island arc and mid ocean ridge volcanism modelled by diapirism from linear source regions abstract the gravitational instability of buoyant regions of partial melt in the upper mantle has been proposed as the underlying cause of regularly spaced volcanism at both island arcs and mid ocean ridges motivated by this suggestion we report a laboratory study of diapirism initiated by the release of buoyant fluid from a linear source into a homogeneous viscous environment fixed volume and fixed flux releases of buoyant fluid are studied a fixed volume release was obtained by rapidly towing a point source of fluid through a body of denser fluid the horizontal cylinder of fluid which was left behind by the source subsequently underwent a gravitational instability yielding plumes with a characteristic spacing expressions for the mean spacing of the plumes and for the growth rate of instability are derived by dimensional arguments and are compared with the experimental results both quantities are found to be independent of the viscosity of the buoyant fluid a surprising result which we have confirmed theoretically in contrast the plume spacing and growth rate produced by the gravitational instability of a layer of buoyant fluid each depend on the cube root of its viscosity in the light of these differing results we conclude that the geometry of the buoyant region of partial melt must be considered when developing gravitational instability models of island arc and mid ocean ridge volcanism we also conclude that if the expressions for a cylindrical region are appropriate then the observed spacing of volcanoes cannot be used to infer the viscosity and hence the melt fraction of the source region a fixed flux release of fluid was achieved by injecting buoyant fluid through a narrow slot in the base of a large tank of denser fluid the interface between the two fluids was found to be unstable and diapirs were formed however the rising diapirs were followed by a continuous uniform sheet of buoyant fluid we suggest therefore that thermal or rheological effects need to be included in fluid mechanical models if volcanism which is periodic in position and time is to be explained

silurian deformation in eastern notre dame bay newfoundland eastern notre dame bay newfoundland is divided into five fault bounded terranes they are from north to south the twillingate terrane the chanceport terrane the new world island terrane the dildo run terrane and the port albert terrane the new world island terrane is characterized by fault repeated sequences of middle ordovician to early silurian turbiditic sandstones sansom formation and conglomerates goldson formation the chanceport terrane has a lower volcanic unit and an upper sedimentary unit consisting of red and green siltstones shales overlain by turdiditic sandstones this sequence is structurally overlain by a mafic and felsic volcanic unit the clastic sedimentary rocks of the chanceport new world island and port albert terranes best record the silurian deformation in the area silurian deformation is divided into two deformation events an early silurian d1 thrusting event and a late silurian d2 dextral ductile faulting event the early silurian joey s cove melange constrains th

relations among subduction parameters clues to the dynamics of the subduction process are found in the many measurable parameters of modern subduction zones based on a critical appraisal of the geophysical and geological literature 26 parameters are estimated for each of 39 modern subduction zones to isolate causal relationships among these parameters multivariate analysis is applied to this data set this analysis yields empirical quantitative relations that predict strain regime and strike slip faulting in the overriding plate maximum earthquake magnitude benioff zone length slab dip arc trench gap and maximum trench depth excellent correlation is found between length of the benioff zone and the product of convergence rate and age of the downgoing slab this relationship is consistent with the conductive heating model of molnar et al 1979 if the model is modified in one respect the rate of heating of the slab is not constant it is substantially slower during passage of the slab beneath the accretionary prism and overriding plate the structural style in the overriding plate is determined by its stress state though the stress state of overriding plates cannot be quantified one can classify each individual subduction zone into one of seven semiquantitative strain classes that form a continuum from strongly extensional class 1 back arc spreading to strongly compressional class 7 active folding and thrusting this analysis indicates that strain class is probably determined by a linear combination of convergence rate slab age and shallow slab dip interplate coupling controlled by convergence rate and slab age is an important control on strain regime and the primary control on earthquake magnitude arc parallel strike slip faulting is a common feature of convergent margins forming a forearc sliver between the strike slip fault and trench optimum conditions for the development of forearc slivers are oblique convergence a compressional environment and a continental overriding plate the primary factor controlling presence of strike slip faulting is coupling strongly oblique convergence is not required the rate of strike slip faulting is affected by both convergence obliquity and convergence rate maximum trench depth is a response to flexure of the underthrusting plate the amount of flexural deflection at the trench depends on the vertical component of slab pull force which is very sensitive to slab age and shallow slab dip shallow slab dip conforms to the cross sectional shape of the overriding plate which is controlled by width of the accretionary prism and duration of subduction deep slab dip is affected by the mantle trajectory established at shallow depth but may be modified by mantle flow much of the structural complexity of convergent margins is probably attributable to terrane juxtaposition associated with temporal changes in both forearc strike slip faulting and strain regime empirical equations relating subduction parameters can provide both a focus for future theoretical studies and a conceptual and kinematic link between plate tectonics and the geology of subduction zones

tectonics and topography for a lithosphere containing density heterogeneities the purpose of this paper is to clarify the dynamic role of lithospheric density heterogeneities in particular with respect to mountain building and other processes of intraplate deformation density anomalies within or just beneath the lithosphere constitute major sources for tectonic stress fields the product of their magnitude by their depth is shown to characterize their ability to induce deformation this rule of the density moment directly yields the lithospheric thickening or thinning rate when applied to structures of large lateral extent for anomalies of lateral extent that is small in comparison with their depth the deformation is vertically inhomogeneous and has been computed with the help of simple physical models of a stratified viscous newtonian lithosphere the analytical treatment is based on fourier transform continent continent collision thickens not only the crust but the entire lithosphere the cold root underlying a mountain chain induces strong regional compressive stresses able to sustain the mountain bulding process without further help from forces transmitted from far away thus the continental lithosphere is in a somewhat metastable mechanical state adiabatic i e rapid thickening or thinning tends to grow further once initiated tectonic phases of strong compression correspond to the climax of such instabilities the response of models with cold lithospheric roots of various intensities has been computed both in two and three dimensions they yield velocity distributions and stress fields instructive comparisons are made with earthquake focal mechanisms and in situ stress measurements in the alpine and appalachian regions in the presence of lateral variations of the mechanical properties of the lithosphere the tectonic style is not only function of the local topography and of the nature of its compensation deformations in neighbouring provinces are coupled as shown by 3 dimensional models for example thickening sustained by a cold lithospheric root may generate extension in peripheral zones of weakness these last results illustrate the point that the computation of regional tectonic stresses requires the knowledge of the density anomalies within the lithosphere on the one hand and of geometrical constraints related to lateral mechanical heterogeneities on the other

kinematic setting and structural control of arc volcanism abstract the evaluation of the kinematic setting and the structural control of volcanic arcs are important in defining the tectono magmatic processes along convergent plate boundaries however our knowledge is fragmented and the available data highlight different behaviours this study analyzes the kinematic setting the structural control and the volcanic productivity of 16 arcs these arcs are characterized by predominant extensional compressional strike slip or oblique motions there is an overall coupling between the normal vs parallel motions along the arc and those of the underlying slab therefore the higher the trench normal or parallel component of the subduction rate the higher is the amount of arc normal or parallel motion this relation confirms that strain partitioning is in general feasible at many convergent settings involving also the volcanic arc portion the arc normal motion may be characterized by extension or compression as a function of the increase in the trench normal convergence or subduction velocity the lack of an evident relation between the subduction rate or convergence rate and the volcanic productivity of the arc is partly inconsistent with previous studies which highlight a relation between the subduction rate and melt production below arcs this discrepancy mainly suggests that different processes control the generation rise and eruption of magma at different depths varying the intrusive extrusive ratio along arcs the structure of the arc does not control the distribution of the volcanoes however it does control the volcanic output through different processes even though regional or local extension associated with strike slip or compressive structures is the ultimate requisite in any setting in general the higher the amount of extension the higher is the output rate along the arc

