deformation at an archean subprovince boundary northern minnesota structural analysis in the northern margin of the quetico subprovince part of the archean superior province of the canadian shield in minnesota reveals that the main deformation involved polyphase folding f1 recumbent and nappe like and f2 upright east west trending and tight to isoclinal during regional ductile transpression and amphibolite facies metamorphism a younger deformation developed during the latter stages of regional transpression resulted in the generation of localized ultraphyllonites along the steeply dipping rainy lake seine river fault rlsrf the major fault separating the quetico subprovince from the rainy lake wrench zone a wedge shaped block between the quetico and wabigoon subprovinces the transpression involved north south shortening and east west dextral shear the presence of shear zones in amphibolite facies wall rock south of the fault and in lower grade ultraphyllonites within the rlsrf suggests that localization of shear occurred by work and or reaction softe

structural evolution of archean rocks in the western wawa subprovince minnesota refolding of precleavage nappes during d2 transpression recent mapping in the western wawa subprovince the vermilion district and its westward extensions in minnesota has identified a major northeast trending stratotectonic break informally called the leech lake structural disconformity llsd that separates two contrasting terranes north of the llsd are elongate east northeast trending fault bounded panels of volcanic rocks which are mostly north topping and homoclinal south of the llsd large scale northwest trending folds involve basaltic sequences that are stratigraphically overlain by thick sections of dacitic volcaniclastic and turbiditic rocks however the most prominent outcrop scale deformational features are northeast trending vertical folds and associated axial planar cleavage related to transpression in d2 d1 minor folds and cleavage are rare new field data indicate that the large folds in a predominantly sedimentary part of the southern terrane are early formed d0 d1 and nappe like the precise form of the early folds is largely ob

neoproterozoic iron formation an evaluation of its temporal environmental and tectonic significance article neoproterozoic iron formation nif provides evidence for the widespread return of anoxic and ferruginous ba sins during a time period associated with major changes in climate tectonics and biogeochemistry of the oceans here we summarize the stratigraphic contextof neoproterozoic iron formation and its geographic and temporal dis tribution it is evident that most nif is associated with the earlier cryogenian sturtian glacial epoch although it is possible that some nif may be ediacaran there is no incontrovertible evidence to support this age assignment the paleogeographic distribution of nif is consistent with anoxic and ferruginous conditions occurring in basins within rodinia or in rift basins developed on its margins consequently nif does not require whole ocean anoxia simple calculations using modern day iron fluxes suggest that only models that invoke hydrothermal and or detrital sourcesofiron are capable ofsupplying sufficient ironto account forthe mass of the largernif occurrences this con clusion is reinforced by the available geochemical data that imply nif record is a mixture of hydrothermal and de trital components a common thread that appears to link most if not all nif is an association with mafi cv olcanics

effect of lithospheric thickness on the formation of high and low angle normal faults it is accepted that extension of a homogeneous brittle layer should produce high angle normal faults the rotation of the upper parts of such a fault to a low dip requires fault offset that is greater than the local lithospheric thickness new model calculations indicate that a fault cutting lithosphere thicker than 10 20 km can build up only a few kilometres horizontal throw before a new fault replaces it such a small offset leaves the fault little rotated from its initial high dip angle for thinner lithosphere the fault offset may be much larger leading to rotation of the upper inactive parts of a high angle fault to a low dip angle in continents this can happen where thin upper crustal lithosphere is decoupled from mantle lithosphere by weak lower crust the calculations suggest that normal faults begin with high dip angles and only in areas of very thin lithosphere can they be offset enough to produce low angle fault structures this prediction is consistent with the occurrence of low angle faults in areas with higher than normal heat flow

discovery of a magma chamber and faults beneath a mid atlantic ridge hydrothermal field crust at slow spreading ridges is formed by a combination of magmatic and tectonic processes with magmatic accretion possibly involving short lived crustal magma chambers the reflections of seismic waves from crustal magma chambers have been observed beneath intermediate and fast spreading centres but it has been difficult to image such magma chambers beneath slow spreading centres owing to rough seafloor topography and associated seafloor scattering in the absence of any images of magma chambers or of subsurface near axis faults it has been difficult to characterize the interplay of magmatic and tectonic processes in crustal accretion and hydrothermal circulation at slow spreading ridges here we report the presence of a crustal magma chamber beneath the slow spreading lucky strike segment of the mid atlantic ridge the reflection from the top of the magma chamber centred beneath the lucky strike volcano and hydrothermal field is approximately 3 km beneath the sea floor 3 4 km wide and extends up to 7 km along axis we suggest that this magma chamber provides the heat for the active hydrothermal vent field above it we also observe axial valley bounding faults that seem to penetrate down to the magma chamber depth as well as a set of inward dipping faults cutting through the volcanic edifice suggesting continuous interactions between tectonic and magmatic processes

role of melt supply in oceanic detachment faulting and formation of megamullions normal faults are ubiquitous on mid ocean ridges and are expected to develop increasing offset with reduced spreading rate as the proportion of tectonic extension increases numerous long lived detachment faults that form megamullions with large scale corrugations have been identifi ed on magma poor mid ocean ridges but recent studies suggest counterintuitively that they may be associated with elevated magmatism we present numerical models and geological data to show that these detachments occur when 30 50 of total extension is accommodated by magmatic accretion and that there is signifi cant magmatic accretion in the fault footwalls under these low melt conditions magmatism may focus unevenly along the spreading axis to create an irregular brittle plastic transition where detachments root thus explaining the origin of the enigmatic corrugations morphological and compositional characteristics of the oceanic lithosphere suggested by this study provide important new constraints to assess the distribution of magmatic versus tectonic extension along mid ocean ridges

rhyolite geochemical signatures and association with volcanogenic massive sulfide deposits examples from the abitibi belt canada the relationship between rhyolite geochemistry and volcanogenic massive sulfide vms mineralization has been proposed as an exploration tool to discriminate prospective felsic volcanic centers the most widely used classification discriminates between four types of rhyolite fi fii fiiia and fiiib the fi rhyolites are calc alkaline with strongly fractionated ree patterns and strongly negative ta and nb anomalies they are usually considered barren unless associated with fii or fiii felsic volcanic rocks the fii rhyolites are calc alkaline to transitional with moderately fractionated ree patterns and moderate ta and nb anomalies they range from barren to having a high potential to host vms mineralization the fiiia and fiiib rhyolites are tholeiitic and show weakly fractionated ree patterns and weak to absent nb and ta anomalies they have the highest potential to host vms mineralization the fiiib rhyolites are high temperature rhyolites with flat ree patterns and no ta or nb anomalies the abitibi greenstone belt especially in quebec is well known for its abundant and diverse vms deposits representative samples of vms associated rhyolites within and outside of mining districts including the classic noranda vms district were analyzed for major and trace elements to validate their proposed favorability for hosting vms deposits results indicate that all of the rhyolite types are prospective but mineralization may differ from the classic noranda type vms deposit the fi type rhyolites appear to be particularly associated with gold rich vms deposits such as the world class laronde deposit and are more prospective for cu au replacement and vein type deposits the fii type rhyolites account for about 70 percent of rhyolites in the abitibi belt although considered less prospective some districts dominated by fii rhyolites such as val d or and selbaie have collectively produced in excess of 100 million metric tons mt of ore deposits in these districts mainly consist of sulfide veins and disseminated ore with low cu and zn grades and are associated with abundant and highly vesicular volcaniclastic rocks that display a compositional continuum from andesite to rhyolite other weakly mineralized fii districts e g hunter mine gemini turgeon are characterized instead by bimodal flow dome sequences the fiiia type rhyolites occur mainly in the noranda district and form flow dome complexes in bimodal sequences with associated noranda type vms mineralization in small felsic centers joutel normetal chibougamau quevillon that show a volcanic evolution from fiiia to fii to fi affinities vms deposits are directly associated with fiiia rhyolites thus demonstrating the usefulness of rhyolite geochemistry for exploration in these areas the fiiib rhyolites are rare in the abitibi belt with most occurring in the matagami district where they are associated with zn cu vms deposits based on this analysis we suggest that a combination of rhyolite geochemistry volcanic facies and style of the mineralization may be more meaningfully applied in exploration than rhyolite type alone particularly in the case of fi and fii rhyolites

correlation of early recumbent and younger upright folding across the boundary between an archean gneiss belt and greenstone terrane northeastern minnesota correlation of an f 1 recumbent fold and an upright f 2 antiform across the boundary between an archean migmatite terrane and the adjacent volcanic plutonic belt indicates a common tectonic evolution for both terranes since the earliest stages of folding the local boundary between amphibolite facies schists and migmatites of the southern vermilion granitic complex vgc and low grade metagraywackes of the adjacent vermilion district is marked by nearly vertical dip slip faulting along the east trending axial trace of a regional f 2 antiform the migmatites in the southern vgc have been folded into s symmetry folds that mark the northern limb of the major antiform these folds are correlated with f 2 folds of z symmetry on the southern limb of the fold in adjacent downfaulted rocks of the vermilion district structural facing near the boundary between the two subprovinces is downward on both limbs of the major f 2 structure which is interpreted to be part of the lower overturned limb of a large scale f 1 recumbent fold a change to upright facing strata farther south indicates a crossing onto the upper limb of the structure the f 1 folding is tentatively attributed to gravitational spreading off the southern margin of the rising lac la croix granite of the east central vgc

footwall rotation in an oceanic core complex quantified using reoriented integrated ocean drilling program core samples oceanic core complexes expose lower crustal and upper mantle rocks on the seafloor by tectonic unroofing in the footwalls of large slip detachment faults the common occurrence of these structures in slow and ultra slow spread oceanic crust suggests that they accommodate a significant component of plate divergence however the subsurface geometry of detachment faults in oceanic core complexes remains unclear competing models involve either a displacement on planar low angle faults with little tectonic rotation or b progressive shallowing by rotation of initially steeply dipping faults as a result of flexural unloading the rolling hinge model we address this debate using palaeomagnetic remanences as markers for tectonic rotation within a unique 1 4 km long footwall section of gabbroic rocks recovered by integrated ocean drilling program iodp sampling at atlantis massif oceanic core complex on the mid atlantic ridge mar these rocks contain a complex record of multipolarity magnetizations that are unrelated to alteration and igneous stratigraphy in the sampled section and are inferred to result from progressive cooling of the footwall section over geomagnetic polarity chrons c1r 2r c1r 1n jaramillo and c1r 1r for the first time we have independently reoriented drill core samples of lower crustal gabbros that were initially azimuthally unconstrained to a true geographic reference frame by correlating structures in individual core pieces with those identified from oriented imagery of the borehole wall this allows reorientation of the palaeomagnetic data placing far more rigorous constraints on the tectonic history than those possible using only palaeomagnetic inclination data analysis of the reoriented high temperature reversed component of magnetization indicates a 46 6 anticlockwise rotation of the footwall around a mar parallel horizontal axis trending 011 6 reoriented lower temperature components of normal and reversed polarity suggest that much of this rotation occurred after the end of the jaramillo chron 0 99 ma the data provide unequivocal confirmation of the key prediction of flexural rolling hinge models for oceanic core complexes whereby oceanic detachment faults initiate at higher dips and rotate to their present day low angle geometries as displacement increases

