potassic dyke swarm in the sapucai graben eastern paraguay petrographical mineralogical and geochemical outlines abstract the western side of the parana basin of brazil extends to central paraguay where repeated and widespread magmatic activity developed from lower cretaceous to oligocene associated with late mesozoic crustal extension trending ne sw in central paraguay this trend is characterized by a zone of nw se normal faults which formed the asuncion sapucai graben up to 45 km wide and 200 km long where alkaline rocks occur as volcanic domes complexes lava flows and dykes these rocks 128 ma aged are dominantly potassic and ne normative a swarm of at least 200 mainley nw se trending dykes occurs in the sapucai region and seems to be formed by two main lineages tephrite to phonolite and peralkaline phonolite and alkali basalt to trachyphonolite they are characterized by ubiquitous diopside to ferrosalite consistently yielding al enrichment trends common olivine fo 81 69 in tephrites and alkaline basalts and up to fo 65 in phonolites zoned megacrysts of hastingsitic hornblende core to kaersutite rim associated with accessory groundmass pargasite in tephrites and phonotephrites k rich hastingsite and k rich ferroan pargasite in the phonolites accessory groundmass mica falls in the annite phlogopite range and consistently yields insufficient si al to satisfy the expected t site occupancy of 8 00 a f u fe ti oxides are ti magnetite rarely ilmenite or haematite phenocrystal i e xenocrystal plagioclase is an 70 20 and an 74 42 in the tephrites and phonolites respectively coexisting groundmass microlites are an 22 14 associated with sodasanidine and sanidine feldspathoids include analcimized leucite and nepheline accessories ti andradite and sphene the two main lineages recognized by distinctive mineralogical variations are consistent with the petrochemical variations complex interaction of discrete and independently evolving magma batches are indicated by intra and or interphase chemical variations suggesting multiple equilibrations of the crystallizing phases under shallow level volcanic pressure regime the observed geochemical trends are quite similar to those of roman region type magma with the same negative anomalies of ta nb zr and ti the most likely mantle source is a garnet peridotite characterized by different enrichment in incompatible elements and which suffered low degree of partial melting 3 7 which has geochemical and isotopic features distinct from those of the adjoining tholeiitic basalts 130 ma and nephelinites 61 39 ma the similarities of the sapucai dyke suite with barton s roman region type magma supports the view that this magma type may not be formed as a result of orogenic and or subduction driven activity in this region therefore a causal relationship of the latter activity with roman region type magma is not supported and remains questionable

evolution characteristics of transtensional faults and their impacts on hydrocarbon migration and accumulation a case study from the huimin depression bohai bay basin eastern china abstract the study of transtensional faults is significantly important for analysing migration and accumulation of hydrocarbons in petroliferous basins the purpose of this study is to present disparities in the temporal and spatial evolution of two boundary transtensional faults northern linshang fault and southern xiakou fault in the huimin depression by 2d structural restoration and quantitative calculations of fault activity this paper also reveals the controls of transtensional faults on the migration and accumulation of hydrocarbons by considering the stress normal to the fault plane p the shale gouge ratio sgr of the fault zone and models of migration pathways the results show that the transtensional systems within the huimin depression were generated from the reactivation of basement faults under an oblique extensional stress field the linshang fault linked two initially separated segments and then behaved as a single fault since the ek period whereas the xiakou fault retained significant three segment growth properties during the paleogene during the hydrocarbon charging period the weak activity and strong vertical sealing restricted the occurrence of vertical migration around the xiakou fault while the relatively strong movement and weak vertical sealing resulted in multilayer hydrocarbon accumulations around the linshang fault meanwhile the vertically closed fault with high sgr 0 6 acted as barriers and trapped hydrocarbons on the lateral migration pathways whereas the vertically closed fault with low sgr

226ra 230th 238u disequilibria of historical kilauea lavas 1790 1982 and the dynamics of mantle melting within the hawaiian plume abstract the geochemical variations of kilauea s historical summit lavas 1790 1982 document a rapid fluctuation in the mantle source and melting history of this volcano these lavas span nearly the entire known range of source composition for kilauea in only 200 yr and record a factor of 2 change in the degree of partial melting in this study we use high precision measurements of the u series isotope abundances of kilauea s historical summit lavas and two ingrowth models dynamic and equilibrium percolation melting to focus on the process of melt generation at this volcano our results show that the 226 ra 230 th 238 u disequilibria of these lavas have remained relatively small and constant with 12 4 excess 226 ra and 2 5 1 6 excess 230 th both are 2 model calculations based mostly on subtle variations in the 230 th 238 u disequilibria suggest that lavas from the 19th to early 20th centuries formed at significantly higher rates of mantle melting and upwelling up to a factor of 10 compared to lavas from 1790 and the late 20th century the shift to higher values for these parameters correlates with a short term decrease in the size of the melting region sampled by the volcano which is consistent with fluid dynamical models that predict an exponential increase in the upwelling rate and thus the melting rate towards the core of the hawaiian plume the pb sr and nd isotope ratios of lavas derived from the smallest source volumes correspond to the kilauea end member of hawaiian volcanoes whereas lavas derived from the largest source volumes overlap isotopically with recent loihi tholeiitic basalts this behavior probably arises from the more effective blending of small scale source heterogeneities as the melting region sampled by kilauea increases in size the source that was preferentially tapped during the early 20th century when the melt fractions were lowest is more chemically and isotopically depleted than the source of the early 19th and late 20th century lavas which formed by the highest melt fractions this inverse relationship between the magnitude of source depletion and melt fraction suggests that source fertility i e lithology controls the degree of partial melting at kilauea thus rapid changes in the size of the melting region sampled by the volcano in the presence of these small scale heterogeneities may regulate most of the source and melting related geochemical variations observed at kilauea over time scales of decades to centuries

segmented lateral dyke growth in a rifting event at barðarbunga volcanic system iceland crust at many divergent plate boundaries forms primarily by the injection of vertical sheet like dykes some tens of kilometres long1 previous models of rifting events indicate either lateral dyke growth away from a feeding source with propagation rates decreasing as the dyke lengthens2 3 4 or magma flowing vertically into dykes from an underlying source5 6 with the role of topography on the evolution of lateral dykes not clear here we show how a recent segmented dyke intrusion in the bar arbunga volcanic system grew laterally for more than 45 kilometres at a variable rate with topography influencing the direction of propagation barriers at the ends of each segment were overcome by the build up of pressure in the dyke end then a new segment formed and dyke lengthening temporarily peaked the dyke evolution which occurred primarily over 14 days was revealed by propagating seismicity ground deformation mapped by global positioning system gps interferometric analysis of satellite radar images insar and graben formation the strike of the dyke segments varies from an initially radial direction away from the bar arbunga caldera towards alignment with that expected from regional stress at the distal end a model minimizing the combined strain and gravitational potential energy explains the propagation path dyke opening and seismicity focused at the most distal segment at any given time and were simultaneous with magma source deflation and slow collapse at the bar arbunga caldera accompanied by a series of magnitude m 5 earthquakes dyke growth was slowed down by an effusive fissure eruption near the end of the dyke lateral dyke growth with segment barrier breaking by pressure build up in the dyke distal end explains how focused upwelling of magma under central volcanoes is effectively redistributed over long distances to create new upper crust at divergent plate boundaries

brazilian structural provinces an introduction abstract the territory of brazil coincides almost entirely with the south american platform the crystalline core of the continent its basement is composed of ancient metamorphic and igneous rocks and it has not suffered any tectonic regeneration since the beginning of the phanerozoic sedimentary rocks with almost horizontal bedding cover this crystalline basement this latter shows ages as old as early precambrian although ages between 500 and 1000 m y are conspicuously frequent the cratonic areas became consolidated more than 1700 m y ago whereas the fold belts formed essentially between 500 and 1700 m y ago the sedimentary cover accumulated from the early silurian in three large intracratonic basins until the platform became completely stabilized a late jurassic to early cretaceous reactivation caused by the break up of the ancient gondwana continent created another basin sequence chiefly along the atlantic continental margin based on the nature of the crystalline basement rocks and the sedimentary cover ten structural provinces can be distinguished 1 rio branco province in the north of the country occupied by the guyana shield and still only poorly known with an important fold belt developed during the so called transamazonian cycle 2000 200m y with high grade metamorphic rocks and only slight influence of later events 2 tapajos province in central brazil corresponding to the amazonas or guaporecraton showing chiefly precambrian crystalline rocks and behaving as an cratonic area during the phanerozoic 3 sa o francisco province located on the atlantic shield with its basement covered by rocks of different ages chiefly affected by the brasiliano cycle between 1000 and 500 m y and constituting another cratonic area 4 tocantins province between the amazonas and sa o francisco cratons with the oldest rocks in its centre ages over 2600 m y and at the eastern and western borders metamorphic sequences of various fold belts and almost no phanerozoic deposits 5 mantiqueira province located along the southern part of the atlantic coast affected chiefly by the brasiliano folding cycle 6 borborema province in the northeast brazilian fold belt affected by the brasiliano cycle in a very complex way and with important faulted zones it was reactivated in phanerozoic times when also sedimentary covers accumulated 7 amazonas province represented by the amazonas sedimentary basin syneclise and subdivided into four parts by three important arcs 8 parnaiba province coinciding with the piauf maranha o syneclise and filled with a rather thick sedimentary sequence 9 paranaprovince the sedimentary basin of southern brazil in which the well known late paleozoic glaciation features are found and which possesses an extensive cover of basaltic rocks of late jurassic to early cretaceous age 10 coastal province and continental margin the youngest structural unit developed during the separation of the continent and represented by rift valleys and coastal basins filled with mesozoic cenozoic deposits of various kinds

