southern kermadec submarine caldera arc volcanoes sw pacific caldera formation by effusive and pyroclastic eruption abstract three newly identified submarine caldera volcanoes brothers healy and rumble ii west of the southern kermadec arc within water depths 80 and low density 0 3 0 5 g cm 3 erupted pumiceous clasts these observations are interpreted as evidence of subaqueously quenched pyroclastic eruption s sheathed by ambient water flashing to steam discharging some 5 km3 of pyroclastic material from healy volcano caldera collapse is inferred to be penecontemporeous with the pyroclastic eruption s although it is possible formation as an explosive crater occurred but we consider the latter unlikely given the volume of erupted material and water depth extrapolation of these observations to other submarine arcs indicates pyroclastic eruption of silicic magmas and syn eruptive caldera collapse is possible within water depths of 500 1000 m

venus trough and ridge tessera analog to earth oceanic crust formed at spreading centers one type of tessera terrain on venus the trough and ridge tessera exhibits a distinctive morphology composed of throughgoing generally parallel linear valleys and shorter orthogonal valleys and ridges the similarity of this pattern to oceanic crustal topography and morphology is examined oceanic crust on earth is characterized by a distinctive orthogonal pattern with transforms and fracture zones in one direction and linear abyssal hills developed parallel to the rise crest in the other similarities between the components of the tessera terrain and terrestrial seafloor fabric and structure are found in terms of linearity parallelism length width spacing orthogonality and general morphology a difference is that trough and ridge tessera tend to occur as plateau like regions at intermediate elevations average 2 km above the surrounding plains while terrestrial seafloor ranges in elevation from that observed at rise crests and oceanic plateaus such as iceland down to depths of several kilometers on the abyssal floors where the orthogonal pattern is often masked by sedimentation on the basis of the similarities it is proposed that trough and ridge tessera may have originated through processes analogous to those responsible for the ocean floor fabric on earth forming at a rise crest similar to divergent boundaries on the earth s seafloor and evolving to its present morphology and configuration through processes of crustal spreading the plateau like nature of the trough and ridge tessera distribution could be the result of localized crustal thickening in the spreading process producing elevated iceland like plateaus whose texture is preserved from subsequent lowland volcanic flooding some regions of more complex tessera patterns may result from deformation of the basic trough and ridge pattern of tessera this hypothesis can be tested with global high resolution imaging and altimetry data and gravity data from the magellan mission

fault trends on the seaward slope of the aleutian trench implications for a laterally changing stress field tied to a westward increase in oblique convergence 1 normal faults along the seaward trench slope sts commonly strike parallel to the trench in response to bending of the oceanic plate into the subduction zone this is not the circumstance for the aleutian trench where the direction of convergence gradually changes westward from normal to transform motion gloria side scan sonar images document that the aleutian sts is dominated by faults striking oblique to the trench west of 179 e and east of 172 w these images also show a pattern of east west trending seafloor faults that are aligned parallel to the spreading fabric defined by magnetic anomalies the stress strain field along the sts is divided into two domains west and east respectively of 179 e over the western domain sts faults and nodal planes of earthquakes are oriented oblique 9 46 to the trench axis and 69 90 to the magnetic fabric west of 179 e sts fault strikes change by 36 from the e w trend of sts where the trench parallel slip gets larger than its orthogonal component of convergence this rotation indicates that horizontal stresses along the western domain of the sts are deflected by the increasing obliquity in convergence an analytical model supports the idea that strikes of sts faults result from a superposition of stresses associated with the dextral shear couple of the oblique convergence and stresses caused by plate bending for the eastern domain most nodal planes of earthquakes strike parallel to the outer rise indicating bending as the prevailing mechanism causing normal faulting east of 172 w sts faults strike parallel to the magnetic fabric but oblique 10 26 to the axis of the trench on the basis of a coulomb failure criterion the trench oblique strikes probably result from reactivation of crustal faults generated by spreading

long submarine lava flows observations and results from numerical modeling long 100 km lava flows are relatively common on mars and venus and have been identified on the moon but they are rarely documented on earth however although 75 of the earth s surface is covered by water only a small percentage of the ocean floor has been investigated at a resolution sufficient to unequivocally identify the boundaries of long submarine lava flows even so basaltic lava flows as long as 110 km have been identified on the deep 1500 m seafloor near hawaii and the east pacific rise ambient conditions on the deep ocean floor may favor the development of long lava flows for the following reasons first high pressures 15 mpa keep volatiles dissolved in basaltic lavas preventing viscosity increases associated with exsolution and vesiculation second seawater rapidly quenches the surface of submarine basalt flows so that an insulating glass layer 1 5 cm thick encases submarine flows within seconds after their emplacement this glass rind effectively insulates the molten flow interior from additional heat loss making submarine basalt flows behave as well insulated subaerial tube fed flows thus for identical basalt flows emplaced on the deep seafloor and subaerially a submarine flow could advance farther before stopping results of numerical modeling indicate that thin 1 m submarine basalt flows behave similarly to identical subaerial flows but thicker submarine flows may advance significantly farther than their subaerial counterparts

the archean cu zn magnetite rich gossan hill volcanic hosted massive sulfide deposit western australia genesis of a multistage hydrothermal system the gossan hill volcanic hosted massive sulfide vhms deposit is an archean cu zn magnetite rich deposit located in the warriedar fold belt of the yilgarn craton western australia the deposit is hosted by redeposited rhyodacitic tuffaceous volcaniclastics of the golden grove formation and is overlain by felsic volcanic rocks of the scuddles formation the deposit consists of two separate subvertical ore zones the stratigraphically lower cu rich ore zone 7 0 mt 3 4 cu is strata bound and varies from podiform massive pyrite chalcopyrite pyrrhotite magnetite to sheetlike massive magnetite carbonate chlorite talc the upper zn cu ore zone 2 2 mt 11 3 zn 0 3 cu 1 5 g t au and 102 g t ag contains strata bound massive sphalerite pyrite chalcopyrite that overlies discordant moundlike massive pyrite pyrrhotite chalcopyrite magnetite sulfide stockwork connects the upper and lower ore zones metal zonation varies from cu au in the lower ore zone to zn cu at the base of the upper ore zone which grades upward and laterally to zn ag au cu pb multiphase deformation and greenschist facies metamorphism overprint the deposit the relationship between host rocks and sulfide indicates that mineralization was broadly codepositional with sedimentation of the golden grove formation however gradational and interdigitating contacts between these volcaniclastic rocks and magnetite support the formation of massive magnetite by subsea floor replacement sulfides replace and vein the massive magnetite with inferred synchronous formation of the upper and lower sulfide ore zones although these sulfide ore zones formed mainly by subsea floor replacement stratiform hydrothermal chert sulfide sediment layers within and adjacent to the upper zn rich ore zone attest to some local exhalation the thickest development of massive magnetite massive sulfide and stringer stockwork occurs in the north of the deposit and supports a common feeder during massive magnetite and sulfide formation furthermore local chlorite quartz hydrothermal alteration massive magnetite massive sulfide and stockwork all form asymmetric zones that thin southward these attributes indicate strong synvolcanic structural control during mineralization the relict synvolcanic growth structure is obscured and probably occupied and by a dacite dome of the hanging wall scuddles formation thermodynamic considerations suggest that massive magnetite and sulfide formed from similar high temperature 300 degrees celsius slightly acidic low fo2 hydrothermal fluids massive magnetite formed from h2s poor fluids whereas massive sulfide formed from relatively rich h2s fluids physicochemical changes associated with interaction between upwelling h2s bearing fluids and preexisting massive magnetite may have resulted in the subsea floor precipitation of sulfide in the lower cu rich ore zone the gossan hill vhms deposit represents a multistage hydrothermal system within an environment characterized by rapid volcaniclastic sedimentation and changing structural and magmatic processes the six main stages were as follows 1 initiation of the hydrothermal system 2 sedimentation metasomatism and progressive heating of convecting fluids 3 deposition of massive magnetite from h2s poor fluids by subsea floor replacement above a buried synvolcanic conduit 4 structural reactivation tapping deeper h2s and metalbearing fluids 5 subsea floor replacement and minor exhalative sulfide mineralization with 6 burial and preservation of the deposit resulting from proximal felsic volcanism

fault and stratigraphic controls on volcanogenic massive sulphide deposits in the strelley belt pilbara craton western australia abstract early archaean fe zn cu volcanogenic massive sulphide deposits of the strelley belt pilbara craton occur at the top of a volcanic dominated sequence at the interface of felsic volcanic rocks and siliceous laminites beneath an unconformity overlain by clastic sedimentary rocks the structure of the sulphur springs and kangaroo caves vms deposits is relatively simple with the present morphology reflecting original deposition rather than significant structural modification the rocks have been tilted giving an oblique cross sectional view of discordant high angle deep penetrating faults in the footwall which splay close to the zones of volcanogenic massive sulphide mineralization faults do not extend far into the overlying sedimentary cover indicating their syn volcanic and syn mineralization timing both the sulphur springs and kangaroo caves sulphide deposits are located within elevated grabens in a setting similar to massive sulphide mineralization in modern back are environments mineralization at sulphur springs and kangaroo caves is located at the edge of the grabens at the site of intersecting syn volcanic extensional faults

suppression of multiple reflections in gloria sidescan sonar imagery the gloria side scan sonar system is an invaluable tool for studying the texture of the sea floor a substantial set of computer algorithms now exist for radiometric and geometric correction of the raw gloria imagery one type of artefact not presently removed is bright lines parallel to the ship track due to multiple reflections between the seabed and the sea surface these are common where the sea floor is flat and can lead to misinterpretation a method is presented here to suppress multiple reflections in gloria and other side scan sonar imagery as a by product of the suppression method a value for the seabed backscattering coefficient can be calculated giving some extra insight regarding sea floor type for sedimentary areas

