a mechanism for magmatic accretion under spreading centres the most active volcanic zone on earth occurs along the global mid ocean ridge system where the lithosphere is pulled apart and new oceanic crust is created at rates between 10 and 200 km myr 1 this volcanism is segmented and is concentrated along the axial valleys and highs of slow and fast spreading ridges respectively with transform fault zones separating and generally offsetting each active ridge segment volcanic activity may be nearly absent at the ridge transform intersection where a slow spreading ridge is offset and greatly reduced where a fast spreading ridge is offset implying thinner crust in these regions1 the source of magmas erupted along ocean ridges is the underlying mantle which undergoes decompression melting when rising to fill the gap between the spreading plates the resulting magma aggregates in the upper mantle and ascends to the crust due to its low density compared with the parent mantle rock the melt is believed to pool in crustal magma chambers whence it periodically erupts to the surface typically the formation ascent and aggregation of magmas along the mid ocean ridges is regarded two dimensionally and modelled in a section through the crust and mantle across the strike of the ridges see ref 2 here however we consider the problem with a three dimensional model for which we provide supporting geological evidence in which the magmas rise as a result of a gravitational instability modified rayleigh taylor in the underlying partial melt zone

giant bauxite deposits of south china multistage formation linked to late paleozoic ice age lpia eustatic fluctuations abstract the lower carboniferous jiujialu formation bauxite deposits at zunyi northern guizhou are distinguished by their great thickness to 110 m and cyclic alternation of bauxite ore and bauxitic claystone layers these features are typical of the kazakhstan subtype of karst bauxite deposits although the genesis of this type of deposit is poorly understood here we undertook a petrological mineralogical and geochemical study of the zunyi deposit in drillcore zk5600 to gain insights into its formation history the zunyi profile contains seven bauxite cycles as recorded by petrological mineralogical and geochemical features indicating a multistage formation history each cycle is composed of a lower bauxitic claystone layer and an upper bauxite ore layer the bauxite ore is characterized by high al2o3 high al mineral diaspore and low sio2 content positive ce anomalies high cia values 90 and strong depletion of major elements indicate that the bauxite ore formed through intense chemical weathering and leaching in a well drained soil environment the bauxitic claystone is characterized by relatively lower al2o3 and higher sio2 and clay mineral mostly illite content this lithology exhibits both negative and positive ce anomalies moderate cia values 70 80 and enrichment of some elements e g fe k indicating formation through moderate chemical weathering in a poorly drained soil environment secular variations in chemical weathering intensity and soil drainage conditions were linked to regional hydrologic changes during the early carboniferous the zunyi area was located in a karstified coastal upland setting and its hydrologic system was affected by glacio eustatic fluctuations high groundwater table elevations during eustatic highstands led to formation of bauxitic claystone layers whereas low groundwater table elevations during eustatic lowstands led to formation of bauxite ore layers

along axis variability in crustal accretion at the mid atlantic ridge results from the ocean study the ocean experiment is an integrated geophysical study of a region of the cape verde abyssal plain that formed at 140 ma deep seismic reflection and ocean bottom hydrophone obh refraction data were acquired along lines parallel and perpendicular to the paleoridge axis trend identified from a detailed magnetic anomaly survey the igneous basement is overlain by about 1 3 km of sediment which enables improved imaging of intracrustal structure beyond that possible near the mid atlantic ridge axis we describe the results of a 150 km long profile oriented parallel to magnetic anomalies m15 and m16 along which deep seismic reflection data collected by the british institutions reflection profiling syndicate are complemented by refraction data constrained by four obhs the line spans an entire spreading segment between two fracture zones the northern of which has an offset of 40 km and the other central has an offset of only 10 km away from the fracture zones the mean igneous crustal thickness is 7 2 km near both fracture zones thinning of up to 4 km is observed giving a mean igneous crustal thickness over the whole segment of approximately 6 5 km differences are seen between the two fracture zones in their seismic velocity structure in the associated basement topography and in the presence of a strong reflection extending into the mantle beneath the northern fracture zone the boundary between oceanic layers 2 and 3 correlates with variably coherent normal incidence reflections and a change in the character of the reflectivity a number of planar reflections up to 10 km in length are present within the middle and lower crust dipping outward from beneath low amplitude basement highs at 15 these appear to be present only within layer 3 the moho has several expressions in the reflection data including isolated reflection events a local increase in reflected amplitudes and a downward decrease in coherent reflections at the center of the segment there is a zone at the base of the crust within which both high and low velocity materials are present this zone shows an enhanced level of discontinuous normal incidence reflectivity and may represent an initial fractionation event as melt was emplaced at the spreading ridge

on the strength of oceanic fracture zones and their influence on the intraplate stress field we use the locations and source mechanisms of oceanic intraplate earthquakes to test the hypothesis that the strength of oceanic fracture zones is less than that of normal oceanic lithosphere if fracture zones are weaker than the surrounding lithosphere they should be sites of concentrated intraplate seismicity and the lithospheric stress field should be perturbed in their vicinity the 77 earthquakes selected for the study have well determined focal mechanisms and epicenters in regions where fracture zones are well mapped we have searched for dependence of faulting style fault orientation or principal stress direction on the distance from the nearest fracture zone if fracture zones were generally weaker than the surrounding lithosphere one of the principal horizontal stresses would be oriented nearly perpendicular to the fracture zone we find no evidence that principal stresses near fracture zones are oriented preferentially in this manner there is a slight tendency for earthquakes to occur near fracture zones and patterns of fault orientation and sense of slip support the view that differential cooling and horizontal contraction on fracture zones may contribute seismogenic stress these patterns are not demonstrable at high significance however if oceanic transform fault zones are weak relative to the surrounding lithosphere these results suggest that the strength of the fracture zones increases within a few million years after cessation of transform motion to levels comparable with that of normal oceanic lithosphere

strength of fracture zones from their bathymetric and gravitational evolution fracture zone evolution is investigated using dynamic models that allow the fault zones to freely slip this is an improvement over past formulations where bathymetric offsets were imposed kinematically the models use a viscoelastoplastic rheology that incorporates the influence of fault friction on fracture zone slip history using viscoelastic plates we assess the role of small scale convection on removal of the lowermost thermal lithosphere beneath fracture zones through a comparison of synthetic gravity to free air gravity across fracture zones we find that the amplitude of the gravity jump across fracture zones is best fit by models with weak faults that have depth averaged yield strengths 10 mpa fracture zones with such low strengths can convert to subduction zones with 100 km of convergence many fracture zones do not fit plate subsidence models with locked or slipping faults but are better fit by systems that are tectonically deformed by modest amounts of extension

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

intraplate seismicity and stresses in young oceanic lithosphere we have conducted a systematic study of oceanic intraplate seismicity in the period 1964 1983 occurring in lithosphere younger than 35 million years focal mechanisms determined for 26 events using first motions p wave modeling and surface wave amplitude radiation patterns along with 13 previously determined mechanisms indicate a diversity of faulting styles and stress orientations in young oceanic lithosphere there is no evidence for a worldwide transition from extension to compression at a specific lithospheric age tensional axes of normal faulting events are oriented oblique to the spreading direction and do not show extension in the spreading direction congressional axes of thrust events show a weak preferred orientation in the spreading direction presumably due to ridge push stresses however stress axes of thrust events in very young lithosphere age 9 ma do not show a correlation with the spreading direction suggesting that the ridge push force is small near the ridge these observations indicate that tensional stress in the spreading direction is concentrated in a very narrow zone at the ridge axis in agreement with models which assume that the axial region has very little tensile strength normal faulting events occur at greater depths and temperatures than thrust events possibly because rocks under tensional stress have strength maxima at higher temperatures alternatively the depths of the normal faulting events may be a result of the higher thermal gradients at depth in young lithosphere if thermoelastic stresses are important in generating this seismicity differential thermal contraction along fracture zones does not seem to be a significant source of near ridge intraplate seismicity high seismicity rates in young lithosphere noted previously on a global scale seem characteristic of all three major ocean basins additionally most of the total oceanic intraplate moment release takes place in young lithosphere however most of the events and more than 96 of the total moment release of the near ridge intraplate seismicity are contained in five areas two zones in the indian ocean one near chagos bank and the other in the amsterdam st paul region indicate tension parallel to the trend of the plate boundary a region of high seismicity near the east pacific rise between 3 and 7 n shows both normal and thrust faulting while another region on the cocos plate west of the panama fracture zone is characterized by strike slip faulting thrust strike slip and normal faulting events are found in a small seismically active zone of the south central pacific the extremely uneven distribution of seismicity if not a result of the relatively short time period studied 20 years indicates local perturbations strongly affect near ridge seismicity