the rate of oceanic detachment faulting at atlantis bank sw indian ridge abstract the rates of slip on oceanic detachment faults and how those rates compare to sea floor spreading rates constitute fundamental data required to constrain how oceanic core complexes form and their role during crustal accretion we combine sea surface magnetic data with the magnetic polarity of shallow core samples and pb u shrimp ages of igneous zircon to determine the time averaged half spreading rate during oceanic detachment faulting at atlantis bank 100 km south of the ultraslow spreading southwest indian ridge swir the pb u zircon ages correlate well with the magnetic ages and so highlight that magmatic accretion and faulting were coeval for over 2 myr creating and exposing a 1 5 km thick layer of gabbro for 35 km parallel to spreading we use bivariate linear regression of distance age data and forward modeling of magnetic anomaly data to calculate a half spreading rate during detachment faulting of 14 1 1 8 1 5 km myr 95 confidence limits when integrated with regional constraints on spreading history we note that detachment faulting coincided with a short lived regional increase in the full spreading rate along the swir and for the ridge segment containing atlantis bank spreading was highly asymmetric with 80 of plate motion accommodated by detachment faulting consequently the detachment fault effectively formed the plate boundary at the surface in this spreading segment highly asymmetric spreading was confined to the spreading segment containing atlantis bank and to the duration of detachment faulting so the ridge segment containing atlantis bank migrated northward relative to its symmetrically spreading eastern neighbour such that the intervening non transform discontinuity shortened we suggest that the highly asymmetric spreading may be a characteristic feature of oceanic detachment faulting an inference supported by more poorly constrained half spreading rates determined at several other oceanic core complexes

new insights into archean crustal development from geochronology in the rainy lake area superior province canada precise isotopic analyses of zircon titanite and rutile from units in the rainy lake area of northwest ontario are combined with structural investigations to develop new stratigraphic interpretations in which lithotectonic sequences were juxtaposed by thrusting prior to steepening by wrenching ages for the oldest volcanic and plutonic rocks in the area range from 2725 pm 2 ma to 2728 4 3 ma the age of metagray wackes of the coutchiching group is bracketed between 2704 pm 3 ma and 2692 pm 2 ma by detrital zircon and a crosscutting intrusion alluvial conglomerate of the seine group is bracketed between 2696 5 3 ma the age of a clast and 2686 2 1 ma the age of the undeformed ottertail lake pluton the coutchiching group is younger than structurally overlying metavolcanic rocks and is therefore allochthon ous these data suggest that sedimentation and deformation were controlled by collisional plate interactions and subsequent transpression at the subprovince boundary combined with dat

causes and consequences of variations in faulting style at the mid atlantic ridge both volcanism and faulting contribute to the rugged topography that is created at the mid atlantic ridge mar and preserved off axis in atlantic abyssal hill terrain distinguishing volcanic from fault generated topography is essential to understanding the variations in these processes and how these variations are affected by the three dimensional pattern of mantle upwelling ridge segmentation and offsets here we describe a new quantitative method for identifying fault generated topography in swath bathymetry data by measuring topographic curvature the curvature method can distinguish large normal faults from volcanic features whereas slope methods cannot because both faults and volcanic constructs can produce steep slopes the combination of curvature and slope information allows inward and outward facing fault faces to be mapped we apply the method to sea beam data collected along the mar between 28 and 29 30 n the fault styles mapped in this way are strongly correlated with their location within the ridge segmentation framework long linear small throw faults occur toward segment centers while shorter larger throw curved faults occur toward ends these variations reflect those of active faults within the axial valley we investigate two different physical mechanisms that could affect fault interactions and thus underlie variations in abyssal hill topography at the mar in the first model only one fault is active at a time on each side of the rift valley each fault grows while migrating away from the volcanic center due to dike injection extension across the fault causes a flexural rotation of nearby inactive faults the amount of stress necessary to displace the fault increases as the fault grows when reaching a critical size the fault stops growing as fault activity jumps inward as a new fault starts its growth near the rift valley this model yields a realistic terracelike morphology from the rift valley floor into the rift mountains the relief is caused by the net rotation accumulated in the lithosphere from the active faults e g 10 reached 20 km from the active fault fault spacing is controlled by lithospheric thickness fault angle and the ratio of amagmatic to magmatic extension we hypothesize that this mechanism may be dominant toward ridge segment offsets an alternative model considers multiple active faults each fault relieves stresses as it grows and inhibits the growth of nearby faults causing a characteristic fault spacing such fault interactions would occur in a region of necking instability involving deformation over an extended area this mode of extension would drive a feedback mechanism that would act to regulate the size of nearby faults we hypothesize that this mechanism may be active in the relatively weak regions of strong mantle upwelling near segment midpoints causing the homogeneous abyssal hill fabric in these regions

oxygen isotope zonation and alteration in the northern noranda district quebec evidence for hydrothermal fluid flow volcanic rocks of the new vauze norbec area central noranda volcanic complex consist of andesites and rhyolites that have been chemically and isotopically altered by synvolcanic semiconformable alteration processes including spilitization chloritization silicification and epidote quartz alteration andesites have whole rock delta 18 o values of 1 9 to 9 7 per mil those with megascopic mineralogical evidence of hydrothermal alteration have ones of 5 1 to 9 7 per mil rhyolites also show a broad range in delta 18 o values 4 4 to 13 7 per mil all of the volcanic rocks are either enriched or depleted in 18 o with respect to fresh rhyolite 8ppm or fresh andesite 6ppm a marked upward stratigraphic increase in delta 18 o values and the strata bound occurrence and regional zoning of alteration minerals provide evidence of district scale hydrothermal alteration these alteration patterns formed under the conditions of an increasing thermal gradient and a downward decrease in water rock ratio controlled by the cooling of the flavrian lake synvolcanic intrusion

modes of faulting at mid ocean ridges abyssal hill bounding faults that pervade the oceanic crust are the most common tectonic feature on the surface of the earth the recognition that these faults form at plate spreading centres came with the plate tectonic revolution recent observations reveal a large range of fault sizes and orientations numerical models of plate separation dyke intrusion and faulting require at least two distinct mechanisms of fault formation at ridges to explain these observations plate unbending with distance from the top of an axial high reproduces the observed dip directions and offsets of faults formed at fast spreading centres conversely plate stretching with differing amounts of constant rate magmatic dyke intrusion can explain the great variety of fault offset seen at slow spreading ridges very large offset normal faults only form when about half the plate separation at a ridge is accommodated by dyke intrusion

petrology and oxygen isotope geochemistry of a fossil seawater hydrothermal system within the solea graben northern troodos ophiolite cyprus hydrothermal mineral zonations and o isotope patterns of the northern troodos complex do not parallel the ophiolite pseudostratigraphy but reflect the convective geometry of an upper cretaceous seawater hydrothermal system large areas of the sheeted intrusive complex sic including the subaxial region of the solea graben are composed of 18o rich subgreenschist mineral assemblages and may represent regions of diffuse seawater recharge other areas of the sic are recrystallized to distinctive epidosite rocks granular high variance assemblages of epidote quartz chlorite that are depleted in 18o al2o3 na2o k2o zr cu and zn and are enriched in cao and sr compared with other mafic volcanic and dike rocks of the solea graben epidosite alteration occurred at temperatures of 310 370 c and involved fluids with 18o values and salinities similar to those of upper cretaceous seawater the epidosite zones are discordant with earlier mineral o isotope zonations and with the axis of spreading in the solea graben suggesting a postspreading off axis origin the seawater hydrothermal system responsible for solea graben massive sulfide deposits was probably driven by hypabyssal intrusions not exposed emplaced in a terminal failed spreading episode the geometries of o isotope surfaces within the solea graben imply that the epidosites formed in fossil upflow and deep recharge conduits depletions in base metals show that epidosite alteration liberated cu and zn to mineralizing fluids within the fossil upflow zone

the structure of oceanic core complexes controlled by the depth distribution of magma emplacement extension at mid ocean ridges can be accommodated by detachment faults forming oceanic core complexes that develop under low rates of magma intrusion modelling reveals that oceanic core complexes can also form under high rates of magma intrusion if the magma is injected into the lower ductile layer of the crust

seafloor expression of oceanic detachment faulting reflects gradients in mid ocean ridge magma supply abstract oceanic detachment faulting is a major mode of seafloor accretion at slow and ultraslow spreading mid ocean ridges and is associated with dramatic changes in seafloor morphology detachments form expansive dome structures with corrugated surfaces known as oceanic core complexes occs and often transition to multiple regularly spaced normal faults that form abyssal hills parallel to the spreading axis previous studies have attributed these changes to along axis gradients in lithospheric strength or magma supply however despite the recognition that magma supply can influence fault style and seafloor morphology the mechanics controlling the transition from oceanic detachment faults to abyssal hill faults and the relationship to along axis variations in magma supply remain poorly understood this study investigates this issue using two complementary modeling approaches the first consists of semi analytical two dimensional 2 d cross axis models designed to address the fundamental mechanical controls on the longevity of normal faults these 2 d model sections are juxtaposed in the along axis direction to examine the response of the plan view pattern of faults to along axis variations in magmatic accretion in the absence of along axis mechanical coupling the second approach uses three dimensional 3 d time dependent numerical models that simulate faulting and magma intrusion in a visco elasto plastic continuum the primary variable studied through both approaches is the along axis gradient in the fraction m of seafloor spreading that is accommodated by magmatism the 2 d and 3 d results predict different abyssal hill spacing and orientation however the plan view geometry of self emerging detachment faults predicted by the 3 d numerical models are well explained by the juxtaposed 2 d models this indicates a first order control by cross axis effects of changing values of m these models are also shown to explain the along axis extent and plan view curvature of the well developed 13 20 n and mt dent occs mid atlantic ridge and cayman rise in terms of quantifiable along axis gradients in magma emplacement rates

numerical heat and fluid flow modeling of the panorama volcanic hosted massive sulfide district western australia exceptional exposure of the archean pilbara block in western australia reveals a cross section through an archean massive sulfide hosting volcanic succession with underlying subvolcanic intrusion in the panorama district a numerical model based on available detailed geologic information has been constructed to simulate heat and fluid flow in the panorama district the modeling provides insight into the evolution of the hydrothermal system and evaluates key geologic parameters and their influence on fluid flow hydrothermal circulation and the genesis of massive sulfide orebodies the model simulates important aspects of the panorama massive sulfide district such as temperature distribution relative alteration zonation and the size and distribution of orebodies predicted temperatures ranging from 150oc at the top of the volcanic pile to 400oc at the andesite diorite interface are comparable to temperature estimates based on previously published oxygen isotope mapping modeled fluid discharge temperatures are highest for the sulphur springs deposit 300o 400oc and lower for the kangaroo caves and other deposits 250o 350oc the most favorable conditions to reproduce the orebodies and their related alteration zonation occur at anisotropic rock permeabilities comparable to the upper oceanic crust 10 15 10 14 m2 and higher fault permeabilities 10 14 10 13 m2 with a specific fault arrangement similar to that mapped in the field the 4 6 million metric tons mt sulphur springs orebody is predicted to form in less than 5 000 yr assuming a hydrothermal fluid with seawater salinity 10 ppm base metal concentration and a low deposition efficiency 10 other deposits form above the faults under similar conditions a large range of base metal concentrations in the fluids can account for the known orebodies but high temperatures 250oc and high flow velocities 10 7 m s are necessary to produce the observed alteration patterns and distribution of ore deposits results indicate that the establishment of a significant hydrothermal system capable of forming economic massive sulfide deposits is favored in fresh volcanic rock packages that have not been affected by earlier compaction or alteration under these conditions economic massive sulfide orebodies 5 mt of 10 zn cu may form in a few thousand years although the overall lifespan of the hydrothermal system may be between 30 000 and 200 000 yr depending on the variations in rock and fault permeability with time