new high resolution acoustic data from the braided system of the orinoco deep sea fan abstract the interpretation of approximately 100 km of em12 multibeam bathymetry and backscatter data together with high resolution topas profiles obtained during a transit over the orinoco deep sea fan near the barbados accretionary prism shows that the braided drainage pattern seen on gloria images by belderson et al 1984 corresponds to wide 10 km low relief

a comparison of the horne volcanogenic massive sulfide deposit and intracauldron deposits of the mine sequence noranda quebec most of the massive sulfide deposits of the rouyn noranda area are associated with rhyolite and andesite flows of the mine sequence and occur within the noranda cauldron a volcanic subsidence structure related to partial evacuation of an underlying magma chamber the flavrian pluton the horne massive sulfide orebodies lie within a sequence of rhyolitic breccias located just south of the inferred margin of the subsidence structure and are separated from the mine sequence by the horne creek fault traditionally the horne deposits and the intracauldron deposits have been interpreted to occur within the same stratigraphic interval even though they share some characteristics such as mineral zoning patterns alteration assemblages and an association with synvolcanic faults the two deposit groups are clearly distinguishable intracauldron massive sulfide deposits are small 5 mt cu zn rich concordant mounds associated with interflow horizons and are closely linked to vent zones where synvolcanic faulting provided hydrothermal fluids with cross stratal structural permeability the three major sulfide deposits at the horne mine are much larger 20 150 mt and formed in a fault bounded sediment filled graben on the flank of a rhyolitic vent complex the orebodies are au cu rich but zn poor and have bowl shaped morphologies which resulted from extensive subsea floor sulfide precipitation whereas the intracauldron sulfide deposits lie within a bimodal flow dominated succession consisting mainly of andesite basalt and high silica rhyolite with volumetrically insignificant pyroclastic rocks the horne orebodies occur in felsic fragmental rocks and rhyolite flows which are part of a stratigraphic sequence which contains virtually no mafic volcanic rocks also compared to all intracauldron rhyolites the horne rhyolites show a fundamental depletion in incompatible trace elements such as the high field strength and rare earth elements which implies that the horne rhyolites may have followed a different magmatic evolutionary path such differences in deposit characteristics their paleoenvironments of formation and the physical volcanology and geochemistry of their host volcanic successions suggest that contrary to previous interpretations the massive sulfide deposits of the intracauldron mine sequence and those of the horne sequence may have formed at different stages in the evolution of the noranda cauldron

seafloor hydrothermal fluids ben nevis area abitibi greenstone belt implications for archean 2 7ga seawater properties abstract a central part of the 2 7 ga abitibi greenstone belt between the destor porcupine and cadillac larder lake structures has been subjected to sub greenschist facies metamorphism t the majority of observed fluid inclusions are two phase and liquid dominated with a vapor bubble volume of 10 microthermometry on 58 samples 1100 fluid inclusions and eutectic temperatures between 45 and 30 c indicate the presence of at least one significant divalent cation in the fluids hence salinity is calculated as wt cacl2 equiv the peak of the salinity distribution of inclusions from vesicles and interstitial and drainage cavity quartz falls between 4 and 10 wt cacl2 equiv a small subset of inclusions yielded salinities between 18 and 22 wt cacl2 equiv most homogenization temperatures fall in an interval between 110 and 160 c a geothermal gradient estimated from the data of 10 15 c km is reasonable and consistent with the interpretation of the area as a regional seafloor recharge zone primary porosity in the modern ocean floor does not show the rapid precipitation of quartz that is indicated for the archean however the global occurrence of silica rich banded iron formations and the lack of silica consuming marine organisms in the precambrian indicates that marine geochemistry and sio2 precipitation mechanisms may have been different during the precambrian the total salinity of a fluid is dependent primarily on its total chlorinity hence water rock interactions with ion exchange cannot be the only reason for the higher salinities encountered in this study indications for boiling or high temperature phase separation are not evident suggesting that these options are not a reason for the observed high salinities our data are in agreement with six other data sets from the 3 75 ga isua greenstone belt w greenland the 3 24 ga pilbara region w australia the 3 2 ga barberton greenstone belt south africa 2 the 2 7 ga matagami area abitibi greenstone belt canada and the proterozoic 2 2 ga ongeluk formation south africa the consistency of fluid inclusion data suggests that archean and early proterozoic seawater may have been more saline and potentially stratified compared to modern seawater

cathodoluminescence characteristics of archean volcanogenic massive sulfide related quartz noranda ben nevis and matagami districts abitibi subprovince canada 1 the cathodoluminescence cl of quartz from ore stockwork veins and interstitial fillings between lava pillows from the 2 7 ga noranda ben nevis and matagami volcanogenic massive sulfide vms districts abitibi greenstone belt has been investigated using the hot cathode technique hc1 lm system to assess the potential of these various sample types to host primary seafloor vms related fluids trapped as inclusions in minerals with primary depositional morphologies the cl responses indicate that the various quartz types are of hydrothermal origin and are therefore a potential host for primary hydrothermal fluid inclusions most notable is a transient t 120 s blue cl characteristic of hydrothermal quartz observed in most samples cl characteristics are similar over 250 km indicating coherent nonrandom behavior furthermore in ore and stockwork material from the matagami and noranda districts respectively cl reveals primary concentric growth zoned quartz that predates pulses of sulfide deposition clear evidence that the quartz is undeformed and directly related to vms mineralization these growth zones are not apparent in transmitted light in addition ore and stockwork quartz commonly show a very unstable t 30 s yellow cl coincident with microfractures and grain boundaries defining areas affected by secondary hydrothermal activity in ore from matagami local zones of nontransient brown cl may reflect strain zones associated with the deformation and recrystallization of the massive sulfide mound and indicate that such modifications can be recognized and are minor in the investigated cases cl clearly reveals pseudo hexagonal apparently zoned structures in sulfide mineralized breccia pipe quartz from the ben nevis area these structures and their host quartz characterized by a very unstable t 20 s bright yellow cl are interpreted as recrystallized quartz that has undergone rapid growth from a strongly supersaturated solution and noncrystalline precursor the cl also clearly reveals colloform crustiform textures indicative of open space filling these textures are not visible optically

sulfur isotope and trace element data from ore sulfides in the noranda district abitibi canada implications for volcanogenic massive sulfide deposit genesis we examine models for volcanogenic massive sulfide vms mineralization in the 2 7 ga noranda camp abitibi subprovince superior province canada using a combination of multiple sulfur isotope and trace element data from ore sulfide minerals the noranda camp is a well preserved vms deposit rich area that is thought to represent a collapsed volcanic caldera due to its economic value the camp has been studied extensively providing a robust geological framework within which to assess the new data presented in this study we explore previously proposed controls on mineralization within the noranda camp and in particular the exceptional au rich horne and quemont deposits we present multiple sulfur isotope and trace element compositional data for sulfide separates representing 25 different vms deposits and showings within the noranda camp multiple sulfur isotope data for this study have 34sv cdt values of between 1 9 and 2 5 and 33sv cdt values of between 0 59 and 0 03 we interpret the negative 33s values to be due to a contribution of sulfur that originated as seawater sulfate to form the ore sulfides of the noranda camp vms deposits the contribution of seawater sulfate increased with the collapse and subsequent evolution of the noranda caldera an inference supported by select trace and major element analyses in particular higher concentrations of se occur in samples with 33s values closer to 0 as well as lower fe zn ratios in sphalerite suggesting lower pressures and temperatures of formation we also report a relationship between average au grade and 33s values within au rich vms deposits of the noranda camp whereby higher gold grades are associated with near zero 33s values from this we infer a dominance of igneous sulfur in the gold rich deposits either leached from the volcanic pile and or directly degassed from an associated intrusion

a neural network method for analysis of hyperspectral imagery with application to the cassas landslide susa valley nw italy abstract geomorphological analysis is an essential element of many landslide monitoring and hazard assessment studies remote sensing techniques have long been an effective tool for such analyses this paper presents results regarding a geomorphological investigation performed exploiting information derived from high resolution hyper spectral airborne sensor mivis multi spectral infrared and visible imaging spectrometer images the authors propose a workflow based on neural network algorithms used both for image geometric correction and classification the case study area is the middle susa valley italian western alps it includes the cassas landslide a well known complex landslide affecting slopes on the southern side of the valley near the town of salbertrand at this location several lifelines railway main roads hydroelectric tunnels etc are concentrated and they suffer from a constant threat of landslide activity traditional geomorphological characterization of the landslide slope was improved by comparing field campaigns with further information extracted from remote sensed datasets presented results include the identification and characterization of major elements of the present day active unstable slope debris covered areas fractured disjointed rock walls landslide accumulation borders as well as of individual structural features and landforms major faults and fractures trenches elongated depressions counterslope scarps related to long term deep seated gravitational slope deformation

