fluid inclusions and hydrothermal alteration on the dixie valley fault nevada footwall rocks of the 1954 rupture segment of the dixie valley fault show extensive hydrothermal alteration related to fluids that were present on the fault during tectonic events hydrothermal alteration of granitic host rocks consists of temporally and spatially overlapping mineral assemblages an early biotite feldspar assemblage is followed by later fe chlorite and epidote both chlorite and epidote are replaced by hydrothermal sericite and cross cut by calcite hematite and quartz calcite veins biotite is partially replaced by prehnite the latest hydrothermal minerals are stilbite laumontite kaolinite alunite smectite illite and pervasive replacement of rock units with fine grained quartz chalcedony and opal secondary fluid inclusions trapped in healed microfractures in igneous quartz include type i inclusions that contain a moderate salinity aqueous liquid and vapor type ii inclusions that contain a moderate salinity aqueous liquid and co2 type iii inclusions that show eutectic melting temperatures below the nacl h2o eutectic and contain substantial cacl2 and type iv inclusions containing halite and other daughter minerals microthermometric measurements on these inclusions yield variable compositions and homogenization temperatures salinities of type i inclusions vary from 0 1 to 12 9 wt nacl with the mode in the interval 0 to 1 salinities of type ii co2 bearing inclusions range from 0 62 to 6 81 wt nacl relative to h2o and salinities of type iii inclusions with low eutectic melting temperatures are 12 9 to 25 3 nacl equivalent wt salinities of halite bearing inclusions are 30 1 to 39 2 wt nacl homogenization temperatures span the range 120 to 400 c the processes of isochemical cooling with upward displacement of the footwall mixing of cool low salinity water with hotter components and mixing of cool evaporite brine with hotter components could be responsible for variable fluid inclusion compositions homogenization temperatures and densities the p t path of the fault fluids is established by mineral equilibria and fluid inclusion characteristics the path includes a lithostatic fluid pressure at 305 c and 1570 bars along with cooling and escape of co2 from fluids the fluid p t path probably approaches hydrostatic pressure conditions at lower temperatures hydrothermal alteration product minerals fluid temperatures pressures and compositions in the footwall of the dixie valley fault constrain minimum fault age to 20 to 25 ma displacement to 6 km with about 3 km of pre 10 to 13 ma and 3 km of post 10 ma uplift fluid compositions and p t data suggest the following mechanism for rupture initiation and arrest ruptures may be initiated as a result of high fluid pressures then opening of dilatant fractures causes drastic decrease in fluid pressure separation of steam and co2 the drastic reduction in fluid bulk modulus that accompanies volatile phase separation permits propagation of the ruptures even though fluid pressure is reduced in areas where fluid pressure reduction is not accompanied by phase separation fractures are arrested by dilatant hardening

burial history of palaeozoic strata in belgium as revealed by conodont colour alteration data and thickness distributions thickness distributions of devono carboniferous formations and their relationship to conodont colour alteration indices cais from over 500 sample locations in belgium and northern france have been studied to reconstruct the thermal alteration and burial history of palaeozoic rocks the depositional centre during eifelian through visean times was located in the subsiding southern part of the dinant basin forming a thick sedimentary pile of probably more than 3 5 km in order to explain conodont cais chiefly resulting from upper carboniferous sedimentation two different depositional centres are considered giving rise to approximately 4 5 km of sediment in the central part of the dinant and namur basins as a consequence of the northward shifting variscan fold belt the upper carboniferous succession in the southern part of the dinant basin and in the rocroi area was reduced or absent conodont cais together with other thermal alteration data from the condroz massif indicate that this area was a structural high for most of the devonian the brabant massif was probably covered with approximately 1 5 2 0 km of upper carboniferous sediments in the south and less than 1 0 km in the central and northern parts in the central campine basin visean strata could have been buried by 3 5 4 5 km of upper carboniferous rocks

seismicity of the galapagos 95 5 w propagating rift an array of ocean bottom seismometers was deployed for 21 days about the 95 5 w galapagos propagating rift tip to obtain seismic evidence for the propagating rift hypothesis and to investigate the dynamic effects of rift propagation seismicity was fairly constant 10 17 events per day with no indication of swarm activity hypocenters were determined for 304 earthquakes the largest of which is estimated to be mb 2 3 with a seismic moment of 2 9 1020 dyn cm the cumulative seismic moment released by all local earthquakes is estimated to be 6 6 1020 dyn cm a b value of 0 77 0 08 was estimated for the region implying a stress regime dominated by a broad tectonic stress field the distribution of earthquakes is consistent with the propagating rift hypothesis most earthquakes occurred south of the propagating rift within the de steiguer deep seismicity within the transform zone is clustered in such a way as to suggest that two or three discrete fault segments may be active at the same time the seismicity observed along the propagating rift is diffuse earthquakes were observed at the propagating rift tip but not in front of it almost all earthquakes occur south of the northern pseudofault and its extension west of the rift tip that is earthquakes occur almost exclusively in lithosphere that has been or is being overtaken by the propagating rift six composite focal mechanisms are consistent with the propagating rift hypothesis first motions from earthquakes located on the propagating rift indicate extension whereas motions associated with the failing rift area indicate compression a compressional regime is also indicated west of the transform zone twelve earthquakes from the transform zone produce a poorly constrained focal mechanism consistent with right lateral strike slip motion along a fault plane that is roughly parallel to the isochrons of the transferred lithosphere

detection of changes in ridge crest morphology using repeated multibeam sonar surveys a quantitative analysis method is developed to objectively identify significant bathymetric changes between multibeam sonar surveys of the same seafloor area the technique involves mathematical gridding of bathymetric surveys from different years co registration of grids based on prominent features covered in both surveys calculation of slope weighted depth difference grids and determination of the boundaries of statistically significant differences the resulting difference grids are used in interpreting geological processes and calculating the volume of new material added to the seafloor the method is applied to data from the southern juan de fuca ridge where a seafloor volcanic eruption of 0 05 km3 is inferred to have occurred between 1983 and 1987 camera tow data and submersible observations provide ground truth which confirms the validity of the quantitative analysis method and indicates empirically that the detection limit of the method allows new seafloor landforms measuring 1 105 m2 in area or about 200 300 m in diameter and 5 15 m in thickness to be detected through repeated sea beam bathymetric surveys

evolution of abyssal lavas along propagating segments of the galapagos spreading center abstract the unusual petrological diversity of abyssal lavas erupted along some segments of the galapagos spreading center is a direct consequence of the propagation elongation of these segments into older oceanic crust with increasing distance behind propagating rift tips relatively unfractionated morb erupted close to the tips are joined first by feti basalts bimodal assemblage and then by a wide range of basaltic and siliceous lavas further behind propagating rift tips this broad range diminishes again approaching the narrow compositional range of adjacent normal ridge segments these compositional variations reflect the evolution of the subaxial magmatic system beneath the newly forming spreading center as it propagates through a pre existing plate we envisage this evolution as proceeding from small isolated ephemeral magma chambers through increasing numbers of larger increasingly interconnected chambers to the steady state buffered system of a normal ridge throughout this evolution magma supply rates gradually increase and cooling rates of crustal magma bodies decrease high degrees of crystal fractionation are favored only when a delicate balance between cooling rate and resupply rate of primitive magma is achieved at other propagating and non propagating ridge transform intersections the degree to which the balance is achieved and the length of ridge over which it evolves control the distribution of fractionated lavas these effects may be evaluated provided a number of tectonic variables including transform length spreading and propagation rates are taken into account

mid ocean ridges fine scale tectonic volcanic and hydrothermal processes within the plate boundary zone a first order model of spreading centers as idealized linear boundaries of crustal and lithospheric generation provides only a gross understanding of global scale plate kinematics as we attempt to understand the complexity of crustal and lithospheric structure of two thirds of the earth s surface it is becoming increasingly necessary to study the tectonic volcanic and hydro thermal processes within the spreading center plate boundary zone all ocean ic crust bears the imprint of these processes this review focuses on a few selected topics concernining the fine scale tectonics and geophysics of the active axial zone of mid ocean ridges with reference to associated volcanic and hydrothermal processes it draws heavily on recent studies that use deeply towed instrument packages multi beam bathymetric mapping ocean bottom instruments and alvin e g the famous amar rise and galapagos expeditions we begin with a review of the large scale structure of spreading centers we then take a close look at the axial neovolcanic zone and progress away from the axis through the active tectonic zones next we consider the characteristics of the axial magma chamber and associated hydrothermal actvity as well as the generation of magnetic anomaly stripes and their implications for crustal generation one of our findings is that the initial two dimensional model of volcanic and tectonic zones must be expanded upon to allow for variations

crustal rifting and related thermo mechanical processes in the lithosphere beneath iceland recent observations of crustal rifting in the ne iceland axial zone are summarized and discussed the thermal state of the crust in the axial zone is discussed on the basis of thermal gradient measurements magnetotelluric soundings and modelling of the accretion mechanism the three independent methods are in good general agreement thermal modelling indicates that a zone of partial melting generally exists below approximately 6 km depth in the axial zone it is suggested that thermal stresses induced by the cooling of the lithosphere as it moves away from the axis are largely responsible for the off axis volcanism and that the thermal stresses also contribute to enhancing the vertical permeability for geothermal waters at intermediate crustal depths on the flanks of the axial zone

sea beam deep tow investigation of an active oceanic propagating rift system galapagos 95 5 w we have tested and corroborated the propagating rift hypothesis with high resolution sea beam and deep tow data collected over the galapagos 95 5 w propagating rift system the propagating rift is continuously breaking through the cocos plate at a velocity of about 50 km m y with an azimuth of about 273 away from the galapagos hotspot this process transfers cocos lithosphere to the nazca plate including the preexisting spreading center which fails as it is gradually preempted by the propagating rift the spreading center azimuth is being changed by about 8 clockwise by this rift propagation the active propagating and failing rift axes overlap by about 15 km and are joined by a broad and anomalously deep zone of nonrigid plate deformation rather than by a transform fault although rift propagation is continuous on a scale of a few kilometers 50 000 years and lithospheric transferral from one plate to the other occurs mostly continuously as the preexisting rift fails it breaks into discrete segments the geometry of seafloor fabric and tectonic elements such as active and failed rifts the v shaped pattern of pseudofaults and the oblique tectonic fabric in the sheared zone of transferred lithosphere are observed as predicted this quantitative verification provides strong support for the propagating rift hypothesis

the evolution of propagating rifts the magnetic survey carried out by mason and raff1 2 off the north west coast of the us has been central to the development of many of the concepts of seafloor spreading and plate tectonics recently hey and wilson3 have again used this survey to illustrate hey s suggestion4 that ridge segments sometimes increase their length by propagation this process leads to a characteristic pattern of magnetic anomalies offset by discontinuities which hey called pseudofaults because unlike fracture zones they fig 1 were never plate boundaries we demonstrate here how the geometry of the pseudofaults magnetic anomalies and the failed rift can be worked out using velocity triangles and the concept of stability this method was originally developed to understand the evolution of triple junctions between three plates5

detailed tectonics near the tip of the galapagos 95 5 w propagator how the lithosphere tears and a spreading axis develops a model for propagation of an active spreading axis into preexisting lithosphere has been developed based on intensive detailed studies using seamarc ii sea beam deep tow and alvin near the tip of the propagator growing westward at 52 km m y along the east west trending galapagos spreading axis near 95 5 w initial lithospheric rifting appears to be accommodated along reactivated roughly east west abyssal hill faults at the tectonic tip extension leads to downdropping of a keystone block which is pervasively cut by normal faults and fissures the initial volcanism initial volcanic tip occurs about 6 5 km behind the tectonic tip as basalt erupts through these fissures covering most of the keystone block the neovolcanic axial tip occurs another 4 5 km back at the western termination of a well defined axial pillow ridge along which volcanism is localized spreading along the propagator axis accelerates to the full rate 29 mm yr half rate at the full rate tip interpreted to be 10 km behind the neovolcanic axial tip or 21 km behind the tectonic tip a complex pattern of strain in the tip area results from the interaction of plate spreading transform shear stress crack propagation dynamic depression of the western end of the axial valley and the transform zone and variations in the strength thickness heterogeneity and anisotropy of the lithosphere southwesterly curvature of the neovolcanic axis near its tip is inferred to result primarily from the response of young thin weak relatively homogeneous lithosphere to counterclockwise rotation of the maximum tensional stress direction relative to the spreading direction due to the influence of shear stress from the transform zone similar stress interactions likely occur at the tectonic tip but are inferred to be accommodated in this older thick strong heterogeneous and anisotropic lithosphere by oblique slip along a set of left stepping reactivated preexisting east west abyssal hill structures although on a time scale of 106 years the overall process of propagation is mostly continuous rifting at the tectonic tip appears discontinuous at a scale of 1500 m or 30 000 years based on the east west spacing of grabens formed as rifting begins the scale of discontinuity of propagation of the neovolcanic axis is 3 5 km or 50 000 100 000 years based on the distribution of northeast trending features interpreted as beheaded western terminations of paleoneovolcanic axes the extension involved in rifting is 10 20 based on summing the horizontal normal separation of all normal faults and fissures integrated and averaged over the zone of rifting these values are approximately doubled if simple shear listric faulting is dominant rather than pure shear normal faulting