pre rift tectonic scenario of the eo cretaceous gondwana break up along se brazil sw africa insights from tholeiitic mafic dyke swarms

polygenetic quaternary volcanism at crater flat nevada abstract alkali basalts erupted during the quaternary at crater flat nevada record a complex history of polycyclic and polygenetic volcanism magmas from the two main centers black cone and red cone are petrographically and geochemically similar although field evidence suggests a number of separate eruptive events high incompatible element concentrations low nb la and high zr y indicate that the magmas were derived by small degrees of partial melting from the lithospheric mantle at red cone a significant range of sr la ce ba and th concentrations is observed with time e g sr range 1308 1848 ppm the youngest samples having the more elevated values however there is only limited variation in the compatible trace elements e g sc and ni the array of compositions at red cone could not have been produced by changes in the degree of partial melting or by fractional crystallization rather a model of magma mixing is proposed between relatively enriched and depleted end members the cluster of black cone data falls consistently at the least enriched end of the red cone sample arrays suggesting that the black cone magma represents one of the mixing end members the modeling indicates that the magmatic plumbing systems of the two centers were linked at least during the early stages of volcanism moreover volcanic activity may have occurred at a number of sites along the length of the magmatic feeder zone during a single eruptive phase this could have significant implications for volcanic hazard assessment in the region around yucca mountain and the proposed nuclear waste repository

reappraisal of the significance of volcanic fields abstract volcanic field is a term commonly used to loosely describe a group of volcanoes often it is implicitly assumed that the volcanoes on a volcanic field are small monogenetic and dominantly basaltic but none of those attributes is indispensable on some definitions of the term actually the term volcanic field can be used to describe a group of purely monogenetic edifices a group of mixed monogenetic and polygenetic edifices or even a group formed only by purely polygenetic edifices differences between each of those alternatives might be important but the extent to which those differences are truly relevant remains still to be explored furthermore there are several limitations on the current knowledge of this type of volcanic activity that explain the lack of a comprehensive effort to study volcanic fields in global contexts in this work issues concerning current definitions of a volcanic field are examined and some criteria that can be used to distinguish volcanic fields from non field volcanoes are suggested special attention is given to the role played by spatial scale on such a distinction also the tectonic implications of their spatial distribution are explored in particular it is shown that volcanic fields are an important component of volcanic activity at a global scale that is closely associated to diffuse plate boundaries and might well be considered the archetypical volcanic form of such tectonic scenarios

transverse versus longitudinal extension in the foredeep peripheral bulge system role of cretaceous structural inheritances during early miocene extensional faulting in inner central apennines belt this study reports on the rocca di cave extensional fault system which is located in the prenestini mountains central italy of the apennines fold and thrust belt and presently represents the oldest exposure of early orogenic foreland fault system of the apennines this fault system was oriented perpendicular to the strike of the foredeep peripheral bulge system indicating that the onset of convergence was marked by a foredeep parallel extension instead of the commonly observed foredeep perpendicular stretching associated with lithosphere flexuring the studied fault system is formed by a 10 km long and 2 km wide e w elongated area that includes two mutually orthogonal sets of faults oriented about e w and n s respectively e w and mainly n s striking faults developed during a preorogenic santonian to campanian in age e w oriented extension kinematics which caused first uplift and erosion and then drowning of the cretaceous carbonate platform and the onset of a pelagic environment both fault sets were later reactivated in the early miocene when the area was forming part of the foreland region ahead of the eastward migrating apennines fold and thrust belt and was undergoing local n s oriented i e foredeep parallel stretching this extensional stage implied the sedimentation of hundreds of meters of synkinematic strata in the northern block of the rocca di cave fault system coevally with episodes of subaerial erosion in areas of the southern block further extensional and right lateral reactivation of inherited faults occurred in the late miocene during the progressive incorporation of the area into the apennines belt

styles of positive inversion tectonics in the central apennines and in the adriatic foreland implications for the evolution of the apennine chain italy abstract integration of new field structural and geophysical data with existing information from the apennines chain in italy and its adjacent adriatic foreland indicates that the styles of positive inversion tectonics and the modes of interaction between the extensional and the subsequent compressive structures vary starting from the cretaceous the contractional deformation induced by the mainly north directed convergence of africa adria with respect to the european plate promoted the closure of various arms of the atlantic and the neo tethys oceans which opened in different times and with distinct orientations the mosaic of continental blocks carbonate platforms rift basins and oceanic domains with several geometries and orientations with respect to the axis of the subsequent compression and the resulting heterogeneities within the shallow sedimentary cover and the overall lithosphere strongly influenced both the structural evolution of the apennine orogenic belt and the intra continental deformation within the adriatic foreland field observations reveal that the steeply e and w dipping mesozoic cenozoic normal faults are systematically decapitated by sub horizontal or gently west dipping thrusts propagating with short cut trajectories pre thrusting normal faults were commonly deformed by later thrusts but little evidence seems to support their entire reactivation as high angle reverse faults this suggests that these shallow and steeply dipping discontinuities were not suitable to be reutilized by the superficial thin skinned thrust faults propagating within the sedimentary cover in contrast presumably late paleozoic and mesozoic w dipping normal faults appear moderately reactivated in the adriatic foreland and strong positive inversion tectonics affect the deeper and buried structural levels of the apennine chain within the latter the syn rift sediments in the hangingwall blocks of the fault bounded basins were totally extruded and generated the strong uplift of the thinned adria continental crust finally the contrasting styles of interactions of the pre existing normal faults with later thrusts i e passive truncation or positive reactivation strictly result from the different evolution of the apennine chain and the combined thin and thick skinned modes of deformation of the stretched lithosphere of the adria plate

magma rates in feeding conduits of different volcanic centres abstract a quasi stationary magma flow rate in asthenospheric and crustal conduits of central type volcanoes and volcanic centres was studied analytically under the following conditions magma rises through cylindrical channels in which the magma temperature does not change with time but the wall rocks are gradually heated the magma rates were calculated for basaltic andesitic and dacitic volcanoes using the continental and oceanic geotherms it follows from these calculations that the magma supply rate may determine the kind of activity of a volcanic centre being constant for large and very active volcanoes intermittent for usual volcanic centres of island arcs or sporadic for volcamic fields clusters of cinder cones and areal volcanism theoretical conclusions are consistent with observational data

comparative velocity structure of active hawaiian volcanoes from 3 d onshore offshore seismic tomography abstract we present a 3 d p wave velocity model of the combined subaerial and submarine portions of the southeastern part of the island of hawaii based on first arrival seismic tomography of marine airgun shots recorded by the onland seismic network our model shows that high velocity materials 6 5 7 0 km s lie beneath kilauea s summit koae fault zone and the upper southwest rift zone swrz and upper and middle east rift zone erz indicative of magma cumulates within the volcanic edifice a separate high velocity body of 6 5 6 9 km s within kilauea s lower erz and upper puna ridge suggests a distinct body of magma cumulates possibly connected to the summit magma cumulates at depth the two cumulate bodies within kilauea s erz may have undergone separate ductile flow seaward influencing the submarine morphology of kilauea s south flank low velocities 5 0 6 3 km s seaward of kilauea s hilina fault zone and along mauna loa s seaward facing kao iki fault zone are attributed to thick piles of volcaniclastic sediments deposited on the submarine flanks loihi seamount shows high velocity anomalies beneath the summit and along the rift zones similar to the interpreted magma cumulates below mauna loa and kilauea volcanoes and a low velocity anomaly beneath the oceanic crust probably indicative of melt within the upper mantle around kilauea s submarine flank a high velocity anomaly beneath the outer bench suggests the presence of an ancient seamount that may obstruct outward spreading of the flank mauna loa s southeast flank is also marked by a large anomalously high velocity feature 7 0 7 4 km s interpreted to define an inactive buried volcanic rift zone which might provide a new explanation for the westward migration of mauna loa s current swrz and the growth of kilauea s swrz

tectonic and magmatic controls on the location of post subduction monogenetic volcanoes in baja california mexico revealed through spatial analysis of eruptive vents post subduction 12 5 ma to less than 1 ma monogenetic volcanism on the baja california peninsula mexico formed one of the densest intra continental areas of eruptive vents on earth it includes about 900 vents within an area 700 km long n s and 70 to 150 km wide w e this study shows that post subduction volcanic activity was distributed along this arc and that modes exist in the volcano distribution indicating that productivity of the magma source region was not uniform along the length of the arc vent clustering vent alignments and cone elongations were measured within eight monogenetic volcanic fields located along the peninsula results indicate that on a regional scale vent clustering varies from north to south with denser spatial clustering in the north on the order of 1 9 10 1 vents km2 to less dense clustering in the south on the order of 7 8 10 2 vents km2 san quintin san carlos jaraguay and santa clara are spatially distinct volcanic fields with higher eruptive vent densities suggesting the existence of individual melt columns that may have persisted over time in contrast the san borja vizcaino san ignacio and la purisima vent fields show lower degrees of vent clustering and no obvious spatial gaps between fields thus indicating an area of more distributed volcanism insight into the lithospheric stress field can be gained from vent alignments and vent elongation measurements within the fields located along the extinct subduction related volcanic arc elongation patterns of cinder cones and fissure fed spatter cones vent clusters and vent alignments trend nw se and n s within the santa clara field located more to the west within the forearc elongation patterns of the same volcanic features trend ne sw these patterns suggest that magmatism was more focused in the forearc and in the northern part of baja california than in its southern region within the extinct arc magma ascent created volcano alignments and elongate cones parallel to nw se to n s oriented tectonic structures in the forearc the existence of n s and ne sw oriented volcanic features indicates a rotation in the stress field orientation compared to the arc