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

shearing of partially consolidated sediments in a lower trench slope setting shimanto belt sw japan abstract detailed mapping of a coastal platform in shikoku sw japan provides evidence for progressive deformation in partially lithified sediments the eocene sediments involved are interpreted as lower slope basin deposits an assemblage of listric normal faults sheath folds broken formations and late stage faulting has developed during the sediments burial and uplift history these structures are typical of many other areas in the shimanto belt of shikoku despite the soft sediment style of deformation the consistency of the fold orientations relative to the regional foliation suggests that they are valid kinematic indicators a sequence of extensional faulting overprinted by synchronous folding and shearing is recognized this is interpreted as the response of the sediments to shape changes in the accretionary basement induced by shortening a general model has been constructed for the evolution of the structures it is proposed that early listric normal faults are subsequently deformed either by shearing along planar surfaces or by motion over frontal and lateral ramps back rotation of sediments during progressive shortening near the front of the prism tightens the fold hinges and rotates the fold axes towards the local shear direction alternative sequences which could account for the observed geometries are also discussed

subaqueous calderas in the archean abitibi greenstone belt an overview and new ideas abstract the 300 700 km abitibi greenstone belt in canada contains numerous world class archean volcanogenic massive sulfide vms deposits yet documentation of archean subaqueous calderas hosting such deposits is lacking the modern sunrise deposit in the myojin knoll caldera of the izu bonin arc shows that submarine calderas are first order sites for vms the hunter mine and normetal calderas as well as the blake river megacaldera complex are reviewed with respect to geometry regional geology physical volcanology dyke emplacement and hydrothermal carbonate alteration these subaqueous calderas are placed into the geodynamic context of the abitibi greenstone belt the abitibi belt displays a complex history of oceanic arc formation evolution collision and fragmentation with numerous types of volcanic edifices and sedimentary basins controlled by oblique subduction and mantle plumes analogue models yielding new insights into the sequence of caldera forming events and their synvolcanic fault patterns are applied to explain archean examples underpressure experiments develop a consistent sequence of fault patterns that compare favourably with natural caldera events during subsidence two sets of faults propagate from the magma chamber margin 1 early reverse outward dipping faults and 2 normal inward dipping faults the largest massive sulfide deposits form preferentially at the caldera margin based on experimental studies and detailed facies mapping the hunter mine caldera is of the segmented piecemeal variety and the normetal caldera is akin to a piston structure the blake river megacaldera contains 1 the e w trending misema 2 the nw trending new senator and 3 the ne trending noranda calderas with the latter two being nested graben type calderas consistent with oblique archean subduction the poorly documented hydrothermal carbonate alteration is discussed and a new exploration model for calderas is presented the alteration study was evaluated at three levels microprobe analysis geochemical staining of drill core and whole rock analyses there were two principal stages of alteration related to vms formation an early silicification phase developed either at depth and or at the seawater interface whereby silicification was bedding parallel in fine grained volcaniclastic deposits near the water surface a pervasive and extensive semi conformable carbonate alteration zone with a discordant focused root zone along faults evolved subsequently three distinct carbonate pairings were observed 1 proximal siderite sideroplesite fe ankerite next to the vms deposit 2 an intermediate ankerite fe dolomite zone and 3 a distal calcite dolomite zone the carbonate phase is associated with the mineralizing event transitions between carbonate species are subtle and changes are indicated by mineral assemblage overlap the silica carbonate hydrothermal alteration pattern is a replacement product and is far more widespread than has been documented this alteration can be easily confused with primary carbonates and chemical precipitation deposits e g banded iron formation if detailed volcano sedimentary facies mapping with alteration has not been conducted and if outcrop is insufficient

the role of granites in volcanic hosted massive sulphide ore forming systems an assessment of magmatic hydrothermal contributions assessment of geological geochemical and isotopic data indicates that a significant subgroup of volcanic hosted massive sulphide vhms deposits has a major or dominant magmatic hydrothermal source of ore fluids and metals this group which is typically characterised by high cu and au grades includes deposits such as those in the neoarchean doyon bousquet laronde and cambrian mount lyell districts these deposits are distinguished by aluminous advanced argillic alteration assemblages or metamorphosed equivalents intimately associated with ore zones in many of these deposits 34ssulphide is low with a major population below 3 34ssulphate differs from coexisting seawater and 34ssulphate sulphide 20 30 these characteristics are interpreted as the consequence of disproportionation of magmatic so2 as magmatic hydrothermal fluids ascended and cooled and as a definitive evidence for a significant magmatic hydrothermal contribution other characteristics that we consider diagnostic of significant magmatic hydrothermal input into vhms ore fluids include uniformly high 3 times modern seawater values salinities or very 18o enriched 18o 5 ore fluids we do not consider other criteria e g variable salinity moderately high 18ofluid 2 5 34ssulphide near 0 metal assemblages or a spatial association with porphyry cu or other clearly magmatic hydrothermal deposits that have been used previously to advocate significant magmatic hydrothermal contributions to be diagnostic as they can be produced by non magmatic processes known to occur in vhms mineral systems however in general a small magmatic hydrothermal contribution cannot be excluded in most vhms systems considered conclusive data that imply minimal magmatic hydrothermal contributions are only available in the paleoarchean panorama district where coeval seawater dominated and magmatic hydrothermal systems appear to have been physically separated this district which is characterised by chloritic and sericitic alteration assemblages and lacks aluminous advanced argillic alteration assemblages is typical of many vhms deposits around the world suggesting that for garden variety vhms deposits a significant magmatic hydrothermal contribution is not required other than deposits associated with advanced argillic alteration assemblages the only deposit for which we ascribe a major magmatic hydrothermal contribution is the devonian neves corvo deposit this deposit differs from other deposits in the iberian pyrite belt and around the world in being extremely sn rich with the sn closely associated with cu and in having formed from high 18o rich fluids 18ofluid 8 5 we consider these characteristics particularly the last as diagnostic of a significant magmatic hydrothermal contribution our analysis indicates that two subgroups of vhms deposits have a major magmatic hydrothermal contribution cu au rich deposits with aluminous alteration assemblages and reduced very sn rich deposits in which sn was introduced in a high temperature ore assemblage comparison with normal vhms deposits suggests that these subgroups of vhms deposits may form in specialised tectonic environments the cu au rich deposits appear to form adjacent to magmatic arcs an environment conducive to the generation of hydrous oxidised melts by melting metasomatised mantle in the wedge above the subducting slab this contrasts with the back arc setting of normal vhms deposits in which relatively dry granites in this contribution we use the term granite sensu latto formed by decompression melting drive seawater dominated hydrothermal circulation the tectonic setting of highly sn rich vhms deposits such as neves corvo is less clear however thick continental crust below the ore hosting basin may be critical as it is in other sn deposits

the gold content of volcanogenic massive sulfide deposits volcanogenic massive sulfide deposits contain variable amounts of gold both in terms of average grade and total gold content with some vms deposits hosting world class gold mines with more than 100 t au previous studies have identified gold rich vms as having an average gold grade expressed in g t exceeding the total abundance of base metals expressed in wt however statistically meaningful criteria for the identification of truly anomalous deposits have not been established this paper presents a more extensive analysis of gold grades and tonnages of 513 vms deposits worldwide revealing a number of important features in the distribution of the data a large proportion of deposits are characterized by a relatively low gold grade 2 g t with a gradual decrease in frequency towards maximum gold grades defining a log normal distribution in the analysis presented in this paper the geometric mean and geometric standard deviation appear to be the simplest metric for identifying subclasses of vms deposits based on gold grade especially when comparing deposits within individual belts and districts the geometric mean gold grade of 513 vms deposits worldwide is 0 76 g t the geometric standard deviation is 2 70 g t au in this analysis deposits with more than 3 46 g t au geometric mean plus one geometric standard deviation are considered auriferous the geometric mean gold content is 4 7 t au with a geometric standard deviation of 26 3 t au deposits containing 31 t au or more geometric mean plus one geometric standard deviation are also considered to be anomalous in terms of gold content irrespective of the gold grade deposits with more than 3 46 g t au and 31 t au are considered gold rich vms a large proportion of the total gold hosted in vms worldwide is found in a relatively small number of such deposits the identification of these truly anomalous systems helps shed light on the geological parameters that control unusual enrichment of gold in vms at the district scale the gold rich deposits occupy a stratigraphic position and volcanic setting that commonly differs from other deposits of the district possibly due to a step change in the geodynamic and magmatic evolution of local volcanic complexes the gold rich vms are commonly associated with transitional to calc alkaline intermediate to felsic volcanic rocks which may reflect a particularly fertile geodynamic setting and or timing e g early arc rifting or rifting front at the deposit scale uncommon alteration assemblages e g advanced argillic aluminous strongly siliceous or potassium feldspar alteration and trace element signatures may be recognized e g au ag as sb bi hg te suggesting a direct magmatic input in some systems

fracture analysis of a volcanogenic massive sulfide related hydrothermal cracking zone upper bell river complex matagami quebec application of permeability tensor theory located in the matagami mining district of the abitibi greenstone belt quebec the bell river complex is a 5 km thick tholeiitic layered gabbro and or anorthosite body 2724 6 2 5 1 9 mau pb which likely acted as the heat source that drove hydrothermal convection and volcanogenic massive sulfide vms hydrothermal mineralization in the area abundant fractures and veins crosscutting the western lobe of the bell river complex formed over a range of temperatures from 250 to 700 c the 250 to 400 c assemblage quartz epidote sericite chlorite plagioclase is the most widespread and occurs as orthogonal anastomosing and random vein sets vein densities average between 15 and 25 veins per m2 locally reaching as high as 40 to 60 veins per m2 these veins typically 1 to 3 mm wide are interpreted to represent thermal cracking associated with hydrothermal fluid mineralization in the district furthermore they crosscut earlier higher temperature pyroxene plagioclase 600 c and magnetite rich 300 600 c veins 1 10 mm wide densities commonly 0 5 veins per m2 detailed field measurements of quartz epidote vein geometries coupled with permeability tensor theory have produced a first order approximation of the maximum model permeability structure veins unfilled of the hydrothermal cracking zone representative district wide values indicate maximum model bulk permeabilities of 10 10 to 10 8 m2 for the hydrothermal cracking zone similar to permeabilities calculated for the fractured sheeted dike complexes of the semail and troodos ophiolites however a high flow zone located within the central parts of the hydrothermal cracking zone is characterized by a maximum model permeability of 10 7 m2 locally 1 m2 the high flow zone reaches maximum model permeability values as high as 10 6 to 10 5 m2 where two or more veins occur with apertures in excess of 2 cm mapping has shown that the hydrothermal cracking zone is confined to an 350 m thick interval located within a strongly layered gabbro and or anorthosite horizon of the layered zone upper bell river complex the base of this interval is 1 000 m below the top of the bell river complex furthermore in and around the town of matagami the dominant orientation of quartz epidote veins parallels the orientation of the layering 110 15 this parallelism is further reflected in the orientation of the calculated quartz epidote vein permeability tensors 94 30 1 n 71 the parallelism suggests that heterogeneities associated with the layer contacts provided planes of low tensile strength along which the hydrothermal cracks preferentially developed such an interpretation further explains why the hydrothermal cracking zone appears to be restricted to the strongly layered zone there are many more low tensile strength planes zones of quartz epidote veining approximately orthogonal to layering are also present the orthogonal veins are usually less continuous and commonly truncated but not crosscut by layer parallel veins the orthogonal subset of veins is interpreted to represent short pathways that allowed fluids to travel between adjacent layer parallel veins however locally longer layering orthogonal quartz epidote veins showing offsets of 10 to 30 cm normal to the layering of the bell river complex may represent initial conduits that allowed fluids to travel into overlying stratigraphy and through to the paleosea floor although volumetrically small in comparison with the permeability structure of the bulk sea floor the significantly higher permeability of the hydrothermal cracking zone indicates its importance in controlling the flow paths and fluxes of fluids deep within the hydrothermal system