the rio grande fracture zone in the western south atlantic and its tectonic implications abstract a broad zone of linear mappable basement structures is observed north and northeast of the rio grande rise in the south atlantic ocean these structures lie along the same flow line as the sa o paulo ridge the florianopolis high and onshore lineaments suggesting that they all comprise the same tectonic trend the rio grande fracture zone the morphology developed along this fracture zone during the early opening of the south atlantic ocean formed a barrier to open ocean circulation during the aptian and allowed the formation of extensive evaporite deposits to the north of it

potassium distribution and metasomatism in pelites and schists how and when relation to postdepositional events a large global database for over 9000 archean to holocene pelites in numerous basins has been developed to evaluate diagenetically unaltered and altered compositions the mineralogy and geochemistry of quaternary muds and ancient mudstones provide first order controls on diagenetic potassium metasomatism in pelites clay mineralogy of quaternary muds is dominated by smectite and kaolinite with minor detrital mica and illite thus k2o is 4 k al is 0 25 and k rb is 220 in most quaternary muds with diagenetic alteration of clays to illite smectite and illite k2o content increases to 4 8 and k al rises to 0 2 0 5 or more in extreme potassium metasomatism authigenic k feldspar develops sources of potassium are internal to shale sandstone units wherein detrital k feldspar is dissolved during diagenesis providing k for clay illitization in closed system isochemical alteration as in some gulf coast and north sea basin cases potassium in amounts greater than that available from closed system alteration is provided from extraformational circulating brines in open system diagenesis k rb with diagenesis varies according to whether the metasomatizing fluid was relatively poor in rb seawater derived brines or richer in rb as in many hydrothermal waters diagenetic fluids may originate from seawater or meteoric water enriched in cations by water rock interactions along their subsurface migration paths and or by dissolution of evaporites potassium metasomatizing fluids penetrate low permeability pelites through faults fractures and microfractures and by diffusion in the mineral matrix typically k metasomatism possibly with pb zn cu ba and f mineralization occurs long after sediment deposition and burial at temperatures of 50 to 150 c related to tectonic activity and fluid movement different modes of metasomatism are recognized and correlated with geological setting in the passive margin gulf coast tertiary basin open system k metasomatism where present is limited to areas adjacent to steeply dipping growth faults which act as conduits for episodic pulses of altering fluids migrated upward from deeper basin sources in contrast widespread pervasive open system k metasomatism produced by continuous long term gravity advective and diffusive flow of brine from the late paleozoic appalachian ouachita wichita mountains highland to altered earlier paleozoic shales plus sandstones bentonites and carbonates in the illinois and anadarko ardmore foreland basins this mode of k alteration following major mountain building events is characteristic of many precambrian and paleozoic collision orogens and associated foreland basins another mode is in rift basins with no external drainage in which indigenous waters are evaporated to brines these brines cause albitization of k feldspar to yield potassium for illitization of detrital clays

are segmented fracture zones weak analytical and numerical models constrain anomalous bathymetry at the clarion and murray fracture zones abstract between 150 w and 135 w the clarion fracture zone fz coalesces from six discrete fz traces to a single fz over a period of 30 million years coinciding with a change in plate motion that placed the associated transform fault tf in compression between 160 and 157 the murray fz is also comprised of several closely spaced

the spacing of rift axis highs dependence upon diapiric processes in the underlying asthenosphere abstract detailed bathymetric mapping has revealed a near harmonic distribution of intermediate wavelength morphologic highs on slow to fast spreading centers one model suggests that the highs may result from the thermal instabilities generated by the interaction of cold fracture zone walls with the axial asthenosphere this may force a deepening of the isotherms near the fracture zone and an uplift of the isotherms midway between the fracture zones a preferred model based on newly accumulated data suggests that the highs may be a result of upwelling within the asthenosphere underlying the ridge axis where a linear region of instability develops at the base of a partial melting zone less dense material at depths of 40 80 km may penetrate the overlying material at discrete and ideally evenly spaced points harmonically spaced upwellings should generate excess magma at discrete points on the ridge axis creating volcanic highs the spacing between highs may depend upon the continuity and thickness of the deep axial instability layer the temperature and viscosity ratios within the magma and the distance between major transform faults instability of the diapirs may stimulate the migration of the highs through time

the east pacific rise near 21 n 13 n and 20 s inferences for along strike variability of axial processes of the mid ocean ridge abstract data obtained along various segments of the mid ocean ridge mor are used to construct an idealized model for crustal accretion the model seeks to predict the topographic volcanic tectonic and hydrothermal characteristics of any given spreading segment of the mor as a function of distance away from the bounding transform faults this model is based on a series of detailed mapping efforts carried out on segments of the mor having a broad range of spreading rates between 2 and 16 cm yr this paper includes the results of the french searise program carried out during the summer of 1980 aboard the n o jean charcot and two american cruises conducted in 1981 aboard the r v melville

a simplified dissolution precipitation model of the smectite to illite transformation and its application 1 by combining the compaction effect ec from a dissolution precipitation model dpm and the thermal effect et from a kinetic model km a simplified dpm is presented a pair of apparent activation energy e and frequency factor a to be used is proposed in the model which is chosen to cause the predicted smectite to fit x ray diffraction xrd data the proposed model is then applied to the illite smectite interstratifications of mudstone in well tazhong4 of the tarim basin in western china a km is also applied for the sake of comparison this case study shows that the most sensitive parameter governing smectite to illite s i transformation is the pair of e and a to calculate an accurate s i transformation history it is vital therefore to calibrate the e and a by using xrd data as a constraint in this specific case study the calibrated e ranges between 5 and 10 28 kcal mol 1 while a is between 9 0623 10 11 and 3 10 7 s 1 l mol 1 for the simplified dpm and e ranges between 3 and 5 while a is between 1 3888 10 11 and 3 5052 10 9 for the km the relation of e versus ln a appears nonlinear by using these e and a the present day values of the calculated smectite history agree well with the xrd determined data with mean absolute relative residuals of 4 45 for the simplified dpm and 5 67 for the km moreover the case study indicates that ec and et have the same importance for s i transformation as contributions of ec of 43 5 and of et of 56 5

a diagenesis model for geomechanical simulations formulation and implications for pore pressure and development of geological structures forward basin modelling is routinely used in many geological applications with the critical limitation that chemical diagenetic reactions are often neglected or poorly represented here a new temperature dependent kinetic diagenesis model is formulated and implemented within a hydromechanical framework the model simulates the macroscopic effects of diagenesis on 1 porosity loss 2 sediment strength 3 sediment stiffness and compressibility 4 change in elastic properties 5 increase in tensile strength due to cementation and 6 overpressure generation a brief overview of the main diagenetic reactions relevant to basin modelling is presented and the model calibration procedure is demonstrated using published data for the kimmeridge clay formation the calibrated model is used to show the implications of diagenesis on prediction of overpressure development and structural deformation the incorporation of diagenesis in a uniaxial burial model results in an increase in overpressure of up to 9 mpa due to both stress independent porosity loss and overpressure generated by disequilibrium compaction caused by a reduction in permeability finally a compressional model is used to show that the incorporation of diagenesis within geomechanical models allows the transition from ductile to brittle behaviour to be captured due to the increase in strength that results in an over consolidated stress state this is illustrated by comparison of the present day structures predicted by a geomechanical only model where a ductile fold forms and a geomechanical model accounting for diagenesis in which a brittle thrust structure is predicted