hydrocarbon accumulation model for neogene traps in the chengdao area bohai bay basin china abstract chengdao is an offshore area in the bohai bay basin that contains approximately 25 7 10 8 bbl of oil and gas reserves within the sandstone reservoirs in neogene strata however previous predictions of hydrocarbon accumulation in neogene traps are inaccurate resulting in a current failure rate of 50 when drilling for hydrocarbons in this area to build an improved exploration model for neogene traps we select 92 traps from neogene strata in the chengdao area to quantify the filling degree which is an indicator of hydrocarbon accumulation efficiency the quantified filling degree is based on actual geological and exploration data and differs significantly among various trap types the filling degree of traps also varies significantly with their structural locations and decreases generally from the northwest to the southeast along the chengbei fault zone vertically the filling degree is highly heterogeneous initially increasing from the bottom to the middle of neogene strata and then decreasing towards the top of the strata these neogene hydrocarbon reservoirs are sourced from the paleogene and as they lay vertically away from the source rocks their hydrocarbon enrichment is constrained largely by hydrocarbon migration distance and vertical migration pathways the sealing capacity of faults and cap rocks sandbody orientation and reservoir sedimentary facies determine the maximum column height which in turn affects the amount of hydrocarbon accumulation within these traps a scatter plot analysis of individual controls and volumetric filling for each trap type is compiled using multivariate linear regression analysis to quantify controls and the dominant control of hydrocarbon accumulation is determined

flexural isostasy and uplift of the sierra nevada of california the sierra nevada of california has experienced several kilometers of uplift in the last 10 m y but its present elevation is isostatically compensated by a crustal root that has probably existed since late cretaceous flexural isostasy can reconcile these observations basin and range faulting and extension starting 10 m y ago allowed rapid uplift of an eroded and regionally compensated magmatic arc underlain by an excess crustal root a quantitative model of an unbroken elastic plate buoyed up by the excess root provides an explanation for the estimated 10 m y paleotopography of the range the same model fits the present topography when broken shear and fiber stresses not transmitted at the location of the owens valley fault zone the broken plate model predicts the appropriate amount of westward tilt of the sierra nevada and can explain the pronounced eastward tilt of the white and inyo mountains the position where paleo and present drainages in the sierra cross over in elevation is the most sensitive indicator of the elastic thickness required of the lithosphere an effective elastic thickness of 50 km best fits the observed crossover distances and is consistent with the amount of post 10 m y uplift calculated fiber and shear stresses suggest that actual extension of the crust is more important in permitting the uplift than the mere presence of normal faults part of the residual isostatic gravity anomaly pattern high gravity over the eastern great valley and western sierra nevada low residual gravity over the high sierra may be explained by the continuing flexural support of departure from local isostatic equilibrium the model implies that before 10 m y ago the sierra should have been marked by a negative isostatic gravity anomaly of more than 100 m gal flanked by isostatic gravity highs such large isostatic anomalies have not been observed to date over eroded arcs but the sierra has uniquely low heat flow and presumably therefore high lithospheric strength and the sierran root is at the upper limit of size that could be isostatically suppressed

felsic mineral crystallization trends in differentiating alkaline basic magmas mineral compositional relationships have been studied in a variety of alkaline basic rocks from small highlevel intrusions these small intrusions must have cooled quickly as the phases are all zoned especially those forming the matrix of some of the rocks which seem to have formed under conditions of almost perfect fractional crystallization

seismic structure beneath the rivera subduction zone from finite frequency seismic tomography 1 the subduction zone of western mexico is a unique region on earth where microplate capture and overriding plate disruption are occurring today the young small rivera plate and the adjacent cocos plate are subducting beneath the jalisco block of mexico here we present a p wave tomographic model of the upper mantle to 400 km depth beneath the jalisco block and surrounding regions using teleseismic p waves recorded by the mapping the rivera subduction zone mars and colima volcano deep seismic experiment codex seismic arrays the inversion used 12 188 p wave residuals and finite frequency theory to backproject the 3 d traveltime sensitivity kernels through the model below a depth of 150 km the tomography model shows a clear gap between the rivera and cocos slabs that increases in size with depth the gap between the plates lies beneath the northern part of the colima graben and may be responsible for the location of colima volcano the images indicate that the deep rivera plate is subducting more steeply than does the adjacent cocos plate and also has a more northerly trajection at a depth of about 100 km both the rivera and cocos slabs have increased dips such that the slabs are deeper than 200 km beneath the trans mexican volcanic belt tmvb it is also found that the rivera plate is at roughly 140 km depth beneath the young central jalisco volcanic lineament our images suggest that the rivera plate and westernmost cocos plate have recently rolled back toward the trench this scenario may explain the unusual magmatic activity seen in the tmvb

contrasting tectonic and sedimentary history of the central and eastern parts of the polish carpathian foredeep basin results of seismic data interpretation polish carpathian foredeep basin pcfb is located at the junction between western and eastern european platforms such location caused different evolution of various parts of this basin in the central part of the pcfb growth fault propagation folds related to latest badenian stages of thrusting in this segment of the carpathians developed only minor normal faulting related to miocene reactivation of older basement faults was detected paleovalleys mapped within the mesozoic basement were interpreted as palaeogene tectonically controlled erosional features developed after inversion of the polish trough in the central pcfb post evaporitic sediments were supplied into the foredeep basin from south from the eroded carpathians in the eastern part of the pcfb a system of large normal faults related to miocene flexure induced reactivation of inherited mesozoic faults was identified within the hangingwall of this system reverse faults rotated blocks and basement pop up structures were described their formation was related to combined effect of compressional stresses transferred from the carpathian collision zone and reactivation of older large scale fault caused by both flexural extension and strike slip movements during deposition of the miocene foredeep sediments differential compaction also played an important role gentle onlap of the middle badenian evaporites by post evaporitic sediments towards the nw was interpreted as a result of tectonically driven relative sea level rise and re establishment of marine connections with eastern paratethys

the plate tectonic approximation plate nonrigidity diffuse plate boundaries and global plate reconstructions the central assumption of plate tectonics that plate interiors are rigid remains a useful but uncertain approximation strain rates of stable plate interiors are bounded between 10 i12 10 i11 year i1 and 4 10 i10 year i1 the narrowness of all plate boundaries the other main assumption of plate tectonics as originally conceived is contradicted by many observations both in the continents and in the oceans some diffuse plate boundaries in both continents and oceans exceed dimensions of 1000 km on a side diffuse plate boundaries cover 15 of earth s surface the maximum speed of relative plate motion across any one diffuse plate boundary ranges from 2t o 15 mm year which is faster than some upper bounds on intraplate motion across stable plate interiors 2m m year i1 strain rates in diffuse plate boundaries can be as high as 10 i8 year i1 25 times higher than the upper bound on strain rates of stable plate interiors but 600 times lower than the lowest strain rates across typical narrow plate boundaries the poles of rotation of the plates flanking a diffuse oceanic plate boundary tend to be located in the diffuse boundary which is a consequence of the strong coupling across the boundary

fault growth mechanisms and scaling properties in foreland basin system the case study of monte alpi southern apennines italy abstract the present work focuses on the comprehension of the growth mechanisms and dimensional properties of the pre pliocene monte alpi fault network by mean of integrated field and laboratory analyses after this multidisciplinary approach detailed throw profiles are computed for each fault belonging to the monte alpi network subsequently the dimensional properties of the individual faults are calculated data obtained from this work are discussed in terms of fault growth mechanisms and timing of deformation and reported in a conceptual tectonic evolution model of the messinian foreland basin system data suggest that originally isolated fault segments developed during the earliest stages of foreland basin formation by re activation of a pre existing early cretaceous fault set and formation of an orthogonal one these two fault sets formed a scale independent network differently the same fault sets were also active during its subsequent development forming a scale dependent fault network characterized by t and y shaped fault intersections this deformation controlled the half graben depocenters evolution filled by up to ca 600 meters of terrigenous deposits during the late messinian time

seismic characteristics and identification of negative flower structures positive flower structures and positive structural inversion negative and positive flower structures and positive inverted structures imply specific modes of formation and their distinctive characteristics make them important criteria for the identification of certain structural styles a negative flower structure from the andaman sea consists of a shallow synform bounded by upward spreading strands of a wrench fault that have mostly normal separations paralleling monoclines and oblique en echelon normal faults flank the divergent wrench fault a positive flower structure from the ardmore basin oklahoma consists of a shallow antiform displaced by the upward diverging strands of a wrench fault that have mostly reverse separations en echelon folds are present on either side of this convergent wrench fault flower structures may resemble extensional or contractional fault blocks on individual seismic profiles flower structures are identified by their unique internal fault and fold architecture by their association with a straight and throughgoing zone of deformation and by their lack of a consistent sense of separation or regional upthrown side positive structural inversion at the rambutan oil field south sumatra basin has formed a shallow anticlinorium and has partly uplifted the underlying graben deeper fault segments bounding the graben have retained their normal fault profiles but at shallow levels some of these faults have reverse separations similar changes of separation with depth may be produced by strike slip but sets of faults with listric profiles and nearby blocklike trend patterns identify the inversion as a dip slip event where correlation of reflections is uncertain or deep control is unobtainable positive inverted structures may also resemble contrational fault blocks detached normal fault assemblages and certain negative inverted structures