melange development in the boones point complex north central newfoundland the boones point complex in north central newfoundland is a narrow melange belt separating roberts arm terrain volcanic and volcaniclastic rocks to the north from upper ordovician westerly derived

fossil evidence for fault derived stratigraphic repetition in the northeastern newfoundland appalachians two types of bedding parallel faults are common in the dunnage zone of eastern notre dame bay they are i early thrusts which together with bedding were rotated by later folding into steep attitudes and ii postfolding transcurrent faults both types of faults occur at all scales and give rise to repetitions in the stratigraphic sequence a compilation of the ages of various rock types based on fossil evidence indicates a relatively simple stratigraphy for the dunnage zone this interpretation appears to be at variance with many reported stratigraphic successions until repetitions by bedding parallel faulting are taken into account examples are given of sequences in which the repetition of various rock types dated by means of fossils is due to bedding parallel faulting we believe that the simple lithostratigraphy is a reliable aid for structural interpretations in areas where fossils are scarce or absent

convection in the mantle wedge above the slab and tectonic processes in subduction zones some nonconductive heat transport in the mantle wedge above the subducting lithosphere is necessary to explain the volcanic and thermal activities in subduction zones the physical mechanisms that have been proposed to explain the observed thermal processes are classified into the following three categories 1 secondary convection mechanically driven by the motion of the slab 2 thermal convection due to a large horizontal temperature gradient associated with the presence of the cool slab and 3 flow induced by ascending mantle diapirs containing low density melt the relative importance of these three mechanisms is evaluated using a simple model in which the conduit of the convective flow is prescribed the first mechanism of mechanically driven flow dominates the other two when the effective viscosity of the mantle wedge exceeds 1020 pa s 1021 p known estimates of wedge viscosity suggest that either the second or third mechanism is more likely to be the one that actually governs the tectonic process it is proposed that both the second and third mechanisms are realized in different stages of a single tectonic sequence and that arc volcanism and back arc spreading occur corresponding to these different modes of convection

small scale instabilities below the cooling oceanic lithosphere summary interpretation of satellite altimetry data as well as ship bathymetry data revealed strongly elongated anomalies roughly perpendicular to the mid ocean ridges in the indian and east pacific oceans a spectral analysis of gravity altimetry data along profiles parallel to the east pacific rise indicated wavelengths of about 150 180 km close to the ridge and about 250 km further away a simple model of rayleigh taylor instabilities developing at the base of the cooling lithosphere is discussed and applied to the data by considering thermal divusion and comparing rayleigh taylor growth rates to the velocity of the thermal front in the cooling lithosphere we are able to explain the observed anomalies by instabilities developing below the lithosphere in a layer with a viscosity of about 1019 pa s above an asthenospheric layer with a viscosity reduction of 2 3 orders of magnitude

sediment slump structures a review of diagnostic criteria and application to an example from newfoundland abstract in the study of the history of deformed sedimentary rocks it is important that the cause of the deformation and the timing of deformation relative to metamorphic and tectonic events be assessed there is a continuous gradation between the deformation of freshly deposited sediments by gravitational forces and the deformation of well lithified sediments by tectonic forces so determining the degree to which gravity tectonism and lithification influenced deformation can be very difficult the characteristics that may be considered in determining the origin of deformational structures in sedimentary rocks can be divided into five categories ductile deformation structures 1 can provide definitive criteria for recognizing post lithification deformation but not pre lithification deformation this also holds for 2 brittle deformation structures and decollements overprinting of sediment reworking or remobilization structures 3 are the best criteria for recognizing pre lithification structures fabrics 4 can be very useful but are as yet poorly understood and there are misconceptions in the literature about what sort of fabrics are or are not found in unlithified sediments spatial relationships 5 may also tell us a great deal but are most useful when taken in conjunction with other types of evidence complex fold patterns in sandstones on farmer head north central newfoundland provide a case study for criteria that may be used to determine the degree of lithification during deformation although these folds have been interpreted as slump folds by earlier workers fabrics and spatial relationships point to a tectonic post lithification origin for the structures a conclusion which has significant implications for the interpretation of the regional geology

seismic response to slab rupture and variation in lithospheric structure beneath the savu sea indonesia article i nfo variations in seismic moment release and stress state across the transition from subduction of oceanic crust to arc continent collision in the banda arc are constrained by focal mechanism solutions from the cmt earthquake catalogue in particular the slab under the western savu sea is unusual in that intermediate depth 70 300 km events indicate that at this depth range the slab is largely in down dip compression this contrasts with the intermediate depth down dip tension that typifies the sunda slab to the west and the far eastern banda slab to the east down dip compression beneath the savu sea reflects subduction of transitional crust of the scott plateau more buoyant than the indian ocean crust subducting further west in this region enhanced magma flux is indicated by unusually narrow volcano spacing in the overlying arc and suggests that down dip compression reflects not only more buoyant transitional crust but also a reduction in slab wedge coupling induced by enhanced magma flux east of the savu sea the near complete absence of intermediate depth seismicity is attributed to a slab window that has opened where australian continental crust has collided with the arc differences in seismic moment release around this slab window indicate asymmetric rupture propagating to the east at a much faster rate than to the west

silurian deformation in eastern notre dame bay newfoundland discussion i southeast thrusting and ocean closure across the exploits subzone resembles that of pickering et al 1988 but lafrance and williams assume deposition in a single marine basin on the gondwanan side of iapetus and postdepositional thrusting i rather than deposition in several basins on the laurentian side i of iapetus and syndepositional thrusting arnott et al 1985 pickering et al 1988 we wish to comment on the terrane analysis offered by them and to point out a number of errors i 1 in their presentation of the five terranes defined in the paper two twillingate and chanceport are known ordovician suspect terranes the new world island and port albert terranes while suspect to north america during the early and middle ordovician share the same late ordovician to early silurian rocks and stratigraphy virtually all previous workers have also recognized late ordovician early silurian sedimentologic links between the new world island terrane exploits subzone and the twillingate chanceport terranes notre dame subzone indicating middle ordovician accretion to north america kay 1967 1969 helwig and sarpi 1969 nelson and casey 1979 nelson 1981 arnott 1983a 19836 arnott et al 1985 pickering et al 1988 williams et al 1988 and others these data imply that any silurian basin s in this region must have been on the laurentian margin of iapetus rather than the gondwanan margin the authors dildo run terrane coextensive with the dunnage mclange is interpreted as being faulted against the new world island terrane following kay 1970 and van der pluijm 1986 however this contact has also been interpreted as stratigraphic home 1969 hibbard and williams 1979 the authors present no evidence that any of their terranes were distinctive in the silurian time period of chief interest to their paper according to the authors the twillingate and dunnage terranes were affected by the earlier event however they conclude that these terranes evolved on opposite sides of an ordovician ocean hence there is no reason to suppose that pre silurian events can be correlated in any meaningful sense

the role of transfer structures on volcanic activity at campi flegrei southern italy abstract the tyrrhenian margin of central italy underwent extension during pliocene and quaternary extension occurred mainly through nw se normal faults bordering a sequence of plio quaternary basins these basins are offset by coeval ne sw faults which show strike slip and normal motions and have been interpreted as transfer faults plio quaternary volcanic activity along the margin occurred along a nw se belt systematically in correspondence with ne sw transverse systems the campi flegrei volcanic district cfvd on the southern tyrrhenian margin consists of an active ne sw volcanic ridge developed along ne sw fractures we performed a structural field analysis with analogue and mechanical models to investigate the role of transverse structures upon volcanism at campi flegrei field analysis at campi flegrei recognized ne sw and to a lesser extent coeval nw se active fractures analogue experiments have simulated the development of transfer fault systems in brittle extensional domains the experiments show that subvertical transfer faults connect offset adjacent normal faults dipping 60 the mechanical model is based on the stress equations in uniaxial lithostatic conditions and absence of regional stresses it shows how pre existing subvertical fractures require the smallest magmatic pressures to be penetrated for a given magmatic pressure subvertical fractures might be penetrated more deeply tapping more easily primitive magmas these results suggest that the cfvd is located along a ne sw transfer zone connecting nw se regional normal faults volcanic activity along such ne sw trend would be induced by the subvertical dip of the transfer faults the subvertical dip of transfer faults also suggests an explanation for the emission of the more primitive products along ne sw systems at campi flegrei these considerations find a wider application on the remaining volcanic districts of the margin located in the same overall structural setting