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

a mid ocean ridge thermal model constraints on the volume of axial hydrothermal heat flux the depth to the axial magma chamber at three oceanic spreading centers as indicated by seismic reflection surveys is greater than that predicted by conductive cooling thermal models of ridge crests the additional cooling at the ridge axis is owed to the circulation of seawater through the shallow crust a theoretical model for the temperature distribution at spreading centers which includes distributed heat sources and sinks is presented by representing the hydrothermal heat loss as a series of heat sinks the depths to the axial magma chamber at the east pacific rise at latitude 9 n the southern juan de fuca ridge and the lau basin spreading center are modeled heat sinks are added to the input model to increase the solidus depth until the computed magma chamber depth matches the magma chamber depth determined by seismic reflection surveys the thermal modeling indicates that the axial hydrothermal heat flux is between 7 1 106 and 13 8 106 mj m2 or 10 to 20 of the total missing heat at the ridge axis requiring extensive low temperature circulation off axis high temperature vents that are spaced 1 km apart and are active between 4 and 10 of the time would account for the axial hydrothermal heat loss

deep tow studies of the vema fracture zone 1 tectonics of a major slow slipping transform fault and its intersection with the mid atlantic ridge the vema transform fault which slips at a rate of 24 mm yr displaces the mid atlantic ridge mar 320 km in a left lateral sense high resolution deep tow studies of the vema ridge transform intersection rti and the eastern 130 km of the active transform fault reveal a complex pattern of dip slip and strike slip faults which evolve in time and space at the intersection both the neovolcanic zone and the west wall of the mar rift valley curve counterclockwise toward the transform fault along trends approximately 30 oblique to the regional north south trend of the spreading axis the curving of extensional structures in the rift valley such as normal faults and the axial zone of dike injection appears to be related to transmission of transform related shear stresses into the spreading center domain intermittent locking of the american and african lithospheric plates across the rti causes shear stresses to penetrate up to 4 km into the mar axial neovolcanic zone where the lithosphere is relatively thin and up to 12 km into the block faulted west wall of the rift valley where the lithosphere is thicker the degree of shear coupling across the rti may vary with time due to changes in the thickness of the lithosphere along the axis 0 10 km the strength of a mantle weld at depth and the presence or absence of an axial magma chamber so that extensional structures at the rti may be either spreading center parallel when coupling is weak or oblique when coupling is strong oblique extension across the rti in addition to other factors may account for some of the down dropping of lithosphere within the deep nodal basin the easternmost 20 km of the active transform fault zone near the rti displays a braided network of three to nine tectonically active grabens and v shaped furrows in a zone 2 4 km wide interpreted to consist of interwoven riedel shears p shears and oblique normal faults clay cake deformation experiments and deep tow observations suggest that p shears and r shears which are 10 20 oblique to the transform slip direction develop during the initial stages of transform faulting near the rti as the newly accreted lithosphere accelerates to full plate velocity some of the r shears propagate along strike and intercept the oblique normal faults resulting in sharply curving scarps at the rti subsequent to this merging of the two fault types some of the r shears develop a significant component of dip slip while other r shears merge with p shears creating a complex anastomosing fault pattern up to 4 km wide a continuous strand within this braided pattern of faults is interpreted to be the principal transform displacement zone near the rti twenty kilometers west of the rti the active transform fault zone narrows to a furrow generally less than 100 m wide with only a few short discontinuous splays this narrow groove cuts through thinly sedimented basalt 20 40 km west of the rti and continues as a narrow furrow less than 100 m wide through up to 1 5 km of layered turbidite fill most of the way to the western rti such a narrow zone of deformation typifies the mature stages of transform faulting where the lithosphere on both sides of the transform fault is relatively old thick and rigid and has completed its acceleration to full plate velocity the transform fault zone is closely associated with a partially buried median ridge and widens to 1 2 km where it transects exposed portions of the ridge the transform parallel median and transverse ridges create the highest topography associated with the transform fault and may be serpentinized ultramafic intrusions capped by displaced crustal blocks of gabbro metagabbro and basalt

thermal parameters of the oceanic lithosphere estimated from geoid height data geoid height anomalies derived from seasat altimeter data have been analyzed across fracture zones and over ocean ridges in two limited regions of the south pacific eltanin fracture zone system east pacific rise and southeast indian ocean observed geoid height age and geoid height derivative with respect to age age relationships have been established comparison with theoretical relationships computed for the plate model of lithospheric cooling permits an estimation of the thermal parameters entering into the model two quantities can be derived the product tm thermal diffusivity volume coefficient of thermal expansion tm bottom boundary temperature and the thickness h of the plate the best fitting values for the south pacific and the southeast indian ocean are tm 0 52 10 3 cm2 s 1 and h in the range 50 70 km for ages less than 30 m y whereas for larger ages 30 m y geoid observations are better explained by a larger h value in the range 70 90 km the depth age relationship observed in the south pacific region is consistent with these parameters on the other hand data for the northeast pacific argue rather for a lithospheric thickness larger than 100 km this suggests that local and regional variations exist in the thermal properties of the oceanic lithosphere

geology and alteration geochemistry of the porphyry cu au deposit at bajo de la alumbrera argentina this paper describes the igneous geology alteration mineralogy and geochemistry as well as the ore mineralization history of bajo de la alumbrera a world class porphyry copper gold deposit in northwestern argentina the deposit occurs in the k rich calc alkaline farallon negro volcanic complex located 200 km east of the main andean porphyry copper belt of chile the deposit consists of a composite stock of dacitic porphyry intrusions and extends into the surrounding andesitic volcanic rocks two of the earliest intrusions now occupying the center of the intrusive complex show intense hydrothermal alteration and mineralization these two porphyries host the highest ore grades and were subsequently intruded by several weakly mineralized or barren porphyries intense quartz magnetite k feldspar alteration pervades the first mineralized porphyry whereas similar but lower intensity alteration affected the second laterally this alteration grades into potassic alteration secondary k feldspar and biotite which affects all porphyries except the latest dikes andesitic wall rocks within a hundred meters of the intrusions are dominated by dark hydrothermal biotite an outer halo of propylitic alteration epidote chlorite albite calcite extends up to 1 km into the andesites feldspar destructive alteration sericite pyrite clay minerals gypsum overprints the potassic and propylitic alteration in the volcanic rocks and all porphyries the alteration is locally controlled by faults and late fractures and developed most pervasively upon the outer part of the potassic zone and toward the transition to propylitic alteration in the upper part of the deposit mass balance calculations show small positive changes in net volume for the potassic alteration and minor volume decreases for the chlorite epidote and feldspar destructive alteration the intensely quartz magnetite veined sample of quartz magnetite alteration indicates a total volume increase of 350 percent copper and gold are intimately associated at all scales they occur as composite gold sulfide grains chalcopyrite rarely bornite show a close correlation of ore grades in sample assays and have a closely overlapping distribution on the mine scale a well developed ore shell coincides with potassic alteration and partly with the distribution of intense quartz magnetite veins even though the bulk of the ore minerals now occupy texturally late positions in vugs and fractures a barren zone with intense quartz magnetite potassic alteration and veins but little or no sulfides is interpreted as the focus of upflow of high temperature ore fluids prior to ore mineral saturation and coprecipitation of copper sulfides and gold in the ore shell

thermal bending stresses and vertical motion on oceanic transform fault the conductive cooling of new oceanic lithosphere causes both horizontal and vertical contraction along transform faults and fracture zones the contraction of material on each side of the fault leads to differential subsidence since juxtaposed seafloor of different ages subside at different rates horizontal contraction causes accumulation of thermal bending stresses which are partly relieved by flexural bending along the transform the relative vertical motion implied by plate flexure at the transform is more important than that predicted by differential subsidence when combined this net vertical slip rate may exceed 5 of the full spreading rate for slow spreading ridges the high vertical to horizontal slip ratios implied may explain anomalously high subsidence rates for seafloor in the proximity of ridge transform intersections departure from horizontal strike slip inferred from transform fault focal mechanisms may possibly be used to put additional constraints on the isotherm controlling the transition from brittle to ductile failure in the oceanic lithosphere

thermal uplift and erosion across the continent ocean transform boundary of the southern exmouth plateau thermal evolution of the continental lithosphere at a continent ocean transform margin is examined using a two dimensional heat conduction model all heating is assumed to result from the emplacement of new oceanic ridge against the continent the assumed initial continental temperature gradient is probably best applied to regions having experienced finite duration periods of rifting lasting tens of millions of years a deep seismic reflection profile across the southern paleo transform margin of the exmouth plateau northwest australia tests the predictions of the model using this reflection profile it is estimated that up to 3 5 km of sediments have been eroded from the continental rim diminishing to almost no erosion at 60 km from the continent ocean transform boundary this trend and these values for erosion can be matched approximately using the model under the condition of local isostasy a finite difference scheme is employed where the surface elevation is the result of the competing 1 thermal uplift up 2 surficial erosion down and 3 local isostatic rebound up in response to the erosion the model predicts that most of the erosion ceases by 40 ma after ridge emplacement and that of the order of 1000 km3 eroded sediments are shed for every 10 km of transform length

diagenesis of the purington shale in the illinois basin and implications for the diagenetic state of sedimentary rocks of shallow paleozoic basins abstract the clay minerals micas and feldspars of the pennsylvanian age purington shale have been more diagenetically active than generally recognized they have undergone diagenetic changes comparable to those of cenozoic and mesozoic sedimentary rocks buried three to four times as deeply and heated to approximately twice the temperature the purington shale on the western shelf of the illinois basin has experienced maximum burial of 1 0 km and was never heated to more than 60 c except for a very brief time at about 80 c the illite smectite i s of the purington shale has 90 illite the chemical compositions of the feldspar assemblage in the 0 09 0 063 mm fraction differ from most modern detrital suites of feldspars but are similar to diagenetic sequences described by others from much greater burial depths and are extensively but not completely albitized the simplest interpretation of the apparently advanced stage of diagenesis is that the diagenetic processes have been operating at less press