role of a ductile decollement in the development of pull apart basins experimental results and natural examples abstract traditional models describe pull apart basins as graben or half graben basins with normal or normal oblique slip master faults analogous to death valley california yet many pull aparts are characterized by asymmetric basins with strike slip master faults indicating that not all pull apart basins conform to the simple death valley models we present analogue modelling results that show developmental sequences and structural styles of pull aparts are dramatically different when overburden rides over a ductile horizon and that thickness of the ductile horizon exerts control on basin development in our models synthetic and antithetic strike slip faults control basin geometries while localized normal faulting and local oblique slip on strike slip faults accommodate basin subsidence faults evolve from initial strike slip to normal oblique and normal dip slip to form a system of either isolated sub basins in the case of thick ductile layers or coalescing sub basins in the case of thin ductile layers these results demonstrate distinct differences between non ductile and ductile decollement pull apart structures basin boundaries dominated by normal faults suggest a decollement within or at the base of a non ductile layer similar to death valley california basin bounding faults dominated by strike slip and oblique slip faults indicate basin formation over a ductile layer similar to the gulf of elat aqaba or gulf of paria venezuela and trinidad

alkaline rocks from continental brazil abstract the present literature review on brazilian alkaline rocks stresses mainly the petrographic and geologic aspects at least eight different rock associations are represented agewise four groups are found among southern brazilian outcrops an older one about 240 m y only that is still poorly known the massifs clustered around jacupiranga about 130 m y old a few occurrences 100 110 m y old tunas itapirapua and the youngest and most important one with an age spread from about 80 m y to about 53 m y recent research shows a clear tectonic control both on local and on regional scales of alkaline occurrences in southern brazil alkaline magmatism was set off by pre drift and drift activity and is at its onset geologically related to basaltic activity emplacement of many massifs is also locally controlled by old lineaments e g in the minas gerais goias belt as is emphasized by geophysical data on a petrographic and chronologic basis several alkaline provinces are distinguished in southern brazil preliminary data on the cambrian precambrian rocks from southern bahia state a province with undersaturated syenites and litchfieldites and from cratonic areas in northern brazil are also presented

structural evolution of horst and half graben structures proximal to a transtensional fault system determined using 3d seismic data from the shipwreck trough offshore otway basin australia abstract using a three dimensional 3d seismic reflection dataset from the shipwreck trough offshore otway basin southern australia we aim to characterise and understand the structural evolution of the shipwreck fault zone sfz and associated extensional structures the sfz is a key structural element of the otway basin which is a nw striking upper jurassic cenozoic rift to passive margin basin that formed due to the breakup of australia and antarctica the sfz marks the transition from the major southern margin rift system to the west and the oceanic continental transform margin to the se the sfz formed and bounds the shipwreck trough on the eastern side and is interpreted as a n s striking transtensional fault zone representing the northern en echelon extension of the transform margin to the south the shipwreck trough is the downward thrown western side of the sfz and is host to an array of horst and half graben structures two of which contain the producing geographe and thylacine gas fields the shipwreck trough and sfz are imaged by the investigator 3d seismic dataset which has previously been studied to conduct 2d restorations and interpret the timing magnitude orientation and nature of the structural events of the shipwreck trough our study adds to this previous research by using spectral decomposition and coherence volumes to further characterise the sfz and associated igneous features basement structural elements and late cretaceous horst and half graben structures we have identified examples of releasing bend releasing jog and restraining jog structures along the sfz that are indicative of sinistral transtensional deformation and have highlighted areas of increased basement fault block relief which have driven extensional faulting in the cover we have also conducted throw distance and throw depth analysis on four horst and half graben structures and shown that the associated normal faults have had continuous late cretaceous growth with the structures being formed through incidental linkage of normal fault segments finally our two way time twt and isochronal mapping of the entire 3d survey shows the development of the late cretaceous rifting event in the shipwreck trough and highlights numerous other structural closures similar to and with close vicinity of the thylacine and geographe gas fields providing implications for prospectivity

an overview of the petroleum system of maracaibo basin the geologically complex maracaibo basin in northwestern venezuela is one of the most prolific hydrocarbon basins in the world having a basinal area of 50 000 km2 19 300 mi2 the basin has produced more than 30 billion bbl of oil with estimated recoverable oil reserves of more than 44 billion bbl the central elements of the petroleum system of the basin include 1 a world class source rock upper cretaceous la luna formation deposited on a shelf to slope environment under anoxic conditions and modified by intermittent oxygenated periods and tectonic events 2 high quality clastic reservoir rocks deposited in eocene and miocene fluviodeltaic settings 3 two main periods of rapid tectonic subsidence responsible for two pulses of voluminous hydrocarbon generation first during paleogene caribbean south american oblique plate collision and second during the neogene uplift of the sierra de perij mrida andes and 4 lateral and vertical migration of oil along strike slip normal and inverted faults as well as a regional unconformity of late eocene oligocene age the maturation migration and trapping of hydrocarbons were closely controlled by the tectonic evolution of the maracaibo basin during the paleogene the development of a foredeep along the northeastern margin of the basin and the strike slip reactivation of the rift related jurassic faults on the maracaibo platform controlled the early structural setting of the source and reservoir rocks hydrocarbons migrated updip from source rocks beneath the north northeastern margin of the basin along north south strike slip faults and into overlying eocene clastic reservoirs in the south central parts of the basin the second period of the maracaibo basin petroleum system developed during subaerial exposure of most of the maracaibo basin during oligocene miocene uplift of the adjacent sierra de perij and mrida andes uplift of mountain ranges surrounding the basin folded and depressed the interior of the basin to form the extensive maracaibo syncline because of the formation of the maracaibo syncline oil generation began in the central and southern parts of the synclinal basin and migrated northward hydrocarbons migrated up the flanks of the maracaibo syncline along reactivated strike slip faults and into miocene rocks adjacent to the uplifted mountain ranges escaping oil has formed numerous surface seeps along the edges of the maracaibo basin

ilchulbong tuff cone jeju island korea revisited a compound monogenetic volcano involving multiple magma pulses shifting vents and discrete eruptive phases ilchulbong sunrise peak tuff cone is a united nations educational scientific and cultural organization unesco world heritage site that owes its scientific importance to the outstanding coastal exposures that surround it it is a type locality for the sedimentary evidence of pyroclastic transport and depositional processes that occur during phreatomagmatic basaltic eruptions its classically shaped cone morphology was long considered to result from a single short lived eruption reanalysis of the sedimentary sequence has now revealed two subtle paraconformities in the deposits indicating that the eruption occurred in three separate phases the break between phases i to ii was long i e days to weeks during which erosion and compaction of the lower cone occurred leading to a volcanowide discontinuity surface and reworked deposits in the lower flanks possibly following cooling and solidification of the initial shallow conduit system the phase ii eruption resumed from a new vent location 600 m away before being terminated by a very brief interval and final eruption of phase iii from the same general site the second pause led to a more subtle discontinuity marked by differences in deposit character above and below it and it was associated with a period of seismicity induced slumping and slipping of deposits detailed geochemical study of the juvenile clasts through the sequence reveals that three separate alkali basaltic magma pulses were erupted corresponding directly to the three eruption phases magmas 1 and 3 may be genetically related with the former showing evidence for longer periods of shallow level fractionation whereas magma 2 originated from a slightly different mantle domain although at a similar depth these results demonstrate that breaks in monogenetic eruption sequences may represent deep magma system processes and they raise fundamental questions concerning the mechanisms of triggering and eruption of multiple magma pulses in small volume basaltic volcanoes this study shows the importance of recognizing sedimentary evidence of time breaks in such eruption sequences as potential indications of shifts in eruption chemistry and vent location these have important implications for the controls of vent migration at other monogenetic volcanoes and for hazard planning during future similar eruptions

seismic constraints on magma chambers at hekla and torfajokull volcanoes iceland hekla and torfajokull are active volcanoes at a rift transform junction in south iceland despite their location next to each other they are physically and geologically very different hekla is an elongate stratovolcano built mainly of basaltic andesite torfajokull is a prominent rhyolitic centre with a 12 km diameter caldera and extensive geothermal activity the scope of this study is to examine the propagation of body waves of local earthquakes across the hekla torfajokull area and look for volumes of anomalous s wave attenuation which can be evidence of magma chambers so far the magma chamber under hekla has been modelled with various geophysical means and its depth has been estimated to be 5 9 km a data set of 118 local earthquakes providing 663 seismic rays scanning hekla and torfajokull was used in this study the major part 650 seismograms did not show evidence for s wave attenuation under these volcanoes only six seismograms had clear signs of s wave attenuation and seven seismograms were uncertain cases the data set samples hekla well at depths of 8 14 km and south part of it also at 4 8 km and 14 16 km western torfajokull is sampled well at depths of 4 14 km eastern and southern torfajokull at 6 12 km conclusions cannot be drawn regarding the existence of magma beyond these depth ranges also magma volumes of smaller dimensions than about 800 m cannot be detected with this method if a considerable molten volume exists under hekla it must be located either above 4 km or below 14 km the former possibility seems unlikely because hekla lacks geothermal activity and persistent seismicity usually taken as expressions of a shallow magma chamber an aseismic volume with a diameter of 4 km at the depth of 8 km in the west part of torfajokull has been inferred in earlier studies and interpreted as evidence for a cooling magma chamber our results indicate that this volume cannot be molten to a great extent because s waves travelling through it are not attenuated intense geothermal activity and low frequency earthquakes are possibly signs of magma in the south part of torfajokull but a magma chamber was not detected there in the areas sampled by this study