hot fingers in the mantle wedge new insights into magma genesis in subduction zones abstract quaternary volcanoes in the northeast japan arc can be grouped into 10 volcanic clusters striking transverse to the arc these have an average width of 50 km and are separated by parallel gaps 30 75 km wide this clustering of volcanic centres topographic profiles low velocity regions in the mantle wedge and local negative bouguer gravity anomalies along the japan sea side of the volcanic arc are closely correlated all these observations may be related to locally developed hot regions within the mantle wedge that have the form of inclined 50 km wide fingers each of the 10 fingers recognised extends from deep mantle 150 km below the back arc region towards the shallower mantle 50 km beneath the volcanic front quaternary volcanoes are built immediately above the hot mantle fingers the volcanic basement along the fingers has been uplifted by repeated injection of magmas into the crust accompanied by quaternary volcanic activity at the surface although volcanic activity is rare along the japan sea coast tomographic results show that hot low velocity mantle fingers exist within the mantle wedge the negative bouguer anomalies at the rear of the volcanic arc could be caused by magmas supplied from the hot mantle fingers these have not yet been erupted but have accumulated at the moho discontinuity

evidence for motion between nubia and somalia along the southwest indian ridge the east african rift marks the northern boundary of the nubian west african and somalian east african plates and has formed by horizontal stretching due to the separation of these plates1 south of 20 s any expression of deformation or seismicity due to the relative motion of these two distinct plates vanishes although the boundary must continue until it intersects another plate boundary the nearest such boundary is that of the antarctic plate marked by the southwest indian ridge but previous analyses of plate motion data have indicated no significant difference between nubia antarctica and somalia antarctica motion2 3 here we show using a large compilation of plate motion data that nubia antarctica motion does differ from somalia antarctica motion and we determine a relative angular velocity of the two plates that has compact confidence limits our analysis places the pole of rotation near to the southern limit of african seismicity implying that the southern part of the nubian somalian plate boundary is a diffuse zone of convergence up to 2 mm yr 1 whereas up to 6 mm yr 1 of separation is accommodated across the east african rift about half the separation rate of the slowest mid ocean ridge

cenozoic thermal mechanical and tectonic evolution of the rio grande rift careful documentation of the cenozoic geologic history of the rio grande rift in new mexico reveals a complex sequence of events at least two phases of extension have been identified an early phase of extension began in the mid oligocene about 30 ma and may have continued to the early miocene about 18 ma this phase of extension was characterized by local high strain extension events locally 50 100 regionally 30 50 low angle faulting and the development of broad relatively shallow basins all indicating an approximately ne sw 25 extension direction consistent with the regional stress field at that time extension events were not synchronous during early phase extension and were often temporally and spatially associated with major magmatism a late phase of extension occurred primarily in the late miocene 10 5 ma with minor extension continuing to the present it was characterized by apparently synchronous high angle faulting giving large vertical strains with relatively minor lateral strain 5 20 which produced the modern rio grande rift morphology extension direction was approximately e w consistent with the contemporary regional stress field late phase graben or half graben basins cut and often obscure early phase broad basins early phase extensional style and basin formation indicate a ductile lithosphere and this extension occurred during the climax of paleogene magmatic activity in this zone late phase extensional style indicates a more brittle lithosphere and this extension followed a middle miocene lull in volcanism regional uplift of about 1 km appears to have accompanied late phase extension and relatively minor volcanism has continued to the present we have estimated geotherms and calculated lithospheric strength curves for the two phases of extension using geologic data to constrain earlier events and geophysical data to constrain the modern geotherm and crustal structure a high geotherm was deduced for early phase extension resulting in a shallow crustal brittle ductile transition and negligible mantle strength the lithosphere cooled after early phase extension resulting in a deeper crustal brittle ductile transition and perhaps more significantly a considerable zone of mantle strength immediately beneath the moho these results indicate that early phase extensional style was controlled by a crustal decollement near the brittle ductile transition which was prevented during late phase extension by significant strength in the uppermost mantle late cenozoic uplift of the rift zone cannot be explained by crustal thinning during extension and geotherm evolution predicted from simple cooling however this uplift does not appear to be restricted to the rift zone and pliocene to recent volcanism and heat flow data suggest that uplift may be caused by magmatic thickening of the crust perhaps unrelated to rifting the complex interrelationship among regional and local prerifting synrifting and postrifting events in the rio grande rift suggests that rifting at least in this region should not be considered in isolation of other geologic events

synthesis of palaeozoic deformational events and terrane accretion in the canadian appalachians plate tectonic theory predicts that most deformation is associated with subduction and terrane accretion with some deformation associated with transform transcurrent movements deformation associated with subduction varies between two end members 1 where the tectonic regime is dominated by subduction of oceanic lithosphere containing small terranes a narrow surface zone of accretionary deformation along the subduction zone starts diachronously on the subducting plate at the trench as material is transferred from the subducting plate to the over riding plate and 2 where continent continent collision is occurring a wide surface zone of accretionary deformation starts synchronously or with limited diachronism palaeozoic deformational events in the canadian appalachians correspond to narrow diachronous events in the ordovician and silurian whereas devonian carboniferous and permian deformational events are widespread and broadly synchronous along the western side of the canadian appalachians the taconian deformational event starts diachronously throughout the ordovician and corresponds to the north north west accretion of the notre dame ascot weedon st victor and various ophiolitic massifs volcanic arc and peri arc terranes over cratonic north america within the eastern half of the central mobile belt the late cambrian early ordovician penobscotian deformational event corresponds to the south easterly accretion of the exploits subzone various volcanic are and peri arc terranes over the gander zone continental rise in the centre of the orogen the late ordovician silurian beothukan deformational event corresponds to the south easterly accretion of the notre dame over the exploits gander subzones along the south eastern side of the central mobile belt the silurian ganderian deformational event corresponds to the north north east sinistral transcurrent accretion of the avalon composite terrane microcontinent over the gander exploits zones along the south eastern half of the orogen the late silurian middle devonian acadian deformation event corresponds to the westerly accretion of the meguma terrane intradeep or continental rise over the avalon composite terrane affecting the entire orogen the late devonian carboniferous and permian acadian alleghanian deformational events correspond to the east west convergence between laurentia and gondwana continent continent collision

wet sediment deformation in the upper ordovician point leamington formation an active thrust imbricate system during sedimentation notre dame bay north central newfoundland

the mantle wedge s transient 3 d flow regime and thermal structure arc volcanism volatile cycling mineralization and continental crust formation are likely regulated by the mantle wedge s flow regime and thermal structure wedge flow is often assumed to follow a regular corner flow pattern however studies that incorporate a hydrated rheology and thermal buoyancy predict internal small scale convection ssc here we systematically explore mantle wedge dynamics in 3 d simulations we find that longitudinal richter rolls of ssc with trench perpendicular axes commonly occur if wedge hydration reduces viscosities to 1 1019 pa s although transient transverse rolls with trench parallel axes can dominate at viscosities of 5 1018 1 1019 pa s rolls below the arc and back arc differ subarc rolls have similar trench parallel and trench perpendicular dimensions of 100 150 km and evolve on a 1 5 myr time scale subback arc instabilities on the other hand coalesce into elongated sheets usually with a preferential trench perpendicular alignment display a wavelength of 150 400 km and vary on a 5 10 myr time scale the modulating influence of subback arc ridges on the subarc system increases with stronger wedge hydration higher subduction velocity and thicker upper plates we find that trench parallel averages of wedge velocities and temperature are consistent with those predicted in 2 d models however lithospheric thinning through ssc is somewhat enhanced in 3 d thus expanding hydrous melting regions and shifting dehydration boundaries subarc richter rolls generate time dependent trench parallel temperature variations of up to 150 k which exceed the transient 50 100 k variations predicted in 2 d and may contribute to arc volcano spacing and the variable seismic velocity structures imaged beneath some arcs