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

thermal contraction and the state of stress in the oceanic lithosphere nonuniform contraction of the oceanic lithosphere as it cools and thickens following its formation at the axis of a spreading center results in a complex three dimensional state of deviatoric stress which can be separated into two parts a thermal bending stress due to changes in the vertical temperature distribution and a thermal contraction stress due to lateral variations in the vertically averaged temperature we examine thermal contraction stresses due to temperature changes in a thin semi infinite rectangular plate bounded by the spreading center axis two stress free boundaries representing transforms or fracture zones define the plate width or ridge segment length l with the bottom of the elastic plate defined by a prescribed temperature the plate thickens as the square root of age as expected for a thermal boundary layer due to vertical conductive cooling above this prescribed temperature elastic stresses are assumed to relax quickly stresses are obtained by properly accounting for the rate of accumulation of the vertically averaged stress as initially stress free material is added to bottom of the cooling thickening plate the state of thermal contraction stress calculated from this model is characterized by large tensile stresses at the boundaries of the plate and relatively low stresses in the plate interior at the ridge axis ridge parallel tensile stresses are about 300 mpa the same as if the plate were not allowed to contract in this direction along the transform boundary the maximum transform parallel tensile stress occurs at a distance l 2 from the ridge transform intersection where its magnitude is comparable to the stress at the ridge axis the tangential stresses at the plate boundary decrease rapidly with distance from the boundary at a distance of l 4 from the ridge axis the ridge axis parallel stresses are one tenth of their ridge axis magnitude the stress magnitudes are independent of both spreading rate and ridge segment length a large transform parallel tensile stress may control the length of transform offsets thermal bending moments are influenced by the large thermal contraction stresses near the ridge axis however a short distance from the ridge these moments attain their free horizontal contraction values which previous studies have shown to cause observable bending of the plate and a geoid anomaly at fracture zones flexure due to thermal bending moments will concentrate bending stresses at a distance from the fracture zone determined by the flexural length of the plate thus providing a natural length scale controlling the spacing of transforms

anisotropic horizontal thermal contraction of young oceanic lithosphere inferred from stress release due to oceanic intraplate earthquakes how freely the oceanic lithosphere contracts horizontally due to thermal contraction is important information because it reflects the boundary condition of the oceanic lithosphere which includes information regarding the magnitude of driving resisting forces of plate tectonics we investigated the horizontal thermal contraction of young oceanic lithosphere using an analysis of the intraplate stress release due to oceanic intraplate earthquakes oceqs and numerical simulations the stress release due to oceqs in young oceanic lithosphere 5 15 ma shows significant differences between the spreading directional component and the ridge parallel component the extensional stress release of the ridge parallel component is six times as large as that of the spreading directional component while the compressional stress release of the ridge parallel component is one seventh that of the spreading directional component we conducted a numerical simulation of the thermal stress evolution of the oceanic lithosphere to investigate how the difference in the horizontal contraction rates between the spreading direction and the ridge parallel direction can explain the observed anisotropic stress release the result indicates that young oceanic lithosphere 5 15 ma barely contracts in the ridge parallel direction only 0 30 of the spreading directional contraction rate while it contracts freely in the spreading direction due to the weakness of the oceanic ridge strength and the low viscosity asthenosphere from the results we constrained the magnitude of the basal traction working on the bottom of the oceanic lithosphere to be smaller than 0 44 mpa

potassium sources and illitization in texas gulf coast shale diagenesis abstract the aim of this paper is to identify the controls on the rates of illitization in texas gulf coast shales under diagenetic conditions two different mineral transformations are considered the transformation of smectite to nearly illite and the precipitation of discrete illite i e the formation of pure illite analyses of potassium feldspar and kaolinite abundance in addition to k2o al2o3 and tio2 contents in shales from three wells are used to supplement published data in order to identify the mineral reactions responsible for the smectite to illite transformation in shales involving illite smectite i s mineral series these data indicate that the first stage of the smectite to illite reaction in shales r 0 structures ca 0 50 illite content is controlled by the presence of feldspar and hence also by the dissolution kinetics of feldspar the empirically derived activation energy for the first i s transformation in shales is similar to that for k feldspar dissolution kaolinite is not involved in this transformation in the shales investigated the second stage of the reaction sequence r 1 structures ca 50 100 illite content occurs after potassium feldspar has largely or completely disappeared from the shale system this indicates that the feldspar no longer controls the second stage of the smectite to illite transformation and suggests another external source of potassium for the reaction in the shales this assumption is supported by the relative increase of potassium content of the shales at least in two of the three studied wells kaolinite does not seem to be involved either in this second stage of the i s reaction sequence the formation of discrete illite crystallites in shales is independent of the i s transformation reactions and takes place only after a very significant period of time has elapsed the slow rate of this reaction reflects the low activity of potassium in solutions that are in equilibrium with shale in comparison with sandstones where a rapid precipitation of pure illite can in some cases be observed it is suggested that the different reactions in the smectite to illite transformation necessitate different sources of potassium either from within or outside an argillaceous whole rock system these different sources of potassium give rise to different rates for the i s conversion reaction and the formation of illite

mechanisms for the origin of mid ocean ridge axial topography implications for the thermal and mechanical structure of accreting plate boundaries to better understand the implications of ridge axis morphology for the thermal and mechanical structure of accreting plate boundaries we examine mechanisms for the creation of stress supported axial topography and its dependence on parameters such as spreading rate asthenosphere viscosity and plate thickness near the ridge axis three basic mechanisms are considered 1 asthenospheric upwelling in a narrow steady state conduit beneath a ridge axis 2 mantle flow stresses due to plate spreading extending ridge axis lithosphere or melt segregation and associated mantle compaction and 3 topography supported by moments due to horizontal extensional stresses in a thickening lithosphere if mid ocean ridge topography is due to asthenosphere rising in a narrow conduit at the ridge axis then the conduit structure cannot be thermally controlled as the associated thermal structure has relatively broad flat isotherms instead of a conduitlike shape mantle flow associated with plate spreading alone induces no stress supported topography thus the intuitive notion that diverging flow at the ridge axis due solely to plate spreading produces the axial valley is incorrect vertical mantle flow at the base of a stretching accreting lithosphere and mantle flow associated with melt segregation can however lead to stress supported axial topography however for these mantle flow related processes to produce significant amounts of axial relief the mantle viscosity beneath the ridge axis must be on the order of 1020 pa s 1021 p poise significantly higher than generally accepted values of 1018 pa s 1019 p horizontal extensional stresses in a strong brittle lithosphere that thickens with distance from the ridge axis can produce axial topography of the form and amplitude observed on slow spreading ridges a continuum idealization of deformation due to faulting is used to examine this mechanism this model predicts the axial valley width is controlled by the plate thickness near the ridge axis an 8 km thick plate near the ridge axis would produce the 30 km wide axial valley observed at slow spreading ridges an increased lithosphere thickness of only several kilometers over the half width of the axial valley can produce kilometer scale relief to examine whether a plate 8 10 km thick at the ridge axis that thickens by several kilometers within 15 km of the axis is plausible at a slow spreading ridge thermal models that include both the heat of magmatic crustal accretion on the ridge axis and upwelling mantle flow beneath the axis are formulated hydrothermal cooling that increases the effective conductivity by about a factor of 10 is required for an 8 km thick plate to exist at a slow spreading ridge axis however even with this enhanced hydrothermal cooling the plate thickness at a fast spreading ridge axis and the resulting horizontal force in the plate are too small to result in appreciable topography while mid ocean ridge axial topography is strongly influenced by spreading rate other factors also shape axial structure for example the anomalously shallow slow spreading reykjanes ridge has an axial structure more like that of a fast spreading ridge and the anomalously deep intermediate spreading australian antarctic discordance has a median valley axial structure typical of a slower spreading ridge in addition within a given spreading segment there is often a systematic deepening of the ridge axis toward transform offsets these non spreading rate dependent axial topographic variations may be produced by variations in magmatic heat input associated with crustal emplacement which shapes the thermal structure and lithosphere thickness near the ridge axis