structural interpretation of hydrocarbon traps sealed by basement normal block faults at stable flank of foredeep basins and at rift basins closures sealed by basement normal block faults are most commonly formed by a grid of faults with multiple orientations and by a favorable dip of the reservoir strata hydrocarbons are trapped updip by structural upturns along the edge of the downdropped blocks or at the upthrown or downthrown edge of blocks where block tilt is away from the sealing faults closure parallel with reservoir strike is provided by 1 updip junction of primary oblique or oblique and longitudinal block bounding faults 2 internal secondary faults that trend transverse or oblique to the block boundaries 3 positive bend of reservoir strata 4 plunge of block edge folds and or 5 stratigraphic terminations of the reservoir fault seal upward along the faults is most likely where a thick section of impervious strata above the reservoir is in contact with itself across the fault or where the fault does not extend upward into shallower permeable zones seal across the fault surface is most certain where impermeable rocks are laterally juxtaposed against the reservoir impervious material within the fault zone such as shaly smear gouge cataclastic gouge mineral deposits and asphalt or tar impregnations can be effective barriers where reservoir facies are juxtaposed across the fault the traps can form a major hydrocarbon play on the stable shelves of foredeep or foreland basins as demonstrated by the san joaquin valley and eastern venezuela basins the traps provide secondary prospects in rift basins uch as the viking graben and vienna basin

fissural volcanism polygenetic volcanic fields and crustal thickness in the payen volcanic complex on the central andes foreland mendoza argentina shield volcanoes caldera bearing stratovolcanoes and monogenetic cones compose the large fissural payen volcanic complex located in the andes foreland between latitude 35 s and 38 s the late pliocene pleistocene and recent volcanic activity along e w trending eruptive fissures produced basaltic lavas showing a within plate geochemical signature the spatial distribution of fractures and monogenetic vents is characterized by self similar clustering with well defined power law distributions vents have average spacing of 1 27 km and fractal exponent d 1 33 defined in the range 0 7 49 3 km the fractal exponent of fractures is 1 62 in the range 1 5 48 1 km the upper cutoffs of fractures and vent fractal distributions about 48 49 km scale to the crustal thickness in the area as derived from geophysical data this analysis determines fractured media crust thickness associated with basaltic retroarc eruptions we propose that the payen volcanic complex was and is still active under an e w crustal shortening regime

mineralogical petrological and geochemical aspects of alkaline and alkaline carbonatite associations from brazil abstract a general description of mesozoic and tertiary fortaleza brazilian alkaline and alkaline carbonatite districts is presented with reference to mineralogy petrology geochemistry and geochronology it mainly refers to scientific results obtained during the last decade by an italo brazilian research team alkaline occurrences are distributed across brazilian territory from the southern piratini rio grande do sul state to the northeastern fortaleza ceara state regions and are mainly concentrated along the borders of the parana basin generally coinciding with important tectonic lineaments the most noteworthy characteristics of these alkaline and alkaline carbonatite suites are i prevalence of intrusive forms ii abundance of cumulate assemblages minor dunites frequent clinopyroxenites and members of the ijolite series and iii abundance of evolved rock types many data demonstrate that crystal fractionation was the main process responsible for magma evolution of all brazilian alkaline rocks a hypothesis is proposed for the genesis of carbonatite liquids by immiscibility processes the incidence of ree and trace elements for different major groups of lithotypes belonging both to carbonatite bearing and carbonatite free districts are documented sr and preliminary nd isotopic data are indicative of a mantle origin for the least evolved magmas of all the studied occurrences mantle source material and melting models for the generation of the brazilian alkaline magma types are also discussed

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

sm nd isotopic investigation of neoproterozoic and cretaceous igneous rocks from southern brazil a study of magmatic processes abstract nd evolutionary paths for diversified igneous suites from southern brazil are here re evaluated using published results we interpret the end paths considering the secondary fractionation of 147sm 144nd due to major petrogenetic processes the inclusion of nd isotopes and geochemical data for precambrian and mesozoic basic rocks allow improving the discussion on the subcontinental lithosphere beneath southern brazil late neoproterozoic rocks mostly granitoids are exposed in two regions of the southern brazilian shield an eastern collisional belt and a western foreland the latter included two geotectonic domains amalgamated at this time the sao gabriel arc 900 700 ma and the taquarembo cratonic block magma genesis mainly involved mixture of crustal and incompatible element enriched mantle components both with a long residence time continental segments are the neoarchaean paleoproterozoic lower crust ca 2 55 ga in the western foreland and paleoproterozoic neoproterozoic recycled crust 2 1 0 8 ga in the collisional belt granitoids with a single crustal derivation are limited in the southern brazilian shield mixing processes are well registered in the western foreland where the re enriched old mantle was probably mixed with a 900 700 ma old subducted lithosphere and a 2 55 ga old lower crust the contribution of the latter increased from the early 605 580 ma to the later 575 550 ma neoproterozoic events which may be due either to crustal thickening or to delamination of the lithosphere magma sources were diversified in the 660 630 ma collisional belt initially they involved the mixing between two components with similar nd isotopic ratios a 2 1 0 8 ga old recycled crust and a subduction processed old mantle regional heating and abundant production of granitic melts with diversified contribution of enriched mantle components mark the end of the collisional period at 630 580 ma we can also attribute this to the delamination of the lithosphere so that the same geodynamic process may explain the magmatism in the whole shield at the end of the dom feliciano orogeny mesozoic rocks include flood basalts from the cretaceous parana province and sub coeval alkalic suites multiple processes of metasomatism affected the lithospheric mantle resulting in some complexity but they mainly register two enriched mantle components both generated during neoarchaean paleoproterozoic events one end member has a more pronounced subduction signature the other one probably resulted from the re enrichment of the first component at the end of the camboriu collisional orogeny 2 0 ga

location and orientation of eruptive fissures and feederdykes at mount etna influence of gravitational and regional tectonic stress regimes abstract lava extrusion from fissures on the flanks of the etna volcano is a common feature of recent activity as evidenced by the eruptions of 1971 1981 1983 and 1985 and was of similar importance during prehistoric times we propose here a model within which the location and orientations of the large majority of historic eruptive fissures on etna s flanks together with their feeder dykes are explained within the framework of a gravitational stress regime stress orientations within this regime are controlled largely by the gross morphology of the volcano but are significantly modified by local topographic anomalies we propose that the gravitational stress regime largely supercedes an earlier developed regional tectonic stress regime which still operates in the sub volcanic basement and may remain the dominant control of eruptive fissure and feeder dyke disposition at lower altitudes on the flanks the proposed model is supported by 1 detailed examination of fissure locations and orientations and the relationship between fissure azimuths and topography 2 precise horizontal distance measurement and vertical ground deformation studies of a recent eruption and 3 qualitative physical modelling experiments in which coloured water is injected into solidified gelatine solutions representing variously shaped volcanic constructs a cautious approach is advocated when extrapolating trends of spatially limited fissure segments for the purpose of locating potential sites of magma ascent and implications of the model for hazard analysis are outlined

petrogenesis of coexisting sio2 undersaturated to sio2 oversaturated felsic igneous rocks the alkaline complex of itatiaia southeastern brazil abstract the itatiaia alkaline complex is a late cretaceous intrusion 72 myr made up of felsic differentiates with syenitic rocks dominant throughout and with presence of both nepheline and quartz rich varieties dykes with phonolitic or trachytic composition cross cut the coarse grained facies the rocks are arranged concentrically with the core of the complex being formed by sio 2 oversaturated syenites with a small outcrop of granites and are radially displaced by faults related to regional tectonic lineaments the minerals show gradual but significant changes in composition salitic and augitic to aegirine rich pyroxenes hastingsite and actinolite to richterite and arfvedsonite amphiboles sodic plagioclase to orthoclase feldspars and so on and the whole rock trends are broadly consistent with fractional crystallization processes dominated by alkali feldspar removal sr isotopic data indicate more radiogenic ratios for the sio 2 oversaturated rocks 0 7062 0 7067 against 0 7048 0 7054 for the sio 2 undersaturated syenites consistent with small amounts of crustal input the favored hypothesis for the petrogenesis of the different syenitic groups is the prolonged differentiation starting from differently sio 2 undersaturated mafic parental magmas potassic alkali basalts to ankaratrites present in the late cretaceous dyke swarms of the area accompanied by variable crustal contamination prior to the final emplacement the lack of carbonatite as a significant lithotype the potassic affinity of the itatiaia complex and the relatively high sr isotopic ratios match the characteristics of the other complexes of the rio de janeiro sa o paulo states coastline and confirm the ultimate derivation of these differentiated rocks from an enriched lithospheric mantle source

evaluation of geological factors in characterizing fault connectivity during hydrocarbon migration application to the bohai bay basin faults play an intricate role in hydrocarbon migration and accumulation since they can serve either as a conduit or a seal quantitative evaluation of fault opening sealing properties requires the selection of valid and optimal parameters among numerous geological factors to characterize the hydraulic behaviors of faults the present study focuses on the chengbei step fault area in the qikou depression bohai bay basin ne china because hydrocarbon migration and accumulation in this area occurred in a relatively short period so that accumulated hydrocarbons can be used as an indicator to deduce hydraulic connectivity of a fault zone between two sites various geological parameters pertinent to a fault such as burial depth dip angle throw strike percentage of sandstone of faulted intervals fluid pressure in faulted mudstone stress normal to the fault plane and shale gouge ratio are analyzed to assess their effectiveness in characterizing fault connectivity an index the fault connectivity probability np is proposed to evaluate the possibility that a fault has been once serving as a migration pathway the statistical relationship between np and any a geological parameter may be used to indicate the effectiveness of this parameter in characterizing the connectivity of a fault during hydrocarbon migration the correlation coefficient of a relationship is a good indicator of the effectiveness and the results are generally in agreement with qualitative assessments parameters representing a single geological factor are generally ineffective whereas those representing implicitly or explicitly two or more factors such as shale gouge ratio stress normal to the fault plane and fault opening index are more effective