regional evolution of hydrothermal fluids in the noranda district quebec evidence from delta 18 o values from volcanogenic massive sulfide deposits the results of oxygen isotope analysis of hydrothermally altered volcanic rocks are used to define the thermal and isotopic characteristics of the ore forming fluids which produced several volcanogenic massive sulfide deposits in the noranda district these deposits corbet ansil norbec horne and mobrun formed during temporally distinct events that span much of the volcanic stratigraphy in the district the data show that the delta 18 o values of the altered rocks and the size economic tonnage of each deposit increase upward through the volcanic stratigraphy from low delta 18 o values at the corbet 2 2 to 4 8ppm and ansil 0 8 to 5 0ppm deposits to intermediate delta 18 o values at the amulet 3 6 6 7ppm and norbec 3 6 10 5ppm deposits to high delta 18 o values at the horne 4 2 11 6ppm and mobrun 6 0 13 8ppm deposits a corresponding increase is indicated for the delta 18 o values of the discharging hydrothermal fluids from 2 or 2 per mil at the corbet deposit to 3 0 or 1 5 per mil at the horne deposit alteration in most of the deposits is shown to have resulted from the flow of fluids having temperatures of 300 degrees or 50 degrees c with the exception of the pyrite rich mobrun deposit where slightly lower temperatures 200 degrees or 50 degrees c are indicated these observations are interpreted to indicate that the duration of hydrothermal discharge as indicated by deposit size at a given site increased upward through the volcanic stratigraphy since the fluid and thus the altered rock delta 18 o values normally evolve from low to high with time in a volcanogenic massive sulfide forming system e g cathles 1983 the duration of fluid discharge is a primary factor in controlling the isotopic signature retained at a given deposit in the noranda district the duration of fluid discharge depended strongly on the rate of volcanic accumulation with short lived systems e g corbet ansil forming during periods of rapid accumulation of extrusive volcanic rocks and longer lived systems forming during more quiescent periods e g norbec or at sites where the accumulation of extrusive material was limited by topography e g horne

detachment and steep normal faulting in atlantic oceanic crust west of africa improved images of the internal structure of early cretaceous north atlantic crust reveal both probable detachment faults and more steeply dipping normal faults the detachment faults occur as subhorizontal structures passing 1 5 km beneath fault blocks without offset several steeper block bounding faults appear to detach onto these structures however the detachments are bounded to the west ridgeward by presumably younger more steeply west dipping normal faults in one possible interpretation the detachment and the steep faults belong to the same rolling hinge extension system an intriguing alternative is that a phase of detachment faulting perhaps related to increased magmatic activity was succeeded by localized amagmatic extension along steeper and more deeply penetrating faults

assessment of the relationship between emplacement of the algoman plutons and regional deformation in the rainy lake region ontario the algoman suite of granites and granodiorites located within the rainy lake region of the superior province intruded the wabigoon quetico subprovince boundary about 2686 ma deformation occurred during the late archean collision between the wabigoon and wawa volcanic arcs the algoman plutons are the youngest rocks in the region their apparent lack of macroscopic deformation fabrics has led previous researchers to use their age as a constraint for the cessation of regional deformation however contrasting competence between the relatively rigid granites and surrounding volcanic and metasedimentary rocks suggests that obvious deformation fabrics may not be observed in the plutons even if they were present during deformation a field anisotropy of magnetic susceptibility ams gravity inversion model and microstructural study were undertaken on two neighboring algoman plutons to determine their emplacement history relative to the regional deformation there is a steep macroscopic foliation in the adjacent rocks and in parts of the plutons the magnetic foliation is very strong and roughly coincident with the field foliations the gravity inversion models indicate that the plutons are relatively shallow 1 km and 4 5 km at their deepest points and their shapes are consistent with emplacement during dextral transpression quartz microstructures such as undulose extinction and subgrain formation indicate that there was some solid state deformation accommodated by dislocation creep mechanisms the field ams gravity and microstructural evidence indicate that the plutons were most likely emplaced syntectonically thus the pluton age does not provide a conclusive constraint to the cessation of regional deformation

composite synvolcanic intrusions associated with precambrian vms related hydrothermal systems large subvolcanic intrusions are recognized within most precambrian vms camps of these 80 are quartz diorite tonalite trondhjemite composite intrusions the vms camps spatially associated with composite intrusions account for 90 of the aggregate sulfide tonnage of all the precambrian intrusion related vms camps these low alumina low k and high na composite intrusions contain early phases of quartz diorite and tonalite followed by more voluminous trondhjemite they have a high proportion of high silica 74 sio2 trondhjemite which is compositionally similar to the vms hosting rhyolites within the volcanic host rock successions the quartz diorite and possibly tonalite phases follow tholeiitic fractionation trends whereas the trondhjemites fall within the composition field for primitive crustal melts these transitional m i type primitive intrusive suites are associated with extensional regimes within oceanic arc environments subvolcanic composite intrusions related to the archean sturgeon lake and noranda and paleoproterozoic snow lake vms camps range in volume from 300 to 1 000 km3 three have a sill morphology with strike lengths between 15 and 22 km and an average thickness between 1 500 and 2 000 m the fourth has a gross stock like shape the vms deposits are principally restricted to the volcanic strata above the strike length of the intrusions as are areally extensive thin exhalite units the composite intrusions contain numerous internal phases which are commonly clustered within certain parts of the composite intrusion these clusters underlie eruptive centers surrounded by areas of hydrothermal alteration and which contain most of the vms deposits early quartz diorite and tonalite phases appear to have intruded in rapid succession evidence includes gradational contacts magma mixing and disequilibrium textures they appear to have been emplaced as sill dike swarms these early phases are present as pendants and xenoliths within later trondhjemite phases the trondhjemite phases contain numerous internal contacts indicating emplacement as composite sills common structural features of the composite intrusions include early xenolith phases abundant small comagmatic dikes fractures and veins and in places columnar jointing internal phases may differ greatly in texture from fine to coarse grained aphyric and granophyric through seriate to porphyritic mineralogical and isotopic evidence indicates that early phases of each composite intrusion are affected by pervasive to fracture controlled high temperature 350 450 c alteration reflecting seawater rock interaction trondhjemite phases contain hydrothermal magmatic alteration assemblages within miarolitic cavities hydrothermal breccias and veins this hydrothermal magmatic alteration may in part be inherited from previously altered wall rocks two of the four intrusions are host to cu mo rich intrusive breccias and porphyry type mineralization which formed as much as 14 ma after the main subvolcanic magmatic activity the recognition of these precambrian subvolcanic composite intrusions is important for greenfields vms exploration as they define the location of thermal corridors within extensional oceanic arc regimes which have the greatest potential for significant vms mineralization the vms mineralization may occur for 2 000 m above the intrusions in some cases vms mineralization has been truncated or enveloped by late trondhjemite phases of the composite intrusions evidence that much of the trondhjemitic magmatism postdates the principal vms activity is a critical factor when developing heat and fluid flow models for these subseafloor magmatic hydrothermal systems

trace element geochemistry of ore associated and barren felsic metavolcanic rocks in the superior province canada reply archaean felsic metavolcanic rocks in the superior province of the canadian shield may be divided into three major groups on the basis of trace element abundances and ratios 1 fi felsic metavolcanic rocks are dacites and rhyodacites characterized by steep ree patterns with weakly negative to moderately positive eu anomalies high zr y low abundances of high field strength elements e g hree y zr hf and high abundances of sr examples occur in the bowman subgroup and skead group in the abitibi belt in the kakagi lake lake of the woods shoal lake and sturgeon lake areas of the wabigoon belt and in the confederation lake area of the uchi belt none of these horizons as known hosts base metal sulphide deposits 2 fii felsic metavolcanic rocks are rhyodacites and rhyolites characterized by gently sloping ree patterns with variable eu anomalies moderate zr y and intermediate abundances of hfs elements and sr examples occur in the misema subgroup of the abitibi belt in the wabigoon lake a

self consistent rolling hinge model for the evolution of large offset low angle normal faults the nature of the physical processes responsible for the formation of continental and oceanic metamorphic core complexes is widely debated the controversy focuses primarily on whether the low angle normal faults observed in these environments formed and slipped at low angles or were rotated from an original high angle orientation after large offsets we describe a self consistent numerical model for the extension of a brittle layer that can spontaneously produce normal fault structures in our formulation a fault or faults form because strength is locally reduced with increasing strain if the reduction in fault strength is 10 of the total strength of the layer then faults lock after an offset smaller than the layer thickness and new faults form larger strength reduction leads to single faults that continue to slip no matter how large the fault offset if the strength reduction occurs by the loss of cohesion then we see the unlimited offset faults for layers 11 22 km thick for reasonable values of cohesion the key result of this study is that structures very similar to those observed in both oceanic and continental core complexes are produced by rotation of the inactive part of the model fault after very large offset

effects of serpentinization on the lithospheric strength and the style of normal faulting at slow spreading ridges rock deformation experiments indicate that serpentinization can strongly influence the strength and tectonics of the oceanic lithosphere strength versus depth profiles calculated for conditions appropriate for slow spreading ridges indicate that the presence of serpentinite can reduce the integrated strength of the lithosphere by up to 30 results from flexural fault models indicate that if serpentinization is isolated to fault zones strain localization should be enhanced providing an explanation for the variations in the style of normal faulting along slow spreading ridge segments at segment centers where serpentinites are scarce deformation is accommodated on closely spaced faults with small throws at the segment ends where serpentinites are most abundant faults are widely spaced and have large throws q 1997 elsevier science b v

the 3 d geometry of detachment faulting at mid ocean ridges seismic images of cretaceous slow spreading crust from the eastern central atlantic provide new constraints on the process of seafloor spreading and the importance of detachment faulting the seismic depth sections image detachment faults that appear to have exhumed footwall massifs of similar geometry to massifs of plutonic and mantle rocks mapped at the present mid atlantic ridge the detachments are consistent with the structure and microearthquakes of the mid atlantic ridge at 26 n with the footwall rotation inferred from paleomagnetic data and with numerical modeling of oceanic detachments other seismically imaged detachments have similar dimensions and geometry but are covered by a layer of small fault blocks the detachment types differ in whether or not the fault locks up in the subsurface probably controlled by the fault strength the elastic thickness and whether the exhumed footwall is partly covered by basalts toward the segment middle decreasing mantle serpentinization decreasing elastic thickness and thicker median valley basalts all increase the likelihood that the fault locks up and a new fault propagates upwards from the still active root zone transferring a slice of the hanging wall to the footwall to be rafted with the footwall out of the median valley as a result an oceanic detachment fault exhuming the footwall at a segment end to form an oceanic core complex may disappear laterally beneath rafted blocks detachment faulting may be more widespread at slow spreading ridges than interpreted from seafloor mapping

thermomechanics of mid ocean ridge segmentation the mechanics responsible for the initiation of the orthogonal pattern characterizing mid ocean ridges and transform faults are studied using numerical models the driving forces are thermal stresses arising from the cooling of young oceanic crust and extensional kinematic boundary conditions thermal stress can exert ridge parallel tension comparable in magnitude to spreading induced tension when selectively released by ridges and ridge parallel structure two modes of ridge segment growth have been identified in plan view an overlapping mode where ridge segments overlap and bend toward each other and a connecting mode where two ridge segments are connected by a transform like fault as the ratio of thermal stress to spreading induced stress increases the patterns of localized plastic strain change from the overlapping to connecting mode the orthogonal pattern marks the transition from one mode to the other besides the amount of stress from each driving force the rate of stress accumulation is crucial in determining the emergent pattern this rate dependence is characterized by the spreading rate normalized by a reference cooling rate pe when pe is paired with the ratio of thermal stress to the reference spreading induced stress they unambiguously define stability fields of the two modes the obliquely connecting the orthogonally connecting and the overlapping mode are similar to ridge transform fault intersections observed in ultra slow slow to intermediate and fast spreading centers respectively the patterns are also sensitive to the strain weakening rate fracture zones were created in part as a response to thermal stress

