ppm sm or tm the ree concentration at which dree becomes constant increases with increasing temperature decreasing ree ionic radius and increasing ca content of the garnet partitioning behaviour of ce sm and tm between a natural pyrope rich garnet and hydrous liquid is analogous to that in the synthetic systems and substantiates the substitution model proposed by harrison and wood 1980 values of dreegarnet liquid for which henry s law is obeyed are systematically higher for grossular than for pyrope dpyrope liquid 0 067 ce 0 108 sm 0 155 tm and dgrossular liquid 0 65 ce 0 75 sm 4 55 tm the implications of non henry s law partitioning of ree for models of basalt petrogenesis involving garnet are far ranging deviations from henry s law permit refinements to be made to calculated ree abundances once basic model parameters have been defined

models of crustal deformation this review is principally concerned with theoretical models of crustal deformation developed during this quadrennium observational data as well as topics in lithospheric flexure and isostasy are covered in companion reviews in this volume progress has been made toward constructing both new inelastic and inhomogeneous elastic models of earthquake induced crustal deformation most of the inelastic models involve dislocations or green s functions in an elastic or anelastic lithosphere which overlies a maxwell viscoelastic asthenosphere

volcanic geology of furnas volcano sao miguel azores abstract furnas is the easternmost of the three active central volcanoes on the island of sao miguel in the azores unlike the other two central volcanoes sete cidades and fogo furnas does not have a well developed edifice but consists of a steep sided caldera complex 8 5 km across it is built on the outer flanks of the povoacao nordeste lava complex that forms the eastern end of sao miguel constructive flanks to the volcano exist on the southern side where they form the coastal cliffs and to the west the caldera margins tend to reflect the regional local tectonic pattern which has also controlled the distribution of vents within the caldera and areas of thermal springs activity at furnas has been essentially explosive erupting materials of trachytic composition products associated with the volcano include plinian and sub plinian pumice deposits ignimbrites and surge deposits phreatomagmatic ashes block and ash deposits and dome materials most of the activity has occurred from vents within the caldera or on the caldera margin although strombolian eruptions with aa flows of ankaramite and hawaiite have occurred outside the caldera the eruptive history consists of at least two major caldera collapses followed by caldera infilling based on 14 c dates it appears that the youngest major collapse occurred about 12 000 10 000 years bp new 14 c dates for a densely welded ignimbrite suggest that a potential caldera forming eruption occurred at about 30 000 years bp recent eruptions

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

constitutive relations for fault slip and earthquake instabilities constitutive relations for fault slip are described and adopted as a basis for analyzing slip motion and its instability in the form of earthquakes on crustal faults the constitutive relations discussed include simple rate independent slip weakening models in which shear strength degrades with ongoing slip to a residual frictional strength and also more realistic but as yet less extensively applied slip rate and surface state dependent relations for the latter the state of the surface is characterized by one or more variables that evolve with ongoing slip seeking values consistent with the current slip rate models of crustal faults range from simple single degree of freedom spring slider systems to more complex continuous systems that incorporate nonuniform slip and locked patches on faults of depth dependent constitutive properties within elastic lithospheric plates that may be coupled to a viscoelastic asthenosphere

distributed damage faulting and friction we present a formulation for mechanical modeling of geological processes in the seismogenic crust using damage rheology the seismogenic layer is treated as an elastic medium where distributed damage modifying the elastic stiffness evolves as a function of the deformation history the model damage rheology is based on thermodynamic principles and fundamental observations of rock deformation the theoretical analysis leads to a kinetic equation for damage evolution having two principal coefficients the first is a criterion for the transition between strength degradation and recovering healing and is related to friction the second is a rate coefficient of damage evolution which can have different values or functional forms for positive degradation and negative healing evolution we constrain these coefficients by fitting model predictions to laboratory data including coefficient of friction in sawcut setting intact strength in fracture experiments first yielding in faulting experiments under three dimensional strain onset and evolution of acoustic emission and dynamic instability the model damage rheology accounts for many realistic features of three dimensional deformation fields associated with an earthquake cycle these include aseismic deformation gradual strength degradation development of process zones and branching faults around high damage areas strain localization brittle failure and state dependent friction some properties of the model damage rheology e g cyclic stick slip behavior with possible accompanying creep are illustrated with simplified analytical results the developments of the paper provide an internally consistent framework for simulating long histories of crustal deformation and studying the coupled evolution of regional earthquakes and faults this is done in a follow up work