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

a model for calculating eruptive volumes for monogenetic volcanoes implication for the quaternary auckland volcanic field new zealand abstract monogenetic basaltic volcanism is characterised by a complex array of behaviours in the spatial distribution of magma output and also temporal variability in magma flux and eruptive frequency investigating this in detail is hindered by the difficulty in evaluating ages of volcanic events as well as volumes erupted in each volcano eruptive volumes are an important input parameter for volcanic hazard assessment and may control eruptive scenarios especially transitions between explosive and effusive behaviour and the length of eruptions erosion superposition and lack of exposure limit the accuracy of volume determination even for very young volcanoes in this study a systematic volume estimation model is developed and applied to the auckland volcanic field in new zealand in this model a basaltic monogenetic volcano is categorised in six parts subsurface portions of volcanoes such as diatremes beneath phreatomagmatic volcanoes or crater infills are approximated by geometrical considerations based on exposed analogue volcanoes positive volcanic landforms such as scoria spatter cones tephras rings and lava flow were defined by using a light detection and ranging lidar survey based digital surface model dsm finally the distal tephra associated with explosive eruptions was approximated using published relationships that relate original crater size to ejecta volumes considering only those parts with high reliability the overall magma output converted to dense rock equivalent for the post 250 ka active auckland volcanic field in new zealand is a minimum of 1 704 km3 this is made up of 1 329 km3 in lava flows 0 067 km3 in phreatomagmatic crater lava infills 0 090 km3 within tephra tuff rings 0 112 km3 inside crater lava infills and 0 104 km3 within scoria cones using the minimum eruptive volumes the spatial and temporal magma fluxes are estimated at 0 005 km3 km2 and 0 007 km3 ka the temporal volumetric evolution of auckland is characterised by an increasing magma flux in the last 40 ky which is inferred to be triggered by plate tectonics processes e g increased asthenospheric shearing and backarc spreading of underneath the auckland region

three dimensional seismic velocity structure beneath aso volcano kyushu japan tomographic results for p and s wave velocity structure beneath the active aso volcano kyushu japan using 800 well recorded earthquakes and ten shots recorded by an eight station seismic network are presented a 68 variance reduction was achieved upon simultaneous inversion for hypocenter and velocity structure well resolved velocity anomalies associated with the active crater reveal heterogeneity up to 26 slower and 18 faster in p velocity and up to 31 slower and 22 faster in s velocity than the one dimensional model the largest anomaly is seen over the upper 11 km in the central and northern parts beneath the central cones two low velocity regions are imaged the first region a 10 15 km region encompassing the upper 3 km centered near the caldera wall at tateno valley is characterized by p velocities up to 19 slower 20 for s the second low velocity region is associated with the central cones and active magma conduit system at 6 km depth velocities as low as 4 3 km s up to 26 in p and 2 km s 31 slower in s characterize the 7 km wide volume the magma chamber is roughly spherical in shape centered at 6 km depth flattens at 10 km depth and is located between mt kishima mt eboshi and mt naka the present focus of magmatism a sharp velocity contrast at the depth of 3 km with high velocities to the southwest and lower velocities to the northeast characterizes different abutting structures associated with the oita kumamoto tectonic line

spatial distribution of cones and satellite detected lineaments in the pali aike volcanic field southernmost patagonia insights into the tectonic setting of a neogene rift system abstract the relationships between the distribution and morphometric features of eruptive structures scoria and spatter cones maar tuff rings and the fracture network were investigated in the pliocene quaternary pali aike volcanic field southernmost patagonia argentina chile the alkali basaltic basanitic magmas which erupted in this area have nearly primary magma compositions and often bear mantle xenoliths hence magma ascent from deep seated reservoirs was probably very fast with no significant stagnation at crustal levels field surveys and satellite image analysis led to the identification of up to 467 eruptive structures and four main nw se ne sw e w and n s fracture systems the spatial distribution of eruptive cones and fractures was investigated through the computation of power law exponents df for self similar clustering the self similarity of cones and fractures was defined between lower and upper cut offs which were in turn related to the thickness of the fractured mechanical layer the fractal character of cones and fracture distribution clustering in the pali aike volcanic field area was thus correlated with crustal thickness the self similarity of fractures was used to establish the relative chronology of the detected fracture systems the self similar clustering exponent is highest in the e w and nw se fracture systems df 1 78 and 1 77 respectively and lowest in the n s system df 1 65 the self similar clustering of eruptive structures is well defined df 1 45 the intense volcano tectonic activity in the pali aike area marks a major pliocene quaternary phase in the development of the magellan neogene rift system

distinct element modeling of structures formed in sedimentary overburden by extensional reactivation of basement normal faults fault and fold patterns in sedimentary rocks have been modeled using a computer code based on the distinct element method dem this numerical method considers the relative motion of disconnected elastic elements governed by newton s laws and frictional sliding criteria forces are transmitted between elements at contact points by changing element size ratios assemblages of elements can simulate different kinematic behavior from localized faulting to distributed flow as an application of the method we investigate folding and faulting patterns that develop in the sedimentary overburden above basement normal faults the number position orientation and sense of slip on faults as well as the fold geometry depend on the basement fault dip and basement rotation the dem model results have practical implications for predicting the evolving characteristics of structural traps and their petroleum potential

are flood basalt eruptions monogenetic or polygenetic a fundamental classification of volcanoes divides them into monogenetic and polygenetic we discuss whether flood basalt fields the largest volcanic provinces are monogenetic or polygenetic a polygenetic volcano whether a shield volcano or a stratovolcano erupts from the same dominant conduit for millions of years excepting volumetrically small flank eruptions a flood basalt province built from different eruptive fissures dispersed over wide areas can be considered a polygenetic volcano without any dominant vent however in the same characteristic a flood basalt province resembles a monogenetic volcanic field with only the difference that individual eruptions in the latter are much smaller this leads to the question how a flood basalt province can be two very different phenomena at the same time individual flood basalt eruptions have previously been considered monogenetic contrasted by only their high magma output and lava fluidity with typical small volume monogenetic volcanoes field data from hawaiian shield volcanoes iceland and the deccan traps show that whereas many feeder dykes were single magma injections and the eruptions can be considered large monogenetic eruptions multiple dykes are equally abundant they indicate that the same dyke fissure repeatedly transported separate magma batches feeding an eruption which was thus polygenetic by even the restricted definition the same magma conduit this recognition helps in understanding the volcanological stratigraphic and geochemical complexity of flood basalts the need for clear concepts and terminology is however strong we give reasons for replacing monogenetic volcanic fields with diffuse volcanic fields and for dropping the term polygenetic and describing such volcanoes simply and specifically as shield volcanoes stratovolcanoes and flood basalt fields

deep magma transport at kilauea volcano hawaii the shallow part of kilauea s magma system is conceptually well understood long period and short period brittle failure earthquake swarms outline a near vertical magma transport path beneath kilauea s summit to 20 km depth a gravity high centered above the magma transport path demonstrates that kilauea s shallow magma system established early in the volcano s history has remained fixed in place low seismicity at 4 7 km outlines a storage region from which magma is supplied for eruptions and intrusions brittle failure earthquake swarms shallower than 5 km beneath the rift zones accompany dike emplacement sparse earthquakes extend to a decollement at 10 12 km along which the south flank of kilauea is sliding seaward this zone below 5 km can sustain aseismic magma transport consistent with recent tomographic studies long period earthquake clusters deeper than 40 km occur parallel to and offshore of kilauea s south coast defining the deepest seismic response to magma transport from the hawaiian hot spot a path connecting the shallow and deep long period earthquakes is defined by mainshock aftershock locations of brittle failure earthquakes unique to kilauea whose hypocenters are deeper than 25 km with magnitudes from 4 4 to 5 2 separation of deep and shallow long period clusters occurs as the shallow plumbing moves with the volcanic edifice while the deep plumbing is centered over the hotspot recent gps data agrees with the volcano propagation vector from kauai to maui suggesting that pacific plate motion azimuth 293 58 and rate of 7 4 cm yr has been constant over kilauea s lifetime however volcano propagation on the island of hawaii azimuth 3258 rate 13 cm yr requires southwesterly migration of the locus of melting within the broad hotspot deep long period earthquakes lie west of the extrapolated position of kilauea backward in time along a plate motion vector requiring southwesterly migration of kilauea s magma source assumed ages of 0 4 my for kilauea and 0 8 my for mauna loa are consistent with this model younger ages would apply if kilauea began its growth south of the locus of maximum melting as is true for loihi seamount we conclude that kilauea is fed from below the eastern end of the zone of deep long period earthquakes magma transport is vertical below 30 km then sub horizontal following the oceanic mantle boundary separating plagioclase and spinel peridotite then near vertical beneath kilauea s summit the migration of the melting region within the hotspot and kilauea s sampling of different sources within the melting region can explain 1 the long term geochemical separation of

monogenetic volcanism personal views and discussion monogenetic volcanism produces small volume volcanoes with a wide range of eruptive styles lithological features and geomorphic architectures they are classified as spatter cones scoria or cinder cones tuff rings maars maar diatremes and tuff cones based on the magma water ratio dominant eruption styles and their typical surface morphotypes the common interplay between internal such as the physical chemical characteristics of magma and external parameters such as groundwater flow substrate characteristics or topography plays an important role in creating small volume volcanoes with diverse architectures which can give the impression of complexity and of similarities to large volume polygenetic volcanoes in spite of this volcanic facies complexity we defend the term monogenetic volcano and highlight the term s value especially to express volcano morphotypes this study defines a monogenetic volcano a volcanic edifice with a small cumulative volume typically 1 km3 that has been built up by one continuous or many discontinuous small eruptions fed from one or multiple magma batches this definition provides a reasonable explanation of the recently recognized chemical diversities of this type of volcanism