some comments on two layer extensional models for the evolution of sedimentary basins simple extensional models that involve listric faulting in the brittle upper crust and plastic flow in the lower lithosphere have been shown to account for the subsidence history of various continental sedimentary basins and continental shelves in this paper we present structural evidence that extension in the continental lithosphere during rifting is not uniform we review evidence that the rifting process in major intracratonic rift basins is spasmodic rather than continuous that extension and subsidence within the rift during a tectonic phase is generally accompanied by uplift and erosion of the rift flanks that subcrustal heat input during rifting occurs over an area much broader than the rift proper and that there may be broad regional uplift before and or during rifting to explain the uplift of the flanks during rifting and their postrift subsidence we invoke two mechanisms subcrustal thinning by extension below the flanks and tilted major normal faults in the crust between graben and flanks the uplift and erosion of the flanks due to tilted faults probably do not significantly affect estimates of extension from subsidence analysis but uplift due to both sources can in certain cases be incorrectly attributed to regional uplift or doming we present a simplified analysis of the two layer extensional model for the elementary case in which extension is instantaneous and the crust is thinned by a different amount from the subcrustal lithosphere we provide a simple graphical method of data analysis to obtain the parameters that describe thinning of the crust and lithosphere during extension from the subsidence history application of this method to the central part of the pattani trough in the intracratonic gulf of thailand shows that extension and thinning of the lithosphere of a factor of 2 since late eocene can account for much of the subsidence history and the present day heat flow our analysis suggests that thinning of the crust in this area is 20 greater than thinning of the whole lithosphere assuming that none of the lithosphere is removed from the region of extension during rifting we show by simple geometric examples that a process of nonuniform extension implies that much greater thicknesses of sediment can be deposited in the center of a young basin than in the case of uniform extension of both crust and subcrustal lithosphere such an extensional process produces significant uplift of the flanks of the rift and as a result of erosion of the uplifted areas the effective cross sectional area of the basin can be increased by as much as 10 20 depending on the rate of erosion compared to the area that would have been created by a process of uniform extension finally we show that it may ultimately be possible to compute erosion rates from seismic cross sections of basins

multistage dolomitization in rainbow buildups middle devonian keg river formation alberta canada abstract more than 80 partially to completely dolomitized buildups of middle devonian keg river formation occur in the rainbow sub basin of northwestern alberta in six buildups a b e f g and tehze four petrographic types of dolomites are identified gray fine crystalline dolomite floating dolomite rhombs patches matrix dolomite and saddle dolomite the petrographic and geochemical data indicate that these dolomites probably formed in three principle stages of dolomitization during progressive burial gray fine crystalline dolomite less than 1 of total dolomite by volume is interpreted as penecontemporaneously dolomitized lime muds by either normal marine waters or evaporitic brines during early exposure the following evidence suggests that this dolomite probably formed early 1 it usually occurs in fractures and breccias presumably related to the early exposure of the buildups 2 it is absent in rugs and molds that formed in the subsurface environment and 3 locally clasts of gray fine crystalline dolomite are embedded in marine limestones rhomb patch dolomite is the major type of dolomite in dolomitic limestones and is widespread throughout the limestone buildups matrix dolomite is fabric destructive and is most abundant in the dolostone buildups both these dolomites probably formed during shallow to intermediate burial because they 1 postdate stylolites 2 are widespread throughout the buildups and crosscut various facies and 3 have light oxygen isotope compositions and low sr but high fe and mn concentrations dolomitizing fluids for dolomite rhombs patches and matrix dolomites were possibly derived from 1 mechanical compaction 2 dehydration of the adjacent muskeg gypsum at relatively shallow burial depths and 3 intermediate depth basinal fluids conducted updip along porous and permeable conduits during d vonian and carly carboniferous times compaction fluids were probably responsible for the early dolomite rhombs and patches however the main phase of matrix dolomitization took place later probably during late mississippian to jurassic time when the east side of the basin was intermittently tilted and uplifted causing updip flow of intermediate depth basinal brines saddle dolomite occurs as late stage vug and fracture fillings it is most depleted in 18o but most enriched in mn and fe suggesting precipitation during deeper burial from warm basinal fluids these fluids were expelled during upper cretaceous sedimentary and tectonic loading of the western side of the basin and then moved up the east side of the basin along existing platform fault and fracture conduits the source of the mg is uncertain some or most of the mg may have resulted from chemical compaction of the earlier matrix dolomite

gold quartz veins in metamorphic terranes and their bearing on the role of fluids in faulting gold quartz vein fields in metamorphic terranes such as greenstone belts provide evidence for the involvement of large volumes of fluids during faulting and may be products of seismic processes near the base of the seismogenic regime in the val d or district of the abitibi greenstone belt canada quartz tourmaline carbonate veins form a vein field 30 15 km in the hanging wall of a crustal scale fault zone which was the main channelway for upward migration of the deeply generated fluids the veins occur in small high angle reverse faults and in adjacent horizontal extensional fractures extending up to 75 m in intact rocks they have formed incrementally during active reverse faulting in response to crustal shortening at depths corresponding to those at the base of the seismogenic zone in actively deforming crust detailed structural and fluid inclusion studies provide evidence for generally lithostatic but fluctuating fluid pressures p of the order of 200 mpa and for cyclic stress reversals during vein formation and provide good support for the fault valve model a comparison of vein characteristics with standard earthquake rupture parameters suggests that each slip increment along veins in reverse faults was accompanied by a small earthquake 4 m 3 or less the large vein field thus represents both the extent of fluid dispersion in the hanging wall of a crustal scale channelway and the distribution of small earthquakes integrated over the lifetime of the hydrothermal system it is proposed that such small earthquakes along veins in reverse faults are related to large earthquakes m 6 nucleating near the base of the seismogenic regime along the nearby crustal scale fault either as aftershocks or as a precursory smarm

petrologic consequences of rift propagation on oceanic spreading ridges abstract the production of anomalously differentiated lava compositions at several mid ocean spreading centers can be attributed to magmatic processes associated with propagating rifts the degree of differentiation attained by magmas beneath oceanic spreading ridges depends mainly on the balance between cooling rate and the supply rate of new magma to shallow chambers low supply rates and moderate cooling rates allow advanced degrees of closed system fractionation to occur high supply rates result in open systems in which magma compositions are buffered by frequent replenishment with new hot magma propagating rift tips are a special class of ridge transform intersection in which the balance between cooling and supply rates is conducive to the development of advanced degrees of differentiation over an expanded length of ridge this balance is affected by the spreading rate the propagation rate of the rift the length of the bounding transform and proximity to hotspots maximum compositional variability and maximum degree of differentiation occur within 50 km of propagating rift tips and subsequently diminish with increasing distance rifts that propagate through plates in directions approximating their absolute motion relative to the lower mantle are characterized by the presence of anomalously differentiated lavas over longer ridge segments than are rifts that propagate against their absolute motion geochemical anomalies may persist though changing in degree and extent for several million years on ridge segments that stop propagating the concept of magnetic telechemistry is generally supported by our study but in the vicinity of hotspots magnetic anomaly amplitude may be controlled more by bathymetric and or thickened magnetic layer effects than by geochemistry

crustal processes at spreading centers recent discoveries and research on mid ocean ridges mor s in the last four years have greatly increased our knowledge of this fundamental type of plate boundary the last iugg review of mor research was in 1975 aumento et al 1975 and its omission as a topic in 1978 may have reflected a feeling that spreading centers were reasonably well understood since that time however there has been a surfeit of important new observations and exciting research and there is now a turmoil of innovative ideas about the processes which create and leave their imprint on two thirds of the earth s crust new seismic measurements have been made and interpreted as evidence for the existence of shallow axial magma chambers amc beneath intermediate total opening rate 5 9 cm yr and fast 9 18 cm yr spreading centers fig 1 yet there is controversy as to whether they are steady state whether they persist beneath slow ridges as well in the steady state what the dimensions and shape of the chamber are its relation to oceanic crustal stratigraphy and its relation to observations in ophiolites black smoker hydrothermal vents have been discovered in the pacific spewing forth 350 c fluids blackened by polymetallic sulfide precipitates this discovery is embroiled in debate as to how long the vents last whether they occur at slow spreading rates as well whether the enormous heat flux of the vents freeze the magma chamber even at fast spreading rates their influence on geochemical balances in the ocean and the value of the sulfide deposits commercially it has been found that our initial view of mor s as two dimensional linear structures has served us well as an initial working model but must be significantly modified to account for variations in tectonic volcanic and hydrothermal activity along strike as well as for episodicity of these processes deep sea drilling project dsdp samples have raised fundamental questions about formation of the magnetic source layer hydrothermal circulation and fault mechanisms along the mor spreading centers which penetrate existing lithosphere along strike as propagating rifts have modified our views of how plate boundaries reorient themselves and how v shaped magnetic and topographic trends are formed we have also found that transform faults may modify spreading center processes for some distance away from their intersections with mor s and that rigid plate transform faults may fail to develop at fast spreading centers under certain conditions resulting in en echelon overlapping of spreading centers this review will focus on these developments and several others only example references to these topics will be given in the text and many important topics will not be discussed directly e g petrology and geochemistry mantle convection research on lithosphere older than 1 m y however i have tried to compile and have included an extensive bibliography for u s research on a broader range of mor topics for mid 1978 to mid 1982 which is the main purpose of this report pre 1978 references are occasionally included where appropriate abstracts are not included

magmatic subsidence of the east pacific rise epr at 18 14 s revealed through fault restoration of ridge crest bathymetry 1 the fault restoration technique of de chabalier and avouac 1994 is applied to an ultra high resolution bathymetry data set from the east pacific rise epr at 18 14 s fault offsets are calculated and subtracted from the original seafloor bathymetry to examine the morphology of the unfaulted seafloor surface within an area encompassing the ridge axis 400 1600 m in dimension the restored topography reveals a gently sloping seafloor 200 m wide which slopes 5 inward toward the spreading axis we attribute this inward sloping seafloor to subsidence within the axial trough due to subsurface magmatic deflation the vertical deformation field extracted from the bathymetry is used to characterize the ridge axis fault population present in the area median fault throws 9 m for inward facing and 8 m for outward facing faults are comparable to values measured for nearby mature abyssal hill fault populations suggesting a genetic link however median fault spacings 70 and 46 m are an order of magnitude smaller and estimated total extensional strain is 3 4 greater than values measured for ridge flank faults these differences indicate the axial fault population at 18 14 s cannot be rafted onto the ridge flanks to form abyssal hill faults without significant modification presumably via volcanic burial we attribute the dense faulting observed in this area to slip on axial fissures during subsidence of the crestal region the surface subsidence of a volcanic edifice can be modeled in terms of volume change in the magma source reservoir and volume of magma withdrawn from the reservoir using the relationship derived for a sill like magma body we estimate that the axial depression at 18 14 s could represent magma withdrawal associated with a small number 4 22 of the frequent dike injection and eruption events required to build the upper crust during the evolution of the trough the subsidence volumes represented by the axial topography at 18 14 s are a small percentage of the underlying upper crustal sections 3 4 suggesting that only a minor mismatch between magma recharge and withdrawal from the axial melt lens during ongoing plate separation could account for this pronounced axial depression

litho stratigraphic effect on variscan fluid flow within the prague synform barrandian evidence based on c o sr isotopes and fluid inclusions the palaeozoic sedimentary sequence of the prague synform ordovician devonian in the centre of the bohemian massif underwent variscan deformation and thermal overprint events variscan veins widespread throughout the sedimentary strata have precipitated from syntectonic aqueous and hydrocarbon rich fluids homogenization temperatures of aqueous inclusions increased from 70 up to 226 degc in the cambrian rocks underlying the prague synform seawater modified due to intensive water rock interaction was the main fluid component fluid flow was limited and restricted to the lithostratigraphic compartments forming a rock buffered system stable isotopic modelling c o and final interpretations of the confined hydrostratigraphic fluid migration was supported by the 87sr 86sr ratio in veins and wall rocks siliciclastic cambrian and ordovician rocks and the associated intersecting veins yielded similar isotopic signatures and consequently the fluid migration is restricted to layer and formation scales gradually downwards increasing fluid temperature and compositional changes reflect burial at oil window conditions an open fluid system could be expected in proximity of major faults of the prague syncline and at the top of the sedimentary sequence