southwest new world island stratigraphy new fossil data new implications for the history of the central mobile belt newfoundland the identification of fossils from 16 new localities on southwestern new world island has resulted in the refinement and simplification of local stratigraphy and in the correlation of rock units across an intensely deformed area complex stratigraphic classifications of earlier workers are replaced by a simpler scheme consisting of four formations the summerford formation oldest rogers cove formation sansom formation and goldson formation mclange units are fault related and cannot be included in the stratigraphy fossil and lithological data provided here confirm previous indications that the rocks of the summerford formation are the remains of a long lived volcanic island complex this complex is overlain by a markedly diachronous coarsening upwards sequence of marine clastic sediments now represented by the rogers cove sansom and goldson formations the new stratigraphic data combined with structural evidence indicate that the stratigraphy is repeated across several bedding parallel faults significant diachroneity of formation boundaries and the absence of syndepositional high angle faults suggest deposition in one large rnid paleozoic basin rather than in a number of small dynamic basins

distribution of volcanoes in active margins we report a comprehensive study of the distribution of volcanoes in 16 active plate margins corresponding to a total of 479 volcanic systems the active volcanic arcs are found to have a ribbon geometry with an average length width ratio of around 10 the shape of the volcanic arc is compared to the shape of the associated trench by projecting one onto the other the projection direction agrees with the direction of plate convergence which shows that plate motion is the main factor constraining magma generation on the lithospheric scale there is no characteristic spacing between volcanoes in each arc the distribution of volcano spacing is best represented by a gamma distribution which corresponds to randomly generated points in the same geometrical conditions in order to explain how such distributions may be generated we have investigated in the laboratory the gravitational instability of a layer of buoyant liquid which is fed at a constant rate at the bottom of denser fluid different dynamic regimes are reached for different values of the viscosity ratio between the two fluids for low viscosity contrasts there is only one initial instability event with buoyant plumes organized in a periodic pattern these plumes are fed continuously by the source even at large times for high viscosity contrasts which is relevant to the geological problem plumes are produced intermittently by individual instability events they take the form of cavity plumes fed by narrow tails once the plumes have reached the surface they continue to be fed from below by material rising through their shrinking tail in successive instability events plume generation is not repeated at the same locations which leads to a complex plan form with time and hence with increasing number of instability events the cumulative distribution of plume spacing changes from periodic to random material from a single plume is able to sustain volcanic eruptions for a period longer than the time between two instability events and hence the distribution of active volcanic centers reflects the cumulative distribution of several instability events this is consistent with the observed distributions

granite magma formation transport and emplacement in the earth s crust the origin of granites was once a question solely for petrologists and geochemists but in recent years a consensus has emerged that recognizes the essential role of deformation in the segregation transport and emplacement of silica rich melts in the continental crust accepted petrological models are being questioned either because they require unrealistic rheological behaviours of rocks and magmas or because they do not satisfactorily explain the available structural or geophysical data provided flow is continuous mechanical considerations suggest that far from being geologically sluggish granite magmatism is a rapid dynamic process operating at timescales of or 100 000 years irrespective of tectonic setting

petrologic diversity in mount st helens dacites during the last 4 000 years implications for magma mixing mount st helens has explosively erupted dacitic magma discontinuously over the last 40 000 years and detailed stratigraphic data are available for the past 4 000 years during this last time period the major element composition of the dacites has ranged from mafic 62 64 wt sio2 to felsic 65 67 wt sio2 temperature has varied by about 150 c 770 920 c and crystallinity has ranged between 20 and 55 water content of these dacites has also fluctuated greatly although the source for the dacitic magmas is probably partial melting of lower crustal rocks there is strong physical evidence such as banded pumices thermal heterogeneities in single pumices phenocryst disequilibrium contrasts between compositions of glass inclusions and host matrix glass and amphibole reaction rims that suggests that magma mixing has been prominent in the dacitic reservoir indeed we suggest that the variations in major and trace element abundances in mount st helens dacites indicate that magma mixing between felsic dacite and mafic magma has controlled the petrologic diversity of the dacitic magmas magma mixing has also controlled the composition of andesites erupted at mount st helens and thus it appears that the continuum of magmatic composition erupted at the volcano is controlled by mixing between felsic dacite or possibly rhyodacite and basalt the flux of the felsic endmember to the reservior appears to have been relatively constant whereas the flux of basalt may have increased in the past 4 000 years as suggested by the apparently increased abundance of mafic dacite and andesite erupted in this period

hydrothermal activity and volcano distribution along the mariana arc 1 submarine volcanic arcs extend for fully a third of the length of the mid ocean ridge system but systematic investigation of hydrothermal venting on those arcs is sparse here we report on the distribution of hydrothermal activity along the intraoceanic mariana arc we identify 76 volcanic edifices along 1370 km of arc grouped into 60 volcanic centers of which at least 26 20 submarine are hydrothermally or volcanically active the overall volcanic center density is 4 4 100 km of arc and that of active centers is 1 9 100 km an equal length of the tonga kermadec arc holds fewer centers 2 9 100 km but a similar density of active centers 1 8 100 km the lower percentage of active centers on the mariana arc results from its having both fewer caldera volcanoes since on both arcs calderas are twice as likely as cones to be active and a lower percentage of active calderas and cones active centers are found from 80 to 230 km above the subducting pacific slab and a quarter lie behind the arc front the frequency distribution of center spacing along the arc front peaks between 20 and 30 km and shows the asymmetric long tail shape typical for many other arcs especially those with volcano populations 20 there is no evidence for a regular spacing of volcanic centers using the new mariana data and recent data from the tonga kermadec arc we estimate that all intraoceanic arcs combined may contribute hydrothermal emissions equal to 10 of that from the global mid ocean ridge

cladistic analysis applied to the classification of volcanoes cladistics is a systematic method of classification that groups entities on the basis of sharing similar characteristics in the most parsimonious manner here cladistics is applied to the classification of volcanoes using a dataset of 59 quaternary volcanoes and 129 volcanic edifices of the tohoku region northeast japan volcano and edifice characteristics recorded in the database include attributes of volcano size chemical composition dominant eruptive products volcano morphology dominant landforms volcano age and eruptive history without characteristics related to time the volcanic edifices divide into two groups with characters related to volcano size dominant composition and edifice morphology being the most diagnostic analysis including time based characteristics yields four groups with a good correlation between these groups and the two groups from the analysis without time for 108 out of 129 volcanic edifices thus when characters are slightly changed the volcanoes still form similar groupings analysis of the volcanoes both with and without time yields three groups based on compositional eruptive products and morphological characters spatial clusters of volcanic centres have been recognised in the tohoku region by tamura et al earth planet sci lett 197 105 106 2002 the groups identified by cladistic analysis are distributed unevenly between the clusters indicating a tendency for individual clusters to form similar kinds of volcanoes with distinctive but coherent styles of volcanism uneven distribution of volcano types between clusters can be explained by variations in dominant magma compositions through time which are reflected in eruption products and volcanic landforms cladistic analysis can be a useful tool for elucidating dynamic igneous processes that could be applied to other regions and globally our exploratory study indicates that cladistics has promise as a method for classifying volcanoes and potentially elucidating dynamic and evolutionary volcanic processes cladistics may also have utility in hazards assessment where spatial distributions and robust definitions of a volcano are important as in locating sensitive facilities such as nuclear reactors and repositories