geometrical statistical modelling of systems of fracture zones along oceanic ridges summary a mechanical statistical model is presented that aims to help to understand the history and geometry of the process of formation of fracture zones along oceanic ridges it uses ideas of statistical fracture theory used in engineering namely the weibull fracture model the approximate parallelism of the fracture zones along ridges makes it possible to use a onedimensional point process model with points along the ridge axes which represent the transform faults the ratios of the lengths of the corresponding fracture zones to the ocean width are used to obtain a rough estimate of the weibull modulus which is an important material parameter in fracture theory the theory is refined by introducing a hard core point process model the corresponding positive minimum distance between subsequent fracture zones results from stress relaxation in the vicinity of a given fracture zone

thermoelastic stress how important as a cause of earthquakes in young oceanic lithosphere we present a series of exploratory models which test the hypothesis that thermoelastic stress is a significant contributor to the state of stress in young oceanic lithosphere less than 35 m y in age in support of this hypothesis is the concentration of seismicity in lithosphere younger than 15 m y where cooling rates are relatively high most near ridge earthquakes have focal depths between the moho and the depth of the 800 c isotherm thrust and strike slip events dominate the shallowest seismicity while the focal mechanisms of the deeper events are generally characterized by normal faulting to assess the importance of thermal stress in the generation of near ridge earthquakes we model the lithosphere as an elastic half space in which the response to cooling is computed using the method of thermoelastic displacement potentials a key assumption in the models is that stress is relieved on time scales generally short compared with the age of the lithosphere a further assumption in some models is that material will not contribute thermal stress until it has cooled below an elastic blocking temperature normal faulting is predicted throughout the lithosphere in thermal stress models based on simple half space cooling models in which a cooled surface layer is incorporated to simulate the effect of hydrothermal circulation on shallow thermal structure predict stresses that can match the locations of both thrust and normal faulting earthquakes near mid ocean ridges the addition of a ridge push stress to the computed thermal stress enhances the likelihood of thrust faulting in the uppermost lithosphere at ages greater than about 15 m y but probably contributes little to the stress field at younger ages though these simple models are limited by several simplifying assumptions particularly in the immediate vicinity of the ridge axis they support the hypothesis that thermoelastic stress can play a major role in the tectonics of young oceanic lithosphere

the largest recorded earthquake swarm intraplate faulting near the southwest indian ridge the most prolific source of moderate and large earthquakes along the mid ocean ridge system is located near the atlantis ii transform along the southwest indian ridge a remarkable swarm of 48 earthquakes was recorded from 1925 to 1933 which included 29 events with m 6 0 and 4 events with m 7 0 examination of the global earthquake catalog suggests this is the largest known earthquake swarm relocated events cluster around 33 8 s 58 0 e approximately 100 km east of the southern intersection of the atlantis ii transform and the spreading ridge and epicentral uncertainties derived from both standard least squares and resampling techniques preclude locations along the plate boundary multiple event relocation shows that seven 1977 1983 events mb 5 1 5 6 are distributed across a 40 km diameter region at the location of the 1925 1933 swarm events mechanisms determined for three of the recent events using waveform inversion indicate normal faulting with variable strike slip components at depths of 12 17 km neither the nodal planes nor the tensional axes show consistency between the three mechanisms grid search fits to body waveforms from the larger 1925 1933 swarm earthquakes suggest normal faulting mechanisms similar to the recent events with depths of 11 20 km and seismic moments ranging up to 1 8 1027 dyn cm the earthquake depths and tensional mechanisms are consistent with release of thermoelastic stress which may be enhanced at this location by thermal bending moments associated with the transform fault the total moment release of the 1925 1933 swarm 7 1027 dyn cm is greater than the cumulative moment of all known oceanic intraplate seismicity since 1963 3 1027 dyn cm this suggests that intraplate stress release is dominated by infrequent but highly energetic episodes of seismicity and that estimates of intraplate moment release taken from observations over a limited time period underestimate intraplate seismicity levels

problems with argon k ar ages in gulf coast shales abstract k ar ages of gulf coast shales do not correspond to diagenetic events but are a combination of inherited detrital ages and authigenic overprints the key to understanding k ar ages of shales is the fate of the 40 ar when the host minerals dissolve or react during the illitisation reaction is 40 ar incorporated within the illite or is it lost to the porewaters ages are modelled as due to a single illitisation reaction assuming a uniform precursor sediment and open system import of k both whole rock and clay mineral fraction data can be reproduced by modelling the 40 ar incorporated into illite is a surprisingly high proportion 75 100 of that produced within detrital phases smectite k feldspar mica perhaps implying that the illitisation reaction is a solid state transformation or that 40 ar is imported into the shales along with incoming k the model does not provide a good fit to the data below 4000 m when the illitisation reaction is apparently complete but shales lose 40 ar by solid state diffusion or recrystallisation

seismic structure of 15 ma oceanic crust formed at an ultrafast spreading east pacific rise evidence for kilometer scale fracturing from dipping reflectors 1 a grid of seismic reflection data from 15 ma cocos plate offers an unprecedented view of the structure of mature oceanic crust formed at an ultrafast spreading 200 mm yr full rate east pacific rise epr the data contain an unexpected quantity of bright reflectors throughout the crust generally 5 km in length significant reflection dip angles 10 30 are seen only on the isochron profiles and several reflectors have a confirmed isochron dip orientation certain upper crustal reflectors ucrs appear to form a single network with isochron dip the most prominent example apparent reflection coefficient 0 2 projects toward a 70 m disruption in the igneous seafloor and waveform analysis demonstrates that the reflector is likely the product of a low velocity zone on the order of tens of meters thick our observations suggest that the isochron dipping ucrs represent small slip kilometer scale faults fractures and none are primarily related to the seismic 2 3 boundary or to the sheeted dike gabbro transition lower crustal reflectors lcrs have no preferred orientation or regular pattern on the 30 km scale of this survey certain lcrs may be similar to the ucrs in origin while others may originate from near axis ductile flow lithologic banding off axis magmatism or some combination thereof overall reflectors in this area are abundant and complex consistent with previous off axis studies but at odds with observations of young crust 5 ma near the epr this dichotomy may be resolved by a delay in reflector formation through some combination of postaccretionary tectonics magmatism and hydrothermal alteration

oceanic transform earthquakes with unusual mechanisms or locations relation to fault geometry and state of stress in the adjacent lithosphere on oceanic transforms most earthquakes are expected to occur on the principal transform displacement zone ptdz and to have strike slip mechanisms consistent with transform parallel motion we conducted a search for transform earthquakes departing from this pattern on the basis of source mechanisms and locations taken from the harvard centroid moment tensor catalogue and the bulletin of the international seismological centre respectively events with unusual mechanisms occur on several transforms we have determined the source mechanisms and centroid depths of 10 such earthquakes on the st paul s marathon owen heezen tharp menard and rivera transforms from inversions of long period body waveforms relative locations of earthquakes along these transforms have been determined with a multiple event relocation technique much of the anomalous earthquake activity on oceanic transforms is associated with complexities in the geometry of the ptdz or the presence of large structural features that may influence slip on the fault reverse faulting earthquakes occur at a compressional bend in the owen transform in the area of mount error and at the st paul s transform near st peter s and st paul s rocks a normal faulting earthquake on the heezen transform is located at the edge of a pull apart basin marking an extensional offset of the fault normal faulting earthquakes along the tharp menard and rivera transforms may also be related to extensional offsets some events with unusual mechanisms occur outside of the transform fault zone however and do not appear to be related to fault zone geometry for instance earthquakes with mechanisms indicating reverse faulting on ridge parallel fault planes are located near the ridge transform intersections of the st paul s and the marathon transforms possible additional contributors to the occurrence of anomalous earthquakes include recent changes in plate motion differential lithospheric cooling and the development of a zone of weakness along the fault zone but we do not find strong evidence to confirm the influence of these processes