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

constraints on deformation conditions and the origin of oceanic detachments the mid atlantic ridge core complex at 15 45 n 1 deformed rocks sampled from a corrugated detachment fault surface near the mid atlantic ridge 15 45 n constrain the conditions of deformation and strain localization samples recovered in situ record deformation restricted to the cold shallow lithosphere greenschist facies with no evidence for significant high temperature deformation either at the fault zone or in the footwall near it high temperature deformation 720 750 c is observed only at two sites and cannot be directly linked to the detachment detachment faulting was coeval with dyke intrusions that cross cut it as demonstrated by the presence of undeformed and highly deformed diabase found in shear zones and by the presence of chill margins in diabase against fault rock basalts are very scarce and restricted to clasts in breccias with no evidence of pillows or extrusive structures gabbros crop out along mass wasted and fault scarps structurally below the detachment footwall rocks show little or no deformation due to strain localization along a narrow shear zone 200 m with fluid flow as required to form talc and amphibole schists after an ultramafic protolith we speculate that the alteration front in a heterogeneous lithosphere may be a rheological boundary that may localize deformation during long periods of time our observations and other geological evidence elsewhere suggest that this detachment model limited to the cold shallow lithosphere is applicable to other corrugated surfaces along slow and intermediate spreading ridges these observations preclude detachment models rooting in melt rich zones i e atlantis bank southwest indian ridge or recording high temperature deformation we infer that oceanic detachment faults 1 localize strain at t 500 300 c 2 persist during active magmatism and 3 root at shallow rheological boundaries such as a melt rich zone or magma chamber hot detachments or an alteration front cold detachments

internal structure of a spreading segment of mesozoic oceanic crust we define the crustal architecture and internal structure of a 70 km long segment of mesozoic oceanic crust combined reflection and wide angle refraction profiles show the relations between reflectivity and seismic velocity within the crust the crust thins by 4 km from the segment center to the bounding fracture zones a 2 km thick lens at the base of the crust in the segment center exhibits enhanced reflectivity in the otherwise mainly opaque lower crust which coincides with abnormally high seismic velocities of 7 5 km s we attribute these characteristics to the presence of a melt injection region the reflection profiles in the 4 km spaced grid show that there are two distinct approximately orthogonal sets of dipping reflectors within the crust ridge normal reflectors dip at typically 25 in both directions the majority dip west toward the spreading center and their common association with basement steps suggests that they represent normal faults they terminate above or near the base of the crust ridge parallel reflectors are shallower dipping averaging 15 and planar many are restricted to the middle crust and we speculate that they represent magmatic rather than tectonic features a prominent dipping reflector beneath a fracture zone cuts at 20 into the upper mantle similar reflectors are found elsewhere beneath fracture zones in the north atlantic and may be caused by thermal stresses generated as the lithosphere cooled and moved away from the spreading axis

factors controlling normal fault offset in an ideal brittle layer we study the physical processes controlling the development and evolution of normal faults by analyzing numerical experiments of extension of an ideal two dimensional elastic plastic brittle layer floating on an inviscid fluid the yield stress of the layer is the sum of the layer cohesion and its frictional stress faults are initiated by a small plastic flaw in the layer we get finite fault offset when we make fault cohesion decrease with strain even in this highly idealized system we vary six physical parameters the initial cohesion of the layer the thickness of the layer the rate of cohesion reduction with plastic strain the friction coefficient the flaw size and the fault width we obtain two main types of faulting behavior 1 multiple major faults with small offset and 2 single major fault that can develop very large offset we show that only two parameters control these different types of faulting patterns 1 the brittle layer thickness for a given cohesion and 2 the rate of cohesion reduction with strain for a large brittle layer thickness 22 km with 44 mpa of cohesion extension always leads to multiple faults distributed over the width of the layer for a smaller brittle layer thickness the fault pattern is dependent on the rate of fault weakening a very slow rate of weakening leads to a very large offset fault and a fast rate of weakening leads to an asymmetric graben and eventually to a very large offset fault when the offset is very large the model produces major features of the pattern of topography and faulting seen in some metamorphic core complexes

3 d p wave velocity structure of oceanic core complexes at 13 n on the mid atlantic ridge the mid atlantic ridge at 13 n is regarded as a type locality for oceanic core complexes occs as it contains within 70 km along the spreading axis four that are at different stages of their life cycle the wealth of existing seabed observations and sampling makes this an ideal target to resolve contradictions between the existing models of occ development here we describe the results of p wave seismic tomographic modelling within a 60 60 km footprint containing several occs the ridge axis and both flanks which determines occ crustal structure detachment geometry and occ interconnectivity along axis a grid of wide angle seismic refraction data was acquired along a series of 17 transects within which a network of 46 ocean bottom seismographs was deployed approximately 130 000 first arrival travel times together with sparse moho reflections have been modelled constraining the crust and uppermost mantle to a depth of 10 km below sea level depth slices through this 3 d model reveal several independent structures each with a higher p wave velocity vp than its surrounds at the seafloor these features correspond to the occs adjacent to the axial valley walls at 13 20 n and 13 30 n and off axis at 13 25 n these high vp features display dipping trends into the deeper crust consistent with the surface expression of each occ s detachment implying that rocks of the mid to lower crust and uppermost mantle within the footwall are juxtaposed against lower vp material in the hanging wall the neovolcanic zone of the ridge axis has systematically lower vp than the surrounding crust at all depths and is wider between occs on average throughout the 13 n region the crust is 6 km thick however beneath a deep lava floored basin between axial occs the crust is thinner and is more characteristically oceanic in layering and velocity depth structure thicker crust at the ridge axis suggests a more magmatic phase of current crustal formation while modelling of the sparse moho reflections suggests the crust mantle boundary is a transition zone throughout most of the 13 n segment our results support a model in which occs are bounded by independent detachment faults whose dip increases with depth and is variable with azimuth around each occ suggesting a geometry and mechanism of faulting that is more complicated than previously thought the steepness of the northern flank of the 13 20 n detachment suggests that it represents a transfer zone between different faulting regimes to the south and north we propose that individual detachments may not be linked along axis and that occs act as transfer zones linking areas of normal spreading and detachment faulting along ridge variation in magma supply influences the nature of this detachment faulting consequently not only does magma supply control how detachments rotate and migrate off axis before finally becoming inactive but also how when and where new occs are created

oxygen isotope alteration in the noranda mining district abitibi greenstone belt quebec a regional whole rock oxygen isotope survey carried out in the noranda district shows how oxygen isotope alteration can be used to guide exploration for massive sulfide deposits the survey is based on 588 outcrop whole rock oxygen isotope determinations from 478 sites in a approximately 35 by 50 km area between the destor porcupine and cadillac larder lake breaks from east of the horne mine to west of the magusi river mine the mean isotopic composition of all samples is 7 4 or 2 per mil smow nested analysis of variance of 478 isotopic determinations from 371 of these sites shows that 74 percent of the total variance in whole rock delta 18 o r r whole rock is associated with outcrop location 21 percent with outcrop sampling and 5 percent with sample analysis an analysis of 259 determinations indicates that rock type is statistically no more important than outcrop sampling the statistical analysis indicates that isotopic variations greater than or 1 per mil can be used to map the hydrothermal circulation responsible for massive sulfide mineralization specific traverses visually confirm the lack of 18 o dependence on rock type and the utility of a 2 per mil interpretive band the large scale pattern of isotopic alteration in the noranda district is dominated by a nearly continuous 2 to 3 km wide annulus at the margins of the flavrian pluton of delta 18 o r 18 o r 18 o r depletion is surrounded up section by a approximately 10 km wide band of patchy but intense 18 o enrichment 8 5 14ppm the combined pattern of concentric light and heavy alteration has a diameter of approximately 30 km the clericy intrusion shows a similar pattern of isotope alteration on a smaller 10 km diam scale the lake dufault pluton seems to have been isotopically altered by continuing convection driven by the flavrian hydrothermal system modeling suggests that the 18 o depletion is produced by water circulating toward and along the edges of an intrusion and that the surrounding heavy alteration is produced either by seawater recharge or limited hydrothermal discharge the large scale and coherency of the alteration pattern centered on the flavrian pluton is remarkable and indicates that this pluton drove a major long lived approximately 10 m y hydrothermal system in which convection penetrated to an 8 kin depth the intensity and coherency of the 18 o depletion correlates with the tonnage of associated massive sulfide mineralization oxygen isotope depletion may in general mark the margins of plutons that have driven sufficient hydrothermal circulation to produce economic massive sulfide deposits up section it may be possible to screen plutons for up section massive sulfide exploration potential by collecting and isotopically analyzing approximately 50 samples from five traverses across a pluton9s margins additional samples may provide details light fingers that guide exploration to particular deposits

seismic evidence for large scale compositional heterogeneity of oceanic core complexes long lived detachment faults at mid ocean ridges exhume deep seated rocks to form oceanic core complexes occs using large offset 6 km multichannel seismic data we have derived two dimensional seismic tomography models for three of the best developed occs on the mid atlantic ridge our results show that large lateral variations in p wave velocity occur within the upper 0 5 1 7 km of the lithosphere we observe good correlations between velocity structure and lithology as documented by in situ geological samples and seafloor morphology and we use these correlations to show that gabbros are heterogeneously distributed as large tens to 100 km2 bodies within serpentinized peridotites neither the gabbros nor the serpentinites show any systematic distribution with respect to along isochron position within the enclosing spreading segment indicating that melt extraction from the mantle is not necessarily focused at segment centers as has been commonly inferred in the spreading direction gabbros are consistently present toward the terminations of the detachment faults this suggests enhanced magmatism during the late stage of occ formation due either to natural variability in the magmatic cycle or to decompression melting during footwall exhumation heat introduced into the rift valley by flow and crystallization of this melt could weaken the axial lithosphere and result in formation of new faults and it therefore may explain eventual abandonment of detachments that form occs detailed seismic studies of the kind described here when constrained by seafloor morphology and geological samples can distinguish between major lithological units such as volcanics gabbros and serpentinized peridotites at lateral scales of a few kilometers thus such studies have tremendous potential to elucidate the internal structure of the shallow lithosphere and to help us understand the tectonic and magmatic processes by which they were emplaced