late metamorphic veins record deep ingression of meteoric water a la icpms fluid inclusion study from the fold and thrust belt of the rhenish massif germany abstract the fluid evolution of late metamorphic quartz vein systems in the fold and thrust belt of the rhenish massif has been studied by microthermometry and la icpms microanalysis the quartz veins are hosted by very low grade organic matter rich metasiltstones and metapelites of the hunsruck nappe complex the veins record two major stages of textural evolution a massive vein filling assemblage with elongate blocky quartz chlorite albite and apatite and a later open space filling assemblage with euhedral crystals of quartz ankerite dolomite and minor calcite and sulfides the euhedral quartz crystals host well preserved fluid inclusion assemblages which made it possible to reconstruct the chemical evolution of the fluid system with time the fluid inclusions are all aqueous two phase with low salinity and three successive fluid generations record a systematic decrease in salinity from 4 7 5 3 to 3 1 3 9 and then to 1 4 1 7 wt eqv nacl the salinity decrease is paralleled by a decrease in homogenization temperatures from 210 225 c to 148 164 c and 124 139 c using la icpms microanalysis of individual fluid inclusions reproducible elemental concentrations of li na k rb cs mg ca ba sr b al as sb s cl and br could be determined the concentrations of the alkali metals and of boron arsenic and antimony are correlated with the fluid salinity element concentrations are highest in the early fluid generation and decrease systematically in the latter two generations the systematic changes in fluid composition salinity homogenization temperature and elemental concentrations are best explained by fluid mixing between a hot metamorphic fluid of moderate salinity that carried elevated concentrations of b as and sb and a cooler dilute fluid that was of meteoric origin the fluid evolution therefore signals deep ingression of meteoric water during late metamorphic exhumation and uplift the molar cl br ratios of the fluid inclusions lie on a linear array that extends from close to seawater to values that are substantially below seawater because evaporitic sources are conspicuously absent in the shallow water marine sequence of the rhenish massif the elevated br concentrations and cl br ratios below seawater are best explained by fluid rock interaction and liberation of br from the organic matter in the metasediments

evolution of metamorphic fluids at the variscan fold and thrust belt in eastern belgium abstract quartz veins in the southern greenschist facies metamorphic border of the stavelot venn massif belgium have been examined a detailed petrographical study of these quartz veins indicates the presence of eleven quartz stages a microthermometric analysis shows the expulsion of three main fluid types during the palaeozoic in the first quartz stage i of caledonian age mixed aqueous gaseous h 2 o co 2 nacl fluid inclusions are present the post caledonian and pre variscan second stage ii is characterised by gaseous co 2 n 2 inclusions occurring along inter crystal grain boundaries and by aqueous h 2 o nacl intra crystal inclusions the quartz stages iii vi which formed during the variscan period contain primary mixed aqueous gaseous h 2 o co 2 nacl inclusions all with an equal bulk chemical composition post variscan quartz stages vii xi could not be investigated microthermometrically the combination of microthermometry chlorite geothermometry and geothermal gradients reveals minimum trapping conditions of 200 mpa and 200 c for stage i quartz and 175 275 mpa and 385 435 c for quartz stages iii vi the trapping conditions of stage ii quartz could not be determined due to the strong recrystallisation of the quartz veins at the lower metamorphic variscan thrust front distinct temperature anomalies are associated with thrust faults as indicated by conodont colour alteration index cai vitrinite reflectance data chlorite geothermometry and fluid inclusion microthermometry the expulsion of metamorphic fluids generated at the southern part of the stavelot venn massif could have caused the distinct temperature anomalies along variscan thrust faults alternatively some of these anomalies could have been induced by deeply circulating meteoric fluids

multiscale spectral analysis of bathymetry on the flank of the mid atlantic ridge modification of the seafloor by mass wasting and sedimentation the results of a multiscale spectral analysis of bathymetric data on the flank of the mid atlantic ridge are described data were collected during two cruises using hydrosweep multibeam tens of kilometers to 0 2 km scale range and mesotech scanning pencil beam sonar attached to remotely operated vehicle jason 1 km to 0 5 m scale range these data are augmented by visual data which enabled us to identify bathymetric profiles which are over unsedimented or thinly sedimented crust our analysis therefore is focused primarily on statistical characterization of basement morphology work is concentrated at two sites site b on 24 ma crust in an outside corner setting and site d on 3 ma crust in an inside corner setting at site b we find that an anisotropic band limited fractal model i e the von karman model proposed for abyssal hill morphology by goff and jordan 1988 is not sufficient to describe the full range of scales observed in this study our observations differ from this model in two ways 1 strike and cross strike dip spectral properties converge for wavelengths smaller than 300 m and 2 in both strike and dip directions the fractal dimension changes at 10 m wavelength from 1 27 at larger scales to 1 0 at smaller scales the convergence of strike and dip spectral properties appears to be associated with destruction of ridge parallel fault scarps by mass wasting which develops canyon like incisions that cross scarps at high angles the change in fractal dimension at 10 m scale appears to be related to a minimum spacing of significant slope breaks associated with scarps which are created by faulting and mass wasting at site d although there is no significant abyssal hill anisotropy the spectral properties at all scales are consistent with the von karman model the fractal dimension at this site 1 15 is less than at site b this difference may be reflect different morphology related to crustal formation at inside corner versus outside corner position or more likely differences in the degree of mass wasting the smoothing of seafloor morphology by sediments is evident in hydrosweep periodograms where relative to basement roughness spectral power decreases progressively with decreasing wavelength

fluid evolution at the variscan front in the vicinity of the aachen thrust quartz carbonate chlorite veins were studied in borehole samples of the rwth 1 well in aachen veins formed in devonian rocks in the footwall of the aachen thrust during variscan deformation and associated fluid flow primary fluid inclusions indicate subsolvus unmixing of a homogenous h2o co2 ch4 n2 na k cl fluid into a h2o na k cl solution and a vapour rich co2 h2o ch4 n2 fluid the aqueous end member composition resembles that of metamorphic fluids of the variscan front zone with salinities ranging from 4 to 7 nacl equiv and maximum homogenisation temperatures of close to 400 c pressure estimates indicate a burial depth between 4 500 and 8 000 m at geothermal gradients between 50 and 75 c 26 mpa but pressure decrease to sublithostatic conditions is also indicated probably as a consequence of fracture opening during episodic seismic activity a second fluid system mainly preserved in pseudo secondary and secondary fluid inclusions is characterised by fluid temperatures between 200 and 250 c and salinities of 5 nacl equiv bulk stable isotope analyses of fluids released from vein quartz calcite and dolomite by decrepitation yielded dh2o values from 89 to 113 13cch4 from 26 9 to 28 9 vpdb and 13cco2 from 12 8 to 23 3 vpdb the low d and 13c range of the fluids is considered to be due to interaction with cracked hydrocarbons the second fluid influx caused partial isotope exchange and disequilibrium it is envisaged that an initial short lived flux of hot metamorphic fluids expelled from the epizonal metamorphic domains of the stavelot venn massif the metamorphic fluid was focused along major thrust faults of the variscan front zone such as the aachen thrust a second fluid influx was introduced from formation waters in the footwall of the aachen thrust as a consequence of progressive deformation mixing of the cooler and lower salinity formation water with the hot metamorphic fluid during episodic fluid trapping resulted in an evolving range of physicochemical fluid inclusion characteristics

three dimensional flow beneath a slow spreading ridge axis a dynamic contribution to the deepening of the median valley toward fracture zones the axial valley along a slow spreading mid ocean ridge may be explained by a vertical pressure gradient due to viscous flow of asthenosphere upwelling in a relatively narrow conduit beneath the ridge axis along a ridge segment the axial valley floor deepens by as much as 2 3 km over distances of several tens of kilometers approaching a ridge transform intersection this deepening may be explained in part by a pressure gradient associated with horizontal flow in the ridge axis conduit vanishing vertical velocity on a conduit end wall at a ridge transform intersection results in reduced vertical flow within about one conduit width of the intersection horizontal flow along the conduit toward the intersection must occur to form lithosphere at a uniform rate along the ridge axis a simple model with a conduit of uniform width that terminates with vertical planar endwalls at ridge transform intersections is considered the model is based on an analytical solution for flow in a narrow conduit and boundary layer approximations for the flow structure near the vertical end walls this model indicates that the induced horizontal flow and pressure gradient extend for a distance along the ridge axis of several lithosphere thicknesses from an intersection for reasonable values of the conduit width and asthenosphere viscosity the model predicts that induced horizontal flow can contribute significantly to the deepening of an axial valley approaching an intersection

the application of a multichannel laser raman microprobe microdil 28 to the analysis of fluid inclusions abstract the application to the analysis of fluid inclusions of laser excited raman microspectrometry with a multichannel detection system has two important features connected with the very short time needed to acquire one single spectrum 1 5 s the accumulation of a great number of the same spectral range results in an averaged spectrum with a very high signal to noise ratio with corresponding low detection limits and good counting statistics on the other hand the rapid acquisition of spectra enables the analysis of many individual inclusions in a relatively short time

a three dimensional gravity study of the 95 5 w propagating rift in the galapagos spreading center abstract seafloor at the galapagos 95 5 w propagating rift pr has a varied morphological expression that can be spatially correlated with the predicted kinematic history of the pr a median valley like depression occurs near the tip of the growing ridge axis to test if this bathymetry is a dynamic feature supported by mantle or lithosphere strength or if it is due to isostatically compensated crustal thickness variations we use three dimensional gravity modelling to constrain the crustal structure in this region from data collected by hey in 1979 and 1982 the gravity anomaly at the pr tip depression suggests that the tip depression is not caused by crustal thinning the data are consistent with a stress supported pr tip depression caused by asthenospheric along axis flow into the growing ridge axis phipps morgan and parmentier 1 in contrast to the tip depression seafloor in the sheared zone of material transferred through transform migration from the cocos to nazca plate is anomalously shallow and has a pronounced regional 300 400 m tilt towards the growing ridge axis over the 20 km width of the sheared zone the gravity data also suggest that the sheared zone is not compensated by crustal thickening

application of least squares collocation to gravity and height variations associated with a recent rifting process abstract the paper describes a technique that utilizes least squares collocation to produce a common and continuous model of gravity variations with time and vertical crustal movements the data used are the results of repeated gravity measurements and levelling a profile time system has been introduced as reference for the sample points where gravity and or height have been observed the essential problem is the determination of the covariance functions these functions have been analysed regarding the isotropy and used for the prediction of the variations the developed model has been applied successfully to data obtained in northern iceland test fields established for controlling a recent rifting episode

the boudinage question in the high ardenne slate belt belgium a combined structural and fluid inclusion approach abstract the terms boudin and boudinage 1 were first introduced as descriptive terms for particular geometric features associated with quartz veins in competent metaquartzitic layers in the high ardenne slate belt belgium the occurrence of the boudins has subsequently been used to draw kinematic conclusions on a pre variscan extensional event in the rhenohercynian belt in this paper a combined structural and fluid inclusion approach allows proposing a new and more representative kinematic model for the development of the veins and associated structures moreover the kinematic significance of interboudin veins is emphasized in this regard microthermometry of primary co 2 n 2 fluid inclusions in the quartz of the interboudin veins suggests that the origin of vein development is hydraulic fracturing in an already compressional setting this suggestion implies the presence of fluid and near lithostatic fluid pressures to permit fracturing at great depths where the rocks otherwise would be either stable or in the realm of ductile behaviour the vein filling fluid had a local origin and was most likely related to the formation of bedding parallel dissolution seams within the host rock veining is followed by layer parallel shortening resulting in the development of double sided mullions at both interfaces of metaquartzitic layers pinned by pre existing veins subsequently these polyphase structures behave passively in the variscan deformation

bathymetric artifacts in sea beam data how to recognize them and what causes them sea beam multibeam bathymetric data have greatly advanced understanding of the deep seafloor however several types of bathymetric artifacts have been identified in sea beam s contoured output surveys with many overlapping swaths and digital recording on magnetic tape of sea beam s 16 acoustic returns made it possible to evaluate actual system performance the artifacts are not due to the contouring algorithm used rather they result from errors in echo detection and processing these errors are due to internal factors such as side lobe interference bottom tracking gate malfunctions or external interference from other sound sources e g 3 5 khz echo sounders or seismic sound sources although many artifacts are obviously spurious and would be disregarded some particularly the omega effects described in this paper are more subtle and could mislead the unwary observer artifacts observed could be mistaken for volcanic constructs abyssal hill trends hydrothermal mounds slump blocks or channels and could seriously affect volcanic tectonic or sedimentological interpretations misinterpretation of these artifacts may result in positioning errors when seafloor bathymetry is used to navigate the ship considering these possible geological misinterpretations a clear understanding of the sea beam system s capabilities and limitations is deemed essential