intraslab seismicity and thermal stress in the subducted cocos plate beneath central mexico we present a model of the subducting cocos slab beneath central mexico that provides an explanation for stresses causing the occurrence of the majority of the intraslab earthquakes which are concentrated in a long flat segment based on the recently developed thermal models for the central mexico subduction zone the thermal stresses due to non uniform temperature contrast in the subducting slab are calculated using a finite element approach the slab is considered purely elastic but due to high temperature at its bottom the behavior is considered as ductile creep the calculation results show a 20 km slab core characterized by a tensional state of stress with stresses up to 70 mpa on the other hand the top of the slab experiences high compressive thermal stresses up to 110 mpa depending on the elastic constants used and location along the flat part of the subducting plate these compressive stresses at the top of the slab are not consistent with the exclusive normal fault intraslab earthquakes and two different sources of stress are proposed the trenchward migration of the mexican volcanic arc for the last 7 ma indicates an increase of the slab dip through time this observation suggests that the gravity torque might exceed the suction torque considering the flat slab as an embedded plate subject to an applied clockwise net torque of 0 5 10 16 1 5 10 16 n m the upper half would exhibit tensional stresses of 40 110 mpa that can actually balance the compressive thermally induced stresses an alternative stress source might come from the slab pull force caused by the slab positive density anomaly based on our density anomaly estimations 75 20 kg m 3 a 350 km slab length dipping at 20 into the asthenosphere induces a slab pull force of 1 7 10 12 4 6 10 12 n m this force produces a tensional stress of 41 114 mpa sufficient to balance the compressive thermal stresses at the top of the flat slab the linear superposition of the thermally and torque or slab pull induced stresses shows tensile stresses up to 60 180 mpa inside the flat slab core also our results suggest that the majority of the intraslab earthquakes inside the flat slab are situated where the resultant stresses are larger than 40 80 mpa this study provides a reasonable explanation for the existence of exclusively normal fault intraslab earthquakes in the flat slab beneath central mexico and also it shows that thermal stresses due to non uniform reheating of subducting slabs play a considerable role in the total stress field

thermal stresses in the oceanic lithosphere evidence from geoid anomalies at fracture zones models for the thermal and mechanical evolution of the oceanic lithosphere predict the progressive development of large thermal stresses in the thickening plate however there has so far been little direct evidence for the magnitude and distribution of thermal stresses we present theoretical models which examine the effect of thermal stresses at fracture zones and show that an anomaly of the predicted form can be observed in geoid profiles which cross fracture zones specifically our models predict the development of thermal bending moments which depend on lithosphere thickness or age and therefore change across fracture zones including the effect of varying thermal bending moments thin plate theory predicts vertical nonisostatic displacements of the lithosphere by plate flexure the predicted amplitude of the resulting geoid anomaly is large enough to be observed in seasat altimeter profiles furthermore the general form of this anomaly differs sufficiently from other predicted components of the geoid anomaly at fracture zones to be discernable the anomaly due to thermal stresses has been clearly identified in geoid profiles across the clarion and the udintsev fracture zones the amplitude of this observed anomaly is well predicted if cooling lithosphere begins to accumulate elastic stresses at a temperature of 700 c consistent with the maximum depth of seismicity in the oceanic lithosphere the distribution of thermal stresses with depth is also consistent with focal mechanisms of intraplate earthquakes

abyssal hill deflections at pacific antarctic ridge transform intersections nearly complete coverage of shipboard multibeam bathymetry data at the right stepping menard and pitman fracture zones allowed us to map abyssal hill deviations along their traces in this study we distinguish between 1 j shaped curvatures at their origin where modeling is addressing primary volcanism and faulting following a curved zone and 2 straight abyssal hills getting bent in anti j shaped curvatures in response to increased coupling across the transform fault after they were formed we compared the mapped abyssal hill deflections to a detailed plate motion model for the pacific antarctic ridge to test how abyssal hill curvature correlates to changes in plate motion direction which lead to periods of transtension or transpression this test was based on the number and size of the abyssal hill deflections the observations show a high abundance of j shaped abyssal hills during periods of significant clockwise change in plate motion direction which leads to transtension the tip of the ridge axis can deflect up to 60 into the transform fault in response to changes in the stress field at ridge transform intersections this is observed in particular at the pitman fracture zone where there has been a 15 clockwise rotation of the spreading direction azimuth during the last 9 myr in addition we observed anti j shaped curvatures at menard pitman and heirtzler fracture zones during periods of transpression when increased coupling across an oceanic transform fault is partially accommodated by distributed strike slip deformation rather than solely by discontinuous displacement at the transform fault anti j shaped deflections typically develop in seafloor less than 2 myr old when the oceanic lithosphere is thin

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

thermal stress and the spacings of transform faults bathymetric charts are used with satellite altimeter profiles to locate major ridge transform intersections along five spreading ridges the ridges are the mid atlantic ridge the east pacific rise the chile rise the pacific antarctic rise and the southeast indian ridge analysis of these data show spacings between transform faults w increase linearly with spreading rate w 6 28 m y this linear correlation is explained by a thermoelastic model of a cooling strip of lithosphere spreading at a rate the traction free boundaries of the thin elastic strip simulate cracks in the lithosphere at transform faults a two dimensional thermoelastic solution for the in plane stress shows the largest stress component is tensional and parallel to the ridge stresses are zero at the ridge and increase as age to a maximum value at an age of w 4 all stress components are small for ages greater than w when the transform spacing is large compared with the spreading rate w 100 m y thermal stresses exceed the strength of the lithosphere for ages between 0 and 30 ma the observed maximum ratio of transform spacing to spreading rate w 10 m y results in low thermal stresses that only exceed the strength of the lithosphere for ages less than 1 ma thus transform faults relieve most of the thermal stress model predictions also agree with earthquake studies showing that normal faults in young lithosphere have tensional axes aligned with the ridge moreover oceanic intraplate earthquakes rarely occur in lithosphere older than 30 ma as predicted by the model these and other geophysical observations confirm turcotte s hypothesis that transform faults are thermal contraction cracks

microearthquakes beneath the median valley of the mid atlantic ridge near 23 n hypocenters and focal mechanisms we report hypocenters and focal mechanisms of microearthquakes located by an ocean bottom seismic network deployed in the median valley of the mid atlantic ridge near 23 n during a 3 week period in early 1982 the network consisted of seven ocean bottom hydrophones and three three component ocean bottom seismometers the instrument coordinates were acoustically determined to within 25 m at the 1 standard deviation level of confidence the hypocentral parameters of the 26 largest microearthquakes are reported 18 of these events have epicenters arid focal depths which are resolvable to within 1 km formal error at the 95 confidence level microearthquakes occur beneath the inner floor of the median valley and have focal depths generally between 5 and 8 km beneath the seafloor composite fault plane solutions for two spatially related groups of microearthquakes beneath the inner floor indicate normal faulting along fault planes that dip at angles of 30 or more these solutions are similar to the mechanisms of nearby large earthquakes microearthquakes also occur beneath the steep eastern inner rift mountains the rift mountain earthquakes have nominal focal depths of 5 7 km and epicenters as distant as 10 15 km from the center of the median valley but these hypocenters have larger uncertainties because of the possible effects of large topographic relief and associated lateral heterogeneity in velocity structure we interpret the depth distribution and source mechanisms of these microearthquakes to indicate that this segment of ridge axis is undergoing brittle failure under extension to a depth of at least 7 8 km we infer that the entire crustal column has been cooled to temperatures within the brittle field of behavior and that significant time has elapsed since the most recent episode of volcanic or shallow magmatic activity along this ridge segment

the rigid plate and shrinking plate hypotheses implications for the azimuths of transform faults the rigid plate hypothesis implies that oceanic lithosphere does not contract horizontally as it cools hereinafter rigid plate an alternative hypothesis that vertically averaged tensional thermal stress in the competent lithosphere is fully relieved by horizontal thermal contraction hereinafter shrinking plate predicts subtly different azimuths for transform faults the size of the predicted difference is as large as 2 44 with a mean and median of 0 46 and 0 31 respectively and changes sign between right lateral rl slipping and left lateral ll slipping faults for the morvel transform fault data set all six plate pairs with both rl and ll slipping faults differ in the predicted sense with the observed difference averaging 1 4 0 9 95 confidence limits which is consistent with the predicted difference of 0 9 the sum squared normalized misfit r to global transform fault azimuths is minimized for 0 8 0 4 95 confidence limits where is the fractional multiple of the predicted difference in azimuth between the shrinking plate 1 and rigid plate 0 hypotheses thus observed transform azimuths differ significantly between rl slipping and ll slipping faults which is inconsistent with the rigid plate hypothesis but consistent with the shrinking plate hypothesis which indicates horizontal shrinking rates of 2 ma 1 for newly created lithosphere 1 ma 1 for 0 1 ma old lithosphere 0 2 ma 1 for 1 ma old lithosphere and 0 02 ma 1 for 10 ma old lithosphere which are orders of magnitude higher than the mean intraplate seismic strain rate of 10 6 ma 1 5 10 19 s 1