magmatic and tectonic extension at mid ocean ridges 1 controls on fault characteristics we use 2 d numerical models to explore the thermal and mechanical effects of magma intrusion on fault initiation and growth at slow and intermediate spreading ridges magma intrusion is simulated by widening a vertical column of model elements located within the lithosphere at a rate equal to a fraction m of the total spreading rate i e m 1 for fully magmatic spreading heat is added in proportion to the rate of intrusion to simulate the thermal effects of magma crystallization and the injection of hot magma into the crust we examine a range of intrusion rates and axial thermal structures by varying m spreading rate and the efficiency of crustal cooling by conduction and hydrothermal circulation fault development proceeds in a sequential manner with deformation focused on a single active normal fault whose location alternates between the two sides of the ridge axis fault spacing and heave are primarily sensitive to m and secondarily sensitive to axial lithosphere thickness and the rate that the lithosphere thickens with distance from the axis contrary to what is often cited in the literature but consistent with prior results of mechanical modeling we find that thicker axial lithosphere tends to reduce fault spacing and heave in addition fault spacing and heave are predicted to increase with decreasing rates of off axis lithospheric thickening the combination of low m particularly when m approaches 0 5 as well as a reduced rate of off axis lithospheric thickening produces long lived large offset faults similar to oceanic core complexes such long lived faults produce a highly asymmetric axial thermal structure with thinner lithosphere on the side with the active fault this across axis variation in thermal structure may tend to stabilize the active fault for longer periods of time and could concentrate hydrothermal circulation in the footwall of oceanic core complexes

emplacement and deformation features of syntectonic leucocratic veins from rainy lake zone western superior province canada abstract networks of leucocratic veins are widespread in the rice bay redgut bay domain of the rainy lake zone at the wabigoon quetico subprovince boundary archean superior province canada vein geometries and their cross cutting relationships with host rocks are indicative of syntectonic emplacement within a regional late archean transpressional deformation event kenoran orogeny the relative folding and boudinage of the veins compared to their host rocks indicate that they were emplaced syntectonically with the d2 deformation semi quantitative strain and vorticity analyses confirm the syn d2 timing of the veins the observed structural features are interpreted as being formed during magma emplacement along dilatant fractures and subsequent solid state deformation under overall ductile deformation conditions the structures related to emplacement are better preserved in domains of low post emplacement strain and veins intruded into competent host rocks whereas the folds and boudins related to post emplacement deformation are well developed in veins intruded into relatively incompetent host rocks in domains of high strain the primary intrusive structures are modified or even locally obliterated due to increasing deformation it is envisaged that regional kinematics and the rheological state of host rocks are the main controls on the style of vein emplacement and deformation the variety of observed structures is considered the result of deformation and lithological heterogeneity rather than several distinct magmatic events

time transgressive archean unconformities underlying molasse basin fill successions of dissected oceanic arcs superior province canada abstract late archean diachronous unconformities underlying tectonically controlled coarse clastic successions are located throughout the superior province of canada examples studied in detail include hiatuses at the base of the timmins and kirkland basins in the abitibi greenstone belt and the crowduck and stormy basins in the wabigoon subprovince the timmins molasse basin fill succession overlies a high angle subaerial unconformity separating deepwater turbiditic deposits from fluvial conglomerates a subaerial erosional unconformity defines the contact between mafic flows and overlying felsic volcaniclastic deposits and fluvial conglomerates of the kirkland basin the crowduck and stormy successions are characterized by shallow water and subaerial unconformities between quartz feldspar porphyry stocks and clastic deposits the pronounced characteristics of the molasse basins which include 1 basal unconformities with older sedimentary volcanic or plutonic rocks 2 bounding fault systems 3 ba

on spreading modes and magma supply at slow and ultraslow mid ocean ridges abstract the ultraslow eastern southwest indian ridge swir offers an opportunity to study the effect of magma supply on an ultraslow mid ocean ridge starting from quasi melt free detachment dominated spreading and transitioning to volcanic spreading as one nears prominent axial volcanos detachments in the quasi melt free mode extend along axis 60 to 95 km and have a lifetime of 0 6 to 1 5 myrs they cut into their predecessor s footwall with an opposite polarity causing part of the footwall lithosphere to experience further deformation hydrothermal alteration sparse magmatism and possibly thermal rejuvenation in a hanging wall position the accretion of the oceanic lithosphere in this context therefore occurs in two distinct stages over the lifetime of two successive detachment faults we examine the transition from this nearly amagmatic detachment dominated mode to the more common volcanic mode of spreading showing that it occurs along axis over distances 30 km it involves a significant thinning of the axial lithosphere and a gradual decrease of the amount of tectonic displacement on faults as the magmatic contribution to the divergence of the two plates increases we develop a conceptual model of this transition in which magma plays a double role it fills the space between the diverging plates thus reducing the need for displacement along faults and it modifies the thermal state and the rheology of the plate boundary affecting its thickness and its tectonic response to plate divergence based on a comparison of the ultraslow eastern swir with the faster spreading mid atlantic ridge we show that the activation of the volcanic or of the detachment dominated modes of spreading is connected with the volume of magma supplied per increment of plate separation over a range of axial lithosphere thickness and therefore over a range of the m ratio defined by buck et al 2005 as the relative contribution of magma and faults to plate divergence m is smaller for a given volume of melt per increment of plate separation if the plate is thicker we therefore propose that m does not fully explain the variability in faulting styles observed at slow and ultraslow ridges and propose that rheological changes induced by magma also play a key role melt itself is weak hydrothermally altered gabbro peridotite mixtures are weak and melt heat sustains more vigorous hydrothermal circulation resulting in contrasted potentials for strain localization footwall flexure on faults and the development of detachment faults

how long can a hydrothermal system be sustained by a single intrusive event published numerical calculations show that the convective systems which produce near surface geothermal systems or ore deposits are likely to cool their intrusion heat source in at most a few tens of thousands of years even if the intrusion is large generally hydrothermal circulation near surface geothermal activity and magmatic intrusion should be closely tied in time and space long lived approximately 1 m y hydrothermal activity normally suggests multiple pulses of intrusion with associated pulses of hydrothermal circulation however if the permeability of the intruded environment is just above that which allows convection and the volume of the intrusion is very large calculations presented here show that a single episode of intrusion can sustain hydrothermal circulation and near surface geothermal activity for approximately 800 000 yr for a deep sill heat source these unusually long lived hydrothermal systems can vent through a few very widely separated discharge sites and have the potential to produce isolated massive sulfide deposits of unusually large size

constraints on water depth of massive sulfide formation evidence from modern seafloor hydrothermal systems in arc related settings the results of three decades of seafloor research provide the most reliable information on the importance of water depth in massive sulfide formation available data from over 130 occurrences show that water depths of seafloor vent sites vary with plate tectonic setting and the regional magmatic and volcanic environment the shallowest hydrothermal systems in subduction related settings are hosted by arc volcanoes these shallow vent sites have a number of features in common with subaerial epithermal systems massive sulfide occurrences in arc related rifts the most likely setting for many ancient analogs are generally restricted to water depths from 700 to 2 000 m with rifts developing within old arc crust at the deeper end of this range back arc spreading centers proximal to arcs host massive sulfide deposits at depths of 1 500 to 2 000 m the deepest hydrothermal systems occur along mature back arc spreading centers distal to volcanic arcs where water depths range from 2 000 to 3 700 m these deeper vent sites probably represent the best modern analogues of ophiolite hosted massive sulfide deposits boiling of the hydrothermal fluids is common at volcanic arcs and in arc related rifts in these environments elevated magmatic gas contents of the hydrothermal fluids can contribute to the widespread occurrence of phase separation and associated gas loss by contrast the high ambient pressures in deep marine hydrothermal systems along mature back arc spreading centers prevent fluids from boiling during their ascent to the seafloor boiling controls the maximum temperature at which hydrothermal fluids discharge at the seafloor and therefore influences the metal content of seafloor sulfide deposits copper rich massive sulfides typically occur at water depths exceeding 1 000 m whereas zn and pb rich occurrences may form at any water depth boiling can be an important control on ag and au grades but is not the only factor controlling precious metal enrichment in massive sulfides shallow marine hot spring deposits can be highly enriched in trace metals such as as hg and sb submarine volcanic arc and back arc settings are geologically complex and significant variations in water depth can occur over short distances paleoenvironmental reconstruction of these environments in ancient volcanic terranes is hampered by the lack of unequivocal volcanological or sedimentological criteria that indicate water depth the relationships established here using modern seafloor observations provide important constraints on the paleoenvironmental setting of ancient volcanic hosted massive sulfide deposits

magmatic and tectonic extension at mid ocean ridges 2 origin of axial morphology 1 we investigate the origin of mid ocean ridge morphology with numerical models that successfully predict axial topographic highs axial valleys and the transition between the two the models are time dependent simulating alternating tectonic and magmatic periods where far field extension is accommodated by faulting and by magmatism respectively during tectonic phases models predict faults to grow on either side of the ridge axis and axial height to decrease during magmatic phases models simulate magmatic extension by allowing the axial lithosphere to open freely in response to extension results show that fault size and spacing decreases with increasing time fraction spent in the magmatic phase fm magmatic phases also simulate the growth of topography in response to local buoyancy forces the fundamental variable that controls the transition between axial highs and valleys is the rise sink ratio fm ft t m where fm ft is the ratio of the time spent in the magmatic and tectonic periods and t m is the ratio of the characteristic rates for growing topography during magmatic phases 1 m and for reducing topography during tectonic phases 1 t models predict the tallest axial highs when fm ft t m 1 faulted topography without a high or valley when fm ft t m 1 and the deepest median valleys when fm ft m t 1 new scaling laws explain a global negative correlation between axial topography and lithosphere thickness as measured by the depths of axial magma lenses and microearthquakes exceptions to this trend reveal the importance of other behaviors such as a predicted inverse relation between axial topography and spreading rate as evident along the lau spreading center still other factors related to the frequency and spatial pervasiveness of magmatic intrusions and eruptions as evident at the mid atlantic and juan de fuca ridges influence the rise sink ratio fm ft t m and thus axial morphology

three dimensional numerical simulation of the hydrothermal system within tag like sulfide mounds a 3 d finite element algorithm with incorporated explicit pipe like elements is developed to solve the coupled time dependent heat transport and fluid flow differential equations and yield the internal temperature and fluid velocity fields for tag like mounds a range of models with permeable and impermeable upper boundaries and different thermal conductivities are investigated the solutions have three common features a central high temperature zone which coincides with a non magnetic zone inferred from magnetic data fluid upflow along the vertical high permeability column surrounding a central pipe and a maximum fluid velocity of the order of the observed maximum value at the central black smokers thermal conductivity variations strongly influence the temperature distribution but not the fluid velocity pattern