migrating transform zone and lithospheric transfer at the galapagos 95 5 w propagator at the 95 5 w propagator system along the cocos nazca plate boundary the spreading axis is offset left laterally about 35 km 1 m y this offset migrates westward 52 km m y relative to points along the nearly east west trending ridge axis transferring lithosphere from the cocos to the nazca plate as it does so the required 58 km m y of right lateral motion across this migrating transform is apparently not accommodated by a throughgoing north south transform fault but rather by an approximately 15 to 20 km wide zone of shear deformation dominated by eastward curving of east west structures to a northwest southeast trend this pattern of curvature the coinciding zone of high seismicity and the abundance of talus in the area suggest that shear is distributed over this entire zone the deformation is inferred to be taken up by a bookshelf mechanism whereby faults oriented at high angles to the shear couple have left lateral slip antithetic to the right lateral shear couple rotating individual blocks clockwise these are interpreted as reactivated abyssal hill bounding normal faults probably dipping 45 60 northeast southwest shortening due to the rotation appears to be accommodated by reverse slip on the faults and may account for the uplift of de steiguer ridge in the zone of transferred lithosphere these faults have apparently had a very complex history initially normal faults formed as abyssal hills were created at the spreading axis and subsequently they had left lateral strike slip motion in the transform zone and reverse motion uplifting de steiguer ridge strain analysis based on observed structural curvature suggests that shear is distributed nonuniformly and lithospheric transfer requires 0 4 m y

conditions for hydrofracture and the fluid permeability of accretionary wedges abstract the dewatering of fine grained sediment piles in accretionary wedges is controlled by zones of active deformation episodic hydrofracture in these zones enhances permeability enabling fluid flow to be faster than the speed of underthrusting here i investigate the mechanics of simultaneous shear failure and hydrofracture in muddy rocks as a function of the faulting mode the physical properties of model rock types are chosen to represent uncemented mud or cherty marly mudstones that have undergone significant diagenesis during thrust faulting hydrofracture always requires pore fluid pressures in excess of the lithostatic load whereas shallow level wrenching or normal faulting permits hydrofracture at hydrostatic pressures significantly lower than lithostatic load all three faulting modes are expected to occur in an accretionary wedge attempting to dynamically maintain its critical taper the model computations indicate that in the upper 80 120 m of a section of soft mudstone hydrofracture inevitably accompanies wrench or normal faulting hard mudstones inevitably hydrofracture to depths of up to 1400 m during normal faulting these differences in mechanical behaviour have profound consequences for fluid venting capabilities of accretionary wedges they influence hydrocarbon trapping and play a role in the localisation of mud diapirs and mud volcanism

the geometry of propagating rifts abstract the kinematics of two different processes are investigated both of which have been described as rift propagation courtillot uses this term to describe the change from distributed to localised extension which occurs during the early development of an ocean basin the term localisation is instead used here to describe this process to distinguish it from hey s type of propagation localisation generally leads to rotation of the direction of magnetisation to hey propagation means the extension of a rift into the undeformed plate beyond a transform fault detail surveys of the galapagos rift have shown that the propagating and failing rifts are not connected by a single transform fault but by a zone which is undergoing shear the principal deformation is simple shear and the kinematics of this deformation are investigated in some detail the strike of most of the lineations observed in the area can be produced by such deformation the mode of extension on the propagating rift appears to be localised for some periods but to be distributed for others neither simple kinematic arguments nor stretching of the lithosphere with conservation of crust can account for the observed variations in water depth

empirical calibration of the oxygen isotope fractionation between quartz and fe mg chlorite abstract an empirical calibration has been made of the oxygen isotope fractionation between chlorite and quartz using samples from localities in metamorphic terrains where the formation temperature has been constrained by independent methods e g fluid inclusion homogenization temperatures mineral assemblages and petrology of the host rocks to the veins thermodynamic modeling and raman spectrometry textural equilibrium of chlorite and quartz was a pre requisite and the chemical composition of chlorite was determined the chlorite analyzed had a range in fe mg varying between 0 35 and 0 7 except for pure mg chlorite fe mg substitution within the range measured does not change the oxygen isotope fractionation between quartz and chlorite measurably the results support a linear correlation for chlorite quartz fractionation over the temperature range given by this study 240 550 c 1000 ln 0 01323 0 002 t c 10 97 0 92 the presently established geothermometer can be applied to quartz chlorite fractionations in low grade metamorphic rocks and may be extrapolated to diagenetic systems

evolution of palaeofluids at the variscan thrust front in eastern belgium the geochemical evolution of the fluids migra ting at the variscan thrust front in eastern belgium has been investigated by a petrographic mineralogical and geoche mical study of ankerite quartz and ferroan calcite veins hosted by lower devonian rocks three vein generations have been recognized the first generation consists of quartz chlorite and ankerite filling pre to early variscan extensional fractures the second generation is present as shear veins of variscan age and contains quartz chlorite and ferroan calcite the third generation consists of ankerite filling post variscan fractures the oxygen and carbon isotopic composition of the two ankerite phases and of the ferroan calcites are respectively between 16 4 and 11 4 pdb between 17 8 and 1 7 pdb this range is greater than that of calcite nodules in the lower devonian siliciclastic sediments 18o 15 6 to 11 1 pdb and 13c 13 4 to 10 2 pdb this suggests precipitation of the carbonate veins from a fluid which was at most only partly isotopically buffered by the calcite nodules in the host rock the calculated oxygen isotopic composition of the ambient fluid from which the calcite veins formed is between 7 8 and 10 0 smow two main fluid types have been recognized in fluid inclusions in the quartz and carbonates the first fluid type is present as secondary fluid inclusions in the first and second vein generations the fluid has a salinity of 0 5 7 2 eq wt nacl and a high but variable homogenization temperature th 124 188 c two origins can be proposed for this fluid it could have been expelled from the lower devonian or could have been derived from the metamorphic zone to the south of the area studied taking into account the microthermometric and stable isotope data and the regional geological setting the fluid most likely originated from metamorphic rocks and interacted with the lower devonian along its migration path this is in agreement with numerical simulations of the palaeofluid and especially the palaeotempera ture field which is based on chlorite geothermometry and vitrinite reflectance data the second fluid type occurs as secondary inclusions in the shear veins and as fluid inclusions of unknown origin in post variscan ankerite veins therefore it has a post variscan age the inclusions are characterized by a high salinity 18 6 22 9 eq wt cacl2 the composition of the fluid is similar to that which caused the development of mississippi valley type pb zn deposits in belgium

fluid evolution in a slate belt gold deposit a fluid inclusion study of the hill end goldfield nsw australia auriferous quartz veins in the hill end goldfield nsw australia comprise bedding parallel vein sets and minor extension and fault controlled veins which are hosted by a multiply deformed late silurian slate metagreywacke turbidite sequence fluid inclusions in quartz either from bedding parallel veins or from narrow steeply n dipping veins leader veins indicate a similar range in homogenisation temperatures th from 350 c to 110 c within this range th data demonstrate five groupings in the temperature intervals 350 280 c 280 250 c 250 190 c 190 150 c and 150 110 c corresponding to a variety of primary and secondary inclusions developed during five periods of vein growth under a generally declining temperature regime inclusion fluids are characterised by a low salinity of around 0 1 to 3 6 wt nacl equivalent laser raman microprobe inclusion analysis indicates that gas phase compositions relate to the paragenetic stage of the host quartz h2o g and n2 dominate in the primary inclusions from barren stage i quartz ch4 and ch4 h2o g are important in inclusions related to the early gold forming events equivalent to stages ii and iii quartz but inclusions developed during the last episode of gold deposition are characterised by h2o g co2 rich and liquid co2 bearing fluids precipitation of gold was aided by sulphidation reactions or phase separation in response to periods of vein opening late in the paragenesis gold deposition may have been promoted by oxidation of the ore fluid

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

fluid evolution during burial and variscan deformation in the lower devonian rocks of the high ardenne slate belt belgium sources and causes of high salinity and c o h n fluids fluid inclusions in quartz veins of the high ardenne slate belt have preserved remnants of prograde and retrograde metamorphic fluids these fluids were examined by petrography microthermometry and raman analysis to define the chemical and spatial evolution of the fluids that circulated through the metamorphic area of the high ardenne slate belt the earliest fluid type was a mixed aqueous gaseous fluid h2o nacl co2 ch4 n2 occurring in growth zones and as isolated fluid inclusions in both the epizonal and anchizonal part of the metamorphic area in the central part of the metamorphic area epizone in addition to this mixed aqueous gaseous fluid primary and isolated fluid inclusions are also filled with a purely gaseous fluid co2 n2 ch4 during the variscan orogeny the chemical composition of gaseous fluids circulating through the lower devonian rocks in the epizonal part of the slate belt evolved from an earlier co2 ch4 n2 composition to a later composition enriched in n2 finally a late variscan aqueous fluid system with a h2o nacl composition migrated through the lower devonian rocks this latest type of fluid can be observed in and outside the epizonal metamorphic part of the high ardenne slate belt the chemical composition of the fluids throughout the metamorphic area shows a direct correlation with the metamorphic grade of the host rock in general the proportion of non polar species i e co2 ch4 n2 with respect to water and the proportion of non polar species other than co2 increase with increasing metamorphic grade within the slate belt in addition to this spatial evolution of the fluids the temporal evolution of the gaseous fluids is indicative for a gradual maturation due to metamorphism in the central part of the basin in addition to the maturity of the metamorphic fluids the salinity of the aqueous fluids also shows a link with the metamorphic grade of the host rock for the earliest and latest fluid inclusions in the anchizonal part of the high ardenne slate belt the salinity varies respectively between 0 and 3 5 eq wt nacl and between 0 and 2 7 eq wt nacl while in the epizonal part the salinity varies between 0 6 and 17 eq wt nacl and between 3 and 10 6 eq wt for the earliest and latest aqueous fluid inclusions respectively although high salinity fluids are often attributed to the original sedimentary setting the increasing salinity of the fluids that circulated through the lower devonian rocks in the high ardenne slate belt can be directly attributed to regional metamorphism more specifically the salinity of the primary fluid inclusions is related to hydrolysis reactions of cl bearing minerals during prograde metamorphism while the salinity of the secondary fluid inclusions is rather related to hydration reactions during retrograde metamorphism the temporal and spatial distribution of the fluids in the high ardenne slate belt are indicative for a closed fluid flow system present in the lower devonian rocks during burial and variscan deformation where fluids were in thermal and chemical equilibrium with the host rock such a closed fluid flow system is confirmed by stable isotope study of the veins and their adjacent host rock for which uniform 180 values of both the veins and their host rock demonstrate a rock buffered fluid flow system

rifting of old oceanic lithosphere geophysical data from five regions in the pacific and indian oceans reveal that long distance 400 km spreading center jumps have occurred in the past the present day seafloor morphology is used to develop a scenario for a spreading center jump the major events are 1 thinning and weakening of the lithosphere at the future rifting site 2 rifting of the weakened lithosphere during rifting the crack is filled from above by normal faulting and wedge subsidence viscous upwelling fills the crack from below 3 spreading at the rift site results in a ridge bounded by two troughs spreading ceases at the dying spreading center resulting in a deep central graben surrounded by flexural ridges periods of slow spreading at both spreading centers produce rough topography 4 ageing and cooling that produce a general deepening of the abandoned spreading ridge and also reduce the thermal contrast across the fossil rifting site the new spreading center develops into a normal spreading rift the major topographic expressions apparent in the seafloor today are the deep trough of the abandoned spreading center and the proximal and distal troughs which formed when the emerging spreading center bisected the fossil rifting site the proximal trough nearer the new spreading ridge and the distal trough farther from the new ridge are first order topographic features 100 1000 km long and 300 km wide resembling fracture zones with which they are often confused they share with fracture zones the characteristic of bringing together fragments of lithosphere of different ages but unlike fracture zones they are generally parallel to magnetic lineations

propagation and linkage of oceanic ridge segments we have investigated the propagation of spreading ridges and the development of structures that link ridge segments using an analogy between ridges and cracks in elastic plates the ridge propagation force and a path factor that controls propagation direction were calculated for echelon ridge segments propagating toward each other the ridge propagation force increases as ridge ends approach but then declines sharply as the ends pass so ridge segments may overlap somewhat the sign of the path factor changes as ridge ends approach and pass so the overlapping ridge ends may diverge and then converge following a hook shaped path the magnitudes of shear stresses in the plane of the plate and orientations of maximum shear planes between adjacent ridge segments were calculated to study transform faulting for different loading conditions simulating ridge push plate pull and ridge suction we identify a zone of intense mechanical interaction between adjacent ridge ends in which stresses are concentrated for all conditions the shear stress in the interaction zone increases as ends approach and remains large as the ends overlap thus crust in this zone may fracture and weaken in preparation for the formation of a through going transform fault the calculated shear planes rotate toward an orientation about 90 from the strike of ridges as the ends pass thus favoring the orthogonal arrangement of ridges and transforms the magnitudes of mean stresses in the plane of the plate and orientations of principal stress planes were also calculated the mean stress is tensile in the interaction zone so basins may form there except in the case of ridge push loading the planes across which the maximum tension acts are oblique to ridges thus favoring obliquely oriented normal faults bounding the transform valley