geochemistry of the 1984 mauna loa eruption implications for magma storage and supply the 1984 eruption of mauna loa volcano hawaii produced about 220 106 m3 of lava of remarkably homogeneous composition no significant compositional changes occurred during the 22 day eruption and along the 20 km vent system even though crystallinity increased substantially accompanying a decline in the eruption rate such homogeneity in lava composition is typical of many historic mauna loa eruptions the 1984 lava along with most other homogeneous mauna loa lavas is multiply saturated crystallizing olivine clinopyroxene and plagioclase bulk compositions are perched at the intersection of an olivine control trend with a trend for more differentiated lavas there is no evidence to indicate that the 1984 lava is a differentiate of the 1975 magma or that it results from mixing a new batch of magma with differentiated melt remaining in the magmatic plumbing system after the 1975 eruption it is proposed that these lavas are erupted directly from a small continuously replenished magma reservoir located about 3 4 km beneath the summit caldera successive eruptions therefore provide a sampling of magma chamber processes and small but discernible changes in lava composition reflect the competing processes of magma supply differentiation and eruption

elliptical boundary of an intraplate volcanic field auckland new zealand abstract the outer vents in the auckland volcanic field lie within 19 to 559 m mean 224 75 m of a best fit ellipse with a 28 9 km long major axis trending almost north south and a minor axis 16 5 km long the ellipse has formed the outer boundary of the field since the inception of volcanism 140 000 years ago we present the following testable hypotheses as an explanation of this pattern the boundary is the expression of a corresponding elliptical source area at depth in the lithospheric mantle possibly asthenosphere material trapped at this level the ellipse may represent a depth contour on a very small upper mantle dome or of a lens intruding into a neck of an extensional structure alternatively it could be the boundary of a flat elliptical area where tensional stresses allow decompressional melting the elliptical tensional region may either have developed in a releasing bend during strike slip faulting along a fundamental lithosphere structure inherited from mesozoic tectonics associated with the nnw trending dun mountain ophiolite belt or may represent the tip of a fracture along which the auckland volcanic province is propagating northward

migrating cretaceous eocene magmatismin the serra do mar alkaline province se brazil melts from the deflected trindade mantle plume department of geological sciences university of durham south road durham dh1 3le uk department of earth sciences university of cambridge downing street cambridge cb2 3eq uk department of physics the university newcastle upon tyne ne1 7ru uk department of geology mcmaster university 1280 main street west hamilton ont canada l8s 4m1 departmento de geoquimica e recursos minerais instituto de geociencias universidade de brasilia

graben hydrocarbon occurrences and structural style major hydrocarbon occurrences types of traps and structural styles have been synthesized from the sirte basin the suez and viking grabens and from other normal faulted regions hydrocarbons occur in a stacked succession of one or more basins pregraben graben fill and interior sag preservation of pregraben accumulations depends on late initiation of crustal arching and limitation of uplift to the graben shoulders furthermore a stable pregraben tectonic environment is required to ensure internal continuity of subsequent fault block closures hydrocarbon traps in the pregraben and graben fill sediments are primarily dependent on the multidirectional orientation of normal faults tilting of fault blocks and in many cases flexing or erosion parallel with block edges the fault pattern is dominated by longitudinal faults subparallel with the graben axis and secondarily by oblique faults the direction and degree of block rotation are influenced by fault profile planar or listric degree of extension fault pattern doglegs junctions terminations downwarping of the subsequent sag basin and isostatic adjustments between large blocks fold closures result from the upward termination of faults into forced folds which may be accentuated by drag during renewed faulting passive drape and differential compaction extend he folds to shallower depths because they form along block edges the flexures have trends that mimic the multidirectional fault patterns truncation traps are usually developed from the erosional retreat of fault scarps and they can also mimic the fault patterns where the sedimentary succession is complete an interior sag basin is superimposed across the graben its lower part is structured mostly by drape and differential compaction across the underlying graben fault blocks and fault block topography the upper part is usually an unsegmented syncline and hydrocarbon traps at this level include depositional buildups reefs turbidite mounds and salt diapirs or other structures caused by superimposed deformation unrelated to graben tectonics

petrogenesis and tectonic setting of the roman volcanic province italy abstract the volcanism in the roman province of italy can be modelled by the partial melting of heterogeneously enriched mantle sources the heterogeneity was created by materials derived from a subducted slab which can still be traced geophysically beneath the central apennines new petrographical and chemical data are presented for the high k calc alkaline and the shoshonitic volcanics of the campania region primary magmas are present only locally the existence of spatial zonation in the volcanism of campania is documented for the first time the shoshonitic leucite basanitic and leucititic volcanics of the phlegraean fields procida ischia and the somma vesuvius areas are at similar degrees of evolution about two times richer in nb and ba than those of northwestern campania and the latium part of the roman province accordingly distinct north western and south eastern subprovinces can be defined the evaluation of enrichment factors that is the abundance ratio between the average contents of each element in the relatively primitive lavas of the low k and the high k suites shows that the mantle sources prior to k metasomatism were different in the two sub provinces of the roman province in the north western one they resembled the sources of ocean island tholeiites and moderately enriched mor basalts in the south east they were similar to those of ocean island alkaline lavas and enriched morb s modelling based on k p ce sr rb ba th sm eu gd y nb and 87 sr 86 sr was carried out it indicates that the range of mantle sources of the volcanics in northwestern campania and latium can be modelled by the addition of 3 to 20 of materials derived by partial melting of carbonaceous pelites to a sr enriched mantle wedge comparable to the honolulu mantle source least enriched in nb the production of sr enriched mantle wedge requires either the action of fluids produced by dehydration of subducted oceanic crust or a small amount of metasomatism caused by the presence of carbonatite melts the near absence of ti ta nb yb and the highly fractionated ree in the metasomatizing component requires the presence of residual garnet and accessory ti rich minerals during the partial melting of the subducted sediments the writers propose that the mantle wedge overlying the subducted slab was hybridized by melts produced by partial fusion of subducted material derived from the continental crust probably sediments this process played a dominant role in the generation of the mantle sources from which the high k calc alkaline shoshonitic leucite basanitic and leucititic magmas of the roman province were derived

akaso field nigeria use of integrated 3 d seismic fault slicing clay smearing and rft pressure data on fault trapping and dynamic leakage the akaso field stoiip 78 106 m3 401 million stb of the eastern niger delta is separated from the large cawthorne channel field stoiip 208 106 m3 1307 million stb by a major antithetic counterregional normal fault in 1989 three dimensional 3 d seismic data acquired and interpreted over the field revealed a subtle splinter or backsplit fault off this major antithetic fault west of the main akaso field behind this backsplit fault we observed stacked amplitude anomalies these anomalies extended to the structural spillpoint defined against the akaso backsplit fault fault slicing and clay smear studies of the backsplit and main antithetic fault indicated strong seal potential with possible leak windows along the fault planes in 1989 the sealing potentials of the western end of the akaso boundary fault were fully confirmed by a deviated appraisal well which found a stoiip of 29 106 m3 183 million stb in a series of stacked footwall closures the initial trapping potential of this major antithetic normal fault can be contrasted with its subsequent behavior during depletion by using rft data acquired in a series of development wells drilled in 1990 in the main akaso structure at the eastern end of the antithetic fault here major stacked columns of oil and gas are trapped and sealed by the antithetic fault but we identified evidence of subsequent leakage due to production effects from the adjacent cawthorne channel field depletion of the unproduced akaso g sands was found to be due to production from the juxtaposed e sands of the adjacent cawthorne channel field thus we inferred dynamic fault leakage we observed a dynamic pressure drop of 400 2144 kpa 58 311 psi between the juxtaposed akaso and cawthorne channel sand due to active crossflow the akaso field therefore represents a prime example of the trapping potential of a clay smeared fault and its subsequent leakage during differential depletion our observations methodology and implications described herein may have an important bearing on reservoir management philosophies in similar structural settings

the origin of a large 3 km maar volcano by coalescence of multiple shallow craters lake purrumbete maar southeastern australia lake purrumbete maar is located in the intraplate monogenetic newer volcanics province in southeastern australia the extremely large crater of 3000 m in diameter formed on an intersection of two fault lines and comprises at least three coalesced vents the evolution of these vents is controlled by the interaction of the tectonic setting and the properties of both hard and soft rock aquifers lithics in the maar deposits originate from country rock formations less than 300 m deep indicating that the large size of the crater cannot only be the result of the downwards migration of the explosion foci in a single vent vertical crater walls and primary inward dipping beds evidence that the original size of the crater has been largely preserved detailed mapping of the facies distributions the direction of transport of base surges and pyroclastic flows and the distribution of ballistic block fields form the basis for the reconstruction of the complex eruption history which is characterised by alternations of the eruption style between relatively dry and wet phreatomagmatic conditions and migration of the vent location along tectonic structures three temporally separated eruption phases are recognised each starting at the same crater located directly at the intersection of two local fault lines activity then moved quickly to different locations a significant volcanic hiatus between two of the three phases shows that the magmatic system was reactivated the enlargement of especially the main crater by both lateral and vertical growth led to the interception of the individual craters and the formation of the large circular crater lake purrumbete maar is an excellent example of how complicated the evolution of large seemingly simple circular maar volcanoes can be and raises the question if these systems are actually monogenetic