dynamics of sheared convective boundary layer entrainment part i methodological background and large eddy simulations abstract the reported study examines the dynamics of entrainment and its effects on the evolution of the dry atmospheric convective boundary layer cbl when wind shear is present the sheared cbl can be studied by means of direct measurements in the atmosphere laboratory studies and numerical techniques the advantages and disadvantages of each technique are discussed in the present paper which also describes the methodological background for studying the dynamics of entrainment in sheared cbls for the reported study large eddy simulation les was chosen as the primary method of convective entrainment investigation twenty four les runs were conducted for cbls growing under varying conditions of surface buoyancy flux free atmospheric stratification and wind shear the simulations were divided into three categories cbl with no mean wind ns cbl with a height constant geostrophic wind of 20 m s 1 gc and cbl with geostrophic wind shear gs in the simulated cases the sheared cbls grew fastes

mineral deposits of canada a synthesis of major deposit types district metallogeny the evolution of geological provinces and exploration methods wayne d goodfellow editor 1 068 pp geological association of canada mineral deposits division special publication no 5 2007 hardcover isbn 13 978 1 897095 24 9 price members cdn 60 00 non members cdn 80 00 following in the footsteps of other landmark reviews of major ore producing countries such as australia hughes 1990 solomon and groves 1994 and south africa anhaeusser and maske 1986 this substantial 1 000 pages monograph is a great achievement that should be in every geologic research library in the world in many ways it sets a new standard most notably in that all figures and photographs are in color and that the entire volume is available in electronic form both as an included dvd and mostly on the gac website it is common in such compendia to have a spectacular cover photo or frontispiece to catch the reader s attention and this book does not disappoint the frontispiece photo is of a 61 8 kg sample from the dome mine timmins ontario and is stated to contain 20 wt percent gold at today s prices the gold alone would be worth 1 5 million dollars

mariana blueschist mud volcanism implications for conditions within the subduction zone several recently discovered active mud volcanoes on the nonaccretionary mariana convergent plate margin are erupting slab derived fluids serpentine mud and metamorphosed rocks from depths of as great as 25 km blueschist materials from the metamorphosed subducted plate are contained in the muds pore fluids indicate a depth dependence for decarbonation the mud volcanoes record in situ conditions along the decollement including pressure and temperature conditions and physical properties within the subduction zone similar mud flow material occurs worldwide as sedimentary serpentinite deposits in accreted fragments of former convergent margins making this kind of mud volcanism a more important phenomenon in convergent margins than previously recognized

geological analysis of aeromagnetic data from southwestern alaska implications for exploration in the area of the pebble porphyry cu au mo deposit aeromagnetic data are used to better understand the geology and mineral resources near the late cretaceous pebble porphyry cu au mo deposit in southwestern alaska the reduced to pole rtp transformation of regional scale aeromagnetic data shows that the pebble deposit is within a cluster of magnetic anomaly highs similar to pebble the iliamna kijik and neacola porphyry copper occurrences are in magnetic highs that trend northeast along the crustal scale lake clark fault a high amplitude short to moderate wavelength anomaly is centered over the kemuk occurrence an alaska type ultramafic complex similar anomalies are found west and north of kemuk a moderate amplitude moderate wavelength magnetic low surrounded by a moderate amplitude short wavelength magnetic high is associated with the gold bearing shotgun intrusive complex the rtp transformation of the district scale aeromagnetic data acquired over pebble permits differentiation of a variety of jurassic to tertiary magmatic rock suites jurassic cretaceous basalt and gabbro units and late cretaceous biotite pyroxenite and granodiorite rocks produce magnetic highs tertiary basalt units also produce magnetic highs but appear to be volumetrically minor eocene monzonite units have associated magnetic lows the rtp data do not suggest a magnetite rich hydrothermal system at the pebble deposit the 10 km upward continuation transformation of the regional scale data shows a linear northeast trend of magnetic anomaly highs these anomalies are spatially correlated with late cretaceous igneous rocks and in the pebble district are centered over the granodiorite rocks genetically related to porphyry copper systems the spacing of these anomalies is similar to patterns shown by the numerous porphyry copper deposits in northern chile these anomalies are interpreted to reflect a late cretaceous magmatic arc that is favorable for additional discoveries of late cretaceous porphyry copper systems in southwestern alaska

structural analysis of aeromagnetic data from the rainy river block wabigoon subprovince minnesota usa and ontario canada strain partitioning along a neoarchean terrane boundary and implications for mineral exploration abstract high resolution aeromagnetic data are used to constrain the structural architecture and mineral potential of the neoarchean rainy river block rrb a sigma shaped wedge of till covered volcano plutonic rocks that form the southern margin the wabigoon subprovince and span the international border between northern minnesota usa and western ontario canada reduced to pole transformed upward continued and derivative enhanced aeromagnetic data show that the rrb can be divided into three structural domains the rainy lake wrench zone rlwz border zone bz and oaks belt ob strain intensity decreased from east strongly deformed rlwz to west moderately deformed ob the rlwz experienced superimposed folding that resulted in complex rock geometries and sigmoidal patterns in the aeromagnetic data a large northeast trending syncline partially represented by an aeromagnetic low that is cut by similarly oriented shear zones and faults comprises the bz the ob is interpreted to occupy a regional scale strain shadow and is composed of a relatively well preserved and magnetically heterogeneous volcanic pile and subvolcanic intrusion that have been overturned and are steeply north dipping as the rrb is fault bounded terrane boundary and hosts numerous mafic intrusions it was a regional zone of synvolcanic and syntectonic paleo permeability and therefore has potential for hosting orogenic gold and volcanogenic massive sulfide vms deposits however linear fault geometries throughout most of the rrb were not conducive to formation of large orogenic gold deposits and pervasive deformation in the rlwz may have compromised preservation of hypogene vms mineralized zones thus the structurally preserved ob is the most prospective area for hosting vms deposits in the rrb whereas the bz and surrounding rocks are prospective for hosting orogenic gold deposits near a 30 km long restraining bend in the quetico fault

strain in an archean greenstone belt of minnesota abstract we measured strain at more than 60 locations in metasedimentary and metavolcanic rocks of the vermilion district an e w trending archean greenstone belt in minnesota strain ellipsoid orientations and shapes correlate strongly with n s location in the belt but magnitudes do not flattening strains occur near the present vermilion fault which bounds the greenstone belt to the north with constrictional strains to the south the observed strain patterns can be mathematically modeled by deformation paths which produce the flattening strains with west plunging 1 axes by dextral shear of the constrictional strains with east plunging 1 axes using reasonable geologic constraints the shear plane must dip to the north with a subhorizontal shear direction structures throughout the district also indicate dextral shear a geometrical finite element program uses the measured strains to destrain the rocks and find the configuration which most closely satisfies strain compatibility equations the linear e w strain patterns and minor rotations about horizontal axes during the deformation preclude origin of the greenstone belt by infolding and shear off the flanks of a rising granitic diapir by accounting for rotations which result in the deformed curvature of the original surface a true estimate of 50 n s shortening across the belt can be made the data and deformation models favor the origin of the vermilion district rocks at a convergent margin most likely as a n dipping subduction zone complex with shallow slab dip the origin of the constrictional strains remains enigmatic

tectonic versus magmatic extension in the presence of core complexes at slow spreading ridges from a visualization of faulted seafloor topography we develop a forward model of the generation of faulted seafloor topography visualization to estimate the relative roles of tectonic and magmatic extension in the presence of core complexes at slow spreading ridges the visualization assumes flexural rotation of 60 normal faults a constant effective elastic thickness te of young lithosphere and a continuous infill of the depressed hanging wall by lava flowing from the spreading axis we obtain a new estimate of te 0 5 1 km from the shapes of the toes of 6 well documented oceanic core complexes we model an 80 km long bathymetric profile in the equatorial atlantic across a core complex and the ridge axis at 13 20 n and estimate the variation in tectonic extension which yields the variation in the fraction of upper crust extension m by magmatic diking at the ridge axis core complex formation appears to be stable for all values of m 0 5 the visualization shows how gabbro emplaced at the base of the lithosphere during extension by magmatic diking is partitioned to each side of the spreading axis and predicts a high probability of finding gabbros in the domes of core complexes

widespread active detachment faulting and core complex formation near 13 n on the mid atlantic ridge oceanic core complexes are massifs in which lower crustal and upper mantle rocks are exposed at the sea floor1 2 3 they form at mid ocean ridges through slip on detachment faults rooted below the spreading axis2 3 4 5 6 to date most studies of core complexes have been based on isolated inactive massifs that have spread away from ridge axes here we present a survey of the mid atlantic ridge near 13 n containing a segment in which a number of linked detachment faults extend for 75 km along one flank of the spreading axis the detachment faults are apparently all currently active and at various stages of development a field of extinct core complexes extends away from the axis for at least 100 km our observations reveal the topographic characteristics of actively forming core complexes and their evolution from initiation within the axial valley floor to maturity and eventual inactivity within the surrounding region there is a strong correlation between detachment fault morphology at the ridge axis and high rates of hydroacoustically recorded earthquake seismicity preliminary examination of seismicity and seafloor morphology farther north along the mid atlantic ridge suggests that active detachment faulting is occurring in many segments and that detachment faulting is more important in the generation of ocean crust at this slow spreading ridge than previously suspected