stress state evolution of the brittle upper crust during compressional tectonic inversion as defined by successive quartz vein types high ardenne slate belt germany abstract in the frontal part of the rhenohercynian fold and thrust belt high ardenne slate belt germany two successive types of quartz veins oriented normal and parallel to bedding respectively are interpreted to reflect the early variscan compressional tectonic inversion of the ardenne eifel sedimentary basin fracturing and sealing occurred in lower devonian siliciclastic multilayers under very low grade metamorphic conditions in a brittle upper crust a geometrical and microthermometric analysis of these veins has helped to constrain the kinematic and pressure temperature conditions of both vein types allowing the reconstruction of the stress state evolution in a basin during tectonic inversion it is demonstrated that bedding normal extension veins which developed under low differential stresses and repeatedly opened and sealed crack seal under near lithostatic fluid pressures reflect the latest stage of an extensional stress regime bedding parallel veins which developed at differential stresses that were still low enough to allow the formation of extension veins cross cut the bedding normal veins and preceded the regional fold and cleavage development these veins show a pronounced bedding parallel fabric reflecting bedding normal uplift and bedding parallel shearing under lithostatic to supra lithostatic fluid pressures during the early stages of a compressional stress regime this kinematic history corroborates that fluid overpressures are easy to maintain during compressional tectonic inversion at the onset of orogeny

gloria observations of the propagating rift at 95 50w on the cocos nazca spreading center in this paper we describe a gloria long range sidescan sonar survey of the area around 95 5 w on the cocos nazca rise where a propagating rift has been predicted to occur the sonar images clearly show a wedge of new crust that has been emplaced by propagation into 1 ma old crust the new crust is up to 800 m deeper than the surrounding seafloor although the elevation difference appears to decay with time the pseudofaults which bound this new crust have been observed from the rift tip back to crustal isochron 0 65 ma near the rift tip they consist of scarps several hundred meters high with narrow shallow valleys at their feet the sheared zone between the southern pseudofault and the trace of the failed rift to the south is characterized by abnormally shallow seafloor and nnw sse trending lineaments these may have been produced by the rotation of tectonic fabric formed at the dying rift perhaps accompanied by other deformation the youngest tectonic and volcanic lineaments on both spreading segments strike 273 but on the dying rift short spreading segments are offset en echelon so that the regional trend of the rise axis is 265 older lineaments throughout the area strike 265 the propagating rift is thus replacing an oblique rise axis by one which is orthogonal to the spreading direction and it is suggested that this is occurring in response to a recent change in spreading direction from 355 to 003 variations in the strikes of the pseudofaults indicate that the propagation direction has varied but that the rift has propagated most rapidly when following the 273 direction which we take to be normal to the new spreading direction

tectonic evolution of ridge axis discontinuities by the meeting linking or self decapitation of neighboring ridge segments recent deep tow sea beam and seamarc ii studies indicate that ridge axis discontinuities along the east pacific rise evolve in several different ways where magmatic pulses along the spreading center happen to meet head on a low point or saddle point occurs along the axial depth profile of the spreading center where the magmatic pulses misalign overlapping spreading centers develop and two distinct evolutionary paths are possible one spreading center tip may cut through to and link with the adjacent en echelon ridge chopping off the opposing ridge tip as proposed by macdonald and fox 1983 in addition either spreading center may cut inside or outside of itself repeatedly decapitating its own ridge tip crack propagation studies show that the crack propagation force g drops significantly when the ratio of crack overlap to crack offset l w exceeds 3 applying this relation to spreading centers we suggest that propagation of the individual spreading center tips may stall when l w is greater than 3 if linkage has not yet occurred then the next magmatic pulse that propagates along the ridge may be deflected away from the path of the existing ridge tip decapitating the ridge tip in a process we call self decapitation subsequent magmatic pulses may be deflected or derailed from the existing path of the ridge because freezing of the axial magma chamber near ridge axis discontinuities creates a core of coherent unfaulted gabbroic rock along the spreading axis which is strong relative to the intensely faulted lithosphere on either side of the frozen magma chamber in addition the local stress field rotates near the discontinuity so that subsequent magma pulses and associated cracking fronts will tend to deflect away from the preexisting path of the ridge for propagating rifts which represent first order changes in plate boundary geometry l w remains 1 5 so that g is maintained near its maximum value and episodes of continued propagation in the same direction are enhanced

quantitative methods for analyzing the roughness of the seafloor the bathymetry of the world ocean has been mapped using a variety of acoustic sounding instruments and traditional contouring methods prior to this study relatively few quantitative studies have been focused on the detailed morphology of the seafloor the work of most investigators has generally dealt with the analysis of seafloor relief without regard to spatial frequency or concern for obvious morphologic anisotropies a brief review of previous work is presented and is accompanied by an identification of the specific applications and limitations of those approaches the availability of more precise digital sounding data coupled with the development of higher resolution sonar mapping instruments e g deeptow seamarc now make it possible to investigate quantitatively many important aspects of seafloor relief and the geological processes responsible for its formation typical bathymetric contour maps represent a low frequency deterministic model of the seafloor to describe the higher frequency variability or roughness of the seafloor requires the development of an appropriate statistical method for generating a valid stochastic model new methods are developed herein which allow valid statistical models of the variability of oceanic depths to be derived from existing digital bathymetric soundings the smooth contoured surface often preserved as a geographic grid of depths when supplemented by such a roughness model can provide an essentially complete statistical description of the relief statistical models of seafloor roughness are also valuable tools for predicting acoustic scattering and bottom loss and in addition contain a wealth of information for more comprehensive interpretations of deep sea relief forming geological processes to allow the variability of depths to be described as a function of scale spatial frequency the amplitude spectrum is employed as the fundamental statistic underlying the model since the validity of the amplitude spectrum depends upon the assumption of a statistically stationary sample space a computer algorithm operating in the spatial domain was developed which delineates geographic provinces of limited statistical heterogeneity within each of these provinces a spectral model is derived by fitting the amplitude estimates with one or two two parameter power law functions using specialized regression techniques the distribution of resultant model parameters is examined for a large test area adjacent to the coast of oregon 42 45 n 130 124 w which includes several contrasting geologic environments the distribution of roughness corresponds generally with the various physiographic provinces observed in the region within some provinces additional complexities are apparent in the roughness model which cannot be inferred by simply studying the bathymetry these patterns are related to a variety of geological processes operating in the region such as the convergence of the continental margin and the presence of a propagating rift on the northern gorda rise in contrast a very large area of the continental margin off the east coast of the united states was found to have in common a single distinct amplitude spectrum this amplitude spectrum is almost identical to that found from the tufts abyssal plain region off the west coast of the united states spectra from both of these areas can be clearly separated into two straight line segments of different slope the two segments of these spectra are interpreted as reflecting two dominant relief forming processes the higher frequency band representing a sedimentary regime and the lower frequency band representing an underlying tectonic volcanogenic regime in many cases the calculated roughness statistics are not constant for data collected along different ship track directions this is due to the anisotropic nature of the seafloor relief a simple model is developed which describes the roughness statistics as a function of azimuth the parameters of this model quantify the anistropy of the seafloor allowing insight into the directionality of the corresponding relief forming processes and the physical meaning of the derived model statistical parameters finally the model is used to successfully predict the roughness of a surface at scales much smaller than those resolvable by surface sonar systems the model regression line derived from a hull mounted sonar is compared to data from deep towed sonars and bottom photographs the amplitude of roughness is predicted to within half an order of magnitude over five decades of spatial frequency and this prediction capability can probably be improved even further the stochastic models presented also demonstrate the potential for closely approximating the full two dimensional nature of some areas of the seafloor with only four statistical parameters these parameters can be estimated from random ship track data given a sufficient number of tracks and range of headings

practical geological comparison of some seafloor survey instruments seafloor survey instruments are integral to the study of marine geology because understanding their resolution and limitations is critical we compare how different survey systems represent the seafloor coincident data collected at the galapagos propagator gloria seamarc ii sea beam deep tow camera sled and alvin allow comparisons of how well seafloor features e g faults and volcanoes observed and characterized in high resolution data are represented in lower resolution coarser scale data sets our reported values for the minimum sizes of detected and well represented features show that practical geological resolutions are generally 2 10 times lower than theoretical resolutions care must be taken in evaluating which system to use to address a particular problem

syn to late orogenic quartz veins marking a retrograde deformation path in a slate belt examples from the high ardenne slate belt belgium abstract an integrated structural petrographic and microthermometric methodology has been applied on syn to late orogenic quartz veins from the high ardenne slate belt belgium to define their relative timing the quartz precipitates which do not show any mutual cross cutting relationships represent the meso scale brittle accommodation during fold initiation amplification and locking crystal plastic deformation structures and p t trapping conditions indicate that the different processes accommodating folding occurred in a progressive manner along a retrograde deformation path associated with the gradual exhumation of the slate belt from ca 7 5 to 6 km depth successive veining occurred from peak metamorphic conditions ca 300 c and 190 mpa measured in extrados veins to subsequent lower p t conditions in the periphery of a lenticular vein ca 275 c and 180 mpa late orogenic saddle reef filling ca 245 c and 160 mpa and the core of a lenticular vein ca 220 c and 150 mpa open cavity growth and fluid assisted brecciation indicate that veining occurred under overpressured fluid conditions it is therefore fair to assume that the high ardenne slate belt acted at least episodically as a mid crustal overpressured fluid reservoir not only in the earliest and latest stages but also during the main stage of orogeny

a fluid inclusion study of fluid pressure and salinity variations in the footwall of the rector branch thrust north carolina u s a abstract last melting and homogenization temperatures of fluid inclusions from plastically deformed bedding parallel quartz veins in the footwall of the rector branch thrust north carolina were studied as a function of distance from the thrust fluid inclusions and microstructures in mylonitic rocks within the thrust zone were also examined fluid inclusions in quartz veins which display evidence for intracrystalline plasticity e g subgrain polygonization occur along subgrain boundaries and have higher homogenization temperatures tn and a wider range 120 320 c compared to less deformed samples maximum th values which approach the temperature of deformation 300 20 c apparently reflect leakage of inclusions along subgrain boundaries minimum th values 120 160 c on the other hand record near lithostatic conditions 2 6 kb at 300 c maximum last melting temperatures tm increase from 20 to 4 c with decreasing distance to the thrust corresponding to a decrease in salinity of the fluid from 23 to 3 wt nacl equivalent the decrease in salinity towards the fault is interpreted as due to infiltration of the fault at depth to approximately 10 km by surface derived waters during periods of fault zone dilatancy inclusions along healed microcracks in quartz from the fault zone display higher salinity 17 26 wt nacl equiv and are interpreted to reflect enhanced fluid rock interaction in the fault zone due to hydration reactions the fluid pressure and salinity variations are consistent with a combined dilatancy hydraulic fracturing model for the rector branch thrust previously documented bulk rock volume losses for this fault zone are inferred to have been produced by the fluxing of the fault zone with undersaturated surface derived fluids

gravity and height variations connected with the current rifting episode in northern iceland abstract following a first gravity survey in 1938 high precision gravity measurements with lacoste romberg gravimeters have been carried out in northern iceland in 1965 1970 1971 and 1975 in order to detect eventual secular gravity variations at that supramarine part of a constructive plate boundary since 1975 when an active rifting episode started there gravity and height variations have been observed in annual intervals these surveys concentrated on the approx 100 km long and 5 km wide fissure zone associated with the krafla central volcano which is the center of the present activity the observations which have been performed at an accuracy level of 0 01 mgal resp 0 03 m give significant information about the long term behaviour of the vertical mass shifts occuring during the rifting process a steep gravity variation gradient observed south of the krafla caldera between 1965 and 1975 may be interpreted as some kind of precursor of the current process since 1975 the rifting event is characterized by shifting activity areas with gravity decrease and elevation at the flanks and gravity increase and subsidence at narrow central parts of the krafla fissure zone the annual variations reaching 0 1 mgal resp 0 5 m and more since 1978 the activity has concentrated on the gjastikki region north of the krafla volcano with decreasing variations in the southern and northern parts of the fissure swarm the average gravity height variation ratio calculated for two profiles south and north of krafla over the observation periods between 1975 and 1979 amounts to 0 2 mgal m but extreme values are found at recently activated areas