caldera forming processes and the origin of submarine volcanogenic massive sulfide deposits certain volcanogenic massive sulfide vms ore deposits form in submarine calderas this association is well known but the link between caldera formation and the origins of the deposits remains poorly understood here we show that the size and location of a vms deposit within a submarine caldera may be determined by how and when the caldera formed these spatial temporal conditions control development of the hydrothermal system associated with the vms deposit we propose that caldera opening along outward dipping faults promotes magma degassing seawater influx and high temperature leaching resulting in a metal rich hydrothermal fluid these outward dipping faults are considered to provide critical pathways for ore forming fluids responsible for some caldera hosted vms deposits and may also be fundamentally important for the formation of many other caldera hosted ore deposit types

long range sonar observations of sedimentary facies patterns in and around the amirante trench western indian ocean long range side scan sonographs are used to map two distinct acoustic facies in the amirante trench western indian ocean a structural barrier formed by a seamount within the trench separates the two facies types the acoustic facies developed s of the seamount is characterized by weak reflectivity returned from sediments deposited by the flow of the deep western boundary undercurrent through the channels of the amirante passage into the southern part of the amirante trench n of the seamount the acoustic facies is characterized by high reflectivity and is confined to the trench axis it arises from sediments episodically deposited by turbidity currents or debris flows possibly originated by slumping of reef talus from the atolls surmounting the adjacent amirante ridge

morphology of abyssal mudwaves at project mudwaves sites in the argentine basin abstract extensive fields of mudwaves are present in the argentine basin mudwaves and water parameters at three sites were studied to provide detailed information on bathymetry mudwave structure sediment parameters and bottom water flow characteristics two sites 5 and 6 are discussed in this paper crests of mudwaves at site 5 on the north flank of the zapiola drift are oriented 45 anti clockwise from the measured westward flow direction and migrate to the left of the flow direction mudwaves at site 6 on the south flank of the zapiola drift are oriented 20 anti clockwise from the inferred southeast bottom flow direction and also migrate to the left of direction evidence from current meters and surface sediments suggests that the mudwaves at site 5 are active today while those at site 6 have been active in the holocene but perhaps not within the last 100 years mudwave orientation anti clockwise to mean flow preferential sediment accumulation on the left wave flank and apparent sediment accumulation on the wave crest agree with a model for mudwave growth presented by blumsack and weatherly 1989 deep sea research 36 1327 1339 cross wave asymmetry in sediment accumulation pattern is sufficient to cause a 10 change in surface sediment bulk density over the wave profile the record of mudwave migration obtained from sites 5 and 6 in the southern argentine basin and from site 7 in the northern argentine basin suggests that bottom water flow has changed over a number of time scales during the last 200 000 years thus the analysis of abyssal bed forms can provide independent evidence about past changes in bottom flow

submarine geology of the hilina slump and morpho structural evolution of kilauea volcano hawaii abstract marine geophysical data including sea beam bathymetry hawaii mr1 sidescan and seismic reflection profiles along with recent robot submersible observations and samples were acquired over the offshore continuation of the mobile kilauea volcano south flank this slope comprises the three active hot spot volcanoes mauna loa kilauea and loihi seamount and is the locus of the hawaiian hot spot the south flank is the site of frequent low intensity seismicity as well as episodic large magnitude earthquakes its sub aerial portion creeps seaward at a rate of approximately 10 cm year the hilina slump is the only large submarine landslide in the hawaiian archipelago thought to be active and this study is one of the first to more highly resolve submarine slide features there the slump is classified into four distinct zones from nearshore to the island s base estimates of size based on these data indicate a slumped area of 2100 km 2 and a volume of 10 000 12 000 km 3 equivalent to about 10 of the entire island edifice the overall picture gained from these data sets is one of mass wasting of the neovolcanic terrain as it builds upward and seaward though reinforcement by young and pre hawaii seamounts adjacent to the pedestal is apparent extensive lava delta deposits are formed by hyaloclastites and detritus from recent lava flows into the sea these deposits dominate the upper submarine slope offshore of kilauea with pillow breccia revealed at mid depths along the lower flanks massive outcrops of volcanically derived sedimentary rocks were found underlying kilauea thus necessitating a rethinking of previous models of volcanic island development the morphologic and structural evolutionary model for kilauea volcano and the hilina slump proposed here attempts to incorporate this revelation a hazard assessment for the hilina slump is presented where it is suggested that displacement of the south flank to date has been restrained by a still developing northeast lateral submarine boundary when it does fully mature the south flank may be more subject to land slips triggered by large long duration earthquakes and thus kilauea may undergo more frequent episodes of failure with increased displacements

strike slip fault styles in slow slipping oceanic transform faults evidence from gloria surveys of atlantis and romanche fracture zones long range sidescan sonar data covering active sections of two transform fault zones are presented to illustrate the range of structures observed during the surveying of several atlantic fracture zones the trace of the actively slipping component the principal transform displacement zone is identified and its configuration interpreted in terms of either extensional compressional stress systems or oblique motion at the plate boundaries

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

mineralization and alteration of a modern seafloor massive sulfide deposit hosted in mafic volcaniclastic rocks tinakula is the first seafloor massive sulfide deposit described in the jean charcot troughs and is the first such deposit described in the solomon islands on land or the seabed the deposit is hosted by mafic basaltic andesitic volcaniclastic rocks within a series of cinder cones along a single eruptive fissure extensive mapping and sampling by remotely operated vehicle together with shallow drilling provide insights into deposit geology and especially hydrothermal processes operating in the shallow subsurface on the seafloor mostly inactive chimneys and mounds cover an area of 77 000 m2 and are partially buried by volcaniclastic sand mineralization is characterized by abundant barite and sulfide rich chimneys that formed by low temperature 250 c venting over 5 600 years barite rich samples have high sio2 pb and hg contents the sulfide chimneys are dominated by low fe sphalerite and are high in cd ge sb and ag few high temperature chimneys including zoned chalcopyrite sphalerite samples and rare massive chalcopyrite are rich in as mo in and au up to 9 26 ppm locally as visible gold below the seafloor the mineralization includes buried intervals of sulfide rich talus with disseminated sulfides in volcaniclastic rocks consisting mainly of lapillistone with minor tuffaceous beds and autobreccias the volcaniclastic rocks are intensely altered and variably cemented by anhydrite with crosscutting sulfate minor sulfide veins fluid inclusions in anhydrite and sphalerite from the footwall to 19 3 m below seafloor m b s f have trapping temperatures of up to 298 c with salinities close to but slightly higher than that of seawater 2 8 4 5 wt nacl equiv these temperatures are 10 to 20 c lower than the minimum temperature of boiling at this depth 1 070 1 204 m below sea level m b s l suggesting that the highest temperature fluids boiled below the seafloor the alteration is distributed in broadly conformable zones expressed in order of increasing depth and temperature as 1 montmorillonite nontronite 2 nontronite corrensite 3 illite smectite pyrite 4 illite smectite chamosite and 5 chamosite corrensite zones of argillic alteration are distinguished from chloritic alteration by large positive mass changes in k2o enriched in illite smectite mgo enriched in chamosite and corrensite and fe2o3 enriched in pyrite associated with illite smectite alteration the 18o and d values of clay minerals confirm increasing temperature with depth from 124 to 256 c and interaction with seawater dominated hydrothermal fluids at high water rock ratios leaching of the volcanic host rocks and thermochemical reduction of seawater sulfate are the primary sources of sulfur with 34s values of sulfides from 0 8 to 3 4 and those of sulfate minerals close to seawater sulfate from 19 3 to 22 5

distributed extension at the indian ocean triple junction the indian ocean triple junction is a ridge ridge ridge type joining two medium spreading ridges with one slow spreading ridge gloria long range side scan sonar images show that while the axial valleys of the two medium spreading ridges are almost colinear apart from a small 5 km offset the valley of the slow spreading third axis does not meet the other two in a simple fashion the axis of this slow spreading southwest indian ridge swir beyond the rift valley walls of the other two ridges steadily deepens by over 1000 m away from the triple junction to reach 5000 m at 35 km the gloria images show large normal faults around the deep swir rift valley which increase in heave away from the triple junction and crosscut the abyssal hills formed at the faster spreading central indian ridge indicating that extension across the swir is distributed over a zone 10 km or more wide this zone also shows no evidence for the formation of new oceanic crust suggesting that the extension may be amagmatic near the triple junction the high relief of the swir rift flanks containing tilted seafloor of the other two ridges may be an isostatic response of the lithosphere to the deep valley produced by this rifting these observations which suggest the progressive development of a propagating swir rift by the extension of preexisting seafloor may have general implications for the dynamics of oceanic spreading centers in particular the deepening and widening of the valley away from the triple junction is attributed to the competing effects of tectonic thinning and lithospheric cooling thermal models of mid ocean ridges spreading at 30 mm yr e g lin and parmentier 1989 predict that the depth to the 700 c isotherm increases with distance from the spreading axis by a proportion similar to the observed doubling in the total width of the deformation with distance from 10 to 35 km away from the triple junction this rate of widening of the deformation zone may therefore support lithospheric necking models for oceanic rift valleys if the width and depth of the deforming lithosphere are related in a simple manner