a model for diagenetic mass transfer between adjacent sandstone and shale abstract there are numerous suggestions that mass transfer between sandstone and shale is an important factor in the diagenesis of sandstone reservoirs however advective transport models for mass transport have not produced results that support these proposals a model is developed that overcomes the problems of advective transport models by assuming that diffusive mass transfer is the principle transport mechanism the process occurs when the rate of dissolution of reactive detrital components exceeds the local rate of diffusive transport at that point in the burial history units with greater amounts of a reactive component will generate more solute per unit time than units with less reactive component the process forms chemical gradients that can rapidly transfer mass equivalent to several vol of rock over distances of at least 5 m the observed transport of potassium and silica between adjacent sandstone and shale in the north sea during late stage illitization was correctly predicted using this conceptual approach the conceptual framework from the illite model can be extended to other diagenetic reactions and suggests that the burial of adjacent sandstone shale units could produce episodic diffusion gradients that transfer mass and form multiple generations of cement in sandstone reservoirs

age dependence of oceanic intraplate seismicity and implications for lithospheric evolution we have determined depths for 16 oceanic intraplate earthquakes using body wave modeling a data set composed of these and 11 other well constrained depths shows that the maximum depth of seismicity deepens with increasing lithospheric age and appears to be bounded by a 700 800 c isotherm the maximum depth to which intraplate earthquakes occur is approximately equal to the flexural elastic thickness but much less than the seismic thickness from surface wave dispersion the maximum faulting depth is consistent with the predictions of rapid weakening at high temperatures for dry olivine rheologies determined by experimental rock mechanics oceanic intraplate earthquakes can occur at greater depths and temperatures than has previously been reported for continental crustal events presumably because oceanic lithosphere is stronger at high temperatures both the number of earthquakes and the cumulative seismic moment decrease with lithospheric age the cause of this trend is unclear possibilities include nonrandom location of preexisting weak zones with respect to the age provinces fault strengthening with age or a decrease in the stress level a decrease in intraplate stress with age is opposite that expected from ridge push stresses which increase with age but may result either from thermoelastic stresses which are greatest in young lithosphere or near ridge stress concentrations due to local heterogeneities in the spreading process

geoid height age relation from seasat altimeter profiles across the mendocino fracture zone twenty eight seasat altimeter profiles crossing the mendocino fracture zone are used together with seafloor ages determined from magnetic lineations to estimate the change in oceanic geoid height with age between ages of 15 and 135 m y an unbiased estimate of the overall geoid offset along each profile is determined from a least squares fit of the along track derivative of the geoid to the geoid slope predicted from a simple two layer gravitational edge effect model uncertainties based upon the statistical properties of each profile are also determined a geoid slope age relation is constructed by normalizing the geoid offsets and uncertainties by the age offsets the results are in agreement with geoid slope age relations determined from symmetrically spreading ridges sandwell and schubert 1980 however the fracture zone estimates have smaller uncertainties and show less scatter a comparison of these results with the geoid slope age prediction of the boundary layer cooling model shows that the thermal structure begins to deviate from this model at an early age 20 40 m y a plate cooling model with a thickness of 125 km is most compatible with the geoid slope age estimates although significant deviations occur these may indicate that the lithospheric thermal structure is not entirely age dependent

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

thermal stresses due to cooling of a viscoelastic oceanic lithosphere theories based upon thermal contraction of cooling oceanic lithosphere provide a successful basis for correlating seafloor bathymetry and heat flow the horizontal components of the contraction of the lithosphere as it cools potentially give rise to large thermal stresses current methods to calculate these stresses assume that on the time scales of cooling the lithosphere initially behaves as an inviscid fluid and instantly freezes into an elastic solid at some critical temperature these instant freezing methods inaccurately predict transient thermal stresses in rapidly cooling silicate glass plates because of the temperature dependent rheology of the material the temperature dependent rheology of the lithosphere may affect the transient thermal stress distribution in a similar way and for this reason the authors use a thermoviscoelastic model to estimate thermal stresses in young oceanic lithosphere this theory is formulated here for linear creep processes that have an arrhenius rate dependence on temperature results show that the stress differences between instant freezing and linear thermoviscoelastic theory are most pronounced at early times 0 20 m y when the instant freezing stresses may be twice as large the solutions for the two methods asymptotically approach the same solution with time a comparison with intraplate seismicity shows that bothmore methods underestimate the depth of compressional stresses inferred from the seismicity in a systematic way less

thermoelastic stress in oceanic lithosphere due to hotspot reheating we investigate the effect of hotspot reheating on the intraplate stress field by modeling the three dimensional thermal stress field produced by nonuniform temperature changes in an elastic plate temperature perturbations are calculated assuming that the lithosphere is heated by a source in the lower part of the thermal lithosphere a thermal stress model for the elastic lithosphere is calculated by superposing the stress fields resulting from temperature changes in small individual elements the stress in an elastic plate resulting from a temperature change in each small element is expressed as an infinite series wherein each term is a source or an image modified from a closed form half space solution we apply the thermal stress solution to midplate swells in oceanic lithosphere with various thermal structures and plate velocities our results predict a stress field with a maximum deviatoric stress on the order of 100 mpa 1 kbar covering a broad area around the hotspot plume the predicted principal stress orientations show a complicated geographical pattern with horizontal extension perpendicular to the hotspot track at shallow depths and compression along the track near the bottom of the elastic lithosphere although stress data near oceanic swells are limited the source parameters of intraplate earthquakes near several hotspots are consistent with the thermal stress model these results indicate that thermal stress due to reheating may be an important contributor to stress fields near hotspots in old oceanic lithosphere

active margin processes along the antarctic peninsula abstract the antarctic peninsula has a remarkable record of active margin processes which include subduction with progressive ridge trench collisions margin segmentation by major fracture zones rifting in a hybrid back arc and sheared plate margin context fore arc basin development and glacial marine controlled trench fill processes several facets of these active margin processes both of internal crustal dynamic and external origin climate controlled have been documented by a geophysical survey during the antarktis vi 2 cruise of r v polarstem october december 1987 reflection seismic profiles have been shot over the rift basin of bransfield strait over an elongated sediment filled trough interpreted as a fore arc basin over accretional and progradational slopes over recent and ancient trench environments and over the facing oceanic domain in this oceanic domain different fracture zones have highly contrasting morphological and geophysical expressions the subduction of a fracture zone like hero f z characterized by a significant relief possibly related to the presence of buoyant serpentinite ridges may have been a factor of subduction termination for the last segment of the aluk drake plate it may also have played a role in the separation of a blueschist bearing fragment smith island from the base of the accretionary plate margin and in its lift to the surface the magnetic anomaly pattern of the oceanic slabs facing the northwestern peninsula margin shows evidence of an intriguing spreading acceleration which apparently preceded ridge trench collision the same anomaly pattern provides a clue to the stratigraphie interpretation of the oceanic sediment cover and of the frontal part of the prograding now passive margin south of the south shetland island arc an apparently broken and tilted oceanic plate fragment squeezed between the south shetland trench and shackleton fracture zone may argue for the role of transpression associated with the oblique convergence of the antarctic and scotia plates

compaction processes and rock properties in uplifted clay dominated units the egersund basin norwegian north sea abstract a thick succession of fine grained sediments in any sedimentary basin can serve as source if organic rich and top seals both sealing and petroleum generation capacity of any fine grained succession is dependent on compaction history mechanical and chemical compactions of the sediments in the egersund basin a thick succession of fine grained sediments of upper jurassic lower cretaceous age overly middle jurassic reservoir sands the egersund basin however has experienced a complex depositional and tectonic history particularly late exhumation neogene which complicates the overall burial thermal and pressure history in this study petrophysical and acoustic properties bulk density and p wave velocity of 10 exploration wells from two neighboring blocks in the egersund basin were utilized to investigate compaction processes in the area in addition x ray diffraction xrd and scanning electron microscopy sem analyses from selected wells and available total organic carbon toc data were used as guide to interpret the compaction processes and resulting rock properties the results show that the transition from mechanical to chemical compaction is not sharp the onset of chemical compaction imposes a measurable deviation in the burial depth trends of the petrophysical properties expected from purely mechanical compaction the continuation of the depth related trends however indicates that chemical compaction may occur in concurrent with continuing mechanical compaction the observed compaction varies between the different clay dominated stratigraphic units indicating that the major control on both mechanical and chemical compaction is the primary sediment composition