a basin model for the silurian of the midland valley of scotland and ireland identification of terranes is frequently based on the apparent disparity of source area for sediment supply palaeontological evidence and the presence of faults whose sense and amount of displacement is sometimes conjectural as an example of such superficially unrelated stratigraphies the silurian successions of the midland valley of scotland and the northern part of ireland are compared a diachronous late llandovery early middle telychian transgressive episode which is earlier in the midland valley inliers is common to many of the successions the transgression resulted in the development of comparable sedimentary facies and fossil assemblages the clast compositions of llandovery and wenlock conglomerates suggest erosion of a common provenance of volcanic rocks founded on a metaquartzite basement areas of active volcanism appear to have migrated in a north westerly direction with time a similar sequence is evident in canada it is suggested that the common features of the scottish irish successions may be accommodated in a unified basin model for silurian sedimentation and volcanism such a basin may have formed in an intra arc environment and been controlled by oblique slip fault mechanisms although evidence for the presence of a silurian arc is equivocal

insights into the dynamics of planetary interiors obtained through the study of global distribution of volcanoes ii tectonic implications from venus abstract the distribution of volcanic features is ultimately controlled by processes taking place beneath the surface of a planet for this reason characterization of volcano distribution at a global scale can be used to obtain insights concerning dynamic aspects of planetary interiors until present studies of this type commonly have focused on volcanic features of a specific type e g large volcanoes in venus or hot spot volcanism on earth or have concentrated on relatively small regions i e vent distribution within individual volcanic fields but no comparison of extensive databases has been made by using the same tools in both planets in this work the description of the distribution of volcanic features observed over the entire surface of venus is made using the same tool used for earth and is applied to an extensive database the analysis is based on density contours obtained with the fisher kernel as a result several groupings of volcanoes are identified refining the already documented concentration of volcanoes on the bat zone in particular some doughnut like patterns are observed that might be related to the action of mantle plumes the occurrence of such features on earth as well as the existence of a uniform distribution of background volcanism on both planets suggests similarities on their geodynamic behavior that had not been identified previously

cinder cone clustering in the transmexican volcanic belt implications for structural and petrologic models cinder cone distributions have most often been characterized using univariate statistics here a new technique to volcano distribution studies cluster analysis is applied to cinder cone distribution in the central transmexican volcanic belt tmvb a total of 1016 cinder cones are identified over an area of approximately 60 000 km2 application of cluster analysis reveals structure in cinder cone distribution using a search radius parameter of 16 km 75 of the cinder cones within the central tmvb are found within eight clusters of 45 159 cones each these clusters are each 2000 to 5000 km2 in area only 22 cones are found within clusters of three or fewer cinder cones indicating that clustering is a pervasive phenomenon some petrologic variation is evident among clusters low mg alkaline cinder cones are found within a single cluster 360 400 km from the trench application of alignment analysis techniques including the two point azimuth method the hough transform and two dimensional fourier analysis demonstrates that cinder cone alignments have common orientations on a regional scale within three clusters all located in the southernmost part of the michoacan guanajuato volcanic field in the western portion of the study area these alignments consist of tens of cinder cones are 20 50 km long and are all oriented with azimuths of 020 040 parallel to the direction of plate convergence high mg lavas which last fractionated at pressures in excess of 8 kbar are only found associated with these alignments indicating that magma transport is significantly enhanced in these areas although local alignments of three to six cinder cones occur within mapped fault zones that transect the area regional cinder cone alignment patterns and the distribution of clusters themselves do not appear to be affected by the presence of these fault zones the cluster analysis the alignment analysis and some petrologic data support the hypothesis that the tmvb is segmented near 101 w

successive orthogonal and oblique extension episodes in a rift zone laboratory experiments with application to the ethiopian rift small scale modeling was performed to examine the effects of the superposition of two successive extensional phases from orthogonal to oblique type 1 and from oblique to orthogonal type 2 in both the type 1 and type 2 models faults produced during the first stage strongly control fault development during the second stage in type 1 models the oblique faults developed during the second oblique phase are confined within a first phase graben whereas in type 2 models the oblique faults produced during the first phase continue to develop during orthogonal extension and connect with each other to give sigmoidal fault blocks type 1 models are compared with the structural setting of the ethiopian rift the evolution of the rift is related to a recent extensional event whose principal direction of stretching trends at around 50 to preexisting major normal faults type 1 laboratory models are fairly comparable to the northern sector of the ethiopian rift referred to here as mer they account for both the development of the en echelon oblique faults of the wonji fault belt and the sinistral shear gradient running parallel to the eastern border of the mer which formed during an oblique rifting extension the statistical analysis of the whole ethiopian rift fault pattern by reference to the experimental data allows the determination of a n100 n110 mean direction of stretching

effects of volcano loading on dike propagation in an elastic half space we use laboratory experiments and numerical models to examine the effects of volcano loading on the propagation of buoyant dikes in a two dimensional elastic half space in laboratory experiments we simulate the propagation of buoyant dikes in an isotropic regional stress field by injecting air into tanks of solidified gelatin a weight resting on the surface of the gelatin represents a volcanic load a numerical model is used to simulate these experiments both experiments and numerical simulations show that as a dike ascends it begins to curve toward the load in response to the local stress field imposed by the load the lateral distance over which dikes curve to the load increases with the ratio of average pressure at the base of the load to the dike driving pressure for realistic volcano and dike dimensions this pressure ratio is going to be large suggesting that dikes can converge to a volcano over lateral distances several times the load width numerical calculations involving an anisotropic regional stress field however predict that the lateral extent of dike attraction shrinks as the regional horizontal compressive stress decreases relative to the vertical compressive stress dike focusing will be substantial if the regional differential stresses are less than the average pressure at the base of the load if this is the case then our models predict a positive feedback between the size of volcanoes and the area of dike attraction this feedback may promote the development of large discrete volcanoes and also predicts a positive correlation between the spacing and sizes of adjacent volcanoes to test this prediction we examine nearest neighbor pairs of the 21 largest volcanoes in the cascade range the 14 pairs examined show a large range in volcano spacing 6 115 km and a statistically significant correlation between spacing and average volcano height this result is consistent with our model results and suggests that the local compressive stress induced by these volcanoes may be an important factor in controlling magma transport in the lithosphere

continental rift architecture and patterns of magma migration a dynamic analysis based on centrifuge models 1 small scale centrifuge models were used to investigate the role of continental rift structure in controlling patterns of magma migration and emplacement experiments considered the reactivation of weakness zones in the lower crust and the presence of magma at moho depths results suggest that surface deformation which reflects the weakness zone geometry exerts a major control on patterns of magma migration in the case of a single rift segment the experimental lower crust and magma were both transferred in an extension parallel direction toward the rift flanks this lateral migration reflected the dominance of far field stresses over extension induced buoyancy forces local pressure gradients favored the raise of experimental magma in correspondence of marginal grabens the lateral migration gave rise to major accumulations below the footwall of major boundary faults providing the magma source able to feed off axis volcanoes in nature as inferred for the main ethiopian rift in the case of two offset rift segments a major transfer zone developed this transfer zone was characterized by prominent experimental lower crust doming and strong magma accumulation dynamic analysis showed that the transfer zone development caused a strong pressure difference in a rift parallel direction which dominated over the far field thinning owing to this pressure gradient almost all the underplated experimental magma collected below the lower crust dome suggesting a rift parallel extension orthogonal migration this process has a direct relevance for the localization of magmatic activity at transfer zones in natural continental rifts such as in the western branch of the east african rift system