pelagic sedimentation on rough seafloor topography 2 inversion results from the north atlantic acoustic reverberation corridor seafloor slope distributions are sensitive to basement topography the surface shape of sediment distributions and the average thickness of sediment within a region by comparing slope distributions computed from bathymetric data to distributions from realistic models of sedimented topography we determine how characteristic values of average sediment thickness l rms basement variability h and apparent diffusivity vary throughout the office of naval research acoustic reverberation corridor located on the western flank of the mid atlantic ridge north of the kane fracture zone 25 5 27 25 n 44 5 49 w values of l from basin centered subregions show thinner sediments near the axis thickening westward with maximum values found on 10 20 ma seafloor lower values of l are found for older late oligocene early miocene seafloor values of l on young seafloor suggest a sediment accumulation rate of 6 5 2 3 m m y since 10 ma values of h computed from hill centered regions range from 217 to 306 m with an average of 267 27 m hills are generally larger to the north of a low running through the study area values of range from 0 12 to 0 77 m2 yr with an average of 0 32 0 19 m2 yr lower values of kbetter describe the surface shapes of individual high standing ponds while higher values are needed to model the surface shapes of thick deposits in regional lows this difference in with geological setting is probably due to variations in sediment transport mechanisms

burial diagenesis and porosity evolution upper jurassic smackover arkansas and louisiana the jurassic upper smackover ooid lime grainstone of the central gulf coast has been a major hydrocarbon exploration target for nearly half a century study of the geologic and sedimentologic framework petrography two phase fluid inclusions and trace element and stable isotope chemistry indicates the following diagenetic history the upper smackover is a blanket ooid grainstone ranging in thickness from 300 to 400 ft 91 to 122 m and covering at least 4 000 sq mi 10 400 sq km in the northern part of the arkansas shelf a regional meteoric water system was established in the grainstone shortly after deposition and prior to significant burial this meteoric water system gave way to conditions which were increasingly more marine toward the south near the louisiana arkansas state line the fresh to marine hydrologic gradient is responsible for the strong precompaction regional diagenetic overprint that eventually controlled available diagenetic pathways for the unit during burial three diagenetic zones are present 1 a northern zone dominated by oomoldic porosity and preburial spar cementation 2 a sou hern zone characterized by compaction effects late subsurface cement primary porosity preservation and late secondary subsurface porosity generation and 3 a transitional zone having characteristics common to both the northern and southern zones during the shallow burial phase compaction became increasingly more important to the south in zones dominated by marine connate fluid whereas rock integrity and a secondary oomoldic porosity system had already been established in the northern zone as burial increased brine originating in the underlying louann salt reached the shelf grainstone and began to displace the original connate fluid the brine triggered the inversion to calcite of remaining grains and cement composed of unstable mineralogy when the louann brine had displaced the original connate fluid caso4 replacement as gypsum and anhydrite was common across all three diagenetic zones also precompaction fine cement associated with oomoldic porosity was recrystallized and reequilibrated with the concentrate brine localized replacement dolomitization may well have developed throughout the burial history of the sequence but was most commonly postcompactional postcompaction poikilitic calcite cement and baroque dolomite were precipitated from the louann brine under conditions of elevated temperature and pressure as subsidence continued organic material from the basinward equivalents of the upper smackover lime grainstone arrived on the shelf and catalyzed so4 reduction resulting in pyrite and galena replacement in the southern and transitional zones as hydrocarbon maturation continued in the fine grained equivalents of the upper smackover h2s and co2 by products of these processes began to increase in the brine of the southern and transitional zones ultimately causing regional dissolution of existing carbonate and significantly enhancing preserved porosity porosity in the northern zone was established early and preserved through moderate burial although porosity is well developed permeability has been largely destroyed by recrystallization dolomitization is a prerequisite for economic reservoir development in the northern zone porosity in the southern zone is solution enhanced preserved primary intergranular porosity compaction is the most important porosity destruction process compaction in the absence of an early diagenetic overprint is affected primarily by grain type and secondarily by size and sorting pellets and oncolites compact easily yet destroy porosity during burial ooids and rhodolites resist compaction during burial therefore are related to porosity preservation grain type distribution as well as size and sorting are controlled by depositional environment processes with the most favorable areas for porosity development concentrations of ooids and rhodolites associated with early salt related structural movement porosity distribution in the southern zone therefore can be outli ed with the aid of geophysical end page 597 techniques porosity in the transitional zone is early secondary or preserved primary intergranular or a combination porosity in this zone therefore is highly unpredictable and a strong potential exists for diagenetic traps unrelated to structure or original depositional environment documentation of significant deep subsurface secondary porosity possibly associated with hydrocarbon maturation opens the potential for hydrocarbon exploration in deeply buried carbonate dominated basins this would include carbonate rocks heretofore thought to be well below the depth of carbonate porosity maintenance because of lithostatic load

tectonics of the failing spreading system associated with the 95 5 w galapagos propagator the failing spreading system associated with the galapagos 95 5 w propagator system comprises a complex set of volcanic and structural provinces data from detailed surveys using gloria seamarc ii sea beam deep tow the submersible alvin magnetics and bottom cameras constrain the plate boundary geometry and evolution of this area although the failing spreading system may involve diffuse spreading the patterns of sedimentation volcanism and faulting suggest localization the failed spreading axis composed of a set of right stepping en echelon basins extends and ages southeastward from offset and overlapping doomed failing axes the failing axis is offset 13 km left laterally from the doomed axis although there may have been a preexisting offset in the doomed axis lack of structural evidence for this and lack of an offset of this magnitude in the brunhes matuyama magnetic reversal argue against this possibility a model whereby the offset is transient does not explain the pattern of failed spreading basins which leads directly away from the currently failing axis detailed structural and volcanic patterns suggest that this offset may be caused by a westward progression of northward jumps in the location of the failing axis like a secondary propagator probably in association with the migrating strain stress field of the migrating transform zone of the propagator system this is modelled as a migrating extensional relay zone merz similar to the relay zones observed at some fast slipping transform faults except that it propagates westward with the overall system bathymetric and magnetic evidence for tectonic elements associated with the merz supports this model although the expected extinct trace of the doomed axis is not obvious in the present incomplete data set unlike other models for the plate boundary geometry the merz model does not require serendipity for temporal and spatial juxtaposition of the primary propagator and the offset within the failing spreading system observation of a similar geometry in the lau basin supports this preferred model present data from the galapagos failing spreading system are inconclusive however and the merz model remains speculative if valid this model implies that in at least some cases paired propagators act together the merz is too short and deep to drive its own propagation according to standard cracking models

punctiform initiation of seafloor spreading in the red sea during transition from a continental to an oceanic rift transition from a continental to an oceanic rift occurs in the red sea by initial emplacement of oceanic crust in regularly spaced hot points which serve as nuclei for axial propagation into segments of oceanic crust accretion and for initiation of seafloor spreading hot points develop above upwelling mantle diapirs caused by density viscosity inversion beneath continental and oceanic rifts

comparison of a stochastic seafloor model with seamarc ii bathymetry and sea beam data near the east pacific rise 13 15 n a recent seamarc ii survey along the hanks and crest of the east pacific rise between 13 and 15 n edwards et al 1988 provides us with a unique opportunity to compare stochastic models of the seafloor against bathymetric data stochastic models can be used to quantitatively characterize intermediate to small scale 100 km seafloor features such as abyssal hills in a recent paper goff and jordan 1988 introduced a model for the two point covariance function having five free parameters which describe the amplitude orientation characteristic wavelengths and hausdorff fractal dimension of seafloor topography these parameters can be estimated from multibeam data such as sea beam by inverting auto and cross covariance functions for the beams the stochastic model can be used to generate synthetic topography to arbitrary resolution and scale by assuming appropriate sea beam noise and response characteristics we can use the synthetic topography to produce synthetic data sets comparison of real and synthetic data is an important means of assessing the performance of the stochastic model unfortunately the limited spatial extent of sea beam in the across track direction and lack of 100 coverage over large areas severely limit our ability to compare the full two dimensional stochastic model with real seafloor morphology especially with respect to abyssal hill length characteristics the recent seamarc ii survey included sufficient off axis topography over 100 km 70 000 km2 to permit a comparison of a complete two dimensional synthetic topographic field with a region of abyssal hill terrain that has close to 100 data coverage we have estimated stochastic models for various regions of the seamarc ii survey area from several sea beam tracks which cross it synthetic fields are generated from these models using a fourier method these comparisons indicate the extent to which the model succeeds in characterizing seafloor topography and the directions we will need to take to improve our modeling a change in ridge complexity at 14 n and the location of a propagator pseudofault appear to be correlated with changes in stochastic character modeled in this region such correlations may provide important new constraints on the understanding of ridge processes

elastic deformation models of krafla volcano iceland for the decade 1975 through 1985 the dynamics of the shallow magma reservoir at krafla volcano in ne iceland have been analyzed with three types of elastic models based on over 70 surveys of tilt and displacement made from 1975 to 1985 a period of continuous volcano tectonic activity modeling results are integrated with geophysical and geological information to estimate the position geometry and volume change of magma reservoir domains subjected to periodic inflation and deflation dominating influences on magma reservoir dynamics are examined in the context of activity in krafla s associated rift system and the deformation of its caldera from 1975 1985 rather than the fluid filled cavity concept of mogi 1958 and most recent workers our models are idealized as strained regions with spherical double spherical or general ellipsoidal symmetry the models are mathematically generalized from that of mogi 1958 and are derived by inversion of displacements model results from different displacement components are remarkably consistent although models based on vertical displacements typically have errors of fit much closer to expected measurement errors than those based on tilt or horizontal displacements about one half of the double sphere and ellipsoid models have significantly better fits than single sphere models double sphere model results are consistent with a 2 to 3 km center to center separation of magma storage zones from at least 2 km to 4 km depth by portions of the fissure system as implied by s wave attenuation patterns for 1976 1977 however all models suggest pressurization zones of more limited extent than possible domains of storage inferred from s wave attenuation ellipsoid models typically implied unrealistically shallow depths of magma storage caldera inflation rates decreased after january 1978 when the caldera periodically reinflated to its level prior to the initial december 1975 deflation event from 1975 1980 the volume and duration of inflation between deflation events was strongly correlated with the volume of the previous deflation after 1980 there was a significant increase in the duration of inflation periods and a decrease in rates of caldera inflation and fissure system widening consistent with these results the magma reservoir is conceptualized as a hot rock mass containing numerous magma chambers and pressure sensitive conduits connecting the chambers and deep magma sources magma is injected into the fissure system at critical pressures determined by the confining stress and rock mass strength the duration and volume of inflation required to reach a critical pressure threshold is largely dependent on the volume of magma released in the previous deflation reduction of the extension rate across the krafla fissure system after 1980 suggests that extensional forces were also reduced a resultant rise of confining pressure on the magma reservoir and a reduced capacity of the fissure system to accommodate dike injection in combination would have increased critical stress levels for reservoir deflation and reduced the pressure gradient driving magma supply from deep sources

mechanisms of fine extinction band development in vein quartz new insights from correlative light and electron microscopy fine extinction bands febs also known as deformation lamellae visible with polarized light microscopy in quartz consist of a range of nanostructures inferring different formation processes previous transmission electron microscopy studies have shown that most feb nanostructures in naturally deformed quartz are elongated subgrains formed by recovery of dislocation slip bands here we show that three types of feb nanostructure occur in naturally deformed vein quartz from the low grade metamorphic high ardenne slate belt belgium prismatic oriented febs are defined by bands of dislocation walls dauphine twin boundaries present along the feb boundaries probably formed after feb formation in an example of two sub rhombohedral oriented febs developed as two sets in one grain the finer feb set consists of elongated subgrains similar to febs described in previous transmission electron microscopy studies the second wider feb set consists of bands with different dislocation density and fluid inclusion content the wider feb set is interpreted as bands with different plastic strain associated with the primary growth banding of the vein quartz grain the nanometre scale fluid inclusions are interpreted to have formed from structurally bounded hydroxyl groups that moreover facilitated formation of the elongate subgrains larger fluid inclusions aligned along febs are explained by fluid inclusion redistribution along dislocation cores the prismatic feb nanostructure and the relation between febs and growth bands have not been recognized before although related structures have been reported in experimentally deformed quartz

a chlorite solid solution geothermometer the los azufres mexico geothermal system chlorite constitutes a major hydrothermal alteration product of metamorphism of andesites in the active geothermal system of los azufres mexico electron microprobe analyses performed on a set of crystals from each sample show wide variations in composition correlation coefficients among chemical constituents were calculated it is shown that the tetrahedral charge is positively correlated with the octahedral vacancy and negatively with the iron content and there is almost no correlation with the octahedral aluminium and magnesium content a procedure is proposed to select end members and substitution vectors and to give a general formula for these chlorites