rare earth element geochemistry of garnet lherzolite and megacrystalline nodules from minette of the colorado plateau province abstract mineral and whole rock ree abundances in garnet lherzolite and megacrystalline nodules from the thumb display broad correlations with major element compositions lherzolites with 12 modal clinopyroxene plus garnet high ca al lherzolites have relatively flat chondrite normalized whole rock ree patterns lherzolites poor in clinopyroxene and garnet low ca al lherzolites have lower hree in clinopyroxenes and garnets and higher whole rock lree hree it is concluded that the low ca al lherzolites may have undergone lree metasomatism after depletion of the major element compositions by partial melting and that much of the garnet now present was originally dissolved in aluminous orthopyroxene the high ca al lherzolites may be interpreted either as primordial mantle samples or as products of equilibration with very lree enriched liquids the megacrystalline nodules are medium to ultracoarse grained intergrowths and megacrysts with mineral compositions similar to discrete nodule suites in kimberlites the ree abundances of the megacrystalline minerals are consistent with an origin as cumulates from magma with extremely fractionated ree similar to minette or kimberlite the patterns of correlation of ree and major elements in this inclusion suite are similar to the patterns observed in the garnet lherzolite and discrete nodule suites of southern african kimberlites both of the subcontinental mantle provinces represented by these suites contain three distinct petrogenetic components refractory garnet lherzolite enriched in lree and depleted in hree fertile garnet lherzolite with generally chondritic ree abundances and a suite of ultracoarse minerals precipitated from magma with extremely fractionated ree generally similar to the host magmas

long term average mineral weathering rates from watershed geochemical mass balance methods using mineral modal abundances to solve more equations in more unknowns abstract the number of phases for which weathering rates can be determined by watershed geochemical mass balance is limited by the number of equations that can be constructed from elemental flux losses from the watershed and mineral stoichiometries mass balance studies of watershed weathering rates routinely use the flux losses of the six major cations sio 2 al na k mg and ca analyses of these species in water are common but following matrix algebraic methods limits the number of weathering rates that can be calculated to six for the brubaker run watershed located in the northern piedmont physiographic province of pennsylvania usa long term 10 3 10 6 year watershed chemical flux losses have been determined using 10 be derived total denudation rates and zirconium normalized total chemical concentrations from bedrock and soils chemical flux losses calculated from solid phase data have three advantages they 1 permit generation of a relatively large number of equations because both major and trace analyses are included 2 eliminate the need for many years of regular e g weekly sampling and chemical analyses of stream water and atmospheric precipitation and measurement of hydrologic parameters i e precipitation stream discharge etc and 3 long term weathering rate calculations need not address biomass for brubaker run eight minerals are involved in weathering the five primary minerals are ree rich epidote ankerite almandine spessartine garnet muscovite chlorite and the three secondary products are weathered muscovite kaolinite and gibbsite the long term average weathering rates of these minerals were calculated using the six major cations and two trace elements selected from rb sr ba la pr nd sm gd and dy despite having the eight equations needed geochemically reasonable weathering rates e g positive primary mineral rates that reflect destruction could not be achieved regardless of the two trace elements used in the mass balance calculations for brubaker run this is primarily attributable to the natural heterogeneity of the trace element concentrations within the host mineral grains with trace element stoichiometries in some minerals varying by as much as an order of magnitude because the trace elements are hosted by a relatively small number of minerals the computed weathering rates of other minerals become very sensitive to small variations in trace cation stoichiometry ree rich epidote garnet and ankerite within the brubaker run watershed together host nearly all of the ca in the bedrock and completely dissolve at or near the weathering front consequently approximately all of the ca in bedrock is lost from the regolith in bedrock the mole percentages of ca hosted by ree rich epidote garnet and ankerite are 49 mol 4 mol and 43 mol respectively and are determined by the modal abundance of the mineral in the bedrock and its ca stoichiometry the weathering rates of ree rich epidote garnet and ankerite can be determined by distributing to each mineral that fraction of the total watershed ca flux loss for which it is responsible based on its mole percent ca in bedrock by using a base cation that is completely lost from the regolith and knowing the mole percentage of that element in the mineral s undergoing weathering additional equations may be added to the mass balance matrix we term this technique the flux distribution method the flux distribution method eliminates the need for additional equations established using trace elements based on the mineral weathering rates for the brubaker run watershed determined using the flux distribution method the rates at which the weathering front penetrated the bedrock the saprolitization rate are 4 5 m myr 1 and 6 5 m myr 1 for chlorite and muscovite respectively these measured long term average saprolitization rates compare very favorably with published theoretical values for the nearby northern maryland piedmont which range from 2 2 to 5 3 m myr 1