exceptional preservation of expandable clay minerals in the ca 2 1 ga black shales of the francevillian basin gabon and its implication for atmospheric oxygen accumulation abstract clay minerals are exceptionally well preserved in marine black shale of the ca 2 1 ga francevillian group in southeastern gabon the fb formation of the francevillian group is characterized by smectite rich clay minerals including randomly ordered r0 type and ordered r1 type mixed layer illite smectite i s the preservation of r0 type clay minerals suggests unexpectedly slow mineral transformation and a moderate degree of diagenesis which is unique considering the paleoproterozoic age of the sedimentary rocks r0 and r1 type smectite rich particles occur in stratigraphic intervals with high organic carbon content and are associated with carbonaceous filamentous structures suggesting formation of clay organic matter complexes our data suggests that clay minerals may have enhanced organic matter preservation providing the oldest example where a link between clay minerals and organic matter sequestration can be established our findings are consistent with the hypothesis that clay minerals enhanced organic carbon burial and aided in atmospheric oxygen accumulation through time

an analysis of geoid anomalies across the mendocino fracture zone implications for thermal models of the lithosphere geoid height increases in a steplike fashion across oceanic fracture zones owing to the juxtaposition of lithosphere with different ages and density structures at a ridge transform intersection the initial geoid offset depends on the age difference across the fault and is approximately equal to 0 15 m m y of age offset the amplitude and shape of this anomaly will change along the length of a fracture zone depending on how the lithosphere cools if the lithosphere thickens indefinitely boundary layer model then the geoid offset will remain constant although the width of the transition zone from one side of the fracture zone to the other will increase owing to both heat transfer across the fault and an increase in the average depth of compensation if however the lithosphere eventually approaches a constant thickness plate model the total geoid offset will decrease with time an analysis of 18 seasat altimetry profiles across the mendocino fracture zone offset 25 30 m y indicates geoid anomaly amplitudes systematically decrease from more than 4 m across the eastern younger part of the fracture zone to about 1 m across the western older part of the fault although the geoid offsets across the older portion of the mendocino fracture zone may have been underestimated by as much as 20 25 this large decrease in geoid offset strongly suggests that the lithosphere approaches a constant thickness along the older part of the fracture zone a plate thickness of 100 km fits the observed change in geoid offset along the mendocino but with uncertainties in the offset determination the age difference across the older part of the fault and the presence of areas of anomalous depths on both sides of the fracture zone this probably represents a minimum estimate of the total plate thickness

source mechanisms of earthquakes near mid ocean ridges from body waveform inversion implications for the early evolution of oceanic lithosphere to investigate the early tectonic evolution of oceanic lithosphere we present a synthesis of the characteristics of near ridge earthquakes i e earthquakes which occur off the axis of major mid ocean spreading centers but in lithosphere less than 35 m y old near ridge seismicity for the past two decades has been rather evenly distributed along the major ridge systems with the exception of the central indian ocean and the eastern part of the cocos plate which have experienced a high rate of earthquake activity during that time for 32 near ridge earthquakes we determined the double couple orientation seismic moment centroid depth and source time function using a formal inversion technique based on matching synthetic and observed p and sh waveforms all types of faulting styles are observed seismogenic deformation in young oceanic lithosphere is concentrated in the first 15 m y and occurs almost entirely in the uppermost mantle from just below the moho to depths at which the temperature estimated from standard thermal models exceeds 800 c thrust faulting is not observed at depths greater than 10 km below the seafloor the deepest near ridge earthquakes are generally characterized by normal faulting focal mechanisms with the t axis oblique or perpendicular to the local spreading direction most of these earthquakes occur in the central indian ocean the transition from mechanisms characteristic of ridge axes to those characteristic of the near ridge environment occurs within the first few million years for lithosphere less than 35 m y old horizontal compressive stress associated with the cooling and subsidence of the lithosphere ridge push does not appear to be the dominant source of stress released in earthquakes this result may be used to place a lower bound of several hundred bars on the typical stress difference in young oceanic lithosphere this bound together with the observed depth of faulting implies that the strain rate associated with seismic deformation is locally at least 10 15 s 1 we propose that thermoelastic stresses associated with plate cooling play an important role in near ridge earthquakes this source of stress can account for the mechanisms and depths of many near ridge earthquakes and for the concentration of activity in very young lithosphere the comparative aseismicity of the oceanic crust may reflect the concentration of thermoelastic stress below the moho due to different material properties for the crust and upper mantle or the early release of thermoelastic stresses through hydrothermal circulation and cracking throughout the crust at or near the ridge axis the higher level of near ridge seismicity in the indian ocean may be the result of modification or enhancement of the more typical near ridge stress state by such local processes as the continental collision between india and asia or active secondary convection beneath the young lithosphere of the indian plate

thermal cracking and the deep hydration of oceanic lithosphere a key to the generation of plate tectonics 1 the development of transient thermal stress in suboceanic mantle is investigated on the basis of two dimensional thermoviscoelastic models incorporating composite rheology appropriate for dry oceanic lithosphere thermal stress is shown to be sufficiently high to deeply fracture the coldest part of lithosphere e g to the depth of at least 30 km and possibly down to 50 km in 100 ma old lithosphere the release of thermal stress by tension cracking is limited to the vicinity of cracks and the cascade crack system is suggested to be required given the finite fracture strength of mantle materials possible physical and chemical consequences of deep thermal cracking are also discussed the rheological evolution of oceanic lithosphere is likely to be affected by thermal cracking and subsequent serpentinization which introduces the localized zones of weakness in the otherwise stiffest part of lithosphere this localized weakening may help to explain why plate tectonic convection not stagnant lid convection operates in earth s mantle

how element translocation by plants may stabilize illitic clays in the surface of temperate soils abstract it has been described several times that plant activity enriches the upper part of soil profiles in the inorganic elements they use for their growth si k ca mg this phenomenon called nutrient uplift or element translocation can counteract element loss via leaching in surface soils and may therefore impact clay mineral stability in upper soil horizons however apart from some studies in tropical ecosystems the link between element translocation and clay mineral stability has been poorly investigated yet the present work explores the consequences of element translocation on clay mineral stability in temperate grassland soils through the use of a geochemical model initially studies indicating that silicon and potassium translocation stabilizes 2 1 clay minerals and especially illitic clay minerals in surface soil were compiled using this analysis as a basis we propose a simple biogeochemical model linking plant primary productivity and clay mineral thermodynamic stability that allowed explaining how plant activity may stabilize clay minerals in the upper part of temperate grassland soils the influence of various parameters such as rainfall primary production elemental composition of plants or acidifying activity of roots was also investigated using the model the outputs of the model suggested that plant activity under certain conditions may well stabilize illitic clay in surface soils which opens wide perspectives for modellers and experimentalists

clay mineral diagenesis and source rock assessment in the bornu basin nigeria implications for thermal maturity and source rock potential abstract an understanding of the mineral and organic matter modifications during burial are critical in assessments of potential source rocks in petroleum exploration studies here we examined the relationships between clay mineral diagenesis and organic thermal indicators vitrinite reflectance and rock eval tmax to constrain the thermal maturity and hydrocarbon potential of source rocks in the mbeji 1 well in the northeastern part of the cretaceous tertiary bornu basin x ray diffraction mineralogical data show that the

differential thermal stresses in the earth summary rocks have a significant thermal expansion coefficient and a high modulus of elasticity small thermal changes can therefore generate very substantial stresses a convenient idealized example is the mechanically free cooling thermal boundary layer in a material that suddenly turns from plastic to rigid in cooling through a rigidus temperature this simplified case can be solved analytically for both a linear and an error function temperature profile a compressed layer develops near the surface with a logarithmic singularity of stress at the boundary at a depth of 0 2 of the elastic thickness the stress changes to tensile and peaks at 15 per cent of the confined shrinkage stress in the middle of the rigid layer the situation is similar to the stresses in one half of a piece of tempered glass like the tempered glass both the cooling lithosphere and a plated boundary to a magma chamber are subject to breakage when cracks are induced in the tensile layer positive pore magma pressure is required to overcome confining pressure at depth in the lithosphere but cracking could become catastrophic in the plated boundary to a near surface magma chamber