causes and rate limiting mechanisms of ridge propagation a fracture mechanics model all known propagating rifts pr s are growing away from shallow ridge axis seafloor we examine a model for rift propagation in which the ridge axis conduit like a crack in the lithosphere propagates in response to a stress concentration at its tip the stress concentration near the tip which causes progressive failure of the lithosphere is primarily due to excess gravity spreading stresses associated with anomalously shallow ridge axis along the growing ridge this stress concentration near the tip is characterized by a stress intensity factor k which must exceed a threshold value to cause failure of the plate a remotely applied tensile stress would make a positive contribution to k but would act equally on both ridge segments and not favor the propagation of either the pressure in the crack may depending on its distribution make either a positive or negative contribution to k for a high enough propagation rate viscous flow induced suction due to the growth of the ridge axis as well as the reduced vertical flow near the pr tip would make k 0 this simple model thus includes a mechanism to control the speed of propagation and is consistent with the axial morphology of the spreading segment associated with the well studied 95 5 w pr of the galapagos spreading center analytical solutions for the contribution to the stress intensity factor of viscous forces resisting ridge axis growth and causing the pr axial topography are consistent with the observed propagation rate for asthenosphere viscosities of about 1019 pa s 1020 p we also examine a more general model of a ridge transform ridge system on the basis of energy conservation in terms of energetics this more general model reduces to that of a single propagating ridge axis crack when both the work done by forces driving propagation and the viscous dissipation due to forces resisting propagation are generated along the growing ridge axis however the more general model also includes mechanisms that may stop ridge propagation propagation will stop either when the energy needed for transform migration is larger than that available for ridge axis growth or when the strain energy absorbed at the dying ridge axis is comparable to that released at the growing ridge axis i e if the stress intensity factors for both ridge axes are similar both of these possibilities may occur when the propagating rift completely propagates through a ridge segment as at 85 w in the galapagos finally in this model the slight systematic differences in ridge axis orientation of the growing and dying ridges are a consequence rather than a cause of ridge propagation

quartz veins with associated sb pb ag au mineralization in the schwarzwald sw germany a record of metamorphic cooling tectonic rifting and element remobilization processes in the variscan belt a combination of textural observations from 38 permian sb pb ag au bearing quartz veins in the schwarzwald sw germany with isotopic fluid inclusion and geochemical data allows to refine genetic models for this common type of mineralization to understand the origin of mineralogical diversity and the correlation with large scale tectonic events textures record four main mineralization stages i fe as sb au ii pb zn cu iii pb sb and iv ag sb stage i sulfides dominated by pyrite and arsenopyrite formed due to cooling of low salinity metamorphic fluids during the permian with maximum homogenization temperatures of 400 c invisible gold in arsenopyrite pyrite marcasite and stibnite was remobilized and locally precipitated as electrum at the end of stage i minerals of stages ii and iii comprise a rich diversity of sb bearing sulfosalts including e g bournonite zinkenite and jamesonite this assemblage formed during jurassic times due to mixing of high salinity mid and upper crustal fluids up to 27 wt nacl cacl2 with homogenization temperatures between 50 and 250 c stage iii is marked by a rich variety of pb sb sulfosalts its local abundance is directly related to the presence of stage i mineralization and its extent of remobilization during a significant influx of pb in the jurassic cretaceous the formation of the upper rhine graben during the tertiary reactivated especially ne sw oriented veins percolating ag rich fluids reacted with earlier stage iii pb sb sulfosalts forming ag rich minerals stage iv such as miargyrite pyrargyrite and stephanite the transition from metamorphic fluids to basinal saline e g pb bearing brines over hundreds of millions of years as is shown in this study and present in other variscan occurrences indicates a possibly typical poly stage characteristic of sb deposits worldwide which has however not been investigated in detail so far

sulphur isotope geochemistry of black shale hosted antimony mineralization arnsberg northern rhenish massif germany implications for late stage fluid flow during the variscan orogeny vein type and bedding concordant mesothermal 180 410 c stibnite sulphosalt mineralization at arnsberg ne rhenish massif germany is hosted by carboniferous pyrite rich black shales and siliceous limestones a detailed sulphur isotope study of the stibnite sulphosalt mineralization and pyrite from a variety of regional host rock lithologies has been carried out using an in situ laser combustion technique the 34 s values of stibnite of various textural types are distinctly negative and lie in a narrow range between 23 9 and 17 1 mean 20 1 in contrast regional sedimentary diagenetic pyrites display a large variation of their 34 s values between 45 4 and 9 3 there is little evidence for significant modification of the hydrothermal fluid during deposition and the s isotope signatures suggest that the sulphur of the stibnite mineralization was not locally derived the 34 s values of pyrite in givetian shales display a significantly narrower range of 28 2 to 7 5 and their mean composition 17 1 is close to the 34 s values of the arnsberg stibnite deposits considering the temperature dependent isotopic fractionation between stibnite and reduced sulfur species the 34 s values of the mineralizing fluid 16 8 200 c and the givetian rock source are essentially identical therefore we propose a model of leaching and isotopic homogenization of sulphur from the middle devonian shales and a subsequent northward migration of these fluids the fluids were trapped in permeability controlled positions within anticlinal zones where fluid cooling induced deposition of stibnite and sulphosalts

a characterization of the spectral density of residual ocean floor topography ocean floor topography can be characterized as a signal related to lithospheric cooling and a residual the residual can be further modeled by comparing the shape of its power spectrum with that of a fractal process power spectra of seabeam profiles along a flowline in the south atlantic are calculated the data are resampled pre whitened by first differencing tapered with a hanning window and passed to an fft the effects of the pre whitening are removed and the spectra are smoothed the spectra of ocean floor topographic residuals are curved power increases less rapidly with increasing wavelength at wavelengths greater than several 10 s of kilometers and straight at shorter wavelengths

epigenetic to low grade tourmaline in the gdoumont metaconglomerates belgium a sensitive probe of its chemical environment of formation abstract this study reports the first observation and compositional data of tourmaline overgrowths in the post caledonian basal gdoumont conglomerates of the belgian part of the rhenohercynian basin authigenic tourmaline occurs as monopolar hemimorphic and bipolar asymmetrical foitite overgrowths within fractures and along the margins of detrital tourmaline grains and tourmalinite pebbles formulae calculated from electron microprobe analyses reveal high amounts of vacancies in the x site 61 to 84 placing this tourmaline amongst the most alkali deficient tourmaline recorded to date besides the alkali deficiency gdoumont overgrowths are characterized by high al low na and ca and low to moderate mg fe mg ratios these characteristics reflect the composition of the host conglomerates and suggest local derivation of the chemical components slightly differing zoning patterns sample scale and differences between localities regional scale indicate heterogeneity of the low grade host rock host tourmaline grains as well as tourmaline occurring in quartz veins crosscutting the gdoumont metasediments have distinctly different compositions the absence of discontinuous zoning suggests growth of the authigenic tourmaline in just one stage it is most likely that tourmaline formed from boron released by metamorphic breakdown of detrital minerals e g illite and or k feldspar based on their crystal chemistry and the observed optical properties the overgrowths must have developed during diagenetic to lower greenschist facies conditions scarce concurrency of bipolar overgrowths isolated neoblasts and cryptocrystalline nests of tourmaline indicates limited growth until middle greenschists facies conditions

pelagic sedimentation on rough seafloor topography 1 forward model this is the first paper in a four part sequence exploring the factors that govern the distribution of pelagic sediments in regions of the world mid ocean ridge system where basement outcrops and sediment deposits both contribute to local topography in this paper we present a simple numerical model of pelagic sedimentation in which sediments travel from topographic highs to lows with flux regulated by a diffusivity parameter by applying the forward model to realistic models of basement topography we investigate how various basement structures sediment rain rates and values of affect the morphology of the seafloor through time we quantify the effect of sedimentation as a nonlinear smoothing filter on topography which acts to reduce spectral power at high wavenumber to decrease the root mean square variability of topography to increase the spacing between hills and to lower the relative proportion of high slope seafloor we find that local accumulation rate is highly dependent on local topography and varies greatly with time we also find that sediment compaction does not contribute directly to the surface concavity exhibited by many intrahill sediment ponds

lithospheric stress near a ridge transform intersection oceanic lithosphere at a ridge transform intersection has distinct structural characteristics the spreading axis and normal fault scarps which parallel the ridge axis away from the intersection locally curve towards the transform fault this implies that the least compressive stress direction becomes oblique to the spreading direction near a ridge transform intersection this stress field may be the result of a tensile ridge axis stress resisting plate separation and a transform shear stress resisting relative plate motion a plane stress plate model with prescribed plate boundary stresses is formulated to explore this hypothesis the orientation of the least compressive horizontal stress near an intersection depends strongly on the ratio of the applied boundary stresses thus ridge axis curvature may place an important constraint on plate boundary forces

stochastic modeling of seafloor morphology inversion of sea beam data for second order statistics at scale lengths less than 100 km or so statistical descriptions of seafloor morphology can be usefully employed to characterize ridge crest processes off ridge tectonics and vulcanism sedimentation and postdepositional transport we seek to develop methods for the estimation of seafloor statistics that take into account the finite precision resolution and sampling obtained by actual echo sounding systems in this initial paper we restrict our attention to the problem of recovering second order statistics from data sets collected by multibeam devices such as sea beam the seafloor is modeled as a stationary zero mean gaussian random field completely specified by its two point covariance function we introduce an anisotropic two point covariance function that has five free parameters describing the amplitude orientation characteristic wave numbers and hausdorff fractal dimension of seafloor topography we formulate the general forward problem relating this model to the statistics of an ideal multibeam echo sounder in particular the along track autocovariance functions of individual beams and the cross covariance functions between beams of arbitrary separation using these second moments as data functionals we then pose the inverse problem of estimating the seafloor parameters from realistic noisy data sets with finite sampling and beamwidth and we solve this inverse problem by an iterative linearized least squares method the inversion method is applied to sea beam transit data from both the pacific and atlantic oceans sea beam system noise stands out as a sharp spike on the along track autocovariance function and can be modeled as a white noise process whose amplitude generally increases with beam angle the five parameters in our second order model can be estimated from the inversion of data sets comprising 100 200 km of track length in general the cross track wave number is the most poorly determined although uncertainties in the assumed sea beam response may bias the values of the fractal dimension using the assumed beamwidth the measured noise values and the seafloor parameters recovered from the inversion we generate sea beam synthetics whose statistical character can be directly compared with raw sea beam data for most of the track segments we have processed thus far the synthetics are similar to the data in the case of one atlantic profile however the comparison clearly indicates the necessity of incorporating higher order statistics the space domain procedures described in this paper can be extended for this purpose

mixing of h2o co2 in fluid inclusions geobarometry and archean gold deposits abstract a modified redlich kwong mrk equation of state for h 2 o and co 2 was used to calculate bulk molar volumes mole fractions and co 2 densities for mixed volatile fluid inclusions the results allow very useful p t diagrams to be constructed for fixed co 2 density and contoured with volume percent co 2 observed in the laboratory these diagrams permit rapid qualitative or quantitative determination of the p t path for a bulk inclusion isochore above the solvus in the system h 2 o co 2 nacl the calculations suggest peak metamorphic pressures of 3 5 kb for some archean greenstone gold deposits insights gained from this work may require reinterpretation of pressure estimates in published fluid inclusion studies

the use of a combined structural stable isotope and fluid inclusion study to constrain the kinematic history at the northern variscan front zone bettrechies northern france abstract a detailed structural stable isotope and fluid inclusion study of distinct vein generations has been performed in a kilometre scale syncline to correlate the temperature pressure conditions of vein formation with specific deformation episodes the structural relationship of the veins with cleavage folds faults and with one another allows the identification of pre syn and post variscan carbonate vein generations the stable isotope composition of the vein calcites and dolomites is very similar to that of the surrounding limestones and dolostones respectively this indicates that the cements were precipitated from fluids buffered by the host rock and implies that the temperature pressure characteristics of the fluids reflect the thermal history of the deformation history within the syncline microthermometric data of primary fluid inclusions were used to estimate the temperature pressure conditions at precipitation time and thus of the deformation pressure corrected trapping temperatures for the veins show a temperature evolution from pre variscan 310 c to variscan 260 200 c and post variscan 75 c this evolution indicates that the syncline developed not at a specific depth but rather as an active progressive deformation process during transport along the northern variscan thrust front

regional and local vertical crustal movements in northern iceland 1965 1980 since 1965 repeated gravity and height measurements have been carried out in northern iceland across the exposed part of the constructive plate boundary in order to detect vertical crustal movements regional movements have been observed across the neovolcanic zone between the western and eastern tertiary basalt zones the center of activity is situated below the krafla caldera and since 1975 a rift process is affecting the 100 km long and 5 km wide fissure zone associated with this central volcano local vertical movements have been investigated by reobserving transverse profiles approximately 10 km south and 15 km north of krafla the recent rifting process is characterized by regional uplift and gravity decrease on the flanks and local subsidence and gravity increase in the central fissure zone the results of repeated levelings and gravity measurements are used to produce common and continuous models of vertical crustal movements