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

stiffness method for anticipating earthquakes in strain softening earthquake models the single variable r kf ks measures the nearness to earthquake instability kf and ks are stiffnesses of the fault zone and elastic surroundings instability occurs when increasing r reaches unity for a simple two dimensional antiplane model r is a function of d e du and h where e is shear strain at the ground surface and u is remote displacement observed at a distance h from the fault r is independent of the fault zone constitutive law crustal rigidity and shear stress magnitude

serreta 1998 2001 submarine volcanic eruption offshore terceira azores characterization of the vent and inferences about the eruptive dynamics abstract high resolution bathymetric data and seafloor sampling were used to characterize the most recent volcanic eruption in the azores region the 1998 2001 serreta submarine eruption the vent of the eruption is proposed to be an asymmetric topographic high composed of two coalescing volcanic cones underlying the location where lava balloons had been observed at the sea surface during the eruption the volcanic products related to the 1998 2001 eruption are constrained to an area of 0 5 km2 around the proposed vent position a submarine strombolian style eruption producing basaltic lava balloons ash and coarse scoriaceous materials with limited lateral dispersion led to the buildup of the cones the 1998 2001 serreta eruption shares many similarities with other intermediate depth lava balloon forming eruptions e g the 1891 eruption offshore pantelleria and the 2011 2012 eruption south of el hierro revealing the particular conditions needed for the production of this unusual and scarcely documented volcanic product

stratification of the earth beneath the azores from p and s receiver functions abstract seismic recordings of iris ida gsn station cmla and of several temporary stations in the azores archipelago are processed with p and s receiver function prf and srf techniques contrary to regional seismic tomography these methods provide estimates of the absolute velocities and of the vp vs ratio up to a depth of 300 km joint inversion of prfs and srfs for a few data sets consistently reveals a division of the subsurface medium into four zones with a distinctly different vp vs ratio the crust 20 km thick with a ratio of 1 9 in the lower crust the high vs mantle lid with a strongly reduced vp vs velocity ratio relative to the standard 1 8 the low velocity zone lvz with a velocity ratio of 2 0 and the underlying upper mantle layer with a standard velocity ratio our estimates of crustal thickness greatly exceed previous estimates 10 km the base of the high vs lid the gutenberg discontinuity is at a depth of 80 km the lvz with a reduction of s velocity of 15 relative to the standard iasp91 model is terminated at a depth of 200 km the average thickness of the mantle transition zone tz is evaluated from the time difference between the s410p and sks660p seismic phases that are robustly detected in the s and sks receiver functions this thickness is practically similar to the standard iasp91 value of 250 km and is characteristic of a large region of the north atlantic outside the azores plateau our data are indicative of a reduction of the s wave velocity of several percent relative to the standard velocity in a depth interval from 460 to 500 km this reduction is found in the nearest vicinities of the azores in the region sampled by the prfs but as evidenced by srfs it is missing at a distance of a few hundred kilometers from the islands we speculate that this anomaly may correspond to the source of a plume which generated the azores hotspot previously a low s velocity in this depth range was found with srf techniques beneath a few other hotspots