velocity structure and a seismic model for nevado del ruiz volcano colombia abstract a seismic tomographic study was performed for nevado del ruiz volcano nrv using more than 1500 high quality local and regional three low p wave velocity low v p and low s wave velocity low v s zones were found one low v s zone at depths 2 4 km located beneath the volcano a second low v p and low v s zone at depths 5 10 km located beneath the crater elongated and dipping to the e se and a third low v p and low v s zone at 10 to 12 km farther to the east these three low v p and low v s zones are believed to be the location of heat sources a high velocity zone for both p and s waves was found at shallow depths 0 5 km around the active crater the upper part 0 2 km depth of the high v p and high v s zone was characterized by low v p v s ratios v p v s ratios 1 80 the low v p v s zone is correlated with a steam dominated geothermal system the high v p v s is interpreted as an intrusive body of magmatic origin which includes partial melting zones associated with low v s anomalies a small low v p zone in which long period lp earthquakes were clustered was found to the southwest of the volcano based on the data obtained with the tomography in combination with seismicity geochemistry geology and gravimetry we suggest a model for the seismic activity of nrv volcano tectonic vt earthquakes that occur very often in swarm like patterns located in several clusters around the volcano seem to be due to changes in stress produced by the passing of fluids and or gas through many small cracks a fault and a caldera like structure separate the vt swarms located to the west of the volcano from the source of lp earthquakes

longevity of a small shield volcano revealed by crypto tephra studies rangitoto volcano new zealand change in eruptive behavior of a basaltic field abstract the life span of small volcanoes in terrestrial basaltic fields commonly considered monogenetic can be difficult to assess due to a paucity of datable materials capable of providing a 102 103 year age resolution we have used microscopic tephra layers crypto tephra in lake sediments to determine the longevity of rangitoto volcano a small shield that represents the most recent volcanism in the auckland volcanic field avf new zealand previous studies suggested construction in a relatively short interval at 550 500 cal yrs bp in contrast the tephra record shows evidence of intermittent activity from 1498 140 to at least 504 6 cal yrs bp a longevity of 1000 years rangitoto volcano is thought to represent about half the magma erupted in the 250 ka history of avf thus the avf has experienced a dramatic shift to prolonged and voluminous central vent volcanism in its most recent history this demonstrates the difficulty in determining time erupted volume relationships in such fields previous avf hazard risk modeling based on isolated short lived

a magnetite apatite deposit in the fanshan alkaline ultramafic complex northern china the fanshan alkaline ultramafic complex which has a length of 6 km and a width of 5 km in plan view consists of three zones from its core to margin zones 1 3 the middle zone zone 2 is the oldest intrusive phase and is ultramafic the outer zone zone 3 and the inner zone zone 1 are alkaline the three zones have essentially the same mineralogy but zone 2 is distinct in its compositional layering and cumulate texture the magnetite apatite orebodies occur in the middle part of zone 2 and are characterized by typically two or three layers of apatite rock each less than 3 m thick and one layer of magnetite apatite rock about 7 m thick the complex is characterized by high k2o contents up to 12 wt but low sio2 50 wt except for the apatite layers whole rock major elements of the three zones show coherent variations on harker diagrams and all rocks show quite similar chondrite normalized rare earth element ree patterns suggesting that they were derived from a common parent magma however the contacts between the three zones indicate that several batches of magma were intruded to form the three zones mineral compositions and ree patterns support in situ fractional crystallization as the origin for the oxide apatite layers the strongly variable oxide apatite ratios argue against liquid immiscibility being directly responsible for the formation of the oxide apatite rocks however liquid immiscibility may have taken place in the magma chamber under high pressure and temperature conditions at depth resulting in a low silica and an al fe mg ca p and volatile rich immiscible melt being intruded first as zone 2

what drives centuries long polygenetic scoria cone activity at barren island volcano abstract barren island in the andaman sea is an active mafic stratovolcano which had explosive and effusive eruptions followed by caldera formation in prehistoric time poorly dated a scoria cone within the caldera marking volcanic resurgence was active periodically from 1787 to 1832 the historic eruptions since 1991 the same scoria cone has produced six eruptions commonly including lava flows links between barren island s eruptions and giant earthquakes such as the 26 december 2004 great sumatra megathrust earthquake have been suggested though there is no general correlation between them the 227 year long activity of the scoria cone named here shanku cone is normally driven by purely magmatic processes i present a source to surface model for barren island and shanku including the source region deeper and shallow magma chambers volcanotectonics dyking from magma chambers and eruptions and eruptive style as controlled by crustal stresses composition and volatile content calculations show that dykes 0 5 m thick and a few hundred meters long originating from shallow level magma chambers 5 km deep are suitable feeders of the shanku eruptions shanku a polygenetic scoria cone at least 13 eruptions since 1787 has three excellent analogues namely anak krakatau 40 eruptions since 1927 cerro negro 23 eruptions since 1850 and yasur persistent activity for the past hundreds of years this is an important category of volcanoes gradational between small monogenetic scoria cones and larger polygenetic volcanoes

spatial distribution and alignments of volcanic centers clues to the formation of monogenetic volcanic fields monogenetic basaltic volcanic fields occur worldwide in tectonic environments ranging from extensional to convergent understanding similarities and differences between these fields may help to characterize key controls on their generation such volcanic fields consist of numerous volcanic centers each of which represents a pathway of magma from its source to the surface we analyzed the spatial distribution of volcanic centers in 37 monogenetic volcanic fields and assuming that the distribution of volcanic centers relative to each other is matched by a similar source pattern within the mantle applied the following methods for each 1 the poisson nearest neighbor pnn analysis representing the degree to which the distribution of the volcanic centers departs from a predicted poisson distribution and 2 a volcanic alignment analysis to ascertain the preferential pathways if any used by the magma to reach the surface this is the first comprehensive global comparison of such analyses magma pathways within the brittle upper crust are influenced to various degrees by two end member situations 1 formation of new extension fractures perpendicular to the least compressive stress 3 and 2 re activation of pre existing fractures that are near parallel to the maximum principal stress 1 the results of the pnn analysis show that independently of the tectonic environment most volcanic fields display a clustered distribution of their volcanic centers alignment analysis shows that either the ambient tectonic environment exerts a strong influence on the preferential orientations of the volcanic alignments or that it is in competition with other factors e g pre existing structures local stress changes due to older intrusions overall these results indicate that the propagation of the magma and therefore the spatial distribution of the volcanic centers within volcanic fields is the product of an interplay between deep level influences i e nature of the magma source and shallow level influences i e stress field pre existing crustal structures of the lithosphere

phoscorites of the salitre i complex origin and petrogenetic implications abstract the salitre i complex is a kidney shaped alkaline carbonatite phoscorite complex part of the late cretaceous alto paranaiba igneous province central brazil this complex is a key example where the phoscorite carbonatite association was preserved from metasomatism by late stage fluids and thus provides an opportunity to address the genesis of phoscorites salitre i is composed of two main coalescing intrusions a a northern one dominantly containing perovskite bebedourites b1 with swarms of ring dykes of carbonatites and phoscorites and b a southern one containing melanite bebedourites b2 salitre i phoscorites can be subdivided into p1 and p2 based on modal abundance mineral texture chemistry and mode of emplacement p1 phoscorites contain essential olivine phlogopite and apatite with accessory magnetite and traces of perovskite whereas p2 phoscorites are composed of the same essential phases but also contain carbonate rich pockets accessory pyrochlore and rare phlogopite carbonate c o isotopes and whole rock sr nd show that salitre i phoscorites crystallized from mantle derived parental magmas similar to other apip carbonatite complexes however chemical and textural discrepancies between p1 and p2 suggest derivation from different magma sources and through distinct processes for instance p1 al and ti rich phlogopite accessory perovskite and general trace element geochemical behaviour can be associated with the evolution of b1 perovskite bebedourites i e genesis through fractional crystallization from a bebedourite magma p2 phoscorites on the other hand have their evolution linked to a magmatic phase that underwent melt immiscibility as suggested by trace element fractionation additionally p2 contains pyrochlore which is generally lacking in bebedourites therefore whereas p1 phoscorites might have derived from a bebedourite magma p2 phoscorites were generated from a carbonated magma enriched in phosphate and iron oxides

global tectonics and space geodesy much of the success of plate tectonics can be attributed to the near rigidity of tectonic plates and the availability of data that describe the rates and directions of motion across narrow plate boundaries m 1 to 60 kilometers wide nonetheless many plate boundaries in both continental and oceanic lithosphere are not narrow but are hundreds to thousands of kilometers wide wide plate boundary zones cover m 15 percent of earth s surface area space geodesy which includes very long baseline radio interferometry satellite laser ranging and the global positioning system is providing the accurate long distance measurements needed to estimate the present motion across and within wide plate boundary zones space geodetic data show that plate velocities averaged over years are remarkably similar to velocities averaged over millions of years

the origin of along rift variations in faulting and magmatism in the ethiopian rift the geological record at rifts and margins worldwide often reveals considerable along strike variations in volumes of extruded and intruded igneous rocks these variations may be the result of asthenospheric heterogeneity variations in rate and timing of extension alternatively pre existing plate architecture and or the evolving kinematics of extension during breakup may exert first order control on magmatism the main ethiopian rift mer in east africa provides an excellent opportunity to address this dichotomy it exposes along strike several sectors of asynchronous rift development from continental rifting in the south to incipient oceanic spreading in the north here we perform studies of volcanic cone density and rift obliquity along strike in the mer by synthesizing these new data in light of existing geophysical geochemical and petrological constraints on magma generation and emplacement we are able to discriminate between tectonic and mantle geodynamic controls on the geological record of a newly forming magmatic rifted margin the timing of rift sector development the three dimensional focusing of melt and the ponding of plume material where the rift dramatically narrows each influence igneous intrusion and volcanism along the mer however rifting obliquity plays an important role in localizing intrusion into the crust beneath en echelon volcanic segments along strike variations in volumes and types of igneous rocks found at rifted margins thus likely carry information about the development of strain during rifting as well as the physical state of the convecting mantle at the time of breakup