the geological interpretation of side scan sonar recent developments in side scan sonar these parameters play a critical role in our ability to technology have increased the potential for fundamental calibrate and ultimately to interpret the new pictures of the changes in our understanding of ocean basins developed ocean floor acoustic image processing is a new applica in the late 1960s side looking sonars have been widely tion of an old and well established technique digital used for the last two decades to obtain qualitative estimates optical images have benefited from several decades of of the acoustic properties of the materials of the seafloor modern developments in the ability to obtain spatially correct digital data from side scan sonar systems have resulted in images that can be subsequently processed enhanced and quantified with appropriate processing these acoustic images can be made to resemble easily recognizable optical photographs any geological interpretation of these images requires an understanding of the inherent limitations of the data acquisition system when imagery is collected these limitations are largely centered on the concept of resolution in side scan sonar images there are several different types of resolution including along and across track resolution display the geology of the seafloor resolution and absolute instrumental resolution all of development in processing techniques these relatively sophisticated techniques have been applied to photographic images from satellites and spacecraft images which are noisy and difficult to obtain but extremely valuable side scan sonar systems on the other hand have only recently been able to produce spatially correct digital images of the seafloor the application of digital signal processing techniques to side scan sonar data willnow allow us to quantify what had been previously very subjective and qualitative interpretations of images of the seafloor the goal of all this processing of acoustic images remains clear the development of an interpretable map of featureless static environment has undergone dramatic modification in the last 50 years early depth sounding with mechanical devices and even early wide beam acoustic echo sounders gave us an extremely low resolu tion picture of the ocean basins this early image of the ocean bottom consisted largely of a flat lying seafloor with few hills and ridges of any consequence completely covered with a thick layer of sediment and only an occasional inexplicable outcrop of hard rock the few recovered rock samples were of little value in understand ing the scientific processes of the deep sea because they could not be placed within any sort of geological and morphological context acceptance of seafloor spreading and the vine matthews hypothesis in the 1960s altered forever our perception that the floor of the ocean basins was unchang ing at least on a geological time scale the initial drilling efforts of the deep sea drilling project also modified our view about the uniformity of the seafloor even on a scale of a few hundreds and perhaps tens of kilometers higher resolution maps using multiple narrow beam echo sounders strongly reinforced this newperception of a nonuniform and scientifically interest ing seafloor tyce 1986 the initial interest grew to excitement as our perspective was extended by visual observation in a very few places down to the scale of a few meters by early submersible expeditions to mid ocean ridge spreading centers ballard and van andel 1977 clearly a new tool beyond the wide beam echo sounder was needed to map features of the seafloor and to understand the processes at work there to be effective this would have to be a tool which had both a sufficiently wide view for tectonic context and adequate resolution for interpretation in terms of geological processes the efficiency of sound transmission in seawater and the development of both electronic and digital techniques capable of rapidly processing the high data rates necessary to generate images dictated that this tool would be some form of acoustic swath mapping the scientific need for

the tectonic evolution of the indian ocean triple junction anomaly 6 to present the tectonic evolution of the indian ocean ridge ridge ridge triple junction the junction of the central indian ridge cir southeast indian ridge seir and southwest indian ridge swir is inferred by mapping structures along its traces on the three plates with a long range side scan sonar geological long range inclined asdic gloria the sonar images to the west of the triple junction show two different styles of tectonic evolution some sections of the trace on the african plate are marked by a swir facing scarp containing fine lineaments that are probably normal faults these correspond with sections of the trace on the antarctic plate of the same age that have a sharp intersection of swir and seir abyssal hills that is to be expected from a simple ridge ridge ridge triple junction evolution a herringbone pattern of isochrons in contrast there are other sections of the trace on the african plate that have a blocky structure with no definite termination of cir abyssal hills against those of the swir the corresponding regions of the antarctic plate show large 10 to 30 km long overlapping fault scarps tapering to the northeast and ending in asymmetric valleys to the southwest these major normal faults are each offset by 1 6 km forming an en echelon series the two different styles of tectonic structures indicate two different modes of evolution although why the triple junction has evolved with two different modes is unclear periods of asymmetric spreading in the cir magnetic anomaly sequence collected on the african plate may possibly indicate that the cir and seir have been offset by a fracture zone at the triple junction during one of these two modes echo sounder traverses on the antarctic and african plates commonly show a steady shallowing of the cir and seir seafloor toward the traces a steep scarp facing the swir and a broad marginal deep at the base of this scarp this rift flank shape resembles that of the swir valley s rift shoulders close to the present triple junction mitchell 1991a suggesting that the mechanical evolution has been similar to the present day and the relief due to the rift flank uplift has been preserved by the cooling and strengthening of the lithosphere with age away from the ridge axes if the shape of the rift shoulder uplift along the trace on the antarctic plate is interpreted with a simple flexural model involving an end loaded elastic plate the inferred rigidity 1019 1021 n m or 2 6 km effective elastic thickness is consistent with the suggestion that the isostatic uplift occurred in young weak lithosphere close to the triple junction interestingly the observed 1 km relief of the trace escarpment and the estimated rigidity suggest that the uplift was produced by a load 2 5 1011 n m that is equivalent to half the present cross sectional area of the swir valley 35 km from the triple junction

quantitative fault studies on the east pacific rise a comparison of sonar imaging techniques a synthesis is presented of published statistics on the lengths scarp heights and spacing of normal faults on the flanks of the east pacific rise epr between 19 s and 21 n nine data sets are included which were collected at different times employing one of four different sonar mapping systems the scripps deep tow instrument seamarc i seamarc ii and gloria with widely ranging data collection capabilities the epr in this region is morphologically quite similar and in some cases the data sets were obtained from the same area of the seafloor it is assumed therefore that data collection procedure and system resolution have had a greater influence on the fault statistics observed than varying tectonic processes we try to account for these sampling effects such that quantitative comparisons of fault development at midocean ridges may then be made we discuss the problem of identifying faults in sonar images and the errors associated with measurements of the relevant parameters we also investigate the statistical properties of the fault populations and derive expressions for correcting observed values of the population means at finite resolution to obtain the corresponding values at infinite resolution the theoretical predictions compare well with the results of a decimation procedure applied to real data sets to simulate diminishing resolution we also discuss the effects that surveying geometry can have on the interpretation of fault lengths the data synthesis is augmented by an analysis of the correlation between scarp height and fault length for areas near 3 5 s and 12 n on the epr the correlation which we find to be linear is used to estimate 1 the minimum operational fault target size for each of the four mapping systems and 2 the topography due to faulting in an area where only fault lengths are known

volcanic and structural reconstruction of the deformed and metamorphosed ming volcanogenic massive sulfide deposit canada implications for ore zone geometry and metal distribution

resolving the relative timing of au enrichment in volcanogenic massive sulfide deposits using scanning electron microscopy mineral liberation analyzer empirical evidence from the ming deposit newfoundland canada the metamorphosed cambro ordovician ming volcanogenic massive sulfide deposit in northern newfoundland canada is locally overlain by a unit consisting of mafic to intermediate medium to coarse grained volcaniclastic breccia with up to 10 vol sulfide clasts analysis via mineral liberation analyzer of two sulfide clasts completed using scanning electron microscope observations allowed the identification of a number of microscopic and submicroscopic electrum grains these electrum grains occur in three types of textural settings 1 free electrum grains with tellurides within gangue minerals 2 inclusions of electrum with tellurides in pyrrhotite grains and 3 free electrum grains with base metals and tellurides interstitial between base metals and along cataclastic fractures in pyrite these three textural settings are similar to those in the underlying massive sulfide orebodies that represent very likely sources to the sulfide clasts the polylithic nature and angularity of the volcaniclastic fragments in the breccia suggest a postmineralization gravity controlled debris flow proximal to its source the mineral assemblage and textures of electrum in the sulfide clasts implies evidence in support of syngenetic and predeformation au introduction in the volcanogenic massive sulfide deposit and argues against an orogenic overprint as the cause for au enrichment