nonlinear strain buildup and the earthquake cycle on the san andreas fault two contrasting models of the earthquake deformation cycle on strike slip faults predict significant temporal declines in shear strain rate near the fault accompanied by a progressive broadening of the zone of deformation adjacent to it in the thin lithosphere model transient deformation results from flow in the asthenosphere due to stress relaxation following faulting through most or all of the lithosphere for an earth model with a thick elastic lithosphere plate thickness depth of seismic slip transient motions are due to postearthquake aseisrnic slip below the coseismic fault plane data from the san andreas fault indicate a long term temporal decrease in strain rate that persists for at least 30 years and may extend through the entire earthquake cycle observations support a cycle long rate decrease and a temporal spreading of the deformation profile only if movement cycles on the northern and southern locked sections of the fault are basically similar if so the usually lower strain rates and broader deformation zone currently observed on the southern san andreas represent a later evolutionary stage of the northern locked section where a great earthquake is a more recent occurrence although the data allow some extreme models to be discarded no sufficiently strong constraints exist to decide between the thin and thick lithosphere models regardless of the appropriate model the geodetic observations themselves indicate that strain buildup is sufficiently nonlinear to cause significant departures from recurrence estimates based on linear strain accumulation and the time predictable model

patterns of stress at midocean ridges and their offsets due to seafloor subsidence the effect of the seafloor subsidence on the horizontal stress field is investigated by combining the finite element method with a formulation that allows us to compute the two dimensional 2d horizontal stresses arising from isostatically compensated vertical loads the topographic load created by the elevation of midocean ridges relative to old ocean floor is shown to be a significant source of ridge parallel tensile stresses these may predominate over the ridge perpendicular stresses and explain observations at midocean ridge offsets such as 1 oblique normal faulting at ridge transform intersections trending up to 60 relative to the ridge axis and 2 nontransform offsets consisting of structures oriented at 45 relative to the ridge trend at midocean ridge overlaps rotation of the ridge parallel tensile stresses favours rift propagation at more than 45 relative to the ridge trend it is suggested that propagating rift tips that bend abruptly lead to partially unlocked offsets and as a result large overlaps may eventually start to rotate and evolve into a microplate

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

the picton and varty lake ultramafic dikes jurassic magmatism in the st lawrence platform near belleville ontario the picton and varty lake ultramafic dikes that intrude ordovician limestone near belleville ontario consist of olivine or serpentinized pseudomorphs and ti phlogopite phenocrysts set in a groundmass of spinel perovskite serpentine calcite apatite and chlorite phlogopite exhibits a complex chemical variation and late stage ba enrichment up to 6 8 wt bao spinels are chemically zoned and commonly exhibit an atoll texture in which a magnesian aluminous chromite core is separated by a silicate zone from late stage magnesian aluminous titanomagnetite chemically similar to kimberlitic spinel atoll spinels from the picton dike also have a zone of titanian pleonaste surrounding a central grain of magnesian aluminous chromite at the centre of the atoll structures both dikes are extremely silica undersaturated with high k na and mg mg fe2 ratios petrographic and chemical features of the picton and varty lake dikes preclude simple classification as both dikes have aspects indicative of kimberl

three dimensional passive flow and temperature structure beneath oceanic ridge ridge ridge triple junctions abstract this study uses numerical modeling to investigate generalized characteristics of mantle flow and thermal structure in the vicinity of ridge ridge ridge rrr triple junctions oceanic triple junctions present a unique opportunity to study three dimensional 3d mantle dynamics in a tectonic setting considerably different than where only two plates diverge in many prominent oceanic triple junctions including rodrigues azores and galapagos the slowest spreading ridge branch intersects the near collinear faster spreading branches quasi orthogonally this study focuses on triple junctions free of influence from nearby hotspots similar to the geometry of the rodrigues triple junction a finite element model was used to calculate the steady state 3d velocity flow field and temperature patterns resulting from advective and conductive heat transfer for the slowest spreading branch model results predict a strong component of along axis velocity directed away from the triple junction both upwelling velocity and temperature are calculated to increase along the slowest spreading ridge toward the triple junction approaching the upwelling rate and temperature of the fastest spreading branch within 200 km of the triple junction upwelling velocity is predicted to increase more than threefold along the slowest spreading ridge in contrast the calculated upwelling velocity and temperature for the fastest spreading branch are not significantly different from the case of a single spreading ridge for triple junctions where the three ridges spread faster than the rodrigues triple junction system such as the galapagos triple junction the contrast in axial upwelling velocity and temperature between the slowest and fastest spreading ridges is predicted to be less significant however for triple junctions with overall slower spreading velocities such as the azores triple junction this contrast is more pronounced