the bousquet 2 dumagami world class archean au rich volcanogenic massive sulfide deposit abitibi quebec metamorphosed submarine advanced argillic alteration footprint and genesis the archean bousquet 2 dumagami deposit is an au rich volcanogenic massive sulfide deposit vms with a total production of 3 87 moz au 2 77 moz ag 80 000 metric tons t cu and 5 000 t zn the deposit is located within the doyon bousquet laronde mining camp in northwestern quebec and hosted by the 2704 to 2695 ma blake river group the world s most productive volcanic assemblage for au rich vms deposits the bousquet 2 dumagami deposit consists of stacked deformed and transposed semimassive to massive pyrite rich lenses breccia zones and associated sulfide veins and stringer zones hosted by the upper member of the bousquet formation 50 to 100 m stratigraphically below the 2687 ma cadillac group sedimentary rocks the main ore zone is known as the massive hangingwall zone at the bousquet 2 mine and zone 5 at the dumagami mine another semimassive to disseminated pyrite rich auriferous zone with coarsely recrystallized massive pyrite is present in the footwall massive footwall zone the massive hangingwall zone is an au ag cu zn sheet like semimassive to massive pyrite rich sulfide lens intermixed with vein and breccia zones the dominant ore type consists of au cu mineralization but its upper and eastern parts are enriched in zn the ore consists of a complex assemblage of sulfides sulfosalts and native gold including abundant pyrite sphalerite a few percent of chalcopyrite bornite and galena with some visible gold the massive hangingwall zone was formed by subsea floor replacement of footwall calc alkaline dacitic volcaniclastic rocks and hanging wall blue quartz phyric rhyolite despite significant north south shortening and metamorphism which was responsible for transposition flattening folding and recrystallization mineralogical gradients related to alteration induced compositional variations can still be identified a number of different metamorphic mineral assemblages can be mapped over several tens of meters from distal to proximal to the ore 1 quartz muscovite mn garnet biotite chlorite 2 quartz muscovite pyrite 3 quartz muscovite andalusite pyrophyllite pyrite with topaz and diaspore and 4 massive quartz pyrite a quartz carbonate biotite assemblage occurs in the hanging wall of the zn rich massive hangingwall zone and is hosted by andesitic sills the thickness of each of these assemblages varies from a few meters to tens of meters all metamorphosed alteration assemblages are characterized by strong progressive na2o depletion gains in mno fe2o3 total mgo and cao are recorded in the quartz muscovite mngarnet biotite chlorite assemblage whereas gains in k2o and losses in cao occur in the quartz muscovite pyrite assemblage in the quartz muscovite andalusite pyrophyllite pyrite and the proximal massive quartzpyrite assemblages all oxides except sio2 fe2o3 total and tio2 were strongly to almost entirely leached the andalusite kyanite pyrophyllite bearing aluminous assemblages are interpreted to represent metamorphosed equivalents of synvolcanic alteration produced by acidic and oxidizing hydrothermal fluids i e metamorphosed advanced argillic style alteration whereas the massive quartz pyrite assemblage is similar to the massive silicic alteration commonly associated with advanced argillic alteration the timing of au mineralization is considered to be close to the age of the host rhyolite 2697 8 1 ma and the age of the overlying felsic volcanic rocks 2697 5 1 1 ma the major au endowment of the doyon bousquet laronde mining camp may be related to favorable source rock or au reservoirs specific to the lower crust or upper mantle beneath the eastern archean blake river group exploration for additional au rich vms in this environment should focus on distal quartz and mn rich garnet biotite and proximal aluminous assemblages with anomalously high au and or cu and zn in intermediate to felsic transitional to calc alkaline volcanic or volcaniclastic rocks located underneath a younger sedimentary cover

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

paleomagnetic evidence of large footwall rotations associated with low angle faults at the mid atlantic ridge exposures of gabbros and mantle derived peridotites at slow spreading oceanic ridges have been attributed to extension on long lived low angle detachment faults similar to those described in continental metamorphic core complexes in continental settings such detachments have been interpreted as having originated and remained active at shallow dips alternatively currently shallow dipping fault surfaces may have originated at moderate to steep dips and been fl attened by subsequent fl exure and isostatic uplift while the latter interpretation would be more con sistent with andersonian faulting theory it predicts large footwall tilts that have not been observed in continental detachment faults here we use the magnetization of oceanic gabbro and peridotite samples exposed near the fifteen twenty fracture zone on the mid atlantic ridge to demonstrate that substantial footwall rotations have occurred widespread rotations ranging from 50 to 80 indicate that original fault orientations dipped steeply toward the spreading axis

the s reflector west of galicia the seismic signature of a detachment fault the so called s reflector is a group of bright continuous reflections underlying the landward tilted fault blocks of the west galicia rifted margin s has been interpreted as the brittle ductile transition the lop of an intrusion a detachment fault and the crust mantle boundary to constrain the internal structure of the reflector we have carried out detailed analyses of these reflections we compare the waveforms of the seafloor reflection and its first multiple both to determine the amplitude of the seafloor reflection and to show hat the seafloor is in effect a spike in the reflectivity series so that the seafloor reflection can be used as the far field wavelet including both source and receiver ghosts we compare he waveform of the seafloor and 5 and show that within the resolution of our data s is a reflection from a step increase in acoustic impedance this result is confirmed through complex trace analysis and in particular the determination of the apparent polarity of s and the examination of the instantaneous frequency function s is consistently positive polarity and shows no significant frequency anomaly simple modelling shows that s is very unlikely to be a reflection from a thin layer we thus conclude that s is probably a single steplike interface from the varying frequency content of the data we determine a value for the effective q between s and the seafloor and use this to assess the amplitude loss due to attenuation and scattering we use a comparison between the seafloor and the s reflection to constrain the amplitude of s and estimate a reflection coefficient for s of at least 0 2 in places decreasing landwards by analogy with structures developed in the highly extended regions of the western united states we consider that the most likely interpretation of s is as a sharp west dipping detachment fault separating a granitic upper plate from a higher velocity lower plate locally probably serpentinized mantle

protracted construction of gabbroic crust at a slow spreading ridge constraints from 206pb 238u zircon ages from atlantis massif and iodp hole u1309d 30 n mar sensitive high resolution ion microprobe shrimp u pb zircon ages of 24 samples from oceanic crust recovered in integrated ocean drilling program iodp hole u1309d and from the surface of atlantis massif mid atlantic ridge mar 30 n document a protracted history of accretion in the footwall to an oceanic detachment fault ages for 18 samples of evolved fe ti oxide gabbro and felsic dikes collected 40 1415 m below seafloor in u1309d yield a weighted mean of 1 20 0 03 ma mean square of weighted deviates 7 1 however the ages range from 1 08 0 07 ma and 1 28 0 05 ma indicating crustal construction occurred over a minimum of 100 200 ka the zircon ages along with petrologic observations indicate at least 2 major periods of intrusive activity with age peaks separated by 70 ka the oldest ages are observed below 600 mbsf an observation inconsistent with models requiring constant depth melt intrusion beneath a detachment fault the data are most consistent with a multiple sill model whereby sills intrude at random depths below the ridge axis over a length scale greater than 1 4 km zircon ages from broadly spaced samples collected along the southern ridge of atlantis massif yield a detachment fault slip rate of 28 7 6 7 mm a and imply significant asymmetric plate spreading up to 100 on the north american plate for at least 200 ka during core complex formation

evidence from gravity data for focused magmatic accretion along the mid atlantic ridge gravity data from the mid atlantic ridge between latitudes 27 50 n and 30 40 n show that the accretion of magma at the ridge is focused at discrete centres along the spreading axis large positive gravity anomalies equivalent to reductions of almost 50 in crustal thickness are observed over non transform discontinuities bounding spreading segments

three dimensional structure of oceanic core complexes effects on gravity signature and ridge flank morphology mid atlantic ridge 30 n 1 our gravity modeling of oceanic core complexes formed at the mid atlantic ridge near 30 n suggests that their shallow domal cores could be dominated by mafic intrusive rocks consistent with recent drilling results at atlantis massif the three dimensional gravity analysis incorporates additional underway geophysics data in a new compilation and uses a higher resolution bathymetry model to remove the gravity contribution of seafloor topography the additional detail is required in order to confidently relate few kilometer scale gravity anomalies to specific morphologic tectonic blocks different models of subseafloor core complex structure and density are tested to determine which minimizes the local gravity anomaly a 3 d core with density 2900 kg m3 as measured in the gabbroic section drilled at the central dome and juxtaposed 3 d hanging wall of fractured basalt density 2600 kg m3 satisfactorily explains most of the bouguer gravity anomaly at atlantis massif the capping detachment fault terminates or plunges northward beneath the seafloor at the northern limit of the central dome the southwest shoulder of the massif has lower density consistent with an upper crustal section 1 km thick whereas the summit and southeastern shoulder have overall density similar to the central dome the older core complexes distributed along atlantis fracture zone are similar in size depth and distance of their summit from the transform fault however weathering alteration probably has reduced their density somewhat compared to atlantis massif bathymetric embayments occur adjacent to the fracture zone in several places on the ridge flanks and are consistently associated with core complexes

extreme crustal thinning in the south porcupine basin and the nature of the porcupine median high implications for the formation of non volcanic rifted margins the southern porcupine basin is characterized by axial stretching factors that are greater than six and typical of rifted margins as such the basin can be regarded as a natural laboratory to investigate the evolution and symmetry of rifting leading towards continental separation and breakup a bright reflection here named p cuts down to the west from the base of the sedimentary section is overlain by small fault blocks and appears to represent a detachment fault p may in part follow the top of partially serpentinized mantle this interpretation is consistent with gravity modelling and with numerical models of crustal embrittlement and mantle serpentinization during extension furthermore p closely resembles the s reflection west of iberia where such serpentinites are well documented although overall the basin remains symmetrical the consistent westward structural dip of the detachment implies that at high stretching factors extension became asymmetric farther south the porcupine median high appearing lens shaped in cross section overlies the tilted fault blocks and is onlapped by postrift sediment despite no evidence for synrift magmatism this high has previously been interpreted as a basaltic structure however it develops above the line of intersection of the crust mantle boundary with the p detachment and hence may be related to the spatial limit of serpentinization the median high may represent a serpentinite mud volcano or diapir we suggest that such structures produce the serpentinite breccias found within the rifted continent ocean transition of nonvolcanic margins

entrainment results including advection and case studies from the flatland boundary layer experiments boundary layer top entrainment measurements from the 1996 flatland boundary layer experiment flatland96 were presented by angevine et al 1998a at that time no reliable estimates of temperature advection which could affect the heat budget approach used were available this paper presents the entrainment results recalculated with advection estimated from the national centers for environmental prediction mesoscale eta numerical model estimates of the contribution to entrainment by mechanical turbulence shear and of the shear free entrainment ratio are also calculated a number of case study examples are also presented to elucidate further the complex interactions that govern entrainment accounting for advection in the budgets results in a slightly smaller estimate of the entrainment ratio a r the mean a r is 0 47 0 11 and the median is 0 40 these results are still somewhat larger than long accepted values and comparable to results from some other studies in comparable conditions the experiments took place in the area near the flatland atmospheric observatory near champaign urbana illinois in june august 1996 the site is interesting because it is extraordinarily flat has uniform land use and is situated in a prime agricultural area

controls on the seafloor exposure of detachment fault surfaces highlights a small fraction of corrugated detachment fault surfaces is eventually exposed at the seafloor seafloor slopes indicate effective friction of 0 2 on shallow part of detachments moderate offset detachment faults may be largely blanketed by hanging wall material seafloor shaping processes profoundly alter the morphology of oceanic core complexes abstract while oceanic detachment faults have been proposed to account for the accretion of 40 of new seafloor in the north atlantic ocean clear exposures of large offset often corrugated fault surfaces remain scarce and spatially limited to help resolve this paradox we examine the conditions under which detachment fault growth may or may not lead to extensive exposure of corrugated fault planes at the seafloor using high resolution bathymetry from four detachment faults at the northern mid atlantic ridge we investigate the rafting of hanging wall derived debris over emerging fault scarps which can lead to covering shallow dipping corrugated fault surfaces we model this process using critical taper theory and infer low effective friction coefficients 0 2 on the shallowest portion of detachment faults a corollary to this result is that detachments emerging from the seafloor at angles 13 are more likely to become blanketed under an apron of hanging wall material we generalize these findings as a simple model for the progressive exposure and flexural rotation of detachment footwalls which accounts for the continued action of seafloor shaping processes our model suggests that many moderate offset hidden detachment faults may exist along slow mid ocean ridges and do not feature an exposed fault surface

geologic interpretation of aeromagnetic and chemical data from the oaks belt wabigoon subprovince minnesota implications for au rich vms deposit exploration the oaks belt ob is a neoarchean volcanic complex located in northwestern minnesota usa it is part of the wabigoon granite greenstone terrane that hosts the world class rainy river gold deposit