insights into the dynamics of planetary interiors obtained through the study of global distribution of volcanoes i empirical calibration on earth abstract the distribution of volcanic features is ultimately controlled by processes taking place beneath the surface of a planet for this reason characterization of volcano distribution at a global scale can be used to obtain insights concerning dynamic aspects of planetary interiors until present studies of this type have focused on volcanic features of a specific type or have concentrated on relatively small regions in this paper the first of a series of three papers we describe the distribution of volcanic features observed over the entire surface of the earth combining an extensive database of submarine and subaerial volcanoes the analysis is based on spatial density contours obtained with the fisher kernel based on an empirical approach that makes no a priori assumptions concerning the number of modes that should characterize the density distribution of volcanism we identified the most significant modes using those modes as a base the relevant distance for the formation of clusters of volcanoes is constrained to be on the order of 100 to 200 km in addition it is noted that the most significant modes lead to the identification of clusters that outline the most important tectonic margins on earth without the need of making any ad hoc assumptions consequently we suggest that this method has the potential of yielding insights about the probable occurrence of tectonic features within other planets

paleoecology of selected conodontophorid species from the cobbs arm formation middle ordovician new world island north central newfoundland the upper chazyan llandeilian cobbs arm formation of new world island north central newfoundland consists of pure limestones and limestones with varying amounts of volcaniclastic admixture inte

the tectonics and depositional history of the ordovician and silurian rocks of notre dame bay newfoundland reply in the notre dame bay region ophiolitic rocks underlie a thick sequence of lower ordovician volcanic arc rocks to the north of the lobster cove chanceport fault neither this fault nor the lukes arm sops head fault shows evidence of very large strike slip movements as parts of the same arc together with much arc derived detritus straddle both faults towards the east this arc derived detritus becomes more distal in aspect and passes laterally into the dunnage melange during the middle ordovician epoch late llandeilo and early caradoc most areas show a marked decrease in volcanic activity and in the amount of coarse detritus deposited coarse turbidites reappear at different times in different areas during the late ordovician these are related to several fault bounded basins and to movements on the lukes arm sops head fault many of these faults particularly in the east are marked by olistostromes several of which can be dated by fossils as late ordovician and early silurian the whole

rheological conditions for emplacement of ural alaskan type ultramafic complexes article i nfo ural alaskan or alaskan type complexes correspond to a particular class of ultramafic intrusions that attract particular attention due to their deep mantle origin and their platinum group element pge mineralization when defined as massifs of dunite clinopyroxenite only forty six complexes are reported in the literature these large scale dunite pipe like structures are rarely isolated and they even can appear in clusters to better understand genesis of these relatively young b460 ma complexes a worldwide compilation has been built and three categories have been defined single circular or elliptical bodies twin bodies with similar shapes and dismembered dunite bodies pge enrichment in alaskan type complexes is highest for the second category where twin bodies are interpreted as horizontal sections of y shaped dunite pipes to constrain mechanical properties of the lithosphere allowing emplacement of the alaskan type complexes the forceful diapiric ascent hypothesis is investigated through numerical thermo mechanical models one hundred high resolution experi ments accounting for realistic phase changes and softening mechanisms have been performed the experiments show that with no rheological softening of the host rock and in case of a relatively weak ductile lower crust the uprising magma tends to spread laterally without reaching the surface to account for the forceful ascent of deep magmas it is hence necessary to assume a strong lower crust rheology and strong local softening mechanisms besidesreproducingtheclustereddistributionoftheweaknesszonesrepresentingmagmapathways theselatter experiments reproduce large scale pipe like cylindrical structures y shaped and funnel shaped bodies and laterally shifted structures interestingly zones of highest strain rates are located at the bottom parts of the inclined edges of y shaped and funnel shaped bodies the restricted age range of alaskan type complexes b460 ma would mean that prior to this time the lower crust was less resistant due to the hotter geotherm prohibiting the possibility of alaskan type magmatism

a new regime of slab mantle coupling at the plate interface and its possible implications for the distribution of volcanoes abstract we investigate the effects of a thin low viscosity layer just above the subducting slab on 3d thermal and flow structure in the mantle wedge by taking northeast japan as an example the low viscosity layer assumed here is needed to explain the observed low surface heat flow and low seismic attenuation in the forearc by decoupling the mantle from the subducting slab we find that when the viscosity in the low viscosity layer is sufficiently low along arc component of the flow arises inside the layer and produces along arc temperature variation it can also be considered as the along arc changes in the degree of slab mantle coupling at the plate interface the onset time and the characteristic wavelength of the 3d flow depend on the viscosity and the extent of the low viscosity layer in order to explain the observed spatial and temporal changes in the distribution of quaternary volcanoes in northeast japan the viscosity and the thickness of the low viscosity layer need to be 5 10 18 pa s and 6 km respectively the model proposed here is based on an assumption which is simpler and better constrained by observations compared to previous models therefore it could be an alternative explanation of the distribution of volcanoes in northeast japan

timing and spatial distribution of deformation in the newfoundland appalachians a multi stage collision history van der plmjm b a 1987 timing and spatial distribution of deformation in the newfoundland appalachians a multi stage collision history in h j zwart m martens i van der molen c w passchier c spiers and r l m vissers editors tectonic and structural processes on a macro meso and micro scale tectonophysics 135 15 24 the newfoundland appalachians have been interpreted as an area where lower paleozoic plate convergence culminated in collision between an ordovician volcanic chain and the north american craton in middle ordovician times closure of the intervening proto atlantic iapetus ocean was considered incomplete subsequent deformation gave rise to regional folding and faulting recent studies in the newfoundland dunnage zone have revealed that the deformation history is far more complex than previously recognized large scale thrusting folding and faulting occurred in silurian devonian times furthermore it has been suggested that the dunnage zone is an allochthonous terrane underlain by dominantly continental crust rather than representing remnants of a rooted ocean basin in view of these results a revision of tectonic scenarios and zonal subdivision is warranted and a multi stage collision history will be discussed with emphasis on the spatial distribution and significance of silurian devonian deformation in central newfoundland subduction in lower paleozoic times gave rise to the formation of a volcanic terrane concurrently to the southeast a marginal sea was formed mariana type subduction in middle ordovician times the volcanic terrane collided with the north american craton first stage collision and back arc spreading terminated continued crustal shortening resulted in the formation of a silurian accretionary terrane telescoped marginal sea and its subsequent deformation second stage collision devonian carboniferous strike slip faulting represents the third stage in the collision history the model is applicable to large tracts of the caledonian appalachian chain its main characteristics are a the revised zonal subdivision of the area is based on characteristics of silurian and older rocks rather than middle ordovician and older rocks only b the central part of the orogen represents a telescoped marginal sea that formed to the southeast of the ordovician volcanic chain rather than a remnant of the incompletely closed iapetus ocean c the earliest deformation is progressively younger toward the southeast d the appalachian collision history is a result of the activity of a single deformation regime over a long period of at least 75 ma

physical characteristics and evolutionary trends of continental rifts rifts may be defined as zones beneath which the entire lithosphere has ruptured in extension they are widespread and occur in a variety of tectonic settings and range up to 2 600 m y in age the object of this review is to highlight characteristic features of modern and ancient rifts to emphasize differences and similarities in order to help characterize evolutionary trends to identify physical conditions favorable for initiation as well as termination of rifting and to provide constraints for future modeling studies of rifting rifts are characterized on the basis of their structural geomorphic magmatic and geophysical features and the diverse character of these features and their evolutionary trends through time are discussed mechanisms of rifting are critically examined in terms of the physical characteristics and evolutionary trends of rifts and it is concluded that while simple models can give valuable insight into specific processes of rifting individual rifts can rarely if ever be characterized by well defined trends predicted by these models more data are required to clearly define evolutionary trends and the models require development to incorporate the effects of lithospheric heterogeneities and complex geologic histories