catastrophic flank collapses and slumping in pico island during the last 130 kyr pico faial ridge azores triple junction abstract the pico island constitutes the easternmost sub aerial domain of a steep wnw ese volcanic ridge which has developed within the nubia eurasia diffuse plate boundary azores triple junction the island comprises three volcanic systems from older to younger the topo volcano the fissural system and the pico stratovolcano from a high resolution digital elevation model 10 m and new bathymetric stratigraphic structural and high precision k ar data we reconstruct the main successive stages of growth and partial destruction of the island over the last 200 kyr we especially concentrate on the central sector of the island which has recorded gradual movements through slumping and catastrophic flank collapses since ca 130 kyr the remmants of the topo volcano are partly exposed on pico s se flank and are here dated between 186 5 and 115 4 ka topo was significantly destroyed by n and s directed large scale flank collapses between ca 125 and 70 ka on pico s n flank collapse seems to have removed all the unstable material but in the s the collapse structure is composite including a major flank collapse and a remnant slump complex that is still active a first episode of deformation occurred between ca 125 and 115 ka along the master fault of the slump between ca 115 and 69 ka most of the unstable material was removed by a major flank collapse leaving behind a still considerable volume of unstable material that comprises the active slump this first collapse was catastrophic and generated a large debris deposit recognized on the high resolution bathymetry with a minimum run out of ca 17 km the scar was partially filled by volcanic products erupted from volcanic cones developed within the slump depression and possibly also from the early wnw ese fissural system subsequent deformation in the slump area affected in part the filling units leading to the individualization of secondary curved faults younger volcanic products have gradually masked the mass wasting scars unlike the well known hilina slump hawaii pico s slump evolution might be controlled by an active regional tectonics

submarine sedimentation processes in the southeastern terceira rift sao miguel region azores abstract most case studies dealing with the evolution of volcanic islands concentrate on the onshore domain in contrast the interplay of constructional and destructive processes in the offshore domain is often paid less attention although they commonly represent most part of the edifice combining bathymetric and multi channel seismic data this study focuses on the impact of volcanism erosion tectonics time variant sediment supply and climatic oceanographic conditions on the submarine sedimentation processes in the southeastern terceira rift azores plateau where the major onshore volcanic systems at sao miguel island the largest volcanic edifice in the southeastern terceira rift evolved between the last and the previous glacio eustatic sea level minimum an average depth value of 142 m of the corresponding shelf segments indicates a formation during the last glacial maximum and a long term subsidence of approximately 0 6 mm a below the shelf edge the western flank is characterized by submarine volcanism while the northern slope is mostly affected by strong turbidity events favored by the north coast s exposure to north atlantic storm swell these turbidity currents pass the shelf break by retrogressive canyons gullies and follow a widespread system of channels towards the deep sea where gullies are not connected to channel systems or where turbidity currents overspill the channel walls stacked lobe deposits are formed at the southern slope a more constant downslope sediment transport leads to the formation of cyclic steps chutes and pools and further distal turbidity lobes turbidity currents follow morphological features and enter the basins via structural bottlenecks in the basins bottom currents are controlled deviated by volcanic highs or fault scarps where they cause the deposition of drift bodies high local accumulations of sediments caused by a channelized sediment flux bottom currents or onshore volcanism were partly remobilized resulting in slumping sliding and a volcanic ridge partly collapsed due to tectonic stress and or gravity spreading