subsea floor replacement in volcanic hosted massive sulfide deposits abstract recent research on volcanic hosted massive sulfide vms deposits indicates that syngenetic subsea floor replacement ores form an important component of many deposits in the context of vms deposits subsea floor replacement can be defined as the syn volcanic formation of sulfide minerals within pre existing volcanic or sedimentary deposits by infiltration and precipitation in open spaces fractures inter and intra granular porosity as well as replacement of solid materials there are five criteria for distinguishing subsea floor replacement in massive sulfide deposits 1 mineralized intervals are enclosed within rapidly emplaced volcanic or sedimentary facies lavas intrusions subaqueous mass flow deposits pyroclastic fallout 2 relics of the host facies occur within the mineral deposit 3 replacement fronts occur between the mineral deposit and the host lithofacies 4 the mineral deposit is discordant to bedding and 5 strong hydrothermal alteration continues into the hanging wall without an abrupt break in intensity criteria 1 3 are diagnostic of replacement whereas criteria 4 and 5 may suggest replacement but are not alone diagnostic because clastic sulfide ores contain accessory rock fragments collected by the parent sediment gravity flow s during transport criteria 2 can only be applied to massive semi massive disseminated or vein style deposits and not clastic ores the spectrum of vms deposit types includes deposits that have accumulated largely subsea floor and others in which sedimentation and volcanism were synchronous with hydrothermal activity and precipitation of sulfides occurred at and below the sea floor over the life of the hydrothermal system deposits that formed largely subsea floor are mainly hosted by syn eruptive or post eruptive volcaniclastic facies gravity flow deposits water settled fall autoclastic breccia however some subsea floor replacement vms deposits are hosted by lavas and syn volcanic intrusions sills domes cryptodomes burial of sea floor massive sulfide by lavas or sediment gravity flow deposits can interrupt sea floor mineralization and promote subsea floor replacement and zone refining the distance below the sea floor at which infiltration and replacement took place is rarely well constrained with published estimates ranging from less than 1 to more than 500 m but mainly in the range 10 200 m the upper few tens to hundreds of metres in the volcano sedimentary pile are the favoured position for replacement as clastic facies are wet porous and poorly consolidated in this zone and at greater depths become progressively more compacted dewatered altered and less amenable to large scale infiltration and replacement by hydrothermal fluids furthermore sustained mixing between the upwelling hydrothermal fluid and cold seawater is regarded as a major cause of sulfide precipitation in vms systems and this mixing process generally becomes less effective with increasing depth in the volcanic pile the relative importance of subsea floor replacement in vms systems is related principally to four factors the permeability and porosity patterns of host lithofacies sedimentation rate the relative ease of replacement of host lithofacies especially glassy materials and early formed alteration minerals during hydrothermal attack and physiochemical characteristics of the hydrothermal fluid

sonar acoustic facies and sediment distribution on an area of the deep ocean floor abstract long range sidescan sonar can be used to map sediment distributions over wide expanses of deep ocean floor seven acoustic facies that arise from differing sediment or rock types have been mapped over the low relief saharan continental rise and madeira abyssal plain these have been calibrated with sampling profiling and camera studies and the facies can be traced confidently on a regional scale using the sidescan data the mapping of the sediment distribution shows that a complex interplay of turbidity current and debris flow processes can occur at a continental rise abysaal plain transition over 1000 km from the nearest continental slope

low temperature alteration at the world class laronde penna archean au rich volcanogenic massive sulfide deposit abitibi subprovince quebec canada evidence from whole rock oxygen isotopes the oxygen isotope composition of 169 whole rock samples from the world class laronde penna au rich volcanogenic massive sulfide deposit range from 6 5 to 22 0 fractional crystallization from basalt to rhyolite accounts for a variation of approximately 2 in primary whole rock 18o values the remaining variance in whole rock 18o values found at laronde penna must therefore be ascribed to water rock exchange with hydrothermal fluids the metamorphosed hydrothermally altered host rocks have been subdivided according to mineralogical assemblages which display weak covariance with 18o values and whole rock geochemical alteration indices water rock equilibrium exchange models indicate that the high 18o values that characterize the laronde penna deposit are compatible with low temperature 150oc hydrothermal alteration at high water rock oxygen atomic ratios 1 50 zones of lower 18o values stratigraphically beneath the principal ore lens 20 north indicate local zones of higher temperature hydrothermal alteration the oxygen isotope composition of the hydrothermal fluid is estimated to have been 5 as a consequence of mixing between seawater 18o 0 and a component of magmatic water degassed from the volcanic and intrusive rocks associated with the laronde penna deposit

accretion of the south florida platform late quaternary development stratigraphic information from high resolution seismic data obtained across the southwest florida platform indicates that the modern shelf is a constructional platform with pliocene pleistocene and holocene sediments resting on an eroded karstic miocene platform the miocene surface dips away from the coastline with significant breaks in slope occurring at the center of the shelf and at the shelf edge at the southwest corner of the platform this surface crops out to form a terrace this terrace lies along the west facing continental slope of the florida shelf and is progressively buried to the south by younger deposits reefs and sediment so that it has no surface expression in the florida straits a paired reef complex rests on the thickest post miocene sediments t at mark the edge of the modern shelf the deepest reef forms a well developed escarpment with its crest buried by approximately 15 m 50 ft of sediment the shallower reef is a low swale over most of its extent but develops into a large reef spit complex howell hook in the central part of the study area within the pliocene pleistocene and holocene sediments two stratigraphic units can be delineated 1 a lower progradational unit of pliocene pleistocene age that can be traced under the shelf edge reef and continuously onlaps the miocene surface and 2 an upper unit of late pleistocene holocene age which is composed of reef and pelagic sediment two sedimentary fans have been identified on the northern slope and floor of the florida straits the apices are set in at gaps in the carbonate ridge rimming the southern florida shelf at least nine complex sigmoidal oblique sequences are identified within the fans a reef buried by sediment of the last two sequences correlates with howell hook radiometric analyses of material dredged from this feature and from the outcrops of the second sequence yield ages of 7 7 11 ka indicating a holocene age for these sediments and suggesting a very high rate of shelf accretion

the evolution of craters and calderas on young seamounts insights from sea marc i and sea beam sonar surveys of a small seamount group near the axis of the east pacific rise at 10 n a northwest trending group of seamounts located just west of the east pacific rise epr axis has been mapped using high resolution sea marc i side scan sonar and sea beam multibeam bathymetry these seamounts have 9 13 km basal diameters and the shapes of the volcanos change from conical domes near the epr axis to truncated cones and larger elliptical truncated cones farther away from the rise axis the detailed resolution of constructional volcanic features on these seamounts and the progressive development of summit depressions craters to large complex calderas and summit plateaus suggest to us a succession of evolutionary stages through which seamounts grow our evolutionary scheme for constructing these volcanos calls for the development of satellite extrusive centers around the base of a growing seamount which serve to drain off magmas and precipitate summit collapse the creation of a summit depression on a seamount will structurally constrain the style of eruptive volcanism to be principally along ring fractures which localize vents around the margin of a crater or caldera the concentration and channelization of volcanic eruptions around the top of the volcano lead to a progradation of the summit area and the establishment of a summit plateau the steep sides of a seamount reflect growth of the summit area out over the upper flanks of the volcano caused by continued eruption of lavas around the margin of the caldera the steep constructional slopes are sites for vigorous mass wasting that shapes and modifies flank terrain as a seamount evolves the crestal topography is the product of many volcanic cycles characterized by filling and subsequent draining of the caldera and expansion of caldera area through progressive collapse and migration of primary magmatic conduits the final phase of volcanism may be a calderafilling episode that builds a rough conical mount over the summit plateau

submarine central volcanoes on the nazca plate high resolution sonar observations abstract this paper describes observations of 214 submarine central volcanoes made with long range side scan sonar and narrow beam echosounder between peru and the east pacific rise they are all circular in plan with basal diameters ranging from less than 1 to over 10 km profiles of four volcanoes were obtained with the narrow beam echosounder and those of a further nine were inferred from their acoustic shadows all the profiles show steep mostly concave sides a distinct summit region could be identified on about two thirds of the volcanoes in just under half of these the volcano culminates in a simple crater but the rest have broad slightly convex summit plateaus with or without craters almost half of all the volcanoes have one or more craters summit plateaus are relatively more common on the larger volcanoes so that for central volcanoes with a basal diameter greater than 4 km the commonest morphology is a steep sided truncated cone with a slightly domed summit plateau in most cases this form must represent the mode of formation not the result of subsequent wave erosion it is suggested that this form is the result of relatively fluid lavas travelling outward on low slopes from a central vent but terminating in steeply dipping hyaloclastites and volcanic breccias the spatial distribution of volcanoes is very non random in the area studied the east pacific rise out to 6 5 ma has less than one volcano per 10 4 km 2 whereas a field hundreds of kilometers across with a peak density of over 60 per 10 4 km 2 was seen north of mendana fracture zone on 6 5 26 ma old crust

contemporary channel levee systems in active borderland basin plains california continental borderland abstract long range large scale side scan gloria information other seismic reflection profiling studies and data from cores in the california continental borderland have defined active levee channel systems extending basinward from the lower fan of hueneme mugu submarine fan redondo submarine fan and santa cruz canyon fan in santa monica san pedro and santa cruz basins respectively the holocene distributaries have been created by a series of turbidity current events these distributaries range in length from 10 25 km and are wide 2 5 km and low relief 1 10 m in their distal parts they are also active conduits for nepheloid flows distributions of sedimentological parameters typically mimic the pattern of levee channel systems organic carbon and biogenic carbonate content roughly outline the systems channels are incised in the upper to middle fan areas and become constructional leveed channels in the lower fan and basin plain as the channel gradient adjusts to maintain a graded profile thus sediment gravity flows are generally confined to channels in the upper fan zones but deposit both channelized and over bank deposits on the lower fan and basin floor the deposits show that the canyon fan activity has continued during a rising sea level phase it is evident that canyon headward erosion rates have been equal to or greater than the transgression rate and that the canyon fan systems have remained linked with their sediment sources frequency of events was probably higher and volumes of the events were often larger during the glacially lowered sea level episodes however turbidity currents of sufficient volume to reach the basin floors continue to occur at century or multi century intervals as one progresses headward in each system the number of flows per length of core increases small flows that do not pass beyond the distributaries are much more frequent and may be decadal in frequency or even more frequent in the santa monica basin system these california borderland basins are probably typical of narrow active margins where rate of lateral sea level transgression is less than or equal to the rate of canyon headward erosion the canyons maintain connections with sediment sources during sea level rise and the systems therefore are active during the entire sea level cycle thus sediment supply is not a simple function of eustacy this contrasts with the simplified sequence model developed on passive margins where canyons turn off as sea level rises