effects of axially variable diking rates on faulting at slow spreading mid ocean ridges abstract magma supply for dike injection can be highly variable within a segment of a slow spreading mid ocean ridge but the tectonic impact of this variability is not fully understood here we use three dimensional numerical models to quantify the effects of variable diking rates on the faulting mode at a 20 km long slow spreading ridge segment in addition to end member faulting modes in which long lived detachment faults or short lived normal faults form along the whole segment we newly identify a transitional mode in which a detachment and a short lived normal fault form simultaneously but in respective domains separated by a transfer fault different faulting modes can be better correlated with the average dike intrusion rate rather than the highest or lowest rate along the segment along axis stress coupling tends to homogenize fault offset along the segment inhibiting the domination of a particular faulting mode associated with an extreme local diking rate this homogenizing effect explains why detachment faults can sometimes form even in the regions previously considered as unfavorable our results further suggest that a long 15 km and continuous detachment partially overlain by younger faults can create an oceanic core complex when faults weaken fast and diking rate is low when faults weaken slow and diking rate is moderate however faulting occurs in the transitional mode producing a detachment over only a part of the segment length

direct geological evidence for oceanic detachment faulting the mid atlantic ridge 15 45 n from a detailed survey and sampling study of corrugated massifs north of the fifteen twenty fracture zone on the mid atlantic ridge we demonstrate that their surfaces are low angle detachment fault planes as proposed but not previously verified spreading direction parallel striations on the massifs occur at wavelengths from kilometers to centimeters oriented drill core samples from the striated surfaces are dominated by fault rocks with low angle shear planes and highly deformed greenschist facies assemblages that include talc chlorite tremolite and serpentine deformation was very localized and occurred in the brittle regime no evidence is seen for ductile deformation of the footwall synkinematic emplacement of diabase dikes into the fault zone from an immediately subjacent gabbro pluton implies that the detachment must have been active as a low angle fault surface at very shallow levels directly beneath the ridge axis strain localization occurred in response to the weakening of a range of hydrous secondary minerals at a very early stage and was highly efficient

strain analysis and rheology contrasts in polymictic conglomerates an example from the seine metaconglomerates superior province canada deformed conglomerates typically display contrasting strains between different lithological clast populations and matrix which can be exploited to provide information about effective viscosity contrasts we analyzed three different clast populations granitoid mafic volcanic and felsic volcanic across a strain gradient in the seine metaconglomerates of ontario canada we calculated finite strain using the rf 4 method and bootstrap statistics granitoid clasts exhibited the least amount of strain while the mafic and felsic clasts had similar greater strains we assumed that the bulk finite strain was recorded by the mafic clasts given their large proportion of the rock and their similar composition to the matrix we calculated effective viscosity contrasts using the finite strain information assuming newtonian viscous rheology the two volcanic clast populations have similar effective viscosities regardless of strain magnitude with ratios of felsic mafic ranging from 0 27 to 2 12 and average ratio of 1 19 the granitoid clasts are significantly more competent than the mafic clasts with effective viscosity ratios ranging from 2 35 to 12 39 and average ratio of 5 61 the results are consistent with a qualitative

testing models for obliquely plunging lineations in transpression a natural example and theoretical discussion theory predicts that stretching lineations in an ideal vertical transpressional zone should be either vertical or horizontal many field descriptions of transpressional zones however indicate a range of lineation orientations between these extremes several theoretical models have been developed to explain such departures from expected lineation orientation and we discuss these in the context of a field example from the archean superior province in the north american craton existing models are insufficient to explain obliquely plunging lineations in this example because 1 obliquely plunging lineations cannot be accounted for by shear zone boundary effects imposed by a no slip condition 2 foliations and lineations vary independently 3 the vorticity normal section is subhorizontal limiting possibilities for inclined simple shear 4 high vorticity is needed for finite strains and lineations to match previously proposed triclinic models but vorticity is relatively low and 5 juxtaposed east and west plunging lineations are unlikely in the previously proposed triclinic models because existing theoretical models are not applicable to our field example we contemplate a new model to explain obliquely plunging lineations within quasi homogeneous transpression

seismicity of the atlantis massif detachment fault 30 n at the mid atlantic ridge 1 at the oceanic core complex that forms the atlantis massif at 30 n on the mid atlantic ridge slip along the detachment fault for the last 1 5 2 ma has brought lower crust and mantle rocks to the seafloor hydroacoustic data collected between 1999 and 2003 suggest that seismicity occurred near the top of the massif mostly on the southeastern section while detected seismicity along the adjacent ridge axis was sparse in 2005 five short period ocean bottom seismographs obs were deployed on and around the massif as a pilot experiment to help constrain the distribution of seismicity in this region analysis of six months of obs data indicates that in contrast to the results of the earlier hydroacoustic study the vast majority of the seismicity is located within the axial valley during the obs deployment and within the array seismicity was primarily composed of a relatively constant background rate and two large aftershock sequences that included 5 teleseismic events with magnitudes between 4 0 and 4 5 the aftershock sequences were located on the western side of the axial valley adjacent to the atlantis massif and close to the ridge transform intersection they follow omori s law and constitute more than half of the detected earthquakes the obs data also indicate a low but persistent level of seismicity associated with active faulting within the atlantis massif in the same region as the hydroacoustically detected seismicity within the massif the data indicate a north south striking normal fault and a left lateral strike slip fault near a prominent transform parallel north facing scarp both features could be explained by changes in the stress field at the inside corner associated with weak coupling on the atlantis transform alternatively the normal faulting within the massif might indicate deformation of the detachment surface as it rolls over to near horizontal from an initial dip of about 60 beneath the axis and the strike slip events may indicate transform parallel movement on adjacent detachment surfaces

geologic continuous casting below continental and deep sea detachment faults and at the striated extrusion of sacsayhuaman peru in one common type of industrial continuous casting partially molten metal is extruded from a vessel through a shaped orifice called a mold in which the metal assumes the cross sectional form of the mold as it cools and solidifies continuous casting can be sustained as long as molten metal is supplied and thermal conditions are maintained i propose that a similar process produced parallel sets of grooves in three geologic settings as follows 1 corrugated metamorphic core complexes where mylonitized mid crustal rocks were exhumed by movement along low angle normal faults known as detachment faults 2 corrugated submarine surfaces where ultramafic and mafic rocks were exhumed by normal faulting within oceanic spreading centers and 3 striated magma extrusions exemplified by the famous grooved outcrops at the inca fortress of sacsayhuaman in peru in each case rocks inferred to have overlain the corrugated surface during corrugation genesis molded and shaped a plastic to partially molten rock mass as it was extruded from a moderate to high temperature reservoir

controls on the magmatic fraction of extension at mid ocean ridges abstract the fraction m of plate separation accommodated by magma emplacement at mid ocean ridges has been recognized as the main control on seafloor spreading modes yet the factors that control m itself are poorly understood here we put forward a simple theoretical framework explaining m in terms of short term cycles of earthquakes and dike intrusions interacting with one another by modulating the stress state of the ridge axis axial lithospheric thickness and the rate of pressure build up in shallow replenishing magma sills are the two main parameters controlling m in our simulations combined with plausible scenarios for the increase of pressure build up rate and the decrease of lithospheric thickness with increasing spreading rate our model appropriately brackets available measurements of m from slow intermediate and fast spreading ridges our model further suggests that the transitions between major modes of seafloor spreading detachment faulting symmetric faulting and fully magmatic correspond to thresholds in axial lithospheric thickness and that the great variability in m at slow and ultraslow ridges directly reflects along axis variability in thermal structure more generally this implies that the balance between bottom up magmatic heating and top down hydrothermal cooling fully determines the time averaged rate of magmatic intrusions in the brittle lithosphere and thus the modes of mid ocean ridge faulting which shape 2 3 of earth s surface

geophysical signatures of oceanic core complexes author posting the authors 2009 this article is posted here by permission of john wiley sons for personal use not for redistribution the definitive version was published in geophysical journal international 178 2009 593 613 doi 10 1111 j 1365 246x 2009 04184 x

detachment faulting at ocean core complexes intracrustal reflections from a cretaceous inside corner are interpreted as a detachment fault that dips gently within the basement and can be followed up the flank of a domed high the fault heave is sufficient for ridge normal extension to have exhumed deep crustal and probably mantle rocks as sampled at some inside corner domed massifs the morphology of these massifs closely resembles that of the cretaceous high implying that the structures are analogous and supporting the notion that modern inside corner massifs are oceanic core complexes formed by unroofing the footwalls of similar detachment faults partially exposed as corrugated slip surfaces this is the first demonstration of the overall geometry of an oceanic core complex detachment fault

crustal thickness on the mid atlantic ridge bull s eye gravity anomalies and focused accretion spreading segments of the mid atlantic ridge show negative bull9s eye anomalies in the mantle bouguer gravity field seismic refraction results from 33 s indicate that these anomalies can be accounted for by variations in crustal thickness along a segment the crust is thicker in the center and thinner at the end of the spreading segment and these changes are attributable to variations in the thickness of layer 3 the results show that accretion is focused at a slow spreading ridge that axial valley depth reflects the thickness of the underlying crust and that along axis density variations should be considered in the interpretation of gravity data

along and across axis variations in crustal thickness and structure at the mid atlantic ridge at 5 s obtained from wide angle seismic tomography implications for ridge segmentation two end member styles of crustal accretion are observed at two adjacent spreading segments at the mid atlantic ridge at 5 s focused accretion to the segment center with rapid crustal thinning toward the transform in the northern segment and crustal thickening toward the transform at an oceanic core complex in the southern segment our results were obtained by tomographic inversion of wide angle seismic reflection and refraction data collected along three intersecting profiles the segment north of the 5 s fracture zone is characterized by a well developed median valley with a pronounced seafloor bulge in the segment center a discrete portion of anomalously low velocities 0 4 to 0 5 km s relative to average off axis structure at depths of 2 5 km beneath this bulge is possibly related to the presence of elevated temperatures and perhaps small portions of partial melt this suggests that this segment is currently in a magmatically active period which is confirmed by the observation of fresh lava flows and ongoing high temperature hydrothermal activity at the seafloor close to the current spreading axis the crust thins rapidly from 8 5 km beneath the segment center to less than 3 km beneath the transform fault which indicates that melt supply here is strongly focused to the segment center the reduction in crustal thickness is almost exclusively accommodated by the thinning of velocity portions indicative of seismic layer 3 the transform fault is characterized by more uniform velocity gradients throughout the entire crustal section and very low upper mantle velocities of 7 2 7 3 km s indicating that serpentinization could be as much as 25 at 3 5 km depth in contrast 4 1 ma old crust of the northern segment shows only minor thinning from the segment center toward the segment end here the transform is characterized by a normal seismic layer 2 3 transition suggesting robust melt supply to the segment end at those times in the adjacent southern segment the crust thickens from 2 5 km beneath the flank of an oceanic core complex to 5 0 km at the segment boundary the observed changes in crustal thickness show a significant temporal and lateral variability in melt supply and suggest a more complex crustal emplacement process than predicted by models of focused melt supply to the segment centers