volcano tectonic evolution of a linear volcanic ridge pico faial ridge azores triple junction assessed by paleomagnetic studies abstract the morphology of volcanic oceanic islands results from the interplay between constructive and destructive processes and tectonics in this study the analysis of the paleomagnetic directions obtained on well dated volcanic rocks is used as a tool to assess tilting related to tectonics and large scale volcano instability along the pico faial linear volcanic ridge azores triple junction central north atlantic for this purpose 530 specimens from 46 lava flows and one dyke from pico and faial islands were submitted to thermal and alternating magnetic fields demagnetizations detailed rock magnetic analyses including thermomagnetic analyses and classical high magnetic field experiments revealed titanomagnetites with different ti content as the primary magnetic carrier capable of recording stable remanent magnetizations in both islands the paleomagnetic analysis yields a characteristic remanent magnetization which presents island mean direction with normal and reversed polarities in agreement with the islands location and the age of the studied lava flows indicating a primary thermo remanent magnetization field observations and paleomagnetic data show that lava flows were emplaced on pre existing slopes and were later affected by significant tilting in faial island magmatic inflation and normal faults making up an island scale graben can be responsible for the tilting in pico island inflation related to magma intrusion during flow emplacement can be at the origin of the inferred tilting whereas gradual downward movement of the se flank by slumping processes appears mostly translational

control on seafloor spreading geometries by stress and strain induced lithospheric weakening abstract seafloor spreading typically occurs along ridge segments oriented at right angles to plate motion and offset by orthogonal transform faults few regions exhibit different patterns such as the east pacific rise epr which additionally displays overlapping spreading centres oscs and microplates we introduce a dynamical model using two independent scalar types of damage in an elastic plate that generates most observed spreading geometries as natural failure modes suggesting that the dynamics of the underlying mantle have only a minor influence on accretionary plate margins the elastic modulus that is affected by the damage determines the type of localized deformation damage reducing the bulk modulus tends to result in tensile fractures while a reduction in shear modulus leads to shear fractures the damage source determines the fracture orientation material weakening in tension results in fractures perpendicular to the most tensile principal stress while shear weakening results in two conjugate fractures at 45 relative to the applied stress strain or energy dependent damage results in propagating localized fractures stress dependent damage tends to evolve into diffuse regions that may eventually focus into narrow zones starting from small perturbations with reduced elastic moduli as nucleation points all ridge geometries start with ridge propagation caused by tensile energy reducing both elastic moduli by a model of damage caused by tensile energy reducing both moduli orthogonal transform faults develop in regions between offset segments if there is an additional reduction in shear modulus due to shear stress the orthogonality of the transform faults does not derive from the local stress orientation but from the dynamics of damage focusing which causes the fault to converge towards an optimal geometry that concentrates nearly all deformation into damaged zones oscs form when the shear damage leading to transform faults is suppressed while microplate formation requires additional reduction of the shear modulus by tensile energy oblique spreading at 45 recently discovered along ultraslow spreading ridges occurs when both moduli are weakened by shear energy a parameter regime exists in which ridge transform patterns develop at low applied tension while microplates form at higher stresses these results indicate that at least three different micromechanical processes operate with different evolution rates oscs and oblique spreading require different material properties

preseismic rupture progression and great earthquake instabilities at plate boundaries we present a procedure for modeling the initially quasi static upward progression of a zone of slip from some depth in the lithosphere toward the earth s surface along a transform plate margin culminating in a great crustal earthquake stress transmission in the lithosphere is analyzed with a generalized elsasser model in which elastic lithospheric plates undergo plane stress deformation and are coupled by an elementary foundation model to a maxwellian viscoelastic asthenosphere upward progression of rupture over a finite length of plate boundary corresponding to a seismic gap along strike is analyzed by a method based on the line spring concept whereby a two dimensional antiplane analysis of the upward progression provides the relation between lithospheric thickness averaged stress and slip used as a boundary condition in the generalized elsasser plate model the formulation results in a nonlinear integral equation for the rupture progression as a function of time and distance along strike a simpler approximate single degree of freedom analysis procedure is described and shown to lead to instability results that can be formulated in terms of the slip softening slope at the boundary falling below the elastic unloading stiffness of the surroundings the results also indicate a delay of ultimate seismic instability due to the stiffer short versus long time asthenospheric response and predict a final period of self driven creep toward instability the procedures for prediction of rupture progression and instability are illustrated in detail for an elastic brittle crack model of slip zone advance and parameters of the model are chosen consistently with great earthquake slips and stress drops for example an effective crack fracture energy of the order 4 106 j m2 at the peak 7 to 10 km below surface of a gaussian bell shaped distribution of fracture energy with depth with variance of the order 5 km simulating strength build up in a seismogenic layer leads to prediction of nominal seismic stress drops of 30 to 60 bars and slips of 2 to 5 m in great strike slip earthquake ruptures breaking 100 to 400 km along strike precursory surface straining in the self driven stage is predicted to proceed at a distinctly higher rate over time intervals beginning 3 to 10 months before such an earthquake this interval being greater for longer distances along strike over which the preseismic upward rupture progression takes place