major element constraints on melting differentiation and mixing of magmas from the galapagos 95 5 w propagating rift system projections in the pseudo quaternary system diopside plagioclase olivine quartz di pl ol q are used in this paper as a base on which to plot additional compositional petrologic or tectonic variables used in this manner the projections provide a sensitive means of displaying and evaluating the chemical variability of mid ocean ridge basalt morb suites such as that of the galapagos 95 5 w propagating rift system and allow mantle controls on the chemical variability of morb to be distinguished from the predominant effects of shallow level crystal fractionation and mixing primitive lavas mg 63 from the 95 5 w region form a broadly linear array within the di pl ol q tetrahedron parallel to the di ol join in terms of two recent high pressure studies of morb petrogenesis stolper 1980 takahashi and kushiro 1983 the primitive liquids in this array are not primary but have evolved by less than 10 olivine fractionation from an array of primary liquids which separated from residual mantle over a range of pressures between 10 20 kb with appropriate assumptions as to residual lithology and mg of residual olivine the nature and origin of 95 5 w primary magmas can be deduced 1 lavas of the normal rift are derived from a broad range of primary magmas that separated from compositionally uniform in terms of major elements mantle over a pressure range of 10 20 kb 2 beneath the newly forming propagating rift melting is initiated perhaps by pressure release at shallow depth 9 11 kb degree of melting and source composition appear similar to those for the normal rift within a few km of the propagating rift tip this shallow source is joined by a second deeper 11 12 kb more iron rich source both sources persist for at least 100 km along the propagating rift but the deeper source appears to become dominant farther behind the tip maximum pressure for melt separation anywhere beneath the propagating rift is less than 15 kb in addition to these source effects the projections allow a more complete description of the shallow level fractionation and mixing effects along the propagating rift primitive lavas of restricted composition erupted near the rift tip give way within 2 3 km to an increasingly diverse suite of lavas characterized by variable extents of fractionation and by numerous single mixing events consistent with magmatic evolution in small only occasionally interconnected magma bodies beyond 15 km the diversity gradually declines toward a restricted population of cpx saturated lavas which characterizes the propagating rift beyond 75 km and is consistent with the presence of a large well mixed magma chamber in this region

preferential rifting of continents a source of displaced terranes lithospheric rifting while prevalent in the continents rarely occurs in oceanic regions to explain this preferential rifting of continents we compare the total strength of different lithospheres by integrating the limits of lithospheric stress with depth comparisons of total strength indicate that continental lithosphere is weaker than oceanic lithosphere by about a factor of 3 also a thickened crust can halve the total strength of normal continental lithosphere because the weakest area acts as a stress guide any rifting close to an ocean continent boundary would prefer a continental pathway this results in the formation of small continental fragments or microplates that once accreted back to a continent during subduction are seen as displaced terranes in addition the large crustal thicknesses associated with suture zones would ironically make such areas likely locations for future rifting episodes this results in the tendency described as the wilson cycle of new oceans to open along the suture where a former ocean had closed

eruptive history of the rhyolitic kane springs wash volcanic center nevada the 19 13 km kane springs wash caldera formed 14 ma ago following eruption of the three youngest members of the kane wash tuff two older cooling units of the kane wash tuff are dated 15 7 0 2 ma and 14 7 0 2 ma and apparently erupted from source areas west of the exposed caldera volcanic eruptions from the kane springs wash center began about 14 ma ago with extrusion of trachyte lavas now preserved north of the caldera member v1 of the kane wash tuff overlies the trachytes and is zoned from rhyolite to trachyte in contrast overlying members v2 and v3 are slightly peralkaline rhyolites comendite eruption of these tuffs 14 1 0 2 ma ago initiated caldera collapse following collapse trachyte lavas indistinguishable in age from the ash flow tuffs erupted within the caldera forming a 5 m diameter cumulodome whose interior portions crystallized to syenite this magmatic resurgence formed an extrusive complex within the caldera rather than doming the cauldron block as in classic resurgent calderas metaluminous ferroedenite rhyolite lavas and associated ash flow tuffs then erupted in the moat surrounding the central trachyte syenite complex pyroclastic deposits from the domes were covered by trachyandesite and basalt lava flows about 13 4 ma ago biotiterhyolite domes dated 13 4 0 2 ma continued to erupt in the northeastern moat and appear to be the last silicic eruptions from the kane springs wash magma system a 13 4 ma metaluminous biotite rhyolite dome containing vapor phase topaz overlies the trachyandesite lavas and appears to be the youngest biotite rhyolite dome these intracaldera units filled the moat resulting in a caldera filled with its own eruptive products local basalt flows covered both intracaldera units and outflow sheets between 12 7 and 11 5 ma ago prior to basin and range faulting the dominant volume of the system was trachytic as trachyte lavas erupted both prior to and following caldera collapse and trachyte inclusions occur as scoriaceous bombs at the top of member v2 fractionation of this trachyte magma produced the slightly peralkaline rhyolitic magma that erupted as the kane wash tuff the shift from an anhydrous mineral assemblage in the tuffs to hydrous assemblages in the later intracaldera rhyolites shows that the system was cooling and was possibly more hydrous in its late stages petrographic characteristics of trachyandesite lavas erupted late in the history of the center suggest that they represent basaltic magmas contaminated with silicic material

quantitative comparison of bathymetric survey systems spatially coincident deep tow sea beam and seamarc ii bathymetric data are statistically analyzed to determine the scales at which each system is reliable for mapping seafloor features the methods we employ including ruler measures of contours and profiles and the covariance function and power spectrum of profiles allow us to investigate the size of features measured as a function of scale

submarine salt flows in the central red sea the central red sea an oceanic basin floored by miocene evaporites reaching kilome ters in thickness in places is at an early stage of development where seafl oor spreading has geologically only recently replaced continental rifting surveys using a high resolution multibeam echo sounder around thetis deep a new spreading center have revealed a remarkable series of structures resembling viscous gravity fl ows which are here interpreted as originating from fl owage of the evaporites laterally unloaded by axial rifting and other processes developing the relief of the deep the fl ow margins are marked by stream wise lineaments and some apparently rotated markers their fronts in the fl oor of the deep are rounded in plan view and profi le their surfaces contain small closely spaced features resembling extensional faults in one area below declining gradients the surface contains along slope ridges and valleys typical of compression folds ogives flowparallel lineaments and extensional faults lie respectively parallel and orthogonal to the direction of maximum seabed gradient movement is apparently heterogeneous at least in part because of varied blocking by relief in underlying basement observed protruding between fl ows flowage is currently transporting materials into the fl oor of the deep where they have the potential to become incorporated into the young oceanic crust by repeated eruption of axial lavas over them in the light of these new data we reexamine the possibility and implications of fl owage in the south atlantic marginal evaporites in particular whether fl owage contaminated early oceanic crust in such areas

late variscan fluid migration at the variscan thrust front of eastern belgium numerical modelling of the palaeothermal and fluid flow field modelling of the palaeothermal field at the variscan thrust front in eastern belgium indicates significant temperature modifications by late variscan palaeofluids migrating from internal to peripheral parts of the orogen a detailed set of calibration data chlorite geothermometry microthermometry organic rank gives evidence of temporary palaeotemperature variations at the variscan thrust front obviously connected to the migration of hot low saline palaeofluids these thermal events likely enhanced organic maturation vitrinite reflectance conodont alteration of devonian and carboniferous sediments which accumulated long before the variscan orogeny occurred numerical simulation 2d finite element method of the palaeothermal field includes coupled heat transport by thermal conduction and fluid flow palaeothermal scenarios yield successive palaeotemperatures 200 300 c which are indicated by the control data due to relatively short term fluid ascent along the detachment and the imbricate thrust front the simulated flow velocities are up to tens of metre per year lasting several thousand years non steady state solution in the scenarios modelled these thermal events occur in a realm of enhanced bulk temperatures due to elevated basal heat flow densities 90 mw m 2 and an additional burial depth of some kilometres the simulated temperature enhancement due to fluids ascending at the variscan thrust front is several tens degrees the scenarios demonstrate long distance fluid migration during or after deformation of the palaeozoic basin and its effect on the palaeothermal field

three dimensional magnetic modeling of a propagating rift galapagos 95 30 w a combined deep tow and seabeam investigation at 95 30 w on the cocos nazca spreading center has revealed the crustal contact between the propagating rift and the dying rift systems the observed 27 km offset between the axes creates an ideal situation for the application of magnetic methods the normally magnetized crust of the propagating rift tip penetrates into older crust which was created when the earth s main field was reversed as the v shaped structure of the rift clearly violates the conventional two dimensional assumptions a full three dimensional analysis is required in addition the distorting influence of over 1000 m bathymetric relief must be removed from the observed field inversions have been performed on the gridded representations of the observed magnetic field and bathymetry working in the fourier domain the result is a gridded rock magnetization distribution the inversion of the surface data covers a large area 6000 km2 and demonstrates close agreement with magnetization amplitudes of rock samples at existing dredge sites in general the propagating rift process appears to be much more orderly than the dying rift process the magnetic polarity transition widths are narrower and the boundaries have fewer undulations than the dying rift which appears to be quite episodic in behavior the average propagation rate is 52 mm yr compared to the average spreading half rate of 29 mm yr the locations of the boundaries suggest that the acceleration to the normal spreading rate on the propagating rift requires about 250 000 years the inversion of the deep tow data near the seafloor provides a high resolution definition of the location of the tip of the propagating rift at 2 38 1 n 95 30 0 w

rifting history of the northern mariana trough seamarc ii and seismic reflection surveys seamarc ii sidescan sonar data and seismic reflection profiles of the northernmost portion of the mariana trough suggest a rifting rather than a seafloor spreading origin for lithosphere within this active backarc basin of the western pacific variations in morphology along the strike of the backarc basin suggest both a spatial and a temporal variation in the style of extension of the basin the northern mariana trough widens from 70 km near 23 n to 135 km near 21 30 n major active fault zones bound both sides of the backarc basin active rifting and volcanism are predominant in a roughly 20 km wide active rift zone along the eastern arc side of the basin from 24 n to 22 n the locus of active rifting has remained near the volcanic front of the arc during extension and is probably controlled by lithospheric weakness which is greatest at the volcanic front the zone of rifting intersects the active volcanic arc at nikko seamount the distinction between the mechanism for formation of the mariana trough proposed here and that of slow mid ocean ridge spreading is that no true plate boundary has developed in the northernmost mariana trough rather the backarc basin is composed of arc lithosphere intruded by arc and backarc basin magmas volcanism both in the rift zone and throughout the basin is fault controlled south of 22 n the zone of active rifting diverges westward from the arc as the transition to true seafloor spreading is approached the transition to true mid ocean ridge spreading occurs to the south of the survey area described here

cenozoic tectonics on the hautes fagnes plateau belgium abstract morphological features under which deformations of oligocene surfaces are included demonstrate the varied tectonic activity of the hautes fagnes plateau during cenozoic times beyond the general uplift of the region a local movement raised the baraque michel massif by about 100 m above the nearby crest of la vecquee moreover several zones of radial nw se faulting are active on the plateau they are related to the opening of the lower rhine embayment and represent the ultimate sw extension of the fault system associated with this opening

mantle viscosity beneath the galapagos 95 50 w propagating rift detailed geophysical surveys in the vicinity of the galapagos 95 5 w propagating rift tip establish the opening history of the rift and its velocity of propagation these data together with a theory for mantle upwelling into slowly widening lithospheric cracks constrain the viscosity of the asthenosphere beneath the propagating rift to be less than about 1017 to 1018 pa s

contemporary plate motion and crustal deformation the measurement of relative plate velocities during the past few years is a signal accomplishment in earth science leading to refinement of the precepts of steady motion of plate interiors and cyclic deformation along plate margins regrettably deformation premonitory to an earthquake has yet to be detected with confidence delineation of the spatial and temporal buildup of strain between earthquakes however has put limits on models of the earthquake cycle a diverse set of fault structures and crustal rheologic conditions can explain the pattern of surface strain accumulation and release along strike slip faults in contrast the geometry of thrusts and normal faults revealed by earthquake deformation has been found to differ markedly from expectations geodetic observations have proved vital to monitor the ascent of magma through the earth s crust and to predict volcanic eruptions at the earth s surface episodic vertical and horizontal deformation in southern california remains a subject of dispute if anything it is less certain than it once seemed

comment on the geometry of propagating rifts by d p mckenzie commentaire sur l article de d p mckenzie the geometry of propagating rifts earth planet sci lett vol 77 pp 176 186