stress transfer between magma bodies influence of intrusions prior to 2010 eruptions at eyjafjallajokull volcano iceland stress transfer between separate magma bodies is evaluated by considering how pressure changes related to magma accumulation propagation influence the stability of a separate nearby magma body three dimensional numerical models are used to evaluate the stability evolution of a magma body through the calculation of two variables i the variation of the threshold pressure needed to cause failure around the magma body and ii the magma pressure change a parametric study indicates that stress interactions are strongly dependent on the distance between magma bodies as well as the body s shape such models are then applied to evaluate stress influence of intrusive activity in 1994 1999 and 2010 at eyjafjallajokull volcano which preceded two eruptions there in 2010 two cases are considered influence of these intrusions on i a magma reservoir at 20 km distance under the katla volcano and ii a silicic magma body under eyjafjallajokull the distance between the eyjafjallajokull intrusions and the katla reservoir is sufficiently long to reduce the stress interaction to insignificant levels with an amplitude of the same order as earth tides a few kilopascals however cumulative stress transfer due to the intrusions to a remnant silicic shallow body situated below the eyjafjallajokull is much larger 0 5 2 5 mpa this mechanical transfer could have contributed to the failure of the silicic body and promoted the chemical mixing mingling between different magma types which is commonly interpreted as the main cause of the 2010 explosive eruption of eyjafjallajokull

predicting gold rich epithermal and porphyry systems in the central andes with a continental scale metallogenic gis brgm s gis andes a comprehensive continental scale metallogenic information system for the entire andes cordillera is based on original syntheses structured into thematic layers the aim of developing the gis was to produce an integrated tool to understand ore deposit localization in the andes a fundamental question arising at the outset was whether the tool would be suitable for producing predictive mineral resource maps at the continental scale considering that previous predictive studies focus only on the regional scale the continental scale synthesis implied working with heterogeneous data in terms of distribution quantity quality and in particular accuracy the benefit is the ability to include uncommon parameters linked to the geodynamic evolution of the active margin and significant only at the continental scale in view of the particularities of the gis dataset an expert guided data driven approach was adopted for the multicriteria processing an approach that combined expert knowledge and the use of elementary statistics allowing to provide a link between the tectonic development of the whole andean margin and the spatial and temporal distribution of individual mining districts this study was purposely restricted to assessing the distribution of neogene gold in the central andes between lat 3 and 33 s thus a incorporating well constrained data on the present morphology of the convergent margin and b avoiding ambiguities in the less well constrained older history of the complex evolution of the andean margin five regional parameters were selected and were considered to have a significant influence on the neogene magmatic hydrothermal ore formation at continental scale the five parameters i host rock lithostratigraphy ii lithostratigraphic contacts iii structural discontinuities and iv depth and v dip of the wadati benioff zone modeled from seismic data had assigned favorability scores from 0 to 2 based on their associated metal content with respect to a set of identified neogene gold deposits the next step was to calculate favorability maps for each criterion that were combined to create an overall cumulative favorability map or predictive gold map verifying the predictive map against known gold deposits it was found that the cumulative favorability score of 4 out of 10 maximum located about two thirds of the known gold bearing epithermal and porphyry deposits and 95 of the metal content a cumulative favorability score of 5 reduced these figures to 50 of the deposits and 71 of the metal content and that of 6 relocated 24 of the deposits and 51 of the metal content in addition to verifying the method the predictive map outlines new potentially favorable gold areas and even indicates that some known districts could well host yet undiscovered mineralization

sedimentology of upper ordovician silurian sequences on new world island newfoundland separate fault controlled basins remapping of northeast new world island newfoundland demonstrates that two major faults separate three distinct sedimentary sequences paleontology and sedimentology indicate that these sequences are partly equivalent in age but were deposited in separate basins of deposition that were adjacent to each other active silurian faults the boyds island and byrne cove faults new names bounded the margins of these basins and directly influenced sedimentation by uplifting ordovician volcanics limestone and black shale which are found both in situ and as blocks within silurian sediments silurian sediments deposited adjacent to these faults are dominated by pebbly mudstones and chaotic bedding interpreted as debris flow deposits and slumped horizons away from the fault scarps sedimentation was predominantly axial it comprises resedimented conglomerates and thick and thin bedded sandstone turbidites west of new world island similar synsedimentary faults are confined to a narrow belt south of the lukes

on the relationship between surface uplift and gravitational extension this paper highlights the relationship between isostatically supported surface elevation and the lateral buoyancy forces of an orogen the former is a linear function the latter is a quadratic function of the thickness of the lithosphere thus surface elevation and lateral buoyancy forces may have differing evolutions in the development of an orogen indeed during the evolution of collisional orogens the qualitative relationship between these two parameters may reverse so that only in the early orogenic evolution topographically high regions exert a net lateral buoyancy force on topographically low regions later in the evolution this may change to a counterintuitive scenario in which there is a net lateral buoyancy force from the topographically lower regions toward the topographically higher regions to illustrate this fact we use a simple one dimensional kinematic model for an orogen which undergoes simultaneous thickening due to convergence and erosion at the surface it can be shown that for reasonable erosion and thickening rates surface elevation increases through time until a steady state is reached while the lateral buoyancy force changes from positive to negative giving rise to the inward collapse of an orogen in the field this may be recognized as the occurrence of two successive coaxial convergent deformation events present day examples may be the formation of back arc basins where extension in the low lying hinterland of an andean type mountain belt is caused by a gravitational compressive force exerted by the hinterland onto a belt of significantly higher surface elevation but possibly of lower potential energy

the volcanic evolution of martinique island insights from k ar dating into the lesser antilles arc migration since the oligocene abstract the lesser antilles island arc bifurcates into two lines in its northern part with an old branch to the east and a recent active branch to the west martinique is located at the southern tip of the separation the two arcs diverge northward and at maximum divergence are separated by a 50 km wide depression despite this separation which suggests a jump in volcanism activity has been almost continuous in martinique island with a slow displacement of the eruptive centers to the west considering timing of emplacement previous authors defined three cycles of activity the old intermediate and recent arcs of late oligocene early miocene mid miocene and late miocene to present ages respectively the present study investigates the timing of emplacement of the volcanic units in martinique island in order to constrain the activity of the old and intermediate lesser antilles arcs as recorded on this island unspiked k ar age determinations on groundmass and plagioclase separates cassignol gillot technique were conducted on 20 samples from the old and intermediate volcanic chains martinique has evolved as eight distinct volcanic centers 1 basal complex and sainte anne series 24 8 0 4 20 8 0 4 ma for the old arc 2 vauclin pitault chain 16 1 0 2 8 44 0 12 ma and 3 south western volcanism 9 18 0 16 7 10 0 10 ma for the intermediate arc and 4 morne jacob volcano 5 14 0 07 1 54 0 03 ma 5 trois ilets volcanism 2 358 0 034 ma and 346 27 ka 6 carbet complex 998 14 to 322 6 ka 7 mount conil 543 8 to 127 2 ka and 8 mount pelee 126 2 ka to present for the recent arc germa et al 2010 germa et al 2011a we propose migration rates of 1 1 1 4 km myr westward toward the back arc region throughout the whole volcanic history of martinique island these rates together with geochemical evidence for a more enriched signature in the youngest magmas are consistent with a geodynamic evolution involving the migration of the northern volcanic front away from the trench in response to the subduction of an aseismic ridge possibly influenced by convergence between north american and south american plates