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

slip along the san andreas fault associated with the great 1857 earthquake historical records indicate that several meters of lateral slip along the san andreas fault accompanied the great 1857 earthquake in central and southern california these records together with dendrochronological evidence suggest that the rupture occurred along 360 to 400 km of the fault including several tens of kilometers of the currently creeping reach in central california geomorphic expressions of late holocene right lateral offsets are abundant along the 1857 rupture along 300 kilometers of the 1857 rupture between cholame and wrightwood the youngest discernible offset ranges from 3 to 9 1 2 meters dormancy of the fault since 1857 almost certainly indicates that this latest offset was created in 1857 fault slip apparently associated with the 1857 earthquake varies in a broadly systematic way along the trace of the fault it is relatively uniform along each of several long segments but changes rather abruptly in value between these segments this nonuniform displacement pattern may imply that some segments of the fault rupture more frequently or experience a slower long term slip rate than others the 1857 offsets indicate a seismic moment m o between 5 3 and 8 7 10 27 dyne cm assuming a 10 to 15 km depth of rupture and relatively uniform slip as a function of depth a comparison with the rupture length average slip value and tectonic setting of the california earthquake of 1906 m s 8 1 4 indicates a value of m 8 1 4 for the 1857 event

paleomagnetism of mesozoic kimberlites in southern africa and the cretaceous apparent polar wander curve for africa including those previously published good paleomagnetic data have been obtained from 48 of 51 sites in 28 separate kimberlites radiometric dates by one or more methods are available for 17 of these bodies and range from 81 to 145 ma with maxima around 90 and 120 ma these ages correlate with geochemical contrasts distinguishing what have been designated group i younger and ii older kimberlites smith et al 1985b stable magnetization appears to be associated with magnetite developed deuterically after emplacement but in most bodies secular variation has not been completely averaged in the remanence acquired the distribution of vgps calculated is distinctly but not sequentially streaked yet correlates broadly with the older and younger petrochemical types mean north poles calculated on this basis yield kp1 n 14 81 100 ma 64 1 n 226 1 e a95 5 2 and kp2 n 10 113 145 ma 47 6 n 269 9 e a95 9 7 with the exception of one undated pipe meltonwold which was not included in the averages and the melilititic male fissure at swartruggens in group ii average all the kimberlites sampled are normally magnetized including these new data 10 of 17 published african cretaceous poles meet acceptability criteria specified and define an apparent polar wander apw curve from 130 to 75 ma the curve appears to consist of two segments trending latitudinally before 100 ma and meridionally thereafter the latter trend is evident even in the distribution of the individual kp1 poles and may be related to the true polar wander speculated by gordon and livermore 1987 to have occurred in that interval

the blue ball arkansas kimberlite mineralogy petrology and geochemistry a study of the mica peridotite in blue ball scott county arkansas reveals that on the basis of mineralogy and whole rock chemistry including the rare earth elements this occurrence is a micaceous kimberlite micas occur as groundmass and phenocryst phases both are compositionally similar and characterized by reversely pleochroic rims and patches on cores with intermediate pleochroism reverse pleochroism is associated with sharp increases in feo total fe and delta t 8 si al and a sharp decrease in al 2 o 3 features consistent with the presence of tetrahedral fe 3 reverse pleochroism in the phlogopite phenocrysts is associated with resorption features mineralogical and compositional similarities between these phlogopites and those of the cumulate glimmerites including the marid nodules allows us to suggest a genetic relationship between them