the tectonic fabric of the barbados ridge accretionary complex abstract a map showing near surface structure and other surficial geological features on the barbados ridge accretionary complex has been constructed from gloria long range side scan sonographs detailed bathymetric surveys using seabeam and single and multichannel seismic reflection sections the map shows major variations in the size orientation and style of structures across the accretionary complex that are principally related to the increasing thickness of the complex and the sediment drape upon it and variations along the complex that derive from the northward thinning of sediment accreted from the ocean floor and the effects of ridges in the oceanic basement the development of crossfaulting is mainly associated with basement ridges mud diapirism is abundant in the southern part of the complex and is characteristic of that part of the complex into which turbidites from the orinoco submarine fan are accreted slumping on a variety of scales is another feature of the complex the map is presented at a scale of 1 1063415 4 inches per degree of longitude and also shows bathymetry at 200 m contour intervals

the filon norte orebody tharsis iberian pyrite belt a proximal low temperature shale hosted massive sulphide in a thin skinned tectonic belt the filon norte orebody tharsis iberian pyrite belt is one of the largest pyrite rich massive sulphide deposits of the world the present structure of the mineralization consists of an internally complex low angle north dipping thrust system of variscan age there are three major tectonic units separated by thick fault zones each unit with its own lithologic and hydrothermal features they are internally organized in a hinterland dipping duplex sequence with high angle horses of competent rocks igneous and detritic rocks and massive sulphides bounded by phyllonites the mineralization is within the lower unit and is composed of several stacked sheets of massive sulphides and shales hosting a stockwork zone with no obvious zonation the intermediate unit is made up of pervasively ankeritized shales and basalts spilites here hydrothermal breccias are abundant the upper unit is the less hydrothermally altered one and consists of silicified dacites and a diabase sill the tectonic reconstruction suggests that the sequence is inverted and the altered igneous rocks were originally below the orebody carbon oxygen and sulphur isotopes in the massive sulphides and hydrothermal rocks as well as the mineral assemblage and the paragenetic succession suggest that the sulphide precipitation in the sea floor took place at a low temperature 150 c without indication at least in the exposed section of a high temperature copper rich event sporadic deep subsea floor boiling is probably responsible for the formation of hydrothermal breccias and the wide extension of the stockwork its co au enrichment is interpreted as being related with the superposition of some critical factors such as the relationship with black shales the low temperature of formation and the boiling of hydrothermal fluids the present configuration and thickness of the orebody is due to the tectonic stacking of a thin and extensive blanket 2 4 km2 of massive sulphides with low aspect ratio they were formed by poorly focused venting of hot modified seawater equilibrated with underlying rocks into the seafloor massive sulphide precipitation took place by hydrothermal fluid quenching bacteriogenic activity and particle settling in an unusual restricted euxinic and shallow basin brine pool with a low detritic input but with important hydrothermal activity related to synsedimentary extensional faulting resedimentation of sulphides seems to be of major importance and responsible for the observed well mixed proximal and distal facies the tectonic deformation is largely heterogeneous and has been mostly channelled along the phyllonitic tectonized shales deformation bands thus sedimentary and diagenetic textures are relatively well preserved outside the deformation bands in the massive sulphides superimposed variscan recrystallization is not very important and only some early textures are replaced by metamorphic tectonic ones the stockwork is much more deformed than the massive sulphides the deformation has a critical effect on the present morphology of the orebody and the distribution of the ore minerals this deposit is a typical example of the sheet like shale hosted anoxic low temperature and zn rich massive sulphides developed in a ensialic extensional basin

pleistocene terraced fluvial succession northern slope of torino hill abstract a pleistocene terraced fluvial succession is found on the northern slope of the torino hill this succession consists of many relics of flat surfaces separated by scarps often associated with sandy and silty deposits perched from 30 to 400 m on the po plain terraced surfaces at high levels are more deformed dissected and mostly deprived of their original sediments lower ones are less deformed more continuous and preserved in the western sector of the area these surfaces form a declining terraced succession towards the plain in the eastern sector they form a terraced succession entrenched within the relief the succession is not linked either to the present hilly watercourses or to the po river now developed to the north of hilly relief because during the pleistocene the river flowed south of the torino hill on the contrary it is connected with modification by ancient northern piemonte plain watercourses this succession is the result of deformation of original plain sectors prior to the po river shifting north of the relief the comparison of chronological reference of fluvial morphological features and sediments with their altimetric distribution suggests a mean uplift of the torino hill of about 1 mm year the present morphology connected with this evolution favours landslide phenomena

density and size distribution of seamounts in the eastern pacific inferred from wide beam sounding data theoretical expressions relating the distribution of apparent seamount heights per unit length of ship track to the distribution of actual seamount heights per unit area of seafloor are derived for both narrow beam and wide beam echo sounders the shapes of seamounts are approximated as truncated right circular cones of constant slope e height to radius ratio and flatness f i e a seamount of basal radius r has a height h r a flat top of radius fr and a slope angle of arctan e the set of all seamounts in a given physiographic province is assumed to be a poissonian ensemble specified by a function v r the average number of seamounts per unit area for which r r a simple choice for the size distribution of seamounts and one shown to be consistent with data in the eastern pacific is to specify that it decays exponentially with r v r vo exp r in the case of a profile of length l taken in water of depth d with a wide beam echo sounder half beam width the shallow slope deep water approximation e 1 e d yields that the expected number of seamounts with apparent heights h h is n h l 2 1 ed 2 fh lv0e h we apply this sampling theory to apparent height data measured from 44 020 km of wide beam profiles on pacific crust of post mesozoic age although the spatial distribution of seamounts in the eastern pacific is certainly not poissonian the abundance statistics do not involve spatial correlations and we show they are robust with respect to deviations from the poisson model provided the statistics are interpreted as averages over the area actually sampled by the track lines the observed falloff of apparent heights in the eastern pacific is very nearly exponential justifying our use of an exponential model of seamount size distribution the shape parameters are estimated from multibeam surveys of pacific seamounts an analysis of 30 features in the height range 200 4000 m yields 0 21 0 011 and f 0 21 0 028 using these parameters we obtain vo 4 0 1 1 10 9 m 2 and 6 3 0 8 10 4 m 1 these values imply that per 106 km2 in the eastern pacific there are 1600 400 seamounts with summit heights greater than 300 m and 200 60 greater than 1000 m integrated over the entire size distribution seamounts occupy about 6 of the seafloor area and constitute about 0 4 of the oceanic crustal volume our estimates of seamount abundance are appreciably greater than those of most other authors a discrepancy we attribute primarily to the sampling bias of previous studies which generally used uncorrected map counts this hypothesis is corroborated by a comparison of our statistics with map count data for seamounts of heights exceeding 2000 m compiled for the eastern central pacific from the scripps charts above 3000 m the map counts are consistent with the predictions of the exponential model but below this height they are biased to low values

global comparisons of volcanic associated massive sulphide districts although volcanic associated massive sulphide vms deposits have been studied extensively the geodynamic processes that control their genesis location and timing remain poorly understood compar

slumping and shallow faulting related to the presence of salt on the continental slope and rise off north carolina abstract seismic reflection profiles and long and medium range sidescan sonar were used to investigate a salt diapir complex and area of slope instability near the base of the continental slope off north carolina within the area of investigation three diapirs are bounded on their upslope side by a scarp 60 m high and 50 km long the slope above the scarp is characterized by a series of shallow rotational normal faults the bottom below the scarp is furrowed by slide tracks which were probably carved by large blocks that broke off the scarp face and slid downslope leaving rubble and scree lobes extensive slumping in this area appears to be a result of uplift and faulting associated with salt intrusion which has fractured and oversteepened the slope leading to instability and failure sharply defined slide tracks suggest that slope failure above the breached diapir complex is a continuing process in contrast to much of the surrounding slope area where few instability features were observed

sea floor drainage features of cascadia basin and the adjacent continental slope northeast pacific ocean abstract sea floor drainage features of cascadia basin and the adjacent continental slope include canyons primary fan valleys deep sea valleys and remnant valley segments long range sidescan sonographs and associated seismic reflection profiles indicate that the canyons may originate along a mid slope escarpment and grow upslope by mass wasting and downslope by valley erosion or aggradation most canyons are partly filled with sediment and quillayute canyon is almost completely filled under normal growth conditions the larger canyons connect with primary fan valleys or deep sea valleys in cascadia basin but development of accretionary ridges blocks or re routes most canyons forcing abandonment of the associated valleys in the basin astoria fan has a primary fan valley that connects with astoria canyon at the fan apex the fan valley is bordered by parallel levees on the upper fan but becomes obscure on the lower fan where a few valley segments appear on the sonographs apparently nitinat fan does not presently have a primary fan valley none of the numerous valleys on the fan connect with a canyon the willapa cascadia vancouver juan de fuca deep sea valley system bypasses the submarine fans and includes deeply incised valleys to broad shallow swales as well as within valley terraces and hanging valley confluences