subduction of a segmented ridge along a curved continental margin variations between the western and eastern sanak baranof belt southern alaska abstract between the western and the eastern parts of the southern alaskan sanak baranof belt there exist variations in pluton spacing geochronology geochemistry and metamorphism all such characteristics can be explained by increase in heat flux towards the eastern parts of the belt despite such differences the initiation of magmatism propagated eastward at a constant rate of 19 cm year throughout the entire belt the average spacing of magmatic centers changes from 165 88 km in the west to 67 41 km in the east the duration of magmatism as measured by u pb zircon and monazite ages increases from 1 2 ma in the west to 4 5 ma in the east the duration of ar ar biotite cooling ages also increases to the east in agreement with the extended cooling times large regions of greenschist and amphibolite facies metamorphism are present in the eastern segments whereas outside of narrow pluton aureoles high temperature metamorphism is not present in the western sanak baranof belt finally intrusions with adakitic characteristics such as high sr y ratios are present in the east but not in the west suggesting that some degree of slab melting occurred in the eastern sanak baranof belt we suggest that the above east west thermal and magmatic variations can be explained by a decreasing rate of subduction and a persistent slab window in the eastern sanak baranof belt caused by the oblique subduction of a segmented spreading ridge along a curved continental margin and that this model is compatible with the resurrection plate hypothesis

structural evolution of a transpression zone in north central newfoundland abstract a dextral transpression zone in notre dame bay north central newfoundland was active in middle to late silurian times following the closure of a marginal basin the deformation in the transpression zone was complex and involved several generations of structures deformation was initially distributed over wide low strain zones through the formation of a penetrative regional cleavage with steepening and overturning of the strata the orientation of the sequence was favorable for strike slip bedding parallel movement and the deformation became more efficiently accommodated by shear within the incompetent lithologic horizons forming localized shear zones in high strain zones the sheared incompetent horizons define a belt of melange the boones point complex centered on the contact between a volcanic island arc terrane to the north and a marginal basin terrane to the south two cleavages with similar orientation and morphology were generated at different times during the structural evolution of the transpression zone one of the cleavages is regional in distribution and is transposed by the shear zones folds with no axial planar cleavage but a cleavage cutting obliquely across their hinges are reported they have the geometry of transected folds but are clearly the result of overprinting by a younger generation of cleavage

proximal to distal sedimentological variation in a linear turbidite trough implications for the fan model determination of geographically dependent sedimentological variation proximality in ancient flysch deposits formed an important part of early turbidite studies attempts to quantify this variation highlighted anomalies which were neatly resolved by application of vertical sequence analysis and the use of fan models however there are many turbidite formations such as the lower cretaceous cumberland bay formation cbf of south georgia which cannot be described in terms of existing fan models but show strong proximal to distal sedimentological changes the cbf is a thick sequence of volcaniclastic sandstone turbidites deposited in a linear back arc basin principally by currents flowing wnw parallel to the basin margin four lithofacies associations are recognized on the basis of sandstone shale ratio the two finergrained associations are constant in character across the cbf outcrop in the coarse grained associations there is a change in character wnw down the palaeocurrent direction this is brought out by decreasing sandstone bed thickness and percentage amalgamation but these changes are not always marked or consistent in contrast the internal character of the sandstone beds changes strongly with a marked proportional increase in tb and tc divisions within the bed downcurrent the evidence suggests that the system was aggradational rather than progradational tectonic control of the basin margins prevented major migration of the depositional system and most areas remained in the same position relative to source through time comparison of the cbf with other turbidite formations suggests two end member states which will produce radically different vertical sequences progradational systems will produce strong vertical facies changes where beds deposited in distal environments are overlain by beds deposited in environments progressively nearer source however there will be no lateral change in the character of any particular facies type in contrast in aggradational systems the major sedimentological variation will be lateral rather than vertical

temporal evolution of granitic magmas in the luanchuan metallogenic belt east qinling orogen central china implications for mo metallogenesis abstract the luanchuan metallogenic belt located within the eastern part of the qinling orogen central china hosts a number of world class mo deposits that are closely related to small late mesozoic granitic plutons zircon u pb dating of distinct plutons in the luanchuan metallogenic belt has yielded ages of 153 1 154 2 152 2 and 148 1 ma molybdenite re os isotopic compositions of yuku ore district in the southern part of luanchuan metallogenic belt has yielded an isochron age of 146 1 ma which is consistent with the large scale mineralization ages in the northern part of the luanchuan metallogenic belt a combination of previous studies and new geochronological and isotopic data show a concordant temporal and genetic link between granitic magmatism and mo mineralization in the luanchuan metallogenic belt suggesting that this mineralization episode formed the most extensive mo mineralization belt in the east qinling orogen zircon grains from mo related granitic plutons show similar trace element distributions high precision multi collector inductively coupled plasma mass spectrometry mc icp ms pb isotope analysis of k feldspar megacrysts from mineralization related granites suggest that they were derived from the lower crust similarly the pb isotopic compositions of pyrite coprecipitated with molybdenite also suggest that the metals were derived form the lower crust with probably minor mantle contribution a continuum mineralization model that describes the sourcing of mo from an evolving granitic magma over successive differentiation events possibly in separate but connected magma chambers could explain the remarkable mo enrichment in the luanchuan metallogenic belt the volatile and mo bearing granitic magmas ascended as diapirs from the deep crust and were emplaced as dikes in the upper crust lithological differences between these mo bearing granites may relate to different stages in the evolution of individual magmas finally ore forming fluids were exsolved from the granitic melts in shallow crustal magma chambers resulting in the formation of the mo deposits of the luanchuan metallogenic belt this model may account for the widespread late mesozoic mo deposits of eastern china

the specific case of the mid proterozoic rapakivi granites and associated suite within the context of the columbia supercontinent abstract rapakivi granites and the associated suite anorthosite mangerite and charnockite that developed during the mid proterozoic escape the global rules that control usual granitic magma formation about 40 points that characterise the proterozoic magmatism confirm the originality of the magmatism rapakivi granites developed without a global orogenic context within the supercontinent columbia the intrusions extend from the east european shield to the western us through fennoscandia greenland and labrador other occurrences in the amazonian shield australia or south china are less well documented they show no definite trend in age or chemistry that would explain large scale mantle plume effects a mantle upwelling of material is contradictory with the re assembly of a supercontinent because it occurred before this magmatic episode 1 9 1 8 ga at 1 7 ga the supercontinent columbia amalgamated with the collision of the yapavai and mazatzal provinces a model is suggested that takes into account the supercontinent re assembly defining a downwelling flow in the mantle that anchors the continent above it in contrast to a material flow heat is still delivered to the base of the continental lithosphere and is focused toward the juvenile suture zone the base of the crust must reach 1200 1300 c before producing anorthosite magmas under such a high temperature gradient lasting over a long time magmas are transferred toward the upper crust giving the thin 5 km and square shape 100 km that the intrusions presently have heat delivery is essentially conductive leading to long time spans for intrusions the presence of the supercontinent immobile over a descending cell poloidal mode of convection developed a tangential force toroidal mode of convection that partly split the continent through strike slip deformation due to plate rotation it developed progressively between 1 57 and 1 3 ga starting from fennoscandia and then passing to amazonia western us and labrador in a clockwise sense the associated rotation induced sinistral shear manifested by small scale shear zones and the orientation of late magmatic facies topaz bearing granites in each province the proterozoic magmatism appears to be unique because it requires a supercontinent with a zone of juvenile crust surrounded by older cratons the present moho still shows remnants of this process having bumpy undulations that may reach 22 km in amplitude over a distance of 200 km

silurian llandovery graptolites from the bay of exploits north central newfoundland and their geological significance only one specimen of a silurian graptolite has ever been recorded from newfoundland and it was not identifiable below generic level the graptolite assemblage discussed and figured here from a sequence of turbidites on upper black island north central newfoundland includes the first positively identified silurian taxa from the province and provides the first unequivocal evidence of silurian oceanic sedimentation in the dunnage zone the graptolite taxa include rastrites peregrinus barrande coronograptus sp cf c gregarius lapworth monograptus austerus sequens hutt orthograptus insectiformis nicholson monograptus spp indet climacograptus sp and glyptograptus sp this assemblage demonstrates that the strata previously assigned to the middle ordovician lawrence harbour formation are actually aeronian middle llandovery in age and that the turbidites considerably postdate both the lawrence harbour and point leamington formations of the exploits subzone furthermore the graptoli