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

on the recurrence time of great earthquakes on a long transform fault a maxwell viscoelastic model is proposed for the lithosphere in the proximity of a long vertical strike slip fault in this framework formulas for the recurrence time of great earthquakes are derived by imposing boundary conditions of 1 uniform strain rate in the fault region or 2 steady aseismic sliding at depth in both cases a minimum effective viscosity of the fault region is found in order that a critical stress for earthquake occurrence can be reached at higher viscosities the recurrence time approaches a minimum limiting value application of the model to the northern locked segment of the san andreas fault would indicate a recurrence time significantly longer than the often quoted 100 year period these results differ considerably from those obtained from similar models employing boundary conditions of assigned stress at large distance from the fault

submarine explosive volcanism in the southeastern terceira rift sao miguel region azores abstract morphologic studies with sonar data and in situ observations of modern eruptions have revealed some information suggesting how submarine volcanic cones develop but the information only addresses the modern surfaces of these features here we describe a study combining morphological data with high resolution seismic reflection data collected over cones within the southeastern terceira rift a succession of deep basins volcanic bathymetric highs and islands e g sao miguel representing the westernmost part of the eurasian nubian plate boundary the cones 252 are distributed in depths down to 3200 m and exhibit an average diameter of 743 m an average slope of 20 and heights mainly between 50 and 200 m the cones are here classified into three different categories by physiographic or tectonic setting we find no particular morphometric differences in cone shapes between these areas first numerous cones located at the submarine flanks of sao miguel s sete cidades and fogo volcano are considered to be parasitic structures second in the southeast of the island they form a superstructure possibly reflecting an early submarine stadium of a posterior subaerial stratovolcano third some cones are controlled by faults mostly in a graben system southwest of the island high resolution multichannel seismic data indicates that the graben cones evolved synchronously with the graben formation bottom currents then probably removed the surficial fine grain size fraction leaving rough surface textures of the cones which backscatter sonar signals strongly in the data recorded here however a young cone investigated in detail is characterized by a smooth surface a marked increase of internal stratification with increasing distance from the summit and upwards concave flanks others exhibit central craters suggesting an explosive than an effusive evolution of these structures the morphological characteristics of these submarine cones show that they have similar sizes and shapes to cinder cones onshore sao miguel

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

crystallization of groundmass spinel in kimberlite groundmass spinel grains in 46 kimberlite and related rocks have been analyzed and compared the majority of the spinel analyses are classified as high chromium chromite chr and magnesioulvo spinel magnetite mum and represent two significant stages of spinel growth there are also a significant number of spinel grains that are classified as xenocryst spinel xen pleonaste spinel ple and magnetite mag eight different spinel zoning trends are identified the majority of the chr spinel grains are interpreted as a primary phase that crystallized as small octahedra from kimberlite magma on the journey from the upper mantle to the final resting place in the upper crust three zoning trends lead directly away from primary chromite the major zoning trend trend 1 is from chromite to magnesio ulvo spinel magnetite this zoning trend is unique to spinel in kimberlite carbonatites and lamprophyres we suggest that this somewhat oxidizing and more magnesian trend was influenced by the high carbonate content of group i kimberlites and the rapid crystallization of the minerals during the evolution of volatiles the zoningtrend 2 involves increasing titanium and ferric iron as a function of increasing fe 2 fe 2 mg this trend is similar to the zoning of spinel in basalt and is thought to be due to co crystallization of magnesium and aluminum rich silicate minerals such as olivine and phlogopite in kimberlites or pyroxene and plagioclase in basalt zoning trend 3 in kimberlite leads away from primary chromite and towards an aluminous pleonaste ple spinel this trend is characterized by a large decease of cr cr al parallel to so called olivine spinel iso potential lines similar trends of lesser magnitude and cyclic al cr zoning have been identified in basaltic spinel this trend is thought to be due to very rapid crystallization under conditions of supersaturation where the crystallization of spinel affects the local environment ahead of the growing spinel crystal i e diffusion controlled crystallization the tendency for immiscibility between ferrite or titanate rich spinel and aluminate rich spinel pleonaste has a great influence on trends 1 and 3 zoning and also on atoll spinel formation very local conditions such as nucleation or lack of nucleation of other minerals can influence both the textural environment and composition of kimberlitic spinel