depositional gaps in abitibi greenstone belt stratigraphy a key to exploration for syngenetic mineralization models of greenstone belt development are crucial for exploration allochthonous models predict belts to be a collage of unrelated fragments whereas autochthonous models allow for prediction of syngenetic mineral deposits within specific stratigraphic intervals superior province greenstone belts consist of mainly volcanic units unconformably overlain by largely sedimentary timiskaming style assemblages and field and geochronological data indicate that the abitibi greenstone belt developed autochthonously we describe major revisions to stratigraphy of the abitibi greenstone belt and the implications of an autochthonous development of the volcanic stratigraphy for exploration for syngenetic mineralization the abitibi greenstone belt is subdivided into seven discrete volcanic stratigraphic episodes on the basis of groupings of numerous u pb zircon ages of pre 2750 2750 to 2735 2734 to 2724 2723 to 2720 2719 to 2711 2710 to 2704 and 2704 to 2695 ma we present revised lithotectonic and or stratigraphic nomenclature using these time intervals including 1 isotopic inheritance in younger episodes which indicates that the older episodes 2750 2735 and 2734 2724 ma had greater extent than is presently seen 2 dikes feeding younger volcanic episodes 2706 ma cutting older volcanic units 2734 2724 ma and 3 2710 to 2704 ma mafic to ultramafic sills intruding the 2719 to 2711 ma episode changes to the nomenclature include the identification of pre 2750 ma volcanic episode supracrustal fragments in the northern and southern abitibi greenstone belt and subdivision of the 2719 to 2711 ma 2710 to 2704 ma and 2704 to 2695 ma episodes into lower and upper parts we present the results of this lithostratigraphic subdivision as the first geochronologically constrained stratigraphic and or lithotectonic map of the abitibi greenstone belt many of the volcanic episodes are intercalated with and capped by a relatively thin sedimentary interface zone dominated by chemical sedimentary rocks stratigraphic and geochronological analysis of these zones indicates discontinuous deposition with localized gaps of 2 to 27 m y between volcanic episodes the zones consist of up to 200 m of iron formation chert breccia heterolithic debris flows of volcanic provenance sandstone and or argillite and conglomerate modeling of the time required for deposition of the volcanic units based on rates of magma production in modern arc and plume environments is on the order of 103 to 104 years whereas the time interval between basalt rhyolite cycles is 106 years the sedimentary interface zones are therefore interpreted as condensed sections zones with very low rates of sedimentation in a basinal setting or zones with negligible rates of sedimentation marked by silicification of existing rock types the sedimentary interface zones are therefore considered submarine correlative conformities disconformities or unconformities separating the equivalent of group level stratigraphic and lithotectonic units the unconformity bounded stratigraphic model provides a new regional to deposit scale interpretive model for use in exploration for syngenetic mineralization

three dimensional visualization of the archean horne and quemont au bearing volcanogenic massive sulfide hydrothermal systems blake river group quebec the hydrothermal system architecture related to the formation of the contemporaneous au bearing horne and quemont volcanogenic massive sulfide vms deposits was visualized by employing kriging methods to map whole rock oxygen isotope compositions zones of silica addition and loss and water contents in two and three dimension zones of alteration were mapped in three dimensions in the vicinity of the steeply dipping horne deposit to depths of as much as 2 km in all nearly 300 samples were analyzed for oxygen isotopes and supplemented by previously published whole rock analyses contents of sio2 h2o mgo al2o3 and s from chemical analyses of nearly 5 000 samples within the two and three dimensional study regions were used separately and in combination with the oxygen isotope data for modeling and hydrothermal mapping purposes the horne and quemont deposits formed within a similar time frame but in different magmatic hydrothermal systems distinguished by their mapped hydrothermal architecture the quemont deposit appears to be centered on the powell pluton which intruded late into an apparent volcanic filled rift graben structure although structural complexities are apparent we infer mineralizing high temperature upflow in the footwall of the quemont deposit to have emanated from a reaction zone above the powell pluton and its precursors beneath a zone of extensive silicification faulting on the andesite fault and horne creek faults plus erosion has removed evidence of the upflow zone in the hydrothermal system of the horne deposit areas of silicification correspond in general with isotopic evidence of lower temperature alteration such alteration east of the quemont deposit signaled the waning of hydrothermal activity the suggested cooling for the most part promoted the precipitation of silica in the case of the horne deposit mixing of metalliferous hydrothermal fluid with cold seawater in the permeable footwall rocks in an apparently relatively stratigraphically stable and long lived hydrothermal system evidently led to marked footwall silicification the silicified footwall may have contributed to an increased efficiency of sulfide precipitation in the horne deposit continued intrusion and some post vms hydrothermal activity is recorded in the hanging wall section to the horne deposit our data suggest that deposition of the 10 million ounces moz of au within the horne deposit was syngenetic and not the product of subsequent hydrothermal activity

a gloria traverse over the marginal fracture zone and continental margin of se africa abstract a geological and physiographical interpretation of nearly 60 000 km 2 of the continental margin of se africa 21 20 e to 31 e has been made using gloria sonographs and seismic and bathymetric records these data have enhanced our understanding of the structural complexity of basement and sedimentary features in particular the marginal fracture ridge mfr which forms part of the agulhas falkland fracture zone affz most of the 500 km long exposed section can be traced in the sonographs and it is concluded that the mfr is not a single lineament but consists of two en echelon basement ridges that overlap by approximately 20 km in the vicinity of the scrutton ridge 23 5 e the latter is an enigmatic basement sliver that may be a narrow upthrust block east of 26 e a major slump and canyon province has been identified in the southwestern part of the natal valley here we record three previously undescribed canyons padrone boesmans and fish and extensive compressional structures in the distal parts of a large slump lobe

the first decade of gloria in 10 years of operation gloria has been used on 26 cruises and has inonified more than 1 of the ocean floor the paper reviews the scientific achievements of this new tool and assesses the contributions which it has made to understanding the geological features and the processes creating them a complete bibliography of gloria papers is included in the references and the best examples of sonographs obtained over different terrain are illustrated

mudwaves on the gardar sediment drift ne atlantic small amplitude 15 m mudwaves were mapped on the gardar sediment drift using the hydrosweep ds multibeam echosounder system on the r v maurice ewing improved data processing techniques allowed the swath mapped bathymetry to image features smaller than the nominal resolution 14 m of hydrosweep mudwave heights wavelengths and orientations were determined from the hydrosweep data and in conjunction with 3 5 khz profiles these mudwave fields were analyzed with regard to present models of mudwave formation mudwave crest orientation was clockwise to mean bottom flow direction and migration was to the right of measured currents for two areas on the drift this is consistent with models developed on large amplitude mudwaves using mudwave orientation and migration direction in a third area bottom current directions were inferred

synvolcanic and younger plutonic rocks from the blake river group implications for regional metallogenesis the blake river group of the abitibi greenstone belt is host to the well known volcanogenic massive sulfide vms deposits of the noranda camp quebec canada in the noranda camp massive sulfide deposits are spatially associated with the coeval flavrian powell intrusive complex which is interpreted to have driven hydrothermal circulation that formed the deposits in the camp in the western blake river group in ontario there are also abundant intrusive rocks yet relatively insignificant accumulations of vms deposits these intrusive rocks are younger 2686 9 1 2 and 2688 5 2 3 ma and are associated with porphyry style cu mo au mineralization with re os molybdenite ages of 2682 4 5 ma the intrusions are also smaller in areal extent and do not have a temporally extended polyphase history like the flavrian powell intrusive complex the intrusions of the western blake river group can also be distinguished by lithogeochemical attributes and a distinct petrological history compared to the flavrian powell intrusive complex they have lower total y yb th hfse and ree with higher zr y la yb sm yb al2o3 yb and la smucn ratios and lower zr nb zr th zr la zr tio2 and nb thucn ratios petrologic modeling is consistent with the western blake river group intrusive rocks being generated as relatively low temperature melts 40 km in the crust where garnet is stable in the residue e g garnet amphibolite residue in contrast modeling of the flavrian powell intrusive complex is consistent with melting at shallower levels in the crust e g 800o to 850oc the occurrence of high temperature magmatism at high levels within this crust that is coeval with volcanic activity explains the occurrence of numerous vms deposits with the flavrian powell intrusive complex as this intrusive complex would have generated abundant heat at high levels in the crust to drive hydrothermal circulation in contrast the intrusions of the western blake river group were generated at depth and at lower temperature increasing the probability of heat loss upon emplacement from depth hence decreasing the probability of hydrothermal circulation at high levels in the crust and likely explaining the paucity of vms mineralization associated with these intrusive rocks the petrological attributes of the flavrian powell intrusive complex and western blake river group may be useful in delineating potentially fertile versus less prospective intrusive complexes in greenfields exploration areas notably the western blake river group intrusions have geochemical features similar to some phanerozoic porphyry cu mo au associated intrusive systems consistent with the observed mineralization and highlighting the potential of porphyry cu mo au systems of late archean age elsewhere in the abitibi

fault strain and seismic coupling on mid ocean ridges the contribution of extensional faulting to seafloor spreading along the east pacific rise epr axis near 3 s and between 13 n and 15 n is calculated using data on the displacement and length distributions of faults obtained from side scan sonar and bathymetric data it is found that faulting may account for of the order of 5 10 of the total spreading rate which is comparable to a previous estimate from the epr near 19 s given the paucity of normal faulting earthquakes on the epr axis a maximum estimate of the seismic moment release shows that seismicity can account for only 1 of the strain due to faulting this result leads us to conclude that most of the slip on active faults must be occurring by stable sliding laboratory observations of the stability of frictional sliding show that increasing normal stress promotes unstable sliding while increasing temperature promotes stable sliding by applying a simple frictional model to mid ocean ridge faults it is shown that at fast spreading ridges 90 mm yr the seismic portion of a fault ws is a small proportion of the total downdip width of the fault w the ratio ws w interpreted as the seismic coupling coefficient x and in this case x 0 in contrast at slow spreading rates 40 mm yr ws w and therefore x 1 which is consistent with the occurrence of large magnitude earthquakes mb 5 0 to 6 0 occurring for example along the mid atlantic ridge axis

the active part of charlie gibbs fracture zone a study using sonar and other geophysical techniques a short survey with gloria side scan sonar and other geophysical instruments has revealed new information about charlie gibbs fracture zone between 29 and 36 w the traces of two transform faults have been clearly delineated they fit small circles about the pole of rotation with an rms error of only about 1 km but they do not always follow the deepest parts of the transform valleys the transforms are joined by a short spreading center at 31 45 w the median transverse ridge appears to have been produced by normal seafloor spreading at this center and bears identifiable vine matthews magnetic anomalies a transverse ridge along the eastern inactive part of the northern transform may be an intrusive feature considerable thicknesses of sediment appear to have been deposited in the northern transform valley from norwegian sea overflow water passing through the fracture zone but transverse ridges have prevented the sediment reaching the southern valley